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Visualized Degradation of CO 2 -Based Unsaturated Polyesters toward Structure-Controlled and High-Value-Added Fluorophores

2021; Chinese Chemical Society; Volume: 4; Issue: 1 Linguagem: Inglês

10.31635/ccschem.021.202000669

2096-5745

Bo Song, Tianwen Bai, Dongming Liu, Rong Hu, Dan Lu, Anjun Qin, Jun Ling, Ben Zhong Tang,

Luminescence and Fluorescent Materials

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Visualized Degradation of CO2-Based Unsaturated Polyesters toward Structure-Controlled and High-Value-Added Fluorophores Bo Song, Tianwen Bai, Dongming Liu, Rong Hu, Dan Lu, Anjun Qin, Jun Ling and Ben Zhong Tang Bo Song State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 , Tianwen Bai MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 , Dongming Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 , Rong Hu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 , Dan Lu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 , Anjun Qin *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 , Jun Ling *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, SCUT-HKUST Joint Research Institute, AIE Institute, Center for Aggregation-Induced Emission, South China University of Technology (SCUT), Guangzhou 510640 Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, Institute for Advanced Study, and Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong 999077 https://doi.org/10.31635/ccschem.021.202000669 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail In comparing with traditional polymer degradation toward unknown or unvalued products (i.e., downcycling), new methods to degrade polymers into monomers or high-value-added materials (i.e., upcycling) are preferable for sustainable developments. In this work, CO2-based linear and cross-linked unsaturated polyesters, that is, poly(alkynoate)s, were successfully degraded by benzamidine into diols and high-value-added pyrimidone or imidazolone derivatives through controlled reaction conditions. The degradation process could be visualized under UV light via gradually increased luminescent intensity of the reaction mixtures and fluorescence patterns could be generated through microcontact printing of the stamp on the thin films of poly(alkynoate)s. Moreover, the pyrimidone derivatives could be applied in bioimaging and inhibiting bacteria, and the imidazolone derivatives could be used for the detection and recovery of gold(III) ions from electronic waste and serve as the fluorescent sensor to detect in situ-generated amines from food spoilage. Thus, this work presents a visualized and high value product-selective degradation to solve the end-of-life issue of polymers. Download figure Download PowerPoint Introduction Many synthetic polymeric materials are designed and synthesized nowadays, not only serving as plastics, rubbers, and fibers, but also being used for organic light-emitting diodes (OLEDs), organic photovoltaics, biomedical materials, and so forth.1–5 However, these polymeric materials are predominantly based on petroleum resources and designed for superior performance, and the degradability and recyclability are sometimes ignored, which has led to the tremendous growth of polymer waste. Contaminated polymer waste is now ubiquitous on the earth, including freshwater systems, terrestrial habitats, and all major ocean basins, deeply affecting humans and the ecosystem.6–8 Undoubtedly, developing new degradable polymeric materials and new methods for polymer degradation to solve the end-of-life issue of polymers are highly desirable. Degradable polymeric materials have greatly impacted the advancement of many fields, such as modern medicine, sensing, and nanotechnology. For example, a wide range of synthetic degradable polymers have been investigated for biomedical applications, such as tissue engineering and drug delivery.9 Thus, various degradable polymers have appeared over the past few decades with the incorporation of reactive groups, which can be cleaved under appropriate conditions.10,11 However, most of them suffer from one or more of the following issues, such as slow degradation rate, incomplete degradation, unknown, or unvalued degraded products (Figure 1).12,13 In recent years, the Chen, Hong, Lu, Li, Moore, and Hillmyer groups have developed a series of chemically recyclable polymers that could be depolymerized to monomers under certain conditions, respectively.14–23 These works