Photoinduced electron transfer in tetrathiafulvalene-porphyrin-fullerene molecular triads

The two molecular triads 1a and 1b consisting of a porphyrin (P) covalently linked to a fullerene (C₆₀) electron acceptor and tetrathiafulvalene (TTF) electron-donor moiety were synthesized, and their photochemical properties were determined by transient absorption and emission techniques. Excitation of the freebase-porphyrin moiety of the TTF-P_2H-C₆₀ triad 1a in tetrahydro-2-methylfuran solution yields the porphyrin first excited singlet state TTF-¹P_2H-C₆₀, which undergoes photoinduced electron transfer with a time constant of 25 ps to give TTF-P_2H^·+-C₆₀^·-. This intermediate charge-separated state has a lifetime of 230 ps, decaying mainly by a charge-shift reaction to yield a final state, TTF^·+-P_2H-C₆₀^·-. The final state has a lifetime of 660 ns, is formed with an overall yield of 92%, and preserves ca. 1.0 eV of the 1.9 eV inherent in the porphyrin excited state. Similar behavior is observed for the zinc analog 1b. The TTF-P_Zn^·+-C₆₀^·- state is formed by ultrafast electron transfer from the porphyrinatozinc excited singlet state with a time constant of 1.5 ps. The final TTF^·+-P_Zn-C₆₀^·- state is generated with a yield of 16%, and also has a lifetime of 660 ns. Although charge recombination to yield a triplet has been observed in related donor-acceptor systems, the TTF^·+-P-C₆₀^·- states recombine to the ground state, because the molecule lacks low-energy triplet states. This structural feature leads to a longer lifetime for the final charge-separated state, during which the stored energy could be harvested for solar-energy conversion or molecular optoelectronic applications.