Influence of benzene ring number attached on non-conjugated 3,4-ethylenedioxythiophene derivatives for solid-state polymerization

Abstract

Based on an EDOT–CH₂–N(R)–CH₂–EDOT platform, we investigated the effect of the substituent on the nitrogen atom on solid-state polymerization. Detailed characterizations of the corresponding polymers were carried out and crystals of all the monomers were obtained for structural analysis. Our study revealed that the introduction of aromatic moieties makes SSP successful, which would greatly widen the scope of monomers. In addition, besides linker distance, the substitution on the CH₂–N(R)–CH₂ linker also plays an important role in the SSP behavior of monomers.

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Introduction

Conjugated polymers have been a type of attractive material since they were invented at the end of the 1970s.¹ They are outstanding functional materials because of their wide-ranging applications in many organic optical devices, ranging from light-emitting devices, supercapacitors, and electrochromic devices to organic photovoltaic cells.^2–5 In addition, they can be easily tailored by rational design and synthesis. It is noted that solid-state polymerization (SSP) has been successfully applied widely in industry since it was invented in the early 1940s, especially for hundred-million-ton scale non-conjugated polymers such as polyesters and polyamides.^6–8

However, as far as conjugated polymers are concerned, except for the typically investigated polydiacetylene⁹ and (SN)_n,¹⁰ few samples or systems have been polymerized by such a facile method, which has special features such as being solvent-free and oxidant-free and has no necessity for an additional external applied potential. After the first successful synthesis of polythiophene using SSP that was realized in 2003,¹¹ many groups have subsequently focused on the investigation of suitable monomers,¹² analysis of their crystal structures and their application in electro-optical devices.¹³

Generally, according to the studies on the development of all these monomers to date, monomers are mainly modified along the longitudinal or parallel direction of 3,4-ethylenedioxythiophene (EDOT) molecules. The latter strategy was first proposed by our group through introducing a flexible one-atom linker between EDOT units and then generating a conjugated quinonoid structure of poly(bis-3,4-ethylenedioxythiophene-methine).¹⁴ Subsequently, we successfully developed a non-conjugated EDOT–CH₂–N(R)–CH₂–EDOT platform for SSP.^14b To further examine the effects of substitution and to determine SSP boundary conditions, we chose a large platform of EDOT–CH₂–N(R)–CH₂–EDOT and introduced bulky groups to study the different effects of the number of benzene rings in the linker. Moreover, we would like to emphasize two points here. First, taking the introduced non-conjugated flexible linker in most cases into consideration, our proposed strategy still has the shortcomings of poor conductivity when compared with traditional conductive PEDOT. Probably a conjugated linker would work well in SSP, which needs to be verified. Second, at present, we are more interested in developing new synthesis methods rather than their application.

Experimental

Materials

Chemicals were purchased from Wuhan Shenshi Chemicals Co., Ltd and were used without further purification unless otherwise noted. 3,4-Ethylenedioxythiophene (EDOT) was purchased from J & K. 5-Iodo-2,3-dihydrothieno[3,4-b][1,4]dioxine¹⁵ was synthesized according to a previous report.

Monomer synthesis and solid-state polymerization

Synthesis of bis(thien-2-ylmethyl) Mannich bases. General method. Monomers were synthesized in accordance with a previous method¹⁶ shown in Scheme 1. Iodo-monosubstituted EDOT (2.0 eq.) was dissolved in glacial acetic acid. To an equal volume of glacial acetic acid, an amine (1.0 eq.) and aqueous formaldehyde (37%, 2.0 eq.) were added with ice cooling. The solutions were fed separately and simultaneously into glacial acetic acid using a syringe pump with stirring. After the addition was completed, the solution was stirred for further 12 hours. The solvent was removed in vacuo to obtain a dark oily residue. Purification by column chromatography (alumina; light petroleum–EtOAc, 5 : 1) gave the required Mannich base.

Scheme 1 Synthesis of the monomers and corresponding polymers and digital images of the monomers and respective polymers produced through SSP.

Bis-(7-iodo-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl-methyl)-phenyl-amine (I₂-3-Ph-EDOT). Orange powder, 50%. ¹H NMR: δ (CDCl₃) 7.23–7.20 (2H), 6.89–6.86 (2H), 6.79 (1H), 4.52 (4H), 4.52–4.21 (8H). ¹³C NMR: δ (CDCl₃) 147.6, 143.6, 137.8, 129.1, 120.3, 118.5, 114.3, 65.3, 64.6, 47.0, 46.6.

