Polyarylcyanation of Diyne: A One-Pot Three-Component Convenient

Nov 29, 2016 - Telephone: +852-2358-7242 (7375). .... Recent New Methodologies for Acetylenic Polymers with Advanced Functionalities. Zijie Qiu , Ting...
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Polyarylcyanation of Diyne: A One-Pot Three-Component Convenient Route for In Situ Generation of Polymers with AIE Characteristics Zijie Qiu,†,‡ Ting Han,†,‡ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,*,†,‡,§ and Ben Zhong Tang*,†,‡,§ †

HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen 518057, China Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong China § Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: A facile, one-pot, three-component polymerization route for in situ generation of polymers with aggregation-induced emission (AIE) characteristics was developed. The polycoupling of dibromoarenes, internal diynes and potassium ferrocyanide was catalyzed by palladium acetate and sodium bicarbonate and proceeded smoothly in dimethylacetamide under nitrogen at 120 °C, producing poly(triphenylacrylonitrite)s (PTPANs) with high weight-average molecular weights of up to 223000 in high yields of up to 84%. This polymerization method enjoys the remarkable advantages of high reaction rate and efficiency and broad monomer scope. Model reaction was carried out to aid the structure characterization and property investigation of the obtained polymers. All the polymers show remarkable thermal stability, losing merely 5% of their weight at high temperature of up to 513 °C. They are soluble in common organic solvents and their spin-coated thin films exhibit high refractive indices (1.6482− 1.7682). Thanks to the triphenylethene chromophore in situ generated during the polymerization, all the polymers are AIE-active and show strong light emission in the solid state. While UV irradiation of the polymer thin films in air through upper masks photo-oxidizes the exposed parts and quenches their light emission, the unexposed parts remain emissive. Two-dimensional fluorescent patterns with good resolution are thus generated.

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INTRODUCTION The development of efficient luminescent materials in the solid state has attracted much attention for their fundamental importance and practical implication.1−4 However, one problem which hinders such development is that the light emission of conventional luminescent materials is often partially or completely quenched when aggregated. This aggregation-caused quenching (ACQ) must be solved as luminescent materials are widely used as solid thin films for their real-world applications. Recently, a new class of luminogens is found to be weakly or nonemissive in solution but display enhanced emission upon aggregation. This unusual phenomenon is known as aggregation-induced emission (AIE) and is the exact opposite of the ACQ effect.5−7 Till now, a variety of AIE luminogens (AIEgens) with diverse structures and functionalities have been developed. Typical AIEgens, such as tetraphenylethene,8 triphenylethene (TriPE),9 and hexaphenylsilole,10,11 possess a common feature of propeller-shaped structure with freely rotatable peripheral aromatic rings. © XXXX American Chemical Society

Generally, when AIEgens are incorporated into the polymer structure, macromolecules with AIE properties are obtained. Compared to their small molecular weight counterparts, AIE polymers show many unique advantages, such as tunable structure and topology and ease modification for multiple functionalities.12,13 Besides, AIE polymers usually possess good processability and thus can be fabricated into large-area thin solid films and devices by simple and energy-saving techniques. These unique properties enable AIE polymers to find a variety of high-tech applications in the aggregate or solid state, such as sensitive and selective fluorescent chemosensors,14 efficient emitters in organic light-emitting diodes,15 multiresponsive materials,16,17 photopatterning,18 biological imaging,19 etc. Attracted by such prospective, various kinds of AIE polymers, such as polyacetylenes, polyphenylenes, polytriazoles and Received: September 26, 2016 Revised: November 19, 2016

A

DOI: 10.1021/acs.macromol.6b02098 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Strategies for in-Situ Generation of AIE Polymers

poly(phenyleneethylene)s have been reported, thanks to the elaborate efforts by the polymer scientists.20−22 These macromolecules are generally obtained by (1) polymerizing monomers with AIEgens,23 (2) attaching AIEgens to polymerizable monomers, whose homopolymerization or copolymerization affords polymers with AIEgens in the side chains,24 or (3) using AIEgens as polymerization initiators to produce polymers with AIEgens on the terminal.25 While AIE polymers with different architectures and properties can be readily obtained through these strategies, monomers carrying AIEgens need to be presynthesized by time-consuming synthetic routes. Recently, a new strategy for generating AIE polymers was proposed. Instead of inherited from the monomers, the AIEgens are generated in situ by reaction of non-AIE reactants during the polymerization. Such synthetic route is novel as the design and synthesis on monomers can be greatly simplified. Besides, it is even possible to utilize fluorescence to monitor the polymerization process. However, it is challenging to develop such system and only a few examples are available in the recent literature (Scheme 1).26−28 This is presumably due to limited organic reactions with mild reaction condition, high efficiency and broad reaction scope toward AIEgens. Although functional polymers with AIE properties are generated in satisfactory yields by these strategies, their molecular weights are only moderate (Mw less than 25000). TriPE is one of the well-known AIEgens and can be prepared by addition reaction of pheny rings to internal alkyne monomers. In 2008, Cheng et al. reported an efficient, threecomponent coupling of aryl bromides, internal alkynes, and potassium ferrocyanide to synthesize β-arylalkenylnitriles (Scheme 2).29 This reaction is highly regioselective, producing solely the cis-addition product. The reaction proceeds through the formation of a vinylic palladium intermediate, whose ligand exchange followed by reductive elimination, generating fully substituted α,β-unsaturated nitriles. As the products, namely βarylalkenylnitriles, are derivatives of triphenylethene, they are

