Photoresponsive Liquid Crystal Research Articles

Liquid Crystal Soft Robot: An Untethered Magnetic‐ and Light‐Responsive Rotary Gripper: Shedding Light on Photoresponsive Liquid Crystal Actuators (Advanced Optical Materials 7/2019)

Liquid Crystal Soft Robot: An Untethered Magnetic‐ and Light‐Responsive Rotary Gripper: Shedding Light on Photoresponsive Liquid Crystal Actuators (Advanced Optical Materials 7/2019)

Photo-responsive dye-doped liquid crystals for smart windows.

An electric field drives most dye-doped liquid crystal devices. Here, we demonstrate a new photo-responsive dye-doped self-organized cholesteric liquid crystal device. Upon UV or blue light exposure, the helical twisting power of the chiral azobenzene changes because of the trans-cis isomerization. As a result, the initially vertically aligned liquid crystal directors and dye molecules will change from transparent state to dark state. Such a polarizer-free photo-activated dimmer can be used for wide range of applications, such as diffractive photonic devices, portable information system, vehicular head-up displays, and as a smart window for energy-saving buildings.

An Untethered Magnetic‐ and Light‐Responsive Rotary Gripper: Shedding Light on Photoresponsive Liquid Crystal Actuators

AbstractHere, a remotely controlled dual magneto‐ and photoresponsive soft robotic gripper is reported, capable of loading, transport, rotation, and release of cargo. The untethered soft actuator consists of a magnetically responsive polydimethylsiloxane layer containing magnetic iron powder coated onto the central region of a light‐responsive liquid crystal polymer film hosting photochromic azobenzene dyes. Light is used to trigger the actuator to autonomously grab and pick up cargo with a high degree of control. Magnetic response is employed to conduct the locomotion as magnetic guidance, allowing the gripper to have both translational freedom and rotational freedom in its locomotion, differentiating the device from other soft robotic grippers. Control can be attained even in enclosed and/or confined spaces, through solely remote actuation. Through combined video, mechanical, and thermal analyses, the actuation mechanism of the light‐responsive liquid crystal network is investigated, shedding light on the decisive role of the temperature evolution in governing both rate of motion and deformation amplitude of the light‐responsive soft actuator.

On Untethered, Dual Magneto‐ and Photoresponsive Liquid Crystal Bilayer Actuators Showing Bending and Rotating Motion

AbstractThe integration of untethered, multi‐stimuli responsive actuation into soft microrobotic devices is a goal in the development of “smart” materials. This manuscript reports on a dual‐stimuli responsive bilayer actuator consisting of a light responsive liquid crystal network (LCN) and a magnetic responsive polydimethylsiloxane (PDMS) composite. This design is of facile fabrication with ample design freedom, using no additional adhesion layers. Untethered control of the bilayer permits motions including bending and rotation, steered individually or in synchronization. Through a systematic study the direct impact of the PDMS layer was elucidated on the light triggered rate of actuation and maximum deformation amplitude of the LCN film. The alignment (homeotropic or planar) of the LCN has a profound effect on the resulting bilayer actuation. It is demonstrated, both experimentally and theoretically, that the rates of sample heating and actuation are directly correlated and highlight the critical role of the PDMS as a heat sink. The maximum amplitude of displacement of the bilayer is tied to the stiffness, being inversely correlated to the PDMS thickness to the third power. These results give insights and provide straightforward design rules to fabricate bilayer actuators with programmed multi‐responsive properties.

Light-regulated molecular diffusion in a liquid crystal network.

Photo-responsive liquid crystal polymer networks offer promising means to generate useful functional devices, but many of them focus on their mechanical response so as to generate surface features or shape change. Here, we investigate the photomechanical effect of the polymer network for molecular transport purposes. Dual wavelength illumination of an azobenzene-functionalized cholesteric liquid crystal polymer film produces excess free volume within the film, which results in an accelerated molecular diffusion through the film. Moreover, the polarization of the UV light exposure on the cholesteric network plays an important role in a remarkable enhancement of molecular diffusion. When linearly polarized UV light rotates along with the twist of the helical axis of the cholesteric polymer, excess free volume forms sequentially from the diffusion network toward the dry network in the polymer. It works in concert with the concentration gradient of the diffusant and greatly improves the diffusion through the film.

Light-controlled tunable terahertz filters based on photoresponsive liquid crystals

We theoretically propose and investigate a light-controllable narrow-band terahertz (THz) filter and a dual-wavelength filter employing a square lattice photonic crystal with point and line defects. The point defects filled the photoresponsive liquid crystal. The change of the refractive index of the liquid crystal mixture material caused by the linearly polarized ultraviolet (UV) light and visible light results in a shift of the resonant wavelength of the cavity in the filter. We calculate transmittance spectra for the proposed THz wave filters, which have a full width at half maximum of 10 GHz and a transmittance of 95% at the resonant frequencies. By changing the polarized directions of UV and visible light, the performance characteristics of the filters can be adjusted.

