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 mimic for nature's solar cells : Simple mixing of nonaporphyrin macrocycle N‐( 1 –Zn) 3 and acceptor ligand C 60 –ZnP–Tripod affords a supramolecular architecture (see scheme), in which the excitation energy collected by the macrocycle is transferred efficiently to the central ZnP acceptor, inducing charge separation between the ZnP and C 60 sites. magnified image 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 fulleropyrrolidine 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 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 1.4×10 7 and 1.6×10 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 60 moiety, which afforded the stable charge‐separated state ( Φ CS >90 %).