Scheme 2. Palladium-Catalyzed Three-Component Arylcyanation of Internal Alkyne with Aryl Bromides and Potassium Ferrocyanide

anticipated to be AIE-active. Besides, due to the presence of electron-accepting cyano group, their light-emitting property should be readily tuned to make them to exhibit redder emission than pure TriPE once an electron-donating unit is incorporated. With such regard, in this paper, we tried to develop such efficient organic reaction into a useful tool for the synthesis of new AIE polymers. Making use of commercial available and handy precursors, we have successfully developed an efficient, one-pot, threecomponent polymerization tool to poly(triphenylacrylonitrite)s (PTPANs) with AIE characteristics and diverse structures. The polymerization can be carried out under mild conditions, producing high molecular weight PTPANs with good solubility, excellent thermal stability and novel optical properties in satisfactory yields. Such attributes make them promising as luminescent materials for various high-tech applications.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Dibromobenzene, 4,4′-dibromobenzophenone, 4,4′-oxybis(bromobenzene), potassium ferrocyanide, palladium(II) acetate, and other reagents were purchased from Aldrich and used as received without further purification. All the organic solvents such as tetrahydrofuran (THF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP) and dimethyformamide (DMF) were distilled prior to use. Monomers 1b and 2a−b were prepared according to the reported procedures.26 Gel permeation chromatography (GPC) was performed in THF at 40 °C at an elution rate of 1.0 mL min−1 on a Waters GPC system equipped with a Waters 515 HPLC pump, a Waters 486 UV−vis detector, a column temperature controller and a set of Styragel B

DOI: 10.1021/acs.macromol.6b02098 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Concentration Effect on the Polymerizationa

a

entry

[1a] (M)

[2a] (M)

[3] (M)

yield (%)

Mn

Mw

Mw/Mn

1 2 3 4 5

0.375 0.25 0.20 0.15 0.10

0.375 0.25 0.20 0.15 0.10

0.225 0.15 0.12 0.09 0.06

gel gel 81 75 60

9400 7700 4100

45900 25700 7300

4.9 3.3 1.8

Carried out in dimethylacetamide under nitrogen at 120 °C for 6 h in the presence of 4 mol % of Pd(OAc)2 and 2.5 equiv of Na2CO3.

