Weak Epitaxy Growth of Phthalocyanine on 2,5-Bis(4-1,1′:4′,1

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Weak Epitaxy Growth of Phthalocyanine on 2,5-Bis(4-1,1′:4′,1′′-terphenyl)-thiophene and the Effect of Phase State of Inducing Layer Tong Wang, Lizhen Huang, Junliang Yang, Hongkun Tian, Yanhou Geng, and Donghang Yan* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: June 23, 2010; ReVised Manuscript ReceiVed: October 28, 2010

The growth of ultrathin films of 2,5-bis(4-1,1′:4′,1′′-terphenyl)-thiophene (3PT) and weak epitaxy growth (WEG) behavior of phthalocyanines (H2Pc and ZnPc) on 3PT ultrathin films were investigated by atomic force microscopy, X-ray diffraction (XRD), and select area electron diffraction (SAED). Domain size of the monolayer films can reach 10 µm at the substrate temperature of 190 °C. The second layer films begin to show Volmer-Weber growth mode. This growth mode transformed without a concomitant phase transition. The XRD and SAED measurements indicate the monolayer and double-layer films of 3PT have identical in-plane and out-of-plane structure. The epitaxial ZnPc films have a similar orientation or intertexture shape utilizing 3PT monolayer and double-layer films as the inducing layer. By comparison with utilizing parasexiphenyl (p-6P) as the inducing layer, the morphology with clear borderline of epitaxial films with different orientations on joint of neighboring 3PT monolayer domains disclosed that the 3PT monolayer films have characteristics of crystal. Meanwhile the distinguishing effects of inducing layer phase state on morphology of epitaxial films were shown, respectively. Introduction Thin films of organic semiconductor molecules attract a significant degree of attention due to their considerable promise in organic electronic and optoelectronic devices with recommendable flexibility and modifying in properties.1,2 The morphology and crystal quality of organic thin films dominate the performance of organic electronic devices.3 Highly ordered, large-sized, and continuous organic semiconductor films are advantageous for charge transport in electronic devices.4 For the purpose of improving thin-film quality and further increasing carrier mobility, many approaches was developed.5,6 Recently, weak epitaxy growth (WEG) has developed and succeeded in fabricating high-quality organic semiconductor thin films.7-11 “Weak” means to decrease the interaction between the molecules and the substrate by way of introducing a new substrate and elevating substrate temperature. Thus, phthalocyanine molecules are upright on the substrate, and the π-π conjugated direction is parallel to the film plane. It is different from conventional organic molecular beam epitaxy film in which molecules tends to lie flat on the substrate due to the strong interaction between molecules and substrate. By the WEG method, the mobility of organic electronic device with phthalocyanine (Pc) on psexiphenyl (p-6P) as active layer achieved the same level as the corresponding single crystals.12 The rodlike molecule p-6P monolayer films have liquid-crystal-like characteristics, and the double-layer films correspond to the approximate β crystal phase.8,13 The epitaxial phthalocyanine films using p-6P monolayer and double-layer films as the inducing layer exhibit different morphology. While for the BP2T with zigzag shape molecule, the phase transition from thin film phase to bulk phase occurs from the fourth layer. The WEG behavior of phthalocyanine films also depends on the different layers and phase * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-431-85262266. Phone: +86-431-85262165.

state of BP2T inducing layer.14 The early stages of organic film growth can determine the subsequent growth.15,16 Furthermore, the inducing layer plays key roles in controlling the orientation and quality of the epitaxial films. Recently, we introduced a new inducing layer to epitaxy growth H2Pc and ZnPc, 2,5-bis(41,1′:4′,1′′-terphenyl)-thiophene (3PT) with bend shape molecule. Highly oriented, defect-free films and some particular phenomena were expected based on the liquid crystal property, inherent polarity, and strongly intermolecular interaction of bend shape molecule, i.e., the banana-shaped compound m-OSB.17,18 First, the film growth of 3PT was investigated using atomic force microscopy (AFM) and selected area electron diffraction (SAED). The 3PT films with large-size monolayer domains were fabricated. The X-ray diffraction (XRD) and SAED measurements indicate that the 3PT monolayer and double-layer films have identical in-plane and out-of-plane structures. Not only the scope of films systems prepared by WEG method has been expanded but also some highly oriented phthalocyanine films were obtained. The epitaxial ZnPc films have similar orientation or intertexture shape grown on 3PT monolayer and double-layer films. The incommensurate and commensurate epitaxial relations between highly oriented phthalocyanine (H2Pc and ZnPc) and 3PT monolayer film were identified. In addition, we discuss the phase behavior shown in different films systems during the epitaxial growth, and give a reasonable explanation. From comparison between p-6P and 3PT as inducing layer, we disclose the 3PT films at the early stage have characteristics of crystal, further show the distinguishing effect of inducing layer phase state on morphology of epitaxial films, respectively. The first two layers of some organic thin films such as p-6P may have different epitaxy effect because of their crystal or liquid crystal in nature. One advantage of 3PT as inducing layer is that we can obtain pure one type of epitaxy films. Because the first two layers both are crystal, and the substrate can be fully covered by inducing layer even the second layer appears. For

