Mechanical stimulus-responsive luminescence: dynamic and durable mechanoluminescence based on PTZ derivatives

Abstract

Unlike previous structures, the compound PTZ-2BP, with benzoyl groups at positions 3 and 7 of PTZ, is reported and demonstrates dynamic and durable ML. Careful analysis of the experimental results indicates that the dynamic ML could be attributed to changes in molecular conformations and energy transfer under the mechanical stimulus.

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Mechanoluminescence (ML) or triboluminescence (TL), a special stimuli-responsive photon emission triggered by mechanical stimulus,¹ has experienced new developments and challenges in organic materials recently since its first report in 1605.² Unlike photoluminescence (PL) induced by UV irradiation, ML emission exhibits unique excitation through mechanical stimuli such as pressing, shaking, or even ultrasound. This distinctive feature underscores its significant potential in stress sensing, optical anti-counterfeiting, biomedicine, and related applications.³ In the exploration of organic ML materials, the relationship between emissive properties and mechanical forces is particularly important. There are various ML emissions, such as fluorescence,⁴ phosphorescence⁵ and delayed fluorescence (DF),⁶ which have been researched. However, there have been limited investigations from the perspective of the excited mechanical force due to the fragile organic crystal and instantaneous ML process.⁷ Dynamic ML dependent on stimulus time could exhibit changeable ML colour under a continuous mechanical force and could be advantageous in the investigation of ML excited mechanical force and bring new development in applications.

Among the several reported dynamic ML, molecules were generally constructed by introducing aryl substituents into the nitrogen atoms of phenothiazine (PTZ) to create the donor–acceptor (D–A) structures.⁸ Additionally, their conformational transitions could be easily realized by grinding the solid sample, thus leading to changeable ML emissions. However, no other novel structures with dynamic ML properties have been reported. Distinct from the previous structures, we designed and synthesized the compound PTZ-2BP, in which benzoyl groups were introduced at positions 3 and 7 in PTZ to construct twisted D–A structures and increase molecular polarity, thereby facilitating the generation of ML.⁹ Furthermore, the introduced alkyl substituent on the nitrogen atom of PTZ could enrich intermolecular interactions, restrict nonradiative transitions and enhance crystallinity, thus promoting the ML emission (Fig. 1a).¹⁰ Another molecule, PTZ-2FBP, with a fluorine substituent, was also synthesized for comparison, as the fluorine atom could enhance molecular polarity.

Fig. 1 (a) Molecular structures of PTZ derivatives and the calculated conformation of PTZ-2BP. (b) ML wavelength of PTZ-2BP under continuous mechanical stimulus within 30 s. (c) Normalized ML spectra of PTZ-2BP at different times. (d) Calculated CIE coordinates in the CIE 1931 colour space chromaticity diagram based on the ML spectra at different times: 0.5 s, 1.0 s, 3.0 s, 10.5 s, 20.5 s, and L–R.

Especially, PTZ-2BP demonstrates changeable ML properties under continuous mechanical stimulus, while PTZ-2FBP shows no ML. Careful analyses of experimental results, coupled with theoretical calculations, indicate that the twisted polar conformation and stable packing of molecules in the crystal should be the main reasons for ML activity. The dynamic ML effect is possibly induced by conformation change and energy transfer from the crystal to an amorphous form during the ML process. Additionally, the good mechanical properties of the PTZ-2BP crystal endow the sensitivity and partial sustainability of the ML process. Herein, we present the synthesis, photophysical properties, crystal structure, mechanical properties, and theoretical calculations of PTZ-2BP and PTZ-2FBP in detail to thoroughly understand the interesting ML process and the relationship between structure and properties.

PTZ-2BP and PTZ-2FBP were conveniently obtained through two-step syntheses (Scheme S1, ESI†).¹¹ The two PTZ derivatives were characterized by ¹H and ¹³C NMR, high resolution mass spectrometry (HRMS) and high-performance liquid chromatogram (HPLC) spectra to certify their chemical structures and purity (Fig. S16–S21, ESI†). The thermogravimetric analysis (TGA) indicated the good thermal stability of PTZ-2BP and PTZ-2FBP, with decomposition temperatures (T_d) of 349 °C and 398 °C, respectively (Fig. S18, ESI†). The UV-vis absorption and PL spectra of PTZ-2BP and PTZ-2FBP measured in a dilute tetrahydrofuran (THF) solution (Fig. S1, ESI†) showed nearly overlapped absorption spectra and PL spectra. The PL quantum yields and lifetimes in THF were also very similar (Fig. S2 and Table S1, ESI†), which indicated their similar molecular conformations and electronic properties at single molecular states in the solution.

