& Liquid Crystals Apolar Bimesogens and the Incidence of the Twist–Bend Nematic Phase Richard J. Mandle,* Edward J. Davis, Craig T. Archbold, Constantin C. A. Voll, Jessica L. Andrews, Stephen J. Cowling, and John W. Goodby [a] Abstract: The nematic twist–bend phase (N TB ) was, until re- cently, only observed for polar mesogenic dimers, trimers or bent-core compounds. In this article, we report a comprehen- sive study on novel apolar materials that also exhibit N TB phases. The N TB phase was observed for materials containing phenyl, cyclohexyl or bicyclooctyl rings in their rigid-core units. However, for materials with long (> C7) terminal chains or mesogenic core units comprising three ring units, the N TB phase was not observed and instead the materials exhibited smectic phases. One compound was found to ex- hibit a transition from the N TB phase to an anticlinic smectic C phase; this is the first example of this polymorphism. In- corporation of lateral substitution with respect to the central core unit led to reductions in transition temperatures; how- ever, the N TB phase was still found to occur. Conversely, util- ising branched terminal groups rendered the materials non- mesogenic. Overall, it appears that it is the gross molecular topology that drives the incidence of the N TB phase rather than simple dipolar considerations. Furthermore, dimers lacking any polar groups, which were prepared to test this hypothesis, were found to be non mesogenic, indicating that at the extremes of polarity these effects can dominate over topology. Introduction Questions concerning the structure and nature of the twist– bend nematic (N TB ) phase have reinvigorated the study of mesogenic dimers in recent years. [1–17] A consensus has emerged that the heliconical model of Dozov [3, 11] is a good de- scriptor of the phase, although this has been rightly scru- tinised. [18–20] In particular, the freeze–fracture transmission elec- tron microscopy (FF-TEM) method that is often used to meas- ure the N TB heliconical pitch length has been questioned, be- cause similar periodic length scales exist in the solid state of 4’,4’’’-(nonane-1,9-diyl)bis(([1,1’-biphenyl]-4-carbonitrile)) (CB9CB). [19] However, measurements of the electroclinic effect in 4’,4’’’-(heptane-1,7-diyl)bis(([1,1’-biphenyl]-4-carbonitrile)) (CB7CB) appear to support the idea of the N TB phase having a pitch length of a few nanometers. [11] Although the heliconical model appears to provide a suitable theoretical treatment of the N TB phase, the relationship be- tween molecular structure (and the molecular and bulk proper- ties that result) and the N TB phase is only poorly understood at present. This is illustrated by the original model proposed by Dozov, which predicts for the N TB phase, the bend elastic con- stant K 33 is negative. [3] Experimental studies have shown that although K 33 is small it is not, nor does it ever become, nega- tive for CB11CB. [21] It is not presently known how the bulk properties of the N TB phase, such as the heliconical pitch length and the correlation length, are linked to molecular structure, or what the upper limits for these properties are. By drawing structure–property correlations new “N TB ” materials can be obtained by rational design rather than chance discov- ery, while also allowing the impact of specific structural fea- tures upon bulk properties to be elucidated. [22] The most general structure–property correlation that exists for the N TB phase concerns the angle between the two meso- genic units. An overall bent shape is currently seen as a pre- requisite, with atomistic modelling showing that only dimers with odd spacer parity possess a sufficiently bent geometry to exhibit this phase. [9] However, the system need not be methyl- ene linked as illustrated by the observation of this phase in bent-core materials [23] as well as dimers containing imine, [24] and ether [25, 26] linking groups. A supermolecular hydrogen- bonded trimer also has been shown to exhibit the N TB phase, [27] and a trimer comprised of two cyanobiphenyl units and a bent core has recently been reported to exhibit a room temperature N TB phase. [28] In this paper, the roles of a variety of modifications to the molecular structures with respect to the thermal stability of the N TB phase will be explored, expanding upon a recent article in which we introduced a number of new materials that ex- hibit the N TB phase. [29] These include the incorporation of non- aromatic rings, the effect of terminal chain length, studies on the length-to-breadth ratio and the study of materials of ex- tremely low polarity. For the sake of brevity, only symmetrical [a] R. J. Mandle, E. J. Davis, C. T. Archbold, C. C. A. Voll, J. L. Andrews, S. J. Cowling, J. W. Goodby Department of Chemistry, The University of York Heslington, York, YO10 5DD (UK) E-mail : richard.mandle@york.ac.uk Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201500423. Chem. Eur. J. 2015, 21, 1 – 11 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 && These are not the final page numbers! ÞÞ Full Paper DOI: 10.1002/chem.201500423