Benzhydryl-bis-(7-iodo-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl-methyl)-amine (I₂-3-di-Ph-EDOT). White powder, 50%. ¹H NMR: δ (CDCl₃) 7.43–7.40 (4H), 7.35–7.30 (4H), 7.25 (2H), 5.03 (1H), 4.21–4.10 (8H), 3.67 (4H). ¹³C NMR: δ (CDCl₃) 143.7, 140.1, 138.3, 128.9, 128.3, 127.1, 121.7, 68.1, 65.3, 64.4, 47.5, 45.1.

General solid-state polymerization. The SSP procedure was according to previously reported methods.^11,14 In a closed 5 mL vial, the monomers (300 mg) were incubated at 60–100 °C, respectively, for 2–24 h.

Crystal structure determination. Intensity data for all four crystals were collected using Mo Kα radiation (λ = 0.7107 Å) on a Bruker SMART APEX diffractometer equipped with a CCD area detector at room temperature. The crystallographic data and details of data collection for the two monomers are given in Table 1. Data set reduction and integration were performed using the software package SAINT PLUS.¹⁷ The crystal structure was solved by direct methods and refined using the SHELXTL 97 software package.¹⁸

Table 1 Crystallographic data and structure refinement of the synthesized monomers

Parameter	I2-3-Ph-EDOT	I2-3-di-Ph-EDOT
Empirical formula	C20H17I2NO4S2	C27H23I2NO4S2
fw	653.27	743.38
Crystal system	Tetragonal	Monoclinic
Space group	Pbca	P21/c
a (Å)	16.591	20.806 (3)
b (Å)	16.098 (3)	18.527 (3)
c (Å)	16.591 (2)	15.312 (2)
α (degree)	90.00	90.00
β (degree)	90.00	109.866 (4)
γ (degree)	90.00	90.00
V (Å^3)	4431.2 (10)	5551.2 (14)
Z	8	8
Dcalcd (g cm^−3)	1.958	1.779
Crystal size (mm^3)	0.22 × 0.10 × 0.10	0.15 × 0.18 × 0.20
Diffractometer	Bruker APEX-II CCD	Bruker APEX-II CCD
F (000)	2512	2896
T (K)	293 (2)	293 (2)
θmax	29.24	26.56
Reflections collected	4320	11 520
Independent reflections	2810	8836
Parameters refined	262	669
R1, wR2	0.0475, 0.1101	0.0369, 0.0811
GOF (F2)	1.014	1.019

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Other characterizations. IR spectra for the characterization of the resulting polymers were recorded on a PerkinElmer FTIR spectrometer. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation at room temperature. Thermogravimetric analysis (TGA) data were obtained from a SETSYS 16 with a heating rate of 10 °C min in a nitrogen atmosphere.

Results and discussion

Synthesis of monomers and solid-state polymerization

Monomers were synthesized through Mannich reaction and subsequently were polymerized through SSP, as shown in Scheme 1. Triphenyl and adamantyl groups were too bulky to be introduced into the platform of EDOT–CH₂–N(R)–CH₂–EDOT. However, such a platform would be widely used not only for alkyl amines^14b but also for aromatic amines. In addition, these obtained monomers can successfully form the corresponding polymers via SSP and their onset temperature of SSP (T_onset) is around 80 °C.

XRD patterns of monomers and respective polymers

As shown in Fig. 1a, the monomer of I₂-3-Ph-EDOT exhibits typical sharp peaks (2θ = 10.8°, 19.64°, 20.06°, 22.08°, 24.64° and 27.58°) in the range of 10°–50° and their typical phases were assigned according to their simulated values. These peak values are considerably consistent with the simulated results. Moreover, in the case of I₂-3-di-Ph-EDOT, there are peaks around 2θ of 9.34°, 13.28°, 15.74°, 16.94°, and 19.24°, which can be assigned to (020), (012), (031), (131) and (040) phases, respectively. After polymerization, all the polymers are in an amorphous phase due to the release of halogen, which results in the collapse or degradation of the crystals. To further investigate what happened during the SSP procedure, in situ XRD measurements were carried out for these monomers. As shown in Fig. 1a, it is obvious that a drastic decrease in the intensity of the (050), (020) and (040) phases was observed after the initial one hour of polymerization for the I₂-3-Ph-EDOT monomer, which reveals that initially polymerization occurs in the ac-plane of the crystal, perpendicular to the b-axis. In the case of I₂-3-di-Ph-EDOT, a similar change has occurred because of the observation of a decrease in the intensity of the (012) phase within the first hour of heat treatment. In addition, the intensity of the (031), (131) and (040) phases decreases drastically, which reveals that there is a large change in these phases.

