Fluorescence Turn-On Visualization of Microscopic Processes for Self

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Fluorescence Turn-on Visualization of Microscopic Processes for Self-healing Gels by AIEgens and Anti-counterfeiting Application Jiangman Sun, Jianguo Wang, Ming Chen, Xiong Pu, Guan Wang, Lin Li, Guanyu Chen, Yuanjing Cai, Xinggui Gu, and Ben Zhong Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01611 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Chemistry of Materials

Fluorescence Turn-on Visualization of Microscopic Processes for Selfhealing Gels by AIEgens and Anti-counterfeiting Application Jiangman Sun,† Jianguo Wang,‡ Ming Chen,‡ Xiong Pu,‖ Guan Wang,† Lin Li,† Guanyu Chen,† Yuanjing Cai,† Xinggui Gu,*,† and Ben Zhong Tang*,‡,§ †Beijing

Advanced Innovation Center for Soft Matter Science and Engineering, College of Materials Science and Engineering, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China. ‡Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, and Institute for Advanced Study, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. §Center for Aggregation-Induced Emission, SCUT-HKUST Joint Research Institute, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. ‖CAS

Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China.

ABSTRACT: Self-healing gels are emerging as promising materials for various human-interfaced applications. Unveiling microscopic processes for self-healing gels is crucial for not only understanding self-healing mechanisms but also guiding materials design, which is, however, hardly achieved due to the lack of direct and effective approaches. Herein, covalent bond-induced emission of AIEgen is, for the first time, utilized for establishing a fluorescence turn-on strategy to visualize microscope processes for self-healing gels. Such strategy allows the in-situ monitoring of gelation and self-healing processes with extremely low background. Besides, the fluorescence also endows self-healing gels with diverse emission, especially white light emission. By coding these multi-color fluorescent self-healing gels, fluorescent codes with stealth effect are successfully fabricated with features of stretchability, wearability, reusability and diversity, performing superiorly in anti-counterfeiting. These results give rise to a deep insight into self-healing mechanism and will potentially boost the development of new multifunctional self-healing gels.

INTRODUCTION Self-healing is originated from nature and has been generally considered as a promising concept for designing highperformance materials, because it reforms the traditional materials with the capability of spontaneously restoring their functionalities and structures after damage.1,2 In the past decades, owning to the development of molecular design and synthetic technology, various self-healing materials, possessing novel optical, electrical, magnetical or thermal functionalities, have been developed extensively.3-7 Among them, emerging self-healing gels, constructed by dynamic covalent or non-covalent chemistry, have aroused growing interest and demonstrated many high-tech applications in industry and biomedical fields.8-11 Nevertheless, it remains to be a major challenge to experimentally elucidate the microscopic healing dynamics in situ with high spatial and temporal resolution, which is not only crucial to understand self-healing mechanisms but could also aid in the rational design of new self-healing gels.12-14 To date, some approaches, including laser speckle imaging (LSI),14 scanning electron microscopy (SEM),15,16 atomic force microscopy (AFM)17 and scanning electrochemical microscopy (SECM),18 have been extended to study the self-healing processes. However, their own intrinsic limitations such as high vacuum, timeconsuming operation, invasive interference, and special

requirements have also seriously retarded the exploitation of their potential in real-timely monitoring the microscopic processes.8 Therefore, more direct and effective approaches are urgently demanded to in situ and noninvasively monitor the microscopic dynamics of self-healing gels. Recent years have witnessed the unique advantages of fluorescence techniques compared to conventional methods, such as high sensitivity, large contrast and fast response.19,20 These superiorities have enabled the fluorescence techniques to realize the real-time visualization of microscopic processes in situ for both materials science21-23 and biomedicine.24 The effectiveness of the fluorescence techniques is strongly determined by the performance of fluorophores. However, conventional fluorophores usually suffer from the notorious aggregation-caused quenching effect (ACQ),25,26 seriously limiting their service in fluorescence techniques. While luminogens with aggregation-induced emission feature (AIEgens) have exhibited excellent emission properties to overcome the ACQ effect intrinsically, undergoing the mechanism of the restriction of intramolecular motions (RIM).27,28 It thus greatly promotes the applications of fluorophores in various areas including optoelectronics,29-32 bioimaging,33-35 display36-38 and so on. Recently, the development of AIEgens for monitoring microscopic processes has become a research subject intensively studied,

