Structure and fluorescent properties of merocyanines based on N,N-diethylthiobarbituric acid

Fluorescent properties of a series of positively and negatively solvatochromic merocyanines based on N,N-diethylthiobarbituric acid containing heterocycles of various electron-donating properties are explored in solvents of various polarity. It is shown, that their solvatofluorochromic range is less than the solvatochromic range. A lengthening of the polymethine chain causes a growth of the vinylene shifts, a decrease of the deviations and a narrowing of the fluorescent spectral bands, as opposed to the absorption ones. The transition to the higher vinylogues is also accompanied by a decrease of the Stokes shifts and an increase of the fluorescent quantum yields. However, a growth of the latter for merocyanines absorbing light in the near-IR range is levelled by an amplification of internal conversion. An absence of a mirror symmetry of the absorption and fluorescence spectra has been revealed. The quantum-chemical analysis of an electronic structure of the merocyanines by the semi-empirical method AM1 is also carried out. The transitions between their ideal electronic states – non-polar polyene, polymethine and dipolar polyene – are investigated. It is shown, that the electronic structure of the fluorescent state of merocyanines approaches the cyanine limit at polymethine chain lengthening.