Proton-transfer kinetics of photoexcited 2-hydroxybiphenyl in aqueous methanol solutions

2-Hydroxybiphenyl 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 methanol up to a mole fraction of about 0.5, have been evaluated as a function of methanol concentration. At mole fractions above 0.51 of methanol, proton transfer does not occur. At mole fractions below 0.5, steady state and pulsed-source fluorimetry show that the rate constant for dissociation decreases exponentially with increasing mole fraction of methanol. This is believed to be due to penetration of the aqueous solvent cage of the 2-hydroxybiphenyl by methanol. 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.