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Rewritable paper, as an environment-friendly approach of information transmission, has potential possibility to conserve energy and promote a sustainable development of our society. Recently, photonic crystals (PCs) have become a research hotspot in the development of rewritable paper. However, there are still many shortcomings that limit the further application of PC paper, such as slow response sensitivity, short-cycle lifetime, poor storage stability, and so on. Herein, we constructed an optically rewritable azobenzene inverse opals (AZOIOs) with a thin film (ca. 1 μm) plated on an inverse opal structure based on the UV/vis switchable structure color of the sample. The top thin film acts as a protective layer to avoid the large deformation of the pore structure and the bottom inverse opal structure with refractive index/pore structure change that provides reversible structure color. Large, reversible, and rapid bandgap shift (ca. 60 nm, 2 s) of AZOIOs can be repeated more than 100 times under alternating UV/vis irradiation based on isomerization of high content of the azobenzene group. On-demand long-time preservation pattern can be obtained by the appearance of azobenzene's intrinsic color. The proof of concept for rewritable PC paper is demonstrated herein. Such inkless rewritable colorful paper paves a way for developing novel display technology.

We prepared crosslinked azobenzene main-chain liquid-crystalline polymers (MCLCPs), in which the exchange of crosslinks can be brought about by transesterification reactions. The exchangeab...

We prepared 2 types of crosslinked liquid-crystalline polymers (CLCPs) (polyacrylates) with the same side-chain length but a different position of an azobenzene moiety in the side chain. We...

The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin–retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

Composite systems of liquid crystals (LCs) and polymers have been extensively studied in various forms such as polymer-dispersed LCs and polymer-stabilised LCs. Blending LCs and polymers is an effective approach to design functional materials with stimuli responsiveness and durability. Here, we show the development of a novel composite material with crosslinked azobenzene LC polymer (PAzo) and poly(dimethylsiloxane) (PDMS) aiming at the improvement of photomobile polymer materials in mechanical properties. The composite films were prepared by photopolymerisation of azobenzene monomers in PDMS elastomers. By forming phase separated structure, in which PAzo chains were dispersed in PDMS matrix, fracture strain of photomobile polymer materials was significantly enhanced. The composite film showed macroscopic bending upon irradiation with UV light. This result implies that PAzo domains in polymer matrix can work as microactuators to afford macroscopic deformation of the whole film. The further improve...

Crosslinked liquid-crystalline polymers (CLCPs) containing azobenzene moieties show reversible macroscopic deformation upon irradiation with light. The deformation of CLCPs strongly depends on alig...

We developed photomobile polymer materials with interpenetrating polymer network (IPN) structures composed of crosslinked liquid-crystalline azobenzene polymer (PAzo) and poly(dimethylsiloxane) (PDMS). By introducing PDMS into a PAzo template network, IPN was formed without disturbing the alignment of mesogens in the PAzo network. The films showed macroscopic bending behavior upon irradiation with UV and visible light. Although the IPN film showed a phase separated structure, the bending speed was significantly enhanced compared with the pristine film of PAzo, thanks to the soft nature of PDMS. The present method of preparing IPNs can be a promising approach to combine PAzo with various polymers and enhance the mechanical and photoresponsive properties.

We developed interpenetrating polymer networks (IPNs) composed of a crosslinked azobenzene liquid-crystalline polymer (PAzo) and poly(alkyl methacrylate)s (PAMAs). When irradiated with UV and visible light, the IPN films showed reversible bending. The photoresponsive and mechanical properties of PAzo films were significantly enhanced by incorporation of the PAMA components.

We prepared photomobile polymer materials composed of crosslinked azobenzene liquid-crystalline polymers (LCPs) and methacrylates with interpenetrating polymer network (IPN) structures, and explored their photoresponsive behavior. The IPN films exhibited reversible bending upon photoirradiation. In addition, the durability of the IPN films was much higher than that of the pristine crosslinked azobenzene LCP films.

