Synthesis of indole-based functional polymers with well-defined structures via a catalyst-free C–N coupling reaction

Abstract

Poly(N-arylene diindolylmethane)s (PMDINs) with precise structures and high molecular weights (M_w up to 389 200) in high yields (up to 95%) were synthesized by the catalyst-free nucleophilic substitution polycondensation of 3,3′-diindolylmethane with different activated difluoro monomers via a C–N coupling reaction process. A model reaction was carried out to assist in determining the optimal reaction conditions for the polymerization and to elucidate the chemical structures of the polymers. The resulting polymers exhibited good thermal stability with high decomposition temperature (T_5% ≥ 377 °C). Fluorescent spectral studies indicated that all these PMDINs had strong solid-state fluorescence. Especially, the polymer PMDIN-3 carrying sulfonyl units was a good blue-light emitter with high quantum yields (21.6%, determined against quinine sulfate). The results obtained by cyclic voltammetry suggested that PMDINs possessed good electroactivity. In addition, owing to the electrochemical activity of the indole rings at the 2-position, PMDIN-3 was readily cross-linked by electrochemical oxidation and the cross-linking film was characterized by scanning electron microscopy. High molecular weights and good comprehensive performance of the indole-based polymers suggested that the catalyst-free C–N coupling reaction of indole derivatives with difluoro monomers could be considered as an effective polymerization route for the synthesis of new functional polymers with well-defined structures.

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Introduction

Indole is ubiquitous in biological and biochemical structures, and has attracted extensive interest in the construction of pharmaceuticals, fragrances, agrochemicals, dyes and materials in the past decades due to its unique electrical, chemical, and optical properties.^1,2 There are multiple reactive sites in the indole group e.g. 1-hydrogen position, 2-position, 3-position, etc. (see numbering of indole nucleus in Scheme 1), which can be used to design numerous significant bioactive natural products and chemical intermediates through C–N (ref. 3–5) or C–C (ref. 6 and 7) bond-forming reactions. The reactive sites in indole group could also be used to construct indole-based polymers for the applications in electronics, electrocatalysis, anode materials, anticorrosion coatings and biological areas, such as polyindole, which is usually obtained by polymerization of indole on its 2,3-position through the synthetic strategy of chemical oxidation or electrochemical polymerization.^8–10 Despite considerable efforts, the availability of such polymers is limited to a certain extent for a variety of reasons, including molecular weights, solubility in common organic solvents like CHCl₃, mechanical strength and thermal stability, etc.^8d,11 Therefore, the development of a new strategy for construction of soluble, heat-resistant and functional polyindoles with high molecular weights through chemical polymerisation still remains an attractive and promising goal.

Scheme 1 Synthesis of poly(N-arylene diindolylmethane)s (PMDINs) via C–N coupling reaction.

In recent years, the catalytic N-arylation of indole using transition-metal^3–5 has become an important synthetic procedure for series of indole-based compounds. In view of some drawbacks in industrial applications of these transition-metal catalysts, such as expensive, oxygen sensitive, difficult to remove after the reaction and apt to leave toxic trace metal contaminants. Therefore, the development of a mild and efficient method for constructing N-arylindole motifs from indoles is highly desirable. In this study, we explore a facile and easier controlled polymerization technique for the synthesis of novel poly(N-arylene diindolylmethane)s (PMDINs) with structural regularity by a catalyst-free C–N coupling polycondensation reaction. 3,3′-Diindolylmethane with two functional groups is rationally designed to react with activated difluoro monomers, which could proceed smoothly under milder nucleophilic substitution polycondensation reaction conditions compared with our previous reports¹² and successfully lead to chain propagation to give high molecular weight polymers (Scheme 1). Since the new indole-based polymers possess indole rings in the main chain, they are expected to be endowed with high thermal stability, outstanding photoluminescence and good electroactivity, which would warrant their potential of applications for advanced high-performance materials.

Experimental section

Materials

1,3-Bis(4′-fluorobenzoyl)benzene and indole were purchased from Sigma-Aldrich Chemicals and used without further purification. 4,4′-Difluorobenzophenone and 4,4′-difluorodiphenyl sulphone were purchased from J & K Technology Co., Ltd., and were purified by recrystallization from alcohol twice before use. 3,3′-Diindolylmethane was synthesized according to the procedures reported by Pool et al.¹³ The rest of materials and reagents were obtained from different commercial sources and used without further purification.

