Fluorescence and flash photolysis comparison of 1-hydroxynaphthalene-2-sulphonate and 1-hydroxynaphthalene-4-sulphonate ions in aqueous solutions of various acidities

Abstract

The pH dependences of fluorescence intensities and flash photolysis transients are reported for 1-hydroxynaphthalene-2-sulphonate and 1-hydroxynaphthalene-4-sulphonate in aqueous solution. From the fluorescence and phosphorescence spectra and pK(S₀) values of 9.58 (2-sulphonate) and 8.27 (4-sulphonate), pK(S₁) values of 0.4 and –0.1 respectively, and pK(T₁) values of 7.9 and 7.2, are estimated. The pK(T₁) estimates agree moderately well with the direct (but approximate) values 7.5 and 7.1 from the flash photolysis transient intensities, and the pK(S₁) estimates are shown to be acceptably consistent with pK(S₁)– log₁₀τ/τ′ determinations from the fluorescence intensities, τ and τ′ being the lifetimes of the naphthol and naphtholate forms respectively. Though both forms of the 2-sulphonate fluoresce, thus resembling 2-naphthol rather than 1-naphthol, the fluorescence intensity of the naphthol form behaves anomalously in remaining constant throughout the pH range 1–8, even when that of the naphtholate form is changing markedly. This and other peculiarities of 1-hydroxynaphthalene-2-sulphonate suggest the presence of an internally hydrogen-bonded form which fluoresces because it avoids the quenching attributable to hydrogen bonding to the solvent.

An increase in the fluorescence intensity of the naphthol forms at higher acid concentrations is shown to be caused by the added anions, which presumably interfere with the solvent quenching mechanism. The perchlorate ion has a particularly large enhancing effect, similar to that of alcohols.