Synthesis and investigation of novel thiophene derivatives containing heteroatom linkers for solid state polymerization

Abstract

Several thiophene derivatives containing different heteroatom linkers were rationally designed, synthesized and studied as potential candidates for solid state polymerization (SSP). Due to the flexible short single heteroatom linkers, these monomers may rotate freely under heating to meet the requirement needed for SSP. Meanwhile, our novel design strategy has been proved to be an amazing protocol and will pave the way to explore more systems in the polythiophene family by using such a facile SSP method.

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Since they were invented in 1977, conductive polymers have become attractive materials.¹ After over three decades, they are still an emerging research area due to their wide application in light-emitting devices,² supercapacitors, electrochromic devices and organic photovoltaic cells.³

Although most conductive polymers are mainly prepared by chemical polymerization, electropolymerization and photopolymerization, recently some groups have paid much attention on solid state polymerization (SSP), which appeared in the early 1960s.⁴ This method attracts much attention due to its solvent free, oxidant free and/or external applied potential free special features, which are required by traditional methods. Until now, only a few examples of thiophene derivatives which are suitable for SSP were successfully realized since the pioneering work by Wudl and co-workers, who were the first to successfully make poly(3,4-ethylenedioxythiophene) (PEDOT) by spontaneous SSP in 2003.^5a

Generally, among all the monomers developed so far, symmetrical dihalogen-substituted monomers⁶ and a 3,4-ethylenedioxythiophene (EDOT) framework are necessary for SSP, i.e., EDOT is indispensable and all successful dihalogen-substituted monomers are modified through the EDOT molecule's vertical direction. In addition, the modification of EDOT, based on either the EDOT framework or its derivative, is burdensome work which involves a multi-step synthesis. Due to restrictions on the present design strategy of monomers, we herein present the successful design of monomers with parallel EDOT direction by introducing flexible linkers between the EDOT units and obtaining corresponding conductive polymers by SSP.

The SSP procedure was conducted according to a previous method.⁵ In a closed vial, the brominated or iodinated compounds (300 mg) were incubated for 2–96 h at 60 °C or 80 °C, respectively. Intensity data for all four crystals were collected by using Mo Kα radiation (λ = 0.7107 Å) on a Bruker SMART APEX diffractometer equipped with a CCD area detector at rt. Data set reduction and integration were performed by using the software package SAINT PLUS.⁷ The crystal structure was solved by direct methods and refined by using the SHELXTL 97 software package.⁸

Scheme 1 demonstrates the designed monomers and their behavior under SSP. After SSP at 60 °C overnight, colorless or yellowish crystals of these monomers changed to be sky-blue or black powder, which are typical colors of p-type conductive polymers. According to the SSP results, except for monomer 2 and 5, the monomers can successfully form conducting polymers through SSP, indicating that single heteroatoms are effective linkers in our design strategy. Surprisingly, dibromo-substituted 2 and 5 failed to form respective polymers under SSP while diiodo-substituted monomers (4, 6) worked well. Previous studies^5,6 showed that in the case of halogen substituted EDOT derivatives or analogs, bromo or iodo substitution did not present a big difference because of the formation of the same polymer matrix under SSP. However, it is not true in the case of our examined X₂–Si–EDOT or X₂–P–EDOT systems.

Scheme 1 (a) Designed monomers and corresponding polymers and (b) photographs of crystals of monomers and respective polymers, formed on heating the crystals of the monomers at 60 °C.

Because of their insolubility, the polymers' structural information was obtained by using a Fourier transform infrared (FTIR) spectrometer, as shown in Fig. 1. P(Si–EDOT) and P(C–EDOT) have peaks around 1520, 1470, 1340, and 1200 cm⁻¹ which originate from the stretching of C=C and C–C in the thiophene ring.^9,10 While in the case of P(P–EDOT), the strong intensity peaks appear in the region of 1200–1500 cm⁻¹, due to the benzene ring's contribution, excluding the peak around 1520 cm⁻¹. Meanwhile, all polymers demonstrate peaks around 690 cm⁻¹, which are related to the in-plane deformation of C–S–C of the thiophene ring.⁹ Because of the existence of heteroatoms in these polymers, IR spectra have distinct fingerprints. For instance, P(Si–EDOT) shows peaks at 1242 cm⁻¹ and 984 cm⁻¹ which originate from the Si–CH₃ symmetric deformation vibration and Si–CH₃ rocking or wagging, and another peak at 839 cm⁻¹ is assigned to Si–C stretching.^11,12 Meanwhile, P(P–EDOT) exhibits a characteristic vibration mode of P–C asymmetrical stretching at 748 cm⁻¹ and stretching vibration at 1437 cm⁻¹.^13,14 In addition, due to the benzene ring in the P(P–EDOT) polymer chain, three peaks in the region of 2800–3000 cm⁻¹, which are assigned to the benzene ring's C–C–H stretch, are observed.

Fig. 1 (a) FTIR spectra for the polymers. (b) Expansion of (a) for the region from 400 to 1700 cm⁻¹.

Among all three polymers' CV curves, P(C–EDOT) and P(Si–EDOT) show typical conductive polymer behavior. As shown in Fig. 2a and b, P(Si–EDOT) has a similar electrochemical behavior to that of P(C–EDOT) except that its reduction peak shifts 0.1 V positively. However, P(P–EDOT) shows the narrowest peak or region in its CV curves. Except for the slowest scan rate of 25 mV s⁻¹, it is difficult to observe typical redox peaks in the total scanning region. Such a special phenomena might be due to the fact that P atoms in the polymer chain are easily oxidized, which means that P atoms may gain or loose an electron especially in the region of 0–0.8 V during an applied potential. In addition, the oxidation is irreversible as the oxidation process has been observed without a typical reduction process in the reverse scan. This explanation is well supported by the evidence of an abrupt reduction current in the region of −0.4 to −0.8 V. Therefore, such a polymer is not a traditional conducting polymer because P atoms act as the second redox centers in the polymer chain.