Measurements

FT-IR spectra were recorded on a Nicolet 6700 FTIR spectrometer. ¹H NMR and ¹³C NMR spectra were recorded using Bruker AMX600 MHz NMR spectrometers in CDCl₃ or DMSO-d₆, depending on the solubility of the material being analyzed. The elemental analysis characterization technique was performed using a Vario EL III apparatus. The molecular weights and molecular weight distributions were estimated by gel permeation chromatography (GPC) on a Wyatt DAWN HELEOS using N,N-dimethylformamide (DMF) as an eluent, testing temperature 50 °C. Thermogravimetric analysis (TGA) was performed on a Setaram TG-92 at a heating rate of 10 °C min⁻¹ under air and nitrogen atmosphere. Absorption spectra were detected on a SHIMADZU UV-3150 UV-vis-NIR spectrophotometer. Fluorescent emission spectra were collected on a PerkinElmer LS-55 fluorescence spectrometer. The cyclic voltammograms were recorded with a Zennium IM6 electrochemical workstation (Zahner, Germany) using a normal three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode in 1.0 M H₂SO₄ at a scan rate of 20 mV s⁻¹. Electrochemical oxidation and deposition of polymers on stainless steel substrate were carried out using PARSTAT 2273 potentiostat controlled by PowerSuite software (Princeton Applied Research, USA) with polymer solution (30 mM in N-methylpyrrolidinone) with tetrabutylammonium chloride as supporting electrolyte (80 mM). Scanning electron microscopy (SEM) measurements were taken using a ZEISS EVO18 instrument.

General synthetic procedure for model compounds (3–5)

A typical synthetic procedure for model compound 3 is illustrated as an example. An oven-dried resealable Schlenk flask was evacuated and backfilled with argon. The flask was charged with indole (2.2 mmol), 4,4′-difluorobenzophenone (1.0 mmol), K₂CO₃ (2 mmol), and N-methylpyrrolidinone (NMP) (5 mL). The reaction mixture was evacuated and flushed with high-purity argon. This procedure was repeated three times. The flask was immersed with continuous stirring in a 160 °C oil bath for 4 h. The resulting solution was allowed to slowly cool to room temperature and subsequently poured into cold water, filtered through Celite, and dried. The crude material was purified by flash chromatography on silica gel and recrystallized from N,N-dimethylacetamide (DMAc) to afford 3 as light yellow crystals (0.41 g, yield 98%). Syntheses of 4 and 5 were carried out in the same manner with yields all above 95%.

3. Yield: 98%; M.p. 204 °C; FT-IR spectrum (KBr pellet, cm⁻¹): 3042, 1652, 1600, 1510, 1455, 1335, 755; ¹H NMR (600 MHz, DMSO-d₆): δ = 6.80 (d, J = 1.8 Hz, 2H), 7.18 (t, J = 7.8 Hz, 7.2 Hz, 2H), 7.26 (t, J = 7.8 Hz, 7.2 Hz, 2H), 7.69 (d, J = 7.8 Hz, 2H), 7.75 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 3.0 Hz, 2H), 7.85 (d, J = 8.4 Hz, 4H), 8.02 (d, J = 8.4 Hz, 4H) ppm; ¹³C NMR (150 MHz, DMSO-d₆): δ = 104.9, 110.7, 120.9, 121.2, 122.8, 123.0, 128.2, 129.6, 131.6, 134.2, 134.8, 142.6, 193.6 ppm; anal. calcd for C₂₉H₂₀N₂O: C, 84.44; H, 4.89; N, 6.79; found: C, 84.16; H, 4.92; N, 6.70.

