Electrochemical Polymerization of o-Dihydroxybene and

Poly(3,4-ethylenedioxythiophene) (PEDOT, Scheme 1) has attracted much attention among conducting polymers (CPs) because of its high conductivity, good...
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J. Phys. Chem. C 2007, 111, 6889-6896

6889

Electrochemical Polymerization of o-Dihydroxybene and Characterization of Its Polymers as Polyacetylene Derivatives Mulin Ma,†,‡ Houting Liu,† Jingkun Xu,*,† Yuzhen Li,† and Yiqun Wan‡ Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal UniVersity, Nanchang 330013, China, and Department of Chemistry, Nanchang UniVersity, Nanchang 330047, China ReceiVed: January 27, 2007; In Final Form: March 10, 2007

A novel conducting polymer poly(o-dihydroxybenzene) (PoDHB) film was synthesized electrochemically by the direct anodic oxidation of o-dihydroxybenzene (oDHB) in boron trifuoride diethyl etherate. The oxidation potential onset of PoDHB in this medium was measured to be only 0.66 V versus a saturated calomel electrode (SCE), which was much lower than that determined in acetonitrile containing 0.1 mol L-1 tetrabutylammonium tetrafluoroborate (1.1 V vs SCE). PoDHB films obtained from this medium showed good electrochemical behavior and good thermal stability. IR, NMR, and theoretical studies showed that the polymerization of oDHB occurred at the 4,5-position, which made PoDHB a derivative of polyacetylene. The PoDHB film was thoroughly soluble in the strong polar organic solvents dimethyl sulfoxide, ammonia, and tetrahydrofuran and partly soluble in pure ethanol. Fluorescent spectral studies indicated that PoDHB was a good blue-light emitter, with an excitation wavelength maximum of 350 nm. The thermoelectric properties of as-formed PoDHB were also determined. In comparison with oDHB, the electrochemical polymerization of its isomers m-dihydroxybenzene, p-dihydroxybenzene, and hydroxybenzene failed under the same conditions. To the best of our knowledge, this is the first paper reporting that high-quality conducting polymer films of PoDHB can be electrodeposited.

1. Introduction Poly(3,4-ethylenedioxythiophene) (PEDOT, Scheme 1) has attracted much attention among conducting polymers (CPs) because of its high conductivity, good processing ability, good mechanical property, and nice environmental stability.1 The 3,4alkylenedioxy substitution at the β-position on the thiophene ring declines the coupling defect through the -R-β-positions, producing regiosymmetric high-quality polymers. Therefore, PEDOT has been widely applied in various areas, such as smart windows, potentiometric sensors, ion selective sensors, solar cells, photoelectrochemical cells, anticorrosion coating, antistatic coating, supercapacitors, and electrode materials.2 The leading problem that limits the further application of PEDOT is the cost of synthesizing 3,4-ethylenedioxythiophene (EDOT). Therefore, the use of other alkylenedioxy-substituted monomers is of great interest. 1,2-Dimethyoxybenzene (DMB) and 1,2-methylenedioxybenzene (MDOB) are cheaper monomers.3 MDOB can be obtained at a much lower price. Furthermore, the properties of the poly(1,2-methylenedioxybenzene) (PMDOB) film are fairly good. The PMDOB film has good electric conductivity, nice thermal stability, and good electrochromic properties whose color transforms from opaque sap green (doped) to transparent baby blue (dedoped). Unlike PEDOT and poly(p-phenylene) (PPP),4 which generally is para-polymerized, PMDOB has main backbones similar to polyacetylene, one of the simplest CPs.5 There are also other reports that the anodic oxidation of MDOB can lead to the formation of a trimer,3 whose main backbone was also similar to polyacetylene. * Corresponding author. Tel.: 86-791-3805183. Fax: 86-791-3826894. E-mail: [email protected]. † Jiangxi Science and Technology Normal University. ‡ Nanchang University.

