Role of Hydrogen Abstraction Reaction in Photocatalytic

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The Role of Hydrogen Abstraction Reaction in Photocatalytic Decomposition of High Energy Density Materials Roman V. Tsyshevsky, Anton S Zverev, Anatoly Y. Mitrofanov, Natalya N. Ilyakova, Mikhail V. Kostyanko, Sergey V. Luzgarev, Guzel G. Garifzianova, and Maija M. Kuklja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08042 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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The Role of Hydrogen Abstraction Reaction in Photocatalytic Decomposition of High Energy Density Materials Roman Tsyshevsky1, Anton S. Zverev2,3, Anatoly Y. Mitrofanov2,3, Natalya N. Ilyakova2, 5

Mikhail V. Kostyanko2,4, Sergey V. Luzgarev2, Guzel G. Garifzianova , and Maija M. Kuklja1,* 1

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA 2 Department of Organic and Physical Chemistry, Kemerovo State University, Kemerovo, Russia 3

4

Yurga Institute of Technology, National Research Tomsk Polytechnic University, Yurga, Russia

Federal Research Center of Coal and Coal Chemistry, Siberian Branch of the Russian Academy of Sciences, Kemerovo, Russia 5 Department of Catalysis, Kazan National Research Technological University, Kazan, Russia

Abstract Explosive phenomena includes a stunningly wide range of diverse manifestations, such as supernova remnant shocks and solar flares, violent decomposition chemistry and synthesis of superior materials under extreme conditions, weapons, missiles, and high velocity impact damage, fuels for space rocket engines, festive fireworks, and applications of detonation waves in construction industry and micro-shocks in medicine. With earliest stages of explosive chemistry in energetic materials remaining poorly understood and constantly posing new science questions, an achievement of a controllable initiation of detonation process represents a particular challenge. Precise tuning of sensitivity to initiation of detonation via photo-excitation appears unreachable because all known secondary explosives are wide band gap insulators. This research demonstrates how YAG:Nd laser irradiation triggers explosive decomposition of PQ-PETN composites formed by pentaerythritol tetranitrate (PETN), high energy density material, mixed with photosensitive 9,10-phenanthrenequinone (PQ).

We suggest, explore, and validate a feasible mechanism of

photocatalytic decomposition of explosives activated by the laser excitation with the energy of 1.17 - 2.3 eV and the wavelength of 1064-532 nm.

*

Corresponding author, [email protected] with cc [email protected]; tel: 703-292-4940

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INTRODUCTION

Reactions involving inter- and intramolecular hydrogen transfer play a crucial role in catalysis1,2,3,4,5, biochemistry6,7,8,9,10,11, microbiology12,13, chemistry of organic ions14,15,16,17,18,19, chemistry and physics of proton-conducting materials for fuel-cell applications20,21 and a rapidly emerging field of photopolymerizarion,22 which finds applications in the development of optical devices23,24, electronics25, dental restorative26 and biomaterials27. Many investigations have been also carried out to reveal the role of multistep reactions involving hydrogen transfer in decomposition of high energy density nitro materials28,29,30,31,32,33. A high activation barrier however usually precludes from consideration of these mechanisms as feasible channels for thermal fragmentation34, except for trinitrotoluene28,29 and its model compound o-NT35,36,37. Optical initiation of chemistry of energetic materials is compelling because it opens new ways for safe handling and storage of high explosives38,39 and bestows new opportunities to use high density of energy stowed in these materials for storage and conversion. Despite this, laser irradiation has been mainly perceived as a source of heat40,41 for vibrational excitation42,43,44 rather than viable means of photo-stimulated initiation45,46,47,48 of energy release. The reason is quite understandable as most secondary explosives are wide band gap dielectrics and do not absorb light with the wavelength of the available laser light sources (1060 and 530 nm)49. The transparency of energetics to the low energy (long wavelength) excitation is usually viewed as a main drawback of the design and application of optical initiating devices. Recently, it was demonstrated that laser (Nd:YAG) excitation is able to trigger a decomposition reaction in PETN when it was mixed with MgO additives50 despite wide band gaps characteristic for both individual materials. Quantum-chemical calculations lifted the contradiction and suggested that PETN - MgO interfaces, formed in the mixture, absorb light with the low energy of 1.3 eV due to oxygen vacancy-adsorbent interactions and electronic states generated in the band gap51,52. This absorption triggers the decomposition chemistry of PETN with a much lower activation barrier than the conventional thermal initiation52. The similar situation was observed in the alumina-PETN mixtures53. It was established that the interaction of energetics with defects leads to a formation of a charged or excited radicals that govern photodecomposition. These developments provoked a set of outstanding questions, such as whether a photocatalytic additive can be deliberately chosen (for instance, as a dopant) to facilitate conventional decomposition reactions or initiate an unusual photodecomposition pathway in energetic materials. To answer this, viable ways of such initiation reactions have to be established. Here, we explore a 1

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feasibility of the photo-catalyzed hydrogen abstraction from a high energy density material through a formation of highly reactive radical species leading to immediate decomposition of the energetic molecule with high energy release. In our study, we employed a widely studied energetic material pentaerythritol tetranitrate (PETN, Figure 1a) and well-known photoinitiator compound - 9,10phenanthrenequinone22,54 (PQ, Figure 1b). (Ortho-benzoquinone (BQ, Figure 1c) was chosen as the model photo initiating molecule for solid state calculations due to its structural resemblance of 9,10phenanthrenequinone and a relatively small size to make calculations affordable). It is determined from DSC measurements55 and density functional theory-based calculations56,57 that the thermal decomposition of PETN requires overcoming an activation barrier of 33-35 kcal/mol, which proceeds through the O-NO2 bond cleavage and produces the NO2 moiety as a primary product. The slow exothermic HONO elimination requires a comparable energy34,56. In this research, we found that laser irradiation is able to initiate photo decomposition PETN samples doped with small quantities of PQ with a dramatically reduced activation energy.

