The effect of solvent and micelles on the rate of excited-state deprotonation

This article describes a demonstration of the effect of solvent and micelles on the rate of excited-state deprotonation of the 8-hydroxyprene-1,3,6-trisulfonate (HPTS) ion. As is typical of hydroxy-aromatic molecules and ions, this ion is a weak acid in its ground electronic state but becomes a much stronger acid in its lowest excited singlet state. The ground and excited state pKas of the HPTS ion are 7.4 and 0.4, respectively. Thus, at pHs in the range between 3 and 6, the acid form of the HPTS ion tends to deprotonate when it becomes electronically excited. However, the rate of this excited-state deprotonation is very solvent dependent. In aqueous solution, the rate of excited-state deprotonation of the acid form of the ion is much faster than fluorescence and nonradiative decay. Thus, the ion emits almost exclusively from its conjugate base form which exhibits a green emission with a lamda max at 510 nm. In solvents with high ethanol content, the rate of excited-state deprotonation is much slower than that in aqueous solution and emission occurs mainly from the acid form of the ion. In such solvents, HPTS exhibits a violet emission with a lambda max at 455 nm which is characteristic of the acid form of the ion. The demonstration described in this article illustrates the colorful changes that occur in the fluorescence of HPTS when the solvent is changed from a mostly alcohol to a mostly aqueous solution. It also illustrates the effect that cetyltrimethylammonium bromide (CTAB) micelles have on the fluorescence of this ion.