A “simple” donor–acceptor AIEgen with multi-stimuli responsive behavior

Abstract

There is still an urgent demand for novel smart materials that can achieve a diverse range of practical applications in the synthetic material area. Herein, we developed a simple but versatile aggregation-induced emission luminogen (AIEgen, 1). Compound 1 was sensitive to an electric stimulus and displayed reversibly three-color switched electrochromism and on-to-off electroluminochromism. Such properties allowed the fabrication of high-performance non-doped OLEDs with a high external quantum efficiency of 5.22%. Due to its AIE property and remarkable sensitive color change in response to polarity change, it can serve as a unique imaging probe for detecting environmental polarity in cells and selective visualization of lipid droplets in live tissues. More impressively, compound 1 exhibited a wide range of thermoresponsive behaviors with a ratiometric luminescence change and noticeable fluorescence color switching. As another remarkable feature, it can respond to anisotropic shearing force and isotropic hydrostatic pressure with prominent and contrasting luminescence conversion due to the distinct disturbance of the weak intermolecular interactions and charge transfer process. The present results may offer an important guideline for multifunctional molecular design and provide an important step forward to expand the real-life applications of smart materials.

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New concepts

At present, there is still an urgent demand for novel smart materials that are able to support more efficient technologies and achieve a diverse range of practical applications. Generally, the reported multiple responsive systems are constructed by assembling different structural components with special functions into composite systems. Thus, precise control of each unit and even intricate and time-consuming organic synthesis are required. Herein, we developed a very simple but quite versatile multi-functional material without involving any tedious synthetic tasks and complicated component integration. Another merit of this material over existing multi-functional systems lies in its much richer function diversity. The facile realization of up to six different kinds of responsive functions in a single simple material is unprecedented. Of note, its unique functions in terms of environmental polarity probing in cells and selective visualization of lipid droplets in live tissues, ratiometric and wide-range thermoresponsive behavior with noticeable color switching, and remarkable but high contrast luminescence conversions in response to anisotropic shearing force and isotropic hydrostatic pressure are rarely reported. Our present material really achieved the goal of atom economy and presented enormous potential to meet multifarious real-life application requirements, including electroswitchable electrochromism and electroluminochromism for information recording, storage devices and OLEDs, biological polarity detection and selective visualization in live tissues, wide-range liquid thermometer, security inks and papers. This material breaks the application limitations of a myriad of single-function smart materials. This concept offers an important guideline for multifunctional molecular design and provides an important step forward to expand the real-life applications of smart materials.

Introduction

In this amazing and beautiful world, living systems, plants or animals, show various adaptive behaviors and a wide variety of intriguing ways to response to stimuli in their surrounding environment.¹ Today, scientists and engineers are fascinated by these distinctive stimuli-responsive behaviors because investigation of these behaviors may bestow new inspirations for developing diverse bio-inspired and smart materials for real-world applications.² One common principle learned from these natural creatures is that their unique and complex physiological functions are derived from their selective integration capability of each particular function related to different kinds of specialized cells.³ And this principle has been efficiently utilized for the fabrication of various novel materials by researchers through assembling different structural components with special function into composite systems at the molecular level to achieve multi-functional materials.^4–7

So far, there is still an urgent demand for novel smart materials that are able to support more efficient technologies and to achieve a diverse range of practical applications in the synthetic material area.^8–15 By utilizing the integration strategy, synthetic scientists have developed a wide variety of new smart systems that are able to response to multiple environmental stimuli.^16–18 However, these multiple responsive systems are generally constructed by the integration of multiple components with specific responsive ability. Thus precise control of each unit and even intricate and time-consuming organic synthesis are required.¹⁹

Owing to the vacant p-orbital on the central boron atom, triarylborons (TABs) serve as excellent electron acceptors. When associated with amine-based electron donors, the incorporated donor–acceptor (D–A) small-molecule systems show unprecedented photophysical and photochemical properties resulting from the eminent and unique intramolecular charge transfer (ICT), and relevant extensive applications in optical storage and memory, optoelectronic and display devices, chemical sensors, security inks and papers, etc. have been developed.^20–27 Indeed, many pioneering and elegant TAB–amine systems with intriguing stimuli-responsive properties have been developed by researchers through rational design.^28–32 While a majority of them displayed only one specific responsive function, little effort had been placed to explore their versatility and capacity in multifarious applications. In this respect, it would be desirable and amazing if we could incorporate these individual responsive properties into a single small-molecule system without involving tedious synthetic tasks and complicated component integration. More importantly, developing simple and versatile small molecule materials with various kinds of environmental responses would solve the limitations of smart materials in practical applications to further expand their application scope.

