Theoretical insights into photoinduced proton transfer of 7-hydroxyquinoline via intermolecular hydrogen-bonded wire of mixed methanol and water

Proton transfer (PT) reactions of 7-hydroxyquinoline (7HQ) via intermolecular hydrogen-bonded wire of methanol, water and mixed methanol–water in ground (S) and first excited (S) states, [7HQ(X) when X = M-methanol and W-water], have been studied by using density functional theory (DFT) at B3LYP and its time-dependent DFT with 6-31+G(d,p) basis set. For all complexes, the intermolecular hydrogen bonds become shorter and the O–H stretching vibrational frequencies shift to lower frequencies in the S state, which confirm that the hydrogen bonding interaction is stronger in the S state. Moreover, the frontier molecular orbitals of all complexes were analyzed to confirm the PT reactions (ππ*). The simulated absorption and emission spectra of 7HQ(MMM) are in good agreement with the experimental data. In addition, the potential energy curves along the PT reaction coordinates of all complexes both in S and S states were scanned by constrained optimizations fixing the O–H bond distance (a proton donor site of 7HQ) to investigate the effect of different intermolecular hydrogen-bonded solvent wires surrounding 7HQ. PT reactions are found to be favorable in S state due to the low PT energy barrier. For pure solvent, the excited-stated proton transfer (ESPT) occurs faster via methanol than water. For mixed solvent, when replacing methanol with one up to three water molecules, PT energy barrier is found to be higher than that of 7HQ(MMM); therefore, water may block the ESPT reaction.