Unusual molecular packing within the low symmetry mesophases of alkyl- and alkoxy-acylhydrazinatonickel(II) liquid crystals

Abstract

Three series of novel thermotropic metallomesogens have been synthesized and characterized using microscopy and DSC. Unexpected X-ray scattering is observed within all of the observed mesophases. Diffuse in-layer reflections are observed both at angles corresponding to approximately twice the molecular width and to the side-by-side separation of molecules within the nematic, SmA and SmC mesophases. Furthermore, the tilt angles within the SmC phases are observed to decrease as the terminal chain lengths decrease. Conoscopic observations show the SmC phases to be strongly biaxial as expected, but surprisingly, weak biaxiality is also observed within both the SmA and nematic mesophases. A model to explain the results is proposed, in which the cores remain orthogonal to the layers, whilst the overall molecular tilt necessary for SmC phase formation is provided by the contribution of the terminal alkyl chains to the overall molecular shape. IR spectroscopy is used to confirm the model.