are groundbreaking in solving the end-of-life issue of polymers. However, there are still some issues to be addressed with these newly-developed polymers. For example, the abilities of polymerization and depolymerization are opposite each other. Thus, balancing the polymers' depolymerizability and their properties is difficult. Moreover, the some polymer depolymerization needs catalysts or harsh reaction conditions, such as elevated temperatures up to 300 °C, which might hamper practical applications. Figure 1 | Schematic comparisons between conventional and our new degradation method. Download figure Download PowerPoint Polyester is a large class of synthetic polymers, which is widely used as engineering plastics, textile fibers, adhesives, and so forth. Poly(alkynoate)s are an emerging kind of unsaturated polyester, which contains both ester and ethnynyl groups in the main chains.24–27 They are generated from the renewable monomer CO2, so they could be regarded as a kind of sustainable polymer. Poly(alkynoate)s possess excellent solubility and good thermostability. They can be degraded in strong basic conditions, but it is an incomplete process toward unknown and unvalued products (i.e., downcycling).24 Thus, a new controllable degradation method for poly(alkynoate)s is highly desired. Considering that ethyl phenylpropiolate could react with benzamidine to generate N-heterocyclic compounds and ethanol,28,29 we envisaged using this addition–heterocyclization–cleavage reaction to degrade poly(alkynoate)s in a repurposing process (i.e., upcycling) (Figure 1). Indeed, poly(alkynoate) P 1 could successfully be degraded by adding benzamidine with 100% conversion into diols and high-value-added N-heterocyclic compounds, among which pyrimidone derivatives were obtained at 110 °C (Figure 2, path A) while imidazolone derivatives were obtained at 60 °C (Figure 2, path B) or at room temperature upon addition of the Bu3P catalyst (Figure 2, path C). Thermoset poly(alkynoate) P 2 was also successfully degraded to imidazolone derivatives in the presence of Bu3P catalyst. In situ Fourier transform infrared (FT-IR) spectroscopy and density functional theory (DFT) calculation well revealed the reaction kinetics and mechanism. Notably, based on the emissive properties of the degraded compounds, the degradation process could be visualized under UV light via gradually increased luminescent intensity of the reaction mixtures and also introduced into microcontact printing (μCP) to generate fluorescence patterns. The N-heterocyclic compounds obtained from P 1 possess unique aggregation-induced emission (AIE) characteristics and could be applied in diverse areas. Figure 2 | Product-selective degradation of P1 toward diols and N-heterocyclic compounds. Download figure Download PowerPoint Experimental Methods The procedures for the product selectivity controlled degradation of P 1 are given below. Path A Into a 10 mL tube equipped with a magnetic stirrer P 1 (57.8 mg, 0.1 mmol, Mw = 80 300 g/mol, Mw/Mn = 2.56) and 3 (24.2 mg, 0.2 mmol) were placed. Toluene (1 mL) was then injected into the tube. The resulting mixture was stirred at 110 °C under air for 12 h. Then the reaction mixture was cooled to room temperature. After filtration, the precipitate was washed with ethanol to give the degraded product 4. The filtered solution was concentrated via rotary evaporator to give the degraded product of 1,8-octanediol. Total yield: 92%. 4: Proton nuclear magnetic resonance (1H NMR) [500 MHz, dimethyl sulfoxide (DMSO)-d6] δ [tetramethylsilane (TMS), ppm]: 12.71 (s, 2H), 8.36–7.91 (m, 8H), 7.69–7.42 (m, 6H), 7.34–6.91 (m, 14H), 6.85 (s, 2H). Carbon nuclear magnetic resonance (13C NMR) (125 MHz, DMSO-d6), δ (TMS, ppm): 142.88, 140.82, 134.55, 131.26, 131.22, 130.89, 130.84, 130.81, 128.72, 128.21, 128.06, 127.97, 126.93, 126.74. High-resolution mass spectrometry (HRMS) [matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)] m/z: M+ calcd for C46H32N4O2, 672.2525; found, 672.2528. Path B Into a 10 mL tube equipped with a magnetic stirrer P 1 (57.8 mg, 0.1 mmol, Mw = 80 300 g/mol, Mw/Mn = 2.56) and 3 (24.2 mg, 0.2 mmol) were placed. Toluene (1 mL) was then injected into the tube. The resulting mixture was stirred at 60 °C under air for 12 h. Then the reaction mixture was cooled to room temperature. After filtration, the precipitate was washed with ethanol to give the degraded product 5. The filtered solution was concentrated via rotary evaporator to give the degraded product of 1,8-octanediol. Total yield: 90%. 