Room Temperature Multicomponent Polymerizations of Alkynes

Aug 11, 2017 - Multicomponent polymerization (MCP) is a fascinating synthetic method for the construction of polymers with diverse structures and ...
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Room Temperature Multicomponent Polymerizations of Alkynes, Sulfonyl Azides, and Iminophosphorane toward Heteroatom-Rich Multifunctional Poly(phosphorus amidine)s Liguo Xu,† Rongrong Hu,*,† and Ben Zhong Tang*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong

‡

S Supporting Information *

ABSTRACT: Multicomponent polymerization (MCP) is a fascinating synthetic method for the construction of polymers with diverse structures and multifunctionalities. As a rapidly developing field, MCP begins to show great impact in polymer chemistry and polymer materials, which attract scientists’ attention by their high convenience and efficiency, great structure diversity, high atom economy, and environmental benefit. In this work, a facile one-pot three-component polymerization of diynes, disulfonyl azides, and iminophosphorane is developed to construct N, O, S, and P-containing heteroatom-rich poly(phosphorus amidine)s with advanced functionalities. The optimized MCP proceeds at room temperature in THF under the catalysis of CuI, generating polymers with high molecular weights (up to 85 600 g/mol) in excellent yields (up to 92%). The MCP enjoys general applicability of various monomers including aromatic and aliphatic alkynes, and the only byproduct generated from the polymerization is nitrogen gas, demonstrating high atom economy and environmental benefit. Interestingly, the phosphorus amidine model compounds were found to possess both aggregation-induced emission behavior and thermally activated delayed fluorescence, indicating unique feature of the corresponding polymer materials. The polymers generally enjoy good solubility in polar organic solvents, good film-forming ability, satisfactory thermal stability, and high refractivity. They can also function as fluorescent chemosensors for Pd2+ ions detection with high sensitivity (Kq up to 207 600 M−1) and selectivity. This MCP provides an efficient approach for the synthesis of heteroatom-rich multifunctional polymer materials, which directly construct the luminescent phosphorus amidine moiety in situ, demonstrating high synthetic efficiency and the potential application in material science.

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INTRODUCTION Multicomponent reactions (MCRs) have attracted much attention due to their high atom economy, simple procedure, diverse product structure, and the ability to construct complex structures efficiently and economically.1,2 Since the first MCR, the Strecker reaction which generates α-amino acids from hydrogen cyanide, amines, and aldehydes/ketones,3 was reported in 1850, a great number of MCRs have been developed such as Hantzsch,4 Mannich,5 Passerini,6 Ugi,7 and A3-coupling reactions,8 which have played an important role in organic synthesis.9,10 Recently, polymer chemists have taken advantage of MCRs and developed a series of efficient multicomponent polymerizations (MCPs) to directly produce polymer materials with well-defined structures.11 For example, Meier’s and Li’s group have developed Passerini threecomponent polymerizations of isocyanide, aldehyde, and © XXXX American Chemical Society

carboxylic acid for the synthesis of polyesters or poly(ester− amide)s, respectively.12,13 After that, Ugi four-component polymerizations of isocyanide, aldehyde, acid, and amine,14 Biginelli three-component polymerization of aldehyde, β-keto ester, and (thio)urea,15 and transition metal-catalyzed A3polycoupling of alkyne, aldehyde, and amine are reported.16 Most recently, we have developed a facile catalyst-free MCP of aromatic alkyne, elemental sulfur, and aliphatic amine for the efficient preparation of polythioamide with well-defined structure and good solubility.17 A series of multicomponent tandem polymerizations (MCTPs) of alkyne, carbonyl chloride, and thiol/hydrazine have also been developed for the functionReceived: May 27, 2017 Revised: July 21, 2017

