Single-crystal-to-single-crystal MOF encapsulation of copper azide to prepare laser-sensitive primary explosives

Abstract

Laser-sensitive primary explosives (LSPEs) are crucial material bases of advanced laser initiation technology. Copper azide (CA), a primary explosive with excellent detonation properties, is limited in preparation and application owing to its extremely high sensitivity. Thus, incorporating CA into LSPEs relies on precise desensitisation strategies. This study successfully implemented a strategy involving sensitive-unit molecular-scale encapsulation. A 2D energetic metal–organic framework (EMOF) [Cu(ATRZ)(N₃)₂]_n (CA-ATRZ) (ATRZ = 4,4′-azo-1,2,4-triazole) was designed and synthesized via a safe and facile single-crystal-to-single-crystal (SCSC) transformation from a 3D EMOF [Cu(ATRZ)₃(NO₃)₂]_n. Leveraging its distinctive structural attributes of encapsulated confinement, CA-ATRZ is substantially improved in terms of safety compared to CA, while maintaining its superior detonation performance. Furthermore, CA-ATRZ obtained by combining MOFs with CA has outstanding ultrafast direct laser initiation characteristics, is free of toxic metals and perchlorate, has high initiating ability, and has decent thermal stability. This strategy could pave the way for developing advanced high-energy LSPEs.

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Introduction

The design and creation of energetic materials (EMs) are challenging and fascinating, and their human pursuit has never stopped since the discovery of black powder in ancient China.¹ Primary explosives represent a category of EMs that are easily initiated by external stimuli, which undergo swift deflagration-to-detonation transition (DDT) upon external energy input, initiating secondary explosives.^2–4 As advancements in defence technology persist, coupled with the continuous evolution of weapons and ammunition, traditional primary explosives and initiation methods prove inadequate to meet the demands of contemporary weaponry.^5–8 Exploring novel EMs and initiation methods is an urgent research objective aimed at achieving the safe and excellent initiation ability of primary explosives simultaneously.^9–11 In contrast to conventional initiation methods (flame, heat, impact, shock, electric spark, etc.), laser initiation is regarded as a safer and more dependable technique. This approach mitigates the risk of inadvertent detonations attributable to factors like electromagnetic pulse, high-power microwave, intense radio frequency, and static electricity by isolating explosive components from potential interference signals.^12–14 The swift progression of laser initiation technology has resulted in the emergence of numerous novel laser-sensitive primary explosives (LSPEs). Nevertheless, existing LSPEs with a low initiation energy threshold tend to detonate inadvertently upon exposure to minor drops, impacts, or static discharge like other primary explosives; this inherent vulnerability constitutes a substantial safety issue. Thus, designing and fabricating novel LSPEs that combine superior safety attributes and exceptional initiation performance is an extremely critical task.

Copper azide (CA) is an energetic complex known for its efficient initiation capabilities and green explosive products,^15,16 and has been extensively researched for applications in micro-electro-mechanical systems (MEMS) initiating explosive devices and self-destruct modules in information storage systems.^17,18 However, CA is one of the most dangerous EMs due to high static electricity and mechanical sensitivity leading to extremely poor safety. Therefore, research is focused on achieving the safe preparation and application of CA by exploring the symbiotic relationship between its sensitivity and energy. Fig. 1a illustrates that the synthesis pathways of CA primarily involve gas–solid or liquid–solid phase in situ reactions of Cu or CuO nanowires, microspheres, or thin films with HN₃ gas to obtain products that are suitable for MEMS-initiating explosive devices.^16,19–21 This can avoid the hazards arising from direct contact with CA, but the complexity of the process and the low conversion rate limit its scale-up production. Encapsulation of CA by porous carbon skeletons can effectively reduce the mechanical and electrostatic susceptibility. However, this composite encapsulation strategy usually lacks molecular structure control on a more microscopic scale, and the low-energy carbon skeleton affects the detonation performance of the product.^15,22–25 Therefore, it is important to develop new techniques to reduce the sensitivity of CA precisely while enabling them to exhibit excellent initiation characteristics in LSPEs.

