Excited-state dynamics and electron transfer process of 1,3,5-triamino-2,4,6-trinitrobenzene

Abstract

Insights into the excited-state dynamics and electron transfer processes of nitro explosives offer an efficient tool for unravelling ultrafast and complex detonation physics. In this work, the excited state dynamics and electron transfer processes of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were studied using femtosecond transient absorption spectroscopy and time-dependent density functional theory. The de-excitation of TATB after excitation at 400 nm involves an equilibrium between the vibrationally hot S₁ (S^*₁) and S₁ states, with lifetimes of 0.64 and 6 ps, respectively. After vertical excitation, the electron density is transferred from the C-ring and NH₂ group to NO₂ groups. The excitation energy activates the nitro groups, and energy is redistributed via their nuclear motion. The relaxation of the initial S^*₁ state shows an apparent structural change occurring at one activated nitro group, which becomes further twisted from the planar benzene ring and relaxes to the S₁–T₁ conical intersection. The T₁ state is populated via the minimal intersystem crossing through the S₁–T₁ intersection. The nitro group charge transfer process is then explored following the ultrafast structural change. This study advances our understanding of photo-initiated reactions as well as the ignition of energetic materials.

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1. Introduction

Molecules exhibit different behaviours in the excited state compared to the ground state.^1–4 As a matter of general interest, ultrafast and complex photo-initiated reactions can be mechanistically probed by effectively exploiting ultrafast excited-state dynamics and electron transfer processes.^5–9 Excited-state behaviour has been extensively studied using ultrafast pump-probe spectroscopy.^10,11 Detailed investigations of the electronic, structural, geometric, and chemical changes that take place when a molecule is promoted to the excited state can enable us to understand the inherent physical and chemical processes that ensue.¹²

Energetic materials (EMs) are organic compounds with substantial amounts of stored chemical energy that can be released under extreme conditions, such as exposure to photons, shock, or rapid heating.^13,14 EMs, which include fuels and explosives, are widely utilized in many fields; however, their useful energy conversion features have associated problems of safety, reliability, and application.¹⁵ On the molecular level, very little is known about the dynamic responses of these materials. For example, electronic excitation plays a crucial role in the energy conversion processes of EMs. Insights into the excited-state dynamics of these materials and their electron transfer behaviours would potentially allow us to unravel these inherent mechanisms, and would also have implications for the behaviour of other nitro derivatives.

Among EMs, nitro explosives constitute the major class. The introduction of a nitro group into a molecule greatly changes its photo-physical behaviour, and can impact the molecular stability and impact sensitivity.^16,17 The electronically excited state of a nitro-containing species undergoes ultrafast intersystem crossing and exhibits a high quantum yield of the triplet state. For example, de-excitation of the nitrostilbene derivative, 2,2′,4,4′,6,6′-hexanitrostilbene (HNS), was revealed to involve a major change from the S₁ to the T₁ state with a lifetime of 6 ps.¹⁸ It is essential to understand the photo-induced behaviour of nitro-containing species. As a type of nitro explosive, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is of particular interest because, although it is relatively stable to external stimuli such as heat, impact, and shock,^19–21 it is a photo-sensitive EM that exhibits colour changes under light irradiation.²² A study of the evolution of the excited states and photo-induced electron transfer of TATB is crucial to understanding the importance of nitro groups and EMs ignition.

In this work, we present an experimental and theoretical investigation of the excited-state dynamics and electron transfer processes of TATB. The initial S^*₁ state of TATB and its evolution into the S₁ state with lifetimes of 0.64 and 6 ps, respectively, were observed using femtosecond transient absorption spectroscopy at an excitation wavelength of 400 nm. The structural relaxation that occurs as the S^*₁ state evolves to the S₁ state, with the apparent structural change occurring at a nitro group, was investigated using the time-dependent density functional theory (TD-DFT) method at the B3LYP/6-311++G(d,p) level. An S₁–T₁ conical intersection was suggested to enhance the efficiency of intersystem crossing. The electron distribution was also evaluated in order to explain the structural change. Ultrafast nitro group charge transfer (Q_nitro), which occurs subsequent to the structural changes, was also explored. The study advances our understanding of ultrafast electron transfer in EMs.

