Metal-Organic Frameworks in Motion.

During the last two decades, engineering motion with small-scale matter has received much attention in several areas of research, ranging from supramolecular chemistry and colloidal science to robotics and automation. The numerous discoveries and innovative concepts realized in motile micro- and nanostructures have converged in the field of small-scale swimmers. These man-made micro- and nanomachines can move in fluids by transforming different forms of energy to mechanical motion. Recently, metal-organic frameworks (MOFs), which are crystalline coordination polymers with high porosity, have been proposed as key building blocks in several small-scale swimmer designs. These materials possess the required features for motile micro- and nanodevices, such as high cargo-loading capacity, biodegradability, biocompatibility, and stimuli-responsiveness. In this review, we take a journey through the major breakthroughs and milestones realized in the area of MOF-based small-scale swimmers. First, a brief introduction to the field of small-scale swimmers is provided. Next, we review different strategies that have been reported for imparting motion to MOFs. Finally, we emphasize the incorporation of molecular machines into the MOF's architecture as the means to create highly integrated small-scale swimmers. The strategies and developments explored in this review pave the way toward the use of motile MOFs for a variety of applications in the fields of biomedicine, environmental remediation, and on-the-fly chemistry.

Characterization of Host-Guest Complexes of Supramolecular Self-Assembled Cages Using EPR Spectroscopy.

Host-guest interactions between nitroxide stable radicals and supramolecular coordination cages were investigated using electron paramagnetic resonance (EPR) spectroscopy in water and acetonitrile. TEMPO showed negligible association with the cages in water, while 4-oxo-TEMPO bound with a strength comparable to that previously reported for related ketones. Carboxylic acid-functionalized nitroxides bound strongly to the acetonitrile-soluble coordination cages. In all cases, host-guest complex formation resulted in significant decreases in the molecular tumbling rate of the guests, with tumbling becoming strongly anisotropic. The polarity of the cage environment in both solvents was found to be intermediate between water and acetonitrile.