Sequential energy and electron transfer in an artificial reaction center: Formation of a long-lived charge-separated state

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-C-60 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 x 10(10) s(-1)) is followed by a sequential electron-transfer relay evolving from the generated singlet excited state of H2P to yield ZnP-H2P.+-C-60(.-) and subsequently ZnP.+-H2P-C-60(.-) with rate constants of 7.0 x 10(9) s(-1) and 2.2 x 10(9) s(-1), respectively. The final charge-separated state, formed in high yield (0.4), gives rise to a remarkable lifetime of 21 mu s 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.