Polyfluorene Derivatives with Hydroxyl and Carboxyl Substitution

May 8, 2009 - Fax: 86 −791-3823357., †. Jiangxi Science and Technology Normal University. , ‡. Chengdu University of Technology. Cite this:J. Ph...
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J. Phys. Chem. C 2009, 113, 9900–9910

Polyfluorene Derivatives with Hydroxyl and Carboxyl Substitution: Electrosynthesis and Characterization Changli Fan,†,‡ Jingkun Xu,*,† Wen Chen,‡ Baoyang Lu,† Huaming Miao,† Congcong Liu,† and Guodong Liu† Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal UniVersity, Nanchang 330013, China, and College of Materials, Chemistry and Chemical Engineering, Chengdu UniVersity of Technology, Chengdu 610059, China ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: April 18, 2009

Novel high-quality poly(9-hydroxyfluorene) (PHF), poly(9-fluorenecarboxylic acid) (PFCA), and poly(9hydroxyl-9-fluorenecarboxylic acid) (PHFCA) films can be easily electrodeposited by low-potential anodic oxidation of fluorene derivatives, which contain either the electron-withdrawing carboxyl group (-COOH) or the electron-donating hydroxyl group (-OH), using a midstrength Lewis acid boron trifluoride diethyl etherate (BFEE) as the solvent and supporting electrolyte. The complexing reaction between the -COOH or -OH groups at the C(9) position of these fluorene derivatives and BFEE increased the ionic conductivity of the BFEE system as an electrolyte significantly. The 13C NMR spectra clearly demonstrates that the chemical shift of carbons at the C(9) and C(14) positions has made a low-field shift. This change in the chemical shift of the carbon atoms confirms the formation of complexion cations when FCA or HFCA was mixed with BFEE. The as-formed polymer films showed good redox behavior and thermal stability. FTIR, 1H NMR, and theoretical investigations indicated that the polymerization of HF, FCA, and HFCA monomers occurred mainly at the C(2) and C(7) positions. The fluorescence properties of the polymers were greatly improved in comparison with those of the monomers, implying that those polymers were good blue light emitters. SEM results indicated that PHF film is a nanomaterial. 1. Introduction Polyfluorenes (PFs) as π-conjugated polymers are among the blue light-emitting polymers that have been widely investigated because of their large photoluminescence (PL) and electroluminescence (EL) quantum efficiencies and excellent chemical and thermal stability.1 Since 1997, papers focused on PFs have been greatly increased.1–6 In an attempt to tune the physical properties of PFs by molecular structure modification, one problem may be the possibility that remote functionalization occurs at the C(9) position. Modification of the chemical structure by substitution of the fluorene unit has enabled synthesis of a large number of derivatives, resulting in numerous polymers with different degrees of stability, conductivity, solubility, and band gap.1,4,5,7 PFs were mainly synthesized by chemical approaches via Suzuki, Yamamoto, and Stille reactions1,8 and by electrochemical oxidations.9–13 As a synthetic method compared with chemical polymerization, there are several advantages of electrochemical polymerization such as rapid analyses, accuracy, precision, and requiring small amounts of material.14,15 However, monomers with pendant groups can also be electrochemically polymerized. In electrochemical polymerization processes, the choice of solvents that decides the properties of polymers is very important. According to the literature,16,17 most PFs, which were electrosynthesized in common organic solvents such as CH2Cl2 and CH3CN do not display satisfactory properties. They are * Corresponding author. E-mails: [email protected] or [email protected]. Telephone: 86-791-3805183. Fax: 86 -7913823357. † Jiangxi Science and Technology Normal University. ‡ Chengdu University of Technology.

brittle, insoluble, intractable, and often decompose before melting. The main reason for the unsatisfactory properties is that the potential applied for polymerization is much higher than the reversible redox potential of the PFs, resulting in destruction of the conjugated structure of the polymers and degradation of their properties.15 Recently, our research group reported that the quality of PF films can be improved using boron trifluoride diethyl etherate (BFEE) as the solvent and supporting electrolyte because of its catalytic effect.18–20 Moreover, BFEE has been found to be an excellent electrolyte for the electrochemical polymerization of other compounds such as benzene and thiophene,21–25 especially for fused-ring compounds such as indole,26 carbazole,27 naphthalene,24 and their derivatives.28–39 High-quality conjugated polymer films can be obtained from this electrolyte. In addition, high-quality PFs can be obtained that substitute with functional groups such as amino,40 halogen,41,42 carboxyl,43,44 and carbonyl groups20,45 at the C(9) position (Scheme 1). However, there was no report on PFs substituted with a hydroxyl group or with both a carboxyl group and a hydroxyl group. There are several advantages for the substitution with groups such as carboxyl and hydroxyl on the conjugated backbone of PFs. First, the presence of pendant lyophilic groups such as hydroxyl groups and carboxyl groups would improve the solubility of the formed polymer, which is beneficial to the structural characterization of this polymer and would facilitate its applications because most conducting polymers are typically inherently insoluble and intractable.11 Second, conducting polymers substituted with a carboxyl group obtained by electropolymerization can afford higher electrode stability and reproducibility when compared with those of unsubstituted polymers, which can be used to prepare the glucose oxidase-

10.1021/jp900323w CCC: $40.75  2009 American Chemical Society Published on Web 05/08/2009