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Figure 1. The molecular structures of a) PETN, b) PQ, and c) BQ, a PQ simulant.

2. 2.1

METHODS

Computational Details

All molecular calculations were carried out within the GAUSSIAN0958 code. Equilibrium ground state structures, electronic properties and reaction pathways were studied using density functional theory59,60 (DFT) – based hybrid PBE061 functional with split-valence 6-31+G(d,p) basis set. According to our recent studies, PBE0/6-31+G(d,p) approximation is sufficiently accurate for predictions of the ground state equilibrium structure of PETN, its electronic structure, optical properties, and thermal stability.

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Vertical excitation energies were computed using time dependent TD PBE0 method62,63. In addition, singlet-triplet (S0→T1) excitation energies were obtained by the ∆SCF method based on differences of total energies in accordance with Frank-Condon principle:

E(S0→T1) = E1(T1) – E0(S0)

(1),

where E(S0→T1) is the energy of the vertical S0→T1 excitation, E0(S0) is the total energy of the ground state equilibrium PETN, and E1(T1) is the total energy of PETN in its triplet state, with the structure corresponding to the ground state equilibrium molecule. All of the stationary points have been positively identified for minimum energy with no imaginary frequencies and the transition states with one imaginary frequency. IRC (intrinsic reaction coordinate) analysis was performed using the Hessian-based Predictor-Corrector integrator algorithm64,65 for each transition state to ensure that the transition state connects desired reactants and products. Solid state periodic calculations were performed by density functional theory (DFT) with optPBE-vDW66,67,68,69,70, which includes corrections for van der Waals interactions and projector augmented-wave (PAW) pseudo-potentials71 as implemented in the VASP code72,73,74. PBE exchange correlation functional75 was employed to simulate hydrogen abstraction reactions in solid state. Minimal energy paths in the VASP periodic calculations were obtained with the nudged elastic band method76 with five intermediate images. Atomic positions were relaxed using conjugate gradient and quasi-Newtonian methods within a force tolerance of 0.05 Å/eV. In simulating an ideal PETN crystal, we used 2×2×2 Monkhorst−Pack k-point mesh with a kinetic energy cut-off of 520 eV. Atomic coordinates and lattice constants were allowed to simultaneously relax without any symmetry constraints. The convergence criterion for electronic steps was set to 10-5 eV, and the maximum force acting on any atom was set not to exceed 0.02 eV/Å. The calculated lattice constants of the tetragonal PETN unit cell, a=b= 9.331 Å, c= 6.624 Å agree with the experimental lattice vectors of a=b= 9.38 Å, c= 6.71 Å

77

within ~1%. To correct the

significantly underestimated band gap energies obtained from vDW-DF, a self-consistent single point calculation with the hybrid PBE0 functional was performed for each configuration using Γ-point only and the kinetic energy cut-off of 520 eV. Energies of the lowest S0→T1 transition in the periodic

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calculations were obtained from Eq. (1). Detailed description and coordinates of surface slab models are collected in Supplementary Information.

2.2

Experiment

To obtain the PQ doped PETN samples (thereafter referred to as PQ-PETN composites), pentaerythritol tetranitrate and 9,10-phenanthrenequinone were dissolved in acetone and precipitated by the fast pouring of the resulting solution into the water. The precipitate was filtered, washed with a small amount of distilled water and dried in air at room temperature. The resulting material was a light yellow powder. Actual measurements were performed on the PQ-PETN composite pressed tablets. To obtain such samples, 14 mg of the composite powder was placed in a pressing tool and pressed at 200 MPa for 5 minutes. The resulting samples were in the form of glassy tablets of 3 mm in diameter and 1 mm thickness. They were placed in the center aperture of the steel shell with 1 mm thickness. Further, the shell with a tablet was placed in the explosion setup (Figure 3), used for the laser initiation.

Figure 2. Micro-photograph of the PQ-PETN composite tablet. Yellow inclusions are the PQ crystals. The 1 mm thick samples had intensive scattering, which makes them unusable for optical measurements. The thinner tablets (0.3 mm thick) with 4 mg weight were fabricated for microscopy and optical measurements. The micrograph of the obtained samples shows (Figure 2) well-defined small crystals of PQ in the tablets bulk. Therefore, the initial composition is a mechanical mixture of PETN and PQ crystals. Optical absorption spectrum was measured in the range of 190 to 1100 nm with Shimadzu UV1700 spectrophotometer. The initiation of samples was achieved using YAG:Nd laser LDPL10M 5

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equipped with the second harmonic generator (SHG) with 532 nm wavelength and 14 ns laser pulse. The scheme of explosion setup is shown in Fig. 3a. The probability of explosion (a percentage of exploded samples vs all probed samples) was measured as a function of energy density. Ten samples were initiated in a series of experiments for each energy density. The energy density was registered by the pyroelectric head PE50BF-DIF-C (Ophir Photonics). In order to achieve the necessary maximum energy density (H, J/cm2), laser beam was focused by a 1.75 and 1.0 D lenses for initiation of PETN and PQ-PETN composite samples, respectively. Steel base

Acoustic sensor

Shell

Sample Glass plate hν

Tektronix TDS3032B

SHG

LDPL10M laser

Figure 3. The experimental setup.