SCHEME 1: Chemical Structures of EDOT, MDOB, oDHB, and Electropolymerization of oDHB

What about the polymers prepared from o-dihydroxybenzene (oDHB) (Scheme 1), whose structure is similar to MDOB without C7? Are its polymers like PMDOB or like PPP? Two electron-donating groups of -OH on benzene may contribute to the polymerization of oDHB. The literature reports that oDHB can be polymerized on a silver surface in the presence of a nonaggregated silver colloid6 or in the presence of an enzyme.7 However, the electrochemical polymerization of oDHB has not been studied until now. The electrochemical polymerization of oDHB was performed in a neutral organic solvent (acetonitrile) using TBATFB as the electrolyte. However, it cannot be electrochemically polymerized in this medium. The literature reports that the use of a middle strong Lewis acid (BFEE) can greatly lower the oxidation potential onset of the aromatic monomer owing to interactions between BFEE and aromatic ring, making the electrodeposition

10.1021/jp070710s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

6890 J. Phys. Chem. C, Vol. 111, No. 18, 2007 of high-quality CPs films possible. In BFEE and its mixed electrolytes, many aromatic monomers, such as thiophene and its derivatives8 pyrrole,9 furan,10 and benzene,11 together with fused ring aromatic monomers such as indole and its derivatives12 (benzofuran,13 fluoranthene,14 carbazole,15 etc.) can be electrochemically polymerized, and a corresponding high-quality CP film had been electrodeposited. Therefore, BFEE may be the suitable media for the electrochemical polymerization of oDHB. In this paper, poly(o-dihydroxybenzene) (PoDHB) can be easily electrodeposited on a stainless steel electrode by direct anodic oxidation of the oDHB monomer in pure BFEE. The structure of PoDHB was determined by IR, 1H NMR, 13C NMR, and quantum chemistry calculations as one of the derivatives of polyacetylene. 2. Experimental Procedures 2.1. Materials. BFEE (Beijing Changyang Chemical Plant) was distilled and stored at -20 °C before use. oDHB (analytical grade; Tianjin Bodi Chemicals Co., Ltd.), m-dihydroxybenzene (mDHB) (analytical grade; Beijing East Longshun Chemical Plant), p-dihydroxybenzene (pDHB) (analytical grade; Tianjin Bodi Chemicals Co., Ltd.), hydroxybenzene (HB) (analytical grade; Tianjin Bodi Chemicals Co., Ltd.), and commercial highperformance liquid chromatography grade acetonitrile (ACN; Beijing East Longshun Chemical Plant) were used as received without further purification. Bu4NBF4 (TBATFB) (95%, Acros Organics) was dried in vacuum at 60 °C for 24 h before use. Deuterium-substituted dimethyl sulfoxide (CD3SOCD3) was made by the Beijing Chemical Plant. Dimethyl sulfoxide (DMSO; analytical grade) was a product of Tianjin Bodi Chemicals Co., Ltd. 2.2. Electrosyntheses of PoDHB Films. Electrochemical syntheses and examinations were performed in a one-compartment cell with the use of a Model 263 potentiostat/galvanostat (EG&G Princeton Applied Research) under computer control. For electrochemical examinations, the working and counter electrodes were Pt wire with a diameter of 0.5 mm and a stainless steel wire with the same diameter, respectively. They were placed 5 mm apart during the experiment. To obtain a sufficient amount of the polymer for characterization, one stainless steel sheet with surface areas of 12 cm2 was employed as the counter electrode. Instead, another stainless steel sheet with surface areas of 10 cm2 was also employed as the working electrode, in comparison with the Pt electrode. Generally, ironporphyrins are prone to undergo oxidation giving highly reactive iron(IV)-porphyrin cation radical species, which can catalyze the oxidation of phenol substrates into free radicals with both C-C and C-O-C coupling mechanisms of the dimer.16 According to the literature,17 the use of Pt or a stainless steel sheet has little impact on the electrochemical polymerization. The electrodes mentioned previously were carefully polished with abrasive paper (1500 mesh) and cleaned with water and acetone successively before each examination. All potentials were referred to a saturated calomel electrode (SCE). The typical electrolytic solution was BFEE and 0.2 mol L-1 oDHB or another monomer. All solutions were deaerated by a dry argon stream and maintained at a slight argon overpressure during the experiments. The amount of the polymer deposited on the electrode was controlled by the integrated current passed through the cell. To remove the electrolyte, oligomers, and monomer, the electropolymerized films were rinsed with acetone and pure water. The PoDHB film was in a doped state and was a metallic dark color. Finally, it was vacuum-dried at 60 °C for 24 h.

