Nonlinear octupolar systems

An essential-state description has been employed to analyze the linear and nonlinear optical properties of octupolar systems in comparison with their dipolar analogs. This approach, which accounts for couplings of electrons to molecular vibrations as well as for solvent effects has shown that the TPA intensity per branch is amplified by a factor of 2 when going from dipolar to octupolar species, in agreement with TDDFT calculations of TPA intensities. 

In 2006, Bernard et al. also examined centrosymmetric boron cluster-containing molecules. Although these molecules did not show second-order nonlinear optical effects because of their cen-trosymmetry, they are interesting candidates for third-order TPA nonlinear processes. Centrosymmetric materials are more suitable for TPA applications in the visible range than noncentrosymmetric ones, because of their lower coloration (Wang et al., 2001). The presence of donor substituents at each arm of tripodal systems increases the TPA properties (Brunei et al., 2001). Bernard et al. desaibed, for the first time, branched octupolar systems with three dodecaborate units (Figure 13.16). 

Octupolar molecules, instead, may be centrosymmetric but they imply the existence of twofold (D2) or threefold (D3) rotational axes. They are characterized by multidirectional CT excitations. The theoretical description of nonlinearity of such systems implies, even in the simplest case, a three-level approach. The prototypical example of an octupolar molecule is represented by 1,3,5-triamino-2,4,6-trinitrobenzene. 

Zyss, J-, Octupolar organic systems in quadratic nonlinear optics molecules and materials. Nonlinear Opt., 1, 1, 1991. 

Table IX collects data as the square root of the orientationally averaged square of the nonlinearity, permitting a comparison between linear and trigonal ( octupo-lar ) complexes. Within the error margins of the experiment (+10 15%), proceeding from linear to octupolar complexes (Fig. 9) results in a tripling of NLO response, consistent with the increase in number of ligated metal centers, but extending the 7i-system by replacing the tra/J5-disposed chloro by a phenylalkynyl ligand results in no further increase.

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