PF Derivatives with OH and COOH Substitution immobilized glucose sensors.46 Finally, the polymer film has a self-doping nature and such a self-doping polymer possessing an anionic dopant ion covalently attached to the polymer backbone forces predominant cation movement during the doping and dedoping processes.47,48 However, it has been reported that carboxyl groups and hydroxyl groups make the formation of polymer films very difficult.49 For these reasons, the preparation of high-quality poly(9-hydroxylfluorene) (PHF), poly(9-fluorenecarboxylic acid) (PFCA), and poly(9-hydroxylfluorene-9-carboxylic acid) (PHFCA) is very interesting and significant but still a great challenge. In this study, the electropolymerization of 9-hydroxylfluorene (HF), 9-fluorenecarboxylic acid (FCA), and 9-hydroxyl-fluorene9-carboxylic acid (HFCA) was successfully achieved, and highquality PHF, PFCA, and PHFCA films were easily electrodeposited. The electrochemical properties, spectroscopic properties, thermal stability, morphology, and electrical conductivity of the as-prepared PHF, PFCA, and PHFCA films were studied in detail. Their structures and polymerization mechanisms were also investigated using ultraviolet-visible (UV-vis), Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (NMR) spectroscopy, and quantum chemistry calculations. 2. Experimental Section 2.1. Reagents and Treatment. 9-Hydroxy fluorene (HF), 9-fluorenecarboxylic acid (FCA), and 9-hydroxy-9-fluorenecarboxylic acid (HFCA) were bought from Acros Organics and were used directly. Boron trifluoride diethyl etherate (BFEE) with a BF3 content of 48.24% and a water content of 0.24% (by volume) was purchased from Beijing Changyang Chemical Plant, which was distilled and stored at 0 °C before use. Commercial high-performance liquid chromatography (HPLC) grade acetonitrile (CH3CN, East Longshun Chemical Plant, Beijing) was used directly without further purification. LiClO4 and Bu4NBF4 (95%, Acros Organics) were dried under vacuum at 60 °C for 24 h before use. Dichloromethane (CH2Cl2), chloroform (CHCl3), and dimethyl sulfoxide (DMSO) were purchased from East Longshun Chemical Plant (analytical grade) and used directly. Trifluoroacetic acid (TFA, 99%, Nankai University Fine Chemical Laboratories, Tianjin) was used directly. 2.2. Electrosynthesis of PHF, PFCA, and PHFCA Films. Electrochemical polymerization and examinations of HF, FCA, and HFCA were performed in a one-compartment cell with the use of a Model 263 A potentiostat-galvanostat (EG&G Princeton Applied Research) under computer control at room temperature. The working and counter electrodes for cyclic voltammetric experiments were platinum wire and stainless steel wire with diameters of 0.5 mm and 1 mm, respectively, which were placed 0.5 cm apart. To obtain a sufficient amount of polymer for characterization, we employed an indium tin oxide (ITO) sheet with surface area of 1 cm2 as the working electrode. A stainless steel sheet with a surface area of 1.5 cm2 was used as the counter electrode. All potentials were referred to a saturated calomel electrode (SCE). Electrochemical deposition procedures for PHF and PFCA were similar to those of PHFCA, except for the typical electrolytic solutions. PHF was obtained from distilled CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE (by volume) containing 0.03 mol L-1 HF. PFCA was prepared from distilled BFEE and BFEE + 30% TFA, containing 0.02 mol L-1 FCA. PHFCA was obtained from distilled BFEE containing 0.01 mol L-1 HFCA. During the experiment, all solutions were deaerated by a dry argon stream and maintained at a slight argon overpressure.

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9901 SCHEME 1: Chemical Structures of Fluorene and Its Derivatives

The amount of the polymer film deposited on the electrode was controlled by the integrated current passed through the cell. In order to remove the electrolyte, oligomers, and monomer, the polymer films were rinsed with anhydrous diethyl ether, and then they were dried at 60 °C for 3 h. 2.3. Characterization. The ionic conductivity of the electrolyte system was measured by using a REX model DDS-307 ionic conductivity meter. The electrical conductivity of polymer films was measured on a pressed pellet of the polymer by the conventional four-probe technique. Infrared (IR) spectra were recorded by using KBr pellets on the Bruker Vertex 70 FTIR spectrometer. UV-vis spectra were obtained by using a PerkinElmer Lamda 900 UV-vis-near-infrared (NIR) spectrophotometer. The fluorescence spectra were obtained with a F-4500 fluorescence spectrophotometer (Hitachi). 1H NMR spectra and 13C NMR spectra were recorded on a Bruker AV 400 NMR spectrometer. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were performed with a thermal analyzer Netzsch TG209. Scanning electron microscopic (SEM) measurements were performed using a Hitachi S-4000N scanning electron microscope. 3. Results and Discussion 3.1. Monomer Solubility and Ionic Conductivity of the Mixed Electrolytes. HF can hardly be dissolved in BFEE but can be thoroughly dissolved in many common organic solvents, for example: CH2Cl2 and CHCl3. FCA and HFCA can be dissolved in BFEE and many common organic solvents. However, it is limited for the dissolution of monomers in above solvents. For this reason, the electrolyte system containing CH2Cl2 or CHCl3 as solvent and 5% BFEE as supporting electrolyte was chosen for the polymerization of HF. For the polymerization of FCA and HFCA, BFEE was chosen as the solvent and supporting electrolyte. However, the largest solubility of HF monomer is 0.2 mol L-1 in CH2Cl2 + 5% BFEE and 0.12 mol L-1 in CHCl3 + 5% BFEE. While the maximum values of FCA and HFCA monomer dissolving in BFEE are 0.15 mol L-1 and 0.3 mol L-1, respectively. The choice of electrolytes is listed in S-table 1 of the Supporting Information. The literature reported that alcohol and acid can interact with BFEE, which led to a higher ionic conductivity of the mixed electrolyte than that of BFEE (Scheme 2),50–52 where HF contains a -OH group, FCA contains a -COOH group, and HFCA contains a -OH group and a -COOH group at one time. They should interact with BFEE because of the above reasons. Therefore, we investigated the effect of HF (a,b), FCA (c), HFCA (d) concentrations on the ionic conductivity (σ) of BFEE-

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Fan et al.

