Manipulating Energetic and Electronic Performance in Multicomponent Crystals through Discrete and Continuous Compositional Variation

Abstract

Organic crystalline materials demonstrate a diversity of properties that arise from a balance between the composition and structural phase of the solid. While deconvoluting the effects that chemical composition, molecular conformation, intermolecular interactions, and packing forces can have on performance is a powerful tool towards developing solid-state design strategies for improved performance, separating the independent effects of these factors on bulk properties is a challenge. In rare cases, the composition of a crystalline material can be held constant while the structural phase is modified (polymorphism) or, conversely, the structural phase kept unaltered while the composition is varied. It is from these systems that a deeper mechanistic understanding of the physical origins that give rise to bulk properties can be extracted. This dissertation focuses on the identification of multicomponent crystalline systems that accept compositional variation, allowing the effects of composition and structural phase on performance to be separated. These studies have enabled reinvestigation of paradigms held in the areas of energetic hydrates and charge-transfer ferroelectrics. Crystalline hydrates are multicomponent crystals in which one or more molecules of water occupy defined sites within the crystal lattice. The formation of crystalline hydrates has long been recognized to erode performance of traditional energetic materials (e.g. HMX, CL-20, tetryl) and is becoming a much more pervasive challenge for modern heterocyclic energetics. Unfortunately no single energetic compound was known to form a great enough diversity of crystalline hydrates to enable meaningful investigation of the effects of hydration on energetic performance. The energetic compound 5,5’-dinitro-2H,2H’-3,3’-bi-1,2,4-triazole, DNBT, is found to produce a remarkable number of anhydrous and hydrate crystal forms, which is exploited to investigate the effects of hydration on energetic performance. It is found that hydrate formation leads to degradation of the energetic performance by decreasing the crystallographic density of the solid form without contributing to the heat release. Ferroelectric behavior is exceedingly rare in organic charge-transfer (CT) cocrystals, a class of multicomponent crystalline materials formed between π-electron donating (D) and accepting (A) species. In the solid state, D and A demonstrate unequal sharing of electron density, leading to predominantly neutral (< 0.5 electron) or predominantly ionic (> 0.5 electron) charge-transfer states. Since the mid-1980s, ferroelectricity in this class of materials has been attributed to temperature-dependent transitions between neutral and ionic CT states, which has since served as a basis for design strategies towards the discovery of novel CT ferroelectrics. Unfortunately, this approach has failed to produce CT ferroelectrics that demonstrate room-temperature ferroelectricity. The CT cocrystal formed between acenaphthene (AN) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is found to demonstrate room-temperature ferroelectricity without transitioning between neutral and ionic CT states. The absence of a neutral-ionic transition in AN-F4TCNQ is further investigated through the formation of solid solutions based on the AN-F4TCNQ structural phase. Ionic CT states are doped into AN-F4TCNQ through the inclusion of dihydronaphtho[1,8bc]furan and dihydronaphtho[1,8bc]-thiophene, which is found to significantly increase the average ionicity but does not lead to a neutral-ionic transition. These results demonstrate that a material with the appropriate symmetry changes for ferroelectricity and intermediate ionicity does not necessarily show competitive ferroelectric performance, indicating the failure of conventional design strategies towards ferroelectric CT cocrystals to fully capture the factors relevant to ferroelectricity in this class of materials.