4. Yield: 95%; M.p. 133 °C; FT-IR spectrum (KBr pellet, cm⁻¹): 3052, 1658, 1599, 1516, 1455, 1338, 743; ¹H NMR (600 MHz, DMSO-d₆): δ = 6.78 (d, J = 2.4 Hz, 2H), 7.16 (t, J = 4.2 Hz, 3.6 Hz, 4H), 7.68 (m, 4H), 7.78 (d, J = 2.4 Hz, 2H), 7.83 (m, 5H), 8.03 (d, J = 8.4 Hz, 4H), 8.13 (d, J = 6.6 Hz, 3H) ppm; ¹³C NMR (150 MHz, DMSO-d₆): δ = 105.0, 110.7, 120.9, 121.2, 122.8, 123.0, 128.2, 129.3, 129.6, 130.5, 131.8, 133.3, 133.6, 134.7, 137.2, 142.9, 194.0 ppm; anal. calcd for C₃₆H₂₄N₂O₂: C, 83.70; H, 4.68; N, 5.42; found: C, 83.36; H, 4.70; N, 5.38.

5. Yield: 97%; M.p. 198 °C; FT-IR spectrum (KBr pellet, cm⁻¹): 3048, 1592, 1520, 1453, 1336, 763; ¹H NMR (600 MHz, DMSO-d₆): δ = 6.78 (d, J = 3.0 Hz, 2H), 7.16 (t, J = 7.2 Hz, 2H), 7.23 (t, J = 7.8 Hz, 7.2 Hz, 2H), 7.67 (d, J = 7.8 Hz, 2H), 7.70 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 3.0 Hz, 2H), 7.90 (d, J = 8.4 Hz, 4H), 8.19 (d, J = 8.4 Hz, 4H) ppm; ¹³C NMR (150 MHz, DMSO-d₆): δ = 105.3, 110.7, 121.1, 121.2, 123.0, 124.0, 128.2, 129.4, 129.7, 134.6, 137.8, 134.3 ppm; anal. calcd for C₂₈H₂₀N₂O₂S: C, 74.98; H, 4.49; N, 6.25; found: C, 74.73; H, 4.52; N, 6.19.

Synthesis of poly(N-arylene diindolylmethane)s (PMDINs)

A typical synthetic procedure for polymer PMDIN-1 is illustrated as an example. To a three-necked flask (25 mL) equipped with magnetic stirrer, an argon outlet, inlet, and water-cooled condenser, 3,3′-diindolylmethane (4.0 mmol), 4,4′-difluorobenzophenone (4.0 mmol), K₂CO₃ (8 mmol), and NMP (10.0 mL) were added. The reaction mixture was evacuated and flushed with high-purity argon. This procedure was repeated three times. The reaction mixture was heated to 170 °C and maintained at this temperature for 6 h. The resulting polymer solution was allowed to slowly cool to room temperature, and subsequently poured into cold water, filtered, washed with water and methanol, and then dried at 100 °C under vacuum (1.645 g, yield 97%). Syntheses of other PMDINs were carried out in the same manner with yields all above 95%.

PMDIN-1. Yield: 97%; FT-IR spectrum (KBr pellet, cm⁻¹): 3044, 2916, 1659, 1598, 1512, 1454, 1249, 745; ¹H NMR (600 MHz, CDCl₃): δ = 4.32 (s, 2H), 7.20 (s, 6H), 7.52 (m, 4H), 7.66 (m, 4H), 7.91 (m, 4H) ppm; anal. calcd for (C₃₀H₂₀N₂O)_n (424.49)_n: C, 84.88; H, 4.75; N, 6.60; found: C, 82.76; H, 4.83; N, 6.48.

PMDIN-2. Yield: 95%; FT-IR spectrum (KBr pellet, cm⁻¹): 3047, 2900, 1677, 1591, 1499, 1454, 1307, 1152, 747; ¹H NMR (600 MHz, CDCl₃): δ = 4.30 (s, 2H), 7.14 (m, 6H), 7.54 (d, J = 12.0 Hz, 4H), 7.61 (m, 5H), 7.91 (d, J = 12.0 Hz, 4H), 7.99 (d, J = 10.8 Hz, 2H), 8.21 (s, 1H) ppm; anal. calcd for (C₃₇H₂₄N₂O₂)_n (528.60)_n: C, 84.07; H, 4.58; N, 5.30; found: C, 83.66; H, 4.62; N, 5.13.

PMDIN-3. Yield: 96%; FT-IR spectrum (KBr pellet, cm⁻¹): 3055, 2918, 1655, 1594, 1511, 1456, 1270, 930, 743; ¹H NMR was not recorded due to the poor solubility of the polymer in DMSO-d₆ and CDCl₃ at room temperature; anal. calcd for (C₂₉H₂₀N₂O₂S)_n (460.55)_n: C, 75.63; H, 4.38; N, 6.08; found: C, 73.96; H, 4.40; N, 6.00.

