Photoinduced electron-transfer dynamics and long-lived CS states of donor-acceptor linked dyads and a triad containing a gold porphyrin in nonpolar solvents

An electron donor-acceptor linked compound containing Zn(II) and Au(III) porphyrins (ZnPQ - AuPQ⁺ PF₆^-) has been designed and synthesized to examine its electrochemical and photophysical properties. Time-resolved transient absorption spectra of ZnPQ-AuPQ⁺ were measured by nanosecond laser photolysis in toluene and cyclohexane. The observed transient absorption bands at 600-800 nm are assigned to the charge-shift state (ZnPQ^{{radical dot}+}-AuPQ). The charge-shifted state decays via back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band at 750 nm due to ZnPQ^{{radical dot}+} in ZnPQ^{{radical dot}+}-AuPQ. The decay of the absorption band obeys first-order kinetics. The k_BET value in cyclohexane is determined as 1.0 × 10⁵ s^-1, which corresponds to a lifetime of 10μs, whereas the lifetime becomes much shorter (250 ps) in benzonitrile. The charge-shift state is also observed for a zinc porphyrin-free base porphyrin-gold porphyrin triad (ZnPQ-2HPQ-AuPQ⁺), which has a longer lifetime of 14 μs in cyclohexane. The detailed photodynamics of ZnPQ-AuPQ⁺ and ZnPQ-H₂PQ-AuPQ⁺ are also reported. We have successfully attained long-lived charge-shift states in nonpolar solvents using the ZnPQ-AuPQ⁺ dyad and the ZnPQ-H₂PQ-AuPQ⁺ triad. This photoinduced electron-transfer in low dielectric environment mimics a key feature of photosynthetic reaction centers.