π-π Stacking Contributing to the Low or Reduced Impact Sensitivity of Energetic Materials

pi-pi 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 pi-pi it stacking of EMs, covering traditional energetic crystals with homogeneous neutral molecules, energetic cocrystals (ECCs), and energetic ionic salts (EISs). A rather large pi-bond is a requisite for the pi-pi stacking, and the pi-pi 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 pi-pi 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.