SCHEME 2: Proposed Complexion Mechanisms of FCA and HFCA and BFEE

based electrolyte systems (Figure 1), the 1H NMR spectra of mixed electrolyte systems containing FCA (a) and HFCA and BFEE (b) (Figure 2), and the 13C NMR spectra of the FCA or HFCA and BFEE. The CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE solutions were colorless, and the color of both solutions was pink when a small amount of HF monomer (0.01 mol L-1) was added. But the color of solution became darker (from pink to blue violet) with increasing HF concentration. Usually, BFEE was colorless. After adding FCA or HFCA monomers, the color of the mixed electrolyte solutions became slighly tea brown and then a deeper and deeper brown. In addition, the ionic conductivity of BFEE-based electrolyte systems increased rapidly (Figure 1a-d) when small amounts of monomers were

Figure 1. Effective concentration of HF (a,b), FCA (c), and HFCA (d) on the ionic conductivity (σ) of BFEE-based electrolyte systems. Electrolyte systems are CH2Cl2 + 5% BFEE (a), CHCl3 + 5% BFEE (b) (by volume), and BFEE (c,d).

introduced into the mixed electrolyte. Finally, the ionic conductivities of the mixed electrolytes became almost stable when the concentration of these monomers increased to their maximum solubility. These results implied that HF, FCA, and HFCA can interact with BFEE. The proposed complexion mechanism of HF, FCA, and HFCA and BFEE are shown in Scheme 2. Some complexes (Scheme 2A-C2) were generated due to the complexion of HF,

Figure 2. 1H NMR spectra of the mixed electrolyte containing FCA (a) and HFCA and BFEE (b). Solvent is CDCl3.

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FCA, or HFCA and BFEE.51,53–55 After that, a large number of anions (Scheme 2a-d) and protons were generated in the mixed electrolytes. Because the proton cannot exist alone, cations were formed (Scheme 2e-h). A large number of anions and cations resulted in significant increases in the ionic conductivity of the mixed electrolyte systems (Figure 1). Generally, by increasing the number of conductive ions in solution, the ionic conductivity of the electrolyte solution will increase. When the concentration of conductive ions increases to a certain degree, the Coulomb force between anions and cations increases, which could impede the movement of ions, resulting in stabilizing or declining ion conductivity (Figure 1). In order to further explore the complexion mechanism of FCA, and HFCA and BFEE, the 1H NMR spectra of the mixed electrolyte systems containing FCA (a) and HFCA and BFEE (b) are shown in Figure 2. There were two different types of protons in the mixed electrolyte system, H+ of the monomer and the conjugate acid proton (Scheme 2e-h). The chemical shift has a different value. But in the 1H NMR spectra, they could not be independently observed because of the exchange among themselves. Only one peak was observed according to the well-known method based on eq 1

δ ) δAxA + δBxB

(1)

Here, δ is the observed proton chemical shift value, δA and δB are the chemical shift values of different types, and xA and xB are the mole fraction of the corresponding type protons. As shown in Figure 2, with an increase in FCA and the HFCA monomer dissolving in BFEE, the chemical shift of the proton made a low-field shift, from 10.24 to 10.78 ppm (Figure 2a) and from 10.23 to 10.66 ppm (Figure 2b), which implied the concentration of cations (Scheme 2e-h) also was rising. In order to further explore the structure of the cations, the 13 C NMR spectra of the FCA monomer and BFEE containing 0.15 mol L-1 FCA were shown in Figure 3a, and the 13C NMR spectra of the HFCA monomer and BFEE containing 0.25 mol L-1 HFCA were shown in Figure 3b. It is clearly seen that the chemical shift of carbons at the C(9) and C(14) positions (Scheme 1) made a low-field shift. This change in the chemical shift of carbon atoms confirms the formation of complexion cations when FCA or HFCA was mixed with BFEE. These results indicate a further complex reaction between BFEE and three monomers, which may be helpful for the electrochemical polymerization of these monomers. 3.2. Electrochemical Polymerization of PHF, PFCA, and PHFCA Film. The electrochemical oxidation of HF, FCA, and HFCA was investigated, along with their onset oxidation potentials in different electrolyte systems (S-Table 1 of the Supporting Information). It was found that the onset oxidation potential of HF in CH2Cl2 or CHCl3 + 5% BFEE (by volume) was much lower than that of HF in other electrolyte systems. The oxidation onset of FCA was initiated at 1.44 V in BFEE, which was much lower than that in CH3CN + 0.1 mol L-1 Bu4NBF4 (1.71 V). Meanwhile, the addition of TFA into BFEE resulted in a decrease in the onset oxidation potential of FCA, which was beneficial to reduce the overoxidation effect, thus making high quality PFCA film possible. The oxidation onset of HFCA was initiated at 1.53 V in BFEE, which was also relatively lower than that of HFCA in CH3CN + 0.1 mol L-1 Bu4NBF4 (1.71 V). Therefore, CH2Cl2 or CHCl3 + 5% BFEE, BFEE and BFEE + 30% TFA, and BFEE were the best choice for electrosynthesis of PH, PFCA, and PFCA films, respectively. Cyclic voltammetry (CV) is a very useful method that qualitatively reveals the reversibility of electron transfer during

Figure 3. 13C NMR spectra of monomers and mixed electrolytecontaining monomers. (a) FCA monomer and BFEE containing 0.15 mol L-1 FCA. (b) HFCA monomer and BFEE containing 0.25 mol L-1 HFCA.