5: 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 12.07 (s, 2H), 8.24–7.96 (m, 8H), 7.67–7.46 (m, 6H), 7.27–6.95 (m, 14H), 6.92 (s, 2H). 13C NMR (125 MHz, DMSO-d6), δ (TMS, ppm): 171.85, 160.56, 144.81, 142.65, 140.95, 140.33, 132.85, 132.45, 132.41, 131.55, 131.18, 131.16, 130.75, 130.70, 128.92, 128.88, 128.06, 127.87, 127.79, 127.32, 124.28, 124.26. HRMS (MALDI-TOF) m/z: M+ calcd for C46H32N4O2, 672.2525; found, 672.2555. Path C Into a 10 mL tube equipped with a magnetic stirrer P 1 (57.8 mg, 0.1 mmol, Mw = 80 300 g/mol, Mw/Mn = 2.56) and 3 (24.2 mg, 0.2 mmol) and Bu3P (8.1 mg, 0.04 mmol) were placed. Toluene (1 mL) was then injected into the tube. The resulting mixture was stirred at room temperature under air for 12 h. After filtration, the precipitate was washed with ethanol to give the degraded product 5. The filtered solution was concentrated via rotary evaporator and washed with petroleum ether to give the degraded product of 1,8-octanediol. Total yield: 94%. Results and Discussion Reaction condition-oriented degradation of poly(alkynoate)s The poly(alkynoate)s P 1 containing both ester and ethnynyl groups in the main chains was synthesized via the Ag2WO4-catalyzed three component polymerization of CO2, diyne 1, and alkyl dihalide 2 according to our previously reported procedures.24 Its structure was unambiguously confirmed by NMR spectroscopies. To achieve product-selective degradation of poly(alkynoate)s via benzamidine 3 to obtain N-heterocyclic compounds 4 and 5, the model reactions of 3 and 6 carried out at different temperatures were first investigated ( Supporting Information Table S1). Results showed that only a pyrimidone derivative 7 was obtained at 110 °C while an imidazolone derivative 8 was obtained at 60 °C. Interestingly, within these two temperatures, a mixture of 7 and 8 was obtained. When Bu3P was used as the catalyst, only 8 was obtained with no dependence on temperature ( Supporting Information Table S2). These results demonstrate a reaction condition orientation. The structures of 7 and 8 were confirmed via NMR spectroscopies and single-crystal X-ray diffraction patterns ( Supporting Information Figures S1–S3, CCDC 1916544 and 1916517). Encouraged by the results of the model reaction, the degradation of P 1 was investigated. When P 1 reacted with 3 in toluene at 110 °C for 12 h, a yellow precipitate of pyrimidone derivative 4 was obtained after simple filtration and 1,8-octanediol was also yielded via the concentration of the filtered solution. The total yield of pyrimidone derivative 4 and 1,8-octanediol is 92% (Figure 2, path A). However, when P 1 reacted with 3 in toluene at 60 °C for 12 h, the orange precipitate of imidazolone derivative 5 and 1,8-octanediol were obtained via the same operation as path A with a total yield of 90% (Figure 2, path B). Interestingly, when a catalytic amount of Bu3P was added, the degradation of P 1 could be carried out at room temperature and the same degraded products as path B were obtained with the total yield of 94% (Figure 2, path C). The gel permeation chromatography (GPC) analysis of the degradation process showed a gradual shift of the peaks to the late elution time. Only a narrow single peak could be observed after 8 h, confirming complete degradation of P 1 (Figure 3a). Figure 3 | (a) Overlay of the GPC curves at different degradation times (condition: 60 °C, [P1] = [3] = 0.1 M), FT-IR spectra of (b) P1, (c) 4, and (d) 5. 1H NMR spectra of (e) model compound 4-1, (f) recycled 4 from P1, (g) model compound 5-1, and (h) recycled 5 from P1 in DMSO-d6. The solvent peaks are marked with asterisks. Download figure Download PowerPoint The structures of the degraded products were characterized by FT-IR spectroscopy and 1H and 13C NMR spectroscopies, and satisfactory structural analysis data were obtained. The FT-IR spectra of P 1 and degraded products 4 and 5 provided in Figures 3b–3d show that the C≡C stretching vibrations at 2211 cm−1 in the spectrum of P 1 could not be observed in the spectra of 4 and 5. Meanwhile, the absorption bands of P 1, 4, and 5 associated with C=O stretching vibrations could be observed at 1710, 1651, and 1701 cm−1, respectively. To facilitate the structural characterization, another model reaction of 9 and 3 was carried out to prepare model compounds 4-1 and 5-1 ( Supporting Information Scheme S1). As shown in the 1H NMR spectra (Figures 3e–3h), the N-H protons