Electrochemical Polymerization and Properties of Poly (triphenylene

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Electrochemical Polymerization and Properties of Poly(triphenylene), an Excellent Blue-Green-Light Emitter Qingli Hao,*,‡ Xingen Xie,†,‡ Wu Lei,† Mingzhu Xia,† Fengyun Wang,*,† and Xin Wang‡ Key Laboratory of Soft Chemistry and Functional Materials, Nanjing UniVersity of Science and Technology, Ministry of Education, Nanjing 210094, China, and Institute of Industrial Chemistry, Nanjing UniVersity of Science and Technology, Nanjing 210094, China ReceiVed: January 7, 2010; ReVised Manuscript ReceiVed: April 21, 2010

Semiconducting polytriphenylene (PTP), a bright blue-green-light emitter, with good electrochemical behavior, excellent thermal stability, and the electrical conductivity of 10-3 S cm-1 was synthesized successfully by direct anodic oxidation of triphenylene (TP) in different media. The oxidation potential onset of TP in boron trifluoride diethyl etherate containing 3% concentrated sulfuric acid (SA) was reduced to 0.94 V, compared with other electrolyte media. The possible structure of the obtained PTP was investigated by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, and 1D and 2D NMR techniques, respectively. The results showed that the PTP was grown via the coupling of the monomer mainly at the meta position of the benzene ring in TP. Fluorescence spectra indicated that the electrochemically polymerized PTP was an excellent blue-green-light-emitting material with fluorescence quantum yield as high as 0.43. In addition, the morphology and thermal stability of the polymers were also explored by means of scanning electron microscopy and thermal analyzer. The average molecular weight (19 980 g mol-1) and its polydispersity of dedoped PTP were determined by gel permeation chromatography. 1. Introduction As “ synthetic metals ”, conducting polymers (CPs) have been fascinating considerable attention since they were discovered approximately thirty years ago.1 Because of their good conductivity, excellent optical properties, and other attractive properties, CPs hold a special and important position in the field of material science, and have been extensively studied in many applications such as rechargeable batteries,2 light-emitting diodes (LEDs),3,4 organic transistors,5 sensors,6 electrochromic displays, and electroluminescent applications.7-9 Moreover, more and more new derivatives of the common CPs, novel CPs, or their copolymers with good fluorescence property, high electrical conductivity, satisfied thermal stability, or other properties have been developed, such as conjugated substituted polyacetylenes or polycarbazole,10,11 oligoquinolines,12 and poly(9-fluorenecarboxylic acid).13 In the past quarter century, electrochemical polymerization of aromatic heterocyclic compounds has been proven to be an especially useful method for the preparation of CP films, and the studies of high-quality CPs are mainly concentrated on simple heteroaromatic compounds such as polyaniline,14,15 pyrrole,16 and thiophene and its derivatives.17-20 The electrolytes used for electrochemical polymerization may be aqueous14-17 or organic solutions.19-22 Nowadays, the pursuit of high-quality polymer films by using fused aromatic compounds is still one of the main goals in the research and development of inherently conducting polymers (CPs). Also, as an excellent solvent and electrolyte,22 BFEE has been proved to be the appropriate electrolyte to polymerize the fused aromatic compounds, such as phenanthrene, pyrene,

fluoranthene, benzanthrone, and their derivatives.23-27 Moreover, the addition of a certain amount of strong acid to BFEE, such as concentrated sulfuric acid (SA) or trifluoroacetic acid (TFA), can further decrease the oxidation potentials of the monomers, and the quality of corresponding polymer films can also be greatly improved.20,27,28 Xu’s group has done lots of valuable research work. Their results show most of these new CPs exhibit excellent optical properties, and most of them are blue-lightemitting materials.23-25,28-30 For instance, polyphenanthrene and its derivatives can be applied in light-emitting diodes (LEDs).23,29 A triphenylene (TP) molecule is constructed of a benzene group and a phenanthrene group. Also, TP and its derivatives have attracted much research interest for their good optical properties. The TP-based chromophores are chosen mainly due to the following reasons:31-34 (1) TP-based π-conjugated systems are known to exhibit longer excited state lifetimes than their phenyl analogues; (2) TP rings have numerous sites available for substitution; and (3) TP derivatives are one of the most common discotic mesogens, which tend to form p-stacked discotic liquid crystalline phases that facilitate charge transport. However, to date, the electropolymerization of TP and its derivatives has not been reported. In this paper, polytriphenylene (PTP) films with high quality were successfully electrodeposited by direct anodic oxidation of TP both in the binary system consisting of BFEE and additional concentrated SA and in acetonitrile containing 0.1 mol L-1 LiClO4. The electrochemical behavior, thermal stability, electrical conductivity, fluorescence properties, structure, and morphology of the as-prepared PTP films were investigated in detail.