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, i.e., CV diagram. Successive cyclic voltammetrys (CVs) of 0.03 mol L-1 HF in CH2Cl2 + 5% BFEE (a) and CHCl3 + 5% BFEE (b) on a Pt electrode are shown in Figure 4. As shown in Figure 4, CVs of HF showed characteristic features similar to other conducting polymers such as polypyrrole and polythiophene during potentiodynamic synthesis. As the CV scan continued, the color of the solution close to the working electrode changed slightly from maple to dark brown, which indicated that part of the monomer was oxidized into oligomers, which were dissolved or dispersed into the solvent. Simultaneously, a polymer film was also formed on the working electrode surface. As shown in Figure 4a, PHF can be reduced and oxidized between 0.4 and 1.2 V. Otherwise, as shown in Figure 4b, the potential of redox was very high, which can be reduced and oxidized between 0.5 and 1.4 V. On the basis of these phenomena, it is possible that PHF films can be easily prepared from CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE. Nie et al. illustrated successive CVs of 0.02 mol L-1 FCA in different mixed electrolytes (BFEE and BFEE + 30% TFA) on a Pt electrode.44 PFCA can be reduced and oxidized between 0.68 and 1.61 V in BFEE. In addition, PFCA can be reduced and oxidized between 0.67 and 1.35 V in BFEE + 30% TFA. High-quality PFCA films were formed on the working electrode. Similarly, successive CVs of 0.01 mol L-1 HFCA in BFEE are shown in Figure 4c. PHFCA can be reduced and oxidized between 0.88 and 1.51 V. The broad redox waves of as-formed

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Figure 4. Cyclic voltammograms of 0.03 mol L-1 HF in CH2Cl2 + 5% BFEE (a), CHCl3 + 5% BFEE (by volume) (b), and 0.01 mol L-1 HFCA in BFEE (c) on the Pt electrode. Potential scan rate is 50 mV s-1.

PHFCA films may be ascribable to the wide distribution of the polymer chain length56 or the conversion of conductive species on a polymer main chain from the neutral state to polarons, from polarons to bipolarons, and finally from bipolarons to the metallic state.57 Two pairs of reduced peaks and oxidized peaks were also found. The first redox reaction at the higher potential can be ascribed to the redox reaction of the oligomer, which leads to the change in color of the electrolyte solution from achromatism to salmon pink.58 The second redox reaction at the lower potential can be ascribed to the redox reaction of the polymer, which is insoluble in solution.59 All of these phenomena indicate a high-quality conducting PHFCA film was formed on the working electrode. In addition, the range in which PFs were reduced and oxidized was broad in the successive CVs of fluorene derivates in BFEEbased electrolyte systems, and there were obvious nucleation processes.18–20,32,40–42,44 3.3. Electrochemical Behavior of PHF, PFCA, and PHFCA Film. PHFs were obtained from distilled CH2Cl2 + 5% BFEE (by volume) and CHCl3 + 5% BFEE (by volume) containing 0.03 mol L-1 HF. PFCAs were prepared from distilled BFEE and BFEE + 30% TFA containing 0.02 mol L-1 FCA. PHFCAs were obtained from distilled BFEE containing 0.01 mol L-1 HFCA. The electrochemical behavior of the PHF films deposited electrochemically from CH2Cl2 + 5% BFEE was studied in monomer-free CH2Cl2 + 0.1 mol L-1 Bu4NBF4 (S-Figure 1a of the Supporting Information) and CHCl3 + 0.1 mol L-1 Bu4NBF4 (S-Figure 1b of the Supporting Information). PHF films deposited electrochemically from CHCl3 + 5% BFEE showed good redox activity in CHCl3 + 0.1 mol L-1 Bu4NBF4 (S-Figure 1c of the Supporting Information). Similar to the results of other conducting polymers such as polypyrrole and polythiophene the steady-state CVs repre-

Fan et al. sented broad anodic and cathodic peaks. Furthermore, these films could be cycled repeatedly between the conducting (oxidized) and insulating (neutral) states without significant decomposition of the materials, indicating the high stability of the PH film. In CHCl3 + 0.1 mol L-1 Bu4NBF4, the anodic and cathodic potential range was much broader for the PHF film prepared in CHCl3 + 5% BFEE (by volume) (Figure 5c) than those produced in CH2Cl2 + 5% BFEE (by volume) (Figure 5b). The redox potentials showed a slightly negative shift for PHF films prepared in CHCl3 + 5% BFEE than those prepared in CH2Cl2 + 5% BFEE. Similarly, the electrochemical behavior of PHFCA films was studied in monomer-free BFEE (Figure 5a) and in concentrated sulfuric acid (Figure 5b) at different potential scan rates. PHFCA films can be cycled repeatedly between conducting and insulating states without significant decomposition, which indicated their good electrochemical and structural stability. According to Figure 4a, PHFCA films can be oxidized and reduced between 0.50 and 1.70 V in monomer-free BFEE. The redox potential window of PHFCA in concentrated sulfuric acid is between 0.59 and 1.19 V (Figure 5b). The difference (Epa - Epc) related to the kinetics of the doping-dedoping reaction was equal to 0.78 V in monomer-free BFEE and 0.31 V in concentrated sulfuric acid. Otherwise, Nie et al. reported that PFCA films deposited from a BFEE + 30% TFA solution had good electrochemical and environmental stabilities in monomer-free BFEE and in concentrated sulfuric acid.44 From these results, it can be reasonably concluded that the doping-dedoping reaction of PFCA and PHFCA in concentrated sulfuric acid was faster than that in monomer-free BFEE, which may be attributed to the different doping anions and the protonation of polymer during the process of successive CV scans. The doping anions (HSO4or SO42-) in concentrated sulfuric acid move more easily into and out of the polymer film than [BF3OC2H5]-, the conducting anions in BFEE. As shown in Figure 5a, two electrochemical redox peaks (Q1,R1) and (Q2,R2) were present. The existence of -COOH or -OH groups complexes BF3 into [RCOOBF3]- H+ or [ROBF3]H+, which furnishes a conducting medium and leads to mild acidity. On the basis of this discussion, at the higher potential, the peaks were related to protonic and electronic exchanges between the polymer and electrolytic solution, which included the self-doping of PHFCA and exchange of the cation between the oxidized and reduced forms of the polymer. The first redox reaction at the lower potential can be ascribed to the electron exchanges, which were the charge-discharge characteristics of PHFCA. However, the CVs show a pronounced hysteresis, i.e., a considerable difference between the anodic and cathodic peak potentials. The peak potential shift of the polymer CVs is hardly explained by conventional kinetic limitations such as ion diffusion or interfacial charge transfer processes. The main reasons accounting for this phenomenon are usually slow heterogeneous electron transfer, effects of local rearrangements of polymer chains, slow mutual transformations of various electronic species, and the electronic charging of a sum of two interfacial exchanges corresponding to the metal-polymer and polymer-solution interfaces, etc. Furthermore, two electrochemical redox peaks (M1, N1) and (M2, N2) associated with the PHFCA films were also present in concentrated sulfuric acid (Figure 5b). However, the redox potentials of the polymer were stable in concentrated sulfuric acid and were lower than those in BFEE. On the basis of these discussions, it can be reasonably concluded that the doping-dedoping reaction of PHFCA was

