Photochemical Strategies Enable the Synthesis of Tunable Azetidine-Based Energetic Materials.

Despite their favorable properties, azetidines are often overlooked as lead compounds across multiple industries. This is often attributed to the challenging synthesis of densely functionalized azetidines in an efficient manner. In this work, we report the scalable synthesis and characterization of seven azetidines with varying regio- and stereochemistry and their application as novel azetidine-based energetic materials, enabled by the visible-light-mediated aza Paternò-Büchi reaction. The performance and stark differences in the physical properties of these new compounds make them excellent potential candidates as novel solid melt-castable explosive materials, as well as potential liquid propellant plasticizers. This work highlights the scalability and utility of the visible-light aza Paternò-Büchi reaction and demonstrates the impact of stereochemical considerations on the physical properties of azetidine-based energetics. Considering the versatility and efficiency of the presented synthetic strategies, we expect that this work will guide the development of new azetidine-based materials in the energetics space as well as other industries.

Impact of Stereo- and Regiochemistry on Energetic Materials.

The synthesis, physical properties, and calculated performances of six stereo- and regioisomeric cyclobutane nitric ester materials are described. While the calculated performances of these isomers, as expected, were similar, their physical properties were found to be extremely different. By alteration of the stereo- and regiochemistry, complete tunability in the form of low- or high-melting solids, stand-alone melt-castable explosives, melt-castable explosive eutectic compounds, and liquid propellant materials was obtained. This demonstrates that theoretical calculations should not be the main factor in driving the design of new materials and that stereo- and regiochemistry matter in the design of compounds of potential relevance to energetic formulators.

Laser-shocked energetic materials with metal additives: evaluation of chemistry and detonation performance.

A focused, nanosecond-pulsed laser has been used to ablate, atomize, ionize, and excite milligram quantities of metal-doped energetic materials that undergo exothermic reactions in the laser-induced plasma. The subsequent shock wave expansion in the air above the sample has been monitored using high-speed schlieren imaging in a recently developed technique, laser-induced air shock from energetic materials (LASEM). The method enables the estimation of detonation velocities based on the measured laser-induced air-shock velocities and has previously been demonstrated for organic military explosives. Here, the LASEM technique has been extended to explosive formulations with metal additives. A comparison of the measured laser-induced air-shock velocities for TNT, RDX, DNTF, and LLM-172 doped with Al or B to the detonation velocities predicted by the thermochemical code CHEETAH for inert or active metal participation demonstrates that LASEM has potential for predicting the early time (<10 µs) participation of metal additives in detonation events. The LASEM results show that while Al is mostly inert at early times in the detonation event (confirmed from large-scale detonation testing), B is active-and reducing the amount of hydrogen present during the early chemical reactions increases the resulting estimated detonation velocities.

Bis(1,2,4-oxadiazolyl) Furoxan: A Promising Melt-Castable Eutectic Material of Low Sensitivity.

A scalable synthesis of bis(1,2,4-oxadiazoyl) furoxan, C6 H2 N6 O4 , its physical properties, and its theoretical performance values are described. Previous attempts to synthesize this compound required expensive reagents, and/or time-consuming synthesis processes and low overall yields. In addition to disclosing a streamlined synthesis of bis(1,2,4-oxadiazolyl) furoxan, we report its molecular configuration and crystal structure, as well as its correct melting point. Bis(1,2,4-oxadiazolyl) furoxan exhibits a very insensitive behavior to impact, friction, and electrostatic discharge, with a calculated detonation pressure 20 % higher than that of TNT. Given its physical properties and theoretical performance values, this material can be classified as a promising ingredient in the development of melt-castable eutectic technology.

A new strategy for folding oligo(m-phenylene ethynylenes).

Backbone-rigidified oligo(m-phenylene ethynylenes) fold into crescent or helical conformations in non-polar organic solvents.

Minimizing urine autofluorescence under multi-photon excitation conditions.

We report on the effects of excitation wavelength, laser power, and phase resolution on the multi-photon-excited autofluorescence (background) from human urine. When compared to the autofluorescence under one-photon excitation conditions (lambda(ex) = 260-480 nm), the urine multi-photon-excited autofluorescence (lambda(ex) = 725-950 nm) can be less complicated. However, at higher laser powers, the multi-photon-excited autofluorescence spectra that are produced by excitation above ~775 nm are more complex in comparison to the corresponding one-photon-excited autofluorescence. The origin of these more complex spectra arises from simultaneous two- and three-photon-driven excitation of intrinsic luminescent species within the urine. At lower laser powers, three-photon-driven processes are minimized and the autofluorescence spectrum is simplified. Phase resolution is used to further minimize the urine autofluorescence, but it cannot fully eliminate autofluorescence even when excitation is performed under multi-photon conditions at 950 nm. For detecting 250 nM Rhodamine 6G (a mock analyte) dissolved in urine, we find that the two-photon excitation is superior in comparison to one-photon excitation by 5- to 70-fold, depending on the excitation wavelength. Phase resolution combined with two-photon excitation leads to an additional 5- to 7-fold improvement in signal-to-background ratios in comparison to steady-state two-photon excitation.