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driven by their outstanding performance of superior photostability, low background and unique fluorescence turnon manner in contrast to conventional counterparts.39-44 Therefore, we envision that the marriage of AIE characteristic and self-healing gels would provide a promising fluorescence turn-on method for in situ investigation of microscopic processes of self-healing gels. Meanwhile, the emission of fluorophores will endow self-healing gels with multifunctionalities for practical applications.

Figure 1. (a) Design rationale for visualization of self-healing PDMS gel based on AIEgen. The reversible imine bonds formed in the Schiff base reaction between bis(amino)-terminated poly(dimethylsiloxane)(NH2-PDMS-NH2) and AIEgen-4CHO restrict the intramolecular motions of AIEgen, resulting in the fluorescence turn on during gelation. (b) 1H NMR of TPE-4CHO in the absence and presence of NH2-PDMS-NH2 in CD2Cl2 for 5 h.

Herein, we put forward a simple but effective strategy to construct a fluorescence turn-on approach for visualizing the gelation and self-healing processes of self-healing gels, through the covalent bond-induced emission of AIEgens in self-healing gels network. As a concept of proof, well-known self-healing PDMS gels based on the covalent dynamic imine bonds are used as shown in Figure 1a.15 By incorporation of AIEgen-4CHO (such as TPE-CHO, Figure 1b) into the gelation, the fluorescence of AIEgen-4CHO would be turned on because of the effective restriction of intramolecular motions by imine bonds to inhibit the non-radiative relaxation (Figure 1a). Thus, the gelation process could be directly monitored by the gradually enhanced fluorescence signal under photoluminescence spectrometer (PL). Moreover, the self-healing process including both diffusion and breakingreformation of imine bond could be also tracked in situ by this strategy for self-healing PDMS gel, with the healing efficiency evaluated quantitatively. Besides, the employment of blue-, green- and red-emission fluorophores to the self-healing PDMS gels, multicolor and white light fluorescent self-healing gels were successfully prepared with the features of transparence, flexibility and stretch. Further, fluorescent codes with stealth effect were creatively realized for anti-

