Photophysical properties of coumarins in relation to the nature of their third and seventh substituents

The spectral, luminescent, and lasing properties of four derivatives of 7-hydroxycoumarin: 7-hydroxy-3-(4-methylthiazolyl)-coumarin (CI), 3-(4-methylthiazolyl)-2-oxocoumarin-furan-2-carboxylate (CII), 3-(4-(benzo1,3]dioxolyl)-thiazolyl)-2-oxocoumarin-furan-2-carboxylate (CIII), and 3-(4-(benzo1,3]dioxolyl)-thiazolyl)-2-hydroxycoumarin (CIV), as well as of their protolytic forms, are studied experimentally and theoretically by the INDO/S method. The luminescence quantum yield of the CIII and CIV compounds, having bulky substituents at position 3, is shown to strongly depend on the polarity of a solvent. It is suggested that, in polar solvents, these substituents are displaced out of the plane of the skeleton of the electronically excited molecule. The quantum-chemical calculations of such structures confirmed their weak fluorescence ability due to intense intersystem crossing. The lasing ability of the protolytic forms of the compounds considered is studied. The reason for the absence of lasing for the neutral compounds is discussed.