Investigation of ultrafast excited state dynamics of 2,2′,4,4′,6,6′-hexanitrostilbene using femtosecond transient absorption spectroscopy

Abstract

A study on the dynamics and structures of the excited states of 2,2′,4,4′,6,6′-hexanitrostilbene shows equilibrium between vibrationally hot S₁ (S^*₁) and S₁ states with lifetimes of 0.8 and 6 ps, respectively. The T₁ state has a lifetime of about 4 ns. It advances understanding of energetic materials ignition.

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Excited states play a crucial role in the energy conversion of energetic materials,^1,2 and these excited states could be initiated by a number of different ignition processes such as photons, sparks, or shocks.^3,4 Moreover, fast non-radiative transitions of excited states can lead to local heating effect called “hot spots” in liquids or crystals, which can accelerate chain reactions of energetic materials.² Excited state decomposition can convert chemical energy into mechanical energy in the sub-picosecond (ps) timescale, which has been extensively investigated by ultrafast pump-probe techniques.^1,5–9 Representing the crossing of multidimensional electronic potential energy surfaces, conical intersections (CIs) are well established as controlling factors in the excited state decomposition of energetic material molecules.¹ The response to ultrafast laser shock loading of liquids has been monitored to reveal the existence of excited states occurring in the first 350 ps following the shock front.¹⁰ To the best of our knowledge, however, the dynamical and spectral behaviours of the excited state of energetic materials have been rarely observed.

Stilbene is a prototypical molecule showing isomerization in the lowest excited state (S₁)¹¹ and its rotational mechanism has been explained using CIs at the perpendicular configuration, which can be accessed from either the trans- or cis- side on the S₁ potential surface.¹¹ Isomerization dynamics have also been observed for azobenzene and its derivatives in the nπ* (S₁) and ππ* (S₂) states.^12,13 Such processes may be quenched by strong intramolecular steric hindrance by increasing multiple nitro functional groups in the molecule. As a derivative of nitrostilbene, 2,2′,4,4′,6,6′-hexanitro-stilbene (HNS, Fig. S1†) is a well-known explosive for good performance with excellent thermal stability, impact and shock insensitivity.

Fig. 1 Steady-state absorption spectrum of HNS in CH₃CN. The inset dynamical scheme describes the HNS excited state processes upon excitation.

To continue the effort on understanding the energy transfer process of energetic materials, we present an experimental investigation on the excited states of HNS using femtosecond (fs) transient absorption spectroscopy. We establish a cascading de-excitation S₂ → S^*₁ → S₁ → T₁ model to interpret the dynamical and structural behaviours of the HNS excited states. The results show equilibrium between S^*₁ and S₁ states, which have lifetimes of 0.8 and 6 ps, respectively. The T₁ state has a long lifetime of about 4 ns. The results of this study advance the understanding of energetic materials ignition.

In the steady-state absorption spectrum of HNS in CH₃CN in Fig. 1, there is a wide band centred at 266 nm, which extends to the 400 nm range.¹⁴ After identifying the effects of the solvent, the wide band can be assigned to π → π* (S₂ state) and n → π* (S₁ state) transitions. The vertical transition energies of the S₂ and S₁ states are calculated to be 4.75 eV (∼261 nm) and 3.55 eV (∼358 nm), respectively, at the level of B3LYP/6-311++G(d,p), which are consistent with experimental results. The oscillator strengths of π → π* and n → π* transitions are computed to be 0.17 and 0.20, respectively. These two states are the Franck–Condon states accessible from the ground electronic state configuration. According to the steady-state absorption spectrum and theoretical calculations, HNS molecules primarily populate lower vibrational manifolds of the S₂ surface by ultraviolet excitation at 266 nm.

The distinct excited state absorption of HNS is exhibited from femtosecond-resolved absorption spectra in Fig. 2. The peaks of the S₁ fluorescence and T₁ phosphorescence of HNS are located at 421 and 412 nm in Fig. S2,† respectively. There have been no reports on S₂ fluorescence and higher-energy T_n phosphorescence of HNS, and the quantum yields of such radiative transitions may be beyond the detection limit. Thus, ground state absorption and stimulated emission are not evident in the transient absorption spectra of HNS.

