Azobenzene-doped Liquid Crystal Research Articles

Light-induced bi-directional switching of thermal conductivity in azobenzene-doped liquid crystal mesophases.

The development of systems that can be switched between states with different thermal conductivities is one of the current challenges in materials science. Despite their enormous diversity and chemical richness, molecular materials have been only scarcely explored in this regard. Here, we report a reversible, light-triggered thermal conductivity switching of ≈30-40% in mesophases of pure 4,4'-dialkyloxy-3-methylazobenzene. By doping a liquid crystal matrix with the azobenzene molecules, reversible and bidirectional switching of the thermal conductivity can be achieved by UV/Vis-light irradiation. Given the enormous variety of photoactive molecules and chemically compatible liquid crystal mesophases, this approach opens unforeseen possibilities for developing effective thermal switches based on molecular materials.

Photocontrolled capacitive biosensor based on photoresponsive azobenzene-doped liquid crystals for label-free protein assay

Azobenzene-doped liquid crystals (ADLCs) have found widespread applications in fields such as holographic technology, photoactuators, and smart windows, but their potential in biosensing has not been explored. In this study, ADLCs were prepared by doping the nematic liquid crystal (LC) host E7 with a photochromic azobenzene dye ChAD-3C-S, which, upon exposure to ultraviolet (UV) light, underwent trans - cis photoisomerization and induced the phase transition of ADLCs from the cholesteric to the nematic state. The isomerization and phase transition are reversed when the nematic ADLCs are irradiated with green light. A biosensing platform based on the photoresponsive ADLCs for the detection and quantitation of bovine serum albumin (BSA) at the LC-glass interface was constructed by assembling a sandwiched LC cell with a pair of glass substrates modified with the homeotropic alignment reagent dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP). By immobilizing BSA on one of the DMOAP-coated glass substrates, the concentration-dependent effect of the biomolecule on the physical properties of ADLCs was analyzed quantitatively through multiple modes of detection including optical texture observation under a polarizing optical microscope as well as transmission spectrometry and capacitance measurements. In the photocontrolled ADLC-based capacitive biosensor, BSA was quantitated by measuring the time-evolved capacitance of an ADLC during the trans–cis-trans isomerization of ChAD-3C-S by alternating the wavelength of incident light, which improved the limit of detection (LOD) to 8.8 × 10−3 μg/ml, an order of magnitude lower than that obtained by spectrometric analysis. Results from this study suggest that the photocontrollable and fast-response features unique to ADLCs may create new possibilities for the development of innovative LC-based biosensing technologies.

Modeling the combined photo-chemo/thermo-mechanical actuation in azobenzene-doped liquid crystal thin films

Azo-doped liquid crystal polymer network (LCN) thin films are promising candidates for the development of soft-robots. These azo-doped LCNs exhibit either photo-chemo-mechanical or photo-thermo-mechanical or both deformations based on the type of azo-molecule used for doping, the type of external stimulus, and the surrounding environment. This paper presents a computational model capable of simulating this combined photo-chemo/thermo-mechanical deformation phenomena. Moreover, the influence of attenuation of light through the thickness and the influence of variation in the through-thickness alignment of mesogens are considered in the model. A detailed investigation of the influence of isomerization parameters, light intensity, and thermal parameters on light-induced temperature rise and the resultant deformations is also presented. The developed model shows good agreement with the experimental measurements available in the literature for the temperatures and deformations attained by azo-doped LCN films under illumination while placed in air/water.

