A chemical approach towards the photosynthetic reaction center

Abstract

Phenylene-bridged zinc diporphyrin (D) - monoporphyrin (M) - pyromellitimide (I) triads were prepared as models for the photosynthetic reaction center (RC). With D and I fixed, the central monoporphyrin subunit is tuned from octaalkyl-substituted zinc porphyrin (M0) to a doubly strapped metal-free porphyrin (SH), a β-unsubstituted metal-free porphyrin (H), and a β-unsubstituted zinc porphyrin (Z) in order to achieve a RC-type sequential ET relay. In D - M0 - I and D - SH - I, the charge separation (CS) between 1M0* and I or 1SH* and I and a subsequent hole transfer to D provide D+ - M0 - I and D+ - SH - I, respectively. Upon excitation of D - H - I, an effective CS between the porphyrin pigments provides D+ - H - I which is converted to D+ - H - I by a subsequent charge shift reaction in 0.8 overall quantum yield in a manner analogous to that in RC. D - Z - I gives D+ - Z - I in 0.4 overall quantum yield both in DMF and THF but the transient absorption spectra revealed that a stepwise ET relay of 1D* - Z - I → D+ - Z - I → D+ - Z - I in DMF, while superexchange mediated long-distance electron transfer is suggested in THF.