Herein, we reported a simple but versatile TAB-containing molecule with D–A structure, aggregation-induced emission (AIE)^33,34 and pronounced ICT effect (Fig. 1A). This luminogen is sensitive to multiple stimuli, including electric field, polarity, temperature, mechanical shearing force and hydrostatic pressure. Each of them could be specifically visualized by the prominent photoluminescence (PL) color change. To the best of our knowledge, the realization of such multifunctional properties, including electrochromism, electroluminochromism and electroluminescence, solvatochromic PL and further environmental polarity detection in cells, thermochromic PL and mechanochromic PL in a single small molecule has not been reported to date, especially the latter three prominent and intriguing aspects. We will elaborate their stories in the forthcoming contents.

Fig. 1 (A) The molecular structure of compound 1. (B) PL spectra of 1 (1.0 × 10⁻⁵ M) in DMF/water mixtures with different water fraction (f_w). λ_ex = 390 nm. (C) Plot of relative PL intensity (α_AIE) at 500 nm versus f_w of the DMF/water mixtures of 1, where α_AIE = I/I₀ and I₀ = emission intensity in pure DMF solution. Inset: photos of DMF solution and DMF/water mixture (f_w = 90%) of 1 taken under 365 nm UV light.

Results and discussion

The target compound 1 was facilely prepared in a high yield of 79% via a simple one-step reaction along the synthetic route as presented in Scheme S1 (ESI†). The product was well characterized by NMR and high-resolution mass spectroscopy with satisfactory results. Detailed information is provided in the ESI† (Fig. S1–S3). The product and the corresponding solutions in common organic solvents showed high stability under ambient conditions. It also exhibited high thermal stability and the corresponding decomposition temperature is 376 °C (Fig. S4, ESI†). The structure of 1 was further confirmed by single crystal X-ray diffraction (details see below) and the associated data are summarized in Table S1 (ESI†).

As anticipated, compound 1 featuring propeller shape exhibited typical AIE properties as shown in Fig. 1.³⁵ A diluted dimethylformamide (DMF) solution of 1 was almost non-emissive. However, upon the addition of 40 vol% of water into the DMF solution, the PL intensity was increased ∼27 000-fold due to formation of aggregates of 1. Interestingly, the PL intensity remained almost unchanged with further increasing the water fraction. As demonstrated in our previous work, the single crystal structure of 1 exhibited a twisted molecular conformation (Fig. S5, ESI†). The active intramolecular rotation of 1 in DMF was restricted in aggregates, thus allowing the luminogen to show strong PL; this claim was further verified by the viscosity-dependent PL measurement (Fig. S6, ESI†).^35,36

