We have studied the characteristic features of the spectral fluorescence properties of natural compounds (umbelliferone and herniarin) in aprotic and proton-donor solvents of different polarities. Based on quantum chemical analysis by the AM1 semiempirical method of the changes in the electronic charge and bond orders in the ground state and the first excited state, we have interpreted the anomalously high Stokes shifts and mirror symmetry of the absorption and fluorescence bands of these compounds in aprotic solvents. We have established that the fluorescence spectra of umbelliferone in water, ethanol, alkaline and acidic solutions differ considerably from the analogous spectra of herniarin. This is connected with detachment and transfer of a proton in the neutral form of umbelliferone upon photoexcitation, with possible formation (depending on the pH of the medium) of an anion, cation, or tautomer. Good agreement was achieved between the calculated and experimental values of the maxima for the electronic bands of these forms.

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