Introduction

Mechanochromic luminescence (MCL) molecules, as a new type of smart material, have been widely applied in fields such as sensors, optoelectronic devices, and anti-counterfeiting inks, which has attracted extensive attention1,2,3,4,5. Under external force stimuli, such as pressing, grinding, friction, and shearing, these materials can exhibit unique luminescence changes. In the past few years, significant progress has been made in MCL materials. At the same time, metal-free modern luminescent organic materials with thermally activated delayed fluorescence (TADF) behavior have become one of the most attractive luminescent materials in the field of organic light-emitting diodes (OLEDs)6,7,8,9,10,11. Such materials can achieve an internal quantum efficiency (IQE) of 100%. The extremely small energy gap (\(\Delta {E}_{ST}\)) between the lowest singlet state (S1) and the triplet state (T1) of the excited state can increase the reverse intersystem crossing rate constant (\({k}_{RISC}\)) from T1 to S1.

Integrating the functions of MCL and TADF into a single molecule will provide great opportunities for the multifunctional applications of organic emitters. In 2015, Chi et al. reported a single-molecule TADF white-emitting molecule that exhibited obvious MCL behavior12. In 2017, Swager and his colleagues introduced a series of TADF emitters, which showed significant two-color MCL responses13. In the same year, Takeda and his colleagues developed the first molecule with both TADF and multi-chromic MCL14. In 2018, Grazulevicius and his colleagues reported a series of TADF molecules with mechanochromic luminescence properties and reversible TADF turn on/off properties in solid state that were induced by the transition between amorphous and crystalline states15. Recently, Wang and his colleagues reported a novel MCL-active TADF material based on changes in packing arrangements16. However, despite these advances, due to the lack of a systematic molecular design, the research on efficient TADF with multicolor MCL materials is still in its infancy.

The 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) exemplifies a thermally activated delayed fluorescence (TADF) molecule characterized by a twisted donor–acceptor (D–A) molecular architecture6,17. Within this structure, carbazole units function as electron donors while the dicyanobenzene moiety serves as the electron acceptor (Fig. 1). In this work, we explored the mechano-responsive luminescence characteristics of 4CzTPN with in situ high-pressure photoluminescence (PL), time-resolved PL measurements, UV–vis spectra and infrared spectra measurements. In situ steady state PL data show that the PL emission of 4CzTPN exhibits red-shift under high pressure. However, PL emission of 4CzTPN exhibits blue-shift upon mechanical grinding. And time-resolved PL measurements data display that the average short PL lifetime decreases when the pressure is from 0 GPa to 10.0 GPa. Moreover, average long PL lifetime disappears when the pressure is beyond 3.0 GPa. In order to further analyze the properties of mechano-responsive luminescence characteristics of 4CzTPN, the powerful infrared spectra measurements are applied to investigate structural change. This work gives deep insight into the interesting mechanoresponsive behavior of 4CzTPN crystals from the structural point of view.

Fig. 1
figure 1

Chemical structure of 4CzTPN.

