Structure and fluorescence properties of merocyanine dyes derived from dimethylbarbituric acid

The fluorescence properties of positively and negatively solvatochromic di-, tetra-, and hexamethinemerocyanines derived from 1,3-dimethylbarbituric acid in solvents of various polarities were studied. The range of solvatofluorochromic effect for these compounds is narrower than the range of solvatochromic effect. Extension of the polymethine chain of these compounds causes in the fluorescence spectra, in contrast to the absorption spectra, an increase in vinylene shifts, a decrease in deviations, and band narrowing. The electronic structure of the merocyanines was analyzed by the AM1 method. Transitions between the ideal states (neutral polyene, polymethine, and charged polyene) were examined. The electronic structure of the merocyanines in the excited state was found to approach the cyanine limit. Its attainment accounts for a sharp increase in the quantum yields of the fluorescence and a decrease in the Stokes shifts in going to higher vinylogs, and also with an increase in the solvent polarity for positively solvatochromic merocyanines and with its decrease for the negatively solvatochromic derivatives.