2. Experimental and theoretical section

TATB powder (purity > 99.5%) was synthesized at the Institute of Chemical Materials at the China Academy of Engineering Physics. For the transient absorption spectroscopy experiments, the concentration of TATB in dimethyl sulfoxide (DMSO, 99% purity) was 1 mmol L⁻¹. The solution was flowed at room temperature using a slow-flow device to avoid decomposition induced by the ultraviolet excitation at 400 nm.

The typical equipment used to obtain transient absorption spectra has been described previously.^11,23 Briefly, a laser system generates fundamental femtosecond pulses at 800 nm, with a pulse duration of 35 fs, energy per pulse of 1 mJ, and repetition rate of 1 kHz. As the pump beam, the third harmonics (266 nm) of the fundamental pulses were attenuated to ∼5 μJ. The white-light-continuum (WLC) pulses from a CaF₂ crystal were used as the probe beam, with a spectral range of 440–640 nm. The pump-probe pulses were organized as follows: the pump-pulse beam was lead and collimated onto the sample cell (1 mm optical pathlength). The probe-pulse beam passed through an optical delay line, and then was split into two parts by a metallic beam splitter. One part was collimated and overlapped with the pump-pulse in the sample cell. The other was collimated into the sample cell at a different spot as a reference signal, to improve sensitivity. The polarization plane of the probe pulses was rotated by a λ/2 plate, and a magic angle (54.7°) configuration between the pump and probe pulses was adopted in all the measurements. Given the speed of light (3 × 10⁸ m s⁻¹), the delay time was varied up to 500 ps with a minimum interval of 2.08 fs. The probe-pulse beam was dispersed by a grating inside the spectrometer (Princeton, SpectraPro 2500i) and detected by a two-dimensional CCD detector (PI-MAX, 1024 × 256 pixel array). On the CCD surface, two image signal stripes were formed, one from the probe and the other from the reference. Each of these signals was sent to a computer through a 16-bit analog-to-digital converter (ADC). The data acquisition and experiment control were performed via the LabView program. A low noise level (<2 mOD) could be achieved by averaging approximately 1600 pulses. The instrumental response of the system was determined to be ∼180 fs by cross-section measurements between the pump and probe pulses. The chirp of the WLC was measured at the position of the sample cell by using the optical Kerr effect of the solvent; this value was used to correct the dispersion of the relative delay time in the time-resolved data. The stimulated emission, ground state bleach, and dark response were eliminated by logical collections, by the use of two shutters. The absorption of excited states, transient products, and long-lived molecular states such as triplet state would acquire ΔA > 0, while the bleaching of the ground state absorption and stimulated emission are obtained for ΔA < 0. Therefore, the spectral changes could be sometimes complicated and required special methods to accurately analyse the time-resolved data.

The TD-DFT method at the B3LYP/6-311++g(d,p) level was employed to calculate the vertical excitation energies and optimize the structures in the ground (S₀) and first singlet excited (S₁) states.²⁴ The first triplet excited (T₁) state was also obtained at this level by changing the spin multiplicity. The natural population analysis (NPA) charge was utilized in this study. The absorption spectra were obtained at the wb97xd/cc-pVTZ level. The Kohn–Sham orbitals, orbital energies, and density-of-states (DOS) spectra were also obtained at this level, to avoid the effects of a diffuse basis function. All the calculations were performed using Gaussian 09 software.²⁵ Although TD-DFT was not as accurate as multi-configuration methods such as the complete-active-space self-consistent field (CASSCF) method, TD-DFT provided reliable results for the related states of this molecule, according to our tests, because it had no multi-configuration effects. It should be pointed out that the TD-B3LYP method is not well suited for searching for an intersection point, and the CASSCF method is too expensive for performing such calculations. The molecular orbitals, electron distributions, and DOS spectra were obtained using a wave function analyser (Multi-wfn).²⁶

3. Experimental and theoretical results

3.1. Kohn–Sham orbitals, DOS, and steady-state absorption spectra

The structure of TATB in the S₀ state (TA-S₀) and the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs, respectively) are shown in Fig. 1. The electron density in HOMO-2 is mainly distributed between C5 and N16, and is mainly distributed among C1, C3, and N19 in HOMO-1. Both orbitals exhibit non-bonding (n) character. HOMO-1 and HOMO-2 have the same energy of −8.99 eV. The electron density in LUMO-1, which shows anti-bonding (π*) character, is distributed equally among the three NO₂ groups, whereas the electron density in LUMO-2 is distributed primarily and symmetrically between one NO₂ and NH₂ group.²⁷ LUMO-1 and LUMO-2 have energies of −0.90 and −0.67 eV, respectively. From the orbital analysis, it can be inferred that the low-lying excited states are accessed through HOMOs → LUMOs transitions that can be assigned as n → π* transitions.

