Concerted Electron-Nuclear Motion in Proton-Coupled Electron Transfer-Driven Grotthuss-Type Proton Translocation

Photoinduced proton-coupled electron transfer and long-range two-protontransport via a Grotthuss-type mechanism are investigated in a biomimetic construct. Theultrafast, nonequilibrium dynamics are assessed via two-dimensional electronic vibrationalspectroscopy, in concert with electrochemical and computational techniques. A low-frequency mode is identified experimentally and found to promote double proton andelectron transfer, supported by recent theoretical simulations of a similar but abbreviated(non-photoactive) system. Excitation frequency peak evolution and center line slopedynamics show direct evidence of strongly coupled nuclear and electronic degrees offreedom, from which we can conclude that the double proton and electron transferprocesses are concerted (up to an uncertainty of 24 fs). The nonequilibrium pathway fromthe photoexcited Franck-Condon region to the E2PT state is characterized by an similar to 110 fstime scale. This study and the tools presented herein constitute a new window into hotcharge transfer processes involving an electron and multiple protons.