3.

RESULTS

Hydrogen abstraction reactions by radicals or photo excited molecules play an important role in organic chemistry and therefore mechanisms of these reactions are being carefully studied78,79. The photochemical reactivity of aromatic ketones is mainly controlled by the nature of the reactive excited state and therefore the chemistry of the PQ-PETN complex should be defined by the interactions of the components. Detailed studies on energetic molecules with photo-sensitive catalyst additives are however largely lacking. Hence we started our modeling with the determination of the nature of the lowest excited states of PQ-PETN, as well as of the PQ and PETN molecules, and their electronic transitions as an important element in understanding catalytic ketone reactivity.

3.1 Optical transitions of the PQ-PETN complex Optical properties of both PETN and PQ have been previously investigated experimentally and theoretically. The absorption spectrum of PETN consists of three main bands at 193.5 nm (>6.41 eV), 260 nm (4.77 eV) and 290 nm (4.27 eV)80,81,82,83,84. The excitation of gas phase PETN molecules by

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laser beam with energies of 5.0, 5.25, and 5.48 eV and pulse duration of 8 ns initiates decomposition releasing the nitric oxide as an initial product81. The absorption spectrum of PQ in acetonitrile shows a weak shoulder at 2.48 eV followed by two bands of medium intensity with maxima at 3.0 and 3.97 eV and strong absorptions at 4.68 and 5.90 eV85. Additionally, the weak signal at 2.16 eV was registered in magnetic circular dichroism spectra of PQ85. It is well established that the chemical reactivity of the 3(n,π*) and 3(π,π*) triplets of ketones differ markedly79 and the reactive (n,π*) triplet is a very electron-deficient species. The interplay between two lowest singlet-triplet transitions, 3(n→π*) and 3(π→π*) of PQ has been thoroughly studied86,87,88,89,90 due to their role in chemical reactivity and distinct features in the light emission spectra of photo excited PQ. The T1 state of PQ has predominantly 3(π,π*) character in polar solvents while it is an 3(n,π*) in nonpolar solvents87,89. Thus, for instance, in nonpolar CCl4 the lowest triplet state is 3(n,π*), and the second excited 3(π,π*) triplet state is almost isoenergetic with an energy that is only 0.25 kcal/mol higher. In polar MeCN, the lowest 3(π,π*) state is 2.4 kcal/mol higher than second 3(n,π*) state89. The 3(n,π*) triplet state was concluded to be both reactive and emissive state regardless of the polarity of the solvent89,90. Furthermore, the rate of the photoinduced hydrogen abstraction by PQ from 2-propanol in CCl4 and MeCN solvents was reported to be proportional to the population of the reactive 3(n,π*) state90 whereas no reactivity of the 3(π,π*) state triplet was observed even in MeCN. Details of our calculations reproduce electronic properties of PETN and PQ molecules well and are illustrated in Supplementary Materials. Now, in simulating a PQ-PETN complex, we found that the HOMO and LUMO states are fully localized on the PQ molecule (Figure 4) and they fall directly in the PETN gap (Figure 5 a-c). The HOMO of PQ of the PQ-PETN complex is 1.89 eV above the PETN HOMO. Besides HOMO, the HOMO-1–HOMO-5 states of PQ molecule (Figures 4 and 5c) in PQ-PETN complex lie above PETN HOMO. The LUMO of PQ is positioned 1.42 eV below the PETN LUMO (Figure 5c). The resulting HOMO-LUMO gap of the system is 3.76 eV (Figure 5c), which is significantly lower than the individual gap of PETN (7.37 eV, Figure 5a) and almost coincides with that of the PQ molecule (3.91 eV, Figure 5b). Further, two low energy vertical singlet-triplet transitions in PQ-PETN complex, S0→T1 3(n→π*) and S0→T2 3(π→π*), have the energies of 1.90 and 2.18 eV (Figure 5c, Table 1), respectively. These 7

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energies are consistent with calculated energies of the S0→T1 and S0→T2 transitions in PQ molecule (1.78 and 2.25 eV, Figure 5b) and with the experimentally measured energy of the 3(n→π*) transition at 2.16 eV85 (Table 1). The vertical S0→T2 3(π→π*) transition appears to require 0.38 eV higher energy than the vertical S0→T1 3(n→π*) transition. At the same time, the energy splitting between the 3

(π,π*) and 3(n,π*) energy minimums in the relaxed configurations is only 0.04 eV (0.9 kcal/mol)

with 3(π,π*) state being the lowest triplet state (Table 1). On one hand, this finding is consistent with the time-resolved resonance Raman spectroscopy study, which reported that the 3(n,π*) triplet state of PQ in nonpolar carbon tetrachloride solvent is positioned slightly above the 3(π,π*) state86. On the other hand, an earlier phosphorescence of PQ investigation suggested the opposite consequence of transitions and indicated the energy difference of 0.25 kcal/mol89. The first two singlet-triplet transitions in PQ-PETN are followed by the singlet-singlet S0→S1 1

