Molecular Machines

Molecular switches and machines may be thought of as the engines driving “nanoscience” and “nanoengineering” applications. This Special Issue celebrates the continued and increasing vitality of this important area of molecular and physical sciences and especially recent developments in bio-relevant applications, with 27 contributions from across North America, Asia, and Europe. The breath that the field has reached is demonstrated amply in the six Minireviews that set the stage for the main themes addressed in the 21 original research articles. In this issue, Fraser Stoddart and co-workers lift the bonnet of molecular machines to review the challenges and opportunities now available to us in getting molecular machines to work at the macroscopic level. Meanwhile, the fundamental concept of what it means to do work at the nanoscale, and especially the meaning of the oft used term “the power stroke”, is tackled by Dean Astumian, Shayantani Mukherjee, and Arieh Warshel in their excellent essay on the subject, in which they delve deep into the thermodynamic and kinetic aspects that are as pertinent to small molecules as they are for relatively massive biological protein-based machines. Indeed, physical and theoretical/computational chemistry are increasingly taking center stage, both in our efforts to understand as well as to develop new systems. Experimental and theoretical aspects of rotaxane chemistry and physics are the focus of several contributions to this issue. Alberto Credi, Ettore Fois, and co-workers focus on rotaxane threading and dethreading, while Davide Williams and co-workers explore the power produced by rotaxane motors and Jose Berna and co-workers delve into the conformational dynamics of rotaxane switching in a combined NMR and DFT study. Frédéric Coutrot and co-workers carry on the journey over conformational space in their study of the anomeric effect on shuttling. Rotaxane shuttling is also the focus of contributions by Stephen J. Loeb and co-workers and by Juli Jiang, Leyong Wang and co-workers, who explore new recognition motifs as stations. The ground- and excited-state motion of molecular switches and motors is explored by Ben Feringa and co-workers, who examine the role of viscosity in activation energies for switching. Jürgen Heck, Denis Jacquemin, and their co-workers explore into the excited-state pathways that are central to photochemical switching of dithienylethenes and the recently discovered donor–acceptor Stenhouse adduct (DASA) switches, respectively, which highlight both the challenges and the opportunities available now towards truly bottom-up-designed light-driven molecular machines. Of course, every motor needs an energy source to drive its function, and the search for new and innovative approaches to achieve this is unending. Nathan McClenaghan and co-workers review recent advances in the use of multiphotochromes and resonant electronic energy transfer (REET) to achieve ever more complex functions. The full versatility of this approach necessitates rational design, and the tuning of switching wavelengths is a key aspect that is addressed by Françisco Raymo′s team for oxazine photochromes. Meanwhile, alternative approaches to driving molecular machines are reported by Browne and co-workers in efforts towards redox-driven, rather than photochemically driven, switching of E–Z isomerization in molecular rotary motors, and by David Leigh, Fred Brouwer, and co-workers, describe the use of one- and two-electron reductions of stations in a multistation rotaxane to achieve successive translocation of a bound shuttle. Minireviews by Da-Hui Qu and by Gwénaël Rapenne and co-workers open up the topic of interfacing molecular machines with surfaces and the challenges and opportunities that the approach presents in connecting molecular systems to the macroscopic world. Meanwhile, Jeffery Zink and co-workers review the challenges and opportunities presented in immobilization of nanomachines on silica nanoparticles for remotely controlled cargo release. These discussions set the stage for several of the original reports focused on surface- and nanoparticle-immobilized switches and machines. Rainer Herges and his co-workers′ efforts to mimic the action of cilia by self-assembly of molecular photoswitches on surfaces as well as the work by Marta Mas-Torrent and colleagues show how redox switching can be used to control surface properties externally. Immobilization of switches on nanoparticles is the focus of contributions by Ying-Wei Yang, Rafal Klajn, and their co-workers. Klajn and colleagues show that immobilization of spiropyrans on gold can be achieved without loss of functionality due to quenching by the gold nanoclusters, while Yang and co-workers report a dual responsive nanoparticle system that shows promise as a selective autonomous drug-delivery vehicle. The drive to develop biohybrid systems continues to inspire novel functional systems, and the team of Nathalie Katsonis show how molecular switches with on–off fluorescence functionality can be combined with bacterial nanocages to provide new opportunities in super-resolution imaging. On the subject of luminescence, the teams of Jérôme Lacour and Luisa De Cola joined forces to develop a high-contrast ratiometric (two-color) luminescent sensor for potassium ions, which presents immediate opportunities in bioimaging applications, while Xiang Ma and co-workers report a novel cucurbiturile-based rotaxane, which achieves high-contrast on and off pH switching of phosphorescence in response to acid and base stimuli. Molecular switching can be seen as focused on changes to physical properties rather than mechanical changes. However, as discussed by Junqiu Liu and co-workers, the mechanical action of bringing components together and moving them apart is an equally important field both for sensing and in the emerging field of switchable catalysis. Maurizio Prato, Aurelio Mateo-Alonso, and co-workers show how molecular shuttling can be used to control the outcome of bis-additions to C60 derivatives. Overall, we hope that you agree that the breadth of physical, chemical, and biological sciences that are encompassed by the field of molecular machines is wide, but nevertheless, many common themes bind the field together. Both as Guest Editors to this Special Issue and enthusiastic molecular mechanics ourselves, we thank all of those that have contributed to making this Special Issue a fantastic jumping-off point for those eager to enter the world of molecular switches, motors, and machines. The field is expanding continuously, and as our ability to both realize ever more complex functional molecular architectures and interface molecular-based systems to the macroscopic world increases, through international as much as interdisciplinary cooperation, so will the opportunities for nanomolecular systems to hit the big time also increase.

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