Electronic Structure and Charge Transport in Conductive Metal–Organic Frameworks

This dissertation described the synthesis and characterization of various conductive metal–organic frameworks. The systematic investigation of changes in the electronic structure of these frameworks as a function of metal ion or unit cell charge provides insights into the mechanism of charge transport in metal–organic materials and new design principles for the synthesis of frameworks with specific electronic or magnetic properties./nChapter 1 provides a general introduction to the challenges associated with engineering charge transport in metal–organic materials. The design strategies for accessing high conductivities are discussed, and benchmark materials for each design strategy are provided. The biggest remaining questions facing the field are outlined, illustrating the need for improved systematic and mechanistic studies./nChapter 2 describes the synthesis of a series of layered frameworks containing early, first row transition metal ions and redox-active ligands. The combination of vibrational and electronic spectroscopy and magnetic susceptibility measurements reveal differing degrees of metal-to-ligand electron transfer across the series. These electron transfers cause the magnetic properties to switch from frustrated antiferromagnetism for titanium to ferrimagnetism for chromium. Further, the charge transport properties of these materials are shown to correlate with the energy alignment of metal and ligand orbitals, suggesting this is the critical factor for optimization in the design of new frameworks./nChapter 3 describes the electrochemical characterization of a layered iron framework containing redox-active ligands. The electrochemical capacity of this framework is shown to rival those of transition metal oxides. This capacity is further shown to be unique amongst frameworks of this structure type, where other metal ion congeners display half the capacity of the iron framework. Spectroscopic characterization demonstrates that close matching of metal and ligand redox potentials is critical to this boosted capacity, providing a new design principle for framework-based cathodes./nChapter 4 describes the synthesis of niobium and molybdenum frameworks containing redox-active ligands. The use of metal-to-ligand electron transfer to generate rare frameworks based on second-row metal ions is demonstrated. Despite being isoelectronic, these frameworks are shown to possess different electronic structures, with the niobium phase being nearly insulating while the molybdenum phase is highly conductive. Solution-phase exfoliation of the molybdenum phase is demonstrated, illustrating its potential as a new 2D material./nChapter 5 describes detailed spectroscopic and magnetic characterization of a framework containing one dimensional chains of iron ions as a function of reduction. The reduction is shown to unexpectedly possess substantial azolate character. Magnetic properties reveal that the mixed-valent frameworks show signatures of itinerant ferromagnetic interactions at high temperature, creating a Griffiths phase at room temperature with a cluster spin glass ground state. The observation of special behavior near half and full reduction suggests the possibility of more exotic electronic phenomena.

Advisor: Long, Jeffrey R