FLUORESCENCE PROPERTIES AND EXCITED STATE INTERACTIONS OF 7-HYDROXY-4-METHYLCOUMARIN LASER DYE

Abstract The effect of solvent on some photophysical properties of 7-hydroxy-4-methylcoumarin (4-methylumbelliferone) is examined. Light absorption results in the population of a locally excited (LE) first singlet state (S 1 , ππ*) mixed with nπ*. This is characterized by extensive non-radiative energy dissipation on the subnanosecond time scale, which shows a sensitivity to environmental factors (solvent polarity and hydrogen-bonding ability) that may be exploited profitably in various systems of industrial and biological importance. Moreover, excellent linear correlations are established between some photophysical properties of 7-hydroxy-4-methylcoumarin and some solvent properties. The results of discussed on the basis of quantum chemical data obtained by application of the CNDO method. The fluorescence spectra in alcoholic solvents and water show complex formation via hydrogen-bonding interactions. The equilibrium constants K for complex formation (dye—water followed by ultrafast proton transfer) are determined at various temperatures. Small K values in the range 13 - 19 1 mol −1 are obtained. Some thermodynamic parameters are also determined. The results indicate an associative excited state complex with a dissociative ground state. The weak excited state complex formed with water is characterized by an extended strong blue-green fluorescence which can be attributed to tautomer formation via intramolecular proton transfer.