Solvent dependence of pyranine fluorescence and UV-visible absorption spectra

Fluorescence and UV-visible absorption spectra of HPTS (8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt, pyranine) were measured in a variety of solvents. Fluorescence maxima (in kcal mol^-1) can be correlated with the Kosower Z parameter (r = 0.901), the Dimroth-Reichardt E_T(30) parameter (r = 0.900) and the Winstein Y parameter (r = 0.916) using one-parameter fits. Good correlations (r = 0.98) were obtained for HPTS fluorescence in ethanol-water mixtures using the Y, Y_OTs and Z parameters. Fluorescence maxima of HPTS in aqueous sulphuric acid solutions gave an excellent correlation with Y_OTs (r = 0.991). Multi-parameter correlations, indicating the significance of specific solvent interactions, were also studied. In addition, fluorescence maxima correlate well with maximum/minimum ratios obtained from UV-visible absorption experiments. Results can be applied in the use of HPTS as a molecular probe of solvent environments and for extension of the Y_OTs scale in acidic solutions. HPTS is a unique molecular probe, not only because of its photoacidic properties and its widespread use as a pH-sensitive biosensor, but also because of its relative stability in acidic environments.