Excited state reaction kinetics of 4-hydroxy-1-naphthalenesulphonate in various alcohol-water mixtures

Abstract

Proton transfer reaction from the excited state of 4-hydroxy-1-naphthalenesulphonate has been investigated in methanol-water, ethanol-water, 1-propanol-water mixtures at different temperatures. A mode-locked Nd:YAG laser in combination with a cavity-dumped synchronously-pumped dye laser is used to excite the probe. Fluorescence decay and time-resolved spectra are recorded with picosecond photon counting spectrometer. The decay is single exponential in all solutions and proton transfer rate is found to increase with increasing temperature and water concentration. A cluster of two water molecules is found to be the proton acceptor. Two new models are proposed to explain the proton transfer process of the probe molecule in methanol- and ethanol-water mixtures and 1-propanol-water mixture. The addition of calcium chloride to the solvent mixtures is observed to decrease the dissociation rate of excited 4-hydroxy-1-naphthalenesulphonate. The faster dissociation rate of the excited probe in the presence of urea is due to the acceptance of the dissociated proton by urea. Based on time-resolved spectra, two possible mechanisms have been proposed to explain the proton transfer process to water in methanol- and ethanol-water mixtures and 1-propanol-water mixture.