Ma et al. SCHEME 2: Complexion of oDHB and BFEE

2.3. Characterization. The electrical conductivity of PoDHB was measured by a conventional four-probe technique. Ultraviolet-visible (UV-vis) spectra were taken with a PerkinElmer Lamda 900 -UV-vis near-infrared spectrometer. Infrared spectra were recorded on a Bruker Vertex 70 FT-IR with KBr pellets. The fluorescence spectrum was determined with an F-4500 fluorescence spectrophotometer (Hitachi). The 1H NMR and 13H NMR spectra were recorded on a Bruker AV 400 NMR spectrometer, and CD3SOCD3 was used as the solvent. The thermogravimetric analysis (TGA) was performed with a Pyris Diamond TG/DTA (PerkinElmer) thermal analyzer. Scanning electron microscopy (SEM) measurements were taken with a JEOL JSM-6360 LA analytical scanning electron microscope. A Soartron 7070 computing voltmeter and Keithley 220 programmable current source were used to record the thermoelectric properties. The fluorescence quantum yields (φoverall) of PoDHB in solution were measured to be 0.065 using anthracene in ACN (standard, φref ) 0.27)18 as a reference and were calculated according to the well-known method given as (eq 1)

φoverall )

n2ArefI nref2AIref

φref

(1)

Here, n, A, and I denote the refractive index of the solvent, the absorbance at the excitation wavelength, and the intensity of the emission spectrum, respectively. Absorbance of the samples and the standard should be similar.19 3. Results and Discussion 3.1. Ionic Conductivity of the Electrolyte. Literature reported that alcohol can interact with BFEE, which led to a higher ionic conductivity of the mixed electrolyte than that of BFEE (Scheme 2) 20,21 oDHB contains two -OH groups. Therefore, it should interact with BFEE in the electrolyte. Figure 1 shows ionic conductivity of oDHB dissolved in BFEE. The ionic conductivity of pure BFEE is relatively low, about 400 µS/cm. With the introduction of oDHB, the ionic conductivity of the electrolyte increased gradually, to its maximum of about 3100 µS/cm, with an increase of the oDHB concentration. The main reason for this phenomenon can be ascribed to the complexation of the monomer and BFEE and the ionic conductive species of HOPhOBF3- that formed (Scheme 2). 3.2. Electrochemical Syntheses of PoDHB Films. Figure 2 shows the anodic polarization curves of oDHB (a), mDHB (b), pDHB (c), and HB (d) in pure BFEE and in ACN containing 0.1 mol L-1 TBATFB. In pure BFEE (Figure 2A), the oxidation onset of oDHB was initiated at 0.66 V versus SCE, lower than that of mDHB (1.14 V), pDHB (0.90 V), and HB (1.21 V). All these values were much lower than those of oDHB (1.10 V), mDHB (1.31 V), pDHB (0.99 V), and HB (1.42 V) in ACN containing 0.1 mol L-1 TBATFB, respectively (Figure 2B). This

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Figure 3. Cyclic voltammograms of 0.2 mol L-1 oDHB in pure BFEE (potential scanning rates: 50 mV/s). Figure 1. Effect of oDHB concentration on the ionic conductivity (σ) of BFEE.