Various fabrication methods of photomobile polymer materials containing photochromic moieties have been developed to improve their photoresponsive and mechanical properties. Here, we newly prepared photomobile polymer materials with interpenetrating polymer network (IPN) structures. The IPN films, which are composed of azobenzene liquid crystalline polymer (LCP) and polymethacrylates, showed reversible bending behavior upon exposure to UV and visible light. Bending speed of the IPN films became higher with decreasing glass-transition temperature of polymethacrylate components. The IPN film containing poly(dodecyl methacrylate) showed faster bending than the pristine azobenzene LCP film.

The synthesis and characterization of organic nanoparticles composed of a polymer network of azobenzene moieties and capable of reproducible, photoinduced mechanical actuation are reported. The molecules within the nanoparticles undergo co-ordinated, reversible isomerization between cis- and trans- conformations in response to ultraviolet and visible electromagnetic radiation, resulting in a reversible 20% height contraction of nanoparticles adsorbed on a substrate. The kinetics of the actuation response as a function of light intensity and duration are reported and closely match the molecular kinetics of azobenzene photoisomerization. The results support the proposed mechanism of co-ordinated molecular conformational changes resulting in observable nanoscale actuation.

Incorporation of dynamic covalent bonds into photomobile liquid-crystalline elastomers with a polysiloxane backbone enables the alignment of mesogens and macroscopic shapes to be controlled through the rearrangement of the network topology, even after the formation of chemically crosslinked networks. The reshaped samples show various sophisticated 3D motions upon irradiation with UV and visible light, depending on their initial shapes.

We present a simple way to control the photoinduced bending direction of azobenzene-containing cross-linked liquid-crystalline polymers. By changing the location of the photoactive azobenzene moieties from cross-links to side-chains, the bending direction of the sample is reversed under identical irradiation conditions. In addition to providing a versatile route toward directionality control of the photoinduced macroscopic motions, this observation highlights the complicated nature of the photomechanical response of azobenzene-containing cross-linked liquid-crystalline polymers, showing that the photomobile behavior can be determined by seemingly small details on the materials design.

We have investigated the photoresponsive behavior of homogeneously aligned low-azobenzene-concentration crosslinked liquid-crystalline polymers. We compared the thermal contraction and photocontraction of the films, and show that upon UV irradiation, they bend efficiently towards the actinic light source. The photoinduced bending is studied both under continuous-wave irradiation and irradiation with nanosecond pulses. Pulsed irradiation revealed that the photomechanical response can take place within sub-millisecond time scale, and that even a single laser pulse can cause the sample to bend.

Crosslinked azobenzene liquid-crystalline polymer films have been prepared by in-situ photopolymerization, and their photoinduced changes in birefringence and stress on macroscopic motion were investigated. Upon exposure to UV light, the film bent toward an actinic light source along the rubbing direction, and the bent film also reverted to the initial flat state after exposure to visible light at room temperature. Moreover, the birefringence simultaneously decreased and the stress was generated upon UV irradiation. These results indicate that the molecular alignment in the film becomes disordered by trans-cis photoisomerization of the azobenzene moieties and the resultant contraction of the film generates the stress.

We prepared crosslinked azobenzene liquid-crystalline polymer brushes on flexible polymer films by surface-initiated atom transfer radical polymerization and a subsequent crosslinking reaction. Reversible isomerization of azobenzene moieties was observed in the crosslinked films upon irradiation with UV and visible light. Upon exposure to linearly polarized UV light, the films bent slightly along the polarization direction of light toward an actinic light source. After turning off the UV light, the films maintained the bent shape, and then reverted to the initial form by irradiation with visible light. We have successfully developed a novel photomobile material with crosslinked azobenzene liquid-crystalline polymer brushes.

Uniaxially aligned azobenzene liquid-crystalline polymers crosslinked by hydrogen bonding can deform in response to actinic light. The polymers can be precisely shaped due to high processability of hydrogen-bonded polymers. The mechanical properties of the polymer fibers were improved by introducing single-walled carbon nanotubes as a filler material. Because of weak hydrogen bonding, starting materials can be recovered, reused and recycled, which is very useful in view of chemical resource utilization.

