Jun-ichi Mamiya

Photomobile Polymer Materials : Towards Light-Driven Plastic Motors

As light is a good energy source that can be controlled remotely, instantly, and precisely, light-driven soft actuators could play an important role for novel applications in wideranging industrial and medical fields. Liquid-crystalline elastomers (LCEs) are unique materials having both properties of liquid crystals (LCs) and elastomers, and a large deformation can be generated in LCEs, such as reversible contraction and expansion, and even bending, by incorporating photochromic molecules, such as an azobenzene, with the aid of photochemical reactions of these chromophores. Herein we demonstrate new sophisticated motions of LCEs and their composite materials: a plastic motor driven only by light. If materials absorb light and change their shape or volume, they can convert light energy directly into mechanical work (the photomechanical effect) and could be very efficient as a single-step energy conversion. Furthermore, these photomobile materials would be widely applicable because they can be controlled remotely just by manipulating the irradiation conditions. LCEs show an anisotropic order of mesogens with a cooperative effect, which leads them to undergo an anisotropic contraction along the alignment direction of mesogens when heated above their LC-isotropic(I) phase transition temperatures (TLC-I) and an expansion by lowering the temperature below TLC-I. [1, 13–18] The expansion and contraction is due to the microscopic change in alignment of mesogens, followed by the significant macroscopic change in order through the cooperative movement of mesogens and polymer segments. It is well known that when azobenzene derivatives are incorporated into LCs, the LC-I phase transition can be induced isothermally by irradiation with UV light to cause trans–cis photoisomerization, and the I-LC reverse-phase transition by irradiation with visible light to cause cis–trans back-isomerization. This photoinduced phase transition (or photoinduced reduction of LC order) has led successfully to a reversible deformation of LCEs containing azobenzene chromophores just by changing the wavelength of actinic light. Although the photoinduced deformation of LCEs previously reported is large and interesting, it is limited to contraction/expansion and bending, preventing them from being used for actual applications. Herein we report potentially applicable rotational motions of azobenzene-containing LCEs and their composite materials, including a first lightdriven plastic motor with laminated films composed of an LCE film and a flexible polyethylene (PE) sheet. The LCE films were prepared by photopolymerization of a mixture of an LC monomer containing an azobenzene moiety (molecule 1 shown in Scheme 1) and an LC diacrylate with an azobenzene moiety (2 in Scheme 1) with a ratio of 20/ 80 mol/mol, containing 2 mol% of a photoinitiator in a glass cell coated with rubbed polyimide alignment layers. The photopolymerization was conducted at a temperature at which the mixture exhibited a smectic phase. The glasstransition temperature of the LCE films is at about room temperature, allowing the LCE films to work at room temperature in air, as the films are flexible enough at this temperature. We prepared a continuous ring of the LCE film by connecting both ends of the film. The azobenzene mesogens were aligned along the circular direction of the ring. Upon exposure to UV light from the downside right and visible light from the upside right simultaneously (Figure 1), the ring

Photo-mechanical effects in azobenzene-containing soft materials

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.

Photomobile polymer materials—various three-dimensional movements

The composition of a crosslinked azobenzene liquid-crystalline polymer and a flexible polymer film can provide a variety of simple devices that can walk in one direction like an ‘inchworm’ and move like a ‘robotic arm’ induced by light.

Azobenzene photomechanics: prospects and potential applications

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. This class of reversible light-switchable molecules includes molecules that photo-dimerize, such as coumarins and anthracenes; those that allow intra-molecular photo-induced bond formation, such as fulgides, spiro-pyrans, and diarylethenes; and those that exhibit photo-isomerization, such as stilbenes, crowded alkenes, and azobenzenes. The most ubiquitous natural molecule for reversible shape change, however, and perhaps the inspiration for all artificial bio-mimics, is the rhodopsin/retinal protein system that enables vision, and this is the quintessential reversible photo-switch for performance and robustness. Here, the small retinal molecule embedded in a cage of rhodopsin helices isomerizes from a cis geometry to a trans geometry around a C=C double bond with the absorption of just a single photon. The modest shape change of just a few angstroms is quickly amplified and sets off a cascade of larger shape and chemical changes, eventually culminating in an electrical signal to the brain of a vision event, the energy of the input photon amplified many thousands of times in the process. Complicated biochemical pathways then revert the trans isomer back to cis, and set the system back up for another cascade upon subsequent absorption. The reversibility is complete, and many subsequent cycles are possible. The reversion mechanism back to the initial cis state is complex and enzymatic, hence direct application of the retinal/rhodopsin photo-switch to engineering systems is difficult. Perhaps the best artificial mimic of this strong photo-switching effect however in terms of reversibility, speed, and simplicity of incorporation, is azobenzene. Trans and cis states can be switched in microseconds with low-power light, reversibility of 105 and 106 cycles is routine before chemical fatigue, and a wide variety of molecular architectures is available to the synthetic materials chemist, permitting facile anchoring and compatibility, as well as chemical and physical amplification of the simple geometric change. This review article focuses on photo-mechanical effect taking place in various material systems incorporating azobenzene. The photo-mechanical effect can be defined as reversible change in shape by absorption of light, which results in a significant macroscopic mechanical deformation, and reversible mechanical actuation, of the host material. Thus, we exclude simple thermal expansion effects, reversible but non-mechanical photo-switching or photo-chemistry, as well as the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere. Azobenzene-based material systems are also of great interest for light energy harvesting applications across much of the solar spectrum, yet this emerging field is still in an early enough stage of research output as to not yet warrant review, but we hope that some of the ideas put forward here toward promising future directions of research, will help guide the field.

