Caught in the act: Excited-state Proton Transfer to the solvent followed in real time by non-linear THz spectroscopy

We have investigated the solvent response after photoexcitation using Optical Pump THz Probe (OPTP) spectroscopy with sub-ps time resolution in combination with quantum-chemical modeling. Excited State Proton Transfer (ESPT) is found to be facilitated by excitation of an intermolecular stretch (H_pyranine-O_water) triggered by an instantaneous (<; 100 fs) redistribution of charge density. Initiation of proton transfer is observed on a time scale of 0.4 ps, i.e. on a much shorter time scale than anticipated before. Our results demonstrate the exceptional sensitivity of OPTP, which enables us to follow the proton motion into the hydrogen bond network of water exploring charge transport in transient solvent configurations in real time.