Electrosynthesis and Characterization of Water-Soluble Poly(9

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J. Phys. Chem. C 2008, 112, 12012–12017

Electrosynthesis and Characterization of Water-Soluble Poly(9-aminofluorene) with Good Fluorescence Properties Changli Fan,†,‡ Jingkun Xu,*,† Wen Chen,‡ and Bin Dong†,§ Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal UniVersity, Nanchang 330013, China, College of Materials, Chemistry and Chemical Engineering, Chengdu UniVersity of Technology, Chengdu 610059, China, and Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Jinan 250100, China ReceiVed: February 26, 2008; ReVised Manuscript ReceiVed: May 6, 2008

The electrochemical synthesis of a novel semiconducting polymer, water-soluble poly(9-aminofluorene) (P9AF), with good fluorescence properties was successfully achieved in pure boron trifluoride diethyl etherate (BFEE) by the direct anodic oxidation of 9-aminofluorene (9AF). The amino group substitution makes P9AF highly soluble in water, facilitating potential applications as a blue-light-emitting material. Fourier transform infrared spectroscopy together with theoretical calculations show that polymerization of 9AF occurs mainly at the C(2) and C(7) positions. The fluorescence spectra indicate that doped P9AF films in water are blue-light emitters with a fluorescence quantum yield of 0.44. Increasing the concentration of hydrochloric acid decreases the solubility of the doped P9AF films but increases that of dedoped P9AF films because of the different complexation of the sNH2 group on P9AF with BF3 or H+. The doped and dedoped P9AF films in 0.5 mol L-1 hydrochloric acid are blue-light emitters with fluorescence quantum yields of 0.40 and 0.17, respectively. 1. Introduction In recent years, polyfluorene (PF) and its derivatives have been considered promising candidates as blue-light-emitting materials. Because of their highly efficient photoluminescence (PL) and electroluminescence (EL), thermal and oxidative stability, good solubility, and emission of polarized blue light,1–9 they are especially suitable for applications to polymer lightemitting diodes. The fluorene structural moiety provides a rigidly planar biphenyl unit within the molecular backbone. Therefore, during past decades remarkable advances have been made in the production of high-molecular-weight and structurally welldefined PF derivatives.10 However, stiff conjugated polymers with increasing conjugation efficiencies are intractable and insoluble. A successful approach to controlling polymer properties such as solubility, emission wavelengths, and processability and mediating the potential interchain interactions in polymer films is substituent derivatization at the C(9) position of the monomeric fluorine.4,11–17 Recently, water-soluble semiconducting polymers have attracted much attention. Generally, they are synthesized by the incorporation of polar functional groups, such as carboxyl groups, anionic sulfonates, and quaternary ammonium salts, onto the polymer backbone.18–23 Most importantly, water-soluble polymers have also been extensively investigated in innocuous and environmentally friendly solvents, such as water and alcohols.23–25 However, few have been synthesized by electrochemical polymerization.21,26,27 This is one of the most useful and proven approaches for the synthesis of semiconducting polymers, providing several advantages such as the small amounts of materials required, rapid analyses, accuracy, and * To whom correspondence should be addressed. Phone: +86-07913805183. Fax: +86-0791-3826894. E-mail: [email protected] and [email protected]. † Jiangxi Science and Technology Normal University. ‡ Chengdu University of Technology. § Shandong University.

