Violation of Ideal Polymethine State in Merocyanines with a Long Chromophore
Keywords:merocyanine dyes, absorption spectra, quantum chemical calculations, “cyanine limit”
The spectral research of a number of merocyanine dyes, the derivatives of aminocoumarin and the corresponding vinylog series of symmetric anionic polymethine dyes, has been carried out. A special attention is focused on the sensitivity of the absorption curve shape to the solvent polarity, which introduces a substantial correction to the spectral properties of polymethine dyes. It is shown that the analyzed compounds reach an ideal polymethine state, the so-called “cyanine limit”, in weakly polar solvents. When changing to solvents with a higher polarity, a considerable growth of the spectral absorption band width is observed, as well as the disappearance of the vibrational transition peak, with the main maximum shifting toward short waves at that. It is found that the emergence of an additional wide short-wave band in the spectra of dyes that absorb in the near IR interval is associated with the violation of the charge distribution symmetry in the polymethine chain. It is also found that, besides the similarity between the absorption spectra of cyanines and corresponding symmetric dyes, the mobility of a charge distribution in the polymethine chromophore for those compounds is possible. Moreover, in strongly polar solvents, they can exist in two forms, which manifests itself in the appearance of an additional absorption band.
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