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Fluorescence quenching of the N-methylquinolinium cation by pairs of water or alcohol molecules

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Abstract

N-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 systems.

Graphical abstract: Fluorescence quenching of the N-methylquinolinium cation by pairs of water or alcohol molecules

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Publication details

The article was received on 16 Oct 2017, accepted on 23 Nov 2017 and first published on 24 Nov 2017


Article type: Paper
DOI: 10.1039/C7CP07057H
Citation: Phys. Chem. Chem. Phys., 2017, Advance Article
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    Fluorescence quenching of the N-methylquinolinium cation by pairs of water or alcohol molecules

    F. Rodríguez-Prieto, C. C. Corbelle, B. Fernández, J. A. Pedro, M. C. Ríos Rodríguez and M. Mosquera, Phys. Chem. Chem. Phys., 2017, Advance Article , DOI: 10.1039/C7CP07057H

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