UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS QUÍMICAS DEPARTAMENTO DE QUÍMICA ORGÁNICA I © Valentina Sacchetti, 2018 TESIS DOCTORAL Covalent and supramolecular wires in the search for electrical and thermoelectrical properties Cables covalentes y supremoleculares en la búsqueda de propiedades eléctricas y termoeléctricas MEMORIA PARA OPTAR AL GRADO DE DOCTORA PRESENTADA POR Valentina Sacchetti DIRECTORES Beatriz María illescas Martínez Nazario Martín León Madrid, 2019 UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS QUÍMICAS Departamento de Química Orgánica I COVALENT AND SUPRAMOLECULAR WIRES IN THE SEARCH FOR ELECTRICAL AND THERMOELECTRICAL PROPERTIES CABLES COVALENTES Y SUPRAMOLECULARES EN LA BÚSQUEDA DE PROPIEDADES ELÉCTRICAS Y TERMOELÉCTRICAS TESIS DOCTORAL Valentina Sacchetti Madrid, 2018 COVALENT AND SUPRAMOLECULAR WIRES IN THE SEARCH FOR ELECTRICAL AND THERMOELECTRICAL PROPERTIES CABLES COVALENTES Y SUPRAMOLECULARES EN LA BÚSQUEDA DE PROPIEDADES ELÉCTRICAS Y TERMOELÉCTRICAS Directores: Dra. Beatriz M. Illescas Martínez Dr. Nazario Martín León Memoria que para optar al grado de DOCTOR EN CIENCIAS QUÍMICAS presenta Valentina Sacchetti Madrid, 2018 Valentina Sacchetti benefited from a Marie Curie fellowship as ESR in the EU Marie Curie Initial Training Network (ITN) MOLECULAR- SCALE ELECTRONICS: “MOLESCO” (project Number 606728, from January 1, 2014 until December 31, 2017). She attended special courses and conferences on topics of general European interests presented by international experts: Media Training Workshop, St John’s College, Durham, UK (13-15 July 2016) - Introduction to the Mass Media world - Media materials and their use; - Production planning; - Basics of storytelling structuring a film and storyboarding; - Basic skills to shoot your own short film (tips on how to capture pictures and sound); - Personal short science film shooting. Complementary Skills Training Course, IBM Research – Zürich, Rüschlikon, Switzerland (18-22 April 2016). - Introduction into Intellectual Property; - Eyes on research in firms; - Start-ups and Spin-offs introduction; - Improving communication skills such as, interpersonal relationships, giving talks, writing papers. Training School on Quantum Transport Simulation Techniques,, University of Oviedo, Spain (18-22 May 2015) - Introduction to Tight-Binding scattering Theory: - Hands on: First tight-binding examples; - Cursory introduction to Gollum; - Hands on: Advanced tight-binding examples; - Cursory introduction to DFT, SIESTA for Gollum; - Hands on: First DFT-Gollum examples, Advanced DFT-Gollum examples, Advanced DFT-Gollum examples. Transferable Skills Training Course, University of Durham (16-21 November 2014) - Focus on communication, impact, personal effectiveness, collaborative working, time management and project management skills. Workshop on Molecular Scale Electronics, Muggendorf, Germany. (24-27 September 2017) Topic: Workshop on Single Molecule Electronics and Nanoscale Systems Graphene based Talk: Electronic Properties of Supramolecular Systems and Carbon Nanoforms www.dur.ac.uk/chemistry/molesco/meetings/muggendorf/ MOLESCO meeting, Lancaster, United Kingdom (15-19 May 2017) Topic: Workshop on Molecular Electronics Talk: Wires based on Amidinium-Carboxylate salt-bridgeinteraction: single molecule-junction approach 3rd Annual Meeting and Workshop, Granada, Spain (17-21 January 2017) Topic: Electron Transport at Molecular Level Talk: Graphene Based Nanostructures for Novel Heterojunctions www.dur.ac.uk/chemistry/molesco/meetings/granada17/ Joint MOLESCO-iSwitch Workshop, Freiburg, Germany (13-17 September 2016) http://www.dur.ac.uk/chemistry/molesco/meetings/muggendorf/ http://www.dur.ac.uk/chemistry/molesco/meetings/granada17/ Summer School on the topics: - Chemistry at the single-molecule scale - Electron Transfer versus Electron Transport - Molecular switches and machines Poster: Supramolecular Wires for Molecular Charge Transport www.dur.ac.uk/chemistry/molesco/meetings/iswitch/ “A Journey through Carbon Nanostructures: from Fullerenes to Graphene”, Toledo, Spain (27-28 June 2016) Symposium in Honor of Nazario Martín and Maurizio Prato Poster: Molecular Charge Transport Properties Affected by Medium Features http://www.journeynanostructures.com/ MOLESCO Workshop , IBM Research , Zürich, Rüschlikon, Switzerland (18-22 April 2016) Topic: Single Molecule Conductance. Talk: Amidinium-Carboxylate based Supramolecular Wires www.dur.ac.uk/chemistry/molesco/meetings/workshop2016/ 2nd Annual Meeting, Workshop, and mid-term Review (MTR), Delft, Netherlands (11 - 15 January 2016). Topic: Electric components built from single molecules and mid-term review of the Marie Curie project MOLESCO. Talk and Poster Presentation www.dur.ac.uk/chemistry/molesco/meetings/mtr/ Joint MOLESCO-DFG Network Workshop on Single-Molecule Electronics, Regensburg, Germany, (15-18 September 2015). Topic: Single-Molecule Electronics Talk: Chemically Bound Fullerene-Graphene Carbon Hybrids Poster: Chemically Bound Fullerene-Graphene Carbon Hybrids www.physik.uni- regensburg.de/forschung/gk_carbonano/ws_home_Sep15.phtml http://www.dur.ac.uk/chemistry/molesco/meetings/iswitch/ http://www.journeynanostructures.com/ http://www.dur.ac.uk/chemistry/molesco/meetings/workshop2016/ http://www.dur.ac.uk/chemistry/molesco/meetings/mtr/ http://www.physik.uni-regensburg.de/forschung/gk_carbonano/ws_home_Sep15.phtml http://www.physik.uni-regensburg.de/forschung/gk_carbonano/ws_home_Sep15.phtml ISNA16-16th International Symposium on novel aromatic compounds , Madrid, Spain (6-10 July 2015) Topic: Novel Aromatic Compounds Theory, Synthesis, and Experiments Poster: Solvent Effects on Molecular Charge Transport Properties http://isna2015.com Workshop and Training Course, Oviedo, Spain (18-22 May 2015) Topic: Molecular Electronics and its applications http://www.cinn.es/molesco-workshop-training-course/ 1st Annual Meeting and Joint MOLESCO-SYMONE Workshop, Engelberg, Switzerland, (8-11 February 2015) Topic: Molecular Electronics Poster: Graphene Covalent Functionalization by Fullerene Derivatives www.dur.ac.uk/resources/chemistry/SYMOLESCOAbstractbooklet.pdf International Workshop and MOLESCO Meeting, Konstanz, Germany (Sept 29th - Oct 3rd 2014) Topic: Controlled Charge and Heat Transport at the Molecular Scale Poster: Covalent Chemical Modification of Graphene www.dur.ac.uk/chemistry/molesco/meetings/konstanz/ Kick-Off Meeting and Workshop, Madrid, Spain (March 31st - April 2nd, 2014) Topic: Molecular-Scale Electronics and Business Meeting-Procedures; Recruitment of ESRs in the Marie Curie Project Talk: Gating Charge Recombination Rates through Dynamic Bridges in Tetrathiafulvalene–Fullerene Architectures www.dur.ac.uk/chemistry/molesco/meetings/kick-off-meeting/ “Quimica respuestas para un mundo mejor” , Madrid, Spain (8-9 October 2014) Topic: International Symposium http://isna2015.com/ http://www.dur.ac.uk/resources/chemistry/SYMOLESCOAbstractbooklet.pdf http://www.dur.ac.uk/chemistry/molesco/meetings/konstanz/ http://www.dur.ac.uk/chemistry/molesco/meetings/kick-off-meeting/ http://www.fundacionareces.es/fundacionareces/cargarAplicacionAgen daEventos.do?identificador=1683 In the Seventh Framework Programme (FP7-PEOPLE) of the “MOLESCO” project, Valentina Sacchetti spent a secondments period, consisting of a stage and formation experience of overall 360 hours. These secondments period was realized under the supervision of Dr. Peter Nirmalraj from the 25 th of November to the 18 th of December 2015 and the supervion of Dr. Bernd Gotsmann from the 5 th to the 30 th of September 2016, at IBM Research Laboratories, Rüschlikon, Zurich, Switzerland. The main target of the undertaken secondments was the learning of the basic operating principle and of the STM and AFM use for the physical characterizations and conductance studies of some of the molecules reported in the present memory. http://www.fundacionareces.es/fundacionareces/cargarAplicacionAgendaEventos.do?identificador=1683 http://www.fundacionareces.es/fundacionareces/cargarAplicacionAgendaEventos.do?identificador=1683 Acknowledgements The present work was realized in the Organic Chemistry Department of the Complutense University of Madrid under the supervision of Prof. Nazario Martín and Prof. Beatriz Illescas. I want to thank both of them, Nazario and Beti, for giving me the opportunity to carry out this PhD in the framework of the European project “MOLESCO”. Thanks to the latter, I had the opportunity to meet world-renowned scientists, to participate in different conferences of European and international importance and to make friends with the fantastic persons composing the group of ESR of the project (Maria, Nico, Laerte, Kevin, Marcus, Sara, Ania, Joseph, Simon, Byron). In the realization of this work have also participated other research groups who I want to give thanks for their contribution. The group of Prof. Nicolás Agraït in the IMDEA Nanoscience Institute (Madrid). Particularly, thanks to Teresa González and Simon Svatek, respectively, for the study of the conductance behaviour of the amidinium-carboxylate based linear supramolecular wires and the thermopower’s investigation of the new nanocomposites based on fullerene derivatives. The group of Prof. Fabian Pauly of the University of Konstanz (Germany) for the studies of electronic transmission through DFT of the amidinium-carboxylate based linear supramolecular wires. The IBM Reseach Institute (Zurich), where I spent a really interesting and pleasant instance of around two months, my sincere thanks to Dr. Peter Nirmalraj, Dr. Bernd Gotsmann and Nico Mosso for teaching me the basic principles in the use of two fundamental techniques in the field of Nanoscience, the STM and AFM. The group of Prof. Colin Lambert of the University of Lancaster (England), and specially to Steven Bailey, for the studies of the energetics and resulting conformations of the fullerene derivatives bonding to a graphene surface through the vdW-DF implementation in SIESTA. I would like to thank the staff of the different CAI of the Faculty of Chemical Sciences. Many thanks to Lola, Helena and Ángel, of the Laboratory of NMR, for their constant availability, as well as to the people of the Laboratory of MS, Nour, Cristina and Estefanía. Thanks to Francisco Javier García for the FTIR spectra included in this Thesis. Ahora, en vez, me gustaria pasar a agradecer todas las personas con las cuales he tenido el placer de compartir de formas distintas estos cuatros años madrileños esperando no olvidar nadie. A todos los compañeros de laboratorio que ya se fueron (Tony, Javi, Luis, Sarita, María, Juan, “Muchachito”, Silvia, Sonia, Jaime, Inés, Rafa) y los que todavia quedan, Andrés (eres un gran amigo!), Paul y Chus (gracias por los muchos interesantes debates quimicos tenidos!), Marta (muchas gracias por enseñarme mal el español... sin ti hubiese hablado como Cervantes jajaja!), Alicia (gracias, entre otras cosas, por haberme recuperado aquel domingo que se me cerró la puerta del labo!), Antonio (a las nuestras variadas conversaciones y tu soporte morál!), Rosa, Marta, Mikiko, Marina, Alfonso, Javi Sevilla, Agus, Jose, Laura, gracias por todos los buenos momentos tanto dentro como fuera del laboratorio. A los “mayores”, Salvo (muchas gracias por tu disponibilidad en escuchar mis dudas quimicas y tus consejos. Me ayudaban mucho en el pensar, no obstante no los seguia siempre! En fín, ahora, espero que te convenciste de que no soy el 1 sobre 10!!), Carmen (muy amable siempre, cualquier cosa se te pregunte), Mª Ángeles (gracias por tu dedicatión al grupo y tu precisión, eres un punto de referencia unico), Luis Sánchez (gracias, junto con Salvo, por tus charlas sobre la importancia de los movimientos intestinales), Margarita, Andreas, Ángel, y David, simplemente gracias. Muchas gracias también a Helena, Virginia y Ana por todas las cuestiones, cuando más cuando menos, burocraticas. Gracias además a toda la otra gente del departamento para los cafes de la mañana o los mágicos viernes por la tarde y las demás ocasiones de fiesta, Paula, Alberto, Jorge, Sergio, Julia, Yerai, Yago, Dani, Helena, Alba, Maika, Borja, Fer, Ana, Paloma, Luke, Javi, Nora, Sara. Andrés, Jorge, Alicia, gracias por todos los buenos ratos que hemos compartimos, echo mucho de menos los intentos de ir al cine acabados en cerve... Sin duda, luego, no puedo olvidarme de las muchas maravillosas compañeras de piso con las cuales compartí mis espacios a lo largo de esto camino de cuatros años. En primer lugar, gracias a Laura (imprescidible compañera de fiestas, viajes y miles de aventuras) que junta con Elisella y Simoncella han sido y siguen siendo super buenas amigas. Luego, gracias también a Elly, Yu y Myriam (y Yago también, compañero de piso “part-time”!) por compartir momentos buenisimos y culturas diferentes. Fabri, amigo mio, sin ti Madrid hubiese sido seguramente menos divertida, grazie per avermi fatto conoscere questa meravigliosa città in tutti i suoi bares y garitos vari, i pranzi della domenica e tutto il resto ovviamente! Un grazie speciale, poi, va a la mia famiglia (nonni sicuramente inclusi), mamma, papà, Diego, Mara e Fabio, siete stati e sarete sempre un attracco sicuro. Infine, grazie a chi ha reso possibile il più bel capolavoro della mia vita, Ivan. Abbreviations and Acronyms In addition to the standard abbreviations and acronyms in organic chemistry (as defined in the Journal of Organic Chemistry author guidelines,http://pubs.acs.org/paragonplus/submission/joceah/joceah_a uthguide.pdf) the following terms have been used in this manuscript: Å anhyd atm AFM Angstrom(s) Anhydrous Atmosphere Atomic force microscopy BH Bulk heterojunction BJ BPDN Break junction Bipyridyl-dinitrooligophenylene–ethynylene dithiol COSY CT CVD CV Correlation spectroscopy Charge transfer Chemical vapor deposition Cyclic voltammetry δ ammmm D-A DAE DBA DCM dd DEPT DFT DMF DMSO Chemical shift in parts per million downfield from tetramethylsilane Donor-Acceptor Diarylethene Donor-bridge-acceptor Dichloromethane Double dobled Distortionless enhancement by polarization transfer Density functional theory N,N-Dimethylformamide Dimethyl sulfoxide DNA Deoxyribonucleic acid http://pubs.acs.org/paragonplus/submission/joceah/joceah_authguide.pdf http://pubs.acs.org/paragonplus/submission/joceah/joceah_authguide.pdf EF Fermi energy ESI EI ET Electrospray ionization Electron impact Electron transport ex-TTF Extended tetrathiafulvalene FLG FMO F4TCNQ Few layer graphene Frontier molecular orbital Tetrafluorotetracyano-p-quinodimethane FTIR GO Fourier transformed infrared spectroscopy Graphene oxide GOS Graphene on surface GQDs Graphene quantum dots h Hex HMQC HOMO Hour(s) Hexane Heteronuclear multiple quantum correlation Highest occupied molecular orbital HOPG Highly oriented pyrolytic graphite HOPG Horseradish peroxidase Hz IPR Hertz Isolated pentagon rule J Coupling constant (in NMR spectroscopy) K Ka LUMO µ m M Kelvin Association constant Lowest unoccupied molecular orbital Micro Multiplet (spectral); meter(s); milli Molar (moles per liter) MALDI MBE MCBJ MS NEGF NLO nm Matrix-assisted laser desorption ionization Molecule based electronics Mechanically controlled breack junction Mass spectroscopy Non-equilibrium Green’s function Nonlinear optical properties Nanometer NMR o-DCB OFETs OPE P Pc PC61BM PC71BM PCE PET PMα-CD PCTE PPA PPP PTB7-Th Nuclear magnetic resonance ortho-dichlorobenzene Organic field effect transistors Oligo(p-phenylene ethynylene) Porphyrin Phthalocyanine (6,6)phenyl C61 butyric acid methyl ester (6,6)phenyl C71 butyric acid methyl ester Power conversion efficiency Photo-induced electron transfer Permethylated α-cyclodextrin Polycarbonate track etched Polyphosphoric acid Poly(p-phenylene) Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5-b′] dithiophene-co-3-fluorothieno[3,4-b]thiophene- 2-carboxylate] PTFE PV Py QD Polytetrafluoroethylene Photovoltaic Pyridine Quantum dot rf-cCVD Radiofrequency catalytic chemical vapour deposition RT Room temperature s Singlet SLG Single layer graphene SPM Scanning probe microscopy STHM Scanning tunneling hydrogen microscopy STM Scanning tunneling microscopy STMBJ Scanning tunneling microscopy break junction t TCNQ Triplet 7,7’,8,8’-tetracyano-p-quinodimethane TEM Transmission electron microscopy TTF TGA THF TLC TPPO Tol UME UPy UV V Tetrathiafulvalene Thermogravimetric analysis Tetrahydrofuran Thin-layer chromatography Triphenylphosphine oxide Toluene Unimolecular electronics Ureido pyrimidinedione Ultraviolet Volts vs. Versus (“against”) XPS X-ray photoelectron spectroscopy Table of Contents Introduction 1 I. Nanoscience and Nanotechnology 1 II. Molecular-Scale Electronics 3 Chapter 1. 1. Background 11 1.1. Molecular Wires 11 1.1.1. Charge Transfer Mechanisms 11 1.1.1.1. Super-exchange Mechanism 12 1.1.1.2. Hopping Mechanism 13 1.1.2. Approaches 14 1.1.3. Scanning Probe Microscopy (SPM) 16 1.1.3.1. STM and AFM techniques 17 1.1.4. Molecular Wires: Origins and Development 22 1.1.5. Supramolecular Interactions in Single Molecular Junctions 26 1.1.5.1. π-π Stacking Interactions 28 1.1.5.2. Hydrogen Bonding 31 1.1.5.3. Host–Guest and Charge-Transfer (CT) Interactions 33 1.1.6. Single Molecule Switches 36 2. Objectives 47 3. Results and Discussion 53 3.1. Amidinium-Carboxylate Based Supramolecular Wires 53 3.1.1. Fullerene Dumbbells Series 53 3.1.1.1. Synthesis of Proton-Donor and Proton-Acceptor Fulleropyrrolidines 54 3.1.1.2. Formation and Characterization of Supramolecular Ensembles 61 3.1.1.3. Supramolecular Ensembles: Physical Characterization 63 3.1.2. Linear Supramolecular Wires Series 66 3.1.2.1. Synthesis of Proton-Donor and Proton-Acceptor Moieties for Linear Supramolecular Wires 66 3.1.2.2. Formation and Characterization of Supramolecular Ensembles 68 3.1.2.3. Supramolecular Ensembles: Physical Characterization 71 3.1.2.4. Theoretical Calculations of Supramolecular Ensembles 76 3.2. Crown Ether Switching Systems as Supramolecular Wires 79 3.2.1. Trans-4,5'-diaminodibenzo[18]crown-6 81 3.2.1.1. Synthesis of trans-4,5’-diaminodibenzo[18]crown-6 82 3.2.2. Oligo-phenylene ethynylene (OPE) and Poly(p- phenylene)(PPP) Crown Ether Molecular Wires Series 83 3.2.2.1. Synthesis of OPE and PPP Crown Ether Molecular Wires Series 84 3.2.2.2. Formation and Characterization of the Complexed PPP Crown Ether Molecular Wires Series 87 3.2.2.3. Complexed PPP Crown Ether Molecular Wires Series: Physical Characterization 89 4. Conclusions 93 5. Experimental Part 977 5.1. General Methods and Techniques 977 5.2. Proton-Donor and Proton-Acceptor Fulleropyrrolidines 99 5.3. Linear Supramolecular Wires 1166 5.4. Crown Ethers Switching Systems 11919 6. Appendix 1311 6.1. NMR titration experiments 1311 Chapter 2. 7. Background 139 7.1. Carbon Nanostructures 139 7.1.1. [60]Fullerene: Structure and Properties 1400 7.1.1.1. [60]Fullerene: Chemical Reactivity 1433 7.1.1.2. [60]Fullerene: Applications 1466 7.1.2. Graphene: Structure and Properties 14949 7.1.2.1. Graphene: Chemical Reactivity 15656 7.1.2.2. Graphene- and Graphene Oxide-Based-C60 Hybrid Materials 168 7.2. Thermopower Measurements In Molecular Junctions 1711 8. Objectives 179 8.1. Graphene On Surface (GOS)-Based Materials 17979 9. Results and Discussion 183 9.1. Retrosynthetic Analysis of the GOS-Based Materials 183 9.1.1. Synthesis of the Diamino-Diphenylmethano[60]Fullerene Building Blocks 184 9.1.1.1. Tour’s Reaction to the GOS-Based Materials 18787 9.2. GOS-Based Materials Characterization 192 9.3. Physical Characterization 198 9.4. Theoretical Calculations 201 10. Conclusions 209 11. Experimental Part 213 11.1. General Methods and Techniques 213 11.2. GOS-Based Materials 214 11.3. Synthesis of FLG-Covalent Hybrids 218 11.4. Synthesis of the Covalent GOS-Based Materials 220 11.5. Control Reaction on FLG 221 Summary 225 Resumen 235 Bibliography 245 A Beatrice Introduction Introduction 1 Introduction I. Nanoscience and Nanotechnology Scientists have investigated nanoscale materials for at least 150 years, ever since the sizes of atoms were first determined. 1 Specifically, the study of the unique size-dependent properties of solid-state materials and the discovery of the rules that govern these properties define the field of nanoscience. Conversely, nanotechnology can be considered, strictly speaking, as technology using single nanoscale building blocks, and Feynman's lectures 2 or Norio Taniguchi's original use of the term 3 are often mentioned as the starting points for the concept. Its basic essence relies on the ability of manipulating matter atom-by-atom or molecule-by-molecule, but, in more informal words, nanotechnology can be thought as the exploitation of the findings of nanoscience. In this respect, nanotechnology became really possible only with the advent of single-molecule manipulation techniques, such as the scanning tunnelling microscope (STM) 4 and later the atomic force microscope (AFM). 5 The invention of these techniques allowed scientists the ability not just to see molecules, but to pick them up and to move them as single building blocks. Indeed, many products either enabled or improved by nanotechnology have already been on the market from 2005 (Table 1). 6 1 a) M. Faraday, Philos. Trans. R. Soc. London 1857, 147, 145; b) G. Mie, Ann. Phys. 1908, 25, 377. 2 R. Feynmann, Plenty of Room at the Bottom, American Physical Society annual meeting, CalTech, 1959. 3 N. Taniguchi, On the Basic Concept of Nano-Technology, Proc. ICPE, Tokyo, Part II, Japan Society of Precision Engineering, 1974 ; “nano-technology mainly consists of the processing of separation, consolidation, and deformation of materials by one atom or one molecule”. 4 G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Appl. Phys. Lett. 1982, 40, 178. 5 G. Binnig, C. F. Quate, C. Gerber, Phys. Rev. Lett. 1986, 56, 930. 6 Meridien Institute, Nanotechnology and the Poor, Opportunities and Risks, 2005, http://www.merid.org. Introduction 2 What makes the nanometre scale intriguing is that material properties at this level differ from those in bulk and from those of molecules or their clusters of small size. Indeed, any matter in nano-sized form exhibits a change of properties which is considered attributable to the quantum- sized confinement effects and the increased surface-to-volume ratio compared to bulk materials. Therefore, the nanoscale is two-faced like the Roman god Janus and can be considered the doorway between the Table 1: Existing and near-term applications of nanotechnology across 12 different sectors. Ref 6. Automotive Industry Chemical Industry Construction Cosmetics -Lightweight construction -Painting -Catalysts -Tires (fillers) -Sensors -Coatings for windshields and auto bodies -Fillers for paints -Composite materials -Impregnation of papers -Adhesives -Magnetic fluid -Materials -Insulation -Flame retardants -Surface coatings for wood, floors, stone, tiles, roofing, etc. -Mortar -Sunscreens -Lipsticks -Skin creams -Toothpaste Electronics Energy Engineering Food&Drinks -Displays -Data memory -Laser diodes -Fiber optics -Optical switches -Filters -Conductive, antistatic coatings -Fuel cells -Solar cells -Batteries -Capacitors -Protective coatings for tools and machines -Lubricant-free bearings -Packaging -Sensors for storage life -Additives -Clarifiers (for juices) Household Medicine Sports/Outdoors Textile -Ceramic coatings for irons -Odour removers -Cleaners for glass, ceramics, metals, etc. -Drug delivery systems -Contrast media -Rapid testing systems -Prostheses and implants -Antimicrobial agents -In-body diagnostic -Ski wax -Tennis rackets, golf clubs -Tennis balls -Antifouling coatings for boats -Antifogging coatings systems for glasses/goggles -Surface coatings -Smart textiles Introduction 3 bulk and the molecular states. Thus, being the nano-world bursting with promising discoveries of physical phenomena that are not accessible using traditional materials or approaches, it is comprehensible the optimism manifested by nanoscientists around the new horizons (in health care, materials, energy and so on) opened by the exploitation of these properties. However, apart from this enthusiastic point of view, the hype surrounding this new field has also naturally led to some cynicism among the non-nanoscientists, but more alarming is the rising of violent antinanotechnology activisms, 7 which fear environmental disaster and sci-fi apocalyptic scenarios. Obviously, because of the interdisciplinary nature of nanotechnology, it exists a higher probability of rising philosophical and ethical questions. Therefore, it is surely required providing nanoscientists with a multi-disciplinary scientific formation, which would also be implemented with an ethical education in the direction of the potential societal implications. On the other hand, though, it is fundamental a major scientific divulgation focused on the topic, to avoid that any scientifically ignorant activism can proliferate, and all the possible concerns about the future of nanotechnologies remain within the confines of a democratic debate about technology, science, and ethics. II. Molecular-Scale Electronics 8 The integrated electronics field, with its specific market demand for further miniaturization of the electronic devices, perfectly fits the current trend of technology towards the reduction of the size, which is clearly stimulated by the exciting different properties displayed by nanoscaled structures. Over the past few decades, traditional transistors have shrunk dramatically reaching a dimension of less than 10 nm, 9 but, due to either technique limitations and lack of fundamental 7 In 2011, for example, a group calling itself the “ELF Switzerland Earth Liberation Front” was caught in Switzerland trying to bomb an IBM nanotechnology facility while it was under construction. The same year, a group in Mexico, known as “Individualidades Tendiendo a lo Salvaje” sent letter bombs to nanotechnology researchers injuring several people. 8 D. Xiang, X. Wang, C. Jia, T. Lee, X. Guo, Chem. Rev. 2016, 116, 4318. 9 ja) E. Lortscher, Nat. Nanotechnol. 2013, 8, 381; b) L. Wilson, International Technology Roadmap for Semiconductors (ITRS); Semiconductor Industry Association, 2013. Introduction 4 understanding of the transport mechanisms, 10 the further downscaling remains extremely challenging. In this context, the bottom-up approach to the creation of functional electrical circuits exploiting the intrinsic properties of individual molecules (or their ensembles) displays several advantages compared to traditional silicon-based electronic components. First, being molecules the smallest building blocks of matter, electrical components made only by them would enable heightened capacities reaching faster performance. Second, the flexibility of the chemical design allows the tailoring of the desired or required properties. Third, being chemically identical, molecules can be synthesized in bulk, thus lowering the cost of manufacturing. Therefore, the use of single molecules or nanoscale collections of single molecules as electronic components (concept which is at the base of the long history of the “molecular-scale electronics”, Figure 1), besides showing the potential to partly replace traditional solid-state devices, provides also an ideal window to investigate the intrinsic properties of nano-reduced materials. Nonetheless, the commercial outlet of molecular-scale electronic devices seems to be still far. In fact, from the exact determination of the 10 A. T. Haedler, K. Kreger, A. Issac, B. Wittmann, M. Kivala, N. Hammer, J. Kohler, H. W. Schmidt, R. Hildner, Nature 2015, 523, 196. Figure 1: Summary chart of the long history of the molecular-scale electronics. Introduction 5 structure-function relationship to the lack of a standardized process for the molecular fabrication and the too high costs for a bulk production, numerous challenges need to be addressed before devices based on single-molecules can be developed. Indeed, over the past decade, the comprehension of the working principles of conceptually simple molecular junctions was considerably enhanced through significant improvements in both experiments and theory. 11 However, there is still much to do in this respect and the field of molecular electronics remains a seductive playground for scientists to explore new fundamental concepts and new applications. 11 a) J. M. Tour, W. A. Reinerth, L. Jones, T. P. Burgin, C. W. Zhou, C. J. Muller, M. R.; Deshpande, M. A. Reed, Ann. N. Y. Acad. Sci. 1998, 852, 197; b) A. H. Flood, J. F. Stoddart, D. W. Steuerman, J. R. Heath, Science 2004, 306, 2055; c) R. L. McCreery, A. J. Bergren, Adv. Mater. 2009, 21, 4303; d) K. Moth-Poulsen, T. Bjornholm, Nat. Nanotechnol. 2009, 4, 551; e) Q. Shen, X. F. Guo, M. L. Steigerwald, C. Nuckolls, Chem. Asian J. 2010, 5, 1040; f) M. G. Schultz, Phys. Rev. B 2010, 82, 155408; g) J. C. Cuevas, E. Scheer, Molecular Electronics: An Introduction to Theory and Experiment; World Scientific: River Edge, NJ, 2010; h) D. Natelson, Y. J. Li, J. B. Herzog, Phys. Chem. Chem. Phys. 2013, 15, 5262; i) L. Sun, Y. A. Diaz-Fernandez, T. A. Gschneidtner, F. Westerlund, S. Lara-Avila, K. Moth-Poulsen, Chem. Soc. Rev. 2014, 43, 7378; l) L. Sánchez, R. Otero, J. M. Gallego, R. Miranda, N. Martín, Chem. Rev., 2009, 109, 2081; m) D. M. Guldi, B. M. Illescas, C. M. Atienza, M. Wielopolski, N. Martín, Chem. Soc. Rev., 2009, 38, 1587. Chapter 1. 1. Background Chapter 1. Background 11 1. Background 1.1. Molecular Wires A molecular wire is the most elementary building block for molecular nanoscale devices. It is a structure which can serve as a transport for electrons, generally composed by a molecule-bridge connected to a source and drain of electrons that can be two molecules themselves, the donor and acceptor moieties respectively, or rather two metal electrodes. Different definitions from literature have outlined the concept of molecular wire over time. It has been described as "a molecule connected between two reservoirs of electrons", 12 or whose behaviour is referred to "a molecule that conducts electrical current between two electrodes", 13 or, even more strictly, as a device that conducts in a regime "wherein the distance dependence of electron transfer may be very weak". 14 Several studies have shown the influences of different structural parameters on the properties of molecular wires. Their structure– property relationships can be determined by probing the electron transfer (ET) process through such systems. 1.1.1. Charge Transfer Mechanisms At macroscopic level, the charge transport in conductors obeys Ohm's law and their resistance is proportional to their length. However, these laws are no more valid at nanometric scale, where the conductance is quantized. The charge transfer process in a molecular wire at long distance is an inherent non-adiabatic process 15 whose rate is distance dependent. The tunneling effect or “super-exchange electron transfer” is favorable if the molecular bridge’s length is small, whilst if the molecular bridge’s length is large, the hopping mechanism is preferred. 12 E. G. Emberly, G. Kirczenow, Phys. Rev. B 1998, 58, 10911. 13 A. Nitzan, M. A. Ratner, Science 2003, 300, 1384. 14 W. B. Davis, W. A. Svec, M. A. Ratner, M. R. Wasielewski, Nature 1998, 396, 60. 15 J. Jortner, J. Chem. Phys. 1976, 64, 4860. Chapter 1. Background 12 Anyway, the quality of a molecular wire as a charge carrier can be obtained by the Equation 1, where the kinetic constant of the electron transfer process is given by: In which k0 is the temperature-dependent prefactor, it is typical for the deactivation of charge transfer processes and depends on the donor- acceptor distance (rDA). β, the so called “attenuation factor”, is independent of the distance and it is employed to define the quality of the wire. The smaller the β value, the weaker is the distance dependence of the wire or, in other words, the longer is the distance that the charge can transfer without any penalty. This value of β can be considered as a parameter for the whole DBA system, where donor and acceptor are molecular units or metallic contacts and it is strictly not referred to the bridge itself. Therefore, for a fixed bridge, the attenuation factor is a result of the existing difference between the energetic levels of the subunits forming the bridge, 16 that means, in conclusion, that β value depends on the electronic coupling between the different components of the systems under study. 1.1.1.1. Super-exchange Mechanism During the tunneling effect (denominated for the first time by Kramers 17 and then by Anderson) 18 the electron transfer occurs from the donor to the acceptor through a tunnel in the energy barrier (Figure 2). During the electron transfer, the orbitals of the bridge are only used as a mean of coupling with no nuclear movement along the bridge (no charge resides on the bridge). This mechanism, no temperature dependent, occurs when the bridge orbitals (above all the LUMO) are not energetically accessible. 16 M. P. Eng, B. Albinsson, Angew. Chem. Int. Ed. 2006, 45, 5626. 17 H. A. Kramers, Physica 1934, 1, 182. 18 a) P. W. Anderson, Phys. Rev. 1950, 79, 350; b) P. W. Anderson, Phys. Rev. 1959, 115, 2. 𝑘𝑒𝑡 = 𝑘0exp⁡(−𝛽𝑟𝐷𝐴) Equation 1. Equation for the kinetic constant of ET processes. Chapter 1. Background 13 1.1.1.2. Hopping Mechanism In case that hopping mechanism is prevailing, there are real energetically affordable intermediate states that actively transport the electron or hole along the wire. Changes in the molecular geometry can take place due to the formation of a charged intermediate excited state which evolves to the more stabilized species where positive charge resides on the donor and negative charge on the acceptor. The bridge is thus acting as a pathway for the electron transport (Figure 3); the electronic coupling between the bridge and terminal moieties is now considerable and the orbitals of the bridge (especially LUMO) are energetically accessible to promote an electron transfer along the bridge itself. This phenomenon is temperature dependent, and is frequently named as thermally activated hopping mechanism. In general, in systems in which there is a large energy gap between the initial and intermediate states, super-exchange will be the dominant mechanism. However, if this energy gap is comparable to the reorganization energy or electronic coupling, hopping mechanism can compete. In many real cases the transfer is expected to be governed by a mixture of the two mechanisms. Figure 2: Coherent charge transport or super-exchange mechanism. Figure 3: Incoherent charge transport or hopping mechanism. Chapter 1. Background 14 1.1.2. Approaches To probe the ET process through a wire's structure three different approaches are typically employed:  The "donor-bridge-acceptor systems" (DBA) method, where the photoinduced electron-transfer processes are studied by measuring the kinetics of electron transfer process by the photophysical study of the decay of the relevant transient species;  The "molecule based electronics" (MBE) approach, which studies electrical processes in molecular assemblies of any scale, creating a molecular sandwich between two electrodes; 19  The "unimolecular electronics" (UME) approach, which studies the conductance properties at the single-molecule level by interfacing a molecule between two metal electrodes. 20 Although the concept of molecular electronics was initially promising because of a visionary future where silicon-based technology would be replaced by the use of single molecules or nanoscale collections of single molecules as electronic components, real commercial implementation of molecular based devices appear to be a distant dream. Instead, something fundamentally different from the initial perspective seems to promise the UME strategy. Beyond the electronic transport that led the initial interest, researchers start now to focus on the rich physics of metal–molecule–metal junctions, in addition to explore new paradigms taking full advantage from the quantum properties of molecules, window into non-equilibrium processes, that have no analogues in conventional electronics. Generally, at single-molecule level, the structure–property relationships of a junction can be discussed by deconstructing the junction into three distinct components: the electrode, the anchor group, and the molecular bridge (Figure 4). 19 J. M. Tour, M. Kozaki, J. M. Seminario, J. Am. Chem. Soc. 1998, 120, 8486. 20 R. M. Metzger, NATO ASI Ser. 1991, B248, 659. Chapter 1. Background 15 Electron transport through molecules in nanoscale junctions is sensitive to very small changes in atomic configuration. 21 The specific anchoring group, whose role is fundamental to define the interface interaction of the molecule with the electrodes, 22 but also variations in the type of electrode material, 23 different molecular geometries 24 and conformations 25 deeply influence measurable properties of the metal– molecule-metal junction. Powerful techniques have been developed to characterize and manipulate the conductance properties of single molecules, including mechanical break junctions, 26 electrochemical deposition, 27 electromigration, 28 electron beam lithography, 29 shadow 21 L. Sun, Y. A. Diaz-Fernandez, T. A. Gschneidtner, F. Westerlund, S. Lara-Avilab, K. Moth-Poulsen, Chem. Soc. Rev. 2014, 43, 7378. 