Fig. 1 Comparison between this work and previous works. (a) Traditional CA synthesis pathway (gas–solid phase in situ reaction) and carbon skeleton encapsulation strategy. (b) Assembly and laser initiation of LSPEs and subsequent initiation of secondary explosives. (c) Synthesis of CA-ATRZ through SCSC and molecular-scale encapsulation of N₃⁻.

Energetic metal–organic frameworks (EMOFs) are important EMs, which have great potential for the development of new primary explosives.²⁶ Research indicates that lasers with specific threshold values can reliably initiate certain EMOFs.²⁷ The desired energy and laser initiation performance can be achieved by adjusting the central metal (Fe, Cu, Ag, Pb, etc.), energetic ligand molecules (such as some nitrogen-rich heterocyclic moieties),²⁸ and counterbalancing anionic building blocks (ClO₃⁻, ClO₄⁻, BrO₃⁻, etc.) (Fig. 1b).^12,29–31 On this basis, we consider the single-crystal-to-single-crystal (SCSC) transformation to be an effective method for safely obtaining CA-based EMOFs with high energy density, exceptional safety, photosensitivity, and the ability to be used as LSPEs. This method has been widely used in preparing MOFs with functions such as gas adsorption and chemical catalysis.^32–36 The SCSC transformation allows for the precise control of the structure and morphology of MOFs, resulting in well-defined crystal structures with high purity.^37,38 Additionally, the SCSC process has unique advantages in the in situ functionalization of MOFs, personalized customization, product consistency, and understanding of reaction mechanisms. For example, anion exchange in energetic MOF pores can be accomplished by SCSC transformation.^39,40 This study proposes a strategy to utilize the SCSC transformation for molecular-level encapsulation of sensitive units to construct high-performance CA-based photosensitive EMOFs (Fig. 1c): (i) Cu²⁺ is used due to its obvious laser absorption effect and electron transfer ability as the metal centre. (ii) To achieve a symbiosis of energy and safety, a nitrogen-rich second ligand can be incorporated into the sensitive CA, and high-energy MOFs with ultrafast direct laser-responsive characteristics can be constructed. The sensitivity of CA can be reduced through the fixation of N₃⁻ and the slip between 2D layers. (iii) Indirect synthesis of the target product can be achieved through SCSC transition between EMOFs, avoiding the generation of highly hazardous byproducts such as CA and HN₃ in direct preparation.

Herein, we report a two-dimensional (2D) CA-based EMOF [Cu(ATRZ)(N₃)₂]_n (CA-ATRZ), which has a forward design adopting a molecular-scale sensitive-unit encapsulation strategy and was synthesised through a simple one-step SCSC transition from the precursor [Cu(ATRZ)₃(NO₃)₂]_n (Fig. 1c). The all-nitrogen high-energy first ligand N₃⁻ replaced the free NO₃⁻, and the nitrogen-rich ATRZ molecule with rich N–N and N=N bonds served as the second energetic ligand, coordinating with Cu²⁺ to construct the main framework. Dual-site confinement encapsulation of the highly sensitive N₃⁻ was performed at the molecular level. Notably, this approach ensures the high energy density and excellent thermal stability of the product CA-ATRZ while reducing the mechanical and electrostatic sensitivity compared to CA. Importantly, CA-ATRZ is proven to have excellent laser detonation characteristics and a low initiation energy threshold due to rational assembly regulation of EMOF building blocks, rapidly initiating and undergoing deflagration-to-detonation transition (DDT) upon 800 nm pulse laser irradiation in an extremely short time. Therefore, CA-ATRZ has great potential for practical application as a high-energy laser-sensitive primary explosive.

