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

Abstract

The synthesis of a new series of electron donor−acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor—C₆₀—and the electron donor—π-extended tetrathiafulvalene (exTTF)—are bridged 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-TOF spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature ¹H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5 greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays a decisive role in governing excited-state interactions in the corresponding electron donor−acceptor conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged by just one aromatic ring, displays—exclusively and independent of the concentration (10⁻⁶ to 10⁻⁴ M)—efficient intramolecular electron transfer events on the basis of a “through-bond” mechanism. On the contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10⁻⁶ M) “through space” intramolecular electron transfer events. These are, however, quite ineffective and, in turn, lead to excited-state deactivations that are at high concentrations (10⁻⁴ M) dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C₆₀ of another molecule, and that stabilize the resulting radical ion pair state with lifetimes reaching 4.0 μs.