counterfeiting by combination of fluorescence and self-healing properties. Different from most of previous reports,21-23 AIEgens with reactive sites could only be induced to be emissive when the covalent dynamic bonds are formed, thus provides deeper insight into the self-healing mechanism for self-healing gels, and the introduction of fluorescence will promote the development of multifunctional self-healing gels. RESULTS AND DISCUSSION Three aldehyde-containing fluorophores TPP-4CHO, TPE4CHO45 and BT-TPE-4CHO were synthesized as depicted in Schemes S1-S3, and the corresponding characterizations for the intermediates and products were shown in Figures S1-S4. Then, their photo-physical properties in THF solution and aggregation were carefully investigated. TPE-4CHO absorbed at 330 nm in THF solution (Figure S5a) and emitted gradually at 525 nm upon aggregation in THF/H2O mixture when increasing the fraction of poor solvent H2O (fw) from 0 to 90% (Figure S5b and S5c), which was also observed by the inset fluorescence photos with turn-on green emission after aggregation in Figure S5b, reflecting a typical AIE characteristic.28 For the solution of TPP-4CHO and BT-TPE4CHO, their absorption maximums were at 359 and 510 nm, respectively (Figure. S6a and S6d), and their corresponding emission was at 465 and 619 nm (Figure S6b and S6e). During their aggregation in THF/H2O mixture, the emission intensity of TPP-4CHO increased under the fw of 60% but decreased over 60% (Figure S6c). For BT-TPE-4CHO (Figure S6f), the initial red emission was very strong while it decreased and red-shifted gradually upon the aggregation, which might be attributed to the emission-detrimental π-π stacking originating from the strong π-π stacking ability of benzothiadiazole (BT) unite.26 Such feature of emission could be assigned to the ACQ effect. Then, AIE-active TPE-4CHO was used to fabricate AIEgencontaining self-healing PDMS gel. As previously reported,15,46 imine bond could be formed under ambient condition between aldehyde and amine, and has been adopted as dynamic covalent chemistry for the preparation of self-healing gels.6 Thus, TPE-4CHO-containing self-healing PDMS gel was constructed by the intensive mixture of bis(amino)-terminated poly(dimethylsiloxane) (NH2-PDMS-NH2, Mn: 5000-7000), 1,3,5-triformylbenzene (BTA, 2.1%, m/m) and TPE-4CHO (0.002%, m/m) in dimethylformamide (DMF) solution. After several hours, the mixture was converted from sol to gel (Figure S7). Subsequently, the fresh gel was dried at room temperature for 2 days and then heated at 70 oC for 5 h to remove the residue of DMF. Within the self-healing gel, the PDMS chains were speculated to be cross-linked by the formation of the dynamic imine bonds via the reversible condensation of reactive amine and aldehyde of NH2-PDMSNH2, BTA and TPE-4CHO. The imine bond between NH2PDMS-NH2 and BTA has been reported previously15,46. To verify the Schiff base reaction between NH2-PDMS-NH2 and TPE-4CHO, 1H NMR titration was carried out in Figure 1b. The chemical shift of 9.87 ppm assigned to the signal of proton (labeled with Ha) at aldehyde of TPE-4CHO was disappeared while a new signal at 8.17 ppm (labeled with Hb) was emerged when excess NH2-PDMS-NH2 was added into the CD2Cl2 solution of TPE-4CHO for about 5h, suggesting the formation of imine bond. In addition, the chemical shifts for the part of PDMS chain remained the same before and after reaction (Figure S8). These results evidently supported the


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Chemistry of Materials formation of dynamic imine bond between TPE-4CHO and NH2-PDMS-NH2 in the self-healing PDMS gel. To demonstrate the potential of AIEgen as a probe to track the formation of self-healing gels, the fluorescence of TPE4CHO during the whole gelation process was monitored closely. As the fluorescence photos displayed in Figure 2a (Up), the precursor mixture of NH2-PDMS-NH2, BTA, and TPE-4CHO in DMF solution showed almost no emission. AIE-active TPE-4CHO was not induced to be emissive due to the activation of non-radiative relaxation to consume the excited-state energy by intramolecular motions, which is attributed to its good solubility in DMF solution supported by the negligible hydrodynamic diameter from dynamic light scattering (DLS) (Figure S9). Then the green emission of TPE-4CHO emerged and was enhanced gradually under 365 nm UV radiation as the time of gelation progresses, in according with the mechanism of RIM28 that the imine bonds restricted the intramolecular motions and inhibited the nonradiative relaxation of TPE-4CHO effectively. The PL spectra of fluorescence variation during the formation of gel were recorded continuously in Figure 2b. At the beginning, the fluorescence signal of the mixture was negligibly detectable. Subsequently, the fluorescence intensity was enhanced gradually during the gelation. The tendency of the enhancement of fluorescence intensity against the gelation time was further depicted in Figure 2d. In the initial 10 min., the fluorescence intensity increased quickly and then its growth speed overall appeared to be slowing. After 210 min, the fluorescence intensity was up to a steady platform, implying the complete formation of self-healing PDMS gel. The gelation process of self-healing PDMS gel was precisely reflected in high contrast under the assistance of TPE-4CHO. Without TPE-4CHO, the PL spectra of the precursor mixture of NH2-PDMS-NH2 and BTA were also recorded in Figure S10a. There was almost no emission observed during the whole gelation, leading to the failure of fluorescent visualization of gelation process. Additionally, non-AIE-active BT-TPE-4CHO was also utilized to probe the gelation process of self-healing PDMS gel. As shown in Figure 2a (Down), the red emission of BT-TPE-4CHO was always observed before and after the formation of gel with extremely low contrast. The PL spectra suggested the initial fluorescence of BT-TPE4CHO was very strong (Figure 2c) and the enhancement of fluorescence intensity was much less than that of TPE-4CHO due to the lack of the assistance of AIE characteristic (Figure 2d). Such largely impeded the monitoring of the gelation process. Thus, extremely low background for AIE-active TPE4CHO stemmed from the fluorescence turn on exhibited the unique superiority in visualizing the gelation process.