(n→π*) transition the energy of which, 2.40 eV (Figure 5c, Table 1), agrees well with the calculated

(2.31 eV, Figure 5b) and experimentally measured (2.48 eV)85 energies of the lowest singlet-singlet transition in the PQ molecule. The calculated relaxation energy is 0.07 eV, and the lowest S1 singlet 1

(n,π*) state lies 2.33 eV above the equilibrium ground state configuration (Table 1). The S0→S1

transition is a weak, symmetry forbidden transition. In the absorption spectrum of PQ in acetonitrile, it appeared as a weak shoulder85. The calculated oscillator strength (f~10-4) of PQ-PETN complex also points to low intensity of this transition. The first S0→S1 transition is followed by the singletsinglet S0→S2 1(π→π*) transition of moderate intensity (f~0.03) at 2.86 eV (Figure 5c, Table 1). The calculated energy of the second singlet-singlet transition in the PQ-PETN complex is consistent with the calculated energy of the S0→S2 transition of the PQ molecule (3.03 eV, Figure 5b) and with the experimentally measured energy of the 1(π→π*) transition (3.0 eV, Table 1). The S2 state in its minimum energy configuration lies 2.44 eV above S0. The further analysis of electronic excitations of the PQ-PETN complex revealed S0→Tn and S0→Sn transitions with the energies of 3.92 and 4.88 eV (Figure 5c) corresponding to the lowest PETN S0→T1 and S0→S1 transitions (Figure 5a). The transition from PQ HOMO to PETN LUMO (S0→Sm, Figure 5c) with the energy of 4.48 eV was also found. Our modeling shows that in the PQ-PETN complex, both HOMO and LUMO are localized on the PQ molecule. The calculated HOMO-LUMO gap (3.76 eV) is consistent with that of a single PQ molecule (3.91 eV) and ~3.5 eV lower than the HOMO-LUMO gap of PETN (7.37 eV) as illustrated in Figures 5 a-c. 8

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The TD PBE0 calculations of vertical electronic transitions of the PQ-PETN complex show a series of the low singlet-singlet and singlet-triplet excitations localized on PQ (Figure 5c) with the energies close to the energies of the vertical electronic transitions of the isolated PQ molecule (Figure 5c). The transitions fully localized on PETN (S0→Tn and S0→Sn, Figure 5a and c) and excitations from PQ HOMO to PETN LUMO (S0→Sm, Figure 5c) have noticeably higher energies (3.9-4.9 eV, Figure 5c) than the transitions fully localized on PQ.

Table 1. Calculated energies of vertical excitations (Evert, in eV), adiabatic excitations (Eadiabatic, eV), and the corresponding relaxation energies (∆Erelax, in eV) of the low excited states of the PQ-PETN complex. Excited State Character Evert ∆Erelax Eadiabatic Experiment 3 1.90 0.07 1.83 (2.16)a T1 (n,π*) 3 2.18 0.39 1.79 T2 (π,π*) 1 b 2.40 0.07 2.33 2.40 / (2.48a) S1 (n,π*) 1 2.86 0.42 2.44 2.75b/ (3.0a) S2 (π,π*) a b

Energies of vertical excitations of the PQ molecule were taken from ref.85 Energies of vertical excitations were measured for PQ-PETN composite samples in this study.

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Figure 4. Molecular orbitals of PQ-PETN complex.

Figure 5. The energy diagrams showing relative positions of molecular orbitals and energies of vertical electronic transitions of individual molecules of a) PETN, b) PQ, and c) PQ-PETN complex. 3.2

Photo-induced decomposition of PQ-PETN

To elucidate a viable decomposition mechanism of the PQ-PETN composite under irradiation with either first (1.17 eV) or second harmonic (2.33 eV) of Nd:YAG laser, we simulated possible fragmentation pathways of the PQ-PETN complex on the potential surface of its ground and excited

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triplet 3(n,π*) states. We focused only on these states, bearing in mind that PQ’s 3(n,π*) triplet is much more electron-deficient and hence more chemically active than 3(π,π*) triplet79,90. We suggest that a possible mechanism of PQ catalyzed decomposition of PETN via the photoinduced hydrogen abstraction reaction has two main steps: (1) Photo excitation populates the reactive 3

(n,π*) triplet state of PQ (Eq. 2) and (2) The excited PQ molecule abstract a hydrogen from PETN

accompanied by breaking one of the O-NO2 bonds of PETN (Eq. 3), as illustrated in Figure 6. C5H8N4O12 + C14H8O2 + hν → C5H8N4O12 + (C14H8O2)*

(2)

C5H8N4O12 + (C14H8O2)* → C5H7N3O10 + (C14H8O2-H)● + (NO2)●+Q

(3)

The population of the lowest T2 3(n,π*) triplet might proceed via either direct vertical S0→T2 excitation or intersystem crossing from one of the excited singlet Sn 1(n,π*) states. In PQ phosphorescence experiments of the lowest triplet states, the population of the 3(n,π*) and 3(π,π*) triplets usually proceeds via intersystem crossing after initial excitation of the molecule to one of its 1