Figure 4. Cyclic voltammograms of PoDHB in pure BFEE at potential scanning rates of 50, 100, 150, 200, and 250 mV/s. The PoDHB film was synthesized electrochemically in pure BFEE at a constant applied potential of 1.13 V vs SCE. j, jp, jp.a, and jp.c are defined as the current densities, peak current densities, anodic peak current densities, and cathodic peak current densities, respectively.

Figure 2. Anodic polarization curves of oDHB (a), mDHB (b), pDHB (c), and HB (d) in pure BFEE (A) and ACN + 0.1 mol L-1 TBATFB (B) (potential scanning rate: 20 mV/s). j: current density.

implied that the oxidation of oDHB and its isomers in pure BFEE was much easier than that in ACN + TBATFB due to interactions between BFEE and aromatic ring. After these experiments, a dark polymer film could be found on the electrode surface in BFEE when oDHB was the monomer. However, no polymer films were electrodeposited when mDHB, pDHB, or HB was used as the monomer. This may result from a large steric effect if mDHB and pDHB were polymerized and HB was polymerized at the o-position. Therefore, the following experiments mainly concentrated on the electrochemical polymerization of oDHB and the characterizations of its polymers.

It should be noted here that BEFF was electrochemically silent in the whole potential range.22 The successive cyclic voltammograms (CVs) of 0.2 mol L-1 oDHB in pure BFEE (Figure 3) showed characteristic features of other conducting polymers, such as polythiophene8 and polypyrrole,9 during potentiodynamic syntheses. As the CV scan continued, a polymer film was also formed on the working electrode surface. In BFEE, PoDHB was reduced and oxidized between -0.1 and 0.8 V versus SCE (Figure 3). The increase in the redox wave currents implied that the amount of the polymer on the electrode was increasing. The broad redox waves of the PoDHB film may be ascribable to the wide distribution of the polymer chain length23 or the conversion of the conductive species on the polymer main chain from the neutral state to polarons, from polarons to bipolarons, and finally from bipolarons to the metallic state.24 The potential shift of the wave current maximum provides information about the increase in the electrical resistance in the polymer film and the overpotential needed to overcome the resistance.25 All these phenomena indicated that a high-quality conducting PoDHB film was formed on the working electrode. 3.3. Electrochemistry of PoDHB Films. The electrochemical behavior of the PoDHB deposited electrochemically from pure BFEE containing 0.2 mol L-1 monomer (Figure 4) was studied in monomer-free BFEE. Similar to results in the literature,26 the steady-state cyclic voltammograms represented broad anodic and cathodic peaks. The polymer film can be oxidized and reduced from 1.07 V (anodic peak potential, Ea) to 0.34 V (cathodic peak potential, Ec). The peak current densities were both proportional to the scanning rates (Figure 4, inset),

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Figure 5. UV-vis spectra of oDHB in DMSO (A), doped PoDHB in DMSO (B), and doped PoDHB on ITO electrode (C). The polymer film was prepared in pure BFEE potentiostatically at 1.13 V vs SCE.