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Macromolecules oriented synthesis of conjugated polymers,18,19 and a metal-free MCTP of activated alkyne, aromatic amine, and formaldehyde is developed to successfully synthesis structure- and sequencecontrolled luminescent polyheterocycles.20 With the rapid development of the field, MCP has proved to be a powerful and efficient tool for the construction of advanced functional polymer materials, which have opened an access to polymer materials with complicated structures and advanced functionalities demanded by the fast development of science and technology. Heteroatom-rich polymers that usually possess specific and diverse properties are desirable materials. For example, phosphorus-containing polymers have been reported to be utilized in water treatment, biomedical field, and halogen-free retardant coating because of their aptitude to bind metals,21,22 biocompatibility, hemocompatibility, protein adsorption resistance,23 and flame retardancy.24 Sulfur-rich polymers generally display high optical performance, good chemical resistance, metal absorbance, thermal stability, etc.25,26 Nitrogen-rich polymers can be used in organic adsorbents for CO2 capture, metal ion sensors, wastewater treatment, etc.27−29 The combination of these structure features into a single polymer might bring new opportunity and may lead to novel structure and functionality.30−32 However, the syntheses of these polymers with complicated structures are quite challenging which generally require tedious synthetic routes, harsh reaction conditions, and troublesome isolations. As a promising synthetic method, MCP can provide a perfect solution for the preparation of a large variety of heteroatom-rich polymers with multiple structural elements and functionalities. In 2005, Chang et al. reported a highly efficient Cu(I)catalyzed MCR of alkyne, sulfonyl azide, and amine to produce a library of N-sulfonylamidines.33 Afterward, a series of MCR of alkyne, sulfonyl azide, and other nuclephiles such as alcohols and water have been explored, which proved to be a group of highly efficient and widely applicable MCRs.34−36 Based on these reactions, a class of efficient Cu(I)-catalyzed MCPs involving alkyne and sulfonyl azide monomers were developed, including MCPs of alkynes, sulfonyl azides, and amines/ alcohols/water to produce polyenamines/polyimidates37,38 and MCPs of alkynes, sulfonyl azides, and salicylaldehyde or ohydroxylacetophenone to produce poly(iminocoumarin)s.39 Besides the nucleophiles that serve as the third component in the series of MCRs, iminophosphorane is a reactant with bipolar nature whose MCR with alkyne and sulfonyl azide can generate unique phosphorus amidine structure in situ under mild conditions.40 The mechanism is proposed in Scheme 1:

Cu(I)-catalyzed cycloaddition reaction of alkyne A and sulfonyl azide B first takes place to produce triazolyl Cu-species E, which then releases N2 to generate the key intermediate keteneimine F.41 The resonance structure G of iminophosphorane then reacts with keteneimine F to form 1,2-phosphazetidine intermediate H, followed by a ring-opening reaction to afford the phosphorus amidine product D. Inspired by the fascinating feature of this reaction, especially the heteroatom-rich product structure, in this work, a threecomponent polymerization of diynes, disulfonyl azides, and iminophosphorane has been developed to prepare poly(phosphorus amidine)s. After optimization of the polymerization conditions, polymers with high molecular weight and well-defined structure were obtained in high yield at room temperature in THF in the presence of CuI, and nitrogen gas is the only byproduct. The small molecular phosphorus amidine model compounds were found to possess both aggregationinduced emission (AIE) and thermally activated delayed fluorescence (TADF) feature. The unique structure of phosphorus amidine also endow polymer product with a series of advanced functionalities such as high refractivity, fluorescence, and sensitive and selective detection of Pd2+.

