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 be en measured. The effect of varying the donor from dialkylamino to N-carbazo lyl 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 substituen ts was detected. Quantum mechanical calculations indicate that the decrease in beta values results from the presence of two overlapping transitions th at contribute to beta with opposite signs. Thus, the charge transfer band g ives a positive beta (zzz) contribution along the long molecular axis, whil e a transition essentially within the carbazole group provides a negative b eta (zxx) contribution to beta (EFISH). Therefore, these molecules can only be described with a two-dimensional model as opposed to the 'classical' on e-dimensional model for NLO chromophores. Experimental verification of this hypothesis was provided by a combination of two experimental methods, EFIS H and hyper Rayleigh scattering (HRS). Each technique probes a different co mbination of the beta tensor elements, so we could deduce the two contribut ions. (C) 2000 Elsevier Science S.A. All rights reserved.