Nondipolar structures with threefold symmetry for nonlinear optics

The second-order polarizabilities beta(-2 omega;omega,omega) of six nondipolar NLO chromophores-1,3,5-tris(isopropylamino)-2,4,6-trinitrobenzene (1), 1,3,5-tris(isopropylamino)-2,4,6-tris(trifluoro methylsulfonyl)benzene (2), tris(dimethylimoniomethyl)methide diperchlorate (3), 2,4,6-tris[4-(N,N-diethylamino)phenylethynyl]-1,3,5-triazine (4), 2,4,6-tris[4(N,N-diethylamino)phenyl]-1 ,3,5-triazine (5), and tris(2-dimethylaminoethenyl)methinium perchlorate (6)-were studied by polarized hyper-Rayleigh scattering (HRS). The NLO activity of these two-dimensional NLO chromophores is attributed to three-level contributions to the beta values, for which different optimization strategies are investigated. Exceptionally large second-order polarizabilities were observed for the conjugationally elongated structures: the beta value of triazine 4 is 15 times larger than that of the one-dimensional two-level chromophore 4-nitroaniline. Results obtained with the polarized HRS experiment are discussed with respect to local field corrections, calibration, and systematic sources of error.