Octupolar system

The UV-Vis data suggest that the subchromophores in 135 and 136 are near to being electronically independent. Derivative 135 has a small dipole moment and can be considered as an almost purely octupolar system, as is also the case for 136. Compared with their tin analogs, the phosphorus derivatives 135 and 136 have higher /3 values owing to the more efficient acceptor nature of the phospho-nium moiety [73 a]. Furthermore, the NLO activity of the octupolar compound 135 is almost three times larger than that of the dipolar subchromophore 137... 

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). 

Although a strong increase was predicted in these systems by theoretical calculations [124], a recent paper demonstrated that Zn(ll) does not produce an enhancement of the TPA efficiency with respect to that of the ligand in octupolar complexes of Fig. 21 [125] both complexes Cl and C2 feature ajPA values (214 and 530 GM, respectively) close to 3-times those of hgands Lgl and Lg2 (69 and 190 GM, respectively). 

In the field of metallic multibranched molecules for TPA, Humphrey et al. developed several series of organo-metallic complexes (Fig. 22). The iron octupolar complex Hulb exhibits a TPA cross-section of 920 GM at 695 nm, which corresponds to an enhanced value with respect to that of the linear system Hula (66 GM) in a ratio of 14 between TPA cross-sections of both structures (Table 4) it must be noted that this efficiency increase is obtained with a red shift of the linear absorption (460 against 436 nm, respectively, as reported in Table 4) [126]. 

Experimental values of appA obtained for crystal violet CV and brilliant green BG (Fig. 23) confirm the potentialities of octupolar molecules, for which TPA cross-sections of 1980 and 762 GM have been reported respectively by excitation in the state 2A the efficiency of CV has been found comparable to that of dipolar or quadrupolar systems [117]. 

Species of the form RC CRu(P P)2X, where P P represents a chelating bis(phosphine), R can be a variety of TT-systems, and X can be a halide or another acetylide ligand, have been incorporated into a wide range of dipolar,quadrupolar, and octupolar architectures. Selected NLO data for some of these compounds are summarized in Table 3 and some dipolar and quadrupolar structures are shown in Figure 14, with octupolar species in Figure 15. 

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. 

It gives information on the geometry of the chromophore, the part of the molecule responsible for the NLO response (for an ideal donor/acceptor one-dimensional system DR = 5, for an octupolar molecule, DR = 1.5 whereas for a A-shape molecule, the amplitude of DR depends on the angle between the chromophore as well as on the D/A groups). On the other hand, the EFISHG measurements give information on the projection of the vector part of p on the dipole moment vector ... 

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.

---