Investigation of excited-state proton transfer in 2-naphthol derivatives containing a carboxyl group in organic solvents and in methanol–water mixtures

Abstract

Excited-state proton transfer (ESPT) in 2-naphthol derivatives with electron withdrawing substituent (SO 2NH) dissolved in methanol and methanol–water mixtures is investigated. The analogs studied contain also a carboxyl group, situated at various distances from the proton donating phenolic group. The method of investigation consists in measuring the steady state fluorescence spectra. The parameter R of these spectra (ratio of fluorescence quantum yields at two defined wavelengths) given by Weller’s equation was fitted to the experimental values of R as a function of water concentration in the mixtures. From this procedure, ESPT rate constant and other parameters of this reaction were obtained. It was found that the carboxyl group contained in the molecule of 2-naphthol derivative greatly enhances ESPT reaction in methanol. This effect depends on the flexibility of a chain linking the aromatic system with the carboxyl group. ESPT reaction in five various analogs of 2-naphthol with an electron withdrawing substituent and a carboxyl group was compared. Influence of water addition and the fluorophore concentration, on ESPT was studied in detail. The effect of organic acid addition to the methanolic solution was also investigated. Influence of other organic solvents on ESPT is compared to that of methanol. Our results indicate that proton transfer to the carboxyl group and a preferential binding of other polar molecules play an essential role in the kinetics of proton transfer reaction The role of these catalytic effects is more pronounced in low polar media than in water solutions. By analogy to our model systems, conclusions concerning the proton transfer reaction in biological energy transformation systems can be drawn.