Salt effects on steady-state quantum yields of ultrafast, diffusion-influenced, reversible photoacid dissociation reactions

The steady-state fluorescence quantum yields of the protonated and deprotonated forms of 8-hydroxypyrene-1,3,6-trisulfonate are determined in three solvents as a function of NaNO₃ concentration. The measured quantum yields, and therefore also the overall dissociation rate coefficients, decrease with increasing salt concentration, in contrast to Brönsted's classical model for primary salt effects. This is a consequence of the reversibility and the nearly diffusion-controlled recombination rate of the proton in the reaction studied. A quantitative analysis is based on newly derived results for reversible diffusion-influenced reactions. Comparison with experiment is made for two screened potentials. Numerical integration with the Debye-Hückel potential indicates that it overestimates the salt effect. A second model (the "naive approximation") is based on a simple screened potential from which analytical expressions for reaction yields and rates are derived. It shows better agreement with experiment in the range 20-200 mM salt.