Dimers to Trimers and Oligomers in Twist-Bend Liquid Crystals
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This is a repository copy of Progression from nano to macro science in soft matter systems: : dimers to trimers and oligomers in twist-bend liquid crystals. White Rose Research Online URL for this paper: https://eprints.whiterose.ac.uk/102010/ Version: Published Version Article: Mandle, Richard orcid.org/0000-0001-9816-9661 and Goodby, John William (2016) Progression from nano to macro science in soft matter systems: : dimers to trimers and oligomers in twist-bend liquid crystals. RSC Advances. pp. 1-9. ISSN 2046-2069 https://doi.org/10.1039/c6ra03594a Reuse This article is distributed under the terms of the Creative Commons Attribution (CC BY) licence. This licence allows you to distribute, remix, tweak, and build upon the work, even commercially, as long as you credit the authors for the original work. More information and the full terms of the licence here: https://creativecommons.org/licenses/ Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ RSC Advances View Article Online PAPER View Journal | View Issue Progression from nano to macro science in soft matter systems: dimers to trimers and oligomers in Cite this: RSC Adv.,2016,6, 34885 twist-bend liquid crystals† Richard J. Mandle* and John W. Goodby In this article we report on the characterization and properties of several unsymmetrical phenyl-benzoate bimesogens that exhibit the soft-matter, twist-bend nematic (NTB) phase. We use this study as a basis to examine the phase behaviour of associated novel trimeric and tetrameric materials, in order to investigate the potential for oligomeric materials to form the NTB phase. Based on our results we Received 8th February 2016 hypothesise that higher oligomers and even polymers are highly likely to exhibit the N phase, provided Accepted 30th March 2016 TB they retain a gross bent structure between consecutive mesogenic units. Thus we show at the level of DOI: 10.1039/c6ra03594a nanoscale organization, dimers can template with respect to one another to form mesophases that are www.rsc.org/advances also found in macromolecular systems. Creative Commons Attribution 3.0 Unported Licence. Introduction groups can, through dipole–dipole interactions, lead to anti- parallel pairing of molecules,32–34 with the extent to which this The discovery of a new phase of matter lacking lamellar occurs being functional group dependent. This article covers ordering of the molecules while at the same time exhibiting two closely related topics. Firstly we describe how, by moving properties associated with such ordering, has motivated a great from symmetrical bimesogens to unsymmetrical, the possibility deal of study into liquid crystal bimesogens and their behav- of stabilising/destabilising the NTB phase in methylene linked iour.1–19 The presently favoured model for this mesophase is bimesogens is possible by manipulating the strength of inter- This article is licensed under a that of the ‘heliconical twist-bend nematic’ phase, predicted molecular interactions, as shown by the target materials in independently by Meyer and Dozov,1,2 in which there exists Scheme 1. a helix with extremely tight pitch with the nematic director Secondly, given that the chemical space surrounding meso- 2,9–11 Open Access Article. Published on 04 April 2016. Downloaded 14/09/2016 15:07:47. inclined from the heli-axis by an acute angle. This model is genic dimers that exhibit the twist-bend phase has been experimentally supported by both freeze-fracture TEM studies somewhat explored, we considered targeting the two oligomeric and carbon K-edge SAXS on CB7CB, which allow direct materials shown in Scheme 2 in the context of the formation 10,19 measurement of the nanoscale helix. The orientational order and stability of the NTB phase. parameters of the NTB phase, determined by polarised Raman spectroscopy as well as 2H and 129Xe NMR,15,16 appear to be consistent with the heliconical model of Dosov and Meyer. It Experimental should, however, be noted that studies on surface freezing20 and – 2H NMR have resulted in alternative models.21 23 Full experimental details for the synthesis and characterization A growing number of materials exist that are known to of products 1 to 9 are provided in the ESI.