Energetic Chromophores: Low-Energy Laser Initiation in Explosive Fe(II) Tetrazine Complexes

High-Energy-Density Material with Magnetically Modulated Ignition

Preparation of a redox-frustrated high-energy-density energetic material is achieved by gentle protolysis of Mn[N(SiMe3)2]2 with the perchlorate salt of the tetrazolamide [H2NtBuMeTz]ClO4 (Tz = tetrazole), yielding the Mn6N6 hexagonal prismatic cluster, Mn6(μ3-NTztBuMe)6(ClO4)6. Quantum mechanics-based molecular dynamics simulations of the decomposition of this molecule predict that magnetic ordering of the d5 Mn2+ ions influences the pathway and rates of decomposition, suggesting that the initiation of decomposition of the bulk material might be significantly retarded by an applied magnetic field. We report here experimental tests of the prediction showing that the presence of a 0.5 T magnetic field modulates the ignition onset temperature by +10.4 ± 3.9 °C (from 414 ± 4 °C), demonstrating the first example of a magnetically modulated explosive.

Construction of High Energy Nanoscale Lead Azide Composite with Improved Flame Sensibility from Intercalated Hydroxide

It is of great significance to develop efficient methods for preparing high-content modified nanoscale lead azide (LA) composites used in microinitiating devices. In this work, a structurally controllable salicylate-intercalated lead hydroxide with a nanoscale mesoporous structure is designed. Using it as a precursor, carbon-based lead azide (LA/C) and salicylate-based lead azide (LA/SA) are fabricated by the gas-solid azidation of the framework (GAF) method within 3 h, greatly reducing the preparation time of nano-LA composites. The characterizations of the composites demonstrate that the Pb in the precursors is transformed into nanoscale LA attached to the salicylate radical or its carbonized skeleton. Due to the unique embedded nanostructures and excellent electrical and thermal conductivity of salicylate-derived carbon materials, LA/C exhibits excellent electrostatic safety (E50 = 0.25 J) and flame sensitivity (H50 = 28 cm). The adjustable organic-inorganic ratio of intercalated hydroxides allows the LA content in LA/C to reach as high as 92.5%, enabling 6.50 mg of LA/C to successfully detonate secondary explosive CL-20 in a microinitiating device, demonstrating an amazing detonation ability superior to other reported LA complexes. The research provides a new perspective for the development of nanoscale LA composites with high LA content and appropriate sensitivity.

A three-dimensional energetic coordination compound (BLG-1) with excellent initiating ability for lead-free primary explosives

Exploration of advanced lead-free primary explosives is a challenging issue in the field of energetic materials. Herein, we designed and synthesized a novel N-rich copper bromate energetic coordination compound (ECC) [Cu(ATRZ)(BrO3)2]n (BLG-1, ATRZ: 4,4'-azo-1,2,4-triazole) by a simple one-step reaction. BLG-1 is the first reported three-dimensional (3D) N-rich copper bromate ECC. Its interesting 3D reticular architecture contributed to its highest thermal decomposition temperature (Td: 226 °C) and crystal density (ρ: 2.69 g cm-3) among N-rich copper bromate ECCs. More importantly, a primary charge of BLG-1 as little as 3 mg could reliably detonate compressed RDX, and 1 mg could detonate CL-20. These incredible values indicated that BLG-1 had an ultra-powerful initiating ability far superior to that of previously reported primary explosives. BLG-1 had improved mechanical sensitivities (IS: 13 J; FS: 1 N) and electrostatic sensitivity (EDS: 240 mJ) compared with those of the typical lead-based primary explosive, lead azide (IS: 4J; FS: 0.75N; EDS: 5 mJ). In particular, BLG-1 had a low laser-initiation threshold of 13 mJ at 808 nm, suggesting that it could serve as a laser-ignitable primary explosive. This work suggests that BLG-1 is a promising candidate with engreat practical application prospects for lead-free primary explosives.

A promising perovskite primary explosive

A primary explosive is an ideal chemical substance for performing ignition in military and commercial applications. For over 150 years, nearly all of the developed primary explosives have suffered from various issues, such as troublesome syntheses, high toxicity, poor stability or/and weak ignition performance. Now we report an interesting example of a primary explosive with double perovskite framework, {(C6H14N2)2[Na(NH4)(IO4)6]}n (DPPE-1), which was synthesized using a simple green one-pot method in an aqueous solution at room temperature. DPPE-1 is free of heavy metals, toxic organic components, and doesn't involve any explosive precursors. It exhibits good stability towards air, moisture, sunlight, and heat and has acceptable mechanical sensitivities. It affords ignition performance on par with the most powerful primary explosives reported to date. DPPE-1 promises to meet the challenges existing with current primary explosives, and this work could trigger more extensive applications of perovskite.