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Figure 5. Cyclic voltammograms of PHFCA films in monomer-free BFEE (a) and in concentrated sulfuric acid (b) at different potential scan rates from 50-300 mV s-1.

very fast, which also indicated the formation of high-quality PHFCA films. According to the above results and literature, PFs prepared from BFEE-based electrolyte systems showed good redox activity and nice structural stability.18–20,32,40–42,44 3.4. Structural Characterization. As-prepared PHF films from CH2Cl2 + 5% BFEE or CHCl3 + 5% BFEE were in a doped state and metallic dark in color. Both doped PHF films were partly soluble in strong polar solvent DMSO but not in general organic solvents. During potentiostatic deposition of PHFCA in BFEE, we formed polymer films on the stainless steel electrode surface (from filemot to henna). The electrolyte solution was changed gradually from colorless to yellow brown due to the part solution of oligomers during the process of electropolymerization. PHFCA could be thoroughly dissolved in DMSO and partly dissolved in CH3CN and acetone but could not be dissolved in CH2Cl2 and CHCl3. Moreover, PFCA film was brown in the doped state and dark green in the dedoped state, and its solubility was poor.44 Therefore, the UV-vis, FTIR, and fluorescence spectra of PHF, PFCA, and PHFCA films were determined in the solid state. The UV-vis spectra of the HF and HFCA monomers and doped PHF and PHFCA films deposited onto the ITO electrode were examined carefully (Figure 6). The spectrum of the HF monomer shows characteristic absorption at 288, 296, and 307 nm (Figure 6a, insert), and the UV-vis spectrum of the HFCA monomer shows absorptions at 262, 277, and 310 nm (Figure 6b, insert). Moreover, all of these corresponding polymers had strong absorption assigned to the πfπ* interband transition, i.e., from 299 to 614 nm (Figure 6c), 299 to 600 nm (Figure 6d), and 300 to 664 nm (Figure 6e), due to the wide molar mass distribution of these polymers. In comparison with those of the monomers, these polymers also showed obvious red shifts. The doped PHF also had a strong absorption centered at 660 nm (Figure 6c) or centered at 540 nm (Figure 6d), which is due to the conductive species on the polymer main chain.

Figure 6. UV-vis spectra of HF (a) and HFCA (b) in DMSO and the solid-state UV-vis spectra of the PHF films prepared from CH2Cl2 + 5% BFEE (c) and CHCl3 + 5% BFEE (d) and the PHFCA films (e) prepared from BFEE on the ITO electrode.

Simultaneously, the new broad band from 614 to 796 nm (Figure 6c) was the result of conducting species such as polarons or bipolarons in the doped state. Additionally, these results show a slightly blue shift for PHF films prepared in CH2Cl2 + 5% BFEE compared with those prepared in CHCl3 + 5% BFEE, well accordance with the results of the electrochemistry of the PHF films. In Figure 6e, the doped PHFCA films showed a broad band centered at 1050 nm, which indicated the formation of bipolarons. The spectra of the doped polyfluorene showed a much broader absorption from 300 to about 425 nm.19 From this point, the actual band positions of PHFCA were red shifted compared with those of the polyfluorene film. The red shift was induced by the presence of -COOH and -OH groups in the polymers. The effect of -COOH and -OH groups were beneficial

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Fan et al. TABLE 1: Main Atomic Electron Density Populations for HF, FCA, and HFCA HF

FCA

HFCA

atom electric charge atom electric charge atom electric charge 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) O(14) Figure 7. FTIR spectra of the HF monomer (a), PHF films obtained from CH2Cl2 + 5% BFEE (b) and CHCl3 + 5% BFEE (c), HFCA monomer (d), and PHFCA (e).

to the effective dispersion of the electrical charge in the conjugated polymeric system. Otherwise, the UV-vis spectra of dedoped PFCA films showed a much broader absorption band from 360 to 700 nm. Therefore, these spectral results confirmed the use of PHF, PFCA, and PHFCA for conjugated polymer formation with a broad molar mass distribution. 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 PHF, PFCA, and PHFCA, vibrational spectra are unique because they may be used to interpret the polymerization mechanism. Figure 7 shows the transmittance FTIR spectra of the HF monomer (a), PHF films obtained from CH2Cl2 + 5% BFEE (b) and CHCl3 + 5% BFEE (c), HFCA monomer (d), and PHFCA fims (e). The broad peak at 3285 cm-1 is the characteristic absorption of the O-H bond, which shifted to nearly 3200 cm-1 in the spectrum of the PHF film (Figure 7b,c). This band, together with the band at 1604 cm-1 (Figure 7a) and 1610 cm-1 (Figure 7b,c), can be ascribed to the elongation and deformation vibration of the O-H bond, which implies that the hydroxyl group still exists on the PHF main chain. As can be seen in Figure 7d, the sharp and strong band at 3445 cm-1 is attributed to the mode of the phenolic hydroxyl group, and the broad band centered at 3230 cm-1 is generally attributed to the O-H stretching vibration caused by the intermolecular hydrogen bond. The bands around 3060 and 3015 cm-1 can be assigned to the dC-H stretching vibration of asymmetric and symmetric motions of the aromatic ring. However, the strong band at 1724 cm-1 is ascribed to the CdO stretching vibration of the uncoordinated carboxylic acid. The CdC vibration of the aromatic ring shows from 1608 to 1450 cm-1. The bands between 1368 and 1242 cm-1 are attributed to the C-H and O-H deformation vibration. Furthermore, the medium intense band appearing in the region from 1200 to 1067 cm-1 can be assigned to the C-O and C-C stretching vibration of the aromatic ring. In the spectrum of PHFCA, the characteristic bands of the benzene ring, -OH group, and -COOH group are still found (Figure 7e). Similar to other fluorene derivates, the characteristic bands of the monomers at 736 and 763 cm-1 (Figure 7a) or at 732