Photocontrol of helix handedness in curled liquid crystal elastomers

ABSTRACTIn this manuscript, we describe a photo-responsive dual-layer polysiloxane-based liquid crystal elastomer (LCE) soft actuator with switchable handedness of helical curling motions. Benefited from the embedded organic near-infrared (NIR) absorbing dye, the LCE ribbon can perform an impressive one-handed helical curling deformation under NIR light irradiation, meanwhile execute another-handed helical curling deformation under ultraviolet (UV) light irradiation due to the trans-cis isomerisation effect of azobenzene. This work demonstrates that the handedness of helical curling motions of LCE soft actuators can be reversibly altered by modulating the wavelength band of light stimuli (UV vs. NIR), which might have potential applications in control devices.

All-Optical Control of Shape.

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large-scale, macroscopic deformations when exposed to light. Here, surface-aligned, azobenzene-functionalized LCEs are prepared via a radical-mediated, thiol-acrylate chain transfer reaction. A long-lived, macroscopic shape deformation is realized in an LCE composed with an o-fluorinated azobenzene (oF-azo) monomer. Under UV irradiation, the oF-azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF-azo LCE to analogs prepared from classical and m-fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light-induced deformations, oF-azo LCEs are demonstrated to undergo all-optical control of shape deformation and shape restoration.

Compliance‐Mediated Topographic Oscillation of Polarized Light Triggered Liquid Crystal Coating

AbstractThe ability to induce oscillating surface topographies in light‐responsive liquid crystal networks on‐demand by light is interesting for applications in soft robotics, self‐cleaning surfaces, and haptics. However, the common height of these surface features is in the range of tens of nanometer, which limits their applications. Here a photoresponsive liquid crystal network coating with a patterned director motive exhibiting surface features that oscillate dynamically when addressed by light with modulated polarization is reported. By utilizing a compliant intermediate layer, the surface topographies increase with a factor 10, from roughly 70–100 nm to 1 µm. This increase in topography height is accompanied by a superimposed dynamic oscillation with an amplitude of ≈100 nm. These values can be translated to a 16.7% average static strain with 3.3% oscillations with respect to the coating thickness. Moreover, utilizing the complying support increases the maximum rotation speeds with an in‐phase response from 2.5 up to 25° s−1. However, at this maximized rotation speed the oscillation amplitude decreases to about half of the initial value.

Photomodulated Electro-optical Response in Self-Supporting Liquid Crystalline Physical Gels.

Photoresponsive liquid crystal (LC) physical gels have attracted more and more attention because of the nature of strong response via light stimulus. Although many efforts on the breaking and recovering of physical gels through photoisomerization have been focused, fast electro-optical response and high mechanical properties even upon light irradiations are difficult to achieve at the same time. In this work, two kinds of azobenzene-containing gelators (AG1 and AG2) with different terminal groups were designed and synthesized. Both gelators could induce the nematic LC P0616A self-assemble into anisotropic phase-separated LC physical gels at low contents. Their phase-transition behavior, thermal stability, microstructure, and mechanical strength were systematically studied. Compared with AG2 in P0616A, the P0616A/AG1 gels showed better mechanical property. When the gelator content was above 3 wt %, the P0616A/AG1 gels possessed good self-supporting ability with a storage modulus more than 104 Pa. Thus, the photoresponsive electro-optical properties and structures of P0616A/AG1 gels were focused in detail. It was surprising that the electro-optical response speed of the P0616A/AG1 gels could be promoted upon UV irradiation. In particular, the decay time (τoff) was only about half when compared with the initial state, whereas the gels still exhibited good self-supporting ability; also the network of the LC physical gels had no change at macro- and microstructural levels. These exciting results would open a door for the application of this material in electro-optical devices.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Self-Assembly of Azobenzene Derivatives into Organogels and Photoresponsive Liquid Crystals.

A new class of coil-rod-coil molecules with an azobenzene core was synthesized. They were found to form robust organogels in several organic solvents. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), FTIR spectroscopy, UV/Vis absorption spectroscopy, 1 H NMR spectroscopy, and X-ray diffraction (XRD) revealed that in these organogels, the molecules self-assembled into a nanofiber network with an H-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen bonding, and van der Waals forces. Interestingly, the incorporation of the azobenzene mesogene into the rigid core led to photoisomerizable liquid crystal materials, which exhibited quick responsiveness to light and temperature, along with the trans-cis transition stimulated by UV light and heating.

Topographical changes in photo-responsive liquid crystal films: a computational analysis.

