Conformation-determined emission enhancement of phenothiazine derivatives under high pressure

Abstract

For organic luminescent materials, molecular conformation is a basic parameter, which significantly influences their photophysical properties and has been a subject of investigation. In this work, we found that the phenothiazine derivative of PTZ-DP-F crystal presented a continuous emission enhancement until the pressure was up to 2.31 GPa. Unlike the crystal state, the short wavelength emission derived from the quasi-axial conformation of the ground powder state was still enhanced at a relatively lower pressure range, revealing that the pressure-induced emission enhancement (PIEE) behaviors are highly related to the conformation discrepancy. Then, combined with the theoretical calculation results, it further confirmed that the enhanced emission at the initial stage should be ascribed to the restriction of intramolecular rotation and vibrations as a result of the enhanced intra-/inter-molecular interactions under high pressure. Additionally, the other phenothiazine derivatives were also chosen for high-pressure experiments to further prove the conformation-dependent pressure-responsive behaviors. Our study provides deep insights into the essential role of intra-/inter-molecular interactions in photo-physical properties to develop a mechanism of PIEE determined by molecular conformation for phenothiazine derivatives and successfully forging new paths for the designing of materials with high luminous efficiency by regulating the intramolecular interactions.

---

Introduction

Organic luminescent materials in aggregate have drawn considerable attention for their wide applications such as sensors, displays, and data storage.^1–9 As is well known, different aggregation states could form distinct molecular conformations synchronously.^10–13 Actually, conformation is a basic parameter of organic luminescent compounds, which considerably creates an effect on their physicochemical properties. The importance of molecular conformation in crystals has long been recognized, and the links between conformation and photophysical properties have also been a subject of investigation.^14–17 Over the past few decades, several strategies have been developed to study the impact of molecular conformation on photophysical properties, such as the modification of molecular structure and polymorphs. For example, Fang et al. chose a rotatable acceptor fragment, o-carborane, to enrich conformational diversities in the crystalline state and generate conformation-dependent multicolor emissions, which afford wide-range excitation-dependent emissions spanning over 230 nm.¹⁸ Zhao et al. reported a symmetric p-bis(2,2-dicyanovinyl)benzene-based fluorophore and found that the relatively planar conformation of molecules in polymorph-O contributed to the better intermolecular charge transfer (ICT) effect and large conjugation, resulting in a red-shifted emission than polymorph-G.¹⁹ Thus, previous studies on molecular conformation demonstrated that the distinct luminescent properties are highly related to different conformations, which act as a specific form of molecular existence. This also confirms the importance of further understanding the relationship between the conformation and photophysical properties.

Proverbially, phenothiazine is a butterfly-shaped molecule endowed with remarkable electronic properties, which usually behave in double conformations: quasi-axial (ax) and quasi-equatorial (eq).^20–25 Previously, Xu et al. explored two phenothiazine derivatives with force-induced delayed fluorescence through the transition of their conformations from ax-conformation to eq-conformation.²⁶ Our group reported the interestingly conformation-dependent room temperature phosphorescence (RTP) effect of phenothiazine derivatives,²⁷ in addition to distinct photo-physical properties of two different conformations.^28–30 Moreover, we found a phenothiazine derivative of (4-(10H-phenothiazin-10-yl)phenyl)(4-fluorophenyl)methanone (PTZ-DP-F) with unique dynamic mechanoluminescence (ML) properties. Its ML behaviors dramatically change from blue to white, and finally to yellow,³¹ which was assigned to the molecular conformation transition of the phenothiazine group from quasi-axial to quasi-equatorial (Fig. S1, ESI†), arising out of the disruption of intermolecular interactions and molecular packing.

Moreover, hydrostatic compression performed by diamond anvil cell (DAC) equipment can help tune emissions in a wide range and give insights into the structure–property relationships of materials in a more controllable way, thus providing a simple and convenient strategy to achieve continuous changes of molecular conformation and arrangements.^32–42 Herein, using hydrostatic pressure, we chose PTZ-DP-F with two distinct conformations as a research object to further understand the conformation-dependent luminescence mechanism. Quite interestingly, a pressure-induced emission enhancement (PIEE) phenomenon was directly observed, which is highly related to the conformation discrepancy (Fig. 1). We applied in situ high-pressure fluorescence and Raman spectroscopy measurements combined with theoretical calculations and Hirshfeld surface analysis to systematically discuss the distinct pressure response of PTZ-DP-F.