Periodate-Based Perovskite Energetic Materials: A Strategy for High-Energy Primary Explosives

The requirements for high energy and green primary explosives are more and more stringent because of the rising demand in the application of micro initiation explosive devices. Four new energetic compounds with powerful initiation ability are reported and their performances are experimentally proven as designed, including non-perovskites ([H2 DABCO](H4 IO6 )2 ·2H2 O, named TDPI-0) and perovskitoid energetic materials (PEMs) ([H2 DABCO][M(IO4 )3 ]; DABCO=1,4-Diazabicyclo[2.2.2]octane, M=Na+ , K+ , and NH4 + for TDPI-1, -2, and -4, respectively). The tolerance factor is first introduced to guide the design of perovskitoid energetic materials (PEMs). In conjunction with [H2 DABCO](ClO4 )2 ·H2 O (DAP-0) and [H2 DABCO][M(ClO4 )3 ] (M=Na+ , K+ , and NH4 + for DAP-1, -2, and -4), the physiochemical properties of the two series are investigated between PEMs and non-perovskites (TDPI-0 and DAP-0). The experimental results show that PEMs have great advantages in improving the thermal stability, detonation performance, initiation capability, and regulating sensitivity. The influence of X-site replacement is illustrated by hard-soft-acid-base (HSAB) theory. Especially, TDPIs possess much stronger initiation capability than DAPs, which indicates that periodate salts are in favor of deflagration-to-detonation transition. Therefore, PEMs provide a simple and feasible method for designing advanced high energy materials with adjustable properties.

Graphene Oxide-Intercalated Tetrazole-Based Coordination Polymers: Thermally Stable Hybrid Energetic Crystals with Enhanced Photosensitivity

High-energy-density photosensitive pyrotechnics with good thermal stability have been in increasing demand in recent years. In this paper, graphene oxide (GO)-intercalated energetic coordination polymers (ECPs) are prepared with improved thermostability but great photosensitivity by using high nitrogen compounds azotetrazole (AT) and 5,5'-bistetrazole-1,1'-diolate dehydrate (BTO) as ligands. The decomposition activation energy (E_a) of Cu-AT has been increased from 135.7 to 151.9 kJ·mol^-1 after intercalating 5 wt% GO, and in the meantime, the exothermic peak temperature (T_p) was increased by 12.6 °C. However, the decomposition E_a of Cu-BTO decreased under the effect of the same amount of GO with little effect on T_p. This confirms that GO has stabilization effects on the Cu-AT crystal, whereas the catalytic effects on Cu-BTO would dominate after dehydration with its crystal lattice collapse. Also, when the content of GO was 3%, the resultant GO_0.03-Cu-AT exhibits a higher density (2.88 g·cm^-3) and good thermostability (T_p = 293.7 °C). This ECP shows excellent low-energy laser ignition performance, which can be ignited with an energy of less than 1 mJ at a wavelength of 976 nm. Low-energy laser initiation is considered to be a safer but more reliable method than the traditional electrical-based ones.

Coordination polymerization of nitrogen-rich linkers and dicyanamide anions toward energetic coordination polymers with low sensitivities

The design and synthesis of energetic materials (EMs) with high energy and reliable stabilities has attracted much attention in the field of EMs. In this work, we employed a strategy of the coordination polymerization of mild dicyanamide ions (DCA^-), two isomeric ligands 1-methyl-5-aminotetrazole (1-MAT) and 2-methyl-5-aminotetrazole (2-MAT) to construct energetic coordination polymers (ECPs). Four new ECPs {[Co(DCA)₂(1-MAT)₂]·H₂O}_n1, [Cu(DCA)₂(1-MAT)]_n2, [Cd(DCA)₂(1-MAT)₂]_n3 and [Cd(DCA)₂(2-MAT)₂]_n4 were successfully synthesized through solvent evaporation routes. Compounds 1 and 4 display 1D chains, while 2 and 3 exhibit 2D-layered structures. Compounds 1-3 with the 1-MAT ligand all exhibit reliable thermal stabilities (> 200 °C). The calculated heats of detonation (ΔH_det) of 1-3 are all higher than 1.4 kJ g^-1, which are higher than traditional explosive TNT (1.22 kJ g^-1) and the reported ECP AgMtta (HMtta = 5-methyl-1H-tetrazole, ΔH_det = 1.32 kJ g^-1). Furthermore, sensitivity testing demonstrates that 1-4 features low mechanical sensitivity to external mechanical action in contrast with the extremely sensitive azide-based ECPs [Cu₃(2-MAT)₂(N₃)₆]_n. In addition, compound 2 shows hypergolic properties via an 'oxidizer-fuel' drop experiment, demonstrating its application prospects in the field of propellants. This work details an approach of synthesizing multipurpose ECPs with reliable stabilities by introducing mild dicyanamide anions into nitrogen-rich skeletons.