Results and discussion

Preparation

The synthetic route of CA-ATRZ is shown in Fig. 2a. The energetic ligand ATRZ and intermediate [Cu(ATRZ)₃(NO₃)₂]_n were synthesized according to methods in the literature.^41,42 [Cu(ATRZ)₃(NO₃)₂]_n undergoes an SCSC transition in aqueous sodium azide (NaN₃) solution, with blue crystals changing to smaller brown crystals. Following simple filtration, washing, and drying, pure target [Cu(ATRZ)(N₃)₂]_n (CA-ATRZ) was obtained, which means that we have achieved encapsulation of highly dangerous N₃⁻ ions and transformation of the 3D MOF to the 2D MOF. Notably, this process did not generate highly hazardous by-products such as N₂O₄, HN₃, and CA, effectively reducing risks during the experimental procedures. Furthermore, the filtered NaN₃ solution obtained can be reused, effectively reducing the emission of pollutants during the preparation process, in line with the requirements of green chemistry.

Fig. 2 SCSC preparation process of CA-ATRZ. (a) Synthesis route of CA-ATRZ. (b–d) OM images of the SCSC transformation process. (e–g) SEM images of the SCSC transformation process. (h and i) PXRD patterns and FT-IR spectra of the samples taken at different time points during the SCSC transformation. (j) Schematic illustration of an SCSC transformation.

The complete SCSC process from [Cu(ATRZ)₃(NO₃)₂]_n to CA-ATRZ was monitored by powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectrometry, scanning electron microscopy (SEM) and optical microscopy (OM). As shown in Fig. 2b, c, e and f, during the initial stage of immersion, [Cu(ATRZ)₃(NO₃)₂]_n has lost the original single-crystal state and generated polycrystals, ultimately leading to a pronounced manifestation of an extremely irregular and textured surface, accompanied by a colour alteration to a brown shade. The PXRD pattern reveals prominent new peaks at 11.15°, 15.8°, 18.02°, and 24.30° 2θ angles, indicative of novel crystalline phases or structural features (Fig. 2h). Concurrently, in the FTIR spectrum, a highly characteristic peak corresponding to the N₃⁻ moiety is evident at a wavenumber of 2100 cm⁻¹ (Fig. 2i), indicating that there has been a process of partial decomplexation of lattice-coordinated ATRZ molecules within the crystal lattice, which subsequently dissolved out of the parent crystal. Meanwhile, the azide anion was incorporated into the crystal structure to promote SCSC conversion, resulting in CA-ATRZ. Finally, a complete transformation of the initial blue crystals into brown crystals was observed (Fig. 2d and g). The PXRD pattern of the final experimental product closely matches the simulated curve of CA-ATRZ and does not match the standard card for CA (JCPDS no. 48-1548) (Fig. S2†), indicating the absence of highly sensitive CA in the experimental product. This confirms the effectiveness of the SCSC strategy in terms of personalization and product consistency, thus reducing risk and improving efficiency in the preparation of CA derivatives. Importantly, the SCSC preparation of CA-ATRZ provides a reference for the precise design and efficient construction of new EMOFs.

Single-crystal structure

The structure of CA-ATRZ was determined through single-crystal X-ray diffraction (SCXRD) studies on crystals, which were obtained via SCSC transformation at room temperature. The results indicate that CA-ATRZ belongs to the monoclinic crystal system, P1̄ space group, with a crystal density of 2.133 g cm⁻³ (293 K) (Table S1†). As shown in Fig. 3, the crystal structure of CA-ATRZ consists of a basic coordination unit comprising a Cu²⁺ ion, an ATRZ molecule, and two N₃⁻ ions, with no remaining solvent molecules. Cu²⁺ generates a distorted octahedral configuration linked with six surrounding nitrogen atoms, where four N atoms are provided by four N₃⁻ ions, and the other two N atoms are from two ATRZ molecules (Fig. 3a). The bond lengths of Cu–N range from 1.979(2) Å to 2.609 Å (Table S2†), and the N–Cu–N angles fluctuate from 86.85(9) to 180.0(10)° (Table S3†). Adjacent Cu²⁺ ions are linked by ATRZ to form a 1D linear structure that extends infinitely along the [011] direction. N₃⁻ is constrained between adjacent linear structures through coordination and exhibits infinite extension along the [100] direction, forming a planar structure (Fig. 3b). Each pair of approximately parallel N₃⁻ connects two Cu²⁺, and the average distance between the two N₃⁻ is 3.26 Å. The bond-length difference of the two N–N bonds in CA-ATRZ is 0.023 Å, lower than the values of 0.052 Å and 0.115 Å observed in CA. Multiple 2D planar structures are stacked via face-to-face π–π interactions between interlayer triazole rings (3.599 Å) to constitute a 3D supramolecular network (Fig. 3c). Therefore, CA-ATRZ molecules exhibit long-range order in all three crystallographic directions.

