Preface

The Earth largely consists of open areas, but it also contains large porous spaces such as cavities, caves, and caverns. These recesses often harbor unique types of chemistry, life, and bacteria. On a smaller scale, chemists have long been fascinated by the concept of nanometer-scale pores in materials, since these features can be utilized to introduce various desirable properties. The expansive fields of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) include tens of thousands of materials with permanent porosity and high surface areas, which can be especially useful for applications in gas storage, gas separation, water capture, energy conversion, energy storage, chemical sensing, drug delivery, luminescence, and proton conduction, among other fields. Highly ordered porous structures featuring strong bonds can be rationally designed; the crucial chemical linkages are between metal and organic linkers in MOFs, and between organic linkers in COFs. There is a significant interest in establishing firm structure–property links in MOFs and COFs to facilitate the future design of tailored materials that address specific applications.

MOFs and COFs typically exhibit a higher degree of long-range order; thus single crystal or powder X-ray diffraction approaches are the usual first choices for atomic-level structural characterization. Other techniques including electron microscopy, three-dimensional electron diffraction, surface area analysis, thermogravimetric analysis, and X-ray absorption spectroscopy are commonly used to provide additional structural information. Solid-state nuclear magnetic resonance (SSNMR) is a powerful characterization method that provides detailed information on structure and order in MOFs and COFs, particularly because these materials incorporate a broad variety of NMR-active nuclei from across the periodic table. SSNMR has been used to investigate the local chemical environment, bond connectivity, dynamics, local disorder, and host–guest interactions in MOFs and COFs. For amorphous MOFs and COFs exhibiting lower long-range order (e.g., some two-dimensional variants), SSNMR is an effective route for obtaining structural data. There have been many elegant studies employing SSNMR characterization of MOFs and COFs, especially with the advent of novel experimental approaches and advanced hardware in recent years. The large body of work in this field has necessitated a summary to guide the inquisitive chemist.

This book consists of five chapters and is designed for a broad audience including non-experts. In Chapter 1, we briefly introduce the history of MOFs and COFs, and then provide a general background of NMR concepts along with some typical SSNMR experimental approaches. In Chapter 2, studies employing SSNMR to investigate metal centers and dopant metals in MOFs and COFs over the past 15 years are summarized. Many metals used in the construction and doping of MOFs and COFs are NMR-active, rendering them excellent probes for SSNMR structural investigation and characterization. As pulse sequences and NMR hardware have advanced, metal nuclei previously considered extremely challenging (e.g., ²⁵Mg, ⁶⁷Zn, and ⁹¹Zr) have become feasible targets for SSNMR experiments. Chapter 3 contains a review of recent SSNMR applications for the characterization of organic linkers in MOFs and COFs. These materials feature structurally and compositionally diverse organic components that are framework backbones and can act as functional sites. Many organic elements have at least one NMR-active isotope, so multinuclear SSNMR can offer several unique perspectives for examining the local linker structure and chemical environment in MOFs and COFs. In Chapter 4, we discuss the use of SSNMR to shed light on host–guest interactions in MOFs and COFs; understanding host–guest interactions is critical for establishing firm structure–property relationships. SSNMR can provide rich information regarding guest binding site locations, guest molecular orientations, guest dynamics, host–guest interaction strengths, and guest-induced changes in the host structure. Chapter 5 begins with an introduction to dynamic nuclear polarization (DNP), which offers significant gains in SSNMR experimental sensitivity and unlocks access to many additional challenging NMR nuclei and experiments that would otherwise be unfeasible. We provide a brief history of DNP NMR, describe common DNP instrumentation, and discuss studies related to the development and applications of DNP NMR in MOFs and COFs.

We would like to thank the Royal Society of Chemistry for the opportunity to edit this book, and the authors who contributed to each chapter. At the outset of this project, the COVID-19 pandemic was having severe repercussions worldwide. All contributors, along with our neighbors and communities, showed remarkable resilience and determination throughout this time. Our sincere gratitude goes to everybody who helped transform this book from a proposal into its finished form.

Wei Wang

Yining Huang

Shoushun Chen