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 (PZn)-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 P-1(Zn)-P-2H-C-60 is followed by singlet-singlet energy transfer to the free base porphyrin excitation energy trap (tau = 59 ps), yielding P-Zn-P-1(2H)-C-60. The free base porphyrin first excited singlet state decays by photoinduced electron transfer to the fullerene (tau = 25 ps), producing a P-Zn-P-2H(.+)-C-60(.-) charge-separated state. Charge shift (tau = 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 (tau = 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 (tau = 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.