Parallel design strategy and rational study of crystal engineering of novel 3,4-ethylenedioxythiophene derivatives for solid state polymerization

Abstract

Several 3,4-ethylenedioxythiophene (EDOT) derivatives containing different heteroatom linkers were rationally designed along parallel directions. They were synthesized and studied as potential candidates for solid state polymerization (SSP). The detailed characterization of these corresponding polymers was carried out using UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and several key crystal structures were analyzed. Our study reveals that heteroatoms like C, Si and P are good linkers between EDOT units, and most monomers can be spontaneously polymerized successfully by annealing at a moderate temperature. Due to the flexible short single heteroatom linkers, these monomers may rotate freely under heating, to meet the requirement for the success of SSP. By carefully analyzing typical crystal structures, direct evidence for the success of SSP was obtained for better understanding of the SSP process. In addition, effective Hal/Hal distance was defined and a new SSP model was proposed.

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1. Introduction

Conductive polymer has been an attractive material since it was first invented in 1977.¹ After over three decades, it is still an extensively researched material due to its wide application in light-emitting devices,^2,3 supercapacitors, electrochromic devices and organic photovoltaic cells.⁴

Although most conductive polymers are mainly prepared by chemical polymerization, electropolymerization and photo-polymerization, some groups have been focusing on the solid state polymerization (SSP) process recently, which was discovered in the early 1960s⁵ for the synthesis of polythiophenes. This method deserves much attention due to its special features, such as being solvent-free and oxidant- or external applied potential-free, conditions that are necessarily required through traditional methods. To date, only a few examples of thiophene derivatives were successfully realized by SSP since the pioneering work, which was performed by Wudl and co-workers, who first successfully synthesized poly(3,4-ethylenedioxythiophene) (PEDOT) through spontaneous SSP in 2003.⁶

Subsequently, several groups investigated similar 5,7-halogen substituted thiophene derivatives⁷ through the thiophene ring’s longitudinal direction and verified the success of their SSP (Fig.1). Moreover, we and others used this unique method to prepare PEDOT counter electrodes or hole conductor for dye-sensitized solar cells.⁸ In our previous communication, we successfully designed EDOT monomers along parallel direction by introducing flexible linkers between EDOT units and obtained the corresponding conductive polymers by SSP.⁹ Our results verify that this new monomer design strategy works very well, and single heteroatoms are effective flexible linkers between EDOT units, which can guarantee their SSP success. In this article, we report the details of the characterizations of the corresponding polymers, the temperature-dependent polymer yield, and further the discussion on crystal analysis for a better understanding of the SSP process.

Fig. 1 Longditudinal and parallel directions in thiophene derivatives.

2. Experimental section

Materials and characterization

All chemicals were purchased from Wuhan Shenshi Chemicals Co., Ltd. and were used without further purification unless stated otherwise. EDOT was purchased from J & K. Anhydrous diethyl ether solvent was distilled from commercial diethyl ether with CaH₂. 5-Bromo-3,4-ethylenedioxythiophene¹⁰ was synthesized according to our previous report. 5-Iodo-2,3-dihydro-thieno[3,4-b][1,4]dioxine¹¹ was synthesized similar to 5-bromo-3,4-ethylenedioxythiophene, except that 5,7-diiodo-3,4-ethylenedioxythiophene was used during reduction by n-butyllithium.

Monomers synthesis and solid state polymerization

Bis(7-bromo-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl) methane (1, Br₂-C-EDOT). Monomer 1 (Br₂-C-EDOT) was synthesized in accordance with a previous method,¹² as shown in Scheme 1. 5-Bromo-3,4-ethylenedioxythiophene was added over several minutes to a stirred solution of zinc chloride in hydrochloric acid, at a temperature between −8 °C and −10 °C. Formaldehyde (37%) was added drop wise over 60 min while maintaining the temperature around −10 °C. The reaction mixture was stirred 60 min while maintaining this temperature, and then quenched with water and extracted with ether. The organic extract was washed with 5% sodium bicarbonate solution, dried over magnesium sulfate, and then the solvent was evaporated. Purification by column chromatography (silica gel, light petroleum–CH₂Cl₂, 2 : 1) afforded a white solid 1 (60%). ¹H NMR: δ (CDCl₃) 4.25 (m, 2H); 4.22 (m, 2H); 3.89 (s, 2H). ¹³C NMR: δ (CDCl₃) 139.7, 137.7, 114.9, 84.4, 65.3, 64.7, 22.2. CI-MS: calcd, 451.8; found (M + 2)⁺, 453.8.