Switchable materials in response to external stimuli serve as building blocks to construct microscale functionalized actuators and sensors. Azobenzene-modified liquid crystal (LC) polymeric networks, that combine liquid crystalline orientational order and elasticity, reversibly undergo conformational changes powered by light. We present a computational framework to describe photo-induced topographical transformations of azobenzene-modified LC glassy polymer coatings. A nonlinear light penetration model is combined with an opto-mechanical constitutive relation to simulate various ordered and corrugated topographical textures resulting from aligned or randomly distributed LC molecule orientations. Our results shed light on the fundamental physical mechanisms of light-triggered surface undulations and can be used as guidelines to optimize surface modulation and roughness in emerging fields that involve haptics interfacing, friction control and wetting manipulation.

Photo-responsive liquid crystals derived from azobenzene centered cholesterol-based tetramers

Azobenzene centered cholesterol based tetramers showing spherulitic domains and photoresponsive behaviour in solution as well as Langmuir monolayers.

Thue-Morse nanostructures for tunable light extraction in the visible region

Controlling light propagation at the nanoscale is a fascinating opportunity offered by modern photonics, more than a challenge to face off. This study is aimed at investigating a particular kind of nanocomposite and reconfigurable optical metamaterials that can be exploited for the realization of a new class of switchable photonic devices, representing a breakthrough with respect to the state of the art. Existing photonic devices exhibit, in general, a drawback in the absence of tunability; this work aims to the design and characterization of metamaterials exploiting reconfigurable media, like LCs, which enable realization of a tunable, high quality, photonic quasi-crystal based switchable mode selector. It turned out that, starting from an unpolarized white light source, through a light extraction mechanism based on the diffraction of light, the high quality structure, combined with a uniformly aligned Photo-responsive Liquid Crystal (PLC), is able to give rise to an extremely narrow (FWHM ≈5 nm) and linearly polarized single mode peak of the extracted light intensity. Moreover, we have shown that the spectral properties (switching) of the samples can be finely controlled by using both an external applied voltage and a suitable pump light source with a maximum increase of 45% of the extracted light. Finally, both Scanning Electron Microscopy (SEM) and Far Field Diffraction (FFD) analysis have shown the high quality morphology of the realized structure.

Microscopic Observation of Phase Transition Behavior Induced Photochemically in Photoresponsive Liquid Crystals with Azobenzene Derivatives

Microscopic Observation of Phase Transition Behavior Induced Photochemically in Photoresponsive Liquid Crystals with Azobenzene Derivatives

Switchable Photonic Crystals Using One-Dimensional Confined Liquid Crystals for Photonic Device Application

Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photoisomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

Synthesis and phase transitions of monomers carrying a biphenyleneazobenzene or an azotolane group. Precursors of photo-responsive liquid crystal polymers

Four functional methacrylate monomers, two of them carrying a biphenyleneazobenzene group, and the other two an azotolane group, were synthesized. The synthetic route involved several reactions, among which are the Suzuki-Miyaura and Sonogashira cross-coupling reactions that allowed us to prepare long rigid rodlike azobenzene cores. The chemical structure of these new azo-monomers was confirmed by proton nuclear magnetic resonance (1H NMR) spectroscopy, and the thermotropic liquid-crystalline behavior was determined from differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses. The four monomers showed a mesomorphic behavior that extends over a broad temperature range. The phase transitions of the monomers carrying a biphenyleneazobenzene core were much broader and weaker than those of monomers carrying an azotolane group. Also, the former showed no crystallization in cooling to room temperature. The four studied monomers displayed liquid crystal phases with lamellar order (SmA and SmC), but only those carrying an azotolane core developed an additional nematic phase. The lamellar stacking of monomers is discussed in terms of lateral interactions between the rigid rodlike cores. Both types of monomers showed a trans to cis photo-conversion of around 90% when irradiated with UV–light. These new monomers are precursors of photo-responsive liquid crystal polymers.

Enhancing and reducing chirality by opposite circularly-polarized light irradiation on crystalline chiral domains consisting of nonchiral photoresponsive W-shaped liquid crystal molecules.

We found possible chirality enhancement and reduction in chiral domains formed by photoresponsive W-shaped molecules by irradiation with circularly polarized light (CPL). The W-shaped molecules exhibit a unique smectic phase with spontaneously segregated chiral domains, although the molecules are nonchiral. The chirality control was generated in the crystalline phase, which shows chiral segregation as in the upper smectic phase, and the result appeared to be as follows: for a certain chiral domain, right-CPL stimuli enhanced the chirality, while left-CPL stimuli reduced the chirality, and vice versa for another chiral domain. Interestingly, no domain-size change could be observed after CPL irradiation, suggesting some changes in the causes of chirality. In this way, the present system can recognize the handedness of the applied chiral stimuli. In other words, the present material can be used as a sensitive chiral-stimuli-recognizing material and should find invaluable applications, including in chiroptical switches, sensors, and memories as well as in chiral recognition.