As expected, compound 1 showed response to an external electric stimulus due to the introduction of a redox-active arylamine donor and exhibited remarkable changes in its electronic spectra in the near-infrared region (details see Fig. 2 and Fig. S7, ESI†). Completely reversible conversion among three different colors, i.e. light yellow, vivid green and dark green corresponding to neutral, monocationic and dicationic states, respectively, could be readily achieved by modulating the redox potentials of the cell. And these changes could be directly observed by the naked eye (Fig. 2A and B). Therefore, the above distinct property of 1 indicates that this compound has great potential to be used as an electroswitchable electrochromic material. Regarding its emission spectra, it can also realize a transformation from a turn-on state with organic luminescence to a turn-off state upon slow oxidation to the mono-cationic and dicationic states, where its initial orange luminescence with an emission peak centered at 572 nm was gradually annihilated (Fig. 2C) due to the formation of the non-emissive cation-radical,¹² also pointing to its potential application in information recording and storage devices. In light of its excellent luminescent behavior of 1 in the solid state, we were accordingly encouraged to evaluate its potential application as a solid-state emitter. Considering that this compound is stable enough for thermal evaporation and exhibited a bright yellow-green fluorescence in the film state (details see Fig. S8 and Table S2, ESI†), we fabricated a non-doped organic light-emitting diode (OLED) to study its electroluminescence (EL). The key device performances of this device are presented in Fig. 2D, Fig. S9 and Table S3 (ESI†). The EL spectra of compound 1 are close to the nondoped PL spectrum (vacuum-deposited) and very stable at various driving voltages, indicating that the emissive excitons were well confined in the emitting layer. The EL shows a bright green emission peak of 516 nm with CIE coordinates of (0.289, 0.551). The maximum luminance, current efficiency, power efficiency, and external quantum efficiency (EQE) values are as high as 4622 cd m⁻², 16.23 cd A⁻¹, 11.69 lm W⁻¹, and 5.22%, respectively. Impressively, the EQE value (5.22%) was practically reaching the theoretical limit value of traditional organic emitters, demonstrating that compound 1 is a good emitter for fabricating efficient OLEDs.

Fig. 2 (A and B) Electrochromism and (C) electroluminochromism of 1 in THF/0.1 M n-Bu₄NPF₆ at 298 K. Inset: photos of 1 at different oxidation states taken under (A and B) day light and (C) 365 nm UV light. (D) EQE versus brightness curve (inset: EL spectrum at 10 mA cm⁻²) of an EL device with a configuration of ITO/HATCN (5 nm)/TAPC (25 nm)/1 (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al.

Due to its typical D–A structure, compound 1 demonstrated a strong solvatochromic effect.³⁷ When the solvent polarity was increased gradually from low-polarity hexane to high-polarity acetonitrile, the emission of its solution exhibited a dramatic bathochromic shift and the intensity gradually weakened (details see Table S4, ESI†). Meanwhile, its solution colors changed from bright blue in nonpolar hexane over green, yellow and orange in lower polarity solvents and then to dark red in high polarity solvents, thus covering the whole visible region as shown in the CIE diagram (Fig. S10, ESI†), and allowing even a visual estimate of the solvent polarity. In sharp contrast, its absorption spectra displayed no obvious change as the solvent polarity increased (Fig. S11, ESI†). Quantitatively, the relationship between the Stokes shift (v_a − v_f) of the luminogen and solvent parameters, or the orientation polarizability f was described by the Lippert–Mataga equation.²⁰ It obeys a good linear relationship and the corresponding excited-state dipole, μ_e, was calculated to be 18.6 D (Fig. S10, ESI†), further confirming its typical TICT character.^20,22 Additionally, the obvious charge separation in its natural transition orbitals of the first singlet excited state also verified the large molecular dipole of compound 1 further reflecting its ICT character (Fig. S12, ESI†).

Herein, it is noteworthy to mention that polarity is an utmost important parameter in chemistry, nanotechnology, and even life science.³⁸ In biological systems, especially at the cellular level, polarity determines the interaction activity of large numbers of proteins and enzymes or reflects the permeability of membrane compartments. Furthermore, abnormal changes in polarity are closely linked with disorders and diseases (e.g., diabetes, liver cirrhosis).^39,40 However, the environmental polarity change in biological systems is relatively subtle, and thus it is very difficult to realize its measurement in a straightforward manner, let alone its macroscopic visualization. Therefore, our present AIE-active system, featuring remarkable polarity dependence and obvious fluorescence change, is expected to provide a valuable tool for the detection of the biological environmental polarity change.⁴¹