Scheme 1 Synthesis of the monomers and corresponding polymers.

Bis-(7-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-dimethyl-silane (2, Br₂-Si-EDOT). Monomer 2 (Br₂-Si-EDOT) was synthesized by modification of a previously reported method.¹³ Anhydrous THF (30 ml) was slowly added drop wise to a solution of lithium diisopropylamide (prepared by addition of n-butyllithium to diisopropylamine, 10 mmol) while the temperature was maintained around −15 °C. The reaction mixture was cooled to −60 °C, and a solution of 2-bromo-3,4-ethylenedioxythiophene (2.21 g, 10 mmol) in anhydrous THF (10 ml) was slowly added drop wise while the temperature was maintained around −50 °C. Then, the reaction mixture was warmed up to −20 °C, and stirred at this temperature for 15 min. After that, the reaction mixture was cooled to −78 °C, and dichlorodimethylsilane (0.65 g, 5 mmol) was added. Then, the cooling bath was removed and the temperature was increased to room temperature. After completion of the reaction, the solution was poured into a mixture of ice water (50 ml), containing 2 ml 1 M HCl and freshly distilled ether (100 ml). The organic layer was washed with water and dried with Na₂SO₄. Purification by column chromatography (silica gel, light petroleum–CH₂Cl₂, 2 : 1) afforded a colorless crystal 2 (50%). ¹H NMR: δ (CDCl₃) 4.24 (m, 2H); 4.21 (m, 2H); 0.55 (s, 6H). ¹³C NMR: δ (d-DMSO) 147.6, 140.9, 107.6, 91.9, 64.5, −1.55. Anal. calcd For C₁₄H₁₄Br₂O₄S₂Si: C, 33.75%; H, 2.83%. Found: C, 32.95%; H, 2.96%.

Bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-dimethyl-silane (3, Si-EDOT). To a solution of EDOT (2.10 g, 14.8 mmol) dissolved in 50 ml of THF under argon, 6.2 ml (14.8 mmol) of n-BuLi (2.4 M) was added drop wise. After stirring for 1 h at room temperature, the mixture was cooled to −78 °C. Dichlorodimethylsilane (0.98 g, 7.6 mmol) was added drop wise, and the mixture was stirred for a further 3 h at −78 °C, followed by 21 h at room temperature. The mixture was then poured into 100 ml of water, and the product was extracted twice with 100 ml of CH₂Cl₂. The organic layers were combined and dried over MgSO₄. After evaporation of the solvent, the product was purified by chromatography on silica gel using CH₂Cl₂ as eluent. The final product was a colorless crystalline solid (1.90 g, yield 75%). ¹H NMR (CDCl₃, ppm): 6.58 (s, 2H), 4.19 (m, 8H), 0.58 (s, 6H), which is consistent with an earlier report.¹⁴

Bis-(7-iodo-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-dimethyl-silane (4, I₂-Si-EDOT). To a solution of 3 (0.67 g, 2 mmol) in anhydrous THF at −78 °C, n-BuLi (2.4 M in hexane; 1.8 ml, 4.3 mmol) in THF (10 ml) was added. The reaction mixture was allowed to warm up to 0 °C for 30 min, and cool to −78 °C, and then iodine crystals (1.1 g, 4.2 mmol) were added in one portion. The reaction mixture was allowed to warm up to 20 °C overnight, the solvent was removed in vacuum, and the residue was recovered in CHCl₃. After washing with KI solution (to remove excess iodine) and water, the organic layer was concentrated in vacuum, and purified by flash chromatography on silica with petroleum–CH₂Cl₂ (2 : 1 v/v) as eluent, followed by recrystallization from EtOH, which yielded pure monomer 4 as light-yellow crystals (0.71 g, 60%).¹H NMR: δ (CDCl₃) 4.24 (m, 2H); 4.20 (m, 2H); 0.54 (s, 6H). ¹³C NMR: δ (CDCl₃) 147.1, 145.0, 114.8, 65.2, 64.7, 56.2, −1.37. Anal. calcd For C₁₄H₁₄I₂O₄S₂Si: C, 28.39%; H, 2.38%. Found: C, 27.78%; H, 2.21%.