Figure 2. (a) The fluorescence photos under 365 nm UV lamp of the gelation processes for self-healing PDMS gels with TPE4CHO (Up) or BT-TPE-4CHO (Down), and (b) and (c) their corresponding PL spectra. (d) Plots of the PL intensity against the reaction time for self-healing PDMS gels with TPE-4CHO and BT-TPE-4CHO, respectively. (e) PL spectra of the self-healing PDMS gel with TPE-4CHO after the addition of acetic acid for different period of time. Inset photos show the state and emission of self-healing PDMS gel before and after adding acetic acid.

To get insight into the gelation process that the imine bonds play the key role to light up the emission of TPE-4CHO, another aldehyde-free AIE-featured TPE-4OCH3 without the ability to form imine bonds was introduced into the precursor mixture of NH2-PDMS-NH2 and BTA. As shown in Figure S10b, there was almost no emission even after the complete formation of gel, because the micro-environment within gel was not rigid enough to restrict the intramolecular motions of AIE-featured TPE-4OCH3 in the absence of covalent bonds. Further, the acetic acid was also utilized to destroy the imine bonds.46 As shown in Figure 2e, the fluorescence intensity decreased quickly after the addition of acetic acid and nearly disappeared at 120 min., associating with the degradation of the gel. This was also confirmed by naked eye in the inset photos. Consequently, the synergistic effect of both the AIE characteristic of TPE-4CHO and the dynamic imine bonds between NH2-PDMS-NH2 and TPE-4CHO facilitated the establishment of the fluorescence turn-on manner for monitoring the gelation process of self-healing gel in situ. This success powerfully demonstrated the promising of AIEgens in non-invasively and in situ probing the formation of selfhealing materials of interest.



Figure 3. (a) Bright-field and fluorescent images of self-healing process at the crack formed in TPE-4CHO-containing self-healing PDMS gel. Scale bar: 10 μm. (b) The uniaxial tensile stress-strain proﬁles of initial (TPE-4CHO) and healable TPE-4CHOcontaining PDMS self-healing gels (TPE-4CHO-after healing). (c) PL spectra at the crack (marked with red-color “+”) of the selfhealing PDMS gel. Inset plots of the healing efficiency against healing time according to the fluorescence change at the crack. Taking the PL intensity of 4.5 h (I4.5 h) as the completed healing, the healing efficiency was defined as the value of I/I4.5 h.