The ML properties were investigated through grinding the recrystallized samples of two PTZ derivatives with a glass rod. Only PTZ-2BP exhibits bright green ML, while PTZ-2FBP has no ML. Also, the ML emission could be repeated through a simple rotary evaporation or recrystallization using dichloromethane. As expected, the ML emission could change from green to yellow under continuous mechanical stimulus (Fig. 1b and Fig. S3, ESI†), indicating a dynamic ML property. Within 30 s, the ML emission of PTZ-2BP could constantly red-shift from the initial 520 nm to 556 nm and finally stabilize at around 556 nm under continuous mechanical stimulus (Table S2, ESI†). As shown in Fig. 1c, five intense ML spectra at different times were selected to conveniently analyse the dynamic emission. From the calculations based on these selected ML spectra of PTZ-2BP, it could be found that their CIE coordinates were largely tuned from (0.33, 0.56) (green) to (0.41, 0.53) (yellow) (Fig. 1d). To obtain insights into the dynamic ML, PL properties under different mechanical grinding states were carefully studied. Coincidentally, the PL emissions exhibit a similar red-shift from 509 nm of PTZ-2BP-o to 546 nm PTZ-2BP-g3 with the increasing grinding time, and the relative PL lifetimes significantly increase from 6.17 ns to 1331.43 μs, and the PL quantum yields decrease from 54.18% to 16.21% (Fig. 2a and Table 1). The delayed PL spectra of different grinding states all show stable long-lived emission peaks at around 550 nm with lifetimes over 900 μs (Fig. 2b and Fig. S4, ESI† and Table 1), indicating the possible radiative transition of triplet excitons. Thus, with the increased grinding degree, the crystallinity of the original sample decreased, and the ratio of the amorphous powder versus the crystal increased, resulting in a long-lived and red-shifted emission. As for PTZ-2FBP, there is no obvious change in the PL spectra after grinding its recrystallized sample, and either no long-lived emissions (Fig. S5, S6 and Table S3, ESI†). The phosphorescence spectra at 77 K indicate that the long-lived emission of PTZ-2BP may be phosphorescence.

Fig. 2 (a) Normalized PL spectra and (b) delayed spectra of PTZ-2BP in different states (o: the as-prepared state; g1: ground state of o; g2; ground state of g1; and g3: ground state of g2.). ML spectra of (c) the PTZ-2BP crystal sample and (d) powder sample under continuous mechanical stimulus within 30 s.

Table 1 Optical properties of PTZ-2BP in the o, g1, g2 and g3 states

state	o	g1	g2	g3
a The maximum emission wavelength (λPL) of photoluminescence and corresponding lifetime (τPL). b The maximum emission wavelength (λDelay) of delayed photoluminescence and corresponding lifetime (τDelay). c The photoluminescence quantum yield (Φ). (All the data were collected at room temperature in air.)
λ PL	509 nm	495 nm	534 nm	546 nm
λ Delay	558 nm	551 nm	548 nm	546 nm
τ PL	6.17 ns	5.57 ns	872.07 μs	1331.43 μs
τ Delay	901.54 μs	1147.28 μs	1278.75 μs	1331.43 μs
Φ	54.18%	45.12%	32.00%	16.21%

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Compared to the similarity of ML and PL spectra of PTZ-2BP, it is considered that the changeable ratio of amorphous powder versus crystal during the grinding process should be the main reason for mechanical stimulus-responsive dynamic ML emission. To verify this point, crystal and ground powder samples were prepared for the ML test, respectively. Totally different ML spectra of stable emissive wavelength at around 520 nm were observed when lightly grinding the crystal sample (Fig. 2c). Furthermore, when heavily grinding the powder sample, the ML spectra would stabilize at around 570 nm (Fig. 2d). Therefore, the ML emission of PTZ-2BP is highly influenced by the formation of sample states, as there might be energy transfer from the crystal to amorphous powder during the ML process coupled with possible conformation changes.