* To whom correspondence should be addressed. Tel/Fax: 86-25-84315054 (Q.H.) and 86-25-84315190 (F.W.). E-mail: [email protected] (Q.H.) and [email protected] (F.W.). † Institute of Industrial Chemistry. ‡ Key Laboratory of Soft Chemistry and Functional Materials.

2. Experimental Section 2.1. Materials. Boron trifluoride diethyl etherate (BFEE, Sinapharm Chemical Reagent Co., Ltd.) was purified by distillation before use. Triphenylene (Meryer, 98%), LiClO4, di-

10.1021/jp100150a  2010 American Chemical Society Published on Web 05/12/2010

Polymerization and Properties of Poly(triphenylene)

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methyl sulfoxide (DMSO), acetonitrile (ACN), and acetone were all A.R. grade and used directly without further purification. Concentrated sulfuric acid (SA) and 25% ammonia were used as received. 2.2. Electrosynthesis of PTP Films. All electrochemical experiments were performed in a one-compartment cell by the use of a CHI660B workstation (Shanghai, Chenhua). Platinum wires or foils were used as working and counter electrodes. For electrochemical examinations, the working and counter electrodes were wire with a diameter of 0.3 and 0.5 mm, respectively. They were placed 5 mm apart during the experiment. Prior to each experiment, these electrodes mentioned above were carefully cleaned successively with water and acetone, and then dried in air. To obtain a sufficient amount of polymer for characterization, a Pt foil of 2 cm2 surface area was used as the working electrode and a 4 cm2 foil was used as the counter electrode. All potentials were referred to Ag/AgCl. The amount of polymer deposited on the electrode was controlled by the total charge passed through the cell, which was read directly from the current-time curves. To remove the monomer, electrolyte, and oligomer, the electrodes coated with PTP films were washed thoroughly with anhydrous ethyl ether and Millipore water, respectively. The formed polymer was scraped from the Pt foils. For spectral analysis, the scraped polymer was dedoped with 25% ammonia for 3 days and then washed repeatedly with Millipore water. Finally, it was dried under vacuum at 60 °C for 24 h. 2.3. Characterization. Ultraviolet-visible (UV-vis) spectra were taken with a Perkin-Elmer Lamda 900 spectrophotometer. Fourier transform infrared (FT-IR) spectra were recorded with a Bruker Vector 22 FT-IR spectrometer in the region of 400-4000 cm-1, using KBr pellets. Field emission scanning electron microscopy (FE-SEM) was carried out with JEOL JSM6380LV electron microscopes. The 1H and 13C NMR spectra were recorded on a Bruker AV 400 NMR spectrometer with d6-DMSO as the solvent and tetramethylsilane as an internal standard (TMS, singlet, chemical shift: 0.0 ppm). The molecular weight of the polymer was analyzed with a Wyatt multiangle laser scattering-gel permeation chromatography (MALS-GPC) system. A PSS SDV column was used with tetrahydrofuran as eluent at a flow rate of 1 mL min-1. The conductivity of asformed PTP films was measured by a RTS-9 4-point probes measurement system. Thermogravimetric analysis (DTA) was performed with a Pyris Diamond TG/DTA thermal analyzer (Perkin-Elmer). The fluorescence spectra were determined with a FL3-TCSPC fluorescence spectrophotometer (France). The fluorescence quantum yield (Φoverall) of the soluble PTP samples was measured by using anthracene in acetonitrile (standard, Φref ) 0.27)35 as a reference and calculated according to the wellknown method based on the following expression:

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. 3. Results and Discussion 3.1. Electrochemical Synthesis of the PTP Film. TP can dissolve in BFEE containing a certain amount of SA to form a stable solution. Therefore, the electrochemical polymerization of TP was carefully studied in BFEE + SA. CPs can be

Figure 1. Anodic polarization curves of TP in mixed electrolytes of BFEE containing 0% (A), 3% (B), 5% (C), 10% (D), and 18% (E) SA (by volume), and in CH3CN + 0.1 mol L-1 LiClO4 (insert). Potential scan rate, 50 mV s-1.

generated by the anodic oxidation of suitable monomers such as thiophene, pyrrole, and other carbocyclic or heterocyclic π-systems.36 Therefore, the anodic polarization curves of TP in the mixed electrolytes of BFEE containing different amounts of SA were investigated in Figure 1. The oxidation onset potential of TP was initiated at 0.97 V in pure BFEE without SA (Figure 1A). This is much lower than that of TP in CH3CN + 0.1 mol L-1 LiClO4 (1.37 V, Figure 1, insert). The oxidation potentials of TP in BFEE containing 3% and 5% SA were 0.94 and 0.96 V, respectively (Figure 1B,C). Generally, the introduction of a small amount of SA has a positive effect on the electrochemical polymerization of the derivatives of benzene. This is mainly because SA increases the ionic conductivity of the electrolyte and the interactions between SA and TP monomer.27 However, the oxidation potential of TP rose to 1.0 and 1.03 V when the concentration of SA increased to 10% and 18%, respectively (Figure 1D,E). This is mainly because of the overoxidation effect of the strong acid (SA). Generally, the lower the monomer oxidation potential is, the less the possibility of side reactions during the polymerization is. On the basis of these aforementioned considerations, BFEE + 3% SA was chosen as the best electrosynthesis of conducting PTP films. Cyclic voltammetry is a very useful method that qualitatively reveals the reversibility of electron transfer during the electropolymerization and also examines the electroactivity of the polymer film because the oxidation and reduction can be monitored in the form of a current-potential diagram, that is, cyclic voltammogram (CV).37 The successive cyclic voltammograms (CVs) of 0.01 mol L-1 TP in different media are shown in Figure 2. In the medium of CH3CN containing 0.1 mol L-1 LiClO4 (Figure 2A), a strong irreversible oxidation of the TP can be found in the potential range from 1.38 to 1.86 V on the first cycle, and the anodic current wave decreases quickly with increases in the scan cycles. This is mainly due to the poor conductivity of the polymer formed on the electrode surface. Moreover, no apparent redox waves of the polymer are found. The main reason for this phenomenon is the high oxidation potential of TP in CH3CN media, which leads to the overoxidation of PTP and many defects on the polymer backbone. Figure 2B shows the CVs of TP in BFEE containing different amounts of SA. In the medium of pure BFEE (Figure 2B(a)), the CVs of TP exhibit characteristic features as other conducting polymers such as polypyrrole and polythiophene during potientiodynamic synthesis. As the CV scans continued, a polymer film formed and steadily adhered on the Pt electrode surface; no visible film dissolved in the solution. The polymer color

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Figure 2. Cyclic voltammograms of 0.01 mol L-1 TP in CH3CN + 0.1 mol L-1 LiClO4 (A), pure BFEE [B (a)], BFEE + 3% SA [B (b)], BFEE + 5% SA [B (c)], and BFEE + 10% SA [B (d)]. Potential scan rate, 100 mV s-1.

