Poling behaviour of two-dimensional charge transfer molecules

Evolution of absorption spectra with molecular alignment induced by electric field poling in two-dimensional charge-transfer molecules has been studied experimentally and theoretically, and compared with that in one-dimensional charge-transfer molecules. In contrast to 3-monosubstituted carbazoles which possess one-dimensional charge-transfer character and in which the absorbance at λ_max decreases after poling, an increase in the absorbance at λ_max after poling has been observed in 3,6-disubstituted carbazoles with two-dimensional charge transfer character. We have performed molecular orbital calculations for these molecules, and obtained molecular polarizability spectra for random and uniaxial orientations. The theoretical results were able to explain the poling behaviour in terms of the vector directions of the transition and ground-state dipole moments of the molecules. Conformers were also studied theoretically, and it has been found that the absorption spectra of disubstituted carbazoles and their evolution upon poling are strongly affected by the conformation of the acceptor groups.