We proposed a new approach to fabricate bilayer films with a crosslinked azobenzene liquid-crystalline (LC) polymer. The polyethylene (PE) substrates were coated with linear azobenzene LC polymers, and then the azobenzene layers were crosslinked by irradiation with electron beams (EBs). In the EB-irradiated films with ordered azobenzene moieties, exposure to UV light caused trans-cis isomerization and the change in alignment of azobenzene moieties. Bending of the bilayer films could be induced toward an actinic light source when they were irradiated from the side of the azobenzene layer, while they bent away from the actinic light source upon UV irradiation from the side of the PE layer. In both cases, the bent films reverted to the initial flat state by turning off the light, which could be repeated at room temperature.

Crosslinked liquid-crystalline polymer films with a different content of an azobenzene moiety were prepared by in-situ photopolymerization, and their photoresponsive behavior was investigated. Observation of the films by polarized absorption spectroscopy and polarizing optical microscopy has revealed that the azobenzene moieties are aligned along the rubbing direction in the films. Upon irradiation with UV light, a film with 5 mol% of the azobenzene bent toward an actinic light source along the rubbing direction. The bent film reverted completely to the initial state upon irradiation with visible light. On the other hand, films with 3 and 1 mol% of the azobenzene showed no obvious bending by continuous irradiation with UV light. The bent film also reverted to the flat state upon continuous irradiation with UV light.

A telechelic poly(N-isopropylacrylamide) (PNIPAM) with azobenzene end-groups (telechelic Az-PNIPAM) was prepared by reversible addition-fragmentation chain transfer (RAFT) free radical polymerization of NIPAM. This polymer self-assembles in cold water forming nanoparticles with a hydrodynamic radius (R H) of 8 nm and an aggregation number of 29 chains. The thermoresponsive behavior of Az-PNIPAM in water was investigated by turbidimetry and light scattering. In addition, the photoresponsive behavior of aqueous Az-PNIPAM was investigated by monitoring the changes with temperature of the solution and transmittance at 650 nm before and after irradiation at 366 nm.

We studied the effects of concentration and location of azobenzene chromophores on the photoinduced deformation of crosslinked liquid-crystalline polymers (CLCPs). The concentration of azobenzene chromophores in CLCP affects the degree of isomerization of azobenzene moieties and the macroscopic deformation behaviour of the films while the location of azobenzene moieties determines the contraction force and length.

Two azobenzene-containing block copolymers with an oxyethylene (P1) or an alkyl (P2) spacer in the side chain were prepared by atom transfer radical polymerization (ATRP). Neither of the block copolymers with a low azobenzene content showed a liquid-crystalline phase. However, their glass transition temperatures (T gs) were lowered by introducing flexible spacers. In particular, T g was lowered by about 30°C in P1. Their photoalignment and holographic behavior was investigated at various temperatures in thin films. Rapid changes in refractive index were induced in the P1 films upon irradiation at a temperature below T g. But the photoinduced birefringence in the P1 film was lower than that in the P2 films. Similar results were obtained in holographic recording. The azobenzene-containing block copolymer with the oxyethylene spacer (P1) used in this study has potential application for dynamic hologram.

We prepared homogeneously aligned azobenzene liquid-crystalline elastomer (LCE) films with low T g and explored their photomechanical properties. Upon irradiation with UV light, the films bent toward a light source at room temperature. The mechanical force generated upon exposure to UV light was evaluated by thermomechanical analysis. It was found that the mechanical force generated by photo-irradiation increased with an increase in the cross-linking density. In particular, an LCE film containing 80 mol% azobenzene cross-linker produced a force of over 1 MPa by photo-irradiation, which enabled the film to lift an object 20 times heavier than itself. The degree of contraction by photo-irradiation increased with an increase in temperature and light intensity. Bending could be brought about by more than 5000 times with periodic irradiation. Furthermore, it was found that the LCE films exhibited bending and unbending behaviour by irradiation with sunlight.