An Iron-NDC Framework with a Cage Structure and an Optothermal Conversion in NIR Window

Pursuing novel materials with efficient photothermal conversion under irradiation at the near-infrared region windows (NIR, 750-850 nm; NIR-I and NIR-II, 1000-1320 nm)) is of great importance due to their irreplaceable applications, especially in the biomedical field. Herein, on the basis of a coordination chemistry strategy, an iron-based metal-organic framework (MOF) of [N(CH₃)₄]₂[Fe₃(NDC)₄]·DMF·3H₂O (Fe-NDC, 1,4-H₂NDC = 1,4-naphthalenedicarboxylic acid, N(CH₃)₄⁺ = tetramethyl-ammonium, and DMF = N,N-dimethylformamide) was prepared and characterized. Due to the d-d transition effect introduced by coordination with the transition-metal ion of iron and the highly conjugated naphthalenic moiety in 1,4-H₂NDC, guaranteeing an energy transfer between iron and the organic module, Fe-NDC shows a remarkable broad absorption, which could be extended into the NIR-II section. As a result, Fe-NDC could be irradiated by NIR laser (both 808 and 1064 nm) to achieve photothermal conversion. This work sets a good example to inspire the future designation of NIR light-irradiated photothermal materials based on the first-row transition metals.

Improving Near-Infrared Emission of meso-Aryldipyrrin Indium(III) Complexes via Annulation Bridging: Excited-State Dynamics

Using non-adiabatic dynamics and Redfield theory, we predicted the optical spectra, radiative and nonradiative decay rates, and photoluminescence quantum yields (PLQYs) for In(III) dipyrrin-based complexes (i) with electron-withdrawing (EW) or electron-donating (ED) substituents on the meso-phenyl group and (ii) upon fusing the pyrrin and phenyl rings via saturated or unsaturated bridging to increase structural rigidity. The ED groups lead to a primary π,π* character with a minor intraligand charge transfer (ILCT) contribution to the emissive state, while EW groups increase the ILCT contribution and red-shift the luminescence to ∼1.5 eV. Saturated annulation enhances the PLQYs for complexes with primary π,π* character compared to those of the non-annulated and unsaturated-annulated complexes, while both unsaturated and saturated annulation decrease the PLQYs for complexes with primary ILCT character. We found that PLQY improvement goes beyond a simple concept of structural rigidity. In contrast, the charge transfer character of excitonic states is a key parameter for engineering the NIR emission of In(III) dipyrrin complexes.

Synthesis, thermal behaviors, and energetic properties of asymmetrically substituted tetrazine-based energetic materials

1,2,4,5-tetrazine ring is a common structure for the construction of energy-containing compounds, and its high nitrogen content and large conjugation effect give it the advantage of a good balance between energy and mechanical stability as a high-nitrogen energy-containing material. However, most of the reported works about tetrazine energetic materials (EMs) are symmetrically substituted tetrazines due to their easy accessibility. A small number of reports show that asymmetrically substituted tetrazines also have good properties, such as high density and generation of enthalpy and energy. Herein, two asymmetrically substituted tetrazines and their five energetic salts were prepared and fully characterized by IR spectroscopy, NMR spectra, elemental analysis, and differential scanning calorimetry (DSC). The structure of the two compounds was further confirmed by single-crystal X-ray diffraction studies. The thermal behaviors and thermodynamic parameters were determined and calculated. In addition, the energetic properties and impact sensitivities of all the compounds were obtained to assess their application potential. The results show that compounds 2–4 and 7–9 show higher detonation velocities than TNT, and the hydrazinium salt 9 possesses the best detonation properties (D = 8,232 m s⁻¹ and p = 23.6 GPa). Except for 4 and 3, all the other compounds are insensitive, which may be applied as insensitive explosives. Noncovalent interaction analysis was further carried out, and the result shows that the strong and high proportion of hydrogen bonds may contribute to the low-impact sensitivity.

