A chemical approach towards the photosynthetic reaction center

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 (M-0) to a doubly strapped metal-free porphyrin (SH), a beta-unsubstituted metal-free porphyrin (H), and a beta-unsubstituted zinc porphyrin (Z) in order to achieve a RC-type sequential ET relay. In D-M-0-I and D-SH-I, the charge separation (CS) between M-1(0)* and I or (SH)-S-1* and I and a subsequent hole transfer to D provide D+-M-0-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 D-1*-Z-I --> D+-Z(-)-I --> D+-Z-I- in DMF, while superexchange mediated long-distance electron transfer is suggested in THF.