The crystallinity changes in solid states during the grinding process of PTZ-2BP and PTZ-2FBP were monitored by powder X-ray diffraction (PXRD). There were sharp peaks for both as-prepared samples of PTZ-2BP and PTZ-2FBP, illustrating their good crystallinity. However, the diffraction peaks showed different changes upon mechanical grinding for the two samples, indicating their crystalline structures have undergone different fracture processes, which may affect the mechanical stimulus-responsive luminescence properties. For PTZ-2BP, the diffraction peaks nearly remain unchanged after the first and second mechanical grinding (g1, g2). It was only after prolonged grinding to the g3 state that the peaks exhibited significant broadening and intensity reduction (Fig. 3a). Thus, the as-prepared sample PTZ-2BP-o contains a large amount of crystalline state, which is hardly destroyed under continuous mechanical grinding and retains good crystallinity to an extent. After the first grinding of the as-prepared sample PTZ-2FBP-o, the diffraction peaks significantly decreased or even disappeared, indicating that its crystalline state is unstable and easily destroyed by mechanical force (Fig. 3d). To further study the mechanical properties of the samples, the nanoindentation test, an advanced technology that can provide quantitative results at a more microscopic level, was employed to characterize the single crystals (Fig. 3b and Fig. S8, ESI†). The average indentation moduli and average hardness of PTZ-2BP are 25.9 GPa and 1.05 GPa, respectively, which are much higher than that of PTZ-2FBP (7.45 GPa and 0.34 GPa). Obviously, PTZ-2BP has a stronger mechanical strength compared to PTZ-2FBP, which would endure more stress and is harder to break under mechanical stimulus. Thus, the significant difference in mechanical strength and crystal stability between PTZ-2BP and PTZ-2FBP may have a profound influence on their different mechanical stimulus-responsive luminescence properties. The as-prepared sample of PTZ-2BP exhibits good mechanical stability, which facilitates the occurrence of ML phenomena. Profiting from that, after several mechanical grinding cycles, it still exhibits a yellow ML emission (Fig. 2d and Video S1, ESI†).

Fig. 3 (a) PXRD patterns of PTZ-2BP in different states. (b) Modulus distributions and (c) hardness distributions of PTZ-2BP in the o state; (d) PXRD patterns of PTZ-2FBP in different states; (e) modulus distributions and (f) hardness distributions of PTZ-2FBP in the o state.

To further investigate the crystal properties of the two compounds, single X-ray diffraction was carried out. The Cambridge Crystallographic Data Centre (CCDC) deposition numbers of PTZ-2FBP and PTZ-2BP are 2391557 and 2391558, respectively.† By analysing their single crystal structures in detail, it is found that the single molecules demonstrate totally different conformations in PTZ-2BP and PTZ-2FBP (Fig. 4 and Fig. S9, ESI†). As for PTZ-2BP, the PTZ unit is much twisted with a fold angle of 136.657°, and the ethyl substituent presents a torsion angle of 43.749°, relative to phenothiazine, presenting a q_a conformation. On the contrary, PTZ-2FBP has a relatively planar single molecular conformation with a fold angle of 168.258°, and the torsion angle between the ethyl substituent and phenothiazine is 84.061° as a q_e conformation. Besides, the side benzoyl/fluorobenzoyl groups in the two crystals exhibit twisted conformations with the torsion angles of around 60°. Also, there are non-parallel stacked molecular dimers in PTZ-2BP with efficient intermolecular interactions evenly distributed on the π system, including 2 π…π (3.654–3.879 Å), 8 C–H…π (2.998–3.934 Å), 3 C–H…N (3.301–3.800 Å), 5 C–H…S (2.763–3.924 Å) and 1 C–H…O (2.985 Å) interactions (Table S5, ESI†). However, the two dimers in PTZ-2FBP both display a parallel stack with no π…π, 16/12 C–H…π (2.894–3.997 Å), 6/4 C–H…N (3.168–3.950 Å), 4/4 C–H…S (3.161–3.784 Å), 4/2 C–H…F (3.145–3.978 Å) interactions, respectively (Tables S6 and S7, ESI†), which are mostly provided by the alkyl substituent. Thus, compared to PTZ-2BP, molecules in the PTZ-2FBP crystal are more easily able to slide under mechanical stimulus and consume energy through a non-radiative relaxation. Also, molecules in the PTZ-2BP crystal could endure more mechanical force to an extent and keep stable packing until the crystal breaks, facilitating its ML properties. The calculated noncovalent interactions (NCI) in the molecular dimer illustrated more interactions between two molecules in the PTZ-2BP dimer, which also indicated that PTZ-2BP has more intermolecular interactions than PTZ-2FBP, benefiting its ML property. Moreover, different from PTZ-2FBP, PTZ-2BP has a non-centrosymmetric space group of Pna2₁, in which the piezoelectric effect could occur and contribute much to the ML emission.

