Tunable piezochromic luminescence via isomer control in N,N′-azotriazole energetic molecules

Abstract

Herein, distinct pressure-dependent emission shifts in N,N′-azotriazole energetic materials are realized through isomer control. 4,4′-azo-1,2,4-triazole exhibits blue-shifted luminescence under compression, and by contrast, 1,1′-azo-1,2,3-triazole exhibits a red-shift. This behavior stems from the different molecular arrangements and dominant interactions present in the isomers, which result in differences in the ways the bandgaps change at high pressure.

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Piezochromic materials, in which the luminescence can be modulated by external pressure, are attracting considerable attention owing to their promise in photonics, pressure sensing, and data storage.^1–4 Elucidating the fundamental principles by which piezochromic materials function and devising strategies for the rational design of luminophores with tunable piezochromic behavior remain central challenges in materials science.⁵ In the field of organic light-emitting materials, both the luminescence color and intensity are highly sensitive to molecular packing and intermolecular interactions.^6–8 Pressure serves as a direct and effective means to modulate intermolecular forces and molecular aggregation,^9,10 as does restricting intramolecular rotation (RIR) and intramolecular vibration (RIV).^11–14 With increasing pressure, organic luminescent materials typically display a gradual red-shift of their emissions accompanied by luminescence quenching. In contrast, pressure-induced blue-shifts and fluorescence enhancement are rarely observed in compressed luminescent materials.¹⁵ The prevalence of the red-shift can be explained by different mechanisms;^16–18 however, the mechanism underlying this unusual blue-shifted luminescence remains elusive.^19,20

Double-heterocyclic azo compounds, which are widely used in photochromic materials, molecular photoswitches and other related fields, are electron-rich and have extended π-conjugated heterocycles.^21–26 The nitrogen atoms in the –N–N=N–N– linkage form tetrazene bonds, which not only enable the formation of nitrogen chains with four or more directly connected nitrogen atoms but also enhance the conjugation.^27,28 These motifs are of particular relevance for the development of high-energy materials and photochromicity.²⁹ Typically, these –N–N=N–N– bridged azotriazole compounds are represented by two isomers, 1,1′-azo-1,2,3-triazole and 4,4′-azo-1,2,4-triazole. The novelty of these nitrogen-rich structures and their different molecular arrangements contribute to the growing interest in their research.^30–32

In this study, the isomers 4,4′-azo-1,2,4-triazole and 1,1′-azo-1,2,3-triazole, which possess distinct structural arrangements, are shown to display different fluorescence responses under pressure. For 1,1′-azo-1,2,3-triazole, subjecting it to greater pressure induces a gradual photoluminescence (PL) red-shift. In contrast, 4,4′-azo-1,2,4-triazole exhibits an unusual blueshift luminescence with enhanced emission intensity when exposed to pressures below 5.2 GPa. These contrasting behaviors could be ascribed to the differences in intermolecular interactions and bandgap evolution under compression. Beyond elucidating the pressure-dependent luminescence of these two isomers, this work offers new insights into the design of nitrogen-rich luminophores and the broader optical properties of energetic materials.

Under ambient conditions (1 atm), the 4,4′-azo-1,2,4-triazole and 1,1′-azo-1,2,3-triazole isomers exhibit different packing motifs. 4,4′-Azo-1,2,4-triazole adopts a planar conformation and belongs to the P2₁/n space group, characteristic of a monoclinic crystal system with each unit cell containing four molecules, whilst it displays a partial face-to-face and partial perpendicular packing arrangement (Fig. 1a).³⁰ Conversely, 1,1′-azo-1,2,3-triazole adopts a planar conformation and belongs to the P2₁/n space group, characteristic of a monoclinic crystal system with each unit cell containing two molecules, and crystallizes in a face-to-face parallel packing arrangement only (Fig. 1d).³²

Fig. 1 (a) Molecular packing of 4,4′-azo-1,2,4-triazole at 1 atm. (b) Pressure-dependent PL of 4,4′-azo-1,2,4-triazole from 1 atm to 5.2 GPa. (c) Pressure-dependent PL maximum of 4,4′-azo-1,2,4-triazole. (d) Molecular packing of 1,1′-azo-1,2,3-triazole at 1 atm. (e) Pressure-dependent PL of 1,1′-azo-1,2,3-triazole. (f) High-pressure PL maximum of 1,1′-azo-1,2,3-triazole. (g) Corresponding PL photographs of 4,4′-azo-1,2,4-triazole at different pressures. (h) Corresponding PL photographs of 1,1′-azo-1,2,3-triazole at different pressures.