Bis-(7-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-phenyl-phosphane (5, Br₂-P-EDOT). This compound was synthesized by a procedure similar to that used for 2 with one modification, i.e., dichlorophenyl phosphine (0.89 g) was added instead of dichlorodimethylsilane. Yield: 2.57 g (40%); ¹H NMR: δ (CDCl₃) 7.8–7.9 (m, 2H), 7.6 (d, 1H), 7.5 (m, 2H), 4.24–4.26 (m, 4H). ¹³C NMR: δ (d-DMSO) 146.4, 146.2, 140.6, 134.8, 134.7, 131.8, 131.6, 129.4, 128.8, 128.7, 92.5, 64.7. Anal. calcd for C₁₈H₁₃Br₂O₄PS₂: C, 39.44%; H, 2.39%. Found: C, 39.38%; H, 2.31%.

Bis-(7-iodo-2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-phenyl-phosphane (6, I₂-P-EDOT). This compound was synthesized by a procedure similar to that used for 5 with one modification, i.e., 5-iodo-2,3-dihydro-thieno[3,4-b][1,4]dioxine was added instead of 5-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxine. Yield: 40%. ¹H NMR: δ (CDCl₃) 7.3–7.5 (m, 5H), 4.24–4.26 (m, 4H). ¹³C NMR: δ (d-DMSO) 146.1, 145.9, 144.5, 135.2, 131.7, 131.5, 129.2, 128.7, 111.9, 111.6, 64.6, 59.5. ESI-MS: calcd, 642.2; found (M + 2)⁺, 644.6. Anal. calcd for C₁₈H₁₃I₂O₄PS₂: C, 33.66%; H, 2.04%. Found: C, 34.2%; H, 2.26%.

General solid state polymerization

The SSP procedure was conducted according to a previously reported method.⁶ In a closed vial, the brominated or iodinated compounds (300 mg) were incubated at 60 °C or 80 °C, respectively, for 2–96 h. The grounded polymers were dried in vacuum at 80 °C overnight, and then stirred with hydrazine hydrate (50% aqueous solution in CH₃OH) overnight. This solution was filtered and washed with CH₃OH and vacuum-dried to yield nearly fully dedoped corresponding polymers.

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 four monomers are given in Table 1. Data sets of reduction and integration were obtained using the software package SAINT PLUS.¹⁵ The crystal structure was solved by direct methods and refined using the SHELXTL 97 software package.¹⁶

Table 1 Crystallographic data and structure refinement of the synthesized monomers

Parameter	Br2-C-EDOT	I2-Si-EDOT	I2-P-EDOT	Br2-Si-EDOT
Empirical formula	C13H10Br2O4S2	C14H14I2O4S2Si	C18H13I2O4S2P	C14H14Br2O4S2Si
fw	454.15	592.26	642.17	498.28
Cryst. syst.	Monoclinic	Orthorhombic	Monoclinic	Orthorhombic
Space group	P2(1)/c	Iba2	P2(1)/c	Pbca
a (Å)	12.665(3)	16.903(8)	14.6939(12)	13.2636(18)
b (Å)	8.8588(19)	28.697(13)	9.2288(7)	10.1957(13)
c (Å)	14.424(3)	8.005(4)	15.8097(12)	27.784(4)
α (deg)	90	90	90	90
β (deg)	110.803(3)	90	104.924(1)	90
γ (deg)	90	90	90	90
V (Å^3)	1512.8(5)	3883(3)	2071.6(3)	3757.3(9)
Z	4	8	4	8
Dcalcd (g cm^−3)	1.994	2.026	2.059	1.762
Cryst. size (mm^3)	0.32 × 0.22 × 0.20	0.29 × 0.22 × 0.20	0.3 × 0.29 × 0.28	0.29 × 0.28 × 0.26
Diffractometer	SMART CCD	SMART CCD	SMART CCD	SMART CCD
F (000)	888	2240.0	1224	1936.0
T (K)	296(2)	296(2)	296(2)	293(2)
θmax	26.5	26.00	26.50	26.50
Reflns collected	9083	11 749	13 288	23 315
Indep. reflns	3134	3775	4288	3895
Param. refined	191	210	245	210
R1, wR2	0.0643, 0.2005	0.0741, 0.2056	0.0253, 0.0532	0.0517, 0.1223
GOF (F2)	1.054	1.050	1.019	1.01

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Other characterizations

Absorption spectra were measured on a Unicam UV 300 spectrophotometer at wavelengths from 300 to 1000 nm. Monomers deposited on fluorine-doped tin oxide (FTO) substrates or slide glasses were prepared by spin-coating or drop-casting with 0.5–3 wt% of CHCl₃ monomer solution. These monomer-coated substrates were employed for SSP (80 °C for I₂-P-EDOT, 60 °C for the others). The resultant polymers and polymer/FTO substrates were used for XRD and UV-Vis measurements. X-ray diffraction (XRD) patterns were obtained by a Bruker D8 advanced X-ray diffractometer using Cu Ka radiation at room temperature. The surface morphologies of the monomers and SSP polymers were analyzed using field-emission scanning electron microscopy (JEOL, JSM-6700F).

