Modern optics and photonics constantly require break‐through materials and designs in order to achieve miniature, lightweight, highly tunable, and effective optical devices. One of the basic optical ...components is the diffraction grating (DG), widely used for the dispersion of light, beam steering, etc. This review gathers research efforts on diffractive optical elements based on cholesteric liquid crystal (CLC) materials with a supramolecular helical architecture. All main types and fabrication approaches of periodic diffractive structures from CLCs are classified and described. Key optical properties of DGs, their advantages and drawbacks are considered. Special attention is paid on the tunability of DGs including design principles and prospective chiral materials. The review consists of three parts divided according to the formation mechanism of diffractive structures: i) the spontaneously formed periodic structures from CLCs confined in cells with hybrid or homeotropic boundary conditions; ii) DGs generated by external electric field applied to CLCs layers; iii) light‐generated DGs (e.g., obtained by holography, mask exposure, photoalignment). The review also aims to initiate and gain collaborations between physicists, engineers and organic chemists to combine novel chiral photoswitches and molecular motors with sophisticated optical design paving the way towards novel smart optical materials.
Cholesteric materials possessing supramolecular helical architecture are a field of great interest for tunable diffractive optics. The review gathers research efforts on smart organic optics from cholesteric liquid crystals. Main types and fabrication approaches of diffractive structures are classified and described. Special attention is paid to the electro‐ and phototunability of optical elements including basic design principles and perspective chiral materials such as photoswitches and molecular motors.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Interfacing molecular photoswitches with liquid crystal polymers enables the amplification of their nanoscale motion into macroscopic shape transformations. Typically, the mechanism responsible for ...actuation involves light-induced molecular disorder. Here, we demonstrate that bistable hydrazones can drive (chiral) shape transformations in liquid crystal polymer networks, with photogenerated polymer shapes displaying a long-term stability that mirrors that of the switches. The mechanism involves a photoinduced buildup of tension in the polymer, with a negligible influence on the liquid crystalline order. Hydrazone-doped liquid crystal systems thus diversify the toolbox available to the field of light-adaptive molecular actuators and hold promise in terms of soft robotics.
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Essentially, all motion in living organisms emerges from the collective action of biological molecular machines transforming chemical energy, originally harvested from light, into ordered activity. ...As a man-made counterpart to nature’s biomolecular machines, chemists have created artificial molecular machines that display controlled and even directional motion in response to light. However, to be of practical value, the motion of these light-fuelled molecular machines will have to be coupled to the rest of the world. Inspired by the complex functional movement seen in the plant and animal world, chemists have undertaken the challenge to harness molecular motion and, so, they have set artificial molecular motors and switches to work and perform useful mechanical action at the macroscopic level. Here, we review these recent developments. We show how modern research has embraced the full complexity of the molecular world by aiming at the design of autonomous, and sometimes adaptive, molecular systems that work continuously under the effect of illumination. We report evidence that molecular motion can be engineered into highly sophisticated movements and that, from a fundamental point of view, continuous movement can only emerge when man-made molecules cooperate, in space and time. Eventually, unravelling the rules of molecular motion will support the creation of molecular materials that produce work continuously under a constant input of energy.The natural world has long provided inspiration for the production of artificial, adaptive materials. This Review discusses how unravelling the rules of molecular motion has enabled integration of the cooperative, and sometimes synchronized, operation of light-responsive molecular machines, across length scales, into responsive and autonomous matter.
The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together ...to produce a contraction. In spite of this progress, harnessing cooperative molecular motion remains a challenge in this field. Here, we show how the light-induced action of artificial molecular switches modifies not only the shape but also, simultaneously, the stiffness of soft materials. The heterogeneous design of these materials features inclusions of free liquid crystal in a liquid crystal polymer network. When the magnitude of the intrinsic interfacial tension is modified by the action of the switches, photo-stiffening is observed, in analogy with the mechanical response of activated muscle fibers, and in contrast to melting mechanisms reported so far. Mechanoadaptive materials that are capable of active tuning of rigidity will likely contribute to a bottom-up approach towards human-friendly and soft robotics.
Self-reproducing molecules abound in nature where they support growth and motion of living systems. In artificial settings, chemical reactions can also show complex kinetics of reproduction, however ...integrating self-reproducing molecules into larger chemical systems remains a challenge towards achieving higher order functionality. Here, we show that self-reproducing lipids can initiate, sustain and accelerate the movement of octanol droplets in water. Reciprocally, the chemotactic movement of the octanol droplets increases the rate of lipid reproduction substantially. Reciprocal coupling between bond-forming chemistry and droplet motility is thus established as an effect of the interplay between molecular-scale events (the self-reproduction of lipid molecules) and microscopic events (the chemotactic movement of the droplets). This coupling between molecular chemistry and microscopic motility offers alternative means of performing work and catalysis in micro-heterogeneous environments.
