Simulation of Pump–Push Molecular Dynamics in the Heptazine–H₂O Complex

Pump-push-probe spectroscopy was employed for the exploration of charge-separation processes in organic photovoltaic blends as well as for proton-coupled electron-transfer (PCET) reactions in hydrogen-bonded complexes of trianisole-heptazine with substituted phenols in organic solvents. In the present work, the electron and proton transfer dynamics driven by a femtosecond pump pulse and a time-delayed femtosecond push pulse has been studied with <i>ab initio</i> on-the-fly non-adiabatic trajectory calculations for the hydrogen-bonded heptazine-H₂O complex. While the dynamics following the pump pulse is dominated by ultrafast radiationless energy relaxation to the long-lived lowest singlet excited state (S₁) of the heptazine chromophore with only minor PCET reactivity, the re-excitation of the transient S₁ population by the push pulse results in a much higher PCET reaction probability. These results illustrate that pump-push excitation has the potential to unravel the individual electron and proton transfer processes of PCET reactions on femtosecond time scales.