3. Results and discussion

Synthesis of monomers and their behavior under SSP

All monomers were synthesized as shown in Scheme 1. Due to the difficulty in the final bromination step, the monobromo-substituted EDOT was synthesized as the starting material in the case of Si, C or P linker-containing monomers. Monomer 1 was obtained through Blanc chloromethylation of 5-bromo-3,4-ethylenedioxythiophene. Monomer 2 or 3 was obtained by the reaction of EDOT derivatives with Me₂SiCl₂ with the help of Li reagent. Further treatment of 3 with n-BuLi, followed by iodine, provided the desired iodo-substituted monomer 4. The synthesis of 5 and 6 was similar to that of Si-based monomer 2.

Finally, these obtained dihalogen-substituted monomers were examined under SSP and these samples successfully released bromine or iodine vapor. According to their SSP results, except for monomer 2 and 5, the other monomers successfully formed conducting polymers through SSP, indicating that single hetero atoms are effective linkers in our design strategy. Surprisingly, dibromo-substituted 2 and 5 failed to form respective polymers under SSP, whereas diiodo-substituted monomers 4 and 6 worked well. It should be noted that previous studies showed that there is almost no difference between bromo and iodo substituted EDOT derivatives or analogs due to the formation of the same polymer matrix under SSP; however, this is not true in our case of the examined X₂-Si-EDOT or X₂-P-EDOT systems. Thus, according to these observations, it is clear that iodo-substituted monomers tend to form the corresponding polymer under SSP, whereas sometimes bromo-substituted monomers cannot form the corresponding polymers. These results disappointed us because bromination is an easier method and even cheaper method compared with iodination. The details are discussed in the following section of crystal analysis.

XRD patterns of the monomers and their respective polymers

Fig. 2 shows the XRD patterns of the synthesized monomers, and their respective polymers formed by SSP, and their detailed XRD peaks positions are summarized in Table 2. As shown in Fig. 2, all monomers show sharp peaks in the range of 10°–60°, which are characteristic peaks for crystal compounds, and they are the indication of relatively long-range orders. However, most sharp peaks disappear, except for a wide broad peak around 22°–26°, once they were changed into their respective polymers through SSP, which indicates the existence of an amorphous phase in the polymer matrix.

Fig. 2 XRD spectra of (a) Br₂-C-EDOT and P(C-EDOT), (b) I₂-Si-EDOT and P(Si-EDOT),and (c) I₂-P-EDOT and P(P-EDOT).

Table 2 Peak positions derived from the X-ray diffraction patterns of the monomers and their respective polymers (s^1st: first strong peak)

Monomers	XRD peak positions (°)	Polymers	XRD peak positions
Br2(C-EDOT)	12.2, 13.9, 14.9, 16.5 (s^2nd), 20.9, 23.2 (s^3rd), 24.0 (s^1st), 25.7, 27.3, 28.3, 32.8	P(C-EDOT)	12.4, 14.0, 23.4, 24.0
I2(Si-EDOT)	15.5 (s^3rd), 16.0, 21.0, 22.2 (s^1st), 23.0, 24.0, 24.7, 26.6 (s^2nd), 27.7, 29.3, 34.3	P(Si-EDOT)	24.0 (broad)
I2(P-EDOT)	11.6 (s^1st), 15.1, 19.4, 21.1, 22.6 (s^2nd), 23.3 (s^3rd), 24.5, 26.6, 29.7	P(P-EDOT)	22.5 (broad)

