Kulinich A. Synthesis and spectral properties of merocyanine dyes based on malononitrile and barbituric acid

Series of new di- tetra- and hexamethine merocyanine dyes with heterocycles of weak, medium and strong electron donating abilities based on malononitrile and barbituric acid are synthesized. The dependence of the spectral-fluorescent properties of merocyanines on donor-acceptor properties of their terminal groups, polymethine chain length, and the solvent nature is ascertained. It was pioneered the analysis of the band shape in the electronic spectra of merocyanine. The influence of the chemical structure and the solvent nature on the electronic structure of the synthesized dyes and their spectral properties is characterized. The thermochromism and thermofluorochromism of the merocyanines in comparison with the corresponding cationic and anionic polymethines is explored. For the first time the fluorescent properties of merocyanines are systematically investigated. The violation of the mirror symmetry law for absorption and fluorescence spectra of merocyanines, the approach of the excited state of merocyanines to the cyanine limit, and the drastic growth of the fluorescence quantum yields with the polymethine chain lengthening are shown. The conclusions about the electronic structure of the merocyanines are verified with the data of NMR spectroscopy, X-ray diffraction analysis, and also quantum chemical calculations.