Photoplasticization Behavior and Photoinduced Pressure-Sensitive Adhesion Properties of Various Polymers Containing an Azobenzene-Doped Liquid Crystal

Abstract Photochemical control of the hardness and fluidity of polymeric materials is applicable to the development of photoreversible adhesives. Here, we report that common polymers show photoplasticization behavior, which is the photoinduced reduction of the glass transition temperature, just by adding an azobenzene-doped liquid crystal (azo-doped LC). We found that the photoplasticization effect that occurred in amorphous polymers with an azo-doped LC was due to a microscopic phase separation of the azo-doped LC from the polymers. In contrast, we did not observe any photoplasticization behavior in crystalline polymers because of the macroscopic segregation of the azo-doped LC from the polymers and high rigidity of the crystalline regions of the polymers. Based on the photoplasticization effect, we successfully achieved photochemical induction of tackiness, which is a pressure-sensitive adhesion property. Furthermore, the maximum tack strength was enhanced upon photoirradiation of the polymers. We inferred that this behavior originated from an increase in the interaction between the samples and the test probe based on the change in polarity of the samples by the photoisomerization of azobenzene. We conclude that “photoplastic” polymers composed of common materials can be used for photoresponsive pressure-sensitive adhesives.

Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Summary Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast ( 6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Real-time monitoring of the uniformity of planar textures and pitches of cholesteric liquid crystals

ABSTRACTTwo methods are proposed to evaluate the uniformity of planar textures (UoPT) and the uniformity of pitches (UoP) of cholesteric liquid crystals (CLCs). First, the real-time monitoring of UoPT and UoP of an area in CLCs is feasible by measuring the degree of linear polarisation (DoLP) of the output linearly polarised lights passing through the same CLC area. Second, the UoPT and the UoP can be evaluated by directly observing the uniformity of the transmission colour across the entire CLC sample sandwiched between two polarisers. The method that uses liquid crystals (LCs) (Δε < 0) to distinguish low DoLP values caused by non-UoPT or non-UoP in chiral azobenzene-doped LCs is also proposed.

Unravelling the photothermal and photomechanical contributions to actuation of azobenzene-doped liquid crystal polymers in air and water

Design guidelines to aid the choice of azobenzene derivatives for light responsive polymer actuators in dry and wet environments.

Light-driven topographical morphing of azobenzene-doped liquid crystal polymer films via tunable photo-polymerization induced diffusion

Photo-polymerization is a convenient way to fabricate azobenzene-doped liquid crystal glassy polymeric networks. During the photo-polymerization process, monomer diffusion is initiated if mesogens with different reactivities are blended in the monomer mixture and the polymerization light intensity is not uniform. Upon the diffusion of liquid crystal monomers and azobenzenes, compositional inhomogeneities are introduced in the polymerized network, leading to stiffness and photo-responsivity gradients. However, the relation between this diffusion process, the resulting compositional gradients and the final light-triggered deformation is not well-understood. Here we developed a computational model to simulate the monomer diffusion and its effect on the mechanics of light-induced topographical switching. By demonstrating light-switchable folding motions and surface transformations of liquid crystal films with various molecular alignments, the photo-responsivity gradient is found to play a major role in modulating the switching performance. Furthermore, the photo-polymerization diffusion can be an effective approach to introduce spatially-varying material properties and to tailor the surface and film deformations by varying the wavelength, illumination area and polarization of the incoming light. The proposed framework can be used to evaluate and control photo-polymerization-induced monomer diffusion to tune light-triggered topographical morphing.

Dynamics of pitch change in chiral azobenzene-doped liquid crystals

Dynamics of pitch change in chiral azobenzene-doped liquid crystals (CAdLCs) has been investigated. Theoretically, the pitch change in CAdLCs is discontinuous. Completed behavior of the discontinuous pitch jump in a fixed thickness cell is analyzed based on the calibration term, which is a key length parameter of the gradually increasing elastic potential energy with time, according to Hooke's law. At a specific time, the energy reaches maximum value and then releases to provide a compressed/extended force to discontinuously change pitch to minimize the energy of the whole system. At other times, the compressed/extended force disturbs the uniformity of planar texture if the surface anchoring energy is weak. Discontinuous pitch jump can be observed in CAdLCs with a few turns of helix. Thus, a novel method to evaluate the pitch of CAdLCs with only two turns of the helix after UV illumination is feasible. In contrast, discontinuous pitch jump in the previous work is hard to be noticed if CAdLCs has a large number of turns of helix. The behavior of pitch change in CAdLCs on the bases of the number of turns of helix and the cell thickness will be discussed in detail.

