Poling behavior of optical absorption spectra in carbazoles with one- and two-dimensional charge-transfer character

Spectral change in optical absorption due to molecular alignment induced by electric poling has been studied experimentally and theoretically, in one- and two-dimensional charge-transfer carbazole molecules. After poling, an increase in absorbance at λ_max was observed in 3,6-dinitrocarbazoles which present a two-dimensional charge-transfer character, while the absorbance at λ_max decreased in 3-monosubstituted carbazoles which possess a one-dimensional charge-transfer character. From molecular-orbital calculations and evaluations of the molecular polarizability spectra for random and uniaxial orientations of the molecules, an explanation is provided for the poling behavior in terms of the vector directions of the transition and ground-state dipole moments of the molecules. Theoretical investigation of various conformers has been made, resulting in the suggestion that the poling behavior of absorption spectra should be strongly affected by the conformation of the acceptor groups in the case of disubstituted carbazoles.