Long-lived photoproduced radical ions in tetrathiafulvalenes covalently tethered to C60

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Abstract
C60, or [60]fullerene, a reversible one- to six-electron acceptor with moderate first electron affinity, was covalently linked, via a 1,3-dipolar addition reaction using azomethine ylides and two flexible insulating σ-chains of different lengths, to a tetrathioalkyltetrathiafulvalene, a reversible one- to two-electron donor with low first ionization potential, yielding molecules 1 and 2. The electrochemical oxidation and reduction waves are the same as those of the separate components; UV-VIS spectra indicate no appreciable charge transfer in the ground state between the donor and acceptor moieties of these D–σ–A systems 1 and 2: there is only a weak shoulder at 800 nm (ε ≈ 200 L mol–1 cm–1), which could be the intervalence transfer band. These same molecules, as well as their donor and acceptor components taken separately, were electrochemically oxidized/reduced in liquid solutions, and also irradiated with laser light in low-temperature glasses. The electron paramagnetic resonance (EPR) spectra revealed photoexcited electron transfer at 77 K, with resulting S = 1/2 radical cation and radical anion states. In a glass at 77 K these radical signals survive a long time (up to several days) after the end of light irradiation. This may be separately solvated pairs of long-lived radicals D˙+–σ–A and D–σ–A˙– or, less likely, a long-lived excited-state zwitterionic biradical D˙+–σ–A˙–. With increasing temperature and the onset of diffusional motion, the EPR signals disappear.

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