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Studies of 2-azaazulenium derivatives: unsymmetrical trimethine cyanine dyes bearing a 2-azaazulenium moiety as one of the terminal groups.

We report here the synthesis of a series of symmetrical and unsymmetrical trimethine cyanine dyes derived from 2-azaazulene, combined spectral and quantum-chemical investigations of their molecular geometry and electron structure, as well as the nature of the lowest electron transitions. Based on the analysis of both calculations and experimental data obtained from absorption and (13)C NMR spectra, it was concluded that the 2-azaazulene residue can be treated as a weakly basic terminal group; its donor properties are provided with the participation of the HOMO-1, in contrast to the typical Brooker's terminal residues with their donor HOMOs. The new classification of the terminal groups of cyanine dyes, and hence the classification of types of unsymmetrical cyanines, is proposed. It is shown that the nature of the higher electron transitions (delocalized or local) in the cyanine dyes depends on their type. In the unsymmetrical trimethine cyanine of the mixed type, negative deviations are observed in their absorption spectra.