Theoretical Insight into Proton-Coupled Energy Transfer in an Anthracene-Phenol-Pyridine Triad

Proton-coupled energy transfer (PCEnT) refers to a photochemical process in which the transfer of electronic excitation energy between donor and acceptor molecules is coupled to a proton transfer reaction. Herein, we apply our recently developed nonadiabatic PCEnT theory to an anthracene-phenol-pyridine triad system. For this system, electronic excitation energy is transferred from the anthracene to the phenol-pyridine, in conjunction with proton transfer from the phenol to the pyridine. Thus, the PCEnT reaction corresponds to the transition from the local excited state of the anthracene (LES) to the local electron-proton transfer (LEPT) state of the phenol-pyridine. With most input quantities determined from first-principles calculations, our theory reproduces the experimentally measured PCEnT rate constant. We analyzed the contributions from different electron-proton vibronic states to the PCEnT rate constant (the LES to LEPT state) and the acceptor absorption spectrum (the ground state to LEPT state). For the triad, PCEnT occurs in the absence of detectable spectral overlap between the donor emission and acceptor absorption spectra, even though conventional energy transfer theories require adequate spectral overlap. In this case, the LEPT vibronic state that dominates the PCEnT process contributes negligibly to the acceptor absorption spectrum due to the small Franck-Condon overlap associated with proton transfer. Energy transfer in PCEnT requires accompanying proton transfer to lower the energy of the LEPT state and enable sufficient spectral overlap that overcomes the small proton vibrational wave function overlap associated with proton transfer. This work enhances fundamental understanding of the PCEnT process and provides guidance for the design of other types of PCEnT systems.