(π→π*) singlet states with the photon energy of 3-3.5 eV86,87,88,89,90. We note that the first harmonic

(1.17 eV) of Nd:YAG laser has too low energy to be useful here as it would require at least three photons. The second harmonic (2.33 eV) is sufficient for the direct S0→T1 (1.90 eV) excitation and is close to the energy of the S0→S1 (2.40 eV, Table 1, Figure 5c) transition. Therefore, such irradiation would serve to populate the T1 3(n,π*) triplet state of interest. The photo-induced hydrogen abstraction reaction from PETN by the PQ 3(n,π*) triplet was modeled using ∆SCF approach. The energy of the lowest vertical 3(n→π*) singlet-triplet transition obtained from Eq. 1 is 2.02 eV (Table 1), which agrees well with the TD PBE0 estimate (1.90 eV, Figure 5c, Table 1) and with the experimentally measured energy of the 3(n→π*) transition at 2.16 eV. The relaxation towards energy minimum is small, 0.07 eV, and ∆SCF-calculated (n,π*) triplet state lies 1.95 eV above the equilibrium ground state configuration (Figure 6). Once the PQ-PETN 3(n,π*) triplet is formed, the transfer of hydrogen from PETN carbon to PQ oxygen, which requires 0.42 eV (9.6 kcal/mol) and releases 1.63 eV (37.6 kcal/mol) of heat Q (Eq. 3), is accompanied by an immediate loss of the NO2 group from PETN (Figure 6). An analysis of the PQ-PETN molecular orbitals in its equilibrium 3(n,π*) triplet configuration and at the transition state revealed some overlap between the PQ’s (2p) functions of oxygen and PETN’s carbon (2p) and hydrogen (1s) atoms (Figure S5 of Supplementary Information), which is a good 11

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indication of chemical bonding between PQ and PETN. Such an overlap of atomic functions is consistent with the commonly accepted mechanisms of the hydrogen abstraction reaction by radicals and photo excited molecules78. Figure 7a shows the equilibrium configuration of the PQ-PETN complex’s triplet. The spin density is mainly localized on the PQ molecule’s O-C-C-O fragment (Figure 7a and b). The transition state (TS) structure is characterized by a charge transfer; Figure 7c demonstrates that -0.23 e is localized on the PQ molecule, whereas a positive charge in the same amount of 0.23 e is localized on the migrating hydrogen. The C5H7N4O12 fragment corresponding to the PETN molecule without the hydrogen is neutral. An analysis of spin densities indicates a negligible negative spin density (-0.04 e) localized on the migrating hydrogen atom while 1.55 e is localized on the PQ molecule and 0.49 e is localized on PETN’s residue C5H7N4O12 , as depicted in Figure 7c and d. Therefore, an analysis of Mulliken charges and spin densities suggests the following mechanism of the hydrogen abstraction reaction. The PQ molecule in its electron deficient 3(n,π*) triplet state withdraws some electron density from the PETN molecule leaving a hole state. The electron transfer is followed by (or proceeds simultaneously with) the proton migration from PETN to PQ. The electron and hole pair results in the formation of two radicals, C14H8O2-H● (PQ-H) and C5H7N4O12●. Once the C5H7N4O12 radical is formed, the NO2 group splits off with a negligibly small activation energy required for this dissociation process. We note here that the hydrogen atom transfer process requires much higher energy, 2.28 eV (52.7 kcal/mol, Figure 6), when proceeding on the ground state potential surface. Furthermore, the hydrogen abstraction from the ground singlet state leads to the formation of new chemical bond between PETN carbon and PQ oxygen atoms (Figure 6 and Figures S6 and S7 of Supplementary Information) and does not lead to the NO2 loss from PETN. The activation barrier of hydrogen atom abstraction in the ground state (52.7 kcal/mol, Figure 6) is significantly higher than the energy required for the homolytic O-NO2 bond cleavage, 32.6 kcal/mol, measured in DSC experiments55 and 34.9 kcal/mol calculated from DFT modeling56. We remark that we found no overlap between molecular orbitals and hence no bonding states in the PQ-PETN equilibrium ground state configuration in contrast to the 3(n,π*) triplet configuration. Our calculations convincingly demonstrate that the photo-stimulated H-abstraction reaction, widely used in organic chemistry, can be employed for selective, catalytic optically initiated decomposition of energetic molecules and materials. We propose that a photoinitiator (e.g., PQ) 12

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introduced in the matrix of an energetic material will likely catalyze the initiation of detonation that can be triggered by the laser beam with an unusually low energy (and precise wavelength). What is most important here, such a reaction can be controlled with high precision and allows for a great deal of variability by combining appropriate and desirable system components (energetic material, catalytic dopant, and laser irradiation power).

Figure 6. Schematic energy diagram representing hydrogen abstraction reaction from PETN by PQ molecule in its excited triplet 3(n,π*) and ground singlet states. Energies in parentheses were obtained using ∆SCF approach.

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Figure 7. a) The Mulliken atomic charges and b) the spin density distribution in the equilibrium configuration of the triplet PQ-PETN complex, c) The Mulliken atomic charges and d) the spin density distribution at the transition state of the hydrogen abstraction reaction.

4.

