Tuning self-assembly of rod-shaped liquid crystals

Liquid crystals (LCs) exhibit a unique combination of an ordered supramolecular structure and a dynamic nature, which makes them attractive for a wide range of applications. Rod-shaped molecules can self-assemble into numerous types of liquid crystalline phases including lamellar phases with varying degrees of order and fluidity. The suitability of an LC material for a given application is strongly dependent on the type of phases, the phase sequence, and their thermal stabilities. Since these three factors are highly sensitive to molecular structure, it is imperative to possess a deep understanding of their structure-property relationships in order to rationally design materials with desirable properties for a given application. The studies herein investigate how changes in molecular structure can be employed to tune the self-assembly and opto-electronic properties of LC materials. The first part of this thesis explores “molecular symmetry breaking” to improve the thermal stability of LC phases. Two series of compounds were studied: 2,6-di(4ʹ-n-alkoxybenzoyloxy)naphthalenes, which form relatively disordered phases, and 4,4’-dialkanoyloxybiphenyls, which form highly ordered phases. The degree of symmetry was varied by appending terminal alkyl chains of different lengths. A systematic comparison of the LC phase behaviour revealed that symmetry breaking leads to a pronounced depression in the melting point with a limited effect on the clearing point, resulting in broader LC phase ranges for less symmetric isomers. This presents a strategy to tune the LC properties of a material while maintaining the inherent opto-electronic properties. The second part of this thesis focuses on strategic molecular design to optimize LC materials for organic semiconductors. Initially, the effect of replacing the central thiophene in 5,5”-dialkyl-α-terthiophene with an oxadiazole or thiadiazole ring was explored. The oxadiazole analogue is not LC whereas the thiadiazole analogue exhibits several potential advantages in LC phase behaviour compared to the parent terthiophene derivative. Inspired by these results, we studied a series of 2,5-bis(2,2’-bithiophene-5-yl)-1,3,4-thiadiazole derivatives, unsymmetrically substituted with an alkyl chain on one side and an aromatic ring on the other. Through variation of the aromatic ring, both the LC and opto-electronic properties can be tailored, making these compounds highly tunable materials.