Effect of carbazole as a donor moiety on the second order nonlinearity of organic molecules

The second order nonlinearity (β) of conjugated organic molecules involving 1,3 indandione derivatives as an acceptor moiety has been studied. Varying the donor from dialkylamino to the chemically similar substituent, N-carbazolyl resulted in a drastic reduction of electric field induced second harmonic (EFISH) β values. For some molecules, even a small negative value of β was received. Quantum chemical calculations indicate that the decrease occurs as a result of two overlapping transitions, which contribute to β with opposite signs. The charge transfer band gives a positive β_zzz along the molecular long axis, while a transition essentially within the carbazole moiety provides a negative β_zxx contribution to β^EFISH. Thus, these molecules must be described with a two-dimensional model as opposed to the 'classical' model of one-dimensional nonlinear optical chromophores. The prediction of the 2-D model was verified experimentally by using a combination of two methods, EFISH and Hyper-Rayleigh Scattering, which probe different combination of the β tensor elements.