22 a) B. Xu and N. J. Tao, Science 2003, 301, 1221; b) L. A. Zotti, T. Kirchner, J. C. Cuevas, F. Pauly, T. Huhn, E. Scheer, A. Erbe, Small 2010, 6, 1529; c) C. Jia, X. Guo, Chem. Soc. Rev. 2013, 42, 5642; d) E. Leary, A. La Rosa, M. T. González, G. Rubio- Bollinger, N. Agraït, N. Martín, Chem. Soc. Rev. 2015, 44, 920. 23 a) A. Danilov, S. Kubatkin, S. Kafanov, P. Hedegard, N. Stuhr-Hansen, K. Moth- Poulsen, T. Bjornholm, Nano Lett. 2008, 8, 1; b) Y. Cao, S. Dong, S. Liu, Z. Liu, X. Guo, Angew. Chem. Int. Ed. 2013, 52, 3906; c) X. Y. Zhou, Z. L. Peng, Y. Y. Sun, L. N. Wang, Z. J. Niu, X. S. Zhou, Nanotechnology 2013, 24, 465204. 24 a) M. Kamenetska, S. Y. Quek, A. C. Whalley, M. L. Steigerwald, H. J. Choi, S. G. Louie, C. Nuckolls, M. S. Hybertsen, J. B. Neaton, L. Venkataraman, J. Am. Chem. Soc. 2010, 132, 6817; b) S. V. Aradhya, L. Venkataraman, Nat. Nanotechnol. 2013, 8, 399. 25 L. Venkataraman, J. E. Klare, C. Nuckolls, M. S. Hybertsen, M. L. Steigerwald, Nature, 2006, 442, 904. 26 M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, J. M. Tour, Science 1997, 278, 252. 27 A. F. Morpurgo, C. M. Marcus and D. B. Robinson, Appl. Phys. Lett. 1999, 74, 2084. 28 H. Park, A. K. L. Lim, A. P. Alivisatos, J. Park and P. L. McEuen, Appl. Phys. Lett. 1999, 75, 301. 29 A. Bezryadin, C. Dekker and G. Schmid, Appl. Phys. Lett. 1997, 71, 1273. Figure 4: Scheme of a single-molecule junction. The bridge unit can be further deconstructed into backbone (light orange block) and substituent (green triangles) subunits. I, is the current. Chapter 1. Background 16 mask evaporation, 30 scanning probe techniques, 31 on-wire lithography 32 and molecular rulers. 33 Specifically, the focus of the present memory is to investigate the electrical properties of different sets of novel compounds as molecular wires, on the basis of the UME approach, and probe the molecular conductance for the developed wire systems by methods based on scanning probe techniques. 1.1.3. Scanning Probe Microscopy (SPM) The invention of the scanning tunneling microscope (STM) and the atomic force microscope (AFM) that gave birth to the SPM techniques, is regarded as a milestone in the history of molecular electronics, because its great contribution to the development of the field continues to promote advances still now. The attractive features of STM and AFM is that they make interrogation of matter with atomic-scale resolution simple and affordable. Previous to the invention of those techniques, defining the structure of materials with atomic-scale precision was troublesome, expensive and difficult. In addition to the interrogation at the nanoscale, the SPM can be also used to manipulate and control various properties of matter in the nanoworld, together with the ability to operate in different environments The 2000 U.S. National Nanotechnology Initiative evidenced the importance of the development of these techniques by saying “STMs, AFMs, and near-field microscopes provide the eyes and fingers required for nanostructure manipulation and measurement”. 34 30 S. Kubatkin, A. Danilov, M. Hjort, J. Cornil, J. L. Bredas, N. Stuhr-Hansen, P. Hedegard, T. Bjornholm, Nature 2003, 425, 698. 31 X. D. Cui, A. Primak, X. Zarate, J. Tomfohr, O. F. Sankey, A. L. Moore, T. A. Moore, D. Gust, G. Harris, S. M. Lindsay, Science 2001, 294, 571. 32 L. Qin, S. Park, L. Huang, C. A. Mirkin, Science 2005, 309, 113. 33 T. Dadosh, Y. Gordin, R. Krahne, I. Khivrich, D. Mahalu, V. Frydman, J. Sperling, A. Yacoby, I. Bar-Joseph, Nature 2005, 436, 677. 34 National Science and Technology Council, Committee on Technology, Subcommittee on Nanoscale Science, Engineering and Technology, “National nanotechnology initiative: The initiative and its implementation plan”, July 2000. [Online]. Available: http://www.nano.gov/nni2.pdf Chapter 1. Background 17 The common feature of SPM techniques is that they share the operating principles of the STM and AFM, which is the use of a probe and the ability to position the material under investigation in relation to the probe with atomic-scale precision. 1.1.3.1. STM and AFM techniques In 1981, Binnig and Rohrer conducted the first successful tunneling experiment 4 giving birth to the scanning tunneling microscopy (STM). Some years later, in 1986, Binnig, Quate, and Gerber 5 developed the atomic force microscopy (AFM), following the necessity of an instrument that displayed the same performance of the STM but going beyond its intrinsic limit of being applied only to conductor and semiconductor materials. They made possible imaging with atomic- scale resolution also of insulators. A general SPM setup is based on the combination of two electrodes, the top electrode is a tip of different possible materials and the down electrode is a surface where the sample of molecules is deposited. Different operating principles are at the base of functioning for the two equipments. STM working principle uses the quantum-mechanical phenomenon of tunneling current which corresponds to a flow of electrons from the surface of one material to the surface of another, even when the surfaces are not in direct contact. The magnitude of this phenomenon is appreciable when the separation between the surfaces is of just few nanometers. A criterion of paramount importance is, for that, to maintain at really small distances the separation between the probe and the surface of the material investigated. The introduction of a feedback loop that allowed to control the gap between the surfaces of the probe and the sample was indeed the crucial step in the success of the tunneling experiment of the two IBM's scientists in 1981 (Figure 5). Chapter 1. Background 18 On the other side, in AFM, force instead of current is used to control the tip-positioning. Its principle of operation consists in the ability to sense the small forces exerted on the tip by the atoms in the sample. The specific probe is composed by a microcantilever provided with a sharp tip. A fundamental requisite is that the cantilever probe must be insensitive to external interferences from the surrounding environment. A control system for the cantilever is given by a laser beam incident on it, which reflects onto a split photodiode which has the responsibility of measuring the deflection of the cantilever itself. The cantilever deflects, due to the experienced forces between the atoms on the surface of the sample and the atoms on the surface of the tip, changing, for that, the angle that the incident laser beam makes with its surface. In turn, the incidence position of the laser on the photodiode changes and this variation is registered as a change in the photodiode voltage (Figure 6). Figure 5: STM schematic representation. Chapter 1. Background 19 Allowing for the direct observation of the system under investigation and permitting simultaneously the realization of other types of studies, such as electron transport, since their conception, SPM-based techniques have become the main experimental techniques in nanoscience to perform single-molecule electronic investigation by forming a metal–molecule–metal junction between the metallic tip and the substrate. To form the aforementioned molecular junction, the tip or substrate can be either immersed in the target molecule solution during the experiments or decorated with molecules before measurements. A unique role in the field of molecular electronics is covered by a specific STM technique called scanning tunneling microscopy break junctions (STMBJ) (Figure 7). Figure 6: Atomic force microscope schematic representation. Chapter 1. Background 20 In the STMBJ technique, a voltage bias is applied between a sharp tip of an STM, typically made of gold, and a substrate, also made of gold, covered with a layer of molecules that are chemically bound to the substrate via anchoring groups that can be of different nature. The tip is brought in contact with the substrate while the electrical conductance between the tip and the sample is being monitored. The formation of Au–Au atomic contacts is established when an electrical conductance of ∼5 G0 or greater is observed. At this point the "crashed" tip is progressively withdrawn, until just a single atom of gold remains in the junction, just before the breakage. During this process, in addition to the formation of Au–Au contacts, also some deposited molecules can statistically bridge the electrodes. This means that in the withdrawal process of the tip, a single-molecule junction can be formed, instead of the single gold atom connection, before the rupture of the electrodes union. During this process the electrical conductance of the tip- molecule-substrate junctions is monitored by conductance-distance measurements. In Figure 8 is reported an example of the relative curves. 22a Figure 7: Working principle of STMBJ technique. Chapter 1. Background 21 The presence of a plateau (as in the left side A) or a peak (as in the right side B) around G0 corresponds to the formation of a single gold atom junction, which indicates atomically sharp electrodes. Typically, thousands of stretching traces are recorded and only curves with clear plateaus are selected for the subsequent statistical analysis based on the criterion of stretching distance. Recent progress on SPM-based techniques were also achieved in the imaging of organic molecules. For the first time, the individual atoms within the molecules were resolved, indicating the potential of this technique not only in resolving molecular structure, but also bonding within and between molecules, molecular conformational changes and chemical reactions at the single-molecule level. 35 For both techniques the key step consists in the atomic functionalization of the probe. This fundamental step corresponds to place a well-defined atom or molecule at the tip of the scanning probe. In the case of AFM, the tip is modified by deliberately picking up one molecule of carbon monoxide (Figure 35 L. Gross, Nature Chemistry 2011, 3, 273. Figure 8: (A,C,E) Single-molecule conductance measurement. (B,D,F) Corresponding conductance histogram illustrate well-defined peaks near 1 G0, 2 G0, and 3 G0 due to the conductance quantization. After the gold−gold contact is completely broken, a new series of conductance steps appear due to the formation of the molecular junctions. Ref 22a. Chapter 1. Background 22 9), 36 whereas in the case of scanning tunneling hydrogen microscopy (STHM), the resolution was increased after filling the gap between the tip and the sample by the introduction of a single molecular hydrogen. 37 1.1.4. Molecular Wires: Origins and Development The concept of molecular wires has evolved starting from the design of linear species covalently bonded, whose constitutive structural core consists in bridge systems composed by identical repetitive molecular motifs of π-conjugated units. The choice of π-conjugated bridges resides in many convenient aspects, which go from their seeming rigidity, to their rod-like structure and more importantly their high degree of electron delocalization. The electronic communication between the individual bridge units has been gaining fundamental relevance in building efficient wire systems. Specifically, the electron delocalization ability is substantial to achieve a favorable mediation of charge/energy transfer over long distances. A systematic experiment to prove the ability of a bridge to sustain charge transport corresponds to the distance-dependence studies. ET reactions are traditionally triggered using light excitation by selectively exciting either the donor or the acceptor unit. These reactions are called photo-induced electron 36 L. Gross, F. Mohn, N. Moll, G. Meyer, R. Ebel, W. M. Abdel-Mageed, M. Jaspars Nature Chem. 2010, 2, 821. 37 J. I. Martínez, E. Abad, C. González, F. Flores, J. Ortega, Phys. Rev. Lett. 2012, 108, 246102. Figure 9: a) Schematic representation of the imaging process. b) Molecule imaged with atomic resolution using non contact AFM-CO-functionalized tips. c) Correspondence of the imaged molecule with the theoretical structure. Ref 36a. Chapter 1. Background 23 transfer (PET) reactions and can be considered the ancestors of single molecule experiments. Indeed, valuable knowledge on wire-like properties has been acquired by experimentally studying excited state properties and their relative ET processes. The results of such investigation are, for convenience, often summarized in the already mentioned parameter called attenuation factor, β. Representative examples of different bridge structures drawn from literature are reported in Table 2. 38 A variety of β values for covalently bonded donor-acceptor systems of different nature are given (Table 2). 38 a) F. Giacalone, J. L. Segura, N. Martín, D. M. Guldi, J. Am. Chem. Soc. 2004, 126, 5340; b) G. de la Torre, F. Giacalone, J. L. Segura, N. Martín, D. M. Guldi, Chem. Eur. J. 2005, 11, 1267; c) K. Pettersson, J. Wiberg, T. Ljungdahl, J. Mårtensson, B. Albinsson, J. Phys. Chem. A 2005, 110, 319; d) C. Atienza, N. Martín, M. Wielopolski, N. Haworth, T. Clark, D. M. Guldi, Chem. Commun. 2006, 0, 3202; e) E. A. Weiss, M. J. Ahrens, L. E. Sinks, A. V. Gusev, M. A. Ratner, M. R. Wasielewski, J. Am. Chem. Soc. 2004, 126, 5577; f) C. Schubert, M. Wielopolski, L. H. Mewes, G. de Miguel Rojas, C. van der Pol, K. C. Moss, M. R. Bryce, J. E. Moser, T. Clark, D. M. Guldi, Chem. Eur. J. 2013, 19, 7575; g) C. Atienza, M. Wielopolski, D. M. Guldi, C. van der Pol, M. R. Bryce, S. Filippone, N. Martín, Chem. Commun. 2007, 0, 5164; h) A. B. Ricks, K. E. Brown, M. Wenninger, S. D. Karlen, Y. A. Berlin, D. T. Co, M. R. Wasielewski, J. Am. Chem. Soc. 2012, 134, 4581. Chapter 1. Background 24 Table 2: List of structures and relative β values for different DBA systems. Bridge D A βCS /A-1 Ref. OPV exTTFa C60 0.01 Guldi-Martín (37a) ZnIITPPb C60 0.03 Guldi-Martín (37b) OPE ZnIIP AuIIIP 0.31 Albisson (37c) exTTFa C60 0.2 Guldi (37d) Phn (or PP) PTZc PDId 0.46 Wasielewski (37e) xyn RuII PTZc 0.77 Wenger (37f) fln exTTFa C60 0.09 Martín (37g) FNn DMJ- Ane NIf 0.34 Wasielewski (37h) a) Extended tetrathiafulvalene. b) Tetraphenylporphyrin. c) Phenotiazine. d) Perylene- 3,4:9,10-bis(dicarboximide). e) 3,5-Dimethyl-4-(9-anthracenyl)julolidine. f) Naphthalene- 1,8:4,5-bis(dicarboximide). Ref 37. Chapter 1. Background 25 Many different examples along the years have demonstrated that employing covalent bonds to link the different components of a molecular wire structure corresponds to very good performances of conductivities, high rationalization of structure–property relationships 39 and of key parameters, such as the mode of transport. 40 One step further in molecular electronics was accomplished studying how the introduction of weak bonding forces can influence the electron flow through molecular wire systems. Among the many different non- covalent interactions, the hydrogen bonding (H-bonding) has been the most investigated. One of the first examples, in which hydrogen bonds were shown to control electron transfer was reported by Therien et al. for bis-chromophoric systems of zinc(II) and iron(III) porphyrins. 41 Hirsch and co-workers in 2010, studying a non-covalent system constituted by a Hamilton receptor/cyanuric acid motif to assemble metalloporphyrins with C60 derivatives, determined a β factor of 0.1 Å -1 intermediate between the extremes of covalent p- phenyleneethynylene 38b,c,d and fluorene systems. 38g,h,42 More recently, Martín et al. reported hybrid covalent/supramolecular porphyrin– [60]fullerene structures, based on the amidinium-carboxylate H-bond interaction, as highly efficient molecular wires with a remarkably low attenuation factor (β = 0.07 ± 0.01 Å -1 ). 43 The supramolecular systems studied by Hirsch and Martín are shown in Figure 10. 39 G. J. Ashwell, P. Wierzchowiec, L. J. Phillips, C. J. Collins, J. Gigon, B. J. Robinson, C. M. Finch, I. R. Grace, C. Lambert, P. D. Buckle, K. Ford, B. J. Wood, I. R. Gentle, Phys. Chem. Chem. Phys. 2008, 10, 1859. 40 S. H. Choi, C. Risko, M. C. R. Delgado, B. S. Kim, J. L. Brédas, C. D. Frisbie, J. Am. Chem. Soc. 2010, 132, 4358. 41 P. J. F. de Rege, S. A. Williams, M. J. Therien, Science 1995, 269, 1409. 42 F. Wessendorf, B. Grimm, D. M. Guldi, A. Hirsch, J. Am. Chem. Soc. 2010, 132, 10786. 43 S. Vela, S. Bauroth, C. Atienza, A. Molina-Ontoria, D. M. Guldi, N. Martín, Angew. Chem. Int. Ed. 2016, 55, 15076. Chapter 1. Background 26 Anyway, the first purpose in molecular electronics remains the understanding and manipulation of the charge transport phenomenon through single-molecules. Therefore, the obvious direction in the evolution of PET reactions to investigate electron transfer was the substitution of the acceptor and donor moieties with the direct placing of single molecules between two metallic electrodes. UME approach is indeed considered to have more practical applications, because it makes directly use of the molecule as an electrical interconnect between two devices. In this regard single-molecule break junction (BJ) methods, such as the STMBJ, turn out to be suitable techniques for characterizing the conductance of unimolecular junctions. 1.1.5. Supramolecular Interactions in Single Molecular Junctions Significant progress has been made during the past years in many aspects of UME area defining, for example, the dependence of conductance with anchoring groups, 22 molecular length, 44 44 B. Capozzi, E. J. Dell, T. C. Berkelbach, D. R. Reichman, L. Venkataraman, L. M. Campos, J. Am. Chem. Soc. 2014, 136, 10486. Figure 10: Supramolecular DBA systems from a) Hirsch study (Ref 41) and b) Martín work (Ref. 42). Chapter 1. Background 27 electrochemical gating, 45 quantum interference effect, 46 contacts geometries, 47 and the influence of molecular conformation. 48 Specifically, the technical progress of BJ-methods opens the opportunity to study charge transport phenomena in more complex junction systems. Recent reports revealed the application of these techniques, for example, to the investigation of molecular systems bridged through non-covalent interactions. 49,46b Non-covalent interactions are comparatively weaker forces than the covalent ones (Figure 11). Their interactions are based on some peculiar features such as self-assembly, self-organization, molecular recognition, reversibility and tunable strength. Basically, two are the interesting aspects which make these supramolecular interactions appealing with respect to the more stable covalent ones: i) on one side, the high selectivity in the connections developed between building blocks, ii) on the other side, the thermodynamic reversibility, which makes recognition process during the assembly one of the possible finely tuned processes. 45 a) Y. Li, M. Baghernejad, A. G. Qusiy, D.Z. Manrique, G. Zhang, J. Hamill, Y. Fu, P. Broekmann, W. J. Hong, T. Wandlowski, D. Zhang, C. Lambert, Angew. Chem. Int. Ed. 2015, 54, 13586; b) H. M. Osorio, S. Catarelli, P. Cea, J. B. Gluyas, F. Hartl, S. J. Higgins, E. Leary, P. J. Low, S. Martin, R. J. Nichols, J. Tory, J. Ulstrup, A. Vezzoli, D. C. Milan, Q. Zeng, J. Am. Chem. Soc. 2015, 137, 14319. 46 a) C. R. Arroyo, S. Tarkuc, R. Frisenda, J. S. Seldenthuis, C. H. Woerde, R. Eelkema, F. C. Grozema, H. S. J. van der Zant, Angew. Chem. Int. Ed. 2013, 52, 3152; b) R. Frisenda, V. A. E. C. Janssen, F. C. Grozema, H. S. J. van der Zant, N. Renaud, Nature Chemistry 2016, 8, 1099. 47 X. L. Li, J. He, J. Hihath, B. Q. Xu, S. M. Lindsay, N. J. Tao, J. Am. Chem. Soc. 2006, 128, 2135. 48 H. Li, M. H. Garner, Z. Shangguan, Q. Zheng, T. A. Su, M. Neupane, P. Li, A. Velian, M. L. Steigerwald, S. Xiao, C. Nuckolls, G. C. Solomon, L. Venkataraman, Chem. Sci. 2016, 7, 5657. 49 a) P. T. Bui, T. Nishino, Y. Yamamoto, H. Shiigi, J. Am. Chem. Soc. 2013, 135, 5238; b) L. Wang, Z. L. Gong , S. Y. Li, W. Hong, Y. W. Zhong, D. Wang, L. J. Wan, Angew. Chem. Int. Ed. 2016, 55, 1; c) W. Zhang, S. Gan, A. Vezzoli, R. J. Davidson, D. C. Milan, K. V. Luzyanin, S. J. Higgins, R. J. Nichols, A. Beeby, P. J. Low, L. Buyi, L. Niu, ACS Nano 2016, 10, 5212. Chapter 1. Background 28 However, the degree of knowledge concerning electron transfer at the single molecule level in non-covalently interacting systems, remains considerably lacking. 50 Therefore, understanding charge transfer within a single supramolecular assembly is a pivotal point for building functional electronic devices, where spontaneous recognition plays a central role in their construction, as well as in conferring unique electronic functionalities. Building aligned species is of particular interest regarding the development of electronic devices, especially for contacting with the electrodes. In this regard, there are a variety of strategies to obtain efficient molecular wires. The strategy of choice relies on the basis of the type of electro or photoactive moieties to be incorporated, because these units define the wire’s electronic properties. Once defined the wire backbone structure, the weak forces can geometrically fine-tune the assemblies. 1.1.5.1. π-π Stacking Interactions The double stranded organization of DNA and the tertiary structure of proteins are examples of structures stabilized by the intervention of the π-π interactions. Indeed, this kind of connections play a significant role in biological structures and, owing to their large amount of delocalized π electron density, result of special interest in molecular electronics. 50 X. Li, D. Hu, Z. Tan, J. B. Zongyuan, X. Y. Yang, J. Shi, W. Hong, Top Curr. Chem. (Z) 2017, 375, 42. Figure 11: Non-covalent vs. covalent bonds strength. Chapter 1. Background 29 Therefore, the understanding of the ET process through π-π stacking is critical in the search for the fabrication of biomolecule-based devices. In a pioneering work, Wu et al. showed that π-π stacking interactions result strong enough to form a molecular junction. 51 Using the mechanically controlled break junction (MCBJ) technique, they studied a series of oligo-phenyleneethynylene (OPE) derivatives. Molecules 1 and 2 possess two active anchoring groups, while the nitrogen atom of the pyridine structure in molecule 3 is hidden, providing a much less probable binding site to the electrode. Molecule 4 directly lacks any efficient second anchoring group (Figure 12). 51 S. Wu, M. T. González, R. Huber, S. Grunder, M. Mayor, C. Schoenenberger, M. Calame, Nat. Nanotechnol. 2008, 3, 569. Figure 12: Conductance peak of the corresponding molecules investigated by Wu. Ref 50. Chapter 1. Background 30 Interestingly, molecules 3 and 4 exhibit a conductance one order of magnitude lower than that of molecules 1 and 2. But even more surprising is that they exhibit an identical magnitude and width in the conductance peak respect to molecules 1 and 2, suggesting that they bind in the junction with a similar probability as molecules with an anchoring group on both ends. Therefore, considering the single anchoring group and the lower conductance, they speculated that the formation of the molecular junctions is ascribed to the π-π stacking interaction between two adjacent molecules. Further investigations have been performed to prove this natural tendency of aromatic systems to self-assemble and form aggregates. Martin et al. demonstrated that the presence of steric hindrance impedes the formation of a stacking arrangement, determining the disappearance of the corresponding conductance peak. 52 González et al. investigated the formation of π-π stacked molecular junctions as a function of the environmental conditions, demonstrating that the presence of polar molecules enhance their organization. 53 Moreover, unique electrical characteristics are exhibited by the inclusion complexes formed when π-π stacked systems (stacked guests) are locked in self-assembled cages (hosts). Using STMBJ technique, Fujii et al. observed that inclusion complexes containing two identical aromatics (homocomplex) and different aromatics (heterocomplex) exhibit, the first, a high conductance value, while the second shows two peaks one of which appears at lower conductance. 54 The presence of these two peaks, in the case of the heterocomplex, was attributed to two different electron transport directions at a fixed bias voltage. Statistical analysis of the I-V curves confirmed experimentally that the heterocomplex junction shows rectification properties (Figure 13). 52 S. Martin, I. Grace, M. R. Bryce, C. Wang, R. Jitchati, A. S. Batsanov, S. J. Higgins, C. Lambert, R. J. Nichols, J. Am. Chem. Soc. 2010, 132, 9157. 53 M. T. González, E. Leary, R. l. García, P. Verma, M. A. N. Herranz, G. Rubio- Bollinger, N. Martín, N. S. Agraït, J. Phys. Chem. C 2011, 115, 17973. 54 S. Fujii, T. Tada, Y. Komoto, T. Osuga, T. Murase, M. Fujita, M. Kiguchi, J. Am. Chem. Soc. 2015, 137, 5939. Chapter 1. Background 31 1.1.5.2. Hydrogen Bonding Hydrogen bonding is by far the most widely investigated supramolecular interaction. Many are the reasons why this bond results so appealing. Firstly, because the electronic communication through systems connected by such interaction shows a strong and directional component comparable to σ or π bonds. 55 Secondly, because the strength of this connection can be tuned availing of external stimuli that cause the alteration of the transport properties at molecular-scale, influencing their conductance and junction formation. 56 Early investigation of conductance properties at single-molecule level of such interaction was performed in 2013. Employing a STM-based I(t) technique, Nishino showed that the carboxylic dimers interacting through H-bonds exhibit a better conductance than the corresponding alkanes of similar molecular length, even if it decays steeply as the ET pathway becomes longer (Figure 14). 57 55 L. Sánchez, M. Sierra, N. Martín, A. J. Myles, T. J. Dale, J. Rebek, Jr., W. Seitz, D. Guldi, Angew. Chem. Int. Ed. 2006, 45, 4637. 56 M. Kotiuga, P. Darancet, C. R. Arroyo, L. Venkataraman, J. B. Neaton, Nano Lett. 2015, 15, 4498. 57 T. Nishino, N. Hayashi, P. T. Bui, J. Am. Chem. Soc. 2013, 135, 4592. Figure 13: Illustration of chemical structures and molecular junctions of homocomplex (left) and heterocomplex (right). Ref 53. Chapter 1. Background 32 To investigate the electron transport through multiple hydrogen bonds, Wang et al. studied a series of ureido pyrimidinedione (UPy) derivatives (Figure 15). 58 The results of this study show that the thiol terminated quadruple-hydrogen-bonded dimer presents a high conductance value which is comparable to conjugated oligo(phenylene- ethynylene) junctions, while the conductance of the pyridyl and amino terminated dimers result considerably lower, owing to the weaker N– Au bond. However, hydrogen bonds are fundamental interactions present in biological systems. They preserve, for instance, the double-stranded 58 L. Wang, Z. L. Gong, S. Y. Li, W. J. Hong, Y. W. Zhong, D. Wang, L. J. Wan, Angew. Chem. Int. Ed. 2016, 55, 12393. Figure 14: Scheme of the formation of a single molecular junction based on the hydrogen bond interaction between two carboxylic units (right); distance dependence of conductance of molecular junction consisting of H-bonded CnCOOH dimers (pink) and alkanedithiolates (blue); nc represents the number of C-atoms in the molecular junction (left). Ref 56. Figure 15: Molecular structures and self-assembly mode of UPy derivatives (left); Single- molecule conductance results from STMBJ experiments for the Upy terminated via thiol groups. Ref 57. Chapter 1. Background 33 structure of DNA and its unique characteristics, such as self-recognition and self-assembly, make it a promising material in molecular electronics. 59 With this aim, the understanding of charge transport properties involving the hydrogen bonds of the DNA structure is a matter of crucial relevance. Particularly interesting is the detection of mutations in the double-stranded DNA (dsDNA) structure through the application of electric-based methods. A good example is the work by Hihath et al., in which STMBJ is used to identify the presence of even a single base pair mismatch by evidencing that the conductance of dsDNA can vary as much as one order of magnitude. 60 1.1.5.3. Host–Guest and Charge-Transfer (CT) Interactions The wide application that host-guest complexes find in many aspects of life, ranging from biological systems to pharmaceuticals, and chemical sensors, make them considerably attractive. A common host in these complexations is represented by cyclodextrins. These compounds are a group of cyclic glucose oligosaccharides of different diameter, characterized by an amphipathic nature, due to the hydrophobic internal cavity and the hydrophilic outer surface. On the basis of these attractive characteristics, Kiguchi et al., using the STMBJ technique, decided to investigate the conductance properties of a π-conjugated molecular junction covered by a permethylated α- cyclodextrin (PM α-CD) (Figure 16). 61 59 G. I. Livshits, A. Stern, D. Rotem, N. Borovok, G. Eidelshtein, A. Migliore, E. Penzo, S. J. Wind, R. Di Felice, S. S. Skourtis, J. C. Cuevas, L. Gurevich, A. B. Kotlyar, D. Porath, Nat. Nanotech. 2014, 9, 1040. 60 J. Hihath, B. Xu, P. Zhang, N. Tao, PNAS 2005, 102, 47, 16979. 61 M. Kiguchi, S. Nakashima, T. Tada, S. Watanabe, S. Tsuda, Y. Tsuji, J. Terao, Small 2012, 8, 5, 726. Chapter 1. Background 34 Kiguchi discovered that inserting a single π-conjugated molecule into a host molecule like CDs is a promising technique. It permits to study the charge transport properties of single-molecule devices suppressing conductance fluctuations with respect to the uncovered π-conjugated molecule. Charge transfer interactions are of course the main central topic of the field. For this reason, thanks to their electronic versatility, complexes generated from the attractive interaction between donor and acceptor molecules have become currently subject of great interest. The discovery of tetrathiafulvalene-tetracyano-p-quinodimethane (TTF- TCNQ) charge-transfer complex in 1973, and its high electrical conductivity, make it an interesting candidate in the field of molecular electronics. 62 García et al. investigated the charge transport of the complex formed between an extended-TTF (ex-TTF) and a tetrafluorotetracyano-p-quinodimethane (F4TCNQ). 63 No conductance 62 P. W. Anderson, P. A. Lee, M. Saitoh, Solid State Commun. 1973, 13, 595. 63 R. García, M. A. Herranz, E. Leary, M. T. González, G. Rubio Bollinger, M. Buerkle, L. A. Zotti, Y. Asai, F. Pauly, J. C. Cuevas, N. Agraït, N. Martín, Beilstein J. Org. Chem. 2015, 11, 1068. Figure 16: Structures of covered 5 and uncovered 6 molecules, conductance traces of covered (thick line) and uncovered (thin line) molecules. Ref 60. Chapter 1. Background 35 signal was registered for the ex-TTF, which according to theoretical calculations results under the measurement sensitivity. In contrast, two distinct conductance plateaus were observed for the complex. The higher was attributed to that of F4TCNQ, while the lower was assigned to the exTTF-F4TCNQ complex (Figure 17). Electron acceptor properties of [60]fullerene and donor behavior of porphyrin have made these molecules fundamental building blocks in molecular electronics. Nishino et al. investigated, for example, the electronic properties of a single fullerene-porphyrin complex by STM. 64 Functionalizing the tip of the STM with fullerene derivatives, they demonstrated that the interaction with a porphyrin derivative, deposited on a highly oriented pyrolytic graphite (HOPG) substrate, results in asymmetric I-V curves. Therefore, single fullerene-porphyrin complex exhibits an obvious rectifying behavior (Figure 18). 64 T. Nishino, T. Ito, Y. Umezawa, PNAS 2005, 102 ,5659. Figure 17: 2D histograms of ex-TTF (a), F4TCNQ (d), all traces (b) and the ones for CT complex between ex-TTF and F4TCNQ (c). Ref 62. Figure 18: Fullerene-porphyrin complex and their I-V curves. Ref 63. Chapter 1. Background 36 1.1.6. Single Molecule Switches The technical progress of BJ-techniques has also allowed the study of one of the most appealing functional elements in a molecular electronic device, the switch. This electronic element has its molecular correspondent in a compound whose electronic communication can be controlled externally. 65 The interest in these molecules is that their “on- off” control could, in principle, introduce relevant applications as, for example, memory elements. Anyway, a molecular functional element with switchable properties, once attached to the electrodes, can preserve or not its functionality. Remarkably, the opposite phenomenon does exist as well. A device whose core is not constituted by a molecule with switchable properties in solution, once pinned to the metal electrodes, can exhibit an unexpected switching activation (Figure 19). Therefore, single molecules in solid-state devices can be divided into: i) intrinsic switches, when the molecule preserves its switching ability, ii) extrinsic switches, 66 when a non-switching molecule may be activated by contacting with the electrodes. The ability to exist in two distinguishable stable forms and to turn each one into the other, under the effect of some external stimuli, is the main feature of molecular switches. Roughly, two modes of switching can be identified for this conversion mechanism, the conformational and the redox type. In the first case, an isomerization reaction takes place, where the three- dimensional structure of the molecule undergoes fundamental changes. In the second case, a molecule can modify its charge state by taking up 65 J. F. Stoddart, H. M. Colquhoun, Tetrahedron 2008, 64, 8231. 66 a) B. de Boer, M. M. Frank, Y. J. Chabal, W. Jiang, E. Garfunkel, Z. Bao, Langmuir 2004, 20, 1539; b) J. M. Beebe, J. G. Kushmerick, Appl. Phys. Lett. 2007, 90, 083117. Figure 19: Scheme of how coupling can influence the performance of a molecular device. Chapter 1. Background 37 (or giving up) an electron. Obviously the redox mechanism can include some minor basic structural changes in the molecule, but, of course, the conformational mechanism, on its side, does not involve changing in the molecular charge. A required condition for a useful bistable molecule is that the energy barrier between the two states, ΔEact, is much larger than the thermal energy kBT (Figure 20). Therefore, switching from one isomer to the other needs to overcome this barrier and two are the ways that can be taken. The molecule can be brought to an excited state, e.g. by illumination. This would result into a relaxation process over the excited potential energy surface (which mean on the top of the blue curve) and the probabilistic conversion into the other form. Clearly, that this could happen or not is a matter of probability. Alternatively, the probabilities may change noticeably when the molecule is attached to the electrodes. To this purpose, STM techniques become a powerful tool for the selective single molecule manipulation. Indeed, it is possible to directly drive the molecule over the ground state barrier by means of various tip- molecule interactions, such as the electric field, tunneling electrons or tip-molecule interaction forces. Therefore, STM can induce single molecule switching by different mechanisms: dipole switching, spin switching, conformational switching, charge switching, bonds switching, etc. Liljeroth et al. presented an elegant example of conformational switching based on a structural isomerization mechanism. 67 By positioning the tip above a naphthalocyanine and increasing the bias 67 P. Liljeroth, J. Repp, G. Meyer, Science 2007, 317, 1203. Figure 20: Potential energy surface for switching mechanism between two states. Chapter 1. Background 38 above the LUMO resonance, the current-induced hydrogen tautomerization of the molecule results in the back and forth switching of the tunneling current between two well defined levels (Figure 21). Wang et al., on the other side, demonstrated a dipole switching mechanism through the conversion of a Sn ion, inside a tin phthalocyanine (SnPc) molecule adsorbed on Ag surface, between two conformations, Sn-up and Sn-down (Figure 22). 68 The mechanism postulated by Wang was that this switching occurs via hole attachment. The resonant tunneling from the highest occupied 68 Y. Wang, J. Kroger, R. Berndt, W. A. Hofer, J. Am. Chem. Soc. 2009, 131, 3639. Figure 21: Up:( Left) Current-trace by tip-molecule interaction. (Right) Orbital images showing the orientation of the LUMO. Down: Scheme of the hydrogen tautomerization reaction responsible for the switching. Ref 66. Figure 22: STM images of SnPc. The single arrow between (a) and (d) illustrates the irreversible switching from Sn-up to Sn-down conformations, while the double arrow between (b) and (e) illustrates the reversible switching. (c) and (f) correspond to a schematic representation of Sn-up and Sn-down respectively. Ref 67. Chapter 1. Background 39 molecular orbital (HOMO) of Sn 2+ starts generating a transient oxidized Sn 3+ state, smaller than Sn 2+ . The applied negative pulse voltage supplies the required energy to move the Sn 3+ under the substrate surface. Whilst, by applying a positive sample bias, the electron may deposit energy to vibrational degrees of freedom of the molecule thus allowing the backward switching process from Sn 3+ to Sn 2+ . Another fascinating example of a switching process is the one presented by bipyridyl-dinitrooligophenylene–ethynylene dithiol (BPDN) molecule. By using MCBJ, Lörtscher et al. showed that it is possible to reversibly control the BPDN switching between two conductance states (Figure 23). 