Self-Association and Electron Transfer in Donor−Acceptor Dyads Connected by <i>meta</i>-Substituted Oligomers

Abstract

The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) inwhich the electron acceptorsC 60sand the electron donorsπ-extended tetrathiafulvalene (exTTF)sarebridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOFspectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature 1 H NMR experiments. Furthermore, the ability of these new conjugates to form wirelikestructures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light oftheir photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysicalexperiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the associationconstants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also playsa decisive role in governing excited-state interactions in the corresponding electron donor-acceptorconjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridgedby just one aromatic ring, displayssexclusively and independent of the concentration (10-6 to 10-4M)sefficient intramolecular electron transfer events on the basis of a “through-bond” mechanism. On thecontrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (threem-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10-6M) “through space” intramolecular electron transfer events. These are, however, quite ineffective and, inturn, lead to excited-state deactivations that are at high concentrations (10-4 M) dominated by intracomplexelectron transfer events, namely, between exTTF of one molecule and C 60 of another molecule, and thatstabilize the resulting radical ion pair state with lifetimes reaching 4.0 µs.