Three‐Dimensional Photomobility of Crosslinked Azobenzene Liquid‐Crystalline Polymer Fibers

Human skeletal muscles are composed of many bundles of fibers and their crucial function to convert chemical energy into mechanical work is achieved by generating smooth motion and inducing high stress by external stimuli. Recently, there has been a considerable effort to develop artificial muscles or actuators that can mimic muscle performance, and various materials that resemble human muscles have been reported such as shapememory alloys, polymer gels, conducting polymers, carbon nanotubes, and dielectric elastomers. To achieve smooth motion as in human muscles, it is most desirable to use soft materials with high mechanical flexibility. Crosslinked liquid-crystalline polymers (CLCPs) are unique materials with properties of both of liquid crystals (LCs) and elastomers and especially promising for applications in actuators due to the self-organization nature of LC systems. CLCPs responding to external stimuli in the form of fibers were also reported for artificial muscles. By incorporating photochromic molecules such as azobenzenemoieties into CLCPs, largemotions can be induced by photochemical reactions of these azobenzene chromophores. Soft actuators driven by light could play an important role for novel applications in a wide range of industrial and medical fields, because light is a clean energy source and can be controlled rapidly and remotely. In our previous work, we have developed photomobile materials with CLCPs containing azobenzene moieties. A bending of the CLCP films composed only of azobenzene mesogens has been observed by irradiation with UV light. The CLCP films can generate surface deformation caused by a change in alignment of LCs upon exposure to UV light, which contributes to the bending. We have also demonstrated new threedimensional movements of the CLCP and their composite materials driven only by light: a light-driven plastic motor, an inchworm walk, and a flexible robotic armmotion. They can convert light energy directly into mechanical work without the aid of batteries, electric wires, or gears. With CLCP fibers containing azobenzene moieties, one may expect the change in alignment of LC mesogens upon exposure to UV light. In this Communication, we report a precise directional control of photomobility in the CLCP fibers. We were able to induce three-dimensional movement of the CLCP fibers only by light. The structures of LCmonomers (A6AB6 andA6AB6OH) and a crosslinker, 4,40-methylenebis(phenyl isocyanate) (MDI) used in this study are shown in Figure 1a. A6AB6 was synthesized according to a procedure similar to that in the literature. The CLCP fibers were prepared by two-step reactions, as previously reported. Firstly, the LC monomers were polymerized by radical polymerization. Then the obtained copolymers were mixed with MDI, and the mixtures were formed into fibers by dipping a tip of a toothpick into the mixture and pulling the mixtures with the toothpick as quickly as possible. Thermal and optical properties of the CLCP fibers were investigated by differential scanning calorimetry (DSC), IR absorption spectroscopy, and polarizing optical microscopy (POM). By DSC measurements, it was found that the CLCP fibers exhibited a glass-transition temperature (Tg) of around 60 8C. In IR spectra of the CLCP fibers, the absorption band corresponding to the N H stretch of the urethane bond was observed at around 3500 cm .

Effect of concentration of photoactive chromophores on photomechanical properties of crosslinked azobenzene liquid-crystalline polymers

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.

Can sunlight drive the photoinduced bending of polymer films

Photoinduced bending and unbending behavior of crosslinked liquid-crystalline polymers containing azotolane moieties in side chains occurred upon irradiation with sunlight, according to the trans–cisphotoisomerization of the azotolane moieties.

Is chemical crosslinking necessary for the photoinduced bending of polymer films

Freestanding crosslinked liquid-crystalline polymer films obtained by self-assembly through intermolecular hydrogen bonding showed photoinduced bending and unbending. The structural change at the microscopic level, caused by trans–cisphotoisomerization of the azobenzene moieties at the hydrogen-bonded crosslinks, is successfully converted into a macroscopic deformation in the liquid-crystalline polymer films.

Simultaneous Analysis of Optical and Mechanical Properties of Cross-Linked Azobenzene-Containing Liquid-Crystalline Polymer Films

The photomechanical behavior of cross-linked azobenzene-containing liquid-crystalline polymer films was investigated by means of simultaneous measurement of their optical and mechanical properties. The connection between photoisomerization of the azobenzene moieties, photoinduced change in molecular alignment, photoinduced stress generation, and macroscopic bending was analyzed. Upon UV irradiation, the films exhibited bending due to gradient in cis-azobenzene content, and subsequent unbending when cis-azobenzene content became uniform throughout the film. The maximum photoinduced stress was generated in the same time scale as the time required to reach photostationary state in the cis-azobenzene concentration. The maximum values of photogenerated stress strongly depended on the crosslinker concentration, even if the azobenzene concentration and the cis-azobenzene content in the photostationary state were similar for all the polymer films. The stress is connected to the initial Youngs modulus and also to the photoinduced change in birefringence of the polymer films. In addition, a significant photoinduced decrease in Youngs modulus was for the first time observed in cross-linked azobenzene-containing liquid-crystalline polymers, which is likely to be an important factor in dictating their photomechanical behavior.

High-Contrast Photoswitching of Nonlinear Optical Response in Crosslinked Ferroelectric Liquid-Crystalline Polymers

A conceptually novel materials design, based on crosslinked ferroelectric liquid-crystalline polymers, is demonstrated for efficient switching of a second-order nonlinear optical (NLO) response in the solid state. By controlling the molecular alignment of the NLO moieties through two-photon isomerization of azobenzene molecules, reversible isothermal photocontrol of second-harmonic generation is achieved with contrast of up to 20.