Fig. 1 The structure of TATB in the S₀ state (TA-S₀), and the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs).

The total, partial, and overlap density-of-states (TDOS, PDOS, and OPDOS) for TATB are shown in Fig. 2. In the TDOS map, each discrete vertical line corresponds to a molecular orbital (MO). The curve for the TDOS was simulated based on the distribution of MO energy levels. From the PDOS, it is clear that the nitro groups contribute the most to the valence MOs. In the range from −10.0 to −7.5 eV, there are comparable contributions from the C-ring and NH₂ groups. However, the contributions are reduced to a low level in the range from −3.0 to 0.0 eV. The OPDOS is negative in the −3.0–0.0 eV range, which indicates the anti-bonding orbitals between the NO₂ groups and other moieties. The DOS curves also show that the low-lying excitation process can be assigned as an n → π* transition.

Fig. 2 Total, partial, and overlap density-of-states (TDOS, PDOS, and OPDOS) for TATB.

In the experimentally obtained steady-state absorption spectrum of TATB in DMSO, there is a wide band centred at 357 nm, which extends to the 450 nm range²⁸ (Fig. 3). The extension of the experimental absorption results from the “hot band” effect that occurs at room temperature and weak interactions in solution, such as intramolecular and intermolecular H bonds. The band is centred at 301 nm in the calculated spectrum, which is lower than that observed experimentally. This may be explained by an overestimation of the excitation energy by the TD-DFT method. However, this discrepancy does not affect the general discussion in this work. From the orbital information above, the peak can be assigned as an n → π* transition (S₁ state),which is Franck–Condon-state-accessible from the vibrational ground state configuration. At 400 nm, the strong absorption efficiency indicates the vertical excitation from the contribution of MO pairs of the low-lying transition dipole moment. Thus, from the calculations in this paper, the ultraviolet excitation at 400 nm (3.10 eV) suggests that TATB molecules are vertically excited to the S₁ surface from the vibrational ground state.

Fig. 3 Measured and calculated steady-state absorption spectra of TATB in DMSO.

3.2. Transient absorption spectra and the dynamic process

The evolution-associated difference spectra (EADS, Fig. 4) were obtained from a global analysis of the transient absorption experiments for TATB. The background level is shown at a time delay of −0.4 ps before the overlap of the pump-probe pulse. The first EADS evolves into the second one in 0.1 ps, which is characterized by a strong transient absorption. This corresponds to the population of the initial excited state, which can be assigned as the S^*₁ state. In addition, there are two peaks centred at 470 and 600 nm, respectively, indicative of the two absorption bands of the S^*₁ state. The evolution of the third EADS takes place in 4 ps, and corresponds to the decrease of the excited state absorption at 600 nm. Moreover, a peak shift from 470 to 450 nm is observed, which can be assigned to the vibrational cooling of the S^*₁ state to the S₁ state. The fourth EADS appears with the decay of the S₁ state to the background level in 20 ps.

Fig. 4 (a) Evolution-associated difference spectra (EADS) resulting from a global analysis of the transient absorption experiments on TATB. Dynamics traces recorded at (b) 600 and (c) 450 nm.

Dynamics traces recorded at 600 nm (upper panel) and 450 nm (lower panel) are presented in Fig. 4b and c, respectively. Here, the lifetimes were fitted with a convolution of a Gaussian-type instrumental response function and an exponential decay rate. The obtained lifetimes for the S^*₁ and S₁ states are 0.64 and 6.0 ps, respectively, from the best fits of the two curves. The former value is in agreement with those observed for the S^*₁ → S₁ transitions of HNS¹⁸ (0.8 ps), carotenoids¹¹ (0.9 ps), and trans-4-aminoazobeneze²³ (0.7 ps). The S₁ state lifetime accords well with that observed for the S₁ → T₁ transition of HNS¹⁸ (6 ps) and the S₁ → S* transition of o-nitroaniline²⁹ (6.4 ps).