–0.118 –0.088 –0.088 –0.131 –0.130 –0.089 –0.087 –0.134 0.039 0.062 0.072 0.073 0.063 –0.526

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) O(15) O(16)

–0.037 –0.085 –0.089 –0.038 –0.038 –0.090 –0.087 –0.041 –0.192 –0.037 –0.035 –0.021 –0.099 0.387 –0.337 –0.312

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) O(15) O(16) O(17)

–0.015 –0.086 –0.086 –0.037 –0.031 –0.087 –0.085 –0.047 –0.124 0.051 –0.041 –0.022 0.023 0.394 –0.306 –0.351 –0.373

and 765 cm-1 (Figure 7d) indicate that the adjacent positions in the aromatic ring are substituted.60 In contrast, the absorption of PHF and PHFCA showed three peaks at 736, 763, and 816 cm-1 (Figure 7b,c) and at 738, 768, and 820 cm-1 (Figure 7e), respectively. This result is similar to 1,2,4-trisubstituted benzene rings indicating that the position of polymerization mainly occurred at the C(2) and C(7) positions (Scheme 1).60 For more information on the structure of polymers and the polymerization mechanism, we calculated the atomic electron density population (Table 1) and proportion of the frontier orbitals (Figure 8) of the HF, FCA, and HFCA monomers using the B3LYP/6-31G(d,p) level of Gaussian 03 software. As shown in Table 1, the results of the main atomic electron density populations revealed negative electric charges at the C(2), C(3), C(6), and C(7) positions in aromatic biphenyl, which implied that these atoms may donate electrons during electrochemical polymerization through radical cation intermediates. According to the molecular orbital theory, the reaction among the active molecules mainly happens on the frontier molecular orbital and the near orbital. For HF, FCA, and HFCA, the proportions of atoms at the C(2), C(7), C(10), C(11), C(12), and C(13) positions in the HOMO were higher than those of other atoms in Figure 8. Meanwhile, the C(2), C(7), C(10), C(11), C(12), and C(13) positions also had rich negative charges. However, the electron spin density is the main factor in controlling the electropolymerization according to many authors.61,62 Therefore, the monomer radical cation was calculated at the B3LYP/6-31G(d,p)

Figure 8. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of HF, FCA, and HFCA.

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Figure 9. Isovalent surfaces (0.005 electron/bohr3) of spin electron density (blue) for HF, FCA, and HFCA radical cations.

TABLE 2: 1H NMR Data of δH (ppm) for the HFCA Monomer and PHFCA atom

HFCA

PHFCA

H(1), H(8) H(2), H(7) H(3), H(6) H(4), H(5) H(17) H(16)

7.46 (d, 2H, JHH ) 8.0 Hz) 7.30 (t, 2H, 3JHH ) 6.96 Hz) 7.40 (t, 2H, 3JHH ) 6.84 Hz) 7.78 (d, 2H, 3JHH ) 7.92 Hz) 6.45 (s, 1H) 12.65 (s, 1H)

7.64 7.41 7.95 6.56 -

3

level for electron spin density,58 and the isovalent surfaces of the electron spin density for these monomers are observed in Figure 9 and S-Table 2 of the Supporting Information. The relatively higher atomic electron density could be seen at the C(2), C(7), C(10), C(11), C(12), and C(13) positions, which implied the electropolymerization of HF, FCA, and HFCA probably occurs at these positions. However, there will be a more significant steric hindrance effect when the coupling occurs at the C(10), C(11), C(12), and C(13) positions than at the C(2) and C(7) positions. Therefore, the polymerization between the monomer would happen preferentially at the C(2) and C(7) positions, which is well in accordance with the FTIR results mentioned previously. PHF and PFCA films were partly soluble in strong polar solvent DMSO and insoluble in general organic solvents. PHFCA could be totally dissolved in strong polar solvents such as DMSO but only partly dissolved in CH3CN, acetone, and so on. Thus, we investigated 1H NMR spectra of doped PHFCA in d6-DMSO (S-Figure 2 of the Supporting Information). There were six group peaks in the spectra of the monomer as shown in Table 2. Some peaks disappeared, and most of the peaks moved to lower fields after polymerization (Table 2). Generally, with introducing longer conjugation length into the polymer main backbone, the chemical shifts of hydrogen atoms on the aromatic rings usually move to lower fields.60,63 This phenomenon can be ascribed to the introduction of higher conjugation length in the PHFCA main chain. Otherwise, these proton lines of PHFCA became broader than the corresponding proton lines of the HFCA monomer due to the formation of a new bond between monomers and PHFCA. The chemical shift at 6.45 ppm (s, 1H) could be ascribed to the proton of the O-H bond at the

Figure 10. Emission spectra of the HF monomer (a), FCA monomer (b), and HFCA monomer (c) in DMSO and the photoluminescence properties of the solid PHF films prepared from CH2Cl2 + 5% BFEE (d) and CHCl3 + 5% BFEE (e), solid PFCA films (f), and solid PHFCA films (g) on the ITO electrode.

