One-Pot Three-Component Tandem Polymerization Toward

Article pubs.acs.org/Macromolecules

One-Pot Three-Component Tandem Polymerization Toward Functional Poly(arylene thiophenylene) with Aggregation-Enhanced Emission Characteristics Haiqin Deng,†,‡,⊥ Rongrong Hu,†,‡,⊥ Engui Zhao,†,‡ Carrie Y. K. Chan,†,‡ Jacky W. Y. Lam,*,†,‡ and Ben Zhong Tang*,†,‡,§ †

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

S Supporting Information *

ABSTRACT: The development of efficient multicomponent tandem polymerization is attractive but challenging, owing to the limitations such as the required strict stoichiometric balance, the poor solubility and low molecular weight of the polymer products, etc. In this work, an efficient one-pot three-component polymerization of alkyne, carbonyl chloride and ethyl 2mercaptoacetate was reported. The polymerization of aromatic diyne (1), diaroyl chloride (2), and ethyl 2-mercaptoacetate (3) catalyzed by Pd(PPh3)2Cl2/CuI proceeded smoothly under mild conditions at room temperature without strict stoichiometric balance of the monomers, affording poly(arylene thiophenylene) (P1) with high molecular weights (Mw up to 156 000) in excellent yields (up to 97%). Single crystal structure of model compound 4 was obtained, aiding in verification of the complete transformation to the desired polymer product. The thiophene-containing conjugated polymer possesses good solubility in common organic solvents, good film-forming ability and high thermal stability. Meanwhile, the polymer shows typical aggregation-enhanced emission behavior: its solution is weakly emissive, but turns to be highly emissive when nanoaggregates or thin films are formed. Furthermore, thin film of P1 shows high refractive indices (n = 1.9461−1.6668) in a wide wavelength region of 400−1000 nm, which can be further modulated by UV irradiation. Well-resolved fluorescent photopattern can be generated by exposure of the thin film of P1 under UV irradiation through a copper photomask. The polymer also serves as an efficient fluorescent chemosensor for Ru3+ with high sensitivity and selectivity, and the quenching constants for the sensing are up to 8.8 × 105 L mol−1. This work provides a new polymerization concept and an efficient approach toward functional conjugated polymer materials, overcoming the limitations of multicomponent polymerization.

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INTRODUCTION The development of efficient polymerization method toward macromolecules with novel structures and unique properties is of great academic significance and industrial implication. The ideal polymerizations normally start from simple reactants, through simple, fast, environmentally friendly reactions to obtain the desired polymer products in high yields. Among numerous synthetic strategies, multicomponent tandem reactions stand out with their unique advantages such as high efficiency, functional group tolerance, atom-, and stepeconomy.1 In tandem reactions, multiple reactions are © 2014 American Chemical Society

combined into one synthetic operation and occur in a specific order without isolating intermediates. The reactive intermediates from the first step can directly undergo next in situ reaction, affording complicated structures selectively, making tandem reactions particularly suitable for the unstable intermediates.2 They have hence received much attention among organic chemists because they provide direct and Received: June 9, 2014 Revised: July 11, 2014 Published: July 25, 2014 4920

dx.doi.org/10.1021/ma501190g | Macromolecules 2014, 47, 4920−4929

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useful tools for the synthesis of functional macromolecules with multisubstituted heterocyclics. In this work, we aim to explore the first tandem polymerization for the synthesis of conjugated polymers. The three-component tandem reaction of alkyne, aroyl chloride, and ethyl 2-mercaptoacetate was developed into an efficient polymerization technique for the preparation of polythiophenes with structural regularity, processability and advanced functionality. Tetraphenylethene- (TPE-) containing diyne 1 was designed as the diyne monomer because TPE core structure is a well-known fluorophore with unique aggregation-induced emission (AIE) characteristics.14 The widely accepted AIE mechanism is the restriction of intramolecular motions.15 In solution, the intramolecular free rotation of the phenyl rings on TPE takes place and serves as a nonradiative decay pathway for the excitons. When the molecules start to aggregate in poor solvent, such intramolecular rotation is restricted, which blocks the relaxation channels and turns on the fluorescent emission accordingly. Integration of such AIEgen into the monomer endows the polymer with AIE or aggregation-enhanced emission (AEE) feature. Moreover, the twisted structure of TPE will significantly inhibit the polymer chain packing, and enlarge the intermolecular distance, thus resulting in good solubility of the polymer product. The polymerization of diyne (1), diaroyl chloride (2), and ethyl 2-mercaptoacetate (3) (Scheme 2) can proceed under mild conditions to afford linear

