Caught in the act: Observation of the solvent response triggered by excited-state proton transfer in real time

Real-time observation of the solvent response following Excited State Proton Transfer (ESPT) of the photoacid HPTS into water using Optical Pump THz Probe (OPTP) spectroscopy from 0.1 ps up to 300 ps is reported. Subsequent to an instantaneous (< 0.2 ps) electronic response of the solute to photoexcitation, an oscillation with a period of 4 ps involving an intermolecular H (pyranine) - O (water) mode is observed. While for the methylated derivative, MPTS, and the deprotonated photoacid this oscillation relaxes on a time scale of 1.5 – 2 ps, for HPTS the oscillation decays more rapidly within 0.4 ps, which marks the onset of proton transfer. Energy transfer from the excited solute to the solvent takes place on a time scale of 120 ps and is proportional to the Stokes shift associated with energetic relaxation from the Franck-Condon region to the ground state of the photoexcited HPTS.