Electrochemical Polymerization of Benzanthrone and Characterization

J. Phys. Chem. B 2009, 113, 37–48

37

Electrochemical Polymerization of Benzanthrone and Characterization of its Excellent Green-light-emitting Polymer Baoyang Lu, Jingkun Xu,* Changli Fan, Huaming Miao, and Liang Shen Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal UniVersity, Nanchang 330013, China ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: October 27, 2008

A novel semiconducting polybenzanthrone, with relatively high electrical conductivity and excellent thermal stability, was successfully electrosynthesized by direct anodic oxidation of its monomer benzanthrone in acetonitrile solution containing Bu4NBF4 or boron trifluoride diethyl etherate (BFEE) acting as the supporting electrolyte. As-formed polybenzanthrone films showed good redox activity and nice structural stability even in concentrated sulfuric acid. UV-vis, FT-IR, and 1H NMR spectral analyses and MALDI-TOF MS results, together with quantum chemistry calculations, proved that the polymer chains grew mainly via the coupling of the monomer at C(3) and C(11) positions. The fluorescence properties of both doped and dedoped polybenzanthrone were greatly improved in comparison with that of the monomer. Furthermore, both doped and dedoped polybenzanthrone, dissolved in common organic solvents, with fluorescence quantum yields as high as 0.52, also emitted strong and bright green or yellow-green photoluminescence at excitation of 365 nm UV light. All these results indicate that the striking polybenzanthrone films as obtained have many potential applications in various fields. Introduction Because of their semiconductivity and other fascinating properties, conducting polymers have played indispensable roles in specialized industrial applications in spite of their short history. Now they have been extensively studied in many applications such as sensors, light-emitting diodes (LEDs), secondary batteries, transistors, and photovoltaic cells.1-9 However, the major aspect useful for most applications is not the metal-like electrical property itself, but the combination of electrical conductivity and polymeric properties such as flexibility, low density, and ease of structural modification that suffice for many commercial applications.3 Recently, it was particularly rewarding to see that conjugated polymers with unique and tunable optical properties have been widely applied in fabrication of optoelectronics, such as polymer light-emitting diodes (PLEDs)6 and electrochromic windows.10 To date, much research has been conducted on the synthesis of conjugated polymers exhibiting excellent fluorescence properties, and the vast portfolio of new polymer structures has been obtained. Unfortunately, most of the resulting polymers showed many disadvantages, such as poor solubility, relatively low electrical conductivity, unfavorable thermal stability, rough surfaces with many defects, etc., which limited the practical uses of these polymers in real-world applications. Therefore, the pursuit of novel conducting polymers that own better optical, electrical, and other properties in order to overcome these defects and obstacles is still very necessary and remains a considerable challenge. Electro-oxidative polymerization of heterocycle aromatic compounds with concurrent polymer films deposition has been proved to be an especially useful method for the preparation of conducting polymer films.3,11-15 Conducting polymer films * To whom correspondence should be addressed. Phone: 86-7913805183; fax: 86-791-3823357; e-mail: [email protected], [email protected].

prepared by this technique are usually formed in an oxidized (doped) state and can be used without further treatment.16,17 However, to date, research on the electropolymerization of fused ring compounds is far from maturity. Therefore, the electrodeposition of high-quality conducting polymer films by direct anodic oxidation of fused ring compounds is quite essential. On the other hand, the band gap of conducting polymers, which is the difference in energy (Eg) between the valence band [highest occupied molecular orbital (HOMO)] and the conduction band [lowest unoccupied molecular orbital (LUMO)],18,19 is a very crucial factor for improving the properties of semiconductors. Intrinsic conductors owe their conductivity to the partial filling of the valence band up to the Fermi level. To imitate such a partially filled band with a semiconductor, their band gap should be zero or close to zero. In semiconductors, the valence and conduction bands are curved by space-charge effects, which lead to a diminished band gap energy when the spatial alternation of the levels is taken into account. Conducting polymers are usually semiconductors. Therefore, the lower the band gap is, the higher the electrical conductivities of conducting polymers may be. One of the most amusing properties of conducting polymers is that their band gaps can be tuned by substituents on the conjugated main chain. Generally, the electron-donating group (donor) substitution can increase the HOMO level, whereas the electron-withdrawing group (acceptor) substitution can decrease the LUMO level. Thus, the highlying HOMO of the donor fragment and the low-lying LUMO of the acceptor fragment can yield an unusually small HOMOLUMO separation,20,21 leading to a smaller band gap. Previous studies mainly concentrated on donor-group-substituted conducting polymers, such as poly(3,4-ethylenedioxythiophene)10,13 and poly(3-methylthiophene).17,22 However, there have been few reports on conducting polymers with electron-withdrawing groups because the electron-withdrawing group substitution make the formation of conducting polymer films very difficult.

10.1021/jp804497q CCC: $40.75  2009 American Chemical Society Published on Web 12/09/2008