Unusual photoresponses in the upper critical solution temperature of polymer solutions mediated by changes in intermolecular interactions in an azo-doped liquid crystalline solvent.

Photoinduced changes in the upper critical solution temperature (UCST) were investigated for polymer solutions in an azobenzene-doped liquid crystal solvent. The UCST of poly(methyl methacrylate) (PMMA) and polystyrene (PS) solutions dropped upon irradiation with UV light, which induces trans-cis photoisomerization of the doped azo dye. In the case of PMMA solutions, the photoinduced drop in UCST was significantly larger than that expected from previous studies using azo-based polymers and common solvents. Moreover, the UCST of PS solutions also decreased under photoirradiation, in a direction opposite to that expected from the contribution of polarity. X-ray diffraction data of the solvent suggest that the decreased intermolecular interaction in the solvent (i.e. larger distance between the solvent molecules) is responsible for the photoresponsive behavior of the UCST. The proposed mechanism is consistent with the Flory-Huggins theory. Using such photoresponses in the UCST, the isothermal transition between 2-phase and 1-phase states by photoirradiation was demonstrated.

Photoplasticization Effect of an Azobenzene-doped Liquid Crystal Depending on Phase Structures

A photoresponsive plasticizer for a photo-inactive polymer was developed using an azobenzene-doped liquid crystal (LC). The azo-doped LC was miscible with poly(methyl methacrylate) up to a critical concentration, above which it phase-separated from the polymer. A photoinduced decrease in the glass transition temperature (Tg) of the polymer was accomplished by photochemically changing the phase structures of the phase-separated LC; the large decrease in Tg (ΔTg = −23 °C) was obtained with the addition of 70 wt % azo-doped LC.

Multiscale modeling and its validation of the trans-cis-trans reorientation-based photodeformation in azobenzene-doped liquid crystal polymer

We propose a new model for predicting the trans-cis-trans reorientation (TCTR)-based photodeformation of the azobenzene-doped liquid-crystal polymer (azo-LCP). The model was validated through the results of a bi-directional photobending experiment performed at various temperatures and mole ratios of the azobenzene monomer within the azo-LCP. Through both numerical and experimental investigations, we found that the photobending curvature of the azo-LCP shows a maximum point at certain temperatures, but only shows a proportional relationship with the azobenzene mole ratio within the azo-LCP. We confirmed that this tendency is caused by the change in the polymeric constraint with the temperature and the low light propagation in azo-LCP.

Dual self-healing abilities of composite gels consisting of polymer-brush-afforded particles and an azobenzene-doped liquid crystal.

We prepared the composite gels from polymer-brush-afforded silica particles (P-SiPs) and an azobenzene-doped liquid crystal, and investigated their inner structure, dynamic viscoelastic properties, thermo- and photoresponsive properties, and self-healing behaviors. It was found that the composite gels had a sponge-like inner structure formed with P-SiPs and exhibited good elastic property and shape recoverability. The surface dents made on the composite gel could be repaired spontaneously at room temperature. Moreover, the composite gel exhibited a gel-sol transition induced by the trans-cis photoisomerization of the azo dye, and the transition could be used as a mending mechanism for surface cracks. Consequently, we successfully developed a material exhibiting two types of self-healing abilities simultaneously: (1) spontaneous repair of surface dents by means of the excellent elasticity of the composite gel and (2) light-assisted mending of surface cracks by photoinduced gel-sol transition.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals

The direct optical switching of bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis- isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

Reversible Phase Transition and Rapid Switching of Azobenzene-Doped Cholesteric Liquid Crystals with a Single Laser