Fig. 3 Crystal structure and packing of CA-ATRZ. (a) Coordination mode of Cu(II) in CA-ATRZ. (b) Single-layered structure of CA-ATRZ. (c) Layered stacking of CA-ATRZ.

When immersed in a solution of sodium azide, the dimensionality of the 3D EMOF [Cu(ATRZ)₃(NO₃)₂]_n rapidly reduces as it transforms into a new 2D-layered structure CA-ATRZ, where the free NO₃⁻ is replaced by the coordinating N₃⁻, leading to the encapsulation of N₃⁻ being successfully executed, and the insensitive ATRZ greatly buffering external stimulation after surrounding the sensitive CA, increasing significantly the difficulty of initiating the CA. In addition, the interlayer slip effectively mitigates external stress stimuli, enhancing structural stability, which is an effective method for sensitivity reduction. This meets the structural expectations of the initial design and subsequent performance. Thus, the molecular-scale confinement encapsulation strategy proves effective in the structural and performance modulation of EMOFs, demonstrating the robust and flexible structural tunability of MOF materials.

Thermal stability and sensitivity

Thermal stability is crucial for primary explosives, due to the ongoing storage and utilization of weapons, which may expose them to elevated temperatures, with ammunition operating at temperatures potentially exceeding 70 °C. Thermogravimetric and differential scanning calorimetry (TG-DSC) were used to assess the stability of CA-ATRZ towards heat loading (nitrogen atmosphere, 10 K min⁻¹), and the details are shown in Fig. 4a. The thermal decomposition of CA-ATRZ occurs with an onset temperature of 209 °C, which is slightly higher than that for CA (205 °C), with no significant weight loss having been observed before; this meets the minimum thermal decomposition temperature requirements for green primary explosives (T_d ≥ 150 °C).⁵ The thermal stability of CA-ATRZ surpasses that of the other CA-based energetic complexes CA-MTZ ([Cu(N₃)₂(MTZ)], 148 °C, onset)⁴³ and CA-NET ([Cu(N₃)₂(1-NET)], 122 °C, peak),⁴⁴ and the organic primary explosive diazodinitrophenol (DDNP, 157 °C, onset) (Table 1),⁹ indicating that the choice of the second ligand influences the structure and properties of EMOFs. The presence of ATRZ contributes to the thermal stability of CA-ATRZ due to its large conjugated planar structure with strong π-electron delocalization, resulting in rich π–π interactions within the 2D layers. In addition, the localized encapsulation of N₃⁻ is also an important factor in ensuring the thermal stability of CA-ATRZ. These factors make it satisfactory for more military and civilian applications.

Fig. 4 Thermal stability and sensitivity analysis. (a) TG-DSC curves in thermal stability tests of CA-ATRZ. (b) Sensitivities of CA-ATRZ, several primary explosives, and two CA-based EMOFs (CA-MTZ and CA-NET). (c) Non-covalent interaction analysis for CA-ATRZ. (d) A simplified model based on the single [Cu(ATRZ)(N₃)₂]_n layer shows that the sensitive N₃⁻ ligands are encapsulated between 1D chain-like [Cu-ATRZ].

Table 1 Physical properties of CA-ATRZ and the reference energetic materials

Compound	ρ (g cm^−3)	N (%)	T d ^c (°C)	Δ f H m ^d (kJ mol^−1)	Q (kcal g^−1)	D (m s^−1)	P (GPa)	IS^h (J)	FS^i (N)	EDS^j (mJ)
a Crystal density. b Nitrogen content. c Decomposition temperature. d Heat of formation. e Heat of detonation. f Detonation velocity. g Detonation pressure. h Impact sensitivity. i Friction sensitivity. j Electrostatic discharge sensitivity.
CA-ATRZ	2.13	63.0	209	1752	1.34	8470	34.7	1.5	50	20
ATRZ	1.62	68.3	313	878	—	7520	23.5	14	160	10 J
CA	2.60	56.9	205	—	—	—	—	≤1	≤0.1	0.05
LA	4.80	28.9	315	450	0.375	5920	33.8	2.5–4	0.1–1	<5
DDNP	1.72	26.7	157	321	—	6900	24.2	<1	24.7	1.8