C(9) position in HFCA, which moved to a lower field at 6.56 ppm after polymerization. The chemical shift at 12.65 ppm (s, 1H) can be assigned to the proton of the -COOH group. Moreover, this chemical shift disappeared after polymerization, which was due to doped cations. These results indicated that the C(9) position was not the polymerization site. The chemical shift at 7.30 ppm (t, 2H, 3JHH ) 6.96 Hz) can be ascribed to the protons at the C(2) and C(7) positions of the HFCA monomer, which disappeared in the spectrum of PHFCA. However, the proton signals at 7.46 ppm (d, 2H, 3JHH ) 8.0 Hz), 7.40 (t, 2H, 3 JHH ) 6.84 Hz), and 7.78 ppm (d, 2H, 3JHH ) 7.92 Hz) can be ascribed to the protons at the C(1) and C(8), C(3) and C(6), and C(4) and C(5) positions, which moved to 7.64 ppm, 7.41 ppm, and 7.95 ppm in the spectrum of the polymer, respectively. The above-mentioned results indicate that a new C-C bond between two monomers via the C(2) and C(7) positions was formed (Scheme 1), which was well in accordance with the results of FTIR spectroscopy and theoretical calculations. An important property of fluorene and its derivatives is the fluorescence of their polymers.1 Figure 10 shows the emission spectra of the HF monomer (a), FCA monomer (b), HFCA monomer (c) in DMSO, the PL property of the solid PHF films prepared from CH2Cl2 + 5% BFEE (d) and CHCl3 + 5% BFEE (e), the solid PFCA films (f), and the solid PHFCA films (g) on the ITO electrode. The emission spectrum of the HF monomer is mainly located at 319 nm (Figure 10a), while the emission spectrum of PHF obtained from CH2Cl2 + 5% BFEE features a maximum at 381 nm and extends to 600 nm (Figure 10d), suggesting the presence of radioactive traps of chemical or physical origin.64 This is a common phenomenon of PL properties and solid-state fluorescence spectroscopy of solid PHF films obtained from CHCl3 + 5% BFEE (Figure 10e). The emission spectrum of the FCA monomer is mainly located at 310 nm (Figure 10b), while the emission spectrum of PFCA features a maximum at 430 nm and extends to 650 nm (Figure 10f).44 In addition, the excitation spectra of the HFCA monomer appeared at 293 nm, while the corresponding emission peaks appeared at 330 nm (Figure 10c). In comparison with the HFCA monomer, the obvious excitation peak of doped PHFCA can be found at 353 nm, while the emission peaks can be found at 408 and 542 nm (Figure 10g). The fluorescence quantum yield φoverall of PHFCA was determined to be only 0.045, which was

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Figure 11. TG of PHF films obtained from CH2Cl2 + 5% BFEE (a) and CHCl3 + 5% BFEE (b), PFCA fims (c), and PHFCA films (d) obtained from BFEE.

TABLE 3: Electrical Conductivity of Partly Polyfluorene Derivates electrolyte systems PF PFO PAF PDCF PBF PFCA PHF PHFCA

BFEE BFEE BFEE BFEE BFEE BFEE + 15% TFA BFEE + 30% TFA BFEE CH2Cl2 + 5% BFEE CHCl3 + 5% BFEE BFEE

electrical conductivity (S cm-1) -1

2.5 × 10 7.8 × 10-3 2.8 × 10-1 1.6 × 10-1 1.4 × 10-1 5.4 × 10-2 6.2 × 10-1 10-1 10-2 10-2 10-2

Figure 12. SEM images of PHF films obtained from CH2Cl2 + 5% BFEE (a), PFCA films obtained from BFEE (b) and BFEE + 30% TFA (c), and PHFCA films obtained from BFEE (d) on the ITO electrode.

references 19 20 40 41 41 42 44

the influence of self-quenching of the -COOH group. These results indicate solid PHF, PFCA, and PHFCA films are good blue light-emitting materials, with potential applications as electron transporting materials in light-emitting diodes (LEDs). 3.5. Thermal Stability, Electrical Conductivity, and Morphology. The thermal stability of the PHF films obtained from CH2Cl2 + 5% BFEE (a) and CHCl3 + 5% BFEE (b), PFCA fims (c), and PHFCA films (d) obtained from BFEE were determined by thermogravimetric analysis (Figure 11). The thermal analysis was performed under a nitrogen stream in the temperature range of 293-890 K, with a heating rate of 10 K min-1. As can be seen from Figure 11, the first weight loss of PHF obtained from CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE occurred at 450 and 460 K, respectively. PFCA fims and PHFCA films obtained from BFEE started to lose weight when the temperature reached to 490 and 500 K, respectively, which was lower than that of PF.18,19 These results indicated good thermal stability of PHF, PFCA, and PHFCA films. Electrical conductivity of some PFs were measured by the conventional four-probe technique (Table 3). Electrical conductivities of PHF films obtained from from CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE was 10-2 S cm-1, and the electrical conductivities of PFCA films obtained from BFEE and BFEE + 30% TFA was 10-1 S cm-1 and 6.2 × 10-1 S cm-1,44 respectively. Additionally, the electrical conductivity of PHFCA obtained from BFEE was 10-2 S cm-1. The abovementioned results were close to those of PF,18,19 PBF,42 PDCF,41 PAF,40 and PFO,20,45 and these values were equivalent to the electrical conductivities of other conducting polymers such as polycarbazole (7.5 × 10-3 S cm-1),27,65,66 oligopyrene (10-1 S