We demonstrate the photoinduced phase transition and reversible switching of a cholesteric liquid crystal (CLC) doped with a high concentration of azobenzene LC (azo-LC). The photoinduced isotropic (PHI) state (light “ON”) is created by the irradiation of a 405 nm laser, which generates cis-isomers that destabilize the cholesteric phase and make the LC transmissive. The response time is reduced by increasing the concentration of the doped azo-LC (<10 sec with 15 wt% azo-LC doped sample). A second light source of longer wavelength can be used to initiate a cis-to-trans photoisomerization of azobenzene. However, when a second light source is not used, the isotropic-to-cholesteric phase transition typically takes a long period of time (> 2 hr) for the cis-to-trans thermal isomerization reaction. We demonstrate a rapid (<10 sec) isotropic-to-cholesteric phase transition in a 10 wt% azo-LC doped sample without the use of a second light source. The phototunable cholesteric-isotropic phase transitions of azo-LC doped CLCs are reversible and repeatable. LC alignment is unnecessary in our highly concentrated azo-LC doped CLC recipes. Our study advances the research of optically switchable photonic devices based on LC materials.

Light-Induced Photoluminescence Switching Using Liquid Crystal-Dispersed Quantum Dots

Light-induced photoluminescence (PL) switching using azobenzene-doped liquid crystal (LC)-dispersed quantum dots (QDs) is demonstrated in a format of LC cell and LC-infiltrated tube. Before light irradiation, excitation photons undergo multiple scattering events before emission out of the LC cell, and PL intensity is high. Upon the light irradiation, the trans-to-cis photoisomerization of azobenzene makes the QD-dispersed LCs highly transparent, thus allowing most excitation photons to pass through LCs and decrease PL intensity. This demonstration may allow a new modulating technique for future development of light-switchable, QD-emission-based display or photonic devices.

Light-induced Tuning of Microfiber-knot Resonator Overlaid with Azobenzene-doped Nematic Liquid Crystals

ABSTRACTThis paper demonstrates light-induced tuning of optical spectrum from optical microfiber knot resonator overlaid with an azobenzene-doped nematic liquid crystal (azo-doped NLC). The high-quality fiber resonator is made by drawing the single mode fiber to the micro-size diameter and self-twisting the microfiber as a knot shape. During the UV light irradiation the azobenzene molecules perform trans-to-cis photoisomerization which disrupts the NLC orientation. The disrupted NLC changes the effective refractive index within the LC overlaid fiber area and shifts the optical spectrum of microfiber knot resonator. The 0.25 nm spectral shifting of resonance wavelength was observed under the irradiation of 50 mW UV light.

Optically tuneable blue phase photonic band gaps

This study investigates an optically switchable band gap of photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. The trans-cis photoisomerization of azobenzene deforms the cubic unit cell of the blue phase and shifts the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with different wavelengths light. The crystal structure is verified using Kossel diffraction diagram. An optically addressable blue phase display, based on Bragg reflection from the photonic band gap, is also demonstrated. The tunable ranges are around red, green, and blue wavelengths and exhibit a bright saturated color.

Fast Cis–Trans Isomerization of an Azobenzene Derivative in Liquids and Liquid Crystals under a Low Electric Field

A dramatically increased rate of cis–trans isomerization of an azobenzene derivative dissolved in liquids (e.g. benzonitrile) and liquid crystals relative to the corresponding thermally activated process can be achieved by applying a low static electric field (see picture) without adding any supporting electrolyte. This electric-field-induced isomerization can affect the electrooptical behavior of azobenzene-doped liquid crystals.

All-Optical Active Plasmonics Based on Ordered Au Nanodisk Array Embedded in Photoresponsive Liquid Crystals

ABSTRACTWe propose a new approach towards all-optical active plasmonics based on ordered Au nanodisk arrays embedded in azobenzene-doped liquid crystals (LCs). Upon photoirradiation, the doped LCs went through phase transition induced by trans-cis photoisomerization of the azobenzene molecules. The phase transition led to the change in the refractive index of the LCs experienced by incident light, and enabled reversible tuning of the localized surface plasmon resonance (LSPR) of the embedded Au nanodisks. The tuning utilized the sensitivity of the LSPR of the Au nanodisks to the change in the surroundings' refractive index. Experimental observations on both peak shift and intensity change of the LSPR matched those from discrete dipole approximation (DDA) calculations.