10.1021/jp1058066  2010 American Chemical Society Published on Web 11/18/2010

Weak Epitaxy Growth of Phthalocyanine the inducing layer, the domain size, the coalescence of adjacent domains and the growth mode have close relations to the phase state, which might greatly influence the epitaxy growth behavior. Understanding the phase state of inducing layer and its effect on the epitaxial films growth will help to better understand the epitaxy growth behavior of organic molecules and select the inducing layer materials appropriately. 2. Experimental Section 2.1. Synthesis of 3PT. Toluene was distilled over sodium/ benzophenone. N,N-Dimethyl formamide (DMF) was dried with CaH2 and distilled under reduced pressure. 4-Bromo-p-terphenyl, 2,5-bis(trin-butylstannyl)-thiophene, and tetrakis(triphenylphosphine) palladium(0) (Pd(PPh3)4) were synthesized according to literature procedures.19-21 Into a mixture of 4-bromo-p-terphenyl (2.31 g, 7.47 mmol), 2,5-bis(tri-n-butylstannyl)-thiophene (2.25 g, 3.40 mmol), and Pd(PPh3)4 (85 mg, 0.074 mmol) was added DMF (25 mL) and toluene (100 mL). The reaction mixture was stirred at 90 °C for 2 days under dark. After the mixture was cooled to room temperature, the precipitation was collected by filtration and washed successively with water and acetone to give an orange solid (3.50 g). The product was further purified by vacuum sublimation twice before characterization to afford an orange crystal (1.10 g, 60%). Differential scanning calorimetry (DSC) was run on a Perkin-Elmer DSC7 at a heating/ cooling rate of 10/-10 °C min-1 at a nitrogen flow. Elemental analysis was carried out on a FlashEA1112 elemental analyzer. Mp 435 °C (from DSC measurement). Anal. calcd for C40H28S: C, 88.85; H, 5.22; S, 5.93 Found: C, 88.5; H, 5.11. 2.2. Fabrication of Organic Films. H2Pc and ZnPc were purchased from Aldrich Company (USA), and p-6P was synthesized according to a previous study.22 They were purified twice by thermal gradient sublimation prior to experiments. First, 3PT films with varied thicknesses were deposited on SiO2 substrate (150 nm thermally oxidation SiO2 layer on silicon wafer). Then, H2Pc and ZnPc thin films with 2 nm thickness were deposited on 3PT and p-6P thin films. The thin films were deposited under pressure of 10-4-10-5 Pa at a rate of about 1 nm/min, and the substrate temperature was kept at 190 °C. 2.3. Characterization of Organic Films. AFM Measurements. AFM topography were obtained by a SPI 3800N (Seiko Instruments Inc.) with tapping mode. A 150 µm scanner and a commercially available SiN4 cantilever with a spring constant of 3 N/m were used in all experiments. TEM Measurements. The organic films of 3PT, H2Pc/3PT, and ZnPc/3PT were deposited on SiO2 substrate first and then deposited a carbon film on the organic films which used as support layer, using gold for demarcating if necessary. The films were separated from SiO2 surface by floatation in 10% HF solution. The organic film with the carbon coating was transferred to a copper grid for measurement. The SAED was imaged with a JEOL JEM-1011 transmission electron microscope operated at 100 kV. To provide weaker-intensity beam and higher contrast, dark field was used for experiment. Simultaneously, MoO3 was used to calibrate the rotation angle of the image relative to the diffraction pattern. XRD Measurements. The out-of-plane XRD patterns were taken from a D8 discovery thin-film diffractometer with Cu KR radiation (λ ) 1.54056 Å). The selected voltage and current were 40 kV and 40 mA, respectively. The in-plane X-ray diffraction data were obtained at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility at a wavelength of 1.2398 Å. BL14B1 is a beamline based on bending magnet and a Si(1 1 1) double crystal monochromator was employed to monochromatize the beam.