69 They demonstrated that by controlling the switching behavior of a single BPDN, this molecule can be used as a memory element to write, read and erase bits by simple voltage pulses. They also performed the control study on a related molecule without the nitro groups demonstrating that it did not show any switching behavior. Anyway, despite the indication of the importance of the presence of the nitro 69 E. Lörtscher, J. W. Ciszek, J. Tour, H. Riel, Small 2006, 2, 973. Figure 23: a) BPDN molecule under study with MCBJ. b) Hysteretic IV curves. A positive voltage switches the molecule from the initial OFF state to the ON state, while an applied negative bias reverse the process from ON to OFF state. c) and d) Memory operation based on the hysteretic IV curve. Ref 68. Chapter 1. Background 40 groups, which is the exact mechanism regulating this switching process actually remains unclear. Obviously, the ideal building blocks desired in molecular electronics are represented by molecules that can be reversibly switched between high and low conductance states. To this purpose, diarylethene (DAE), and derivatives, have always been seen as promising molecular units because of their peculiar structure and high lifetimes of the charge- separated states. 70 Such molecules, in fact, can be reversibly switched between an "on-off" state. The "off" state consists in the open form of the molecule characterized by an interrupted π-conjugated structure. Under UV light irradiation, instead, the open state can be switched to the "on" form corresponding to the closed fully conjugated structure. On the opposite, the backwards process is triggered by visible light. Recently, Sendler et al. reported the controlled in situ switching of DAE derivatives from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation (Figure 24). 71 70 S. Castellanos, A. A. Vieira, B. M. Illescas, V. Sacchetti, C. Schubert, J. Moreno, D. M. Guldi, S. Hecht, N. Martín, Angew. Chem. Int. Ed. 2013, 52, 13985. 71 T. Sendler, K. Luka-Guth, M. Wieser, W. J. Lokamani, M. Helm, S. Gemming, J. Kerbusch, E. Scheer, T. Huhn, A. Erbe, Adv. Sci. 2015, 2, 1500017. Figure 24: a) Photocromic switching process of the DAE structure designed by Sendler. b) and c) Conductance−distance traces respectively for the open (“off”) and closed (“on”) form. Ref 70. Chapter 1. Background 41 They demonstrated that before UV irradiation, no clear and defined conductance plateaus are visible, due to the lack of fully π-conjugated molecular states. But, what happen after irradiation is that they start to observe defined conductance plateaus, which indicates a successful switching "on" of the molecular state. Another interesting family of molecules which show a peculiar character of sensors and switches is the one formed by crown ethers derivatives. Crown-ether molecules are well known to selectively bind alkali atoms and an always greater variety of structures, belonging to the family of these compounds, is nowadays available and investigated. 72 The only area which remains to be explored is represented by their conductance properties. Anyway, some theoretical studies revealed that by incorporating these structures within wires, any change in electrical conductance of the wire upon binding would lead to discriminating sensing. A pioneer theoretical work of Liu et al. in 2003, by using density functional theory (DFT) calculations showed how the conductance value of a short molecular wire containing a 18-crown-6 molecule connected via sulfur atoms to two long atomic gold wires changes drastically passing from the empty to the full form of the hole (Figure 25). 73 72 a) M. J. Marsella, T. M. Swager J. Am. Chem. Soc. 1993, 115, 25, 12214; b) P. V. Bernhardt, E. J. Hayes, Inorg. Chem. 2002, 41, 2892; c) J. Guo, J. Lee, C. I. Cu, N. C. Gallego, S. T. Pantelides, S. J. Pennycook, B. A. Moyer, M. F. Chisholm, Nat. Commun. 2014, 5, 5389; d) J. Li, D. Yim, W.D. Jang, J. Yoon, Chem. Soc. Rev. 2017, 46, 2437. 73 C. Liu, D. Walter, D. Neuhauser, R. Baer, J. Am. Chem. Soc. 2003, 125 , 13936. Figure 25: 18-crown-6 conductance values for the empty and complexed with different alkali metals forms. Ref 72. Chapter 1. Background 42 More recently, instead, Ismael et al. using a DFT-based approach to quantum transport, studied the sensor potential abilities of single- molecule junctions formed by crown ethers attached to anthraquinone units, pinned to gold electrodes via alkyl chains. 74 Calculations showed that, due to a charge transfer from the ion to the molecular wire, the molecular resonances shift closer to the electrode Fermi energy and the conductance results enhanced by different amounts by changing the complexed alkali metal (Figure 26). On the other hand, Ponce et al. reported an interesting work studying the effect that a metal coordination has on the conductance of single molecular wires. MCBJ method revealed a variable metal coordination effect depending on the anchoring group. They investigated the conductance properties of a family of rigid, highly conductive ligands using acetylthiols, pyridines and ethynyl groups as anchors. Upon metal coordination, the molecular conductance of thiol and ethynyl 74 A. K. Ismael, A. A. Jobory, I. Grace, C. J. Lambert, J. Chem. Phys. 2017, 146, 064704. Figure 26: a) Anthraquinone molecular wires containing crown ethers. b) Transmission coefficient T(E) of 7 with different alkali metals. Ref 73. Chapter 1. Background 43 derivatives decreases, whereas that of pyridine derivatives increases (Figure 27). 75 In conclusion, all the aforementioned experimental and theoretical findings reveal the degree of development achieved in the so-called molecular electronics and represent continuous and important steps toward the achievement of real molecular electronic devices for eventually realistic applications. However, as stated before, we are still far away from this ultimate goal. 75 J. Ponce, C. R. Arroyo, S. Tatay, R. Frisenda, P. Gaviña, D. Aravena, E.Ruiz, H. S. J. van der Zant, E. Coronado, J. Am. Chem. Soc. 2014, 136, 8314. Figure 27: a) Chemical structures of molecular wires 9-11 and complexes 9c-11c. b) Conductance histograms of the bare and complexed forms. Ref 74. 3. Objectives Chapter 1. Objectives 47 2. Objectives Based on the examples discussed in the previous section about the interesting opportunities displayed by non-covalent interactions, the main objective in the first part of this thesis focuses on the development of supramolecular wire structures whose electronic properties can be investigated and potentially exploited to obtain a functional molecular element of a circuit. In order to simplify the approach to this first chapter of the work, the next section is divided in two main parts involving amidinium-carboxylate and crown ethers, respectively, as common chemical functional groups. • Part 3.1. Amidinium-Carboxylate Based Supramolecular Wires. In this section, we have carried out the synthesis of a new generation of molecular wires sharing the common feature of the amidinium-carboxylate non-covalent interaction as core bridge unit. Our proposal was centered on studying the conductance properties in two target series: a) Fullerene dumbbells series, on one side, constituted by fulleropyrrolidines characterized by various molecular lengths and side chains of different nature (Figure 28). Figure 28: Scheme of the target fullerene dumbbells molecules. Chapter 1. Objectives 48 b) Linear supramolecular wires series, on the other side, composed by aromatic derivatives characterized by different lengths and anchoring groups (Figure 29). • Part 3.2. Crown Ethers Switching Systems as Supramolecular Wires. This chapter focuses on the synthesis of functionalized linear crown ether structures as supramolecular wires with switching power conferred by the metal complexation. Inspired by the variable effect that a metal coordination has on the conductance of single molecular wires depending on their anchoring group 75 and the well-known potentialities of crown ethers as sensors and switches, we decided to investigate the only area which remained unexplored, their conductance properties. 73 Two consecutives projects have been addressed: a) Synthesis of 4,5'-diaminodibenzo[18]crown-6, on one side, to investigate the electronic properties of a short molecular wire containing a 18-crown-6 ether connected via a primary amino group to two gold electrodes (Figure 30); Figure 29: Linear supramolecular wire series based on aromatic compounds Chapter 1. Objectives 49 b) Synthesis of oligo(p-phenylene ethynylene) (OPE) and poly(p- phenylene)(PPP) crown ether molecular wires series, on the other side, to study the effect of a metal coordination on these wire structures, where the crown ether moiety is not directly inserted in the π-conjugated system but laterally connected to the central unit. Different anchoring groups have been proposed to study the variable effect that the anchoring group could have on the conductance upon metal coordination (Figure 31). 75 Figure 30: 4,4'-Diaminodibenzo-18-crown-6, (left) empty, and complexed with potassium (right). Figure 31: OPE and PPP crown ether wires series, (left) empty, complexed with potassium (right). 4. Results and Discussion Chapter 1. Results and Discussion 53 3. Results and Discussion 3.1. Amidinium-Carboxylate Based Supramolecular Wires Among the many possible ways to introduce hydrogen bonds, the choice of using the amidinium carboxylate salt bridge was dictated by the following different considerations: i) The two-point amidinium- carboxylate binding motif guarantees an extraordinary stabilization introducing a strong component of selectivity and directionality in the charge transfer process. ii) A previous work of our group focused on the efficiency of non-covalent donor-acceptor ensembles, composed by the interaction of a porphyrin-amidine and a fulleropyrrolidine- carboxylic acid. 55 iii) The exceptionally strong electronic coupling deriving from such binding, results in remarkably high values of association constant, even in polar solvents (up to 10 5 M -1 in THF), and more efficient formation of longer-lives radical-ion-pair states (~10 µs in THF). iv) A recent investigation of the aforementioned H-bonding motif was also realized in our group in 2016. 43 Interestingly, the molecular wire behaviour of such “supra-system”, was described through the determination of its attenuation factor as a function of the growing length of the C60-p-phenylene carboxylic acid oligomers. The extraordinary small -value obtained (0.07±0.01 Å -1 ) is among the lowest -values reported in the literature. 3.1.1. Fullerene Dumbbells Series Dumbbell-type molecules are symmetric compounds characterized by a typical dumbbell shape, where two identical molecular units are bridged through a spacer. Fullerene dimers, in which two [60]fullerene spheres are covalently connected through a molecular electroactive bridge, are therefore perfect example of dumbbell shape molecules. Dumbbells-shape covalent molecular wires based on fullerenes are synthetically achieved by bifunctional cycloaddition reactions to C60, that means, a derivatization of C60 with bifunctionalized molecules. 76 The first non-covalent H-bonding C60-dimer was firstly reported independently by Hummelen 77 and Martín. 78 76 J. L. Segura, N. Martín, Chem. Soc. Rev. 2000, 29, 13. 77 M. T. Rispens, L. Sánchez, J. Knol, J. C. Hummelen, Chem. Commun. 2001, 0, 161. 78 J. J. González, S. González, E. Priego, C. Luo, D. M. Guldi, J. de Mendoza, N. Martín, Chem. Commun. 2001, 0, 163. Chapter 1. Results and Discussion 54 The compounds proposed in this first part, are non-covalently linked through the two point H-bonding interaction of the functionalized aromatic units of the spacer. Therefore, a stepwise different synthetic strategy requires to be used. However, these compounds own some of the main desirable properties required for the study of molecular wires in a nanoscale approach. Among them, the stiffness, symmetry and linearity of their structures, besides, the visibility on a gold surface for the conductance experiments performed through STM setups, thanks to the bigger size of the fullerene in comparison to other significantly smaller anchoring groups. 11d,79 3.1.1.1. Synthesis of Proton-Donor and Proton-Acceptor Fulleropyrrolidines 2-Substituted N-methyl-3,4-pyrrolidino[3,4:1,2][60]fullerenes (fulleropyrrolidines) are products easily achievable from the decarboxylation of the iminium salts derived from the condensation of substituted amino acids with different starting aldehydes (Scheme 1). The 1,3-dipolar cycloaddition of azomethine ylides to C60, also known as "Prato's reaction", is among the several successful methodologies employed for the functionalization of fullerenes. It is a site-selective reaction, because the addition occurs only at a 6,6 ring junction. 80 The reaction is easily controlled to get the monoadduct compound, and the starting materials (substituted amino acids and aldehydes) are commercially available or readily synthesized from commercial 79 a) C. A. Martin, D. Ding, J. K. Sørensen, T. Bjørnholm, J. M. van Ruitenbeek, H.S. J. van der Zant, J. Am. Chem. Soc. 2008, 130, 13198; b) K. Gillemot, C. Evangeli, E. Leary, A. La Rosa, M. T. González, S. Filippone, I. Grace, G. Rubio-Bollinger, J. Ferrer, N. Martín, C. J. Lambert, N. Agraït, Small 2013, 9, 3812. 80 M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993, 115, 9798. Scheme 1: 1,3-Dipolar cycloaddition of azomethine ylides to C60. Chapter 1. Results and Discussion 55 sources. Another noticeable advantage associated to this powerful methodology is that two functional chains can be simultaneously introduced (R1 and R2), offering the possibility to incorporate, in one step, the desired functionalities along with the role of increasing the solubility of the product. When R2 is not hydrogen, a stereogenic centre is generated at the C2 position in the pyrrolidine ring. Anyway, this is just a minor drawback, because it can jump out only when either R1 or R2 chains contain a chiral group, determining the production of diastereomeric mixtures. Therefore, taking into account the previous Scheme 1, the desired final compounds were obtained as depicted in their retrosynthetic analysis (Scheme 2). Scheme 2: a) Functional group synthesis of the H-donor fulleropyrrolidines. b) Retrosynthetic analytic chart of the target fullerenes dumbbells molecules. Chapter 1. Results and Discussion 56 The synthesis of the H-donor and H-acceptor fulleropyrrolidines can be separated in three main building blocks (Figure 32): a. the side chains (R1) b. the skeleton of the spacer (p-phenylene backbone) c. the functional groups (R2) a. Side Chains The synthesis of the side chains (R1) was achieved from the corresponding α-aminoacids: N-octylglycine and N-(3,6,9- trioxadecyl)glycine (Scheme 3). Initially, it was decided to synthesize the desired fulleropyrrolidines using a simple apolar-substituted α-amino acid. Hence, the choice was N-octylglycine, which is easily achievable in only one synthetic step. Figure 32: Main building blocks for the fullerene dumbbells. Scheme 3: Synthetic routes to the amino acids containing the side chains (R1). Chapter 1. Results and Discussion 57 The commercially available α-iodocarboxylic acid can be converted via substitution reaction to the α-amino acid 1, through nucleophilic attack of commercially available octylamine, through its amino group. In order to increase the solubility of the final compounds in more polar solvents, it was decided to introduce a more polar appendage consisting in a triethyleneglycol chain. The synthetic pathway employed to obtain the more polar α-amino acid consists in a three steps process. The synthesis starts with a Swern oxidation, reaction where the hydroxy group of the easily available triethyleneglycol monomethyl ether is converted into the aldehyde functionality of 2. The following condensation of glycine benzyl ester with aldehyde 2 under reductive conditions (NaBH3CN) yielded the N-Cbz-N-(3,6,9-trioxadecyl)glycine benzyl ester 3, which was finally reduced, by hydrogenolysis of the benzyl ester, to the N-substituted glycine 4. 81 b. Spacer Skeleton The core structure of the wires consists on a p-phenylene backbone of 1 or 2 units. The longer skeleton is extended by a sequence of coupling reactions starting with a Miyaura borylation to obtain the p- functionalized phenylboronic acid pinacol esters 5d-f (Scheme 4). The choice of using boronic esters instead of boronic acids is related to the instability of the latter in not truly anhydrous conditions. They tend 81 T. Da Ros, M. Prato, F. Novello, M. Maggini, E. Banfi, J. Org. Chem. 1996, 61, 9070. Scheme 4: Synthetic route to the elongation of the p-phenylene backbone. Chapter 1. Results and Discussion 58 to trimerize generating cyclic boroxines. 82 Therefore, the commercial starting materials methyl 4-bromobenzoate, 4-bromobenzonitrile and 4- bromobenzoic acid are coupled with bis(pinacolate)diboron to obtain the corresponding phenylboronic acid pinacol esters 5d-f. The latter are then reacted with 4-iodobenzaldehyde in a Pd catalysed Suzuki coupling reaction to obtain the final 4'-formyl-[1,1'-biphenyl]-4- substituted products 6d-f. At this point, compounds 6a-f are employed in the condensation, through Prato's reaction, with N-octylglycine 1 or N-(3,6,9-trioxadecyl)glycine 4 and C60 to give, respectively, the [60]fullerene dumbbells 7a-c and 10a-d (Scheme 5). c. Functional Groups The 4-(1-octyl-3,4-fulleropyrrolidin-2-yl) methyl ester derivatives 7a,c were hydrolysed under basic conditions to achieve the respective acid functionalities of the final products 8a,c (Scheme 6). 82 D. G. Hall, Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine, Wiley-VCH Verlag GmbH & Co. KGaA., 2006. Scheme 5: Fulleropyrrodines’ synthesis. Chapter 1. Results and Discussion 59 The nitrile group of fulleropyrrolidines 7b and 10b,d also requires a further treatment to give the desired conversion into the final amidine products. This final step consists in an adjustment of Garigipati’s method 83 by using the Weinreb’s amide transfer reagent (Scheme 7). 84 The active transfer agent is achieved from the interaction of trimethyl aluminium and ammonium chloride in warm anhydrous toluene under evolution of methane. The nitrile group of the fulleropyrrolidines with 1 or 2 phenylene units readily attacks the methylchloroaluminium amide giving rise to the formation of an aluminium complex, which evolves to the amidinium chloride species under heating. The final basic hydrolysis of the latter compounds transforms the salts into the corresponding neutral desired amidines (Scheme 8). 83 R. A. Garigipati, Tetrahedron Lett. 1990, 31, 1969. 84 J. I. Levin, E. Turos, S. M. Weinreb, Synthetic Communications 1982, 12, 989. Scheme 7: Amidine synthesis from Weinreb’s amide formation. Scheme 6: Hydrolysis of 2-(4-methoxycarbonylphenyl)pyrrolidino[3,4:1,2][60]fullerene. Chapter 1. Results and Discussion 60 All new products were satisfactorily characterized by standard analytical and spectroscopic techniques. A reliable proof for the successful conversion of the nitrile function into the amidinium group is the disappearance of the characteristic band of CN group at 2228 cm - 1 in the FTIR spectra of compounds 8b and 10b,d. Additionally, the appearance of a double peak around 3393 and 3457 cm -1 , corresponding to the asymmetric and symmetric stretching bands of primary amines, part of the amidinium group, confirms this conversion. Interestingly, other characteristic features of the amidinium-containing fulleropyrrolidines can be identified. The wagging band of amine group (-NH2) between 649 and 910 cm -1 , the scissoring band of the same group at 1646 cm -1 and the sp 3 stretching bands of the carbons bonded to hydrogen (C-H) at 2855 and 2923 cm -1 , evident identification of the glycolic chain. As an example, the compared FTIR spectra of molecules 10b and 12b are shown in Figure 33. Scheme 8: Synthesis of amidine-containing fulleropyrrolidine derivatives. Chapter 1. Results and Discussion 61 3.1.1.2. Formation and Characterization of Supramolecular Ensembles Complexation of the supramolecular ensemble was realized starting from the neutral form of the amidinium-containing fulleropyrrolidines. Once converted by basic hydrolysis to the corresponding amidine molecular structures, proton donor compounds 9b and 12b,d were complexed with the proton acceptor molecules 8a,c and 10a,c (Figure 34). Figure 33: Comparison of FTIR spectra of compounds 10b and 12b. Chapter 1. Results and Discussion 62 Considering the hydrogens involved in the supramolecular interaction as axial hydrogens (NHax) and the hydrogens external to the two point connection as equatorial hydrogens (NHeq) then, the creation of the complexes results evidently confirmed by the shift of the amidinic moiety’s protons. The clear signature of salt bridge formation is the concentration-dependent downfield shift of amidinium protons involved in hydrogen bonding to carboxylate (NHax shifts around 0.3 ppm after addition of one equivalent of the carboxylic acid fulleropyrrolidine) and the insensitivity of the chemical shift for the amidinium protons external to the salt bridge (shift of NHeq is smaller than 0.03 ppm). In Figure 35, the qualitative 1 H NMR spectra of the isolated molecules 10a and 12d and the corresponding supramolecular complex formed at equimolar concentration 85 is shown as representative example. 85 It was firstly recorded the 1 H NMR spectra of amidinium half 10a by preparing a solution of the compound at 10 -3 M in CHCl3. Following, an equivalent quantity of the carboxylic unit 12d, corresponding to a 1:1 ratio, was added. The resulting solution of the complex 12d·10a, at the final concentration of 10 -3 M in CHCl3, was formed by Figure 34: Supramolecular dumbbell-type fulleropyrrolidine complexes 9b·8a,c and 12b,d·10a,c. . Chapter 1. Results and Discussion 63 Complete assignment of the protons was realized with additional HMQC, COSY and DEPT experiments. 3.1.1.3. Supramolecular Ensembles: Physical Characterization Physical analysis has allowed a further insight into the characterization of the products and their non-covalent complexes. The morphological study of these supramolecular assemblies on highly ordered pyrolytic graphite (HOPG) supports by drop deposition revealed the sample’s structure dependence on its molecular concentration. Specifically, AFM experiments demonstrated the formation of supramolecular aggregates. The size of these aggregates varies increasing the concentration of the light sonication of the mixture to favour the solubilisation of the proton acceptor moiety into the proton donor solution and finally analyzed. Figure 35: 1 H NMR (CDCl3) spectra of 10a and 12d together with the corresponding supramolecular complex formed at equimolar concentration 12d·10a. Chapter 1. Results and Discussion 64 preformed supramolecular complexes (at 1:1 ratio) 86 which were deposited. Final clusters of variable size between 6 and 20 nm were visible at higher concentration, passing first through the creation of islands of molecules, whose fusion led to one-molecule thickness monolayers (Figure 36). Due to the limited solubility of the N-octylglycine derivatives, it was impossible to perform further analysis related to their conductance properties. This was the main reason why we decided to change the side chain, using the more polar amino acid N-(3,6,9- trioxadecyl)glycine. This change produced the desired increase in the solubility of the products, allowing to test their electronic properties as supramolecular hydrogen bonded wires. The conductance properties of these non-covalent ensembles were proved by STMBJ measurements, carried out separately before complexation on the amidinium and 86 The 1:1 solutions of 9b·8a and 9b·8c were prepared by mixing the volumes’ solutions of the corresponding proton donor and proton acceptor compounds at the final concentration of 10 -6 M and 10 -5 M in CHCl3. Each solution of the preformed complexes was deposited by drop casting method on the surface of the HOPG sample. The solvent on the samples was dried by evaporation in a preheated oven at 60°C overnight and then analyzed at AFM. Figure 36: AFM morphological analysis of HOPG blank (a) and AFM imaging of the supramolecular complex 9b·8a at 10 -6 M (b) and 10 -5 M (c) deposited on HOPG. Chapter 1. Results and Discussion 65 carboxylate derivatives on the one hand and the whole supramolecular assembly on the other hand (Figure 37). 87 From the deposition on gold of different samples of diluted solutions of both the amidinium and carboxylate compounds, two conductance peaks could be identified. The first of these conductance peaks, at higher value of G0, is assigned to the conductance of the C60 moiety and the second one, at lower value of G0, to the molecules connected through the amidine and the carboxylic acid anchoring groups. The STMBJ measurements of the preformed complex (Figure 37) evidenced only the presence of the separated halves (the amidinium and the carboxylate compounds) but not the peak relative to the supramolecular complex itself, probably because the conductance value results lower than the noise cut off of the set up and should, therefore, be smaller than 10 -5 G0. Anyway, as visible from Figure 37, the amidinium half 12b of the complex shows that the conductance associated to the peak 87 The 1:1 solutions of 9b·8a and 9b·8c were prepared by mixing the corresponding volumes of the proton donor and proton acceptor compounds at the final concentration of 10 -6 M in CHCl3. The solution of the separated halves (each one at a concentration of 10 -6 M in CHCl3) and the preformed complexes were deposited by drop casting method on the surface of a gold sample. The samples were dried by evaporating the solvent in air and then analyzed at STMBJ. Figure 37: STMBJ of the supramolecular fulleropyrrolidines 12b·10a. The G0 peak identifies the conductance of one Au atom between the electrodes. Chapter 1. Results and Discussion 66 of the functional group is quite high. Therefore, this result interestingly paves the way to the investigation of the amidiunium/amidine functionalities as efficient and unexplored anchoring groups in molecular electronics. 3.1.2. Linear Supramolecular Wires Series This chapter stemmed from the necessity to get a deeper understanding about the two point amidinium-carboxylate interaction as supramolecular alternative connection to the covalent bonding in molecular wires. Therefore, we decided to synthesize really basic systems, whose key strength exactly relies on a linear design to facilitate the investigation and understanding of conductance properties through their wire structure. To this purpose, the initial choice was to use two well-known aurophilic groups, pyridine (Py) and thiomethyl ether (SMe) as the anchoring groups. Physical conductance measurements revealed lately that the pyridine derivatives result inappropriate to study the ET properties by STMBJ, because we were not able to detect neither the signal of the isolated molecules nor the complex. 3.1.2.1. Synthesis of Proton-Donor and Proton-Acceptor Moieties for Linear Supramolecular Wires The synthesis of the desired products requires just few steps to obtain both the corresponding amidine derivatives and the carboxylic extended structures (Figure 38). Starting from the commercial precursors, 4-(methylthio)benzonitrile and 4-pyridinecarbonitrile, the amidine derivative 14a,b were obtained through the direct conversion of the nitrile group through its reaction Figure 38: Linear supramolecular wires series 15a,b·14a and 16a,b·14b. Chapter 1. Results and Discussion 67 with the preformed methylchloroaluminium complex, by following the adjusted Garigipati’s method 83 previously described for the synthesis of the dumbbells wires series, followed by the neutralization step with NaOH (Scheme 9). On the other side, 4-(methylthio)benzoic acid and isonicotinic acid are products directly available from commercial sources. Furthermore, the spacer structure was easily expanded through a Pd catalysed Suzuki cross-coupling reaction. The potassium pyridine- and (methylthio)phenyl- 4-trifluoroborate salts were coupled with 4- bromobenzoic acid to give the corresponding carboxylic biphenyl derivatives (Scheme 10). The construction of the supramolecular systems was realized in the same way that the formation of fulleropyrrolidine based non-covalent assemblies. The assembly of the tailored molecular building blocks leads to the final desired linear molecular wires (Figure 39). Scheme 9: Amidine synthesis of the pyridine and (methylthio)phenyl derivatives. Scheme 10: Suzuki coupling to achieve the carboxylic biphenyl derivatives of both anchoring groups. Figure 39: Complexes formed from the pyridine and thiomethylether derivatives. Chapter 1. Results and Discussion 68 Because it was not possible to study the conductance properties of the supramolecular wire series having pyridines as anchoring groups, only the thiomethyl ether series was completed with the synthesis of an additional compound, which was employed as reference (Figure 40). The 4,4''-bis(methylthio)terphenyl 17 was identified as the most similar covalent compound to compare the conductance of the supramolecular ensemble 16a·14b. The synthesis was achieved by following one-step procedures. Molecule 17 was obtained by a Suzuki cross-coupling reaction between the commercial products 4-(methylthio)phenylboronic acid and p- dibromobenzene 88 (Scheme 11b). All intermediates, reference compounds and final complexed products were unambiguously characterized by standard spectroscopic and analytical techniques (see experimental section). 3.1.2.2. Formation and Characterization of Supramolecular Ensembles Complexation studies of supramolecular ensembles were carried out by monitoring the changes observed in the chemical shifts (δ) of the proton signals of the amidinium moiety 14b after addition of increasing quantities of the carboxylic derivatives 16a,b. Both titration experiments for the formation of the complexes 16a·14b and 16b·14b 88 Y. Joseph, A. Peic, X. Chen, J. Michl, T. Vossmeyer, A. Yasuda, J. Phys. Chem. C 2007, 111, 12855. Figure 40: Refence compound 17. Scheme 11: Synthesis of the reference compounds 17. Chapter 1. Results and Discussion 69 perfectly show how the CH resonance of the protons of the aromatic ring close to the amidinium group undergo a downfield shift upon increasing the addition of the carboxylate derivatives. These downfield shifts give evidence of the formation of a positive charge in the amidinium group by interaction with the carboxylate (Figure 41). The titration analyses were conducted at room temperature and/or, in some cases, lowering the temperature below zero Celsius degrees. The decision to decrease the temperature is related to the equilibrium nature of supramolecular forces. In fact, the reduction of molecular vibrational degrees permits to block molecules in their more stable conformation. The determination of the association constant of the complex was carried out by 1 H NMR titration experiments. Because of the well- known strength of the amidinium-carboxylate interaction 55 and that, as stated by data from literature, 89 the limit for NMR titration experiments corresponds to around 10 5 M -1 , it was necessary to use a solvent whose polarity was high enough to decrease the value of the corresponding association constant. Therefore, the choice of the solvents were [D7]DMF and [D6]DMSO. [D7]DMF qualitative titration experiments show the clear appearance of the signal relative to the amidinium 89 a) C. S. Wilcox, Frontiers in Supramolecular Organic Chemistry and Photochemistry, H. J. Schneider and H. Dürr, VCH, Weinheim, 1991, pp. 123–144; b) P. Thordarson Chem. Soc. Rev. 2011, 40, 1305. Figure 41: Partial 1 H NMR of 14b (middle) and the 1:1 supramolecular complexes 16a·14b (down) and 16b·14b (up) in [D7]DMF. Chapter 1. Results and Discussion 70 protons involved in the supramolecular interaction. It appears as a broad band changing its width and position with the guest concentration, whose equatorial and axial hydrogen atoms can be resolved by decreasing the temperature to -15 °C (Figure 42). The shift of this broad signal (from 7.1 to 9.9 ppm for 16a·14b), corresponding to the amidinium protons, evidences that the supramolecular interaction is effectively formed. On the other hand, the quantitative titration experiments of the wires complexes 16a·14b and 16b·14b, carried out in [D6]DMSO, allowed to determine Ka values ≥ 10 4 M -1 for both complexes. 90 This result, achieved in so polar solvent, clearly reveals the extraordinary strength of the supramolecular assembly for this specific amidinium-carboxylate units. 90 See in Annex Titration Section for more details. Figure 42: Protons shift and resolution at -15°C of the amidinium moiety of compound 11b in titration experiments in [D7]DMF for the complex 16a·14b. Chapter 1. Results and Discussion 71 3.1.2.3. Supramolecular Ensembles: Physical Characterization Since the main interest of this part of the thesis relies on the investigation and understanding of the ET processes in single molecular systems characterized by weak supramolecular interactions, an accurate physical characterization of the final products was carried out. Separate measurements of charge transfer features of the single units and the pre-formed complexes were conducted in ambient conditions by using a made-in-house STM. The molecules and their complexes were deposited on commercial gold over glass substrates, which serve as the first electrode, while the gold tip (freshly cut from a gold wire), acting as the second electrode of the STM, was repeatedly moved vertically (z) in and out of contact with the substrates covered by molecules. Several thousands of individual conductance (G) traces vs. tip displacement (z) traces were recorded. Regions of constant conductance (plateaus) at values below 1 G0 indicating the formation of a stable molecular junction between the STM tip and substrate were identified in these traces. But because this does not occur in all the G vs. z traces, in order to discriminate the molecular-junction signature, the traces with plateaus were separated from those without them. The results of the many breaking traces, that complied with the previous criterion, were gathered in a conductance histogram, which presents peaks at the conductance of the most probable configurations, and that is normally considered to be the conductance of a single-molecule junction. Also 2D-histograms of G vs. z traces were built giving valuable information about the breaking length, which is related to the length of the molecule. Firstly the amidine half 14b and the complementary carboxylic unit 16a, of the short methylthio-containing wire complex 16a·14b were studied, separately. 