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For instance, Br₂-C-EDOT has peaks at 12.2°, 13.9°, 14.9°, 16.5°, 20.9°, 23.2°, 24.0°, 25.7°, 27.3°, 28.3°, and 32.8°, while P(C-EDOT) has only four weak peaks at 12.4°, 14.0°, 23.4°, and 24.0°. Compared with other polymers, P(C-EDOT) has two additional distinct peaks in the region of 10°–20°, revealing that P(C-EDOT) partially keeps its order in polymer chain matrix to some extent. However, P(P-EDOT) and P(Si-EDOT) demonstrate complete amorphous phases due to their low resolution XRD peaks. Moreover, the C and Si linker-based monomers and polymers (Fig. 2a and b) have similar XRD patterns. In fact, all these monomers have three strong intensity peaks around 16°, 22° and 26°, and their polymers' XRD peak positions are almost in accordance with the highest intensity peaks of the respective monomers. However, a large difference appeared in the case of I₂-P-EDOT (Fig. 2c), which has the strongest peak at 11.6°. In addition, the simulated XRD pattern shows that the strongest peak forms at 6.06° for I₂-Si-EDOT. According to a previous study and the result of the crystal structure analysis,⁶ it is easy to obtain d-spacing based on the strongest peak in the diffractogram. In our case, the positions of the strongest peaks are shifted to 24.0°, 6.06° and 11.6° and the corresponding d-space are 3.70, 14.57 and 7.64 Å after C, Si and P linkers were introduced. Therefore, these linkers change a lot on their crystal frame work. The detailed structure analysis (see Crystallographic section) is presented in the following section.

Absorption of polymers and hydrazine-treated polymers

All UV-Vis absorption spectra of the polymers obtained by SSP and their neutral polymers after treatment with hydrazine are shown in Fig. 3. Due to the existence of a quinonoid structure¹⁷ in the polymer matrix, all of them exhibit typical conjugated polymer properties. In the case of P(Si-EDOT) and P(P-EDOT), they both exhibit similar curves with a typical free carrier tail^18,19 around the near-IR region; moreover, they also show a broad peak around 550 nm after treatment with hydrazine. The latter absorption peak is ascribed to the π–π* electronic transition of dedoped (reduced) polythiophene.^6,18,19 In addition, the near-IR peak around 770 nm of dedoped polymer decreases drastically, indicating that the hydrazine-treated polymer is in the neutral state. However, in the case of P(C-EDOT), it shows a typical curve after hydrazine reduction while apparent oxidation tail is missing after completion of SSP; i.e., the prepared P(C-EDOT) shows quite a different electronic state behavior when compared with traditional polymers obtained through SSP. We attribute this observation to the lower bromine doping level because of the reaction by the CH₂ group with generated bromine in the polymer chain. We can assume that some of the bromine was trapped inside the chains, and it may have little chance to carry out the simultaneous oxidation doping process. In addition, such explanation is supported by the observation of that a small amount of bromine was released during the SSP procedure.

Fig. 3 Absorbance spectra of the monomers, polymers by SSP and dedoped polymers by hydrazine treatment (freshly made).

SEM of the monomers and polymers

Fig. 4a–f shows the SEM surface morphologies of the drop-casted monomer films and respective polymers on an FTO substrate. It is obvious that the morphology significantly changes after SSP. Most monomers have a uniform film with the shape of rods varying between 100 and 300 nm and the length reaching 2 μm and near 1 μm for Br₂-C-EDOT and I₂-Si-EDOT, respectively, whereas I₂-P-EDOT shows typical aggregation of micrometer-sized irregularly shaped sheets. In fact, after the SSP procedure, most polymers show an enlarged size; however, it should be noted that in the case of P(C-EDOT), it shows a different morphology with a long linear rope. Such featured characteristic may be attributed to the quick polymerization process during SSP. This explanation supports the fact that the Br₂-C-EDOT monomer is not stable at room temperature and generates silk polymer. Such featured property may be beneficial to enhance mechanical strength of the polymer.

Fig. 4 (a) Photographs of crystals of monomers and their respective polymers, formed by heating the crystals of the monomers at 60 °C. (a) Br₂-C-EDOT, (b) P(C-EDOT), (c) I₂-Si-EDOT, (d) P(Si-EDOT), (e) I₂-P-EDOT and (f) P(P-EDOT). All these monomers were drop-casted on FTO substrates.

Crystallographic X-ray analysis

Single crystals of the three successful monomers, i.e., 1, 4 and 6, and a typical unsuccessful sample 2 were obtained and studied by X-ray analysis (Fig. 5–8) for further understanding the structural requirements for SSP. Before obtaining the crystal structures of the monomers, we expected that their inter-molecule halogen distances are within double the van der Waals radius of the corresponding halogens, 2r_w, (3.7 Å for Br and 4.0 Å for I) like previous reports.^6,7a However, what surprised us is that the distances between neighboring halogens are a little bit longer. Their nearest halogen distances are 4.296, 4.343, and 4.092 Å for Br₂-C-EDOT (1), I₂-Si-EDOT (4) and I₂-P-EDOT (6), respectively, which can make the SSP successful. As we can see from Fig. 5–7, it is obvious that due to the V-shape of the monomers, the general explanation that is suitable for traditional planar prototype of 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) monomer is useless. Therefore, we discuss their SSP propagation pathways in detail as follows.