Results and discussion

Model reaction

To confirm the activity of the NH group in the nucleus of indole and gain insights into the chemical structures of the polymers, we performed the N-arylation of indole with electron-deficient fluorobenzene as a model reaction. In this work, we here describe a simple, highly efficient catalyst-free N-arylation in which fluorines of fluorobenzene derivatives having additional electron-withdrawing groups like carbonyl and sulfonyl is substituted for indoles. This was determined during a preliminary survey of reaction conditions with use of highly electron-deficient 4,4′-difluorobenzophenone and indole as model arylating agents shown in Table 1. To identify the optimized reaction conditions for the reaction, a series of different organic solvents, bases and reaction temperatures were examined. At low temperature, no or trace amount of product was obtained (Table 1, entries 1 and 2). At the higher temperatures, the yields increased dramatically (compare entries 3 and 4). In addition, we found that the choices of base and solvent were also crucial to the outcome of the reaction. Among the solvents tested, NMP was clearly the best choice, while DMF, DMAc and sulfolane provided slightly low yields (compare entries 4–8), this may be related to the dissolution properties of the products. Of the bases tested, potassium carbonate or cesium carbonate was most effective and efficient for product formation (compare entries 4 and 9–14), and this indicated that indole moiety with negative charge (nucleophile) could be easily formed with potassium carbonate or cesium carbonate being used as the base (Scheme 2).

Table 1 Some representative results from the screening of reaction conditions for the N-arylation of indole with 4,4′-difluorobenzophenone^a

Entry	Solvent	Temp. (°C)	Base	Yield^b (%)
a Reaction conditions: indole (2.2 mmol), 4,4′-difluorobenzophenone (1.0 mmol), and base (2.0 equiv.) in 5.0 mL of solvent under an argon atmosphere for 4 h.b Isolated yields (average of two runs).c Little coupling product was determined. NMP = N-methylpyrrolidone, DMF = N,N′-dimethylformamide, DMAc = N,N′-dimethylacetamide, DMSO = dimethylsulfoxide.
1	NMP	100	K2CO3	^c
2	NMP	120	K2CO3	<8
3	NMP	140	K2CO3	55
4	NMP	160	K2CO3	98
5	DMF	160	K2CO3	92
6	DMA	160	K2CO3	93
7	DMSO	160	K2CO3	80
8	Sulfolane	160	K2CO3	84
9	NMP	160	K3PO4	95
10	NMP	160	KF	21
11	NMP	160	Li2CO3	^c
12	NMP	160	Na2CO3	13
13	NMP	160	Cs2CO3	99
14	NMP	160	NaOtBu	26

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Scheme 2 N-Arylation of indole with electron-deficient fluorobenzene.

With optimized conditions now in hand, we explored the scope of the N-arylation reactions of a series of other electron-deficient halobenzenes with indole in the presence of 2.0 equiv. of K₂CO₃ in NMP at 160 °C under an argon atmosphere, and the results are summarized in Table 2. Gratifyingly, each replacement of fluorine reacted under the optimized reaction conditions to give the corresponding N-arylated indole derivatives in good to excellent yields (up to 98%) (entries 3–5), although chlorobenzene or bromobenzene showed poorer reactive activity and gave lower yield. The model compounds 3–5 were characterized by standard spectroscopic techniques from which satisfactory analysis data corresponding to their expected molecular structures were obtained (see Experimental section for details). Therefore, we envisaged that the present methodology might be a clue for constructing new N-arylindoles as well as various indole-based functional polymer materials.