cm-1),67,68 polyanthracene (10-1 S cm-1),69 and polyphenanthrene (10-1 S cm-1).70,71 This good semiconducting property will facilitate their applications and with the other excellent properties previously described will make PHF, PFCA, and PHFCA very important as active layers in light-emitting diodes, which is one of the most promising applications of semiconducting conjugated polymers. The surface morphology of PHF films obtained from CH2Cl2 + 5% BFEE (a), PFCA films obtained from BFEE (b) and BFEE + 30% TFA (c), and PHFCA films obtained from BFEE (d) on the ITO electrode were observed by scanning electron microscopy (SEM) (Figure 12). Macroscopically, the PHF, PFCA, and PHFCA films appeared to be smooth, homogeneous, and compact, which is extremely beneficial to improving their electrical conductivity and electron transfer capability and also made these polymer films good candidates for applications in ion selective electrodes, ion-sieving films, matrices for hosting catalyst particles, etc.72 Microscopically, even at high magnifications, the surfaces of these polymer films are not smooth and homogeneous. PHF films obtained from CH2Cl2 + 5% BFEE appeared to be uneven at a magnification of 10000 (Figure 12a). As shown in Figure 12a, the morphology of the doped PHF films appeared to be continuous and spherical, with a diameter of nearly 180 nm, which implied PHF films had a nanostructure. As can be seen in Figure 12b, the morphology of doped PFCA films obtained from BFEE appeared spherical and continuous, with a diameter that was fitful at a magnification of 20000. Moreover, a similar phenomenon appeared in Figure 11c, which described the microscopic morphology of doped PFCA films obtained from BFEE + 30% TFA at a magnification of 10000. The SEM images of the PHFCA films prepared in BFEE are shown in Figure 12d. Microscopically, the PHFCA film resembles ordered arrangements of granules. The growth of nuclei was in the form of clusters. The morphology was beneficial to the movement of anions into and out of the PHFCA films during the doping and dedoping process, which were well in accordance with the good redox activity of the PHFCA films. 4. Conclusion In summary, a set of fluorene-based polymers has been synthesized by direct anodic oxidation of HF, FCA, and HFCA

PF Derivatives with OH and COOH Substitution in CH2Cl2 + 5% BFEE and CHCl3 + 5% BFEE (by volume), BFEE and BFEE + 30% TFA, and BFEE solutions. The introduction of these monomers into BFEE-based electrolytes increased the ionic conductivity significantly, making polymerizations possible. It can be clearly seen from 13C NMR spectra that the chemical shift of carbons at the C(9) and C(14) positions (Scheme 1) made a low-field shift. This change in the chemical shift of carbon atoms confirms the formation of complexion cations when FCA or HFCA was mixed with BFEE. They all showed much lower oxidation potentials in BFEE or the binary solvent system additional BFEE as a supporting electrolyte than those in CH3CN. Moreover, these polymer films showed semiconducting properties with poor solubility in common organic solvents such as CH2Cl2 and CHCl3. The as-formed polymer films showed good redox behavior and good thermal stability. UV-vis, FTIR, and 1H NMR spectral analyses together with quantum chemistry calculations determined that the electrochemical polymerization of these monomers mainly occurred via the coupling at the C(2) and C(7) positions. Additionally, the fluorescence properties of the polymers were greatly improved in comparison with those of the monomers, which implied the polymers are good blue light emitters. SEM results indicated that PHF is a nanomaterial, and this morphology of polymers facilitated the movement of doping anions into and out of PFCA and PHFCA films during doping and dedoping process. With so many attractive properties, we believe a wide application for these polymers is probable. Acknowledgment. The National Natural Science Foundation of China (NSFC) (50663001, 50503009), the key scientific project of the Ministry of Education, China (2007-207058), the Natural Science Foundation of Jiangxi Province (2007GZH1091), Jiangxi Provincial Department of Education (GJJ08369) are acknowledged for their financial supports. Supporting Information Available: Onset oxidation potentials of HF, FCA, and HFCA in different electrolytes, main atomic electron spin densities for HF, FCA, and HFCA, 1H NMR spectra of HFCA and PHFCA, and successive cyclic voltammograms of PHF. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scherf, U.; Neher, D. AdV. Polym. Sci. 2008, 212, 1–322. (2) Grimsdale, A. C.; Mullen, K. AdV. Polym. Sci. 2006, 199, 1–82. (3) Neher, D. Macromol. Rapid Commun. 2001, 22, 1366–1385. (4) Leclerc, M. Macromol. Rapid Commun. 2007, 28, 1675–1675. (5) Leclerc, M. J. Polym. Sci. Polym. Chem. 2001, 39, 2867–2873. (6) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477–487. (7) Schutz, P.; Caruso, F. Langmuir 2001, 17, 7670–7674. (8) Tang, C.; Liu, F.; Xu, H.; Huang, W. Prog. Chem. 2007, 19, 1553– 1561. (9) Raultberthelot, J.; Simonet, J. J. Electroanal. Chem. 1985, 182, 187–192. (10) Raultberthelot, J.; Simonet, J. New. J. Chem. 1986, 10, 169–177. (11) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76–95. (12) Sharma, H. S.; Park, S. M. J. Electrochem. Soc. 2004, 151, E61– E68. (13) Gilberto, S.; Gianni, Z. J. Electroanal. Chem. 1985, 186, 191– 199. (14) Groenendaal, L.; Zotti, G.; Aubert, P. H.; Waybright, S. M.; Reynolds, J. R. AdV. Mater. 2003, 15, 855–879. (15) Chen, W.; Xue, G. Prog. Polym. Sci. 2005, 30, 783–811. (16) Banach, M. J.; Friend, R. H.; Sirringhaus, H. Macromolecules 2004, 37, 6079–6085. (17) Asawapirom, U.; Bulut, F.; Farrell, T.; Gadermaier, C.; Gamerith, S.; Guntner, R.; Kietzke, T.; Patil, S.; Piok, T.; Montenegro, R.; Stiller, B.; Tiersch, B.; Landfester, K.; List, E. J. W.; Neher, D.; Torres, C. S.; Scherf, U. Macromol. Symp. 2004, 212, 83–91.

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