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RESULTS AND DISCUSSION Polymerization. To investigate the applicability of this onepot multicomponent polymerization, aromatic diyne 1a and disulfonyl azide 2a were prepared to conduct the polymerization with commercially available iminophosphorane 3.42,43 The MCP was carried out under nitrogen atomosphere at room temperature in the presence of CuI and Et3N to afford heteroatom-rich poly(phosphorus amidine) P1a/2a/3 (Scheme 2). The solvent effect on the polymerization of 1a, 2a, and 3 was first investigated (Table 1). When halogenated solvents such as CH2Cl2 or CHCl3 were used as the solvent, moderate yields and high molecular weights (Mw) above 35 000 g/mol were achieved. When polar solvents such as THF or DMF were used as the solvent, the polymer products could be welldissolved, considering the potential hydrogen bonding formation between the polymer and the solvent molecules. Improved polymerization result could hence achieved with high yields of up to 88% and large Mws of up to 79 500 g/mol. The effect of monomer concentration on the MCP of 1a, 2a, and 3 in THF was then studied with the monomer feeding ratio fixed at [1a]:[2a]:[3] = 1:1:2.5 (Table 2). When high concentration of 1a about 0.2 M was adopted for the MCP, insoluble gel was formed which can be further dissolved in DMF slowly after stirring at room temperature for 4 h, indicating high molecular weight polymer with poor solubility was formed. The optimized monomer concentration of 1a is proved to be 0.1 M, which could produce P1a/2a/3 with the highest Mw among the tested condition. The MCP could still proceed smoothly to afford satisfactory Mws and high yields with lower monomer concentration. A strict stoichiometric balance of 1a and 2a is required to obtain polymer with high molecular weight. Time course of this MCP was also studied based on the optimized solvent and monomer concentration (Table 3). Remarkably, P1a/2a/3 with a Mw of 33 000 g/mol can be produced in 85% yield within 2 h, demonstrating the high efficiency of this MCP. When the polymerization time was increased from 1 to 12 h, the yield and Mw of the product gradually increased to 91% and 85 600 g/mol, respectively. Further prolonging the polymerization time to 24 h results in

Scheme 1. Proposed Mechanism of Cu(I)-Catalyzed MCR of Alkyne, Sulfonyl Azide, and Imimophosphorane

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Macromolecules Scheme 2. Multicomponent Polymerizations of Cu(I)-Catalyzed Diynes, Disulfonyl Azides, and Iminophosphorane

Table 1. Solvent Effect on the MCP of 1a, 2a, and 3a

Table 3. Time Course on the MCP of 1a, 2a, and 3a

entry

solvent

yield (%)

Mwb (g/mol)

Mw/Mnb

entry

t (h)

yield (%)

Mwb (g/mol)

Mw/Mnb

1 2 3 4

CH2Cl2 CHCl3 THF DMF

64 74 88 83

35300 35600 79500 69400

1.89 1.98 2.91 3.06

1 2 3 4 5 6 7

1 2 3 4 8 12 24

55 85 89 86 88 91 gel

9300 33000 63000 74400 79500 85600

1.58 2.21 2.51 2.64 2.91 2.89

a

Carried out at room temperature under nitrogen for 8 h in the presence of CuI and Et3N. [1a] = [2a] = 0.10 M, [3] = 0.25 M, [Et3N] = 0.20 M, [CuI] = 0.02 M. bDetermined by GPC in DMF on the basis of a PMMA calibration.

a

Carried out at room temperature under nitrogen in THF in the presence of CuI and Et3N. [1a] = [2a] = 0.10 M, [3] = 0.25 M, [Et3N] = 0.20 M, [CuI] = 0.02 M. bDetermined by GPC in DMF on the basis of a PMMA calibration.

Table 2. Monomer Concentration Effect on the MCP of 1a, 2a, and 3a entry

[1a] (M)

yield (%)

Mwb (g/mol)

Mw/Mnb

1 2 3 4

0.20 0.10 0.08 0.05

gel 88 87 90

79500 68500 35700

2.91 3.16 1.87

Table 4. Effect of Different Monomers on the MCPa

a

Carried out at room temperature under nitrogen in THF for 8 h in the presence of CuI and Et3N. [1a] = [2a], [3] = 2.5[1a], [Et3N] = 2[1a], [CuI] = 0.2[1a]. bDetermined by GPC in DMF on the basis of a PMMA calibration.

entry

polymer

yield (%)

Mwb (g/mol)

Mw/Mnb

1 2 3 4 5 6 7

P1a/2a/3 P1a/2b/3 P1b/2a/3 P1c/2a/3 P1d/2a/3 P1e/2a/3 P1f/2a/3

91 87 92 81 75 54 50

85600 53500 70400 14400 29300 18700 18800

2.89 2.63 2.49 1.88 2.58 2.09 1.72

a

Carried out at room temperature under nitrogen in THF for 12 h in the presence of CuI and Et3N. [1a−f] = [2a−b] = 0.10 M, [3] = 0.25 M, [Et3N] = 0.20 M, [CuI] = 0.02 M. bDetermined by GPC in DMF on the basis of a PMMA calibration.