columns (HT3, HT4 and HT6; molecular weight range: 102−107). The polymers were dissolved in THF (about 2 mg mL−1) and filtered through a 0.45 μm PTFE filter before being injected into the GPC system. IR spectra were recorded on a PerkinElmer 16 PC FTIR spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker AV 400 spectrometer in deuterated chloroform or dichloromethane using tetramethylsilane (TMS, δ = 0) as internal reference. High-resolution mass spectra (HRMS) were recorded on a GCT premier CAB048 mass spectrometer operating in MALDI−TOF mode. UV spectra were measured on a Milton Ray Spectronic 3000 Array spectrophotometer. Photoluminescence (PL) spectra were recorded on a PerkinElmer LS 55 spectrophotometer. Thermogravimetric analysis (TGA) was performed on a TA TGA Q5000 under nitrogen at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) analysis was performed on a TA Instruments DSC Q1000 at a heating rate of 10 °C min−1. Polymer Synthesis. All the polymerization reactions were performed under nitrogen atmosphere using a standard Schlenk technique. A typical procedure for the polymerization of 1a, 2a, and 3 is given below as an example. Into a 25 mL Schlenk tube with a magnetic stirrer were placed 0.2 mmol of 1a, 0.2 mmol of 2a, 0.12 mmol of 3, 8 × 10−3 mmol (4 mol % to that of 1a) of Pd(OAc)2, and 0.5 mmol of Na2CO3 in 2 mL of distilled DMAc. The solution was stirred under nitrogen at 120 °C for 6 h. The polymerization was terminated by pouring the mixture into 300 mL of methanol through a dropper filled with neutral Al2O3 to remove the residue catalyst and any insoluble substrates if formed. The precipitates were collected by filtration through a sand-core funnel and were dried in vacuum to a constant weight. Yellow powder of polymer P1a/2a/3 was obtained in 60% yield (Table 5, entry 1). Mw 7 300; Mw/Mn 1.8. IR (film), ν (cm−1): 3055, 3032, 2938, 2864, 2206, 1604, 1509, 1472, 1443, 1390, 1288, 1247, 1176, 1113, 1020. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.83−6.28 (aromatic protons), 3.92 (OCH2 protons), 1.76 (CH2 protons), 1.54 (CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 158.67, 139.17, 138.49, 132.30, 131.58, 130.65, 130.37, 129.98, 129.05, 128.55, 127.88, 127.72, 125.92, 119.51, 113.63, 67.30, 28.47, 25.15. Other polymers were prepared by the same procedures and their characterization data were given below. P1b/2a/3. Yellow powder; yield 84% (Table 5, entry 2). Mw 97 500; Mw/Mn 4.7. IR (film), ν (cm−1): 3053, 3026, 2938, 2862, 2207, 1603, 1506, 1443, 1288, 1244, 1177, 1111, 1072, 1022. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.53−6.46 (aromatic protons), 3.98, 3.75 (OCH2 protons), 1.79, 1.57 (CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 159.66, 134.88, 133.16, 132.72, 132.51, 132.36, 131.88, 131.54, 131.29, 130.46, 130.19, 129.07, 120.06, 119.41, 114.87, 113.13, 68.51, 29.67, 26.36. P1c/2a/3. Yellow powder; yield 67% (Table 5, entry 3). Mw 41 000; Mw/Mn 4.1. IR (film), ν (cm−1): 3049, 2934, 2860, 2210, 1663, 1603, 1508, 1246, 1177, 1111, 1018. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.27−6.91 (aromatic protons), 3.96 (OCH2 protons), 1.75, 1.54 (CH2 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 158.69, 132.30, 131.51, 130.66, 130.39, 129.40, 127.72, 127.31, 113.89, 67.31, 28.44, 25.12. P1d/2a/3. Yellow powder; yield 82% (Table 5, entry 4). Mw 45 800; Mw/Mn 3.8. IR (film), ν (cm−1): 3055, 2939, 2862, 2206, 1601, 1503, 1285, 1244. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.45−6.73 (aromatic protons), 3.91 (OCH2 protons), 1.76, 1.57 (CH2 protons). 13 C NMR (CD2Cl2, 100 MHz), δ (ppm): 158.58, 133.65, 131.93,

131.50, 130.32, 130.07, 128.97, 127.84, 118.18, 113.78, 67.28, 28.43, 25.12. P1a/2b/3. Yellow powder; yield 61% (Table 5, entry 5). Mw 11 700; Mw/Mn 2.6. IR (film), ν (cm−1): 3053, 3028, 2210, 1597, 1495, 1443, 1400, 1269, 1182, 1109, 1074, 1022. 1H NMR (CD2Cl2, 400 MHz), δ (ppm): 7.49−7.02 (aromatic protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 130.59, 127.73, 127.12. The IR, 1H NMR, 13C NMR spectra of P1b−d/2a/3 and P1a/2b/3 are shown in Figure S1−S12 in the Supporting Information). Model Reaction. Model compound 6 was synthesized by arylcyanation of 1-methoxy-4-(phenylethynyl)benzene with bromobenzene and potassium ferrocyanide. The experimental procedure was similar to that for preparing P1a/2a/3. Pale yellow solid; yield 50% (after purification by column chromatography and crystallization). IR (film), ν (cm−1): 3051, 3017, 2955, 2928, 2835, 2203, 1603, 1576, 1553, 1506, 1441, 1298, 1254, 1177, 1113, 1078, 1028. 1H NMR (400 MHz, CD2Cl2), δ (ppm): 7.43 (m, 5H, aromatic protons), 7.20 (m, 5H, aromatic protons), 7.02 (m, 2H, aromatic protons), 6.76 (m, 2H, aromatic protons) and 3.77 (s, 3H, CH3 protons). 13C NMR (CD2Cl2, 100 MHz), δ (ppm): 160.17, 156.71, 141.41, 139.95, 131.56, 131.17, 130.35, 130.11, 129.27, 128.93, 128.80, 120.64, 114.36, 112.07, 55.81. HRMS (MALDI−TOF): m/z 311.1304 (M+, calcd 311.1313) (Figure S13 in the Supporting Information). Photopatterning. The photo-oxidation of the polymer film was conducted using 365 nm light obtained from a Spectroline ENF280C/F UV lamp. The procedures were similar to those described in our previous publication.18