Fig. 1 The molecular structure and two possible conformations, and the distinct pressure-responsive behaviors based on the different conformations during the compression process (enhanced emission for quasi-axial and quenched emission for quasi-equatorial).

Results and discussion

Impressively, the fluorescence spectra of PTZ-DP-F crystal upon compression from 0 GPa to 10.19 GPa (Fig. 2A and Fig. S2, ESI†) suggested that PTZ-DP-F crystal mainly experienced two stages of variation under high pressure. At ambient conditions, the PTZ-DP-F crystal displayed blue emission with a fluorescence band at 434 nm. Incredibly, as the pressure increased from 0 GPa to 2.31 GPa, the emission intensity of PTZ-DP-F crystal was enhanced remarkably, together with a slight red-shift from 434 nm to 444 nm (Fig. 2A), and this abnormal change could be visualized in corresponding fluorescence photographs (Fig. 2B), which can be recognized as a PIEE phenomenon. Normally, for the organic luminescent materials, most of them showed a gradual quenched emission with the elevated pressure, probably caused by different mechanisms, such as exciton coupling, orbital overlap, and π–π aggregation.^43–46 This PIEE phenomenon is observed very rarely in organic piezochromic materials (PCMs), and the related investigations are still in the preliminary stage.^47–55 Then, with further compression over 2.31 GPa, the emission band demonstrated a continuous decline of intensity with an obvious red shift from blue to green (Fig. S2, ESI†), and this change of emission could also be observed by the naked eyes, which can be identified as being derived from the pressure-induced conformational planarization caused by the more closely stacking mode and the decrease of intermolecular distances.

Fig. 2 (A) The compression process of PTZ-DP-F crystal at the pressure ranging from 0 to 2.31 GPa (λ_ex = 365 nm); (B) the pressure-dependent intensity (green), wavelength (orange), and corresponding fluorescence images with respect to pressure; (C) fluorescence spectra of ground PTZ-DP-F powder upon compression (λ_ex = 365 nm) in the range of 0–1.10 GPa; (D) the corresponding pressure dependence of the fluorescence intensity and wavelength with fluorescence images.

Note that when the pressure was released, PTZ-DP-F showed incomplete recovery and its luminescence was restored to a light green color, not the initial blue crystal. Moreover, a new emission band at 555 nm suddenly appeared (Fig. S3, ESI†). This drastic irreversibility might be caused by the lattice disturbance under high pressure.⁵⁶ Compared with previous work (Fig. S1, ESI†), this recovered spectrum was consistent with the fluorescence spectrum of PTZ-DP-F upon applying external force. Therefore, the emerging emission band belonged to quasi-equatorial with decreased energy gap, while the short wavelength emission (434 nm) derived from quasi-axial conformation.³¹ Moreover, the potential surface scanning of PTZ-DP-F demonstrates that the ax-conformation is more stable than eq-conformation (Fig. S4, ESI†). Impressively, when the second pressurization was carried out on the same sample, the short wavelength band still exhibited enhanced emission upon the pressure from 0 GPa to 1.03 GPa. Moreover, with further compression, the intensity was reduced, while the other band showed decreased intensity continuously (Fig. S5, ESI†). Upon releasing the pressure, this new emissive peak would still exist stably (Fig. S6, ESI†).

What would be the main reason for its unique PIEE behavior? To search for the possible emissive mechanism, the analysis of the single crystal structure was performed, and the detailed crystal parameters are summarized in Table S1 (ESI†). As shown in Fig. S7 (ESI†), PTZ-DP-F adopts a quasi-axial conformation in the single crystal. Moreover, abundant and efficient intermolecular interactions could be observed, such as C–H⋯O (2.468 Å) and C–H⋯π interactions (distances ranging from 2.834 to 2.892 Å) (Fig. S7, ESI†). Based on the general view, in the initial stage of pressurization, the distance between the adjacent molecules was reduced and the intermolecular interactions were correspondingly enhanced. Accordingly, the distinct PIEE phenomenon in the pressure range of 0–2.31 GPa could be explained by the restriction of intramolecular rotation (RIR) and the restriction of intramolecular vibrations (RIV). This indicated that the restriction of the intramolecular motion (RIM) would result in a decrease in the nonradiative vibration process, thus enhancing the emission intensity.^47–55 It also demonstrates that the enhanced intramolecular interactions could effectively increase the luminescence intensity, guiding the preparation of high-efficiency luminescent materials.