indicating the reversible redox behavior of the polymer. The polymer film can be cycled repeatedly between the conducting (oxidized) and the insulating (neutral) states without significant decomposition of the materials in pure BFEE, implying good stability of the polymer. 3.4. Structural Characterizations. During the potentiostatic process, the color of BFEE containing oDHB progressively darkened with the applied potential. This indicated that there were a few soluble oDHB oligomers formed during anodic oxidation. With the propagation of polymerization, a few of the soluble oligomers became insoluble and were deposited on the working electrode with elongation of the polymer main chain. However, there was still some oDHB oligomers diffused away from the electrode, making the color of the solvent darker. PoDHB was opaque blue in the doped state (Figure S1, see Supporting Information). When dedoped, its color changed to light green (Figure S1). Dedoped PoDHB was thoroughly soluble in a strong polar organic solvent such as DMSO, tetrahydrofuran, and ammonia and partly soluble in ethanol. It can also be dissolved in aqueous ammonia. The UV-vis spectra of oDHB and its polymer are shown in Figure 5. The oDHB monomer showed a characteristic absorption at 282 nm (Figure 5A). On the contrary, the spectra of the dedoped PoDHB films showed not only the absorption at 282 nm but also a broad absorption from 290 to 360 nm (Figure 5B). This implies a higher conjugation length of the PoDHB films. On the other hand, the doped PoDHB film showed another strong absorption on the ITO electrode, from 520 to 920 nm with a peak at 711 nm (Figure 5C). This wide peak can be assigned to the absorption of the conductive species such as polarons and bipolarons on the main backbone of PoDHB in the doped state. The fluorescence spectra of doped PoDHB prepared in BFEE were examined using DMSO as the solvent through the wavelength scans of emission (Figure 6). The emission spectrum maximum was mainly at 385 nm with two shoulders of 374 and 406 nm, when excited at 350 nm. The fluorescence quantum yield was determined to be 0.065. However, the monomer was nonfluorescent. Upon irradiation with 365 nm UV light, the colorless DMSO solution of PoDHB turned to blue immediately (Figure S2, see Supporting Information). This means that the polymer was a good blue-light emitting material. Figure 7 shows the infrared spectra of the oDHB monomer (A) and PoDHB (B) in the doped state. From this figure, great changes can be found between the monomer and the polymer. The peak at 3450 cm-1 in the monomer (Figure 7A) can be

Ma et al.

Figure 6. Emission spectra of PoDHB in DMSO when excited at 350 nm.

Figure 7. FTIR spectra of the oDHB monomer (A) and doped PoDHB films (B) obtained potentiostatically at 1.13 V vs SCE from pure BFEE.

assigned to the stretching of O-H.27 The peaks at 3327 cm-1 in the monomer and 3280 cm-1 in the polymer are the stretching of O-H of the hydrogen bonding association between molecules. The peaks of oDHB, centered at 740 cm-1 and a multiplet from 2000 to 1650 cm-1, were assigned to the 1,2disubstituted benzene ring (Figure 7A). However, these peaks changed dramatically in the spectrum of the polymer (Figure 7B). The peaks at 1722 and 856 cm-1 indicated a 1,2,4,5substituted benzene ring. This implies that the electropolymerization of oDHB might occur at the C4- and C5-position and that the structure of PoDHB may be similar to polyacetylene. The peaks from 1622 to 1470 cm-1 in the monomer and the peaks from 1620 to 1375 cm-1 showed the vibrations of the backbone of benzene. The peaks from 1280 to 1186 cm-1 in the monomer and the peaks from 1238 to 1155 cm-1 showed the vibrations of C-O. The existence of the broad absorption at ca. 3280 cm-1 indicated that O-H still exists after polymerization, indicating that -OH is not the polymerizing site. To further explore the structure of PoDHB and its polymerization mechanism, the atomic electron density population and reactivity of the oDHB (Scheme 1) monomer were calculated at the B3LYP/6-31G(d,p) level using Gaussian 03 software.28 The results of the main atomic electron density populations showed negative electric charges on C(3), C(4), C(5), and C(6) (Table 1), which implies that these atoms will donate electrons during electrochemical polymerization through radical cation intermediates. According to the molecular orbital theory, the reaction between the active molecules mainly happens on the frontier molecular orbital and the near orbital. For oDHB, the

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TABLE 1: Main Atomic Electron Density Populations for oDHB atom

electric charge

atom

electric charge

C(1) C(3) C(5) O(7)

0.287275 -0.110106 -0.098197 -0.594693

C(2) C(4) C(6) O(8)

0.313491 -0.094294 -0.128623 -0.567828

TABLE 2: Main Composition and Proportion of Frontier Orbitals in oDHB (%)a atom

HOMO - 1

HOMO

LUMO

LUMO + 1

C(1) C(2) C(3) C(4) C(5) C(6) O(7) O(8)