changed from brown to black as it deposited. From the CVs in Figure 2B(a), one can see PTP films can be reduced and oxidized between 0.66 and 0.77 V. The increases of the redox wave currents implied that the amount of the polymer on the electrode increased as the CV scan continued. The shift of the peak potential provides information about the increase of the electrical resistance in the polymer film; and the overpotential needs to overcome the resistance.38 When SA was added into BFEE, similar phenomenon could also be observed (Figure 2B(b-d)). The synthesized polymer adhered strongly to the Pt electrode surface, too. Seen from Figure 2B(b-d), the increase of the redox wave current per cycle is the highest in BFEE containing 3% SA and the accompanying redox processes become more reversible (Figure 2B(b)). The main reason for these phenomena can be ascribed to the ionic conductivity increases of the mixed electrolytes of BFEE containing SA. In pure BFEE, the conductivity species are mainly [BF3 · Et3O]- or a small amount of BF4-. The addition of SA into the electrolyte can dissociate

into SO42- and H+ by complexion with BFEE. This results in the increase of the ionic conductivity of mixed electrolyte. As the SA concentration is increased further in the mixed electrolytes, the density of current per cycle decreases. This may be attributed to the fact that the oxidation potential of the monomer increases, together with the strong overoxidation effect of SA. Therefore, the following work is concentrated on the electrosynthesis and characterizations of PTP films prepared in BFEE + 3% SA. 3.2. Electrochemistry of PTP Films. The electrochemical behavior of the PTP films deposited electrochemically from BFEE + 3% SA was investigated carefully in monomer-free SA (A), BFEE + 3% SA (B), and pure BFEE (C), respectively, as shown in Figure 3. Similar to the results in the literature,30 the steady-state CVs represent broad anodic and cathodic peaks. The peak current densities are proportional to the scanning rates (inset of Figure 3, panels A-C), indicating the reversible redox behavior of the polymer.39 Furthermore, these films can be



Figure 3. CVs of PTP films in monomer-free SA (A), BFEE + 3% SA (B), and pure BFEE (C) at a potential scan rate of 50 (a), 100 (b), 150 (c), 200 (d), 250 (e), and 300 (f) mV S-1. The PTP film was synthesized electrochemically in BFEE + 3% SA at a constant applied potential of 1.2 V. Inset: Plots of redox peak current densities vs potential scan rates. jp is the peak current density, and jp,a and jp,c denote the anodic and cathodic peak current densities, respectively.

cycled repeatedly between the conducting (oxidized) and insulating (neutral) state without significant decomposition of the materials, indicating the high structural stability of the polymer. The polymer films obtained from BFEE + 3% SA can be oxidized and reduced from 0.87 V (anodic peak potential, Ea) to 0.66 V (cathodic peak potential, Ec) in monomer-free BFEE at the scan rate of 50 mV S-1 (Figure 3C). With the scan rate increasing, the peak potentials shift positively and negatively to about 0.98 V (Ea) and 0.62 V (Ec), respectively. This mainly results from the slow transfer rate of large doping anions and solvent molecules in BFEE, such as (EtO)3 · BF3-, especially at higher scan rate.40 The polymer films can be oxidized and reduced from 0.58 V (Ea) to 0.4 V (Ec) in concentrated SA at the scan rate of 50 mV S-1 (Figure 3A). This indicates high stability of PTP films even in concentrated SA. Moreover, the small anions in concentrated SA (SO42- or HSO4-) can facilitate the doping and dedoping process.27 This led to the good redox activity of PTP films in concentrated SA. In a word, PTP films show good redox activity even in concentrated sulfuric acid. It should be noted that the stability of both Pt anode and Pt cathode in monomer-free electrolytes of BFEE or SA was also tested (no figures here). Results indicated that the working and counter electrodes (both Pt wire) were very stable during CV experiments, further proving the high quality of PTP films. 3.3. Structural Characterizations. As-prepared PTP films in the mixed electrolytes were in a doped state. When they were dedoped with 25% ammonia, their color changed slightly from brown to dark. What is more, both the doped and dedoped PTP films are partly soluble in the usual solvents such as tetrhydrofuran, DMSO, and CHCl3. The UV-visible spectra of TP, doped PTP, and dedoped PTP dissolved in DMSO are shown in Figure 4. The TP monomer

Figure 4. UV-visible spectra of TP (A), doped PTP (B), and dedoped PTP (C) prepared from a mixed electrolyte of BFEE + 3% SA potentiostatically at 1.2 V. Solvent: DMSO.