Fig. 4 (a) Single molecules and (b) molecular dimers in the crystals of PTZ-2BP and PTZ-2FBP. (c) Calculated conformations based on PTZ-2BP (a) (from single crystal) and PTZ-2BP (b) (from theoretical calculation).

In the crystal, PTZ-2BP molecules show the q_a conformation with a twisted packing mode, which would be destroyed under mechanical stimulus, resulting in a bright green ML. Under continuous mechanical stimulus, the q_a molecules with green ML emission could be converted into the q_e ones, and the dynamic ML from green to yellow was observed. The different PL properties under mechanical grinding could also be explained by the conformation change from q_a to q_e. To further investigate these two molecular conformations, time-dependent density functional theory (TD-DFT) calculations were performed on the monomers and dimers (derived from ground-state geometries in single crystals and optimized molecular structures in the gaseous state). As expected, the optimized molecular structure in the gaseous state presented the q_e conformation of PTZ-2BP(b) (Fig. 4c). In consideration of the damaged intermolecular interaction after being ground heavily, the molecular conformation in the amorphous state should be similar to the optimized q_e one.

For the crystals, a similar orbital delocalization is observed on the whole molecule for monomers in PTZ-2BP and PTZ-2FBP, suggesting their similar electronic properties of the single molecular state (Fig. S11, ESI†). Differently, the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are largely separated in the dimers of PTZ-2BP, with no obvious orbital distribution change from the monomer to the dimer for PTZ-2FBP (Fig. S12, ESI†). Therefore, the unparallel packing mode leads to the stronger intermolecular charge transfer in PTZ-2BP, which might benefit the ML performance. Noticeably, the calculated dipole moment of the dimer in PTZ-2BP is greatly enhanced from 8.39 D of the monomer to 12.85 D, also promoting the intense ML (Fig. 5). On the contrary, the dipole moment of ML-inactive PTZ-2FBP drastically decreased from 7.29/7.27 D (monomer) to 0.01 D (dimer). From the TD-DFT predicted singlet and triplet excited energy levels, there are four possible intersystem crossing (ISC) channels between S₁ and T_n on the dimers in PTZ-2BP with the singlet and triplet splitting energy (ΔE_ST) lower than 0.3 eV, according to the energy gap law (Fig. S13, S14 and Tables S8, S9, ESI†). The efficient ISC processes endowed by small ΔE_ST could populate the triplet excited states and ensure long-lifetime emissions of PTZ-2BP in the solid state.

Fig. 5 The electrostatic potential distribution (blue and red regions correspond to negative and positive potentials, respectively) and dipole moment of PTZ-2BP and PTZ-2FBP.

Conclusions

In summary, we have synthesized two luminogens, PTZ-2BP and PTZ-2FBP, with similar chemical structures but different ML properties. PTZ-2BP exhibits dynamic mechanoluminescence from green to yellow under continuous mechanical stimulus, and the yellow ML could be observed even after prolonged grinding. Through careful analysis and comparison with the PL properties, the D–A structure, coupled with twisted molecular conformations and efficient intermolecular interactions, ensures the ML emission of PTZ-2BP. Especially, its dynamic ML effect could be attributed to molecular conformation changes under mechanical stimulus. This work thoroughly investigated the changeable ML emission under mechanical stimulus and revealed the crucial influence of molecular conformations on ML performance, further increasing the possibility of new applications.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (22105085, 22375077 and 52073122), the start-up funding from Jianghan University and the Research Fund of Jianghan University (Grant No. 2023KJZX04) for support.

