Photo-induced Electron Transfer or Proton-Coupled Electron Transfer in Methylbipyridine/Phenol Complexes: A Time-Dependent Density Functional Theory Investigation.

It is often difficult to assign the nature of an excited-state process unambiguously based on a limited number of experimental evidence. The methylbipyridine/phenol complex is a classic example, where experimental observations support a proton-coupled electron transfer (PCET) or a photo-induced electron transfer (PET) process. Here, we implemented time-dependent density functional theory calculation to elucidate the nature of the process. We found that PCET is possible only when mediated by a H-bond between methylbipyridine and phenol. However, a conventional PET can occur through π-π stacking interaction between the donor and the acceptor. Thus, the photophysical process in the complex is indeed governed by competition of H-bonding versus π-π interaction. Our calculations including the solvent model based on density (SMD) suggest that π-π stacking is more favorable than H-bonding, and hence, conventional PET is a more favorable excited-state process for the methylbipyridine/methoxyphenol complex than PCET.