A soft-bent dimer composite exhibiting twist-bend nematic phase: Photo-driven effects and an optical memory device

Abstract

We report the influence of photoisomerization on a guest-host composite comprising a soft-bent dimer exhibiting the presently sought-after twist-bend nematic (NTB) phase, doped with small amounts of a calamitic photoactive azobenzene-based dimer that assumes a bent shape when photo-driven. Trans-cis photoisomerization of the latter compound leads to a substantial decrease in the nematic-isotropic and nematic-NTB transition temperatures. The results bring out the puzzling feature that the thermal stability of the NTB phase, having a helical structure, despite the molecules being achiral, is influenced more than the regular nematic. Differential influence is also seen in the Frank elastic constants of the regular nematic: while the splay elastic constant is significantly diminished its bend counterpart is hardly affected. Molecular conformational aspects of the entities are proposed to explain these features. Through a judicious combination of this photo-driven effect and an AC electric field, an attractive type of optical memory device is demonstrated.

Methylene- and thioether-linked cyanobiphenyl-based liquid crystal dimers CBnSCB exhibiting room temperature twist-bend nematic phases and glasses

“…To date, a wide variety of bent LC dimers, oligomers, 64,[73][74][75][76][77] polymers, 78 and bent-core molecules 79,80 have been found to exhibit the N TB phase. In addition, N TB materials have been used for diverse applications such as photonic and optical devices, [81][82][83][84] gels, 85 elastic bodies, 86 and photoalignment technologies. 87 For characterization and application purposes, it is necessary to develop materials that exhibit the N TB phase over a wide temperature range, the room-temperature N TB phase, and its glass phase or N TB glass (N TB G).…”

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

“…18 Several techniques, including electro-optical measurements, 19 freeze-fracture transmission electron microscopy, 20,21 and resonant soft, hard, and tender X-ray scattering measurements, [22][23][24][25][26][27] suggest that the helical pitch of an N TB phase formed by several molecules is B10 nm, which is significantly shorter than the pitch of a typical chiral N phase or cholesteric phase exhibiting a helicoidal helix (generally hundreds of nanometers). Materials that exhibit the N TB phase have been employed for various LC applications, such as wavelength-tunable selective light reflection [28][29][30][31] and optical memory 32 devices, LC physical gels, 33 photo-switchable viscoelastic bodies, 34 and photoalignment technology. 35 To date, a number of bent LC dimers composed of two mesogenic structures connected with an odd-numbered oligomethylene spacer have been shown to exhibit the N TB phase.…”

Thioether-linked azobenzene-based liquid crystal dimers exhibiting the twist-bend nematic phase over a wide temperature range

“…In-depth reinvestigation of an LC dimer comprising two 4cyanobiphenyl arms connected by a heptamethylene spacer, 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl)heptane (CB7CB) indicated that the Nx phase is the NTB phase [11] (here, it is to be noted that the Nx phase observed for the tetrapod molecule reported recently by Mandle et al was distinguished from the NTB phase [7]). Thus, identification of the NTB phase inspired extensive study on a large number of molecules that exhibit the NTB phase, such as bent LC dimers , various oligomers [35,[37][38][39][40][41][42][43][44][45][46], polymers [47], and bent-core molecules [48,49], and on related potential applications [50][51][52][53][54]. Electro-optical analysis [55], transmission electron microscopy [56,57], and soft, tender, and hard resonant X-ray scattering [34,[58][59][60][61] confirmed that the helical pitch of the NTB phase is considerably short, typically the length of several molecules or 10-20 nm.…”