Near-Infrared Light Triggered a High Temperature Utilizing Donor-Acceptor Cocrystals

Developing a suitable initiation for the energetic materials that respond to a low-power near-infrared laser can aid in replacing the current expensive and bulky laser-initiation systems. Here, we report on a system of molecularly tailored 1:1 donor-acceptor (D-A) charge-transfer (CT) cocrystals that manifest ultrabroad absorption (200-2500 nm) characteristics as well as noteworthy very fast self-assembly behaviors. The very narrow highest occupied molecular orbital-lowest unoccupied molecular orbital gap enables N,N,N',N'-tetramethyl-p-phenylenediamine and tetrahalo-1,4-benzoquinones (TMPD-TXBQ) cocrystals to have a great light-harvesting ability in the near-infrared range. When irradiated with a low-power hand-held 808 nm laser with an input energy of only 40 mJ or a power density of 260 mW·cm^-2, these TMPD-TXBQ cocrystals immediately undergo an efficient photothermal conversion followed by a dramatic exothermic thermal polymerization reaction due to the face-to-face D-A-D-A stacking in these cocystals to achieve a temperature as high as 318.9 °C. This temperature is high enough for a thermal initiation of most common energetic materials, and thus this TMPD-TXBQ cocrystal can potentially act as a near-infrared laser initiator that is compact, lightweight, and cost-effective.

Facile Synthesis of Energetic Nanoparticles of Copper Azide with High Initiation Ability for Micro-Initiator Applications Using Layered Copper Hydroxide

Copper azide (CA) is one of the preferred primary explosives in the micro-initiating device, and it is of conducive significance to develop high-content CA-modified materials. In this work, we reported two types of CA composites with CA nanorods embedded in carbon nanosheets (CA/C) and CA distributed on salicylic acid (CA/SA) using layered copper hydroxide nanosheets intercalated with salicylic acid as the precursor. The detailed characterizations demonstrated that CA/C exhibits eximious electrostatic sensitivity (1.06 mJ) due to the inherent structural characteristics of CA/C such as the limitation of the free movement of CA by the layered structure and preeminent electrical conductivity of carbon nanosheets. Surprisingly, CA/C with nearly 1.0 mg in the miro-initiating device can reliably detonate Hexanitrohexaazaisowurtzitane (CL-20). CA/C exhibits extremely high CA content (93%), excellent ignition ability, and detonation ability, and its performance is superior to pure CA and most CA-modified materials reported previously. CA/SA also has an excellent detonation ability and its electrostatic sensitivity is as low as 0.92 mJ. These findings provide a new perspective for the development of high-performance primary explosives for the micro-initiating device.

Preparation of Laser Energetic Coordination Polymers Based on Urazine by Self-Crystallization

A practical and brilliant way of preparing laser energetic coordination polymers based on crystallization chemistry and coordination theory is proposed in this paper. Design and successful synthesis of urazine (C₂H₄N₄O₂, H₂ur, 1) by the theory of "cyclization of semicarbazides" are reported. Using the "acid-controlled self-crystallization" synthesis method, with H₂ur as the ligand, we successfully synthesized [Ag(H₂ur)₂ClO₄·H₂O]_n (3) and confirmed its composition and 1D structure. In addition, 3 was subjected to a simple drying operation to obtain a solvent-free [Ag(H₂ur)₂ClO₄]_n (4). Also, 4 has the best abilities in physics and chemistry, such as excellent thermal stability, insensitivity to light, mechanical insensitivity, and good corrosion resistance. In particular, thermogravimetric analysis-differential scanning calorimetry-Fourier transform infrared spectroscopy and powder X-ray diffraction were employed to analyze the thermal decomposition products of 4 and demonstrated that the main decomposed products are AgCl, N₂, and H₂O. Moreover, the calculated predictions show that 4 has an acceptable detonation performance (P = 22.5 GPa; D = 6.9 km s^-1). Furthermore, the hot needle examination and detonation experiment show that 4 can be used as a primary explosive. More importantly, as a laser-detonated light-sensitive material, 4 has a significant application value in safety detonators (E = 100 mJ, P = 10 W, and τ = 10 ms).

New Method Synthesis of Urazine and Dissolve-crystallization of Its Ag(I)-based Laser Energetic Coordination Polymers

Guided by synthetic chemistry, coordination chemistry and crystallization chemistry, a novel synthesis and crystallization strategy of laser complex energetic materials has been established in this article. In this paper, a...

New perspectives on the laser initiation for metal tetrazine complexes: a theoretical study

In order to understand the relationship between laser initiation and charge transfer of metal tetrazine complexes (MTCs), several sets of MTCs with different metals and ligands were designed and their charge transfer (CT) characters were examined using a time-dependent density functional theory method (TD-DFT) in combination with UV-vis spectra, hole-electron distribution, interfragment charge transition, and transition density matrix analyses. Results show that Fe(II), Mn(II), and Cu(II) are suitable divalent transition metal cores in constructing the optical initiation tetrazine complexes. By replacing the divalent metal cores with a monovalent center, new sets of complexes are proved to possess metal-to-ligand charge transfer (MLCT) character and stronger absorption intensity in the near-infrared (NIR) region, which implies that monovalent MTCs are more in favor of low-energy laser initiation than divalent MTCs. Reasonable tuning of the structure of pyrazole substituent can expect to enhance the explosive performance while preserving the optical characteristics, which is an important design principle. This work thoroughly depicts the photoactive states for MTCs and gives a train of thought to explore new desirable laser initiation explosives.

