Synthesis of fluorescent poly(silyl indole)s via borane-catalyzed C–H silylation of indoles

Abstract

The development of a new polymerization strategy is essential for the design and synthesis of polymers with new frameworks, since it would effectively expand the diversity of the structures and properties of polymers. Here we report a metal-free B(C₆F₅)₃-catalyzed step-growth polymerization of indoles and hydrosilanes through regioselective dehydrogenation silylation of indoles. A series of indole and silyl-based monomers were designed and synthesized for the preparation of linear poly(silyl indole)s with good solubility through AA + BB-type or AB-type polymerization. In addition, we employed trisilane and biindole as building blocks to construct hyperbranched polymers with rigid structures. All the obtained polymers were fully structurally characterized and observed to fluorescently emit strong bright blue-violet light in both solution and in the solid state under UV light. These poly(silyl indole)s with their novel type of structure demonstrated very promising application potentials in organic optoelectronic materials.

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Introduction

Introducing small-molecule reactions to polymer synthesis has been a long-standing research topic in polymer science,^1–6 as the incorporated heteroatom^7–12 or heterocycle^13–19 can provide the obtained polymer not only with a new framework but also useful properties.^20–22 In recent years, the family of polymers with silicon atoms in their main chains, such as polysiloxanes, polycarbosilanes and polysilanes, has emerged as one of the most important families of functionalized polymeric materials. They are widely used as oils, rubbers, silicone resins, heat-resistant materials and optoelectronic materials, because of their excellent thermal, electronic and optical properties.^23–26 Progress has been made towards the application of organocatalytic step-growth polymerizations in the synthesis of organosilicon polymers, with such reactions including dehydrogenative coupling,^27–32 hydrosilylation of silanes with ketones, alkenes or alkynes,^33–37 the Piers–Rubinsztajn reaction^38,39 and others.^40,41 Using the indole skeleton to construct polymeric materials has also attracted intense attention. It has done so due to the wide applications of polyindoles and their derivatives in sensors, supercapacitors and electrocatalysts.^42–44 Electropolymerization and chemical oxidation in the presence of metal catalysts are common strategies for the synthesis of such polyheterocycles.^45–47 So far, there are only a few reports on the preparation of polyindoles from organic reactions.^48–50

Recently, by using B(C₆F₅)₃ as a catalyst, our group realized the convergent disproportionation of indoles into a series of C3-silylated (or borylated) indoles in up to 99% yields.^51,52 Being a powerful metal-free catalyst, B(C₆F₅)₃ has been shown to exhibit a long-life catalytic performance, as evidenced by the observation that both catalytic activity and excellent product yields can be maintained even after 10 sequential additions of starting materials. Furthermore, B(C₆F₅)₃ has been employed to achieve highly regioselective C-3 functionalization of indoles and complete reduction of quinoline at room temperature (RT). Mechanistic studies suggested that by sharing ammonium hydridoborate, C-3 functionalization of an indole and complete reduction of a quinoline would each consume a product generated from the other reaction such that they can mutually promote each other, thus representing an example of mutualism applied in organic synthetic chemistry.⁵³ Moreover, the employment of Al(C₆F₅)₃, a congener of B(C₆F₅)₃ but with stronger Lewis acidity, enabled the switchable C–H silylation of indoles through an Al(C₆F₅)₃-based thermally induced frustrated Lewis pair (FLP). Note that all these methods produced C3-silylated indoles in excellent yields, specifically up to 99%, which used to be challenging to accomplish.⁵⁴