insoluble gel, which can be slowly dissolved in DMF, suggesting the formation of high Mw polymer. Last but not least, various aromatic diynes 1a−d, aliphatic diynes 1e−f, and aromatic sulfonyl azides 2a−b were tested for this MCP under the optimized conditions to demonstrate the wide monomer scope of this polymerization and enrich the structure as well as functionality of the poly(phosphorus amidine)s (Scheme 2 and Table 4). When 2a was replaced by 2b with higher steric hindrance, both yield and Mw of the product decreased. In general, the MCP enjoys wide monomer

scope including aromatic and aliphatic alkynes, which generates good to excellent yields of up to 92% and Mws beyond 14 400 g/mol. The aromatic diynes 1a−d possess better performance in this MCP compared with the aliphatic diynes 1e−f, in terms of yields, Mws, and product solubility. The electronically C

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Scheme 3. Synthetic Routes of Model Compounds (A) 6 and 8 and (B) 9; (C) Single Crystal Structure of Compound 8

deficient aromatic diyne 1c is less favored to take this polymerization compared with other aromatic diynes. Structural Characterization. Model compounds 6, 8, and 9 were synthesized through the reported synthetic procedures to help characterize the polymer structure and serve as a comparison for the properties (Scheme 3A,B).40 The 1H NMR and HRMS spectra of these model compounds have been reported (6) or provided in Figures S1−S3 (8, 9), verifying the chemical structures of them as shown in Scheme 3. Moreover, single crystal of 8 was obtained from its dichloromethane/ petroleum solution, which proved the expected structure of 8 and suggested the existence of the strong intramolecular hydrogen bonds between the N−H and SO moieties (Scheme 3C and Table S1). The polymer structures were characterized through the comparison of the standard spectroscopic techniques such as IR as well as 1H, 13C, and 31P NMR spectra of the monomers, model compound 9, and the polymers. In the IR spectra, two absorption bands associated with the CC and C−H stretching vibrations of 1a at 2106 and 3269 cm−1, respectively, together with the N3 stretching vibration peak of 2a at 2142 cm−1 all disappear in the spectra of 9 and P1a/2a/3, indicating the total consumption of alkyne and azide groups from the monomers (Figure 1). Meanwhile, two new peaks located at ~3288 and 1540 cm−1 emerge in the IR spectra of both 9 and

Figure 1. IR spectra of (A) 1a, (B) 2a, (C) 3, (D) 9, and (E) P1a/2a/ 3.

P1a/2a/3, suggestive of the generation of N−H and CN moieties. Similarly, the IR spectra of other polymers all suggest the disappearance of the CC and C−H stretching D

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Furthermore, the 13C NMR spectra of monomers, 9, and P1a/2a/3 are shown in Figure 3. The acetylene carbon

vibrations and the emergence of N−H and CN stretching vibrations (Figure S4). To gain more structural details, 1H NMR spectra of the monomers, 9, and P1a/2a/3 in DMSO-d6 were compared (Figure 2). In the spectra of 9 and P1a/2a/3, the acetylene

Figure 3. 13C NMR spectra of (A) 1a, (B) 2a, (C) 3, (D) 9, and (E) P1a/2a/3 in DMSO-d6. The solvent peaks are marked with asterisks.

