Environment-dependent photophysics of xanthene anionic fluorophores: Eosin Y vs Eosin E

Xanthene derivatives constitute an important class of oxygen-containing heterocycles with broad biological, pharmacological, and photonic relevance. In this study, the photophysical behavior of the anionic fluorescent dyes Eosin Y and Eosin E was systematically investigated across twelve solvents spanning a wide polarity range, with particular emphasis on solvent-induced modulation of their electronic transitions and spectral responses. Linear solvation energy relationship (LSER) analysis based on the Kamlet–Abboud–Taft (KAT) and Catalán models revealed the hydrogen bond acceptor ability (β, or SB depending on the solvatochromic model) as the predominant solvatochromic parameter governing the spectral behavior of both dyes. Excited-state dipole moments, estimated using the Bakshiev method, exhibited pronounced polarity-dependent enhancement, increasing from 1.32 to 5.37 D for Eosin Y and from 1.12 to 3.58 D for Eosin E. These trends are consistent with structural differences and molecular asymmetry introduced by the benzoate and ethyl ester substituents, which modulate electron delocalization and intramolecular proton-transfer dynamics within the xanthene framework. The substantial increase in dipole moment upon excitation provides compelling evidence for efficient charge separation and the involvement of intramolecular proton transfer processes. Combined with the Intrinsic Polarity Descriptor (IPD*), our results underscore the decisive role of solvent polarity in shaping the photophysical properties of xanthene-based dyes and enable a more reliable assessment of their performance as environment-sensitive fluorescent probes for potential applications.