38 J. Phys. Chem. B, Vol. 113, No. 1, 2009 SCHEME 1: Chemical structure of benzanthrone

It has been reported that the electron-withdrawing carbonyl group makes the formation of conducting polymer films very difficult.18 A destabilizing carbonyl substituent inhibits electropolymerization, which usually leads to a higher oxidation potential of the corresponding monomer and poorer polymer film quality, and almost no conducting polymer films can be formed, mainly because the strong electron-withdrawing group can sufficiently stabilize the radical cation intermediates to diffuse away from the electrode, thus no further reactions happen on the anode surface.23 For example, Zecchin et al.24 reported that it was impossible to obtain poly(9-fluorenone) from its monomer 9-fluorenone by oxidative coupling in acetonitrile solutions. On the other hand, due to their electron withdrawing effects, the monomers with carbonyl groups can form electrondeficient conjugated polymers. These polymers have characteristic features, such as high electron affinity and low-lying conduction bands, which make them good candidates for n-type electrical conductors. Thus, an investigation of the synthesis and characterization of carbonyl group substitution on conducting polymers is very necessary and significant. Because of their excellent color characteristics and high photostability both in solution and in the solid state, benzanthrone (Scheme 1) and its derivatives have widespread applications, such as reactive fluorescent dyes in analytical chemistry and biochemistry, daylight fluorescent pigments for synthetic textile materials, for mass and chemical coloration of other polymeric materials, fluorescent probes in molecular and cell biology (and recently for fluorescence lifetime imaging), components in color liquid-crystalline displays (LCDs) of the “guest-host” type, etc.25-27 Benzanthrone structure material is an organic semiconductor oligomer that can provide some merits of self-radiation, flexibility, light weight, easy fabrication, and low cost in photovoltaic and display devices. To date, the benzanthrone molecule, which has the extended π-electron systems and is a very promising block for building high-quality conducting polymers, has not attracted a considerable interest in the field of synthetic metals. It is well-known that polymers or oligomers may possess better optical properties than those of the corresponding monomers because of their longer delocalized π-electron chain sequences, such as polyfluoranthene,28 poly(1,2methylenedioxybenzene),29 poly(o-dihydroxybenzene),30 and oligopyrene.31 Polybenzanthrone and its derivatives hereby deserve much attention, and they are expected to gain better properties such as excellent fluorescence and luminescence properties. However, the polymerization of benzanthrone and its derivatives and characterization of their polymers have not been studied until now. In this paper, benzanthrone (Scheme 1) was chosen as the monomer, and high quality polybenzanthrone films were successfully electrodeposited by direct anodic oxidation of benzanthrone both in acetonitrile containing 0.1 mol L-1 Bu4NBF4 and in the binary solvent system consisting of acetonitrile and additional boron trifluoride diethyl etherate (BFEE). The electrochemical behavior, structural characterization, polymerization mechanism, solubility, spectroscopic properties, thermal stability, electrical conductivity, and morphology of as-formed polybenzanthrone films were investigated in detail.

Lu et al. 2. Experimental Section 2.1. Materials. Benzanthrone (Alfa Aesar), commercial highperformance liquid chromatography grade acetonitrile (Beijing East Longshun Chemical Plant, China) and concentrated sulfuric acid (Ji’nan Chemical Reagent Company, China) were used directly without further purification. Bu4NBF4 (Acros Organics, 98%) was dried under vacuum at 60 °C for 24 h before use. BFEE (Changyang Chemical Plant, Beijing, China) was purified by distillation before use. Dimethyl sulfoxide (DMSO, analytical grade) was a product of Beijing East Longshun Chemical Plant and was used directly. Deuterium-substituted dimethyl sulfoxide (d6-DMSO; CD3SOCD3) was a product of Cambridge Isotope Laboratory. Inc. Other reagents were all A.R. grade and were used as received without further treatment. 2.2. Electrosynthesis and Electrochemical Tests. All the electrochemical experiments of benzanthrone were performed in a one-compartment cell with the use of Model 263A potentiostat-galvanostat (EG&G Princeton Applied Research) under computer control. The working and counter electrodes were platinum (Pt) wire with a diameter of 0.5 mm and stainless steel wire with a diameter of 1 mm, respectively. They were placed 5 mm apart during the examinations. Prior to each experiment, these electrodes mentioned above were carefully polished with abrasive paper (1500 mesh), cleaned successively with water and acetone, and then dried in air. In acetonitrile solution containing 0.1 mol L-1 Bu4NBF4, a Pt wire directly immersed in the solution served as the reference electrode and was calibrated using the ferrocene (Fc/Fc+) redox couple, which has a formal potential E1/2 ) +0.35 V vs Pt, whereas in the binary solvent system of acetonitrile and BFEE, an Ag/AgCl electrode directly immersed in the solution served as the reference electrode, and its electrode potential was calibrated against a saturated calomel electrode; both of them revealed sufficient stability during the experiments. The concentration of benzanthrone used through all the experiments was 0.01 mol L-1. All the solutions were deaerated by a dry nitrogen stream and maintained under a slight overpressure through all the experiments. To obtain a sufficient amount of the polymer films for characterization, an ITO electrode and a stainless steel sheet with surface areas of 4 and 6 cm2 each were employed as the working and counter electrodes, respectively. The polymer films grew potentiostatically, and their thickness was controlled by the total charge passed through the cell that was read directly from the current-time (I-t) curves by computer. After polymerization, polybenzanthrone films were washed repeatedly with anhydrous acetonitrile to remove the electrolyte, monomer, and oligomer. For spectral analyses, they were dedoped with 25% ammonia for 3 days and then washed repeatedly with pure water. Finally, they were dried at 60 °C under vacuum for 24 h. 2.3. Characterization. The electrical conductivity of asformed polybenzanthrone films was determined by applying the conventional four-probe technique with pressed pellets of the sample. Ultraviolet-visible (UV-vis) spectra were measured with a Perkin-Elmer Lambda 900 UV-vis-near-infrared spectrophotometer. Infrared spectra were recorded using a Varian 3100 FT-IR spectrometer with samples in KBr pellets. The 1H 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). MALDI/TOF (matrix assisted laser desorption ionization/timeof-flight) MS spectra were carried out on a Bruker MICROFLEX mass spectrometer. Thermogravimetric analysis (TGA) was performed with a Pyris Diamond TG/DTA thermal analyzer (Perkin-Elmer). Scanning electron microscopy (SEM) measure-

Electrochemical Polymerization of Benzanthrone

J. Phys. Chem. B, Vol. 113, No. 1, 2009 39

Figure 1. Anodic polarization curve (inset) and CVs of 0.01 mol L-1 benzanthrone in acetonitrile containing 0.1 mol L-1 of Bu4NBF4. Potential scan rates: 50 mV s-1 (inset) and 100 mV s-1, respectively.

ments were made with a JEOL JSM-6360LA scanning electron microscope or a Cold Field Emission Electron Microscope S-4300 (Hitachi). With an F-4500 fluorescence spectrophotometer (Hitachi), fluorescence spectra were determined. The fluorescence quantum yields (φoverall) of the soluble polybenzanthrone samples were measured using anthracene in acetonitrile (standard, φref ) 0.27)33 as a reference and calculated according to the well-known method based on the expression:

φoverall )

n2ArefI n2refAIref

× 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.34 3. Results and Discussion 3.1. Electrochemical Polymerization of Benzanthrone. 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).35 We first investigated the electrochemical polymerization of benzanthrone in the traditional organic solvent acetonitrile. Figure 1 shows the anodic polarization curve and CVs of 0.01 mol L-1 benzanthrone in acetonitrile containing 0.1 mol L-1 Bu4NBF4. The oxidation potential of benzanthrone in this medium was determined to be 1.68 V vs SCE (inset of Figure 1), a relatively low value compared with other aromatic compounds in the same medium.28,36-41 The CVs of benzanthrone (Figure 1) showed characteristic features of other conducting polymers or oligomers during potentiodynamic syntheses such as pyrene31 and 1-nitropyrene.42 In the first cycle, the anodic current began to increase at 1.68 V vs SCE, and there was a current loop between 1.68 and 2.20 V vs SCE. The formation of this loop is characteristic of nucleation processes, as reported in the literature.43 Also, the formation of a shiny, compact, and homogeneous polybenzanthrone film brown in color on the Pt electrode surface was observed at potentials greater than 1.68 V vs SCE. As the potential scanning continued, there was an obvious reduction peak near 1.47 V vs SCE. The increases of the redox wave currents implied that the amount

of the polymer on the electrode increased. All these phenomena indicated that benzanthrone can be electropolymerized in the traditional organic solvent acetonitrile. It is well-known that high oxidation potential generally makes the electropolymerization of aromatic monomers quite difficult and also leads to some side reactions. Furthermore, the conducting polymer films produced on the electrode are easily overoxidized during further electropolymerization. Obviously, it is beneficial to the preparation of high-quality conducting polymer films if the polymerization potential can be decreased. In 1995, Shi et al.44 substantially decreased the potential to about 1.0 V vs Ag/AgCl by performing the electropolymerization of thiophene in BFEE solution. With the lower polymerization potential, flexible polythiophene films with strong mechanical properties can be easily electrodeposited. To date, with the use of BFEE, a wide variety of high-quality conducting polymers electrochemically synthesized from heterocyclic or fused ring aromatic monomers have been successfully achieved.13,19,28,30-32,42 From this point of view, BFEE is a very good choice for the electropolymerization of aromatic monomers. Therefore, we carefully examined the electrochemical performance of benzanthrone in BFEE despite its poor solubility. The onset oxidation potential of benzanthrone in this medium was greatly lowered to about 0.88 V vs SCE. However, further experimental results demonstrated that benzanthrone did not exhibit good electrochemical behavior even though a polymer film can be obtained in BFEE. There were no apparent redox waves in the CVs (Figure 1 of the Supporting Information) and the resulting polymer film was thin and discontinuous compared with that synthesized from acetonitrile solution. On the other hand, it was reported that shiny, flexible, and compact polythiophene films can be easily prepared with the use of the binary solvent solution consisting of acetonitrile and BFEE, which turned out to be a good binary electrolyte system.45 For benzanthrone, the good solubility of acetonitrile combined with the lowered onset oxidation potential effect of BFEE makes the binary solvent solution a good candidate for the electropolymerization. Therefore, the electrochemical polymerization of benzanthrone in the mixed electrolytes of acetonitrile (used as the solvent) containing different amount of BFEE was also tested. It should be noted that BFEE was used as the supporting electrolyte in acetonitrile solutions without any other salts. When different volume proportions of BFEE (acetonitrile/BFEE ) 3:1, 2:1, 1:1, 1:2, 1:3) were added to acetonitrile with 0.01 mol L-1 benzanthrone, the onset potential of the electropolymerization of benzanthrone decreased with increasing BFEE concentration, as shown in Figure 2 of the Supporting Information. The onset oxidation potential decreased in the acetonitrile solution from 1.68 V vs SCE without BFEE to 0.88 V vs SCE with 100% BFEE. The potential decrease in the acetonitrile solution with BFEE could be attributed to the catalytic effect of the Lewis acid on the deprotonation of benzanthrone on the electrode.44 The lower oxidation potential of benzanthrone in those systems may have prevented the side reactions between monomer units, thereby imparting a greater linearity to the overall structure of polybenzanthrone, which should have resulted in improved stereoregularity. Meanwhile, BFEE existed as polar molecular Et3O+ and BF3OEt-, which supported the conducting medium. On the basis of these results, obviously, we could regulate the onset potential of the benzanthrone electropolymerization between 0.88 and 1.68 V vs SCE by varying the concentration of BFEE, which would be very useful in the electrochemical copolymerization of benzanthrone with other aromatic monomers.

40 J. Phys. Chem. B, Vol. 113, No. 1, 2009

Figure 2. Cyclic voltammograms of 0.01 mol L-1 benzanthrone in the binary solvent systems of BFEE/acetonitrile ) 3:1 (A), 2:1 (B), 1:1 (C), 1:2 (D), 1:3 (E) (by volume). Potential scan rate: 100 mV s-1.

Figure 2 shows the successive CVs of 0.01 mol L-1 benzanthrone in the mixed electrolytes of acetonitrile and BFEE with different volume ratios (3:1, 2:1, 1:1, 1:2, 1:3) on the Pt electrode. From this figure, it can be clearly seen that better curves with apparent redox waves and larger increasing interval of the peak current densities were found from each volume ratio than those from BFEE or acetonitrile containing 0.1 mol L-1 Bu4NBF4. During the experiments, it was also found that polybenzanthrone films dark in color were easily obtained, indicating the faster polymerization rate of polybenzanthrone in the binary solvent system owing to the catalytic effect of BFEE. Moreover, the obvious 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. Obviously, CVs of benzanthrone tested in the binary solvent system of each volume ratio were also very successful. For comparison, we choose two systems (acetonitrile containing 0.1 mol L-1 Bu4NBF4 and the binary solvent system consisting of acetonitrile and BFEE with a volume ratio of 2:1) to prepare polybenzanthrone films. Further experiments demonstrated that the optimum polymerization potentials needed for the potentiostatic polymerization of benzanthrone in the two systems are 1.9 V vs SCE and 1.75 V vs SCE, respectively. At these two applied potentials, polybenzanthrone films formed with regular morphology and good adherence on the working electrodes. The polybenzanthrone films used for all the characterization mentioned below were all prepared by chronoamperometry at constant applied potentials of 1.9 V vs SCE and 1.75 V vs SCE, respectively. It should be pointed out here that sufficient amounts of polybenzanthrone films were more easily obtained in the binary solvent system than in acetonitrile

Lu et al.

