Proton transfer and acidity constant in the excited state of naphthols by dynamic analyses

Dynamic behavior of the excited singlet state of naphthols in a sulfuric acid-water mixture at 300 K has been studied by means of nanosecond time-resolved spectroscopy and fluorometry. On the basis of dynamic analyses involving proton-induced fluorescence quenching processes, proton dissociation (k1) and association (k2) rate constants at 300 K were determined: α-naphthol, k1 = 8.6(+-0.3) x 1107 s-1, k2 = 1.0(+-0.2) x 11010 M-1s-1, β-naphthol, k1 = 5.8(+-0.3) x 1107 s-1, k2 = 2.7(+-0.3) x 11010 M-1s-1, M-1s-1. Using the values of k1 and k2, the correct values are: pK(αNL) = 2.1 +- 0.2 and pK(βNL) = 2.7 +- 0.2. It is found that proton-induced quenching processes are present both in the excited states of neutral α-naphthol[ksub(q) = 7.5(+-0.5) x 1108 M-1s-1] and α-naphtholate anion [k'sub(q) = 1.2(+-0.1) x 11010 M-1s-1]. However, for β-naphthol the rate constants of ksub(q) and k'sub(q) are negligibly small compared with those of competitive processes showing that a simple acid-base equilibrium is established in the excited state. Isotope effects on the rate constants and pK values were also examined. (orig.)