Table 2 N-Arylation of indoles with electron-deficient halobenzenes^a

Entry	Electron-deficient halobenzenes	Product	Yield^b (%)
a Reaction conditions: indole (2.2 mmol), 2 (1.0 mmol), and 2.0 mmol of K2CO3 in 5 mL of NMP at 160 °C under an argon atmosphere for 4 h.b Isolated yields (average of two runs) based on 2.
1			58
2			26
3			98
4			95
5			97

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Polymerization reaction

To explore the catalyst-free C–N coupling reaction of NH group with activated difluoride into a useful tool for the preparation of sequentially well-defined indole-based polymers, we rationally designed a bifunctional monomer 3,3′-diindolylmethane, and the synthesis of it was carried out as depicted in ESI† for details. The polymerization reaction of 3,3′-diindolylmethane with different aromatic difluoro monomers was performed in NMP, and a series of novel poly(N-arylene diindolylmethane)s (PMDINs) with high molecular weights were obtained in high yields (Scheme 1).

To search for optimum conditions for the polymerization, we first studied temperature effect on the polymerization in NMP. Taking PMDIN-1 as an example, as exhibited in Table 3, the temperature exerted strong effect on the reaction. Upon increasing the temperature from 140 to 170 °C, M_w increased from 10 600 to 141 700 and the resulting polymer was obtained in high yield (97%). Clearly, the polymerization proceeded only when sufficient energy was provided.

Table 3 Temperature effect on the polymerization of 4,4′-difluorobenzophenone with 3,3′-diindolylmethane to form PMDIN-1^a

Entry	Temp. (°C)	Mn^b	Mw^b	Mw/Mn^b	Yield (%)
a Carried out under an argon atmosphere in NMP for 6 h. Concentrations of monomers were 0.4 mol L^−1.b Determined by GPC in DMF on the basis of a polystyrene calibration.
1	140	5100	10 600	2.08	52
2	150	21 300	47 300	2.22	78
3	160	45 000	98 700	2.19	90
4	170	65 600	141 700	2.16	97

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We then investigated the concentration effect on the molecular weights. As shown in Table 4, neither lower (0.3 mol L⁻¹) nor higher (0.5 mol L⁻¹) concentrations of monomers were good for the polymerization. The reaction rates are slower with a lower concentration, and higher concentration promote by-product formation. Instead, a concentration of 0.4 mol L⁻¹ is enough to give a soluble polymer with a high molecular weight in a high yield.

Table 4 Concentration effect on the polymerization of 4,4′-difluorobenzophenone with 3,3′-diindolylmethane to form PMDIN-1^a

Entry	Concn of monomers (mol L^−1)	Mn^b	Mw^b	Mw/Mn^b	Yield (%)
a Carried out under an argon atmosphere in NMP at 170 °C for 6 h.b Determined by GPC in DMF on the basis of a polystyrene calibration.c Data taken from Table 3, entry 4.
1	0.1	9500	19 700	2.07	66
2	0.2	16 700	35 800	2.14	73
3	0.3	42 200	88 300	2.09	84
4^c	0.4	65 600	141 700	2.16	97
5	0.5	46 300	93 600	2.02	89

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The above investigation allowed us to polymerize other monomer combination under optimum conditions. The polymerization results are exhibited in Table 5. All the polymerizations proceed smoothly, affording polymers in high yields (95–97%) with high molecular weights (M_w = 109 100–389 200). Compared with that of traditional polyindole, the synthesis conditions of the acquired PMDINs are milder and the molecular weights and yields are much higher.^8d The polymerization of 3,3′-diindolylmethane with 1,3-bis(4′-fluorobenzoyl) benzene gave much better results: the obtained polymer PMDIN-2 with higher molecular weight up to 389 200 was isolated in higher yield up to 95%. Furthermore, the synthesized polymers were characterized by FT-IR, ¹H NMR and elemental analyses. The satisfactory results were in good agreement with the proposed structures (see Experimental section). The ¹H NMR spectrum of PMDIN-2 is given in Fig. 1 (DMSO-d₆) as an example. There was no peak at δ 10.73 ppm, which was assigned to the resonance of the NH proton of 3,3′-diindolylmethane. It was suggestive of the occurrence of the polymerization. Moreover, the absorption peaks of PMDIN-2 were much broader than those of monomers, indicative of its polymeric nature.

Table 5 Polymerization data for poly(N-arylene diindolylmethane)s (PMDINs)

Polymer code	Mn^a	Mw^a	Mw/Mn^a	Yield (%)
a Determined by GPC in DMF on the basis of a polystyrene calibration.
PMDIN-1	65 600	141 700	2.16	97
PMDIN-2	115 800	389 200	3.36	95
PMDIN-3	56 100	119 100	2.12	96

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Fig. 1 ¹H NMR (600 MHz) spectrum of PMDIN-2 recorded in CDCl₃.