1D and 2D conductance histograms were built with approximately a 15-20% of all the recorded traces for both compounds (Figure 43). Chapter 1. Results and Discussion 72 While the results for the carboxylic derivative agree with the previously reported in the literature, 91 the results presented here for the amidine compound results again, as for the fulleropyrrolidine supramolecular wires, interestingly high and comparable with the one of the carboxylic unit, which in turn is commonly known to be a good anchoring group. Therefore in the present thesis it is shown for the first time that the amidine group can be as efficient linker to gold as the carboxyl group. Figure 43 clearly shows that the conductance values obtained for both compounds are very similar. In particular, a main conductance value of log(G/G0) = -3.0±0.9 for the carboxylic derivative and log(G/G0) = - 2.8±0.5 for the amidine one, where the error is given by the width at half maximum of the peaks observed in the conductance histograms. The typical length of the plateaus for the amidine and carboxylic halves was also determined (0.84 and 0.95 nm, respectively), resulting in good 91 S. Ahn, S. V. Aradhya, R. S. Klausen, B. Capozzi, X. Roy, M. L. Steigerwald, C. Nuckolls, L. Venkataraman, Phys. Chem. Chem. Phys. 2012, 14, 13841 Figure 43: 2D conductance histograms versus relative distance z of a) the amidine derivative 14b, and b) the carboxylic half 16a. Chapter 1. Results and Discussion 73 agreement with the expected theoretical length for each compound of ~0.68 nm. At this point, the measurements carried out on the short methylthio-containing wire complex 16a·14b evidence the presence of two plateaus of different lengths, one at high and the other at low values of G0, centered at log(G/G0) = -6.1±0.7 and -3±0.4 (Figure 44). In order to see these two peaks more clearly, two groups were made with the obtained traces, separating those that have plateaus in the low conductance range (Low-G) from those that do not (High-G). After this separation, the corresponding length distributions for the high and low conductance plateaus were obtained, with typical maximum length values of 0.74 nm (High-G plateaus) and 1.45 nm (Low-G plateaus) (Figure 45). Figure 44: 2D conductance histogram vs. relative distance z of 16a·14b. Figure 45: a) All the 1D conductance histogram traces of the complex 16a·14b, compared with the two separated groups at Low-G and High-G. b) Length distribution of the Low-G and High-G plateaus separately, taking into account the gold retraction after the gold contact breakage. Chapter 1. Results and Discussion 74 These results led us to conclude that the low conductance plateaus, almost doubling the length of the shorter plateau, are due to the molecular junctions formed by the whole complex, while the high conductance plateaus can be due to any of the two halves, which would result from the breakage of the complex, which seems to be partially happening when the complex is deposited over the gold STM substrate. In order to estimate the efficiency for electron conduction of the amidine-carboxylic interaction, the results for the amidine-carboxylic complex 16a·14b were compared with those for the terphenyl used as reference compound 17 (Figure 46). While the length of the plateaus for the amidine-carboxylic complex is only slightly longer, their conductance is more than two orders of magnitude lower, as for compound 17 a conductance of log(G/G0) = - 3.7±0.7 was obtained. Therefore, the conductance value found for the complex results unexpectedly low respect to the previous studies about CT in supramolecular DBA systems connected by the amidinium- carboxylate interaction. 55,56 A possible explanation for this result can be related to the mixed nature of the linkage in the amidinium-carboxylate interaction, which is not merely based on hydrogen bonding interaction but also on electrostatic forces due to the charge separation. Therefore, this charge separation can, in principle, interrupt the CT process along the wire. Figure 46: a) 2D conductance histogram vs. relative distance z of 17. b) Plateaus length distribution comparison of molecule 17 with the complex 16a·14b. Chapter 1. Results and Discussion 75 Finally, it was also studied the longer supramolecular complex 16b·14b. In Figure 47 the 2D histogram for all the traces with plateaus were shown. The highest peak, centered at log(G/G0) = -2.7 ± 0.4, are equivalent to those observed in the shorter complex and were assigned to the amidine derivative 14b. On the other hand, the lowest-G plateaus having a main conductance of log(G/G0) = -7.0 ± 0.6 and a typical maximum length of 1.7 nm, were assigned to junctions formed by the whole longer amidine-carboxyl complex 16b·14b. This reduction of one order of magnitude (from ~10 -6 G0 to ~10 -7 G0) between the short and long complexes corresponds to a calculated decay factor β of 2. This value, compatible with the decay factor reported in the literature for amine- oligophenyl compounds of 1.7 per added phenyl ring, 25 therefore, results in agreement with the elongation of the system. Indeed, when we deposited only the carboxylic part 16b of the longer complex, the same decay in conductance respect to the carboxylic half 16a of the short complex 16a·14b was observed. Figure 48 shows the former plateaus at a mean conductance of log (G/G0) = -4.0±0.6. Figure 47: a) 2D histograms versus for 16b·14b (all histograms present an artifact at -4.5 due to the use of an IV converter with two amplification stages with high gains). b) The length distribution of the Low-G and High-G plateaus, separately, shows the correspondence of the shorter plateaus to conductance of the amidine derivative 14b. Chapter 1. Results and Discussion 76 Summarizing, the results of low conductance for the methylthio ether complexes discouraged the possibility to further investigate the other pyridine linear wire series. In fact, measurements of the conductance of the complexes formed by the interaction of the pyridine derivatives (15a·14a and 15b·14a) did not afford any results. Due to the smaller size of the compounds in comparison to the methylthio ether analogues and the proved low conductance of the last ones, it resulted extremely difficult for the pyridine appended smaller complexes to identify any plateaus in the STMBJ experiments. 3.1.2.4. Theoretical Calculations of Supramolecular Ensembles To understand the low and high conductance behaviour of the STMBJ experiments for the different molecular junctions, electronic structure and transport calculations were also performed. Density functional theory (DFT) was used in combination with the non-equilibrium Green’s function (NEGF) formalism. 92 DFT+Σ method 93 was also employed, to avoid the self-interaction errors and image charge effects, introduced by DFT at the hybrid molecule-metal interfaces. First, the electronic structure of the molecules 14b and 16a and of the complexes 16a·14b and 16b·14b were analyzed in the gas phase (Figure 49). 92 F. Pauly, J. K. Viljas, U. Huniar, M. Häfner, S. Wohlthat, M. Bürkle, J. C. Cuevas, G. Schön, New J. Phys. 2008, 10, 125019. 93 S. Y. Quek, L. Venkataraman, H. J. Choi, S. G. Louie, M. S. Hybertsen, J. B. Neaton, Nano Lett. 2007, 7, 3477. Figure 48: 2D and 1D conductance histograms versus relative distance z of the longer wire complex 16b (a) and (b), respectively. Chapter 1. Results and Discussion 77 The calculated HOMO-LUMO gaps at the DFT level were found to be about 3 eV for 14b and 16a. These gaps were reduced to about 2 eV in the case of 16a·14b and 16b·14b, since the molecular complexes are obviously longer. The calculated binding energy for the assembly 16a·14b, which forms from 14b and 16a, was found to be -0.94 eV. Moreover, it results that the HOMO and LUMO of the short complex 16a·14b was localized on the 16a and 14b parts, respectively. Next step has consisted in the metal-molecule-metal junctions construction. Here molecules and complexes bridge two Au electrodes. After exploring various possible binding configurations, the lowest energy configurations were determined to be S and N, S and O, S and S, S and S binding to undercoordinated Au atoms on both sides for 14b, 16a, 16a·14b and 16b·14b, respectively, as shown in Figure 50. Figure 49: Calculated density of states (DOS) of the free molecules (a) 14b, (b) 16a, (c) 16a·14b and d) 16b·14b. For each molecule, in addition, the spatial distributions of HOMO and LUMO orbital wave functions were presented. Isosurfaces of positive and negative isovalues are shown in red and blue, respectively. Chapter 1. Results and Discussion 78 These findings are in agreement with previous studies on carboxylic and methylsulfide molecular junctions. 94 Low conductance values for the molecular junctions formed by the compounds 14b, 16a, 16a·14b and 16b·14b were derived with the DFT+Σ method. (Table 3) These results suggest a rather big mismatch between the Fermi level of Au and both the HOMO and LUMO levels of the products (Figure 51). 94 D. L. Bao, R. Liu, J. C. Leng, X. Zuo, Y. Jiao, Z. L. Li, C. K. Wang, Phys. Lett. A 2014, 378, 1290. Figure 50: Optimized ground-state geometries of the four different molecular junctions with C in yellow, H in light blue, S in white, N in blue and O in red. Table 3: Calculated conductances by DFT and DFT+Σ methods compared with the experimental results. GDFT GDFT+Σ Gexp 14b 1.2x10-2 1.2x10-3 10-3 16a 8.8x10-3 4.6x10-4 10-3 16a·14b 2.5x10-5 1.0x10-6 10-6 16b·14b 2.0x10-5 5.0x10-7 10-7 Chapter 1. Results and Discussion 79 These transmission curves evidence that molecular transmission resonances occur far away from the Fermi energy (EF). While the transmission curves for 14b and 16a exhibit a rather similar behaviour, the conductance of 14b is a factor of about 3 larger than that of 16a. Visibly, a much lower conductance for the wires complexes 16a·14b and 16b·14b is observed. This reduction is attributed to the longer size of the molecular complexes and an off-resonant transport. In Figure 51 it can also be identified a Fano anti-resonance at about -2 eV with respect to the Fermi level for 16b·14b. This is considered related to an interference effect between a localized molecular orbital and a delocalized one. Overall, the conductance calculated by the DFT+Σ method agrees well with the experimental values, while DFT overestimates the experimental conductances by about one order of magnitude. Therefore, theoretical calculations are in agreement with the experimental study, demonstrating that the high conductance values measured correspond to molecular junctions formed by only one of the two halves (14b, 16a) while the low conductance features, to the molecular junctions formed by the whole complex (16a·14b or 16b·14b respectively). 3.2. Crown Ethers Switching Systems as Supramolecular Wires Crown ethers are ring shape molecular structures containing several ether groups. Their discovery (awarded later in 1987 with the shared Nobel Prize in Chemistry together with Donald J. Cram and Jean-Marie Figure 51: Energy dependent electronic transmission probability relative to EF, calculated by the DFT+Σ method. Chapter 1. Results and Discussion 80 Lehn) was performed by Charles J. Pedersen in 1967, at DuPont, while he was trying to prepare a complexing agent for divalent cations. 95 Part of his research included the design of a synthetic route toward the bis- phenol depicted in Scheme 12. However, a secondary by-product was obtained as white crystals insoluble in hydroxylic solvents due to incomplete protection of starting catechol. 96 Luckily, Pedersen further investigated this by-product discovering that it was a biphenol derivative composed by a 18-membered ring able to accommodate cations in the hole located in the centre of the molecule. The molecular model of dibenzo[18]crown-6 (DB18C6) shows in all its evidence the exactitude of the chosen name to define these systems, which, due to their extraordinary ability to surround the bonded cation, behave as a crown do wreathing the head of a king (Figure 52). 95 C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 2495. 96 C. J. Pedersen, J. Inclusion Phenom. 1988, 6, 337. Scheme 12: The first crown ether obtained as by-product. Figure 52: a) The first crown ether synthesized, DB18C6, in its empty and complexed form. b) Molecular model of DB18C6·Na + complex. Chapter 1. Results and Discussion 81 Therefore, the most significant property of crown ethers consists in their use as hosts for binding inorganic and organic guests, especially cationic ones. 97 The stability of the formed complexes arises from ion- dipole interactions, when complexing metallic cations, or H-bonds, when complexing ammonium cations. This interaction is maximized when the diameter of the unsolvated ion fits exactly the size of the crown ether. However, complexation can be achieved even if the fit is not optimal. If the crown is too large, it can wrap around the cation or can also complex several ions. While, if the crown is too small, it can forms a sandwich system consisting of two crown ethers per cation. 95 Therefore, our interest focuses clearly in taking advantage of their peculiar ability to create stable but reversible supramolecular structures towards the synthesis of wire systems, characterized by a switching power conferred by this ion-dipole interaction. The substantial aim of this chapter, as these systems are already widely investigated, goes in the direction of studying the only aspect remained unexplored, “how their CT properties can be modulated by this phenomenon of complexation”. 3.2.1. Trans-4,5'-diaminodibenzo[18]crown-6 The first project developed in this direction was the synthesis of a simple short molecular wire containing a 18-crown-6 ether connected via amino groups to two gold electrodes (Figure 53). The synthesis of this system was carried out on the basis of a theoretical investigation of 2003 where the same molecular structure, varying just for the anchoring group used, is studied from the point of view of its electronic properties, by complexing with different cations (Li + , Na + , K + and H + ). 75 This theoretical work found that the cationic 97 C. J. Pedersen, J. Am. Chem. Soc. 1967, 89, 7017. Figure 53: 4,5'-diaminodibenzo[18]crown-6 in its empty and complexed form. Chapter 1. Results and Discussion 82 binding significantly lowers the conductance in comparison to the empty form of the crown ether, due to quantum mechanical interference effects. Furthermore, a striking insensitivity of conductance to the type of ligand is predicted. A recent theoretical work of Lambert group also demonstrated that the spectrum of responses from molecules with different crown-ether side groups supplies a fingerprint which can be used for discriminating sensing of alkali metals. 74 Therefore we decided to carry out an experimental work to study this conductance switching behavior of crown ethers. We decided to examine the effect of the only flawless fitting K + cation, whose ionic radius of 1.38 Å, perfectly accommodates in the cavity of 18C6, having a radius of 1.34-1.43 Å. 98 3.2.1.1. Synthesis of trans-4,5’-diaminodibenzo[18]crown-6 To achieve the product 18, a one step reduction from the commercially available trans-4,5'-dinitrodibenzo[18]crown-6, using hydrazine hydrate and nickel Raney as the catalyst, allows to get the final product (Scheme 13). The product was fully characterized by standard spectroscopic and analytical techniques. 1 H NMR analysis of 18, for example, shows a broad signal around δ= 2.74, corresponding to the protons of the amine groups, absent in the starting product. The FTIR spectrum of 18 also showed the characteristic double peaks of primary amines at 3246– 3424 cm -1 (Figure 54). 98 N. K. Dailey, in R. M. Izatt and J.J. Christensen (Eds.), Synthetic Multidentate Macrocyclic Compounds, Academic Press, New York, 1978, 207. Scheme 13: Synthesis of trans-4,5'-diaminodibenzo[18]crown-6 Chapter 1. Results and Discussion 83 Anyway, once the empty crown ether 18 was analyzed by STMBJ technique, unfortunately, the measurements did not evidence any conductance signal. This finding, unexpected from the theoretical study, was justified attending to the lack of conjugation between the two aromatic systems. 3.2.2. Oligo-phenylene ethynylene (OPE) and Poly(p-phenylene) (PPP) Crown Ether Molecular Wires Series The previous results have prompted us to think about a new design for the development of molecular wires including crown ether moieties as a switching unit. We propose a different molecular organization, where it was avoided that the positioning of the crown ether could interrupt the -conjugation along the structure. Therefore, we decided to synthesize fully conjugated OPE and PPP derivatives, where the crown ether ring Figure 54: a) 1 H NMR spectrum in CDCl3 and b) FTIR spectrum of 18. Chapter 1. Results and Discussion 84 is located as pendant on the benzene’s central unit while at the extremities of the wire structure two well-known anchoring groups, the pyridine and the methylthio ether were used. This design was thought to help in evidencing the variations of conductivity produced by metal coordination on systems whose CT properties are renowned. Moreover, the study of the two different anchoring groups offers the possibility to demonstrate the effect that metal coordination has on molecular conductance for each anchoring group, as shown by Ponce’s work. 75 3.2.2.1. Synthesis of OPE and PPP Crown Ether Molecular Wires Series The OPE and PPP crown ether molecular wires derivatives were obtained as depicted in their retrosynthetic analysis. (Scheme 14) The key steps towards the OPE (25a,b) 99 and PPP (26a,b) derivatives are, respectively, Pd catalyzed Sonogashira reaction from derivatives 24 and 20a,b and a Suzuki coupling from derivatives 24 and the commercial boronic acids of pyridine and 4-methylthio benzene (Scheme 14). 99 Y. L. Zhao, L. Liu, W. Zhang, C. H. Sue,Q. Li, O. Š. Miljanid, O. M. Yaghi, J. F. Stoddart, Chem. Eur. J. 2009, 15, 13356. Chapter 1. Results and Discussion 85 To synthesize the desired OPE and PPP derivatives 25a,b and 26a,b it was employed a convergent synthetic route. Important starting materials to these synthesis were 4-[(trimethylsilyl)ethynyl]pyridine (20a) and (4-ethynylphenyl)(methyl)sulfane (20b) (Scheme 15). Scheme 14: Retrosynthetic analytic chart of the target OPE and PPP crown ether wires series. Scheme 15: Synthetic procedure towards 4-[(trimethylsilyl)ethynyl]pyridine 20a and (4-ethynylphenyl)(methyl)sulfane 20b. Chapter 1. Results and Discussion 86 The products were prepared starting by a Sonogashira coupling reaction from commercial 4-bromopyridine hydrochloride and (4-bromophenyl) (methyl)sulfane, respectively, with trimethylsilylacetylene in a really good yield, according to a similar previously reported procedure. 100 Then, the thiomethyl derivative 19, differently from compound 20a, went through a desylilation reaction. The central core of the wire, on the other side, was prepared by a sequence of 5 steps (Scheme 16). 99 1,4-Bis(trimethylsilyl)-2,3-dimethoxybenzene (21) was prepared from 1,2-dimethoxybenzene (veratrol) in two steps. Compound 21 reacted with ICl in DCM giving the ipso electrophilic aromatic substitution on the trimethylsilyl positions, to afford 1,4-diiodo-2,3-dimethoxybenzene (21). Product 22 was treated with the demethylating agent boron tribromide being reduced to the diol 23. At this point, the precursor crown ether 24 was prepared through the macrocyclization of the pentaethylene glycol di(p-toluenesulfonate) with the diol 23. Once the central core structure of both wires series (OPE and PPP derivatives) was synthesized, compound 24 was employed to obtain the final products 25a,b and 26a,b (Scheme 17). 100 Y. Arakawa, S. Kang, H. Tsuji, J. Watanabe, G. Konishi, RSC Adv. 2016, 6, 92845. Scheme 16: Synthesis of the wires core structure. Chapter 1. Results and Discussion 87 Compounds 25a and 25b were prepared through a Pd catalysed Sonogashira reaction where 24 is coupled with 20a and 20b, respectively. Unfortunately, it was impossible to isolate pure final products, due to the formation of a stable electron donor–acceptor complex between the crown ether and triphenylphosphine oxide (TPPO). 101 On the other side, products 26a and 26b were synthesized in high yield by a Suzuki coupling between the molecular central core 24 and side arms represented by two commercial compounds, 4- pyridinylboronic acid and 4-(methylthio)benzeneboronic acid, respectively. All the intermediates and final compounds were unambiguously characterized by the standard analytical and spectroscopic techniques. 3.2.2.2. Formation and Characterization of the Complexed PPP Crown Ether Molecular Wires Series Treatment of methanol solutions containing the PPP crown ether molecular wires 26a,b and KSCN, in a 1:l ratio, leads to the formation of the potassium (K + ) complexes of the final crown ether compounds, 27a,b (Scheme 18). 101 S. Banerjee , P. Dey , S. Basu, Radiochemistry 2010, 52, 2, 162. Scheme 17: Synthesis of OPP 25a,b and PPP 26a,b crown ether molecular wires series. Scheme 18: Synthesis of the K + complexed PPP crown ether molecular wires 27a,b. Chapter 1. Results and Discussion 88 1 H NMR experiments visibly reveal the formation of the complex by comparison of the free host signals 26a,b and its signals after K + complexation (27a,b) (Figure 55). The strong upfield shift of the peaks corresponding to the crown ether moiety and the smaller shift of the signals of the aromatic protons of the PPP backbone are a clear indication of the successful complexation of the metal cation. Further proof that the potassium ion-dipole interaction with the hole of the crown ether occurs effectively was provided by ESI mass spectroscopy (Figure 56). Figure 55: 1 H NMR comparison of methylthio ether crown ether derivatives 26b and its potassium complex 27b. Chapter 1. Results and Discussion 89 The appearance of the peak corresponding to the molecule plus the mass of the potassium at m/z 595,2 for the complex 27b respect to the mass of the empty molecule 26b plus sodium at 579,2 m/z, which is consequence of the analytical technique itself, guarantees the occurrence of the complexation phenomenon. 3.2.2.3. Complexed PPP Crown Ether Molecular Wires Series: Physical Characterization The conductance properties of the synthesized systems was investigated by STMBJ technique. The measurements of the free compounds 26a,b were carried out without any difficulties, corresponding to similar p- terphenylene derivatives systems (Figure 57). 25 Figure 56: Comparison of the ESI mass spectra of empty 26b and complexed 27b. Chapter 1. Results and Discussion 90 While the determination of the conductance features of both empty compounds 26a,b, resulting to be around 10 -4 G0, was accomplished without difficulties, the measurements of complexes 27a,b are presenting some experimental problems. Work is currently in progress to try to determine these experimental values. Figure 57: STMBJ of empty PPP crown ether molecular wires 26a and 26b. 4. Conclusions Chapter 1. Conclusions 93 4. Conclusions The work developed in this first part of the thesis has allowed to further deepen in the knowledge relative to the employment of non-covalent interactions in the formation of systems potentially applicable in the field of molecular electronics.  Amidinium-Carboxylate Based Supramolecular Wires -The systems developed for this study have strengthened the well- known concept of the formation of highly stable non-covalent complexes based on the two point amidinium-carboxylate binding motif (see determination of the Ka in Appendix). -The investigation on their electron transfer properties was satisfactorily performed by the use of the STMBJ method, demonstrating that, through H-bonds, the behaviour of these transfer properties exhibits evident dependence on the junction length. -It also results, from this study, that the amidinium group displays electronics characteristics interesting enough to be further investigated and employed as novel anchoring group. -However, in spite of the aforementioned properties, and the potentiality of the H-bonding interactions to be employed as switching systems by simply changing the polarity of the solvent, the amidinium- carboxylate salt bridge does not compete with covalent spacers, showing conductance values around two orders of magnitude lower than covalent derivatives of comparable length. The low conductance Figure 58: Schematic representation of the supramolecular fulleropyrrolidines dumbbells (9b·8a,c; 12b·10a,c and 12d·10a,c) and supramolecular linear wires (15a,b·14a and 16a,b·14b). Chapter 1. Conclusions 94 values measured for the investigated systems, in fact, clearly evidences that these systems are not suitable to be employed as building blocks for the creation of self-assembling molecular electronic components.  Crown Ethers Switching Systems as Supramolecular Wires The proposed molecular wires, based on a conjugated backbone structure including dangling crown ethers, were successfully synthesized and fully chemically characterized. Studies on their CT properties to determine the effect of the metal complexation and the different anchoring groups, are still in progress. Figure 59: Schematic representation of the wires based on crown ethers switching systems (empty 26a,b and complexed 27a,b). 5. Experimental Part Chapter 1. Experimental Part 97 5. Experimental Part 5.1. General Methods and Techniques -Materials: Reagents for synthesis were purchased from commercial sources and used without further purification. Solvents were dried and distilled using standard techniques. 102 Those reactions requiring an inert atmosphere were carried out using Argon as source. -Analytical thin-layer chromatography (TLC): was performed using aluminum coated Merck Kieselgel 60 F254 plates to check the evolution of reactions. Visualization was made by UV light (λ = 254 or 365 nm). -Purification of crude reaction mixtures: was achieved by flash column chromatography or gravity-fed column chromatography using silica gel. -NMR spectra: were recorded on a Bruker DPX-300, Bruker AV-500 or Bruker AVIII-700 at 298 K, using partially deuterated solvents as internal standards. Chemical shifts (δ) are expressed in ppm and are referred to the residual peak of the solvent. Spin multiplicities are reported as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br), with proton-proton coupling constants (J) given in Hz. -FTIR spectra: were recorded on a Bruker TENSOR 27 (ATR device, 7500-370 cm -1 ). The spectral range was 4000-550 cm -1 , with a resolution of 1 cm -1 . Spectra of pellets of dispersed samples of the corresponding materials were recorded in dried KBr. -Mass spectra: were realized by the mass spectra services at the Universidad Complutense de Madrid. Electronic Impact measurements (EI) were recorded using a HP 5989A apparatus (70 eV, 200 ºC). MALDI-TOF measurements were recorded utilizing a BRUKER- REFLEX III apparatus (matrix: dithranol, N2 laser at 337 nm) or Brucker Ultraflex III apparatus (matrix: trans-2-[3-(4-tert- Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)). 102 W. L. F. Armarego, C. L. L. Chai, Purification of Laboratory Chemicals, Elsevier, 2003. Chapter 1. Experimental Part 98 -Scanning Tunnel Microscopy (STM): Measurements of the Fullerene Dumbbells Series were performed using a custom built STM setup located in IBM's Noise Free Labs 103 . Measurements of the Linear Supramolecular Wires were carried out by the group of Prof. Nicolás Agraït at IMDEA Nanoscience Institute. They were performed using the break-junction technique with a made- in-house scanning tunneling microscope (STM) optimized to work in ambient conditions. Commercial gold over glass substrates (Arrandee) were used, and a freshly cut gold wire as tip. The experiments were performed in air, after immersing the substrates in 1 mM solutions of the compounds in dioxane. A constant bias voltage V of 230 mV was established between the tip and the substrate along the experiments, and a protection 12 MΩ resistor was placed in-series. The current I along the STM substrate-tip circuit was measured with a made-in-house linear current-to-voltage converter with two amplification stages. As gains were used 5 × 10 8 and 2.5 × 10 10 V/A, values which allow to explore a range in conductance G = I/V of 9 orders of magnitude between 10 and 10 -8 G0 (G0 = 2e 2 /h). Several thousands of G vs. z traces were recorded. The criterion used to identify a molecular plateau (regions of constant conductance at values below 1 G0) was that at any conductance below 0.5 G0, a displacement z larger than 8 Å was needed to produce a change in conductance of log(G/G0) = 0.1. As reference, the typical displacement needed for gold-gold tunneling to produce this conductance change in air is 0.2 Å. Using the traces that complied with this criterion, conductance histograms were built by accumulating the number of points measured in fixed log(G/G0) intervals along thousands of traces. 2D conductance-distance histograms were built accumulating the number of points measured in fixed log(G/G0) and z intervals along the traces. In this case, the position at which the conductance 0.5 G0 is reached (just after each gold contact is broken) was used as zero for z. In those conductance values where plateaus are frequent in the G vs z traces, there will be a larger number of points recorded, and a peak will form in the conductance histograms. 103 E. Lörtscher, D Widmer, B. Gotsmann, Nanoscale 2013, 5, 10542. Chapter 1. Experimental Part 99 -Atomic force microscopy (AFM): was performed at IBM Zurich Laboratories, using a VEECO Dimension 3100 instrument under ambient conditions in air. It worked on tapping mode with a PPP- NCHR-W tip (which guarantes radius of curvature below 10 nm taken fresh from the wafer) at a working frequency of 330 KHz. Height and phase images were simultaneously obtained. The samples were prepared by drop-casting on freshly cleaved mica and were dried under ambient conditions for few hours. 5.2. Proton-Donor and Proton-Acceptor Fulleropyrrolidines Synthesis of the amino acids containing the side chains R1: N-octylglycine (1) 42 Octylamine (15.12 g, 117 mmol) was dissolved in a mixture of EtOH (15 mL) and water (12 mL). After cooling to 0°C 2-iodoacetic acid (7.44 mg, 40 mmol) was slowly added portionwise. The reaction mixture was stirred overnight allowing reaching room temperature and afterwards poured into 100 mL acetone. The occurring white precipitate was filtered, washed with acetone and dried in vacum. Yield: 83 %. 1 H NMR (300 MHz, D2O) δ 3.58 (s, 2H), 3.09 – 2.98 (m, 2H), 1.75 – 1.60 (m, 2H), 1.32 (dd, J = 25.9, 11.5 Hz, 10H), 0.85 (t, J = 6.8 Hz, 3H). 3,6,9-Trioxadecanaldehyde (2) 81 To a solution of oxalyl chloride (3 mL) in dichloromethane (75 mL) under argon and cooled in a dry ice-acetone bath 5 mL of dimethyl sulfoxide in 15 mL of DCM were carefully added. The solution was stirred for 10 min, and then a solution of 5 mL (29.7 mmol) of triethylene glycol monomethyl ether in 30 mL of DCM was added dropwise. The mixture was stirred for 15 min, and then triethylamine (20 mL) was added dropwise over a period of 20 min. The reaction Chapter 1. Experimental Part 100 mixture was left for 30 min at -78 °C and then allowed to reach RT. After a standard workup with a saturated aq NaCl solution, the crude was purified by flash chromatography DCM/MeOH (9:1), affording the aldehyde as a light yellow oil. Yield: 45 % 1 H NMR (300 MHz, CDCl3) δ 9.74 (s, 1H), 4.16 (d, 2H), 3.78 – 3.68 (m, 4H), 3.66 (dd, J = 5.3, 3.4 Hz, 2H), 3.56 (dd, J = 6.2, 3.1 Hz, 2H), 3.38 (s, 3H). N-Cbz-N-(3,6,9-trioxadecyl)glycine benzyl ester (3) 81 To a solution of aldehyde 2 (250 mg, 1.5 mmol) and glycine benzyl ester 4-toluenesulfonate (1,4 g, 4.09 mmol) in 30 mL of methanol- acetic acid (99:1) sodium cyanoborohydride (319 mg 5.07 mmol) was added in 30 min. The solution was stirred at RT for 2.5 h and then was poured into 75 mL of saturated NaHCO3 solution. The product was extracted with ethyl acetate, the organic phase washed with brine and dried with anhyd Na2SO4, and the solvent was removed in vacuo. The residue aws redissolved in dioxane and treated with benzyl chloroformate (1 g, 5.03 mmol). The N-Cbz-N-(3,6,9- trioxadecyl)glycine benzyl ester was then purified by column chromatography with AcOEt/petroleum ether (1:1). Yield: 50 % 1 H NMR (300 MHz, CDCl3) δ 7.38 – 7.27 (m, 10H), 5.16 (s, 2H), 5.09 (d, J = 3.2 Hz, 2H), 4.21 (s, 1H), 4.16 (s, 1H), 3.67 – 3.47 (m, 12H), 3.37 – 3.32 (m, 3H). N-(3,6,9-Trioxadecyl)glycine (4) 81 Chapter 1. Experimental Part 101 A solution of 3 (390 mg, 0.88 mmol), cyclohexene (6.26 g, 76.21 mmol) and palladium on carbon 10% (196 mg, 1.84 mmol) in 12 mL of EtOH were heated under reflux. The reaction was followed by TLC and after 24h, if it was not completed the same quantities of cyclohexenne and palladium on carbon were added and it was kept under reflux for other 24h. After filtration on celite, the solvent was evaporated under vacuum leaving a dark oil. Yield: 70% 1 H NMR (300 MHz, CDCl3) δ 3.82 (broad t, 2H), 3.70 – 3.50 (m, 12H), 3.38 (s, 3H), 3.20 (broad t, 2H). FTIR (KBr) ʋ: 3350, 1650, 1111 cm -1 . Synthesis of the spacer skeleton: General Procedure for the Synthesis of p-functionalized phenylboronic acid pinacol esters 5d-f 104 Pd(dppf)Cl2 (0.07 mmol) was added to a mixture of the corresponding aryl bromides (1.4 mmol), bis(pinacolato)diboron (1.5 mmol), and KOAc (402 mmol) in 7 mL of anhyd DMF. The mixture was degassed under argon atm for 1h, then heated at 80°C for 5h. The solvent was removed and the crude material purified by column chromatography on silica gel with Hex/AcOEt (20:1). 4-methoxycarbonylphenylboronic acid pinacol ester (5d) 105 Yield: 86% 1 H NMR (300 MHz, CDCl3) δ 8.08 – 7.98 (m, 2H), 7.88 (d, J = 8.3 Hz, 2H), 3.94 (s, 3H), 1.37 (s, 12H). 104 T. Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995, 60, 23, 7508. 105 T. Yamamoto, T. Morita, J. Takagi, T. Yamakawa, Org. Lett. 2011, 13, 5766. Chapter 1. Experimental Part 102 4-cyanophenylboronic acid pinacol ester (5e) 106 Yield: 92%. 1 H NMR (500 MHz, CDCl3) δ 7.87 (d, J = 8.0 Hz, 2H), 7.62 (d, J = 8.0 Hz, 2H), 1.34 (s, 12 H). 4-carboxylphenylboronic acid pinacol ester (5f) 106 Yield: 74%. 1 H NMR (500 MHz, CDCl3) δ 8.09 (d, J = 8.5 Hz, 2H), 7.90 (d, J = 8.0 Hz, 2H), 1.35 (s, 3H). General Procedure for the Synthesis of 4'-formyl-[1,1'-biphenyl]-4- substituted products 6d-f A mixture of dioxane/H20 (90 mL, 3:1) was degassed for 1h under argon atm. Boronates 5d-f (0.76 mmol) and 4-iodobenzaldehyde (3.8 mmol) were added and the solution was heated to 80°C. After 30 min, Pd(dppf)Cl2 (0.038 mmol) and CsF (2.28 mmol) were added and the mixture was kept at 80ºC for 16 h. The reaction was washed with water and extracted with DCM (3 x 20 mL). The organic phase was dried over MgSO4 and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on silica gel with Hex/DCM (1:1). 106 P. Li, C. Fua, S. Ma, Org. Biomol. Chem. 2014, 12, 3604. Chapter 1. Experimental Part 103 methyl 4'-formyl-[1,1'-biphenyl]-4- carboxylate (6d) 107 Yield: 96% 1 H NMR (300 MHz, CDCl3) δ 10.10 (s, 1H), 8.16 (d, J = 8.2 Hz, 2H), 8.00 (d, J = 8.2 Hz, 2H), 7.81 (d, J = 8.3 Hz, 2H), 7.72 (d, J = 8.2 Hz, 2H), 3.97 (s, 3H). 4'-formyl-[1,1'-biphenyl]-4-carbonitrile (6e) 108 Yield: 93%. 1 H NMR (300 MHz, CDCl3) δ 10.11 (s, 1H), 8.02 (d, J = 8.4 Hz, 2H), 7.86 – 7.68 (m, 6H). 4'-formyl-[1,1'-biphenyl]-4-carboxylic acid (6f) 109 Yield: 74%. 107 Y. L. Zhao, Y. Li, S. M. Li, Y. G. Zhou, F. Y. Sun, L. X. Gao, F. S. Han, Adv. Synth. Catal. 2011, 353, 1543. 108 J. I. Urgel, D. Ecija, W. Auwärter, D. Stassen, D. Bonifazi, J. V. Barth, Angew. Chem. Int. Ed. 2015, 54, 6163. 109 T. Fujimori, P. Wirsching, K. D. Janda, J. Comb. Chem. 2003, 5, 625. Chapter 1. Experimental Part 104 1 H NMR (300 MHz, Acetone-d6) δ 10.13 (s, 1H), 8.17 (d, J = 8.2 Hz, 2H), 8.07 (d, J = 8.3 Hz, 2H), 7.99 (d, J = 8.4 Hz, 2H), 7.92 (d, J = 8.3 Hz, 2H). Synthesis of the Amidinium-Carboxylate Fulleropyrrolidines: General Procedure for the Synthesis of fullerene derivatives 7a-c and 10a-d To a solution of C60 (0.25 mmol) in 80 mL of toluene aldehydes 6d-f (0.25 mmol) were added togheter with N-octylglycine 1 or N-(3,6,9- trioxadecyl)glycine 4 (0.75 mmol). The mixture was heated under reflux for 3h. After cooling, the solvent was removed under reduced pressure. The crude material was purified by column chromatography on silica gel with CS2, and then eluted with Tol/MeOH (9:1). The compounds dissolved in DCM and precipitated by addition of methanol, were recovered as brown solids (racemic form). 4-(N-Octyl-3,4-fulleropyrrolidin-2-yl)benzoic acid methyl ester (7a) Yield: 38% 1 H NMR (300 MHz, CDCl3) δ 8.10 (d, J = 8.4 Hz, 2H), 7.92 (s, 2H), 5.15 (s, 2H), 4.17 (d, J = 9.6 Hz, 1H), 3.91 (s, 3H), 3.20 – 3.09 (m, 1H), 2.60 – 2.42 (m, 1H), 1.94 – 1.79 (m, 2H), 1.51 – 1.17 (m, 10H), 0.88 (s, 3H). 13 C NMR (125 MHz, CDCl3) δ 171.92 165.04, 164.61, 155.75, 155.22, 153.43, 153.14, 152.60, 152.08, 145.72, 144.38, 143.85, 143.62, 143.03, 140.93, 140.41, 138.84, 136.14, 135.09, 132.99, 128.58, 125.82, 125.03, 80.76, 75.52, 67.30, 28.22, 27.89, 26.94, 23.37, 23.20,23.04, 22.88, 22.72, 22.56, 21.18, 14.62 Chapter 1. Experimental Part 105 4-(1-Octyl-3,4-fulleropyrrolidin-2-yl)benzonitrile (7b) Yield: 35% 1 H NMR (300 MHz, CD3OD) δ 8.11 (s, 2H), 7.87 (d, J = 8.4 Hz, 2H), 5.22 (s, 1H), 5.17 (d, J = 9.4 Hz, 1H), 4.21 (d, J = 9.4 Hz, 1H), 2.70 – 2.59 (m, 1H), 2.22 – 2.09 (m, 1H), 1.99 – 1.83 (m, 2H), 1.73 – 1.18 (m, 10H), 0.89 (s, 3H). 13 C NMR (125 MHz, CDCl3) δ 154.17, 152.95, 147.80, 147.76, 146.76, 146.66, 146.64, 146.61, 146.58, 146.42, 146.40, 146.09, 146.03, 145.99, 145.89, 145.82, 145.80, 145.76, 145.72, 145.67, 145.64, 145.17, 144.97, 144.86, 144.76, 143.62, 143.47, 143.17, 143.07, 143.05, 143.01, 142.68, 142.64, 142.60, 142.59, 142.50, 142.47, 142.39, 142.28, 142.23, 142.13, 141.99, 140.69, 140.38, 139.94, 137.58, 136.80, 136.42, 135.94, 132.89, 130.54, 119.03, 112.81, 82.37, 76.57, 69.28, 67.17, 53.79, 32.33, 30.01, 29.71, 28.61, 27.93, 23.12, 14.57. ESI_POS: m/z theoretical for C76H22N2O3 736.29 exp [M+H] + 737.35. FTIR (KBr) ʋ: 2923, 2853, 2798, 2229, 1676, 1607, 1501, 1460, 1299, 1264, 1213, 1178, 1113, 1020, 754 cm -1 . Chapter 1. Experimental Part 106 4'-(1-Octyl-3,4-fulleropyrrolidin-2-yl)biphenyl-4-carboxylic acid methyl ester (7c) 110 Yield: 26% 1 H NMR (300 MHz, CDCl3) δ 8.10 (d, J = 8.5 Hz, 2H), 7.91 (s, 2H), 7.71 (d, J = 8.5 Hz, 4H), 5.14 (s, 2H), 4.17 (d, J = 10.1 Hz, 1H), 3.95 (s, 3H), 3.25 – 3.99 (m, 1H), 2.61 – 2.47 (m, 1H), 1.90 – 1.86 (s, 2H), 1.69 – 1.12 (m, 10H), 0.92 (s, 3H). 