To demonstrate the practical application of 1 as a fluorescent probe for detecting environmental polarity in biological systems, two-photon fluorescence imaging in live cells was performed by using a two-photon microscope and MCF-7 cells as a model. Prior to live cell imaging, the cytotoxicity of 1 was assessed by standard MTT assay and the results indicate that 1 exhibits no cytotoxicity towards MCF-7 cells (Fig. S13, ESI†). When the MCF-7 cells were incubated with 1 for 1 h, weak green fluorescence detected in the range of 495–540 nm of 1 was observed (Fig. S14A, ESI†), and its intensity was greatly enhanced by extending the incubation time to 5 h (Fig. 3A). The co-staining experiment with commercial Lysotracker Red indicated that 1 was mainly localized in the lysosome (Fig. S15A, ESI†). Given that lipid droplets (LDs) are extremely hydrophobic and their polarity is lower than other intracellular regions,⁴¹ we anticipated that the hydrophobic 1 should be more inclined to target LDs. If compound 1 readily targets LDs, the detected fluorescence should be blue-shifted and the corresponding intensity should be much stronger than in lysosome. To verify our presumption, MCF-7 cells were treated with oleinic acid (OA) to induce the formation of LDs intracellularly. As expected, in addition to the original green fluorescence signal, newly appeared and strong blue fluorescence signal in the range of 420–460 nm was detected in the OA-treated MCF-7 cells. More importantly, the blue signal was relatively dominant and the resulting merged images were blue emissive as clearly shown in Fig. 3B and Fig. S14B (ESI†). As shown in Fig. S16 (ESI†), a much better colocalization of 1 with BODIPY, a commercial lipid droplet dye, was observed, further verifying that probe 1 was dramatically enriched in the hydrophobic lipid droplets and thus signals their much lower polarity. Therefore, the above noticeable difference in the fluorescence signals observed before and after OA treatment explicitly demonstrated a discrimination of the cellular environmental polarity. This made probe 1 highly suitable for direct and selective lipid droplet visualization in biological systems. Furthermore, given that two-photon fluorescence microscopy in tissue imaging exhibited remarkable advantages over traditional fluorescence techniques such as deep penetration, high 3D resolution, and in situ visualization with simple operation.^42,43 We further applied compound 1 for two-photon imaging of the excised mesenteric adipose tissues of nude mouse (Fig. 3C and Fig. S17, ESI†). As shown in Fig. 3C, the reconstructed 3D two-photon microscopic images of the ex vivo tissues displayed similar noticeable and dominant blue fluorescence signal to that observed in MCF-7 cells, indicative of the excellent lipid droplet imaging property of probe 1 in tissues.

Fig. 3 (A and B) Two-photon microscopic images of MCF-7 cells treated without (A) or with (B) oleic acid (OA) and then stained with compound 1 for 5 h. (C) Reconstructed 3D two-photon microscopic images of the excised mesenteric adipose tissues of nude mouse stained with compound 1 for 30 min. Two-photon excitation wavelength: 780 nm. The two-photon fluorescence emitted from 1 was captured through two filters: 420–460 nm (blue, A1–C1) and 495–540 nm (green, A2–C2). (A3–C3) Merged bright field images of the blue and green channels. Scale bar: 20 μm.

In general, the luminescence of organic compounds in a solution state is quenched with the increase of temperature. And most of the TICT systems also have the same quenching problem at high temperature.³¹ However, it is intriguing that our present system can achieve a continuously enhanced emission by increasing temperature. From another perspective, this temperature effect is also powerful evidence to reinforce that the TICT process is really involved. Typically, in THF solvent with a moderate polarity, two discernable bands centered at 556 and 596 nm, respectively, could be observed. As demonstrated in Fig. S18 (ESI†), increasing the temperature from 29 °C to 47 °C led to a change of their relative intensities. Additionally, there is an excellent linearity between their intensity ratio (λ₅₅₆/λ₅₉₆) and the temperature in the range from 29 °C to 47 °C, including a vital physiological temperature range, suggesting that the present ratiometric system may be useful for the quantitative determination of temperature.⁴⁴ More interestingly, a noticeable luminescence color transition from dark orange to bright yellow was accompanied by the above temperature increase process. Therefore, our system could realize visual and ratiometric temperature detection. It has been recognized that solvent polarity is heavily temperature-dependent.³⁸ In weak polar toluene, similar intensity-intensified and blue-shifted tendency could be observed with the increase of temperature, while these variations were comparatively more conspicuous and were accompanied by striking color changes in relatively higher polarized solvents, i.e. dichloroethane and o-dichlorobenzene, as shown in Fig. S19 (ESI†).

