A spectroscopic/thermodynamic study of the rhodamine B lactone ⇌ zwitterion equilibrium

Abstract Temperature dependences of the visible absorption spectra of rhodamine B (RB) in water and 18 alcohol solvents have been examined and utilized to estimate thermodynamic parameters (Δ G °, Δ H °, Δ S °) for the RB lactone ⇌ zwitterion (L ⇌ Z) equilibria in these sol From the L ⇌ Z equilibrium in the strong hydrogen-bonding solvent trifluoroethanol an intrinsic RB zwitterion molar absorptivity of e = 1.30 × 1O 5 dm 3 /mol·cm is estimated. The thermodynamic results suggest that the position of the RB L ⇌ Z equilibrium is strongly influence by self-association of the solvent. The position of the equilibrium in aliphatic alcohols can be predicted from simple features of the solvent molecules. Consequences of the results regarding calculation of the natural radiative lifetime of the fluorescent state from the absorption spectrum are discussed.