5.7955 1.7855 29.0230 12.6671 4.0304 30.9833 9.8819 5.7897

16.6694 16.5660 3.5269 10.1501 16.6133 0.3256 14.4415 21.6686

11.6552 8.6088 29.8677 7.1926 7.4356 31.3548 2.1167 1.7255

17.6348 20.0905 0.3801 26.9309 27.1997 0.5600 3.0248 4.1471

Figure 9. 1H NMR spectra of oDHB (A) and PoDHB (B). Solvent: d6-DMSO.

a HOMO, HOMO - 1, LUMO, and LUMO + 1 are defined as highest occupied molecular orbital, next highest occupied molecular orbital, lowest unoccupied molecular orbital, and next lowest unoccupied molecular orbital, respectively.

Figure 10. d6-DMSO.

13

C NMR spectra of oDHB (A) and PoDHB (B). Solvent:

Figure 8. Optimized structures for (oDHB)10 oligomers polymerized at 4.5-position.

proportions of atoms C(1), C(2), C(4), and C(5) in the HOMO were higher than other atoms (Table 2). Therefore, these theoretical results implied that the polymerization between the monomer would happen preferentially on C(4) and C(5). For another way to research the polymerizing mechanism of PoDHB, the optimized structures by the HF/6-31G(d) level of (oDHB)10 are plotted in Figure 8. In Figure 8, the structure of 4,5-polymerization (oDHB)10 was screwed and stable, which is similar to the structure of DNA. To obtain deep insight into the polymer structure and the polymerization mechanism of oDHB, the 1H NMR spectrum of dedoped PoDHB obtained from BFEE was recorded in d6-DMSO, as is illustrated in Figure 9B. For comparison, the 1H NMR spectrum of the monomer was also included, as shown in Figure 9A. Two proton groups were shown in the 1H NMR spectrum of oDHB (Figure 9): 6.61-6.66 and 6.75-6.77 ppm, which can be assigned to the protons at the b- and c-positions, respectively. Because of the spin-spin splitting between protons at the b- and c-positions, they both showed multiple peaks. In the spectrum of PoDHB, there was only one singlet at 7.65 ppm. There may be three polymerization mechanisms for the electrochemical polymerization of oDHB, through the b-b-, b-c-, or c-c-positions. If the polymerization happened through the b-b- or b-c-positions, the proton lines in Figure 9B would be a triplet. Therefore, this singlet proton line at 7.65 ppm can be assigned to the proton at the b-position. This implies that the

Figure 11. TGA curves of doped PoDHB films obtained potentiostatically at 1.13 V vs SCE from pure BFEE.

polymerization site was the c-position (C4 and C5) and that the main backbone of PoDHB was similar to polyacetylene. In addition, the chemical shift at 7.65 ppm moving to the lower field indicated a higher conjugation length in comparison with those of the monomer.29 On the other hand, three group peaks can be found in the 13C NMR spectra of the monomer oDHB: 115.64, 119.44, and 144.98 ppm, which are assigned as peaks b-d (Figure 10 A), respectively. Although the peak intensity became weak after polymerization, three group peaks at 107.81, 121.94, and 145.02 ppm can also be seen in Figure 10B. This was mainly ascribed to the strength of Cc becoming weak following its transition from a tertiary carbon to a quaternary carbon during the polymerization , in good agreement with the results of 1H NMR. On the basis of these results of IR, 1H NMR, and 13C NMR and the theoretical calculations described previously, a conclu-

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Figure 12. SEM micrographs of doped (A) and dedoped (B) PoDHB films deposited on an ITO glass electrode.

