Through rational design, polythiophene can be easily tailored, synthesized and be endowed with special and multi-functional properties for different applications1, 2. Recently, a promising method of solid state polymerization (SSP)3-9 including self-acid-assisted polymerization (SAAP)10-12, attracts much attention from academia due to its special features of solvent, oxidant or noble metal catalyzers free. For instance, the well-known poly(3, 4-ethylenedioxythiophene) through SSP has been employed for photovoltaic devices of dye-sensitized solar cell13-15.
In addition, it is known that solid state polymerization (SSP) is a widely used method in industry16. Taking this into consideration, necessary investigation in this field is needed especially for the conjugated polythiophene synthesis. Though dozens of monomers5-12 have been developed so far by longitudinal design strategy based on 3, 4-ethylenedioxythio-phene (EDOT) or 3, 4-ethylenedioxyselenophene molecule (EDOS), they need multiple synthesis steps due to the difficulty of modification on EDOT or EDOS molecules. Recently, we17-19 and others20 proposed a parallel design strategy with the substantial modification of polymer properties through main-chain, which may shed light on design and on the development of numerous monomers for SSP. Thus, a facile platform of EDOT-CH(R)-EDOT17 is developed and corresponding quinonoid conjugated poly(3, 4-ethylenedioxy-thiophene methine)s derivatives21-26 would be obtained with excellent optical-electronic properties. However, some key issues are not clear, such as the accurate structures requirements for SSP, the in-situ monitoring of SSP process, rearrangement of repeat units conformation in polymer chains, the relationship between solid & melt state polymerization and so on.
In order to further broaden SSPʹs scope and explore related application, it is necessary to effectively tune these conjugated polymersʹ photo-physical and chemical properties based on EDOT-CH(R)-EDOT platform. Moreover, through the modification on the side chain of CH2 linker, the substitutions of R show their great effect on electrochemical and optical properties for the corresponding polymers19, which is not a common phenomenon for those traditional conjugated polymers. In addition, in order to obtain high molecular weight poly(3, 4-ethylenedioxythiophene methine)s, alkyl chains are introduced and then these monomers are investigated under melt-state polymerization.
In this study, among optical-electronic fields & applications common moieties such as thiophene, carbazole and fluorene are introduced as the side chains and their corresponding poly(3, 4-ethylenedioxythiophene methine)s derivatives' photo-physical and electrochemical properties are carefully investigated as well. In addition, the alkyl chain effect on SSP and their molecule weight distribution under reaction conditions are investigated further.
EDOT, thiophene-2-carbaldehyde, were purchased from J & K. 5-Iodo-2, 3-dihydro-thieno[3, 4-b][1,4]dioxine17-20, 27, 5-butylthiophene-2-carbaldehyde28, 9-butyl-9H-carbazole-3-carbaldehyde29, 9, 9-dibutyl-9H-fluorene-2-carbaldehyde30 were synthesized according to previous reports.
Monomers of I2-CH(R)-EDOT were synthesized in accordance to previous method31 as shown in Scheme 1.
White solid (37%), 1H NMR: δ (CDCl3) 6.58 (d, 1H, thiophene-H), 6.57 (d, 1H, thiophene-H), 5.99 (s, 1H, -CH), 4.25 (d, 2H, -CH2), 4.18 (d, 2H, -CH2), 2.74 (t, 2H, -CH2), 1.59 (m, 2H, -CH2), 1.36 (m, 2H, -CH2), 0.92 (t, 3H, -CH3). 13C NMR: δ (CDCl3) 145.7, 144.1, 141.7, 137.5, 125.5, 124.4, 123.6, 65.5, 64.8, 47.8, 36.1, 33.85, 30.1, 22.5, 14.1.
