3.1 Dynamics of Condensed Phase Molecular Systems

Our research on photoinduced proton transfer between acids and bases, with foundations made in a larger set of activities on photo-induced elementary reactions followed with ultrafast infrared spectroscopy, can be divided into three major research periods. In the first period third party funding from the German-Israeli Foundation of Scientific Research (GIF-722; GIF-876), in the second period from the German Research Foundation (DFG NI 492/11-1 + DFG NI 492/13-1). Since 2018 funding comes through an ERC Advanced Grant (XRayProton, Nibbering, 2018-2024) and the German Research Foundation (DFG NI 492/13-2).

Phase 1 (2000-2010):

Photoacid-base neutralization reactions in aqueous solution have been investigated with ultrafast infrared spectroscopy. Proton transfer pathways have been identified, where the photoacid-base encounter pair with one water connecting the reactants have been shown to be of special importance as a pathway with sequential proton transfer.

Phase 2 (2011-2017):

Photoacid base interactions have been explored with IR and soft-x-ray spectroscopies. We derived a correlation between the O-H stretching frequency and hydrogen bond distance for photoacid-base complexes, after correction for solvent-indcued vibrational frequency shifts. We established a novel flatjet technique, enabling soft-x-ray spectroscopy in transmission mode. With this approach hydrogen bonding of amine bases and conjugate protonated amine acids has been investigated.

Phase 3 (2018-...):

Proton transport along solvent bridges is studied exploiting bifunctional photoacids, using a combined experimental (femtosecond UV-IR pump-probe spectroscopy) and ab initio quantum chemical calculations. The transient electronic structure of acids and bases and its associated dynamics will be explored during water mediated proton transport processes with time-resolved soft-X-ray spectroscopy. We have measured the oxygen K-edge of hydrated proton complexes and found an inner H₇O₃⁺ with strongly altered electronic structure surrounded by a hydration shell affected by the Coulombic interactions with the positive charge of the proton. Time-resolved nitrogen K-edge spectroscopy of an amine photoacid shows minor electronic structural changes on the acid side but large ones on the base side of the Förster cycle.