Solvatochromic effects on the fluorescence and triplet–triplet absorption of phenosafranine in protic and aprotic solvents

Photophysical and spectroscopic properties of phenosafranine were investigated in 21 solvents, including pure aprotic solvents and pure and binary protic solvent mixtures. It was found that these properties are highly dependent of the nature of the medium in protic solvents. Characteristic parameters such as the Stokes’ shift, the fluorescence quantum yield and lifetime and the T–T absorption maxima frequencies showed a very clean solvent dependence, with a strong incidence of the specific interactions of phenosafranine with solvent molecules, most likely through hydrogen bonding. On the other hand, in aprotic solvents these parameters are practically insensitive to changes in solvent properties. The experimental results in protic media suggest an increase of the dipole moment of the dye molecule as a consequence of the S0 → S1 transition, while a decrease in the dipole moment come along with the T1 → Tn transition. In selected protic solvents, the photophysical kinetic parameters were obtained from fluorescence quantum yield and lifetime determinations and intersystem-crossing quantum yields. Their solvent dependence is discussed.