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl)benzoic acid (10a) Yield: 22% 1 H NMR (300 MHz, CDCl3) δ 8.15 (d, J = 8.3 Hz, 2H), 7.98 (s, 2H), 5.27 (d, J = 12.6 Hz, 2H), ), 4.37 (d, J = 10.4 Hz, 1H), 4.18 – 3.90 (m, 2H), 3.77 (m, 6H), 3.59 (m, 2H), 3.37 (m, 4H), 2.94 (m, 1H). 110 M. Segura, L. Sánchez, J. de Mendoza, N. Martín, D. M. Guldi J. Am. Chem. Soc. 2003, 125, 49, 15093. Chapter 1. Experimental Part 107 13 C NMR (125 MHz, CDCl3) δ 168.95, 147.32, 146.31, 146.25, 146.21, 146.11, 145.97, 145.72, 145.74, 145.52, 145.46, 145.3, 145.33, 144.72, 143.13, 142.64, 142.56, 142.21, 142.14, 141.86, 141.72, 140.63, 139.11, 137.33, 137.05, 136.51, 136.01, 135.58, 133.51, 132.69, 132.14, 131.53, 131.33, 130.07, 128.75, 128.61, 127.01, 125.79, 125.58, 123.06, 79.83, 73.66, 70.10, 69.94, 69.51, 65.41, 60.81, 57.81, 54.02. ESI_POS: m/z theoretical for C76H23NO5 1029.1576 exp [M+H] + 1030.1690. FTIR (KBr) ʋ: 2923, 2854, 1715, 1694, 1610, 1460, 1424, 1375, 1289, 1251, 1206, 1109, 853, 755 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl) benzonitrile (10b) Yield: 25% 1 H NMR (300 MHz, CDCl3) δ 7.98 (d, J = 7.3 Hz, 2H), 7.73 (d, J = 8.5 Hz, 2H), 5.23 (d, J = 10.6 Hz, 2H), 4.34 (d, J = 9.7 Hz, 1H), 4.10 – 3.93 (m, 2H), 3.87 – 3.68 (m, 6H), 3.58 (m, 2H), 3.43 – 3.30 (m, 4H), 2.99 – 2.86 (m, 1H). 13 C NMR (175 MHz, CDCl3) δ 146.77, 146.64, 146.61, 146.58, 146.46, 146.21, 146.03, 145.99, 145.89, 145.78, 145.77, 145.73, 145.70, 145.67, 145.64, 145.17, 144.97, 144.90, 144.86, 144.76, 144.14, 143.62, 143.01, 142.68, 142.64, 140.63, 139.11, 136.51, 135.58, 133.51, 132.69, 132.14, 131.33, 130.24, 130.07, 129.02, 128.61, 127.01, 125.98, 125.58, 119.12, 110.59, 79.83, 75.66, 70.10, 69.94, 69.71, 65.02, 61.81, 58.39, 54.05. Chapter 1. Experimental Part 108 ESI_POS: m/z theoretical for C76H22N2O3 1010.1630 exp [M+H] + 1011.2. FTIR (KBr) ʋ: 2853, 2228, 1733, 1460, 1372, 1257, 1102, 845, 800, 758 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl)biphenyl- 4-carboxylic acid (10c) 110 Yield: 48% 1 H NMR (300 MHz, CDCl3) δ 8.14 (d, J = 8.4 Hz, 2H), 7.98 – 7.88 (m, 3H), 7.73 (m, 3H), 5.25 (s, 2H), 4.35 (s, 1H), 4.11 – 4.03 (m, 2H), 3.85 – 3.71 (m, 6H), 3.59 (m, 2H), 3.38 (m, 4H), 2.95 – 2.90 (m, 1H). 13 C NMR (175 MHz, CDCl3) δ 168.95, 146.44, 146.32, 146.25, 146.11, 146.00, 145.68, 145.46, 145.39, 145.10, 145.02, 144.95, 144.85, 144.39, 144.23, 144.11, 143.29, 143.24, 143.23, 143.14, 142.57, 141.6, 141.5, 141.44, 140.88, 140.63, 140.34, 140.32, 140.07, 139.67, 139.11, 138.63, 137.11, 136.51, 135.58, 133.51, 132.69, 132.14, 131.33, 130.33, 130.27, 130.07, 129.47, 128.61, 127.37, 127.11, 127.01, 125.79, 125.00, 79.83, 73.66, 70.10, 69.94, 67.51, 63.41, 62.81, 57.81, 53.02. ESI_POS: m/z theoretical for C82H27NO5 1105.1889 exp [M+H] + 1106.1942. FTIR (KBr) ʋ: 2923, 2853, 2798, 2229, 1676, 1607, 1501, 1460, 1299, 1264, 1213, 1178, 1113, 1020, 754 cm -1 . Chapter 1. Experimental Part 109 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl)biphenyl- 4-cyano (10d) Yield: 15% 1 H NMR (300 MHz, CDCl3) δ 7.93 (s, 2H), 7.71 (s, 4H), 7.66 (d, J = 8.4 Hz, 2H), 5.24 (d, J = 6.2 Hz, 2H), 4.34 (d, J = 9.7 Hz, 1H), 4.13 – 3.97 (m, 2H), 3.86 – 3.70 (m, 6H), 3.58 (m, 2H), 3.38 (m, 4H), 2.91 (m, 1H). 13 C NMR (75 MHz, CDCl3) δ 146.74, 146.55, 146.53, 146.47, 146.23, 146.03, 145.61, 145.44, 145.39, 145.32, 145.27, 145.24, 144.93, 144.86, 144.83, 144.76, 143.91, 142.28, 141.68, 140.73, 140.67, 140.63, 140.34, 139.11, 136.81, 135.08, 133.78, 133.11, 132.69, 132.14, 131.33, 130.43, 130.33, 130.07, 128.92, 127.37, 127.01, 125.99, 125.50, 119.12, 115.67, 79.13, 73.65, 70.10, 69.94, 69.51, 65.41, 60.81, 57.81, 53.82. ESI_POS: m/z theoretical for C82H26N2O3 1086.1943 exp [M+H] + 1087.0954. FTIR (KBr) ʋ: 2922, 2854, 2225, 1740, 1462, 1110, 826, 762 cm -1 . General Procedure for the hydrolysis to obtain compounds 8a,c To a sample of 15 mg of the fullerene derivatives 7a,c in THF (5 mL) at room temperature KOH 10% (40 mL) was added. The reaction was heated under reflux for 3 h. Then, HCl (3 mL) was added to neutralize and a precipitate was formed. The precipitate was filtered and washed with water (20 mL), methanol (20 mL) and ethyl ether (20 mL) affording the corresponding carboxylic acids 8a,c. Chapter 1. Experimental Part 110 4-(1-Octyl-3,4-fulleropyrrolidin-2-yl)benzoic acid (8a) 110 Yield: 94% 1 H NMR (300 MHz, CDCl3 ) δ 8.16 (d, J = 8.4 Hz), 7.96 (br s, 2 H), 5.16 (d, J = 9.2 Hz, 1 H), 5.15 (s, 1 H), 4.17 (d, J = 9.2 Hz, 1 H), 3.20 (m, 1 H), 2.57 (m, 1 H), 2.00 (m, 2 H), 1.42-1.27 (m, 10 H), 0.92 (m, 3 H). 4'-(1-Octyl-3,4-fulleropyrrolidin-2-yl)biphenyl-4-carboxylic acid (8c) 110 Yield: 72% 1 H NMR (300 MHz, CDCl3 ) δ 8.16 (d, J = 8.4 Hz, 2 H), 7.93 (br s, 2 H,), 7.74 (m, 4 H,), 5.14 (d, J = 9.0 Hz, 1 H,), 5.13 (s, 1 H,), 4.17 (d, J = 9.0 Hz, 1 H,), 3.32 (m, 1 H,), 2.63 (m, 1 H,), 2.01 (m, 2 H,), 1.90 (m, 2 H,), 1.70-1.26 (m, 8 H,), 0.90 (t, J = 8.4 Hz, 3 H). Chapter 1. Experimental Part 111 General Procedure for the Synthesis of the amidinium fulleropyrrolidine derivatives 8b and 11b,d Aluminium amide complex (1 M): Under scrupulously anhydrous conditions, NH4Cl (0.39 mmol), previously desiccated overnight in a heater at 80°C, was added to 0.2 mL of dry toluene. The suspension was cooled at -79°C and argon/vacuum cycles were repeated 3-5 times. After warming to room temperature, the mixture was cooled to 0°C and a solution of AlMe3 in hexane (2M, 0.2 mL) was slowly added. When the addition was completed, the reaction mixture was allowed to warm to RT and was further stirred for 2 h under argon, until gas evolution had ceased. Amidinium formation: A solution of 7b or 10b,d (0.03 mmol) in dry toluene (1 mL), after 3-5 argon/vacuum cycles at -79°C, was allowed to warm at RT and then heated at 90°C. The previously formed aluminium amide complex was added under argon atm and the resulting solution was stirred for 78h. The heating was discontinued and the reaction solution quenched with MeOH. After evaporation of the solvent under reduced pressure, the compound was purified by column chromatography on silica gel DCM/MeOH (9:1). 4-(1-Octyl-3,4-fulleropyrrolidin-2-yl)benzamidine hydrochloride (8b) Yield: 62% 1 H NMR (300 MHz, CD3OD) δ 8.09 (s, 2H), 7.87 (d, J = 7.5 Hz, 2H), 5.22 (s, 1H), 5.17 (d, J = 9.5 Hz, 1H), 4.21 (d, J = 9.0 Hz, 1H), 2.64 (s, 2H), 1.99 (s, 2H), 1.27 (m, 10H), 0.89 (s, 3H). Chapter 1. Experimental Part 112 13 C NMR (75 MHz, CDCl3) δ 154.23, 153.02, 147.41, 147.26, 146.56, 146.46, 146.21, 146.19, 146.09, 146.03, 145.99, 145.95, 145.84, 145.81, 145.74, 145.69, 145.67, 145.61, 145.07, 144.88, 144.81, 144.72, 143.54, 143.27, 143.12, 143.01, 142.68, 142.64, 142.60, 142.59, 142.40, 142.37, 142.29, 142.28, 141.99, 140.63, 139.11, 138.73, 136.51, 135.58, 133.51, 132.69, 132.14, 131.47, 131.33, 130.07, 130.01, 128.61, 127.01, 125.99, 125.79, 125.58, 79.83, 65.41, 60.81, 54.84, 31.64, 29.06, 28.95, 27.89, 27.55, 22.93, 14.01. MALDI-TOF: m/z theoretical for C77H28ClN3 1029.1972 exp [M-Cl] + 995.2147. FTIR (KBr) ʋ: 3367-2990, 2763, 2328, 1713, 1430, 1312, 1217, 1198, 795, 789, 754 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl) benzamidine hydrochloride (11b) Yield: 20% 1 H NMR (300 MHz, CDCl3) δ 8.05 – 7.94 (m, 2H), 7.73 (d, J = 8.5 Hz, 2H), 5.23 (d, J = 10.5 Hz, 2H), 4.34 (d, J = 9.8 Hz, 1H), 4.09 – 3.93 (m, 2H), 3.76 (m, 6H), 3.58 (m, 2H), 3.37 (m, 4H), 2.98 – 2.91 (m, 1H). 13 C NMR (75 MHz, CDCl3) δ 146.71 146.64, 146.63, 146.54, 146.67, 146.41, 146.13, 145.99, 145.89, 145.78 145.70 145.67, 145.64, 145.17, 144.97, 144.86, 144.76, 143.94, 143.62, 143.01, 142.68, 142.64, 141.63, 140.21, 139.03, 138.23, 136.11, 135.99, 132.51, 132.49, 132.14, 131.57, 130.33, 130.07, 130.01, 128.61, 127.01, 125.99, 125.79, 125.58, 79.83, 72.76, 71.50, 69.84, 69.63, 65.71, 60.05, 57.81, 54.52. Chapter 1. Experimental Part 113 MALDI-TOF: m/z theoretical for C76H26ClN3O3 1063.1663 exp [M- Cl] + 1028.2097. FTIR (KBr) ʋ: 3421-3087, 2923, 2855, 1730, 1621, 1459, 1426, 1382, 1168, 1114, 855, 718, 575, 526 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl)biphenyl- 4-amidine hydrochloride (11d) Yield: 18% 1 H NMR (700 MHz, CDCl3) δ 7.90 (d, J = 8.2 Hz, 4H), 7.79 – 7.64 (m, 4H), 5.26 (d, J = 13.5 Hz, 2H), 4.36 (d, J = 9.7 Hz, 1H), 4.07 (d, 2H), 3.88 – 3.48 (m, 8H), 3.40 (s, 4H), 2.94 (s, 1H). 13 C NMR (175 MHz, CDCl3) δ 146.71 146.64, 146.63, 146.54, 146.67, 146.41, 146.13, 145.99, 145.89, 145.78 145.70 145.67, 145.64, 145.17, 144.97, 144.86, 144.76, 143.94, 143.62, 143.01, 142.68, 142.64, 141.63, 140.21, 139.03, 138.23, 136.11, 135.99, 132.51, 132.49, 132.14, 131.57, 130.33, 130.07, 130.01, 128.61, 127.01, 125.99, 125.79, 125.58, 79.83, 72.76, 71.50, 69.84, 69.63, 65.71, 60.05, 57.81, 54.52. MALDI-TOF: m/z theoretical for C82H30ClN3O3 1139.1976 exp [M- Cl] + 1104.3109. FT-IR (KBr): 3326-2890, 2826, 2802, 2325, 1699, 1482, 1342, 1112, 750 cm -1 . Chapter 1. Experimental Part 114 General Procedure for the neutralization to obtain the amidine derivatives 9b and 12b,d The amidinium salt derivative 8b or 11b,d (10 mg) was washed with an aq solution of NaOH (10%) and extracted with CHCl3 (3 x 30 mL). The organic phase was dried over MgSO4 and the solvent removed under reduced pressure to obtain the corresponding neutral forms 9b and 12b,d. 4-(1-Octyl-3,4-fulleropyrrolidin-2-yl)benzamidine (9b) Yield: 72% 1 H NMR (300 MHz, CD3OD) δ 8.11 (s, 2H), 7.67 (d, J = 7.5 Hz, 2H), 5.16 (s, 1H), 5.07 (d, J = 9.5 Hz, 1H), 4.01 (d, J = 9.5 Hz, 1H), 2.84 (s, 2H), 2.04 (s, 2H), 1.32 (m, 10H), 0.93 (s, 3H). FTIR (KBr) ʋ: 3332, 2986, 2763, 2328, 1713, 1430, 1312, 1217, 1198, 795, 789, 754, 574 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl) benzamidine (12b) Chapter 1. Experimental Part 115 Yield: 80% 1 H NMR (300 MHz, CDCl3) δ 8.01 (m, 2H), 7.73 (d, J = 8.4 Hz, 2H), 5.60 (d, J = 10.2 Hz, 2H), 4.54 (d, J = 9.6 Hz, 1H), 4.11 – 3.93 (2m, 2H), 3.46 (m, 6H), 3.28 (m, 2H), 3.07 (m, 4H), 2.78 – 2.71 (m, 1H). FTIR (KBr) ʋ: 3456, 3393, 2923, 2855, 1730, 1645, 1462, 1372, 1258, 1181, 1112.09, 910, 844, 807, 729, 649 cm -1 . 4'-(N-(3,6,9-Trioxadecyl)glycine-3,4-fulleropyrrolidin-2-yl)biphenyl- 4-amidine (12d) Yield: 68% 1 H NMR (300 MHz, CDCl3) δ 7.88 (d, J = 8.6 Hz, 4H), 7.79 – 7.58 (m, 4H), 5.26 (s, 1H), 5.23 (s, 1H), 4.34 (d, J = 9.5 Hz, 1H), 4.02 (s, 2H), 3.49 (s, 4H), 3.37 (s, 8H), 2.97 – 2.86 (m, 1H). FT-IR (KBr) ʋ: 3103, 3026, 2920, 1942, 1857, 1604, 1494, 1459, 1379, 1178, 1080, 1029, 895, 724, 692 cm -1 . General Procedure for the formation of the amidinium-carboxylate complexes 9b . 8a,c and 12b,d . 10a,c To a solution of the amidine derivatives 9b and 12b,d (1 mmol) in hot CHCl3 (5 mL) the corresponding carboxylic acids 8a,c and 10a,c (1 mmol) were added. The resulting solution was stirred for 1h and then, warmed to room temperature, the solvent was dried under vacuum giving the corresponding complexes 9b . 8a,c and 12b,d . 10a,c. Chapter 1. Experimental Part 116 5.3. Linear Supramolecular Wires Synthesis of the Proton Donor and Proton Acceptor Moieties: For the synthesis of the amidine moieties 14a,b of the supramolecular linear wires, see the General Procedure to obtain the amidinium fulleropyrrolidine derivatives and their neutralization in the previous section (General Procedure for the hydrolysis, p. 92). 4-pyridinecarboamidine hydrochlorid (13a) Yield: 99% 1 H NMR (300 MHz, DMSO-d6) δ 6.93-6.87 (m, 2H), 6.18-6.10 (m, 2H). 13 C NMR (75 MHz, CD3OD) δ 170.12, 149.27, 135.28, 123.43. FTIR (KBr) ʋ: 3367-2860, 1692, 1592, 1558, 1483, 1414, 1355, 1222, 749, 685 cm -1 . 4-pyridinecarboamidine (14b) Yield: 98% 1 H NMR (300 MHz, DMSO-d6) δ 8.90-8-86 (dd, J = 4.5, 1.6 Hz, 2H), 7.78-7.74 (dd, J = 4.5, 1.6 Hz, 2H). 13 C NMR (75 MHz, CD3OD) δ 165.32, 150.84, 144.69, 122.91. FTIR (KBr) ʋ: 3352, 3186, 2926, 2856, 1678,1620, 1551, 1513, 1463, 1407, 1262, 1219, 1067, 998, 847, 757, 617. 4-(methylthio)benzamidine hydrochloride (13b) Chapter 1. Experimental Part 117 Yield: 99% 1 H NMR (300 MHz, CD3OD) δ 6.76 – 6.70 (m, 2H), 6.16 – 6.04 (m, 2H), 2.58 (m, 3H). 13 C NMR (75 MHz, CD3OD) δ 169.93, 139.53, 128.77, 126.76, 124.05, 14.78. FTIR (KBr) ʋ: 3350-2859, 1740, 1677, 1599, 1532, 1479, 1431, 1370, 1263, 1219, 1090, 833, 754, 608 cm -1 . 4-(methylthio)benzamidine (14b) Yield: 90% 1 H NMR (300 MHz, CD3OD) δ 7.72 – 7.60 (m, 2H), 7.36 – 7.22 (m, 2H), 2.51 (s, 3H). 13 C NMR (75 MHz, CD3OD) δ 167.37, 144.69, 131.87, 128.36, 126.58, 14.95. FTIR (KBr) ʋ: 3417, 3237, 2925, 2855, 1672, 1599, 1555,1480, 1409, 1158, 1028, 1013, 731, 718, 733, 675, 645 cm -1 . General Procedure for the synthesis of the carboxylic biphenyl derivatives 15b and 16b Potassium pyridine or (methylthio)phenyl 4-trifluoroborate salts (2.6 mmol) and 4-bromobenzoic acid (4.2 mmol) were added to a mixture of dioxane/H20 (3:1) previously degassed for 1h. The stirring solution was heated at 80°C and Pd(dppf)Cl2 (0.13 mmol) and CsF (7.8 mmol) were added after 30 minutes. The temperature was maintained at 80°C overnight under argon atm. The crude was washed with water and extracted with DCM (3 x 20 mL). The organic phase was dried over MgSO4 and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on silica gel with DCM/MeOH (9:1). The product extracted from the column chromatography was further purified by recrystallization from pyridine to obtain the carboxylic biphenyl derivative 15b and from MeOH to achieve 16b. Chapter 1. Experimental Part 118 4-(pyridin-4-yl)benzoic acid (15b) Yield: 80% 1 H NMR (500 MHz, DMSO-d6): δ 8.67 (dd, 2H, 1.53 Hz, 4.57 Hz), 8.06 (dd, 2H, 1.83 Hz, 6.72 Hz), 7.92 (dd, 2H, 1.83 Hz, 6.72 Hz), 7.76 (dd, 2H, 1.53 Hz, 4.57 Hz). 13 C NMR (175 MHz, DMSO-d6): δ 166.90, 150.33, 145.96, 141.24, 131.26, 130.08, 127.11, 121.46. ESI_POS: m/z theoretical for C12H9NO2 199.06 exp [M+H] + 200.51. FTIR (KBr) ʋ: 2431, 1700, 1604, 1403, 1317, 1296, 1180, 1073, 1006, 828, 733, 742, 701, 668, 519 cm -1 . 4'-(methylthio)-[1,1'-biphenyl]-4-carboxylic acid (16b) Yield:72% 1 H NMR (300 MHz, CDCl3) δ 8.16 (d, J = 10.2 Hz, 2H), 7.68 (d, J = 9.9 Hz, 2H), 7.57 (d, J = 9.6 Hz, 2H), 7.35 (d, J = 10.0 Hz, 2H), 2.54 (s, 3H). 13 C NMR (125 MHz, CDCl3) δ 168.95, 140.88, 138.90, 135.19, 130.27, 129.47, 128.03, 127.72, 127.11, 16.53. FTIR (KBr) ʋ: 3059, 2925, 2852, 1643, 1589, 1439, 1398, 1312, 1176, 1125, 1087, 956, 843, 816, 718, 578, 541 cm -1 . Synthesis of the Reference Compounds for Linear Supramolecular Wires: 4,4''-bis(methylthio)-1,1':4',1''-terphenyl (17) 88 Chapter 1. Experimental Part 119 A mixture of Toluene/H2O (15 mL / 2 mL) was degassed for 1h. 4- (methylthio)phenylboronic acid (500 mg, 2.98 mmol) and p- dibromobenzene (280.7 mg, 1.19 mmol) were added and heated to 95°C under argon atm. After 30 minutes, Na2CO3 (630.6 mg, 5.95 mmol) together with Pd(Ph3)4 (13.87 mg, 0.012 mmol) as a catalyst were added and the temperature was kept overnight. The reaction mixture was cooled down to room temperature before MeOH was added. The mixture was filtered and the precipitate was washed with deionized water and HCl solution (0.5 M) to get rid of the unreacted NaCO3, followed by another washing step with deionized water and finally with MeOH. The precipitate was dried with P2O5 under vacuum. It was purified by column chromatography on SiO2 with hexane/DCM (1:1) as the eluent to obtain 17 as a white solid. Yield: 55% 1 H NMR (300 MHz, CDCl3) δ 7.64 (s, 4H), 7.57 (d, J = 8.3 Hz, 4H), 7.34 (d, J = 8.3 Hz, 4H), 2.53 (s, 6H). General Procedure for the formation of the amidinium-carboxylate complexes 15a,b . 14a and 16a,b . 14b To a solution of the amidine derivatives 14a,b (1 mmol) in hot MeOH (5 mL) the corresponding carboxylic acids 15a,b or 16a,b (1 mmol) were added. The resulting solution was stirred for 1h and then, warmed to room temperature. The solvent was dried under vacuum giving the corresponding complexes 15a,b . 14a and 16a,b . 14b. 5.4. Crown Ethers Switching Systems Synthesis of trans-4,5'-diaminodibenzo[18]crown-6: trans-4,5'-diaminodibenzo[18]crown-6 (18) 111 111 B. R. Pandya, Y. K. Agrawal, Dyes and Pigments 2002, 52, 161. Chapter 1. Experimental Part 120 A mixture of trans-4,5'-dinitrodibenzo[18]crown-6 (0.99 g, 2.2 mmol), Raney nickel (1.5 g) and anhyd ethanol (100 mL) was stirred under reflux as hydrazine hydrate (0.40 g, 8.0 mmol) was added over 20 min. The reflux was continued for 2 h to ensure complete removal of ammonia and the hot mixture was filtered to recover the catalyst. Excess solvent was removed under reduced pressure and on cooling, obtaining product 19. Yield: 60% 1 H NMR (300 MHz, CDCl3) δ 6.72 (d, J = 8.4 Hz, 2H), 6.28 (d, J = 2.4 Hz, 2H), 6.21 (dd, J = 8.4, 2.5 Hz, 2H), 4.10 (dd, J = 4.9, 2.9 Hz, 8H), 4.07 – 3.92 (m, 8H). Synthesis of the “arms” of the OPE and PPP Crown Ether Molecular Wires Series: trimethyl((4-(methylthio)phenyl)ethynyl)silane (19)100 4-bromophenylmethylsulfide (2 g, 8.7 mmol) ws dissolved in DMF (80 mL) and diisopropylamine (20 mL). Under argon atm, trimethylsilylacetylene (1.29 g, 13.1 mmol), P(PPh3)4 (0.5 g, 0.44 mmol), and CuI (0.04 g, 0.44 mmol) were added to the solution. The mixture was then stirred under Ar at 80°C for 48 h. The solvent was then removed in vacuum and the residue dissolved in DCM (50 mL) before being washed with water (2 x 20 mL) and brine (20 mL). The organic phase was then dried over MgSO4. After the solvent was removed in vacuum, column chromatography on SiO2 with hexane/DCM (3:1) was carried out to provide the product 19 as a yellow solid. Yield: 94% 1 H NMR (300 MHz, CDCl3) δ 7.76 – 7.78 ppm (d, J = 8.7 Hz, 2H), 7.50 – 7.52 (d, J = 8.7 Hz, 2H), 2.39 (s, 3H), 0.26 (s, 9H). 4-((trimethylsilyl)ethynyl)pyridine (20a) 100 Chapter 1. Experimental Part 121 A mixture of 4-bromopyridine hydrochloride (3.00 g, 15.4 mmol), PdCl2(PPh3)2 (271 mg, 0.39 mmol), and CuI (74 mg, 0.39 mmol) was placed in a 100 mL three-neck round-bottom flask. The flask was flushed with argon, and degassed diisopropylamine (30 mL) was added into the flask while stirring at 40 °C, followed by the addition of (trimethylsilyl)acetylene (4.44 mL, 30.9 mmol). After 12 h, the reaction mixture was diluted with water (1 mL), extracted with DCM, dried over MgSO4, and filtered. The solvent was removed under reduced pressure and the crude oil was purified by column chromatography, eluting with a hexane/ethyl acetate (89:11) mixture to collect the product. After removal of the solvent, the product was obtained as a dark brown oil. Yield: 94% 1 H NMR (300 MHz, CDCl3) δ 8.56 (d, J = 5.2 Hz, 2H), 7.30 (dd, J = 4.6, 1.4 Hz, 2H), 0.31 – 0.22 (m, 9H). (4-ethynylphenyl)(methyl)sulfane (20b) 100 Compound 19 (0.81 g, 3.49 mmol) was dissolved in a mixture of MeOH (50 mL) and DCM (50 mL), and K2CO3 (2.42 g, 17.5 mmol) was added. The reaction mixture was purged with Ar flow for 15 min and stirred at room temperature for 2 h. Most of solvents were removed under reduced pressure and water (50 mL) was added to the reaction mixture. The mixture was extracted with Et2O (2 x 50 mL), and then the combined organic extracts were washed with brine (3 x 50 mL). The combined organic extracts were then dried over MgSO4 and filtered, and the solvent was evaporated in vacuum. Column chromatography on SiO2 with hexane/DCM (3:1) was carried out to afford the product 20b. 1 H NMR (300 MHz, CDCl3) δ 7.96 – 7.99 (d, J = 8.7 Hz, 2H), 7.53– 7.55 (d, J = 8.7 Hz, 2H), 3.92 (s, 3H), 3.23 (s, 1H). Chapter 1. Experimental Part 122 Synthesis of the core structure of the OPE and PPP Crown Ether Molecular Wires Series: (2,3-dimethoxyphenyl)trimethylsilane 99 Veratrol (2.5 g, 18.09 mmol) was dissolved in a mixture of dry hexane (6.25 mL) and TMEDA (2.5 mL). nBuLi (1.6 M in hexane, 12.5 mL, 20 mmol) was added dropwise at room temperature. The reaction was stirred at room temperature for 28 h and cooled to -78°C. ClSiMe3 (2.81 mL) was slowly added and the reaction mixture was allowed to warm to room temperature over 5 h. H2O was added and the reaction mixture was extracted with hexane. The organic layer was separated and dried over MgSO4. Solvent was removed in vacuum and the residue was purified by flash chromatography on SiO2 with hexane/DCM (10:1) to give the product as a colourless oil. Yield: 73% 1 H NMR (300 MHz, CDCl3) δ 6.97 (t, J = 7.6 Hz, 1H), 6.87 (ddd, J = 7.6, 5.6, 1.7 Hz, 2H), 3.78 (d, J = 0.8 Hz, 6H), 0.25 (s, 9H). (2,3-dimethoxy-1,4-phenylene)bis(trimethylsilane) (21) 99 (2,3-dimethoxyphenyl)trimethylsilane (2.76 g, 13.14 mmol) was dissolved in TMEDA (2.5 mL) and cooled to 0°C. nBuLi in hexane (1.6 M, 12.5 mL, 20 mmol) was added dropwise. The reaction mixture was stirred at room temperature for 28 h and then cooled to -78°C. After ClSiMe3 (2.81 mL) was added dropwise, the reaction mixture was warmed to room temperature over 5 h. H2O was added and the reaction mixture was extracted with hexane. The organic layer was separated and dried over MgSO4. Solvent was removed in vacuum and the residue was purified by flash chromatography on SiO2 with hexane/DCM (10:1) to give the product as a colourless oil. Chapter 1. Experimental Part 123 Yield: 58% 1 H NMR (300 MHz, CDCl3) δ 7.11 (s, 2H), 3.83 (s, 6H), 0.29 (s, 18H). 1,4-diiodo-2,3-dimethoxybenzene (22) 99 Compound 21 (2.18 g, 7.73 mmol) was dissolved in DCM (11.5 mL) and the solution was cooled to 0°C. A solution of ICl (2.58 g, 15.92 mmol) in DCM (11.5 mL) was slowly added. The reaction mixture was warmed to room temperature, stirred for 30 min, and quenched with an aq solution of Na2S2O3. The organic layer was separated and dried over MgSO4. The solvent was removed in vacuum and the crude product was purified by flash chromatography on SiO2 with hexane/DCM (10:1) to give the product as a yellowish oil that slowly solidify. Yield: 88% 1 H NMR (300 MHz, CDCl3) δ 7.24 (s, 2H), 3.87 (s, 6H). 3,6-diiodobenzene-1,2-diol (23) 99 Compound 22 (2.65 g, 6.79 mmol) was dissolved in DCM (29.5 mL) and the solution was cooled to -78°C. BBr3 (31.17 mL, 7.81 g, 31.17 mmol) was added and the reaction mixture was warmed to room temperature and stirred for 14 h. The reaction was poured into ice/H2O and the mixture was extracted with EtOAc. The organic layer was separated and dried over MgSO4. The solvent was removed in vacuum and the residue was purified by flash chromatography on SiO2 with DCM to afford 23 as a white solid. Yield: 92% 1 H NMR (300 MHz, CDCl3) δ 7.00 (s, 2H), 5.62 (s, 2H). Chapter 1. Experimental Part 124 1,4-diiodiobenzene-2,3-[18]crown-6 (24) Compound 23 (500 mg, 1.38 mmol) was solubilised into anhyd THF under argon atm. NaH (828 mg, 34.5 mmol) was added and after H2 evolution has ceased, Cs2CO3 (273.93 mg, 1.42 mmol) was added to the mixture and the reaction was refluxed for 30 minutes. After this time, a solution of pentaethylene glycol di(p-toluenesulfonate) (798.12 mg, 1.46 mmol) was added dropwise during 1h. After the addition, the reaction mixture was stirred for 78h, cooled to room temperature, and filtered to remove the salts. The solvent was evaporated in vacuum and the residue was purified by flash chromatography on silica with DCM/acetone (9:1) to afford the product as a white solid. Yield: 20% 1 H NMR (300 MHz, CDCl3) δ 7.23 (s, 2H), 4.19 (t, J = 4.9 Hz, 4H), 3.96 (t, J = 4.9 Hz, 4H), 3.79 – 3.71 (m, 8H), 3.68 (s, 4H). 13 C NMR (75 MHz, CDCl3) δ 152.55, 135.70, 93.34, 73.16, 71.04, 70.80, 70.47. ESI_POS: m/z theoretical for C16H22I2O6 563,9506 exp [M+Na] + 586.9398. Synthesis of the final PPP Crown Ether Molecular Wires Series: General Procedure to obtain the PPP Derivatives 26a,b A round bottom flask charged with 1,4-diiodobenzene-2,3-18-crown-6 (0.053 mmol) and pyridineboronic acid or (4-(methylthio)- phenyl)boronic acid (0.159 mmol) were deoxygenated for 1h while simultaneously deoxygenating the mixture dioxane/H20 (90 mL, 3:1). After that time, solvents were added to the solids and the mixture was heated to 80°C. After 30 minutes, always under argon atm, Pd(dppf)Cl2 Chapter 1. Experimental Part 125 (0.003 mmol) and CsF (0.159 mmol) were added and the reaction was stirred overnight. Cooled to room temperature, the reaction mixture was extracted with chloroform and dried over MgSO4. The solvent was then evaporated in vacuum and the residue purified by column chromatography on silica with CH3Cl/acetone (9:1) to afford the corresponding final products 26a and 26b, respectively as yellowish and grey powders. 1,4-di(pyridine-4-yl)benzo[18]crown-6 (26a) Yield: 99% 1 H NMR (300 MHz, CDCl3) δ 8.67 (s, 4H), 7.55 (d, J = 5.4 Hz, 4H), 7.22 (s, 2H), 4.03 (t, J = 5.0 Hz, 4H), 3.82 – 3.57 (m, 16H). 13 C NMR (75 MHz, CDCl3) δ 150.88, 149.99, 145.97, 134.58, 125.88, 124.38, 73.61, 71.32, 71.09, 70.96, 70.59. FT-IR (KBr) ʋ: 3033.27, 2924.79, 2864.30, 1724.87, 1596.54, 1545.85, 1514.97, 1464.81, 1437.44, 1404.58, 1354.92, 1318.38, 1214.39, 1118.31, 1045.52, 999.63, 947.14, 810.08, 755.41, 719.59, 623.00 cm -1 . ESI_POS: m/z theoretical for C26H30N2O6 466,2104 exp [M+Na] + 489.2 1,4-bis(4-(methylthio)phenyl)-benzo[18]crown-6 (26b) Yield: 98% Chapter 1. Experimental Part 126 1 H NMR (300 MHz, CDCl3) δ 7.54 (d, J = 8.3 Hz, 4H), 7.29 (d, J = 8.3 Hz, 4H), 7.15 (d, J = 9.0 Hz, 2H), 3.98 (dd, J = 12.6, 7.6 Hz, 4H), 3.84 – 3.57 (m, 16H), 2.53 (s, 6H). 13 C NMR (75 MHz, CDCl3) δ 150.36, 137.55, 134.99, 134.82, 129.79, 126.21, 125.60, 77.16, 72.82, 70.94, 70.81, 70.78, 70.44, 15.86. FT-IR (KBr) ʋ: 2923.28, 2868.92, 1737.58, 1597.04, 1508.37, 1468.32, 1432.97, 1388.38, 1353.14, 1304.09, 1243.58, 1210.90, 1116.82, 1050.21, 1014.64, 946.79, 808.15 cm -1 . ESI_POS: m/z theoretical for C30H36O6S2 556,1953exp [M+Na] + 579.2. General Procedure to form the complexes 27a,b Compound 26a or 26b (0.018 mmol) and KSCN (0.018 mmol) were dissolved in hot methanol (60°C) for 3h, under stirring. Cooled to room temperature, the solution was filtered and the solvent was evaporated to recover the complexed products 27a,b. Complex 27a Yield: quantitative 1 H NMR (300 MHz, CDCl3) δ 8.72 (d, J = 3.2 Hz, 4H), 7.60 (d, J = 5.4 Hz, 4H), 7.23 (s, 2H), 3.87 (s, 4H), 3.70 (d, J = 10.7 Hz, 12H), 3.58 (d, J = 4.2 Hz, 4H). 13 C NMR (75 MHz, CDCl3) δ 150.19, 149.84, 145.38, 134.20, 126.54, 124.06, 77.16, 73.68, 73.38, 70.45, 70.26. ESI_POS: m/z theoretical for C26H30N2O6 466.2 exp [M+K] + 505.4. Chapter 1. Experimental Part 127 Complex 27b Yield: quantitative 1 H NMR (300 MHz, CDCl3) δ 7.56 (d, J = 8.3 Hz, 4H), 7.35 (d, J = 8.3 Hz, 4H), 7.13 (s, 2H), 3.71 (d, J = 25.2 Hz, 16H), 3.47 (d, J = 3.3 Hz, 4H), 2.54 (s, 6H). 13 C NMR (75 MHz, CDCl3) δ 148.60, 138.55, 134.50, 134.03, 129.27, 126.73, 126.39, 69.84, 68.66, 68.37, 15.50. ESI_POS: m/z theoretical for C30H36O6S2 556,1953 exp [M+K] + 595.2 . 6. Appendix Chapter 1. Appendix 131 6. Appendix 6.1. NMR titration experiments In a typical experiment, a solution of the amidine compound 14b at 1 × 10 -3 M was utilized as solvent in the preparation of the guests solutions (16a or 16b) at 1 × 10 -2 M, to ensure working at constant concentration of the host. Increasing amounts of 16a or 16b solutions (1 × 10 -2 M in [D6]DMSO) were added up to a total of 3 molar equivalents to the host solution of 14b (1 × 10 -3 M in [D6]DMSO) and 1 H NMR spectra were recorded at room temperature on a Bruker DPX-300. Determination of the binding constants for the complexes formed by interaction between 14b and 16a or 16b was carried out by the calculator Bindfit of the website supramolecular.org. The calculator operates by accepting a preformatted excel document with host and guest concentrations at each titration point together with the appropriate signals (e.g., for NMR the chemical shifts). The user, at this point, has to provide an initial guess for the binding constant and the calculator performs a non- linear regression on the given data using the corresponding exact binding equation implemented on the server. In our case, it was chosen to use the curve-fitting algorithm Nelder-Mead and the binding isotherm for a simple 1:1 equilibrium 89b (Equation 2): Here, G is the guest concentration (16a and 16b, respectively), G0 the “initial” guest concentration, H refers to the host concentration (14b), H0 to the “initial” host concentration and Ka represents the association constant. The binding fit program allowed to determine values of Ka ˃ 10 4 M -1 for both complexes. These binding constants were confirmed using Sigma Plot software by fitting the proton signals to a 1:1 association model and, again, in both cases Ka ˃ 10 4 M -1 were obtained. Therefore, more diluted solutions were considered necessary to get a more exact value of K. However, attempts with more diluted samples, always 𝐺 = 1 2 G0 − H0 + 1 𝐾a − G0 − H0 + 1 𝐾a 2 + 4 G0 𝐾a Equation 2: Binding isotherm for a 1:1 equilibrium. Chapter 1. Appendix 132 taking into account the sensitivity’s limits of the NMR technique, did not allowed to get the proper 1 H NMR spectra. Figure 60: NMR titration of a dilute solution of 14b (1 × 10 -3 M) in [D6]DMSO with increasing concentration of 16a in [D6]DMSO (0; 2.66× 10 -4 ; 5.27 × 10 -4 ; 7.85 × 10 -4 ; 1.04 × 10 -3 ; 1.29 × 10 -3 ; 1.54 × 10 -3 ; 1.81 × 10 -3 ; 2.07 × 10 -3 ; 2.32 × 10 -3 ; 2.57 × 10 -3 ; 3.10 × 10 -3 ; 3.61 × 10 -3 ; 4.13 × 10 -3 ; 4.63 × 10 -3 ; 5.17 × 10 -3 ; 5.68 × 10 -3 ; 6.19 × 10 -3 ; 6.69 × 10 -3 ; 7.21 × 10 -3 and 7.73 × 10 -3 M). Chapter 1. Appendix 133 Figure 61: NMR titration of a dilute solution of 14b (1 × 10 -3 M) in [D6]DMSO with increasing concentration of 16a in [D6]DMSO (0; 8.43 × 10 -5 ; 1.7 × 10 -4 ; 2.56 × 10 -4 ; 3.42 × 10 -4 ; 4.28 × 10 -4 ; 4.71 × 10 -4 ; 5.14 × 10 -4 ; 5.57 × 10 -4 ; 6.00 × 10 -4 ; 6.43 × 10 -4 ; 6.86 × 10 -4 ; 7.30 × 10 -4 ; 7.73 × 10 -4 ; 8.16 × 10 -4 ; 8.59 × 10 -4 ; 1.07 × 10 -3 and 1.29 × 10 -3 ). Chapter 2. 7. Background Chapter 2. Background 139 7. Background 7.1. Carbon Nanostructures Although widely distributed, carbon is not really particularly abundant in nature. Actually, this atom constitutes only about 0.025 percent of Earth’s crust. 112 However, from all the elements contained in the Periodic Table, it is the only one able to provide the basis for life on our planet, forming more compounds than all the other elements combined. It exists in a wide variety of nanostructures embracing different allotropic forms characterized by an ample amount of shapes and sizes (Figure 62). For a long time, diamond and graphite represented the only known allotropes of carbon. This situation changed in 1985 with the serendipitous discovery of fullerenes, observed for the first time by H. Kroto, R. Smalley and R. Curl, who received in 1996 the Nobel Prize in 112 HyperPhysics, Georgia State University, Abundance of Elements in Earth's Crust. Figure 62: Allotropes and nanoforms of carbon. Chapter 2. Background 140 Chemistry for this finding. 113 The advent of fullerenes opens the beginning of a discovering era for other amazing carbon nanostructures, such as carbon dots (CDs), carbon nanotubes (CNTs), and graphene as well as other less-explored carbon nanoforms such as graphene quantum dots (GQDs), nanodiamonds, carbon nanoonions, carbon nanohorns, etc. (Figure 62). A most interesting finding is that this structural diversity, guaranteed by sp 3 -, sp 2 -, and sp-hybridized carbon networks, results in sheer endless chemical and physical properties. 7.1.1. [60]Fullerene: Structure and Properties After the fortunate discovery of fullerenes by H. Kroto, R. Smalley and R. Curl in 1985, the first successful preparation of these zero- dimensional structures in macroscopic quantities, by evaporation and recondensation of graphite, was reported five years later by Krätschmer and Huffman. 114 The fullerenes, of which the smallest stable and most prominent is C60, are carbon structures built up of fused pentagons and hexagons. The pentagons are the units responsible for providing curvature to the structure, since a network consisting only by hexagons is planar (see graphene). Specifically, C60, with an outer diameter of 10.34 Å, 115 is made up of a spherical network of sixty structurally equivalent sp 2 -hybridized carbon atoms. The shape of this molecule corresponds to a football (or soccer ball or, more technically, an Ih-symmetrical truncated icosahedron) composed of 12 isolated pentagons and 20 hexagons and two different type of bonds. The bonds at the junctions of two hexagons, [6,6] bonds, are shorter (1.37 Å) than the bonds at the junctions of a hexagon and a pentagon, [5,6] bonds (1.45 Å) (Figure 63). 113 H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162. 114 W. Kratschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman, Nature 1990, 347, 354. 115 a) C. S. Yannoni, P. P. Bernier, D. S. Bethune, G. Meijer, J. R. Salem, J. Am. Chem. Soc. 1991, 113, 3190; b) M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996. Chapter 2. Background 141 This bond-length alternation in C60 shows that the double bonds are located in the [6,6] junctions and there are no double bonds in the pentagonal rings. Due to the curvature of the molecule, each σ-bond presents an intermediate hybridization of sp 2.3 in between the hybridizations of graphite (sp 2 ) and diamond (sp 3 ). 116 Therefore, [60]fullerene behaves as an electron deficient polyene, with the most reactive positions located in the [6,6] bonds. The driving force of the reactivity of fullerenes is due to the release of strain energy stemming from the saturation of one or more double bonds. The stability of C60 has been demonstrated through the isolated pentagon rule (IPR), developed by H. Kroto in 1987. 117,118 It says that the local tension enhances with the number of shared bonds between two pentagon rings, leading to less stable molecules. Owing to this rule, the easier fullerenes to be formed will be those in which pentagons are separated by rings of hexagons. 118 The [6,6] bonds in the C60 cage, due to their shorter length respect to the [5,6] bonds, are considered as double bonds. The six-membered rings, therefore, resemble the rarely encountered cyclohexatriene molecule, with localised multiple bonds, while five-membered rings are topologically related to the [5]radialene. 119 The non-aromaticity in the six-membered fragments of the [60]fullerene cage is a consequence of the substantial deviation of the double bond geometry from the normal planar one, which means a pyramidalization effect. This 116 R. C. Haddon, Acc. Chem. Res. 1992, 25, 127. 117 T. G. Schmalz, W. A. Seitz, D. J. Klein, G. E. Hite, Chem. Phys. Lett. 1986, 130, 203. 118 H. W. Kroto, Nature 1987, 329, 529. 119 A. Hirsch, Fullerenes and Related Structure, Top Curr. Chem. 199, Springer, Berlin Heidelberg New York Springer, 1999. Figure 63: [60]fullerene structure showing the differenttypes of bonds. Chapter 2. Background 142 pyramidalization is the reason for the appearance of properties non- typical of arenes. 120 In spite of considerable pyramidalization of multiple bonds, the six-membered rings maintain a planar geometry, mainly due to the rigidity of the [60]fullerene spherical structure. This blocked-pyramidization induced strain determines the high reactivity of fullerenes compared to other carbon forms. Fullerene C60 is a good electron acceptor. 121,122,123 Theoretical calculations of the molecular orbital levels of C60 show a triply degenerated LUMO comparatively low in energy. 124 It allows the reversible addition of up to six electrons in solution. 125 Cyclic voltammetry (CV) measurements of C60 solutions also supported its facile and stepwise reversible reduction. 126 All the reductions are one-electron transfer processes and are favored by the small reorganization energy of [60]fullerene. 121 However, the reduction potentials are strongly dependent on the nature of the solvent, the electrolyte and the temperature. The six electron acceptor reversible capability of [60]fullerene comes out in experiments at -10°C in a solution of acetonitrile:toluene (Figure 64). 127 120 A. L. Balch, M. M. Olmstead, Chem. Rev. 1998, 98, 2123. 121 D. M. Guldi, Chem. Commun., 2000, 5, 321. 122 D. M. Guldi, M. Prato, Acc. Chem. Res. 2000, 33, 695. 123 N. Martín, L. Sánchez, B. Illescas, I. Pérez, Chem. Rev. 1998, 98, 2527. 124 a) C. A. Reed, R. D. Bolskar, Chem. Rev. 2000, 100, 1075; b) A. D. J. Haymet, Chem. Phys. Lett. 1985, 122, 421; c) P. D. Hale, J. Am. Chem. Soc. 1986 108, 6087. 125 a) F. Arias, Q. Xie, Y. Wu, Q. Lu, S. R. Wilson, L. Echegoyen, J. Am. Chem. Soc. 1994, 116, 6388; b) P.M. Allemand, A. Koch, F. Wudl, Y. Rubin, F. Diederich, M. M. Alvarez, S. J. Anz, R. L. Whetten, J. Am. Chem. Soc. 1991, 113, 1050. 126 R. E. Haufler, J. Conceicao, L. P. F. Chibante, Y. Chai, N. E. Byrne, S. Flanagan, M. M. Haley, S. C. O'Brien, C. Pan, J. Phys. Chem. 1990, 94, 8634. 127 Q. Xie, E. Pérez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 1992, 114, 3978. Chapter 2. Background 143 The oxidation process, as Haddon et al. predicted, 128 is an extremely difficult process, but in 1993 Echegoyen et al. were able to measure the first electrochemically irreversible oxidation of C60 at 1.26 V (vs Fc/Fc + , TBAPF6, 1,1,2,2-tetrachloroethane, 100 mV/s). 129 These pronounced redox activity and excellent electronic properties have led to a notable breakthrough in synthetic electron donor-acceptor systems, using C60 as an outstanding electron acceptor. Especially, under optimal conditions, the small reorganization energies provide an optimal charge-separation kinetics and a decelerated charge- recombination rate. Thus, donor-acceptor arrays containing fullerene have been proposed as models for artificial photosynthetic mimics and as energy storage systems. 