In another aspect, thermochromic solutions of compound 1 also allow a simple quantitative determination of the temperature dependence, thus possessing great potential to be used as a luminescent thermometer. In order to test its response range and facilitate its application, we have selected tetraethylene glycol dimethyl ether (TRIEDM) with a very high boiling point of 275 °C and ideal stability as the solvent. As shown in Fig. 4, its emission band gradually blue shifted and the corresponding intensity was continuously enhanced with the increase of temperature. What is exciting is that its luminescence color also exhibited obvious conversion from orange to bright yellow-green. Due to the limitations of the temperature control device, we only detected up to 175 °C, but it is reasonable to predict that the above change tendency will be continuous if the temperature condition permits and its solution colors probably cover the whole visible region as shown in the CIE diagram in Fig. 4C. Moreover, there is also a good linear relationship between the intensity ratio (λ₅₃₅/λ₆₀₀) and the temperature in a wide range of 25 °C to 175 °C. It should be mentioned that the above temperature-dependent emission evolution and color conversion are completely reversible. Such intriguing properties inspired us to fabricate a simple liquid thermometer by utilizing the above TRIEDM solution system. As illustrated in Fig. 4D, when we heated the above solution from the top and synchronously cooled it from the bottom, an apparent color change pattern, corresponding to a specific temperature gradient, could be directly observed by the naked eye. This is only a very simple trial but it is reasonable to anticipate that the above color switching should be much more prominent if higher temperature can be achieved. In light of the above excellent properties, our system should be a promising candidate for high performance thermometers with a wide detection range and a high upper limit.

Fig. 4 (A) Temperature-dependent fluorescence spectra and (B) plot of the corresponding intensity ratio (λ₅₃₅/λ₆₀₀) of 1 in tetraethylene glycol dimethyl ether with temperature (1 × 10⁻⁵ M, λ_ex = 390 nm, R² = 0.993). (C) CIE chromaticity diagram showing the temperature dependence of the (x, y) color coordinates of 1. (D) The gradient fluorescence of 1 solution in a quartz tube.

We were then interested in unveiling the underlying mechanism of the above polarity-dependent and thermochromic PL behaviors, which was envisaged to provide an initial guideline for the design of new polarity and temperature responsive materials. Taking all the experimental data together, we proposed that increasing temperature will lead to the increase of solvent hydrophobicity, thus benefiting the planar geometry and resulting in intensified and blue-shifted luminescence.^22,45 Given that temperature switching is considered to induce dynamic change in molecular conformation, we then attempted to obtain persuasive evidence from theoretical calculations. Considering that there are multiple flexible aryl rotors of 1, we performed many attempts on various conformations of the excited state S1. Ultimately, we found that the central plane formed by atoms C16, N2 and C25 was the pivot to determine the flexible conformation, and is also the critical position to link the charge separated donor and acceptor units. When excited to S1, the molecule rotated around the N2–C25 bond, and the associated dihedral angle C16–N2–C25–C30 varied from ∼30° to ∼90° to demonstrate a more twisted conformation as shown in Fig. 5A. Combined with the most populated transitions of molecular orbitals, the twisting degree of molecular conformation could be well elucidated by the variation of the central dihedral angle of C16–N2–C25–C30. In order to clarify its influence on the PL, we calculated the PL spectra of eight conformations with different twisting angles of C16–N2–C25–C30 (computational details see the ESI†). As shown in Fig. 5B, a gradual red shift and attenuation of PL were observed when the dihedral angle varied from 0° to 90°. These results revealed that a more planar conformation greatly facilitated the fluorescence emission. The potential energies of these eight conformations in the excited states were also calculated. The potential energy of the planar conformation with 0° is about 14 kcal mol⁻¹ higher than that of the twisted conformation (dihedral angle 90°, Fig. 5C). The results from theoretical calculations together with those from the polarity-dependent and thermochromic studies reinforced our previous envision that both the polarity and temperature strongly influenced the dynamic equilibrium between the planar and the twisted conformations. And the stability of the planar geometry was enhanced by lowering the solvent polarity and raising the temperature.