Figure 3. Cyclic voltammograms of polybenzanthrone films electrochemically synthesized in acetonitrile containing 0.1 mol L-1 Bu4NBF4 on the Pt electrode in concentrated sulfuric acid (A) at different potential scan rates indicated and in monomer-free acetonitrile containing 0.1 mol L-1 Bu4NBF4 at a potential scan rate of 50 mV s-1 (B). 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.

containing 0.1 mol L-1 Bu4NBF4 when benzanthrone of 0.01 mol L-1 was used. Furthermore, in the binary solvent system, polybenzanthrone films can be also easily electrodeposited when a much cheaper stainless steel sheet was used as the working electrode for large-amount polymer film deposition, which facilitates their industrial applications significantly. 3.2. Electrochemistry of Polybenzanthrone Films. The electrochemical behavior of polybenzanthrone films obtained from acetonitrile was determined carefully, as shown in Figure 3. Similar to the results in the literature,3 the steady-state CVs represented broad anodic and cathodic peaks in concentrated sulfuric acid. The peak current densities were proportional to the scanning rates (inset of Figure 3B), indicating the reversible redox behavior of the polymer.46-49 Furthermore, these films could be 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 could be oxidized and reduced from 0.66 V (anodic peak potential, Ea) to 0.48 V vs SCE (cathodic peak potential, Ec) in concentrated sulfuric acid. In a word, polybenzanthrone from acetonitrile showed good redox activity even in concentrated sulfuric acid. These films were also tested in monomer-free acetonitrile solution containing 0.1 mol L-1 Bu4NBF4. In this medium, it was found that polybenzanthrone prepared from the acetonitrile solution is soluble and that the amount of polybenzanthrone on the electrode surface gradually decreases during the CV experiments. The anodic and cathodic peak current densities of polybenzanthrone film also decreased dramatically. As can be seen from Figure 3B, a well-resolved

Electrochemical Polymerization of Benzanthrone

J. Phys. Chem. B, Vol. 113, No. 1, 2009 41 TABLE 1: Assignments of FTIR Spectra of Dedoped Polybenzanthrone band (cm-1)

assignment

3066, 3040 1653, 1644 1616, 1598, 1577, 1541, 1508, 1455 1326, 1302, 1278 1071 1029 941 843 773, 749 700 Figure 4. FTIR spectra of benzanthrone (A), dedoped polybenzanthrone obtained potentiostatically from acetonitrile solution (B) and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (C).

redox wave could be observed on the first cycle of the CV scan, which presented that polybenzanthrone could be oxidized and reduced from 2.22 to 1.48 V vs SCE in acetonitrile solution. Figure 3 in the Supporting Information shows the successive CVs of polybenzanthrone films prepared from the binary solvent system in the monomer-free mixed electrolyte of acetonitrile and BFEE with a volume ratio of 2:1 (A), in monomer-free BFEE (B), and in concentrated sulfuric acid (C). From the figures, polybenzanthrone films from the binary solvent system also exhibited good electrochemical activity in all the solvents mentioned above. The potentials needed to oxidize or reduce the polymer film prepared from the binary solvent system were from 1.58 V (Ea) to 1.42 V vs SCE (Ec) in monomer-free binary solvent system, from 1.69 V (Ea) to 1.39 V vs SCE (Ec) in monomer-free BFEE, and from 0.99 V (Ea) to 0.77 V vs SCE (Ec) in concentrated sulfuric acid. On the basis of the previous discussions, it can be reasonably concluded that polybenzanthrone from the two systems both showed good redox activity and nice structural stability. 3.3. Structural Characterization. Vibrational spectra can provide much structural information for conducting polymers, especially for insoluble and infusible polymers. A comparison of the evolution of the vibrational modes appearing in conducting polymers and in some simpler related molecules acting as references usually facilitates the interpretation of the experimental absorption spectra. For polybenzanthrone, vibrational spectra are unique because they may be used to interpret the polymerization mechanism. Figure 4 displays the transmittance FTIR spectra of benzanthrone monomer (A) and dedoped polybenzanthrone obtained from acetonitrile solution (B) and from the binary solvent system (C). From this figure, no apparent difference was found between the FTIR spectra of dedoped polybenzanthrone prepared from both of the two systems, indicating the similar structure of the polymer obtained from different systems. It should also be stressed here that both the FTIR spectra of doped and dedoped polybenzanthrone prepared from the two systems are quite similar, mainly depended on the automatic dedoping process of polybenzanthrone, concerning dopant removal. The details of the band assignments of dedoped polybenzanthrone are given in Table 1. From Figure 4 and Table 1, the CdO stretching vibration and C-C-C (aryl ketones) asymmetric stretching vibration existed in the spectrum of both monomer and polymer, indicating that the CdO double bond was not destroyed during electrochemical polymerization.

C-H ring stretching vibration CdO stretching vibration CdC ring stretching vibration C-C-C (aryl ketones) asymmetric stretching vibration C-H in-plane bending of 1,2,3-trisubstituted phenyl C-H in-plane bending of 1,2,3,4-tetrasubstituted phenyl C-Cd stretching vibration 1,2,3,4-tetrasubstituted, C-H wagging 1,2,3-trisubstituted, C-H wagging 1,2,3-trisubstituted, ring deformation

TABLE 2: Main Atomic Electron Density Populations for Benzanthrone atom

electric charge

atom

electric charge

C(1) C(3) C(5) C(7) C(9) C(11) C(13) C(15) C(17)

-0.154 -0.130 -0.090 0.340 -0.089 -0.138 0.078 0.097 0.019

C(2) C(4) C(6) C(8) C(10) C(12) C(14) C(16) O(18)