resonances of 1a at δ 83.3 and 80.8 have disappeared in the spectra of 9 and P1a/2a/3. The resonances of CH3, tertiary carbon, CH2, and aromatic carbons of 1a at δ 30.6, 41.2, 68.5, and 155.9 and the aromatic carbons of 2a and 3 at δ 160.9 and 151.6, respectively, all retained in the spectra of 9 and P1a/2a/ 3 with slight shift. Most importantly, a new peak emerged at δ 160.1 after the reaction, which is associated with the newly formed CN group, proving the expected polymer structure.40 In addition, the 31P NMR spectra of 3, 9, and P1a/2a/3 suggested that the resonance of phosphorus atom in 3 at δ 0.7 has shifted to δ 18.8 in the spectra of 9 and P1a/2a/3 after the reaction (Figure S6). Differently, two peaks representing phosphorus atom are observed at δ 26.6 and 21.8 in the spectrum of P1e/2a/3, again proving the existence of the tautomers in the polymer prepared from aliphatic diynes. Solubility and Thermal Stability. In these heteroatomrich partially conjugated polymers, despite the existence of various intermolecular interactions such as hydrogen bonding and n → π* interactions, they can still be well dissolved in polar solvents such as DMF and DMSO. Thermogravimetric analysis of them suggested satisfactory thermal stability, with their decomposition temperatures at 5 wt % weight loss in the range 219−237 °C under nitrogen (Figure S7).

Figure 2. 1H NMR spectra of (A) 1a, (B) 2a, (C) 3, (D) 9, and (E) P1a/2a/3 in DMSO-d6.

proton resonance of 1a located at δ 4.20 is absent, while the resonances of CH2 and CH3 of 1a located at δ 5.08 and 1.56 have shifted to δ 4.64 and 1.51, respectively. The electrondeficient aromatic protons of 2a with resonances at δ 8.10 and 7.48 have also shifted to higher field after the MCR or MCP. Meanwhile, new peaks at δ 8.34 and 8.20 are found in the spectra of 9 and P1a/2a/3, respectively, suggestive of the generation of N−H proton. The resonance peaks of P1a/2a/3 are broader compared with that of 9, demonstrating its polymeric nature. The 1H NMR spectra analysis of other polymers suggests similar results with newly formed N−H groups located at δ 8.10−10.51 (Figure S5). In particular, two sets of N−H peaks were observed in the spectra of polymers prepared from aliphatic diynes 1e−f, indicating that tautomerization might take place in P1e−f/2a/3 (Figure S5E,F).40 E

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Figure 4. (A) Absorption spectra of 6, 8, 9, and P1a−c/2a/3 in DMSO solution. Concentration: 10 μM. (B) PL spectra of solid powder of 6, 8, 9, and P1a−c/2a/3. Excitation wavelength: 360 nm. Inset: fluorescence photos of solid powder of 6, 8, 9, and P1a−c/2a/3 taken under UV irradiation.

Figure 5. PL spectra of (A) 8 and (C) P1b/2a/3 in DMSO/water mixtures with different water fractions. Plots of relative emission intensity (I/I0) of (B) 6, 8, 9, and (D) P1b/2a/3 versus water fraction of the aqueous mixtures. Inset: fluorescence photos of (B) 8 and (D) P1b/2a/3 in DMSO/ water mixtures with different water fractions. Concentration: 10 μM. Excitation wavelength: 335 nm (8); 355 nm (P1b/2a/3).

value. Indeed, the spin-coated thin films of P1a−d/2a/3 and P1a/2b/3 all exhibit high n values of 1.6287−1.8145 in a wide wavelength region of 400−1700 nm (Figure S8 and Table S2), which are much higher compared with commercial optical plastics such as polycarbonate, polystyrene, polyacrylate, and poly(methyl methacrylate), whose n values generally lie in the

Light Refractivity and Chromatic Dispersion. Polymers with high refractive indices (n) have drawn much attention because of their applications in lenses, prisms, holographic image recording systems, optical waveguides, etc.44,45 The abundant heteroatoms and polarizable aromatic rings of these poly(phosphorus amidine)s may endow them with high n F

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Figure 6. (A) Time-resolved transient PL decay profiles of solid powder of 6 and 8. (B) PL spectra of solid powder of 8 at 300 and 77 K and its 0.5 ms delayed PL spectra at 77 K. Excitation wavelength: 335 nm. The average fluorescence lifetimes (⟨τ⟩) were calculated by ⟨τ⟩ = (A1τ12 + A2τ22)/ (A1τ1 + A2τ2), where A1 and A2 are the pre-exponentials for lifetimes τ1 and τ2.

