title: 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.
abstract: 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 1.4x10(7) and 1.6x10(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 (Phi(CS)>90%).
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Kuramochi
authors.forename: Atula S D
authors.initials: AS
authors.lastname: Sandanayaka
authors.forename: Akiharu
authors.initials: A
authors.lastname: Satake
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Araki
authors.forename: Kazuya
authors.initials: K
authors.lastname: Ogawa
authors.forename: Osamu
authors.initials: O
authors.lastname: Ito
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Kobuke