The relationship between the structure and physical properties of acylhydrazinatometallomesogens

Abstract

Series of novel thermotropic metallomesogenic complexes have been synthesised and their liquid crystalline phases characterised. Anomalous in-plane packing of the molecules in all phases, namely two diffuse in-layer features, is observed by x-ray scattering. The first, at 16A, compares to a molecular width of SA and shows that molecules are crystallographically unequal to their neighbours. The second, at 5A, corresponds to a side-by-side separation of molecules and coupled with conoscopic observations from the (usually uniaxial) nematic and smectic A phases indicates a degree of biaxiality within these phases. Furthermore, the tilt angle measured by x-rays within the smectic C phase decreases with chain length.
A model is proposed for packing the molecules to account for both the scattering and the conoscopic observations. It combines attractive interactions between molecules which slows rotation of the molecules about the long axis, and steric effects both from the core and the terminal chain. Infrared studies, computational analysis and initial powder diffraction of the three different complex types support the model. Given the proposed packing arrangement within the mesophases it is perhaps more appropriate to denote the SmA phases the more esoteric biaxial smectic A (SmAt,) phase. The nematic phases of the alkoxy- complexes also demonstrate biaxiality. These systems provide a very rare example of biaxiality in calamitic molecules.
Mixtures of the complexes and commercially available nematogens were made up to evaluate any potential for device application. Of the characteristics studied, the response sharpness and the light transmission through the cell were able to be improved, however the threshold voltage at which the device switched increased in every case. The inclusion of metallomesogenic complexes to the nematogens thus increased the degree of ordering within the system. Binary mixtures of the liquid crystalline materials were made up in an attempt to reduce the transition temperatures of the systems. A relatively straightforward method of predicting both the transition temperatures and eutectic compositions was used and agreed well with experimentally observed transition temperatures. As a first step towards lowering the melting points significantly this proved successful.