Rodríguez-Prieto, FlorCosta Corbelle, CarlosFernández Rodríguez, BertaRíos Rodríguez, María del CarmenMosquera González, Manuel2018-07-182018-11-242018Rodríguez-Prieto, F., Corbelle, C., Fernández, B., Pedro, J., Ríos Rodríguez, M., & Mosquera, M. (2018). Fluorescence quenching of the N-methylquinolinium cation by pairs of water or alcohol molecules. Physical Chemistry Chemical Physics, 20(1), 307-316. doi: 10.1039/c7cp07057h1463-9076http://hdl.handle.net/10347/17054This is the peer-reviewed version of the following article: Physical Chemistry Chemical Physics 2018, 20, 307–316. DOI: 10.1039/C7CP07057H, which has been published in final form at http://pubs.rsc.org/en/content/articlelanding/2017/cp/c7cp07057h. This article may be used for non-commercial purposes onlyN-Methylquinolinium cation (MQ+) in its first-excited singlet state is a strong oxidant commonly used as a photosensitizer, whose fluorescence is therefore quenched by electron donors. Interestingly, the fluorescence of MQ+ is also quenched by hydroxy compounds such as water and alcohols, more difficult to oxidize. We investigated the quenching mechanism of MQ+ fluorescence by small amounts of water and alcohols in acetonitrile solution. The fluorescence intensities and lifetimes exhibited a nonlinear dependence on the quencher concentration. We found evidence that emissive exciplexes MQ+*-ROH are formed between the excited quinolinium and the hydroxy compounds. An accurate quantitative description of the results was obtained with a model in which the exciplex reacts with a second molecule of the hydroxy compound, which quenches the fluorescence. The rate constant of this process increased as the quencher ionization energy decreased. We showed also that a low basicity of the hydroxy compound inhibits the quenching process. These results are consistent with the existence of a concerted photoinduced proton-coupled electron transfer (PCET) involving an intermediate complex of the excited quinolinium with a H-bonded molecular pair of the hydroxy compounds. In these pairs, a water or an 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. The structure of the intermediate complex was investigated using high-level quantum mechanical calculations. At high water concentrations in acetonitrile/water mixtures, the quenching process is slowed down, indicating that higher water aggregates are less effective for a PCET process. The results obtained may be relevant to the study of water oxidation and electron transfer in biological systemseng© The Royal Society of Chemistry 2018journal article10.1039/C7CP07057H1463-9084open access