of 4 and 5 resonated at 12.71 and 12.07 ppm, respectively. Meanwhile, the ethynyl protons of 4 and 5 resonated at 6.85 and 6.92 ppm, respectively. Notably, the 1H NMR spectra of degraded products 4 and 5 obtained from P 1 are almost the same as those of model compounds 4-1 and 5-1, and the 1H NMR spectrum of 1,8-octanediol obtained from P 1 is also almost the same as that of the standard sample ( Supporting Information Figure S4), confirming the structures of the degraded products. After achieving a complete degradation of soluble linear poly(alkynoate)s, we endeavored to degrade insoluble thermoset poly(alkynoate)s. It is well known that thermoset materials have favorable material properties but are unable to be reprocessed and are difficult to recycle due to their chemically cross-linked structures.30 To tackle this challenge with our strategies, an insoluble cross-linked polymer P 2 was synthesized from the Ag2WO4-catalyzed polymerization of tetrayne ( 10), alkyl dihalide ( 2), and CO2 under mild reaction conditions ( Supporting Information Scheme S2), which could not be dissolved in any solvent. As shown in Supporting Information Scheme S3, when benzamidine 3 and Bu3P were added into the suspension of P 2 in tetrahydrofuran (THF), the polymer disappeared and the reaction mixtures became an orange solution in as short as 10 min, and the degraded products 11 and 1,8-octanediol were obtained. Kinetics and mechanism investigation To better understand the kinetic profile of the degradation, in situ FT-IR spectroscopy was used to monitor the degradation process. When the degradation of P 1 was carried out at 110 °C with 1 equiv of 3 relative to the number of alkynoates moiety, a new absorption band attributed to the C=O stretching vibration of 4 emerged at 1652 cm−1, whose intensity increased over 30 min but remained almost unchanged afterward, indicating the degradation could be completed in 30 min (Figure 4a). When the degradation of P 1 was carried out at 110 °C with 1 equiv of 3 and 0.2 equiv of Bu3P, a new absorption band attributed to the C=O stretching vibration of 5 appeared at 1701 cm−1, which rapidly increased to reach saturation after 5 min, suggestive of a fast rate and high efficiency of the degradation upon adding the Bu3P catalyst (Figure 4b). The investigation of the temperature effects, the molar ratio of 3, and Bu3P on the degradation suggested that the molar ratio of 3 had almost no effect on the degradation rate at 110 °C, but it exerted significant effects at 60 °C during degradation (Figures 4c and 4d). When 2 instead of 1 equiv of 3 were added into the reaction mixtures at 60 °C, the degradation time could be shortened from 500 to 250 min. When Bu3P was used to promote the degradation, its molar ratio affected the degradation rate significantly. The degradation time was extended from 5 to 15 min as the molar ratio of Bu3P decreased from 0.2 to 0.1 equiv (Figure 4e). Moreover, the degradation of P 1 could be completed at room temperature after only 2 h in the presence of Bu3P (Figure 4f). The 1H NMR spectra of the original unseparated reaction mixtures at different degradation conditions indicated that the resonance peak representing the protons of the methylene group adjacent to the ester groups in P 1 completely disappeared after the degradation ( Supporting Information Figure S5), indicating nearly 100% conversion of P 1. Figure 4 | In situ FT-IR profiles of the peaks (a) at 1652 cm−1 for the degradation at 110 °C with 1 equiv of 3, (b) at 1713 and 1701 cm−1 for the degradation at 110 °C with 1 equiv of 3 and 0.2 equiv of Bu3P. (c and d) The time-dependent peak intensity at 1652 cm−1 (110 °C) or at 1701 cm−1 (60 °C) with different equivalents of 3. (d) The time-dependent peak intensity at 1701 cm−1 with different equivalents of 3 and Bu3P at 110 °C. (f) The time-dependent peak intensity at 1701 cm−1 with 1 equiv of 3 and 0.2 equiv of Bu3P at different temperatures. Download figure Download PowerPoint Moreover, a theoretical calculation was employed to show a deeper understanding of the above degradation behaviors. We used ethyl phenylpropiolate 6 and benzamidine 3 as model reagents to simplify the calculation and employed DFT to analyze the mechanisms of three paths of degradation. As shown in Figure 5, for path A, three steps were observed, that is, nucleophilic addition, nitrogen-activated double-bond rotation, and carbonyl addition. First, the ethynyl group was attacked by 3 and formed A_3 