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The safety and reliability of LSPEs are closely associated with non-explosive stimuli (mechanical, electrostatic discharge, etc.) during their production, use, and storage.⁴⁵ The mechanical sensitivity of CA-ATRZ was assessed using BAM standard procedures for impact and friction sensitivities (IS and FS, respectively). To demonstrate the advantages of CA-ATRZ more clearly, we compared the sensitivities of CA-ATRZ with those of several primary explosives, and two CA-based EMOFs (CA-MTZ and CA-NET) based on least-normalized sensitivity values (Fig. 4b). The experimental results show that the impact, friction and electrostatic spark sensitivities of CA-ATRZ are 1.5 J, 50 N and 20 mJ, respectively, which are much higher than those of CA (IS ≤ 1 J, FS ≤ 0.1 N, EDS = 0.05 mJ). This is consistent with our expectations for improved CA sensitivity via localized encapsulation of sensitive groups. In addition, CA-ATRZ has lower sensitivities than the organic primary explosive DDNP (IS = 1 J, FS = 24.7 N, EDS = 1.8 mJ), and shows reduced sensitivity of friction and electrostatic discharge stimuli compared to CA-NET (FS = 1 N, EDS = 14 mJ)⁴³ and widely used military primary explosive LA (FS = 0.1–1 N, EDS < 5 mJ). Thus, CA-ATRZ exhibits auspicious mechanical and electrostatic spark sensitivities and is ideal for most military and civilian applications, which paves a new pathway for the practical application of CA.

The rational use of the sensitive-unit encapsulation strategy enables the sensitive N₃⁻ anions to be fixed in the 2D-layered structure through coordination bonds at both ends, which is the main reason for the reduced sensitivity of CA-ATRZ (Fig. 4c). Compared to reducing CA sensitivity by preparing carbon-based composites (Fig. 1a), the construction of CA-based EMOFs alleviates the safety issues of CA at the molecular scale while ensuring a higher energy level. Sliding between 2D layers under mechanical stimulation to dissipate external stress is also an important factor in the reduction of sensitivity.⁴⁶ Furthermore, the non-covalent interactions significantly impact the structural stability of EMOFs. Using electron density theory from quantum mechanics, the non-covalent interactions (NCI) between ATRZ molecules were analysed through Gaussian 16, Multiwfn 3.8 and VMD v1.93 visualization software (Fig. 4d).^47–49 In the NCI map, blue, green, and red surfaces represent strong attractive interactions, weak interactions, and strong non-bonded overlaps, respectively. Due to the large planar conjugation configuration of ATRZ, there are numerous π–π interactions between adjacent 2D layers (Fig. 4d), creating an energy buffer effect that improves thermal stability and lowers mechanical and electrostatic sensitivity.^50–52 These analyses demonstrated the effectiveness of the 2D EMOF encapsulation of the sensitive device strategy in improving CA safety and provided effective strategies for reducing the sensitivity of EMs.