Thermal stability

The thermal properties of PMDINs were evaluated by thermogravimetric analysis (TGA). As depicted in Fig. 2, all the polymers enjoyed high thermal stability, exhibiting 5% weight loss at temperature (T_d) from 377 to 392 °C. They also carbonized in moderate yields up to ca. 42% when they were heated to 700 °C. The PMDINs showed higher decomposition temperatures in nitrogen than that in oxygen.

Fig. 2 TGA thermograms of PMDINs recorded under nitrogen.

Optical properties

According to our previous published paper,¹² we found that the prepared polyindoles were blue light-emitting polymers, in which there were undergone an intramolecular charge transfer from the electron donor (indole segment) to the acceptor (the electron-deficient group of sulfonyl or carbonyl portion) to result in luminescence. In this paper, our motivation was the introduction of more indole rings into the polymer main chains to construct some new efficient blue photoluminescence polyindoles promising for use in blue PLEDs. UV absorption and fluorescence spectra of dilute solutions of model compounds 3–5 and PMDINs in NMP were recorded and the related photophysical data are summarized in Table 6. Fig. 3 shows the optical absorption and emission spectra of PMDINs. The polymers had two absorption peaks. Among the polymers, the absorption maxima of PMDIN-3 containing the sulfonyl unit exhibited the blue-shifted absorption profile (278 and 338 nm), revealing that it possessed the lowest electronic conjugation (Fig. 3a and Table 6). In addition, model compounds 3–5 gave rise to similar UV absorption bands. 3 and 4 displayed almost the same electronic absorption energies, and the λ_max values were in the range of 288–341 nm. However, the long wavelength absorption maximum (327 nm) of 5 was obviously blue-shifted compared to that of 3 (341 nm) (Fig. S1†). These results demonstrated a good agreement with those of PMDINs. Moreover, the absorption maximum of PMDIN-3 solids was red-shifted by 14 nm compared with the maximum of it in solution. The red shift indicated enhanced intermolecular electronic interactions in the solid state.¹⁴ In contrast, a blue shift of 27 nm observed for PMDIN-2 was larger than that of PMDIN-1 due to the lower polymer coplanarity in the solid state.

Table 6 Optical properties of model compounds, PMDINs in solution^a and in solid-state

Model compound	λab^b (nm)	λem^c (nm)
	Soln	Soln^d (ΦF)
a In dilute NMP solution (10 μM).b Absorption maximum.c Emission maximum.d ΦF = fluorescence quantum yield in NMP solution determined using quinine sulfate (ΦF = 0.55 in 1 M H2SO4) as standard.
3	288, 341	515 (0.53%)
4	289, 337	518 (0.24%)
5	288, 327	453 (15.8%)

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Polymer	λab^b (nm)		λem^c (nm)
	Soln	Solid	Soln^d (ΦF)	Solid
PMDIN-1	289, 357	348	513 (1.2%)	495
PMDIN-2	281, 353	326	463 (0.85%)	480
PMDIN-3	278, 338	352	460 (21.6%)	444

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Fig. 3 (a) UV and (b) fluorescence spectra of PMDINs in NMP solutions. Solution concentration: 10⁻⁵ M; excitation wavelength (nm): 355 (PMDIN-1), 353 (PMDIN-2), and 340 (PMDIN-3); excitation and emission slits (nm): 5.0 nm and 3.0 nm, respectively. (c) Fluorescence images of model compounds and PMDINs in NMP, irradiated at 365 nm; from left to right: 5, 3, 4, PMDIN-3, PMDIN-1, and PMDIN-2. (d) Fluorescence images of 5 and PMDINs (powder, λ_exc = 365 nm). From left to right: PMDIN-3, PMDIN-1, PMDIN-2, and 5.