Based on the unique covalent dynamic bond-induced fluorescence turn-on behavior of TPE-4CHO, the monitoring of self-healing processes and the assessment of healing efficiency at the crack in the self-healing PDMS gel were also conducted byusing a confocal lasing scanning microscopy (CLSM). As suggested in Figure 3a, the self-healing gel was cut off with a deep and clear crack under bright-field and fluorescent windows of CLSM. Then the boundary of the crack became indistinct where the green fluorescence was gradually enhanced from 0 to 4.5 h accompanying with the disappearance of crack. It was mainly ascribed to the zipperlike repair processes involving the fast adhesion of the cut by diffusion and the slow breaking-reformation reaction of dynamic imine bonds between amine and aldehyde groups, which has been proposed by the previous reports.6 After healing for 4.5 h, the wound was cured thoroughly and the mechanical properties were completely recovered as shown in Figure 3b. The uniaxial tensile stress-strain proﬁles of the healable self-healing PDMS gel after cutting were nearly recovered to the initial state. As the fluorescence of TPE-4CHO could only be turned on upon reacting with NH2-PDMS-NH2 in the gel, the variation of fluorescence intensity could be allowed to unveil the microscopic self-healing processes of both diffusion of cut edge and formation of dynamic imine bonds in situ. The PL spectra at the crack marked with red-color “+” were measured continuously (Figure 3c), revealing gradual enhancement against healing time. Therefore, fluorescence technique could

be used to evaluate the healing efficiency in this self-healing PDMS gels, which is defined as the ratio of fluorescence intensity during and after healing. The ratio of I/I4.5 h at 525 nm against healing time was further studied to evaluate the selfhealing process in the inset of Figure 3c, with I and I4.5 h as the corresponding fluorescence intensities during and after healing. The healing efficiency could be assessed precisely, exhibiting the advantages of in-situ and real-time recording, convenience, high sensitivity and accuracy, and fast response compared to the tensile test (Figure S11).14 Interestingly, such profile was well in according with the theoretical equation of polymer diffusion,47,48 thus experimentally revealing the diffusion is the essential stage before the breaking-reformation reaction during the repair of damages.48 In addition, it is worth mentioning that fluorophores such as rhodamine and fluorescein could only be used to track the diffusion during the self-healing processes, due to the incompetent monitoring the important breaking-reformation reaction by the simple mixture with self-healing gels.22,23 However, the fluorescence turn-on emission of AIEgen allows to monitor the formation of dynamic imine bonds which are the key role in the self-healing mechanism, thus providing deeper insight into the self-healing mechanism in self-healing process. Besides the monitoring of self-healing processes by the fluorescence turn-on property of AIEgens, the introduction of fluorophores into the self-healing gels could also endow them with unique functionalities of emission. For instance, the fluorescent self-healing gels with different emission colors were obtained by incorporation of TPP-4CHO, TPE-4CHO and BT-TPE-4CHO covalently. They emitted blue, green and red colors with the peaks at 443 nm, 525 nm, 601 nm, respectively (Figure 4a). Then, a multi-color and stretchable belt could be prepared as desired through the autonomous healing of the self-healingPDMS gels containing TPP-4CHO, TPE-4CHO and BT-TPE-4CHO, respectively (Figure 4b). The imine bonds within the self-healing gels containing TPP4CHO and BT-TPE-4CHO were also checked by NMR in Figure S12. Such imine bonds between NH2-PDMS-NH2 and BT-TPE-4CHO made BT-TPE-4CHO as a network node in gel, effectively blocking the aggregation of BT-TPE-4CHO and avoiding the red-emission-detrimental ACQ effect. Further, the precursor mixture of TPP-4CHO, TPE-4CHO and BT-TPE-4CHO was added into the NH2-PDMS-NH2 matrices to construct the emission-tunable self-healing PDMS gels by easily adjusting their molar ratios (Figure S13). Remarkably, the white light self-healing PDMS gel was successfully achieved when the molar ratio was 1 : 4 : 12 (Inset of the white light film and its PL spectrum in Figure 4c). The white light emission was carefully characterized by the Commission International de L’Eclairage (CIE) chromaticity coordinates (Inset of CIE curve in Figure 4c), and the value of (0.34, 0.34) suggested the pure white color defined by CIE in 1931.49 Notably, these three fluorophores in the white light selfhealing PDMS gel were reasonably speculated to be crosslinked to the NH2-PDMS-NH2 to form network by the dynamic imine bonds not the simple physical mixture (Figure 4c).50,51