precision. Therefore, electrosynthesis of high-quality, watersoluble poly(fluorene) films has become a very great and interesting challenge because the electrochemical polymerization of fluorene and its derivatives has generally led to poor-quality polymers. During the past decade, high-quality semiconducting polymers have been easily produced using boron trifluoride diethyl etherate (BFEE) as the solvent and supporting electrolyte,28–33 especially when fused-ring compounds (such as indole and its derivatives,34,35 anthracene,36 naphthalene,37,38 benzofuran,34 and fluorene and its derivatives39–43) were used as the monomers. Therefore, it is expected that the electrochemical polymerization of 9-aminofluorene (9AF) will also lead to high-quality polymer films. Moreover, substitution of an amino group on the polyfluorene backbone may give this polymer special characteristics, such as good solubility in water. This paper presents, for the first time, the electrochemical polymerization of 9AF in pure BFEE. We show that high-quality and water-soluble doped or dedoped poly(9-aminofluorene) (P9AF) films with good fluorescence properties can be easily electrodeposited. The electrochemistry, thermal stability, and structural characterization of P9AF are presented in detail. 2. Experimental Section 2.1. Materials. 9AF (99%; Acros Organics, Belgium) and commercial HPLC-grade acetonitrile (ACN; Tianjin Kermel Chemical Reagents Research Institute) were used as received. BFEE (Beijing Changyang Chemical Plant, China) was distilled and stored at -20 °C before use. Tetrabutylammonium tetrafluoroborate (Bu4NBF4, 95%; Acros Organics) was dried under vacuum at 60 °C for 24 h before use. 2.2. Electrosynthesis of P9AF Film. Electrochemical polymerization and tests were performed in a one-compartment cell using a model 263A potentiostat-galvanostat (EG & G Princeton Applied Research) under computer control. For electrochemical tests, the working and counter electrodes were

10.1021/jp801673m CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

Characterization of Water-Soluble Poly(9-aminofluorene)

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Figure 1. Cyclic voltammograms of 0.02 mol L-1 9AF in pure BFEE (A) and ACN + 0.1 mol L-1 Bu4NBF4 (B). Potential scan rate )100 mV s-1.

a Pt wire with a diameter of 0.5 mm and a stainless-steel wire with a diameter of 1 mm, respectively. They were placed 5 mm apart during the experiments. To obtain a sufficient amount of polymer for characterization, stainless-steel sheets with a surface area of 10 and 12 cm2 were used as the working and counter electrodes, respectively. These electrodes were carefully polished with abrasive paper (1500 mesh) and then successively cleaned with water and acetone before each test. All potentials were referred to a saturated calomel electrode (SCE). A typical electrolytic solution was pure BFEE containing 0.02 mol L-1 9AF. The amount of polymer film deposited on the electrode was controlled by the integrated current passed through the cell. To remove the electrolyte, oligomers, and monomer, the polymer films were rinsed with diethyl ether. As-formed P9AF films were in the doped state and dark metallic in color. For spectral analyses, the polymer was dedoped with 25% ammonia for 3 days. It was then dried under vacuum at 60 °C for 24 h. 2.3. Characterization. The electrical conductivity of the asformed P9AF films was measured using the conventional fourprobe technique. Ultraviolet-visible (UV-vis) spectra were recorded using a Perkin-Elmer Lambda 900 ultraviolet-visiblenear-infrared spectrophotometer. Infrared spectra were recorded with a Bruker Vertex 70 Fourier transform infrared (FT-IR) spectrometer with samples in KBr pellets. Fluorescence spectra were determined with an F-4500 fluorescence spectrophotometer (Hitachi). Thermogravimetric analysis (TGA) was performed with a Pyris Diamond thermogravimetric-differential thermal analyzer (Perkin-Elmer). Scanning electron microscopy (SEM) measurements were made with a Philips XL 30 ESEM. The fluorescence quantum yields (φoverall) of the soluble P9AF samples were measured using anthracene in ACN (standard, φref ) 0.27)44 as the reference and calculated according to the well-known method based on eq 1

φoverall )

n2Aref I n2ref AIref

φref

(1)

Here, n, A, and I denote the refractive index of the solvent, absorbance at the excitation wavelength, and intensity of the emission spectrum, respectively. The absorbance of the samples and the standard should be similar.45,46 3. Results and Discussion 3.1. Electrochemical Synthesis of P9AF Film. Successive cyclic voltammograms of 0.02 mol L-1 9AF in pure BFEE and ACN + 0.1 mol L-1 Bu4NBF4 on a Pt electrode are shown in Figure 1. From the first cyclic voltammographic cycle it can be seen that oxidation of 9AF in pure BFEE was initiated at 1.58 V. This value was about 0.46 V lower than that for 9AF in

Figure 2. (A) Cyclic voltammograms of P9AF films in pure BFEE at potential scan rates of 20 (a), 50 (b), 100 (c), 150 (d), 200 (e), 250 (f), and 300 mV s-1 (g). (B) Cyclic voltammograms of P9AF films in pure BFEE in the potential range of 0.729-1.38 V at a potential scan rate of 20 mV s-1. The P9AF films were synthesized electrochemically in pure BFEE at a constant applied potential of 1.70 V.

