Phase transitional behaviour of S-shaped oligomers incorporating biphenylene and cholesterol entities

ABSTRACT

A new series of symmetrical S-shaped oligomers 4,4′-bis[(5-cholesteryloxycarbonylpentyl alkoxy)hexyloxy]biphenyl consisting of outer spacers –(COCH₂)_n– as well as an inner spacer –(CH₂)₆– has been synthesised. Their liquid crystalline properties and phase transition temperatures with associated enthalpy changes are recorded. The outer spacers are varied from n = 5–8 to 10 and 11. The compounds with even spacer exhibit enantiotropic phase and oligomers with odd parity display monotropic phase. The oligomers with odd membered n = 5, 7 and 11 exhibit N* and SmC* phases upon cooling. Whilst upon heating, the homologues with even-numbered member n = 6 and 10 show N* phase and upon cooling, both compounds exhibit N* and SmC* phases. However, oligomer with outer spacer n = 8 displays enantiotropic N* and SmC* phases. The temperature range of N* phase for even and odd membered decreased as the outer spacers are increased. The odd–even effect has been found in the I–N* transition temperatures where the odd-parity oligomers exhibit lower values when compared to compounds of an even-parity series. The X-ray diffraction measurements reveal the appearance of SmC* phase that can be associated with the monolayer ordering of these oligomers.

GRAPHICAL ABSTRACT

KEYWORDS:

• Symmetrical S-shaped
• liquid crystalline properties
• outer spacers
• enantiotropic
• monotropic
• odd–even effect

Acknowledgements

The main author (G.-Y.Yeap) would like to acknowledge Universiti Sains Malaysia for funding this project through RU Grant Number: [1001/PKIMIA/811223] and Malaysian Ministry of Education for FRGS Grant Number: [203/PKIMIA/6711422]. F. Osman also wishes to thank the MOHE for the scholar assistantship under the MyBrain15 scheme.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Universiti Sains Malaysia: [Grant Number 1001/PKIMIA/811223]; Malaysian Ministry of Education for FRGS: [Grant Number 203/PKIMIA/6711422].