Direct Construction of Acid-Responsive Poly(indolone)s through

Letter Cite This: ACS Macro Lett. 2019, 8, 569−575

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Direct Construction of Acid-Responsive Poly(indolone)s through Multicomponent Tandem Polymerizations Chunxuan Qi,†,‡ Chao Zheng,†,‡ Rongrong Hu,*,† and Ben Zhong Tang*,†,§ †

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State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510641, China § Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Multicomponent polymerizations (MCPs) as a burgeoning field in polymer chemistry has proved to be a powerful and popular tool for the synthesis of functional polymer materials with diverse and complex structures. To explore the general applicability of MCPs and enrich the product structures of MCPs, multicomponent tandem polymerizations (MCTPs) with great synthetic simplicity and efficiency were pursued. In this work, MCTPs of N-(2iodophenyl)-3-phenyl-N-tosylpropiolamide, aromatic terminal alkynes, and diamines were explored through combining Sonogashira coupling and Michael addition reaction in a one-pot procedure. The MCTPs could proceed efficiently and conveniently under mild conditions with Pd(PPh3)2Cl2, CuI, and i-Pr2NEt, affording 12 poly(indolone)s with unique structures and high Mws (up to 30400 g/mol) in high yields (up to 97%). The poly(indolone)s possess a unique acid-triggered fluorescence “turn-on” response which could realize specific detection of CF3SO3H from other inorganic and organic acids through a rapid acid-catalyzed reaction from enamine to ketone.

T

producing heteroatom-rich poly(N-sulfonylamidine)s with high yields and high molecular weights (Mws), releasing N2 as the only byproduct.27−30 Most recently, we’ve reported the catalyst-free MCPs of elemental sulfur, diamines, and diynes/ diisocyanides to directly convert elemental sulfur to sulfurcontaining functional polythioamides/polythioureas with welldefined structures and good solubility under mild conditions.31,32 The major challenges of developing new MCPs from small molecular organic reactions are the various side reactions that may occur among the multiple monomers, the impure structure embedded in the polymer backbone, the unidentified terminal groups, and the solubility issue of the polymer products, which lead to limited reports of MCPs and their narrow scope of product structures. To explore the general applicability of MCPs and enrich the product structures of MCPs, multicomponent tandem polymerizations (MCTPs) were proposed to combine multiple steps in a one-pot procedure, which avoided the purification of intermediates and directly afforded polymer products with well-defined structures.33 Through MCTP strategy, various reactions with compatible conditions could be integrated in one polymerization to realize highly efficient chemical conversion and

he constant pursuing of new polymer synthetic methodology plays a vital role in the advances of polymer materials. Multicomponent polymerizations (MCPs), as a burgeoning field in polymer chemistry, have gained wide popularity recently among polymer scientists.1−4 Besides the advantages inherited from small molecular multicomponent reactions, such as high atom/step economy, simple and inexpensive reactants, environmental benefits, synthetic efficiency, and operational simplicity,5−7 the unique feature of MCPs compared with other polymerizations is the great structural diversity of the polymer products that could be achieved through various combinations of multiple monomers.8−17 Most importantly, unlike other polymerizations that only link functional monomeric units together in a polymer chain, MCPs could often build new functional units embedded in the polymer mainchain directly from the polymerizations.18,19 With these features, MCPs could serve as a powerful tool for the construction of libraries of functional polymer materials with great synthetic simplicity and efficiency. For example, Li et al. and Meier et al. have reported the Passerini three-component polymerizations and Ugi four-component polymerizaitons for the synthesis of linear and hyperbranched poly(ester−amide)s/polyamides, respectively;20−25 Tang et al. have reported the A3-polycoupling of alkynes, amines, and aldehydes for the synthesis of polyamines;26 Choi et al. and we have also reported a series of Cu(I)-catalyzed room temperature MCPs based on alkynes and sulfonyl azides, respectively, © XXXX American Chemical Society

Received: April 21, 2019 Accepted: April 26, 2019

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DOI: 10.1021/acsmacrolett.9b00297 ACS Macro Lett. 2019, 8, 569−575

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ACS Macro Letters Table 1. Optimization of the Multicomponent Tandem Polymerization of 1, 2a, and 3aa

entry

[1] (M)

T2 (°C)

t2 (h)

yield (%)

