Non-centrosymmetric species

We also note that organoborate anions, in particular BPhr, have also been used as counterions for cationic second-order NLO dyes (e.g., see Refs 51, 236). In principle, non-dipolar contributions to (3 are possible with non-centrosymmetric species such as this such contributions in the inorganic Bp4 ion have been proposed as responsible for the different values of (3 determined for Bp4 and PF6 salts. 

Parity labels only apply to MOs in molecules that possess a centre of inversion (centrosymmetric molecules), e.g. homonuclear X2, octahedral EXg and square planar EX4 molecules. Heteronuclear XY, or tetrahedral EX4 molecules, for example, do not possess a centre of inversion and are called non-centrosymmetric species. 

The main requirement for molecule-based second-order NLO materials is a non-centrosymmetric organization of the constituent active species. Furthermore, organic... 

Most theoretical discussions for molecules concentrate on calculations of second-order nonlinear optical properties. These results can be used equally well for the design of either molecules or molecular fragments. The latter are intended for inclusion in polymers as either a solid solution or side-chains. These are discussed in detail in section 4.3, together with systems in which a crystalline phase is dispersed in a polymer matrix. In molecularly dispersed systems the incorporation and orientation of an active species in a polymer obviates the need for a non-centrosymmetric crystal structure but does require the imposition of a polar state on the polymer (e.g. with an applied electric field). Thus molecular species that as crystals are not useful as second-order nlo materials (because they adopt a centrosymmetric structure) may be applicable in a polymeric system. Though it has received less attention in the past, considerable effort has recently been devoted to theoretical studies of... 

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