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RESULTS AND DISCUSSION Polymerization. To develop the polyarylcyanation of internal diyne with aryl dibromide and K4[Fe(CN)6] into a new synthetic tool for preparing AIE polymer, we first optimized the reaction conditions using 1a, 2a, and 3 as model monomers. We first examined the concentration effect on the polymerization. Surprisingly, when the polymerization was carried out at 1a and 2a concentration of higher than 0.25 M with an excess amount of 3, gelation occurred after 3 h (Table 1, entry 1−2). This demonstrates the fast rate of such polymerization. Dilution helps to solve the problem and a polymer was isolated in a high yield (81%) at a lower concentration of 0.2 M (Table 1, entry 3). Since further lowering the monomer concentration results in reduced molecular weights and yields (Table 1, entry 4−5), we thus chose 0.2 M as the optimum monomer concentration of 1a and 2a for further investigation. Potassium perrocyanide (3) is an inorganic complex and serves as an environmental friendly cyanation reagent. Thus, it is of interest to test its ability to release CN− ion in organic solvent. The polymerization of 0.2 M of 1a and 2a in the presence of 0.033 and 0.1 M of 3, which correspond to 0.5 equiv and 1.5 equiv of required CN ligand, respectively, generates polymers with molecular weights of 5100 and 5400 (Table 2, entry 1−2). Increasing the concentration of K4[Fe(CN)6] to 0.12 M helps boost the molecular weight to 9400 (Table 2, entry 3). Further increments of the C

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Macromolecules Table 2. Concentration Effect of 3 on the Polymerizationa entry

[3] (M)

yield (%)

Mn

Mw

Mw/Mn

1 2 3b 4

0.033 0.10 0.12 0.13

79 81 81 80

5100 5400 9400 9600

10500 11900 45900 46800

2.1 2.2 4.9 4.9

xylene, and chlorobenzene, the polymerization performed in DMAc give a high molecular weight polymer in a high yield. Although a polymer with a high Mw was also isolated in NMP, the yield was rather low. These suggest that the polymerization is better to be carried out in polar solvents, among which DMAc is the best. On the basis of the investigation above, we adopted the optimized reaction conditions and explored the monomer scope from a series of dibromoarenes and internal diynes (Scheme 3). Monomers 1a−d are dibromoarenes with different electron density and conjugation. Monomer 2a is an internal diyne with flexible alkyl chain as spacer but 2b is a rigid TPEcontaining diyne. Since other monomer combinations show higher reactivity than 1a/2a/3, gels are formed under optimized conditions. We thus diluted the monomer concentration by half to 0.1 M and terminated the reaction after 6 h. By doing so, soluble, high molecular weights (Mn = 9900−20600) polymers P1b−d/2a/3 are obtained in satisfactory to high yields (Table 5, entry 2−4). Because P1c/2a/3 and P1d/2a/3 carry electron-withdrawing ketone groups and electron-donating oxygen atoms, this polymerization thus shows a high tolerance to monomers with different electron densities. However, moderate polymerization results are still only obtained in P1a/2b/3 (Table 5, entry 5). This may be due to the bulky and rigid structure of 2b, which imposes a steric effect on the propagation of the polymer chain. Despite its whole conjugated molecular structure, P1a/2b/3 is readily soluble in common organic solvents. Thus, the results given in Table 5 demonstrate the versatile of this polymerization with a wide monomer scope. Model Reaction. To gain an insight into the structures of PTPANs formed, we carried out a model reaction of 1methoxy-4-(phenylethynyl)benzene (5) with bromobenzene (4) and 3 under the same synthetic conditions for the polymers (Scheme 4). Two isomeric compounds (6a and 6b) in a molar ratio of 6a to 6b of 9:1 were obtained according to 1H NMR analysis (Figure 2C). This indicates the high regioselectivity of this arylcyanation reaction. The needle-shaped single crystals of 6a were obtained by slow diffusion of dichloromethane/hexane into a mixture of 6a and 6b at room temperature. Results from spectroscopic analysis and single-crystal X-ray diffraction unambiguously confirm the structure of 6a (CCDC 1506266). Structural Characterization. All the polymers were fully characterized by IR and NMR spectroscopies and gave good results corresponding to their molecular structures (see Experimental Section for details). The IR and NMR spectra of P1a/2a/3 are presented as examples. Figure 1 shows the IR spectra of monomer 1a and 2a, the model compound 6 and the corresponding polymer P1a/2a/3. The small peak at 2201 cm−1 in the spectrum of 2a is due to the stretching vibration of its CC bond (Figure 1B). Although the absorption band of the cyano group occurs at a similar wavenumber of around 2200 cm−1, its intensity is much stronger (Figure 1C and 1D). The strong absorption band located at 2206 cm−1 confirms the presence of CN group in both 6 and P1a/2a/3, denoting the successful integration of CN group into the model compound and the polymer structure. Meanwhile, the IR spectra of P1a/2a/3 is almost the same as that of model compound 6, which further substains the structure of P1a/2a/ 3. The 1H NMR and 13C NMR analyses provide more detailed structural information. Figure 2 shows the 1H NMR spectra of the monomers (1a and 2a), model compound (6) and polymer

