Sequential Energy and Electron Transfer in an Artificial Reaction Center: Formation of a Long-Lived Charge-Separated State

Abstract

A novel molecular triad, representing an artificial reaction center, was synthesized via linking a fullerene moiety to an array of two porphyrins (i.e., a zinc tetraphenyl porphyrin (ZnP) and a free base tetraphenyl porphyrin (H2P)). In this ZnP−H2P−C60 triad, the ZnP performs as an antenna molecule, transferring its singlet excited state energy to the energetically lower lying H2P. In benzonitrile, this energy transfer (k = 1.5 × 1010 s-1) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of H2P to yield ZnP−H2P•+−C60•- and subsequently ZnP•+−H2P−C60•- with rate constants of 7.0 × 109 s-1 and 2.2 × 109 s-1, respectively. The final charge-separated state, formed in high yield (0.4), gives rise to a remarkable lifetime of 21 μs in deoxygenated benzonitrile and decays directly to the singlet ground state. In contrast, in nonpolar toluene solutions the deactivation of the porphyrin chromophores (ZnP and H2P) takes place via singlet−singlet energy transfer leading to the fullerene singlet excited state. This stems from the unfavorable free energy changes for an intramolecular electron-transfer event in toluene from the singlet excited state of H2P to the adjacent fullerene acceptor.