Structure and spectral-luminescent properties of 6-isoxazolyl-7-hydroxycoumarins

Abstract

We have studied the particular features of the spectral and fluorescent properties of 6 isoxazolyl 7 hydroxycoumarin and its 7 methoxy substituted analog in aprotic and proton donor solvents of different polarity. Changes in the electron charge and bond orders in the ground and first excited states have been ana lyzed by the ab initio DFT and TDDFT quantum chemical methods with the B3LYP/6 31G(d,p) basis set. Based on this analysis, we have analyzed anomalously high Stokes shifts, the shape of vibronic bands, and the fluorescence ability of these compounds in aprotic solvents. It has been found that the fluorescence spectra of hetaryl substituted 7 hydroxycoumarin, in contrast with its absorption spectra in motional, DMSO, basic and acidic solutions, strongly differ from the fluorescence spectra of its 7 metoxy analog. We show that this is related to the proton detachment and transfer in the neutral form of 7 hydroxycoumarin upon photoexci tation with the possible formation of anion or tautomer depending on the pH of the medium.