Organic Chemistry Frontiers c6qo00760k 1 Rhodanine-based dyes absorbing in the entire visible spectrum Rafael Sandoval-Torrientes, Joaquín Calbo, David García-Fresnadillo, José Santos, Enrique Ortí* and Nazario Martín* A series of new broad-absorbing dyes based on rhodanine derivatives conjugated with triarylamines through a fluorene backbone were synthesized. Spectroscopic and electrochemical characterizations, along with theoretical calculations, revealed interesting properties of the dyes that efficiently absorb in the entire visible spectrumQ4 . Please check this proof carefully. Our staff will not read it in detail after you have returned it. Translation errors between word-processor files and typesetting systems can occur so the whole proof needs to be read. Please pay particular attention to: tabulated material; equations; numerical data; figures and graphics; and references. 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ORGANIC CHEMISTRY FRONTIERS RESEARCH ARTICLE Cite this: DOI: 10.1039/c6qo00760k Received 28th November 2016, Accepted 3rd February 2017 DOI: 10.1039/c6qo00760k rsc.li/frontiers-organic Rhodanine-based dyes absorbing in the entire visible spectrumQ1 † RafaelQ2 Sandoval-Torrientes,a Joaquín Calbo,b David García-Fresnadillo,c José Santos,a Enrique Ortí*b and Nazario Martín*a,c A series of new broad-absorbing dyes based on rhodanine derivatives conjugated with triarylamines using a fluorene backbone was synthesized. Spectroscopic andQ5 electrochemical characterizations, along with theoretical calculations at the B3LYP/cc-pVDZ level, revealed interesting properties of the dyes, which make the dyes efficiently absorb in the entire visible spectrum. Since environmental problems derived from the overconsump- tion of fossil fuels have started to manifest, many resources have been allocated in search of new environmentally friendly energy sources. Among these sources, solar energy probably represents the most abundant and available source of energy. Over the last 15 years, organic photovoltaics have experienced slow but sustained progress.1 From the earliest poly(p-phenyl- ene vinylene) devices (with the low 3% power conversion efficiency (PCE))2–4 to the poly(3-hexylthiophene-2,5-diyl) devices (with 5% PCE)5–7 and the state-of-the-art donor– acceptor (D–A) photovoltaic devices (with 11% PCE),8–12 the most common feature of these materials is their polymeric nature. However,Q6 in the last few years, more and more systems based on small molecules have been developed with efficiencies comparable to those of their polymer counterparts.13–15 Once the ability ofQ7 small molecules to efficiently perform like polymers was demonstrated, the race to develop new dyes that are able to broadly harvest light in the visible-near infrared region (where most of the solar spectrum is centred) started. Several groups have recently incorporated the electron- deficient, rhodanine end-capping group to a series of electron- rich systems, allowing the fabrication of devices with efficien- cies ranging from 5 to 9%.13,15,16–18 All these systems share A–D–A structures in common, with rhodanines linked to the central core by a Knoevenagel condensation reaction with an aldehyde group. Herein, we report for the first time four new molecules (RFTA-1–4, see Scheme 1) via incorporating rhodanine (R) derivatives into a fluorene (F) backbone (through its C9 position), giving rise to D–A–D structures (employing tri- arylamine (TA) as electron-donating groups) that feature absorp- tion spectra spanning the entire visible region. Spectroscopic and electrochemical studies, complemented by density func- tional theory (DFT) calculations, provide clear insight into the electronic and optical properties of these novel dyes. All the synthesized dyes share a central fluorene core bearing either N-hexyl-2-(1,1-dicyanomethylene)rhodanine or N-(2-ethylhexylrhodanine) on its C9 position and two triaryl- amines (triphenylamine or diphenylaminothiophene) on posi- tions C3 and C6. By employing both Q8electron donors and acceptors of different relative strengths, we were able to fine- tune the light absorption properties of the dyes, which made the dyes cover the whole visible spectral range with different λmax. From the synthetic point of