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 ¹P _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- ¹P _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.