Fig. 1 XRD patterns of monomers and their respective polymers and in situ XRD patterns of monomers under SSP. (a) I₂-3-Ph-EDOT and (b) I₂-3-di-Ph-EDOT.

FTIR spectroscopy

Structural information was obtained using a Fourier transform infrared (FTIR) spectrometer, as shown in Fig. 2. All the polymers exhibit peaks around 1480, 1434, 1362, and 1234 cm⁻¹, which originate from the stretching of C=C and C–C in the thiophene ring.^19,20 Due to the existence of C–N vibrations, a strong peak at 1080 cm⁻¹ was observed.²¹

Fig. 2 FTIR spectra of P(3-Ph-EDOT) and P(3-di-Ph-EDOT).

Crystallographic X-ray analysis

Single crystals of monomers were obtained and studied by X-ray analysis (shown in Fig. 3–5) for further understanding the SSP polymerization pathways. Although these monomers are designed based on the same platform of EDOT–CH₂–N(R)–CH₂–EDOT, their crystal structures are considerably different and their parameters are listed in Table 1. In addition, the substituent groups have a large effect on the halogen/halogen (I/I) distances and angles in the monomers, as shown in Fig. 3. The I/I distances are derived from the molecular structure, and it is found that a bulkier substituent group leads to shorter intermolecular I₁/I₂, S₁/S₂ and C₉/C₆ bridge distances due to the flexible CH₂–N(R)–CH₂ linker, as summarized in Table 2. In addition, the introduction of a diphenylmethyl group results in an increase of 10° in the ∠C₉N₁C₆ angle, and drastically decreases the ∠S₁N₁S₂ and ∠I₁N₁I₂ angles when compared with the phenyl group.

Fig. 3 Crystal structures of (a) I₂-3-Ph-EDOT and (b) I₂-3-di-Ph-EDOT.

Table 2 Intermolecular halogen distances and angles

Monomers	Distance (Å)			Angles (°)
	I1/I2	S1/S2	Bridge length (C–N(R)–C)	∠C–N(R)–C	∠S1N1S2	∠I1N1I2
I2-3-Ph-EDOT	12.218	6.329	3.998 (C9/C6)	108.12 (∠C9N1C6)	159.49	145.76
I2-3-di-Ph-EDOT	8.300	3.699	4.204 (C9/C6)	118.8 (∠C9N1C6)	72.00	81.11

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As shown in Fig. 4a and b, I₂-3-Ph-EDOT has the lowest I/I halogen distance of 4.308 Å at a C1–C14 contact distance of 5.780 Å, followed by the second shortest distance of 4.332 Å at a C1–C14 contact distance of 5.711 Å, whereas the third shortest distance is of 7.735 Å at the shortest C1–C1 contact distance of 4.594 Å. It is easily found that the first and second polymerization pathways (shown in Fig. 4c and d) involve only one effective I/I halogen distance of 4.308 Å and 4.332 Å, respectively, with their corresponding C1/C14 contact distances of 5.780 Å and 5.711 Å. In addition, a third possible polymerization pathway is proposed in Fig. 4e, which involves two I/I distances of 4.308 Å and 4.332 Å. Taking the fact that halogen distances are similar among all these pathways into consideration, it is difficult to determine which way is the preferred way. However, according to the in situ XRD analysis (see Fig. 1a), great changes have occurred in the ac-plane of the crystal, perpendicular to the b-axis. Thus, the first and second pathways would be the preferred ways. Due to this fact, we have selected the shortest distance of 4.308 Å as the effective halogen/halogen distance, as shown in Table 3.