Fig. 2 The CV curves of different polymers in acetonitrile solution containing 0.1 M Bu₄NClO₄ taken at various scan rates. (a) P(C–EDOT), (b) P(Si–EDOT) and (c) P(P–EDOT).

Single crystals of the three successful monomers (1, 4 and 6) and a typical unsuccessful sample 2 were obtained and studied by X-ray analysis (shown in Fig. 3 and the ESI†) for further understanding of the structural requirements for SSP.

Fig. 3 Single-crystal X-ray structure of compound Br₂–C–EDOT (1). (a) View of anti-parallel dimers, (b) view of C11–C11 distance between each third molecule and (c) crystal packing viewed along the a-axis. Hydrogen atoms are omitted for clarity: Br, red; S, yellow and C, gray.

The nearest halogen distances for Br₂–C–EDOT (1), I₂–Si–EDOT (4) and I₂–P–EDOT (6), which are 4.296, 4.343, and 4.092 Å respectively, can lead to a successful SSP. It is obvious that due to the V-shape of the monomers, the general explanation which is suitable for the traditional planar prototype of 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) monomer is not applicable here. We select Br₂–C–EDOT (1) as a typical example to discuss the SSP propagation pathways in detail.

As shown in Fig. 3, 1 forms anti-parallel dimers with the nearest halogen distance of 4.296 Å with the C1–C11 contact of 5.414 Å, followed by the third shortest distance of 4.805 Å with the C1–C11 shortest contact of 3.914 Å. It is noted that the C11–C11 distance between each third molecule in the chain (9.186 Å) is 18% longer than the corresponding distance in the regular polythiophene (7.85 Å).¹⁵ Taking the bulkier monomer 1 into consideration, this distance might be within the range to construct a corresponding polymer. Although the second shortest halogen distance of 4.727 Å was found in the crystal structure, its corresponding C1–C11 contact is 6.651 Å, which is too long to supply a rational pathway for the polymerization. It is noted that the effective Hal–Hal distance (4.805 Å) is much longer (30%) than the double van der Waals radius of bromine (3.7 Å). We attribute such a phenomenon to its flexible linker. Such a featured structure may rotate or elongate freely under heating, resulting in the close proximity to the halogen atom of the neighboring monomer, which plays an important role in SSP, especially in our examined system.

Through crystal structure analysis (Fig. S14 and 15, ESI†), the effective Hal–Hal distances of I₂–Si–EDOT (4.492 Å) and I₂–P–EDOT (4.527 Å) are shown to be 12.3% and 13.2% longer than the double van der Waals radius of iodine (4.0 Å) respectively and all of them succeed in SSP. Taking the heteroatom linkers of C, Si, and P into consideration, these flexible molecules may rotate or elongate freely under heating, which results in the close proximity to the neighboring molecule. Once the intermolecular halogen distances reach or are close to their double van der Waals radius, SSP occurs. However, the Hal–Hal distance of Br₂–Si–EDOT (Fig. S16, ESI†) is shown to be 50% longer than the double van der Waals radius of bromine, which results in the failure of SSP. Our result reveals that the distance requirement of Hal–Hal can be drastically extended to an excess of 30% of the double van der Waals radius of halogen through the introduction of the flexible linker. Moreover, without considering dynamic effects, effective Hal–Hal and C–C contact distances are key factors to determine their SSP behavior.

A conductivity test reveals that most of these polymers show poor conductivity with values of 2–6 orders of magnitude lower than that of the well-known PEDOT (Table S2, see ESI†). P(Si–EDOT) shows the best conductivity of 0.02 S cm⁻¹ among the examined polymers while both P(C–EDOT) and P(P–EDOT) show poor conductivity at 10⁻⁶ to 10⁻⁷ S cm⁻¹. Due to the poor conjugation by the existence of the heteroatom linker along the polymer chain, the P(Si–EDOT)'s conductivity is not so high, which is comparable to those of reported non-conjugated polythiophene derivatives.^16,17 In addition, taking the doping level which directly determines the conjugated polymer's conductivity into consideration, the low conductivity of P(C–EDOT) and P(P–EDOT) might be due to the reaction of bromine or iodine with the active groups attached on the polymer chains, thereby resulting in an extremely low doping level.

In this work, several kinds of novel thiophene containing heteroatom linkers were synthesized and successfully employed in SSP. Among these rationally designed monomers, flexible single heteroatom linkers such as C, Si and P between EDOT units were examined. Interestingly, only iodo substituted monomers can undergo SSP in the case of Si and P as a linker. Combing this result with the crystal structure analysis, it is clear that heteroatom types and substitution groups have a great effect on their SSP behavior and this finding may help us to gain a deeper understanding of SSP. Furthermore, taking the thiophene moiety into consideration, our design strategy, along with the EDOT parallel direction, may broaden the application of SSP, suggesting a promising approach for designing other thiophene monomers suitable for SSP.

This work was supported by the Natural Science Foundation of China (21001084, 21371138), New Teacher Fund for Doctor Station, Ministry of Education (20100141120005) and the Fundamental Research Funds for the Central Universities (1101018) and The Project sponsored by SRF for ROCS, SEM.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, CIF files, crystallographic data, UV-Vis spectra, TGA for corresponding polymers, ¹H, ¹³C NMR and SSP conversion yield of polymers under SSP. CCDC 943874–943877. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra45014g

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