shows a strong absorption band with the absorption maximum at 269 nm (Figure 4A), while spectra of the doped (Figure 4B) and dedoped (Figure 4C) PTP films show a much wider absorption band at around 326 nm except for the similar absorption band at around 278 nm, the overall absorption of PTP tails off to about 551 nm. The absorption maxima of both PTP films red-shift about 8-10 nm. Generally, higher wavelength means higher conjugation length. Therefore, the result of the red-shift of UV-vis spectra of PTP means higher conjugation backbone in comparison with the monomer.41 Figure 5 shows the FTIR spectra of TP (a), doped PTP films (b), and dedoped (c) PTP films. The films were obtained from BFEE + 3% SA. The peaks at 1431 and 1495 cm-1 for TP are assigned to the stretching vibration of CdC of the benzene ring (Figure 5a), while the corresponding vibration peaks for the doped and dedoped PTP films locate at 1431, 1486, and 1614 cm-1,

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Figure 5. FT-IR spectrum of TP (a), doped PTP (b), and dedoped PTP (c).

740 cm-1 in the IR spectrum of TP indicates that existence of a 1,2-disubsituted benzene ring (Figure 5a). In the polymer spectra (Figure 5b,c), the new peaks at 878 and 817 cm-1 may result from the emergence of the 1,2,4-trisubstituted benzene ring. The peak at 752 cm-1 for both doped and dedoped PTP indicates that existence of a 1,2-disubsituted benzene ring (Figure 5b,c). These results show that there are two kinds of benzene rings in the polymer. Moreover, for the doped PTP, there are two band peaks at 1125 and 1086 cm-1, which mainly ascribes to BF3OEt3- or small amount of BF4-. In summary, all of the above indicate that the polymerization of TP occurred mainly at the C2, C3, C6, C7, C10, or C11 positions (Scheme 1A). This verifies that the main component in PTP has the characteristics of typical conjugated polymers.

respectively (Figure 5b,c). The peaks at 1049 and 1241 cm-1 are assigned to the asymmetric stretching vibration of C-C of the benzene ring (Figure 5a). The strong and narrow peak at

To obtain the deep structure and the possible polymerization mechanism of PTP, 2D-NMR techniques such as HSQC and COSY, combined with 1H, 13C NMR spectra of dedoped PTP, were examined and the results are shown in Figure 6, panels

SCHEME 1: Possible Electropolymerization Mechanism of TP



Figure 6. The downfield spectral expansions of 1H and 13C NMR spectra of TP (A) and dedoped PTP (B), and 1H-13C HSQC expanded spectrum of dedoped PTP (C) prepared from BFEE + 3% SA. Solvent: d6-DMSO.

A, B, and C, respectively. The d6-DMSO was used as the solvent and TMS as an internal standard. As shown in Figure 6A, the proton chemical shift of 7.71-7.73 ppm and the carbon signal at 127.55 ppm are ascribed to the protons and carbons of TP at 2-, 3-, 6-, 9-, 10-, 11-position (Scheme 1A); the proton and carbon signals at 8.80-8.82 and 123.50 ppm can be ascribed to the protons and carbons at 1-, 4-, 5-, 8-, 9-, 12-position in TP (Scheme 1A).

In contrast, it is noteworthy that the proton signal of PTP (Figure 6B) becomes much wider and broader than those in the monomer spectrum (Figure 6A), suggesting the former has a wide distribution of molecular weight. That indicates the electrochemical polymerization is successful. But the assignment of the exact structure becomes more difficult due to various polymers with different polymerization degrees or other unknown byproducts in the sample.

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Figure 7. The fluorescence spectra of TP (A), doped PTP (B), and dedoped PTP (C) prepared from BFEE + 3% SA. Solvent: DMSO.