Mw (g/mol)b

Mw/Mnb

1 2 3 4 5 6 7 8 9 10 11

0.025 0.05 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1

70 70 70 70 rt 40 50 55 60 60 60

24 24 24 24 24 24 24 24 24 12 6

98 91 90 91 87 84 84 84 86 83 84

9400 16100 27700 15400 19600 24500 25000 27200 28500 30400 29300

1.21 1.44 1.82 1.47 1.71 2.01 1.90 1.94 1.96 1.99 1.96

a

Monomer 1 was reacted with 2a at room temperature in THF solution under nitrogen in the presence of Pd(PPh3)2Cl2, CuI, and i-Pr2NEt for 24 h. 3a was then added to react at different temperatures (T2) with different times (t2). The polymerization was carried out with 0.2 mmol of 1, [1] = 2[2a] = 2[3a], [Pd(PPh3)2Cl2] = 0.05 [1], [CuI] = 0.1 [1], and [i-Pr2NEt] = 10 [1]. bDetermined by GPC in THF on the basis of a polystyrene calibration.

were generally efficient, which can be conducted under mild conditions, producing poly(indolone)s with complex structures, high yields, and high Mws. The unique chemical structures of these polymers endow them with a fluorescence “turn-on” response toward acids, which could distinguish trifluoromethanesulfonic acid from many other inorganic and organic acids. To develop one-pot two-step three-component tandem polymerization for the preparation of poly(indolone)s, monomer 1 was synthesized (Scheme S2A), electron-deficient aromatic diyne 2a was designed concerning its high reactivity, and secondary diamine 3a was chosen as a reactive nucleophile. The polymerizations of 1, 2a, and 3a were carried out in the presence of Pd(PPh3)2Cl2, CuI, and i-Pr2NEt in THF under nitrogen. In the first step, monomer 1 was reacted with 2a for 24 h at room temperature, and 3a was then added in the second step to proceed with the Michael addition at 60 °C for 12 h to afford polymer P1/2a/3a (Table 1). The reaction conditions of the MCTP such as monomer concentration, reaction temperature, and time were optimized. When the concentration of 1 was increased from 0.025 to 0.2 M, while keeping the monomer loading ratio [1]:[2a]:[3a] = 2:1:1, the polymerization yields were generally high, but significant influence on the Mws of the products was observed and the best result was obtained with a Mw of 27700 g/mol and a yield of 90% from the monomer concentration of 0.1 M (Table 1, entries 1−4). The reaction temperature of the second step after the addition of 3a was then investigated from room temperature to 70 °C (Table 1, entries 3 and 5−9). A gradual increase of both yield and Mw was observed and a product with a Mw of 28500 g/mol was obtained in 86% yield at 60 °C. When the second step was reacted for 6 h, a polymer with a Mw of 29300 g/mol was already produced in 84% yield, indicating the high efficiency of the MCTP. Further increasing the reaction time did not show significant influence on the polymerization result (Table 1, entries 9−11). In addition, the

afford polymers with diverse structures. For example, Sonogashira coupling of alkyne and carbonyl chloride, Michael addition of electron-deficient internal alkyne and phenylhydrazine, as well as the following cyclization, were combined in a MCTP under mild condition at room temperature, generating new aromatic pyrazole rings embedded in the polymer mainchain with excellent yields and high Mws.34 The MCTP of diyne, guanidine hydrochloride, DMSO, and O2 was reported to generate conjugated poly(pyrimidine)s with welldefined and tunable structures through combining Glaser coupling-nucleophilic addition-heterocyclization-oxidation reactions in one pot;35 a metal-free MCTP of activated internal alkyne, formaldehyde, and aromatic diamines was reported to realize the synthesis of sequence-controlled poly(tetrahydropyrimidine)s with high yield and high Mws at room temperature from one single reaction.36,37 These work have demonstrated the great possibility of reactions and polymer product structures from MCTPs. Recently, the consecutive three-component cyclocarbopalladation, Sonogashira coupling, Michael addition reactions of N(2-iodophenyl)-3-phenyl-N-tosylpropiolamide 1, aromatic alkynes, and amines were reported by Müller et al., which were featured with high efficiency, broad scope of substrates, mild conditions, and interesting photophysical property of products.38−43 The mechanism was depicted in Scheme S1: the oxidative addition of A first took place to produce palladium species B, followed by an intramolecular insertion to generate intermediate C. The coupling reaction of C and a terminal alkyne D then occurred to afford intermediate E in the presence of CuI and i-Pr2NEt. Subsequently, the Michael addition of the newly added amine to the CC bond in E proceeded, and the indolone product H was furnished after prototropy. In this work, a series of aromatic diyne monomers were designed and synthesized, and their MCTPs with 1 and commercially available diamines were explored. These MCTPs 570