Carried out in dimethylacetamide under nitrogen at 120 °C for 6 h in the presence of 4 mol % of Pd(OAc)2 and 2.5 equiv of Na2CO3. [1a] = [2a] = 0.2 M. bData taken from Table 1, entry 3. a

concentration, however, exert little improvement on the polymerization results (Table 2, entry 4). On the basis of these experimental data, if it is assumed that all six CN− ligands can be dissociated from [Fe(CN)6]4−, a high improvement in polymerization result should be observed from entry 1 to entry 2 in Table 2. However, the actual case occurs from entry 2 to entry 3. Thus, we proposed the ligand exchange of [Fe(CN)6]4− to [Fe(CN)2Br4]4−, where the four CN− ligands in the planar position were substituted with Br− ligand during the polymerization (Scheme S1). The steric hindrance of the bulky Br− ligand may hamper further ligand exchange. The low solubility of 3 in DMAc and hence its low dissociation in this solvent may also lead to the optimized concentration of 3 (0.12 M) higher than theoretical value (0.1 M). Thus, the concentration of 3 was fixed at 0.12 M. A series of experiments tracking the time course on the polymerization was performed thereafter. While the numberaverage molecular weight (Mn) was enhanced gradually with prolonging the reaction time, the weight-average molecular weight (Mw) increased dramatically, rising the polydispersity index by almost four times from 4.9 at 6 h to 16.7 at 24 h (Table 3, entry 1−4). The isolated yield, on the other hand, Table 3. Time Course on the Polymerizationa entry b

1 2 3 4

time (h)

yield (%)

Mn

Mw

Mw/Mn

6 12 18 24

81 84 81 83

9400 12100 9900 13400

45900 117800 147400 223000

4.9 9.7 14.9 16.6

Carried out in dimethylacetamide under nitrogen at 120 °C in the presence of 4 mol % of Pd(OAc)2 and 2.5 equiv of Na2CO3. [1a] = [2a] = 0.2 M, [3] = 0.12 M. bData taken from Table 2, entry 1.

a

remains almost the same at all the reaction times tested. These phenomena are indicative of a step polymerization with fast monomer consumption but slow polymer growth. The effect of solvent on the polymerization was shown in Table 4. While no polymer was generated in 1,4-dioxane, oTable 4. Solvent Effect on the Polymerizationa entry

solvent

yield (%)

Mn

Mw

Mw/Mn

1 2b 3 4 5 6

NMP DMAc DMF chlorobenzene 1,4-dioxane o-xylene

17 83 54 trace trace trace

10400 13400 4000

134000 223000 7100

12.9 16.6 1.8

Carried out under nitrogen at 120 °C for 24 h in the presence of 4 mol % equiv of Pd(OAc)2 and 2.5 equiv of Na2CO3. [1a] = [2a] = 0.2 M, [3] = 0.12 M. bData taken from Table 3, entry 4.

a

D

DOI: 10.1021/acs.macromol.6b02098 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 3. Synthetic Route to Functional Poly(triphenylacrylonitrite)s

Table 5. Polymerization of Different Monomersa entry b

1 2 3 4 5

monomer

yield (%)

Mn

Mw

Mw/Mn

1a/2a/3 1b/2a/3 1c/2a/3 1d/2a/3 1a/2b/3

60 84 67 82 61

4100 20600 9900 12000 4600

7300 97500 41000 45800 11700

1.8 4.7 4.1 3.8 2.5

(P1a/2a/3). Since this is an arylcyanation addition reaction to internal alkyne, the 1H NMR spectra of 6 and P1a/2a/3 resemble largely 2a. The absorption peaks of P1a/2a/3 are broad due to its high molecular weight. The peaks at around δ 4.00 in the spectra of 2a, 6 and P1a/2a/3 correspond to the d1 and d2 resonances of the protons attached to the oxygen atom. Using the peak integrals in Figure 2C, the molar ratio of 6a and 6b is 9:1, confirming the high regioselectivity of this arylcyanation reaction. The CN group attaches preferentially to an alkene with electron-donating group, corresponding well with the proposed reaction mechanism (Scheme 4).29 Although

a Carried out in dimethylacetamide under nitrogen at 120 °C for 6 h in the presence of 4 mol % of Pd(OAc)2 and 2.5 equiv of Na2CO3. [1] = [2] = 0.10 M, [3] = 0.06 M. bData taken from Table 2, entry 1.

Scheme 4. Synthesis of Model Compound 6 and the Proposed Reaction Mechanism

E

DOI: 10.1021/acs.macromol.6b02098 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. IR spectra of (A) 1a, (B) 2a, (C) 6, and (D) P1a/2a/3. Figure 3. 13C NMR spectra of (A) 1a, (B) 2b, (C) 6, and (D) P1a/ 2a/3 in (A and B) CDCl3 and (C and D) CD2Cl2. The solvent peaks were marked with asterisks.