Also, the high-pressure fluorescence experiment on the PTZ-DP-F powder in the amorphous state by grinding the PTZ-DP-F crystal heavily was performed to further search for the reason for the different pressure-response of dual emission. As shown in Fig. 2C, during the pressure range from 0 to 1.10 GPa, the short wavelength band derived from quasi-axial conformation exhibited the enhanced emission as before, while the fluorescence intensity of the other band assigned to the quasi-equatorial was decreased continuously. Moreover, its emission color changed from yellow-green to light blue (Fig. 2D). When the pressure was further elevated, both of them were obviously quenched with gradual red shifts (Fig. S8, ESI†). Importantly, the ground PTZ-DP-F powder is in an amorphous state without intermolecular interactions. Thus, this result demonstrated that the distinct pressure-responsive behaviors were highly related to the molecular conformation of PTZ-DP-F, that is, within a certain pressure range, quasi-axial conformation affords a PIEE phenomenon, while the quasi-equatorial does not. Moreover, when the pressure was totally released, it could recover to the initial state (Fig. S9, ESI†).

To further gain insights into the mechanism of this PIEE phenomenon, we carried out theoretical calculations and detailed analysis under high pressure based on the structural data in a unit cell of a single crystal of PTZ-DP-F (Fig. 3 and Table S2, ESI†). According to theoretical results, the pressure could significantly reduce the cell volume (Fig. 3A), which effectively shortened the distance among molecules and made them stack more closely, thus enhancing the intermolecular interactions between adjacent molecules. It should be noted that the cell volume of PTZ-DP-F as a function of pressure showed break points appearing at 3.0 GPa, which is consistent with the node of fluorescence intensity changes with pressure (2.31 GPa) in the experiment (Fig. 2B and 3A). This result demonstrated that the emission enhancement at the initial pressure stage was related to the molecular aggregation and intermolecular interactions. Also, it was found that the crystal exhibited anisotropic compression behavior and the b-axis was the most compressible compared with the a and c axes (Fig. 3B). Considering the molecular arrangements, the b-axis of the crystal structure was compressed mostly, meaning that the twisted PTZ-DP-F molecules possibly tended to be flattened. This tendency would be beneficial for the red-shifts of the emission. Subsequently, the Hirshfeld surface under elevated pressure visually described the evolution of intermolecular interactions. At the beginning of compression (0–3.0 GPa), the red regions (① represents C–H⋯O hydrogen bonds and ② represents C–H⋯π interactions, which were enlarged, showing that initial intermolecular interactions became stronger (Fig. 3C). These strengthened intermolecular interactions can effectively limit intramolecular rotation to decrease non-radiative transition loss, thereby enhancing the emission. Upon further compression, π–π interactions (③) increased drastically, resulting in the quenched fluorescence (Fig. S10, ESI†).

Fig. 3 Theoretical results. Corresponding cell volume (A) and axial length (B) as a function of pressure based on the calculated results; (C) Hirshfeld surface for the calculated structure of PTZ-DP-F at 0.0, 1.0, 2.0, and 3.0 GPa mapped with a d-norm distance: ① represents C–H⋯O hydrogen bonds and ② represents C–H⋯π interactions; pressure-dependent C–H⋯H–C distance (D) and dihedral angle (E).

However, as mentioned above, the amorphous state of the ground PTZ-DP-F powder without intermolecular interactions could also display a PIEE at a lower pressure range. In order to verify this doubt, we extracted individual molecules at different pressures for analysis (Fig. 3D and E). It was found that the quasi-axial conformation had obvious C–H⋯H–C interactions. Upon the process of compression, the distance between the two hydrogen atoms was shortened gradually along with the reduced dihedral angle between the benzene ring and phenothiazine plane, which further restricted the intramolecular rotation and vibrations, thus enhancing the emission intensity. In contrast, there were no such intramolecular interactions observed in the quasi-equatorial conformation. To conclude, the PIEE phenomenon of PTZ-DP-F resulted from the restriction of intramolecular motion (RIM) caused by the enhanced intermolecular and intramolecular interactions, which is highly related to the quasi-axial conformation.