Yellow solid (35%), 1H NMR: δ (CDCl3) 6.58 (d, 1H, thiophene-H), 6.57 (d, 1H, thiophene-H), 5.99 (s, 1H, -CH), 4.25 (d, 2H, -CH2), 4.18 (d, 2H, -CH2), 2.74 (t, 2H, -CH2), 1.59 (m, 2H, -CH2), 1.36 (m, 2H, -CH2), 0.92 (t, 3H, -CH3). 13C NMR: δ (CDCl3) 145.7, 144.1, 141.7, 137.5, 125. 5, 124.4, 123.6, 65.5, 64.8, 47.8, 36.1, 33.85, 30.1, 22.5, 14.1. Anal. Calcd for C21H20I2O4S3: C, 36.75%; H, 2.94%; Found: C, 38.12%; H, 2.41%
Light yellow solid (40%), 1H NMR: δ (DMSO-d6) 8.11 (d, 1H, Ar-H), 7.96 (s, 1H, Ar-H), 7.56 (d, 1H, Ar-H), 7.52 (d, 1H, Ar-H), 7.44 (t, 1H, Ar-H), 7.27 (d, 1H, Ar-H), 7.16 (t, 1H, Ar-H), 5.92 (s, 1H, -CH), 4.35 (t, 2H, -CH2), 4.20 (d, 2H, -CH2), 4.16 (d, 2H, -CH2), 1.71 (m, 2H, -CH2), 1.30 (m, 2H, -CH2), 0.88 (t, 3H, -CH3).13C NMR: δ (CDCl3) 143.7, 140.4, 139.3, 136.7, 131.1, 125.3, 125.1, 122.4, 122.3, 120.1, 119.1, 118.3, 108.3, 108.2, 64.9, 64.2, 46.8, 42.5, 40.4, 30.8, 20.2, 13.5. Anal. Calcd for C29H25I2NO4S2: C, 45.27%; H, 3.27%; N, 1.82%. Found: C, 46.85%; H, 3.412%; N, 1.75%.
Light yellow solid (40%), 1H NMR: δ (CDCl3) 8.05 (d, 1H, Ar-H), 7.95 (s, 1H, Ar-H), 7.42 (d, 1H, Ar-H), 7.31-7.38 (m, 3H, Ar-H), 7.20 (d, 1H, Ar-H), 6.02 (s, 1H, -CH), 4.25 (d, 2H, -CH2), 4.16 (d, 2H, -CH2), 4.14 (d, 2H, -CH2), 1.85 (m, 2H, -CH2), 1.24 (m, 26H, -CH2), 0.86 (t, 3H, -CH3). 13C NMR: δ (CDCl3) 144.0, 140.7, 139.6, 137.0, 131.4, 125.6, 125.4, 122.7, 122.6, 120.4, 119.5, 118.6, 108.6, 108.5, 65.2, 64.5, 47.2, 43.1, 40.7, 31.8, 29.6, 29.5, 29.4, 29.3, 29.2, 28.9, 27.3, 22.6, 14.0.
Yellow solid (25%), 1H NMR: δ (CDCl3) 7.66 (d, 1H, Ar-H), 7.61 (d, 1H, Ar-H), 7.29-7.31 (m, 3H, Ar-H), 7.21 (d, 1H, Ar-H), 7.18 (s, 1H, Ar-H), 5.86 (s, 1H, -CH), 4.22 (d, 2H, -CH2), 4.12 (d, 2H, -CH2), 1.96 (m, 4H, -CH2), 1.07 (m, 4H, -CH2), 0.86 (m, 4H, -CH2), 0.68 (m, 6H, -CH3). 13C NMR: δ (CDCl3) 150.9, 150.8, 143.9, 140.7, 140.3, 139.7, 137.2, 126.9, 126.6, 126.5, 124.6, 122.7, 122.4, 119.5, 65.2, 64.4, 54.8, 47.3, 41.1, 40.0, 26.8, 25.8, 22.9, 22.4, 14.1, 12.9.
The SSP procedure was followed according to previous method5-9, 17-20. Due to the fact that hydrogen atom in CH2 bridge was easily removed by the oxidant of iodine, the conjugated poly(bis-ethylenedioxythiophene methine)s with quinoidal structure were obtained. These polymers were further treated with hydrazine hydrate and afforded nearly fully dedoped respective polymers.
Intensity data for all four crystals were collected using Mo Kα radiation (λ = 0.07107 nm) on a Bruker SMART APEX diffractometer equipped with a CCD area detector at rt. Data sets reduction and integration were performed using the software package SAINT PLUS32. The crystal structure is solved by direct methods and refined using the SHELXTL 97 software package33.