Initiation performance

The detonation properties are the most important parameters for evaluating the energetic performance of a primary explosive, where the detonation heat (Q) is directly affected by the heat of formation (Δ_fH_m). The Δ_fH_m of CA-ATRZ was determined by calculating the adiabatic combustion heat (Δ_cU) measured by an oxygen bomb calorimeter, resulting in a high positive value (1752 kJ mol⁻¹), surpassing that of the ligand ATRZ (878 kJ mol⁻¹). Utilizing the crystal density of CA-ATRZ and the computed heat of formation, we derived its detonation characteristics, including detonation heat (Q), detonation velocity (D), and detonation pressure (P), employing the Kamlet–Jacobs equation.⁵³CA-ATRZ exhibits a high detonation heat value (1.34 kcal g⁻¹), which surpasses LA (0.385 kcal g⁻¹), LS (0.475 kcal g⁻¹) and another LSPE [Ag(ATCA)ClO₄]_n (0.511 kcal g⁻¹).³⁰ As shown in Table 1, the calculated detonation velocity and detonation pressure for CA-ATRZ are 8470 m s⁻¹ and 34.7 GPa, respectively, better than those of K₂DNABT (D = 8330 m s⁻¹, P = 31.7 GPa),⁵⁴ DDNP (D = 6900 m s⁻¹, P = 24.2 GPa), LA (D = 5920 m s⁻¹, P = 33.8 GPa), and most of LSPEs, such as BNCP (D = 7210 m s⁻¹), [Ag(ATCA)ClO₄]_n (D = 6800 m s⁻¹), and Cu(NPyz)₄(ClO₄)₂ (D = 7500 m s⁻¹).^55,56 The outstanding detonation performance of CA-ATRZ is attributed to the high nitrogen content (63%) resulting from the two nitrogen-rich high-energy ligands, ATRZ and N₃⁻, which have a huge energy advantage compared to non-energetic carbon skeletons. Meanwhile, this nitrogen-rich strategy is environmentally friendly due to pollution-free detonation products. Noteworthily, the excellent detonation performance of CA-ATRZ is comparable to that of secondary explosives, providing a reference for the development of high-energy primary explosives.

Furthermore, the explosion capacity of CA-ATRZ was visually demonstrated through the modified Koenen test with a Bunsen burner when 5, 10, and 20 mg quantities of CA-ATRZ in a naturally loose state were placed in a sealed aluminium crucible and flame-heated until an explosion occurred. The intense shock waves severely damaged the aluminium crucible, as shown in Fig. 5a–d. Compared to an undamaged aluminium crucible (Fig. 5a), 5 mg of CA-ATRZ induces substantial damage to the sidewalls of the aluminium crucible (Fig. 5b). With 10 mg, any part of the crucible was destroyed (Fig. 5c), and the explosion of 20 mg resulted in the instantaneous fragmentation of the aluminium crucible into minute particles smaller than 1 mm (Fig. 5d). In addition, the CA-ATRZ explosion causes significantly more damage to the aluminium crucible than LA does at an equivalent dosage. This suggests that CA-ATRZ has excellent deflagration-to-detonation transition (DDT) capability and energy output characteristics in a loose state and with a small charge, and the excellent explosion capacity of copper azide is retained in CA-ATRZ.

Fig. 5 Detonation tests of CA-ATRZ. (a–d) Koenen test results of CA-ATRZ and LA of different weights. (e) Structure of the detonator charge of CA-ATRZ. (f) Lead plate reaming with 30 mg of CA-ATRZ and LA initiating RDX.

The initiation performance is a crucial indicator for assessing whether EMs can be used as primary explosives, reflecting their ability to initiate secondary explosives. In this study, the initiation capability of CA-ATRZ was determined using the minimum primary charge (MPC) method. Various masses of CA-ATRZ were loaded into industrial detonators to initiate the secondary explosive RDX and directly act on a lead plate with a thickness of 5 mm (Fig. 5e). The borehole diameter in the lead plate serves as an assessment of its initiation performance, and the test results are shown in Fig. 5f. For CA-ATRZ masses equal to or exceeding 30 mg, initiation of secondary explosive RDX and penetration of the 5 mm lead plate are completed successfully. A 30 mg CA-ATRZ-loaded detonator resulted in a borehole diameter of 9.86 mm in the lead plate. Furthermore, under the same loading conditions, replacing CA-ATRZ with 30 mg of LA resulted in a borehole diameter of 9.84 mm, indicating that CA-ATRZ exhibits excellent initiation performance comparable to LA in initiating RDX, meeting the requirements for weapon systems. Thus, CA-ATRZ is expected to be used as a new green initiating substance in military or civilian detonators.