efficient approaches to access a diversity of complicated molecular structures from simple precursors and procedures, which can be difficult to achieve by other methods.3 With the compelling advantages of tandem reactions, their great potential in polymer synthesis can be foreseen. Some frontier work has been reported using tandem catalysis system for ethylene polymerization, but most of them involve complicated biocatalyst or transition-metal catalysts, which limit their applications.4 A few tandem polymerizations were also reported to synthesize nonconjugated polymers. For example, Ueda et al. reported the preparation of ordered poly(amidethioether) through tandem type polymerization of 2,4dichlorophenyl acrylate, 4,4′-thiobis(benznenthiol), and 4,4′oxidianiline.5 An efficient approach for surface modification of silica nanoparticles through tandem reversible addition− fragmentation chain transfer polymerization and click chemistry was reported by Brittain et al.6 Choi et al. reported the tandem ring-opening/ring-closing metathesis polymerization of various cycloalkenes and terminal alkynes-containing monomers to afford nonconjugated polymers.7 Hoyle and Lowe et al. reported a tandem phosphine-mediated thiol−ene/radicalmediated thiol−yne sequence to prepare multifunctional thioethers.8 To the best of our knowledge, tandem polymerizations have not been developed for the synthesis of conjugated polymers, due to the lack of efficient synthetic approaches. Conjugated polymers, especially heterocyclic-containing polymers, are a group of novel functional materials with unique electronic and photophysical properties, which are in great demand in a wide range of potential high-tech applications such as fluorescent sensors and photoelectronics.9 However, they are synthetically difficult considering their complicated structures which require tedious reaction procedure, harsh reaction condition, painful isolation, severe catalyst poison side-effect, etc.10 Therefore, there are continued strong demands for new synthetic methodologies with maximization of product diversity, reaction efficiency, atom- and step-economical strategies. Organic tandem reactions based on alkynes with rich chemistry and high reactivity have been extensively reported. They show great potential in being developed into facile and efficient polymerization approaches toward structural diversified, heterocyclic-containing functional conjugated polymers.11 In 2012, Müller et al. reported an economical and practical protocol of one-pot three-component consecutive Sonogashira−Fiesselmann cyclocondensation tandem reaction of alkyne, benzoyl chloride and ethyl 2-mercaptoacetate, using Pd(PPh3)2Cl2/CuI as catalyst system to afford 2,4-disubstituted thiophenes with high efficiency (Scheme 1).12 They further enlarged the reaction scope and introduced two functional groups in alkyne or benzoyl chloride monomers to obtain oligothiophenes.13 Despite the advantages of these tandem reactions, little effort has been made to develop them into

Scheme 2. Synthetic Route toward P1 by Three-Component Tandem Polymerization

poly(arylene thiophenylene) (P1) with high molecular weights in high yields, which can also tolerate imprecise stoichiometric monomer feed ratios. As a functional polymer material, P1 exhibits good solubility, satisfactory thermal stability, high light refractivity and AEE property. It can generate well-resolved fluorescent photopattern under UV irradiation and can also be utilized as a selective and sensitive fluorescent chemosensor for the detection of metal ions with a superamplification effect.

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RESULTS AND DISCUSSION Polymerization. To develop the one-pot three-component coupling−addition−cyclocondensation reaction into an efficient polymerization approach as a powerful tool for the preparation of conjugated poly(arylene thiophenylene), monomers with multiple functional groups are designed and prepared. TPE-containing diyne 1 was prepared according to our previous publications.16 Commercially available monomers terephthaloyl dichloride (2) and ethyl 2-mercaptoacetate (3) were chosen as the other two monomer components.

Scheme 1. Three-Component Coupling−Addition− Cyclocondensation Reaction

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remained almost constant afterward. The molecular weight of P1, on the other hand, continuously increased with prolonged polymerization time and the product from 48 h polymerization possessed the highest molecular weight (Mw = 84 900). In general, the increased polymerization time played positive roles in the polymerization. For the three-component tandem polymerization, the molar ratio of each monomer should be strictly controlled to obtain polymer with high molecular weight. The theoretical molar ratio among 1, 2, and 3 should be 1:1:2. To check if the polymerization can proceed without strict stoichiometric control, a series of control experiments were carried out to evaluate the impact of the monomer feed ratio on the polymerization. The concentration of 3 was fixed at 0.18 M and the ratio of monomer 1 and 2 was gradually tuned from 0.95:1 to 1:0.95 (Table 2, no. 1−5). When the monomer feed

The typical polymerization was carried out in THF under nitrogen in the presence of Pd(PPh3)2Cl2, CuI, and Et3N. Diyne 1 was first reacted with dicarbonyl chloride 2 for 3 h at room temperature, monomer 3 was then added with 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) and ethanol to proceed the addition and cyclocondensation reactions to complete the formation of new thiophene rings in polymer P1. Different reaction temperatures of the second step were tested first and the results are shown in Table 1. The polymerization carried Table 1. Effect of Temperature on the Polymerizationa no.