† Compounds 3–7 24–29 exhibit the NTB phase, the bulk of which are symmetrical were synthesised, as shown in Scheme 1, via the EDAC/DMAP bimesogens possessing a methylene spacer of odd parity. One mediated Steglich esterication between compound 1 (4-(9-(4- notable exception is etheric bimesogens,30,31 which can be hydroxyphenyl)nonyl)phenyl 4-cyanobenzoate), prepared as designed to be unsymmetrical in structure by exploiting described previously,29 and an appropriate benzoic acid. bifunctional spacer groups, such as bromoalcohols, during Fortuitously the chemical intermediate required to prepare chemical synthesis.31 Many of the materials known to exhibit unsymmetrical methylene linked phenylbenzoate dimers was the NTB phase contain polar functional groups, most o en also be used as a platform chemical in the synthesis of oligo- nitriles as in the CBnCB series of materials. Polar functional meric materials, as shown in Scheme 2.29 Compound 8 was prepared via the EDAC/DMAP mediated Steglich esterication between compound 1 and 1,4-terephthalic acid (Scheme 2). Department of Chemistry, University of York, York, YO10 5DD, UK. E-mail: richard. [email protected] Compound 9 was prepared via the EDAC/DMAP mediated 0 † Electronic supplementary information (ESI) available. See DOI: Steglich esteri cation of compound 1 and 4,4 -(nonane-1,9-diyl) 10.1039/c6ra03594a dibenzoic acid.26,29 This journal is © The Royal Society of Chemistry 2016 RSC Adv.,2016,6,34885–34893 | 34885 View Article Online RSC Advances Paper Scheme 1 Target dimeric materials. Creative Commons Attribution 3.0 Unported Licence. Scheme 2 Target oligomeric materials. Small angle X-ray scattering was performed on a Bruker D8 possessing 2- and 4-brushes, whereas the smectic C phase Discover equipped with a temperature controlled, bored exhibited a focal-conic texture that was unbroken, which at rst graphite rod furnace, custom built at the University of York. sight could appear as a texture associated with the smectic A This article is licensed under a Computational chemistry was performed at the B3LYP/6-31G(d) phase (Fig. 1a and b). However, X-ray diffraction clearly shows level of DFT in Gaussian G09,35 with Gaussian output les that the smectic phase is tilted (Fig. 1d), and with the combi- rendered using Qutemol.36 Full experimental details, including nation of the POM studies the phase was unequivocally classi- Open Access Article. Published on 04 April 2016. Downloaded 14/09/2016 15:07:47. chemical characterisation and descriptions of instrumentation ed as smectic C. The tilt angle determined from X-ray used, are available in the ESI.† diffraction was found to reach a maximum of 13, although as the sample readily crystallises shortly aer cooling into the Results and discussion smectic C phase this value is not saturated. If we turn now to the properties of the dimers, we can The liquid crystalline properties of the 1 and 2 were studied by compare those of unsymmetrical structure with the parent a combination of polarised optical microscopy (POM) and symmetrical bimesogen 2, which was found to possess differential scanning calorimetry (DSC), the results for which a melting point of 157.6 C, and monotropic phase transitions are presented in Table 1. from the liquid to the nematic phase at 146.6 C, and from the Although compound 1 can be described unequivocally as nematic to the NTB phase at 114.5 C. In calamitic systems is a dimer (or bimesogen), 2 is harder to dene in such absolute well known that nitrile terminated materials have a strong terms due to its ability to form intermolecular hydrogen bonds tendency to exist as antiparallel correlated pairs or n-mers.32,33 between the phenol and nitrile groups leading to possible Contrast this with isothiocyanates, which do not exhibit as large supramolecular oligomeric structures.28,37 Whereas 2 has been a degree of antiparallel correlation as nitriles,38 and with nitro demonstrated to exhibit nematic and twist-bend nematic pha- terminated materials, which have a stronger tendency than ses,26 1 was found to exhibit a rst order nematic to smectic C nitriles to form antiparallel correlated pairs.34 As noted in the phase transition. Phase assignments were made initially by introduction, moving from symmetrical bimesogens to polarised optical microscopy (POM) and then by small angle X- unsymmetrical variants such as compounds 2–6 (Table 2) allows ray scattering (SAXS) see Fig. 3, whereas the associated the possibility of engineering molecules and thus exerting enthalpies and entropies of transitions were determined by a degree of control over the condensed phases they exhibit by differential scanning calorimetry (DSC) as shown in Table 1. controlling the strength and type of intermolecular The defect texture observed for the nematic phase of 1 interactions. showed a classical schlieren texture composed of singularities 34886 | RSC Adv.,2016,6, 34885–34893 This journal is © The Royal Society of Chemistry 2016 View Article Online Paper RSC Advances À Table 1 Transition temperatures (), associated enthalpies of transition [kJ mol 1] and dimensionless entropies of transition {DS/R} for compounds 1 and 2.26,29 Transitions in parenthesis () are monotropic No. R Cr SmC NTB N Iso 1 /H C 109.5 (C 61.6 —— C 71.9) C [47.19] [3.21] [1.32] {14.85} {1.15} {0.46} 157.6 114.5 146.6) 2 C [45.41] —— (C [0.65]C [1.50] C {12.68} {0.20} {0.43} Table 2 Transition temperatures ( C) and scaled NTB–N transition T – T – – temperatures ( NTB N/ N I) for compounds 2 7.