The introduction of indole groups to silicon-containing polymeric frameworks is expected to produce more reactive chemical bonds, which can be post-functionalized with suitable reagents. Furthermore, they may be made luminescent due to the presence of silicon and indole-containing functional groups alternating within the polymer backbones. It is envisioned that the polymers containing both indole and silicon may exhibit properties superior to those of the existing polyindoles or silicon-containing polymers. In combination with the advantages offered by Lewis-acid-catalyzed reactions, such as high catalytic efficiency, mild reaction conditions and no requirement of additives, we expected that such Lewis-acid-catalyzed reactions can be employed as template reactions for post-polymerization modification⁵⁵ and to construct poly(silyl indole)s. Therefore, we focused our research efforts on the application of silylation of indoles to polymer synthesis. In this context, a series of biindoles, bifunctional monomers bearing both silyl and indole groups, and trisilyl monomers were successfully designed and synthesized. B(C₆F₅)₃-catalyzed dehydrosilylation of these monomers afforded a wide variety of linear and cyclic model compounds and poly(silyl indole)s. To the best of our knowledge, this was the first time that both indole and silicon-containing groups were simultaneously introduced to the backbone of a polymer. The obtained polymers were structurally characterized using Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Thermogravimetry analysis (TGA) revealed the high thermal stability levels of the poly(silyl indole)s. Furthermore, these poly(silyl indole)s were observed to emit blue violet light upon being photoexcited, and their photoluminescence behaviors were investigated by carrying out UV-Vis and fluorescence spectroscopy analyses as well.

Results and discussion

Monomer synthesis

Various types of monomers, including biindoles, bifunctional monomers bearing both silyl and indole groups, and trisilanes, were designed and prepared in good to high yields (see ESI† for more details about the syntheses). The biindole monomers (1a–1c) were prepared by carrying out Suzuki–Miyaura coupling reactions between bromo-substituted N-methylindole and indolylboronic acids (Scheme 1A).^56,57 AB-type monomers bearing both silyl and indole groups (2a–2d) were synthesized by carrying out Suzuki–Miyaura cross-coupling reactions between indolylboronic acids and 1-bromo-4-iodobenzene (or 1-bromo-3-iodobenzene) followed by lithiation of indoles bearing the 4-bromophenyl (or 3-bromophenyl) group and then alkylation with Me₂SiClH (Scheme 1B). Similarly, the trisilane 3b was prepared from 1,3,5-tris(4-bromophenyl)benzene and 1,4-bis(dimethylsilyl)benzene by first subjecting these starting materials to lithiation and then subjecting the resulting species to alkylation (Scheme 1C).

Scheme 1 Structures of and synthetic routes to (A) biindoles, (B) bifunctional monomers and (C) trisilane.

Model reaction and polymerization

We also set out to investigate the possibility of achieving a C–H silylation of N-methylindole in a polymer synthesis in the presence of 1 mol% B(C₆F₅)₃. Reacting 1,1′-diethyl-5,5′-biindole (1a) with PhMe₂SiH in a 1 : 2 ratio at 120 °C produced model compound 1 (MC1) as a white solid in an 84% yield, under the previously reported optimal conditions⁵¹ (Scheme 2A). The molecular structure of the product was confirmed using single-crystal X-ray diffraction (Fig. 1A). Under the same reaction conditions, the C–H silylation of 1-methyl-5-phenyl-indole and 1,4-bis(dimethylsilyl)benzene (3a) furnished model compound 2 (MC2) in 82% yield (Scheme 2B).

Scheme 2 Synthetic routes to (A) MC1 and (B) MC2.

Fig. 1 Depictions of X-ray crystal structures of (A) MC1 and (B) MC3, with hydrogen atoms omitted for clarity and ellipsoids drawn at 50% probability.