Notes and references

1. (a) J. Walton, Triboluminescence, Adv. Phys., 1977, 26, 887–948; (b) F. Bacon, The Advancement of Learning, Press of P. F. Collier & Son, New York, 1901, pp. 208–209.
2. (a) J. Zink, Acc. Chem. Res., 1978, 11, 289–295; (b) H. Nakayama, J. Nishida, N. Takada, H. Sato and Y. Yamashita, Chem. Mater., 2012, 24, 671–676.
3. (a) Y. Xie and Z. Li, Triboluminescence: Recalling Interest and New Aspects, Chemistry, 2018, 4, 943–971; (b) W. Li, Q. Huang, Z. Mao, Q. Li, L. Jiang, Z. Xie, R. Xu, Z. Yang, J. Zhao, T. Yu, Y. Zhang, M. P. Aldred and Z. Chi, Angew. Chem., Int. Ed., 2018, 57, 12727–12732; (c) H. Zhang, Y. Wei, X. Huang and W. Huang, Recent Development of Elastico-Mechanoluminescent Phosphors, J. Lumin., 2019, 207, 137–148; (d) C. Wang, Y. Yu, Y. Yuan, C. Ren, Q. Liao, J. Wang, Z. Chai, Q. Li and Z. Li, Heartbeat-Sensing Mechanoluminescent Device Based on a Quantitative Relationship between Pressure and Emissive Intensity, Matter, 2020, 2, 181–193; (e) T. Hou, W. Li, H. Wang, Y. Zheng, C. Chen, H. Zhang, K. Chen, H. Xie, X. Li, S. He, S. Zhang, D. Peng, C. Yang, J. W. Y. Lam, B. Z. Tang and Y. Zi, An Ultra Thin, Bright, and Sensitive Interactive Tactile Display Based on Organic Mechanoluminescence for Dual-Mode Handwriting Identification, InfoMat, 2024, 6, e12523.
4. (a) C. Wang, B. Xu, M. Li, Z. Chi, Y. Xie, Q. Li and Z. Li, A stable tetraphenylethene derivative: aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence properties, Mater. Horiz., 2016, 3, 220–225; (b) J.-I. Nishida, H. Ohura, Y. Kita, H. Hasegawa, T. Kawase, N. Takada, H. Sato, Y. Sei and Y. Yamashita, Phthalimide compounds containing a trifluoromethylphenyl group and electron-donating aryl groups: color-tuning and enhancement of triboluminescence, J. Org. Chem., 2016, 81, 433–441; (c) B. Xu, W. Li, J. He, S. Wu, Q. Zhu, Z. Yang, Y.-C. Wu, Y. Zhang, C. Jin, P.-Y. Lu, Z. Chi, S. Liu, J. Xu and M. R. Bryce, Achieving very bright mechanoluminescence from purely organic luminophores with aggregation-induced emission by crystal design, Chem. Sci., 2016, 7, 5307–5312.
5. (a) G. E. Hardy, J. C. Baldwin, J. I. Zink, W. C. Kaska, P.-H. Liu and L. Dubois, Triboluminescence spectroscopy of aromatic compounds, J. Am. Chem. Soc., 1977, 99, 3552–3558; (b) J. Yang, Z. Ren, Z. Xie, Y. Liu, C. Wang, Y. Xie, Q. Peng, B. Xu, W. Tian, F. Zhang, Z. Chi, Q. Li and Z. Li, AIEgen with Fluorescence-Phosphorescence Dual Mechanoluminescence at Room Temperature, Angew. Chem., Int. Ed., 2017, 56, 880–884; (c) V. C. Wakchaure, K. C. Ranjeesh, G. Goudappagouda, T. Das, K. Vanka, R. Gonnade and S. S. Babu, Mechano-responsive room temperature luminescence variations of boron conjugated pyrene in air, Chem. Commun., 2018, 54, 6028–6031.
6. (a) S. Xu, T. Liu, Y. Mu, Y.-F. Wang, Z. Chi, C.-C. Lo, S. Liu, Y. Zhang, A. Lien and J. Xu, An organic molecule with asymmetric structure exhibiting aggregation-induced emission, delayed fluorescence, and mechanoluminescence, Angew. Chem., Int. Ed., 2015, 54, 874–878; (b) J. Guo, X.-L. Li, H. Nie, W. Luo, S. Gan, S. Hu, R. Hu, A. Qin, Z. Zhao, S.-J. Su and B. Z. Tang, Achieving high-performance nondoped oleds with extremely small efficiency roll-off by combining aggregation-induced emission and thermally activated delayed fluorescence, Adv. Funct. Mater., 2017, 27, 1606458.