IR spectra for the characterization of the resulted polymers were recorded on a Perkin-Elmer FTIR spectrometer. Absorption spectra were measured on a Unicam UV 300 spectrophotometer at wavelengths from 300 to 1000 nm. Monomers were deposited by spin-coated or drop-casted with 0.5%-3% (mass fraction) of CHCl3 monomers solution on fluorine doped tin oxide (FTO) substrate or slide glasses. These monomers coated substrate were employed for SSP and then resulted polymers or polymer/FTO substrates. These products were used for XRD, UV-Vis or as working electrode for electrochemical measurements. For the three-electrode electrochemical measurements in 0.1 mol∙L−1 LiClO4 in acetonitrile, a 1 cm2 area of FTO/Polymer substrate, platinum foil, and Ag/AgCl were served as the working, counter, and reference electrodes, respectively (CH Instruments 604D electrochemical system). X-ray diffraction (XRD) patterns were obtained by Bruker D8 advanced X-ray diffractometer by using Cu-Ka radiation at room temperature. The molecular weight and molecular weight distribution of the polymers were determined by gel permeation chromatography (GPC) equipped with a Waters 2690 separation module and a Waters 2410 refractive index detector (Waters Co., Milford, MA). N, N-Dimethylformamide (DMF) was used as eluent at a flow rate of 0.5 mL∙min−1 with the temperature maintained at 30 ℃, and the results were calibrated against polystyrene standards.
According to the SSP results, all monomers can change into corresponding polymers smoothly and their SSP onset temperatures (Tonset) are not very high around 40-80 ℃. It is interesting that after introducing butyl group, I2-BuTh-EDOT requires lower temperature of 48℃ to trigger SSP while I2-Th-EDOT needs Tonset of 62 ℃. On the other hand, carbazole and fluorene substituted monomers have their Tonset of 72 ℃, 64 ℃ respectively. In addition, the introduction of longest hexadecyl chain results in a little bit higher Tonset of 78℃. Though thiophene, carbazole and fluorene are rigid aromatic rings, their Tonset are quite lower than those of benzene or naphene rings containing monomers based on the same platform19, which may due to the existence of alkyl chains. This interesting phenomenon is beneficial for the synthesis of those soluble polymers to meet the facile processibility and application requirements. Because Tonsetare definitely sensitive to corresponding monomersʹ structures, the detailed discussion will be done in the crystal analysis section as follows. In addition, taking similar properties for P(Carb-EDOT) and P(Carb-C16-EDOT) into account, P(Carb-EDOT) is chosen to measure its electro-optical properties while soluble P(Carb-C16-EDOT) is selected for its molecular weight information.
FTIR spectra of the poly(3, 4-ethylenedioxythiophene methine)s derivatives is shown in Fig. 1. The absorption at 1475, 1434, 1360, and 1259 cm−1 are signed to the stretching of C=C and C-C in the thiophene ring34, 35.Meanwhile, typical peaks around 704 cm−1, which are related to in-plane deformation of C-S-C of thiophene ring, are observed34. In addition, all polymers show distinct strong sharp peaks at 2956, 2926 and 2870 cm−1, which are assigned to CH3 or CH2 stretching modes36. In the case of P(Carb-EDOT), stretching mode at 1099 cm−1 is observed due to the existence of C-N bond37.
As we can see, I2-Th-EDOT shows typical crystalline phase with lots of sharp peaks at the range of 10°-40° and its XRD pattern is in consistent with its simulated results. However, other monomers show broad peaks at the scanned range. We attribute this interesting phenomenon to their amorphous phase at room temperature with the existence of alkyl chains. And it would show more crystal phase under low temperature. After SSP, polymers show broad peak around 25°, 23° and 24° for P(BuTh-EDOT), P(Carb-EDOT) and P(Flu-EDOT) respectively, which reflect their π-π stacking26distance of 0.356-0.386 nm between polymer chains. While in the case of P(Th-EDOT), it shows poor resolution peak in its XRD pattern as shown in Fig. 2(a). It appears that alkyl chains of aromatic substitutions attached on CH(R) bridge are beneficial to chain packing in polymers matrix.
