Photoresponsive Molecular Switches and Machines

Abstract

The field of photoactivatable molecular switches and machines is expanding rapidly with an ever increasing pace of progress. In response to the steeply growing attention and relevance of this topic, we have devoted a special issue of ChemPhotoChem to the field. The issue covers a variety of the most recent developments in light-controlled molecular switches and machines brought forward by leading researchers from across the globe. A diverse set of communications, articles, minireviews, and reviews deliver fresh insights into current hot topics, urgent problems to be solved, and the most important future prospects of the field of light-activated switches and molecular machines. One of the important current trends that is clearly identified in this issue is the expansion of structural space for photoresponsive switching architectures. While the most eminent and established photoswitches, such as azobenzenes, diarylethenes, or spiropyrans, are increasingly used for ever more complex functionalities and applications, new molecular motifs, such as metronidazole, arylazopyrazoles, phenoxyl–imidazolyl radical complexes, hydrazones or indigoid chromophores, have entered the stage. These systems expand the structure and property range of light-responsive molecular tools, and thus will be of high and complementing value for future developments and applications. Another venue where there has been an ardent recent effort in the field is in tackling the persisting deficiency of detailed knowledge about the excited-state processes that govern photoswitching events. A significant body of work is dedicated to the mechanistic understanding of fundamental photophysics behind these processes. Of particular interest are: i) the dynamic processes involved in interconverting ground/stable states into meta/bistable ones, ii) the specifics of the accompanying electronic and geometric changes in the excited state, and iii) the resulting differences in the properties of photochemically interconvertible multistates. It is evident that only a combination of clever chemical structure engineering and sophisticated analytical tools will be needed to solve these important problems. Applying the spatial and temporal resolution of light provides unique opportunities for the rational control of functionality at the molecular, supramolecular, and even larger scale. In this issue the breadth of photoswitch applicability is evident by the great variety of potential uses ranging from biological chemistry, polymers, and liquid crystals, to host–guest systems, molecular machines, and solar energy storage. This diversity of applications is made possible by the inherent properties of reversible light addressability: Creation of responsiveness at the molecular level, and the unique precision of the exerted control. In this manner greater complexity can be generated in a design-driven fashion at the smallest scales, and current developments are clearly headed towards a considerable increase in complexity. Taken together we believe that this issue will not only be an important source of inspiration for specialists, but more so will instill enthusiasm and curiosity about the power of light-addressable molecular systems in the general readership. We hope that the special issue will be as stimulating for chemists, biologists, physicists, and materials scientists as it is for us. Henry Dube received his diploma in chemistry from the LMU Munich in 2004 and his Ph.D. from the ETH Zurich in 2008 in the group of François Diederich. After a postdoctoral stay at the Scripps Research Institute in the group of Julius Rebek, Jr., he started his independent group at the LMU Munich in late 2011 with a Liebig Fellowship of the FCI. Since 2014 he is an Emmy-Noether group leader at the same institution. His research focuses on photochemistry, molecular machines, and supramolecular chemistry. Ivan Aprahamian received all of his degrees (B.Sc. in 1998, M.S. in 2000, and Ph.D. in 2005) from the Hebrew University of Jerusalem, Israel. He carried out his postdoctoral research in Professor Sir Fraser Stoddart′s group at UCLA, and then joined the faculty at Dartmouth College in 2008, where he became a full professor in 2019. His research focuses on hydrazone-based adaptive functional materials, including switches, sensors and fluorophores. Nobuyuki Tamaoki obtained his Ph.D. degree from Chiba University in Japan. After working as a researcher, a senior researcher and a group leader at Japanese government's research institutes including the National Institute of Advanced Industrial Science and Technology, he moved to Hokkaido University as a full professor in 2008. Currently he is also a director of the Green Nanotechnology Center of the University. His research interests include photochemistry, stereochemistry, organic synthesis, molecular switches and motors and self-assembled molecular systems.