A series of novel monomethine cyanine dyes derived from 2‐azaazulene have been synthesized. Combined spectral and quantum‐chemical investigations of their molecular geometries and of their electronic structures and the natures of their lowest electron transitions have been performed. The analysis results obtained by ab initio and semiempirical methods, together with experimental data obtained from absorption and 13C NMR spectra, have shown that there is practically no difference between the charge distributions over the π‐electron systems in the ground state in a reference 2‐azaazulenium salt and in monomethine cyanines bearing 2‐azaazulene as a terminal group, because these are not dependent on the relative locations of molecular orbitals. However, the spectral properties of – and correspondingly the natures of electron transitions in – the chromophores of the salt and of the monomethine cyanine are drastically different, due to the different mechanisms of frontier molecular orbital generation. Similar differences in properties in ground and excited states are observed for unsymmetrical monomethine cyanines. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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.

No abstract