Studies of 2-azaazulenium derivatives – 3: The nature of electron transitions and spectral properties of styryl dyes containing terminal groups of different types

This paper presents the results of a quantum-chemical study of the molecular geometry and electron structure as well as spectral measurements of absorption and 13C NMR spectra of dimethylaminostyryls, methoxystyryls, and methylstyryls bearing 2-azaazlenium (AA) moiety in comparison with analogous dyes containing 2-benzimidazolim and 4-pyrylium residues. Based on the analysis of both calculations and experimental data, it was concluded that these types of extremely unsymmetrical cyanines differ between each other only slightly in the ground state with respect to the charge distribution and molecular geometry while their spectral properties reveal a considerable difference between dimethylaminostyryls and methoxystyryls due to the lowest disposition of the lone electron pair level of the oxygen atom and hence, limiting basicity of the p-methoxyphenylene residue. It was shown that appearance of a high-positioned local level generated by AA terminal group caused the inversion of the delocalized and quasi-local electron transitions in AA-methoxystyryl and hence drastic changes in its absorption spectrum.

► Cyanine dyes explored by spectroscopic methods and quantum chemical calculations.
► Classification of cyanine dyes is proposed basing on the research.
► Features in the absorption spectra of AA dyes caused by the local molecular orbital.