7. (a) C. Wang, Y. Yu, Y. Yuan, C. Ren, Q. Liao, J. Wang, Z. Chai, Q. Li and Z. Li, Heartbeat-Sensing Mechanoluminescent Device Based on a Quantitative Relationship between Pressure and Emissive Intensity, Matter, 2020, 2, 181–193; (b) C. Wang, Y. Yu, Z. Chai, F. He, C. Wu, Y. Gong, M. Han, Q. Li and Z. Li, Recyclable Mechanoluminescent Luminogen: Different Polymorphs, Different Self-Assembly Effects of the Thiophene Moiety and Recovered Molecular Packing via Simple Thermal-Treatment, Mater. Chem. Front., 2019, 3, 32–38; (c) J. Wang, C. Wang, Y. Gong, Q. Liao, M. Han, T. Jiang, Q. Dang, Y. Li, Q. Li and Z. Li, Bromine-Substituted Fluorene: Molecular Structure, Br–Br Interactions, Room-Temperature Phosphorescence, and Tricolor Triboluminescence, Angew. Chem., Int. Ed., 2018, 57, 16821–16826.
8. (a) J. Yang, J. Qin, P. Geng, J. Wang, M. Fang and Z. Li, Molecular Conformation-Dependent Mechanoluminescence: Same Mechanical Stimulus but Different Emissive Color over Time, Angew. Chem., Int. Ed., 2018, 57, 14174–14178; (b) H. Deng, Z. Yang, G. Li, D. Ma, Z. Xie, W. Li, Z. Mao, J. Zhao, Z. Yang, Y. Zhang and Z. Chi, Dynamic organic mechanoluminescence (ML): The roles of Mechano-induced conformational isomer and energy transfer from ML to photolumisscence (PL), Chem. Eng. J., 2022, 438, 135519.
9. (a) Z. Xie, X. Zhang, Y. Xiao, H. Wang, M. Shen, S. Zhang, H. Sun, R. Huang, T. Yu and W. Huang, Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism, Adv. Mater., 2023, 35, 2212273; (b) Z. Liu, X. Ye, L. Chen, L. Niu, W. Jin, S. Zhang and Q. Yang, Spontaneous Symmetry Breaking of Achiral Molecules Leading to the Formation of Homochiral Superstructures that Exhibit Mechanoluminescence, Angew. Chem., Int. Ed., 2024, 63, e202318856; (c) Z. Xie, Y. Xue, X. Zhang, J. Chen, Z. Lin and B. Liu, Isostructural Doping for Organic Persistent Mechanoluminescence, Nat. Commun., 2024, 15, 3668.
10. (a) Y. Yu, Y. Fan, C. Wang, Y. Wei, Q. Liao, Q. Li and Z. Li, Achieving Enhanced ML or RTP Performance: Alkyl Substituent Effect on the Fine-Tuning of Molecular Packing, Mater. Chem. Front., 2021, 5, 817–824; (b) W. Li, Q. Huang, Z. Mao, Q. Li, L. Jiang, Z. Xie, R. Xu, Z. Yang, J. Zhao, T. Yu, Y. Zhang, M. P. Aldred and Z. Chi, Alkyl Chain Introduction: In Situ Solar-Renewable Organic Colorful Mechanoluminescence Materials, Angew. Chem., Int. Ed., 2018, 57, 12727–12732; (c) W. Li, Q. Huang, Z. Yang, X. Zhang, D. Ma, J. Zhao, C. Xu, Z. Mao, Y. Zhang and Z. Chi, Activating Versatile Mechanoluminescence in Organic Host–Guest Crystals by Controlling Exciton Transfer, Angew. Chem., Int. Ed., 2020, 59, 22645–22651.
11. (a) J. Yang, X. Gao, Z. Xie, Y. Gong, M. Fang, Q. Peng, Z. Chi and Z. Li, Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence, Angew. Chem., Int. Ed., 2017, 56, 15299–15303; (b) Q. Liao, Q. Gao, J. Wang, Y. Gong, Q. Peng, Y. Tian, Y. Fan, H. Guo, D. Ding, Q. Li and Z. Li, 9,9-Dimethylxanthene Derivatives with Room-Temperature Phosphorescence: Substituent Effects and Emissive Properties, Angew. Chem., Int. Ed., 2020, 59, 9946–9951; (c) J. Jia and Y. Wu, Synthesis, Crystal Structure and Reversible Mechanofluorochromic Properties of a Novel Phenothiazine Derivative, Dyes Pigm., 2017, 136, 657–662.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, NMR spectral data, photophysical properties, and single crystal X-ray analysis data. CCDC 2391557 and 2391558. See DOI: https://doi.org/10.1039/d4tc04568h

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