The dilute solution is prepared due to its small solubility of P(Th-EDOT). And all polymersʹ solutions absorption spectra are presented in Fig. 3. As we can see, in the case of fresh made polymers solutions absorption spectra, most of them show peaks at 740-760 nm except that P(Carb-EDOT) exhibits a peak at shorter wave length of 670 nm, indicating of p-type featured character. According to the observation of drastic drop of intensity at near-IR region around 600-1000 nm38, 39, we obtained their neutral type polymers after hydrazine treatment. Briefly, compared with P(Th-EDOT) treated with hydrazine, the existence of butyl leads to the disappearance of peak at about 580 nm. At this moment, we cannot understand this interesting phenomenon, which may due to the poor conjugation by the steric effect. However, the introduction of carbazole and fluorene moieties result in a bathochromic peaks at longer wave length which is 612 nm and 640 nm respectively. Their detailed parameters of optical properties are summarized in Table 1. These results indicate that the modification on side chain in poly(bis-3, 4-ethylenedioxy-thiophene methine)s matrix can change their optical properties, revealing these polymers' great potential application in future optical-electronic devices application.
As shown in Fig. 4, the onset of oxidation potentials are 0.93, 0.57, 0.27 and 0.21 V (vs Ag/AgCl) for P(Th-EDOT), P(Th-Bu-EDOT), P(Carb-EDOT) and P(Flu-EDOT) respectively, indicating that the substitution moieties on CH2 bridge have great influence on their electrochemical properties. It is noted that the introduction of butyl on thiophene results in the shift up of HOMO energy level because of the decrease of the initial onset potential of 0.36 V when compared with that of P(Th-EDOT).
Due to the observation of large reduction currents, we attribute these to the reduction process of oxidized polymer, i.e., these polymers can undergo a two-electron non-reversible oxidation processes40. In addition, except for P(Th-EDOT), other polymers show stable CV behaviors. In the case of P(Th-EDOT), as shown in Fig. 4(a), its oxidation currents are not proportional to the scan rates. This special phenomenon indicates that additional electrochemical reaction occurs during the CV measurements. Taking the existence of non-conjugation parts41 in the polymer chains into consideration, we believe CV measurements, especially during the anode scans, will result in the enhancement of conjugation parts in polymer chains. And this explanation is well supported by the fact that P(Th-EDOT) can show high initial oxidation potentials. Thus, in the existence of iodine under SSP, there are more non-conjugated parts in P(Th-EDOT) when compared with those carbazole or fluorene moieties containing polymers.
Monomers of I2-Th-EDOT and I2-Flu-EDOT structures and corresponding SSP pathways are presented in Figs. 5-7. Interestingly, though alkyl fluorene moiety has bulkier size, I2-Flu-EDOT has similar molecule structure with I2-Th-EDOT, especially concerning of its intramolecular I/I, S/S distances and corresponding angles, as shown in Fig. 5 and Table 2. We attribute this result to the fact that the fused ring of fluorene and its alkyl chains are far away from EDOT units, which has little steric effect on the ring of fluorene and its alkyl chains are far away from EDOT units, which has little steric effect on the single molecule framework.
As shown in Fig. 6, monomer of I2-Th-EDOT has the first and the second closest Hal/Hal distance of 0.4397 and 0.4425 nm respectively (Table 3) with corresponding C13-C1 and C13-C1 contact distances of 0.5424 and 0.5244 nm respectively. And the third closest Hal/Hal distance of 0.6191 nm is observed with the shortest C-C (C13-C13) contact distance of 0.4910 nm. Thus, the first polymerization pathway along with a-axil direction (formation of a helix between two columns of monomers, shown in Fig. 6(c)) involves sole I/I distance of 0.4397 nm while the second polymerization pathway along with b-axil direction (shown in Fig. 6(d)) involves sole I/I distance of 0.4425 nm. Because these two pathways have similar key I/I distance and their corresponding C/C distances are 0.5424 nm and 0.5244 nm respectively. At this moment, we cannot determine which the preferred way is. For comparison, the shorter distance of 0.4397 nm is selected as the effective I/I distance as shown in Table 1.
