Excited state proton transfer in 1 : 1 complexes of 2-(oxazol-2-yl)-3-hydroxychromone with water

We extend our earlier theoretical study of excited state proton transfer (ESPT) in the low-lying conformers of 2-(oxazol-2-yl)-3-hydroxychromone (OHC) by exploring the dynamics of water-mediated ESPT in them. In the previous study [D. Sarma and S. G. Ramesh, Phys. Chem. Chem. Phys., 2025, 27, 11659-11672], OHC-A and OHC-B with intra-ring H-bonding showed a larger propensity for proton transfer compared to OHC-C with inter-ring H-bonding. Placing a water molecule between the H-bonding sites of these conformers, we have carried out a computational and dynamical study of the ESPT. Surface hopping simulations initiated on the first bright state of the complexes show that the fractions of water-mediated double proton transfer (DPT) events are roughly similar to the PT fractions of the corresponding monomers. However, the dynamics of the DPT for the water complexes of OHC-A and B are differentiated by whether they are of the donor-first type (OHC to water) or the acceptor-first type (water to OHC) across the H-bond bridge. The former is always found to be near-concerted, i.e. with a short time interval between the two PT events, while the latter shows varied time intervals between the two transfers. In contrast, the OHC-C-water complex shows only donor-first DPT. In the water complexes of OHC-A and B, several single proton transfer (SPT) events take place that show only acceptor-side PT and share many features with the acceptor-first DPT events, while the OHC-C-water complex shows few SPT events. The details of the features associated with each type of trajectory in these OHC-water complexes are discussed.