Laminated films composed of a flexible plastic sheet and a crosslinked azobenzene liquid-crystalline polymer (CLCP) layer with different initial alignment of azobenzene mesogens were prepared by thermal compression bonding of a CLCP film and an unstretched low-density polyethylene (PE) film with an adhesion layer. In both laminated films with a homogeneous alignment of mesogens and a homeotropic one, large movements could be induced by irradiation with UV light at room temperature. A partly laminated film with homogeneous CLCP bent from both ends of the whole film, while one with homeotropic CLCP bent up from the center of the film toward the light source. These results indicate that we enable plastic films with no photoresponsive property to move flexibly upon photoirradiation just by laminating the CLCP layers partly, and the photoinduced motions of the laminated films are completely dependent on the alignment direction of the mesogens in the CLCP layers. In addition, it was found that the maximum stres...

A block copolymer containing an azobenzene (Az) moiety in the side chain was prepared by atom transfer radical polymerization (ATRP). For comparison, a random copolymer with a similar Az content was also designed. Their photoresponsive and holographic behavior was investigated in thin films. The block copolymer film showed microphase separation with cylinder microdomains after annealing, which was then irradiated at 366 nm. It was found that trans-cis photoisomerization in the block copolymer film was slower than that in the random copolymer film. This may be due to the restricted motion of the Az moieties in the Az blocks. The photoinduced birefringence (Δn) of the block copolymer film was higher than that of the random copolymer film upon irradiation with a linearly polarized beam. In holographic recording, similar results were obtained. A block copolymer with 22-mol% Az showed a liquid-crystalline (LC) phase, while a random copolymer with 24-mol% Az exhibited no LC phase. The block copolymer showed an ...

A series of liquid-crystalline (LC) diblock copolymers with an azobenzene moiety in the side chain were prepared by atom transfer radical polymerization (ATRP). Using a bromo-terminated poly(methyl methacrylate) as a macroinitiator, the obtained diblock copolymers showed well-defined structures and narrow molecular-weight distributions. Microphase-separated nanostructures with mesogenic blocks in either continuous or separated phases were observed. Owing to varied content of mesogens in the block copolymers, they showed different behavior of photoinduced alignment and holographic recording. Block copolymers with azobenzenes in the majority phases exhibited a similar performance of photoinduced alignment and refractive-index modulation to the azobenzene-containing homopolymer, but their photoinduced mass transportation was partly suppressed by the microphase separation, leading to a lower surface relief than that of the homopolymer. The photoinduced mass transfer was greatly prohibited in the block copolym...

Plastic films coated with crosslinked liquid-crystalline polymer (CLCP) layers containing azobenzene moieties (plastic/CLCP bilayer films) were prepared by thermal polymerization of mixtures of an azobenzene monomer and a crosslinker placed between a plastic film and a glass substrate. When the glass substrate was untreated, the obtained bilayer film bent slightly away from the UV light source when irradiated on the CLCP layer side. This bending direction was the same as that of a hybridly aligned CLCP film. When the glass substrate was coated with a rubbed polyimide to strengthen the homogeneous alignment of the azobenzene moieties of the CLCP surface layer, the obtained bilayer film bent toward the irradiation direction of the incident UV light. It was the same as a homogeneously aligned CLCP film and the degree of bending was larger than that of the untreated sample. This means that the photoinduced bending behavior of the plastic/CLCP bilayer films strongly depends on the alignment of the azobenzene m...

Three azobenzene acrylate monomers containing alkyl spacers of different length were synthesized. Thermodynamic and mesomorphic properties of the monomers were measured by differential scanning calorimetry and polarizing optical microscopy. A crosslinker with the undecyl spacer was also synthesized to prepare crosslinked liquid-crystalline polymer films by photopolymerization. Polarized UV spectra and polarizing optical micrographs demonstrated that the mesogens are aligned homogeneously in the films. Low glass transition temperature of the films enabled them to bend near room temperature upon UV irradiation. Besides, the films generally bent away from the light source before they at last bent towards the light irradiation.

