Laser Initiation of Fe(II) Complexes of 4-Nitro-pyrazolyl Substituted Tetrazine Ligands.

The synthesis and characterization of new 1,2,4-triazolyl and 4-nitro-pyrazolyl substituted tetrazine ligands has been achieved. The strongly electron deficient 1,2,4-triazolyl substituted ligands did not coordinate Fe(II) metal centers, while the mildly electron deficient 4-nitro-pyrazolyl substituted ligands did coordinate Fe(II) metal centers in a 2:1 ratio of ligand to metal. The thermal stability and mechanical sensitivity characteristics of the complexes are similar to the conventional explosive pentaerythritol tetranitrate. The complexes had strong absorption in the visible region of the spectrum that extended into the near-infrared. In spite of having improved oxygen balances, increased mechanical sensitivity, and similar absorption of NIR light to recently reported Fe(II) tetrazine complexes, these newly synthesized explosives were more difficult to initiate with Nd:YAG pulsed laser light. Specifically, the complexes required lower densities (0.9 g/cm3) to initiate at the same threshold utilized to initiate previous materials at higher densities (1.05 g/cm3).

Cooperative enhancement of the nonlinear optical response in conjugated energetic materials: A TD-DFT study.

Conjugated energetic molecules (CEMs) are a class of explosives with high nitrogen content that posses both enhanced safety and energetic performance properties and are ideal for direct optical initiation. As isolated molecules, they absorb within the range of conventional lasers. Crystalline CEMs are used in practice, however, and their properties can differ due to intermolecular interaction. Herein, time-dependent density functional theory was used to investigate one-photon absorption (OPA) and two-photon absorption (TPA) of monomers and dimers obtained from experimentally determined crystal structures of CEMs. OPA scales linearly with the number of chromophore units, while TPA scales nonlinearly, where a more than 3-fold enhancement in peak intensity, per chromophore unit, is calculated. Cooperative enhancement depends on electronic delocalization spanning both chromophore units. An increase in sensitivity to nonlinear laser initiation makes these materials suitable for practical use. This is the first study predicting a cooperative enhancement of the nonlinear optical response in energetic materials composed of relatively small molecules. The proposed model quantum chemistry is validated by comparison to crystal structure geometries and the optical absorption of these materials dissolved in solution.

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

The synthesis and characterization of air stable Fe(II) coordination complexes with tetrazine and triazolo-tetrazine ligands and perchlorate counteranions have been achieved. Time-dependent density functional theory (TD-DFT) was used to model the structural, electrochemical, and optical properties of these materials. These compounds are secondary explosives that can be initiated with Nd:YAG laser light at lower energy thresholds than those of PETN. Furthermore, these Fe(II) tetrazine complexes have significantly lower sensitivity than PETN toward mechanical stimuli such as impact and friction. The lower threshold for laser initiation was achieved by altering the electronic properties of the ligand scaffold to tune the metal ligand charge transfer (MLCT) bands of these materials from the visible into the near-infrared region of the electromagnetic spectrum. Unprecedented decrease in both the laser initiation threshold and the mechanical sensitivity makes these materials the first explosives that are both safer to handle and easier to initiate than PETN with NIR lasers.

Synthesis and Electrochemical Behavior of Electron-Rich s-Tetrazine and Triazolo-tetrazine Nitrate Esters.

We have prepared energetic nitrate ester derivatives of 1,2,4,5-tetrazine and 1,2,4-triazolo[4,3-b]-[1,2,4,5]-tetrazine ring systems as model compounds to study the electrochemical behavior of tetrazines in the presence of explosive groups. The model compounds showed lower thermal stabilities relative to PETN (pentaerythritol tetranitrate), but slightly improved mechanical sensitivities. The presence of electron-rich amine donors leads to a cathodic shift of the tetrazine redox potentials relative to those of previously reported tetrazine explosives. At these potentials, electron-rich tetrazines with either covalently bound or co-dissolved nitrate ester groups are irreversibly reduced. Effectively, changes in the electronic structure of tetrazines affect their electrochemical response to the presence of nitrate ester groups. Thus, it may be possible to develop tetrazine-based electrochemical sensors for the detection of specific explosives and electrocatalysts for their disposal.

Ultrafast Photodissociation Dynamics of Nitromethane.

Nitromethane (NM), a high explosive (HE) with low sensitivity, is known to undergo photolysis upon ultraviolet (UV) irradiation. The optical transparency, homogeneity, and extensive study of NM make it an ideal system for studying photodissociation mechanisms in conventional HE materials. The photochemical processes involved in the decomposition of NM could be applied to the future design of controllable photoactive HE materials. In this study, the photodecomposition of NM from the nπ* state excited at 266 nm is being investigated on the femtosecond time scale. UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) are combined with nonadiabatic excited state molecular dynamics (NA-ESMD) simulations to provide a unified picture of NM photodecomposition. The FSRS spectrum of the photoproduct exhibits peaks in the NO2 region and slightly shifted C-N vibrational peaks pointing to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an nπ* state lifetime of ∼20 fs were predicted from NA-ESMD simulations. Predicted time scales revealed that NO2 dissociation occurs in 81 ± 4 fs and methyl nitrite formation is much slower having a time scale of 452 ± 9 fs corresponding to the excited state absorption feature with a decay of 480 ± 17 fs observed in the TA spectrum. Although simulations predict C-N bond cleavage as the primary photochemical process, the relative time scales are consistent with isomerization occurring via NO2 dissociation and subsequent rebinding of the methyl radical and nitrogen dioxide.

Two-Photon Absorption in Conjugated Energetic Molecules.