To further elucidate the piezochromism at a molecular level, a high-pressure Raman experiment for PTZ-DP-F was performed. As shown in Fig. S11 (ESI†), most of the Raman peaks showed obvious blue shifts with the increased hydrostatic pressure, which was caused by the decrease of interatomic distances and the increase in the effective force constants with simultaneously reduced intermolecular distance.^57,58 Importantly, the changing trends of the Raman bands at 720 cm⁻¹ and 824 cm⁻¹ should be followed, which are mainly attributed to C–H out-of-plane bending vibrations of the central benzene ring, according to the calculated results (Fig. S12, ESI†). Compared with the normal blue shifts, these two Raman peaks have no obvious changes of shift, but the Raman intensity was enhanced remarkably (Fig. 4) due to the increased rigidity of the skeletal vibration, further demonstrating that molecular vibrations are limited by the close packing under high pressure to avoid the losses of Raman vibration activity. Additionally, when the pressure was released, the Raman spectrum was found to be identical to the initial spectrum (Fig. S13, ESI†), indicating that the compression was reversible and no chemical action occurred upon compression.

Fig. 4 (A) High-pressure Raman spectra (650–850 cm⁻¹) of PTZ-DP-F crystal upon compression (λ_ex = 785 nm); (B) the pressure dependence of Raman peaks and intensities at 720 cm⁻¹ and 824 cm⁻¹.

Additionally, other similar compounds to PTZ-DP-F were chosen for high-pressure experiments for comparison (Fig. 5). By substituting methyl groups in different places, PTZ-1Me-H with ax conformation and PTZ-H-Me with eq conformation were obtained. With the increased pressure, PTZ-1Me-H showed enhanced emission within a certain pressure range, while PTZ-H-Me displayed the quenched emission continuously. Furthermore, the polymorphs with ax and eq conformations of PTZ-3Cl were also obtained, fortunately. During the compression process, the PIEE phenomenon was also observed for PTZ-3Cl-ax at the pressure value from 0 to 0.53 GPa, while PTZ-3Cl-eq exhibited decreased fluorescent intensity. These results are quite consistent with the previously mentioned phenomena of PTZ-DP-F, further confirming that the different conformations of phenothiazine derivatives determine different pressure-responsive behaviors.

Fig. 5 Pressure-responsive behaviors of PTZ-1Me-H-ax (A) and PTZ-H-1Me (B), PTZ-3Cl-ax (C) and PTZ-3Cl-eq (D) during the compression process.

Experimental

Materials

The phenothiazine derivatives used in this work were synthesized and previously reported by us.^30,31,59

Measurements

The high-pressure experiments were carried out using a diamond anvil cell (DAC) with a diameter of 500 μm. T301 stainless steel sheets were used to serve as a sample chamber of 200 μm in diameter. A small ruby chip was loaded into the sample chamber along with the prepared sample for in situ pressure calibration based on the changes in the R1 fluorescence band of ruby. Furthermore, according to Pascal's principle, silicone oil was added in the experiments as a pressure-transmitting medium (PTM) to guarantee hydrostatic pressure. The measurements on the ruby chip were performed using a Horiba Jobin Yvon T64000 Raman spectrometer with a 532 nm laser. High-pressure fluorescence spectra were recorded using the self-built equipment with an optical fiber spectrometer (FX2000 fiber spectrometer, Ideaoptics, China) with a laser of 360 nm, Changchun New Industries Optoelectronics Tech Co., Ltd). High-pressure Raman spectra were acquired using a confocal Raman system (LabRAM Aramis, Horiba Jobin Yvon) with a 785 nm laser as the excitation source. The laser directly focused on the sample through the microscope through a 50×/0.75 NA objective with an integration time of 10 s. All spectra were recorded under the same conditions if not specially pointed out.

Computational methods

The Gaussian 09 program with the basis set level of 6-31G(d) level was utilized to perform the calculation of the potential energy curve. High-pressure theoretical calculations were carried out using the CASTEP package in Materials Studio 2017. The calculation was performed on a plane wave set using norm-conserving pseudopotentials with an energy cut-off of 750 eV. The energy tolerance was set as 2.0 × 10⁻⁵ eV per atom with a force tolerance of 0.05 eV Å⁻¹, a maximum displacement of 2.0 × 10⁻³ Å, and a maximum stress tolerance of 0.10 GPa for achieving the geometry optimization. For self-consistent field (SCF) calculation, the energy tolerance was set to 1.0 × 10⁻⁶ eV per atom. The initial packing geometry from the structural data of a single crystal was fully relaxed under external stress of 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 GPa. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was used to describe the exchange–correlation (XC) effects. The TS scheme was used for dispersion corrections. The Hirshfeld surface (HS) theory was generated by Crystal Explorer 3.1 based on the obtained crystal data of the optimized cell units from Material Studio 2017.