Solubility and Thermal Stability. Although P1/2/3 are constructed mainly from aromatic rings, all show good solubility in common organic solvents such as THF, dichloromethane, chloroform, etc. This is due to the presence of twisted triphenylacrylonitrite units in the polymer backbone, as suggested by the crystal structure of the model compound. Such a twisted conformation endows a large free volume to accommodate more solvent molecules for interaction, thus enhancing the solubility of the polymers. High thermal stability is a key feature of high performance polymeric materials, which allows their applications in real engineering conditions. The thermal stability of P1/2/3 is evaluated by TGA analysis. Figure 4 shows the thermal degradation process of P1/2/3 when being heated from 50 to 800 °C under nitrogen atmosphere. Degradation temperature (Td), which is defined as the temperature for 5% weight loss, is used for the thermal stability study. All the poly-

Figure 2. 1H NMR spectra of (A) 1a, (B) 2b, (C) 6, and (D) P1a/2a/ 3 in (A and B) CDCl3 and (C and D) CD2Cl2. The solvent peaks were marked with asterisks.

it is difficult to elucidate the regioregularity of the polymer due to the broadness of the absorption peak and hence peak overlapping, it is anticipated that P1a/2a/3 adopts also a regioregular structure. More convincing evidence of the proposed polymer structure comes from the 13C NMR analysis (Figure 3). The complete absence of the CC resonance peak in 6 and P1a/2a/3 further confirms the occurrence of the arylcyanation addition reaction. Besides, peaks associated with the resonance of the cyano carbon are observed at δ 120.64 and 119.51 in the spectra of 6 and P1a/2a/3, respectively. This demonstrates the successful incorporation of CN group, well match with the information given by the IR analysis described above. The IR and NMR data of other polymers are summarized in the Experimental Section and all solidly confirm their respective structures (Figures S1−S12).

Figure 4. TGA thermograms of P1/2/3 recorded under nitrogen at a heating rate of 10 °C/min. F

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Macromolecules (triphenylacrylonitrite)s show very high thermal stability with Td of above 410 °C, which can meet various harsh engineering conditions in the fields of semiconductor, high temperature coating, industrial machinery, etc. Notably, the Td of P1a/2b/3 reaches 513 °C, which is comparable to that of Kapton, a wellknown high thermally resistant polyimide.30 Additionally, P1a/ 2b/3 renders 64% of its weight even after heated to 800 °C, possibly due to the thermal-induced polymerization of the cyano group at high temperature. Such a process cross-links the polymer and further enhances the resistance of the resulting material to thermolytic attack. DSC analysis shows that P1a/ 2a/3, P1c/2a/3 and P1d/2a/3 have similar glass transition temperatures (Tg) at around 140 °C, while P1b/2a/3 shows an enhanced Tg at around 202 °C because of the presence of the more bulky TPE unit (Figure S14). No signal related to the glass transition temperature was detected in P1a/2b/3 even when heated to 400 °C, presumably due to its highly rigid structure. The combination of good solubility and excellent thermal stability enables PTPANs as light, versatile alternatives compared to metal, ceramics and old generation polymers. Optical Property. The absorption spectra of dilute THF solutions (10 μM) of 6 and P1a/2a/3 are shown in Figure S15. While 6 exhibits an absorption maximum (λabs) at 330 nm, the λabs of P1a/2a/3 occurs at a longer wavelength (∼350 nm) with a stronger intensity, which is suggestive of its higher conjugation. The light-emitting properties of 6 were investigated in THF and THF/water mixtures with different water fractions ( f w). As a derivative of triphenylethene, 6 is anticipated to be AIE-active as well. As shown in Figure 5A, the pure THF solution of 6 was nonemissive under UV excitation. However, when an increasing amount of poor solvent of water was added into the THF solution, the molecules of 6 start to aggregate. Because of the