range of 1.49−1.58.46 The other critical parameters for optomaterials, chromatic dispersion (D and D′), and Abbé numbers (vD) of the polymers are summarized in Table S2. The results suggest that the chromatic dispersions of the polymers in the IR region lie in the range 0.002−0.011, which are generally smaller compared with those of commercial optical plastics.47 Photophysical Properties. During investigation, fluorescence emission was observed from model compounds 6 and 8. Systematical studies of photophysical properties of model compounds and polymers were then conducted. In dilute DMSO solution, the absorption maxima of model compounds and polymers generally located at ∼335 nm. P1b/2a/3 and P1c/2a/3 possess red-shifted absorption maxima at 355 and 381 nm, respectively, due to the conjugated tetraphenylethene or benzophenone structure of the monomers, while P1e−f/2a/ 3 possess blue-shifted absorption maxima, owing to the nonconjugated monomer structure (Figure 4A and Figure S9). Photoluminescence (PL) behaviors of model compounds and polymers were then investigated in solid state (Figure 4B). The emission maxima of 6 and 8 were located at 510 nm, which were red-shifted compared with that of 9 at 480 nm. XRD characterization suggested that solid powders of 6 and 8 are crystals while the powder of 9 is amorphous (Figure S10), and the intermolecular interaction as well as the immobilized molecular structure in the crystal grids might lead to the bathochromic shift. The emission maximum of P1a/2a/3 is at 480 nm, while those of P1b/2a/3 and P1c/2a/3 are red-shifted to 525 and 545 nm, respectively, owing to the conjugated structure from the diyne monomers. Other polymers did not show obvious emission. The emission behaviors of model compounds and P1a−c/ 2a/3 were then studied in DMSO/water mixtures with different water fractions ( f w), considering that water is a poor solvent for these compounds and might induce molecular aggregation. The PL spectra of 8 are shown in Figure 5A,B as an example. In DMSO solution or DMSO/water mixture with less than 70 vol % water, the compound is almost nonemissive. When f w increased to 80 vol %, a broad emission peak emerged at ∼480 nm, and further increase of water content raises the emission intensity, which reaches maxima at 99 vol % aqueous media, suggesting typical aggregation-induced emission characteristics.48 AIE compounds generally possess a propeller shape, and their intramolecular rotation or vibration might consume excited state energy in the solution state in a nonradiative

manner. Such intramolecular motions are restricted in the aggregated states, and radiative decay takes place to emit light.49 Compounds 6 and 9 possess similar AIE behavior (Figure S11). On the other hand, the DMSO solution of P1b/2a/3 emits weakly at 510 nm, owing to the TPE moiety whose phenyl ring rotation has been partially restricted in the polymer backbone. Gradual addition of water into the solution increases the emission intensity, and when water content is beyond 50 vol %, the emission gradually drops (Figure 5C,D). Similarly, the fluorescence intensities of P1a/2a/3 and P1c/2a/3 were affected by water content in the DMSO/water mixtures, which reach maxima at 30 vol % aqueous solution and pure DMSO solution, respectively (Figure S12). The fluorescence quantum efficiencies of these compounds were measured (Table S3), suggesting the AIE feature of the model compounds and P1b/2a/3 with their aggregated state emission quantum efficiencies of up to 7.8%. The fluorescence lifetimes of the model compounds and polymers were then measured (Table S3). Interestingly, the time-resolved fluorescence spectra of the microcrystals of 6 and 8 suggested a shortly lived emissive species with τ1 at 2.6−5.5 ns and a long-lived emissive species with τ2 at 742.1−1171.2 ns at room temperature (Figure 6A), while the other compounds generally possess solid state lifetime ranging at 1.1−2.3 ns. The emission maxima of 8 recorded at room temperature, 77 K, and delayed for 0.5 ms at 77 K were located at 495, 502, and 515 nm, respectively, revealing that the emission at room temperature is attributed to S1−S0 radiative decay (Figure 6B). The low temperature phosphorescence emission maxima only red-shifted 20 nm compared with its fluorescence emission maxima. These results suggested that 6 and 8 possess thermally activated delayed fluorescence (TADF) characteristics.50 DFT calculation were also performed and the ΔEST values of 6 and 8 were calculated to be 0.21 and 0.20 eV, respectively (Figure S13), indicating a possible reverse intersystem crossing process.51−53 Moreover, the electron distributions of HOMO and LUMO orbitals are clearly separated, which is in accordance with the small overlap of the HOMO and LUMO wave function and small ΔEST value of TADF compounds.54 Palladium Ion Detection. The heteroatom-rich nature of the fluorescent polymers prepared from this MCP might endow these products unique properties. For example, the heteroatoms might be able to interact with metal ions; in particular, the phosphorus-containing moieties can serve as ligand for platinum group metal ions. Palladium is a typical example of G

