Electronic transitions in polymethine dyes involving local and delocalized levels

•Polymethine dyes producing delocalized and local molecular orbitals have been investigated.•The local orbitals are characterized by the charge mainly localized within the terminal groups.•Their existence leads to a generation of the specific quasi-local electronic transitions.•They differ from the transitions between delocalized orbitals by sensitivity to the chain length.•The quasi-local transitions can be experimentally detected by absorption or anisotropy spectra.

Several series of polymethine dyes containing terminal groups, which can generate the local levels close to the HOMO/LUMO energy gap, have been investigated by spectroscopic and quantum-chemical methods. The analysis of the obtained data has shown that the participation of the local levels in the electronic transitions leads to the appearance of the specific quasi-local transitions which differ from the transitions between delocalized molecular orbitals by their sensitivity to the length of the π-conjugated chromophore and to the chemical constitution of the terminal groups. These quasi-local transitions can be experimentally detected by measuring of the ordinary absorption spectra or by the excitation anisotropy spectra, in case when their low-intensive bands are covered by the intensive absorption band. In the unsymmetrical dyes, containing different terminal groups, the delocalized and quasi-local transitions can be mixed producing complicated absorption spectra with two comparatively intensive bands, and their shapes can be gradually transformed upon the lengthening of the π-conjugated chromophore.