ACN + 0.1 mol L-1 Bu4NBF4 (2.04 V), which implies that 9AF was much more easily oxidized in pure BFEE than in ACN-Bu4NBF4 solution.25,47–49 Moreover, the cyclic voltammograms of 9AF in pure BFEE (Figure 1A) showed features characteristic of other semiconducting polymers, such as polythiophene29,50 and poly(9-fluorenone),41 during potentiodynamic synthesis. The peak current densities of the redox system increased regularly during the successive scans, indicating that an electroactive polymer film was formed on the Pt electrode surface. P9AF could be reduced and oxidized between 0.33 and 1.49 V. The increase in the redox wave currents implies that the amount of polymer on the electrode increased. The potential shift in the current wave maximum provides information about the increase in the electrical resistance of the polymer film and the overpotential needed to overcome this resistance.51 All these phenomena indicate that a semiconducting P9AF film was formed on the working electrode. In contrast, the successive cyclic voltammograms of 9AF in ACN + 0.1 mol L-1 Bu4NBF4 were not very successful, although a polymer film was still formed on the working electrode (Figure 1B). The anodic current waves decreased quickly with increasing cyclic voltammographic scan numbers. This is mainly because a thin polymer film was formed on the electrode surface and its conductivity decreased rapidly. No apparent redox waves of the polymer were observed, indicating that only a trace amount of the polymer was formed. From this point of view, pure BFEE is better than ACN as the medium for the electrosynthesis of semiconducting P9AF films. Therefore, the following studies mainly focused on the electrosynthesis and characterization of P9AF in BFEE. 3.2. Electrochemistry of P9AF Films. To gain deeper insight into the electrochemical behavior of P9AF films, the electrochemical properties of the P9AF films deposited electrochemically from BFEE solution were studied in monomer-free BFEE (Figure 2). Similar to results reported previously in the literature,52 the steadystate cyclic voltammograms displayed broad anodic and cathodic peaks. Two anodic peaks were seen at a slow scanning rate (20

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SCHEME 1: Structures of Doped and Dedoped P9AF

mV s-1) (Figure 2B). These anodic peaks were centered at 1.0 and 1.29 V and can be mainly ascribed to oxidation of short-chain polymer. With increases in the scanning rate, these peaks can be no longer distinguished, mainly because the scanning rate was faster than the oxidation rate of the short-chain polymer. The peak current densities were proportional to the scan rates (inset in Figure 2), indicating the reversible redox behavior of the polymer. Furthermore, these films could be repeatedly cycled between the conducting (oxidized) and insulating (neutral) states without significant decomposition, indicating the high stability of the polymer. The polymer film could be oxidized and reduced from 1.45 to 0.74 V in pure BFEE (Figure 2A). All of these results indicate that the as-formed P9AF films show good redox activity and stability. 3.3. Structural Characterization. As-formed P9AF films were in a doped state and dark metallic in color. After dedoping with 25% ammonia for 3 days, significant color changes were observed (from black to brown). Both doped and dedoped P9AF films could be thoroughly dissolved in a polar solvent, such as dimethyl sulfoxide (DMSO), ethanol, ACN, or acetone, but only partly dissolved in tetrahydrofuran. However, they could not be dissolved in CH2Cl2, chloroform, etc. Conversely, doped P9AF films were easily dissolved in water, but the dedoped films were insoluble in water. With an increase in the concentration of hydrochloric acid, the solubility of the doped P9AF films decreased, whereas that of the dedoped P9AF films increased. It is well known that the stability of the following modalities decreases from left to right53

mofluorene),43 cannot be dissolved in water. Therefore, we can reasonably conclude that the amino-group substitution made P9AF highly soluble in water. The UV-vis spectra of the 9AF monomer and the doped and dedoped P9AF were examined in both water and DMSO. As shown in Figure 3, the monomer showed several characteristic absorptions at ca. 267 nm (Figure 3A (a,d)).54 The doped P9AF films showed broader absorption with peak absorption at 325 nm in water (Figure 3A (e)) and at 339 nm in DMSO (Figure 3A (b)). There was no significant change in the spectra of the doped and dedoped polymer films in DMSO (Figure 3A (b,c)) for the autodedoping of the polymer. The red shift of P9AF compared with the monomer reflects the higher conjuga-