Novel wormlike nanostructures were self-assembled in bulk films of a well defined diblock copolymer with azobenzene moieties, which was prepared by atom transfer radical polymerization (ATRP). For comparison, a homopolymer with almost the same repeat units of azobenzene as those in the copolymer was also prepared. They both had well defined structures and exhibited a smectic liquid crystalline phase. Upon annealing the copolymer films, poly (methyl methacrylate) formed a matrix with excellent optical properties, and the azobenzene segment in the minority phase self-assembled into a wormlike mesogenic domain in the bulk films. Such block copolymer films exhibited stability and transparency by eliminating the scattering of visible light, indicating their potential application as photoresponsive functional materials. Although wormlike morphologies have been obtained in micelles from block copolymer solutions, to the best of our knowledge, such wormlike nanostructures have never been explored in bulk films.

After being pre-treated upon irradiation of unpolarized light at 366 nm, an azobenzene-containing liquid-crystalline polymer film was photoinduced alignment by one linearly polarized laser beam at 488 nm, leading to a large birefringence of 0.21. Then holographic gratings were recorded in the pre-irradiated polymer films by interference of two coherent laser beams. Little surface relief was obtained by the holographic recording process, indicating that a refractive-index grating paid most of contributions to the diffraction efficiency. Because the cooperative motion of the azobenzene mesogens were eliminated in the cis-azobenzene-rich isotropic film by the pre-irradiation, multi-processes of the refractive-index gratings were successfully achieved. The obtained grating structures were clearly shown in the polarizing optical microscopic pictures, which were also verified by their diffraction patterns. The azobenzene-containing liquid-crystalline polymer used in this study has potential application in high-...

Grating waveguide couplers with a flat surface were fabricated in an azobenzene liquid-crystalline polymer film by holographic lithography using Ar + laser beams at 488 nm. When a probe beam at 633 nm was incident to one grating of a grating waveguide coupler, the beam propagated in the waveguide and an output beam came out from the other grating with the throughput coupling efficiency of ≈5%. Upon irradiation of the film between two gratings with UV light to cause trans-cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly switched. It was found that the alternating irradiation at 366 and 436 nm induced reversible changes in the intensity of the guided probe beam.

An amphiphilic diblock liquid-crystalline copolymer consisting of poly(ethylene oxide) (PEO) and polymethacrylate containing an azobenzene moiety in the side chain was prepared by atom transfer radical polymerization (ATRP). The specifically-designed chemical constitution and the low molecular-weight polydispersity enable the block copolymer to form a regular microphase-separated nanostructure self-assembled in its bulk films, in which a hexagonal packing of the PEO cylinders normal to the substrates was dispersed in a continuous phase of azobenzene mesogens with out-of-plane alignment. Then a commerically available rubbing technique was first used to control the microphase-separated domains in the copolymer films. By introducing novel supramolecular cooperative motions between ordered azobenzene mesogens and PEO microphase domains, macroscopic array of PEO nanocylinders was regularlly formed along the rubbing direction. Both the dotted and parrallel stripe patterns of PEO nanocylinders were successfully ...

We studied the photo-induced dynamics in a LC polymer film including azobenzene by using a time-resolved and a microscopic technique. The film was confined in a liquid crystal cell, while it is a photomobile film under a free standing condition, which is triggered by the photoisomerization of azobenezene. From the result of the time-resolved measurements, the change inside the film induced by UV irradiation was highly anisotropic polarization change. Microscopic observation revealed that the film was consisted of ordered and disordered region with a patched structure, and the UV induced change was travelled in the ordered region on the order of seconds.

Input and output grating couplers with a grating period of 1.3 µm were fabricated in an azobenzene liquid-crystalline polymer film by holography. When a He-Ne laser beam at 633 nm was incident to the input grating, the beam was coupled, propagated in the polymer film, and then was taken out from the output grating. The coupling efficiency of the input and output coupler was estimated as about 1.1% in TE mode. Upon irradiation of the film with UV light to cause trans-cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly changed.

