Energy and safety are the two most important concerns of energetic materials (EMs), while they usually contradict each other: the high energy typically goes together with low safety. Low sensitivity and highly energetic materials (LSHEMs) balance well the energy and safety and thus are highly desired for extensive applications. Nevertheless, on the whole, the energy–safety contradiction, the energy and component limits, and insufficient knowledge about the relationships among components, structures, and properties and performances of EMs have made the development of LSHEMs, or even the entire group of EMs, evolve slowly. This Perspective focuses upon the current progress in the clarifications of the energy–safety contradiction and the crystal packing–impact sensitivity relationship of EMs. Also, we propose strategies for creating new LSHEMs or desensitized EMs through crystal engineering, covering traditional EMs composed of neutral single-component molecules, energetic cocrystals, and energetic ionic salts. Two levels of intrinsic structures, molecule and crystal, are accounted for in constructing LSHEMs: at the molecular level, it is proposed to store much chemical energy in bonds while avoiding any bond formation in an energetic molecule that is too weak to intrinsically balance the energy and safety; at the level of crystal, it is suggested that intermolecular interactions be enhanced to increase packing compactness and energy density and to strengthen the anisotropy of the intermolecular interactions to facilitate ready shear slide and low mechanical sensitivity; and overall, a big π-bonded energetic molecule with an oxygen balance close to zero and a hydrogen bond-aided face-to-face π–π molecular stacking is preferred as a LSHEM. Hopefully, this Perspective will set a root for establishing a systematic theory for creating LSHEMs.

Alleviating the energy & safety contradiction of energetic materials through crystal engineering.

Cocrystal" is currently an increasingly popular term in the crystal community, as it has already been verified that the cocrystallization to form new crystals can serve as a promising strategy and an efficient technology to modulate and improve properties and performances of materials in the fields of pharmaceuticals and others. Nevertheless, the definition and intetsion of cocrystal still remain debatable. In this Perspective, we redefine cocrystal with a broadened intention as "a cocrystal is a single-phase crystalline solid composed of two or multiple components in a stoichiometric ratio, and the components of a cocrystal can be atoms, molecules, anions and cations in pairs, and/or metallic cations with free electrons shared". Thereby, cocrystals are classified into five types in terms of the kinds of the components and their interactions in crystal entity, including atomic cocrystal, molecular cocrystal, ionic cocrystal, metallic cocrystal, and mixed-type cocrystal. The reasserted definition and broadened intention of cocrystal present a uniform term for all crystalline solids with two or multiple components and help to avoid confusion of numerous existing terms of these solids. Thereby, some terms with narrow intentions are expected to be updated and applied less and less as time goes on.

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Ionization, conjugation, hydrogen bonding, coordination bonding and π–π stacking consolidate the cyclo-N5− caged in salt crystals.