H3N-BF3 > RH2-BF3 > (C2H5)2O-BF3 > H2O - BF3 (2) which implies the possible structure of the doped and dedoped P9AF, shown in Scheme 1. The molecular polarity was enhanced when BF3 was coordinated with R-NH2, which may have contributed to the good water solubility of P9AF in the doped state. However, the quaternary ammonium compound was formed when dedoped polymer films were introduced into 0.5 mol L-1 HCl, which made the dedoped polymer films soluble. However, formation of the quaternary ammonium compound was stunted by BF3, resulting in the lower solubility of the doped films in 0.5 mol L-1 hydrochloric acid. However, other polyfluorene derivatives, such as polyfluorenone,41 poly(9,9-dialkylfluorene),40 and poly(9-bro-

Figure 3. UV-vis spectra of (A) monomer (a), doped (b), and dedoped (c) P9AF films in DMSO solution and monomer (d) and doped P9AF films (e) in aqueous solution. On the ITO electrode (B) doped (a) and dedoped (b) P9AF films in the solid state.

Characterization of Water-Soluble Poly(9-aminofluorene)

J. Phys. Chem. C, Vol. 112, No. 31, 2008 12015 TABLE 1: Main Atomic Electron Density Populations for 9-Aminofluorene atom

electric charge

atom

electric charge

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

-0.123629 -0.090472 -0.129768 -0.087464 -0.078196 0.069049 0.059892

C(2) C(4) C(6) C(8) C(10) C(12) N(14)

-0.088895 -0.131163 -0.091229 -0.138560 0.060953 0.070620 -0.577836

TABLE 2: Main Compositions and Proportions of the Frontier Orbitals in 9-Aminofluorene (%) Figure 4. FT-IR spectra of 9AF monomer (A) and dedoped P9AF film (B). Dedoped P9AF film was obtained potentiostatically at 1.70 V from pure BFEE containing 0.02 mol L-1 9AF.

SCHEME 2: Structural Formula Calculated for the 9AF Monomer

tion length of the backbone.55 However, the doped and dedoped P9AF films showed strong absorption on an indium tin oxide (ITO) electrode, from 280 to 733 nm (Figure 3B), which can be assigned to absorption of conductive species such as polarons and dipolarons on the main backbone of P9AF in the doped and dedoped states.46 The band gap of P9AF obtained from the onset of the optical absorption spectra (Eg) was roughly 3.03 eV (Eg ) 1241/λonset(nm)).45 Figure 4 shows the transmittance FT-IR spectra of the 9AF monomer and dedoped P9AF. The broad peak at 3448 cm-1 observed in the spectrum of 9AF is the characteristic absorption of the N-H bond, which was broad and shifted to 3431 cm-1 in the spectrum of the dedoped P9AF film. This band together with the band at 1591 (Figure 4A) or 1608 cm-1 (Figure 4B) and 1450 cm-1 can be ascribed to elongation and deformation vibrations of the N-H bond, which implies that there were still amino groups on the dedoped P9AF main chain. The characteristic bands of the monomer at 737 and 762 cm-1 are typical of the absorption of ortho-substituted benzene, whereas the band at 1450 cm-1 can be assigned to the skeleton vibration of the aromatic ring of fluorene (Figure 4A). In contrast, absorption of P9AF showed three peaks at 820, 771, and 737 cm-1, which strongly suggest that P9AF was 1,2,4-trisubstituted49 and imply that polymerization mainly occurred at the C(2) and C(7) positions (Scheme 2). To further explore the structure of P9AF and its polymerization mechanism, the atomic electron density population and reactivity of the 9AF monomer (Scheme 2) were calculated at the B3LYP/6-31G(d,p) level using Gaussian 03 software.56 The results for the main atomic electron density populations showed negative electric charges on C(1), C(2), C(3), C(4), C(5), C(6), C(7), C(8), and C(9) (Table 1), which implies that these atoms donate electrons when the 9AF monomer is electrochemically synthesized through radical cation intermediates. According to molecular orbital theory, the reaction between the active molecules happens mainly at the frontier molecular orbital and the near orbital. For 9AF, the proportions of atoms C(2), C(7), C(10), C(11), C(12), and C(13) in the highest occupied molecular orbital (HOMO) were higher than those of other

