Energy Transfer Followed by Electron Transfer in a Porphyrin Macrocycle and Central Acceptor Ligand: A Model for a Photosynthetic Composite of the Light-Harvesting Complex and Reaction Center

A system that models a photosynthetic composite of the light-harvesting complex and reaction center is reported in which light energy collected by cyclic antenna porphyrins is transferred to a central energy-acceptor porphyrin, followed by photoinduced electron transfer to a fullerene positioned above the ring plane. Pyridyl tripodal ligands appended with bis(phenylethynyl)porphyrinatozine(II) (ZnP-Tripod) and additional fullero-pyrrolidine moieties (C-60-ZnP-Tripod) were synthesized as the reaction center units. The tripodal ligand was strongly accommodated by the light-harvesting porphyrin macrocycle N-(1-Zn)(3) (1-Zn - trisporphyrinatozinc(II)) by using three-point coordination of pyridyl to uncoordinated porphyrinatozine sites to afford a stable 1:1 composite. The binding constants for ZnP-Tripod and C., ZnP-Tripod in benzonitrile were estimated from steady-state fluorescence titrations to be 1.4 x 10(7) and 1.6 x 10(7) M-1, respectively. The steady-state fluorescence titration, fluorescence lifetime, and transient absorption studies revealed that in both composites the excitation energy collected by the nine porphyrins of N-(1-Zn)(3) was efficiently transferred to a ZnP moiety by means of a through-space mechanism with a quantum yield of approximately 90%. Furthermore, in the composite with C-60-ZnP Tripod, the converged energy at the ZnP moiety induced electron transfer to the C, moiety, which afforded the stable charge-separated state (phi(CS) > 90%).