Benzene-Templated Model Systems for Photosynthetic Antenna−Reaction Center Function

A synthetic strategy for preparing artificial photosynthetic antenna−reaction center complexes based on formation of a benzene core via a Diels−Alder reaction has been applied to the preparation of a zinc porphyrin (P Zn )−free base porphyrin (P 2H )−fullerene (C 60 ) molecular triad. Spectroscopic studies in 2-methyltetrahydrofuran show that excitation of the zinc porphyrin antenna moiety to form 1 P Zn −P 2H −C 60 is followed by singlet−singlet energy transfer to the free base porphyrin excitation energy trap (τ = 59 ps), yielding P Zn − 1 P 2H −C 60 . The free base porphyrin first excited singlet state decays by photoinduced electron transfer to the fullerene (τ = 25 ps), producing a P Zn −P 2H •+ −C 60 •- charge-separated state. Charge shift (τ = 167 ps) yields P Zn •+ −P 2H −C 60 •- . This final charge-separated state is formed with quantum yields >90% following excitation of any of the three chromophores. Charge recombination in 2-methyltetrahydrofuran (τ = 50 ns) occurs by an apparently endergonic process to give triplet states of the chromophores, rather than the ground state. In benzonitrile, charge recombination yields the ground state (τ = 220 ns). The high efficiencies of the various energy and electron-transfer processes suggest that this molecular architecture will be useful for the design of more complex antenna−reaction center complexes.