Conclusions

In summary, the distinct pressure-responsive behaviors of phenothiazine derivatives depend on their different conformations. PTZ-DP-F with ax-conformation exhibited a rare PIEE phenomenon, while the eq-conformation showed quenched emission. Based on the high-pressure fluorescence contrast experiments between the crystal and amorphous states as well as theoretical results, we propose that the PIEE behavior results from the restriction of intramolecular motion (RIM) caused by the enhanced intra-/inter-molecular interactions. Moreover, other phenothiazine derivative systems displayed similar behaviors to PTZ-DP-F, which indicated the conformation-determined piezochromic behaviors. Actually, the exploitation of PIEE properties would greatly facilitate the potential applications of organic luminescent materials in optical pressure-sensing devices and information storage. Additionally, it further indicates that the appropriate intramolecular interactions could effectively improve the luminous efficiency of the materials, providing a certain reference for the development of materials with high luminous efficiency.

Author contributions

A. Li performed the experiments and measured and analyzed the optical data. Z. Liu, M. Gao and J. Yang provided the compounds. C. Bi and S. Xu carried out the high-pressure fluorescence experiments. J. Wang and Z. Li conceived the project. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (no. 22205157 and 22105143), the starting Grants of Tianjin University and the Tianjin Government for financial support.

Notes and references

1. S. Ma, S. Du, G. Pan, S. Dai, B. Xu and W. Tian, Organic molecular aggregates: From aggregation structure to emission property, Aggregate, 2021, 2, e96.
2. Q. Li and Z. Li, Miracles of molecular uniting, Sci. China Mater., 2019, 63, 177–184.
3. Q. Li and Z. Li, Molecular packing: another key point for the performance of organic and polymeric optoelectronic materials, Acc. Chem. Res., 2020, 53, 962–973.
4. Q. Li and Z. Li, The strong light-emission materials in the aggregated state: what happens from a single molecule to the collective group, Adv. Sci., 2017, 4, 1600484.
5. F. Hu, W. Yang, L. Li, J. Miao, S. Gong, C. Ye, X. Gao and C. Yang, Multifunctional emitters with TADF, AIE, polymorphism and high-contrast multicolor mechanochromism: Efficient organic light-emitting diodes, Chem. Eng. J., 2023, 464, 142678.
6. W. L. Li, Q. Y. Huang, Z. Mao, J. Zhao, H. Y. Wu, J. R. Chen, Z. Yang, Y. Li, Z. Y. Yang, Y. Zhang, M. P. Aldred and Z. G. Chi, Selective expression of chromophores in a single molecule: soft organic crystals exhibiting full-colour tunability and dynamic triplet-exciton behaviours, Angew. Chem., Int. Ed., 2020, 59, 3739–3745.
7. Y. Zhan, Fluorescence response of anthracene modified D-π-A heterocyclic chromophores containing nitrogen atom to mechanical force and acid vapor, Dyes Pigm., 2020, 173, 108002.
8. Y. Fan, Q. Li and Z. Li, Afterglow bio-applications by utilization of triplet excited states of organic materials, Sci. China: Chem., 2023, 66, 2930–2940.
9. X. Lian, J. Sun and Y. Zhan, Dibenzthiophene and carbazole modified dicyanoethylene derivative exhibiting aggregation-induced emission enhancement and mechanochromic luminescence, J. Lumin., 2023, 263, 120023.
10. S. Li, Y. Xie, A. Li, X. Li, W. Che, J. Wang, H. Shi and Z. Li, Different molecular conformation and packing determining mechanochromism and room-temperature phosphorescence, Sci. China Mater., 2021, 64, 2813–2823.
11. Z. Zhang, B. Xu, J. Su, L. P. Shen, Y. Xie and H. Tian, Color-tunable solid-state emission of 2,2′-biindenyl-based fluorophores, Angew. Chem., Int. Ed., 2011, 50, 11654–11657.