via the proton-transfer process. This nucleophilic addition was only allowed in the E-configuration due to the steric hindrance of the ester group. After a typical nitrogen-activated double-bond rotation ( A_TS3), the ester group was attacked by an amidine group ( A_TS4). After equivalent ethanol and proton transfer, the product ( A_7) was yielded with a low ΔG = −50.0 kcal/mol. The highest energy barrier was confirmed at the step of nitrogen-activated double-bond rotation with ΔGA = 44.0 kcal/mol, which is the rate-determining step. Thus, the molar ratio of 3 would not affect the degradation rate at 110 °C, which agreed well with our in situ FT-IR results (Figure 4c). Figure 5 | DFT-calculated profiles of path A (blue) and path B (red). All numbers are given as relative Gibbs free energy (kcal/mol) under tight criteria of M06-2X/6-311++G(d,p) in the solvation model of toluene. Download figure Download PowerPoint In path B, only two main steps were observed, that is, electrophilic addition and carbonyl addition (Figure 5). The ethynyl group was initially attacked by 3 at the ester side. After a similar proton-transfer process ( B_TS2), the ester group was attacked by an amidine group to form a five-membered ring ( B_TS4) and produce B_5. Furthermore, after configuration adjustment, B_6 was obtained with ΔG = −36.4 kcal/mol. The highest energy barrier shifted to the step of electrophilic addition with 3 with ΔGB = 40.6 kcal/mol, due to the absence of nitrogen-activated double-bond rotation. Accordingly, a molar ratio of 3 would significantly affect the degradation rate at 60 °C. This conclusion is also consistent with our in situ FT-IR results (Figure 4d). For the comparison of paths A and B, the latter was observed with lower ΔGB (40.6 kcal/mol) than that of path A (ΔGA = 44.0 kcal/mol), but with a more unstable product ( B_6: −36.4 kcal/mol) than that of path A A_7: −50.0 kcal/mol). The constant (k) of the reaction rate is an important parameter for a chemical reaction. According to Rice–Ramsperger–Kassel–Marcus (RRKM) theory,31 k can be calculated from ΔG‡ according to eq. 1 and the quotients are obtained from ΔΔG‡ according to eq. 2. k = A exp ( − Δ G ‡ R T )(1) k B k A = exp ( − Δ G B ‡ − Δ G A ‡ R T ) = exp ( Δ Δ G A B ‡ R T ) (2)where A is a constant, R is the ideal gas constant (8.314 J·mol−1·K−1), and T is 298.15 K. Therefore, kB/kA could be deduced as 3 × 102, which indicated that only path B could occur under low temperature. Meanwhile, path A is favored at high temperature with a more stable product (Figure 5). Because PMe3 showed the same effect as PBu3 on the degradation,28,29 the former was used as a model catalyst to simplify the calculation ( Supporting Information Scheme S4). The ΔG of PMe3 attacking ethynyl group from the side adjacent to the ester groups is 20.3 kcal/mol, which is +1.6 kcal/mol higher than that away from the ester groups, indicating good region selectivity in this attacking step, so compound 6 was initially attacked by PMe3 to produce a compound C_2, which processed similar electrophilic addition with 3. Then C_5 was yielded after proton transfer. Cleaving PMe3 could either occur before ( C_TS4_L) or after ( C_TS6_As) carbonyl addition. Since the very high energy barrier on C_TS4_As step (60.7 kcal/mol), PMe3 could only be cleaved before carbonyl addition ( C_TS4_L). Thus C_6_L shared the same reaction process as path B ( B_3 to B_6) ( Supporting Information Figure S6). Apparently, PMe3 could greatly reduce the energy barrier compared with path B and the reaction significantly increased, which is also consistent with our in situ FT-IR results (Figures 4e and 4f). Photophysical properties of degraded products Since 4 and 5 contain tetraphenylethene (TPE), a typical moiety featuring AIE characteristics,32–34 their emission behaviors were systematically investigated in DMSO/water and THF/water mixtures with different water fractions (fw; Supporting Information Figures S7 and S8), respectively, and their absolute photoluminescence (PL) quantum yields (ΦF) were also tested ( Supporting Information Table S3). The results showed that both of them are AIE-active and follow the mechanism of restriction of intramolecular motion (RIM).32 They emitted faintly in solution, but adding a poor solvent like water greatly enhanced the emission intensity. The highest emission values were recorded in the DMSO/water mixtures with fw values of 60% for 4 and in THF/water mixtures with fw values of 90% for 