Proton-transfer kinetics of photoexcited 7-hydroxy-1-naphthalenesulphonic acid in aqueous DMSO solutions.

The monoanion of 7-hydroxy-1-naphthalenesulphonic acid (HNS) undergoes pseudo-first-order dissociation and its conjugate base undergoes second-order protonation in the lowest excited singlet state. The proton transfer kinetics in water containing dimethylsulphoxide (DMSO), up to a mole fraction of about 0.4, have been evaluated as a function of DMSO concentration. At mole fractions above 0.5 of DMSO, proton-transfer does not measurably occur. At mole fractions below 0.5, steady-state and pulsed-source fluorimetries show the rate constant for dissociation to decrease exponentially with increasing mole fraction of DMSO. This is believed to be due to penetration and disruption of the aqueous solvent cage of HNS by DMSO, resulting in impairment of the Grotthus proton-transfer mechanism. The rate of neutralization of the conjugate base by hydrogen ion is found to vary only slightly with solvent composition and depends on the bulk dielectric properties of the solvent.