12. K. Wang, H. Zhang, S. Chen, G. Yang, J. Zhang, W. Tian, Z. Su and Y. Wang, Organic polymorphs: one-compound-based crystals with molecular-conformation- and packing-dependent luminescent properties, Adv. Mater., 2014, 26, 6168–6173.
13. F. Nie, B. Zhou and D. P. Yan, Ultralong room temperature phosphorescence and reversible mechanochromic luminescence in ionic crystals with structural isomerism, Chem. Eng. J., 2023, 453, 139806.
14. A. Sussardi, C. L. Hobday, R. J. Marshall, R. S. Forgan, A. C. Jones and S. A. Moggach, Correlating Pressure-induced emission modulation with linker rotation in a photoluminescent MOF, Angew. Chem., Int. Ed., 2020, 59, 8118–8122.
15. W. Yang, Y. Yang, X. Cao, Y. Liu, Z. Chen, Z. Huang, S. Gong and C. Yang, On-off switchable thermally activated delayed fluorescence controlled by multiple channels: Understanding the mechanism behind distinctive polymorph-dependent optical properties, Chem. Eng. J., 2021, 415, 128909.
16. A. Li, H. Liu, C. Song, Y. Geng, S. Xu, H. Zhang, H. Zhang, H. Cui and W. Xu, Flexible control of excited state transition under pressure/temperature: distinct stimuli-responsive behaviours of two ESIPT polymorphs, Mater. Chem. Front., 2019, 3, 2128–2136.
17. H. Wu, W. Chi, G. Baryshnikov, B. Wu, Y. Gong, D. Zheng, X. Li, Y. Zhao, X. Liu, H. Agren and L. Zhu, Crystal multi-conformational control through deformable carbon-sulfur bond for singlet-triplet emissive tuning, Angew. Chem., Int. Ed., 2019, 58, 4328–4333.
18. R. Huang, C. Wang, D. Tan, K. Wang, B. Zou, Y. Shao, T. Liu, H. N. Peng, X. G. Liu and Y. Fang, Single-fluorophore-based organic crystals with distinct conformers enabling wide-range excitation-dependent emissions, Angew. Chem., Int. Ed., 2022, 61, e202211106.
19. J. Zhang, H. Kang, N. Li, S. Zhou, H. Sun, S. Yin, N. Zhao and B. Z. Tang, Organic solid fluorophores regulated by subtle structure modification: color-tunable and aggregation-induced emission, Chem. Sci., 2017, 8, 577–582.
20. N. Wang, Z. Guo, Z. Ni, J. Xu, X. Qiu, J. Ma, P. Wei and Y. Wang, Molecular tailoring of an n/p-type phenothiazine organic scaffold for zinc batteries, Angew. Chem., Int. Ed., 2021, 60, 20826–20832.
21. M. Okazaki, Y. Takeda, P. Data, P. Pander, H. Higginbotham, A. P. Monkman and S. Minakata, Thermally activated delayed fluorescent phenothiazine-dibenzo[a,j]phenazine-phenothiazine triads exhibiting tricolor-changing mechanochromic luminescence, Chem. Sci., 2017, 8, 2677–2686.
22. Y. Rout, A. Ekbote and R. Misra, Recent development on the synthesis, properties and applications of luminescent oxidized phenothiazine derivatives, J. Mater. Chem. C, 2021, 9, 7508–7531.
23. N. Acharya, M. Hasan, S. Dey, S.-C. Lo, E. B. Namdas and D. Ray, Phenothiazine-quinoline conjugates realizing intrinsic thermally activated delayed fluorescence and room-temperature phosphorescence: understanding the mechanism and electroluminescence devices, Adv. Photon. Res., 2021, 2, 2000201.
24. A. Stockmann, J. Kurzawa, N. Fritz, N. Acar, S. Schneider, J. Daub, R. Engl and T. Clark, Conformational control of photoinduced charge separation within phenothiazine-pyrene dyads, J. Phys. Chem. A, 2002, 106, 7958–7970.
25. J. Daub, R. Engl, J. Kurzawa, S. E. Miller, S. Schneider, A. Stockmann and M. R. Wasielewski, Competition between conformational relaxation and intramolecular electron transfer within phenothiazine-pyrene dyads, J. Phys. Chem. A, 2001, 105, 5655–5665.
26. Y. Chen, C. Xu, B. Xu, Z. Mao, J. Li, Z. Yang, N. R. Peethani, C. Liu, G. Shi, F. Gu, Y. Zhang and Z. Chi, Chirality-activated mechanoluminescence from aggregation-induced emission enantiomers with high contrast mechanochromism and force-induced delayed fluorescence, Mater. Chem. Front., 2019, 3, 1800–1806.