Optically transparent thick films of methacrylate copolymer containing donor-acceptor-type azobenzene and cyanobiphenyl moieties were prepared, and their optical properties were explored in terms of a photoinduced change in molecular alignment and the formation of Bragg gratings with angular and polarization multiplicity. Azobenzene moieties substituted with ethoxy, cyano, and nitro groups were employed, and their photoresponsive behavior was investigated. It was found by polarized UV-vis spectroscopy that the alignment of azobenzene moieties in the films was random in the initial state but became perpendicular to the direction of polarization of the writing beam after irradiation with a linearly polarized beam from an Ar+ laser at 514 nm. The film containing the cyanoazobenzene moieties showed the fastest formation of optical anisotropy due to an efficient change in molecular alignment. When two linearly polarized beams from the Ar+ laser were allowed to interfere in the films with a thickness of 200 microm, the diffraction beam was observed immediately and reached 97% upon photoirradiation at 200 mW/cm2 for 5 min. It was clearly observed that the response time became shorter with an increase in the light intensity of the writing beams, maintaining the high diffraction efficiency. Furthermore, it was clearly seen that the diffraction efficiency strongly depended on the direction of polarization of the reading beam. Holographic multiple data storage in a film was demonstrated, and 55 holograms with angular multiplicity were recorded successfully. In addition, multiple data storage using the polarization and angle selectivity simultaneously was first demonstrated.

Highly birefringent liquid-crystalline polymers (LCPs) containing an azobenzene group directly connected to a tolane moiety (azotolane moiety) were synthesized. Attention was focused on the effect of the position of the tolane moiety in the mesogenic azotolane group and the donor−acceptor substituent on photochemical phase transition behavior. In solution, cis−trans back-isomerization of the LCP containing azotolane moieties with a donor−acceptor was faster than that of LCPs having no acceptor groups. All the LCP films showed a homogeneous alignment and very high values of birefringence (about 0.4 at 633 nm). In the LCPs with a conventional azotolane moiety, a large change in birefringence (above 0.35) was induced repeatedly by irradiation of UV light. Furthermore, we investigated photoswitching behavior at telecommunication wavelength (1550 nm) based on the photochemical phase transition by irradiation of UV light.

We studied the effect of the ratio between the monomer and cross-linker molecules in the azobenene included liquid crystal polymer films by using the heterodyne transient grating (HD-TG) technique, which is one of the time-resolved measurement techniques. Depending on the ratio, the magnitude of the refractive index change, its anisotropy, and the lifetime of the cis isomer of azobenzene, generated by a UV pulse irradiation. By increasing the cross-linker ratio, the refractive index change and its anisotropy was reduced, indicating less ability for the motion, while slower lifetime was observed by increasing the monomer ratio, indicating that the film is difficult to return the original shape by a visiblelight irradiation. The obtained dynamics was consistent with the functionality of the films.

The change in transmittance of a 1.55-μm laser beam was investigated by a photochemical phase transition of an azobenzene liquid-crystalline polymer. Effects of the intensity of actinic light and temperature on the photoresponsive behavior were explored using laser beams at 633 nm and 1.55 μm. It was found that the intensity of the actinic light or temperature affected on the response, regardless of the wavelength of the probe beam.

We have synthesized two types of azobenzene liquid-crystalline polymers with a tolane moiety exhibiting high birefringence: one is the copolymer having an azobenzene moiety and a 3-ring tolane group in the side chain (PA-3T) and the other is the polymer containing a long mesogen core, in which the azobenzene group is directly connected to the tolane moiety (P3AT). The values of 0.35 and 0.39 in birefringence were obtained in PA-3T and P3AT, respectively. When the film was irradiated with a writing beam at 488 nm, the change in birefringence was efficiently induced with P3AT compared with PA-3T.

