Comparing Nuclear Reaction Coordinates Driving Photoinduced Proton-Coupled Electron and Energy Transfer

Proton transfer is coupled to electron transfer or electronic excitation energy transfer in proton-coupled electron transfer (PCET) and proton-coupled energy transfer (PCEnT) reactions, respectively. Although photoinduced PCET and PCEnT have been studied extensively in anthracene-phenol-pyridine molecular triads, the nuclear reaction coordinates for these two pathways are still unknown. Herein, we identify the nuclear reaction coordinates for PCET and PCEnT in the triads by sampling the seam space between the relevant intersecting diabatic states. The reaction coordinate has contributions mainly from anthracene and phenol for PCET but involves the entire triad, including pyridine, for PCEnT. Both reaction coordinates involve motions related to contraction of the phenol C-O bond as well as motions within the anthracene rings that differ for PCET and PCEnT due to different charge distributions. The proton acceptor motion contributes more to the PCEnT than the PCET reaction coordinate. Both PCET and PCEnT reactions are concerted in that there is no stable intermediate, but they are both most likely asynchronous. Our analysis also suggests that PCET proceeds through a conical intersection in its minimum energy pathway, whereas PCEnT moves through an avoided crossing. The quantum mechanical treatment of the transferring proton within a vibronically nonadiabatic framework suggests that the kinetic isotope effect may be greater for PCEnT than for PCET, although more extensive calculations are required for confirmation. Understanding the fundamental similarities and differences between PCET and PCEnT mechanisms will help guide the design of proton-coupled systems that favor either charge transfer or energy transfer.