T (°C)

yield (%)

Mwb

Mw/Mnb

1 2 3

0 25 70

97 96 96

106 700 84 900 94 900

3.2 6.1 3.6

a

Carried out in THF under nitrogen in the presence of Pd(PPh3)2Cl2, CuI and Et3N for 48 h. [1] = 0.05 M, [2] = 0.05 M, [3] = 0.18 M, [Pd(PPh3)2Cl2] = 4 mol %, [CuI] = 8 mol %, and [Et3N] = 0.10 M. Monomer 1 was reacted with 2 at room temperature for 3 h prior to the addition of 3. bDetermined by GPC in THF on the basis of a linear polystyrene calibration.

Table 2. Effect of Monomer Feed Ratio on the Polymerizationa

out at 0 °C afforded polymer with high molecular weight (Mw = 106 700) and high yield (97%), which represents the best results among the tested temperatures. Satisfactory results were also obtained at room temperature and 70 °C, indicating small temperature effect on the polymerization yield and molecular weight. The polydispersity index (Mw/Mn) of the polymer obtained at room temperature was found to be higher than that obtained at lower or higher temperatures. The reaction rate of each step of the tandem polymerization might be affected by temperature in different ways, generating polymer intermediates and products with different solubility and hence resulting in the variation of polydispersity index. Regarding energy conservation and simple operation, further optimizations of the polymerization were carried out at room temperature. Polymerization time of the second step reaction after the addition of monomer 3 was then systematically investigated at room temperature. As shown in Figure 1, the yields of P1 increased gradually with the reaction time in the first 24 h and

no.

[1] (M)

[2] (M)

[3] (M)

yield (%)

1 2 3c 4 5 6 7

0.0475 0.0495 0.05 0.05 0.05 0.05 0.05

0.05 0.05 0.05 0.0495 0.0475 0.05 0.05

0.18 0.18 0.18 0.18 0.18 0.11 0.32

86 96 96 95 94 85 91

Mwb 36 79 84 62 49 74 106

800 600 900 000 200 800 400

Mw/Mnb 3.9 5.0 6.1 5.1 4.2 3.8 4.1

Carried out in THF under nitrogen at 25 °C for 48 h in the presence of Pd(PPh3)2Cl2, CuI, and Et3N. [Pd(PPh3)2Cl2] = 4 mol %, [CuI] = 8 mol %, [Et3N] = 0.10 M. Monomer 1 was reacted with 2 for 3 h prior to the addition of 3. bDetermined by GPC in THF on the basis of a linear polystyrene calibration. cData taken from Table 1, no. 2. a

ratio was stoichiometric imbalanced, the yield was barely affected and the Mw is generally high. When the concentrations of 1 and 2 kept constant with the ratio of 1:1, the Mw of polymer increased with higher concentration of 3. These experiments demonstrate that the polymerization is quite efficient even without strict stoichiometric control of monomers. Last but not least, the influence of the monomer concentrations was investigated as shown in Table 3. When the monomer concentrations were 0.10 M for both 1 and 2, the polycoupling rapidly formed insoluble gel even before monomer 3 was added. The concentrations of 1 and 2 were then tuned from 0.05 to 0.02 M while keeping [1]:[2]:[3] = 1:1:3.6. When the concentration of 1 and 2 were both 0.03 M, Table 3. Effect of Monomer Concentration on the Polymerizationa no.

[1] (M)

[2] (M)

[3] (M)

[Et3N] (M)

yield (%)

Mwb

Mw/Mnb

1 2c 3 4

0.10 0.05 0.03 0.02

0.10 0.05 0.03 0.02

0.18 0.11 0.072

0.20 0.10 0.06 0.04

gel 96 93 72

84 900 156 000 17 100

6.1 5.1 1.9

a Carried out in THF under nitrogen at 25 °C for 48 h in the presence of Pd(PPh3)2Cl2, CuI and Et3N. [Pd(PPh3)2Cl2] = 4 mol %, [CuI] = 8 mol %, Monomer 1 was reacted with 2 at room temperature for 3 h prior to the addition of 3. bDetermined by GPC in THF on the basis of a linear polystyrene calibration. cData taken from Table 1, no. 2.

Figure 1. Time course on the polymerization. Carried out in THF under nitrogen at 25 °C in the presence of Pd(PPh3)2Cl2, CuI, and Et3N. [1] = 0.05 M, [2] = 0.05 M, [3] = 0.18 M, [Pd(PPh3)2Cl2] = 4 mol %, [CuI] = 8 mol %, and [Et3N] = 0.10 M. Monomer 1 was reacted with 2 at room temperature for 3 h prior to the addition of 3. 4922


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the polymerization was highly efficient, affording polymer with Mw of 156 000 in 93% yield. When the monomers were further diluted, however, both yield and Mw of P1 were decreased. It is worth mentioning that the sole polymerization between monomer 1 and 2 was reported to afford polymer product with maximum Mw of 39 100, much lower than that of P1.17 The comparison suggests that the addition of the third monomer not only reacts efficiently, but also promotes the polymerization, possibly by endowing the resultant polymers with enhanced solubility. Structural Characterization. Small molecular model reaction was conducted, in order to characterize the polymer structure obtained from the three-component tandem polymerization (Scheme 3). TPE-containing monoyne 5 was reacted Scheme 3. Synthetic Route toward Model Compound 4

Figure 3. FT-IR spectra of (A) 1, (B) 2, (C) 3, (D) 4, and (E) P1.