It can be seen that some new peaks appeared in Figure 6B after polymerization and most of the peaks moved to lower field, which was mainly due to the introduction of higher conjugation length in the PTP main chain.42 The information mentioned right now and the new single peaks at 9.30 ppm indicate that the polymerization position occurred mainly at the C2, C3, C6, C7, C10, or C11 positions (Scheme 1A), in good accordance with the FT-IR results. According to the correlations of H-C HSQC (Figure 6C) and H-H COSY (not shown here), there are eight quaternary carbon atoms in the downfield region (129.7-128.5 ppm) and ten tertiary carbon atoms in the region of 127.8-121.8 ppm as shown in Figure 6B. The quaternary carbon atoms of the polymer are presented in Scheme 1B, including the original six atoms from the monomer (C13-18), and two new quaternary carbon atoms more, C6 and C11. This means TP was successfully polymerized by the electrochemical method. The other carbon atoms are all bonded with one hydrogen atom respectively. To get exact structures of the products, more investigation should be done in further work. Combining all information obtained right now, one of the possible polymerization mechanisms can be proposed as Scheme 1. The meta position of the benzene ring in TP is probably the key point of polymerization. The possible polymerization mechanism of TP is illustrated in Scheme 1. The initiation step involves the anodic oxidation of monomers (Scheme 1A) to the radical cations (Scheme 1B) around the working electrode, followed by coupling and deprotonation. The dimers of TP are produced. Thereafter, the

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Figure 9. TG (a) and DTA (b) curves of dedoped PTP films obtained potentiostatically at 1.2 V from BFEE + 3% SA after treatment with 25% aqueous ammonia for 3 days.

dimer is reoxidized to its radical cation C or D, and then couples with another radical cation B to form the trimeric cation E or F. After deprotonation, the trimers of TP are produced. The above steps proceed continuously with the formation of oligomer and polymer species. Finally, polymeric nuclei deposit on the anode, and continuously electrodeposit to form a stable PTP film on the surface of the electrode. The fluorescence spectra of the TP, doped PTP films, and dedoped PTP films prepared in BFEE + 3% SA were examined in DMSO. As shown in Figure 7, the emission spectra of TP (Figure 7A) have several peaks mainly at 372 and 382 nm, while the emission spectra of doped (Figure 7B) and dedoped (Figure 7C) PTP films have several peaks mainly at 439, 466, and 494 nm, and 439, 466, 494, and 534 nm, respectively. This shows that there is a large bathochromic shift in the emission wavelength between the monomer and the polymer. The redshift of the emission peak is mainly due to the increase of the conjugated chain length, further proving the formation of the conjugated backbone of PTP, in good agreement with the UVs vis spectral results (Figure 4). Meanwhile, the PTP films can emit strong photoluminescence when exposed to 365 nm UV light (Figure 8B). As shown in Figure 8, the solutions of PTP in both doped (Figure 8B(b)) and dedoped (Figure 8B(c)) states exhibit a bright blue-green-light-emitting property, whereas the monomer shows no emissions (Figure 8B(a)). According to eq 1, the fluorescence quantum yield (Φoverall) of TP monomer was

Figure 8. Photoluminescence of TP [panels A (c) and B (c)], doped PTP [panels A (b) and B (b)], and dedoped PTP [panels A (a) and B (a)] under UV light irradiation of 365 nm. Solvent: DMSO.



Figure 10. SEM images of PTP films deposited electrochemically on the ITO electrode for 400 s from BFEE + 3% SA: (A, B, C) doped PTP; (D, E, F) dedoped PTP.

calculated to be only 0.07, while those of the soluble doped and dedoped PTP films prepared from BFEE containing 3% SA were 0.29 and 0.43, respectively, 4 and 6 times more than that of the monomer. This is probably due to the relevant conjugated lengths of the molecules. Compared with other CPs prepared electrochemically, the Φoverall of PTP is much higher than those of the blue-light-emitting poly(9-bromophenanthrene) (0.04),23 poly(1, 5-dihydroxynaphthalene) (0.1),28 and the blue-

green-light-emitting poly(9-cyanophenanthrene) (0.2),29 although it is slight lower than that of poly(benzanthrone) (green-lightemitting, Φoverall ) 0.52).25 These results imply that PTP may be a good candidate in blue-green-light-emitting diodes or other organic electronic devices. 3.4. Molecular Weight and Its Polydispersity. The number and weight average molecular weights (Mn and Mw) were determined by MALS-GPC analysis with polystyrene as a

