Light in control of twisting matter

Abstract

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.