Imbricated structure

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Imbricated structures have been found for many typical nematogens, such as 2,2 -di-bromo-4,4 -bis-(4"-methoxybenzylideneamino)biphenyl [7], (4-ethoxybenzylidene)-4 -butylaniline (EBBA) [8], 4,4 -dihexyloxybenzalazine [9], 4-butylphenyl-4 -butylbenzoyl-oxybenzoate [10], propyloxyazoxybenzene [11], cholesteryl hexyloxybenzoate [12] and so on, and also for unconventional molecules with lateral chains on the core moieties [13]. 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

The crystal structure of the mesogenic 4-(3"-pentenyl)-4 -(ethoxy)-l,l -bicyclohexane (PEBCH) was determined by Nath et al. [78]. This compound exhibits a nematic phase. The molecules are packed parallel to each other in an imbricated mode. 

In 1997, Sridhar et al. [117] investigated the crystal structure of the mesogenic 4-isothio-cyanato phenyl 4-pentylbicyclo[2,2,2]octane-1-carboxyl-ate. The crystal structure of this compound shows a layered packing along the fl-axis and imbrication along the other two axes. 

Thus, tetraalkoxysilanes with an even number of methylene groups like those in the structure series 4 6-0-0 exhibit a normal imbrication order, while tetraalkoxysilanes with an odd number of methylene groups like those in the structure series 4-7-0-0 show an inverse order of imbrication,... 

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