Altering Physical Properties of High-Energy Materials and Other Small Molecules Through Template-Directed and Co-Crystallization Methods
The quality and performance of organic materials shape our everyday life. Organic crystals have the propensity to be polymorphic, and properties between forms may be drastically different. Crystallization of polymorphic compounds in the presence of 2-dimensional templates is one potential means to control the nucleation and growth. CL-20 is a high-energy material that is used in propellants, with its performance linked to phase, particle size/shape, and the presence of defects and/or impurities. Perylene is a well-studied organic semiconductor, and its electrical and optical properties are tied to 3D structure and molecular orientation. Crystallization of perylene and CL-20 with gold-thiol self-assembled monolayers and functionalized siloxane templates was studied for the ability of template-directed growth to affect crystal orientation, morphology and phase relative to conventional solution growth. CL-20 generally yielded mixed phases and morphologies from solution, however selectively grew high density epsilon-CL-20 on all siloxanes in benzene, with preferred orientations varying by siloxane choice. The highest nucleation densities were observed for surfaces capable of hydrogen bonding and carbonyl interactions. For perylene, the majority of solvent/surface combinations yielded concomitant growth of both forms, with preferred (100) orientations. Selective alpha growth was observed for Si-OH in toluene, and selective growth of metastable beta-perylene was obtained on Si-3-NH2 in toluene.Another way to alter physical properties of a material is through co-crystal formation, which has been studied by the pharmaceutical industry with APIs to generate compounds with enhanced stability and bioavailability. The favorable interaction between CL-20 and carbonyl groups observed in the surface study was extended to phosphonyls to form a 1:2 CL-20:TPPO co-crystal. Additionally, co-crystallization was applied to a library of meta-substituted diphenyl ureas which have been shown to crystallize with a strong hydrogen bonding network. Crystallization with stronger hydrogen bond acceptors yielded new compounds which were analyzed for their structural and physical properties. Five new 1:1 co-crystal structures were determined using XRD for mCyPU:TPPO, mClPU;TPPO, mCF3PU:TPPO, mClHPU:TPPO, mNHPU:TPPO, and one 1:1 mCyPU:DMSO solvate. Polymorph solvent screening revealed additional phases for mCyPU:TPPO (1-4), mClPU:TPPO (1-3), mNHPU:TPPO (1-3) and mClHU:TPPO (1-3). Additional DMSO solvates were identified by TGA and 1H-NMR for mNHPU:DMSO, mCF3PU:DMSO and mClHPU:DMSO.
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