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

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C₆₀) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (≥96%). Excitation of the porphyrin moiety of C-P-C₆₀ is followed by a combination of photoinduced electron transfer to give C-P^·+-C₆₀^·- and singlet-singlet energy transfer to yield C-P-¹C₆₀. The fullerene excited state accepts an electron from the porphyrin to also generate C-P^·+-C₆₀^·-. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C^·+-P-C₆₀^·- is at least 60 times faster than recombination of C-P^·+-C₆₀^·-, leading to the overall quantum yield near unity for the final state. 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-C₆₀^·- in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.