Non-centrosymmetric arrangements

Above diffraction patterns of (a) mask representing molecule of durene, 1,2,4,5 tetra-methyl benzene, (6) the same ma k with an extra hole at the centre positive regions are strengthened, negative regions weakened. (Lipson and Cochran, 1953.) Below diffraction patterns of (c) the centrosymmetric arrangement of holes shown inset, and (d) the non-centrosymmetric arrangement of holes shown inset (C. A. Taylor, 1952)... 

An alternative approach is to prepare so-called poled polymer samples. An NLO compound is dispersed into a polymer at a temperature above Tg for the polymer. A large voltage (for example, 10 V/cm) is placed across the material. Since the NLO molecules have large dipole moments, they will tend to align with the field. After adequate alignment is achieved, the temperature is dropped below Tg, locking in the now non-centrosymmetric arrangement of NLO chromophores. This fairly simple procedure can be used to prepare samples with quite acceptable x values. 

In a preceding paragraph we have shown that a necessary condition for second order NLO activity in a material is the absence of the symmetry center. This a stringent requirement and it is perhaps the most difficult feature the chemistry of these materials must face with. To get a non-centrosymmetric arrangement of molecules on a macroscopic scale is a difficult task, and even more difficult is to get a non-centrosymmetric polar structure. A polar structure is that in which there is at least one macroscopic direction (polar axis) which is not changed in the opposite direction by symmetry transformations allowed for the material. With reference to crystals, out of the 32 crystal classes only the following 20 correspond to non-centrosymmetric crystals 1,2, m, 222, mm2,4, -4,422,4mm, -42m, 3, 32, 3m, 6, —6, 622, 6mm, —62m, 23, —43m... 

Fig. 9.7 Two cubic arrangements for Li2ZnGe centrosymmetric Fm3m (a), non-centrosymmetric F43m (b).

Fig. 9.8 Band structure and densities of states calculated for the compound Li2ZnGe in the non-centrosymmetric F43m cubic arrangement. Zn 3d inert orbitals (flat levels at -8 eV in the band structure) are not represented in the DOS.

Stable Z-type LB films have been prepared, however, by the alternative deposition of two different monolayers (A and B) in a head-to-head ABAB arrangement [142]. Two different monolayers can be layered a number of different ways (AABBAA, AABAAB, ABBABB, etc.) and there are a myriad of ways to arrange three or more different monolayers in LB films. These types of LB films have no plane of symmetry (i.e. they are non-centrosymmetric) and manifest non-linear optical behavior [108,143]. Schematics of some of the different types of LB films are illustrated in Fig. 11. 

The EO effect is a second-order nonlinear optical (NLO) effect. Only non-centrosymmetrical materials exhibit second-order NLO effects. This non-centrosymmetry is a condition, both at the macroscopic level of the bulk arrangement of the material and at the microscopic level of the individual molecule. All electro-optic modulators that are presently used by telecom operators are ferro-electric inorganic crystals. The optical nonlinearity in these materials is to a large fraction caused by the nuclear displacement in the applied electric field, and to a smaller fraction by the movement of the electrons. This limits the bandwidth of the modulator. The nonlinear response of organic materials is purely electronic and, therefore, inherently faster. 

Phenyltrichlorostannane reacts with 1 surplus amine to yield the hexacoordinate tin compound 2c only. An elimination of HCl from 2c seems to be impossible. Even reaction with a proton sponge, Ai, A, A, A -tetramethylnaphthalene-1,8-diamine, failed. 2c crystallizes in a non-centrosymmetric monoclinic space group (Cc). The molecule involves the all-d5 configuration of the salen type ligand. The angle (93.9°) of C19-Sn-Cl indicates a real cis arrangement of the chlorine atom and the phenyl group (Fig. 2) [2]. 

Table 1.17. The 230 crystallographic space groups arranged according to seven crystal systems and 32 crystallographic point groups as they are listed in the International Tables for Crystallography, vol. A. The centrosymmetric groups are in bold, while the non-centrosymmetric groups that do not invert an object are in italic. The remaining are non-...

This cell is very different from that of Ooct-OPV3 and contains four symmetry-related molecules. Due to the shorter substituents, the density is much higher. Despite this, there is no Ti-stacking (see Fig. 16-21). The successive /r-systems within one, non-centrosymmetric molecule are arranged in a helical fashion because the dihedral angles all have the same sign. 

E(ind) = —x E + X E - - ). Hence, only molecules that are non-centrosymmetric can have a non-zero (3 and only non-centrosymmetric materials can have non-zero x (i.e., a centrosymmetric arrangement of non-centrosymmetric molecules will have zero xM-... 

P) of the compound in question and on the orientation of the molecules in the crystal. Specifically, a non-centrosymmetric packing arrangement is required for a solid to exhibit NLO properties and, while it is possible to alter the value of P by synthetic modification, encouraging molecules to adopt a non-centrosym-metric packing arrangement is a formidable challenge. 

In order to use the advantages of Y-type multilayers of one single material one must employ an alternative concept for the arrangement of the molecules in order to obtain non-centrosymmetric LB films. We have recently introduced 2-docosylamino-5-nitropyridine (DCANP, Fig.5a), a substance that can easily be deposited onto hydrophobic substrates, yielding LB films of at least up to 1.2 pm thickness. 

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