VALIDATION

We assume that our conclusions regarding a feasibility of the PQ catalyzed decomposition of the energetic molecules will remain valid when applied to the corresponding nitro energetic materials. In the meantime, the described calculations (section 3) were performed using a simplified model of the two-molecule PQ-PETN complex that presumably represents well interactions and processes occurring in the gas phase and in solutions. An extrapolation of these predictions to materials and 14

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realistic samples however requires careful validation with both experiment and theory.

To

substantiate further our findings, we simulated decomposition of PETN crystals and performed laser initiation experiments on the PQ-PETN mixture by second harmonic (2.33 eV) output of Nd:YAG laser.

4.1

Calculations of the model BQ-PETN interface and its decomposition

Our hypothesis for theoretical validation is founded on the quality of the model of the PQ-PETN bimolecular complex and its ability to reproduce properly critical interactions in the system. This is because both PQ and PETN are molecular materials with well localized wave functions hence the molecular model should be sufficiently accurate in its description of photochemistry. We therefore expect that qualitative trends obtained using the PQ-PETN molecular complex are to be similar to those obtained from solid state calculations regardless of possible minor quantitative discrepancies. To simulate decomposition of PETN crystals, we employed a slab model in which the molecular photocatalyst was placed on the (110) PETN surface (Figure 8a). A relatively large linear size of the PQ molecule (~9.5 Å) requires designing a PETN supercell containing approximately 1000 atoms, which is tremendously computationally demanding. Instead, in our solid state calculations, we used a BQ molecule (Figure 1c), a simulant of PQ, which offers a reasonable compromise between sufficient accuracy and computational affordability. With the similar important structural building blocks in these two molecules, the most significant difference between the BQ and PQ molecular structures there is one aromatic ring of BQ versus three rings of PQ. This may induce a small shift in the spectrum of the electronic states but should not affect other obtained conclusions. First, we repeated simulations of the hydrogen transfer reaction using the BQ-PETN bimolecular complex (Figure 8b) to ensure similarities of PQ-PETN and BQ-PETN behavior and, which is the same, to verify that the BQ will, indeed, serve as a reliable model of PQ for reactions proceeding in the solid state of PETN. Similarly to calculations described in section 3, we used ∆SCF method to model lowest triplet states and explore the reaction mechanism of H-abstraction by the BQ molecule adsorbed on the PETN surface. In simulating the BQ-PETN complex, we found that, similarly to the PQ-PETN complex, the HOMO and LUMO states that fall directly in the PETN gap are fully localized on the BQ molecule (Figure 8). The calculated HOMO-LUMO gap is 3.70 eV. The BQ HOMO lies 1.57 eV above the

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PETN HOMO, and the LUMO of BQ is positioned 1.92 eV below the PETN LUMO. The overall set of energies of the lowest singlet-triplet and singlet-singlet transitions in BQ-PETN complex are systematically shifted by ~-0.6 eV relatively to the corresponding energies of the PQ-PETN complex. Thus, the calculated energies of the 3(n→π*), 3(π→π*) and 1(n→π*) vertical transitions (obtained from TD PBE0) are 1.35, 1.41 and 1.94 eV, respectively. These energies are in good agreement with the energies of optical transitions of the BQ molecule (1.30, 1.39 and 1.84 eV, see Supplementary Information). In periodic calculations, the actual supercell slab contained 16 PETN (See Supplementary Information for more details) molecules and one BQ molecule adsorbed on the surface. Such a model corresponds to the concentration of photosensitive additive (PQ or BQ) of 6.25 %91.

Figure 8. a) The structure of the supercell modeling the photo initiating molecule (BQ) on the (110) PETN surface, b) BQ-PETN molecular complex, c) Projected density of states (PDOS of the BQ molecule adsorbed on the (110) surface of PETN, and d) Isosurfaces of BQ-PETN molecular orbitals.

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Both the top of the valence band and the bottom of the conduction band of the supercell (containing the BQ molecule adsorbed on the (110) surface of PETN) are formed by 2p functions of carbon and oxygen atoms of BQ. The resulting BQ-PETN band gap is 3.60 eV (Figure 8c) and essentially coincides with the HOMO-LUMO gap of the isolated BQ molecule, 3.61 eV, which is ~50% lower than the band gap of the pristine PETN surface (~6.8 eV, Figure 8c). The calculated BQPETN HOMO-LUMO gap is 3.70 eV, which is consistent with the solid state estimation (3.60 eV, Figure 8c). The HOMO and LUMO states of the BQ-PETN complex as well as HOMO-1 (Figures 8d) are localized on the BQ molecule, whereas LUMO+1 is formed from 2p atomic functions of ONO2 fragment of PETN (Figures 8d). The calculated energy of the singlet-triplet S0→T1 transition of the supercell is 0.90 eV, in good agreement with 1.0 eV, the obtained energy of the 3(n,π*) T1 state of the BQ-PETN molecular complex. The spin density depicted in Figures 9a and b shows that the excitation is fully localized on the BQ molecule, with the electron and hole components shown in Figures 9c and d. The hydrogen transfer from one of the surface PETN molecules to the BQ molecule in the triplet state requires 0.57 eV, is accompanied by the homolytic O-NO2 bond cleavage in PETN and proceeds with the release of 1.48 eV of heat (Table 2). This amount of energy is sufficient to trigger further thermally-stimulated decomposition of a neighbor PETN molecule from its ground state, which requires ~1.5 eV55,56,57. The activation barrier and reaction energy obtained for the BQ-PETN complex are also consistent with solid state estimations (Table 2). Figure S8 illustrates the overlap between C (2p) and H (1s) atomic functions of PETN and O (2p) functions of BQ in the equilibrium 3

