Solvatochromism and thermochromism of rhodamine solutions

Article abstract of DOI:10.1016/0584-8539(86)80095-2

Electronic absorption spectra of rhodamine B have been examined in 16 protic and 14 aprotic solvents.Rhodamine B exists in solution as a highly colored zwitterion, a colorless lactone and an intensely colored cation.Protic solvents stabilize the zwitterion: the position of the lactone-zwitterion equilibrium depends upon solvent-dye hydrogen bonding and solvent dielectric/polarizability properties.From its temperature dependencene, thermodynamic parameters (ΔG⁰, ΔH⁰, ΔS⁰) for this equilibrium were determined in water, n-propanol and n-octanol.The zwitterion is not stable in aprotic solvents, in which the cation or lactone is favored according to the acidity or basicity of the medium.Absorption spectra of rhodamine 6G, an esterfied dye which does not exhibit the above equilibria, are only modestly altered by most variations of solvent and temperature.