Proton-transfer kinetics of photoexcited 7-hydroxy-1-naphthalenesulfonic acid in aqueous formamide solutions

The monoanion of 7-hydroxy-1-naphthalenesulfonic acid (HNS) undergoes pseudo-first order dissociation and its conjugate base, second order protonation in the lowest excited singlet state. The proton transfer kinetics in water containing formamide up to a mole fraction of about 0.95 have been evaluated as a function of formamide concentration. At mole fractions above 0.95 of formamide, proton-transfer does not measurably occur. At mole fractions below 0.95, steady state and pulsed-source fluorimetries show the rate constant for dissociation to decrease exponentially with increasing mole fraction of formamide. This is believed to be due to penetration and disruption of the aqueous solvent cage of the HNS by formamide, resulting in impairment of the Grotthus proton-transfer mechanism.