3.3. Structures, orbital information, and potential energy surfaces of excited states

Starting from TA-S₀, the low-lying excited states are accessed mainly by the vertical transitions HOMOs → LUMOs, as shown in Fig. 1. The low-lying vertical excitation energy to the S₁ surface was calculated as 3.67 eV, without any geometrical change.²² After its vertical excitation, the electron density is transferred from the C-ring and NH₂ groups to a NO₂ group, as shown in Fig. 2. There is a major contribution from LUMO-1 to the dipole moment, in addition to some contribution from LUMO-2. The subsequent transitions of LUMO-1 proceed via vertical excitations to higher unoccupied molecular orbitals such as LUMO-4 and LUMO-6 (Fig. 5). From the electron distribution spectra, it can be deduced that both of these transitions are highly possible. The electron can be re-excited to higher π* and σ* orbitals, which is indicated by the two absorption peaks at 470 and 600 nm in the transient absorption spectra in Fig. 4.

Fig. 5 (Upper) LUMO-4 and LUMO-6 of TA-S₀, together with (Lower) the geometrical structure, LUMO-1, LUMO-4, and LUMO-6 of TA-S₁.

The initial excitation energy, after making the NO₂ groups active, is redistributed via the nuclear motion of the active nitro group (i.e. relaxation). Relative to TA-S₀, the C(5)–N(10) bond and two N(10)–O(11,12) bonds of TA-S₁ are lengthened by about 0.04 Å, respectively. Also, the energy of TA-S₁ in the S₁ state is 3.42 eV, a reduction of 0.25 eV. The apparent change is suggested to be due primarily the vibrational cooling of the vs. (NO₂) mode. The electron density is mainly distributed in LUMO-1 of the active nitro group in TA-S₁, which is much different from that of TA-S₀; this further indicates an electron has been transferred into the active nitro group in the relaxation process. Thus, it is still feasible that the electron can be re-excited to LUMO-6 from LUMO-1, based on the electron distribution in Fig. 5. However, the re-excitation of LUMO-1 to LUMO-4 is infeasible because of the electron transfer in the relaxation process. Referring to the EADS in Fig. 4, the feasible re-excitation corresponds to the blue shift of the peak at 470 to 450 nm, while the infeasible re-excitation corresponds to the disappearance of the absorption peak at 600 nm.

The bond of the active NO₂ group, which exhibits π* character, becomes further twisted from the planar benzene ring. After optimization, TA-S′₁ in the S₁ state exhibits a dihedral C(4)C(5)N(10)O(12) twist of 69.7° and a dihedral C(6)C(5)N(10)O(11) twist of 104°. The electron density is mainly distributed in the NO₂ group with the deviated structure in Fig. 6, as LUMO-1 of TA-S′₁. The energy of the LUMO is reduced to −2.36 eV, and the energy of TA-S′₁ in the S₁ state is greatly reduced to 2.25 eV. In the meantime, the energy of TA-S′₁ in the T₁ state is calculated as 2.43 eV, slightly higher than that in the S₁ state. This means that the TA-S′₁ structure is close to the point of the S₁–T₁ conical intersection, which is suggested to enhance the efficiency of intersystem crossing and greatly reduce the lifetime of the transition.³⁰ Thus, the T₁ state can be easily populated via a minimal intersystem crossing from the S₁ state. The TA-T₁ structure was also optimized, and displayed the electron distribution in the deviated NO₂ group, in good agreement with the foregoing discussion. The dihedral C(4)C(5)N(10)O(12) twists to 46.9° and the dihedral C(6)C(5)N(10)O(11) twists to 84.2°, such that O(11) is nearly perpendicular to the benzene ring. The energy of TA-T₁ in the T₁ state is 2.30 eV, much lower than that on the S₁ surface.

Fig. 6 (Upper) Geometrical structure and LUMO-1 of TA-S′₁, in which the dihedral C(4)C(5)N(10)O(12) twists to 69.7° and the dihedral C(6)C(5)N(10)O(11) twists to 104°. (Lower) Geometrical structure and HOMO-1 of TA-T₁, in which the dihedral C(4)C(5)N(10)O(12) twists to 46.9°, and the dihedral C(6)C(5)N(10)O(11) twists to 84.2°.

