Nature of the lowest electronic transitions of thia- and quinocyanines with fluorinated polymethine chain

•Thia- and quinocyanines with fluorinated tri- and pentamethine chain were studied.•Fluorine substituents are causes the considerable changes of the atomic charges.•Obtained data show the fluorination lead to the small C–C bond lengths changes.•The S0→S1 transition is more sensitive to the fluorine substituents than the S0→S2.•Changes in spectral characteristics are caused by molecular orbitals shifting.

Quantum-chemical and spectroscopic study of fluorine atoms as substituents in the trimethine and pentamethine chain influence on the electronic structure in polymethine dyes has been performed. According to the calculation data that were experimentally confirmed by RCA and 13C NMR spectroscopy fluorination of polymethine chain leads to the small C–C bond lengths changes in chromophore and appreciably changes of the atomic charges. It was established that the introduction of fluorine atoms in the chromophore is accompanied by the significant shifting of the frontier and nearest levels and hence by changing the electronic transitions energies. The simultaneous analysis of the absorption and fluorescence excitation anisotropy spectra as well as results of the calculations of electronic transitions has shown that the S0 → S1 transition is more sensitive to the fluorine substituents than the S0 → S2 transition.

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