Structure-property relationships in azobenzene-based twist-bend nematogens

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Heliconical smectic phases formed by achiral molecules

M.S. acknowledges the support of the US National Science Foundation I2CAM International Materials Institute Award, Grant DMR-1411344 and NSF grant DMR-1307674. D.P., E.G. acknowledges the support of the National Science Centre (Poland) under the grant no. 2016/22/A/ST5/00319. R.W. gratefully acknowledges the Carnegie Trust for the Universities of Scotland for the award of a PhD studentship. The beamline 11.0.1.2 at the Advanced Light Source at the Lawrence Berkeley National Laboratory is supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02- 05CH11231. Addendum: Heliconical smectic phases formed by achiral molecules Published: 17 July 2018, https://doi.org/10.1038/s41467-018-05334-x "We would like to make our readers aware of the related publications by S.P. Sreenilayam et al. (Nat. Commun. 7, 11369 (2016) and Phys. Rev. Mat. 1, 035604 (2017)), which report the spontaneous helix formation in a polar smectic liquid crystal phase made of achiral bent-core mesogens."Peer reviewedPublisher PD

An FT-IR spectroscopic study of the role of hydrogen bonding in the formation of liquid crystallinity for mixtures containing bipyridines and 4-pentoxybenzoic acid

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Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

<p>The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CB<i>n</i>CB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CB<i>n</i>OCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBO<i>n</i>OCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBO<i>n</i>OCB dimer and lowest for the CB<i>n</i>CB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (<i>T</i>_{N_TB_N}) is higher for the CB<i>n</i>OCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBO<i>n</i>OCB dimers, a virtual value of <i>T</i>_{N_TB_N} was estimated for CBO3OCB and <i>T</i>_{N_TB_N} was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CB<i>n</i>CB or CB<i>n</i>OCB series. The dependence of <i>T</i>_{N_TB_N} on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.</p

Heliconical smectic phases formed by achiral molecules.

Chiral symmetry breaking in soft matter is a hot topic of current research. Recently, such a phenomenon was found in&nbsp;a fluidic phase showing orientational order of molecules-the&nbsp;nematic phase; although built of achiral molecules, the phase can exhibit structural chirality-average molecular direction follows a short-pitch helix. Here, we report a series of achiral asymmetric dimers with an odd number of atoms in the spacer, which form twisted structures in nematic as well as in lamellar phases. The tight pitch heliconical nematic (NTB) phase and heliconical tilted smectic C (SmCTB) phase are formed. The formation of a variety of helical structures is accompanied by a gradual freezing of molecular rotation. In the lowest temperature smectic phase, HexI, the twist is expressed through the formation of hierarchical structure: nanoscale helices and mesoscopic helical filaments. The short-pitch helical structure in the smectic phases is confirmed by resonant X-ray measurements

Structure–property relationships in twist-bend nematogens : the influence of terminal groups

The synthesis and characterisation of a range of non-symmetric liquid crystal dimers designed to exhibit the twist-bend nematic phase is reported. Beginning with 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl) hexane, each nitrile group is exchanged systematically for a methoxy group. The behaviour of these dimers is interpreted in terms of their bent shape being the predominant driving force for the formation of the twist-bend nematic phase, and the small differences between the twist-bend nematic–nematic transition temperatures are attributed to the differences between the interaction strength parameters of the mesogenic units. The 4-alkyloxyphenyl 4-[6-(4′-cyanobiphenyl-4-yl)hexyloxy]benzoates with ethyl, butyl, hexyl and octyl chains show the twist-bend nematic phase, whereas the corresponding 4-alkyloxyphenyl 4-[5-(4′-cyanobiphenyl-4-yloxy)pentyloxy]benzoates do not. This difference in behaviour is attributed to the more bent structure of the former. Increasing the terminal chain length initially decreases the twist-bend nematic–nematic transition temperature and this suggests that the chain disrupts the interactions between the mesogenic units. Subsequent increases in chain length have a smaller effect suggesting that the chain can be accommodated within an intercalated arrangement. The transitional behaviour of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-butyloxybiphenyl-4′-yl) hexane is compared to that of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-((S)-2-methyl)butyloxybiphenyl-4′-yl) hexane, and it is shown that chain branching strongly destabilises the twist-bend nematic phase. This is attributed to difficulties associated with packing the dimers

Azobenzene-based liquid crystal dimers and the twist-bend nematic phase

<p>The synthesis and characterisation of two new sets of non-symmetric liquid crystal dimers is reported, the 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexanes (CB6OABX) and 1-(4-substitutedazobenzene-4′-yloxy)-6-(4-cyanobiphenyl-4′-yloxy)pentanes (CBO5OABX). The terminal substituents are methyl, methoxy, butyl, butyloxy, nitrile and nitro. All the CB6OABX dimers exhibit twist-bend nematic (N_TB) and nematic (N) phases. The CBO5OABX dimers also all show an N phase but only the butyl and butyloxy homologues exhibit the N_TB phase. The transitional behaviour of the non-symmetric dimers is compared to that of the corresponding symmetric dimers, the 1,5-bis(4-substitutedazobenzene-4′-yloxy)pentanes (XABO5OABX) and either 1,7-bis(4-cyanobiphenyl-4′-yl)heptane or 1,5-bis(4-cyanobiphenyl-4′-yloxy)pentane. The XABO5OABX dimers all show a nematic phase and in addition, the butyl homologue exhibits a smectic A phase. The difference in transitional behaviour between the CB6OABX and CBO5OABX dimers is attributed to the difference in their molecular shapes arising from different bond angles between the <i>para</i> axis of the cyanobiphenyl unit and the first bond in the spacer. Specifically, the all-<i>trans</i> conformation of a CBO5OABX dimer is more linear than that of the corresponding CB6OABX dimer. Differences within each set of dimers are attributed to changes in the average molecular shape and the strength of the mixed mesogen interaction on varying the terminal group. Crystal structures are reported for CB6OABOMe, CBO5OABNO₂ and MeOABO5OABOMe.</p