view, the most remarkable achievement was the insertion, for the first time (to the best of our knowledge), of a rhodanine fragment into the C9 posi- tion of the fluorene core. This opened the door to the use of Scheme 1 Rhodanine/fluorene/triarylamine (RFTA) dyes. †Electronic supplementary information (ESI) available: Experimental details, synthesis, structural and photophysical characterization, cyclic voltammograms, NMR spectra, and theoretical calculations. See DOI: 10.1039/c6qo00760k aIMDEA-Nanociencia, Ciudad Universitaria de Cantoblanco, 28049 Madrid, SpainQ3 bInstituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Spain cDepartamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain This journal is © the Partner Organisations 2017 Org. Chem. Front., 2017, 00, 1–5 | 1 1 5 10 15 20 25 30 35 40 45 50 55 1 5 10 15 20 25 30 35 40 45 50 55 www.rsc.li/frontiers-organic rhodanine acceptors as central units in D–A–D type structures, thus complementing their former use as terminal moieties in A–D–A-type systems. Another interesting aspect is the gain in the planarity and rigidity of the rhodanine/fluorene accepting core. To date, in all the previouslyQ9 described dyes containing the rhodanine fragments, the fragments were attached via a Knoevenagel condensation with an aldehyde group on the side of the main molecular skeleton, thus resulting in molecules lacking the rigidity that we herein were able to provide. As depicted in Scheme 2, triarylamine products were obtained by a Suzuki cross-coupling reaction of 3,6-di- bromofluorenone (1)19 with boronic ester 2,20 followed by a Knoevenagel condensation of the resulting product 3 with either rhodanine 6 or 7 to obtain the products RFTA-1 and RFTA-2, respectively. In the case of the diarylaminothiophene systems, its corresponding organotin derivative 10 was pre- pared as depicted in Scheme S2† in the ESI,† and then coupled by a Stille cross-coupling reaction with 1. Further Knoevenagel condensation with the corresponding rhodanine provided products RFTA-3 and RFTA-4. All Knoevenagel reac- tionsQ10 provided products in moderate to low yields. Moreover, theQ11 unreacted starting materials were easily recovered and were repeatedly used again until their full conversion. Full synthetic details are available in the ESI.† All RFTA dyes showed strong absorption in the visible region of the spectrum (Table 1 & Fig. S2, S3 in the ESI†), fea- turing a single broad band that red-shifted and broadened as stronger acceptors and/or donors were attached to the fluorene core (see Fig. 1a). The first compound of the series (RFTA-1), bearing rhodanine as an acceptor and triphenylamine (TPA) as a donor, exhibited peaks at 523 nm with its onset at 653 nm. When the stronger electron-accepting 2-(1,1-dicyanomethyl- ene)rhodanine 7 was introduced (RFTA-2), the molecular absorption caused a 20 nm red shift, peaking at 545 nm with an onset at 692 nm. A more dramatic bathochromic effect was observed with rhodanine as an acceptor and diphenyl- aminothiophene (DPAT) as a donor (RFTA-3). In this case, the absorption was centered at 565 nm, representing a 40 nm shift, with an onset at 748 nm. Finally, RFTA-4 had the strongest donor and acceptor moieties and showed a broad absorption band centered at 590 nm with the onset at 786 nm. The bathochromic shift observed in λmax abs on increasing the donor and acceptor strengths of the substituent groups suggests that this band, showing quite large molar absorption coefficients (1.6–2.3 × 104 M−1 cm−1), may be attributed to an intramolecular charge transfer (ICT) process, according to previously described fluorenone derivatives bearing electron- donating groups.21 Solvatochromic experiments demonstrated noticeable shifts in the lower energy absorption peak (Fig. S2, in the ESI†), thus providing extra evidence of its suggested ICT nature. In addition, the significant broadening of thisScheme 2 Synthetic route to rhodanine-based compounds RFTA-1–4. Table 1 Photophysical properties of RFTA-1–4 dyes and their 3 & 5 precursors measured in CH2Cl2 solution Dye λmax abs [nm] λonset a [nm] ε × 104 [M−1 cm−1] λmax em [nm] E0–0 b [eV] Φem c RFTA-1 523 653 1.64 1019 1.79 0.008 RFTA-2 545 692 2.34 1043 1.73 0.006 RFTA-3 565 748 1.93 1013 1.71 0.011 RFTA-4 590 786 