Exploring the mesomorphic potential of 2,4-disubstituted thiophenes: a structure–property study

Abstract

The synthesis and mesomorphic properties of 25 novel esters derived from 4-substituted thiophene-2-carboxylic acids and either (S)-1-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4′-hydroxybiphenyl-4-carboxylate are described. A structure–property relationship has been deduced, and mesogenic properties are found to be dependent on the total number of rings in the molecular core and the nature of the substituent in the 4-position of the thiophene ring. Four-ring systems have greater thermal stability and are more likely to form mesophases than their three-ring counterparts. A 1,4-phenylene substituent in the 4-position of the thiophene ring (Series I and II) gives better thermal properties than does a 2,5-thienyl substituent (Series III and IV). A rationale is suggested in terms of conjugative effects that may be important in mesophase thermal stability. A preliminary molecular modelling study (B3LYP/6–31G(d)) reveals that 2,4-disubstituted systems offer a slight preference for a folded geometry structure compared with their 2,5-counterparts. Homologues (n = 7–10,12) in Series II show an interesting chiral phase behaviour between the SmA and SmC* ferrielectric phase, tentatively assigned SmX*, which then gives a SmC* antiferroelectric phase. Textures have been characterised by thermal polarising microscopy using a conventional glass slide cover-slip and free-standing film. Although the SmX* phase has still to be fully characterised, this is the first reported occurrence of such a phase in 2,4-thiophene systems.

Keywords:

• 2,4-thiophene
• synthesis
• characterisation
• structure–property
• antiferroelectric
• ferrielectric
• unknown SmX

Acknowledgements

ASM is grateful to Dr Chrissie Grover for synthesis of the compounds discussed. Sincere thanks are also due to Dr Gunnar Andersson for his valuable assistance and guidance with the preliminary electro-optical studies.

Notes

1. I–N and N–K transitions occur simultaneously. However, the existence of the N phase is observed in supercooled unco- vered droplets.

2. Ultra-high vacuum was used to remove both DMF and the tri-n-butyltin bromide by-product. DMF may also be removed by washing with pentane.

3. Melting points were determined using a Mettler FP52 hot-stage. No liquid crystal mesophases were observed, either on heating to the isotropic melt or on cooling to the crystalline state.