Time-dependent density functional theory (TD-DFT) was used to investigate the relationship between molecular structure and the one- and two-photon absorption (OPA and TPA, respectively) properties of novel and recently synthesized conjugated energetic molecules (CEMs). The molecular structures of CEMs can be strategically altered to influence the heat of formation and oxygen balance, two factors that can contribute to the sensitivity and strength of an explosive material. OPA and TPA are sensitive to changes in molecular structure as well, influencing the optical range of excitation. We found calculated vertical excitation energies to be in good agreement with experiment for most molecules. Peak TPA intensities were found to be significant and on the order of 10(2) GM. Natural transition orbitals for essential electronic states defining TPA peaks of relatively large intensity were used to examine the character of relevant transitions. Modification of molecular substituents, such as additional oxygen or other functional groups, produces significant changes in electronic structure, OPA, and TPA and improves oxygen balance. The results show that certain molecules are apt to undergo nonlinear absorption, opening the possibility for controlled, direct optical initiation of CEMs through photochemical pathways.

Independent Control of Optical and Explosive Properties: Pyrazole-Tetrazine Complexes of First Row Transition Metals.

Complexes of 3-amino-6-(3,5-dimethylpyrazole)tetrazine) (NH2TzDMP, 1) and 3-(3,3'-dinitroazetidine)-6-(3,5-dimethylpyrazole)tetrazine) (DNAZTzDMP, 2) with first row transition metal centers were synthesized. Reactions of Fe(II)(H2O)6(BF4)2 and Fe(NO3)3·9H2O with 1 and 2 both led to complexes of the form [(RTzDMP)3Fe]X2 (X = BF4, R = NH2 (3), DNAZ (4); X = NO3, R = NH2 (5), DNAZ (6)), which showed intense MLCT bands in the visible region of the spectrum. Ligands 1 and 2 also reacted with Cu(II)(NO3)2·5/2H2O to form [(RTzDMP)2Cu(NO3)][NO3] (R = NH2 (7), DNAZ (8)) in addition to reacting with Cu(I)(CH3CN)4(PF6) to form [(RTzDMP)2Cu][PF6] (R = NH2 (9), DNAZ (10)). Lastly reactions of 1 and 2 with Co(NO3)2·6H2O and Ni(NO3)2·6H2O led to [(NH2TzDMP)2Co(H2O) (NO3)][NO3] (11), [(DNAZTzDMP)2Co(H2O)2][NO3]2 (12), [(NH2TzDMP)3Ni][NO3]2 (13), and [(DNAZTzDMP)2Ni(H2O)2][NO3]2 (14). The complexes display rich electrochemical and photophysical properties that are unaffected by derivation with explosive groups.

Photoactive high explosives: linear and nonlinear photochemistry of petrin tetrazine chloride.

Pentaerythritol tetranitrate (PETN), a high explosive, initiates with traditional shock and thermal mechanisms. In this study, the tetrazine-substituted derivative of PETN, pentaerythritol trinitrate chlorotetrazine (PetrinTzCl), is being investigated for a photochemical initiation mechanism that could allow control over the chemistry contributing to decomposition leading to initiation. PetrinTzCl exhibits a photochemical quantum yield (QYPC) at 532 nm not evident with PETN. Using static spectroscopic methods, we observe energy absorption on the tetrazine (Tz) ring that results in photodissociation yielding N2, Cl-CN, and Petrin-CN as the major photoproducts. The QYPC was enhanced with increasing irradiation intensity. Experiment and theoretical calculations imply this excitation mechanism follows sequential photon absorption. Dynamic simulations demonstrate that the relaxation mechanism leading to the observed photochemistry in PetrinTzCl is due to vibrational excitation during internal conversion. PetrinTzCl's single photon stability and intensity dependence suggest this material could be stable in ambient lighting, yet possible to initiate with short-pulsed lasers.

CO2 laser heating system for in situ radial x-ray absorption at 16-BM-D at the Advanced Photon Source.

We present a portable CO2 laser heating system for in situ x-ray absorption spectroscopy (XAS) studies at 16-BM-D (High Pressure Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory). Back scattering optical measurements are made possible by the implementation of a Ge beamsplitter. Optical pyrometry is conducted in the near-infrared, and our temperature measurements are free of chromatic aberration due to the implementation of the peak-scaling method [A. Kavner and W. R. Panero, Phys. Earth Planet. Inter. 143-144, 527-539 (2004) and A. Kavner and C. Nugent, Rev. Sci. Instrum. 79, 024902 (2008)] and mode scrambling of the input signal. Laser power stabilization is established using electronic feedback, providing a steady power over second timescales [Childs et al., Rev. Sci. Instrum. 91, 103003 (2020)]-crucial for longer XAS collections. Examples of in situ high pressure-temperature extended x-ray absorption fine structure measurements of ZrO2 are presented to demonstrate this new capability.

Quantum chemistry studies of electronically excited nitrobenzene, TNA, and TNT.

The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S(0) and S(1)) and lowest two triplet (T(1) and T(2)) electronic states are investigated, with particular emphasis on the S(1) relaxation pathway and the nonadiabatic region leading to radiationless decay of S(1) population. In nitrobenzene, relaxation on S(1) occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S(1), S(0), and T(2). Moreover, spin-orbit coupling constants for the S(0)/T(2) and S(1)/T(2) electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the S(1) population should quench primarily to the T(2) state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on S(1) is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO(2). The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S(1) energy pathway.

Portable Raman explosives detection.

Recent advances in portable Raman instruments have dramatically increased their application to emergency response and forensics, as well as homeland defense. This paper reviews the relevant attributes and disadvantages of portable Raman spectroscopy, both essentially and instrumentally, to the task of explosives detection in the field.