130 Also, thanks to this property, C60 or its derivatives may be employed as acceptor components in photovoltaic devices as well as in organic electronics. 7.1.1.1. [60]Fullerene: Chemical Reactivity Because C60 does not possess any kind of functionalization, but double bonds, its reactivity is driven by the transformation of more strained sp 2 into sp 3 carbon atoms. This simple consideration shows the great 128 R. C. Haddon, L. E. Brus, K. Raghavachari, Chem. Phys. Lett. 1986, 125, 459. 129 Q. Xie, F. Arias, L. Echegoyen, J. Am. Chem. Soc. 1993, 115, 9818. 130 H. Imahori, Y. Sakata, Eur. J. Org. Chem. 1999, 1999, 2445. Figure 64: Cyclic voltammogram (a) and square wave voltammogram (b) of C60 with TBAPF6 as supporting electrolyte, at –10° C in toluene:acetonitrile 4:1 at 100 mV/s (V vs Fc/Fc + ). Ref 128. Chapter 2. Background 144 difference between the reactivity of fullerenes and the classical planar aromatic systems. The [60]fullerene chemical behaviour is comparable to an electron poor polyolefin. The driving force of its chemical reactivity is the reduction of the strain energy stored in the spherical carbon framework. Regarding the solubility, [60]fullerene is soluble in aromatic solvents and in carbon disulfide. In polar and H-bonding solvents such as acetone, tetrahydrofuran or methanol, C60 is essentially insoluble. Therefore, chemical functionalization is the key to fully exploit its vast potential, providing the possibility of combining its unique properties with those of other compounds, while increasing its solubility. Among all known organic reactions, the nucleophilic addition and the cycloaddition reactions have been widely employed to modify fullerenes for further practical applications. a) Nucleophilic Additions Nucleophiles of different nature, based on carbon, nitrogen, phosphorous or oxygen, give a wide variety of nucleophilic addition reactions to [60]fullerene. By the initial attack of a nucleophile to C60, the intermediate NunC60 n− is formed. This intermediate is stabilized by: (a) the addition of electrophiles E + , for example H + or carbocations to give C60EnNun species, (b) the addition of neutral electrophiles EX such as alkyl halogenides to give C60EnNun, (c) intramolecular addition to yield methanofullerenes and cyclohexenefullerenes, or by (d) oxidation to obtain compounds of type C60Nu2 (Scheme 19). Although many isomers are possible for additions, the preferred mode of addition is 1,2. The addition pattern changes according to the size of the added groups. For a combination of sterically demanding addends, Scheme 19: General mechanism of the nucleophilic addition to [60] fullerene. Chapter 2. Background 145 1,4 additions and even 1,6 additions can take place alternatively or exclusively. 119,131 Given the large number of reactive double bonds in the molecule, polyadditions are frequent in the chemistry of fullerenes. b) Cycloaddition Reactions Cycloaddition is the most employed reaction in fullerenes chemistry since it leads to stable adducts avoiding the formation of isomeric mixtures, very frequent in nucleophilic addition reactions. A large variety of cycloaddition reactions have been carried out with C60 and the complete characterization of the products, mainly monoadducts, have greatly increased our knowledge of fullerenes chemistry. These chemical transformations provide a powerful tool for the functionalization of the fullerene sphere, because almost any functional group can be covalently linked to C60 (Scheme 20). 131 M. S. Meier, R. G. Bergosh, M. E. Gallagher, H. P. Spielmann, Z. Wang, J. Org. Chem. 2002, 67, 5946. Scheme 20: Schematic representation of cycloaddition reactions on [60]fullerene. Chapter 2. Background 146 7.1.1.2. [60]Fullerene: Applications The possibility to chemically modify the structure of C60 opened the doors to the many possible investigation lines involving this molecule, as well as to its real commercial applications. Sporting goods, such as badminton rackets with fullerenes in the polymer matrix composite, and cosmetics, such as the Vitamin C60 skin creams, are only some suggestive examples made available to consumers in select markets (Figure 65). However, fullerenes research has mainly been focused in the fields of organic electronics and bioscience in the search for potential applications. Molecular electronics is a research area where the outstanding properties of fullerenes, in particular their electrochemical and photophysical properties (favouring the separation of the charges while avoiding a fast recombination) 132,11m find application for the construction of molecular electronic devices such as transistors and rectifiers. Organic field effect transistors (OFETs) and photodetectors performance has been increased indeed due to the n-type semiconducting properties of fullerenes based on C60, C70 along with 132 a) R. A. Marcus, J. Chem. Phys. I 1956, 24, 966. Figure 65: a) and b) Advertisements of cosmetic products based on [60]fullerene. c) Racket frames incorporating nanoclays with fullerenes. Chapter 2. Background 147 C84. 133 A recent work of Qiu et al. showed, for example, that the degree of rectification current of mixed-monolayers of alkanethiolates in tunneling junctions can be controlled by the amount of fullerene incorporated, reaching a maximum value of 940 at 1 V. 134 [60]Fullerene and its derivatives have also found wide application in the nowadays important sector of environment sustainability. Solar cells, alternatives to fossil fuels and water purification systems, are some of the possible uses to which fullerenes are dedicated. In fact, (6,6)phenyl C61 butyric acid methyl ester (PC61BM) and (6,6)phenyl C71 butyric acid methyl ester (PC71BM) acceptors still remain the cornerstone of all fullerene derivatives for photovoltaic (PV) studies. 135 For example, recently, Chen et al. reported a record high power conversion efficiency (PCE) of ~10% for a bulk heterojunction (BH) organic solar cell, obtained for the PTB7-Th polymer with PC71BM acceptor. 136 Fullerene employment as hydrogen gas storage devices for electric vehicles is another interesting green application due to its unique molecular structure, which potentially can be chemically and reversibly hydrogenated and de-hydrogenated. 137 C60H36 isomers, characterized approximately by 5% of hydrogen content, have been developed experimentally. 138 According to theoretical calculations, a C- H bond is 68 kcal/mol, which is about 15 kcal/mol weaker than a C-C bond (83 kcal/mol). 137 This means that, under heating, the C-H bond will break faster than the C-C bond, making fullerene hydrides able to give off hydrogen while preserving their structure. 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Their powerful antioxidant and radical scavenger properties have led to investigate their potential in a wide variety of biological activities including multiple sclerosis, 140 neurodegenerative, 141 anti-HIV activity, 142 anti-Ebola activity, 143 cancer, 144 radiation exposure, 145 ischemia, 146 osteoporosis, 147 general inflammation 148 and selective antimicrobial agents against bacteria, just to name a few. 149 139 S. R. Chae, E. M. Hotze, M. R. Wiesner, Water Sci Technol. 2013, 67, 11, 2582. 140 A. S. Basso, l. D. Frenke, F. J. Quintana, F.A. Costa-Pinto, S. Petrovic-Stojkovic, L. Puckett, A. Monsonego, A. Bar-Shir, Y. Engel, M. Gozin, H. L. Weiner, J. Clin. Invest. 2008, 118, 1532. 141 L. L. Dugan, D. M. Turetsky, C. Du, D.Lobner, M. Wheeler, C. R. Almli, C. K. F. Shen, T. Y. Luh, D. W. Choi, T. S. Lin, PNAS 1997, 94, 9434. 142 S. Bosi, R. T. Da, G. Spalluto, J. Balzarini, M. Prato, Bioorg. Med. Chem. Lett. 2003, 13, 4437; b) S. Marchesan, R. T. Da, G. Spalluto, J. Balzarini, M. 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Graphene: Structure and Properties Graphite, the most abundant natural carbon allotrope due to its thermodynamic stability at ambient temperature and pressure, is characterized by a network of sp 2 hybridized carbons for each single layer, which in turn corresponds to graphene structure. Each atom is bonded to other three, forming hexagons that are fused throughout the layer. The layers are separated by a distance of 3.35 Å and linked by van der Waals interactions generated by the delocalization of π-orbitals. This delocalization generates conjugated π-electrons that are responsible of the electronic properties of graphite. Graphite is anisotropic because of the nature of this binding forces, it presents good electrical and thermal conductivity through the layers and it is a poor electrical and thermal conductor in the perpendicular direction. 150 Although graphene can be considered the mother for all the different allotropes (i.e. fullerenes can be obtained from graphene with the introduction of pentagons, 151 carbon nanotubes by rolling graphene along a given direction and reconnecting the carbon bonds, 152 etc.) and it has been presumably produced every time someone writes with a pencil, it was “curiously” isolated only 440 years after its invention (Figure 66). 153 150 D. D. L. Chung, J. Mater. Sci. 2002, 37, 1475. 151 W. Andreoni, The Physics of Fullerene-Based and Fullerene-Related Materials, Springer, Berlin, 2000. 152 a) R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998; b) J. C. Charlier, X. Blase, S. Roche, Rev. Mod. 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So, depending on its structure, it could be classified in three different types: single layer, bilayer and few-layer graphene. 166 In general, carbon has led to several technological revolutions, from the industrial one, with energy production by burning carbon, in the 19th century 167 , to the revolution in manufacturing industry, in the 20th century, with the introduction of plastics based on carbon. 168 So, why might graphitic carbon not be a key component of a third technological revolution in the 21st century? After all, the number of publications on this topic keeps growing with no sign of slowing down, and the reasons are multiple. First, its unique set of properties can be exploited in many areas. Second, it is an inherently sustainable and economical technology due to its abundance on earth. Third, as planar material results compatible with the established production technologies and integrable with conventional materials, such as Si. Graphene and its relative materials are a perfect example of transferable technology to the industry, in fact, a significant number of global patents around graphene have been filed since 2004 (Figure 68). 169 166 A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183. 167 E. Hobsbawm, The Age of Revolution: Europe 1789-1848, Weidenfeld&Nicolson Ltd., 1962. 168 H. G. Elias, Plastics, General Survey, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. 169 Patent Analysis Report on Graphene, PatSeer Pro, 2017. Figure 68: Filing trends in Graphene during the last 15 years. Ref 170. Chapter 2. Background 153 A very diverse range of applications was found for graphene, including characterization, 170 polymer composites, 171 transparent displays, 172 transistors, 173 capacitors, 174 solar cells, 175 biosensors, 176 conductive inks, 177 windows, 178 saturable absorbers, 179 photodetectors, 180 tennis rackets. 181 However, overall, the graphene patent space comprises patent filings in two main sectors: electronics (e.g. use of graphene for displays, transistors and computer chips) and synthesis (e.g. production of graphene by chemical vapour deposition (CVD), exfoliation, chemical, electrochemical, thermal, or photocatalytic reduction of graphene oxide, etc.). In spite of the great application potential of this material and its derivatives, the competitiveness of graphene in the field of semiconductors and sensors is decreased by the intrinsic zero band gap nature of its pristine form. 182 Therefore, if the initial research efforts were mainly focused on the development of new synthetic routes enabling an effective production of well-defined sheets (such as micromechanical cleavage, 153 chemical vapor deposition, 158 graphitization of silicon carbide, 183 anodic bonding, 184 unzipping of 170 A. Colli, S. A. Awan, A. Lombardo, T. J. Echtermeyer, T. S. Kulmala, A. C. Ferrari, US 20130162333, 2013. 171 O. Rozhin, A. Ferrari, W. I. Milne, US 20100003528, 2010. 172 Y. Woo, S. A. Seo, D. Kim, H. Chung, US 20110089403, 2011. 173 T. Kobayashi, EP 2393107, 2013. 174 L. Song, A. Zhamu, J. Guo, B. Z. Jang, US 7623340, 2009. 175 S. M. Yoon, W. M. Choi, H. J. Shin, J. Y. Choi, EP 2439779, 2014. 176 G. Owen, GB 2471672, 2011. 177 a) B. Z. Jang, A. 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Background 155 To carry out this opening, there are four kinds of methods: heteroatom doping, 196 electrostatic field tuning, 197 cutting graphene into nanoribbons 198 and chemical modification (Figure 69b). 199 Therefore, functionalization schemes confer an additional element of control over the nanomaterial properties by various covalent and non- covalent interaction methods of organic and inorganic molecules. 200 196 M. L. Mueller, X. Yan, B. Dragnea, L. S. Li, Nano Lett. 2011, 11, 56. 197 T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science 2006, 313, 951. 198 L. Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Nature 2009, 458, 877. 199 H. Liu, Y. Liu, D. Zhu, J. Mater. Chem. 2011, 21, 3335. 200 a) Z. Sun, D. K. James, J. M. Tour, J. Phys. Chem. Lett. 2011, 2, 2425; c) M. Quintana, E. Vazquez, M. Prato, Acc. Chem. Res. 2013, 46, 138; d) K. P. Loh, Q. Bao, P. K. Ang, J. Yang, J. Mater. Chem. 2010, 20, 2277; e) K. Dirian, M. A. Herranz, G. Katsukis, J. Malig, L. 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Moreover, from molecular orbitals considerations, the carbon atoms at zigzag borders seem to be the most reactive positions. 202 Graphene, depending on the production method, can also present a different number of defects due to the absence of some sp 2 carbon atoms or the presence of carbons with tetrahedral hybridization. The consequence is obviously an enhanced reactivity of the material because of the chemical activation of the atoms surrounding the structural imperfection. 203 The major part of reviews are dedicated to the reactivity of graphene based on the chemical transformation of readily available graphene oxides. 204 Graphene oxide (GO) is a chemically modified graphene layer. The modification consists in the introduction of epoxide, hydroxyl, carbonyl and carboxyl groups by oxidative treatment of graphite. 205 This treatment consists in the initial intercalation of the 201 S. Ryu, M. Y. Han, J. Maultzsch, T. F. Heinz, P. Kim, M. L. Steigerwald, L. E. 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Sh, Carbon 2013, 64, 225. Chapter 2. Background 157 graphite planes with acid, followed by oxygenation and final delamination of the final more hydrophilic material. Anyway, the major drawbacks of this method are that GO retains significant levels of defects and possess uncontrolled geometrical shapes due to the harsh oxidation conditions. On the other side, stands out the opportunity to directly functionalize the pristine graphene. Obviously, respect to GO, the pristine material possesses a lower reactivity due to the planarity of its structure. Specifically, the chemistry of pristine graphene can be divided in two main approaching types: supramolecular and covalent. The non-covalent functionalization is a physical process that takes advantage of the adsorption of molecules 200f,206 (that can also be characterized by photo- and/or electroactive moieties) onto the graphene surface, through van der Waals forces, electrostatic interactions, hydrogen bonding, coordination bonds, π-π interactions or electron transfer processes, to benefit from eventually induced new properties. The main advantage of this technique is that graphene’s π- natural structure remains unaffected. Covalent Functionalization of Graphene When organic molecules are covalently attached on the graphene surface, its extended aromatic character is perturbed, enabling the control of its electronic properties. To functionalize the pristine material covalently different methodologies were developed, the major part of which were already part of the arsenal of chemical reactions previously used for the functionalization of related fullerenes and carbon nanotubes (Figure 70). 207 206 a) V. 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Background 158 The first covalent reaction conducted on the pristine material produced the so called “graphane”, in which the honeycomb structure of graphene is modified by the covalent attachment of a hydrogen atom for each knot of the lattice. 208 However, this first hydrogenation process, carried out in a cold hydrogen plasma flow, resulted in the production of a high number of defects. Less imperfections were observed with the radiofrequency catalytic chemical vapour deposition Lerner, M. Yan, Nano Lett. 2010, 10, 3754; g) E. Bekyarova, M. E. Itkis, P. Ramesh, C. Berger, M. Sprinkle, W. A. de Heer, R. C. Haddon, J. Am. Chem. Soc. 2009, 131, 1336; h) M. Quintana, A. Montellano, A. E. del Rio Castillo, G. Van Tendeloo, C. Bittencourt, M. Prato, Chem. Commun. 2011, 47, 9330; i) L. Rodríguez-Pérez, M. Á. Herranz, N. Martín, Chem. Commun. 2013, 49, 3721. 208 A. Savchenko, Science 2009, 323, 589. Figure 70: Schematic representation of the principal reactions on pristine graphene. Ref 208. Chapter 2. Background 159 (rf-cCVD) method used later by Biris group. 209 While, other groups, using the wet-chemical method based on a Birch reduction, obtained a slowdown of the process which permits to reduce the competitive process of H2 evolution. 207a,210 Halogenated analogues of graphane were also obtained by different approaches. 211 From theoretical calculations, the fluorinated derivative seems to be the more stable between the five graphene derivatives (graphane, graphene fluoride, bromide, chloride, and iodide). 211a However, hydrogenated and halogenated graphene derivatives result without any exclusion highly insulator. Considering the low reactivity of graphene, particularly suitable to fulfil the task of functionalizing by covalent attachments are the thermal and/or photoinduced radical additions. One of the first examples was carried out on FLG by employing the photodecomposition of benzoyl peroxides. 212 Surely, the most widely used technique to functionalize the pristine surface of graphene consists in the use of the aryl radicals obtained from aryl diazonium salts. This reaction, which is exactly the functionalization method used in this second chapter of the present thesis, is a powerful tool that provides interesting intermediates to introduce further functional groups by simply changing the substituents in the benzene ring. After the use of this reaction to chemically modify the external surface of CNTs 209 L. Zheng, Z. Li, S. Bourdo, F. Watanabe, C. C. Ryersonb, A. S. Biris, Chem. Commun. 2011, 47, 1213. 210 R. A. Schafer, J. M. Englert, P. Wehrfritz, W. Bauer, F. Hauke, T. Seyller, A. Hirsch, Angew. Chem. Int. Ed. 2013, 52, 754. 211 a) R. R. Nair, W. Ren, R. Jalil, I. Riaz, V. G. Kravets, L. Britnell, P. Blake, F. Schedin, A. S. Mayorov, S. Yuan, M. I. Katsnelson, H. Ming Cheng, W. Strupinski, L. G. Bulusheva, A. V. Okotrub, I. V. Grigorieva, A. N. Grigorenko, K. S. Novoselov, A. K. Geim, Small 2010, 6, 2877; b) H. L. Poh, P. Šimek, Z. Sofer, M. Pumera, Chem. Eur. J. 2013, 19, 2655. 212 H. Liu, S. Ryu, Z. Chen, M. L. Steigerwald, C. Nuckolls, L. E. Brus, J. Am. Chem. Soc. 2009, 131, 17099. Chapter 2. Background 160 (Scheme 22), 213 Tour and collaborators also went a step further employing the same reaction on different GO derivatives. 214 The aryl radicals of the Tour’s reaction derives from the thermal decomposition of in situ generated diazonium salts by action of isoamyl nitrite on aniline derivatives. The suggested mechanism for this reaction, still not completely clear, involves three main steps (Scheme 23): 215 213 J. L. Bahr, J. Yang, D. V. Kosynkin, M. J. Bronikowski, R. E. Smalley, J. M. Tour, J. Am. Chem. Soc. 2001, 123, 6536. 214 a) J. R. Lomeda, C. D. Doyle, D. V. Kosynskin, W. F. Hwang, J. M. Tour, J. Am. Chem. Soc. 2008, 130, 16201; b) Z. Jin, J. R. Lomeda, B. K. Price, W. Lu, Y. Zhu, J. M. Tour, Chem. Mater. 2009, 21, 3045. 215 M. E. Lipioska, S. L. H. Rebelo, M. F. R. Pereira, J. A. N. F. Gomes, C. Freire, J. L. Figueiredo, Carbon 2012, 50, 3280. Scheme 22: Covalent functionalization of single wall carbon nanotubes (SWNT) by diazonium salts performed by Tour’s group. Ref 214. Chapter 2. Background 161 a) the aniline derivatives attack nitrite group, giving the diazonium species plus the diazotate (i) and the alcoxide (iii) species (which can also be formed); b) the anionic species (i) and (iii) react with the diazonium salts leading to diazoanhydride and diazoether, respectively. At this point, the formed intermediates undergo a rapid thermal decomposition releasing nitrogen and generating the aryl radicals; c) finally these species readily react with the curved surface of CNTs or the planar one of graphene forming a carbon-radical structure, which in turn can react with other radicals. However, due to the radical nature of the reaction other associated paths can also take place. 215 The first reaction performed involving diazonium salts functionalization on pristine graphene was carried out by Haddon (Scheme 24). 207g Scheme 23: Proposed mechanism for Tour’s reaction: (a) diazotization reaction; (b) formation of aryl radicals and of oxygen radicals that reinitiate the cycle; and (c) recombination of radicals with CNT. Ref 216. Chapter 2. Background 162 Nitrophenyldiazonium salts were employed as radical initiators determining changes in the electronic structure and transport properties of the graphene surface from near-metallic to semiconducting. However, another group arrived to the opposite conclusion studying exactly the same reaction. They claimed that the material, after the modification, exhibits a higher conductivity than its original pristine form resulting from the charge transfer effect of nitrophenyl groups (Figure 71). 216 Hirsch et al. 217 developed a wet chemical approach based on the functionalization of graphene layers derived from an exfoliation process of bulk graphite by activation with a liquid Na/K alloy. The positive potassium cations intercalate the negative charged graphite 216 P. Huang, H. Zhu, L. Jing, Y. Zhao, X. Gao, ACS Nano 2011, 5, 7945. 217 J. M. Englert, C. Dotzer, G. Yang, M. Schmid, C. Papp, J. M. Gottfried, H. P. Steinruck, E. Spiecker, F. Hauke, A. Hirsch, Nat. Chem. 2011, 3, 279. Scheme 24: Haddon’s functionalization of pristine graphene by nitrophenyldiazonium salts. Ref 208g. Figure 71: Raman spectra of pristine and modified graphene, where the asterisks correspond to the peaks introduced by nitrophenyl groups (a) and I/V curve of pristine and modified graphene respectively (b). Ref 217. Chapter 2. Background 163 layers enhancing the interlayer distance. At this point the successful reaction with the radicals generated from aryl diazonium salts is testified by the temperature increase and the liberation of nitrogen. Interestingly, once the negative charged graphene was functionalized the hindrance of the groups present on the surface avoids the π-π stacking reaggregation process. Another example of prevented reaggregation process of graphene layers, mediated by diazonium salts functionalization, was reported by Tour’s group. 218 In this process, thermally expanded graphite is reacted in situ with 4-bromophenyl diazonium salts. After a mild sonication in DMF, the exfoliation of functionalized, especially at the edges, FLGs takes place. This particular higher degree of edge’s functionalization is motivated considering the low internal surface exposure of graphite during the reaction. The diazonium salts functionalization, depending on the substituent of the aryl rings, results also particularly useful to be coupled with other reactions. An interesting example is furnished by the so called “click reaction”, the Cu(I)-catalyzed Huisgen [3+2] cycloaddition between azide and alkyne moieties. 219 Its combination with the diazonium salts reaction on graphene, due to the retaining of the conjugation of the formed triazole ring, results extremely suitable for studying the charge transport properties of different molecular architectures derived from the grafting of chemically or physically active molecules (Figure 72). 219c,220 218 Z. Sun, S. Kohama, Z. Zhang, J. R. Lomeda, J. M. Tour, Nano Res. 2010, 3, 117. 219 a) F. Amblard, J. H. Cho, R. F. Schinazi, Chem. Rev. 2009, 109, 4207; b) H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. Int. Ed. 2001, 40, 2004; c) Z. G. L. C. Paulus, A. J. Hilmer, S. Shimizu, M. S. Strano, Chem. Mater. 2011, 23, 3362. 220 a) H. X. Wang, K. G. Zhou, Y. L. Xie, J. Zeng, N. N. Chai, J. Li, H. L. Zhang, Chem. Commun. 2011, 47, 5747; b) M. Castelaín, G. Martínez, P. Merino, J. A. Martín-Gago, J. L. Segura, G. Ellis, H. J. Salavagione, Chem. Eur. J. 2012, 18, 4965. Chapter 2. Background 164 A good alternative to the aryl diazonium salt covalent modification of graphene edges was the Friedel–Crafts acylation reaction. 207d The reaction is carried out in a mild viscous acid medium, a mixture of polyphosphoric acid (PPA) and phosphorus pentoxide (P2O5), and allows the protonation of the graphitic surface favouring its delamination. The 4-aminobenzoic acid, generating in situ carbonium ions, becomes capable of attacking the defects of the graphite edges. This functionalization, enhancing the solubility of graphite, allows in this way the further penetration of the viscous medium which leads, eventually, to the isolation of exfoliated graphene. Another useful possibility to create covalent interactions with the pristine carbon lattice relies on the cycloaddition processes. The graphene reactivity in these reactions and more specifically in the Diels−Alder cycloaddition is attributable to the particular peculiarity of graphene of degeneracy of the electronic states at the Dirac point. The states close to the Fermi level may give rise to either an antisymmetric or symmetric graphene orbital, making graphene able to act as both donor and acceptor (diene and dienophile) within the frontier molecular orbital (FMO) theory. 221 1,3-Dipolar cycloadditions of azomethine ylides “in situ” generated by thermal condensation of aldehydes and α- amino acids, [2+1] cycloadditions of thermo-derived nitrenes 221 K. Fukui, Theory of Orientation and Stereoselection, Springer-Verlag, Berlin, Germany, 1970. Figure 72: Representation of different graphene materials obtained by using Cu(I)- catalyzed Huisgen 1,3-dipolar cycloaddition reactions. Ref 220c and 221. Chapter 2. Background 165 bromomalonates in the presence of a base and [2+2] cycloadditions of arynes derivatives can be generically considered additions of dienophiles to the sp 2 carbon-carbon bonds of the graphene structure. The [4+2] Diels-Alder cycloaddition reaction makes the only exception, because graphene results able to behave, as diene and dienophile. To prove the efficiency of this reaction, various types of graphene were employed (epitaxial graphene, exfoliated graphene and highly orientated pyrolitic graphite), together with different dienes (9- methylanthracene and 2,3-dimethoxy-1,3-butadiene) and dienophiles (tetracyanoethylene and maleic anhydride). 207b,222 However, different reaction conditions, such as the substrate used, the graphene type and temperature, result to strongly affect the degree of surface functionalization. 207b,222 As a representative example, Quintana et al. 207h demonstrated that the reactive sites of graphene vary depending on the type of covalent chemical modification performed. Making use of the selective binding of gold nanoparticles to the amino groups, Quintana compared the selectivity of the 1,3 dipolar cycloaddition versus the amidation reaction on exfoliated graphene. It was observed that, while the 1,3-dipolar cycloaddition occurs indiscriminately all over the graphene surface, with the amidation reaction, on the other side, the functional groups were deposited mainly at the edges of the graphene layers. (Figure 73) 222 S. Sarkar, E. Bekyarova, R. C. Haddon, Acc. Chem. Res. 2012, 45, 673. Chapter 2. Background 166 Other interesting example, which shows one of the few attempts to carry out a [2+2] cycloaddition reaction on graphene, is furnished by the recent work of García et al. 223 The synthesis of a hybrid all-carbon nanostructure via a C60-aryne building block is described (Figure 74). In conclusion, the chemical modification of pristine graphene, by the covalent attachment of different functionalities, induces expected changes in its original properties. The resulting solubility of this 223 D. García, L. Rodríguez-Pérez, M. A. Herranz, D. Peña, E. Guitián, S. Bailey, Q. Al- Galiby, M. Noori, C. J. Lambert, D. Pérez, N. Martín, Chem. Commun. 2016, 52, 6677. Figure 73: Graphene layers functionalized with Au nanoparticles on the left and TEM photographs of the corresponding composites on the right of the a) 1,3-dipolar cycloaddition and b) the amidation reaction. Ref 208h. Figure 74: TEM images of pristine FLG (a) and FLG-C60 nanoconjugates. Ref 224. Chapter 2. Background 167 hybrids improves in both polar and apolar media if hydrophilic 224 or hydrophobic 214a,225 precursors are employed, respectively. Improved mechanical and thermal properties have been detected in polymeric matrix which make use of graphene as a filler. 226 Moreover, a lot of different modifiers, ranging from synthetic 227 and natural 228 polymers to relatively small organic or inorganic molecules, 229 could be incorporated in the graphene composites to achieve better electrical and photoelectrical properties. These latter properties, together with its many other advantageous characteristics and the always better control of the techniques at the base of its chemical manipulation, rise the interest and expectation towards its still comparatively rare industrial applications. A field which may receive a significant boost by the development of graphene based materials is, for example, the flexible and stretchable electronics area, including semiconducting channels for high-speed transistor and logic circuits, transparent electrodes for flexible energy-harvesting devices, touch panels, rollable displays, and sensing materials for tactile sensors. 230 224 a) F. Liu, J. Y. Choi, T. S. Seo, Chem. Commun. 2010, 46, 2844; b) J. Liu, W. Yang, L. Tao, D. Li, C. Boyer, T. P. Davis, J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 425; c) D. W. Lee, T. Kim, M. Lee, Chem. Commun. 2011, 47, 8259. 225 a) J. F. Shen, Y. H. Hu, C. Li, C. Qin, M. X. Ye, Small 2009, 5, 82; b) E. Y. Choi, T. H. Han, J. Hong, J. E. Kim, S. H. Lee, H. W. Kim, S. O. Kim, J. Mater. Chem. 2010, 20, 1907. 226 a) M. A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z. Z. Yu, N. Koratkar, ACS Nano 2009, 3, 3884; b) E. R. Margine, M. L. Bocquet, X. Blase, Nano Lett. 2008, 8, 3315. 227 Q. Bao, H. Zhang, J. Yang, S. Wang, D. Y. Tang, R. Jose, S. Ramakrishna, C. T. Lim, K. P. Loh, Adv. Funct. Mater. 2010, 20, 782. 228 Q. Yang, X. Pan, F. Huang, K. Li, J. Phys. Chem. C 2010, 114, 3811. 229 a) Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc. 2008, 130, 5856; b) M. Quintana, K. Spyrou, M. Grzelczak, W. R. Browne, P. Rudolf, M. Prato, ACS Nano 2010, 4, 3527; c) Q. Su, S. Pang, V. Alijani, C. Li, X. Feng, K. Mullen, Adv. Mater. 2009, 21, 3191. 230 a) N. Petrone, I. Meric, J. Hone, K. L. Shepard, Nano Lett. 2012, 13, 121; b) D. Kuzum, H. Takano, E. Shim, J. C. Reed, H. Juul, A. G. Richardson, J. de Vries, H. Bink, M. A. Dichter, T. H. Lucas, D. A. Coulter, E. Cubukcu, B. Litt, Nat. Commun. 2014 , 5 , 5259; c) J. Li, J. Liang, L. Li, F. Ren, W. Hu, J. Li, S. Qi, Q. Pei, ACS Nano 2014, 8, 12874; d) B. Zhu, Z. Niu, H. Wang, W. R. Leow, H. Wang, Y. Li, L. Zheng, J. Wei, F. Huo, X. Chen, Small 2014 , 10, 3625. Chapter 2. Background 168 7.1.2.2. Graphene- and Graphene Oxide-Based-C60 Hybrid Materials Both non-covalent stacking and covalent functionalization of graphene with photosensitizers are expected to shift the Fermi level. This shift results in the opening of an electronic gap between the valence and the conduction band of this material, thus allowing the formation of efficient D-A systems. In these nanoconjugates, graphene can play a dual role, either as electron donor or electron acceptor, depending on the relative energy levels of the interacting species. Porphyrins (P) and phthalocyanines (Pc) have been widely employed as organic photosensitizers in the creation of either covalent or non-covalent graphene-based D-A hybrids. 231 Recently, instead, a great interest has been rising towards the development of novel graphene-based composite nanomaterials integrating C60 and its derivatives, widely known in literature as well established molecules for organic photovoltaics applications. 135,136,232 The easiest way to form hybrid materials combining the properties of both carbon allotropes, was to use GO derivatives, due to the presence on their surface of several organic functionalities which could serve as anchoring groups for further chemical functionalization. X. Y. Zhang et al., for example, prepared covalent GO-based-C60 hybrid materials using fulleropyrrolidine 233 and fullerenol. 234 The nonlinear optical properties (NLO) of the new hybrids were investigated revealing an enhanced performance with respect to its parent species (C60 and graphene) for both nanocomposites, which can 231 a) T. Skaltsas, S. Pispas, N. Tagmatarchis, Chem. Eur. J. 2013, 19, 9286; b) Y. Xu, Z. Liu, X. Zhang, Y. Wang, J. Tian, Y. Huang, Y. Ma, X. Zhang, Y. Chen, Adv. Mater. 2009, 21, 1275; c) J. Malig, N. Jux, D. Kiessling, J. J. Cid, P. Vázquez, T. Torres, D. M. Guldi, Angew. Chem. Int. Ed. 2011, 50, 3561; d) L. Brinkhaus, G. Katsukis, J. Malig, R. D. Costa, M. Garcia-Iglesias, P. Vázquez, T. Torres, D. M. Guldi, Small 2013, 9, 2348; e) M. E. Ragoussi, J. Malig, G. Katsukis, B. Butz, E. Spiecker, G. de la Torre, T. Torres, D. M. Guldi, Angew. Chem. Int. Ed. 2012, 51, 6421. 232 T. Zhuang, X. F. Wang, T. Sano, Z. Hong, Y. Yang, J. Kido, Appl. Phys. Lett. 2013, 103, 203301. 233 X. Y. Zhang, Y. Huang, Y. Wang, Y. F. Ma, Z. F. Liu, Y. S. Chen, Carbon 2009, 47, 1, 334. 234 Z. B. Liu, Y. F. Xu, X. Y. Zhang, X. L. Zhang, Y. S. Chen, J. G. Tian, J. Phys. Chem. B 2009, 113, 29, 9681. Chapter 2. Background 169 be at least partially attributable to the photoinduced electron transfer mechanism between the graphene sheet and C60 (Figure 75). 234, 235 In these two attempts, fullerene was only grafted on the edge of graphene, instead, Y. Zhang et al. presented an effective route to simultaneously exfoliate and functionalize graphene. Fullerene molecules were innovatively inserted in between graphene layers of GO via the Fisher esterification of the hydroxyl groups on graphene 235 X. Y. Zhang, Z. B. Liu, Y. Huang, X. J. Wan, J. G. Tian, Y. F. Ma, Y. S. Chen, J. Nanosci. Nanotechnol. 2009, 9, 10, 5752. Figure 75: (a) and (b) GO-based-C60 hybrid materials using fulleropyrrolidine and fullerenol, respectively.(c) Open-aperture Z-scan curves of Graphene-C60, pyrrolidine fullerene, graphene oxide, and fulleropyrrolidine/graphene oxide blend with (1:1 w/w) for 5 ns pulsed laser at 532 nm. (d) Open aperture Z-scan results of fullerenol in DMF, C60 in toluene, GO, Graphene-C60 and GO/fullerenol blend with the same concentration of 0.1 mg/mL for 5 ns pulsed laser at 532 nm. The graphene-C60 has the largest dip among the transmittance curves of these materials, indicating that it has the best NLO effect. Ref 235 and 236. Chapter 2. Background 170 oxide with the carboxyl groups in (1,2-methanofullerene C60)-61- carboxylic acid (Figure 76). 236 All the previous works showed the versatility of the many functional groups present on the surface of GO to synthesize GO-based-C60 hybrid materials. Although less-studied, some examples of all-carbon hybrids based on the functionalization of pristine graphene have also been reported. Yu et al., for example, developed a simple method to achieve a relatively “cleaner” all-carbon hybrid, by using n-BuLi to lithiate graphene, then made to react with C60 by nucleophilic addition (Figure 77). 237 These C60-grafted graphene nanosheets employed as electron acceptors in poly-(3-hexylthiophene)-based BH solar cells resulted to significantly improve the electron transport, and hence the overall device performance, whose PCE is of ∼1.22%. On the other side, García et al. carried out the synthesis of a new really-all-carbon hybrid 236 Y. Zhang, L. Ren, S. Wang, A. Marathe, J. Chaudhuri, G. Li, J. Mater. Chem. 2011, 21, 5386. 