Nondipolar structures with threefold symmetry for nonlinear optics

The second-order polarizabilities β(-2ω;ω,ω) of six nondipolar NLO chromophores - 1,3,5-tris(isopropyl-amino)-2,4,6-trinitrobenzene (1), 1,3,5-tris(isopropylamino)-2,4,6-tris(trifluoromethylsulfonyl)benzene (2), tris(dimethylimoniomethyl)methide diperchlorate (3), 2,4,6-tris[4-(N,N-diethylamino)phenyl ethynyl]-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 β values, for which different optimization strategies are investigated. Exceptionally large second-order polarizabilities were observed for the conjugationally elongated structures: the β 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.