Chelate Coordination Compounds as a New Class of High-Energy Materials: The Case of Nitro-Bis(Acetylacetonato) Complexes

The existence of areas of strongly positive electrostatic potential in the central regions of the molecular surface of high-energy molecules is a strong indicator that these compounds are very sensitive towards detonation. Development of high-energy compounds with reduced sensitivity towards detonation and high efficiency is hard to achieve since the energetic molecules with high performance are usually very sensitive. Here we used Density Functional Theory (DFT) calculations to study a series of bis(acetylacetonato) and nitro-bis(acetylacetonato) complexes and to elucidate their potential application as energy compounds with moderate sensitivities. We calculated electrostatic potential maps for these molecules and analyzed values of positive potential in the central portions of molecular surfaces in the context of their sensitivity towards detonation. Results of the analysis of the electrostatic potential demonstrated that nitro-bis(acetylacetonato) complexes of Cu and Zn have similar values of electrostatic potential in the central regions (25.25 and 25.06 kcal/mol, respectively) as conventional explosives like TNT (23.76 kcal/mol). Results of analysis of electrostatic potentials and bond dissociation energies for the C-NO₂ bond indicate that nitro-bis(acetylacetonato) complexes could be used as potential energetic compounds with satisfactory sensitivity and performance.

Nitratoethyl-5H-tetrazoles: improving the oxygen balance through application of organic nitrates in energetic coordination compounds

1- and 2-Nitratoethyl-5H-tetrazole (1-NET and 2-NET) were prepared through nitration of the respective alkyl alcohol using 100% nitric acid. A mixture of 1- and 2-hydroxyethyl-5H-tetrazole was obtained after alkylation of 1,5H-tetrazole. Also, a one-pot synthesis of 1-hydroxyethyl-5H-tetrazole was developed enabling the selective preparation of 1-NET. Both organic nitrates were characterized by ¹H, ¹³C, and ¹H-¹⁵N HMBC NMR spectroscopy. In addition, calculations using the Hirshfeld method and the EXPLO5 code were performed. Principally, 20 energetic coordination compounds involving the d-metals Mn, Cu, Zn, and Ag, each structurally characterized by low temperature single crystal X-ray diffraction, were prepared based on 1-NET and 2-NET. Of these complexes, 18 were obtained as pure bulk materials, and therefore, characterized for impact, friction, and ball drop impact sensitivity, as well as electrostatic discharge and thermal stability using differential thermal analysis. Hot plate and hot needle tests were performed mostly showing strong deflagrations making the complexes candidates for green combustion catalysts. Furthermore, successful PETN initiation experiments were carried out for several complexes and all ECCs were investigated by laser ignition experiments.

Low-Power Laser Ignition of an Antenna-Type Secondary Energetic Copper Complex: Synthesis, Characterization, Evaluation, and Ignition Mechanism Studies

In recent years, development of new energetic compounds and formulations, suitable for ignition with relatively low-power lasers, is a highly active and competitive field of research. The main goal of these efforts is focused on achieving and providing much safer solutions for various detonator and initiator systems. In this work, we prepared, characterized, and studied thermal and ignition properties of a new laser-ignitable compound, based on the 5,6-bis(ethylnitroamino)-N'2,N'3-dihydroxypyrazine-2,3-bis(carboximidamide) (DS3) proligand. This new energetic proligand was prepared in three steps, starting with 5,6-bis(ethylamino)-pyrazine-2,3-dicarbonitrile. Crystallography studies of the DS3-derived Cu(II) complex (DS4) revealed a unique stacked antenna-type structure of the latter compound. DS4 has an exothermal temperature of 154.5 °C and was calculated to exhibit a velocity of detonation of 6.36 km·s^-1 and a detonation pressure of 15.21 GPa. DS4 showed properties of a secondary explosive, having sensitivity to impact, friction, and electrostatic discharge of 8 J, 360 N, and 12 mJ, respectively. In order to study the mechanism of ignition by a laser (using a diode laser, 915 nm), we conducted a set of experiments that enabled us to characterize a photothermal ignition mechanism. Furthermore, we found that a single pulse, with a time duration of 1 ms and with a total energy of 4.6 mJ, was sufficient for achieving a consistent and full ignition of DS4. Dual-pulse experiments, with variable time intervals between the laser pulses, showed that DS4 undergoes ignition via a photothermal mechanism. Finally, calculating the chemical mechanism of the formation of the complex DS4 and modeling its anhydrous and hydrated crystal structures (density functional theory calculations using Gaussian and HASEM software) allowed us to pinpoint a more precise location of water molecules in experimental crystallographic data. These results suggest that DS4 has potential for further development to a higher technology readiness level and for integration into small-size safe detonator systems as for many civil, aerospace, and defense applications.