To summarize, the NO₂ groups in TATB are activated after vertical excitation to the S₁ state, which show their π* bond character. Then, they are relaxed via the vibrational cooling of one active nitro group, leading to TA-S₁ in the S₁ state. The nitro group is further twisted from the planar benzene ring so as to approach the S₁–T₁ conical intersection. The T₁ state is then populated via minimal intersystem crossing. Overall, the electron distribution spectra provide a new insight into the de-excitation process.

3.4. Q_nitro transfer process

One nitro group of TATB becomes the active site after vertical excitation. On basis of the foregoing discussion, the Q_nitro transfer process is summarized in Table 1.

Table 1 Relative energy (eV) and Q_nitro (−e) values in different structures of TATB

Structure	State	Energy	Qnitro
			Site 1	Site 2	Site 3	Average
TA-S0	S0	0.00	0.397	0.397	0.397	0.397
	S1	3.67	0.459	0.459	0.459	0.459
TA-S1	S0	0.40	0.389	0.392	0.392	0.391
	S1	3.42	0.727	0.339	0.339	0.468
TA-S′1	S1	2.25	0.887	0.363	0.364	0.538
	T1	2.43	0.645	0.375	0.372	0.464
TA-T1	T1	2.30	0.598	0.380	0.381	0.453

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The initial Q_nitro values at the three sites are 0.397, 0.397, and 0.397 (−e), respectively. After excitation, they increase to 0.459, 0.459, and 0.459 (−e), respectively, indicating charge transfer from the benzene ring to the three nitro groups. In the relaxation process of the initial excited state, Q_nitro is transferred from the other two sites to Site 1. The overall Q_nitro does not change significantly in the redistribution process, but Q_nitro at Site 1 increases to as high as 0.727 (−e). It is noted that the Q_nitro distribution at the three sites of TA-S₁ in the S₁ state is different from that in the S₀ state, mainly at Site 1. This result comports with the deviation of the NO₂ group taking place on the S₁ rather than the S₀ surface. Subsequently, the Q_nitro transfer process involves mainly the decrease of Q_nitro at Site 1 via deviation of the NO₂ group. The Q_nitro at Site 1 decreases to 0.645 (−e) in the T₁ state of TA-S′₁, rather than 0.887 (−e) in the S₁ state. Furthermore, the Q_nitro at Site 1 decreases to 0.598 (−e) in succession without significant change at the other two sites.

Overall, the ultrafast Q_nitro transfer process involves (a) an initial increase after excitation, then (b) redistribution from the first two sites to Site 1, and (c) a decrease at Site 1 via the deviation of the NO₂ group. The Q_nitro transfer process is also in accord with the NO₂ group deviation on the S₁ surface rather than on the S₀ surface. This study of the dynamic response of EMs upon excitation shows potential application for the exploration of EMs under other extreme conditions.

4. Discussion

4.1. Assignment of the S₁ → T₁ transition and de-excitation process

The de-excitation of a relaxed S₁ state usually proceeds via fluorescence emission, non-radiative transition to the ground state, or intersystem crossing to the T₁ state. In this work, the fourth EADS in the transient absorption spectra decayed to the background level without providing any additional information about the de-excitation process. Therefore, we must analyse the transition according to the following information: first, TATB is a photosensitive EM which undergoes colour changes from yellow to green or darker under light irradiation/photolysis, which has been attributed to the formation of a long-lived radical in the T₁ state;²² and second, intersystem crossing to the T₁ state is likely to happen in this case. By way of explanation, one may consider that the conical intersection is probably close to TA-S′₁, as discussed in the previous section. The Kohn–Sham orbitals, electron distribution, and orbital energies are in good agreement with this interpretation. Alternatively, a previous study using nanosecond transient absorption spectroscopy under N₂ and O₂ conditions showed that the T₁ state absorption occurs at wavelengths of 340 and 410 nm, respectively.³¹ Unfortunately, those peaks are beyond the present experimental range. Ultimately, the lifetime of the singlet excited state of the nitro-substituted species is greatly reduced and the quantum yield of the triplet state is relatively high.^32,33 TATB exhibits behaviour similar to HNS, which undergoes a major S₁ → T₁ intersystem crossing. Some reports show that there is an intermediate state in the intersystem crossing, when the participating singlet and triplet states are composed of essentially the same orbitals in terms of the El-Sayed rule.²⁹ On the basis of the foregoing arguments, it would be reasonable to assign the decay of the relaxed S₁ state to the S₁ → T₁ transition in this work.