The physico-chemical processes supporting life's purposeful movement remain essentially unknown. Self-propelling chiral droplets offer a minimalistic model of swimming cells and, in surfactant-rich ...water, droplets of chiral nematic liquid crystals follow the threads of a screw. We demonstrate that the geometry of their trajectory is determined by both the number of turns in, and the handedness of, their spiral organization. Using molecular motors as photo-invertible chiral dopants allows converting between right-handed and left-handed trajectories dynamically, and droplets subjected to such an inversion reorient in a direction that is also encoded by the number of spiral turns. This motile behavior stems from dynamic transmission of chirality, from the artificial molecular motors to the liquid crystal in confinement and eventually to the helical trajectory, in analogy with the chirality-operated motion and reorientation of swimming cells and unicellular organisms.
Transferring structural information from the nanoscale to the macroscale is a promising strategy for developing adaptive and dynamic materials. Here we demonstrate that the knotting and unknotting of ...a molecular strand can be used to control, and even invert, the handedness of a helical organization within a liquid crystal. An oligodentate tris(2,6-pyridinedicarboxamide) strand with six point-chiral centres folds into an overhand knot of single handedness upon coordination to lanthanide ions, both in isotropic solutions and in liquid crystals. In achiral liquid crystals, dopant knotted and unknotted strands induce supramolecular helical organizations of opposite handedness, with dynamic switching achievable through in situ knotting and unknotting events. Tying the molecular knot transmits information regarding asymmetry across length scales, from Euclidean point chirality (constitutional chirality) via molecular entanglement (conformation) to liquid-crystal (centimetre-scale) chirality. The magnitude of the effect induced by the tying of the molecular knots is similar to that famously used to rotate a glass rod on the surface of a liquid crystal by synthetic molecular motors.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
All‐optical fabrication of elastic volume diffraction gratings in polydimethylsiloxane (PDMS) is presented. Novel material based on the commercially available PDMS with incorporated benzophenone ...(BPh) photoactive molecules is developed. The gratings are formed by a holographic technique and UV irradiation through an amplitude mask. New material permits to obtain efficient volume gratings with periods ranging from hundreds nanometers to dozens of micrometers in elastic films of different thicknesses. Besides symmetric transmission 1D gratings, slanted and 2D gratings have been fabricated as well. Photoattachment of BPh molecules and their real‐time diffusion within elastic PDMS matrix in accordance with the spatially modulated light is considered as a mechanism of the gratings formation. The refractive index modulation amplitude of about 7.0 × 10−4 is achieved. It is shown that mechanical strain of the elastic volume gratings allows fully reproducible alteration of the gratings parameters. In order to tune the diffracted wavelength over the whole visible spectral range (from 410 up to 700 nm), a strain of about 75% is required. New volume diffraction PDMS gratings represent very successful combination of cheap and widely known materials with one‐step optical fabrication techniques providing high‐performance tunable diffraction elements that can be applied in photonics, sensing, and spectroscopy.
Elastic volume gratings are fabricated based on polydimethylsiloxane modified with photosensitive benzophenone molecules. The proposed material allows the creation of volume gratings by a holographic technique. Due to the elasticity of PDMS gratings, their parameters can be reversibly driven by unidirectional stretching. Such elastic easily tunable diffraction elements can be used in optics, photonics, and spectroscopy.
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A cholesteric mixture based on the nematic liquid crystalline side-chain polymer doped with a chiral-photochromic compound was prepared and used as an active medium for creation of stable ...polarization selective gratings by phototunable modulation of the helix pitch. Such modulation was fabricated in the polymer mixture by a nonpolarized UV-irradiation with spatially modulated intensity that causes E–Z isomerization of a chiral-photochromic dopant, decreasing its helical twisting power. It was shown that the gratings recorded by UV-exposure through a mask are strongly selective to the handedness of circular polarized light. The studied polymer film forms a right-handed helical structure and, correspondingly, the diffraction of only the right-circularly polarized light was found in the transmittance mode. The maximum diffraction efficiencies were found for the wavelength values between the maxima of selective light reflection. The films obtained open very interesting possibilities for further development of materials with stable gratings operating in the entire visible spectral range. Both the position and the width of the spectral range of an efficient diffraction can be easily controlled by the UV exposure and concentration of the dopant. The materials obtained and methods developed can be used for creation of specific diffraction elements for optics and photonics.
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