5, Moreover, the ΦF of 4 in its film state was measured to be 78.2%, which is much higher than that of 5 (20.7%) due to the highly twisted molecular conformation of 4 that hampers the intermolecular π–π stacking interaction in the solid state, suppressing nonradiative transition and activating a radiative transition. In situ visualization of the degradation process Based on the faint emission in solution and intense luminescence in the aggregate state, AIE luminogens (AIEgens) have been successfully used to directly visualize invisible things, such as defects,35 the crystallization process,36 the glass transition temperature (Tg),37 and the polymerization process.38 However, direct visualization of the degradation process of a polymer using AIE technology is virtually unexplored. Since its degraded product 5 enjoys the AIE feature, herein, the degradation of P 1 was visualized for the first time. As shown in Figure 6a, the degradation of P 1 with 3 and Bu3P at room temperature was performed under UV light. The reaction mixture initially showed almost no emission. Then orange luminescence was observed and gradually enhanced with longer time. The reason is that the AIE-active degraded product 5 was continuously precipitated during the degradation, which intensively emits in the aggregate state. The fluorescence in the middle portion of the tube was analyzed at different intervals by ImageJ software, in which, grayscale was automatically calculated at each pixel to represent the brightness or intensity of the fluorescence signal.39 Accordingly, the statistical distribution and average value of grayscale of the selected area were obtained. As an example, Figure 6b demonstrates the results at 60 min. The plot of grayscale versus time shown in Figure 6c gives a positive correlation. The highest value of grayscale appeared at 120 min and then reached saturation, and its shape was similar to that of the in situ FT-IR curve (Figure 4f), indicating that in situ visualization of the degradation process via AIE technology is reliable. Moreover, the degradation process could also be visualized under UV light with a P 1 film slowly immersed into the toluene solution of 3 and Bu3P at room temperature (Figure 6d). Figure 6 | (a) Fluorescence images of the reaction mixture at different times. (b) Grayscale distribution of the selected area at 60 min. Inset: fluorescence image of the reaction mixture at 60 min and the selected area for grayscale analysis. (c) Plot of time against grayscale of the reaction mixture. (d) Photographs of the film of P1 before and after degradation. Download figure Download PowerPoint Microcontact printing As previously mentioned, the Bu3P-catalyzed degradation of P 1 is very fast at room temperature with a fluorescence color change, it is an ideal candidate to be applied in μCP to generate fluorescence patterns. μCP as a kind of soft lithography possessing many advantages, including high resolution, very short reaction times, simple large-area patterning, low cost, and so forth.40–42 Certainly, it can help to solve the disadvantages encountered in microfabrication, which is the basic technology used in making all microelectronic systems, such as high cost, complex facilities and technologies for high-energy radiation needed, and limitations to extremely flat silicon substrates. μCP allows the patterned transfer of a molecular ink onto a surface using an elastomeric stamp. Several reactions, such as Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC), Diels–Alder reactions, thiol-ene, and amino-yne click reactions, have been introduced into μCP.43,44 Herein, Bu3P-catalyzed degradation was applied in this area. First, the thin films of P 1 on quartz substrates (30 × 30 mm) were prepared via spin coating. Then a stamp with "AIE" characters was incubated with the ink, that is, the toluene solution of 3 and Bu3P, and placed on the prepared polymer films. Only after 2 min, an orange "AIE" pattern was obtained under both daylight and UV light which represents the generation of the degraded product 5 (Figures 7a and 7b). Figure 7 | (a and b) Two-dimensional patterns generated by the degradation of P1 film taken under daylight and UV light, respectively. Excitation wavelength: 365 nm. (C) Illustration of μCP and (d) fluorescence microscopy image after μCP. Excitation wavelength: 540–552 nm. Emission wavelength: 575–640 nm. Download figure Download PowerPoint Encouraged by the above results, poly(dimethylsiloxane) (PDM