Physicochemical Methods for the Structuring and Assembly of MOF Crystals.

ConspectusMetal-organic frameworks (MOFs) are promising for various applications through the creation of innovative materials and assemblies. This potential stems from their modular nature, as diverse metal ions and organic linkers can be combined to produce MOFs with unique chemical properties and lattice structures. Following extensive research on the design and postsynthetic chemical modification of MOF lattices at the molecular level, increasing attention is now focused on the next hierarchical level: controlling the morphology of MOF crystals and their subsequent assembly and positioning to create functional composites.Beyond well-established methods to regulate crystal size and shape through nucleation and coordination modulation, physicochemical techniques leveraging wetting effects, interparticle interactions, and magnetic or electric fields offer attractive avenues for the hierarchical structuring and assembly of MOFs. These techniques facilitate crystal alignment and yield unique superstructures. While our research group primarily focuses on directing MOF crystal orientation and positioning using external stimuli such as magnetic and electric fields, we also explore hierarchical MOF synthesis and structuring using liquid interfaces and depletion force-assisted packing.This account highlights our journey and progress in developing methods to regulate the morphology, assembly, orientation, and positioning of MOF crystals, placed in the context of work by other groups. First, we examine commonly utilized structuring methods for MOF crystals that employ liquid-liquid and air-liquid interfaces to spatially confine reactions, allowing us to access unique morphologies such as mushroom-like crystals and Janus particles. We also discuss strategies for concentrating and packing MOF crystals into superstructures, utilizing fluid interfaces for spatial confinement of crystals, depletion forces, entropic effects, and crystal sedimentation.A particularly compelling challenge in expanding the applicability of MOF materials is how to manipulate free-standing MOF crystals. This issue is especially important because MOFs are typically produced as loose powders, and industrial material processing is generally more efficient when the material is fluidized. While extensive research has been conducted regarding MOF growth on substrates with both positional and orientational control, there is a clear need for similar precision with free-standing MOFs dispersed in a fluid matrix. Our group has thus focused on the relatively new, yet powerful approach of using electric and magnetic fields to manipulate MOF crystals, which offers unprecedented control over the orientation and positioning of dispersed MOF crystals, complementing the more well-established methods of MOF growth on substrates. In this Account, we provide foundational background and discussions on the interactions between these external fields and MOF crystals, including critical considerations for effective MOF manipulation using such techniques. We also discuss their unique advantages and applications, and briefly examine potential application areas, such as photonics, smart materials like soft robotics and absorbents, and sensing. This Account highlights the promising potential of well-organized and aligned MOF crystals over randomly oriented ones in various applications, owing to enhanced selectivity and performance. It underscores the importance of specialized assembly methods to advance materials science and engineering, encouraging the reader to explore such approaches.

Transparent PDMS Surfaces with Covalently Attached Lubricants for Enhanced Anti-adhesion Performance.

Liquid-like surfaces featuring slippery, omniphobic, covalently attached liquids (SOCALs) reduce unwanted adhesion by providing a molecularly smooth and slippery surface arising from the high mobility of the liquid chains. Such SOCALs are commonly prepared on hard substrates, such as glass, wafers, or metal oxides, despite the importance of nonpolar elastomeric substrates, such as polydimethylsiloxane (PDMS) in anti-fouling or nonstick applications. Compared to polar elastomers, hydrophobic PDMS elastomer activation and covalent functionalization are significantly more challenging, as PDMS tends to display fast hydrophobic recovery upon activation as well as superficial cracking. Through the extraction of excess PDMS oligomers and fine-tuning of plasma activation parameters, homogeneously functionalized PDMS with fluorinated polysiloxane brushes could be obtained while at the same time reducing crack formation. Polymer brush mobility was increased through the addition of a smaller molecular silane linker to exhibit enhanced dewetting properties and reduced substrate swelling compared to functionalizations featuring hydrocarbon functionalities. Linear polymer brushes were verified by thermogravimetric analysis. The optical properties of PDMS remained unaffected by the activation in high-frequency plasma but were impacted by low-frequency plasma. Drastic decreases in solid adhesion of not just complex contaminants but even ice could be shown in horizontal push tests, demonstrating the potential of SOCAL-functionalized PDMS surfaces for improved nonstick applications.