Network-Forming Metal–Organic Liquids and Glasses

Abstract

Describing the relations and interactions between various entities, networks are critical in mathematics, biology, computer science, social sciences, and business. In materials chemistry, a network refers to a system where directional interactions spatially connect distinct molecular units. Fascinatingly, an underlying network is not unique to ordered crystalline structures. Various melts and glasses adopt network structures, which fundamentally dictates their macroscopic properties and wide-ranging functionalities. In comparison to conventional disordered networks such as glassy SiO2, metal–organic network-forming liquids and glasses contains metal centers bridged by organic ligands instead of a single oxygen anion. The ability to manipulate the directional interactions between metal centers by modifying the functional group, size, and shape of the ligand offers great potential to significantly expand the design space of functional disordered materials with unique network topologies and properties. The work presented in this dissertation describes the design, synthesis, and characterization of a novel class of network-forming liquids and glasses through realizing melting transitions in metal–organic networks at record-low temperatures. Chapter One provides an introduction to metal–organic liquids and glasses as a new class of network-forming materials, with a particular focus on the interconversion between the crystalline, molten, and glassy phases of metal–organic networks. In Chapter Two, efforts to establish general thermodynamics principle to promote reversible, low-temperature melting by decreasing the enthalpy and increasing the entropy of fusion in a series of metal–bis(acetamide) networks are discussed. The network-forming nature of a metal–bis(acetamide) liquid is supported by preserved metal–ligand coordination bonds and extended-range structural ordering, as characterized by extended X-ray fine structure (EXAFS) and pair distribution function (PDF), respectively. In addition, differences in the glass-forming ability of the metal–bis(acetamide) liquid are observed and rationalized through rheological measurements. In Chapter Three, the network-forming nature of metal–bis(acetamide) liquids is further illustrated through a comparative study with molecular metal–acetamide liquids. Despite sharing similar metal–ligand coordination environments, metal–bis(acetamide) liquids exhibit structural ordering on a much longer length scale and significantly higher viscosities relative to their molecular analogs, which contributes to the higher glass-forming ability of the former. Chapter Four describes facile reversible recrystallization and glass transitions in a series of metal–ethylenebis(acetamide) networks. The high tunability of the structure and thermophysical properties of these materials are then demonstrated, either by synthetic modification or by liquid-phase blending to form binary glasses. Notably, a large reflectivity contrast ratio of 4.8 between the glass and crystalline phases of a Co−ethylenebis(acetamide) binary network is demonstrated, which has exciting implications for optical switching and rewritable data storage. Chapter Five discusses melting and glass formation in porous metal–bis(acetamide) frameworks bearing a semi-rigid ligand. While structural transitions and loss of porosity occur concurrently with melting, CO2 pressure and pressure history is found to substantially impact their glass transition behavior, revealing exciting opportunities for using a gas medium to condition structures of metal–organic glasses without changing their compositions.