The ramifications of changing molecular geometry in a series of all-aromatic liquid crystals derived from p-quinquephenyl are reported. Substituting heterocyclic rings such as thiophene, oxadiazole, oxazole, or 1,3-phenylene into the p-quinquephenylene core affects molecular shape changes via the substituent's exocyclic bond angle. In general, we found that introducing nonlinearity into molecules depresses the melting transition temperature. The symmetric (boomerang-shaped) molecules, 2,5-bisbiphenyl-4-yl-1,3,4-oxadiazole, 2,5-bisbiphenyl-4-yl-oxazole, and 1,3-bisbiphenyl-4-yl-benzene, melt into isotropic phases showing small monotropic mesophases. By contrast, the asymmetric (hockey stick-shaped) mesogens, 2-terphenyl-4-yl-5-phenyl thiophene and 2-terphenyl-4-yl-5-phenyl-1,3,4-oxadiazole, exhibit more stable enantiotropic liquid crystalline phases. The hockey stick-shaped mesogens exhibit a smectic phase as well as a nematic phase. High-temperature X-ray determination of the smectic layer spacing gives a...

Javelin-, Hockey Stick-, and Boomerang-Shaped Liquid Crystals. Structural Variations on p-Quinquephenyl†

In this article, we probe the formation of liquid crystal and soft crystal phases as a consequence of minimising the free volume of the system either through design engineering of molecular shape o...

What makes a liquid crystal? The effect of free volume on soft matter

The fabrication of organic semiconductor thin films is extremely important in organic electronic devices. This tutorial review—which should particularly appeal to chemists and physicists interested in organic thin-film growth, organic electronic devices and organic semiconductor materials—summarizes the method of weak epitaxy growth (WEG) and its application in the fabrication of high quality organic semiconductor thin films. WEG achieves the thin-film fabrication of disk-like organic semiconductor molecules with highly structural order, molecular level smoothness and large size domains on amorphous substrate. The organic field-effect transistor devices based on these thin films exhibit a high charge mobility that is comparable with their corresponding single-crystal devices. Moreover, it provides a way to produce organic superlattices.

Weak epitaxy growth of organic semiconductor thin films

Crystallographic studies on hexaphenyl thin films — a review

The lattice theory of hard rods with anisotropic dispersion forces presented recently is extended to the case where the rods are dispersed in a diluent. Two systems are treated: (i) rods of axial ratio x diluted with solvent molecules having x=1, and (ii) rods of axial ratio xα mixed with homologs of axial ratio xβ. Treatment of the latter system can be regarded as an extension of the Flory and Abe theory for ’’athermal’’ polydisperse systems, in which dispersion forces are inconsequential. Solutions of sufficiently long (x≳20) rods subject to anisotropic dispersion forces are predicted to exhibit complex phase equilibria involving a re‐entrant nematic phase (with a critical point) and a triple point, as well as the usual nematic–isotropic coexistence. These deductions should be relevant to thermotropic solutions of rodlike synthetic polymers with phenylene groups or other groups exhibiting anisotropic interactions. Calculations for mixtures of short rods yield more prosaic phase diagrams. It is suggested...

The phase equilibria in thermotropic liquid crystalline systems

Vorlander,1 studying the melting behavior of various polyphenyl compounds, reported that both p-quinquephenyl and p-sexiphenyl exhibited liquid crystalline behavior. Based on these early qualitativ...

C. A. Kovac

Papers

Liquid Crystal Transitions of p-Sexiphenyl