Title: Controlling the Properties of Modular Materials
Description: I introduce the concept of modular materials and give a brief overview of their history and widespread occurrence in many areas of chemistry. I then discuss some of the many applications in which modular materials may find a use and link them to the following chapters. Chapter 1 describes the layered superatomic material Re₆Se₈Cl₂ and the induction of superconducting behavior in its single crystals through a current annealing technique. We suggest that this superconductivity arises through electron doping, as a result of dissociation of the apical Cl atoms from the clusters. Chapters 2-4 explore other types of superatomic materials and their properties, centered on the well-studied Co₆X₈ unit, where X is a chalcogen. Chapter 2 describes a Co₆Te₈-C₇₀ co-crystal that exhibits multiple phase changes with temperature, each giving rise to unique electronic, thermal, and structural properties. Chapter 3 describes a series of “solid solutions” of Co₆Se₈ and Cr₆Te₈ units. By varying the ratios of the component superatoms, transport properties of the crystals can be tuned, and unexpected behavior arises as a result of structural heterogeneity.

Chapter 4 presents another study of Co₆Se₈ co-crystallized with rod-shaped C₁₄₀ fullerenes. The packing and electronic properties are found to be greatly affected by the degree of solvent inclusion. Chapters 5-6 examine another class of cluster-based materials: atomically precise gold nanoparticles. In Chapter 5 the cluster Au₂₁ is shown to self-assemble depending on the surface “hook” ligands, with corresponding differences in electronic transport. Chapter 6 discusses an interesting phase transition and thermally-induced hysteresis observed in crystals of the Au₁₀₃ cluster, also related to the surface ligand configuration. Chapters 7-8 take a different approach to modular materials, in the form of organic polymers. Using the robust, electroactive pigment molecule PDI as a common building block, we synthesize extended networks that are found to be exceptional pseudocapacitive energy storage materials. Chapter 7 introduces the honeycomb-shaped PDI-triptycene polymer, establishes its pseudocapacitive nature, and explores the role of cyclization in tuning its behavior.

Chapter 8 expands upon the concept by combining PDI with hexaazatrinaphthalene to create a “contorted” network with best-in-class energy storage performance. In addition to in-depth kinetic analyses to elucidate the mechanism of storage, we fabricate two-electrode cells to demonstrate the material’s potential in real-world devices.