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Figure 4. (a) Normalized PL spectra of self-healing PDMS gels with different fluorophores of TPP-4CHO (blue), TPE-4CHO (green), and BT-TPE-4CHO (red), respectively. Insets show the corresponding fluorescence photos. (b) Fluorescent photos show the preparation of stretchable multicolor belt by the healing of blue-, green- and red-emission self-healing PDMS gels under UV light. (c) (Left) PL spectrum of the white light film containing TPP-4CHO, TPE-4CHO, and BT-TPE-4CHO with the molar ratio of 1 : 4 : 12. Insets show the Commission Internationale de L’Eclairage (CIE) 1931 coordinates and the white light film under UV light. (Right) The schematic illustration of the existence form of fluorophores in the white light film. (d) The transmittance spectrum for the blank, TPP-4CHO, TPE4CHO, BT-TPE-4CHO and white light films. Insets show these transparent films.

The mechanical properties of these fluorescent self-healing PDMS gels were also investigated (Figure S14). These fluorescent self-healing PDMS gels were highly stretchable. At an uniaxial tensile speed of 30 mm min-1, the ultimate strain to fracture was ranged around 300% for all these fluorescent self-healing PDMS gels (marked as Blank, TPP4CHO, TPE-4CHO, and BT-TPE-4CHO, Figure S14). The self-healing properties were also evaluated by tensile tests of these fluorescent self-healing PDMS gels undergoing an ambient healing process (about 21 °C) for about 5 h after being cut off with a razor blade. For all healable gels (marked as Blank-healing, TPP-4CHO-healing, TPE-4CHO-healing, and BT-TPE-4CHO-healing, Figure S14), the ultimate stress recovered back to about 100% of the pristine value. For the white light self-healing PDMS gel, the stress was also recovered (Figure S14). In addition, all these fluorescent selfhealing PDMS gels were transparent without obvious color. As indicated in Figure 4d, the 400 μm-thick self-healing PDMS gels exhibited a transmittance of almost 100% in the range of visible light, which was also confirmed by the inset digital photos. As a consequence, these fluorescent selfhealing PDMS gels possessed the advantages of stretch, selfhealing, high transparence and tunable emission, which would facilitate promising applications in the high-tech areas of anticounterfeiting, information security and storage technology. To this end, the potential applications of these fluorescent self-healing PDMS gels for anti-counterfeiting were demonstrated in Figure 5. Inspired by jigsaw puzzle, fluorescent 2D codes were designed as illustrated in Figure 5a. Firstly, the fluorescent self-healing PDMS gels were cut into small square pieces with different emission. Then these transparent and colorful square pieces were orderly and

closely arranged like coding to autonomously heal to generate a single pattern with special information stored. Such information could only be read out under UV light irradiation while no information was divulged under daylight. Based on this schematic, a fluorescent 2D code was successfully obtained by using theblue, green, red and white light fluorescent self-healing PDMS gels (Figure 5b). Under daylight, the information was not recognized by the freely available application software of “COLORCODE” downloaded onto a phone,52,53 because the transparence of the code film makes the pattern inside invisible. While a 365 nm UV lamp was used to irradiate, the fluorescent pattern was revealed out clearly and recognized by the software of “COLORCODE”, resulting in the read of the invisible information (Figure 5b, Movie S1). This evidently demonstrated that this fluorescent 2D code based on selfhealing gels would offer a promising candidate for anticounterfeiting technology. Furthermore, this fluorescent 2D code based on the self-healing PDMS gels was soft, it thus could be used as skin-like materials for wearable information storage.15 As shown in Figure 5c, this fluorescent 2D code could be attached on skin conformably for the wearable anticounterfeiting application. In addition, this fluorescent 2D code could also be cutback to small square pieces and recoded to prepare another new fluorescent 2D code for the storage of other information (Figure S15, Movie S2) based on the self-healing property. Actually, the self-healing PDMS gels could be tailored into different sharps to meet the various demands. As an example of fluorescent bar code in Figure 5d, it was fabricated by the autonomous healing of those belts with blue, green and red emission according to the unique sequence. Such fluorescent bar code could be applied to



distinguish the true from the false for wine. Compared to traditional materials for anti-counterfeiting,52,54 the fluorescent self-healing gels intrinsically integrated with the properties of fluorescence and self-healing exhibited unique superiorities of stealth, wearability, reusability and diversity, which

demonstrated huge potential in application. It should be noted that the anticounterfeiting is mainly based on fluorescence, not necessarily requiring AIE fluorescence property such as fluorescence on/off.

