Light in control of twisting matter

Smart materials adapt to, rather than resist, changes to their environment. In Nature, a variety of smart materials in the biological system demonstrate unique and potential functions, which appear as norm in the design of smart synthetic materials. Liquid crystals are very useful in the engineering of smart synthetic materials because of their well-ordered and controllable structure, anisotropy, and high sensitivity to external stimuli. These properties promote the engineering of synthetic materials to display smart and potential functions comparative to those of biological materials. The research presented in this thesis describes strategies to develop new, functional and smart molecular materials by amplifying molecular motion, which is intrinsically limited to the nanoscale, up to the macroscopic level of functional materials. A special focus is given to molecular motion that is triggered by light as an external stimulus, and induces a change in chiral structure of cholesteric liquid crystals at molecular level, which is eventually amplified by cooperativity of the liquid crystals into either a mechanical or an optical output. This work also provides insights into the mechanisms of amplification of molecular movement. While stabilizing the twisted organization of liquid crystals by in-situ polymerization, photo-induced molecular switching lead to disorder in the liquid crystal polymer network, which in turn created strain and was eventually transformed into mechanical motion at the macroscale (chapter 3 and 4), including both helical motion (chapter 3 and 4) and bending motion (chapter 4) which were shown to arise from a twisted geometry. Alternatively, starting from a ground state in which the twisting of a chiral liquid crystal is hampered, amplification of molecular motion was achieved by releasing this frustration, which was translated into original properties for this system (chapter 5 and 6). These investigations also point out that the amplification of molecular motion can be manipulated to reach different ranges, from the microscale (chapter 5 and 6) to the macroscale (chapter 3 and 4), by using an appropriate choice of irradiation with light (with either a local- or a spatial- focus). In addition to amplifying motion, we showed that chirality, an essential property of molecular matter, can be translated (e.g. chapter 3; molecular chirality indirectly amplified into macroscopic chiral shapes) or suppressed (e.g. chapter 5 and 6; supramolecular chirality is suppressed using homeotropic confinement) at different scales. Changes of material design, such as architecture, chemical constitution, fabrication and controlled operation have impact on the amplification of molecular motion, molecular chirality as well as other intrinsic molecular properties to present variously at the level of functional materials (e.g. mechanical output in chapter 3 (helical motion) and chapter 4 (helical- and bending motion with slow shape relaxation), and optical output in chapter 5 (single stimuli-responsiveness) and chapter 6 (dual stimuli-responsiveness)). Notably, taking inspiration from the design of biological materials in nature can lead to effective strategies in engineering new, smart and sophisticated function of materials (e.g. chapter 3). Overall, we have developed light-responsive liquid crystal polymer networks that are capable of performing light-induced helical motion and some of them also retain their light-induced shape. The complex and versatile helix-based behavior they display suggests prospective use in applications such as soft robotics and microfluidic technology. Moreover, light-responsive chiral liquid crystals show original optical properties that could be used for the development of re-writable and photonic technologies

Subnanowatt Opto-Molecular Generation of Localized Defects in Chiral Liquid Crystals

The controlled writing and deleting of topological states in soft matter systems is addressed. The reversible lightinduced topological structuring of chiral liquid crystals at the micrometer scale is reported. Various kinds of localized defect structures are generated at subnanowatt optical power levels, which is done by using chiroptical molecular switches that operate at the molecular scale

Revolving supramolecular chiral structures powered by light in nanomotor-doped liquid crystals

Molecular machines operated by light have been recently shown to be able to produce oriented motion at the molecular scale 1,2 as well as do macroscopic work when embedded in supramolecular structures 3–5. However, any supramolecular movement irremediably ceases as soon as the concentration of the interconverting molecular motors or switches reaches a photo-stationary state 6,7. To circumvent this limitation, researchers have typically relied on establishing oscillating illumination conditions—either by modulating the source intensity 8,9 or by using bespoke illumination arrangements 10–13. In contrast, here we report a supramolecular system in which the emergence of oscillating patterns is encoded at the molecular level. Our system comprises chiral liquid crystal structures that revolve continuously when illuminated, under the action of embedded light-driven molecular motors. The rotation at the supramolecular level is sustained by the diffusion of the motors away from a localized illumination area. Above a critical irradiation power, we observe a spontaneous symmetry breaking that dictates the directionality of the supramolecular rotation. The interplay between the twist of the supramolecu-lar structure and the diffusion 14 of the chiral molecular motors creates continuous, regular and unidirectional rotation of the liquid crystal structure under non-equilibrium conditions.Photo-Engineered Helices in Chiral Liquid CrystalsFree-standing three-dimensional topological structures in geometrically confined chiral nematic liquid crystals: fundamentals and application

Light-activated helical inversion in cholesteric liquid crystal microdroplets

International audienc

Time-programmed helix inversion in phototunable liquid crystals

<p>Doping cholesteric liquid crystals with photo-responsive molecules enables controlling the colour and polarisation of the light they reflect. However, accelerating the rate of relaxation of these photo-controllable liquid crystals remains challenging. Here we show that the relaxation rate of the cholesteric helix is fully determined by helix inversion of the molecular dopants.</p>

Preparation of biomimetic photo-responsive polymer springs

This protocol describes the preparation of polymer springs that twist under irradiation with light, in a manner that mimics how plant tendrils twist and turn under the effect of differential expansion in different sections of the plant. The artificial springs are typically 1 mm in width, 50 μm in thickness and up to 10 mm in length, their length being limited by cell dimensions only. They are made from polymer networks that keep memory of a liquid crystalline order, and in which an azobenzene derivative is introduced covalently as a molecular photo-switch. This liquid crystal polymer is prepared by irradiation of a twist cell filled with a mixture of shape-persistent liquid crystals, liquid crystal having reactive end groups, molecular photo-switches, some chiral dopant and a small amount of photo-initiator. This cell is assembled out of two glass slides separated by a spacer and covered by a thin film of polyimide that was rubbed along the long axis of the cell for the bottom slide, and along the short axis of the cell for the top slide. Once the cell is filled by capillarity, photo-polymerization takes place at 48 ºC and takes approximately 1.5 h. The product is a photo-responsive liquid crystal polymer network that is characterised by optical microscopy, scanning electron microscopy and tensile strength measurements. The film is post-cured overnight at 60ºC. Removing the resulting soft polymer film and cutting out the desired spring-like shape takes ~45 min. The springs operate at ambient temperature, by mimicking the orthogonal contraction mechanism that is at the origin of plant coiling. They are shape shifting under irradiation with ultraviolet light and can be pre-programmed to either wind or unwind, as encoded in their geometry. Once illumination is stopped, the springs return to their initial shape in ambient light conditions

Conversion of light into macroscopic helical motion.

A key goal of nanotechnology is the development of artificial machines capable of converting molecular movement into macroscopic work. Although conversion of light into shape changes has been reported and compared to artificial muscles, real applications require work against an external load. Here, we describe the design, synthesis and operation of spring-like materials capable of converting light energy into mechanical work at the macroscopic scale. These versatile materials consist of molecular switches embedded in liquid-crystalline polymer springs. In these springs, molecular movement is converted and amplified into controlled and reversible twisting motions. The springs display complex motion, which includes winding, unwinding and helix inversion, as dictated by their initial shape. Importantly, they can produce work by moving a macroscopic object and mimicking mechanical movements, such as those used by plant tendrils to help the plant access sunlight. These functional materials have potential applications in micromechanical systems, soft robotics and artificial muscles