RESUMEN
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.
RESUMEN
2-Naphthol (2NOH) was used as a fluorescent probe in order to examine and quantify the changes in hydrogen ion concentration in micelles formed by the zwitterionic 3-(tetradecyldimethylammonium)-propanesulfonate (SB3-14) surfactant or by anionic sodium dodecyl sulfate (SDS). In the presence of SDS, 2NOH is incorporated into the anionic micelle and the neutral form of the probe becomes the dominant species. The results are consistent with a microenvironment probably with a higher acidity and/or lower polarity in the micellar surface. The addition of SB3-14 generates a plateau at pH 3 to 9 with a fluorescent component of low intensity, which indicates the partial formation of 2NO(-)*, promoted by proton transfer to water. Theoretical results provided information on the structural parameters, emission wavelength, and changes in ΔpK(a) values due to the solvent, which are consistent with a solubilization site similar to aqueous ethanol. Zwitterionic surfactants concentrate anions such as trifluoroacetate in zwitterionic micelles, and as a result, the micellar surface charge becomes negative and promotes hydrogen ion incorporation into the micellar surface. Effects observed on the proton transfer between 2NOH* and anions in zwitterionic micellar solutions are complex and, besides the well-known anion incorporation, include changes in the surface potential and acidity of the surface. Zwitterionic micelles are able to emulate the mostly zwitterionic nature of biological membranes, and in the complex nature of zwitterionic micelles, we found reasons for the selection of zwitterionic headgroups in surfactants in natural systems as major components of biological interfaces.