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

The second order nonlinearity (β) of a series of organic π-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) β values for the molecules involving the N-carbazolyl substituents was detected. Quantum mechanical calculations indicate that the decrease in β values results from the presence of two overlapping transitions that contribute to β with opposite signs. Thus, the charge transfer band gives a positive βzzz contribution along the long molecular axis, while a transition essentially within the carbazole group provides a negative βzxx contribution to βEFISH. 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 β tensor elements, so we could deduce the two contributions.