Laser-pointer-induced self-focusing effect in hybrid-aligned dye-doped liquid crystals.

Nonlinear optics deals with phenomena where "light controls light"; e.g., there is mediation by an intensity-dependent medium through which light propagates. This field has attracted much attention for its immense potential in applications dependent on nonlinear processes, such as frequency conversion, multiple-photon absorption, self-phase modulation, and so on. However, such nonlinearities are typically only observed at very high light intensities and thus they require costly lasers. Here, we report on a self-focusing effect induced with a 1 mW handheld laser pointer. We prepared polymer-stabilized dye-doped liquid crystals, in which the molecular director orientation gradually changes from homeotropic at one surface to homogeneous at the other. This is referred to as hybrid alignment. In such films, the threshold intensity needed to form diffraction rings was reduced by a factor of 8.5 compared to that in conventional homeotropic cells, which enabled the induction of the self-focusing effect with a laser pointer.

Photomobile polymer materials: photoresponsive behavior of cross-linked liquid-crystalline polymers with mesomorphic diarylethenes.

Cross-linked liquid-crystalline (LC) polymers with a mesomorphic diarylethene were prepared to demonstrate a versatile strategy for cross-linked photochromic LC polymers as photomobile materials. Upon exposure to UV light to cause photocyclization of the diarylethene chromophore, the cross-linked polymer films bend toward an actinic light source. By irradiation with visible light to cause a closed-ring to open-ring isomerization, the bent films revert to the initial flat state. Without visible-light irradiation, the bent films remain bent even at 120 °C, indicating high thermal stability of the cross-linked diarylethene LC polymers.

Facile strain analysis of largely bending films by a surface-labelled grating method.

Mechanical properties of flexible films, for example surface strain of largely bending films, are key to design of stretchable electronic devices, wearable biointegrated devices, and soft microactuators/robots. However, existing methods are mainly based on strain-gauge measurements that require miniaturized array sensors, lead wires, and complicated calibrations. Here we introduce a facile method, based on surface-labelled gratings, for two-dimensional evaluation of surface strains in largely bending films. With this technique, we demonstrate that soft-matter mechanics can be distinct from the mechanics of hard materials. In particular, liquid-crystalline elastomers may undergo unconventional bending in three dimensions, in which both the inner and outer surfaces of the bending film are compressed. We also show that this method can be applied to amorphous elastomeric films, which highlights the general importance of this new mechanical evaluation tool in designing soft-matter-based electronic/photonic as well as biointegrated materials.

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.

Location of the Azobenzene Moieties within the Cross-Linked Liquid-Crystalline Polymers Can Dictate the Direction of Photoinduced Bending.

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.

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 Young's modulus and also to the photoinduced change in birefringence of the polymer films. In addition, a significant photoinduced decrease in Young's 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.

Solvent induced amplification of the photoresponsive properties of α,ω-di-[4-cyanophenyl-4'-(6-hexyloxy)-azobenzene]-poly(N-isopropylacrylamide) in aqueous media.

Irradiation at room temperature of α,ω-di-[4-cyanophenyl-4'-(6-hexyloxy)-azobenzene]-poly-(N-isopropylacrylamide) (Az(2)-PNIPAM) solutions in water/1,4-dioxane (6 mol% dioxane) reversibly converts a turbid suspension into a clear solution, demonstrating for the first time that cononsolvency of PNIPAM in mixed aqueous solvents in synergy with preferential chromophore solvation can act as actuators of responsive systems.

Photomechanics of liquid-crystalline elastomers and other polymers.

Muscle is a transducer that can convert chemical energy into mechanical motion. To construct artificial muscles, it is desirable to use soft materials with high mechanical flexibility and durability rather than hard materials such as metals. For effective muscle-like actuation, materials with stratified structures and high molecular orders are necessary. Liquid-crystalline elastomers (LCEs) are superior soft materials that possess both the order of liquid crystals and the elasticity of elastomers (as they contain polymer networks). With the aid of LCEs, it is possible to convert small amounts of external energy into macroscopic amounts of mechanical energy. In this Review, we focus on light as an energy source and describe the recent progress in the area of soft materials that can convert light energy into mechanical energy directly (photomechanical effect), especially the photomechanical effects of LCEs with a view to applications for light-driven LCE actuators.

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.

A rodlike organogelator: fibrous aggregation of azobenzene derivatives with a syn-chiral carbonate moiety.

A chiral azobenzene derivative containing a cyclic syn-carbonate moiety functions as a gelator for various organic solvents; the dipole-dipole interaction drives the fibrous self-assembly of the rodlike gelator.