Studies of 2-Azaazulenium Derivatives: The Natures of Electron Transitions in the 2-Azaazulenium Cation and in Monomethine Cyanine Dyes Bearing 2-Azaazulenium Moieties as Terminal Groups

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 C-13 NMR spectra, have shown that there is practically no difference between the charge distributions over the pi-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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)