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Figure 7. (A) PL spectra of P1b/2a/3 in DMSO/water mixture with 50 vol % water in the presence of different amounts of Pd2+. (B) Stern−Volmer plot of relative intensity (I0/I) versus the Pd2+ concentration. I0 = PL intensity without Pd2+. Concentration: 10 μM.

Figure 8. (A) Photographs of P1b/2a/3 in DMSO/water mixtures with 50 vol % water containing different metal ions taken (upper) in daylight and (lower) under UV illumination. (B) Relative intensity (I0/I) of aqueous solution of P1b/2a/3 versus different metal ions. I0 = PL intensity without metal ions. Polymer concentration: 10 μM. Metal ion concentration: 50 μM.

be 2.4 × 10−7 M, suggesting high sensitivity toward Pd2+ ions. Moreover, the PL spectra of P1b/2a/3 in 50 vol % aqueous mixture were studied in the presence of 16 different metal ions including Ru3+, Rh3+, Pd2+, Pt4+, Cr2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Ce3+, and Pb2+ under the same condition for comparison, and only Pd2+ showed remarkable fluorescence response, indicating high selectivity of this fluorescent chemosensor (Figure 8). To investigate the working mechanism of the selective detection for Pd2+, 31P NMR and XPS spectra were measured to prove the interaction between phosphorus atom from the polymer and Pd2+. Through the comparison of the 31P NMR spectra of 8 and P1b/2a/3 without or with the presence of Pd2+, a new peak emerged in the low field at δ ∼25.2 after the addition of Pd2+, indicating the formation of coordination bond between Pd2+ and phosphorus amidine moiety (Figure S14). Furthermore, the XPS spectra for P 2p of P1b/2a/3 without or with the presence of Pd2+ are shown in Figure S15. A new peak emerged at 133.3 eV with higher energy, suggesting the effective binding of phosphorus atoms to Pd2+ ions, which is responsible for the high selectivity of the fluorescent sensor.

platinum group metal, whose complexes are extensively used as catalysts for the synthesis of a great number of materials through coupling, metathesis, oxidation, and reduction reactions.55−57 However, trace amounts of palladium residue can bring large effect on the optoelectronic properties of materials and may cause corrosion and allergenic potential on humans.58 The sensitive and specific detection of Pd2+ is hence an important issue in great demand. The conventional method for Pd2+ detection such as atomic absorption spectrometry and X-ray fluorescence spectrometry is generally expensive and time-consuming.59−61 In this work, the phosphorus-containing fluorescent polymers are explored as a fluorescent chemosensor for Pd2+, which enjoys a series of advantages such as rapid detect, low cost, reliability, high sensitivity, and selectivity. The nanoaggregates of P1b/2a/3 in DMSO/H2O mixtures with 50 vol % water were studied as an example. When aqueous solution of PdCl2 was gradually added into the nanoaggregates, the emission rapidly decreased while the spectral profile remained unchanged (Figure 7A). When the concentration of PdCl2 was increased from 0 to 100 μM, the PL intensity dramatically dropped. The Stern−Volmer plot of relative PL intensity (I0/I) versus the Pd2+ concentration appears as a straight line with a large quenching constant of 207 600 M−1 (Figure 7B), and the detection of limit (LOD) is calculated to H

