Octupolar compounds

The primary objective of this section is to show the importance of lanthanides to nonlinear harmonic generation. This work, which is unpublished, evolved from earlier studies that have shown the correlation of nonlinear optical effects in dipolar and octupolar compounds which involve the use of one or more electron-donating groups connected via a conjugated tt-linker to an electron acceptor group. 

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... 

Ray and Leszcynski172 have investigated the hyperpolarizabilities of ionic octupolar compounds including solvent effects and Ray173 has also analysed the molecular structure and [1 hyperpolarizabilities of a series of ionic organic and organometallic compounds. 

Zojer et al.192 have discussed the limitations of the essential-state model in calculating the TPA of bis(dioxaborine-)-substituted chromophores. Lukes and Breza193 have made an AM1-ZINDO/SOS study of spirobifluorene type molecules. Jha et al.194 have employed their method of obtaining model exact solutions in the PPP approximation to calculate TPA cross-sections for trans-stilbene and 7,8-disubstituted stilbenes. Zhou et al 95 and Liu et al.196 have studied one and two photon absorption in octupolar compounds using DFT and ZINDO/SOS methods and the group has applied similar methods to trigonal dehydrobenzo[l8]annulenes.197... 

Figure 14. Structure of ionic extended conjugated octupolar compounds used in this investigation.

Figure 15. Structure of neutral extended conjugated octupolar compounds used in this calculation a) A = -NO2 b) A = -H. (Reprinted from Chemical Physics Letter, P.C. Ray and J. Leszczynski, (2004) Chem. Phys. Lett.. 399, 162, Copyright (2004), with permission from Elsevier)...

Sissa et al have presented theoretical models for describing the electroabsorption (EA) spectra of dipolar, quadrupolar, and octupolar organic chromophores in solution and therefore for addressing molecular information that go well beyond the linear absorption spectra and are complementary to nonlinear spectroscopic techniques. So, for dipolar molecules EA spectra provide the excited state dipole moment and polarizability whereas for more complex quadrupolar and octupolar compounds, they also give information on the dark states. 

Two-photon absorption properties of pseudo-octupolar compounds containing substituted fluorene arms have been predicted using a SOS... 

Table 6 2PA properties of octupolar and multibranched compounds. The corresponding linear building blocks are included for comparison, when available. The solvent in which the measurement was performed is indicated in parenthesis after the molecular structure... 

Due to their ionic interaction, they conquer the dipole attraction and in fact, ionic dipolar compounds exhibit excellent bulk SHG efficiency [7-10]. Dynamic hyperpolarizabiUties of 1,3,5 substituted benzenes and triazines cations calculated by ZINDO/CV/SCRF method are listed in Table 6. Experimental data [92] for the corresponding neutral octupolar molecules (as shown in Figure 15) are also listed in Table 6. 

In this chapter we report on some novel strategies that have been pursued to obtain efficient second-order nonlinear molecules starting from the well-known phthalocyanines. In principle, these planar centrosymmetric molecules do not present second-order activity and have been extensively studied for third-order applications. In order to induce asymmetry, two main approaches have been followed a) peripheral substitution of the macrocycle with donor and acceptor groups and b) structural modifications of the Pc core to reduce the symmetry, the resulting-noncentrosymmetric compounds (i.e. subphthalocyanines) presenting variable degrees of dipolarity/octupolarity in the nonlinear response. 

The experimental jShrs values (Table 4) for all four isomers are rather similar and comparable to those of the trinitro chlorosubphthalocyanine isomeric mixture la-b (in a 1 to 3 ratio, respectively, see part 4.2), showing the very little influence of the substitution pattern on the octupolar" NLO response. This trend is further confirmed by the similarity between the (i) llOMO-LUMO energy differences (Table 4) and (ii) the SOS-derived values (Table 5) between all four compounds. 

Table 6. Dipolar and octupolar spherical components and molecular anisotropy ratio, p, for compounds la-d...

This work provides a relatively comprehensive review of studies involving ruthenium coordination and organometallic complexes as nonlinear optical (NLO) compounds/materials, including both quadratic (second-order) and cubic (third-order) effects, as well as dipolar and octupolar chromophores. Such complexes can display very large molecular NLO responses, as characterised by hyperpolarizabilities, and bulk effects such as second harmonic generation have also been observed in some instances. The great diversity of ruthenium chemistry provides an unparalleled variety of chromophoric structures, and facile Ru" —> Ru" redox processes can allow reversible and very effective switching of both quadratic and cubic NLO effects... 

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. 

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