Their 1H NMR spectra were also compared in Figure 4. The typical acetylene proton of monomer 1 resonanced at δ 3.03, and the SH proton as well as CH2 protons next to SH group of 3 resonanced at δ 1.94 and 3.17, respectively, which all disappeared in the spectra of 4 and P1. The aromatic protons of 2 resonanced at δ 8.25, which shifted to high field after the reaction. Meanwhile, a new peak emerged at δ 7.28, indicating the formation of new thiophene rings. The absorption peaks of P1 are broader compared with those of 4, suggesting its polymeric nature. Similarly, the 13C NMR spectra of acetylene group of 1, carbonyl groups of 2, and CH2 next to the SH group of 3 resonanced at δ 88.9, 167.7 and 26.5, respectively (Figure S1). These peaks were all absent in the 13C NMR spectra of 4 and P1. Moreover, the carbonyl group of 3 resonanced at δ 170.8 was shifted to δ 162.2 after the new thiophene rings formed. These characterizations proved that P1 has the precise structure as shown in Scheme 2. Solubility and Thermal Stability. Although P1 is composed of conjugated aromatic backbone, the polymer possesses good solubility in common organic solvents, such as dichloromethane, chloroform, THF, etc. The twisted conformation of TPE units embedded in the polymer backbone played an important role in enhancing the polymer solubility.

with commercially available 4-methoxylbenzoyl chloride 6 and 3 under similar reaction condition of the optimized polymerization, affording model compound 4. Single crystals were easily formed in the dichloromethane/hexane solution of 4 as shown in Figure 2A. Under UV irradiation, the crystals emitted bluish green light. X-ray structure analysis is shown in Figure 2B, giving direct evidence of the desired structure of compound 4 (Table S1, Supporting Information). The monomers, model compound and polymer were fully characterized by standard spectroscopic techniques (see Experimental Section), giving satisfactory analysis data corresponding to their expected molecular structures. The IR spectra of them are shown in Figure 3 for comparison. The absorption bands of 1 associating with the C−H and CC stretching vibrations were observed at 3275 and 2106 cm−1, respectively. The S−H stretching vibrations of 3 were observed at 2571 cm−1. In both the spectra of 4 and P1, all these peaks were disappeared, confirming that the terminal triple bond and S−H bonds have been completely consumed by the polymerization.

Figure 2. (A) Fluorescence image and (B) single crystal structure of model compound 4 (CCDC 1006529). The photograph in part A was taken under a fluorescent microscope. Excitation wavelength: 330−385 nm. 4923


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degradation temperature of P1 at its 5% weight loss under nitrogen was 305 °C. Photophysical Properties. P1 is a fully conjugated polymer with thiophene and benzene rings in the polymer backbone, its photophysical properties were thus investigated in comparison with its model compound 4. The absorption spectra of THF solutions of 4 and P1 with a concentration of 10 μM were shown in Figure S3. The absorption maximum of 4 was located at 350 nm, while that of P1 bathochromically shifted about 20 nm, indicating a higher conjugation in the polymer. With the typical AIE-active TPE building blocks embedded in the molecular skeletons, both 4 and P1 were expected to possess AIE characteristics. Their fluorescence photographs were taken in THF and THF/water mixtures as shown in Figure 5A and 6A. While keeping the constant concentration, THF solution of 4 was nonemissive under UV irradiation. When a nonsolvent of 4, water, was added into the solution, the molecules started to aggregate and gradually showed enhanced emission. When more than 70 vol % of water was added, the nanoaggregates formed in the mixed solvent system were brightly emissive. In contrast to the small molecule, the fluorophores in P1 were covalently linked in the polymer backbone, which partially restricted the free rotation of the phenyl rings to some extent even in the solution, thus the THF solution of P1 was faintly emissive. Addition of water induced the polymer chains to aggregate and increase the emission swiftly. Their photoluminescence behaviors were then studied by photoluminescence (PL) spectrometry. The THF solution of 4 showed an almost flat line in the PL spectra. Only when more than 70 vol % water was added, did an obvious emission peak emerge at 502 nm and rise with increasing water content. Eventually, the emission intensity increased by 156-fold in the nanoaggregates formed in 90 vol % aqueous mixture compared with its THF solution, demonstrating typical AIE phenomenon.

Figure 4. 1H NMR spectra of (A) 1, (B) 2, (C) 3, (D) 4, and (E) P1 in chloroform-d. The solvent peaks were marked with asterisks.

