2ND-ORDER NONLINEAR-OPTICAL PROPERTIES OF THE 4 TETRANITROTETRAPROPOXYCALIX[4]ARENE CONFORMERS

This paper reports a systematic experimental and theoretical study of the second-order nonlinear optical properties of multichromophoric mol ecules that range from dipolar symmetry to three-dimensional octupolar symmetry. The four possible conformers of tetranitrotetrapropoxycalix [4]arene (cone, pace, 1,2-air, 1,3-alt) were studied by nanosecond hyp er-Rayleigh scattering and a newly developed time-resolved femtosecond hyper-Rayleigh scattering technique. The latter enables to correct fo r long-lived fluorescence contributions to the second harmonic scatter ing intensity. The depolarization ratios D-x/z prove the (partial) oct upolar symmetry of the 1,2-alt and 1,3-alt conformers, and thus explai n why their hyperpolarizabilities beta(FHRS) are of the same order of magnitude as those of the dipolar cone and pace conformers. The corres ponding theoretical second-order nonlinear optical properties (both be ta(FHRS) and D-x/z) were calculated using the conformations obtained f rom single-crystal X-ray diffraction, molecular mechanics (MM), and mo lecular dynamics (MD) calculations. In contrast with sum-over-state ca lculations presented in the literature, our theoretical method takes a lso into account octupolar contributions by linearly adding the NLO-pr operties of the separate chromophoric groups and using Bersohn's theor y. The agreement between experimental and theoretical results is good both for the conformers having dipolar symmetry and for the conformers having (partly) three-dimensional octupolar symmetry. The 1,2-alt and 1,3-alt conformers of the tetranitrotetrapropoxycalix[4]arene represe nt the first examples of multichromophoric molecules that have high hy perpolarizabilities beta due to 3D octupolar symmetry.