Molecular Flexibility and Bend in Semi‐Rigid Liquid Crystals: Implications for the Heliconical Nematic Ground State

Abstract

The N TB phase phases possess al ocal helicals tructure with ap itch length of af ew nanometers and is typically exhibited by materials consisting of two rigid mesogenic units linked by af lexible oligomethylene spacer of odd parity,g iving ab ent shape. We report the synthesis and characterisation of two novel dimeric liquid crystals, and perform ac omputational study on 10 cyanobiphenyl dimers with varying linking groups, generating al arge libraryo f conformers for each compound;t his allows us to present molecular bend angles as probability weighted averages of many conformers, rather than use as ingle conformer.W e validate conformer libraries by comparison of interproton distances with those obtained from solution-based 1D 1 H NOESYN MR, finding good agreement between experiment and computational work. Conversely,w ef ind that using any singlec onformer fails to reproduce experimental interproton distances. We find the use of as ingle conformers ignificantly overestimatest he molecular bend angle while also ignoring flexibility;i na ddition,w es how that the average bend angle and flexibility are both linkedt ot he relative stabilityo ft he N TB phase. Experimental SectionCompound 1 has been extensively studied, [32] compounds 2-5 were reported in ref.[24],c ompounds 6, 7 and 8 were reported by Arakawa et al. in ref.[27] and [28].C ompounds 9 and 10 were synthesised as part of this work accordingt oS cheme 1. Steglich [a] R.

Ether‐ and Thioether‐Linked Naphthalene‐Based Liquid‐Crystal Dimers: Influence of Chalcogen Linkage and Mesogenic‐Arm Symmetry on the Incidence and Stability of the Twist–Bend Nematic Phase

“…Although the α value of a single conformer with an all‐ trans alkyl spacer is useful for designing new twist–bend nematic dimers, in actuality a conformational distribution with “average molecular bend angle” occurs due to the gauche conformation and flexibility. The former α value is apt to be overestimated from the latter average molecular bend angle . Such deviation in the N TB phase was suggested on the basis of comprehensive XRD measurements …”

“…[44] Although the a value of a single conformer with an all-trans alkyl spaceri su seful for designing new twist-bend nematic dimers, in actuality ac onformationald istribution with "average molecular bend angle" occursd ue to the gauche conformation and flexibility.T he former a value is apt to be overestimated from the latter average molecular benda ngle. [49,50] Such deviation in the N TB phase was suggestedo nt he basis of comprehensive XRD measurements. [51] In marked contrastt ot he most widely reported methylenelinked molecular systems, it remains ac hallenging issue to develop twist-bend nematogens based on versatile chalcogen bis(ether)-linked dimers containinge ther bonds at the linker positions between an odd-numbered alkylene spacer and each mesogenica rm due to the lessb ent geometries that occur,a s describeda bove.V ery few examples of bis(ether)-linked dimers based on fluorinated mesogensa re knownt oe xhibit the N TB phase.…”

Distinct twist-bend nematic phase behaviors associated with the ester-linkage direction of thioether-linked liquid crystal dimers

“…73 Mandle et al comprehensively studied the relationship between the bend angle of several CB dimers with different linkages and their N TB formation using both experimental and quantum calculation approaches. 45,54 In addition, the influence of various combinations of chalcogens and other linkages on the N TB phase incidence and stability has been studied. 52,53 We previously reported 53 a number of systematically designed thioetherlinked LC dimers to ascertain the molecular design necessary to form the N TB phase; unsymmetrical ester-and thioetherlinked CB-based dimers with a butylene spacer, viz.…”

Study of the Experimental and Simulated Vibrational Spectra Together with Conformational Analysis of Thioether Cyanobiphenyl-Based Liquid Crystal Dimers

“…The structure of the NTB phase has primarily been studied using non-resonant (SAXS, WAXS) [ 35 , 36 , 37 ] and resonant X-ray scattering [ 32 , 33 , 38 , 39 , 40 , 41 ] as well as using polarized Raman [ 42 ], infrared [ 43 , 44 , 45 , 46 ] and nuclear magnetic resonance spectroscopy (NMR) [ 47 , 48 , 49 , 50 , 51 ]. Current experimental results show that the formation of the N TB phase is very sensitive to changes in the shape of the molecules [ 52 , 53 , 54 , 55 , 56 , 57 ]; therefore, the key is to carefully define the relationship between the molecular parameters of the molecules and the occurrence of the modulated nematic phase. For this purpose, many authors have used molecular modeling methods, mainly the density functional theory (DFT) [ 28 , 29 , 33 , 34 , 52 , 58 , 59 , 60 , 61 ] or molecular dynamics simulations (MD) [ 62 , 63 , 64 ] to analyze structural and conformational changes of the dimers.…”