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) porphyrinatozinc(II) (ZnP-Tripod) and additional fullero-pyrrolidine moieties (C₆₀-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 porphyrinatozinc sites to afford a stable 1:1 composite. The binding constants for ZnP-Tripod and C ₆₀-ZnP-Tripod in benzonitrile were estimated from steady-state fluorescence titrations to be 1.4 × 10⁷ and 1.6 × 10 ⁷M^-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 C₆₀-ZnP-Tripod, the converged energy at the ZnP moiety induced electron transfer to the C₆₀ moiety, which afforded the stable charge-separated state (φ_CS > 90%).