Molecular engineering room-temperature bent-core nematics

Głębowska, Agata; Vita, Francesco; Francescangeli, O.; Dingemans, Theo J.; Samulski, Edward T.

doi:10.1080/02678292.2014.987709

Cited by 11 publications

(8 citation statements)

References 19 publications

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“…[63][64][65][66][67] This has led to considerably lower transition temperatures compared to the non-methylated analogues; derivatives with three lateral methyl groups supercool to room temperature in the nematic phase ( Figure 1) and XRD studies indicate the presence of cybotactic clusters. In the case of the compound shown below, OC4 2MePh(mono3MeODBP), the nematic phase persists at room temperature for nearly 24 hours before crystallisation occurs.…”

Section: Introductionmentioning

confidence: 96%

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

et al. 2015

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Scharrer (2015): The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals, Liquid Crystals,We have previously demonstrated that the incorporation of lateral methyl groups on oxadiazole-based liquid crystals leads to relatively low-temperature cybotactic nematic phases which, in some cases, supercool to room temperature. We report here the synthesis and phase behaviour of related compounds that possess lateral halogen groups and in some cases, lateral methyl groups as well. Derivatives with three lateral substituents (one halogen and two methyl groups) in a specific pattern supercool in the nematic phase to room temperature. As was the case with the previously reported trimethylated derivatives, the low-temperature nematic phase is glassy in nature. Two of the new trisubstituted derivatives (with bromo and chloro groups) remain in the nematic phase upon subsequent heating until transitioning to the isotropic phase indicating that the low-temperature nematic phase may be more stable than that shown by the trimethylated analogue. Preliminary X-ray diffraction analysis confirms the presence of a tilted cybotactic nematic phase. In addition, the splitting observed in the wide-angle scattering feature is indicative of enhanced local biaxial packing.

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Section: Introductionmentioning

confidence: 96%

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

et al. 2015

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“…This value corresponds to a transverse intermolecular distance dt = 2π/q0 ≈ 4.4 Å, a typical value for both calamitic and bent-core nematics. In particular, the WA reflections do not show the evident splitting observed in some laterally substituted BLCs, indicative of two different transverse intermolecular distances [2,[47][48][49]. The position of the WA reflections, always lying on the equatorial axis, indicates the alignment of the molecular director n along the applied magnetic field B over the whole nematic temperature range.…”

Section: X-ray Diffraction Resultsmentioning

confidence: 87%

Comparative 2H NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

et al. 2020

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Bent-core liquid crystals showing a nematic phase stable at low temperatures are very attractive for applicative purposes in view of the inherent biaxial nature of the nematic phase. In this work, a typical five-ring bent-core mesogen was investigated by means of 2 H NMR spectroscopy and X-ray diffraction (XRD) methods. These techniques provide complementary information on the structural properties of the nematic phase and the average mesogen conformation: small-angle XRD reveals the presence of short-range positional order in the form of skewed cybotaxis, while a comparison of the orientational order parameters measured by wide-angle XRD and NMR provides an estimate of the molecule bend angle. In addition, 2 H NMR puts in evidence the occurrence of an unexpected transition to a low-temperature tilted phase, having a crystalline or smectic-like character. The results were compared with those of previous 13 C NMR investigations.Crystals 2020, 10, 284 2 of 15 structure in nematic domains, which, in BLCs, is not due to pre-transitional local fluctuations in the vicinity of a nematic-smectic phase transitions. Indeed, the accepted model of the biaxial 'cybotactic nematic' phase assumes the existence of a peculiar short-range ordering in the form of nanometer-sized clusters having a layered supramolecular structure (usually tilted, i.e., smectic C-like) with intrinsic biaxial orientational order [15]. The main evidence of this 'cybotactic' local arrangement is given by the characteristic four-spot small-angle (SA) X-ray XRD feature [15][16][17][18], which is sensibly different from the two-spot pattern exhibited by conventional calamitic nematics.The occurrence of nano-sized aligned domains or local clusters in the nematic phase of bent-core LCs was also supported by independent studies based on Dynamic Light Scattering (DLS) [20] and on a combination of NMR experiments performed on similar five-rings banana-shaped molecules [21][22][23]. In particular, 2 H NMR static spectra recorded in the isotropic and in the nematic phase of these BLC nematogens were characterized by an unusual line-broadening [24], related to both dynamic and static reasons, which were further investigated by means of 2 H NMR relaxation times [25], 1 H NMR relaxometry [26] and 1 H NMR diffusometry [27]. All these studies supported the occurrence of local nanosized ordered domains, or clusters, in the isotropic and in the nematic phase formed by this type of BLC nematogens [24]. Additional 2 H NMR studies on molecular probes having a similar structure than the BLC aromatic core (five-or three-rings moieties), dissolved in different nematic phases, confirmed the peculiar ordering properties of BLCs [28,29]. Recent reviews have been published where the main orientational and dynamic properties of nematic phases of typical calamitic LCs and five-rings bent-core liquid crystals were compared [30,31]. Moreover, NMR spectroscopy has been used to investigate the mesophase biaxiality in several types of LC systems [32][33][34], such as organosiloxane tetr...

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“…Five new BC nematic LCs have been added in the growing list of BC nematic LCs, which are currently gaining considerable research interest due to their remarkable properties. [27,28] Unexpected liquid crystalline behaviour of threering bent-core mesogens: bis(4-subst.-phenyl) …”

Section: Discussionmentioning

confidence: 99%

Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

2015

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Isoxazole-based bent-core liquid crystals (LCs) derived from naturally occurring curcumin were synthesised and their LC properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Five compounds, including a branched alkyl chain derivative, were prepared and characterised. These derivatives exhibit enantiotropic mesophases. While lower homologues display widetemperature-range nematic phase, a longer-chain derivative 3d shows smectic C phase in addition to the nematic phase. The bent angle in these compounds is in between calamitic LCs and banana LCs. Therefore, the molecules escape from polar order packing observed in typical bent-core LCs. Increasing the length of alkyl chain reduces both melting and isotropic temperatures in the series. However, the compound with branched alkyl chains exhibits significant reduction in the nematic-isotropic temperature only. Detailed XRD experiments confirm the presence of the N phase in the lower homologues and SmC phase in a higher homologue.

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Molecular engineering room-temperature bent-core nematics

Cited by 11 publications

References 19 publications

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

Comparative 2H NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

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