237 D. Yu, K. Park, M.Durstock, L. Dai, J. Phys. Chem. Lett. 2011, 2, 1113. Figure 76: Functionalization of the graphene oxide via Fisher esterification. Ref 237. Figure 77: Schematic representation of grafting C60 onto graphene through lithiation reaction with n-butyllithium. Ref 238. Chapter 2. Background 171 nanoconjugate by the generation of a [60]fullerene-benzyne building block and its further chemical cycloaddition reaction with exfoliated graphene. 223 7.2. Thermopower Measurements In Molecular Junctions 238 The thermopower and the conductance (G) are characteristic properties of molecular junctions which serve to characterize the electronic transport. The importance of thermopower measurements is due to two main reasons: i) it provides complementary information to the conductance about the electronic structure and transport properties of a junction at molecular scale, and ii) the control of these processes paves the way to the development of new environmentally friendly organic based thermoelectric devices, which can convert directly heat into electricity with high efficiency for energy-harvesting applications or nanoscale cooling systems. The thermoelectric properties of a molecular junction depend on the combination of the charge transport and the thermal transport. Consider a temperature difference (ΔT) applied between two metal electrodes. Due to the Seebeck effect a voltage difference (ΔV), proportional to the temperature difference, is generated in the nanoscale junction (Figure 78). This is expressed as: 𝛥𝑉 = 𝑆𝛥𝑇 238 L. Rincón-García, C. Evangeli, G. Rubio-Bollingerac, N. Agraït, Chem. Soc. Rev. 2016, 45, 4285. Figure 78: Thermovoltage measurement in a molecular junction. Chapter 2. Background 172 Where S is the aforementioned thermopower (or Seebeck coefficient). Therefore, the electrical current flowing into a junction is the sum of the bias-induced current and the temperature-induced current: 𝐼 = 𝐺𝛥𝑉 + 𝐺𝛥𝑇𝑆 This electrical current depends on the transmission probability for an electron, at the Fermi level, to cross from the left-side electrode to the right-side electrode. Because this transmission probability is a function of the electron energy, which in turn depends on the electronic structure of the molecule (molecular orbitals), it is modified by the coupling to the electrodes. The alignment of the HOMO and the LUMO with respect to the Fermi level is crucial for the transport properties of the junction. The sign of thermopower is governed by which molecular orbital is closer to the Fermi level:  S > 0, when the electrons below the Fermi level have a larger transmission probability than those above (the movement occurs from the cold to the hot electrode and the Fermi level is closer to the HOMO);  S < 0, when the electrons above the Fermi level have a larger transmission probability than those below (the movement occurs from the hot to the cold electrode and the Fermi level is closer to the LUMO) (Figure 79). Figure 79: For higher transmission probability below the Fermi level (a), the transport takes place through the HOMO (c) determining a positive sign for S and conversely (b and d). Chapter 2. Background 173 The most widely used method to study the transport processes in molecular junctions is the break junction technique. Specifically, the recent development of a useful STMBJ-based technique permits to measure simultaneously, during the entire evolution of the molecular junction, both conductance and thermopower. 239 In this technique, the movement of the STM tip during its approach and retraction is halted at short time intervals for a few milliseconds and the voltage is ramped to collect hundreds of IV curves for each approaching-retracting cycle. The thermopower S can be measured from such IV curves, since, when there is no temperature difference between the electrodes, IV curves cross the origin. However, if there is a temperature difference between the electrodes a voltage is generated due to the Seebeck effect, and the IV curves are offset by 𝛥𝑉 = 𝑆𝛥𝑇 (see above). By monitoring 𝛥𝑇 in real time during the acquisition of the IV curves, and the measurement of 𝛥𝑉, such as in Figure 80c, the thermopower S can be determined. The thermopower measurements of any given junctions although remarkably stable, can be submitted to some fluctuations due to factors such as the contact geometry, the orbital hybridization and the 239 C. Evangeli, K. Gillemot, E. Leary, M. T. González, G. Rubio-Bollinger, C. J. Lambert, N. Agraït, Nano Lett. 2013, 13, 2141. Figure 80: Tip displacement z (a) and bias voltage applied (b) at the molecular junction as a function of time. Experimental IV curves showing the voltage offset due to the temperature difference (c) and schematic representation of the setup (d). Chapter 2. Background 174 intramolecular interactions which induce variation in the alignment of the orbitals of the molecules with respect to the Fermi level of the electrodes. 240 Different studies also demonstrated that the value of S varies linearly with the length of the molecule. 241 It increases or decreases depending on the nature of the molecular backbone (presence of electron-donating or electron-withdrawing groups, 240 as well as of π- conjugated units or alkyl chains, 242 etc.). Furthermore, tunability of the thermopower can be achieved, with little influence on the conductance, by changing the anchoring group. 242,243 Indeed, the nature of the anchoring group is mainly responsible for the sign of S and, therefore, of the type of transport (p- or n-type, corresponding to transport through the HOMO and the LUMO, respectively). Another important factor which affects the thermopower is the metal constituting the electrodes, since different level alignment of the molecular orbitals can be induced depending on the different work functions of the metal. 242,244,245 In table 4, the conductance G and the thermopower S, obtained for some molecular junctions, are presented to illustrate typical values and how they vary depending on the factors mentioned above. 240 J. A. Malen, P. Doak, K. Baheti, T. D. Tilley, A. Majumdar, R. A. Segalman, Nano Lett. 2009, 9, 3406. 241 P. Reddy, S. Y. Jang, R. A. Segalman, A. Majumdar, Science 2007, 315, 1568. 242 a) J. A. Malen, P. Doak, K. Baheti, T. D. Tilley, R. A. Segalman, A. Majumdar, Nano Lett. 2009, 9, 1164; b) W. B. Chang, C. K. Mai, M. Kotiuga, J. B. Neaton, G. C. Bazan, R. A. Segalman, Chem. Mater. 2014, 26, 7229. 243 J. R. Widawsky, P. Darancet, J. B. Neaton, L. Venkataraman, Nano Lett. 2012, 12, 354. 244 K. Baheti, J.A.Malen, P. Doak, P. Reddy, S. Y. Jang, T. D. Tilley, A. Majumdar, R. A. Segalman, Nano Lett. 2008, 8, 715. 245 S. K. Yee, J. A. Malen, A. Majumdar, R. A. Segalman, Nano Lett. 2011, 11, 4089. Chapter 2. Background 175 Table 4: Typical values of conductance and thermopower for some molecular junctions. Molecule G(G0) S,ΔS(μV.K-1) Ref. 8.7± 2.1 12.9 ± 2.2 12.9 ± 2.2 A. Majumdar (242) 1.05 x 10 -2 1.19 x 10 -2 7.6 x 10 -3 8.7 x 10 -3 7.2 ± 0.2 8.3 ± 0.3 4.0 ± 0.6 5.4 ± 0.4 -1.3 ± 0.5 R. A. Segalman (241) 8.7 ± 0.2 12.9 ± 2.2 14.2 ± 3.2 2.3 ± 0.3 4.9 ± 1.9 6.4 ± 0.4 6.8 ± 0.2 5.5 ± 0.1 5.2 ± 0.4 4.9 ± 0.2 3.3 ± 0.1 2.4 ± 0.4 A. Majumdar (243a) ~7.1 x 10 -4 ~2.5 x 10 -4 ~2.0 x 10 -3 ~1.3 ± 10 -3 ~2.5 ± 10 -3 ~6.3 ± 10 -4 ~6.3 ± 10 -4 ~1.6 ± 10 -3 -8.9 ± 2.2 -14.5 ± 1.2 -29.62 ± 3.4 -7.6 ± 3.2 -16.4 ± 1.6 -30.0 ± 2.6 -8.4 ± 1.9 -20.1 ± 1.4 -33.1 ± 8.8 R. A. Segalman (246) 8. Objectives Chapter 2. Objectives 179 8. Objectives 8.1. Graphene On Surface (GOS)-Based Materials As mentioned before, fullerenes are known to have been largely employed as electron acceptors in DBA systems 133,134 and graphene has demonstrated outstanding potential properties for electronic applications. 11d,161,184,185 However, to enhance this applicability and make it able to be handled, its chemical manipulation is required. In this optic and because of the less number of studies dedicated to the functionalization of the pristine material, due to its lower reactivity than GO, this second chapter of the present thesis has been conceived to study the properties exhibited by carbon nanohybrids based on covalently modified pristine graphene with fullerene derivatives. Specifically, the design of the target nanocomposites was formulated with the aim to get novel heterojunctions where, changing the connection position between the spacer and the graphene layer, would be possible to investigate the modification of the electron transfer path and the thermopower properties by use of a conductive atomic force microscope (c-AFM). The synthesized three covalent GOS-based hybrids (GOS1, GOS2 and GOS3) are diphenylmethanofullerenes appended materials. The fullerene moiety, acting as anchoring group, is covalently attached to a silica-supported single layer graphene (SLG), which acts as the second electrode of the c-AFM, through a bridging spacer, constituted by a diphenylmethane unit. This bridge unit can form a covalent bond with the monolayer through just one or both aromatic rings, using two possible attaching positions, para and meta (Figure 81). Chapter 1. Objectives 180 In GOS1 and GOS2 both phenyl units of the fullerene derivatives are connected to the pristine graphene monolayer but differ from each other by the position of the linkage. GOS1 is a p-substituted fullerene derivative, while GOS2 is bonded through the m-positions. On the other side, GOS1 and GOS3 are both linked through the p-positions of the rings, but differ from each other by the number of rings employed to form this covalent interaction. GOS3, in fact, is attached by just one unit of the diphenylmethane moiety to the pristine graphene sheet and was employed as reference material. Figure 81: GOS-based materials: (a) GOS1, (b) GOS2 and (c) GOS3. Chapter 2. Results and Discussion 183 9. Results and Discussion 9.1. Retrosynthetic Analysis of the GOS-Based Materials As discussed in the background, the covalent modification of pristine graphene, enhancing its properties (like opening its band gap, tuning conductivity, and improving solubility and stability), represents a necessary, very challenging and important research field to make it available for industrial applications. Anyway, a general problem, specifically related to the functionalization of neutral graphene is that, being characterized by a planar structure, results to be a rather chemically inert system. It is not surprising, therefore, that the covalent functionalization of pristine graphene requires mostly reactive species (such as radicals, nitrenes, carbenes, and arynes) that can form covalent adducts with the sp 2 carbons in its structure. In this context, in this second part of the present thesis, it was decided to use the diazonium chemistry to activate, by covalent modification, the structure of a graphene monolayer. The design of the target materials was structured in two main parts, the synthesis of the diamino-diphenylmethano[60]fullerene derivatives and following, their chemical insertion on the graphene monolayer. These methano[60]fullerene derivatives were thought to act as precursors for the in situ generation of the diazonium salts, during the functionalization process, via Tour’s reaction. 213 The selected diamino- diphenylmethano[60]fullerene building blocks and their further radical addition to graphene, resulted in the synthesis of the three aforementioned new all-carbon hybrid materials. GOS1, GOS2 and GOS3 were obtained as depicted in their retrosynthetic analysis (Scheme 25). Chapter 2. Results and Discussion 184 9.1.1. Synthesis of the Diamino-Diphenylmethano[60]Fullerene Building Blocks Functionalization of [60]fullerene was carried out through a Bamford- Stevens reaction, 246 which takes place through the in situ generation of intermediate diazocompounds. 247 Specifically, to justify the reaction product, two possible mechanisms were postulated (Scheme 26): (a) a 1,3-dipolar cycloaddition on a [6,6] bond of C60 to form a pyrazoline which is thermally unstable and loses nitrogen rapidly to give the thermodynamically most stable product, the [6,6]-closed methanofullerene, or (b) the direct extrusion of nitrogen followed by [2+1] cycloaddition reaction of the resulting carbene in a [5,6] bond of C60 to form the product of kinetic control, the [5,6]-open fulleroid. Conversion of the [5,6]-open fulleroid to the [6,6]-isomer is accomplished by heating as well as by photoexcitation. 248 246 W. R. Bamford, T. S. Stevens, J. Chem. Soc. 1952, 4735. 247 R. Gómez, J. L. Segura, N. Martín, Org. Lett. 2005, 7, 717. 248 R. A. J. Janssen, J. C. Hummelen, F. Wudl, J. Am. Chem. Soc. 1995, 117, 544. Scheme 25: Retrosynthetic analytic chart of the GOS-based materials. Chapter 2. Results and Discussion 185 However, the first step towards the synthesis of the desired diphenylmethano[60]fullerene structures 31, 33 and 35 consists in a condensation reaction to get the target tosylhydrazones. While product 30 requires a protection/deprotection strategy, 247 a straightway synthetic path was followed to achieve hydrazones 32 and 34 (Scheme 27). Scheme 26: Cyclopropanation reaction by: 1,3-dipolar cycloaddition to form [6,6]- diphenylmethanofullerenes (DPMs) or by insertion of a carbene to obtain the [5,6]-fulleroid. Scheme 27: Synthesis of the target tosylhydrazone moieties 30 by protection/deprotection strategy (a) and 32 and 34 by direct approach (b). Chapter 2. Results and Discussion 186 These syntheses were started from commercially available 4,4'- diaminobenzophenone, 3,3'-diaminodibenzophenone and 4- aminobenzophenone, respectively. 3,3'-Diaminobenzophenone and 4- aminobenzophenone were directly condensed with p-tosylhydrazine in ethanol to afford the corresponding tosylhydrazones 32 and 34. On the other side, 4,4'-diaminobenzophenone was initially reacted with trifluoracetic anhydride to afford the corresponding N-protected diamido compound 28, then it was treated with p-tosylhydrazine to yield the corresponding p-tosylhydrazone 29 under mild conditions. At this point, the hydrolysis of the trifluoroacetamido-diphenyl tosylhydrazone 29 with potassium carbonate afforded the corresponding diamino-p-tosylhydrazone 30. Thus, these sequential steps of protection and deprotection to obtain the 4,4'- diaminodiphenyltosylhydrazone 30 can be attributed to the increasing electron density in the carbonyl group due to the presence of two electron-donor amino groups in the p-position of the starting product. Following, the treatment of these derivatives (30, 32 and 34) with sodium methoxide in the presence of pyridine and [60]fullerene in refluxing o-dichlorobenzene (o-DCB) affords the final building blocks 31, 33 and 35 (Scheme 28). As aforementioned, although both [6,6]-closed and [5,6]-open isomers could be expected from this cycloaddition reaction, the high temperature at which the reaction proceeds allows to control the exclusive formation of the thermodynamically more stable [6,6]-closed isomer. 1 H NMR and 13 C NMR allow to confirm the formation of the only [6,6]-closed isomers. As example, the comparison between the 1 H Scheme 28: Synthetic route to the desired methano[60]fullerene building blocks. Chapter 2. Results and Discussion 187 NMR spectra of tosylhydrazone 32 and the corresponding 3,3’- diaminodiphenylmethano[60]fullerene 33 is shown in Figure 82. All the synthesized products were satisfactorily characterized either by standard analytical and spectroscopic techniques. 9.1.1.1. Tour’s Reaction to the GOS-Based Materials As discussed in the background, radical species are really useful compounds to overcome the inert character of graphene, allowing its chemical modification. To this purpose, the so called Tour’s reaction is an appropriate way to guarantee the introduction of different compounds with a variety of possible functionalities on the surface of the graphene layer. As shown in the background, the active species in the Tour’s reaction are aryl radicals derived from the in situ generated diazonium salts. Thus, a fundamental prerequisite in employing this reaction is the presence of arylamino groups in the designed starting products. In this way, the arylamino groups can react with the activating isoamyl nitrite leading to the formation of diazonium salts (see Scheme 23 in the previous section 7.1.2.1.). Figure 82: 1 H NMR of tosylhydrazone 32 (down) and the corresponding 3,3’-diamino diphenylmethanofullerene 33 (up). Chapter 2. Results and Discussion 188 The functionalization process with derivatives 31, 33 and 35 was carried out not only on supported single layer graphene (Graphene On Surface, GOS), but also previously on exfoliated-FLGs, thus permitting to test their chemical reactivity. The synthesis of the graphene-fullerene covalent aggregates is described in Scheme 29. Pristine FLGs are obtained through graphite exfoliation in o-DCB following Colleman’s method with high degree of purity. 189 Such FLGs suspension in o-DCB is immediately reacted with the fullerene derivatives 31 and 33 via aryl diazonium chemistry in the presence of isoamyl nitrite following the well-established procedure reported by Tour, 213 providing the FLG1 and FLG2 materials, respectively. On the other side, to successfully functionalize the single layer graphene with the synthesized building blocks, different conditions were tested. When the functionalization of copper supported (Cu- supported-)GOS was tried, each experimental reaction condition used resulted inadequate to the purpose. Scheme 29: Tour’s reaction on FLG and GOS. Chapter 2. Results and Discussion 189 At first, because of the malleability of the copper surface, it was thought to use a special expressly designed reaction equipment (Figure 83). The employed glass vessel was characterized by a mechanical stirrer blade directly connected to the plug on the top and four glass peaks at its bottom. Our principal aim was, by positioning the Cu-supported- GOS at the center of these four peaks, to favour the agitation of the solvent preventing, on the other side, the shifting of the Cu-surface. The reaction was carried out in o-DCB at 90°C for 8 hours, and at the end of this time, the Cu-supported-GOS was recovered in its initial position but, unfortunately, it was not possible to prove the functionalization process due to its folding, in turn attributed to the maybe too strong solvent’s stirring. In a second functionalization sequence, it was decided to slightly change the reaction conditions. It was employed a 1:1 mixture of N- methylpyrolidone (NMP) with CS2, while some drops of trifluoroacetic acid acted as catalytic agent. The first to permit gaining a perfect Figure 83: Special reactor expressly designed. Chapter 2. Results and Discussion 190 solubilization of the molecule and the second one to promote the formation of the aryl radicals. The functionalization was carried out using again Cu-supported-GOS at different reaction temperature (90°C and 60°C) and times (8 and 6 hours). Moreover to prevent the former folding of the Cu-supports, providing agitation without direct stirring of the solvent, another “unconventional” system was developed (Figure 84). Round bottom flasks, filled with the reaction mixture, were positioned on the top of glass vials containing magnetic stirrers. However, in all cases with different reaction times, the Cu-supported-GOS resulted partially corroded. It results probably due to the combination of the acid, even if in catalytic quantities, with the organosulfur compound, which induce the formation of cuprous sulfides and following hydrogen sulfide which induce the formation of cuprous sulfides and following Figure 84: “Unconventional” system to avoid the folding of the Cu-supports. Chapter 2. Results and Discussion 191 hydrogen sulfide which increase the corrosion process of the metal surface. 249 Decisive solution to get out of this impasse situation, overcoming all the different encountered problems, was to carry out the reaction at milder conditions, using graphene deposited single layers on less sensible and more inert supports, such as silica. Reaction times and temperatures were reduced to 4 hours and room temperature. Moreover, it was decided to eliminate the stirring from the solution and to compare the chemical modification of two SiO2/Si-supported-GOS by functionalization in different conditions (Figure 85). A support was directly dipped in the reaction mixture, while on the surface of the other SiO2/Si-supported-GOS a single drop of the reaction mixture was deposited forming a contact angle. Finally, to our delight, the reaction conditions, in both cases, did not compromise the 249 a) A. Fateh, M. Aliofkhazraei, A. R. Rezvanian, Review of corrosive environments for copper and its corrosion inhibitors. Arabian Journal of Chemistry, 2017, doi:10.1016/j.arabjc.2017.05.021; b) L. R. Lewand “The Role of Corrosive Sulfur in Transformers and Transformer Oil”, Proceedings of the Sixty-Ninth Annual International Conference of Doble Clients, Insulating Materials Session, 2002. Figure 85: Graphene’s functionalization on silica support by: (a) immersion in the reaction mixture and (b) exposure to contact angle formation through the mixture and the graphene covered face of the silica support. Chapter 2. Results and Discussion 192 single layer graphenes, which were recovered undamaged together with their SiO2/Si supports. Therefore, when using GOS as starting graphene material, the corresponding aryl diazonium salts were previously generated by adding isoamyl nitrite to a solution of 4, 6 or 8 and subsequently drop casted on the GOS surface, in a procedure slightly modified respect the Tour’s one. 213 9.2. GOS-Based Materials Characterization Several techniques were used as characterization tools for the synthesized graphene-based materials. Raman spectroscopy is, for instance, a powerful instrument for the identification of graphene doping and imperfections. In fact, the chemical introduction of covalently attached species on its surface effectively corresponds to the creation of defects, being responsible for the substantial rehybridation of the sp 2 carbons into sp 3 lattice atoms. Therefore, Raman studies on either FLGs and GOS, before and after chemical treatment, confirm the successfulness of the functionalization process. All aggregates present an increase in the D band associated with a higher defect degree on the nanomaterials. This increase is related presumably with the sp 3 carbon centers produced on the basal plane by the aryl attack of the diphenylmethanofullerene derivatives 31, 33 or 35. The ratio with the G band (ID/IG) is a way to quantify the covalent functionalization of those materials. According to this criterion, in our measurement we found a high degree of functionalization for FLG2 with an ID/IG of 0.29 compared with the 0.14 obtained for FLG1 when the reaction was carried out with dispersion of FLG (Figure 86). Figure 86: Raman spectra of pristine FLG (black), FLG1(red) and FLG2 (blue) under 532 nm laser excitation wavelength. Chapter 2. Results and Discussion 193 An analogous behavior is found with the graphene deposited on silica surface where an ID/IG ratio of 0.54 for GOS2 is the highest among the three different samples being 0.26 and 0.23 the values obtained for GOS1 and the reference hybrid GOS3 respectively (Figure 87). Moreover, a new peak around 1457 cm -1 is clearly observed for all fullerene aggregates which can be assigned to the pentagonal pinch mode [Ag(2)] of C60. 250 Thus, these evidences confirms the covalent attachment of 31, 33 or 35 on the FLG or GOS. Thermogravimetric analysis (TGA) under inert atmosphere was only carried out on the FLG derivatives to estimate the degree of surface functionalization (Figure 88). 250 J. Guan, X. Chen, T. Wei, F. Liu, S. Wang, Qi. Yang, Y. Lua, S. Yang, J. Mater. Chem. A 2015, 3, 4139. Figure 87: Raman spectra of pristine GOS in black and in red GOS1 (a), GOS2 (b) and GOS3 (c), under 532 nm laser excitation wavelength. Chapter 2. Results and Discussion 194 Pristine FLG shows high thermal stability until 800ºC degrees with a slight weight loss due to some physisorbed solvent molecules used in the exfoliation process. For both FLG-C60 covalent compounds, two weight loss processes were observed. The first weight loss step can be assigned to the thermal detaching of the attached C60 moiety followed by the second weight loss where the C60 is decomposed. The total weight loss for FLG2 (34.70 %) is significantly higher compared to the FLG1 (24.80 %) which can be interpreted as a higher degree of functionalization. The average molar content can be estimated for FLG1 in 1 organic molecule per 224 carbon atoms and for FLG2 in 1 organic molecule per 139 carbon atoms. The diphenylmethano[60]fullerene 31 reacted with FLGs in absence of the radical initiator isoamyl nitrite, was used as control reaction, to further prove the real occurrence of a covalent modification and not just a supramolecular interaction between the fullerene derivatives and the graphene layer. Thermogravimetric analysis of the sample, indeed, revealed that the mass loss of the control reaction FLG+31 (grey line) is smaller than the covalent functionalized product FLG1 (red line) but similar to the weight loss of the isolated molecule 31 (blue line) (Figure 89). Figure 88: TGA analysis under inert conditions of exfoliated FLG (black), and the nanoconjugates FLG1 (red) and FLG2 (blue). Chapter 2. Results and Discussion 195 The meaning of this result was further supported by the Fourier- transform infrared spectroscopy (FTIR) by which the characteristic peak for the fullerene moiety was observed in derivative FLG1. Figure 90 shows the FTIR spectroscopy comparison of the FLG1, obtained by following the reported procedure that employs isoamyl nitrite, with the result of the control reaction in its absence, FLG+31. Among the characteristic peaks are the skeletal in plane vibrations of graphene at 1581 cm -1 as well as some vibrational peaks at 2928 and Figure 89: TGA analysis under inert conditions of the control reaction FLG+31 (grey) compared with the exfoliated FLG (black), the isolated molecule 31 (blue) and the nanoconjugate FLG1 (red). Figure 90: FTIR spectra of FLG1 (red) compared to FLG+31 (black). Chapter 2. Results and Discussion 196 2857 cm -1 , that could be related to aliphatic carbon atoms. But more important are the characteristic vibrational peaks of pristine C60 shown around 1454 and 525 cm -1 for the covalent FLG1 that are not present for the control reaction FLG+31. These peaks are also present in the FTIR spectroscopy of the organic molecules 31, 33 and 35. As further evidence, X-ray photoelectron spectroscopy (XPS) analysis was recorded for FLG1 and FLG2. The survey spectra of FLG1 exhibit a main C 1s component together with two more signals assigned to O 1s and N 1s (Figure 91). The 12% oxygen contribution observed for FLG1 is the same obtained for pristine FLG, indicating that no extra oxygen atoms are covalently anchored on the FLG surface during the radical addition reaction with 31. However for both FLG1 and FLG2 the N 1s peak could be related to unreacted amine groups that are still present in the organic molecule being the high resolution spectra of N 1s made up of only one component. The high resolution C 1s core level spectrum of both materials FLG1 and FLG2 presents, besides the four expected components (photoelectrons emitted from sp 2 carbon atoms, sp 3 carbon atoms, oxidized carbon atoms in the C–O bonds and the –* shake up Figure 91: XPS spectra of FLG1 with inset of the high resolution spectra of C 1s (up) and N 1s (down). Chapter 2. Results and Discussion 197 structure of graphene sheets) two new contributions that could be assigned to C 1s shake-up satellite peaks of C60. 250,251 Interestingly, from the transmission electron microscopy (TEM) analysis of FLG1 and FLG2 it can be recognized some spherical species at their edges (Figure 92). 251 C. Enkvist, S. lunell, B. Sjögren, S. Svensson, P. A. Brühwiler, A. Nilsson, A. J. Maxwell, N. Martensson, Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 14629. Figure 92: TEM micrographs of FLG1 with scale bar 200 nm (a), 20 nm (b) and representative image of the width profile of the fullerene attached to the aggregate (c). TEM micrographs of FLG2 with scale bar 100 nm (d), 10 nm (e) and representative image of the width profile of the fullerene attached to the aggregate (f). Chapter 2. Results and Discussion 198 Under identical conditions, the same spherical forms are not observable in pristine FLG TEM micrographs. Thus, the round shape nanoforms could be attributed to the covalently attached organic molecules based on C60, since their diameter (~1 nm) is compatible with the reported diameter for C60. 236,252 9.3. Physical Characterization At first, the morphological characterization of all the novel graphene hybrids materials synthesized reveals the presence of molecular islands, with a height of ~1.2 nm, that partially cover the graphene sheet (Figure 93). Visible are also the characteristic folds of bare graphene, which are known to occur during the transfer process of CVD-graphene from copper. The thermoelectric properties of these graphene-base novel hybrids were determined in ambient conditions by probing these islands through the use of a Pt-coated (Multi75-G from BudgetSensors) AFM tip, while the graphene layer, that is held at high temperature, acts as the opposing electrode. The Pt-coated tip was placed into gentle contact (~30 nN) with the substrate and held still through a feedback loop controller. Figure 94 shows a schematic representation of the measuring technique. 252 X. Zhang, Z. Zhi, Y. Huang, X. Wan, J. Tian, Y. Ma, Y. Chen, J. Nanosci. Nanotech. 2009, 9, 5752. Figure 93: AFM image of GOS2 on SiO2/Si Chapter 2. Results and Discussion 199 As demonstrated in the background, this allows to measure, through a generated non-zero tip-substrate temperature difference (ΔT), IV curves in which the thermoelectric voltage shows up as a voltage-offset (ΔV), 253 that can be related to the Seeback coefficient (S). To facilitate a precise voltage reading, a disconnection of the c-AFM from the voltage source and following connection to a resistor allowed to perform the zero-calibration in short time intervals. At this point, the thermopower S and the conductance G were determined. To obtain statistically significant values for S, more than 600 IV curves at several different points on all samples were recorded for either the empty area, corresponding to the bare graphene layer, and the molecule islands. The result is shown in S-histograms, where S was determined to be the mean of the distribution with an error that is the standard error (Figure 95). 253 L. Dobusch, M. M. Furchi, A. Pospischil, T. Mueller, E. Bertagnolli, A. Lugstein, Appl. Phys. Lett. 2014, 105, 253103. Figure 94: Schematic of the c-AFM setup used. Chapter 2. Results and Discussion 200 Figure 95a shows a histogram of S measured with the tip in contact with the graphene. Since the thermal conductivity of graphene is high and the electrical contact is low-resistive, the thermal gradient, in this case, is not at the tip/graphene junction but has a strong lateral extension. The S value of 2±3 µV/K determined here results, therefore, in good agreement with other experiments where the in-plane thermopower of graphene is found to be positive and <10 µV/K. 254 On the other side, Figure 95b-d shows the S histograms measured when the tip is in contact with a graphene area covered by a molecular island. Interestingly, it is found that when the force with which the tip is pushed towards the surface is increased, the tip can jump into contact with the graphene and the thermopower of graphene, as shown in 254 a) M. Buscema, M. Barkelid, V. Zwiller, H. S. J. van der Zant, G. A. Steele, A. Castellanos-Gomez, Nano Lett. 2013, 13, 358; b) J. Wu; H. Schmidt, K. K. Amara, X. Xu, G. Eda, B. Özyilmaz, Nano Lett. 2014, 14, 2730. Figure 95: Histograms showing the thermopower values for the SiO2/Si-supported graphene monolayer (a) and all the nanocomposites, GOS1 (b), GOS2 (c) and GOS3 (d). Chapter 2. Results and Discussion 201 Figure 95a, is measured. For the mono-linked G-based-C60 hybrid (GOS3, Figure 95d) a S value of 74±19 µV/K is found, which has to be considered exceptionally high in comparison with the values reported for other molecules. 238 The histograms corresponding to the thermopower measured for the p- and m-linked diphenylmethanofullerenes (GOS1 and GOS2, respectively) on their molecular islands evidence the appearance of two peaks, but it has to be noted that for each sample, the single approach in a given area only gives values that correspond to one of the peaks. The value of the positive peak, 74±25 µV/K for GOS1 and 70±35 µV/K for GOS2, results in striking agreement with the high value of S found for GOS3. This correspondence, also supported by the previously reported XPS analysis of GOS1 and GOS2, suggests that the link of one of the aryl units of the p- or m-diphenylmethano[60]fullerene may have not formed, giving rise to the presence of mono-linked G-based-C60 hybrids on these samples. It was suggested, therefore, that the peak at 74 µV/K can be assigned to the mono linked graphene derivative, and therefore the thermopower corresponding to the p-(4) and m-(6) molecules doubly graphene-connected, corresponds to the peaks at -56±21 µV/K and -27±8 µV/K, respectively. These high quantum thermopower values suggest great promise for molecular/graphene composite materials in thermoelectric applications, while the change in thermopower polarity with the number of anchoring groups implies potential for GOS1 and GOS2 to be used in more complex device architectures. 9.4. Theoretical Calculations With the aim of understanding the change in sign and magnitude of the Seebeck coefficient, observed as a function of different bonding geometries between the fullerene derivatives 31, 33 and 35 and the graphene sheet substrate, theoretical calculations were carried out. The energetics and resulting conformations of the fullerene derivatives 31, 33 and 35 bonding to a graphene surface via activated aryl radicals were obtained from the SIESTA 255 implementation of density functional theory where the van der Waals density functional (vdW- 255 J. M. Soler, E. Artacho, J. Gale, A. García, J. Junquera, P. Ordejón, D. Sánchez Portal, J. Phys. Condens. Matter. 2002, 14, 2745. Chapter 2. Results and Discussion 202 DF) was employed. 256 The vdW-DF used in SIESTA is applicable to arbitrary geometries 257 and to ensure accuracy the inter atomic forces were relaxed to optimize the geometry. Once the conformer geometries were optimized, the mean field Hamiltonian and overlap matrices, derived from SIESTA, were used to calculate the electronic and thermoelectronic properties using the quantum transport code GOLLUM (Figure 96). 258 256 a) M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Lett. 2004, 92, 246401; b) D. C. Langreth, M. Dion, H. Rydberg, E. Schröder, P. Hyldgaard, B. I. Lundqvist, J. Quantum Chem. 2005, 101, 599. 257 H. B. Jansen, P. Ross, Chem. Phys. Lett. 1969, 3, 140. 