DOI: 10.1021/acsmacrolett.9b00297 ACS Macro Lett. 2019, 8, 569−575

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ACS Macro Letters

Figure 1. (A) Multicomponent tandem polymerizations of 1, diynes 2a−f, and diamines 3a−g. (B) Chemical structures, yields, and Mws of P1/2a− f/3a−g. Mws were determined by GPC in THF on the basis of a polystyrene calibration.

with the polymerization result obtained under nitrogen, the Mw of the product was decreased to 17600 g/mol when the second

MCTPs of 1, 2a, and 3a were conducted in air to learn the influence of air condition on each step (Table S1). Compared 571

DOI: 10.1021/acsmacrolett.9b00297 ACS Macro Lett. 2019, 8, 569−575

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ACS Macro Letters step reaction after adding 3a was operated in air. When both steps were carried out in air, the Mw of the product further decreased to 12200 g/mol, while the yield did not change much, proving the feasibility of the MCTP in air. To test the applicability of this MCTP, various aromatic diynes 2a−f were designed and synthesized,44 and commercially available primary and secondary aliphatic diamines 3a−f, as well as aromatic diamine 3g, were purchased to polymerize with monomer 1 to produce 12 poly(indolone)s (Figures 1 and S1). Under the optimized conditions, the MCTPs of 1, 2a−f, and 3a−g generally proceeded smoothly, producing polymers with high Mws of up to 30400 g/mol in high yields of up to 97%, indicating a wide monomer scope and the general high efficiency of this polymerization. In particular, the electron-deficient aromatic diyne 2a enjoyed improved Mws of the polymerization product. From the less-favored nucleophiles, such as the secondary diamines 3b and 3c, with large steric hindrance and the aromatic diamine 3g, the Mws of the polymerization products are relatively small. Some of the polymers possess narrow PDI values, probably due to their relatively small polymerization degree and limited soluble range of Mws. The chemical structures of the poly(indolone)s were characterized and a small molecular model compound 4 possessing a similar structure with the repeating unit of the poly(indolone)s was synthesized (Scheme S2B and Figure S2). The comparison of IR, 1H NMR, and 13C NMR spectra of monomers 1, 2a, and 3a, model compound 4, and P1/2a/3a proves the expected structure of the polymer. For example, in the IR spectra of P1/2a/3a, the CC stretching vibration of 1 at 2218 cm−1, the CC and C−H stretching vibrations of 2a at 2103 and 3280 cm−1, respectively, and the N−H stretching vibration of 3a at 3190 cm−1 all disappeared, verifying the consumption of the CC bonds from 1 and 2a and the N−H bond from 3a in the MCTP (Figure S3). Furthermore, the CO stretching vibrations of 1 and 2a at 1680 and 1643 cm−1, respectively, as well as the stretching vibrations of saturated C−H bonds of 3a at 2919, 2844, and 2744 cm−1, all were retained in the spectrum of P1/2a/3a, corresponding to the expected structure of poly(indolone). More structural details could be revealed by the NMR spectra. In the 1H NMR spectrum of P1/2a/3a, the resonance of terminal acetylene proton of 2a at δ 3.26 disappeared, while the peaks from the aromatic protons of 1 at δ 8.10 and 8.03 and the methyl peak at δ 2.47 were all retained (Figure 2 and Figure S4). Most importantly, the resonance peak of the aromatic proton next to the iodine substituted group of 1 at δ 7.42 was shifted to high field at δ 5.24 and 5.42 in the spectra of 4 and P1/2a/3a, respectively, because of the removal of neighboring iodine substituent, the formation of the fused indolone ring, and the shielding effect from the adjacent benzene ring in the products.45 Moreover, new peaks emerged at δ 7.54 and 7.60 in the spectra of 4 and P1/2a/3a, respectively, originated from the newly formed olefin proton in the products. Similarly, in the 13C NMR spectra of P1/2a/3a, the resonances of CC of 1 at δ 81.83 and 93.37, and that of 2a at δ 80.31 and 82.72, together with the resonance of the carbon atom of 1, which was connected with an iodine atom at δ 102.63, all disappeared, suggesting the expected conversion (Figure S5). The characteristic peaks from the model compound 4 all emerged in the 13C NMR spectra of P1/2a/ 3a, proving their similar chemical structures. The resonance peak of the CO group on the indolone ring from 4 at δ

Figure 2. 1H NMR spectra of (A) 1, (B) 4, and (C) P1/2a/3a in CDCl3. The solvent peaks are marked with asterisks.

