Fast‐and‐Giant Photorheological Effect in a Liquid Crystal Dimer

Abstract

stimuli, and these have been the focus of very considerable fundamental and technological interest. The potential applications of this class of materials are wide ranging including as photoresponsive adhesives, brakes, in vibration control, and in fluid-assisted polishing. In electromagneto-rheological fluids, [2][3][4][5] for example, the viscosity can be switched by -two to three orders of magnitude using either electric or magnetic fields. To date, artificial switchable materials with versatile functionality have been designed by utilizing soft matter, such as photoresponsive hydrogels, [6][7][8] polymers, [9][10][11][12][13][14][15][16] liquid crystals (LCs), [14][15][16][17][18][19][20][21][22] photo-or pH-responsive micelles, [23][24][25][26][27] and surfactants. [28] However, these materials often show a tradeoff between the contrast ratio in their viscoelastic properties and the switching speed, i.e., high switching contrast ratio in their viscoelastic properties and fast switching speeds are barely achieved simultaneously. Therefore, designing materials that exhibit good overall performance is highly challenging. To achieve such a system requires a material that exists both as a fluid and a solid and that can be switched between by external stimuli.LCs are ideal candidates for mechanically switchable systems given the large number of liquid crystalline and solid phases known to exist, and that may be tuned by molecular design to achieve self-assembled structures having a range of different length scales. For example, while the highly symmetric fluid nematic (N) phase exhibits low viscosity due to the presence of only short-range spatial order, columnar or smectic phases having lower symmetries show higher viscosities due to 1D and 2D long-range periodicities. Therefore, switching between smectic or columnar, and nematic, or isotropic phases, can result in switchable mechanical materials showing elements of high performance (see, for example, refs. [14-20]). For example, Akiyama and Yoshida [14] designed sugar alcohol derivatives with multiple azobenzene-based arms that exhibited smectic LC phases and underwent room-temperature photoinduced solidliquid transitions via the trans-cis isomerization of the azobenzene fragments. Saito et al. [19] developed anthracene-based photoresponsive columnar LC materials in which a change in molecular planarity in response to light stimuli resulted in light-induced melting. Indeed, this serves as a photoresponsive adhesive. Despite these significant advances in material design, Mechanically responsive organic materials can change their viscoelastic properties in response to external stimuli. However, materials that exhibit highly contrasting viscoelastic properties coupled with fast and reversible switching between the states involved have remained elusive. Here, it is shown that a nonsymmetric photoresponsive liquid crystal dimer exhibits photoswitching of its viscoelastic properties (shear viscosity, storage, and loss moduli) with remarkable contrast of up to 10 6 whil...

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

“…With respect to both scientific viewpoints and practical applications, azobenzene is an important building block for LC materials owing to its ability to undergo trans-cis photoisomerisation [67], leading to light-induced phase transitions [68][69][70][71][72] and development of photo-mobile materials [73][74][75], light-induced alignment technologies [76][77][78][79][80], light-induced chiral segregation [81,82], and molecular designs for supramolecular functional materials [82][83][84][85][86]. To date, several azobenzene-containing twist-bend nematogenic dimers have been developed [24][25][26], and the light-induced phase transition has been verified [24,87]. However, azobenzene-based LC dimers that exhibit the NTB phase over a broad temperature range, including room temperature, have yet to be established.…”