Solvent-dependent steady-state fluorescence spectroscopy for searching ESPT-dyes: solvatochromism of HPTS revisited

We reinvestigated the solvatochromism of 8-hydroxypyrene-1,3,6-trisulfonate (pyranine) in conjunction with that of 8-methoxypyrene-1,3,6-trisulfonate and of 1-hydroxypyrene (pyrenol) by use of 25 different solvents. Conclusions for the prediction of ESPT behaviour of synthetic dyes were drawn by comparison with the solvatochromism of p-hydroxystyryl Bodipy dyes. Solvents were chosen according to their Kamlet-Taft parameters alpha and beta for elucidating the acidicity of the dyes and the basicity of their conjugated bases in the ground and excited state. Comparison of the spectra of pyranine and pyrenol in solvents with varying beta-values revealed that the acidity of both dyes is similar therein. The well-known ESPT behaviour of pyranine in water is assigned to a change of the electronic state at alpha-values similar to 0.7 to 0.8. The high acidity of this excited state also appears in the vanishing solvatochromism of the photoproduct fluorescence. However, prediction of an ESPT tendency of synthetic dyes might fail when only fluorescence emission data are considered. We propose to refer instead to the energetic difference of the 0-0 transition in absorption together with the solvatochromism of the acidic form in aprotic solvents of similar polarity.