Fig. 4 Single-crystal X-ray structure of compound I₂-3-Ph-EDOT. (a) View of the halogen distances, (b) view of the C/C distances, (c and d) crystal packing viewed along the a-axis and C1/C14 distance and (e) crystal packing viewed along the c-axis, C1/C14 distance, distance between each third molecule and the proposed third polymerization pathways. The numbers indicate the corresponding distances in Å. I, purple; S, yellow; N, light blue and C, gray.

Table 3 Selected Hal/Hal and C–C contact distances (Å) for the reported crystals

Molecules parameters	I2-3-Ph-EDOT	I2-3-di-Ph-EDOT
a Effective Hal/Hal distance.b 2rw of iodine = double van der Waals radius of iodine = 4.0 Å.
Shortest Hal/Hal distance	4.308^a	4.055
2^nd shortest Hal/Hal distance	4.332	4.241
3^rd shortest Hal/Hal distance	7.735	4.242^a
C–C shortest contact distance	4.594	5.673
C–C 2^nd shortest contact distance	5.771	5.888
C/C distance between each third molecule	—	11.120
Longer than 2rw of iodine^b	7.7%	6.5%
Tonset of SSP	75 °C	85 °C

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As shown in Fig. 5a and b, I₂-3-di-Ph-EDOT has the lowest halogen distance of 4.055 Å with the shortest C1–C14 contact distance of 5.673 Å, followed by the second shortest distance of 4.241 Å with a C28–C14 contact distance of 5.888 Å and then the third shortest distance of 4.242 Å with a C28–C14 contact distance of 6.216 Å. It is found that the first polymerization pathway (shown in Fig. 5c) involves I/I distances of 4.055 and 4.242 Å, whereas the second polymerization pathway (shown in Fig. 5d) involves I/I distances of 4.241 Å and 4.658 Å, which indicates that the former pathway is the preferred way with an effective halogen distance of 4.242 Å for SSP. This expectation is well consistent with the powder in situ XRD results, as discussed in the above section (Fig. 1b).

Fig. 5 Single-crystal X-ray structure of compound I₂-3-di-Ph-EDOT. (a) View of the halogen distances, (b) view of the C/C distances, (c and d) crystal packing viewed along the b-axis, proposed first and second polymerization pathways. The numbers indicate the corresponding distances in Å. I, purple; S, yellow; N, light blue and C, gray.

In addition, taking the fact that the effective halogen distances in the monomers of I₂-3-Ph-EDOT and I₂-3-di-Ph-EDOT are longer by 7.7% and 6.5%, respectively, than double the van der Waals radius (2r_w) of iodine into consideration, we can efficiently explain the observation of their similar T_onset of 75–85 °C, as shown in Table 3. Interestingly, I₂-3-EDOT^14b exhibits a T_onset of 95 °C with an effective halogen distance of 20.1% longer than 2r_w of iodine. Therefore, it appears that a longer effective halogen distance requires a higher T_onset. Furthermore, although they have rigid benzene rings, the T_onset of these monomers does not change considerably. We attribute this to the presence of a long flexible CH₂–N–CH₂ linker, which results in weak molecular steric or π–π interactions between the benzene rings. However, the effective halogen distance, T_onset and in situ XRD results are the key evidence for understanding SSP.

Thermal stability. TGA analysis

The thermal stability of the polymers was investigated by thermogravimetric analysis (TGA) (Fig. 6). The 5% weight-loss temperatures for I₂-3-Ph-EDOT and I₂-3-di-Ph-EDOT were 145 and 182 °C, respectively, which indicate that the introduction of a bulky group would enhance thermal stability to some extent.

Fig. 6 TGA curves of the polymers at a heating rate of 10 °C min⁻¹ under nitrogen.

Conclusion

In this study, two new EDOT derivatives were synthesized to investigate the steric effect based on an EDOT–CH₂–N(R)–CH₂–EDOT platform for SSP. Their crystal structures were carefully analyzed to understand their polymerization pathways. Our results indicate that an aromatic group can be introduced onto the N atom on such a platform, which would greatly expand the scope of SSP, although it generates non-conjugated polymers. Furthermore, besides linker distance, we can speculate that the substitution on the CH₂–N(R)–CH₂ linker also plays an important role in the SSP behavior of the monomers.

Acknowledgements

This study was supported by the Natural Science Foundation of China (21371138), the Funds for Creative Research Groups of Hubei Province (2014CFA007), the Fundamental Research Funds for the Central Universities (2042015kf0180) of China and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1402762 and 1402763. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra10007k

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