Fluorescence of some simple umbelliferone derivatives

The absorption and fluorescence spectra of umbelliferone were studied; it was found that in the excited state the basicity of the α-pyrone part of the molecule increases to such an extent that it exceeds that of the phenolate oxygen of the molecule. This is responsible for the neutral nondissociated form of umbelliferone being only in nonpolar medium which prevents the dissociation of the phenolic group in the excited state. In nonpolar medium, umbelliferone behaves as a substance with closely lying π → π* and n → π* transitions. Hydrogen bonds affect the fluorescence of 7-methoxycoumarin so that the π → π* transition is preserved as the lowest excited singlet state.