sion can be reasonably drawn that PoDHB is one of the derivatives of polyacetylene, not PPP. 3.5. Thermal Analysis. The thermal analysis of PoDHB was tested under a nitrogen stream from 302.4 to 1073.2 K with a heating rate of 10 K/min (Figure 11). There are two steps in weight loss (Figure 11a). The first one is from 302.4 to 446.1 K, up to 8.4%, which can be ascribed to water evaporation or other moisture trapped in the polymer. The second one occurs from 446.1 to 1073.2 K, up to 39.5%, which results from the degradation of the PoDHB main backbone. From the DTG curve (Figure 11b), the fastest weight loss rate of the PoDHB films occurred at 663 K. All these results indicate that PoDHB has a good thermal stability. 3.6. Morphology. The SEM images of the PoDHB film prepared in BFEE are shown in Figure 12. Macroscopically, the doped PoDHB film resembled ordered arrangements of the granules (Figure 12A). The growth of the nuclei was in the form of clusters. This morphology facilitated the movement of doping anions into and out of the polymer film during doping and dedoping, in good agreement with the high redox activity of the PoDHB films. The dedoped PoDHB film resembled arrayed leaves (Figure 12B). This is because of the migration of counteranions out of the polymer surface, which broke the smooth surface of the granules. This confirms the properties of PoDHB prepared in pure BFEE. 3.7. Thermoelectric Properties of the PoDHB Films. The progress of searching high-quality thermoelectric material has never stopped. The thermoelectric figure of merit ZT can be used to scale these thermoelectric properties (eq 2).30 2

σS ZT ) T κ

(2)

Here, σ, S, κ, and T denote the electrical conductivity, Seeback coefficient, thermal conductivity, and temperature, respectively. Thermoelectric materials can be used in thermoelectric refrigeration and power generation with less environmental pollution. They can transform heat energy into electric energy without media and pollutants, which meets wide applications. Until now, the best thermoelectric materials were alloys of stibium, bismuth, and telluride,31 which have a ZT value of about 1. Almost all thermoelectric materials have been metal or metal oxides in the past 100 years, and there was little concentration on their polymers.32 Therefore, the thermoelectric properties of PoDHB as a conducting polymer were tested.

Figure 13. Effect of temperature on the electrical conductivity of doped PoDHB.

The electrical conductivity of the PoDHB films was measured to be 9.16 × 10-4 S/cm at 292.4 K by the four-probe method after storage for about 1 month. Figure 13 shows the relationship of electrical conductivity of the PoDHB films and temperature. As a conducting polymer, their relationship follows a quasi onedimension variable range hopping model (1-D-VRH),33 which is described as (eq 3)

[()]

σDC(T) ) σ0 exp -

T0 T

1/2

ln σDC (T) ) -(T0/T)-1/2 + ln σ0

(3) (4)

Eq 434 is obtained by eq 3, where σDC(T) is the electrical conductivity, T is temperature, and σ0 and T0 are characteristic constants. The electrical conductivity of the PoDHB films was affected greatly by the environment temperature (Figure 13). The electrical conductivity plot of the polymer films indicates three linear regions (regions 1-3). In region 1, from room temperature to 214 K, the electrical conductivity decreased abruptly with a decrease in the temperature, which showed the property of a semiconductor. However, there was a great change at 214 K. With the temperature lowering continually, the electrical conductivity increased rapidly in region 2 (from 214 to 160 K) and increased slowly after 160 K (region 3). This implies that the polymer films behaved as a conductor. All these results