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standard. The Mw of the soluble part of dedoped PTP is 19980 g/mol. Compared with other electropolymerized CPs, this value is much lower than that of poly(N-alkoxy-(p-ethynylphenyl)carbazole) (148 000 g mol-1)10 and poly(3-methoxythiophene) (184 674 g mol-1),43 but much higher than that of polybenzanthrone (4-12 repetitive units)25 and oligopyrene (2-5 repetitive units).44 This indicates that the PTP prepared in the electrolyte of BFEE containing 3% SA has a long chain or high polymer degree, in good accordance with UV-vis and fluorescence results. The polydispersity index (Mw/Mn) is calculated as 8.24, which means the electropolymerized PTP in this work has a wide distribution of molecular weights. 3.5. Thermal Analysis. The thermal stability of CPs is very important for their potential application. DTA is a significantly dynamic way to detect the degradation behaviors of CPs. The weight loss of a polymer sample is measured continuously, whereas the temperature is changed at a constant rate. The thermal analysis of dedoped PTP films was tested, as shown in Figure 9. The thermal analysis was performed under a nitrogen stream in the temperature range of 323-1073 K, with a heating rate of 10 deg min-1. As seen from Figure 9, there was a slight weight loss from 323 to 474 K, up to 2%, which is ascribed to water evaporation or other moisture trapped in the polymer. The decomposition up to 42% occurred between 474 and 921 K. This weight loss was attributed to the degradation of the skeletal PTP backbone chain structure. The total weight loss was less 50%. All these results indicated that PTP films had good thermal stability. 3.6. Conductivity and Morphology. The electrical conductivity of the pressed pellet of doped PTP films obtained from BFEE + 3% SA was measured to be 10-3 S cm-1 by the conventional four-probe technique. The surface morphologies of PTP films deposited on the ITO electrodes from BFEE + 3% SA were observed by SEM, as shown in Figure 10. Panels A, B, and C of Figure 10 present the SEM images of doped PTP films electropolymerized at 0.94 V for 1 min; panels D, E, and F of Figure 10 demonstrate the morphology of the dedoped PTP film, which was treated with doped PTP films in the monomer-free mixture of BFEE + 3% SA at the potential of -0.2 V until the current density decreased to zero. The morphologies of two kinds of PTP materials are similar from the macroscopic view (Figure 10A,D); however, the interestingly different microstructure can be clearly observed from Figure 10B,C,E,F. Macroscopically, the doped PTP films appear to be smooth and compact (Figure 10A). However, the microstructure of doped PTP exhibits a loosely granular morphology with a small number of laminated plates in it (Figure 10B,C). The appearance of such laminated plates might be attributed to the character of TP derivatives to form p-stacked discotic liquid crystalline phases that facilitate charge transport.32 This morphology facilitates 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 PTP films. However, after being treated at a constant applied potential of -0.2 V for a long time, the laminated plates of the PTP films disappear and a regular arrangement of nanowires with the diameter of about 100 nm and length of 1-2 µm is observed (Figure 10D,E,F). Moreover, the surfaces of the nanowires and laminated plates are covered by many nanoparticles with the avarage size of 15 nm. The morphology difference between the doped and dedoped PTP probably results from two main reasons: one is the treating time and potential; the other is the characteristic of the polymer. The counteranions migrate out of the polymer surface under the condition of the applied negative potential, which led to the rearrangement of the