2.34 1036 1.65 0.010 3 440 — — 707 2.29 0.027 5 483 — — 709 2.16 0.030 a Estimated extrapolation from the absorption feature edge to A = 0. bUncertainty ±5%. cUncertainty ±20%. Fig. 1 (a) Normalized Q12UV-vis absorption spectra of RFTA dyes in CH2Cl2 solution. (b) Molar absorption profiles. Research Article Organic Chemistry Frontiers 2 | Org. Chem. Front., 2017, 00, 1–5 This journal is © the Partner Organisations 2017 1 5 10 15 20 25 30 35 40 45 50 55 1 5 10 15 20 25 30 35 40 45 50 55 absorption band for compounds RFTA-3 and RFTA-4 is note- worthy. As has been discussed below, based on theoretical calculations, this effect was attributed to larger conjugation between the DPAT units and the fluorene core provided by the thiophene rings. Regarding the emission features of RFTA compounds (Table 1, Fig. S4–S6,† in the ESI†), their emission maxima (λmax em ) are strongly red-shifted (Stokes’ shifts of ca. 9000 cm−1 for RFTA-1 and RFTA-2 (ca. 500 nm) and ca. 7500 cm−1 for RFTA-3 and RFTA-4 (ca. 450 nm)) with respect to that of their precursors 3 and 5 (Stokes’ shifts of 8600 cm−1 (267 nm) and 6600 cm−1 (226 nm), respectively; cf. 6800 cm−1 (130 nm) for 9H-fluoren-9-one21d in polar aprotic solvent). This again demonstrates the important role played by the structural fea- tures since RFTA-2 and RFTA-4, bearing (dicyanomethylene) rhodanine moieties, have larger λmax em values. Additionally, the combination of lower LUMO energies for RFTA-2 and RFTA-4 and higher HOMO energies for RFTA-3 and RFTA-4 (including the DPAT feature) explains the lower energy values for the 0–0 transition (E0–0, Table 1) for RFTA-3 and RFTA-4. This is in excellent agreement with the results obtained from electro- chemistry and theoretical calculations, vide infra. On the other hand, the low-emission quantum yields determined for all the RFTA compounds (1% or below) reflect the fact that excited state deactivation by fluorescence is of little importance compared to that by non-radiative processes. This agrees with the energy gap law stating that the lower the gap between the ground and excited state, the less efficient the radiative processes. The electrochemical properties of the new RFTA dyes were studied by cyclic voltammetry (CV). All molecules showed an amphoteric redox behavior, presenting one oxidation and two reduction processes (see Table 2 and Fig. S1, in the ESI†). For RFTA-1 and RFTA-2, both bearing the TPA donor, the oxidation potentials were identical (1.05 V). However, a significant 100 mV anodic shift was observed for the first reduction poten- tial while moving from RFTA-1 (−0.61 V) to RFTA-2 (−0.51 V), bearing a stronger electron-accepting rhodanine group. Similarly, the second reduction potential experienced a moderate 50 mV anodic shift. When the stronger DPAT donor was used (RFTA-3 and RFTA-4), the oxidation potential experienced a 130 and 110 mV cathodic shift, respectively. In the case of RFTA-3, with the same rhodanine acceptor as RFTA-1, there was no change for the first reduction potential (−0.61 V). However, RFTA-4, bearing the strongest acceptor, showed a 30 mV anodic shift compared to RFTA-2 and presented the smallest reduction potential (−0.48 V). This difference was ascribed to experi- mental error. The calculated electrochemical gaps ranged from 1.66 to 1.44 eV, diminishing from RFTA-1 to RFTA-4, which is in good agreement with the trend inferred from the spectro- scopic data (Table 1). Concerning the redox potentials of the excited states (Table 2), while the oxidation process from RFTA* to RFTA+ had similar potentials for all the RFTA dyes, the reduction process from RFTA* to RFTA− seems to be slightly less favored in the case of RFTA-3, lacking the TPA (less coplanar/less electronically coupled donor, vide infra) and dicyanomethylene (stronger acceptor) moieties. To gain insight into the optical and electronic properties of the novel rhodanine-based dyes, theoretical calculations were performed within the density functional theory (DFT) frame- work (see the ESI† for full computational details). Minimum- energy Q13optimized geometries calculated at the B3LYP/cc-pVDZ level in CH2Cl2 indicated that the rhodanine moiety remained coplanar with the fluorene core in all four dyes, constituting both moieties on the central