Synthesis and photochemistry of a carotene-porphyrin-fullerene model photosynthetic reaction center

A new photosynthetic reaction center mimic consisting of a porphyrin (P) linked to both a fullerene electron acceptor (C₆₀) and a carotenoid secondary electron donor (C) was synthesized and studied in 2-methyltetrahydrofuran using transient spectroscopic methods. Excitation of the porphyrin is followed by photoinduced electron transfer to the fullerene (τ = 32 ps) to yield C-P⁺-C₆₀^-. Electron transfer from the carotene to the porphyrin radical cation (τ = 125 ps) gives a final C⁺-P-C₆₀^- state with an overall yield of 0.95. This state decays to give the carotenoid triplet state with a time constant of 57 ns. The molecular triad is highly soluble in organic solvents and readily synthesized. These qualities make the molecule a useful artificial photosynthetic reaction center for a variety of spectroscopic and photochemical investigations.