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Results and discussion

Under ambient conditions, 4CzTPN crystals presented a bright yellow emission with the maximum wavelength (λem) of 580 nm, which was attributed to the S1–S0 transition6. In order to explore the pressure-responsive luminescent behavior of 4CzTPN, we conducted the in-situ high pressure PL measurement. Figure 2f showed the schematic illustration of the in-situ high pressure PL measurements. Upon compression, the PL intensity continuously decreased. With the change of emission intensity, the PL maximum showed a remarkable red-shift of approximately 97 nm, reaching up to 10.6 GPa (Fig. 2a). The pressure coefficient of emission wavelengths was 9.4 nm GPa−1, which could be represented by the slope of linear fitting (Fig. 2b). The red-shift of emission wavelength led to the reduction of S1 energy level. The changes of S1 energy level and T1 energy level were the main reasons for the complex evolution of the PL lifetime. The fluorescence photographs exhibited the pressure-dependent color change process of the 4CzTPN crystal. Emission color changed through yellow to red step-by-step (Fig. 2a). After releasing pressure, the PL spectra reverted to the original wavelength, providing unambiguous evidence of the reversible piezo-chromic behavior of the material. Figure 2c showed the UV–vis absorption spectra of 4CzTPN crystal under high pressure. With the increasing pressure, an obvious red shift was observed, which was consistent with the significant red shift of the photoluminescence (PL) spectra. This distinct red shift phenomenon could be directly reflected by the change of the optical color, transitioning from yellow to red and then to black. The red-shift of UV–vis absorption band resulted in the reduction of S1 energy level. The changes of S1 energy level would be responsible for the complex evolution of the PL lifetime. More interestingly, after being ground, the PL spectrum exhibited a blue shift phenomenon of approximately 16 nm (Fig. 2d). The result was in accordance with the previous report18. The blue shift would be resulted from a phase transition from crystalline state to amorphous state by grinding19,20,21. The XRD patterns of 4CzTPN crystal before and after grinding were exhibited in Fig. S1. After grinding, the disappearance of diffraction peaks suggested the phase transition from crystalline state to amorphous state. Figure 2e showed the PL spectrum of 4CzTPN powder under high pressure. The obvious red-shift phenomenon could be observed.

Fig. 2
figure 2

(a)Pressure-dependent PL emissions of 4CzTPN. Insets illustrate the corresponding PL micrographs with increasing pressure. (b) Pressure-dependent emission wavelengths of 4CzTPN. Green line represents linear fittings of the data to achieve pressure coefficients of emission wavelengths. (c) UV–vis spectra of 4CzTPN under high pressure. Insets illustrate the corresponding sample micrographs with increasing pressure. (d) PL emissions of 4CzTPN upon grinding at ambient conditions. (e) Pressure-dependent PL emissions of 4CzTPN powder. Insets illustrate the corresponding PL micrographs with increasing pressure. (f) Schematic illustration of the in-situ high pressure PL measurements.

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In order to elucidate the pressure-dependent behavior of PL emission as observed, we conducted measurements of the PL decay curves for 4CzTPN crystals under different pressures (as depicted in Fig. 3a, c and Fig. S2). The emission decay profiles were modeled using a biexponential function22,23. Subsequently, the variations in the average lifetime as a function of increasing pressure are illustrated in Fig. 3b. The average lifetime, referred to as the intensity-weighted lifetime \({\tau }_{iw}\), is calculated according to the formula \({\tau }_{iw}=\left(A{\tau }_{1}^{2}+B{\tau }_{2}^{2}\right)/\left(A{\tau }_{1}+B{\tau }_{2}\right)\)22,23,24. Under ambient conditions, there obviously existed two lifetime decay processes: a short PL lifetime of 7.94 ns and a long PL lifetime of 1.68 µs. The long PL lifetime could readily be attributed to the delayed fluorescence. The short PL lifetime was assigned as promoted fluorescence. Upon compression, the average short-lived component decreased sharply (Fig. 3b). More interesting, the average long-lived component disappeared beyond 3.0 GPa (Table 1). The vanishing of delayed fluorescence would be due to the widening of singlet–triplet (S1 and T1) energy gap \(\Delta {E}_{ST}\)25,26. After releasing pressure, the PL decay curves reverted to the original state (Fig. S3).

Fig. 3
figure 3

(a, c) High pressure PL decay curves of the 4CzTPN crystals in the time range of 0–100 ns and 0–2700 ns. (b) The change of average short PL lifetime at different pressure.