Coordination Chemistry of a Bis(Tetrazine) Tweezer: A Case of Host-Guest Behavior with Silver Salts

The carbon-carbon cross-coupling of phenyl s-tetrazine (Tz) units at their ortho-phenyl positions allows the formation of constrained bis(tetrazines) with original tweezer structures. In these compounds, the face-to-face positioning of the central tetrazine cores is reinforced by π-stacking of the electron-poor nitrogen-containing heteroaromatic moieties. The resulting tetra-aromatic structure can be used as a weak coordinating ligand with cationic silver. This coordination generates a set of bis(tetrazine)-silver(I) coordination complexes tolerating a large variety of counter anions of various geometries, namely, PF₆⁻, BF₄⁻, SbF₆⁻, ClO₄⁻, NTf₂⁻, and OTf⁻. These compounds were characterized in the solid state by single-crystal X-ray diffraction (XRD) and diffuse reflectance spectroscopy, and in solution by ¹H-NMR, mass spectrometry, electroanalysis, and UV-visible absorption spectrophotometry. The X-ray crystal structure of complexes {[Ag(3)][PF₆]}_∞ (4) and {[Ag(3)][SbF₆]}_∞ (6), where 3 is 3,3′-[(1,1′-biphenyl)-2,2′-diyl]-6,6′-bis(phenyl)-1,2,4,5-tetrazine, revealed the formation of 1D polymeric chains, characterized by an evolution to a large opening of the original tweezer and a coordination of silver(I) via two chelating nitrogen atom and some C=C π-interactions. Electrochemical and UV spectroscopic properties of the original tweezer and of the corresponding silver complexes are reported and compared. ¹H-NMR titrations with AgNTf₂ allowed the determination of the stoichiometry and apparent stability of two solution species, namely [Ag(3)]⁺ and [Ag(3)₂]²⁺, that formed in CDCl₃/CD₃OD 2:1 v/v mixtures.

Laser ignition of energetic complexes: impact of metal ion on laser initiation ability

Alkali metal-containing energetic complexes were easy to initiate, followed by the free ligand, whereas the alkaline-earth metal complexes exhibited longer initiation delay times.

A series of guanidine salts of 3,6-bis-nitroguanyl-1,2,4,5-tetrazine: green nitrogen-rich gas-generating agent

Nitrogen-rich energetic materials (EMs) have been widely studied because of their high thermal stability, insensitivity, excellent detonation performance and non-toxic characteristics. In particular, these compounds are well applied as gas-generating agents (GGAs). As a nitrogen-rich heterocyclic framework, 1,2,4,5-tetrazine derivatives have shown great potential in the design of GGAs. The guanidine salts of 3,6-bis-nitroguanyl-1,2,4,5-tetrazine (DNGTz), guanidine (G₂DNGTz) (1), aminoguanidine (AG₂DNGTz) (2), diaminoguanidine (DAG₂DNGTz) (3), and triaminoguanidine (TAG₂DNGTz) (4) have been synthesized and characterized by elemental analysis and FT-IR. The crystal structures of 1 and 2 were obtained by X-ray single crystal diffraction. Crystal analysis shows that 1 and 2 arrange through zigzag-chain-like assembly and face-to-face geometries, which is helpful in decreasing mechanical sensitivity. The thermal stability and thermal decomposition kinetics of these four salts were studied by Differential Scanning Calorimetry (DSC). Furthermore, the thermogravimetry-Fourier transform infrared-mass spectrometry (TG-FTIR-MS) analysis of thermal decomposition products reveals that the main decomposition gaseous products are H₂O, N₂O, CO₂, NO, N₂ and NH₃. Then, the cytotoxicity of the four salts was tested by MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) method, and it was found that salts 1–4 show slight cytotoxicity in mouse fibroblasts (L929), at a concentration of 0.125 mg ml⁻¹. The insensitivity, low toxicity, and production of clean gases without solid residue after burning of salt 1 indicate that it can be used as a potential green energetic material for GGAs.

All salts show slight cytotoxicity in mouse fibroblasts and the main decomposition gas products of thermal decomposition are non-toxic.

Incorporation of a nitrogen-rich energetic ligand in a {Yb} complex exhibiting slow relaxation of the magnetisation under an applied field

Though often used as the precursor in energetic materials research and organic synthesis, 3,6-dihydrazinyl-1,2,4,5-tetrazine (dhtz) has not been previously explored in coordination chemistry. Herein, we present the first crystalline coordination compound with dhtz, [Yb₂(dhtz)(fod)₆] (fod^- = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate). This compound exhibits Single-Molecule Magnet behaviour under an applied magnetic field, and m_J states were resolved through luminescence studies.