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CONCLUSIONS In this work, facile and efficient one-pot three-component polymerizations of alkynes, sulfonyl azides, and iminophosphorane were reported for the synthesis of heteroatom-rich multifunctional polymers. The MCP proceeded in mild conditions at room temperature in the presence of CuI and triethylamine in THF, generating poly(phosphorus amidine)s with high molecular weights in excellent yields. Good monomer applicability of this MCP has been proved, and both aromatic diynes and aliphatic diynes can be polymerized to afford product with high molecular weight. This MCP not only serves as a tool to link monomeric units together in polymer chains but also builds new functional units in situ. In this case, phosphorus amidines embedded in the polymer backbone are directly formed through the MCP, which cannot be achieved by other methods. The small molecular phosphorus amidine model compounds generally possess AIE characteristics, and their crystals also show TADF property. The unique heteroatom-rich phosphorus amidine structure endows the polymer products a series of properties such as good solubility in polar solvents, good film-forming ability, satisfactory thermal stability, and high refractivity with small chromatic dispersions. Moreover, the fluorescent poly(phosphorus amidine)s can function as a sensitive and selective chemosensor for Pd2+ ion. The multicomponent polymerization of alkynes, sulfonyl azides, and iminophosphorane has proved to be a convenient and powerful tool for the synthesis of functional polymer materials with unique heteroatom-rich structure. It is anticipated that these multicomponent polymerizations can accelerate the development of polymerization methodology as well as new functional polymer materials.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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and P1b/2a/3 with or without Pd2+, respectively; XPS spectra for P 2p of P1b/2a/3 with and without Pd2+ (PDF) X-ray crystallographic data of compound 8 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*(R.H.) E-mail [email protected]; Tel +86-2223-7066. *(B.Z.T.) E-mail [email protected]; Tel +852-2358-7375. ORCID

Rongrong Hu: 0000-0002-7939-6962 Ben Zhong Tang: 0000-0002-0293-964X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Science Foundation of China (21404041, 21490573, and 21490574), the Young Elite Scientist Sponsorship Program of the China Association of Science and Technology (2015QNRC001), the Guangdong Natural Science Funds for Distinguished Young Scholar (2016A030306045), the Natural Science Foundation of Guangdong Province (2016A030312002), the National Basic Research Program of China (973 Program; 2013CB834701), the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01).

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REFERENCES

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The general procedure for the multicomponent polymerization is shown below with the MCP in Table 1, entry 3, as an example. Diyne 1a (0.10 mmol, 46 mg), disulfonyl azide 2a (0.10 mmol, 38 mg), iminophosphorane 3 (0.25 mmol, 88 mg), and CuI (0.02 mmol, 4 mg) were added into a 10 mL Schlenk tube equipped with a magnetic stirrer. After purging with dry nitrogen 5 times, 1 mL of THF and Et3N (0.20 mmol, 28 μL) were then added. After stirring at room temperature for 8 h, the reaction mixture was diluted by 3 mL of CHCl3 and then precipitated in 200 mL of CHCl3/hexane mixture (v/ v = 1/10) through a cotton-filled dropper. The precipitates were then collected by filtration and dried under vacuum to constant weight. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01096. Materials and instrumentation; experimental procedures and characterization data; single crystal data, 1H and 13C NMR spectra of compound 8; HRMS spectra of compounds 8 and 9; IR and 1H spectra of P1a/2b/3 and P1b−f/2a/3; 31P NMR spectra of 3, 9, P1a/2a/3, and P1e/2a/3; TGA thermograms of P1a−f/2a−b/3; refractive indices and chromatic dispersions of P1a−d/ 2a−b/3; absorption spectra of P1a/2b/3 and P1d−f/ 2a/3; XRD curves of 6, 8, and 9; PL spectra of 6, 9, P1a/ 2a/3, and P1c/2a/3 in DMSO/H2O mixtures with different H2O fractions; photophysical properties of 6, 8, 9, and P1a−c/2a/3; frontier-molecular-orbital distributions and energy levels of 6 and 8; 31P NMR spectra of 8 I

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