Development and Evolution of Energetic Cocrystals.

In spite of the importance of energetic materials to a broad range of military (munitions, missiles) and civilian (mining, space exploration) technologies, the introduction of new chemical entities in the field occurs at a very slow pace. This situation is understandable considering the stringent requirements for cost and safety that must be met for new chemical entities to be fielded. If existing manufacturing infrastructure could be leveraged, then this would offer a fundamental shift in the discovery paradigm. Cocrystallization is an approach poised to realize this goal because it can use existing materials and make new chemical compositions through the assembly of multiple unique components in the solid state. This account describes early proof-of-principle studies with widely used energetics in the field, including 2,4,6-trinitrotoluene (TNT) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), forming cocrystals with nonenergetic coformers that alter key properties such as density, sensitivity, and morphology. The evolution of these studies to produce cocrystals between two energetic components is detailed, including those exploiting new intermolecular interaction motifs that drive assembly such as halogen bonding. Implications of cocrystallization for performance, sensitivity to external stimuli, and manufacturability are explored at each stage. The derivation of many of these cocrystals from energetic materials in common use satisfies the goal of using materials already demonstrated to be cost-effective at scale and with well-understood safety profiles. The account concludes with a discussion of cocrystallizing molecules having excess of oxidizing power with those that are oxygen-deficient to push the limits of explosive performance to unprecedented levels. The purposeful production of an arbitrary combination of two solid components into a cocrystal is far from certain, but the studies described motivate the continued exploration of novel materials and the development of predictive models for identifying crystallization partners. When such cocrystals form, many of their most important properties cannot be predicted, pointing to another challenge for the purposeful development of energetic materials based on cocrystallization.

Thermal decomposition pathways of nitro-functionalized metal-organic frameworks.

The decomposition behavior of high energy metal-organic frameworks (MOFs) with extensive nitration is disclosed. In contrast to the detonation behavior observed in molecular energetic compounds, deflagration transforms cubic MOFs into anisotropic carbon structures with highly dispersed metal. Both the structural metal and intimate mixing are found to be critical.

A melt castable energetic cocrystal.

An energetic cocrystal is described between 3,4-diaminofurazan (DAF) and 4-amino-3,5-dinitropyrazole (ADNP). The material is remarkable because interaction between the components leads to melting behavior and melt-state stabilization (absent in ADNP), that allows a melt castable formulation with explosive performance superior to DAF. This provides another potential advantage of cocrystallization for energetic materials.

Hydrogen Peroxide Solvates of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane.

Two polymorphic hydrogen peroxide solvates of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20; wurtzitane is an alternative name to iceane) were obtained using hydrated α-CL-20 as a guide. These novel H2 O2 solvates have high crystallographic densities (1.96 and 2.03 g cm-3 , respectively), high predicted detonation velocities/pressures (with one solvate performing better than ϵ-CL-20), and a sensitivity similar to that of ϵ-CL-20. The use of hydrated materials as a guide will be important in the development of other energetic materials with hydrogen peroxide. These solvates represent an area of energetic materials that has yet to be explored.

Rendering non-energetic microporous coordination polymers explosive.

Adsorption of oxidizing guest molecules into a non-energetic microporous coordination polymer produces explosives with desirable oxygen balance, high heat released upon decomposition, and suppressed vapor pressure of the guest. Here, this results in primary explosives, materials very sensitive to impact, that have the potential to be used as replacements for lead-based initiators.

Synthesis and photophysical properties of biphenyl and terphenyl arylene-ethynylene macrocycles.

A series of single-walled carbon nanotube precursors, C3h-symmetric cyclotri(ethynylene)(biphenyl-2,4'-diyl) and cyclotri(ethynylene)(p-terphenyl-2,4″-diyl), have been prepared by a linear stepwise oligomerization-cyclization route and by statistical intermolecular cyclooligomerization. In addition to producing these members of a novel class of arylene ethynylene macrocycles, 1 and 2, the latter statistical process produces the smaller cyclic dimer, cyclodi(ethynylene)(p-terphenyl-2,4″-diyl) and the larger cyclic tetramer cyclotetra(ethynylene)(biphenyl-2,4'-diyl). These macrocycles display large Stokes shifts in their fluorescence spectra. Their biphenyl or terphenyl connectivity prevents these macrocycles from achieving full planarity in the ground state, and the ethynylene moieties could provide synthetic access to cyclic arylene oligomers and discrete carbon nanotube segments.