The unusual solvatochromism and solvatofluorochromism of longwave absorbing and emitting barbiturate merocyanine dyes

Spectral-fluorescent properties of a series of merocyanine dyes comprising the barbituric acid residue as the electron-accepting terminal group are investigated in comparison with those of their N,N-methylated analogues in media of various polarity. It is revealed that in polar aprotic solvents the electronic absorption spectra of the studied compounds are influenced dramatically by the formation of hydrogen bonds between the NH-groups of barbituric residue and solvent molecules. An effect of such nucleophilic solvation on the electronic structure of the studied dyes is analysed using both the spectral data obtained and the DFT/B3LYP quantum chemical simulation. It is found also, that solvation has comparatively weak influence on the shape and position of the fluorescence bands of the studied merocyanines while the fluorescence quantum yield changes substantially in solvents of various polarity. (paper)