PoDHB Polymerization/Characterization of Its Polymers described previously indicate that PoDHB changes from a semiconductor to a conductor with a temperature decrease. At the same time, the Seeback coefficient and thermal conductivity were determined to be 80 µV K-1 and 7.165 × 10-4 W cm-1 K-1 at 292.4 K, respectively. Therefore, at 292.4 K, the ZT value of the polymer can be calculated to be 2.39 × 10-6. Although this value is still very low in comparison with other thermoelectric materials, it is still encouraging because the electrical conductivity of the conducting polymers can be easily attained to 102 S/cm. If oDHB is copolymerized with a monomer whose polymer has a very high electrical conductivity, a copolymer film can be expected with a high ZT value. 4. Conclusion In summary, a novel conducting polymer PoDHB, whose main backbone was similar to polyacetylene, was easily deposited electrochemically by direct anodic oxidation of oDHB in pure BFEE. However, by comparison with oDHB, mDHB, pDHB, and HB did not electrochemically polymerize in the same systems. UV-vis, FT-IR, 1H NMR, 13C NMR, and quantum chemistry calculations determined the structure of PoDHB as a polyacetylene derivative. Furthermore, PoDHB has a nice thermal stability and good fluorescence properties. Finally, the thermoelectric properties of PoDHB were determined. Acknowledgment. NSFC (50663001 and 20564001), Funds of Jiangxi Provincial Department of Education ([2006]243), Funds of Ministry of Education, China (2007-207058), and Nanchang Testing Fund (2005013) are acknowledged for their financial support. In addition, the authors thank Dr. Laifeng Li, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, for determination of thermoelectric properties of the polymer sample. Supporting Information Available: Figure S1. UV-vis spectra of oDHB in DMSO (A), doped PoDHB in DMSO (B), and doped PoDHB on ITO electrode (C) prepared in pure BFEE. Opaque blue and light green are the colors of polymer in the doped and dedoped states, respectively. Figure S2. Photographs of PoDHB dissolved in DMSO before (left picture) and after (right picture) UV irradiation (365 nm). Among the two cells of each picture, the left is blank solvent DMSO, and the right is doped polymer in DMSO. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Jonas, F.; Schrader, L. Synth. Met. 1991, 831, 41. (b) Heywang, G.; Jonas, F. AdV. Mater. 1992, 4, 116. (c) Winter, I.; Reece, C.; Hormes, J.; Heywang, G.; Jonas, F. Chem. Phys. 1995, 194, 207. (d) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994, 369, 87. (e) Cutler, C. A.; Bouguettaya, M.; Kang, T. S.; Reynolds, J. R. Macromolecules 2005, 38, 3068. (f) Welsh, D. M.; Kloeppner, L. J.; Madrigal, L.; Pinto, M. R.; Thompson, B. C.; Schanze, K. S.; Abboud, K. A.; Powell, D.; Reynolds, J. R. Macromolecules 2002, 35, 6517. (g) Sotzing, G. A.; Thomas, C. A.; Reynolds, J. R. Macromolecules 1998, 31, 3750. (h) Wang, F.; Wilson, M. S. R.; Rauh, D. Macromolecules 2000, 33, 2083. (i) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Ingana¨s, O.; Johansson, T.; Feidenhans, R. Synth. Met. 1999, 101, 561. (j) Kiebooms, R.; Aleshin, A.; Hutchison, K.; Wudl, F. J. Phys. Chem. B 1997, 101, 11037. (2) (a) Sindhu, S.; Rao, K. N.; Ahuja, S.; Kumar, A.; Gopal, E. S. R. Mater. Sci. Eng. B 2006, 132, 39. (b) Mousavi, Z.; Bobacka, J.; Lewenstam, A.; Ivaska, A. J. Electroanal. Chem. 2006, 593, 219. (c) Va´zquez, M.; Bobacka, J.; Luostarinen, M.; Rissanen, K.; Lewenstam, A.; Ivaska, A. J. Solid State Electrochem. 2005, 9, 312. (d) Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Synth. Met. 2002, 131, 185. (e) Girotto, E. M.; Gazotti, W. A.; De Paoli, M. A. J. Phys. Chem. B 2000, 104, 6124. (f) Bendikov, T. A.; Harmon, T. C. Anal. Chim. Acta 2005, 551, 30. (g) Ocampo, C.; Oliver, R.; Armelin, E.; Alema´n, C.; Estrany, F. J. Polym. Res. 2006, 13, 193. (h) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds,

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