Influence of terminal substituent on non-linear S-shaped oligomers consisting of azobenzene moieties at the peripheral arm: Synthesis, characterisation and phase transition behaviour

ABSTRACT

A series of symmetrical S-shaped mesogens based on 4,4′-bis-(6-bromohexyloxy)biphenyl as a central unit containing two 2-{6-[4-(4-substitutedphenylazo)phenoxy]hexyloxy}phenol as side-chain groups has been successfully synthesised. The terminal substituent was varied from halogen (X = F, Cl, Br and I) to non-halogen (X = C₂H₅ and OC₂H₅). The oligomers with C₂H₅ and OC₂H₅ substituents exhibit predominantly the monotropic nematic (N) phase. The OC₂H₅-containing derivatives possess long-range stability of N phase than its C₂H₅-containing analogue in which it has small range of N phase stability. As for halogen-containing analogues, oligomer with F exhibits monotropic N phase whilst oligomers with Cl and Br exhibit monotropic N and smectic A (SmA) phases. In addition, homologue with Br shows additional phase which is smectic B (SmB) phase upon further cooling. However, the oligomers in which F, Cl and Br were substituted by I exhibits purely monotropic SmA and SmB phases. X-ray diffraction measurements reveal that the smectic phase is inclined to the monolayer structure.

GRAPHICAL ABSTRACT

KEYWORDS:

• S-shaped mesogen
• monotropic
• nematic
• smectic A
• smectic B
• monolayer

Acknowledgements

The main author (G. Y. Yeap) would like to thank Universiti Sains Malaysia (USM) for partial finance support through RU [Grant Number 811223] and the laboratory facilities. F.Osman also acknowledges the Ministry of Higher Education of Malaysia (MOHE) for the research 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 (USM): [Grant Number 811223].