Integrating Macromolecules with Molecular Switches

Abstract

Long before their development by synthetic chemists, molecular machines have been operating in nature, where they perform functions critical to life. Examples of natural molecular machines include adenosine triphosphate (ATP) synthase, which enables endergonic ATP synthesis by coupling to proton concentration gradients, and linear motors, such as kinesins and myosins, whose movement along the surfaces is driven by ATP hydrolysis. Inspired by the elegance with which these and other molecular machines operate in nature, chemists have been involved in synthesizing artificial molecular machines—entities designed to perform desired tasks (such as controlling directional motion and synthesizing other molecules) in the presence of an energy source. The successful development of artificial molecular machines requires integrating switchable elements within larger objects. In that respect, polymers have attracted considerable attention: they can be prepared inexpensively and have a wide range of useful mechanical properties (depending on the structural formulae), and they can be recycled. This Special Issue highlights the diverse ways by which molecular switches can be incorporated into macromolecular architectures, as well as the emerging properties and applications of the resulting materials. Most attention over the past decade has been devoted to molecular switches operated by light, which is reflected in the contents of this Special Issue. Barrett and co-workers discuss the current status of polymers functionalized with the archetypal molecular photoswitch, azobenzene (https://doi.org/10.1002/marc.201700253). These authors review systems that can perform bending motion as well as more complex movements (such as oscillations or helical deformations) and highlight the importance of harvesting sunlight for these operations. At the same time, they point out that the light-to-mechanical energy conversion can result from not only molecular isomerization events, but can also occur because of photothermal effects. A novel example of one such “molecular robotic machine” is reported by Priimagi, Zeng, and co-workers, who incorporated a red-shifted azobenzene into a liquid crystal elastomer (https://doi.org/10.1002/marc.201700224). The authors describe how the movement's directionality of their caterpillar-shaped “robot” can be controlled by the topology of the underlying surface. In a related contribution, Yu, Wei et al. focus on some key engineering aspects of photoactuating polymers, namely, their easy fabrication and the use of low-cost polymer matrices (https://doi.org/10.1002/marc.201700237). By dispersing crosslinked azobenzene polymers within a flexible polyurethane matrix, they were able to effectively transfer light-responsiveness to the otherwise photoinert matrix. The photoinduced bending could be reversed by combining thermal treatment and mesogen realignment, conferring to the films the ability to deform reversibly. The potential of analytical methods—in particular, vibrational (infrared and Raman) spectroscopy—for probing photoswitching in azo-polymers, is highlighted by Pellerin, Vapaavuori, and Bazuin (https://doi.org/10.1002/marc.201700430), who contend that the development of such analytical tools is essential for a better mechanistic understanding of key processes, such as translation of molecular-scale photochemical events into macroscopic motion. Light-triggered changes, such as unfolding/folding of macromolecules, mass-transport, and photoinduced chirality in azo-materials can now be studied in great detail with the help of these techniques, owing to their ability to finely probe the molecular environment, orientation, and chiral order. An important direction in the optical manipulation of molecular systems lies in the development of strategies allowing one to overcome the use of UV irradiation and instead, to operate the systems using visible or near-infrared light. This has several important advantages, including decre