The steric hindrance of such twisted and bulky group resulted in large intermolecular distances and large free volume to accommodate solvent molecules. The conjugated polymer possesses good film-forming ability and can be easily fabricated into tough thin film by spin-coating or drop-casting processes. The conjugated polymer backbone also endowed the polymer with good thermal stability. As shown in Figure S2, the

Figure 5. (A) Photographs of 4 in THF/water mixtures with different water fractions ( f w) taken under 365 nm UV irradiation from a hand-held UV lamp. (B) Emission spectra of 4 in THF/water mixtures with different water fractions. (C) Plot of relative emission intensity (I/I0) versus the water fraction of the aqueous mixtures of 4. Solution concentration, 10 μM; excitation wavelength, 350 nm. 4924


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Figure 6. (A) Photographs of P1 in THF/water mixtures with different water fractions (f w) taken under 365 nm UV irradiation from a hand-held UV lamp. (B) Emission spectra of P1 in THF/water mixtures with different water fractions. (C) Plot of relative emission intensity (I/I0) versus the water fraction of the aqueous mixtures of P1. Solution concentration, 10 μM; excitation wavelength, 370 nm.

the polymer thin film, photo-oxidative reaction took place, which changed the chemical components of the film and thus the n values. The refractive index spectra of P1 before and after UV irradiation were compared in Figure 7. Upon UV exposure

The PL spectra of P1 behaved differently. In dilute THF solution with a concentration of 10 μM, the emission maximum of P1 was located at 517 nm. Addition of water into its THF solution gradually enhanced its light emission without a noticeable change on the emission profile as well as the emission maximum. Such AEE phenomenon was frequently observed in polymers with AIEgens. The highest emission intensity was observed in 80 vol % aqueous mixtures and its intensity enhanced about 5.5-fold compared with that of its THF solution. The emission intensity slightly decreased when more than 90 vol % water was added, probably due to the reduction of the effective polymer concentration caused by the poor solubility in such mixed solvents with high water content. The average sizes of the polymer nanoparticles formed in THF/water mixtures were investigated by particle size analyses as shown in Figure S4. The average sizes for the particles formed in 50, 70, 90, and 95 vol % aqueous mixtures were 222, 206, 185, and 110 nm, respectively, suggesting that the nanoparticles become smaller when water content increases. Light Refraction. Processable polymers with high refractive indices (n) are promising candidates for various practical applications, including lenses, prisms, optical waveguides and holographic image recording systems, etc.18 Moreover, the refractive index modulation is a critical issue in the optical data storage devices such as compact discs, digital versatile discs, and holographic recording materials;19 hence, it is highly rewarding to develop photosensitive polymers with controllable refractive indices.20 Generally, organic polymer materials such as polystyrene, polycarbonate, poly(methyl methacrylate), and polyacrylate, have refractivity lying in the region of 1.49−1.58.21 With its many polarizable aromatic rings, ester groups and heteroatoms, which are well-known contributors for increasing refractivity, high n value can be expected from P1. Indeed, the spin-coated thin film of P1 possessed high n values of 1.9461− 1.6668 in a wide spectral region of 400−1000 nm. Furthermore, with the photosensitive ester groups, P1 was potentially photosensitive. When UV irradiation was applied on

Figure 7. Wavelength dependence of refractive indices of thin films of P1 on the UV irradiation time.

for 40 min, the n values in the same wavelength region dropped to 1.5319−1.5121, and decreased about 0.2145 at 632.8 nm, demonstrating efficient tunability of the thin film refractivity. The Abbé number defined as νD = (nD −1)/(nF −nC), where nD, nF, and nC are the refractive index (RI) values at Fraunhofer D, F, and C lines of 589.3, 486.1, and 656.3 nm, respectively,22 represents the variation or dispersion of n value with wavelength, which is a critical parameter for optomaterials. The νD values of P1 upon different irradiation time (0−40 min) were calculated to be in the range of 7.6430−66.1139, with corresponding D values of 0.1308−0.0151, where D = 1/νD, which is comparably small among all advanced polymer 4925