258 J. Ferrer, C. J. Lambert, V. M. García-Suárez, D. Z. Manrique, D. Visontai, L. Oroszlany, R. Rodríguez-Ferradás, I. Grace, S. W. D. Bailey, K. Gillemot, New J. Phys. 2014, 16, 093029. Figure 96: On the left, the synthesized a) GOS1, b) GOS2 and c) GOS3. On the right, the relaxed geometries from the theoretical calculations. The graphene sheet with 216 carbon atoms is periodic in both the in-plane x,z directions. Chapter 2. Results and Discussion 203 The calculations realized by carefully placing the reactive aryl units close to the pristine graphene surface and replacing the C60 with two hydrogen atoms and reducing the graphene sheet to 96 atoms to save computation time, lead to successfully carrying out the modeled Tour reaction using SIESTA. After many randomly distributed initial starting configurations the calculations show that randomly placing the dibenzyl units over the surface results in over 60% detachment and no covalent bonding observed. Once the relaxed configurations of GOS1,2,3 (in absence of the [60]fullerene moiety) have been found, the C60 is reattached and the graphene sheet is doubled in size to contain 216 carbon atoms. The system is again relaxed using the bonded conformations. To model the STM junction a gold tip is constructed and the most energetically favourable tip to fullerene derivatives 31, 33 and 35 geometry is found. This is shown in Figure 97 where the Au-C bond is shown to form two legs of length d = 2.5 Å placed equidistantly between two carbon atoms on the C60 unit forming the edge of a pentagonal ring. Figure 97: The gold STM tip model showing the most energetically favourable geometry to form the STM gold tip and C60 geometry. Chapter 2. Results and Discussion 204 Finally, the relaxed geometry for the hybrid nanocomposites systems GOS1,2,3 shown in Figure 96 were furnished of the STM tip and the resulting structures are shown in Figure 98. SIESTA then calculates the Hamiltonian of these relaxed structures without further conjugate gradient relaxation. This Hamiltonian and the overlap matrices are then used by the transport code GOLLUM to calculate the transmission coefficients T(E) from which the thermoelectric properties are obtained. The thermopower is calculated as a function of energy for a given temperature dictated by the experimental conditions, in this case 300K. The sign and magnitude of the thermopower are controlled by modulating the contact between the graphene sheet and the benzyl units due to the quantum mechanical effects at the molecular level, for example the sign is given by the negative of the slope of logT(E) at a given energy. At this stage the robustness of the STM tip and C60 contact location was investigated. Locating the tip on the C60 surface at a number of possible pentagon edge sites produced little change in the transmission peaks justifying the decision to use one location and one set of transmission data for each calculation. The calculated values of the Seebeck coefficient S(V/K) at 300K result in good agreement with the observed values, if the energy is selected, and are shown in Table 5. Figure 98: The modelled STM experimental set up to measure the electronic and thermoelectrical properties of GOS1,2,3. Chapter 2. Results and Discussion 205 As demonstrated from Table 5, the values of the Seebeck coefficient, calculated at an energy of -0.44 eV, generally follow the trend of the experimental results, showing, moreover, surprising agreement with them for GOS1 and GOS2, while the calculated S value for GOS3 seems to divert. However, taking into account the histograms of the Seebeck coefficient within the HOMO-LUMO gap, which is defined to lie between -0.5 and -0.2 eV (Figure 99), and considering the most probable values, the results change. On one side, GOS1 and GOS2 maintain their good agreement with the experimental data, in fact, both calculated results of the Seebeck coefficient are negative and included in a range of 25-50 μV/K, while on the other side, GOS3 now shows that the thermopower displays a high probability to assume positive values above 50 μV/K. Table 5: Comparison between the values of the Seebeck coefficient S(V/K) at an energy of -0.44 eV, calculated at 300K, and the correspondingt values experimentally determined. Scalc (300 K) Sexp GOS1 -56 μV/K -48,5 μV/K GOS2 -27 μV/K -29,6 μV/K GOS3 74 μV/K 23,8 μV/K Figure 99: Histograms of the Seebeck coefficient within the HOMO-LUMO gap. Chapter 2. Results and Discussion 206 These results, therefore, suggest that combining density functional theory calculations with thermoelectric characterization experiments consists in a valid method for searching and investigating different type of hybrid materials, which could enhance thermoelectric performances. 9. Results and Discussion 10. Conclusions Chapter 2. Conclusions 209 10. Conclusions The second chapter of the present thesis has illustrated a simple design approach to achieve high thermoelectric performance materials based on novel covalently attached graphene-molecule composites. The thermoelectric properties of these new hybrids were successfully characterized through c-AFM by employing the graphene layer as second electrode to test the connected molecular units. The synthesized novel GOS-materials display superior values of thermopower when compared with metal-molecule-metal arrangements. Interestingly, the stemming conclusion from our charge transport studies is that even minimal structural changes in the linked molecules can produce notable variation in the measured thermoelectricity. Moreover, the magnitude and the polarity of thermopower is conditioned by the nature of the anchoring groups and the DFT method reveals itself to be a well- established and efficient simulation technique to produce reliable predictions. Therefore, with the present thesis work, it is proudly demonstrated that: Figure 94: Thermoelectric properties determination on GOS-based materials. Chapter 1. Conclusions 210  The combination of the density functional theory calculations with the thermoelectric characterization experiments results into an effective way of searching for other material combinations which enhances the thermoelectric performances;  The fullerene covalently anchored graphene derivatives GOS1 and GOS2, generating exceptionally high quantum thermopowers, pave the way to the potential introduction of 2D organic materials in thermoelectric applications operating at ambient conditions, without the necessity of using rare-earth or heavy elements. 11. Experimental Part Chapter 2. Experimental Part 213 11. Experimental Part 11.1. General Methods and Techniques -Materials: Reagents for synthesis were purchased from commercial sources and used without further purification. Solvents were dried and distilled using standard techniques. 102 Those reactions requiring an inert atmosphere were carried out using argon as source. The graphite used for graphene exfoliation was purchased from TIMCAL (TIMREX SFG15, ρ = 2.26 g/cc, particle size = 8.80 μm, surface area = 9.50 m 2 /g, ash ≤ 0.100%, interlaminar distance = 0.3354-0.3358 nm). Vacuum filtrations of graphene materials were carried out with polycarbonate track etched (PCTE) (pore size = 0.2 μm, Φ = 47 cm) and polytetrafluoroethylene (PTFE) (pore size = 0.2 μm, Φ = 47 cm) membranes. -Raman spectra: were recorded on a NT-MDT in Via Microscope at room temperature using an exciting laser source of 532 nm. -Thermogravimetric analyses (TGA): were carried out with a thermobalance TA-TGA-Q-500 under N2 or air atmosphere, depending on the sample. The sample (≈ 0.5 mg) was introduced inside a platinum crucible and equilibrated at 100ºC followed by a 10 ºC/min ramp between 100 and 1000 ºC. -XPS analysis: were carried out using a SPECS GmbH (PHOIBOS 150 9MCD) spectrometer operating in the constant analyzer energy mode. A nonmonochromatic aluminium X-ray source (1486.61 eV) was used with a power of 200 W and voltage of 12 kV. Pass energies of 75 and 25 eV were used for acquiring both survey and high resolution spectra, respectively. Survey data were acquired from kinetic energies of 1487 - 400 eV with an energy step of 1 eV and 100 ms dwell time per point. The high resolution scans were taken around the emission lines of interest with 0.1 eV steps and 100 ms dwell time per point. SpecsLab Version 2.48 software was used for spectrometer control and data handling. The semi-quantitative analyses were performed from the C 1s (284.3 eV) signal. The samples were introduced as pellets of 8 mm diameter. -Transmission electron microscopy (TEM): was performed on a JEOL JEM 2100, with an acceleration voltage of 200 kV equipped with a Chapter 2. Experimental Part 214 camera CCD ORIUS SC1000 (model 832) and the microanalysis were performed by XEDS (OXFORD INCA). The samples were dispersed in o-DCB and dropped onto a holey carbon copper grid (200 mesh), the solvent was removed in a vacuum oven during 48 hours. -Conductive Atomic force microscopy (c-AFM): was performed under ambient conditions using Cervantes Fullmode AFM from Nanotec Electronica SL working on tapping mode at a working frequency of ~70 kHz to obtain height images. The samples of SiO2/Si-supported- SLGs were analyzed as provided after the chemical reaction and following treatment. 11.2. GOS-Based Materials Synthesis of Amino Functionalized Diphenylmethano[60]Fullerene Building Blocks: Protection step 4,4'-(trifluoroacetamido)benzophenone (28) 247 A solution of 4,4'-diaminobenzophenone (848 mg, 4 mmol) in 15 mL of dry THF, 12 mL of trifluoroacetic anhydride and a few drops of dry pyridine were refluxed for 4h under argon atm. After cooling to room temperature, the reaction was quenched by cautiously adding methanol and water and extracted with DCM. The organic phase was dried over MgSO4 and the solvent was then evaporated under reduced pressure. The residue was purified by column chromatography on silica gel with hexane/ethyl acetate (7:3), yielding a white solid. Yield: 85% 1 H NMR (300 MHz, CD3OD) δ 7.98 – 7.73 (m, 8H). General Procedure for the Condensation to obtain p-tosylhydrazones 29, 32 and 34 Under argon atm, a solution of 28, 3,3′-diaminobenzophenone or 4- aminobenzophenone (3.81 mmol) and p-tosylhydrazide (3.81 mmol) in 50 mL of toluene and 7 mL of EtOH was refluxed for 24 h. After Chapter 2. Experimental Part 215 cooling to room temperature, the solvent was evaporated and the crude purified by column chromatography, eluting with hexane/AcOEt (1:1) to obtain p-tosylhydrazones 29, 32 and 34. 4,4'-(trifluoroacetamido)benzophenone-p-tosylhydrazone (29) 247 Yield: 80% 1 H NMR (300 MHz, CD3OD) δ 7.94 (d, J = 8.1 Hz, 2H), 7.83 (d, J = 9.3 Hz, 2H), 7.61 (d, J = 9.3 Hz, 2H), 7.47 (d, J = 8.1 Hz, 2H), 7.33 (d, J = 8.7 Hz, 2H), 7.21 (d, J = 8.7 Hz, 2H), 2.42 (s, 3H). . 3,3'-diaminobenzophenone-p-tosylhydrazone (32) Yield: 79% 1 H NMR (300 MHz, CD3OD) δ 7.84 (d, J = 8.2 Hz, 2H), 7.41 (d, J = 8.0 Hz, 2H), 7.23 (t, J = 7.8 Hz, 1H), 7.01 (t, J = 7.7 Hz, 1H), 6.81 (d, J = 8.1 Hz, 2H), 6.70 (d, J = 7.9 Hz, 2H), 6.46 (s, 1H), 6.39 (d, J = 7.5 Hz, 1H), 2.44 (s, 3H). 13 C NMR (125 MHz, MeOD) δ 159.75, 147.27, 141.65, 140.93, 139.97, 138.16, 129.30, 128.99, 128.27, 118.65, 117.82, 115.23, 21.52. MALDI-TOF: m/z theoretical for C77H25NO5 380.13, exp M + 380.83. Chapter 2. Experimental Part 216 FTIR (KBr) ʋ:3451, 3383, 2924, 2854, 1924, 1624, 1489, 1452, 1381, 1324, 1240, 1165, 1091, 1051, 1016, 900, 790, 701, 670, 582, 548 cm -1 . 4-aminobenzophenone-p-tosylhydrazone (34) 247 Yield: 83% 1 H NMR (300 MHz, CDCl3) δ 7.86 (d, J = 8.3 Hz, 2H), 7.45 (d, J = 7.8 Hz, 2H), 7.26 (m, 5H), 6.93 (d, J = 8.5 Hz, 2H), 6.75 (d, J = 8.5 Hz, 2H), 3.95 (s, 2H), 2.43 (s, 3H). Deprotection step 4,4'-diaminobenzophenone-p-tosylhydrazone (30) 247 A solution of 2 (500 mg, 0.873 mmol) in 30 mL of MeOH was treated with 20 mL of an aq solution of K2CO3 (10%) and stirred for 24 hours at room temperature. The mixture was extracted with DCM, the organic phase dried over anhyd MgSO4 and the solvent evaporated under reduced pressure. The residue was then purified by column chromatography on silica gel, DCM/MeOH (9:1), to yield a yellowish solid. Yield: 95% 1 H NMR (300 MHz, CDCl3) δ 7.82 (d, J = 8.3 Hz, 2H), 7.75 (d, J = 8.3 Hz, 2H), 7.54 (d, J = 8.3 Hz, 2H), 7.39 (d, J = 8.3 Chapter 2. Experimental Part 217 Hz, 2H), 7.08 (d, J = 9.02 Hz, 2H), 6.91 (d, J = 9.02 Hz, 2H), 2.45 (s, 3H). General Procedure for the Bamford-Stevens reaction to obtain the diphenylmethano[60]fullerenes 31, 33 and 35 To a solution of 30, 32 or 34 (0.315 mmol) in 8 mL of dry pyridine, sodium methoxyde (0.315 mmol) was added under argon atm. After stirring at room temperature for 15 minutes, a solution of [60]fullerene (0.227 mmol) in 20 mL of o-DCB was added at once and the mixture was heated at 180°C for 18 h. After cooling to room temperature, the solvent was stripped away in a rotary evaporator and the remaining solid purified by column chromatography on silica gel, eluting first with CS2 and then with toluene/AcOEt (8:2) to afford 31, 33 and 35 as brown solids. p-diaminodiphenylmethano[60]fullerene (31) 247 Yield: 45% 1 H NMR (300 MHz, DMSO-d6) δ 7.91 – 7.82 (d, 4H), 6.73 – 6.65 (d, 4H). m-diaminodiphenylmethano[60]fullerene (33) Chapter 2. Experimental Part 218 Yield: 30% 1 H NMR (300 MHz, DMSO-d6) δ 7.29 (dd, J = 1.9 Hz, 4H), 7.14 (t, J = 8.0 Hz, 2H), 6.57 (d, J = 8.6 Hz, 2H), 5.31 (s, 4H). 13 C NMR (175 MHz, DMSO) δ 149.30, 148.64, 145.35, 144.64, 144.53, 144.32, 144.16, 144.01, 143.65, 143.38, 142.40, 142.37, 142.12, 141.84, 141.63, 140.17, 139.54, 137.42, 129.17, 128.25, 118.63, 116.30, 113.64, 79.69, 59.56, 39.52. MALDI-TOF: m/z theoretical for C77H25NO5 916.10, exp M + 916.16. FTIR (KBr) ʋ: 3451, 3385, 2923, 2854, 1611, 1489, 1456, 1382, 1312, 1277, 1162, 862, 771, 697, 569, 525 cm -1 . p-aminodiphenylmethano[60]fullerene (35) 247 Yield: 22% 1 H NMR (300 MHz, CDCl3) δ 8.07 (d, J= 8.64 Hz, 2H), 7.86 (d, J= 8.64 Hz, 2H), 7.47– 7.44 (m, 2H), 7.40 – 7.33 (m, 1H), 6.76 (d, J = 8.5 Hz, 2H), 3.77 (s, 2H). 11.3. Synthesis of FLG-Covalent Hybrids Chemical exfoliation of graphite: Graphite flakes (100 mg) were dispersed in o-DCB (100 mL) and sonicated at room temperature during 150 minutes in a low-power sonication bath, to obtain homogeneous aggregates. The dispersion was then centrifuged at 500 rpm for 45 minutes. After this process, the decanted supernatant was isolated with a Pasteur pipette and stored in solution. Chapter 2. Experimental Part 219 General Procedure for the Tour’s reaction to obtain FLG1 and FLG2 50 mL of exfoliated graphene suspensions in o-DCB were reacted with a 5 mL solution of the corresponding methano[60]fullerene 31 or 33 (0.065 mmol) in o-DCB. The solution was sonicated while deoxigenating with argon for 90 minutes. Isoamyl nitrite (0.26 mmol) was slowly added to the reaction mixture and heated at 70ºC for 24 hours under inert atm. The final product was sequentially washed in a 0.1 μm size PTFE membrane several times with o-DCB, DCM and MeOH. FLG1 FTIR (KBr) ʋ: 2928, 2857, 1655, 1579, 1444, 1402, 1157, 1068, 756, 525 cm -1 . TGA: weight loss and temperature desorption (organic anchoring groups): 24.82%, 650 °C. Raman: ID/IG = 0.14; XPS: % atomic: C (284.6 eV) = 87.2, O (531.6eV) = 12.1, N (399.6eV) = 0.65. Chapter 2. Experimental Part 220 FLG2 FTIR (KBr): 2928, 2857, 1653, 1579, 1454, 1401, 1038, 784, 525 cm -1 . TGA: weight loss and temperature desorption (organic anchoring groups): 34.70 %, 650 °C. Raman: ID/IG = 0.29. XPS: % atomic: C (284.6 eV) = 80.21, O (532.6 eV) = 19.1, N (399.6 eV) = 0.7. 11.4. Synthesis of the Covalent GOS-Based Materials General Procedure for the Tour’s reaction to obtain GOS1, GOS2, GOS3 A 10 -4 M solution of the corresponding methano[60]fullerene 31, 33 or 35 (0.008 M) in chlorobenzene/CS2 (1:1) was deoxygenated for 1 hour. Isoamyl nitrite in a catalytic quantity was slowly added to the reaction mixture that was subsequently drop casted (one drop) onto the GOS. The material was maintained at room temperature under inert atm for 4 hours and subsequently washed several times with o-DCB, DCM and MeOH. GOS1 Chapter 2. Experimental Part 221 Raman: ID/IG = 0.26; GOS2 Raman: ID/IG = 0.54. GOS3 Raman: ID/IG = 0.23 11.5. Control Reaction on FLG 50 mL of exfoliated graphene suspensions in o-DCB sonicated and deoxigenated with argon for 90 minutes were reacted with a 5 mL solution of the corresponding methano[60]fullerene 31 (0.065 mmol) in o-DCB. The mixture was heated at 70ºC for 24 hours under inert atm. The final product was washed in a 0.1 μm size PTFE membrane several times with o-DCB, DCM and MeOH. FLG-Control FTIR (KBr) ʋ: 2900, 1638, 1579, 1454, 1384, 1157, 1060, 632, 506 cm - 1 ; TGA: weight loss and temperature desorption (organic anchoring groups): 14.69 %, 650 °C. Summary Summary 225 Summary Introduction The field of molecular electronics, based on the use of single molecules or nanoscale collections of single molecules as electronic components, has represented, and still now represents, a seductive playground for scientists where there is still much to do. Over the past decade, experiments and theory have allowed to greatly enhance the comprehension of the working principles at the base of molecular junctions. Thus, the interest has shifted over other less known, investigated or understood, aspects involving structural or theoretical challenges. So far, most of research efforts in this field were mainly devoted to the investigation of the electrical conductance of covalently conjugated molecular wires. Now, instead, more complex structures (i.e. based on supramolecular systems), less known features (namely, the thermoelectric properties of these systems) and hybrid materials begin to grab the scientific attention of the area. Chapter 1. Objectives A new generation of molecular wires characterized by weak intermolecular forces in their core unit has been designed, synthesized and studied. On one hand, the salt bridge connection between the amidinium and carboxylate groups was exploited to the development of a set of supramolecular systems with different length, structure and anchoring groups to get a better knowledge of the charge transfer properties of non-covalently linked molecular wires: Amidinium-Carboxylate Based Supramolecular Wires: a) Fullerene dumbbells series b) Linear supramolecular wires series On the other hand, instead, it was taken advantage of the ion-dipole interaction, which origins between a metallic cation and a crown ether cavity, to study the variable effect that a metal coordination has on the conductance of single molecular wires depending on their anchoring group: Summary 226 Crown Ethers Switching Systems as Supramolecular Wires: a) 4,5'-diaminodibenzo[18]crown-6 b) OPE and PPP crown ether molecular wires series Results and Discussion Amidinium-Carboxylate Based Supramolecular Wires: a) Amidinium-Carboxylate Based Fullerene Dumbbells Series The synthesis of a new family of non-covalent fullerene dumbbells characterized by the two-point supramolecular interaction between the amidinium and carboxylate groups has been carried out. The clear signature of salt bridge formation is the concentration- dependent downfield shift of the amidinium protons involved in the hydrogen bonding to the carboxylate observed in 1 H NMR experiments. It was carried out the morphological study of these supramolecular assemblies as a function of their increasing concentrations and it was demonstrated that initially occurs the formation of molecules islands. Subsequently, blend to led to the creation of one-molecule thickness Figure S1: Supramolecular dumbbell-type fulleropyrrolidine complexes, 9b·8a,c and 12b,d·10a,c. Summary 227 monolayers and finally, at the highest concentration degree, on the top of these monolayers, supramolecular aggregates tend to be formed. Their electronic properties, as supramolecular hydrogen-bonded wires, were proved by STMBJ measurements, which evidenced that the corresponding conductance value was probably too low to be detected by the noise cut off of the set up, because only the peaks of the separated halves were visible. b) Amidinium-Carboxylate Based Linear Supramolecular Wires To go deeper about the two point amidinium-carboxylate interaction as supramolecular alternative connection to the covalent bonding in molecular wires, simple systems, based on a linear design, were synthesized to facilitate the investigation and understanding of conductance properties through their supramolecular structure. The synthesis of a control compound (17) was also carried out to compare the conductance of the supramolecular ensemble 16a . 14b with the most similar covalent analogous. The qualitative 1 H NMR titration experiments clearly demonstrated the formation of the supramolecular complexes, showing a broad band corresponding to the signal of the amidinium protons involved in the non-covalent interaction, modifying its width and position as a function Figure S2: Linear supramolecular wires series 15a,b·14a and 16a,b·14b. Figure S3: Refence compound 17. Summary 228 of the guest concentration. The quantitative experiments, on the other hand, allowed to determine Ka values superior to 10 4 M -1 for all the complexes. The STM measurements revealed three main results: i) the pyridine derivatives have demonstrated inadequate to be investigated, resulting the conductance values of either, both halves and the complexes, impossible to be detected. ii) It was shown for the first time that the amidine group can be as efficient linker to gold as the carboxyl group, infact the conductance values obtained for both separated halves of the short thiomethyl ether complex derivative result very similar between them. iii) The mixed nature of the amidinium-carboxylate salt bridge connection, based on hydrogen bonding interaction and electrostatic forces, which are due to the charge separation, can be, in principle, considered the responsible for the interruption of the CT process along the wire. Interruption which result into a conductance value of the whole short thiomethyl ether complex derivative unexpectedly more than two orders of magnitude lower of the terphenyl used as reference compound (17). Crown Ethers Switching Systems: a) 4,5'-diaminodibenzo[18]crown-6 The synthesis and the full characterization of the 4,5'- diaminodibenzo[18]crown-6 was successfully carried out. However, it resulted impossible to detect the conductance signal corresponding to the empty molecule, probably due to the lack of direct conjugation between the crown ether structure and the aromatic rings. Therefore, the study of the complexed molecules was not attempted. Figure S4: 4,5'-diaminodibenzo[18]crown-6. Summary 229 b) OPE and PPP crown ether molecular wires series On the basis of the previous result, it was planned the synthesis of OPE and PPP derivatives, where the crown ether ring was located as pendant on the benzene’s central unit, in order to avoid that its positioning could interrupt the -conjugation along the wire structure. While the PPP derivatives were successfully synthesized and characterized, the OPE derivatives crown ether wires, unfortunately, were synthesized as irreversible complexes of TPPO. The CT study through the STMBJ technique of the free compounds 26a,b showed, without any surprise, conductance values really similar to p-terphenylene based systems. On the other side, instead, the determination of the electronic properties of the complexed PPP crown ether wires is currently underway. Conclusions It was accomplished the synthesis of novel supramolecular wires, based, on one side, on the amidinium-carboxylate salt bridge while, on the other, on the ion-dipole interaction of a cation with the cavity of a crown ether. The non-covalent interaction between the two-point amidinium-carboxylate connection was further investigated, also from the point of view of its conductance properties. The low values of conductance observed in comparison to the covalent reference compound could be justified by a combination of supramolecular forces and strong charge separation. The electronic properties of the synthesized PPP crown ether derivatives are currently underway. Figure S5: Synthesis of the K + complexed PPP crown ether molecular wires 27a,b. Summary 230 Chapter 2. Objectives Synthesis of Graphene On Surface (GOS)-Based Materials Taking into account the well-known electronic properties of the [60]fullerene and the outstanding and interesting characteristics demonstrated by the graphene sheet, novel junctions based on the combination of those features into nanohybrid structures of covalently modified pristine graphene with fullerene derivatives were developed. Specifically, the design of the target nanocomposites was formulated with the aim of studying the thermoelectric properties of hybrid wire systems whit different connection between the spacer and the graphene layer. Results and Discussion The synthesis of the graphene composites GOS1,2,3, and the control hybrids FLG1,2, was achieved following few fundamental steps, and their chemical, physical and theoretical characterization was positively carried out. The successfulness of the functionalization process was confirmed by: Figure S6: Graphene On Surface (GOS)-Based Materials: a) GOS1, b) GOS2 and c) GOS3. Summary 231 - Raman studies on either FLGs and GOS, before and after chemical treatment, which clearly have shown the increase of the D band associated with a higher defect degree on the nanomaterials. - Thermogravimetric analysis (TGA), only carried out on the FLG derivatives, which has allowed to estimate the degree of surface functionalization as 1 organic molecule per 224 carbon atoms for FLG1 and 1 organic molecule per 139 carbon atoms for FLG2. - FTIR by which the characteristic vibrational peaks of pristine C60 around 1454 and 525 cm -1 could be observed in all the covalently functionalized FLGs derivatives. - XPS analysis, which has evidenced a main C 1s component together with two more signals corresponding to O 1s and N 1s, which, respectively, were assigned to the atmospheric oxygen and the unreacted amine groups, still present in the organic molecule. - TEM analysis has allowed to recognize, at their edges, some spherical species not observable in pristine FLG TEM micrographs. These spherical forms were attributed to the covalently attached organic molecules based on C60, since their diameter (~1 nm) is compatible with the reported diameter for C60. - AFM analysis of the novel GOS-based materials also has revealed the presence of molecular islands, partially covering the graphene sheet, which have a height of ~1.2 nm (in good agreement with the size of a fullerene moiety). - By c-AFM measurements, the thermoelectric properties of the nanohybrids materials synthesized were revealed as surprisingly high respect to other organic molecules. Moreover, it was possible to ascribe the positive S value, of the two which each GOS show (the positive around 74 µV/K and the other negative at -56±21 µV/K and -27±8 µV/K, for GOS1 and GOS2, respectively), to the presence of fullerene derivatives which have reacted with just one of the two amine’s legs. This because, it results in perfect agreement with the value of thermopower derived for the mono-linked G-based-C60 hybrid (GOS3). Summary 232 Conclusions Novel molecular/graphene composite materials, were synthesized. The high quantum thermopower values derived for GOS1 and GOS2 suggest great promise for these hybrid materials in thermoelectric applications, and, moreover, the change in the thermopower polarity with the number of anchoring groups makes them candidates to be potentially used in more complex device architectures. Resumen 235 Resumen Introducción El campo de la electrónica molecular, basado en el uso de moléculas individuales o un conjunto de ellas como componentes de dispositivos electrónicos, ha representado, y todavía representa, un área de trabajo seductora para los científicos en la que todavía hay mucho por hacer. Durante la última década, el desarrollo de procedimientos experimentales junto con la parte teórica han permitido mejorar en gran medida la comprensión de los principios básicos de las uniones moleculares. Por lo tanto, el interés se ha dirigido hacia otros aspectos menos conocidos, investigados o entendidos, que implican desafíos estructurales o teóricos. Hasta ahora en este campo, la mayoría de los esfuerzos en investigatión se han dedicado al estudio de la conductancia eléctrica de cables moleculares conjugados covalentemente. Ahora, en cambio, estructuras más complejas (por ejemplo, basadas en sistemas supramoleculares), características menos conocidas (como las propiedades termoeléctricas de estos sistemas) y nuevos materiales híbridos comienzan a captar la atención científica del área. Chapter 1. Objectivos Se diseñó, sintetizó y estudió una nueva generación de cables moleculares caracterizados por fuerzas intermoleculares no covalentes en su unidad central. Por un lado, la conexión del puente salino entre los grupos amidinio y carboxilato se aprovechó para desarrollar un conjunto de sistemas supramoleculares con diferentes longitudes, estructura y grupos de anclaje de modo que se obtuviese un mejor conocimiento de las propiedades de transferencia de carga de los cables supramoleculares: Cables Supramoleculares Basados en la Unión Amidinio-Carboxilato: a) Serie de derivados de fullereno tipo dumbbells b) Serie de cables supramoleculares lineales Por otro lado, en cambio, se aprovechó la interacción ion-dipolo, que se origina entre un catión metálico y una cavidad de éter corona, para estudiar el efecto variable que la coordinación del metal tiene sobre la Resumen 236 conductancia de cables moleculares individuales, dependiendo de su grupo de anclaje: Interruptores Moleculares Basados en Éteres Corona: a) 4,5'-diaminodibenzo[18]corona-6 b) Serie de cables moleculares derivados de éter corona sobre esqueletos de OPE y PPP Resultados y discusión Cables Supramoleculares Basados en la Unión Amidinio-Carboxilato: a) Serie de derivados de fullereno tipo dumbbell caracterizados por el enlace amidinio-carboxilato Se ha llevado a cabo la síntesis de una nueva familia de derivados de fullereno de tipo duumbbells caracterizados por la interacción supramolecular de dos puntos entre los grupos amidinio y carboxilato. En los experimentos de 1 H NMR se encuentra la prueba efectiva de la formación de los complejos supramoleculares a través del apantallamiento, dependiente de la concentración, de los protones del Figura S1: Complejos supramoleculares derivados de fulleropirrolidina de tipo dumbbells, 9b·8a,c and 12b,d·10a,c. 237 grupo amidinio implicados en el enlace de hidrógeno con el carboxilato. El estudio morfológico con concentración creciente de estos agregados supramoleculares ha demostrado que, primero, se forman islas de moléculas, que posteriormente se fusionan llevando a la creación de monocapas con espesor de una molécula, cuya tendencia, a la más alta concentración, es la de formar agregados supramoleculares de mayor espesor. Además, sus propiedades electrónicas como cables supramoleculares unidos por enlaces de hidrógeno se probaron mediante STMBJ, evidenciando que el valor de conductancia correspondiente era probablemente demasiado bajo respecto al límite de detección del aparato, observandose sólo los picos de las mitades aisladas y no el complejo supramolecular. b) Series de cables supramoleculares lineales caracterizados por el enlace amidinio-carboxilato Para obtener un conocimiento más profundo sobre la interacción amidinio-carboxilato como conexión supramolecular alternativa al enlace covalente en cables moleculares, se sintetizaron sistemas simples basados en un diseño lineal para facilitar la investigación y la comprensión de las propiedades de conductancia a través de su estructura supramolecular. También se llevó a cabo la síntesis de un compuesto de control (17) para comparar la conductancia del conjunto supramolecular 16a . 14b con el análogo covalente más similar. Figura S3: Series de cables supramoleculares lineales 15a,b·14a and 16a,b·14b. Resumen 238 Los experimentos de valoración mediante 1 H NMR demostraron cualitativamente la formación de los complejos supramoleculares mostrando la aparición de una banda ancha, atribuible a la señal de los protones de amidinio implicados en la interacción no covalente, que modifica su anchura y posición en función de la concentración de los huéspedes. Los experimentos cuantitativos, por otro lado, permitieron determinar valores de Ka superiores a 10 4 M -1 para todos los complejos. Los experimentos STM revelaron tres resultados principales. i) los derivados de piridina se han demostrado inadecuados para ser investigados, resultando imposible detectar los valores de conductancia de cualquiera de las dos mitades aisladas y de los complejos. ii) Por primera vez se ha demostrado que el grupo amidino puede ser tan eficaz como el carboxilo como grupo de anclaje, debido a que los valores de conductancia obtenidos para ambas mitades del complejo corto resultan muy similares entre ellos. iii) El valor de conductancia correspondiente al complejo anterior resulta inesperadamente bajo, más de dos órdenes de magnitud inferior al del terfenilo utilizado como compuesto de referencia (17). Por lo tanto, en principio, la naturaleza mixta de la interacción amidinio-carboxilato, basada en enlaces de hidrógeno y fuerzas electrostáticas (debido a su separación de carga) se puede considerar responsable de la interrupción del proceso de CT a lo largo del cable. Interruptores Moleculares Basados en Éteres Corona: a) 4,5'-diaminodibenzo[18]corona-6 La síntesis y la caracterización completa del 4,5'-diaminodibenzo[18] corona-6 se llevó a cabo con éxito. Figura S4: Compuesto de referencia 17. Figura S4: 4,5'-diaminodibenzo[18]corona-6. 239 Sin embargo, resultó imposible detectar la señal de conductancia correspondiente a la molécula vacía, probablemente debido a la falta de conjugación directa entre la estructura del éter corona y los anillos aromáticos. Por lo tanto, el estudio de las moléculas complejadas no pudo llevarse a cabo. b) Series de cables moleculares de éter corona sobre esqueletos de OPE y PPP Sobre la base del resultado anterior, se formuló la síntesis de los derivados de OPE y PPP, con la unidad de éter corona unida únicamente a la unidad central de benceno, para evitar que su posicionamiento interrumpa la conjugación  a lo largo de la estructura del cable. Mientras los derivados de PPP se sintetizaron y caracterizaron con éxito, los derivados de OPE, desafortunadamente, se obtuvieron como complejos irreversibles de TPPO. El estudio STMBJ de transferencia de carga a través de los compuestos sin complejar 26a,b mostró, sin sorpresa, valores de conductancia realmente similares a los sistemas basados en p-terfenilenos. Por otro lado, la determinación de las propiedades electrónicas de los cables complejados 27a,b está actualmente en curso. Conclusiones Se llevó a cabo la síntesis de nuevos cables supramoleculares, basados, por un lado, en la interacción amidinio-carboxilato y, por otro, en la interacción ion-dipolo de un catión con la cavidad de un éter corona. La conexión no covalente entre los grupos amidinio-carboxilato se investigó más profundamente, también desde el punto de vista de sus propiedades de conductancia. Los bajos valores de conductancia Figura S5: Síntesis de los cables moleculares 27a,b derivados de PPP y caracterizados por éteres corona complejados con K + . Resumen 240 observados en comparación con el compuesto de referencia covalente podrían justificarse mediante una combinación de la unión a través de fuerzas supramoleculares con una fuerte separación de cargas. El estudio de las propiedades electrónicas de los derivados de éter de corona-PPP sintetizados está actualmente en curso. Chapter 2. Objetivos Síntesis de Materiales Basados en Grafeno Sobre Superficie Teniendo en cuenta las conocidas propiedades electrónicas del [60]fullereno y las características destacadas e interesantes demostradas por la lámina de grafeno, se desarrollaron nuevas uniones basadas en la combinación de esas características en estructuras nanohíbridas de láminas de grafeno modificado covalentemente con derivados de fullereno. Específicamente, el diseño de los nanomateriales objetivo se formuló con la finalidad de estudiar las propiedades termoeléctricas de sistemas de cables híbridos con diferentes conexiones entre el espaciador y la capa de grafeno. Resultados y discusión La síntesis de los compuestos de grafeno GOS1,2,3, y los híbridos de control FLG1,2, se logró después de algunos pasos fundamentales, y su caracterización química, física y teórica se llevó a cabo positivamente. Figura S6: Materiales híbridos basados en grafeno sobre superficie: a) GOS1, b) GOS2 y c) GOS3. 241 El éxito del proceso de funcionalización fue confirmado por: - Estudios Raman en los FLGs y GOS, antes y después del tratamiento químico, mostrando el aumento en la banda D asociado con un mayor grado de defectos en los nanomateriales. - Análisis termogravimétrico (TGA), llevado a cabo solo en los derivados de FLG, ha permitido estimar el grado de funcionalización de la superficie como 1 molécula orgánica por 224 átomos de carbono para FLG1 y 1 molécula orgánica por 139 átomos de carbono para FLG2. - FTIR, mediante el cual se pudieron observar los picos vibracionales característicos del C60 (alrededor de 1454 y 525 cm -1 ) en todos los derivados de FLG funcionalizados covalentemente. - Análisis XPS, que han evidenciado un componente C 1s principal junto con dos señales más, asignadas a O 1s y N 1s. Estas dos señales, respectivamente, se consideran debidas al oxígeno atmosférico y a los grupos amino sin reaccionar, todavía presentes en la molécula orgánica. - El análisis TEM ha permitido reconocer algunas formas esféricas en los bordes de los derivados de grafeno, no observables en micrografías TEM de FLG prístinos, que se atribuyeron a las compuestos de C60 covalentemente unidos al grafeno, ya que su diámetro (~1 nm) resulta compatible con el diámetro descrito en la bibliografía para moléculas de C60. - El análisis AFM de los nuevos materiales basados en GOS también ha revelado la presencia de islas moleculares, cubriendo parcialmente la lámina de grafeno, con una altura de ~1,2 nm (de acuerdo con el tamaño de una unidad de fullereno). - Mediante medidas de conductancia a través de c-AFM, las propiedades termoeléctricas de los materiales nanohíbridos sintetizados se revelaron como sorprendentemente altas respecto a las de otras moléculas orgánicas. Además, los derivados de [60]fullereno doblemente anclados a la monocapa de grafeno muestran dos valores principales de termopower (S), uno positivo alrededor de 74 μV/K y el otro negativo a -56 ± 21 μV/K y -27 ± 8 μV/K, respectivamente para GOS1 y GOS2. Por lo tanto, el valor positivo, en perfecto acuerdo con el valor de termopower derivado para el híbrido GOS3, se consideró Resumen 242 atribuible a la presencia de derivados de fullereno que reaccionaron con sólo uno de los dos grupos amino. Conclusiones Se han preparado novedosos materiales moleculares hibridos de grafeno y [60]fullereno. Los altos valores de termopower obtenidos para GOS1 y GOS2 sugieren un gran potencial para estos materiales moleculares híbridos de grafeno en aplicaciones termoeléctricas. 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