The published work shows that water (or alcohol) dimers play a fundamental role in the fluorescence quenching of N-methylquinolinium. Our results are consistent with the existence of a concerted photoinduced proton-coupled electron transfer involving an intermediate complex of the excited quinolinium with a H-bonded molecular pair of the hydroxy compounds. In these pairs, a water or alcohol molecule is able to donate an electron to the photoexcited quinolinium cation and a proton to the second H-bonded hydroxy molecule, showing an enhanced reducing power in comparison with the isolated molecule.

Our results may be relevant to the study of renewable energy sources, solar water splitting and solar fuel production, and contribute to understanding the puzzling photorelaxation and electron transfer mechanisms of biomolecules, where water molecules have been shown to play a fundamental role.

Fluorescence quenching of the N‑methylquinolinium cation by pairs of water or alcohol molecules Physical Chemistry Chemical Physics, 2017, DOI: 10.1039/C7CP07057H