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materials (Table 4). The high refractivity, refractive index tunability and low optical dispersion of P1 make it promising

detection, including chromatography, atomic absorption, and inductively coupled plasma mass spectrometry, normally require sophisticated operation, difficult sample preparation, and have limited sensitivity.26 The development of highly sensitive and selective fluorescent sensors for Ru3+ is thus in great demand due to the detrimental effects on human health and environment. P1 with heterocyclic chelating groups and AEE characteristic is thus investigated as a fluorescent chemosensor for Ru3+ ion. THF/water mixture of P1 with 90 vol % water content was used for the detection. Gradual addition of Ru3+ to the nanoparticle suspensions of P1 significantly decreased the fluorescence emission (Figure 9A). When [Ru3+] = 36.7 μM, the PL intensity of P1 dropped to merely ∼4% of its original value. The Stern−Volmer plot of (I0/I − 1) values versus the Ru3+ concentration gave an upward bending curve rather than a straight line, demonstrating a superamplified quenching effect.27 The quenching process was divided into three stages (Figure 9B), and large quenching constants were calculated to be 204 130, 444 500, and 880 430 M−1 for each stage, respectively. When Ru3+ was interacted with the fluorescent polymer through heteroatoms, energy transfer may occur and quench the emission. When the concentration of Ru3+ was increased, it entered into the three-dimensional network of nanoaggregates of P1, and enabled interaction with more binding sites since more surface area was exposed. Meanwhile, the nanoaggregates swelled when more Ru3+ entered, which left more space for the phenyl rings on TPE to rotate and consumed exciton energy by nonradiative decay. Both two effects made the polymer nanoaggregates highly susceptible to Ru3+, leading to the superquenching effect. Its selectivity as a sensitive fluorescent sensor was then studied. The PL quenching of the nanoaggregate of P1 in the presence of a series of common metal ions, including Ca2+, Co2+, Cu2+, Fe2+, Fe3+, Mg2+, Hg2+, Ni2+, Ag+, and Zn2+, was investigated. As shown in Figure 10, compared with Ru3+, the other metal ions all exert little change on the PL of P1, indicating high selectivity toward Ru3+. The relatively higher standard reduction potential of the Ru(III)/Ru(0) couple might have differentiated Ru3+ from other metal ions in the fluorescence quenching process, thus account for the high selectivity.28 The mechanistic reason is still under investigation.

Table 4. Refractive Indices and Chromatic Dispersions of P1a no.

t (min)

n633

νD

D

1 2 3

0 10 40

1.7348 1.6047 1.5204

7.6430 21.5336 66.1139

0.1308 0.0464 0.0151

a Abbreviation: t = irradiation time, n = refractive index, νD = Abbé number = (nD − 1)/(nF − nC), where nD, nF, and nC are the RI values at wavelengths of 589.2, 486.1, and 656.3 nm, respectively, D = chromatic dispersion = 1/νD.

coating material in the advanced optical display systems, such as high-performance complementary metal oxide semiconductor image sensors and microlens components for charge-coupled devices.18 Photopatterning. Fluorescent photopatterns are of great demanding in photonic and electronic devices, biological sensing and probing systems.23 The good film-forming ability, high film emission efficiency, and photosensitivity endow P1 with potential application of luminescent photopatterns through photolithography process. When a solution of P1 was spin-coated on a silicon wafer, an emissive thin film was formed, which was then irradiated by UV light in air for 20 min through a copper photomask. The fluorescence of the exposed regions was quenched due to the photo-oxidative reaction which decomposed the chromophore, whereas the unexposed region remained emissive. As can be seen from Figure 8, a twodimensional photoresist pattern was observed under a fluorescence microscope with high resolution and sharp edges.

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CONCLUSIONS In this work, a one-pot three-component tandem polymerization based on the consecutive coupling-addition-cyclization reactions of alkyne, carbonyl chloride and ethyl 2-mercaptoacetate was reported. The polymerization proceeded efficiently at room temperature with high tolerance to the monomer feed ratio, affording thiophene-containing conjugated polymers with high molecular weights (up to 156 000) and nearly quantitative yields. Single crystal structure of the model compound, together with other characterizations proved that the monomers were transformed to the desired polymer structure completely. Comparison with the previously reported twocomponent polycoupling product from the same monomers 1 and 2 suggested that the latter addition-cyclization reaction not only completely transformed the polycoupling intermediate into the desired thiophene-containing polymer structure, but also promoted the first step of the polycoupling, evidenced by the elongated polymer chain. The thiophene-containing conjugated polymer product enjoyed a series of advantages, including good solubility in common organic solvents, high

Figure 8. Two-dimensional fluorescent photopattern generated by photo-oxidation of P1. The photograph was taken under UV illuminations (330−385 nm).

Metal Ion Detection. Ruthenium complexes are widely used in many applications including catalysts and dye-sensitized solar cells.24 However, extensive use of ruthenium ion could accumulate the harmful wastes to the environment, considering that Ru3+ exhibits various forms of toxicity, such that they are corrosive and harmful to eyes, skin, respiratory tract, and digestive tract, which may cause long-term adverse effects on the aquatic environment.25 Traditional methods of Ru3+ 4926