(n,π*) triplet configuration of the BQ-PETN complex and the transition state structure of the

hydrogen abstraction reaction. The atomic charges and spin densities of the BQ-PETN complex (Figure S9), occurring during the H-abstraction reaction, tend to change in a similar way as those of the PQ-PETN complex (Figure 7). Indeed, the positive charge (0.24 e) on migrating hydrogen atom and the increased spin density (0.48 e) on the C5H7N4O12 fragment at the transition state of the BQ-PETN complex evidences in favor of the electron transfer from PETN to BQ in addition to proton migration. Overall, our calculations of the BQ-PETN supercells show that (1) the lowest triplet state of the system containing the BQ molecule adsorbed on the PETN surface is well-localized on the light absorbing molecule; (2) PETN decomposition via the hydrogen abstraction reaction requires 0.57 eV, which is ~3 times lower than the activation energy of the thermally stimulated PETN decomposition 17

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(1.5 eV)55,56, and proceeds with the heat release in the amount 1.48 eV (Table 2), which is sufficient to initiate thermal decomposition of the neighbor molecules; and (3) The simplified model of the bimolecular complex is suitable for qualitatively accurate predictions of electronic properties and chemical reactivity of photoinitiator-energetic material composite system. Thus, our calculations are validated.

Figure 9 a) The spin density of the triplet state BQ on the (110) PETN surface, b) The spin density of the BQ-PETN molecular complex in the triplet state, c) electron and d) hole components of the 3 (n,π*) T1 state of the BQ-PETN complex as predicted from the ∆SCF simulations. Table 2. The calculated activation barriers (Eb, eV) and reaction energies (Er, eV) of PETN decomposition via the photo induced H-abstraction reaction. Model System BQ-PETN molecular complex BQ on the (110) PETN surfacec PQ-PETN molecular complex

Eb 0.42a (0.36)b 0.57 0.42

Er -1.54 (-1.17) -1.48 -1.63

a

Calculated using hybrid DFT PBE0/6-31+G(d,p) method in the gaseous phase Calculated using standard DFT PBE/6-31+G(d,p) method in the gaseous phase c Solid state estimates were obtained from periodic PBE calculations b

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Laser initiation measurements of the PQ-PETN composite

Our hypothesis for experimental validation is twofold. First, if our conclusions obtained from the computational modeling are correct, the optical absorption spectra of the PQ-PETN composite samples should clearly show the peaks associated with PQ triplet and singlet transitions and PETN transitions. Also, the low energy excitations relevant to the second harmonic radiation of Nd:YAG laser should lie in the range of 1.9-2.5 eV. Second, the pronounced photocatalytic effect of PQ should be manifested in a higher probability of explosive decomposition of PQ-PETN composites than the probability characteristic for pristine PETN samples. The performed measurements are represented in Figure 10, which shows that the PQ-PETN samples begin absorbing light with the energies above ~1.95 eV. The absorption spectra of the PQ in ethanol solution and of the crystalline PETN are depicted in Figure 10a. A comparison of the composite spectra (Figures 10b) and the spectra of individual components superimposed as shown in Figure 10a, offers a transparent, unambiguous interpretation of the measured spectra. In particular, the Figure 10 indicates that the PQ-PETN samples exhibit the same transitions as the PQ molecule with slightly shifted energies (by 0.1-0.3 eV), whereas the PETN samples do not absorb light in the energy range 2.4-3.8 eV. The origin of the experimental PQ-PETN absorption spectra (~2.16 eV) agrees well with the calculated energy, 1.90 eV, of the S0→T1 3(n→π*) excitation (Table 1). Further, the absorption spectrum of the PQ-PETN composite samples has a weak shoulder at ~2.40 eV followed by the band of medium intensity at 2.75 eV and the band of high intensity at 3.56 eV (Figure 10b). The position of the shoulder coincides with the calculated energy of the vertical S0→S1 transition (2.40 eV, Table 1), and the experimentally observed maximum at 2.75 eV is consistent with the energy of the S0→S2 transition at 2.86 eV (Table 1). This clearly illustrates that the PQ serves to absorb laser irradiated photons in the range where the pristine PETN samples are transparent.

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Figure 10. The measured absorption spectrum of a) PQ in ethanol solution and crystalline PETN and b) PQ-PETN samples In the laser initiation experiments performed to probe chemistry from the PQ excited state, the probability of explosion was measured as a function of the radiation exposure and performance of the pure PETN samples was compared to the behavior of the PQ-PETN composites. For initiation of PETN, energy densities were set to 2.70, 3.08 3.25, 3.64, and 4.03 H, J/cm2, whereas initiation of PQPETN composite samples was performed at 1.71, 2.34, 2.49, 2.50, and 2.65 H, J/cm2 The resulting probability curves are shown in Figure 11. Each point represents the proportion of the samples exploded (minimum of 10 samples were initiated in each attempt) for a given radiation exposition.

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PQ-PETN PETN

Figure 11. The probability curves of initiation of pure PETN and PQ-PETN composite.