Fig. 5 (A) Molecular structures of the ground state and the excited state of 1 based on TDDFT calculations at the B3LYP/6-31G(d) level. (B) Simulated fluorescence spectra and (C) calculated potential energies of different conformations in the excited state of 1.

Excitingly, this compound was also sensitive to external shear force stimulus and exhibited attractive tribochromic PL behavior (Fig. 6). We found that compound 1 showed red-shifted and remarkably enhanced emission when its powder was ground. Upon grinding, its emission color changed from blue to yellow-green. The PL maximum shifted from 480 nm to 509 nm, and the corresponding quantum yield also dramatically increased up to 86% from the original 52% (Fig. 6A and C). It's also appealing that the color transition after grinding could be directly observed by the naked eye in daylight (Fig. S20, ESI†). When the ground powder was exposed to dichloromethane (DCM) vapor, the original blue state could be restored completely, indicative of a reversible tribo-responsive process. Given its excellent tribochromic PL behavior, we further explored the practical application in rewritable paper. By immersing the filter paper into the DCM solution of 1 and then drying by a blower, we prepared a blue emissive rewritable paper, on which we can write any legible and yellow-green letters such as “AIE” with a sharp rod. When exposing the filter paper to a DCM atmosphere for a few minutes, the written letters can be easily erased. And the above writing-erasing process could be repeated many times. Accordingly, compound 1 could potentially be used in security inks and papers.

Fig. 6 (A) PL spectra of 1 before grinding, after grinding and after treatment with dichloromethane vapor. λ_ex = 390 nm. (B) Fluorescent photographs of powders of 1 taken under irradiation with 365 nm UV light. (C) Writing and erasing of letters “AIE” on the filter paper using 1 taken under UV light. (D) Fluorescent photos of powder 1 taken under different pressures. (E) Fluorescence spectra of powder 1 during compression and (F) decompression via DAC (diamond anvil cell). Excitation wavelength was 365 nm.

To get an insight into the mechanism of tribochromic PL, powder X-ray diffraction (pXRD) measurement of 1 was performed. As demonstrated in Fig. S21 (ESI†), the pristine powder of 1 exhibited sharp and intense diffraction peaks and the present pattern is well consistent with that simulated from the single crystal X-ray data, suggesting a well-defined crystalline state. However, relatively weak reflections were only observed after grinding, indicative of significant destruction of the crystalline state by mechanical forces. In this state, amorphous species should be predominantly produced. Upon fuming by DCM vapor, the original sharp signals were restored, revealing the recovery of the crystalline state. Therefore, the observed tribochromic PL of 1 should be due to a reversible morphological transformation between the blue crystalline state and the amorphous yellow-green state.⁴⁶

Given that compound 1 can respond to anisotropic shearing force, we also explored its responsive behavior to isotropic hydrostatic pressure. As shown in Fig. 6D, compound 1 showed remarkable and continuous three-color variation from blue to yellow and then to orange with increasing the in situ pressure. Concomitantly, the PL spectrum exhibited a gradual red shift from 481 nm to 588 nm (Fig. 6E), accompanied by a weaker intensity. Such observations are noticeably different from those of the ground state. However, once the pressure was released, the PL spectrum gradually returned to the initial state (Fig. 6F and Fig. S22, ESI†). To probe the structural change during the above piezochromic PL process, an in situ high-pressure Raman experiment was performed.^47,48 As demonstrated in Fig. S23 (ESI†), all the Raman peaks displayed blue-shift when increasing the external pressure, which was presumably attributed to the simultaneously decreased bond length and intermolecular distance at higher pressure. Combined with the DFT calculation results (Table S5, ESI†), the peaks at 709, 723, and 740 cm⁻¹ were attributed to the C–H bond off-plane wagging vibrations, and their respective intensity gradually decreased with increasing pressure. The peaks ascribed to breathing vibrations of a benzene ring at 997 cm⁻¹ (P1, P2 and P4) and 1002 cm⁻¹ (P3 and P5) gradually blue shifted and fused into one single peak at high pressure. Accordingly, it's reasonable to deem that the intermolecular interactions were likely to be enhanced at increased hydrostatic pressure and molecules were squeezed together compactly during the compression process.⁴⁹ Once the pressure was released, it can return to the initial state.

