BINARY SOLVENT EFFECTS ON THE ABSORPTION AND EMISSION OF 7-AMINOCOUMARINS

The photophysical properties of di-, mono- and non-ethyl 7-aminocoumar ins have been investigated in different water/ethanol mixtures. The ef fect of the solvent composition on the spectral shifts and on the radi ationless deactivation process depends on the molecular structure of t he dye. The solvent effect is analyzed in terms of hydrogen bondings b etween the dye and protic solvents. The mechanism for internal convers ion is discussed by two models: via a twisted intramolecular charge tr ansfer (TICT) state, and by an hybridational change of the amino group (open-close umbrella-like motion, ULM model). Both models can explain the experimental results if solute-solvent hydrogen bonds are conside red.