165.60, the CH3 group from 1 at δ 21.79, and the peak of the CO group from 2a at δ 195.02 were retained in the spectrum of P1/2a/3a with a slight shift.46−50 The IR spectra (Figures S6−S8), 1H NMR spectra (Figures S9−S11), and 13C NMR spectra (Figures S12−S14) of the other 11 polymers were also measured, proving their corresponding structures of poly(indolone)s. All these polymers enjoy good to excellent solubility in organic solvents such as chloroform, dichloromethane, tetrahydrofuran, and N,N′-dimethylformamide. Thermogravimetric analysis indicated that all the polymers showed satisfactory thermal stability. Their decomposition temperature at 5 wt % weight loss range is from 242 to 304 °C under N2, and they possess high thermal resistance with a high char yield of up to 65% (Figure S15). The poly(indolone)s were generally nonemissive or faintly emissive in solution or solid states, while the acid-triggered fluorescence “turn-on” response was observed for all the polymers. When a filter paper loaded with P1/2c/3a was fumed by HCl vapor, the appearance of the filter paper turns from red to orange in daylight, and red emission emerged upon UV irradiation (Figure S16). Their UV and photoluminescence spectra before and after the treatment of hydrochloric acid are shown in Figures S17 and S18. The absorption maxima of the THF solutions of the 12 poly(indolone)s were located at 466−489 nm and they were generally nonemissive. For most of the polymers, when HCl was added into the polymer solution, the absorption maxima were dramatically blue-shifted, while a strong fluorescence peak emerged at 433 nm. Besides the influence on UV and PL spectra, the Mws of P1/2a/3a were also dramatically decreased upon the addition of increasing amounts of HCl, indicating the breakage of the polymer chain under acidic condition (Table S2). For the other 11 polymers, a similar acid-triggered molecular weight decrease is observed, indicating that the poly(indolone)s can generally undergo degradation in acidic conditions (Table S3). Furthermore, P1/2c/3a was then investigated as an example for the study of selective fluorescence detection of acids, and 572

DOI: 10.1021/acsmacrolett.9b00297 ACS Macro Lett. 2019, 8, 569−575

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ACS Macro Letters 11 commonly used inorganic and organic acids such as HCl, HNO3, H2SO4, CF3SO3H, CH3SO3H, TsOH, CF3COOH, CH3COOH, HCOOH, PhCOOH, and oxalic acid were tested. As shown in Figure 3, when the same amount of acid was

Figure 4. Acid-catalyzed reaction from enamine to ketone and the single crystal structures of model compounds 4 and 5.

to form imine ion K, which was then attacked by H2O to form intermediate L. The proton was transferred from the oxygen to the nitrogen atom to form M. The diethylamine moiety was then removed as a leaving group, and after deprotonation, the ketone product 5 was furnished. In the single crystal structure of 4, intramolecular π−π stacking interaction was observed between the two parallel phenyl rings with a center distance of 3.588 Å, while in the single crystal structure of 5, these two phenyl rings are separated with a large distance and show no interaction. With this insight, the poly(indolone)s may undergo similar enamine to ketone degradation in acidic conditions, as shown in Scheme S4. The photophysical properties of 4 and 5 were significantly different. The UV absorption maxima of the THF solution of 4 was 482 nm, which was red-shifted 122 nm compared with that of 5, suggesting the elongated conjugation of 4 (Figure S21A). While the THF solutions and the solid powders of 4 and 5 were faintly emissive, their crystals show obvious emission at 604 nm (4) and 506 nm (5), with a fluorescence quantum efficiency of 2.6% (4) and 9.3% (5) (Figure S21B), suggesting typical crystallization-induced emission behavior. The luminescence behavior of compound 5 with stronger emission was also studied in THF and water mixtures with different water contents (Figure S22). The emission intensity of its THF solution at 497 nm was significantly increased with the water content that was increased from 0 to 90 vol %, with a slight shift of the emission maximum to 483 nm, indicating its aggregation-induced emission property. In this work, one-pot, two-step, three-component tandem polymerizations of N-(2-iodophenyl)-3-phenyl-N-tosylpropiolamide 1, terminal diynes 2a−f, and diamines 3a−g were designed through combining Sonogashira coupling reaction, the Michael addition reaction of enyne indolone intermediates, and amine sequentially. The MCTPs could proceed efficiently and conveniently to produce 12 poly(indolone)s with unique and complex structures and high Mws in high yields at mild condition under the catalysis of Pd(PPh3)2Cl2 and CuI. The MCTPs enjoy wide monomer scope, including various aromatic diynes and primary and secondary aliphatic amines, as well as aromatic amines. Furthermore, all the poly(indolone)s show unique acid-triggered fluorescence “turnon” response and specific fluorescence detection of CF3SO3H was realized, owing to the rapid acid-catalyzed reaction from enamine to ketone, which was proved by the small molecular model reactions. It is anticipated that this multicomponent tandem polymerization could provide an efficient tool for the synthesis of functional luminescent materials with unique chemical structures.

