π−π stacking, usually together with the aid of hydrogen bonding (HB), serves as a main characteristic of low impact, sensitive, highly energetic materials (LSHEMs), which are highly desired for application, and attracts considerable attention in designing and synthesizing new EMs. This Perspective highlights the progress of the insights into the π−π stacking of EMs, covering traditional energetic crystals with homogeneous neutral molecules, energetic cocrystals (ECCs), and energetic ionic salts (EISs). A rather large π-bond is a requisite for the π−π stacking, and the π−π stacking can be classified into four patterns, including face-to-face stacking, wavelike stacking, crossing stacking, and mixing stacking, with an increasing difficulty in shear sliding, and HB plays an important role in supporting sliding layers. Straightforwardly, the stacking pattern−impact sensitivity relationship is rooted in the steric hindrance when sliding, and the face-to-face π−π stacking is preferred to design LSHEMs at the crystal level, due to the least steric hindrance or the lowest sliding barrier among the four patterns. This stacking has been extensively observed in traditional EMs, ECCs, and EISs, enlightening us to make a rule for designing new EMs with such stacking. However, it is still difficult to make the rule, attributed to the unclear relationship between molecular and stacking structures. Maybe, it will become increasingly feasible to achieve the rule by establishing a database with detailed information on molecules and related stacking patterns, increasing the amount of data by collecting experimental and predicted results, and combining with advanced machine learning technologies. Combining this article with a recent review of HB in EMs (Cryst. Growth Des. 2019, 19 (10), 5981−5997), an overall perspective of intermolecular interactions in energetic crystals with C, H, O, and N atoms could have been presented.

Hydrogen bonding (HB) universally exists in CHON-containing energetic materials (EMs) and significantly influences their structures, properties, and performances. As time proceeds, some new types of EMs such as energetic cocrystals (ECCs) and energetic ionic salts (EISs) are thriving currently and richening insight into the HB of EMs, and these are reviewed in this article as well. The intramolecular HB mostly exists in stable molecules while seldom in less stable molecules; weak and abundant HBs dominate intermolecular interactions and consolidate crystal packing. For ECCs with neutral heterogeneous molecules, intermolecular HBs serve as one of the strategies for crystal design. In comparison, the HBs in EISs are greatly strengthened as their polarity significantly increases with ionization. A strong intramolecular HB usually enhances molecular stability with large π-bonds and packing coefficients and facilitates reversible H transfer, which is advantageous for low mechanical sensitivity. The intermolecular HB-aided π–π stacking that favors low mechanical sensitivity is observed in all three kinds of EMs, including traditional EMs with neutral homogeneous molecules, ECCs with neutral heterogeneous molecules, and EISs. However, a strong intermolecular HB in an EM causes a ready intermolecular H transfer, thereby worsening thermal stability. Thus, the influence of HBs on the stability of EMs can go both ways, and there should be a balance when new HB-containing EMs are designed.

Understanding intermolecular interactions is fundamental to understanding the molecular stacking structures and some properties of energetic crystals, such as density, energy, mechanics, and sensitivity. The Hirshfeld surface method is a straightforward tool to reveal intermolecular interactions and nowadays has become increasingly popular in the field of energetic materials. This article highlights a wide range of applications of this method in describing intermolecular interactions including hydrogen bonding, π-stacking, halogen bonding, and lone pair−π (n−π) stacking, and molecular stacking patterns, and in predicting shear sliding characteristic and further impact sensitivity. Meanwhile, the roughness of the quantitative description of intermolecular interaction strength of the method, as a main shortcoming, is pointed out herein. Thus, this work is expected to guide the right and full use of the method. Besides, we present a perspective about using the Hirshfeld surface method to rapidly screen the molecular stacking mode and further impact sensitivity; thus, the fast screening of the two most important properties can be implemented, in combination with the existing mature energy prediction methods based on components. Thereby, a more reliable prediction procedure with an additional consideration of molecular stacking pattern will be produced, setting a basis for data-driven and crystal engineering research of energetic materials.

The intermediate structures for an energetic material (EM) loaded until the final decay are often inaccessible and overlooked, while they are a determining factor of property and performance, with a similar importance of the original structure under common conditions. The present work exemplifies the importance by revealing the low impact sensitivity of 2,2-dinitroethylene-1,1-diamine (FOX-7) with a consideration of heat-induced polymorphic transformation. Checking the packing structures of the polymorph at ambient conditions (α-form) and the two heat-induced ones (β- and γ-forms) of FOX-7, we confirm that the heating until the final decay makes the shear sliding increasingly ready. That is, from the α- to β- and γ-FOX-7, the crystal packing varies from a wavelike shape to a face-to-face one, with the increase of molecular planarity, as their maximal torsion angles of O–N–C–C decrease from 35.6 to 25.6 and 20.2°; and their shear-sliding barriers reduce and ready sliding ranges increase in the same order, verified by density functional theory calculations. This heat-induced polymorphic transformation of FOX-7 from wavelike to face-to-face π–π stacking is responsible for its low impact sensitivity, by remedying its disadvantage of relatively low thermal stability. Hardly, we will understand the low impact-sensitivity of FOX-7 if the original α-form is considered alone. This work presents an exact example to show the importance of intermediates produced by external stimuli loaded on an EM for understanding its performance. It also shows the complexity of the sensitivity mechanism of EMs and some possible deficiencies caused by considering the initial unloaded case alone.

Polymorphism is universal in energetic materials, and polymorphic transformation (PT) causes variations in the structure, properties, and performance. This article reviews the polymorphs of six traditional energetic compounds (ECs), including 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (