Experimental and computational studies of protolytic and tautomeric equilibria of Erythrosin B and Eosin Y in water/DMSO

Abstract

Xanthene dyes Eosin Y (EOS) and Erythrosin B (ERY) are photosensitizers that present a complex protolytic system. To understand how the media affects their properties, we correlated the experimental pK_a in water/DMSO with theoretical calculations by molecular modeling approaches based on their tautomer's energy. It shows that in EOS the phenolic group is more acidic than the carboxylic group due to the presence of bromine atoms. The iodine in ERY, through the stability of tautomers involved in the protolytic forms, drives pK_a-COOH < pK_a-OH in water-rich media and the inversion pK_a-OH < pK_a-COOH in DMSO-rich media caused by the solvation that affects its tautomeric equilibria. For EOS, the possible protolytic equilibria are: NEL ⇌ MAF or NEQ ⇌ MAF as pK_a1 = pK_a-OH and MAF ⇌ DA as pK_a2 = pK_a-COOH in the range of 0 to 70% of DMSO in water. For ERY at above 35% DMSO in water, the pK_a-OH < pK_a-COOH came from the large amount of MAF and NEL, indicating that these tautomers may be responsible by the inversion. These effects originate from different electronic delocalizations influenced by the overlap between the σ natural bond orbitals of C–I (ERY) or C–Br (EOS) with π* of C–C, higher for ERY than for EOS. The simulated spectra permitted the confirmation of experimental finds of molar fractions. The analysis of the molecular orbitals confirmed that the main changes in absorption profile are due to HOMO–LUMO π–π* transitions related to the phenolic group. The results allowed understanding of the influence of the environment on preferential tautomers and pK_a.