Photoinduced motions in azo-containing polymers.

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Surface relief structures on azo polymer films

Security inks have become of increasing importance. They are composed of invisible substances that provide printed images that are not able to be photocopied, and are readable only under special environments. Here we report a novel photoluminescent ink for rewritable media that dichroically emits phosphorescence due to a structural bistability of the self-assembled luminophor. Long-lasting images have been developed by using conventional thermal printers, which are readable only on exposure to ultraviolet light, and more importantly, are thermally erasable for rewriting. Although thermally rewritable printing media have already been developed using visible dyes and cholesteric liquid crystals, security inks that allow rewriting of invisible printed images are unprecedented. We realized this unique feature by the control of kinetic and thermodynamic processes that compete with one another in the self-assembly of the luminophor. This strategy can provide an important step towards the next-generation security technology for information handling.

Rewritable phosphorescent paper by the control of competing kinetic and thermodynamic self-assembling events.

Nanofibrillar materials, such as cellulose, chitin and silk, are highly ordered architectures, formed through the self-assembly of repetitive building blocks into higher-order structures, which are stabilized by non-covalent interactions. This hierarchical building principle endows many biological materials with remarkable mechanical strength, anisotropy, flexibility and optical properties, such as structural colour. These features make nanofibrillar biopolymers interesting candidates for the development of strong, sustainable and biocompatible materials for environmental, energy, optical and biomedical applications. However, recreating their architecture is challenging from an engineering perspective. Rational design approaches, applying a combination of theoretical and experimental protocols, have enabled the design of biopolymer-based materials through mimicking nature's multiscale assembly approach. In this Review, we summarize hierarchical design strategies of cellulose, silk and chitin, focusing on nanoconfinement, fibrillar orientation and alignment in 2D and 3D structures. These multiscale architectures are discussed in the context of mechanical and optical properties, and different fabrication strategies for the manufacturing of biopolymer nanofibril-based materials are investigated. We highlight the contribution of rational material design strategies to the development of mechanically anisotropic and responsive materials and examine the future of the material-by-design paradigm. Nanofibrils are abundant and critical structural components in nature that can be exploited for novel and sustainable materials. In this Review, hierarchical design strategies for cellulose, silk and chitin nanofibrils in nature and in materials engineering are discussed.

Nanofibrils in nature and materials engineering

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.

Azobenzene photomechanics: prospects and potential applications

Rewritable full-color recording on a thin solid film of a cholesteric low-molecular-weight compound

New results are presented on optical recording to the films of polymers substituted with azo dyes. Holographic gratings are formed in the polyurethane films by exposure to laser light without any post-treatment as in previous works using polyacrylates, epoxy-based polymers and polyesters substituted with azo dyes. It is shown for the first time that the principal mechanism of optical recording could be the mass transfer of polymers during the cis-trans isomerization of azo dye units on the polymers from illuminated areas to non-illuminated areas. Besides this transport, some irreversible photochemical reactions involving atmospheric oxygen are suggested as another possible origin for the holographic recording.

Photomodification of polymer films: azobenzene-containing polyurethanes

A polymer film surface modified by laser illumination was used as a command surface for nematic liquid crystal (LC). Azobenzene-containing polyurethanes were taken for the material of the film. A relief grating inscribed by laser illumination onto the surface of these polymers induced a reorientation of the LC director from the initial homeotropic alignment. Possible applications of the described processes include command-surface creation for liquid-crystal displays, reversible and adaptable liquid-crystal structures for laser-beam targeting and aiming, etc.

Photoinduced alignment of nematic liquid crystal on the polymer surface microrelief

Nonlinear spatial filtering is realized due to the electro-optical effect in spatial light modulators (SLMs). Optical methods with nonlinear filtering are suggested for various optical applications such as optical inspection of photolithography masks, wavefront sensor, etc. A fast optical response of the SLM allows optical inspection with a video (or faster) rate. The optical system proposed is tolerant to the vibrations and temperature variations because of the dynamic properties of the SLM.

Nonlinear image self-filtering with liquid crystal spatial light modulator

A few optical schemes are considered and discussed for display devices, based on liquid crystal image converters. These schemes promise enhancement in light output and diminishing in system sizes in comparison to well-known ones realized yet.

Alexander V. Parfenov

Papers

Rewritable full-color recording on a thin solid film of a cholesteric low-molecular-weight compound

Photomodification of polymer films: azobenzene-containing polyurethanes

Photoinduced alignment of nematic liquid crystal on the polymer surface microrelief

Nonlinear image self-filtering with liquid crystal spatial light modulator

Advanced optical schemes with liquid crystal image converters for display applications