Hao et al. macrostructure of PTP chains. The exact mechanism of forming the interesting nanostructures is still unknown and will be further studied and discussed in detail. 4. Conclusions Novel PTP films with conductivity of 10-3 S cm-1 have been electrochemically polymerized in the electrolyte of BFEE containing 3% SA (by volume). Such electrolyte medium is favorable for lowering oxidation potential of TP (only 0.94 V). The obtained PTP films exhibit good redox activity and high electrochemical stability. SEM images of the dedoped PTP films demonstrate a regular arrangement of nanowires with a diameter about 100 nm. According to the information obtained from FTIR and NMR spectra, a possible polymerization mechanism is suggested. The fluorescence spectra indicate that PTP is an excellent blue-green-light-emitting material due to its long conjugation polymeric chain and high molecular weight. With so many good properties, a wide range of applications of PTP and its derivatives can probably be achieved. For instance, it will be a good candidate for a luminophore in liquid crystalline systems or light-emitting diodes. Acknowledgment. This work was supported by the Science and Technology Supporting Item of Jiangsu Province, China (BE2009159), the SRF for ROCS, State Education Ministry and Ministry of Personnel of the PRC (2006), and the Excellent Plan Foundation of NUST (2008). We thank Dr. Weiyan Shao for NMR assistance and Prof. Lude Lu for helpful discussions. References and Notes (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene, (CH)x. J. Chem. Soc., Chem. Commun. 1977, 16, 578–580. (2) Saraswathi, R.; Gerard, M.; Malhotra, B. D. Characteristics of aqueous polycarbazole batteries. J. Appl. Polym. Sci. 1999, 74, 145–150. (3) Jin, Y. E.; Kim, J. Y.; Park, S. H.; Kim, J.; Lee, S.; Lee, K.; Suh, H. Syntheses and properties of electroluminescent polyfluorene-based conjugated polymers, containing oxadiazole and carbazole units as pendants, for LEDs. Polymer 2005, 46, 12158–12165. (4) Tokito, S.; Suzuki, M.; Sato, F.; Kamachi, M.; Shirane, K. Highefficiency phosphorescent polymer light-emitting devices. Org. Electron. 2003, 4, 105–111. (5) Horowitz, G. Organic field-effect transistors. AdV. Mater. 1998, 10, 365–377. (6) Hao, Q.; Wang, X.; Lu, L.; Yang, X.; Mirsky, V. M. Electropolymerized multilayer conducting polymers with response to gaseous hydrogen chloride. Macromol. Rapid Commun. 2005, 26, 1099–1103. (7) Donnat-Bouillud, A.; Mazerolle, L.; Gagon, P.; Goldenberg, L.; Petty, M. C.; Leclerc, M. Synthesis, characterization, and processing of new electroactive and photoactive polyesters derived from oligothiophenes. Chem. Mater. 1997, 9, 2815–2821. (8) Morin, J. F.; Boudreault, P. L.; Leclerc, M. Blue-light-emitting conjugated polymers derived from 2,7-carbazoles. Macromol. Rapid Commun. 2003, 23, 1032–1036. (9) Jegadesan, S.; Sindhu, S.; Rigoberto, C. A.; Valiyaveettil, S. Direct electrochemical nanopatterning of polycarbazole monomer and precursor polymer films: ambient formation of thermally stable conducting nanopatterns. Langmuir 2006, 22, 780–786. (10) Fulghum, T.; Karim, S. M. A.; Baba, A.; Taranekar, P.; Nakai, T.; Masuda, T.; Advincula, R. C. Conjugated poly(phenylacetylene) films crosslinked with electropolymerized polycarbazole precursors. Macromolecules 2006, 39, 1467–1473. (11) Liou, G. S.; Hsiao, S. H.; Huang, N. K.; Yang, Y. Z. Synthesis, photophysical, and electrochromic characterization of wholly aromatic polyamide blue-light-emitting materials. Macromolecules 2006, 39, 5337– 5346. (12) Tonzola, C. J.; Kulkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A. Blue-light-emitting oligoquinolines: Synthesis, properties, and high-efficiency blue-light-emitting diodes. AdV. Funct. Mater. 2007, 17, 863–874. ¨ nal, A. M. Electrochemical polymeri(13) Bezgin, B.; Cihaner, A.; O zation of 9-fluorenecarboxylic acid and its electrochromic device application. Thin Solid Films 2008, 516, 7329–7334.

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JP100150A

Electrochemical Polymerization and Properties of Poly (triphenylene

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