atom

HOMO-1

HOMO

LUMO

LUMO+1

6.001373 0.69828 3.359026 9.67867 C(1) C(2) 1.77829 14.66217 15.56573 0.061902 C(3) 1.268136 4.20516 1.295216 10.0649 C(4) 5.185921 5.306027 8.898365 7.750685 C(5) 7.150655 5.297057 9.48826 9.986772 C(6) 2.922372 4.139511 1.481579 12.16719 C(7) 1.294787 14.21686 16.98469 0.076982 C(8) 7.281585 0.390954 4.125589 12.13644 C(9) 2.377521 0.263545 0.176878 4.149463 C(10) 4.007217 10.99647 5.844724 8.335139 C(11) 1.375683 12.03458 11.44565 0.251197 C(12) 1.248587 11.48199 12.8063 0.224734 C(13) 9.472421 11.55142 5.492009 10.90226 N(14) 48.63545 4.755982 3.035977 14.21367 HOMO-1, HOMO, LUMO, and LUMO+1 are defined as the highest occupied molecular orbital, the next highest occupied molecular orbital, the lowest unoccupied molecular orbital, and the next lowest unoccupied molecular orbital, respectively.

Figure 5. Fluorescence spectra of monomer (A) and doped P9AF (B) in aqueous solution.

atoms (Table 2). These theoretical results imply that polymerization between the monomer occurs preferentially at C(2) and C(7) (Scheme 2), in accord with the FT-IR results. The fluorescence spectra of the 9AF monomer and soluble doped P9AF were examined with wavelength scans of excitation-emission using water as the solvent, as shown in Figure 5. The excitation peak of the monomer appeared at 290 nm, whereas the corresponding emission peak appeared at 352 nm (Figure 5A).55 In comparison with those of the monomer, the obvious excitation peak of soluble doped P9AF was at 357 nm and the emission peak at 408 nm (Figure 5B). The emission was in the visible region, which implies favorable blue-light photoluminescence properties of soluble P9AF. Under natural light, doped P9AF was light yellow, whereas the monomer was colorless and transparent (Figure 6A). When exposed to 365

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Figure 6. Photographs of monomer (left) and doped P9AF film (right) dissolved in pure water before (A) and after (B) UV irradiation (365 nm).

TABLE 3: Quantum Yields of 9AF and P9AF P9AF films solvent

9AF

doped

dedoped

water 0.5 M HCl

0.0024 0.0057

0.44 0.4

insoluble 0.17

nm UV light, the former emitted strong blue light whereas the latter had no emission (Figure 6B). Furthermore, the fluores-

Figure 7. TG/DTG curves of dedoped P9AF films produced potentiostatically at 1.70 V vs SCE from pure BFEE after treatment with 25% aqueous ammonia for 3 days.

cence quantum yield φoverall of soluble doped P9AF in water was determined to be 0.44 (R-NH2:BF3). In 0.5 mol L-1 hydrochloric acid the doped and dedoped P9AF films were bluelight emitters with fluorescence quantum yields of 0.40 (RNH2:BF3) and 0.17 (R-NH3+Cl-), respectively, as shown in Table 3. The main reason for the large difference between doped P9AF in water and doped P9AF in 0.5 mol L-1 HCl was the complexation reactions of R-NH2 with BF3 or HCl, as shown in Scheme 1. All these results indicate that P9AF films are good blue-light-emitting materials. 3.4. Thermal Analysis. The thermal stability of semiconducting polymers is very important for their potential applications. TGA is an important dynamic way of detecting degradation behavior. The weight loss of a polymer sample is measured continuously, while the temperature is changed at a constant rate. The thermal stability of the dedoped P9AF film was investigated, as shown in Figure 7. Thermal analysis was performed under a stream of nitrogen in the temperature range 300-1046 K at a heating rate of 10 K min-1. There was a slight weight loss from 300 to 395 K, amounting to 6.3% (Figure 7). This degradation can be ascribed to evaporation of moisture or other volatiles trapped in the polymer.57 Decomposition amounting to 5.0% occurred between 395 and 551 K. This weight loss may be attributed to the P9AF backbone chain structure. The final weight loss was about 19.3% from 551 to 1046 K, possibly

Figure 8. SEM micrographs of the P9AF film deposited on an ITO glass electrode surface from pure BFEE at an applied potential of 1.70 V: (A) doped and (B) dedoped.