The above observations raise a question: why does the mechanical grinding and hydrostatic pressure trigger distinct luminescence alteration? The analysis of the crystal structure was expected to provide some clues for the answers. As shown in Fig. 7A, multiple weak intermolecular C–H⋯π interactions (distances ranging from 3.06 to 3.43 Å) were formed between adjacent molecules, which played a vital role in fixing the orientation of the diamine donor and the triarylboron acceptor. Additionally, the intermolecular amine donor and the boron acceptor units were close to each other (Fig. 7B) to cause intermolecular charge transfer. In the crystal lattice, the molecules form ordered but very loose arrangements (Fig. S24, ESI†). Accordingly, regarding the mechanism of tribochromic PL, we presumed that these weak intermolecular interactions and the intermolecular charge transfer processes as well as conformational planarization will be perturbed by the anisotropic stimulus of mechanical grinding to result in red-shifted and remarkably enhanced emission (Fig. 7C). However, the situation is presumably different for the piezochromic PL process. The isotropic high pressure is strong enough to squeeze the adjacent molecules close together to give rise to strong intermolecular π–π interactions and intermolecular charge transfer processes to lead to red-shifted and annihilated emission (Fig. 7C).^50–52 The above analyses further imply the significant role of molecular packing in controlling the solid-state luminescence.

Fig. 7 (A) Intermolecular interactions of 1 (H atoms except those involved in interactions are omitted for clarity, distance unit: Å). (B) Possible intermolecular charge transfer interactions of 1 (D: donor, A: acceptor). (C) Illustration of the proposed tribochromic PL and piezochromic PL mechanism of 1 (CT: charge transfer).

Conclusions

In this work, we designed and synthesized a twisted AIE-active small molecule containing TAB and amine units with response to multiple external stimuli. It was simply demonstrated that this compound was sensitive to external electric stimuli and displayed three-color switched electrochromism and on-to-off electroluminochromism. And it could serve as an emitter for the fabrication of efficient non-doped OLEDs. Due to its remarkable polarity-dependent behavior accompanied by obvious PL change from blue to red and AIE properties, it could be utilized as a fluorescent probe for detecting environmental polarity in cells and allowed selective visualization of LDs in live tissues. Additionally, the new molecule showed unique thermochromic properties due to its ICT character. It exhibited ratiometric luminescence signal change and noticeable fluorescence color switching at different temperatures. A wide detection range of temperature and a high upper limit were realized. Another equally fascinating behavior was that it could respond to anisotropic shearing force and isotropic high pressure with remarkable but contrast luminescence conversion. XRD results demonstrated that the tribochromic PL involved a reversible morphological transformation between the crystalline state and the amorphous state. Crystal structure analysis combined with in situ Raman spectroscopy further indicated that the emission enhancement induced by grinding was attributed to the perturbation of weak intermolecular interactions, charge transfer processes and conformational planarization, while intermolecular π–π interactions and intermolecular charge transfer processes jointly led to the emission annihilation at high pressure. Therefore, the present system featuring multi-stimuli responsive properties has potential for multifarious real-life applications, including electroswitchable electrochromic and electroluminochromic materials for information recording, storage devices and OLEDs, probing the environmental polarity in biological systems and selective visualization of lipid droplets in live tissues, a wide-range liquid thermometer, security inks and papers. The results presented here are anticipated to provide efficient guidelines for the design of multifunctional AIE-active molecules.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (21788102 and 21472059), the Research Grants Council of Hong Kong (16305618, C6009-17G, and A-HKUST605/16), the Innovation and Technology Commission (ITC-CNERC14SC01 and ITCPD/17-9), the University Grants Committee of Hong Kong (AIE/P-03/08) and the Science and Technology Plan of Shenzhen (JCYJ20160229205601482 and JCYJ20170818113602462).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1884262. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9mh01041f

‡ These two authors contributed equally to this work.

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