Copper(II) Dicyanamide Complexes with N-Substituted Tetrazole Ligands - Energetic Coordination Polymers with Moderate Sensitivities

Following the useful concept of energetic coordination compounds (ECC), copper(II) dicyanamide was used as a building block for the synthesis of eight new complexes. As ligands, six different N-substituted tetrazoles were applied, leading to the formation of high-nitrogen containing complexes. The obtained compounds were characterized in detail by single crystal as well as powder XRD, IR, EA, DTA, and TGA. In addition, the sensitivities towards impact and friction were determined with BAM standard techniques as well as the sensitivity towards electrostatic discharges. All compounds show moderate sensitivities (IS>6, FS>80 N) and energetic properties but differ in their polymeric structures forming polymeric chains or layers up to 3D networks.

1,2,4,5-Tetrazine based ligands and complexes

One of the most intriguing nitrogen based aromatic heterocycles is 1,2,4,5-tetrazine or s-tetrazine (TTZ) thanks to its electron acceptor character and fluorescence properties and the possibilities of functionalization in the 3 and 6 positions allowing access to various ligands. In this review we focus on the two main families of TTZ based ligands, i.e. ditopic symmetric and monotopic non-symmetric, together with their metal complexes, with a special emphasis on their solid state structures and physical properties. After a description of the most representative complexes containing unsubstituted TTZ as a ligand, symmetric TTZ ligands and complexes derived thereof are discussed in the order: 3,6-bis(2-pyridyl)-tetrazine, 3,6-bis(3-pyridyl)-tetrazine, 3,6-bis(4-pyridyl)-tetrazine, 3,6-bis(2-pyrimidyl)-tetrazine, 3,6-bis(2-pyrazinyl)-tetrazine, 3,6-bis(monopicolylamine)-tetrazine, 3,6-bis(vanillin-hydrazinyl)-tetrazine and TTZ containing carboxylic acids. Remarkable results have been obtained in recent years for metal-organic frameworks and magnetic compounds in which magnetic coupling is enhanced when the tetrazine bridge is reduced to radical anions. Non-symmetric ligands, such as dipicolylamine-TTZ and monopicolylamine-TTZ, are comparatively more recent than the symmetric ones. They allow in principle the preparation of mononuclear complexes in a controlled manner, although binuclear complexes have been isolated as well. Moreover, in the monopicolylamine-TTZ-Cl ligand, deprotonation of the amine, thanks to the electron acceptor character of TTZ, afforded a negatively charged ligand equivalent of a guanidinate.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

S,S-Tetrazine-Based Hydrogels with Visible Light Cleavable Properties for On-Demand Anticancer Drug Delivery

Photocleavable hydrogels are of great importance in the field of controlled drug delivery, stem cell fate regulation, surface patterning, and intelligent devices. However, the development of novel photocleavable gel systems by visible light is usually met with challenges such as the lack of efficient and tunable photocleavable groups and reactions. Herein, we reported the facile fabrication of a new type of photocleavable hydrogels by the direct gelation of 4-arm thiol-terminated polyethylene glycol with 3,6-dichloro-1,2,4,5-tetrazine via the formation of S,S-tetrazine linkages. The prepared hydrogels underwent efficient degradation upon irradiation by ultraviolet or green light, and the degradation kinetics could be significantly promoted by hydrogen peroxide. Correspondingly, the hydrogels loaded with calcium peroxide microparticles or glucose oxidase/catalase enzymes enabled the precise and efficient in vivo photocontrol of gel degradation and drug release for cancer treatment. This work offers a promising and facile strategy towards the fabrication of visible light cleavable hydrogels with tunable and on-demand drug release properties.

Seeking a novel energetic co-crystal strategy through the interfacial self-assembly of CL-20 and HMX nanocrystals

Solvent and heat induced self-assembly to CL-20/HMX co-crystals has been investigated. The mechanism towards such process could be concluded to nanoparticle inducing, oriented aggregation, surface integration and co-crystals assembly.

A highly stable octa-coordinated energetic complex

The first octa-coordinated energetic material with excellent thermal stability based on transition metal Cd was reported.