-0.088 -0.125 -0.115 -0.110 -0.085 0.071 0.022 0.023 -0.504

Besides, CdC ring stretching vibration also displayed in the band range from 1616 to 1450 cm-1 in the two spectra (Figure 4, panels A and B). In the spectrum of benzanthrone, the bands of 691 cm-1 were assigned to the ring deformation vibration of 1,2-disubstituted benzene, and the out-of-plane vibration of three adjacent C-H bonds and the ring deformation vibration of 1,2,3trisubstituted benzene are at 796, 781 and 763 cm-1. In contrast, the peaks present at 843 and 773, 700 cm-1 in the polymer spectrum (Figure 4B) manifested the 1,2,3,4-tetrasubstituted and 1,2,3-trisubstituted benzene ring, respectively. The existence of the 749 cm-1 band was also ascribed to the C-H wagging of 1,2,3-trisubstituted benzene. From these results, the electrochemical polymerization of benzanthrone may occur at C(1), C(3), C(4), C(6), C(8), and C(11) positions. On the other hand, the peaks in the spectrum of dedoped polybenzanthrone were obviously broadened in comparison with the monomer, similar to those of other conducting polymers,28-32,48,50 which may be attributable to the wide conjugated chain length distribution of polybenzanthrone. To gain further insight into the structure of polybenzanthrone and the polymerization mechanism, atomic electron density population (Table 2) and proportion of the frontier orbitals (Table 3) of the benzanthrone monomer were calculated at the B3LYP/6-31G(d,p) level using Gaussian 03 software. The results of main atomic electron density populations revealed negative electric charges on C(1), C(3), C(4), C(6), C(8), and C(11) (Table 2), which implied that these atoms may donate electrons during electrochemical polymerization through radical cation intermediates. Among them, C(1), C(3), C(4), and C(11) were the more-negative ones. According to the molecular orbital theory, the reaction between the active molecules mainly happens on the frontier molecular orbital and the near orbital. For benzanthrone, the proportions of atoms C(1), C(3), C(4), C(9), and C(11) in the HOMO were higher than other atoms as listed in Table 3. Meanwhile, C(1), C(3), C(4), and C(11) also

42 J. Phys. Chem. B, Vol. 113, No. 1, 2009

Lu et al.

TABLE 3: Main Composition and Proportion of the Frontier Orbitals in Benzanthrone (%)a atom

HOMO - 1

HOMO

LUMO

LUMO + 1

C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) C(17) O(18)

0.035 0.027 0.190 0.058 0.289 2.822 2.723 2.816 0.290 0.057 1.243 1.091 0.383 1.011 1.209 16.353 16.047 51.519

8.539 5.171 15.347 7.282 2.639 4.043 0.279 2.152 7.705 0.160 5.572 4.158 13.730 1.344 0.034 8.527 4.125 9.168

8.542 1.834 9.995 13.063 1.281 12.317 9.467 2.228 1.987 4.253 0.358 5.576 4.960 0.127 0.094 5.907 4.307 13.676

7.903 0.048 7.481 0.096 3.116 2.489 5.642 19.351 6.868 6.435 19.803 1.766 8.324 0.751 5.177 0.116 0.884 3.714

The 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. a

TABLE 4: Main Atomic Electron Spin Densities for Benzanthrone atom

electron spin density

atom

electron spin density

C(1) C(3) C(5) C(7) C(9) C(11) C(13) C(15) C(17)

0.086 0.271 -0.009 -0.059 0.173 0.117 0.187 -0.072 0.078

C(2) C(4) C(6) C(8) C(10) C(12) C(14) C(16) O(18)

-0.010 0.119 0.028 -0.028 -0.074 0.005 -0.024 0.117 0.127

SCHEME 2: Isovalent surfaces (0.004 electron/bohr3) of spin electron density (in blue) for benzanthrone

had rich negative charges. However, there will be more significant steric hindrance effect when coupling at C(1) and C(11) position than at C(3) and C(4) positions. On the other hand, electron spin density is the main factor controlling the electropolymerization according to many authors.51-53 Therefore, the monomer radical cation was calculated at the B3LYP/ 6-31G(d,p) level for electron spin density,53 as listed in Table 4, and the isovalent surfaces of the electron spin density for benzanthrone monomer is observed in Scheme 2. The relatively higher atomic electron density can be seen at C(3), C(4), C(9), and C(11) positions, implying that the electropolymerization of benzanthrone probably occurs at these positions. Therefore, a reasonable conclusion can be easily drawn from all these theoretical results, that is, the polymerization between the monomer would happen preferentially on C(3), C(4), and C(11), well in accordance with FTIR results mentioned previously.

Figure 5. 1H NMR spectra of benzanthrone (A), dedoped polybenzanthrone prepared from the acetonitrile solution (B), and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (C). Solvent: d6-DMSO.

TABLE 5: 1H NMR Data of δH (ppm) for Benzanthrone and Dedoped Polybenzanthrone atom

benzanthrone

polybenzanthrone

H(1) H(2) H(3) H(4) H(5) H(6) H(8) H(9) H(10) H(11)

8.61 (d, 3JHH) 8.0 Hz) 7.81 (t, 3JHH) 15.2 Hz) 8.22 (d, 3JHH) 8.2 Hz) 8.34 (d, 3JHH) 7.2 Hz) 7.90 (t, 3JHH) 8.0 Hz) 8.75 (d, 3JHH) 8.4 Hz) 8.63 (d, 3JHH) 7.6 Hz) 7.64 (t, 3JHH) 15.2 Hz) 7.86 (t, 3JHH) 15.2 Hz) 8.46 (d, 3JHH) 8.0 Hz)

8.69 (d, 3JHH) 7.6 Hz) 7.79 (d, 3JHH) 8.0 Hz) 8.42 (d, 3JHH) 7.2 Hz) 7.94 (t, 3JHH) 12.8 Hz) 8.97 (d, 3JHH) 8.4 Hz) 8.75 (d, 3JHH) 8.0 Hz) 7.72 (t, 3JHH) 15.2 Hz) 7.85 (t, 3JHH) 15.2 Hz)