While in the case of I2-Flu-EDOT (Fig. 7(a)), the important Hal/Hal and related C-C contact distances are marked in the Fig. 7(a, b). It is obvious that the first polymerization pathway along with a-axil direction (Fig. 7(c)) involves sole halogen distance of 0.5391 nm with the formation of a helix between sole columns of monomers. Though helix structures are observed by us19and other SSP system of quinodimethanes42, such polymerization pathway involving one column monomers is not very common among reported thiophene monomers. In this case, it may help to facilitate their helix formation. In addition, the second polymerization pathway is observed along with the same axil direction as shown in Fig. 7(d) and it involves two I/I distance of 0.5650 nm and 0.8424 nm. Thus, the first pathway is the preferred one with the effective halogen distance of 0.5391 nm, which is quite long19 among reported thiophene derivatives. It appears that the bulkier substitution can increase its effective halogen distances and also the C-C contact distance.
It should be pointed out that taking SSP thermal requirement into consideration, such heat energy drives molecule reaction groups close together and construct corresponding C-C bond. Meanwhile, it drives molecule conformation, i.e. to reach repeat units equilibrium state. Therefore, we mainly discuss the effective halogen distance and ignore their inter-ring torsion angles between neighboring repeat units.
Though we failed to obtain I2-BuTh-EDOT crystal information, it shows lower Tonset when compared with that of non-alkyl substituted I2-Th-EDOT. Moreover, as its effective halogen distance is up to 0.5391 nm, I2-Flu-EDOT shows quite low Tonset of 64 ℃. Furthermore, our previous report20 showed that I2-Pr-EDOT has a little bit lower Tonset of 70 ℃ with its effective halogen distance of 0.4878 nm. Therefore, it appears that long alkyl chain would decrease Tonset drastically. According to these interesting results, our previously proposed equation19 of linear dependence of effective I/I distance with Tonset needs to be modified or refined. As for the fitted dotted line from those alkyl-chain samples, it gives an equation of y = 19.4x − 32.8 as shown in Fig. 8 (data derived from those samples is presented in Table S2). Due to the lacking of enough crystals data at present, moderate R of 0.75 is obtained. Therefore, we can primitively deduce that the increasing 0.1 nm of effective I/I distance needs the elevated Tonset of 19 ℃. It is lower than that from those non-alkyl chain samples19 (0.1 nm for elevated Tonset of 26 ℃). We noticed that I2-Carb(C16)-EDOT with hexadecyl chain shows a little bit higher Tonsetthan that of I2-Carb-EDOT with butyl chain, which might due to the larger effective halogen distance of the former.
In addition, alkyl chain would lower Tonsetaround 30 ℃, revealing that non-harsh SSP condition is needed for monomers with alkyl chains. We think that this interesting phenomenon is from the possibility that alkyl chain will weaken molecule interaction especially those π-π stacking in crystal, which results in the decreasing of Tonset. Furthermore, though it is generally accepted that the organic solid reaction usually occurs within 0.42 nm26 between reaction groups distances, we have got several samples with quite long effective halogen/halogen distances over 0.5 nm. These exciting observations and their SSP success encourage us and reveal that SSP would be a very powerful means in the synthesis of conjugated polythiophene.
Due to the poor solubility of other polymers, P(Flu-EDOT) and P(Carb-C16-EDOT) are selected for the detection of molecular weight and the results are listed in Table 4. Oligomers are obtained at low temperature under SSP. Previous studies18, 19 showed that SSP produces low molecular weight distribution because of the difficulty of the contact and reaction between their terminal groups of iodine atoms in oligomers during SSP procedure. However, large molecular weight of 19 K can be obtained for P(Carb-C16-EDOT) at 150 ℃ for three days under melt-state polymerization. It is noticed that high molecular weight of polythiophene derivatives are frequently obtained by melt state polymerization12, 44. Therefore, low to high molecular weight can be well controlled through melt state polymerization, which is very important for their future application in other optical-electron fields.
In this work, typical electro-optical moieties of thiophene, carbazole and fluorene moieties are introduced to EDOT-CH(R)-EDOT platform and successfully employed in SSP. The results indicate that incorporation of desire moieties is a general means in fine tuning their corresponding polymers properties and paving their future application in photo-electronic devices. In addition, the introduction of alkyl chains would weaken intermolecular interaction and result in the drastically decrease of Tonset. Furthermore, high molecular weight can be obtained through melt state polymerization.
Supporting Information: available free of charge via the internet at http://www.whxb.pku.edu.cn.