The high-pressure PL behavior of 4,4′-azo-1,2,4-triazole is shown in Fig. 1b. When the pressure is increased from 0 to 5.2 GPa, the PL emission displays a gradual blue shift from 534 to 491 nm (Fig. 1c), accompanied by an obvious pressure-induced emission enhancement (PIEE) from 94.5 to a maximum intensity of 277. After exceeding 6.2 GPa (Fig. S1), the PL emission shows a common red-shift, and the PL gradually quenches until it almost disappears. The corresponding pressure-dependent changes in the fluorescence color and intensity are evident in Fig. 1g, where the luminescence shifts from green to blue and gradually diminishes beyond 5.2 GPa. In contrast, 1,1′-azo-1,2,3-triazole exhibits a continuous red-shift in its PL as the pressure increases (Fig. 1f), accompanied by a gradual intensity loss (Fig. 1e and Fig. S2). The fluorescence remains faint blue throughout compression and slowly disappears above 14.0 GPa (Fig. 1h). Meanwhile, as the pressure increases, a new emission peak near 750 nm for 1,1′-azo-1,2,3-triazole is generated. Both 1,1′-azo-1,2,3-triazole (Fig. S3) and 4,4′-azo-1,2,4-triazole (Fig. S4) return to their original states after the pressure is released.

To explore the pressure-dependent bandgap evolution of the 1,1′-azo-1,2,3-triazole and 4,4′-azo-1,2,4-triazole isomers, UV-Vis absorption experiments were carried out. For 4,4′-azo-1,2,4-triazole, the absorption edge exhibits a progressive blueshift up to 5.2 GPa, corresponding to a bandgap widening from 3.46 to 3.63 eV (Fig. 2a and b). This broadening accounts for the anomalous blueshift observed in the PL emission.³³ Above 6.2 GPa (Fig. 2b), the absorption edge undergoes a continuous red-shift accompanied by a pronounced smearing effect, indicative of extensive lattice compression and possible structural changes. In contrast, the absorption edge of 1,1′-azo-1,2,3-triazole shows a red-shift and the corresponding bandgap gradually decreases (Fig. 2c) whilst the absorption edge gradually broadens. These findings can be attributed to reduced intermolecular separations and progressively enhanced intermolecular interactions.^34,35

Fig. 2 High-pressure UV-Vis spectra (a) and the corresponding bandgap values (b) of 4,4′-azo-1,2,4-triazole. Pressure-dependent UV-Vis spectra (c) and bandgap evolution (d) of 1,1′-azo-1,2,3-triazole.

Based on the pressure-dependent optical responses, Fourier-transform infrared spectroscopy (FT-IR) was employed to correlate the PL shifts with the local structural variations, as shown in Fig. 3a and c. The gradual blue shift of the IR vibrational modes with increasing pressure indicates the occurrence of pressure-triggered bond contraction resulting in shortened interatomic distances.³⁶ The continuous spectral evolution further indicates a steadily contracting molecular framework. In the IR spectra of 4,4′-azo-1,2,4-triazole, the C–H band at 3130 cm⁻¹ displays a smaller shift between 1.5 and 4.0 GPa, but undergoes a markedly accelerated shift at higher pressures. Above 16.0 GPa, peak splitting emerges, suggesting the occurrence of pressure-induced molecular distortion.³⁷ In the Raman spectra of 4,4′-azo-1,2,4-triazole (Fig. 3b), the relative peak intensity of the carbon ring stretching vibrations at 1529 cm⁻¹ and 1468 cm⁻¹ becomes stronger as the pressure increases, indicating that the polarizability of the C–N bond increases.³⁸ This observation should be related to the enhancement of hydrogen bonding.³³ In contrast, only the peak widths broaden in the Raman spectra of 1,1′-azo-1,2,3-triazole (Fig. 4d). Additional high-pressure Raman spectra confirm that both compounds undergo continuous structural contraction without distinct phase transitions. Additionally, through PXRD (Fig. S5 and S6) and IR (Fig. S3 and S4) analyses after release of the pressure, it was found that the structure was not changed by the pressurization process.