Figure 5. (a) Schematic illustration of the fabrication procedure for the fluorescent code by self-healing. The demonstration of (b) practical fluorescent 2D code before and after healing, (c) the wearable capability of the soft fluorescent code on skin, and (d) anti-fake fluorescent bar code for wine under daylight and UV light.

CONCLUSION In summary, by covalently introducing AIE-active TPE-4CHO as a probe, the visualization of microscopic processes including gelation and self-healing for self-healing PDMS gel has been achieved successfully. Besides the superiority of AIE property, another key point is that the emission of AIEgens could only be induced upon the formation of dynamic imine bonds not the usual aggregation of molecules in self-healing PDMS gels. The formation of dynamic imine bonds between TPE-4CHO and NH2-PDMS-NH2 was confirmed by 1H NMR. Due to the restriction of the intramolecular motions by such covalent bonds, AIEgen was induced to emit gradually undergoing the mechanism of RIM during the formation of self-healing gels. Thus, the gelation process could be in situ monitored in the fluorescence turn-on manner with extremely low background and high contrast compared to the non-AIEactive fluorophores. Meanwhile, such covalent bond-induced emission of TPE-4CHO can enable to track the microscopic self-healing processes for self-healing PDMS gels. By capturing fluorescent images and recording fluorescence intensity change, both diffusion and breaking-reformation reaction of covalent bonds at the crack were monitored in situ, demonstrating the great potential for the quantitative analysis of self-healing microscopic dynamics and healing efficiency. Unlike the simple observation of the diffusion by mixture of fluorophores in self-healing gels, our strategy would get deeper insight into the breaking-reformation reaction of covalent bonds at the crack. Fluorescence not only facilitates the visualization of microscopic processes but also endows unique emission properties for self-healing gels. The transparent, stretchable, flexible and fluorescent self-healing PDMS gels with blue,

green and red emission were designed by incorporation of TPP-4CHO, TPE-4CHO and BT-TPE-4CHO, respectively. By easily tuning the molar ratios of TPP-4CHO, TPE-4CHO and BT-TPE-4CHO, the white light self-healing PDMS gel could be constructed for potential application of display. Combining the properties of fluorescence and self-healing, fluorescent codes with the stealth information were fabricated by the multi-color fluorescent self-healing PDMS gels. Such colorless and transparent fluorescence pattern of codes was only read out under UV light, demonstrating the promising application in anti-counterfeiting. Collectively, this work allows for the deeper understanding of self-healing mechanism, and also extends the high-tech applications by tailored design of fluorescent self-healing gels.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, synthetic procedures, characterization, preparation of self-healing gels and applications for anticounterfeiting, including Schemes S1-S3 and Figures S1-S14. Movie showing the fluorescent pattern revealed out and recognized by the software of “COLORCODE” (avi).

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions


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Chemistry of Materials All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the National Natural Science Foundation of China Grant (21702016), the grants from the Research Grant Council of Hong Kong (16305518, C600917G and A-HKUST605/16) and the Innovation and Technology Commission (ITC-CNERC149C01 and ITS/254/17). B. Z. Tang acknowledges the financial support from the National Science Foundation of China (21788102). We thank Prof. Z. Gan and Q. Yu for the help of confocal lasing scanning microscope. We also thank the Analytical Instrumentation Center, Peking University, Beijing 100871.

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