acceptor unit of the D–A–D archi- tecture (see Fig. S7 in the ESI†). For RFTA-3 and RTFA-4, the thiophene rings bridging the donor DPAT groups to the accep- tor moiety were calculated to be almost coplanar with the fluorene core, with the largest dihedral angles of 8.9°. In con- trast, the phenyl rings bridging the TPA units in RFTA-1 and RFTA-2 were 30–32° out of the plane of the fluorene core due to the steric hindrance caused by short H⋯H contacts (Fig. S7, in the ESI†). Finally, both TPA and DPAT units were calculated to show the typical mill sails shape. The highest-occupied molecular orbitals (HOMO and HOMO−1) were predicted to be nearly degenerate and are loca- lized on the two electron-donor fragments, either TPA or DPAT (see Fig. 2a for RFTA-3, as a representative example, and Fig. S8,† in the ESI,† for the rest of compounds). The higher conjugation and more planar structure promoted by the less- aromatic thiophene rings in RFTA-3 and RFTA-4 determined that the HOMOs of these dyes were computed to be higher in energy (−5.02 and −5.04 eV, respectively) than the HOMOs of RFTA-1 and RFTA-2 (−5.11 and −5.13 eV, respectively). Moreover, this provokes a certain splitting (0.07 eV) between the HOMO and HOMO−1. The increase of ∼0.1 eV in the HOMO energy confirms DPAT groups as better donors than TPAs and is in good agreement with the lower oxidation poten- tials obtained for RFTA-3 and RFTA-4 (Table 2). The lowest- unoccupied molecular orbital (LUMO) was located on the acceptor rhodanine–fluorene moiety for all four compounds. The LUMO was calculated at −3.07 eV for RFTA-1 and RFTA-3, and it was computed to be ∼0.1 eV lower in energy for RFTA-2 and RFTA-4 due to the presence of the stronger electron- Table 2 Electrochemical data for the ground and excited states of dyes RFTA-1–4 Dye Eox1=2 a [V] Ered11=2 a [V] Ecvgap b [V] EHOMO/ ELUMO c [eV] Eox* d [V] Ered* d [V] RFTA-1 1.05 −0.61 1.66 −5.45/−3.79 −0.74 1.18 RFTA-2 1.05 −0.51 1.56 −5.45/−3.89 −0.68 1.22 RFTA-3 0.92 −0.61 1.53 −5.32/−3.79 −0.74 1.10 RFTA-4 0.94 −0.48 1.42 −5.34/−3.92 −0.71 1.19 aMeasured by CV in CH2Cl2 solution using 0.1 M Bu4NPF6 as the sup- porting electrolyte, glassy carbon as the working electrode, and Pt wires as the reference and counter electrodes; all potentials were obtained vs. Fc/Fc+ as an internal reference. b Electrochemical gap determined as Eox–Ered. cHOMO/LUMO energies estimated according to EHOMO = −[Eox 1=2 + 4.4] eV; ELUMO = EHOMO + Ecv gap. d Excited state redox potentials, uncertainty 5%. Organic Chemistry Frontiers Research Article This journal is © the Partner Organisations 2017 Org. Chem. Front., 2017, 00, 1–5 | 3 1 5 10 15 20 25 30 35 40 45 50 55 1 5 10 15 20 25 30 35 40 45 50 55 accepting 2-(1,1-dicyanomethylene)rhodanine moiety. This result supports the less negative reduction potentials obtained for RFTA-2 and RFTA-4. Time-dependent density functional theory (TD-DFT) calcu- lations were conducted to fully rationalize the nature and trends experimentally obtained for the low-lying absorption bands of the dyes. The 30 lowest-lying singlet excited states (Sn) were computed at the B3LYP/cc-pVDZ level in CH2Cl2 solu- tion, and the main characteristic features of the most relevant excitations are shown in Table 3 for RFTA-3 as a representative example (see Table S1 in the ESI† for all the dyes). The lowest-lying broad absorption band observed in the UV-vis spectra of RFTA-1–4 (Fig. 1) originated from the S0 → S1 and S0 → S2 transitions, which were described by the HOMO→ LUMO and HOMO−1→ LUMO one-electron excitations, respectively. These excitations imply an electron transfer from the donor units, where the HOMO and HOMO−1 are located, to the electron-acceptor rhodanine–fluorene moiety, where the LUMO resides (see Fig. 2a). This confirmed the charge transfer character of the absorption band. The electronic transition to the S1 state was calculated at 1.72 eV for RFTA-1 and experi- enced a red-shift while moving to RFTA-2 (1.63 eV) and RFTA-3 (1.62 eV). The shift was attributed to the insertion of a stronger electron-acceptor group (7) for RFTA-2 and to the enhanced electron-donor character of the DPAT moieties in RFTA-3. The inclusion of both acceptor/donor groups in RFTA-4 led to an additional red-shift of the transition