Driving force and electronic coupling effects on photoinduced electron transfer in a fullerene-based molecular triad

Tuning thermodynamic driving force and electronic coupling through structur al modifications of a carotene (C) porphyrin (P) fullerene (C-60) molecular triad has permitted control of five electron and energy transfer rate cons tants and two excited state lifetimes in order to prepare a high-energy cha rge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (greater than or equal to 96%). Excitation of the porp hyrin moiety of C-P-C-60 is followed by a combination of photoinduced elect ron transfer to give C-P.+-C-60(.-) and singlet-singlet energy transfer to yield C-P-C-1(60). The fullerene excited state accepts an electron from the porphyrin to also generate C-P.+-C-60(.-). Overall, this initial state is formed with a quantum yield of 0.97, Charge shift from the carotenoid to yi eld C.+-P-C-60(.-) is at least 60 times faster than recombination of C-P.+- C-60(.-), leading to the overall quantum yield near unity for the final sta te. Formation of a similar charge-separated species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C. +-P-C-60(.-) in 2-methyltetra-hydrofuran yields the carotenoid triplet stat e, rather than the ground state, Comparison of the results for this triad w ith those for related triads with different structural features provides in formation concerning the effects of driving force and electronic coupling o n each of the electron transfer steps.