Mesomorphic properties of cyanobiphenyl dimers with a substituted central malonate unit: overruling effect of fluorination

ABSTRACT

Two series of C2-substituted dimers carrying a central C2-hexyl- or C2-nonafluorohexyl malonate and two cyanobiphenyls attached via alkoxy spacers of various lengths (n = 6–12) were synthesised, and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopyand X-ray diffraction (wide-angle and small-angle X-ray scattering). The C2-substituent dimers together with the spacer lengths had a pronounced influence on the phase behaviour. In case of C2-hexyl-substituted dimers, shorter spacers favoured N phases (n = 6–9), while longer spacers favoured SmA phases (n ≥ 9). In addition, decreased clearing temperatures were observed. In contrast, C2-nonafluorohexyl-substituted dimers suppressed N phases and displayed only SmA phases. Moreover, clearing temperatures and phase stabilities considerably increased. Based on the experimentally determined layer thickness, a packing model for the SmA phases was proposed, where C2-substituted dimers displaced mesophases with molecules in a Y-conformation. As a result, less stable mesophases for C2-hexyl-substituted dimers due to weakened intermolecular interactions and more stable mesophases for C2-nonafluorohexyl-substituted dimers due to the fluorophobic effect and nanosegregation were obtained.

Graphical Abstract

KEYWORDS:

• Cyanobiphenyl
• dimer
• malonate
• perfluorinated
• nanosegregation
• packing model
• substitution

Acknowledgments

Generous financial support by the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the Bundesministerium für Bildung und Forschung (shared instrumentation grant # 01 RI 05177) and the Carl-Schneider-Stiftung Aalen (shared instrumentation grant) is gratefully acknowledged. We would also like to acknowledge Frank Gießelmann for the use of the DSC device from his research group. Furthermore, we would like to thank Johannes Kästner for his permission to use the computational resources from his working group.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Funding

This work was supported by the Bundesministerium für Bildung und Forschung Carl-Schneider-Stiftung Aalen [# 01 RI 05177].