to the S1 state computed at 1.56 eV, in good agreement with the experimental data. S2 was predicted to follow similar trends (Table S1†). Interestingly, D–A–D dyes containing the DPAT unit (RFTA-3 and RFTA-4) showed a larger separation between S1 and S2 (0.19 eV) compared to the dyes bearing TPA as a donor moiety (0.12 eV in RFTA-1 and RFTA-2). This was ascribed to the more planar and better conjugated structures promoted by the thio- phene rings of the DPAT units, and explains the broadening observed for the lowest-lying CT band while passing from RFTA-1–2 to RFTA-3–4 (Fig. 1). Moving to higher energies, Q14the excited state associated with the HOMO−2 → LUMO excitation (S5 for RFTA-1 and RFTA-3 and S4 for RFTA-2 and RFTA-4) was predicted to influence the shape of the band observed around 450 nm (Fig. 1). This state originated from a one-electron pro- motion centered on the rhodanine–fluorene acceptor moiety and was calculated at similar wavelengths (445–461 nm, Table S1†) for all four dyes. Finally, a large number of elec- tronic excitations with a high multiconfigurational character were predicted in the 300–400 nm range. Specifically, the sharp bands experimentally obtained at 325 nm for RFTA-1 and RFTA-2 and the less-intense bands around 375 nm obtained for RFTA-3 and RFTA-4 (Fig. 1) originated from elec- tronic excitations mainly centered on the donor and fluorene moieties. Conclusions In summary, a new family of dyes bearing rhodanine deriva- tives rigidly linked to a planar fluorene central core were syn- thesized and characterised for the first time. The new dyes show a very broad absorption band that covers all the visible region of the spectrum from 400 to 790 nm. Calculations demonstrated that this absorption implies the electronic tran- sition to the S1 (HOMO → LUMO) and S2 (HOMO−1 → LUMO) excited states, which have a charge-transfer nature. The rela- tively high intensity exhibited by these transitions was due to the effective conjugation between the donor and acceptor Fig. 2 (a) Isovalue contours (±0.025) calculated for the frontier MOs of RFTA-3. Alkyl chains and hydrogen atoms are omitted for better viewing. (b) Frontier MO energies, including the HOMO–LUMO gap, for the four rhodanine-based D–A–D dyes. H and L denote HOMO and LUMO, respectively. Table 3 Singlet excited states calculated for RFTA-3 at the TD-B3LYP/ cc-pVDZ level in CH2Cl2 State E a (eV) E a (nm) f b Descriptionc Natured S1 1.62 767 0.560 H → L D → A S2 1.80 687 0.658 H−1 → L D → A S5 2.75 451 0.653 H−2 → L A → A S6 3.08 403 0.503 H−5 → L F → A S7 3.14 395 0.446 H−1 → L+1 D → A a Vertical excitation energies (in eV and nm). bOscillator strengths. cDescription in terms of monoexcitations; H and L denote HOMO and LUMO, respectively. dNature of the excited state; the labels A, D, and F refer to acceptor, donor, and fluorene-core moieties, respectively. Research Article Organic Chemistry Frontiers 4 | Org. Chem. Front., 2017, 00, 1–5 This journal is © the Partner Organisations 2017 1 5 10 15 20 25 30 35 40 45 50 55 1 5 10 15 20 25 30 35 40 45 50 55 units through the fluorene core. Emission measurements showed that these dyes had large Stokes’ shifts and were poor emitters, due to the strong CT character of the lower energy transition. From our observations, it may be concluded that dyes bearing the dicyanovinylene rhodanine acceptor fragment allowed a much broader absorption of the visible range of light, with electrochemical energy gaps between 1.4 and 1.5 eV, as previously reported.22 Furthermore, it was demonstrated that a thienodiphenylamine donor fragment provided a more effective coupling with the acceptor due to the reduced dihedral angle allowed by the thiophene ring. The newly synthesized D–A–D dyes possess the necessary properties for their use in small molecule organic photovoltaics. Acknowledgements This work was funded by the European Commission (ERC-320441-Chirallcarbon), MINECO of Spain (CTQ2014- 52045-R, CTQ2015-71154-P, Unidad de Excelencia María de Maeztu MDM-2015-0538), Comunidad de Madrid (FOTOCARBON-CM S2013/MIT-2841), Generalitat Valenciana (PROMETEO/2016/135), and European FEDER funds (CTQ2015-71154-P). J. C. thanks the MECD of Spain for a predoctoral FPU grant. 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