NONDIPOLAR STRUCTURES WITH THREEFOLD SYMMETRY FOR NONLINEAR OPTICS

The second-order polarizabilities beta(-2 omega;omega,omega) of six no ndipolar NLO s-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), [4-(N,N-diet hylamino)phenylethynyl]-1,3,5-triazine (4), 2,4,6-tris[4(N,N-diethylam ino)phenyl]-1 ,3,5-triazine (5), and tris(2-dimethylaminoethenyl)methi nium perchlorate (6)-were studied by polarized hyper-Rayleigh scatteri ng (HRS). The NLO activity of these two-dimensional NLO chromophores i s attributed to three-level contributions to the beta values, for whic h different optimization strategies are investigated. Exceptionally la rge second-order polarizabilities were observed for the conjugationall y elongated structures: the beta value of triazine 4 is 15 times large r than that of the one-dimensional two-level chromophore 4-nitroanilin e. Results obtained with the polarized HRS experiment are discussed wi th respect to local field corrections, calibration, and systematic sou rces of error.