4.2. The importance of the nitro group in nitro explosives

The nitro group has been shown herein to be an important factor in the excited-state dynamics and electron transfer in TATB, and its influence can probably be extrapolated as a general rule for nitro explosives.³⁴ The lone pair electrons in the nitro group make an essential contribution to the HOMOs, and the anti-bonding π orbital of the nitro group contributes the most to the LUMOs. When the molecule suffers an external stimulus, an electron is excited and transferred into a low-lying MO of the nitro group via the excitation energy. The nitro group is activated by the π* bonding character, which can be readily observed in the electron distribution spectra. The active nitro group is then perturbed in some typical way, such as through vibrational cooling followed by deviation from coplanarity with the benzene ring. The observed results are in good agreement with the major NO product obtained from the dissociation of EMs in the excited state.¹⁵ The overall process typically occurs on the picosecond or even femtosecond time scale. The extra energy is converted into thermal energy via vibrational and translational motion, which is further transferred into the surroundings via intermolecular interactions.

The nitro group plays an important role in the initial reactions of nitro explosives. The excitation and relaxation processes that take place with other explosives such as RDX, HMX, CL20, and PETN have also been investigated.³⁴ These exhibit similar behaviours, and their impact sensitivity was found to be related to the excitation energy of the relaxed structure. Consequently, other characteristics also influence the impact sensitivity of these EMs, including their crystal, surface, and interfacial structures. These characteristics should clearly be considered in future work.

4.3. Implications for the photo-behaviour of nitroderivatives

The anti-bonding π orbital of the nitro group is large, conjugated with the aromatic ring, and is relatively low in energy. Under light irradiation, electronic excitation couples the excitation energy into the low-lying LUMOs, such as those with π* bonding character in a nitro group. The nitro group is activated by the excitation energy and then relaxes by a typical pathway, such as vibrational cooling or structural deviation. This process is indicated by the greatly reduced lifetime of the singlet excited state of nitro-substituted species and the relatively high quantum yield of the triplet state.^24,32,33,35 Such processes can be observed via the ultrafast pump-probe technique.

Importantly, there are several differences between nitro derivatives and nitro explosives. Nitro explosives usually have a symmetrical structure with many nitro groups. The influence of the nitro group son the photo-physical behaviour of the materials is distinct and clear. Nitro derivatives have fewer nitro groups than nitro explosives. In the low-lying unoccupied orbitals, the nitro group can be conjugated with other groups, such as aromatic rings and NH₂. Such nitro groups exhibit distinct photo-physical behaviours depending on their geometrical arrangement and electron distribution.^24,35,36

Thus, the introduction of a nitro group into a molecule greatly changes the photo-physical behaviour of the derivative, and studies of the excited-state dynamics and electron transfer processes can effectively serve to unravel the ultrafast and inherently complex changes that occur.

5. Conclusions

The combination of femtosecond transient absorption spectroscopy and the TD-DFT method in this work offers a distinctive approach to studying ultrafast excited-state dynamics and electron transfer processes, especially the changes in the π* bonding character of the nitro group in nitro explosives.

The initial S^*₁ state was observed at an excitation of 400 nm, with two peaks centred at 470 and 600 nm. This state evolved into the S₁ state, with a peak at 450 nm from EADS. The lifetimes of the two states were found to be 0.64 and 6 ps, respectively. The electron density was transferred from the C-ring and NH₂ groups to the NO₂ groups after vertical excitation. The excitation energy activated the nitro groups and was then redistributed via their nuclear motion. The structural relaxation of the initial S^*₁ state revealed that the electron density was mainly distributed in the activated nitro group. Then, the active nitro group was further deviated from coplanarity with the benzene ring and relaxed to the S₁–T₁ conical intersection point. The T₁ state was suggested to be populated via minimal intersystem crossing through the S₁–T₁ intersection. The electron distribution spectra provided a new insight into the de-excitation process. This study advances our understanding of photon-initiated reactions and EM ignition processes.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 11504349).

