Properties and the nature of electric-field-induced variations in optical absorption spectra of polymethine dyes in polymer matrices

The effect of a permanent electric field on the absorption spectra of ionic (cationic and anionic) and intraionic symmetric and asymmetric polymethine dyes in poly-N-epoxypropylcarbazole, polyvinylbutyral, and polystyrene polymer films is studied. It is found that the electric field causes an increase in absorption of symmetric dyes in the short-wavelength region and a decrease in their absorption in the long-wavelength region. Asymmetric dyes exhibit opposite properties. These properties are common for these groups of dyes irrespective of their chemical structure and ionic nature. They are retained both in photoconducting polymers and in polymers that are incapable of charge photogeneration. These effects also take place in liquid solutions, although in this case they are much weaker. Various mechanisms of the influence of an electric field on the spectra, such as the electron transfer, Stark and Kerr effects, intermolecular interactions, and dye isomerization, are discussed. It is shown that the spectral effects are caused by the redistribution of the electron density in a chromophore of the dye in the ground state resulting in a change in the probability of vibronic transitions.