Bridge-Clamp Bis(tetrazine)s with [N]8 π-Stacking Interactions and Azido-s-Aryl Tetrazines: Two Classes of Doubly Clickable Tetrazines

Click chemistry at a tetrazine core is useful for bioorthogonal labeling and crosslinking. Introduced here are two new classes of doubly clickable s-aryl tetrazines synthesized by Cu-catalyzed cross-coupling. Homocoupling of o-brominated s-aryl tetrazines leads to bis(tetrazine)s structurally characterized by tetrazine cores arranged face-to-face. [N]₈ π-stacking interactions are essential to the conformation. Upon inverse electron demand Diels-Alder (iEDDA) cycloaddition, the bis(tetrazine)s produce a unique staple structure. The o-azidation of s-aryl tetrazines introduces a second proximal intermolecular clickable function that leads to double click chemistry opportunities. The stepwise introduction of fluorophores and then iEDDA cycloaddition, including bioconjugation to antibodies, was achieved on this class of tetrazines. This method extends to (thio)etherification, phosphination, trifluoromethylation and the introduction of various bioactive nitrogen-based heterocycles.

Evolution of Oxidizing Inorganic Metal Salts: Ultrafast Laser Initiation Materials Based on Energetic Cationic Coordination Polymers

An effective and novel design strategy for ultrafast laser-initiating materials has been established on the basis of coordination chemistry for the first time in the present work. In view of the positive effect of Ag ion and perchlorate on laser sensitivity, silver perchlorate as a representative of oxidizing inorganic metal salts was used to construct energetic cationic coordination polymers (ECCPs), which solved the inconvenient situation caused by the difficulty in applying these salts directly in energetic materials because of the unavoidable hygroscopicity and the inhomogeneity of physical mixtures of oxidants and reductants. With the nonenergetic nitrogen-rich ligand 3-amino-1H-1,2,4-triazole-5-carbohydrazide (ATCA), one new laser-sensitive Ag(I)-based ECCP [Ag(ATCA)ClO₄]_n (1) was successfully synthesized with a compact helical structure proved by X-ray single-diffraction crystal data. The physicochemical property evaluation revealed that this Ag-ECCP was not only completely devoid of the undesirable properties of the silver perchlorate and displayed excellent tolerance to moisture and noncorrosive properties to metal shells, but was also endowed with good thermal stability and excellent safety for mechanical stimulation. Moreover, theoretical calculations based on the standard molar enthalpy of formation and the lead plate explosive test as the actual damage experiment have proved that the compound has a superior detonation performance (up to 6800 m s^-1 and 0.511 kcal g^-1) compared to the traditional primary explosives. More importantly, the laser-initiation-experiment-based femtosecond laser-testing system and high-speed photography demonstrated that this ECCP was an energetic material with great potential for application in the safety detonator as an ultrafast photosensitive initiating material for laser direct initiation, whose initiation delay time is as low as 73 ms using only 200 mJ initiation energy.

A green metal-free fused-ring initiating substance

Over the past century, the search for lead-free, environmentally friendly initiating substances has been a highly challenging task in the field of energetic materials. Here, an organic primary explosive featuring a fused-ring structure, 6-nitro-7-azido-pyrazol[3,4-d][1,2,3]triazine-2-oxide, was designed and synthesized through a facile two-step reaction from commercially available reagents. This organic initiating substance meets nearly all of the stringent criteria of environmentally friendly primary explosives for commercial applications: it is free of toxic metals and perchlorate, has a high density, high priming ability, unusual sensitivities towards non-explosive stimuli, excellent environmental resistance, decent thermal stability, high detonation performance, satisfactory flowability and pressure durability, and is low-cost and easy to scale-up. These combined properties and performance measures surpass the current and widely used organic primary explosive, DDNP. The fused-ring organic primary explosive reported herein may find real-world application as an initiating explosive device in the near future.

Energetic transition metal complexes based on methyl carbazate and picric acid: syntheses, structures, physicochemical properties and catalysis on the thermal decomposition of ammonium perchlorate

Seven energetic transition metal complexes based on methyl carbazate and picric acid were synthesized in this work. All of them exhibit excellent catalytic performance on the thermal decomposition of ammonium perchlorate, especially the Cu(ii) complex.

2-Methyl-substituted monotetrazoles in copper(ii) perchlorate complexes: manipulating coordination chemistry and derived energetic properties

Several new energetic coordination compounds (ECC) have been prepared using two 2-methyl-substituted tetrazoles. By simply changing the reaction conditions, the coordination sphere of the metal center can be manipulated in order to obtain different complexes with varying energetic properties.

Synthesis and properties of insensitive [1,2,4]triazolo[4,3-b]-1,2,4,5-tetrazine explosives

An insensitive explosive 3-nitro[1,2,4]triazolo[4,3-b]1,2,4,5-tetrazin-6-amine (TTNA) was prepared and fully structurally characterized with good detonation parameters (T_d = 213 °C; P = 33.9 GPa; D = 8953 m s⁻¹, IS > 40 J, and FS > 360 N).