Figure 3. (A) UV spectra, (B) PL spectra, and (C) photo of P1/2c/ 3a in THF after 0.5 h of the addition of 10 equivalents of different acids. Concentration: 10 μM (A and C) and 5 μM (B); Excitation wavelength: 485 nm. The pKa values of the acids indicated in (C) were taken from refs 51−56.

added into the THF solution of P1/2c/3a, a dramatic change on UV absorbance and luminescence was observed specifically from CF3SO3H after 0.5 h. Upon the addition of CF3SO3H, the polymer solution color changed from red to light yellow in daylight, the absorption maximum at 489 nm shifted to 460 nm, and the absorptivity significantly decreased, and the polymer solution turned from nonemissive to red-emissive with the emission maximum located at about 619 nm. Other acids did not bring obvious changes in UV and PL spectra in 0.5 h; this is probably due to the strong acidity of CF3SO3H with the lowest pKa value of −15.7 among all the tested acids,51−56 which may be distinguished from other acids by a fast reaction rate in acid-assisted reactions. When the time was elongated from 0.5 to 8 h, other strong acids also show decreased absorptivity and emerged red emission (Figure S19). Interestingly, the change in absorption and emission of P1/2c/ 3a could be used to distinguish HCl, HNO3, and H2SO4 through different responses. When the same amount of these three acids were added into the THF solution of P1/2c/3a, the red color of the blank solution was faded to light yellow in the presence of HCl and H2SO4, but to magenta in the presence of HNO3, while the fluorescence “turn-on” was barely seen for HNO3, but emission can be observed at 639 nm for HCl and 582 nm for H2SO4. To reveal the underlying mechanism of the acid “turn-on” fluorescence response of the polymers, model compound 4 was treated with HCl under similar conditions to study the chemical conversion, and product 5 was isolated and purified (Figures 4 and S20). Both single crystals of 4 and 5 were obtained to confirm the expected structures of these compounds and this acid-catalyzed reaction (Table S4). The possible mechanism was proposed in Scheme S3. The amine site of compound 4 was first protonated in the acidic environment. The protonated enamine J was then isomerized 573

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ACS Macro Letters

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00297. Materials and instruments; Synthesis and characterizations; Reaction mechanisms; FT-IR spectra of monomer 1, 2a, 3a, 4, and P1/2a/3a; FT-IR, 1H and 13 C NMR spectra of P1/2a−f/3a−g; Thermogravimetric analysis of P1/2a−f/3a−g; UV and PL spectra of P1/2a−f/3a−g in THF before and after the addition of HCl; Change of Mw of P1/2a−f/3a−g after the addition of HCl; UV and PL spectra of P1/2c/3a in THF before and after 8 h of the addition of different acids; HRMS spectra and photophysical properties of 4 and 5. (PDF) Single crystal data of 4 (CIF) Single crystal data of 5 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. ORCID

Rongrong Hu: 0000-0002-7939-6962 Ben Zhong Tang: 0000-0002-0293-964X Author Contributions ‡

These authors contributed equally.

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 (21490573, 21822102, 21774034, 21490574, and 21788102), the Natural Science Foundation of Guangdong Province (2016A030306045 and 2016A030312002), Guangdong Special Support Program, and the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01).

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REFERENCES

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Letter

ACS Macro Letters

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DOI: 10.1021/acsmacrolett.9b00297 ACS Macro Lett. 2019, 8, 569−575