The 1H-NMR and quantum chemical analysis of the stability of tautomers of cinnamoyl pyrone derivatives and vinylogs has been studied. The relationship between the structure of the most stable tautomer and its spectral properties has been investigated. It has been determined that the tautomer of highest stability (88–100 molar %) has an α-pyrone structure and exhibits a trans-conformation in the cinnamoyl fragment. An intense fluorescence of dyes has been observed in non-polar solvents with cinnamoyl fragments having electron-donating substituents or several double bonds in the polymethine chain. A gradient in solvent polarity resulted in fluorescence quenching which permits the use of the dyes as intensometric fluorometric probes for medium polarity examination. <alternatives> [...] </alternatives>
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