To further investigate the polymer structure and the polymerization mechanism of benzanthrone, the 1H NMR spectra of benzanthrone (Figure 5 A) and dedoped polybenzanthrone obtained from acetonitrile solution (Figure 5 B) and from the binary solvent system (Figure 5 C) were recorded in d6-DMSO. There were 10 group peaks in the spectra of the monomer as shown in Figure 5A. However, after polymerization, some peaks disappeared, and most of the peaks moved to lower fields (Figure 5B). Generally, with the introduction of longer conjugation length into the polymer main backbone, the chemical shifts of hydrogen atoms on the aromatic rings usually move to lower fields.28 Therefore, this phenomenon can be ascribed to the introduction of higher conjugation length in the polybenzanthrone main chain. As can be apparently seen from Figure 5, panels B and C, the proton chemical shifts of polybenzanthrone prepared from acetonitrile solution were approximately the same as those of polybenzanthrone prepared from the binary solvent system, also indicating the similar structure of the polymer obtained from the two different systems. The assignments of the 1H chemical shifts in the spectrum of both benzanthrone and dedoped polybenzanthrone are listed in Table 5. As shown in Figure 5A, the proton chemical shift at 8.22 ppm (d, 3JHH ) 8.2 Hz) can be ascribed to the proton at the C(3) position, and that at 8.46 ppm (d, 3JHH ) 8.0 Hz) can be ascribed to the proton at the C(11) position (Table 5). It can be clearly seen that they disappeared in the spectrum of polybenzanthrone (Figure 5B), indicating that the polymerization of benzanthrone mainly occurred at these two positions. On the other hand, the proton signals at 7.64 ppm (t, 3JHH ) 15.2 Hz) and at 8.34 ppm (d, 3 JHH ) 7.2 Hz) can be ascribed to the protons at C(9) and C(4) positions, which moved to 7.72 ppm (t, 3JHH ) 15.2 Hz) and 8.42 ppm (d, 3JHH ) 7.2 Hz) in the spectrum of the polymer,

Electrochemical Polymerization of Benzanthrone

J. Phys. Chem. B, Vol. 113, No. 1, 2009 43

SCHEME 3: Possible polymerization mechanism for benzanthrone

respectively. Under these circumstances, the polymerization through C(9) and C(4) positions was denied because the proton lines of these positions still existed in the spectrum of the polymer. The information as mentioned above indicated that a new C-C bond between two monomers via C(3) and C(11) positions was formed, well in accordance with FTIR and quantum chemistry calculation results. On the basis of FTIR, 1H NMR, and theoretical calculation results, it can reasonably be concluded that the electropolymerization of benzanthrone occurs mainly at C(3) and C(11) positions. To our surprise, the structure of the polymer is not similar to poly(para-phenylene) or polyacenaphthene.32 The possible polymerization mechanism of benzanthrone is illustrated in Scheme 3. The initiation step involves the anodic oxidation of monomers to A or B in the vicinity of the anode (Scheme 3). The radical cations then dimerize and deprotonate. After the deprotonation step, the dimer is reoxidized and couples with another radical cation (A or B). Deprotonation and reoxidation follow, and the process continues with the formation of oligomer species. Once the chain length of the oligomers exceeds the solubility limit of the solvent, precipitation occurs and nuclei deposit on the anode. Then the above steps proceed continuously. Finally, polybenzanthrone electrodeposits on the surface of the electrode. The rate of polymerization is limited by the diffusion of monomers to the anode surface during the propagation stage. The deprotonation steps in the above mechanisms are believed to produce an acidic environment in the region of the anode.54 3.4. UV-vis, MS, and Fluorescence Spectra. Polybenzanthrone films prepared from acetonitrile were in the doped state and dark brown in color. When they were dedoped by 25% ammonia, their color changed to brownish yellow. Whereas

Figure 6. UV-vis spectra of benzanthrone (A), dedoped polybenzanthrone obtained from acetonitrile solution (B) and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (C). Solvent: DMSO.

polybenzanthrone films from the binary solvent system were metallic dark in color, they changed to dark brown after dedoping. It is very interesting that both doped and dedoped polybenzanthrone films from the two systems are partly soluble in many common organic solvents, such as acetonitrile, DMSO, dichloromethane, tetrahydrofuran, chloroform, and even methanol, ethanol, acetone, diethyl ether, etc. This nice solubility of polybenzanthrone may facilitate its applications in various fields. The UV-visible spectra of benzanthrone and dedoped polybenzanthrone dissolved in DMSO were carefully examined, as shown in Figure 6. The monomer showed a characteristic absorption peak at 390 nm (Figure 6A), which was assigned to a single-electron π-π* transition of phenyl rings. The spectrum

44 J. Phys. Chem. B, Vol. 113, No. 1, 2009 of dedoped polybenzanthrone films obtained from acetonitrile (Figure 6B) showed a strong absorption band in the same region with a maximum shifted to 419 nm. The overall absorption of the polymer tailed off to about 526 nm in comparison with that of the monomer (455 nm), which is mainly due to the increase of the conjugated chain length. For comparison, the UV-visible spectrum of the polymer prepared from the binary solvent system was also included, as shown in Figure 6C. The maximum absorption shifted to 423 nm with the overall absorption of the polymer tailing off to about 658 nm characterized the spectrum. Generally, longer wavelength in spectra indicates longer polymer sequence.55 The fairly small red shift between the maximum absorption of dedoped polybenzanthrone and that of benzanthrone monomer indicated that the oxidation product has not a long enough polymer sequence. On the other hand, the band gap of solid polybenzanthrone calculated from the onset of the optical absorption spectra (Eg) was roughly 2.35 eV (Eg ) 1241/ λonset) from acetonitrile solution and was 1.89 eV from the binary solvent system.34 Note that the UV-vis spectra of doped polybenzanthrone films on the ITO electrode obtained in the binary solvent system at other applied potentials (1.8 V and 1.9 V vs SCE) were also performed, and similar results were obtained. Therefore, it can be reasonably concluded that polybenzanthrone films with more regioregularity were formed in the binary solvent system than those in acetonitrile, leading to longer absorption wavelength in the UV-vis spectra and smaller band gap. Besides, all these spectral results confirmed the occurrence of the electropolymerization among the monomers and the formation of conjugated polymers in both of the two systems. The composition of soluble polybenzanthrone films in DMSO was characterized by MALDI-TOF mass spectrometry, as shown in Figure 7. As can be seen, the MALDI-TOF mass spectra revealed that the soluble polymer from acetonitrile (Figure 7A) was mainly constituted of tetramer (m/z ) 903.744), octamer (m/z ) 1830.575), and dodecamer (m/z )2751.129), whereas that of polybenzanthrone from the binary solvent system (Figure 7B) was uniformly tetramer (m/z ) 904.088). These results indicated that the soluble polymers were relatively short-chain oligomers (4-12 repetitive units), well in accordance with UV-vis spectral results. However, it is also observed that soluble polybenzanthrone from acetonitrile has a longer polymer chain length than that from the binary solvent system, contrary to the UV-vis spectra. This phenomenon is probably attributable to the better stereoregularity and longer effective conjugated length of polybenzanthrone films prepared from the binary solvent system. Fluorescence spectra of the monomer and dedoped polymer in DMSO were recorded, as shown in Figure 8. For the monomer (Figure 8A), it can be seen that the emission spectrum emerged at 334 nm with a small shoulder (excited at 293 nm). In comparison, a maximum emission at 503 nm characterized the spectrum of dedoped polybenzanthrone from acetonitrile when excited at 432 nm (Figure 8B). The fluorescence spectrum of dedoped polybenzanthrone from the binary solvent system dissolved in DMSO is also given in Figure 8C. A dominant maximum at 521 nm can be clearly seen in the emission spectrum when excited at 429 nm (Figure 8C). Very large red shifts between the monomer and the polymer (about 169 nm for polybenzanthrone from acetonitrile and 187 nm for polybenzanthrone from the binary solvent system) can be clearly seen from the figure, which is mainly attributable to the elongation of the polymer’s delocalized π-electron chain sequence. In addition, it is worth mentioning that the emission