Next, we investigate the photoluminescence properties of model compounds and PMDINs. The fluorescence curves in Fig. 3b show that the emission peaks of PMDINs in dilute NMP solutions at 10 μM were centered at 460–513 nm, whereas those of model compounds 3–5 were observed at the slight shorter wavelengths of 453–518 nm (Fig. S2,† Table 6). 3, 4, PMDIN-1, and PMDIN-2 exhibited weak yellow fluorescent emissions. In contrast, 5 and PMDIN-3 displayed blue-green fluorescent emissions (Fig. 3c). Notably, for PMDIN-1 and PMDIN-3, the emission peaks were close to those of 3 and 5 as shown in Table 6. However, for PMDIN-2, the emission maximum was about a 55 nm blue shift relative to that of 4. We have proposed that the drastic changes in the emission wavelength between polymers and model compounds mainly depended upon the dipole moment at an excited state.¹⁵ It was inferred that the 1,3-bis-benzoyl groups in the polymer PMDIN-2 chains were less suitable than the carbonyl groups or the sulfonyl groups for an perturbation of the dipole moment and electronic status at an excited state, which could give rise to dynamic changes in the emission wavelength. In addition, the fluorescence quantum yields (Φ_F) of PMDINs determined in NMP solutions using quinine sulfate (Φ_F = 0.55 in 1 M H₂SO₄) as standard¹⁶ were 1.2%, 0.85% and 21.6%, respectively, whose values became much higher than those of model compounds (Φ_F = 0.53%, 0.24% and 15.8%, respectively).

It is a challenging project for the design and synthesis of polymers exhibiting highly efficient solid-state luminescence for the development of optoelectronic PLED devices. Conventional luminescent polymers often suffer from strong interstrand interaction in the aggregated state, which quenches and red-shifts their light emission.¹⁷ However, in this work, it is remarkable that model compound 5 and PMDINs all exhibited strong solid-state fluorescence upon excitation with UV light (λ_exc = 365 nm, Fig. 3d). The solid-state emission wavelengths of PMDINs were observed from 444 to 495 nm (Table 6). These results could be attributed to aggregation-induced emission (AIE).¹⁸ We assumed that the indole units in PMDINs were linked together by methylene groups, which could rotate relatively freely in the solution state and the intramolecular rotation could serve as nonradiative pathways for the excitons to decay. In the aggregated state, such rotation was partially restricted, which blocked the relaxation channels and thus turned on the light emission of the luminophore.¹⁹ These characteristics make these PMDINs potential candidates for polymer emitters and solid-state lighting devices.

Electrochemical properties

The energy levels of the HOMO and LUMO were determined by measuring the electrochemical properties of PMDINs using cyclic voltammetry (CV). A platinum (Pt) electrode was modified with a polymer film by means of dip-coating, and was used as the working electrode. A Pt wire was used as the counter electrode, and Ag/AgCl (4.0 M KCl) served as the reference electrode. The CV measurements were carried out in a 1.0 M H₂SO₄ electrolyte at room temperature at a scan rate of 20 mV s⁻¹. For all the polymers, a reversible oxidation process was observed (Fig. 4). The HOMO level of the polymers was deduced based on the oxidation onsets under the assumption that the energy level of ferrocene (Fc) was ca. −4.8 eV.²⁰ In the anodic scan, the onset of oxidation for PMDIN-1, PMDIN-2, and PMDIN-3 occurred at 0.46, 0.48, and 0.45 V, respectively. We assumed that these peaks could be attributed to the oxidation of indole units which are still electrochemically active especially at the 2-positions.^9a,12 The HOMO energy levels of PMDIN-1–3 are −5.26, −5.28, and −5.25 eV, respectively (Table 7). As expected from the structures of the polymers, PMDINs with identical backbone structures have similar HOMO energy values. The LUMO energy levels of the polymers were calculated using their HOMO energies and optical band gaps to be −2.60, −2.51, and −2.10 eV, respectively (Table 7). The lower LUMO level of PMDIN-3 indicated that sulfone moiety was a strong electron accepting building block. In addition, the very similar HOMO and LUMO levels measured for PMDIN-1 and PMDIN-2 demonstrated that the energy levels were mainly determined by their conjugated backbone. For this reason, we assumed that the electron-withdrawing sulfonyl group adopting the tetrahedron structure significantly affected the coplanar structure of the polymer and exerted an important effect on optical properties as well as electrochemical properties.

Fig. 4 Cyclic voltammograms of PMDINs conducted in aqueous H₂SO₄ (1.0 M) at a scan rate of 20 mV s⁻¹.