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Figure 9. (A) Emission spectra of P1 in THF/H2O mixtures (1/9 v/v; concentration =10 μM) with different Ru3+ concentrations. (B) Plots of (I0/I − 1) values versus Ru3+ concentrations in 90% aqueous mixtures of P1. I0 = intensity at [Ru3+] = 0 μM. (TMS; δ = 0 ppm) as internal standard. IR spectra were collected on a PerkinElmer 16 PC FT-IR spectrophotometer. High resolution mass spectra (HRMS) were measured on a GCT Premier CAB 048 mass spectrometer operated in MALDI−TOF mode. Single crystal X-ray diffraction intensity data was recorded at 100 K on a Bruker-Nonices Smart Apex CCD diffractometer with graphite-monochromated Mo Kα radiation. Processing of the intensity data was carried out through the SAINT and SADABS routines and the structure and refinement were obtained employing the SHELTL suite of X-ray programs (version 6.10). The number (Mn) and weight-average (Mw) molecular weights and polydispersity indices (Mw/Mn) of the polymers were estimated by a Waters Associates gel permeation chromatography (GPC) system equipped with UV, RI, and PL detectors. The polymers dissolved in THF (∼2 mg mL−1) were filtered through 0.45 mm PTFE syringe-type filters before injected into the GPC system. THF was used as the eluent in a flow rate of 1.0 mL min−1. A set of monodisperse linear polystyrenes covering the Mw range of 103−107 were utilized for Mw calibration. UV−vis absorption spectra and photoluminescence spectra were recorded on a Milton Roy Spectronic 3000 array spectrophotometer and a PerkinElmer LS 55 spectrofluorometer, respectively. Thermogravimetric analysis (TGA) was carried out under nitrogen on a PerkinElmer TGA 7 analyzer at a heating rate of 10 °C min−1. Particle sizes of the polymer aggregates in THF/water mixtures were measured on a BeCoulter Delsa 440SX Zeta potential analyzer. RI values were estimated by J. A. Woollam M-2000 V multiwavelength ellipsometer in a wavelength region of 400−1000 nm. Photopatterning and RI tuning of the polymer films were conducted on a Spectroline ENF-280C/F UV lamp at a distance of 3 cm as light source. The incident light intensity was ∼18.5 mW cm−2. The film was prepared by spin-coating the polymer solution (10 mg of P1 in 1 mL of 1,2-dichloroethane) at 1000 rpm for 1 min on a silicon wafer. The polymer film was dried in a vacuum oven at room temperature overnight. The pattern was generated by UV irradiation of the polymer film through copper photomask for 20 min followed by development in 1,2-dichloroethane. The photo was taken on an optical microscope (Olympus BX 41) under a UV light source. Polymer Synthesis. All the polymerization reactions were carried out under a nitrogen atmosphere using a standard Schlenk technique. A typical procedure for the polymerization of P1 from Table 3, no. 3, is given below as an example. A 25 mL Schlenk tube equipped with a magnetic stirrer was charged with TPE-containing diyne (1) (76 mg, 0.20 mmol) and terephthaloyl chloride (2) (41 mg, 0.20 mmol). Pd(PPh3)2Cl2 (6 mg, 0.008 mmol), and CuI (3 mg, 0.016 mmol) were added under nitrogen. Then 6.67 mL of THF and 0.06 mL of Et3N were injected. The resulting solution was stirred for 3 h at room temperature. Afterward, ethyl 2-mercaptoacetate (3) (0.08 mL, 0.72 mmol) was added with 0.83 mL of ethanol (Vethanol:VTHF = 1:8), and

Figure 10. Changes in relative emission intensities (I0/I − 1) of P1 in THF/H2O mixtures (1:9 by volume; concentration =10 μM) with various metal ions (2 mM). I0 = intensity in the absence of metal ions.

thermal stability, good film forming ability, unique AEE behavior, large and modulable refractivity, photosensitivity, etc. Moreover, it can serve as an efficient fluorescent sensor toward Ru3+ detection with both high sensitivity and selectivity. This work provides a new tandem polymerization and an efficient approach toward functional conjugated polymer materials, overcoming the limitations of multicomponent polymerization, such as strict stoichiometric balance, poor solubility, and low Mw of the polymer product. We hope the present result will pave the way to facile syntheses of heterocyclic conjugated polymers with novel molecular structures and inspire research enthusiasm for further development of efficient polymerization approaches toward useful polymer materials.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl under a nitrogen atmosphere immediately prior to use. Triethylamine (Et3N), methanol, ethanol and other chemicals and reagents were all purchased from Aldrich or J&K and used as received without further purification. Diyne 1 and monoyne 5 were prepared according to the literature procedures.16 1 H and 13C NMR spectra were recorded on a Bruker ARX 400 NMR spectrometer using CDCl3 as solvent and tetramethylsilane 4927


