Mesophase structure and behaviour in bulk and restricted geometry of a dimeric compound exhibiting a nematic–nematic transition

Sebastián, Nerea; Tamba, Maria-Gabriela; Stannarius, Ralf; Fuente, M. R. de la; Salamończyk, Mirosław; Cukrov, Greta; Gleeson, J. T.; Sprunt, Samuel; Jákli, Antal; Welch, Chris; Ahmed, Ziauddin; Mehl, Georg H.; Eremin, Alexey

doi:10.1039/c6cp03899a

Cited by 45 publications

(58 citation statements)

References 53 publications

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“…S6 in ESI †). The reduction of small angle scattering intensity following the N-TB transition has been reported previously, 21,56,57 and we conjecture this is due to a lack of lamellar (cybotactic) fluctuations in the TB phase. Materials which exhibit transitions from the TB phase into smectic phases would be expected to exhibit cybotactic fluctuations; and consequently they do not show this reduction in scattering intensity.…”

Section: Resultssupporting

confidence: 84%

Molecular shape as a means to control the incidence of the nanostructured twist bend phase

2018

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Liquid crystalline phases with a spontaneous twist-bend modulation are most commonly observed for dimers and bimesogens with nonamethylene spacers. In order to redress this balance we devised a simple chemical intermediate that can be used to prepare unsymmetrical bimesogens; as a proof of concept we prepared and studied eleven novel materials with all found to exhibit the twist-bend phase and exhibit a linear relationship between TN-I and TTB-N. A computational study of the conformational landscape reveals the octamethyleneoxy spacer to have a broader distribution of bend-angles than the nonamethylene equivalent, leading to reductions in the thermal stability of the TB phase. This result indicates that a tight distribution of bend-angles should stabilise the TB phase and lead to direct TB-Iso phase transitions, and conversely a broader distribution should destabilise the TB phase which may allow new states of matter that are occluded by the incidence of this phase to be revealed.

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Section: Resultssupporting

confidence: 84%

Molecular shape as a means to control the incidence of the nanostructured twist bend phase

2018

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“…The rate of the decrease is slowed down near the N-Ntb transition; there seems to be a small plateau or even an increase right before the transition. Such a behavior has been already observed in other studies and is associated with the bent conformations of the dimeric molecules [20,46]. Importantly, the temperature dependence of 33 K in DTC5C9…”

Section: Discussionsupporting

confidence: 78%

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

et al. 2016

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We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay ( 11 K ), twist ( 22 K ), and bend ( 33 K ) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rodlike mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. 33 K decreases strongly to 0.4 pN near the N -Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Keywords: dimeric nematic; elastic constants; birefringence; negative dielectric anisotropy; twist-bend nematic. N phase. Uniform bend, however, cannot be realized in space without other types of deformations, either splay or twist. Thus two different variations of the nematic with spontaneous bend have been proposed by Meyer [8] and Dozov [9]: a twist-bend nematic and a splay-bend nematic (yet to be discovered experimentally). The relative stability of the two is controlled by the ratio of the splay 11 K to twist 22 K constants. Namely, in the twist-bend phase, 11 22 /2 KK  , while in the splay-bend case, 11 22 /2 KK  [9, 10]. As for the ChOH state, Meyer [11] predicted that it can be induced by an external electric or magnetic field in a cholesteric liquid crystal only when 33 22 KK  . In calamitic nematics formed by rod-like molecules, the latter condition is not satisfied; the expected and universally observed trend follows the inequalities 33 22 11 K K K  [13]. Prior research indicates that the dimeric materials feature elastic properties very different from those of rod-like nematogens. Atkinson et al. [14] demonstrated a strongodd-even effect in the behavior of 33 K . Two dimers, one with an even number of alkyl groups in the spacer chain and another one with an odd number, were characterized by the splay Frederiks transition technique. In this approach, 11 K is determined from the threshold field of reorientation, and 33 K is obtained by extrapolating the voltage dependence of capacitance far above the threshold. 33 K of the odd homolog was about (3-4) pN, much smaller (by a factor 10 ) than the corresponding value for the even homolog; the results were in good agreement with the theoretical consideration of the odd-even effect by...

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“…Our measurements of 33 K show an overall decrease upon cooling to 0.38 pN ≈ near the N-N TB transition, after which 33 K grows just before the transition. A slight increase in 33 K was already observed for CB7CB and other dimeric liquid crystals [16,19,[21][22][23]43], with the exception of Ref. [18] that shows a monotonously increasing 33 K with the decreasing temperature.…”

Section: Discussionmentioning

confidence: 73%

Elastic and viscous properties of the nematic dimer CB7CB

et al. 2017

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We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.

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Mesophase structure and behaviour in bulk and restricted geometry of a dimeric compound exhibiting a nematic–nematic transition

Cited by 45 publications

References 53 publications

Molecular shape as a means to control the incidence of the nanostructured twist bend phase

Molecular shape as a means to control the incidence of the nanostructured twist bend phase

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

Elastic and viscous properties of the nematic dimer CB7CB

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