Developments in liquid-crystalline dimers and oligomers

Abstract

Liquid-crystalline dimers and bimesogens have attracted much attention due to their propensity to exhibit the spontaneously chiral twist-bend mesophase (N TB ), most often by dimers with methylene spacers. Despite their relative ease of synthesis, the number of ether-linked twistbend materials significantly lags behind those of methylene-linked compounds. In this work, we have prepared and studied a range of ether-linked bimesogens homologous in structure to the FFO9OCB; as with methylene-linked systems, it appears that it is molecular topology and the gross molecular shape that are the primary drivers for the formation of this phase of matter. Dimers and bimesogens are well studied within the context of the twist-bend phase; however, present understanding of this mesophase in oligomeric systems lags far behind. We report our recent efforts to prepare further examples of oligomeric twist-bend nematogens, including further examples of our 'n+1' methodology, which may allow the synthesis of high-purity, monodisperse materials of any given length to be prepared. We have observed that there is a tendency for these materials to exhibit highly ordered soft-crystalline mesophases as opposed to the twist-bend phase.ARTICLE HISTORY

Physics of liquid crystals of bent-shaped molecules

“…Goodby's group presented an interesting argument in favor of the decisive role of the molecular bend, by noting that for many different N TB -forming materials, the temperature of the N U -N TB transition is linearly proportional to the clearing Goodby 2017) (Simpson et al 2017), concluding that the details of chemical composition of these materials is of much lesser importance than the bent shape in the formation of N TB . This conclusion is supported by a large number of experimental observations on newly synthesized materials with various chemical structures (Mandle, Stevens, and Goodby 2017) (Mandle and Goodby 2016a) (Mandle et al 2016).…”

Isomeric trimesogens exhibiting modulated nematic mesophases

“…In our previous attempt to synthesis A6 3 we were unable to cleave the benzyl ether without accompanying nitrile reduction, 40 in the present work we circumvented this issue by moving to the use of a tert butyldimethylsilyl (TBDMS) protecting group. The phenol group of the intermediate i1, synthesis reported in, 40 was protected with a TBDMS group and subsequent hydrogenation saturated the alkyne and removed the benzyl ether to afford i3. Mitsunobu etherication with i4 gave i5; cleavage of the TBDMS group was achieved using TBAF.…”

“…38,39 Relatively little is known about the relationship between the twist-bend phase and molecular structure within the context of oligomesogenic materials, with only a handful of examples known. [33][34][35][36]40,41 One limitation to developing such a relationship is the difficulty in preparing higher oligomers. Herein we present a simple synthetic route to two isomeric trimesogens, shown in Schemes 1 and 2, which could easily be expanded to give further examples of higher oligomers with homologous structures.…”

An experimental and computational study of calamitic and bimesogenic liquid crystals incorporating an optically active [2,2]-paracyclophane

“…Conversion of i-2 into the triflate followed by Suzuki-Miyaura coupling with i-5 afforded i-3 (80% over both steps); a subsequent Williamson etherification with i-6 -prepared via the Apel bromination of the analogous alcohol described in ref [12] -afforded compound 2 in 77% yield (Scheme 1).…”