restriction of intramolecular motion in the aggregate state, the emission of 6 was increased dramatically at water fraction larger than 80%. At f w of 99%, the emission intensity is 80-fold higher than that in THF solution. Clearly, the emission of 6 is induced by aggregate formation and it is AIE-active. The AIE behavior of 6 can be interpreted by the single-crystal structure analysis. In Figure 5C, the phenyl rings are twisted from the olefin plane and this suggests a twisted molecular conformation. The intermolecular distance is 4.54 Å, which is long enough to prevent emission quenching caused by π−π stacking (Figure 5C). These are typical characteristics for AIE molecules.7 Notably, the crystals of 6 show a much stronger emission than their isolated species in solution and aggregates in the suspension, as demonstrated by the photograph shown in the inset of Figure 5B. The emission maximum of 6 is located at around 500 nm, which is 70 nm red-shifted from that of TriPE due to the intermolecular charge transfer from the electrondonating methoxy group to the electron-withdrawing cyano moiety.31 A similar investigation was also carried out for P1a/2a/3. As illustrated in Figure 6, P1a/2a/3 is also AIE-active, showing stronger green emission in the aggregate state. However, different from 6, whose emission only turns on in THF/water mixture with high water content, the emission of P1a/2a/3 is gradually enhanced with the amount of water added. This is a common phenomenon for AIE polymer since the rotation of the phenyl rings of the AIEgen is restricted by the polymer chain in some extent even in THF solution. Such a common phenomenon further proves the restriction of intramolecular motion as the mechanism for the AIE phenomenon. P1a/2a/3 shows similar emission behavior in different aggregate states with emission spectra all located at 530 nm. The highest emission intensity is achieved at 90% water content, being 16fold higher than that in THF solution. However, at very high water content, the hydrophobic polymer may from large aggregates, which decreases the effective dye concentration in the solution. This is probably the reason for the slightly emission drop at 98% aqueous mixture. Further investigation on the relationship between the polymer structure and the photophysical property was carried out. Different monomers with light-emitting unit (1b and 2b), electron- withdrawing group (1c) and electron-donating group (1d) were utilized to synthesize polymers (P1a/2a/3, P1b/2a/ 3, P1c/2a/3, P1d/2a/3, and P1a/2b/3) (Scheme 3). Among them, P1a/2b/3 is a fully conjugated polymer without flexible alkyl spacer. As expected, all the polymers absorbs at similar wavelength of 340 nm due to the presence of similar chromophore along the polymer chain (Figure 7A). All the polymers are AIE-active, as demonstrated by their emission spectra and fluorescent photographs shown in Figure S16−S19. Their normalized emission spectra are shown in Figure 7B. Surprisingly, when TPE unit (1b), electron-withdrawing carbonyl group (1c) and electron-donating alkoxy group (1d) are introduced, the resulting polymers (P1b/2a/3, P1c/2a/3 and P1d/2a/3) all show bluer emission than P1a/2a/3, presumably due to their lower conjugation. For P1a/2b/3 with fully conjugated structure, its emission occurs at a longerwavelength of 540 nm and shows a strong yellow emission for its extended electron conjugation. On the other hand, the similar spectral pattern between P1c/2a/3 and P1d/2a/3 suggests that D−A interaction between the alkoxy and the cyano groups in P1d/2a/3 is quite weak, which is also supported by its poor solvatochromism (Figure S20). In

Figure 5. (A) Emission spectrum of 6 in THF/water mixtures with different water fractions ( f w). Solution concentration: 10 μM; excitation wavelength: 380 nm. (B) Plot of relative emission intensity (I/I0) versus the composition of the THF/water mixture of 6. Inset in part B: Photographs of 6 in pure THF solution, THF/water mixture (1/99, v/v), and crystal state taken under 365 nm UV irradiation from a hand-held UV lamp. (C) Structure and intermolecular interaction in a crystal of 6 (CCDC 1506266). G

DOI: 10.1021/acs.macromol.6b02098 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. (A) Photographs of P1a/2a/3 in THF/water mixtures with different water fractions ( f w) taken under 365 nm UV irradiation from a handheld UV lamp. (B) Emission spectra of P1a/2a/3 in THF/water mixtures with different water fractions. Solution concentration: 10 μM. Excitation wavelength: 380 nm. (C) Plot of relative emission intensity (I/I0) versus the composition of the THF/water mixture of P1a/2a/3.

Figure 7. (A) Absorption spectra of P1/2/3 in THF solutions. (B) Emission spectra of P1/2/3 in THF/water mixtures (10/90, v/v). Solution concentration: 10 μM. Excitation wavelength: 380 nm.

semiconductor. Thanks to their excellent solubility and filmforming ability, uniform films of P1/2/3 can be readily fabricated on silica wafers by a simple spin-coating technique. Their light refraction was investigated and the results are shown in Figure 8. Basically, the refractive index of a polymer affects by its polarizability, chain flexibility, chain orientation and the presence of heteroatom. Because of the presence of cyano group and polarized aromatic rings, all the thin films of P1/2/3 show high RI values of 1.6482−1.7682 at 632.8 nm. Notably, the fully conjugated P1a/2b/3 shows the highest RI value of 1.7682 among all the polymers. To the best of our knowledge, such high RI value is among the best for pure organic polymer materials.32

conclusion, the emission of P1/2/3 is primarily dominated by the extend of conjugation rather than D−A interaction, and can be fine-tuned from green to yellow by utilizing different monomers. Light Refraction. The refractive index (RI) is a very fundamental yet important criteria for key components, such as lenses and prisms, in any optical instrument that uses refraction. High-refractive-index-polymer is defined as polymeric material whose refractive index greater than 1.50.32 Such polymers are in great demand for advanced optoelectronic fabrication, such as optical adhesives or encapsulants for organic light emitting diode, photoresist for lithography and microlens components for charge coupled devices or complementary metal oxide H

