A novel nematic-like mesophase induced in dimers, trimers and tetramers doped with a high helical twisting power additive

Mapping the local ambidextrous chirality in thin films of NTB phase by circular dichroism spectra

Circular dichroism mapping (CDM) method was introduced by utilizing the highly collimated light beam of synchrotron radiation (SR) available at Diamond Light Source B23 beamline for scanning the thin films of the NTB phase. We apply SR-CDM to two achiral dimeric materials exhibiting the NTB phase: symmetric DTC5C9 and dissymmetric DTC5C9CB. The SR-CDM measurements directly capture the chiral information in the local NTB domains, providing the ultimate complement to the theoretical predictions of the helical structures: the spontaneous symmetry breaking in NTB phase is ambidextrous. The macroscopic chirality of the NTB phase is determined by the combination of doubly degenerate locally chiral domains in the illuminated area. Additionally, we investigate the temperature dependence, as well as the dynamic nature of the local chirality in the NTB phase by in-situ SR-CDM, confirming a chiral conversion influenced by the state of enantiomeric aggregation and a progressive unwinding of the helical structure in left-handed domains as the crystallization temperature is approached.

The Interplay of Spacer Chirality and Parity in Mesogenic Dimers

Introducing chirality into soft materials, including liquid crystals (LCs), profoundly impacts their self-organization and physical properties. In this study, we synthesized a novel series of LC dimers with a chiral center as part of their flexible spacer. The dimers were prepared in racemic and enantiomerically pure forms. Their spacer length and parity were varied to investigate the effect of spacer chirality and parity on mesomorphic behavior and on chiral induction in the nematic phase of achiral mesogens. Our results show that the even-membered chiral dimers only have chiral nematic phases. In contrast, the odd-membered dimers display rich mesomorphism, including the intriguing blue phase (BP) and chiral form of the twist-bend nematic phase (N*TB). The observed significant difference in the 3D surface morphology between the racemic and chiral forms of the N*TB phase suggests that the chiral moiety in the spacer promotes a chiral hierarchy. Furthermore, the chiral dimers show a prominent odd-even effect in the helical twisting power in nematic hosts. These findings highlight the importance of the position of the chiral group within the dimeric molecule and provide new insights into how intrinsic chirality in the spacer affects the overall structural chirality.

Controlling spontaneous chirality in achiral materials: liquid crystal oligomers and the heliconical twist-bend nematic phase

Liquid crystal oligomers, namely dimers, trimers and tetramers, consisting of cyanobiphenyl and benzylideneaniline-based mesogenic units connected by either linear or bent alkoxy or alkyl spacers are reported. These materials, although built from achiral molecules, show the spontaneously chiral heliconical twist-bend nematic (N_TB) phase. We report the relationships between the shape of the oligomer, and the N_TB phase stability, the temperature dependence of the helical pitch length and tilt angle, birefringence, and elastic constants.

Twist-Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene-based Nonsymmetric Dimers

A selection of pyrene-based liquid crystal dimers have been prepared, containing either methylene-ether or diether linked spacers of varying length and parity. All the diether linked materials, CBOnO.Py (n=5, 6, 11, 12), exhibit conventional nematic and smectic A phases, with the exception of CBO11O.Py which is exclusively nematic. The methylene-ether linked dimer, CBnO.Py, with an even-membered spacer (n=5) was solely nematogenic, but odd-members (n=6, 8, 10) exhibited both nematic and twist-bend nematic phases. Replacement of the cyanobiphenyl fragment by cyanoterphenyl giving CT6O.Py, gave elevated melting and nematic-isotropic transition temperatures, and SmA and SmC_A phases were observed on cooling the nematic phase. Intermolecular face-to-face associations of the pyrene moieties drive glass formation, and all these materials have a glass transition temperature at or above room temperature. The stability of the glassy twist-bend nematic phase allowed for its study using AFM, and the helical pitch length, P_TB , was measured as 6.3 and 6.7 nm for CB6O.Py and CB8O.Py, respectively. These values are comparable to the shortest pitch of a twist-bend nematic phase measured to date.

Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic Ab phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so.

Chemically induced splay nematic phase with micron scale periodicity

Nematic liquid crystals lack positional order of their constituent molecules, which share an average orientational order only. Modulated nematic liquid crystal phases also lack positional order, but possess a periodic variation in this direction of average orientation. In the recently discovered splay nematic (NS) phase the average orientational order is augmented with a periodic splay deformation of orientation perpendicular to the director. In this communication we report the first example of a splay nematic phase which is chemically induced by mixing two materials, neither of which exhibit the NS phase. The splay-nematic phase is identified based on its optical textures, X-ray scattering patterns, and small enthalpy of the associated phase transition. We measure the splay periodicity optically, finding it to be ∼9 μm. This unexpected generation of the splay-nematic phase through binary mixtures offers a new route to materials which exhibit this phase which complements ongoing studies into structure-property relationships and could accelerate the development of technologies utilising this remarkable polar nematic variant.

The Chiral Twist-Bend Nematic Phase (N*TB )

The twist-bend nematic, N_TB , phase has been observed for chiral materials in which chirality is introduced through a branched 2-methylbutyl terminal tail. The chiral twist-bend nematic phase, N*_TB , is completely miscible with the N_TB phase of the standard achiral material, CB6OCB. The N*_TB phase exhibits optical textures with lower birefringence than those observed for the achiral N_TB phase, suggesting an additional mechanism of averaging molecular orientations. The N*-N*_TB transition temperatures for the chiral materials are higher than the N_TB -N transition temperatures seen for the corresponding racemic materials. This suggests the double degeneracy of helical twist sense in the N T B * phase is removed by the intrinsic molecular chirality. A square lattice pattern is observed in the N* phase over a temperature range of several degrees above the N*_TB -N phase transition, which may be attributed to a non-monotonic dependence of the bend elastic constant.

Pentaerythritol Derived Tetrapode Exhibiting a Nematic-Like Mesophase at Ambient Temperatures

The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'N_X ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the N_X phase to exhibit an electrooptic response consistent with a nematic-like phase.

Indication of a twist-grain-boundary-twist-bend phase of flexible core bent-shape chiral dimers

The effect of the molecular chirality of chiral additives on the nanostructure of the twist-bend nematic (NTB) liquid crystal phase with ambidextrous chirality and nanoscale pitch due to spontaneous symmetry breaking is studied. It is found that the ambidextrous nanoscale pitch of the NTB phase increases by 50% due to 3% chiral additive, and the chiral transfer among the biphenyl groups disappears in the NTB* phase. Most significantly, a twist-grain boundary (TGB) type phase is found at c > 1.5 wt% chiral additive concentrations below the usual N* phase and above the non-CD active NTB* phase. In such a TGB type phase, the adjacent blocks of pseudo-layers of the nanoscale pitch rotate across the grain boundaries.

Novel V- and Y-Shaped Light-Emitting Liquid Crystals with Pentafluorinated Bistolane-Based Luminophores

Herein, we describe the synthesis of novel light-emitting liquid-crystalline (LC) compounds bearing pentafluorinated bistolane-based luminophores with a V- or a Y-shaped molecular geometry and the evaluation of their LC and photophysical characteristics. The V- or Y-shaped compounds exhibited a unique LC phase and showed photoluminescence (PL) behavior under various circumstances, such as in dilute solution or in the solid state. Notably, PL characteristics were observed even under high-temperature conditions with a crystal (Cr) to LC phase transition, although the PL efficiency (Φ PL) was gradually reduced because of thermal molecular motion. Interestingly, Φ PL was found to be completely recovered through the LC → Cr phase transition during the cooling process; the PL characteristics of the V- or Y-shaped compounds were sensitively changed by external thermal stress, giving these compounds the ability to act as thermoresponsive PL sensing materials.