Table 7 Electrochemical data of polymers

Polymer	λonset (nm)	Eg^a (eV)	Eonset (eV)	HOMO^b (eV)	LUMO^c (eV)
a Optical band gap calculated from the UV-vis absorption onset.b The HOMO energy levels were determined from onset voltage of the first oxidation potential with reference to ferrocene at −4.8 eV.c LUMO levels of the polymers were estimated from the optical band gaps and the HOMO energies.
PMDIN-1	466	2.66	0.46	−5.26	−2.60
PMDIN-2	448	2.77	0.48	−5.28	−2.51
PMDIN-3	394	3.15	0.45	−5.25	−2.10

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To further verify the electrochemically active of indole units at the 2-position, we also investigated the electrochemical deposition of the polymer PMDIN-3. The SEM images of a thin PMDIN-3 film prepared in NMP were shown in Fig. 5. Macroscopically, the film that formed on a stainless steel anode was flat, compact and metallic black in color. Microscopically, the polymer film showed irregular growth of granules and resembled a growth of aggregates shaped as blossoms. It was expected that electrochemical oxidation of this polymer should result in coupling between 2-indolyl position, forming an extended three-dimensional cross-linked polymer network (Scheme 3). This confirmed the good redox activity of PMDIN-3. For comparison, the scanning electron micrograph of the dip coated linear PMDIN-3 film was also tested (Fig. S10†). Interestingly, the films formed by dip coated and electrodeposited had the different surface morphology. In order to further understanding the electrochemical activity of indole units at the 2-position, the IR spectrum of the electrodeposited film was carried out (Fig. S9†). The peak located at 738 cm⁻¹ can be assigned to the out-of-plane deformation of the C₂–H bond. It was obviously present for PMDIN-3 and as lower for the electrodeposited film. This proved that the successful cross-linking of PMDIN-3 occurred at the 2-position. Compared the CV of linear PMDIN-3 with that of the electrodeposited film, the result further confirmed the cross-linking of PMDIN-3, but there still existed some unreactive 2-indolyl position in the film (inset in Fig. 5). All in all, the convenient and economical cross-linking method may be anticipated to construct an array of cross-linked polymer films with unique properties for high-technological applications, especially in advanced optoelectronic devices.

Fig. 5 Scanning electron micrograph of the electrodeposited PMDIN-3. Inset is the cyclic voltammograms of electrodeposited film.

Scheme 3 A schematic representation of the formation of cross-linked polymer network.

Conclusions

In summary, we have demonstrated that nucleophilic substitution reactions with the NH group in the indole molecule as reactive groups gave novel PMDINs. The catalyst-free C–N polycouplings of 3,3′-diindolylmethane with different activated difluoro monomers proceed smoothly in NMP at 170 °C for 6 h, producing polymers PMDIN-1–3 with high molecular weights in high yields. This provides a simple synthetic pathway that will allow the preparation of a great variety of indole-based polymers from commercially available aromatic compounds. Rational model reaction was performed to help elucidate the polymer structures. All these achieved polymers show good thermal stability with high decomposition temperatures (T_ds > 370 °C). Due to the indole moieties in the main chains, these PMDINs are strong UV absorbers and display strong solid-state fluorescence. CV suggests that PMDINs have lower energy levels, particularly for PMDIN-3, which is promising as PLEDs materials. Moreover, owing to the good electroactivity, the polymers can be utilized as processable precursors to cross-linked films. This work does not only deepen our systemic understanding of catalyst-free nucleophilic substitution polycondensation but also provide new perspectives for generating indole-based polymers or cross-linked films with thermal stability and well optical properties. We expect that the present results will inspire research enthusiasm for creation of novel and high-performance polymers with new and/or enhanced functionalities and properties via this facile and easier synthetic strategy for high-technological applications in optoelectronics, automotive, aircraft, and spacecraft industry.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (no. 21202134), the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (11zxfk26), the Research Fund for the Doctoral Program of Southwest University of Science and Technology (no. 12zx7129). We thank the Centre of Testing & Analysis, Southwest University of Science and Technology for UV and fluorescence measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Optical properties, IR spectra for model compounds and PMDINs, copies of ¹H and ¹³C NMR spectra. See DOI: 10.1039/c4ra03602f

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