Laser-sensitive performance

The laser initiation experiment of CA-ATRZ was conducted to measure its response time and minimum detonation energy threshold when exposed to a multi-pulse femtosecond laser at a wavelength of 800 nm. Fig. 6 depicts the entire laser ignition process captured using a high-speed camera operating at 23 000 fps, revealing a rapid DDT process crucial for LSPE initiation. A violent detonation occurred in CA-ATRZ only after 14 ms, and then by completing the DDT in an extremely short time, the total initiation energy is calculated to be 49 mJ, which is slightly higher than the 35 mJ for the excellent Fe-based LSPEs [(TriTzPyr)₃Fe][ClO₄]₂ and [(NH₂TriTzPyr)₃Fe][ClO₄]₂.²⁹ In contrast, pure BNCP can only be excited using near-infrared lasers when doped with carbon black or other sensitising components.⁵⁷ Therefore, CA-ATRZ is an excellent potential material for laser-initiated detonators with low energy thresholds and low delay. The complete process of laser-induced shock-wave growth is shown in Fig. 6. Fig. 6b shows the initial state of the shock wave growth caused by the detonation of CA-ATRZ. After the initiation of 10 μs (Fig. 6c), a lot of window fragments and smoke are ejected with the shock wave, and the shock wave propagates forward by 11.02 mm. Its initial velocity can be calculated as 1102 m s⁻¹, which is higher than the initial velocity of shock waves induced by most high-energy explosives under laser irradiation.⁵⁸ Interestingly, some distortion can be seen in the leading edge of the shock wave in the air, and there is a clear boundary between the radial shock wave and the axial shock wave (Fig. 6d). In the next frame at t = 20 μs, the shock wave almost grows into a whole. At t = 40 μs, the leading edge of the shock wave generated by the CA-ATRZ detonation reaches 36.51 mm from the window and develops into a rapidly propagating full spherical wave (Fig. 6e) with a shock wave velocity of 792.34 m s⁻¹. At t = 80 μs, the shock wave velocity has decayed to 551.2 m s⁻¹, and at t = 100 μs, most of the shock wave has passed out of the field of view. From the evolution of the shock-wave velocity, it can be concluded that CA-ATRZ has extremely high energy-output characteristics, which greatly facilitated the practical application of CA in LSPEs.

Fig. 6 High-speed photographs of the laser initiation process of CA-ATRZ. (a–h) The growth process of shock waves generated by laser initiation of CA-ATRZ.

The absorption of photons by LSPEs promotes electron transition to higher energy levels, which facilitates their laser initiation. The UV-vis spectra of ATRZ, CA-ATRZ, and [Cu(ATRZ)₃(NO₃)₂]_n were collected in the solid state, as depicted in Fig. 7a. The 2D EMOF CA-ATRZ shows significant absorption in the 500–900 nm wavelength range. At the wavelength of 800 nm, corresponding to the wavelength of the laser source, the ligand ATRZ has a certain absorption intensity (A = 0.09), while CA-ATRZ shows a significantly enhanced absorption intensity at this position (A = 0.84), consistent with its laser initiation test results. While ATRZ lacks apparent absorption at 800 nm and cannot be initiated using an 800 nm laser, CA-ATRZ demonstrates excellent laser initiation performance. Furthermore, the 3D EMOF [Cu(ATRZ)₃(NO₃)₂]_n exhibits strong absorption (A = 0.69) at 800 nm, with a higher absorption intensity than CA-ATRZ. However, under the action of an 800 nm laser, it only manifests as decomposition without deflagration or detonation. The encapsulated high-energy ligand N₃⁻ is inferred to contribute to laser-induced DDT in CA-ATRZ.

Fig. 7 Exploration of laser initiation mechanism in CA-ATRZ. (a) Solid-state UV-Vis spectra of ATRZ, [Cu(ATRZ)₃(NO₃)₂]_n, and CA-ATRZ. (b) Charge density difference (CDD) plot of CA-ATRZ.