27. J. Ren, Y. Wang, Y. Tian, Z. Liu, X. Xiao, J. Yang, M. Fang and Z. Li, Force-induced turn-on persistent room-temperature phosphorescence in purely organic luminogen, Angew. Chem., Int. Ed., 2021, 60, 12335–12340.
28. J. Yang, M. Fang and Z. Li, Stimulus-responsive room temperature phosphorescence materials: internal mechanism, design strategy, and potential application, Acc. Mater. Res., 2021, 2, 644–654.
29. 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.
30. M. Gao, Y. Tian, J. Yang, X. Li, M. Fang and Z. Li, The same molecule but a different molecular conformation results in a different room temperature phosphorescence in phenothiazine derivatives, J. Mater. Chem. C, 2021, 9, 15375–15380.
31. J. Yang, J. W. Qin, P. Y. Geng, J. Q. Wang, M. 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.
32. J. Guan, C. Zhang, D. Gao, X. Tang, X. Dong, X. Lin, Y. Wang, X. Wang, L. Wang, H. H. Lee, J. Xu, H. Zheng, K. Li and H. Mao, Drastic photoluminescence modulation of an organic molecular crystal with high pressure, Mater. Chem. Front., 2019, 3, 1510–1517.
33. L. Wang, K. Q. Ye and H. Y. Zhang, Organic materials with hydrostatic pressure induced mechanochromic properties, Chin. Chem. Lett., 2016, 27, 1367–1375.
34. Z. Fu, K. Wang and B. Zou, Recent advances in organic pressure-responsive luminescent materials, Chin. Chem. Lett., 2019, 30, 1883–1894.
35. Z. Man, Z. Lv, Z. Xu, Q. Liao, J. Liu, Y. Liu, L. Fu, M. Liu, S. Bai and H. Fu, Highly sensitive and easily recoverable excitonic piezochromic fluorescent materials for haptic sensors and anti-counterfeiting applications, Adv. Funct. Mater., 2020, 30, 2000105.
36. Z. Yang, Z. Chi, Z. Mao, Y. Zhang, S. Liu, J. Zhao, M. P. Aldred and Z. Chi, Recent advances in mechano-responsive luminescence of tetraphenylethylene derivatives with aggregation-induced emission properties, Mater. Chem. Front., 2018, 2, 861–890.
37. A. Li, S. Xu, C. Bi, Y. Geng, H. Cui and W. Xu, Piezochromic mechanism of organic crystals under hydrostatic pressure, Mater. Chem. Front., 2021, 5, 2588–2606.
38. Z. Ma, Z. Wang, M. Teng, Z. Xu and X. Jia, Mechanically induced multicolor change of luminescent materials, ChemPhysChem, 2015, 16, 1811–1828.
39. Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye, B. Zou and W. Tian, Piezochromic luminescence based on the molecular aggregation of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene, Angew. Chem., Int. Ed., 2012, 51, 10782–10785.
40. X. Su, N. Li, K. Wang, Q. Li, W. Shao, L. Liu, B. Yu, Y. Zhang, T. Lin, B. Zou, Y. Liu and S. X. Zhang, Real time optical monitoring of cascade mechanochemical reactions and capture of ultra-unstable intermediates under hydrostatic pressure, Mater. Chem. Front., 2023, 7, 4040–4049.
41. L. Wang, K. Wang, H. Zhang, C. Jiao, B. Zou, K. Ye, H. Zhang and Y. Wang, The facile realization of RGB luminescence based on one yellow emissive four-coordinate organoboron material, Chem. Commun., 2015, 51, 7701–7704.
42. Y. Zhang, J. Zhang, J. Shen, J. Sun, K. Wang, Z. Xie, H. Gao and B. Zou, Solid-state TICT-emissive cruciform: aggregation-enhanced emission, deep-red to near-infrared piezochromism and imaging In vivo, Adv. Opt. Mater., 2018, 6, 1800956.
43. Z. Gao, K. Wang, F. Liu, C. Feng, X. He, J. Li, B. Yang, B. Zou and P. Lu, Enhanced sensitivity and piezochromic contrast through single-direction extension of molecular structure, Chem. – Eur. J., 2017, 23, 773–777.
44. K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H. Fujihisa, H. Sato, Y. Shimoikeda and S. Yamaguchi, Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of tetrathiazolylthiophene, J. Am. Chem. Soc., 2013, 135, 10322–10325.
