Photoinduced electron transfer in tetrathiafulvalene-porphyrin-fullerene molecular triads

The two molecular triads la and lb consisting of a porphyrin (P) covalently linked to a fullerene (C-60) electron acceptor and tetrathiafulvalene (77F ) electron-donor moiety were synthesized, and their photochemical propertie s were determined by transient absorption and emission techniques. Excitati on of the freebase-porphyrin moiety of the TTF - P-2H - C-60 triad 1a in te trahydro-2-methylfuran solution yields the porphyrin first excited singlet state TTF - P-1(2H) - C-60, which undergoes photoinduced electron transfer with a time constant of 25 ps to give TTF - P-2H(.-) - C-60(.-). This inter mediate charge-separated state has a lifetime of 230 ps, decaying mainly by a charge-shift reaction to yield a final state, TTF.+ - P-2H - C-60(.-). T he 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 exci ted state. Similar behavior is observed for the zinc analog 1b. The TTF-P-Z n(.-) - C-60(.-) state is formed by ultrafast electron transfer from the po rphyrinatozinc excited singlet state with a time constant of 1.5 ps. The fi nal TTF.- - P-Zn - C-60(.-) state is generated with a yield of 16%, and als o has a lifetime of 660 ns. Although charge ;recombination to yield a tripl et has been observed in related donor-acceptor systems, the TTF.- - P - C-6 0(.-) states recombine to the ground state, because the molecule lacks low- energy triplet states. This structural feature leads to a longer lifetime f or the final charge-separated state, during which the stored energy could b e harvested for solar-energy conversion or molecular optoelectronic applica tions.