Next, we employed this metal-free catalyst system and 3a as a common comonomer to expand the scope of biindoles produced. Taking into consideration the characteristics of both the polymerization and organic reaction, we further optimized the reaction conditions for the polymerization. Using 5 mol% catalyst and a prolonged reaction time (72 h), polymerization of 1a and 3a in toluene at 120 °C produced polymer 1 (P1) with a weight-average molecular weight (M_w) of 7.1 kg mol⁻¹ in 82% yield (Table 1, entry 1). Switching to biindole 1b or 1c, polymerization still proceeded smoothly, furnishing polymer 2 (P2) (5.7 kg mol⁻¹) or polymer 3 (P3) (3.9 kg mol⁻¹), respectively (Table 1, entries 2 and 3). In general, step-growth polymerization of monomers bearing bifunctional groups (AB-polymerization) provides a powerful strategy for AA + BB polymerization. AA + BB polymerization, however, is generally notoriously sensitive to monomer purity and causes a stoichiometric imbalance. A monomer composed of both indole and silyl groups was expected to eliminate the requirement of stoichiometric balance and simplify the polymerization procedures. The employment of indoles with the 4-(dimethylsilyl)phenyl group at the C5, C6 or C7 position (2a–2c) yielded the corresponding polymers (P4, 3.9 kg mol⁻¹; P5, 4.8 kg mol⁻¹; P6, 80%), respectively (Table 1, entries 4–6). When 5-(3-(dimethylsilyl)phenyl)-1-methyl-indole (2d) was used, near-quantitative conversion of monomers to only oligomers occurred (Table 1, entry 7). Interestingly, after purification and precipitation, model compound 3 (MC3), a cyclic dimer, was identified to be the major product, which was structurally characterized from NMR and high-resolution mass spectrometry (HRMS) analyses. Its solid-state structure was confirmed from a single-crystal X-ray diffraction study (Fig. 1B). The concentration of 2d was found to have a significant impact on the condensation (Table S1†). When the concentration of 2d was increased from 0.1 to 0.2, 0.4 and 0.6 M, the yield of MC3 gradually decreased from 75% to 70%, 56% and 34%, respectively. Further increasing the concentration of 2d resulted in precipitation, further highlighting the difficulty in synthesizing the polymers. Note that employment of monomers with certain structures and at low concentration tended to result in cyclization rather than intermolecular polymerization. As shown in Table S1,† when polymerization was performed using a monomer concentration of 0.1 M, we obtained cyclic dimer in up to 75% yield, demonstrating that we may easily obtain diverse products through such condensation.

Table 1 Structures of the obtained polymers and polymerization results.^a

Entry	Monomer	Polymer	Yield (%)	M n ^b (kg mol^−1)	M w ^b (kg mol^−1)	PDI^b
a Conditions: 5 mol% B(C6F5)3, 120 °C, 72 h. For entries 1–3, 1 (0.5 mmol) and 3a (0.5 mmol) in toluene (0.5 mL); for entries 4–7, bifunctional monomers (0.3 mmol) in toluene (0.5 mL); for entry 8, 1a (0.5 mmol) and 3b (0.33 mmol) in toluene (0.5 mL). b Determined from GPC relative to polystyrene (PS) standards in THF. c No polymerization.
1	1a + 3a	P1	82	4.7	7.1	1.52
2	1b + 3a	P2	66	3.3	5.7	1.74
3	1c + 3a	P3	78	1.9	3.9	2.02
4	2a	P4	72	2.1	3.9	1.84
5	2b	P5	81	2.7	4.8	1.76
6	2c	P6	80	2.5	3.2	1.27
7^c	2d	—	—	—	—	—
8	1a + 3b	P7	89	2.8	8.1	2.90

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Hyperbranched polymers, constituting an important class of functional polymeric materials, are widely applied in various areas such as coatings, drug delivery carriers, biomaterials, nanomaterials and so on.^58–61 In order to obtain silicon-containing polymers displaying diverse topologies, trisilane 3b was synthesized and copolymerized with biindole 1a for the synthesis of hyperbranched polymers. The polymerization was performed by reacting 1a (0.5 mmol) with 3b (0.33 mmol) in the presence of 5 mol% B(C₆F₅)₃ catalyst at 120 °C, and furnished P7 with a moderate M_w value of 8.1 kg mol⁻¹ in 82% yield. Due to its rigid structure, the obtained transparent block solid (P7) was partially soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, benzene, toluene, and N,N-dimethylformamide.

Structural characterization and thermal stability

Considering the similar structures of these polymers, P1 was selected as an example and structurally characterized using NMR spectroscopy. Based on a comparison of the ¹H NMR spectra (shown in Fig. 2) of P1 (Fig. 2A), model compound MC2 (Fig. 2B), monomers 1a (Fig. 2C) and 3a (Fig. 2D), the characteristic peak centered at δ 4.46 ppm was assigned to the hydrosilane protons of monomer 3a; this peak disappeared in the spectra of MC2 and P1. The doublet centered at δ 0.38 ppm and attributed to the silyl methyl group protons of 3a became a singlet and shifted to δ 0.64 for MC2 and 0.66 ppm for P1. The peak at δ 6.57 ppm, corresponding to the protons at the C3 position of the indole ring in 1a, completely disappeared after production of MC2 and that of P1. Meanwhile, the characteristic [N-CH₃] resonance peak at δ 3.86 ppm in 1a shifted toward high field, reaching 3.83 ppm for MC2 and 3.71 ppm for P1. In combination with the polymeric nature as revealed by the typically broad peaks observed for P1, these results clearly demonstrated that successful copolymerization occurred between 1a and 3a.