Lu et al.

Figure 7. MALDI-TOF mass spectra of soluble polybenzanthrone film prepared from acetonitrile solution (A) and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (B). Solvent: DMSO.

Figure 8. Emission spectra of benzanthrone (A), dedoped polybenzanthrone electrochemically synthesized in acetonitrile solution (B) and in the binary solvent system of acetonitrile and BFEE (2:1, by volume) (C). Solvent: DMSO.

peak in the fluorescence spectra of the doped and dedoped polybenzanthrone from both of the two systems emerged at the same wavelength and appeared in the same shape. All the above results show that polybenzanthrone from both of the two systems is a typical green-light-emitter, implying some potential applications in organic optoelectronics. According to eq 1, the fluorescence quantum yield (φoverall) of benzanthrone monomer was calculated to be only 0.03, whereas that of the soluble doped and dedoped polybenzanthrone films prepared from acetonitrile were 0.15 and 0.16,

Electrochemical Polymerization of Benzanthrone

Figure 9. Photoluminescence of benzanthrone (panels a and e), soluble polybenzanthrone obtained potentiostatically from acetonitrile solution (b: doped; c: dedoped) and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (f: doped; g: dedoped) under UV light irradiation of 365 nm. Solvent: DMSO.

respectively, or 4 times more than that of the monomer. More exhilaratingly, φoverall of the doped and dedoped polybenzanthrone films prepared from the binary solvent system were determined to be 0.49 and 0.52, respectively, or 15 times more than the monomer, and more than 3 times as high as those of polybenzanthrone from actonitrile. The large distance between polybenzanthrone from the binary solvent system and from acetonitrile is mainly attributable to stereoregularity of the polymer backbone described previously. The better regioregularity and more uniform polymer structure of polybenzanthrone films from the binary solvent system, which reduces the fluorescence quenching among the polymer matrix, made them exhibits much better fluorescence properties. In short, these shocking figures demonstrated that polybenzanthrone from the binary sovent system possessed excellent fluorescence properties. Meanwhile, it is also very interesting to find that soluble polybenzanthrone dissolved in common organic solvents, such as methanol, ethanol, acetone, acetonitrile, diethyl ether, DMSO, etc., can all emit strong photoluminescence when exposed to 365 nm UV light. Figure 9 shows the photoluminescence properties of soluble polymer when exposed to 365 nm UV light. As shown in Figure 9, soluble polybenzanthrone from acetonitrile in both doped (Figure 9b) and dedoped (Figure 9c) states exhibited a bright green light-emitting property, whereas the monomer showed no emissions (Figure 9, panels a and e). Also, polybenzanthrone from the binary system in both doped (Figure 9f) and dedoped (Figure 9g) states exhibited a stronger yellowgreen-light-emitting property. It should be noted here that the doped polybenzanthrone from both of the two systems showed slight weaker emission light than the dedoped polybenzanthrone, mainly because the dopants in the doped polybenzanthrone led to the fluorescence quenching among the polymer matrix. On the basis of these striking fluorescent results, as excellent greenlight-emitting polymers, the polybenzanthrone films may find many applications in various fields, such as optoelectronics, and are a very promising candidate for hole-transporting materials in LEDs. It can be expected that polybenzanthrone films are probably operative as active layers in organic light-emitting diodes (OLEDs), owing to their high yield of fluorescence. 3.5. Thermal Analysis. The thermal stability of conducting polymers is very important for their potential applications. It is known that the skeletal decomposition temperatures of conducting polymers are usually low, generally reported to be lower

J. Phys. Chem. B, Vol. 113, No. 1, 2009 45

Figure 10. TG (A) and DTG (B) curves of dedoped polybenzanthrone films obtained potentiostatically from the binary solvent system of acetonitrile and BFEE (2:1, by volume).