To delve deeper into the laser initiation mechanism of CA-ATRZ, simulation calculations were conducted using Gaussian16 and Multiwfn software, employing time-dependent density functional theory (TD-DFT).^47,48,59,60 Focusing on the minimal coordination unit [Cu(ATRZ)(N₃)₂], molecular fragments Cu²⁺, N₃⁻ (1), N₃⁻ (2), and ATRZ were analysed; details are shown in the ESI section 6.† Hole–electron distribution revealed that 23.1% of holes and 44.02% of electrons were concentrated on the central Cu²⁺, indicating copper ions as efficient photon absorbers through excited electron transitions. Other holes primarily resided on two N₃⁻ ligands (40.61% and 35.56%), and other electrons are distributed on both N₃⁻ and ATRZ molecules (14.18% and 15.57%), respectively. This shows that compared with Cu²⁺ and N₃⁻, which act as electron donors and acceptors at the same time, the second ligand ATRZ almost only exists as an electron acceptor.

The interfragment charge transfer (IFCT) method quantitatively assessed electron transfer between segments, revealing charge transfer (CT) mode dominance (74.16%) over the local excitation (LE) mode (25.84%) (Fig. 7b). Notably, Cu²⁺ and N₃⁻ participated in various electron transfer modes, the local excitation in Cu²⁺ and N₃⁻ accounting for 12.9% and 33.8% of electron transition, respectively. The metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), and ligand-to-ligand charge transfer (LLCT) accounted for significant electron transitions (12.9%, 33.8%, and 27.4%, respectively), which occurred between all segments including Cu²⁺, ATRZ, and N₃⁻. This shows that the involvement of high-energy ligand N₃⁻ and conjugate planar ATRZ contributed synergistically to the laser initiation process, and the electron transfer between the central metal Cu²⁺ and the ligand N₃⁻ is dominant. The presence of CA in the structure increases the absorption of near-infrared and visible light by CA-ATRZ, which provides accurate and valid theoretical guidance for the design and synthesis of new CA-based LSPEs.

Conclusions

In conclusion, a laser-sensitive primary explosive CA-ATRZ has been reported, which is a copper azide-based 2D EMOF. CA-ATRZ was designed according to the molecular-scale sensitive-unit confinement strategy, and synthesised pure products via single-crystal-to-single-crystal (SCSC) transformation, confirming for the first time the effectiveness of SCSC for the synthesis of EMOFs. This unique 2D structure exhibits enhanced safety compared to CA, with reduced mechanical sensitivity (IS = 1.5 J, FS = 50 N), electrostatic sensitivity (ESD = 20 mJ), and improved thermal stability (T_d = 209 °C). Interlayer slip effects of CA-ATRZ can weaken stress when subjected to external stimuli, and the confinement encapsulation of the sensitive N₃⁻ unit and the π–π interactions between ATRZ also contribute to the stability of its structure. Detailed studies show that CA-ATRZ demonstrates excellent detonation performance (D = 8470 m s⁻¹, P = 34.7 GPa), better than that of most primary explosives, and its initiation ability is comparable to lead azide, which was confirmed through lead plate experiments and laser initiation experiments. Most impressive is its ultrafast, low-energy threshold laser initiation property (t = 14 ms, E = 49 mJ), which shows the balance between low laser initiation energy and safety. In addition, CA-ATRZ and its detonation products are non-toxic, which makes it both cost-effective and environmentally friendly. These highlights suggest the successful implementation of the molecular-scale sensitive-group encapsulation strategy that leads to CA-based MOFs with both laser detonation performance and acceptable sensitivity, and the CA-ATRZ has great potential as a novel LSPE. Such advancements offer new avenues for future LSPEs and even development of primary explosives. It is expected that additional CA-based laser-sensitive primary explosives will continue to be produced through molecular-scale sensitive-unit confinement pathways.

Author contributions

Investigation, design, and main experimental work: R. L., P. P., Z. L., and Q. L. Calculations: R. L., and Y. W. Writing: R. L. Review and editing work: R. L., Q. Z., and H. D. Single crystals: P. P., and R. L. Supervision and project administration: Q. Z. and H. D.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support by the National Natural Science Foundation of China 21975232 is acknowledged (Q.Z.).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis details for novel materials, FTIR spectroscopy, powder X-ray diffraction (PXRD) spectra, X-ray crystallography, and computational analysis details. CCDC 2341002. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02170c

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