In Search for the “Phenolate” Monoanion of Fluorescein in Solution

Abstract

Abstract Fluorescein is exclusively popular owing to bright fluorescence of its R2− dianion. While the structure of the latter and tautomerism of the H2R form are well established, only the species with COO− and OH groups has been supposed for HR− up to now. Using absorption, emission, and excitation at different pH in non-HBD solvents, DMSO, acetonitrile, and acetone, we reveal that 10–12% of HR− ions nevertheless exist as “phenolate” monoanion with nonionized COOH group.