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Table 1 The value of delay PL emission lifetime at different pressure.
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In-situ high-pressure infrared spectroscopy measurements (Fig. 4) were carried out to understand the unique PL emission behavior based on the structural aspects. It can be seen that the entire infrared peak shifted to higher frequencies, which would promote the non-radiative process. The bands in the range of 1386 to 1536 cm−1 are the C–H wagging (δ(C–H)) of the carbazole segment of 4CzTPN. Under compression, the wavenumber of the C–H bond showed a blue shift, indicating that as the C–H bond continued to be compressed and shortened, the vibration was enhanced, suggesting stronger molecular interactions. The band at 2237 cm−1 can be identified as the stretching vibration of C≡N (ν(C≡N)), which is characteristic of the cyano bond27. The bands in the range of 3000 to 3100 cm−1 are the C–H stretching vibrations (ν(C–H)) of the carbazole segment28. When compressed, the relative intensity of the infrared absorption peak of ν(C–H) changed. The peak at 3023 cm−1 (marked with an asterisk) gradually became stronger and shifted to a higher wavenumber, taking the dominant position. This indicated that with the increase of pressure, the absorption peak of (ν(C–H) changed due to the emergence of many newly formed intramolecular interactions within the reduced spacing29. When the pressure was greater than 3.0 GPa, the pattern of relative intensity for the infrared absorption peak of ν(C–H) transformed significantly, which would be the reason for the disappearance of delayed fluorescence. After the release of pressure, all the peaks in the infrared spectrum completely returned to their original states, which would be responsible for the of reversibility PL emission wavelength.

Fig. 4
figure 4

High Pressure IR spectra of 4CzTPN in the wavenumber region of 500 cm−1–3500 cm−1.

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Conclusion

In summary, we find that the 4CzTPN crystal presents distinct luminescent responses to anisotropic grinding and isotropic compression. Grinding of the crystals leads to a phase transition from crystalline state to amorphous state, which would be responsible for the blue-shift of PL emission. In sharp contrast, high-pressure experiments carry out with DAC demonstrates that the yellow fluorescence of the crystals transform into red fluorescence with an emission wavelength (λem) of 677 nm under a pressure of 10.6 GPa. The red-shift is due to the decrease in the energy level of the S1 state. High-pressure time-resolved measurements shows that the short fluorescence lifetime decrease, and the lifetime of delayed fluorescence disappears above 3.0 GPa. Infrared spectra indicate the pattern of relative intensity for the infrared absorption peak of ν(C–H) transform significantly when the pressure is greater than 3.0 GPa, which would be the cause of the disappearance of delayed fluorescence. This study has expanded the versatility of TADF material. It also indicates that high-pressure treatment is a potential method for controlling the fluorescence lifetime of TADF materials, providing a new approach to improving the performance of TADF-OLEDs. Moreover, the research of pressure-responsive luminescent materials would provide idea for their promising applications in fields like high-pressure detection and monitoring, pressure-modulated smart displays and lighting technologies, as well as secure information encryption.

Methods

Sample preparation and high-pressure generation

4CzTPN was purchased from Xi’an baolaite Technology Ltd and used as received. A symmetric diamond anvil cell (DAC) was used to generate high pressure. A T301 steel gasket was preindented to a thickness of 40 mm. The sample was loaded into a 150 mm size hole of the gasket. A small ruby ball was placed into the hole for in situ pressure calibration according to the R1 ruby fluorescence method. Thick CCl4 (Aldrich) was used as the pressure-transition medium (PTM) in the high-pressure PL, UV–Vis, IR measurement.

Optical measurements

The 355 nm line of a UV DPSS laser was used for PL measurements (Light & Microvision Industrial Technology Co., Ltd). The optical fiber spectrometer is an Ocean Optics QE65Pro spectrometer. The PL micrographs of the samples were captured using a Canon camera equipped on the light path. Time resolved PL data was collected via Edinburgh FLS1000 photoluminescence spectrometer under the laser excitation at 375 nm. The measured PL decay curves were fitted using double exponential functions. The IR absorption modes were detected by a liquid-nitrogen-cooled detector through a microscope spectrometer of SHMADZU, IRTracer-100.