Substituted crown ethers as central units in discotic liquid crystals: effects of crown size and cation uptake

Abstract

In this paper, a series of crown ether-based liquid crystals was synthesised. The effect of different crown ether ring sizes, terminal groups and complexed salts on the mesomorphic properties was investigated. The first example of [12]crown-4 as the central core of a discotic liquid crystal is described. Columnar hexagonal phases were found for the [12]crown-4-ethers carrying o-terphenyl (1) and triphenylene (2) groups. The [24]crown-8 ethers showed columnar hexagonal phases when substituted with o-terphenyl groups (7) while no mesophases could be detected in the case of the triphenylene substituted derivatives 8. It was found that an increase in the size of the macrocycle leads to a decrease in the clearing points and phase stabilities. Depending on the size of the crown ether, uptake of suitable cations can lead to drastic increases in the clearing temperatures and phase stabilities due to structural rearrangements or ion pair formation, as deduced from ¹H and ¹³C NMR spectroscopy.

Keywords:

• columnar phases
• crown compounds
• liquid crystals
• salt effect
• substituent effects

Acknowledgements

Generous financial support by the Deutsche Forschungsgemeinschaft, the Max-Planck-Gesellschaft (International Max Planck Research School for Advanced Materials fellowships for M.K. and P.S.), the Fonds der Chemischen Industrie, the Bundesministerium für Bildung und Forschung and the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg is gratefully acknowledged. We would like to thank Prof. Dr. Clemens Richert for providing the MALDI-TOF MS equipment.