Fig. 2 Two-dimensional transient absorption spectra of HNS in CH₃CN from 380–680 nm with a time delay up to 450 ps following excitation at 266 nm.

A particularly long-lived state is observed to have a timescale on the order of hundreds of picoseconds, which could be assigned to the T₁ state. The T₁ state has been recognized as a prominent feature in the energy conversion of energetic materials.¹⁴ The lifetime of the T₁ state may be greatly reduced by the strong steric hindrance of molecular planarity in HNS molecule. Moreover, the T₁ state of HNS has been observed via nanosecond-resolved transient absorption spectroscopy, where HNS primarily populates the S₁ state at an excitation of 355 nm before undergoing intersystem crossing to the T₁ state for investigation. The T₁ state is further decomposed into radicals. The observable decay rate difference between N₂ and O₂ ambience in Fig. S3† is due to radical absorption rather than T₁ state as ref. 14. The two peaks at 340 and 520 nm are assigned to radical absorption, with born-lifetimes of 10–30 ns. These results indicate that the lifetime of the T₁ state of HNS is shorter than 10 ns. When the HNS solution does not sufficiently flow, a nearly non-decay transient product with lifetime more than 1 ms is observed by the femtosecond absorption method before the overlap of pump-probe pulses (Fig. S4†). This point is consistent with radical absorption of HNS from the T₁ state using the nanosecond-resolved absorption method.

Fig. 3 Spectra and dynamics of the three species obtained using SVD.

Fig. 4 Spectra and dynamics of the three HNS excited states.

From the Jablonski energy level model, the T₁ state is primarily populated from the minimal intersystem crossing of the lowest excited single state (S₁ state). This process usually relaxes in the timescale of several picoseconds. From this point of view, the S₁ state and its exponential decay to T₁ state are expected to be observed in the transient absorption spectra. Furthermore, an ultrafast internal conversion from the initial S₂ state to S₁ state can be also expected in the time scale of 0.1–1 ps.

To analyse the spectra, the singular value decomposition (SVD) method is presented to illustrate the spectra and dynamics of different species. The first three species have reasonable signal-to-noise ratios and are utilized for further investigation, as shown in Fig. 3. The first spectrum is very broad with one band at 400 nm. The dynamical behaviour shows that it increases exponentially after excitation, and then decays slightly in the range of 20–450 ps. Furthermore, this behaviour indicates that there are primarily two processes for this species. Two bands at 480 and 650 nm are observed in the second spectrum, and its dynamics show a major exponential decay. The third spectrum has peaks centred at 400 and 625 nm, respectively. Moreover, its dynamics show a fast decay after excitation and an exponential increase later. Based on the above discussion, we propose a typical three cascading de-excitation model, where S₂ → S₁ → T₁ fits the SVD results and a global fitting is performed, as shown in Fig. 3. The obtained lifetimes for S₂ and S₁ are 0.8 and 6 ps, respectively. The 0.8 ps lifetime is explicitly assigned to the decay of the S^*₁ state to S₁ state involved in the S₂ → S₁ transition, as discussed below. The T₁ state is established to have a long lifetime of 1000 ps, exceeding the experimental range of 450 ps. Associated with nanosecond-resolved absorption spectra (Fig. 3) and sub-picosecond-resolved spectroscopy by Rajchenbach et al.,² we determine that its lifetime is about 4 ns here. The dynamics for the three excited states are exhibited in Fig. 4. The corresponding spectra are present on basis of the de-excitation model and the obtained decay rate (1/lifetime). There are two peaks at 400 and 625 nm in S^*₁ spectrum, and one at 450 nm in S₁ spectrum. A band is centred at 400 nm in the T₁ spectrum.