References

1. N. Tamai and H. Miyasaka, Chem. Rev., 2000, 100, 1875.
2. J. Ma, J. Zhao, P. Yang, D. Huang, C. Zhang and Q. Li, Chem. Commun., 2012, 48, 9720.
3. D. Achey, E. C. Brigham, B. N. DiMarco and G. J. Meyer, Chem. Commun., 2014, 50, 13304.
4. G. A. Parada, T. F. Markle, S. D. Glover, L. Hammarstrom, S. Ott and B. Zietz, Chemistry, 2015, 21, 6362.
5. M. H. Kim, L. Shen, H. Tao, T. J. Martinez and A. G. Suits, Science, 2007, 315, 1561.
6. N. E. Levinger and L. A. Swafford, Annu. Rev. Phys. Chem., 2009, 60, 385.
7. M. J. Jordan and S. H. Kable, Science, 2012, 335, 1054.
8. R. A. Mathies, Nat. Chem., 2015, 7, 945.
9. I. V. Rubtsov, Nat. Chem., 2015, 7, 683.
10. C. L. Evans and X. S. Xie, Annu. Rev. Anal. Chem., 2008, 1, 883.
11. R. Berera, R. van Grondelle and J. T. Kennis, Photosynth. Res., 2009, 101, 105.
12. C. Adamo and D. Jacquemin, Chem. Soc. Rev., 2013, 42, 845.
13. J. E. Field, Acc. Chem. Res., 1992, 25, 489.
14. N. C. Dang, C. A. Bolme, D. S. Moore and S. D. McGrane, J. Phys. Chem. A, 2012, 116, 10301.
15. A. Bhattacharya, Y. Guo and E. R. Bernstein, Acc. Chem. Res., 2010, 43, 1476.
16. C. Zhang, J. Hazard. Mater., 2009, 161, 21–28.
17. C. Zhang, Y. Ma and D. Jiang, J. Mol. Model., 2012, 18, 4831.
18. G. Chu, M. Shui, Y. Xiong, J. Yi, K. Cheng, T. Xu, J. Xin and Y. Gu, RSC Adv., 2014, 4, 60382.
19. C. J. Wu, L. H. Yang, L. E. Fried, J. Quenneville and T. J. Martinez, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 67, 235101.
20. O. U. Ojeda and T. Cagin, J. Phys. Chem. B, 2011, 115, 12085.
21. C. Zhang, B. Kang, X. Cao and B. Xiang, J. Mol. Model., 2012, 18, 2247.
22. Y. Xiong, F. Zhong, T. Xu and K. Cheng, J. Phys. Chem. A, 2014, 18, 6858.
23. Y. P. Wang, S. Zhang, S. M. Sun, K. Liu and B. Zhang, Chin. J. Chem. Phys., 2013, 26, 651.
24. R. A. Vogt, C. Reichardt and C. E. Crespo-Hernandez, J. Phys. Chem. A, 2013, 117, 6580.
25. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Revision B.01, 2010.
26. T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580.
27. S. Kakar, A. J. Nelson, R. Treusch, C. Heske, T. van Buuren, I. Jiménez, P. Pagoria and L. J. Terminello, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 15666.
28. T. Y.-J. Han, P. F. Pagoria, A. E. Gash, A. Maiti, C. A. Orme, A. R. Mitchell and L. E. Fried, New J. Chem., 2009, 33, 50.
29. J. Yi, Y. Xiong, K. Cheng, M. Li, G. Chu, X. Pu and T. Xu, Sci. Rep., 2016, 6, 19364.
30. D. Polli, P. Altoe, O. Weingart, K. M. Spillane, C. Manzoni, D. Brida, G. Tomasello, G. Orlandi, P. Kukura, R. A. Mathies, M. Garavelli and G. Cerullo, Nature, 2010, 467, 440.
31. S. Q. K. Cheng and J. Chen, Chin. J. Energ. Mater., 2002, 10, 13.
32. R. Hurley and A. C. Testa, J. Am. Chem. Soc., 1966, 88, 4330.
33. R. Hurley and A. C. Testa, J. Am. Chem. Soc., 1968, 90, 1949.
34. G. Chu, Z. Yang, J. Xin, T. Xi, Y. Zhao, W. He, M. Shui, Y. Gu, Y. Xiong, K. Cheng and T. Xu, Submitted.
35. R. A. Vogt and C. E. Crespo-Hernandez, J. Phys. Chem. A, 2013, 117, 14100.
36. Y. Sun, Y. Shu, T. Xu, M. Shui, Z. Zhao, Y. Gu and X. Wang, Cent. Eur. J. Energ. Mater., 2012, 9, 411.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11584e

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