2-D effects in the second-order optical nonlinearity of organic molecules incorporating carbazole

The second order nonlinearity (beta) of a series of organic pi -conjugated molecules involving 1,3 indandione derivatives as an acceptor moiety has been measured. The effect of varying the donor from dialkylamino to N-carbazolyl substituents was examined. Despite the chemically similar nature of the two donors, a drastic reduction of electric field induced second harmonic (EFISH) beta values for the molecules involving the N-carbazolyl substituents was detected. Quantum mechanical calculations indicate that the decrease in beta values results from the presence of two overlapping transitions that contribute to beta with opposite signs. Thus, the charge transfer band gives a positive beta (zzz) contribution along the long molecular axis, while a transition essentially within the carbazole group provides a negative beta (zxx) contribution to beta (EFISH). Therefore, these molecules can only be described with a two-dimensional model as opposed to the 'classical' one-dimensional model for NLO chromophores. Experimental verification of this hypothesis was provided by a combination of two experimental methods, EFISH and hyper Rayleigh scattering (HRS). Each technique probes a different combination of the beta tensor elements, so we could deduce the two contributions. (C) 2000 Elsevier Science S.A. All rights reserved.