The initiation threshold energy densities were determined by the Neyer method. Figure 11 demonstrates that the initiation threshold, measured as 50% probability of explosion of the pure PETN samples, which appears at 3.3 J/cm2, is visibly reduced for the PQ-PETN composites, 2.5 J/cm2. The probability of the PQ-PETN samples initiation sharply increases to 100% when the laser energy density reaches 2.5 J/cm2, and it requires not less than 5 J/cm2, i.e. twice higher exposure, to initiate pristine PETN samples. Although tempting, it is difficult to correlate these numbers directly to the calculated activation energies of decomposition (see section 3) through either the thermal or optical excitation. Nevertheless, the observed reduction of energy needed to initiate explosion of PQPETN is certainly consistent with the reduction of activation barrier to initiate the bond dissociation in the composites. Thus, the experimental result is in good agreement with the theoretical formulation and the hypothesis of this article. Apparently, the samples containing PQ particles are more reactive and PQ serves to catalyze the explosive decomposition reaction. This is facilitated by two factors. On one hand, there is a process of the PETN dehydrogenation leading to a decrease of the potential barriers of the further decomposition reaction. On the other hand, PQ particles absorb light more intensely, part of which goes to heating the material and thus also facilitates the further reaction. The difference in the forms of the pure PETN and PQ-PETN curves is likely due to the efficiency (kinetics) of the 21

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explosion reaction centers formation. In the case of pure PETN, the process has a stochastic nature, which leads to a smooth increase of the probability of the sample explosion within the range of energy densities from 2 to 4 J/cm2, and it replicates our earlier result92 of the initiation of PETN with wavelength of 1064 nm under the same conditions. We note that PQ used in the experiments may be not the most effective initiator22,54 as the wavelength of laser light enters the tail of the absorption band. In addition, the concentration dependence of the efficiency of initiation also plays a role. Perhaps, a more homogeneous dopant distribution in the energetic material is desirable, as the reaction is to be facilitated only at the PQPETN interface.

5.

SUMMARY AND CONCLUSIONS

A combined experimental and theoretical study of photo induced decomposition of a high energy density material PETN containing 0.2% of PQ additives shows that the photocatalytic decomposition of energetic materials is an efficient way to trigger an explosive reaction in a controllable way. The second harmonic of the Nd:YAG laser irradiation (2.33 eV), to which the pristine PETN samples are transparent and indifferent, can be used to excite the PQ molecule in the PQ-PETN composite samples from its ground state to highly reactive triplet 3(n,π*) state. Once excited, the PQ triplet abstracts hydrogen from the PETN molecule with the activation barrier of 0.42 eV. The hydrogen transfer is followed by an immediate loss of the NO2 group. The exothermic reaction proceeds with the energy release of 1.63 eV, which is sufficient to induce thermal decomposition of neighbor PETN molecules in its ground state via the conventional cleavage of the O-NO2 bond that requires ~1.5 eV55,56,57. Our theoretically proposed concept and initial (probing) molecular quantum chemical calculations were carefully validated by both modeling and experiments. DFT periodic calculations of PETN crystals containing defects (in this case, a photosensitive molecular impurity) confirmed the suggested photocatalytic decomposition mechanism. Laser initiation measurements corroborated the predicted optical absorption spectra features and the energies of electronic transitions in PQ-PETN samples while DFT calculations helped to interpret the nature of the transitions. The explosion probability curves registered as a function of laser energy density deposited to the samples verified the trends in the energy threshold reduction needed to trigger explosions when photosensitive PQ is added to PETN samples.

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The understanding of decomposition mechanisms of high energy density materials triggered by heat, light, and mechanical impact is not only important for safe storage and handling of these materials, but is also required for designing and improving effective detonating devices. This research demonstrated how the laser excitation can be effectively used for photo-initiation of explosive decomposition and how control (or tuning) over the initiation of explosive decomposition can be gained. Thus, such an approach to photo-initiation of high energy density (explosive) materials described in this paper can serve in a variety of possible applications well beyond conventional high explosions with large release of thermal energy. For example, laser excitation of carefully selected combinations of photo-catalysts and high energy density materials in small quantities is appealing as safe, effective, and cost-efficient means for generating shock waves of microexplosions in medical procedures93,94 including cancer treatment95,96, orthopedics97, fragmentation of kidney stones98 and gallbladder stones99, and an increase the efficiency of genetic transformations in bacteria and fungi95. Acknowledgments: This research is supported in part by the US ONR (Grants N00014-16-1-2069 and N00014-16-1-2346), NSF, and the Ministry of Education and Science of Russian Federation (no. 2014/64 Project 2146). We used NSF XSEDE resources (Grant TG-DMR-130077), the Stampede supercomputing system at TACC/UT Austin (Grant OCI-1134872), and DOE NERSC resources (Contract DE-AC02-05CH11231). MMK is grateful to the Office of the Director of National Science Foundation for support under the IRD program. Any appearance of findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NSF and other funding agencies. Supporting Information: Supporting Information file contains descriptions of 1) computational details and structural models employed for solid state periodic calculations, 2) results and discussion of vertical electronic transitions in PETN, PQ and BQ molecules, 3) structures and XYZ coordinates of reagents, intermediates, molecules at the transitions states, and final products involved in the reaction of PETN decomposition via photo induced hydrogen transfer.

Conflicts of Interest: The authors declare no conflict of interest.

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