This paper reports a systematic experimental and theoretical study of the second-order nonlinear
optical properties of multichromophoric molecules that range from dipolar symmetry to three-dimensional
octupolar symmetry. The four possible conformers of tetranitrotetrapropoxycalix[4]arene (cone, paco, 1,2-alt, 1,3-alt) were studied by nanosecond hyper-Rayleigh scattering and a newly developed time-resolved
femtosecond hyper-Rayleigh scattering technique. The latter enables to correct for long-lived fluorescence
contributions to the second harmonic scattering intensity. The depolarization ratios D
x/z
prove the (partial)
octupolar symmetry of the 1,2-alt and 1,3-alt conformers, and thus explain why their hyperpolarizabilities
βFHRS are of the same order of magnitude as those of the dipolar
cone and paco conformers. The corresponding
theoretical second-order nonlinear optical properties (both βFHRS and D
x/z
) were calculated using the
conformations obtained from single-crystal X-ray diffraction, molecular mechanics (MM), and molecular
dynamics (MD) calculations. In contrast with sum-over-state calculations presented in the literature, our
theoretical method takes also into account octupolar contributions by linearly adding the NLO-properties of
the separate chromophoric groups and using Bersohn's theory. 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 represent the first examples of multichromophoric molecules that have high hyperpolarizabilities β
due to 3D octupolar symmetry.
Operation of an alternative near-field optical microscope is presented. The microscope uses a microfabricated silicon-nitride probe with integrated cantilever, as originally developed for force microscopy. The cantilever allows routine close contact near-field imaging on arbitrary surfaces without tip destruction. The effect of adhesion forces on the coupling to the evanescent wave has been observed. Images with a lateral resolution of about 50 nm are presented and compared with atomic force images. A specific sample area can be selected using an integrated conventional light microscope.
Highly stable films of tetranitrotetrapropoxycalix [4]arene (9) with second-order nonlinear optical (NLO) properties and a noncentrosymmetric structure were obtained by a novel crystallization process at 130 ± 140 8C in a dc electric field. The packing of 9 in these films was elucidated by a combination of X-ray diffraction, angle-dependent second-harmonic generation, and scanning force microscopy (SFM).The experimental results agree well with solid-state molecular dynamics calculations for these films. No crystalline phase was observed for nitrocalix [4]arene derivatives with longer or branched alkyl chains; this explains the limited NLO stability of films of these calixarenes. Scanning force microscopy on the aligned films of 9 showed two distinct surface lattice structures: a rectangular lattice (a 9.3, b 11.7 ) and a pseudohexagonal lattice (d % 11.4 ). The combination of these data with the interlayer distance of 8.9 (X-ray diffraction) allowed the packing of molecules of 9 in these structures to be fully elucidated at the molecular level.
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