than 600 K, which hinder the practical uses of these polymers in various fields. To investigate the thermal stability of polybenzanthrone films, thermogravimetric and differential thermogravimetric analytical experiments were performed under a nitrogen stream at a heating rate of 10 K min-1. Figure 10 shows the TG (A) and DTG (B) curves of dedoped polybenzanthrone films obtained from the binary solvent system. At low temperatures (T < 502 K), the polymer initially underwent a small weight decrease of about 1.63%, which may be attributed to water evaporation or other moisture trapped in the polymer according to many authors.56 With the gradual increasing of the temperature, a more pronounced weight loss step (about 2.95%) was observed for 502 K < T < 640 K, which may be caused by the volatilization of a few oligomers trapped in the films. From 640 to 905 K, the polymer proceeded to a rapid weight loss of 29.33%. Simultaneously, from the DTG curve (Figure 10 B), the corresponding maximal decomposition existed at 729.5 and 801.1 K. Such a prominent weight loss was closely related to the oxidizing decomposition of the skeletal polybenzanthrone backbone chain structure. The following degradation after 905 K was probably caused by the overflow of the oligomers decomposed from polybenzanthrone mentioned previously. In addition, even when the temperature reached 1100 K, the residual weight of polybenzanthrone films were 62.45%, also indicating the high thermal stability of polybenzanthrone films. Similar results were found for polybenzanthrone films prepared from acetonitrile, as shown in Supporting Information Figure 4. All these figures indicated sufficiently that polybenzanthrone has higher thermal stability than most of the conducting polymers reported previously,28-32,42,48,57-60 which can be mainly ascribed to the high thermal stability of benzene ring incorporation into the main chain of as-formed polybenzanthrone films. The high decomposition temperature of the polymer films indicates they can be applied in a wide temperature scale. This property is of special significance for some potential applications such as the candicates of nuclear reactors, where high thermal stability is essential. 3.6. Conductivity and Morphology. The electrical conductivity of the pressed pellet of doped polybenzanthrone films obtained from acetonitrile was measured to be as high as 1.6 S cm-1 by the conventional four-probe technique, whereas that of doped polybenzanthrone films obtained from the binary solvent system was as high as 2.2 S cm-1, both much higher than that of other conducting polymers with good fluorescence properties, such as polyfluorene (2.5 × 10-1 S cm-1),48

46 J. Phys. Chem. B, Vol. 113, No. 1, 2009

Lu et al.

Figure 11. SEM images of polybenzanthrone films deposited electrochemically on the ITO electrode for 400 s from acetonitrile solution (A-D) and from the binary solvent system of acetonitrile and BFEE (2:1, by volume) (E-H). A, B, E, F: doped polybenzanthrone; C, D, G, H: dedoped polybenzanthrone.

polycarbazole (7.5 × 10-3 S cm-1),57 oligopyrene (10-1 S cm-1),42,58 polyanthracene (10-1 S cm-1),59 polyphenanthrene (10-1 S cm-1),60 and some other conducting polymers.28-32 This good semiconducting property will facilitate their applications and, together with other excellent properties described above, will make polybenzanthrone very important in its use as active layers in light-emitting diodes, which is one of the most promising applications of semiconducting conjugated polymers. The surface morphology of polybenzanthrone films deposited on the ITO electrodes from both of the two systems was observed by scanning electron microscope (SEM), as shown in Figure 10. Macroscopically, in both the doped and the dedoped states, polybenzanthrone films from both of the two solvent

systems appeared to be smooth, homogeneous, and compact. Microscopically, even at high magnifications, the surfaces of the doped polymer films obtained from both acetonitrile solution (Figure 11, panels A and B) and the binary solvent system (Figure 11, panels E and F) are still rather smooth, homogeneous, and continuous. The smooth and homogeneous structures of compact polybenzanthrone films were extremely beneficial to improve their electrical conductivity and electron transfer capability and also make them good candidates for applications in ion-selective electrodes, ion-sieving films, and matrices for hosting catalyst particles, etc.61-63 However, after dedoping at a constant applied potential of -0.4 V, the surface of the polybenzanthrone film from acetonitrile became uneven and

Electrochemical Polymerization of Benzanthrone appeared as an irregular arrangement of atactic microrods with diameter in the level of micron but not long enough (Figure 11, panels C and D), quite different from that of the doped polymer. Similar results were found from the films deposited from the binary solvent system. After dedoping, polybenzanthrone films from the binary solvent system were still very smooth but, microscopically, appeared to be uneven at a magnification of 10 000-20 000 (Figure 11, panels G and H) compared with the doped polymer. These differences between the doped and dedoped polybenzanthrone from both of the two systems were mainly due to the migration of counteranion out of the polymer film and the gradual solubility of polybenzanthrone from the electrode to the solution during the dedoping proesses, which broke the smooth surface of doped polybenzanthrone films. 4. Conclusions High quality green-light-emitting polybenzanthrone with relatively high electrical conductivity as 1.6∼2.2 S cm-1 and excellent thermal stability was successfully electrodeposited by direct anodic oxidation of its monomer benzanthrone in both acetonitrile containing 0.1 mol L-1 of Bu4NBF4 and the binary solvent system consisting of actonitrile and additional BFEE. Polybenzanthrone films obtained from both mediums showed good redox activity and structural stability even in concentrated sulfuric acid. As-prepared polybenzanthrone films also showed excellent fluorescence properties, good thermal stability, and nice solubility in common organic solvents. UV-vis, FTIR, and 1H NMR spectral analyses and MALDI-TOF MS results, together with quantum chemistry calculations, determined that the electrochemical polymerization of benzanthrone mainly occurred via the coupling at C(3) and C(11) positions. Moreover, the fluorescence properties of polybenzanthrone were greatly improved in comparison with those of the monomer. Both doped and dedoped polybenzanthrone were excellent strong green or yellow-green photoluminescence at excitation of 365 nm UV light with high fluorescence quantum yields. Besides, SEM results indicated that polybenzanthrone films demonstrated smooth homogeneous surfaces even at high magnifications. With so many attractive properties, we believe, a wide range applications of polybenzanthrone and its derivatives can probably be achieved. For example, the soluble polybenzanthrone, due to their intense fluorescence, high thermal stability, and selfcolored properties, will be a good candidate for luminophore in liquid crystalline systems. Acknowledgment. NSFC (50663001, 50503009), the key scientificprojectfromMinistryofEducation,China(2007-207058), Natural Science Foundation of Jiangxi province (2007GZH1091), and Jiangxi Provincial Department of Education (GJJ08369) are acknowledged for their financial support. Supporting Information Available: Anodic polarization curves and cyclic voltammograms of benzanthrone in BFEE. Cyclic voltammograms of polybenzanthrone films in different mediums as indicated. Thermogravimetric and differential thermogravimetric curves of dedoped polybenzanthrone. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Phys. ReV. Lett. 1977, 39, 1098–1101.

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