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the scope of 9.2636−13.4443 and 0.0744−0.1079, respectively. In combination with their high RI value, these optical data demonstrate the high performance of P1/2/3 as advanced optical materials for industrial applications. Photopatterning. As discussed previously, P1/2/3 are all AIE polymers with excellent solubility, which can be readily fabricated into emissive thin film by spin-coating of their solutions. Such characteristics make P1/2/3 as promising candidates in the field of photolithography. Here P1a/2a/3 and P1a/2b/3 are taken for demonstration. A uniform thin film of P1a/2a/3 was obtained on silicon water by spin-coating of its dichloroethane solution. The formed polymer film was then irradiated with UV excitation in air through a copper photomask. The exposed parts (lines) undergo photo-oxidation in the presence of UV irradiation, which quenches their light emission. However, the unexposed areas (squares) remain intact and emit light when irradiated. Thus, a well-resolved twodimensional fluorescent pattern with green emission was generated without the need of development process (Figure 9A). Such photosensitivity is caused by the fact that the double bonds in the polymers may undergo photodecomposition and subsequent photochemical reaction with oxygen upon UV irradiation.12 This breaks the electronic conjugation, bringing emission quenching as a consequence. Fluorescent patterns with different emission color can be generated from P1a/2b/3 (Figure 9B). Similar fluorescent patterns can also be fabricated from other PTPANs (Figure S21). These distinguish luminescent patterns prove that P1/2/3 can be used for manufacturing high resolution organic semiconductor and photonic devices in a cost-efficient fashion.

Figure 8. Wavelength-dependent refractive indices of thin films of P1/ 2/3.

In addition to refraction, optical dispersion is another crucial phenomenon for optical materials, especially in photographic and microscopic lenses, because dispersion causes chromatic aberration, which decreases the imaging resolution. The Abbé number (vD) of a material serves as a measure of the variation or dispersion in its RI value with wavelength and is defined as (nD − 1)/(nF − nC), where nD, nF, and nC are the RI values at wavelengths of Fraunhofer D, F, and C spectral lines of 589.2, 486.1, and 656.3 nm, respectively. The corresponding D values are calculated as the reciprocal of vD. Consequently, polymer films with small D values are desired for industrial applications. Table 6 shows the vD and D values of P1/2/3, which fall within

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CONCLUSION In this paper, we succeeded in developing an efficient polymerization route for in situ generated of AIE-active PTPANs by palladium-catalyzed three-component polyarylcyanations of diynes. This polymerization route enjoys the advantages of mild reaction conditions, high efficiency, simple starting materials and broad monomer scope. By changing the monomer structure, the emission of P1/2/3 can be tuned from green to yellow. All the synthesized P1/2/3 show high RI values and excellent thermal stability. They are photosensitive and can generate well-resolved fluorescent patterns by photolithography of their emissive thin films. Further research will be conducted to endow them with new functionalities by modifying their structure to widen their real-world applications.

Table 6. Refractive Indices and Dispersion of Films of P1/2/ 3a polymer

n632.8

νD

D

P1a/2a/3 P1b/2a/3 P1c/2a/3 P1d/2a/3 P1a/2b/3

1.6987 1.7217 1.6515 1.6482 1.7682

12.1016 10.3663 13.4443 13.3362 9.2636

0.0826 0.0965 0.0744 0.0750 0.1079

Abbreviation: n = refractive index, νD = Abbé number = (nD − 1)/(nF − nC), where nD, nF, and nC are the RI values at wavelengths of Fraunhofer D, F, and C spectral lines of 589.2, 486.1, and 656.3 nm, respectively, D = 1/νD. a

Figure 9. Photopatterns generated by photolithography of films of (A) P1a/2a/3 and (B) P1a/2b/3 through copper masks taken under UV irradiation. Excitation wavelength: 330−385 nm. I

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02098. HRMS spectrum of 6, absorption spectrum of P1a/2a/3 and 6, structural characterization (IR, 1H NMR and 13C NMR spectra) and PL spectra of P1b−d/2a/3 and P1a/ 2b/3, solvatochromism of P1d/2a/3 and two-dimensional fluorescent photopatterns of P1b−d/2a/3 and P1a/2b/3 (PDF) Crystallographic file for 6a (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.W.Y.L.) E-mail: [email protected]. Telephone: +852-23587242 (7375). Fax: +852-2358-1594. *(B.Z.T. )E-mail: [email protected]. Telephone: +852-23587242 (7375). Fax: +852-2358-1594. ORCID

Zijie Qiu: 0000-0003-0728-1178 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Basic Research Program of China (973 Program, 2013CB834701 and 2013CB834702), the University Grants Committee of Hong Kong (AoE/P-03/08), the National Science Foundation of China (21490570 and 21490574), the Research Grants Council of Hong Kong (16305014, 16303815 and 16308116), Nissan Chemical Industries, Ltd., and the Innovation and Technology Commission (ITC−CNERC14SC01 and ITCPD117-9). B.Z.T. thanks the Guangdong Innovative Ressearch Team Program (201101C0105067115) for support .

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