45. H. Liu, Y. Dai, Y. Gao, H. Gao, L. Yao, S. Zhang, Z. Xie, K. Wang, B. Zou, B. Yang and Y. G. Ma, Monodisperse π–π stacking anthracene dimer under pressure: unique fluorescence behaviors and experimental determination of interplanar distance at excimer equilibrium geometry, Adv. Opt. Mater., 2018, 6, 1800085.
46. A. Li, Y. Liu, L. Han, S. Xu, C. P. Song, Y. Geng, L. Pan, B. Xu, W. Tian, H. Zhang, W. Xu and H. N. Cui, Pressure-induced remarkable luminescence-changing behaviours of 9, 10-distyrylanthracene and its derivatives with distinct substituents, Dyes Pigm., 2019, 161, 182–187.
47. Y. Gu, K. Wang, Y. Dai, G. Xiao, Y. Ma, Y. Qiao and B. Zou, Pressure-induced emission enhancement of carbazole: the restriction of intramolecular vibration, J. Phys. Chem. Lett., 2017, 8, 4191–4196.
48. N. Li, Y. Gu, Y. Chen, L. Zhang, Q. Zeng, T. Geng, L. Wu, L. Jiang, G. Xiao, K. Wang and B. Zou, Pressure-induced emission enhancement and piezochromism of triphenylethylene, J. Phys. Chem. C, 2019, 123, 6763–6767.
49. Y. Gu, H. Liu, R. Qiu, Z. Liu, C. Wang, T. Katsura, H. Zhang, M. Wu, M. Yao, H. Zheng, K. Li, Y. Wang, K. Wang, B. Yang, Y. Ma and B. Zou, Pressure-induced emission enhancement and multicolor emission for 1,2,3,4-tetraphenyl-1,3-cyclopentadiene: controlled structure evolution, J. Phys. Chem. Lett., 2019, 10, 5557–5562.
50. A. Li, J. Wang, Y. Liu, S. P. Xu, N. Chu, Y. J. Geng, B. Li, B. Xu, H. N. Cui and W. Q. Xu, Remarkable pressure-induced emission enhancement based on intermolecular charge transfer in halogen bond-driven dual-component co-crystals, Phys. Chem. Chem. Phys., 2018, 20, 30297–30303.
51. H. Yuan, K. Wang, K. Yang, B. Liu and B. Zou, Luminescence properties of compressed tetraphenylethene: the role of intermolecular interactions, J. Phys. Chem. Lett., 2014, 5, 2968–2973.
52. H. Liu, Y. Gu, Y. Dai, K. Wang, S. Zhang, G. Chen, B. Zou and B. Yang, Pressure-induced blue-shifted and enhanced emission: a cooperative effect between aggregation-induced emission and energy-transfer suppression, J. Am. Chem. Soc., 2020, 142, 1153–1158.
53. M. Xie, X. Chen, K. Wu, Z. Lu, K. Wang, N. Li, R. Wei, S. Zhan, G. Ning, B. Zou and D. Li, Pressure-induced phosphorescence enhancement and piezochromism of a carbazole-based cyclic trinuclear Cu(I) complex, Chem. Sci., 2021, 12, 4425–4431.
54. M. Wu, H. Liu, H. Liu, T. Lu, S. Wang, G. Niu, L. Z. Sui, F. Bai, B. Yang, K. Wang, X. Yang and B. Zou, Pressure-induced restricting intermolecular vibration of a herringbone dimer for significantly enhanced multicolor emission in rotor-free truxene crystals, J. Phys. Chem. Lett., 2022, 13, 2493–2499.
55. A. Li, F. Li, Y. Chen, Y. Xie, X. Li, X. Liu, S. Xu, W. Xu, J. Wang and Z. Li, Flexible and continuous regulation promoting remarkably emission enhancement of planar triphenylene, ACS Mater. Lett., 2022, 4, 2151–2158.
56. Y. Li, Z. Ma, A. Li, W. Xu, Y. Wang, H. Jiang, K. Wang, Y. Zhao and X. Jia, A single crystal with multiple functions of optical waveguide, aggregation-induced emission, and mechanochromism, ACS Appl. Mater. Interfaces, 2017, 9, 8910–8918.
57. L. Kang, K. Wang, X. Li and B. Zou, High pressure structural investigation of benzoic acid: Raman spectroscopy and X-ray diffraction, J. Phys. Chem. C, 2016, 120, 14758–14766.
58. A. Banerji and S. K. Deb, Raman scattering study of high-pressure phase transition in thiourea, J. Phys. Chem. B, 2007, 111, 10915–10919.
59. M. Gao, Y. Tian, X. Li, Y. Gong, M. Fang, J. Yang and Z. Li, The effect of molecular conformations and simulated “self-doping” in phenothiazine derivatives on room-temperature phosphorescence, Angew. Chem., Int. Ed., 2023, 62, e202214908.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00110a

---