Fig. 5 Single-crystal X-ray structure of compound Br₂-C-EDOT (1). (a) View of anti-parallel dimers, (b) view of Br/Br distances, (c) crystal packing viewed along the c-axis and proposed polymerization pathway. Hydrogen atoms are omitted for clarity: Br, red; S, yellow and C, gray.

As shown in Fig. 5, 1 forms anti-parallel dimers with the nearest halogen/halogen (Hal/Hal) distance of 4.296 Å and C1–C11 contact of 5.414 Å, followed by the third shortest Hal/Hal distance of 4.805 Å and the shortest C1–C11 contact of 3.914 Å. It should be noted that the C11/C11 distance between each third molecule in the chain (9.186 Å) is 18% longer than the corresponding distance in regular polythiophene (7.85 Å).²⁰ Taking the bulkier monomer 1 into consideration, this distance might be within the range to construct the corresponding polymer. Although the second shortest Hal/Hal distance of 4.727 Å is found in the crystal, its corresponding C1–C11 contact is 6.651 Å, which is too long to supply a rational pathway for the polymerization. The reasonable polymerization pathway is along the c-axis (blue arrow in Fig. 5c), which involves C11–C1–C11–C1 repeatable chains.

Because there are strong interactions between the halogen atoms and the new C–C bonds formed during SSP, the Hal/Hal distance and corresponding C/C contact distance are very important to deduce the preferred polymerization pathway. Here, we define the minimum Hal/Hal distance involved in all proposed polymerization pathways as the effective distance. Therefore, the effective Hal/Hal distance (4.805 Å) for 1 is much longer (30%) than the 2r_w of bromine (3.7 Å). We attribute this interesting structural information to its flexible linker. Such a structure may rotate or elongate freely under heat treatment, resulting in it moving closer to halogen atom of the neighboring monomer, which plays an important role in SSP, especially in our designed monomer of 1.

As shown in Fig. 6, no dimer exists in the I₂-Si-EDOT crystal with the first and second closest Hal/Hal distances of 4.343 and 4.492 Å, respectively, and C12–C12 and C1–C12 contacts of 5.356 and 5.683 Å, respectively. Although the above C–C contact is a little bit longer, the C1/C12 distance between each third molecule in the chain is 9.521 Å, which is similar to that of Br₂-C-EDOT. There are two possible polymerization pathways, which are shown in Fig. 6c. As the first path involves longer I/I distances of 6.290 and 6.721 Å, it is suggested that the second path is the preferred one with an effective halogen distance of 4.343 Å under SSP.

Fig. 6 Single-crystal X-ray structure of compound I₂-Si-EDOT (4). (a) View of the I/I distances, (b) view of the C/C contact distances, (c) crystal packing viewed along the c-axis and proposed polymerization pathway. Hydrogen atoms are omitted for clarity: I, purple; S, yellow and C, Si gray.

In the case of I₂-P-EDOT (shown in Fig. 7), a dimer forms with a short Hal/Hal distance of 4.092 Å and neighboring C10–C16 contact of 6.303 Å, which is the third shortest one among the crystal structures. Along the b-axis direction, as demonstrated in first polymerization pathway, the C16–C16 contact distance is 4.235 Å with a longer distance of Hal/Hal of 6.683 Å. Moreover, a Hal/Hal of 4.527 Å and neighboring C16–C16 contact of 5.305 Å are found in the crystal. The C16/C16 distance between each third molecule in the chain is 9.229 Å, which is consistent with the above two monomers. The second possible polymerization direction (a-axis) is as follows. Along the a-axis direction, Hal/Hal is 6.481 Å and neighboring C10–C10 contact is 3.965 Å with a C10/C16 distance between each third molecule in the chain of 9.930 Å; moreover, the short distance of Hal/Hal of 4.092 Å and neighboring C10–C16 contact is 6.303 Å is involved as well. Taking the longer neighboring C10–C16 contact of 6.303 Å and the longer Hal/Hal of 6.481 Å into consideration, the former pathway with the effective Hal/Hal distance of 4.527 Å might be a more favorable choice. It should be noted that its in situ XRD patterns (see ESI, Fig. S2†) indicate that a major change in [002] phase occurs in the initial 2 hours of the polymerization procedure, which is consistent with our prediction of the most possible 1^st polymerization pathway.