Characterization of Water-Soluble Poly(9-aminofluorene) attributable to the ashing of carbon and the shell of the amino group. From the differential thermogravimetry (DTG) curve (Figure 7) it can be seen that the rate of weight loss of P9AF was fastest at 505 K. Moreover, only about 30% decomposition occurred, even at temperatures higher than 1000 K. All these results indicate the good thermal stability of the P9AF films. 3.5. Conductivity and Morphology. The electrical conductivity of the P9AF films obtained potentiostatically from BFEE solution was 0.28 S cm-1, which is close to that of polyfluorene.39 These semiconducting properties will be useful in the applications of P9AF films. The properties of semiconducting polymers are strongly dependent on their morphology and structure. Therefore, as shown in Figure 8, scanning electron microphotographs of the P9AF films were examined. The surface of the doped P9AF film was flat and smooth but with some sundries (Figure 8A). A sheet of orange cluster was embedded on the surface during electrodeposition. After electrochemical dedoping at a constant applied potential of -0.17 V, the surfaces of the P9AF films became uneven (Figure 8B). The morphology of the dedoped P9AF films appeared branched and dendritic. 4. Conclusion Water-soluble P9AF films with an electrical conductivity of 0.28 S cm-1 were successfully electrosynthesized in pure BFEE by the direct anodic oxidation of 9AF. The oxidation potential of 9-aminofluorene in this medium was determined to be 1.48 V, which is lower than that in ACN + 0.1 mol L-1 Bu4NBF4 (2.04 V). According to the infrared spectra and quantum chemistry calculations, the P9AF chains grew mainly via the coupling of the monomer at the C(2) and C(7) positions. The fluorescence spectra suggest that soluble P9AF is a good blue-light emitter with fluorescence quantum yields of 0.44 (doped) in water and 0.4 (doped) and 0.17 (dedoped) in 0.5 mol L-1 hydrochloric acid. It is the amino substitution that makes P9AF highly soluble in water. Acknowledgment. The Natural Science Foundation of China (50663001), the Key Scientific Project of the Ministry of Education, China (2007-207058), and the Natural Science Foundation of Jiangxi Province (2007GZH1091) are acknowledged for their financial support. References and Notes (1) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416–7417. (2) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV. Mater. 1997, 9, 798–802. (3) Kreyenschmidt, M.; Klaerner, G.; Fuhrer, T.; et al. Macromolecules 1998, 31, 1099–1103. (4) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; et al. Appl. Phys. Lett. 1998, 73, 629–631. (5) Miteva, T.; Meisel, A.; Knoll, W.; et al. AdV. Mater. 2001, 13, 565–570. (6) Grell, M.; Knoll, W.; Lupo, D.; et al. AdV. Mater. 1999, 11, 671– 675. (7) Cho, N. S.; Hwang, D. H.; Lee, J. I.; et al. Macromolecules 2002, 35, 1224–1228. (8) Lu, H. H.; Liu, C. H. Y.; Chang, C. H. H.; Chen, S. A. AdV. Mater. 2007, 19, 2574–2579. (9) Huang, F.; Niu, Y. H.; Zhang, Y. AdV. Mater. 2007, 19, 2010– 2014. (10) Lim, E.; Jung, B.-J.; Shim, H.-K. Macromolecules 2003, 36, 4288– 4293. (11) Marsitzky, D.; Vestberg, R.; Blainey, P.; et al. J. Am. Chem. Soc. 2001, 123, 6965–6972. (12) Janietz, S.; Bradley, D. D. C.; Grell, M.; et al. Appl. Phys. Lett. 1998, 73, 2453–2459. (13) Yu, W. L.; Pei, Y.; Cao, Y. Chem. Commun. 1999, 18, 1837–1838.

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