Fig. 2 ¹H NMR spectra of (A) P1, (B) MC2, (C) 1a and (D) 3a in CDCl₃.

We also analyzed the FT-IR spectrum of P1 and compared it with those of monomers 1a and 3a (Fig. 3A–C). As seen in Fig. 3A, the characteristic peak of 3a associated with Si–H stretching vibration was observed at 2121 cm⁻¹; this peak almost disappeared in the spectrum of P1 (Fig. 3B), indicating the consumption of the Si–H group of the monomers and occurrence of polymerization. Similar results were observed in the FT-IR spectrum of P4 (Fig. S2†). Furthermore, thermogravimetric analysis (TGA) of the obtained silicon-containing polymers showed the linear polymers displaying high thermal stability (Fig. 3D). There was only a less than 5% weight loss in nitrogen (T_d5) below 205 °C for P1–P6. Note that P1, P2 and P3, composed of AA + BB monomers, each exhibited two stages in the thermal decomposition process. In sharp contrast, P4, P5 and P6 derived from AB-polymerization each showed only one step in the thermal decomposition process and the major weight loss occurred in the range 350–550 °C. P7 underwent a more complex thermal decomposition process, exhibiting a relatively low T_d5 value of 171 °C but high residual mass of 47% at 800 °C.

Fig. 3 FT-IR spectra of (A) 3a, (B) 1a and (C) P1. (D) Thermogravimetric analysis (TGA) results for the obtained polymers P1–P7.

Photophysical properties

Due to the application of the indole derivatives as fluorescent materials, we further investigated the optical properties of the obtained polymers. These polymers were all purple blue solids, and able to fluorescently emit strong bright blue-violet light both in solution and in the solid state when exposed to UV light (Fig. 4A). To get more information about the fluorescence properties of this conjugated alternating copolymer, we compared the UV-Vis absorption spectra of diluted P1–P7 THF solutions (Fig. 4A). It turned out that they all showed strong absorption tailing to 400 nm due to their conjugated units. Fluorescence spectra shown in Fig. 4B revealed that the maximum emission wavelength (λ_max) values of the polymers were between 360 and 380 nm, attributed to the intramolecular charge transfer between indole donor and silane acceptor units in the polymeric backbone of poly(silyl indole)s. All these results demonstrated the very promising application prospects of these polymers with novel polymeric frameworks in organic optoelectronic materials.

Fig. 4 (A) UV-Vis absorption spectra of diluted P1–P7 THF solutions. (B) Photoluminescence (PL) spectra of diluted P1–P7 THF solutions. Inset: photographs of fluorescent P1 in the solid state (left) and in THF solution (right) taken under 365 nm-wavelength UV irradiation.

Conclusions

In summary, we developed a strategy involving metal-free B(C₆F₅)₃-catalyzed step-growth polymerization of indoles and hydrosilanes through highly regioselective silylation of indoles. A broad variety of biindoles and silyl substituted indoles were designed, synthesized and utilized as monomers. In model reactions, both linear and cyclic oligomers can be obtained in high yields by selecting monomers with appropriate structures and adjusting the monomer concentration. Linear poly(silyl indole)s with excellent solubility in common organic solvents can be obtained, specifically through AA + BB polymerization for P1–P3 and AB polymerization for P4–P6. Hyperbranched polymers can also be prepared from copolymerization of trisilane and biindole. These polymers were fully structurally characterized from the results of FT-IR spectroscopy, NMR spectroscopy, GPC and TGA studies. Moreover, the obtained poly(silyl indole)s all exhibited strong bright blue-violet fluorescence in both solution and solid states. Our strategy provided a simple and efficient way to synthesize poly (silyl indole)s with a novel polymeric framework showing promising application prospects in the field of organic optoelectronic materials. Further investigation of the practical applications of these poly(silyl indole)s is underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21871107, 22225104, 22071077 and 21975102).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR spectra, crystal data, bond lengths and angles, and further tabular data. CCDC 2167809 and 2167804. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2py01470j

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