Absorption, fluorescence, and acid-base equilibria of rhodamines in micellar media of sodium dodecyl sulfate

Rhodamine dyes are widely used as molecular probes in different fields of science. The aim of this paper was to ascertain to what extent the structural peculiarities of the compounds influence their absorption, emission, and acid-base properties under unified conditions. The acid-base dissociation (HR⁺ ⇄ R + H⁺) of a series of rhodamine dyes was studied in sodium n-dodecylsulfate micellar solutions. In this media, the form R exists as a zwitterion R^±. The indices of apparent ionization constants of fifteen rhodamine cations HR⁺ with different substituents in the xanthene moiety vary within the range of pK_a^app = 5.04 to 5.53. The distinct dependence of emission of rhodamines bound to micelles on pH of bulk water opens the possibility of using them as fluorescent interfacial acid–base indicators.