Fig. 3 High-pressure IR spectra of 4,4′-azo-1,2,4-triazole (a) and 1,1′-azo-1,2,3-triazole (c) in the pressure range of 1.0 atm −20.0 GPa. Pressure-dependent Raman spectra evolution of 4,4′-azo-1,2,4-triazole (b) and 1,1′-azo-1,2,3-triazole (d).

Fig. 4 High-pressure Hirshfeld surfaces of 4,4′-azo-1,2,4-triazole (a) and 1,1′-azo-1,2,3-triazole (b) in the pressure range of 1.0 atm −25.0 GPa.

As is well established, the luminescence properties of organic molecules are closely related to their molecular arrangement, molecular conformation and intermolecular interactions.³⁴ To verify the evolution of the intermolecular interactions of 1,1′-azo-1,2,3-triazole and 4,4′-azo-1,2,4-triazole molecules under high pressure, Hirshfeld surface analyses were performed and are displayed in Fig. 4a and b. At 1 atm, the interactions occur on the triazole, whilst the weak interaction of the –N–N=N–N– bridged bond occurs only when the pressure rises to 9 GPa, as reflected by the light-colored surfaces and minimal contact forces. This suggests that the stability of the –N–N=N–N– bridged bond under high pressure may be related to these interactions. In addition, for 4,4′-azo-1,2,4-triazole, due to its partial face-to-face and partial perpendicular packing arrangement, the C–H and N interactions on the 1,2,4-triazole ring exhibit additional weak interactions (Fig. 4a). As the pressure increases, the red surface area gradually expands, indicating strengthened interactions. Above 6.0 GPa, the π–π interactions between adjacent molecules become evident and intensify upon further compression. In contrast, due to the exclusively face-to-face parallel arrangement of 1,1′-azo-1,2,3-triazole, the π–π interactions between adjacent molecules become evident at a lower pressure of 3.0 GPa (Fig. 4b) and intensify upon further compression.

To sum up, comparative studies of two azotriazole isomers reveal distinct pressure-dependent luminescence behaviors. Upon increasing the pressure to 5.2 GPa, 4,4′-azo-1,2,4-triazole exhibits a pronounced blueshift in its PL, accompanied by a remarkable PIEE phenomenon. However, 1,1′-azo-1,2,3-triazole displays a continuous red-shift and fluorescence quenching. This behavior arises from the perpendicular molecular arrangement and the dominance of 1,2,4-triazole ring side interactions, which widen the bandgap. Upon further compression, the promoted intermolecular π–π stacking reverses this trend, causing the band gap to continuously narrow. In contrast, due to the existence of only a face-to-face parallel arrangement of 1,1′-azo-1,2,3-triazole, the π–π interactions between adjacent molecules become evident at a lower pressure of 3.0 GPa, thus causing the band gap to continuously narrow. Meanwhile, for the 4,4′-azo-1,2,4-triazole below 5.2 GPa, the intermolecular interactions could remarkably inhibit or restrict the movement of the aromatic moieties, restraining the energy loss by intramolecular motion and thereby enhancing emission. For 1,1′-azo-1,2,3-triazole, the appearance of a peak at 750 nm might be due to the increase in pressure creating strong π–π interactions between the molecules, thus causing them to form a dimer¹⁹ and ultimately resulting in fluorescence quenching. These findings not only provide direct experimental support for the intermolecular interactions of PIEE but also establish a conceptual framework for the rational design of next-generation materials.

This work was supported by the National Natural Science Foundation of China (NO. 22135003). The authors would like to thank Professor Qian Li for her guidance and assistance in rewriting and data analysis. The authors would like to acknowledge the useful comments of the reviewers.

L. Y. C, C. Y. L. and L. Y. H. designed and performed experiments and analyzed data. G. M. Z. assisted in performing the experiments and wrote the manuscript. W. L. L. synthesized the raw materials and assisted in testing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cp00027d.

Notes and references

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Footnote

† Meng-Zhou Guan and Yi-Lin Cao contributed equally to this paper.

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