We now examine the de-excitation process of the S₂ → S₁ transition of HNS. The timescale for this excitation is usually between 0.1–0.2 ps for some kinds of aromatic hydrocarbons in gas phase, such as benzophenone.¹⁵ The lifetime of 0.8 ps for this transition of HNS in CH₃CN is much larger than that in the gas phase.¹⁵ There should be an observation of one particular state assigned to a vibrationally hot S₁ state (so-called S^*₁ state) in the de-excitation S₂ → S^*₁ → S₁ processes. The evolution from the initial S₂ state leads to the S^*₁ state, which indicates that the electronic excitation energy is converted into thermal energy through vibrational and translational energy motion. Such states have been commonly observed in various kinds of photochromic reactions.¹² The de-excitation of S₂ → S^*₁ is fast on the order of ∼40 fs for carotenoid¹⁶ and 70 fs for trans-4-aminoazobenzene.¹⁷ The rapid process is probably due to the S₂–S₁ conical intersection zone, which is close to the steep slope in Franck–Condon region on the S₂ surface.^15,18 However, the ultrafast lifetime of the S₂ → S^*₁ transition for HNS is not observed in this work. For some energetic materials such as nitromethane, there may be also S^*₁ state involvement in the lifetime. The excitation of nitromethane at 271 nm could be assigned to the S₂ state according to the results by Arenas et al.¹⁹ The lifetime of the nπ* state at the excitation of 271 nm was determined to be 36 fs through femtosecond-resolved mass spectrometry by Guo et al.²⁰ The 36 fs lifetime could be assigned to the S₂ → S^*₁ transition rather than to ground state transition through internal conversion.

The relaxation of the S^*₁ state undergoes through an intermolecular energy transfer to the surrounding solvent. The cooling process of the initial S^*₁ state of chrysene in solution has been suggested to lead to the relaxed S₁ state.²¹ Moreover, the lifetimes of the S^*₁ → S₁ transition for carotenoid¹⁶ and trans-4-aminoazobenzene¹⁷ are determined to be 0.9 and 0.7 ps, respectively. The relaxation process of such transition is established to be 0.8 and 1.0 ps for HNS in CH₃CN and 1,4-dioxane (Fig. S5†) in this work, respectively, in agreement with the above results. The increase of lifetime in solution can be ascribed to the viscosity of solvent, which slows the spread of the wavepacket on the S₁ surface from the S₂ surface.¹⁵ In addition, the vibrational cooling dynamics of azobenzene in the vibrationally hot ground electronic state has been determined to be on a time scale of approximately 20 ps via time-resolved IR spectroscopy with femtosecond time resolution.^13,22,23 In the gas phase, the photoexcitation of 1,3,5-trinitro-1,3,5-triazacyclo-hexane (RDX) primarily populates the upper vibrational manifolds of the S₁ surface (S^*₁ state) at the photon energy of 4.8 eV (258 nm).^1,5–9,20 The NO product from S^*₁ state via CIs is observed to be vibrationally hot (1600 K) and rotationally cold (20 K), which is similar behaviour to NO dissociated from S₂ surface (5.5 eV at 226 nm). The overall process should be faster than 180 fs.¹

Our results reveal new classification of excited states of energetic materials introduced by Rajchenbach et al.² First, we observe equilibrium among the S^*₁ state, the rapidly populated S₁ state, and the long-lived T₁ state. We then obtain the absorption spectra for the three states and demonstrate that the S₁ spectrum is similar to that given in ref. 2. Second, the 6 ps lifetime for the non-radiative S₁ → T₁ process of HNS is consistent with the related 4.7 ps lifetime for S₁ → X transition of CH₃NO₂ obtained by the coherent anti-Stokes Raman scattering (CARS) technique. Finally, we determine that more excited states should be involved in the fast non-radiative transition, other than S₁ → T₁ transition. Despite the fast local heating effect and the acceleration of decomposition in liquid or crystal due to the excited states, vibrational cooling dynamics²⁴ and other effects may be important factors for further understanding the ignition of energetic materials.

Conclusions

The dynamics and structures of the HNS excited states have been investigated using femtosecond transient absorption spectroscopy. Via SVD and the global fitting method, three species are analysed, and a cascading de-excitation model (S₂ → S^*₁ → S₁ → T₁) is established. An equilibrium between the S^*₁ state, rapidly populated S₁ state and the long-lived T₁ state is acquired, and the spectrum for each state is exhibited. The lifetimes of S^*₁ and S₁ are about 0.8 and 6 ps, respectively. The T₁ state has a lifetime of about 4 ns. This study reveals new recognition of excited states and understanding of energetic materials.

Acknowledgements

Thanks for the technical support by Professor Bing Zhang, Associate professor Song Zhang and Ph.D. Yaping Wang in Wuhan Institute of Physics and Mathematics, China. This work is supported by the Science Foundation of CAEP' Fund (no. 2011A0302013).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08305a

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