Mechanism of back electron transfer in an intermolecular photoinduced electron transfer reaction: solvent as a charge mediator.

Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile (ACN) by using time-resolved near- and mid-IR spectroscopy. The Py dimer radical cation (Py2(·+)) and DCB radical anion (DCB(·-)) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCB(·-) to the solvent and charge recombination between the resulting ACN dimer anion and Py2(·+). The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye-sensitized solar cells.