Based on a hydrophilic poly(ethylene oxide) macroinitiator (PEOBr), a novel amphiphilic diblock copolymer PEO-block-poly(11-4-cyanobiphenyloxy)undexyl) methacrylate) (PEO-b-PMA(11CB) was prepared by atom transfer radical polymerization (ATRP) using CuCl/1,1,4,7,10,10-hexamethyltriethylenetriamine as a catalyst system. An azobenzene block of poly (11-[4-(4-butylphenylazo)phenoxyl]undecyl methacrylate) was then introduced into the copolymer sequence by a second ATRP to synthesize the corresponding triblock copolymer PEO-b-PMA(11CB)-b-PMA(11Az). Both of the amphiphilic block copolymers had well-defined structures and narrow molecular-weight distributions, and exhibited a smectic liquid-crystalline phase over a wide temperature range.

Azobenzene liquid-crystalline elastomer (LCE) films with homeotropic alignment were prepared by in-situ photopolymerization of mixtures of an LC monomer and a cross linker, both of which contain azobenzene chromophores. Thermodynamic and mesomorphic properties of the monomers and the LCE films were determined by differential scanning calorimertry and polarizing optical microscopy. X-ray diffraction patterns revealed that the LCE films show a smectic A phase. Polarized UV spectra and conoscopic observation with a polarizing optical microscope demonstrated that the azobenzene mesogens are aligned homeotropically in the films.

Cross-linked liquid-crystalline polymer films were prepared by polymerization of mixtures containing azobenzene monomers and cross-linkers with azobenzene moieties. Two types of the cross-linked liquid-crystalline polymer films were prepared: monodomain films and polydomains films. Photoirradiation of the monodomain films with UV linearly polarized light (LPL) gave rise to bending of the films toward actinic light along the polarization direction, when the light polarization was parallel to the rubbing direction. When the two directions were perpendicular to each other, the bending was isotropic without a preferential direction. On the other hand, exposure of the polydomain films to UV LPL led to bending of the films always along the polarization direction of the actinic light. This means that one can induce bending of the film along any direction precisely by choosing the polarization direction of the actinic light.

Optical switching of a 1.55-μm laser beam was explored by means of photochemical phase transition of polymer and low-molecular-weight azobenzene liquid crystals (LCs). It was observed that the 1.55-μm laser beam could be repeatedly switched by turning on and off the actinic light, and a large change in refractive index was induced in the azobenzene LCs at the near infrared region. It was also found that controlling the thickness of LCs could enhance the change in the transmittance of the 1.55-μm laser.

A series of poly(alkyl methacrylate)s having azobenzene and cyanobiphenyl moieties were synthesized and their optical properties were characterized. The effect of content of the photosensitive unit and the length of alkyl chain of a transparent unit on the photoresponsive alignment behavior was investigated. The azobenzene monomer (photoresponsive unit), the cyanobiphenyl monomer (mesogenic unit), and the alkyl methacrylate monomers with methyl, ethyl, n-butyl, hexyl or lauryl group (transparent unit) were copolymerized to yield transparent polymer films. Upon irradiation of LPL at 488 nm, a uniaxial optical anisotropy was generated due to the molecular alignment. The photoinduced alignment behavior was strongly dependent on the alkyl chain length and the intensity of the actinic light.

Oriented liquid-crystalline network films with various cross-linking densities were prepared by polymerization of mixtures of mono- and diacrylates, both of which contain azobenzene chromophores. The optical anisotropy in the films was evaluated by polarizing optical microscopy and polarized IR spectroscopy. The dichroic ratios and the order parameters of the films were estimated from polarized UV absorbance. The free-standing films were found to show photoinduced bending and unbending behavior upon alternate exposure to unpolarized UV light at 366 nm and unpolarized visible light at >540 nm. It was observed that the films with different cross-linking densities exhibited different bending extents and speeds.

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.

Liquid-crystalline gels and elastomers were prepared by polymerization of mixtures containing azobenzene monomers and cross linkers with azobenzene moieties. Oriented liquid-crystalline gels and elastomer films were found to undergo anisotropic bending and unbending behavior only along the rubbing direction, when exposed to alternate irradiation of unpolarized UV and visible light. In the case of polydomain liquid-crystalline elastomer films, the bending and unbending were induced exactly along the polarization direction of incident linearly polarized light. By altering the polarization direction of light, a single film could be bent repeatedly and precisely along any chosen direction.