Fig. 7 Single-crystal X-ray structure of compound I₂-P-EDOT (6). (a) View of the I/I distances, (b) possible polymerization pathway along the b-axis, (c and d) the I/I distances and corresponding C/C contact distances and possible polymerization pathway along the a-axis. Hydrogen atoms are omitted for clarity: I, purple; S, yellow; P, yellowish brown and C, gray.

Although Br₂-Si-EDOT failed in the SSP test, its crystal (shown in Fig. 8) was obtained to understand the crystal structure requirement for a successful SSP. Compared with I₂-Si-EDOT, this analogue belongs to a different space group of Pcba, showing quite a different crystal structure, as summarized in Table 1. An anti-parallel dimer is also found in the crystal with Hal/Hal distance of 4.060 Å along with C1–C14 contact of 5.284 Å and C1–C1 of 5.859 Å; however, the C1–C14 distance between each third molecule in the chain (8.273 Å) is short enough to form a polymer chain. Moreover, it is impossible to polymerize along a-axis because of the extraordinarily long distance of Br/Br, which exceeds 8 Å as shown in Fig. 8c. Based on the values of Br/Br and C/C contact distances, we can deduce two possible polymerization pathways, as shown in Fig. 8b. The first pathway is very simple, containing a single bond of Br/Br distance of 5.228 Å with C1/C1 contact of 5.859 Å, whereas the second one is along C1–C1–C14–C14–C1 repeated subunits, involving two longer Br/Br distances of 5.228 Å (corresponding C1/C1) and 5.859 Å (corresponding C14/C14). In fact, the polymerization will happen within these anti-parallel dimers. It seems that the first pathway is the one in priority if taking no deformation during polymerization into consideration. Therefore, the effective Hal/Hal distance is up to 5.228 Å, which is 50% longer than the 2r_w of bromine (3.7 Å). Moreover, a C1–C11 distance of 10.196 Å between each third molecule in the chain is 7%–11% larger than that for successful monomers (1, 4 and 6), which may contributes the SSP failure. Thus, while the initial C–C bond formation most likely occurs within the dimer, the following polymerization pathway would be terminated due to the excessively long Hal/Hal distance. Therefore, even if there is a flexible –Si–(CH₃)₂ linker, it still has threshold value of Hal/Hal distance for a successful SSP.

Fig. 8 Single-crystal X-ray structure of compound Br₂-Si-EDOT (2). (a) View of the Br/Br distances, (b) view of the corresponding C/C contact distances and possible polymerization pathways, (c) crystal packing viewed along the b-axis. Hydrogen atoms are omitted for clarity: Br, red; S, yellow and C, Si gray.

It should be noted that due to the frequent existence of anti-parallel dimers in most crystals, and the observation of Br₂-P-EDOT dimer at m/z 938.7 [dimer]²⁺ in MALDI-MS, the chemical formation of dimer cannot guarantee the continuation of polymerization. Herein, we define an effective Hal/Hal distance for the successful SSP candidates, and Table 3 summarizes all important distances derived from the crystal analysis. Therefore, the effective Hal/Hal distances for Br₂-C-EDOT, I₂-Si-EDOT, I₂-P-EDOT are longer than 30.0%, 12.3% and 13.2% of their 2r_w, respectively, and all of them lead to a successful SSP. Taking the excellent flexibility of heteroatom linkers of C, Si and P into consideration, these molecules may rotate or elongate freely under heating, which will shorten the distance between neighboring molecules. Once the intermolecular halogen distances reach or are close to the 2r_w of halogen, SSP occurs. However, the 2r_w of Br₂-Si-EDOT is 50% longer than the 2r_w of bromine, resulting in failure of SSP. Our results reveal that the distance requirement of Hal/Hal can be drastically extended to exceed 30% above the 2r_w of halogen by the introduction of a flexible linker. Through the above discussion, we can reach a conclusion that without considering the dynamic effect, effective Hal/Hal and C–C contact distance are key factors to determine their SSP behavior.

