Bent-Core LC Dimers

Sandeep Kumar; Santanu Kumar Pal

doi:10.1017/9781316662120.006

5 - Bent-Core LC Dimers

Published online by Cambridge University Press: 23 July 2017

Sandeep Kumar and

Santanu Kumar Pal

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Sandeep Kumar: Affiliation:
Raman Research Institute, Bangalore, India
Santanu Kumar Pal: Affiliation:
Indian Institute of Science Education and Research, Mohali, India

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Summary

Over the past two decades, bent-core liquid crystals (BLCs) have provided fascinating results to the scientific arena of LC research. The sharp bend within the linkage of the core group leads to the unique physical properties having no complement in conventional calamitic LCs such as occurrence of chiral phases and/ or polar phases even though the constituent molecules are achiral. Specially the bent (angular) molecular architecture leads to the variation in self-assembly processes that demonstrate as the unconventional phase structures. BLCs themselves are primarily comprised of three parts: bent core, rigid arm and the flexible tail chain. Up to now, a rich variety of BLCs with different structures and unique functions have been developed and their phase structures are also even more enriched. LC dimers obtained from monomeric BLC units, interconnecting two mesogenic units through flexible spacers (such as alkylene, siloxane, carbosilane, oxyethylene) are relatively new. The inherent motivation behind the preparation of bent-core dimers was to use the microsegregation to manipulate the phase structure of bent-core molecules. Based on the adjoined mesogenic units, the bent-core dimers can be categorically divided into three sub units:

5.1 Symmetrical bent-core LC dimers

5.2 Nonsymmetrical bent-core LC dimers

5.3 Unconventional bent-core LC dimmers

SYMMETRICAL BENT-CORE LC DIMERS

Introduction

In symmetrical bent-core dimers, both the two mesogenic units joined via flexible spacer are BLC unit. The possible different types of arrangements possible for the bent-core dimers that can be achieved by connecting through a flexible spacer with possible combinations of the bent-core mesogenic units are presented in Figure 5.1.

Structure–Property Relationships

Dimers Based on Siloxane and Alkylene Spacer

The first attempt of bent-core LC dimers consisting of two BLC units connected by flexible oligosiloxane spacer (dimethylsiloxane) units, shown in Figure 5.2, was reported almost a decade ago by G. Dantlgraber and his coauthors.1 The motivation behind the work was initiated by the observation that decoupling of the layers by the microsegregated oligosiloxane units disfavours an antiferroelectric (AF) assembly of the bentcore molecules. Therefore, antiferroelectricity is not solely arising by the compensation of the layer polarization but interlayer fluctuations of the molecules also play a vital role. Hence anticlinic layer arrangement or ferroelectricity can be induced by suppressing of the layer fluctuation. The dimers were synthesized by hydrosilylation of terminally unsaturated bentcore mesogens with hexamethyltrisiloxane (1) or octamethyltetrasiloxane (2) using Karstedt's catalyst.

Information

Type: Chapter
Information: Liquid Crystal Dimers , pp. 185 - 224
DOI: https://doi.org/10.1017/9781316662120.006 [Opens in a new window]
Publisher: Cambridge University Press
Print publication year: 2017

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