Salt formation, single molecular type

The use of salt formation to expand the number of crystals which contain a single molecular type was first applied by Meredith (26), and more recently by Marder et. al. (22). In the latter work, ionic interactions are used to offset dipolar interactions among achiral molecules, which enhances the probability that the resulting crystal will be noncentrosymmetric. In our case, of course, noncentrosymmetry is ensured by the chirality of the molecules involved. It is important to note that, within the picture we have presented, neither the assurance of noncentrosymmetry, nor the enhanced hyperpolarizability of the chiral molecule guarantees that the nonlinearity of any particular chiral organic salt crystal will be large. These properties simply ensure that each crystal so formed has an equal opportunity to express the molecular hyperpolarizability in an optimized way. 

Polyion complexes between acid derivatised polythio-phene (poly(thiophene-3-acetic acid)), PTAA (Figure 14.39), and a stable surface active cation have been used to construct conductive LB films. In this case, the formation of a polyion complex renders PTAA surface active and provides additional control over the molecular orientation of the polymer within the monolayer. The monolayer assembly thus obtained has a structure composed of a well ordered condensed monolayer of the surface active molecule onto which is a single monolayer of PTAA ionically bound. For example, the sodium salt of PTAA can be readily absorbed onto a monolayer of dimethyldioctadecylam-monium bromide. The monolayer assembly of the polyion complex was successfully transferred into LB film in a Z-type manner [294]. 

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