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

Molecules parameters	Br2-C-EDOT	I2-Si-EDOT	I2-P-EDOT	Br2-Si-EDOT
a Effective Hal/Hal distance, not including dimer.b 2rw: double van der Waals radius.
Shortest Hal/Hal distance	4.296	4.343^a	4.092	4.059
2^nd shortest Hal/Hal distance	4.727	4.492	4.527^a	5.228^a
3^rd shortest Hal/Hal distance	4.805^a	4.763	6.481	5.817
C–C contact shortest distance	3.914	5.359	3.965	5.284
C–C contact 2^nd shortest distance	5.414	5.676	4.234	5.416
C/C distance between each third molecule	9.186	9.516	9.229	10.196
2rw of Hal^b	3.7	4.0	4.0	3.7
Longer than 2rw of Hal^b	30.0%	8.6%	13.2%	41.3%

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It should be noted that what we discussed is mainly based on the initial step of polymerization, whereas the middle and in latter steps may be slightly different. Taking the number of repeated units in the polymer chain, which is around 10,^7a into consideration, we can assume that the polymerization process occurs within a short range, i.e., the polymerization occurs simultaneously within 5 nm³–15 nm³ of nanospace with about 3–5 repeated crystal unit cell. Therefore, the polymerization terminates once the neighboring terminal halogen groups in the polymer chain are quite large, which is due to the deformation of crystal frame work. In addition, optical anisotropy would be observed during the crystal to polymer transformation. However, with regard to optical anisotropy change, further monomer design and investigation need to be performed, which is beyond the scope of this study.

Proposed model for the solid-state polymerization

Based on the crystal structure data of the developed monomers, a simple model for the solid-state polymerization of two types of monomer is illustrated in Scheme 2. When the polymerization proceeds in a domino-type reaction mechanism, all the monomer molecules exhibit a conrotatory molecular motion for the formation of the polymer chain. Compared with the well-established solid-state polymerization of diene or diyne,^21,22 which occurs through crystal to polymer crystal transformation, crystalline thiophene-based monomers gradually collapse during the SSP process because of the elimination of bulkier halogen atoms. Compared with these rigid EDOT derivatives, the flexible ones need to rotate to shorten the halogen distance under thermal treatment at initial stage, and then SSP proceeds smoothly. In addition, although anti-parallel dimers exist in most cases in their crystalline form, and some of them may form corresponding chemical dimers, further continuous polymerization may not occur if their effective Hal/Hal or C/C contact distances are far away from the threshold.

Scheme 2 Schematic for the varying distance between halogens of crystals during SSP polymerization. (a) Traditional rigid EDOT derivatives (b) EDOT derivatives containing flexible chains.

SSP yield dependence of reaction time with temperature

Ignoring the oligomer solubility in organic solvent, the yield was calculated by extraction of the crude reaction product (with CHCl₃) and gravimetric analysis of the residue, and the result was presented in Fig. 9. As can be seen in Fig. 9, Br₂-C-EDOT shows higher conversion efficiency at 60 °C, while I₂-Si-EDOT and I₂-P-EDOT show lower conversion efficiency. In addition, it should be noted that I₂-P-EDOT shows the slowest reaction rate under SSP among all monomers. Combing the crystal analysis in the above section and the key distances shown in Table 3, we attribute such behavior to the existence of benzene ring, which would restrict the free rotation of EDOT unit and have strong interactions with neighboring thiophene rings, thus retarding the SSP process. Higher conversion efficiency would be obtained at a higher temperature of 80 °C. Finally, all final conversion values are stabilized at 70%. According to these facts and discussion a successful SSP needs rational molecule design, including heteroatom types, substituted groups on heteroatom and halogen substitution effects on EDOT unit in our system.

Fig. 9 Conversion percentage versus reaction time of SSP of the monomers over a range of temperatures.

4. Conclusions

In this work, several kinds of novel EDOT derivatives, containing heteroatoms linkers such as C, Si and P, were synthesized in parallel direction and successfully employed in SSP. This design strategy allowed us to expand the scope of SSP, and to prepare new types of conductive polymers. Interestingly, only iodo-substituted monomers can undergo SSP in the case of Si and P as linkers. Combined with their crystal structure analysis, it is clear that heteroatom types and substitution groups have significant effects on their SSP behavior. This finding may help us gain a deeper understanding of SSP. Furthermore, through detailed analysis of the crystal structures, the maximum Hal/Hal distance would be enhanced and further tuned by thermal treatment. In addition, we define the required minimum Hal/Hal distance as effective Hal/Hal distance, and propose a new SSP model for these flexible monomers. Compared with these monomers based on previous EDOT longitudinal direction design strategy, the Hal/Hal and C–C contact distances of our new monomers are key factors that determine their SSP behavior.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21371138) and The Project sponsored by SRF for ROCS, SEM.

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Footnote

† Electronic supplementary information (ESI) available: CIF files for monomers of 1, 2, 4 and 6; DSC for some monomers; in situ XRD patterns for I₂-P-EDOT. See DOI: 10.1039/c4ra01688b

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