A Charge Transfer State Induced by Strong Exciton Coupling in a Cofacial mu-Oxo-bridged Porphyrin Heterodimer

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, u-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(III) and octaethylporphyrinatophosphorus(V), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(III) and P(V) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ~130 ps. The high-energy triplet CT state (3CT = 1.68 eV) lasts for nearly 100 us prior relaxing to the ground state.