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Author Contributions

0.12 mL of DBU. The reaction mixture was stirred for 48 h at room temperature under nitrogen atmosphere, and was then added dropwise into 200 mL of methanol via a cotton filter to precipitate the polymer. The precipitate was allowed to stand overnight and then collected by filtration. The polymer was washed with methanol and dried under vacuum at room temperature to a constant weight. A yellow powder was obtained in 93% (133 mg). Mw: 156 000. Mw/Mn: 5.1 (GPC, polystyrene calibration). IR (KBr), υ (cm−1): 3053, 3022, 2978, 2932, 2903, 1715, 1605, 1547, 1495, 1443, 1406, 1371, 1273, 1250, 1209. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.52, 7.41, 7.28, 7.16, 7.11, 7.10, 7.09, 7.06, 4.26, 1.28. 13C NMR (100 MHz, CDCl3), δ (TMS, ppm): 162.23, 149.18, 148.26, 144.57, 143.34, 141.10, 135.66, 132.28, 131.60, 128.89, 128.26, 127.53, 127.21, 126.05, 125.51, 61.19, 14.39. Model Reaction. 3-(4-Methoxyphenyl)-5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2-carboxylate (4) was synthesized as a model compound with the following experimental procedure similar to that of P1 described above. Into a 25 mL Schlenk tube equipped with a magnetic stirrer was placed TPE-containing monoyne (5) (357 mg, 1.00 mmol) and 4-methoxybenzoyl chloride (6) (171 mg, 1.00 mmol). Afterward, Pd(PPh3)2Cl2 (15 mg, 0.02 mmol) and CuI (8 mg, 0.04 mmol) were added under a nitrogen atmosphere and followed by injection of THF/Et3N (10 mL/0.15 mL). The resulting solution was stirred at room temperature for 3 h. Ethyl 2-mercaptoacetate (3) (0.16 mL, 1.20 mmol), 1.00 mL of ethanol, and 0.22 mL of DBU were subsequently injected. The reaction mixture was further stirred for 18 h at room temperature under nitrogen. Then 20 mL of water was added, and the solution was extracted with 30 mL of dichloromethane for three times. The solvent was evaporated and the resultant crude product was purified by a silica-gel chromatography column to give yellow solid in 84% yield (500 mg). IR (KBr), υ (cm−1): 3055, 3020, 2976, 2937, 2902, 2835, 1708, 1608, 1576, 1547, 1497, 1443, 1404, 1371, 1290, 1250, 1211. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.44 (d, J = 8.4 Hz, 2H), 7.39 (d, J = 8.4 Hz, 2H), 7.20 (s, 1H), 7.16− 7.09 (m, 9H), 7.08−7.00 (m, 8H), 6.97−6.91 (d, J = 8.8 Hz, 2H), 4.24 (q, J = 7.1 Hz, 2H), 3.85 (s, 3H), 1.27 (t, J = 7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3), δ (TMS, ppm): 162.35, 159.67, 149.35, 148.14, 144.72, 143.69, 143.58, 141.59, 141.95, 140.31, 132.24, 131.59, 131.52, 131.35, 130.70, 128.39, 128.07, 128.00, 127.88, 127.46, 126.93, 126.85, 126.79, 125.46, 125.19, 113.44, 61.07, 55.51, 14.42. HRMS (MALDI− TOF): m/z 592.2070 [M+, calcd 592.2072]. Preparation of Nanoaggregates. Stock THF solutions of 4 and P1 with a concentration of 1 mM were prepared. Then, 0.1 mL of the stock solutions were transferred to 10 mL volumetric flasks. After adding appropriate amounts of THF, water was added dropwise under vigorous stirring to furnish 10 μM THF/water mixtures with the water fractions ( f w) of 0−95 vol %. The absorption and emission measurements of the resulting solutions were immediately performed. Preparation of Metal-Ion Solutions. Inorganic salts such as CaCl2, CoCl2, CuCl2, FeCl2, FeCl3, MgCl2, HgCl2, NiCl2, RuCl3, AgNO3, and ZnCl2 were dissolved in distilled water (10 mL) to obtain 10 mM aqueous solutions. The stock solutions were then diluted with distilled water to furnish the desired concentrations for further experiments.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work reported in this article has been partially supported by National Basic Research Program of China (973 Program; 2013CB834701), the Research Grants Council of Hong Kong (604711, 602212, HKUST2/CRF/10 and N_HKUST620/11) and the University Grants Committee of Hong Kong (AoE/P03/08). B.Z.T. expresses thanks for the support of the G u a ng d o n g I nn o v a t i v e Re s e a r c h T e a m Pr og r a m (201101C0105067115).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The single crystal data for compound 4 (CCDC 1006529), including a .cif file, 13C NMR spectra of monomers 1−3, model compound 4 and P1, TGA thermogram of P1, absorption spectra of 4 and P1, size distributions of nanoparticles of P1 in THF/water mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

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These authors contributed equally.

AUTHOR INFORMATION

Corresponding Authors

*(B.Z.T.) E-mail: [email protected]. *(J.W.Y.L.) E-mail: [email protected]. Telephone: +852-23587375 (8801). Fax: +852-2358-1594. 4928


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One-Pot Three-Component Tandem Polymerization Toward

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