Electronic properties of polymethine systems 7: soliton symmetry breaking and spectral features of dyes with a long polymethine chain

The dependence of the position and shape of the absorption bands on the length of the polymethine chromophore is quantum-chemically and spectrally investigated for the vinylogous series of the following dyes: pyrido-, thia- and thiapyrylocyanines. It is established that the symmetry breaking occurs in the cyanines containing the comparatively long chain, 4-6 vinylene groups. It is assumed that in the ground state the near-infrared polymethine dyes with the long enough polymethine chain may exist in two charged forms with symmetrical and asymmetrical disposition of charge wave or soliton. The distortion in the electron density distribution and π-bond occupancies in the asymmetrical form leads to the drastic widening of the long wavelength spectral bands and, simultaneously, to considerable decreasing of the absorption band intensity. It is shown that the spectral effects caused by symmetry breaking are strongly sensitive to the solvent polarity.