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 (H 2 P)). In this ZnP−H 2 P−C 60 triad, the ZnP performs as an antenna molecule, transferring its singlet excited state energy to the energetically lower lying H 2 P. In benzonitrile, this energy transfer ( k = 1.5 × 10 10 s - 1 ) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of H 2 P to yield ZnP−H 2 P • + −C 60 • - and subsequently ZnP • + −H 2 P−C 60 • - with rate constants of 7.0 × 10 9 s - 1 and 2.2 × 10 9 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 H 2 P) 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 H 2 P to the adjacent fullerene acceptor.