Pressure-assisted Assembly And Its Applications In Energetic Materials

Supramolecular chemistry is defined as “chemistry beyond the molecule”. The supramoleculararchitecture is the complex and organized assembly built of different building blocks via various noncovalentinteractions. It possesses specific microstructures and macroscopic characteristics. The supramolecularinteractions include hydrogen bonding, π-stacking, cation/anion-π, electrostatic interaction, van der Waalsforce, dipole-dipole, hydrophilic and hydrophobic interactions. It is the cooperativity of various noncovalentinteractions that hold building blocks together to support the system with targeted structures and functions. Therealm of supramolecular chemistry involves designing, synthesis and applications of assemblies. The basicissue is the cooperativity of various noncovalent interactions within assembly process and how to realizeeffective and controllable assembly.Pressure is one of the basic physical parameters. The advent of diamond anvil cell (DAC) has inspiredscientists’interest in high-pressure research. The atomic distances and electron density distribution can betuned by the applied pressure, resulting in novel structures and properties. Especially, the effect of pressure ismore significant for molecular crystals. Angle dispersive X-ray diffraction (ADXRD) can provide structuralinformation, including crystal symmetry, lattice constants, and atomic positions. Raman scattering has provento be a powerful tool for monitoring local chemical environment. Moreover, vibrations of hydrogen donors canoffer information on connectivity of building blocks. Here, pressure is exerted to supramoleculat crystals. Onone hand, we investigate the cooperativity of various noncovalent interactions under high pressure. On theother hand, we explore new assemblies of different building blocks, and try to retrieve the new phase toambient conditions via substituent groups. Following the strategy of pressure-assisted assembly(pressassembly), we study the behaviors of energetic materials and prepare the high-density phase.We propose the concept of the cooperativity of noncovalent interactions at high pressures. Ammoniumsquarate ((NH4)2C4O4, AS) and guanidinium perchlorate (C(NH2)3)+·ClO4-, GP) are chosen as models. AS hasthe bilayered structure, hydrogen bonding and π-stacking are dominant interactions. We perform in situhigh-pressure Raman and ADXRD experiments of AS to the pressure of19GPa. The experimental results reveal that these two noncovalent interactions are “separated” by external pressure. AS undergoes two phasetransitions at2.7GPa and11.1-13.6GPa, respectively. The first phase transition arises from rearrangements ofhydrogen-bonded networks. NH stretching vibrations experience dramatic changes in profiles and intensitydistribution, whereas π-stacking proves not to undergo obvious change. The second one is governed byπ-stacking. The reason is the modes of squarate anions have undergone many changes in this pressure region,and hydrogen bonding evolves into the disordered state. GP exhibits a typical supramolecular structure oftwo-dimensional (2D) hydrogen-bonded ionic networks at ambient conditions. In situ Raman spectroscopy andsynchrotron ADXRD experiments have been performed to investigate the response of GP to high pressures of11GPa. A subtle phase transition, accompanied by the symmetry transformation from R3m to C2, has beenconfirmed by obvious changes in both Raman and ADXRD patterns at4.5GPa. The phase transition isattributed to the competition between hydrogen bonds and close packing of the supramolecular structure athigh pressures. Hydrogen bonds have been demonstrated to evolve into a distorted state through the phasetransition, accompanied by the reduction in separation of oppositely charged ions in adjacent sheet motifs.These studies demonstrate pressure is a useful tool for investigating the cooperativity of noncovalentinteractions within supramolecular architectures.We propose functional supramolecular materials can be synthesized following strategy ofhigh-pressure-assisted assembly (“pressassembly” for describing the process). Guanidinium tetrafluoroborate(C(NH+2)3)·BF-4, GFB) also exhibits typical2D rosette hydrogen-bonded networks, similar with GP. Thelattice modes and ADXRD results reveal GFB is stable over0-26GPa. However, GFB has anisotropiccompressibilities, the compressibiliy of c-axis is much higher than a-axis. The distance betweenhydrogen-bonded layers is decreased significantly, leading to the dramatic reduction in separation between Fand H atoms. This separation is smaller than the sum van der Waals radii (2.55) of F and H above10.2GPa.New hydrogen bond of N-H…F has formed between adjacent layers. Raman spectra also provide strongevidence for new hydrogen bond formation, including emergence of new NH vibration and red shifts of somemodes. Fitting of the third B-M EoS reveals that the structure of GFB tends to soften above10.2GPa. This isascribed to the attraction nature of new hydrogen bonds formed between adjacent layers. There are also newhydrogen bonds formed in GP above13.2GPa, evidenced by new NH vibration and red shift. Guanidiniummethanesulfonate (C(NH+2)3·CH3SO3, GMS) exhibits the representative supramolecular structure of2Dhydrogen-bonded bilayered motifs under ambient conditions. The2D hydrogen-bonded sheet assembles intobilayers as a nonpolar region in which the methyl groups hold the sheets together via van der Waals interactions. On the basis of the experimental results, two phase transitions were identified at0.6and1.5GPa,respectively. The first phase transition, which shows the reconstructive feature, is ascribed to therearrangements of hydrogen-bonded networks, resulting in the symmetry transformation from C2/m to Pnma.The second one proves to be associated with local distortions of methyl groups, accompanied by the symmetrytransformation from Pnma to Pna21. Because of the steric hindrance of methyl groups, GMS is quenched tothe first high-pressure phase upon complete decompression, implying the first phase transition is irreversibleand the second one is reversible. This study suggests the structural stability and the process ofpressure-assisted assembly can be influenced and tuned by substituents.Following the strategy of pressure-assisted assembly, we examine the energetic materials urea nitrate((NH–2)2COH+·NO3, UN) and acetamidinium nitrate (C+-2N2H7·NO3, AN). UN exhibits the typicalsupramolecular structure with uronium cation and nitrate anion held together by multiple hydrogen bonds inthe layer. The irreversible phase transition in the range9-15GPa has been corroborated by experimental resultsand is proposed to stem from rearrangements of hydrogen bonds. Further analysis of XRD patterns indicatesthe new phase (phase II) has Pc symmetry. The retrieved sample is10.6%smaller than the ambient phase involume owing to the transformation from2D hydrogen-bonded networks to three-dimensional (3D) ones. Thedensity in phase Pc has been increased by11.8%compared to the phase P21/c under ambient conditions andtherefore phase Pc is expected to have much higher detonation power. AN crystallizes in space group P21/m.Arrangements of ion pairs construct the hydrogen-bonded array in ac plane. Consideration that the methylgroup in AN does not participate in hydrogen bonds, such tailoring can bring down the stability ofhydrogen-bonded arrays. This offers advantageous conditions for new assembly formation at lower criticalpressure. Both ADXRD and Raman scattering measurements serve as strong proof of the new assemblyformation, accompanied by symmetry transformation from P21/m to P-1. Compared with UN, the criticalpressure has dropped to1.3-3.4GPa. Pressassembly process involves distortions of methyls and sliding ofadjacent ion pairs. Significantly, the new assembly has a9.8%higher density than the ambient one, so as topossess higher detonation velocity. It has proven pressure is indeed a powerful method for creating newassemblies with new functions.