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Figure 1. Molecular structure of 3PT.

Figure 2. AFM topography of (a) 0.21 ML, (b) 0.53 ML, (c) 0.85 ML, (d) 1.06 ML, (e) 1.38 ML, and (f) 1.91 ML 3PT film grown on SiO2 substrate at the temperature of 190 °C; the scale bar is 5 µm. Height profiles of the solid lines in (c) and (f); the height scale bars is 2 nm.

3. Results and Discussion The molecular structure of 3PT is given in Figure 1. Figure 2 shows the morphologies of 3PT ultrathin film with different coverage at substrate temperature of 190 °C. In the early stages, the 3PT film formed fractal islands on the SiO2 substrate (Figure 2a), which changed to dendritic islands (Figure 2b) and then compact islands (Figure 2c) as the coverage increases, similar to the growth behavior of p-6P and BP2T. Simultaneously, when the coverage increases to 0.85 ML, some neighboring compact islands took place coalescence and a small quantity of second layer islands began to grow, which implies that the first layer is two-dimensional growth (layer-by-layer growth). The average single domain size of 3PT film can reach 10 µm. From the height data of the film, the layers’ height is about 3.1 nm, which is consistent with the estimated long axis of 3PT molecules approximately. Hence, 3PT molecules are standing upright with respect to the substrate approximately. With the coverage further increasing to 1.06 ML (Figure 2d), most of the first layer

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Figure 4. (a) TEM dark-field morphology of 1.38 ML 3PT film and (b) corresponding SAED patterns of 3PT double-layer film. The scale bar is 2 µm.

Figure 3. TEM dark-field morphologies and corresponding electron diffraction patterns of (a) 0.9 ML and (b) 1.38 ML 3PT film; dashed lines show the place of adjacent domains coalescing through the joint of cracks of adjacent domains in two directions. The scale bar is 1 µm.

domains began to coalesce. Continue increasing to 1.38 ML, the density and size of the dendritic islands in second layer increased significantly (Figure 2e), nevertheless the first layer have not yet coalesced completely. The average single domain size of the second layer of 3PT film can reach about 5 µm. The 3PT ultrathin films with large size provide platform for WEG as inducing layer. As the second layer growing, the third or fourth layer also appears, reflecting the growth mode change to the island growth (Figure 2f). Figure 3a shows the SAED pattern of 3PT monolayer film. It is orthogonal two-dimensional lattice patterns similar to p-6P. After calculating statistically and demarcating with gold, d(200) ) 3.80 Å and d(010) ) 5.76 Å. Moreover, we observed many parallel cracks along the direction of b* in 3PT films, which formed due to strain coming from the film growth. From the joint of two directions cracks in adjacent domains (Figure 3b), it can be concluded where is the place of domain coalescing, although no evident grain boundary was observed when two single domains coalesce. The indexed ED pattern consists of two [001] zones of 3PT corresponding to neighboring domain 1 and domain 2 denoted in morphology of Figure 3b. Figure 4 shows the TEM dark field morphology of 1.38 ML 3PT film and corresponding SAED pattern of 3PT double-layer film. The indexed ED pattern shows lattice parameters of the bilayer films is same with the monolayer. The in-plane structure evolution was further tested by the grazing incidence refraction (GIXD) measurements using the synchrotron radiation. As Figure 5 shows, the diffraction patterns of all the films show three diffraction peaks corresponding to the (110), (200), and (210). No significant change of the lattice spacing was observed as the coverage change from 1.06 to 4.2 ML, except a slight difference in the monolayer films. The monolayer film shows slightly larger lattice space than the subsequent layers in the in-plane structure, about 0.03 Å, d(200) ) 3.82 Å for 1 ML and d(200) ) 3.79 Å for 4 ML, which might be originated from

Figure 5. The in-plane GIXD patterns of 3PT films with different coverage.

Figure 6. The out-of-plane XRD patterns of 3PT films with different coverage.

the surface tension of the SiO2 substrate. Figure 6 shows the out-of-plane XRD diffraction patterns, from the diffraction peak of (004), there is no shift from 1.9 to 4.2 ML (the small angle diffraction peak of (001), (002), and (003) is influenced by the thickness interference period peak which makes it seem to have an upward shift). The monolayer film can only show thickness interference information. The out-of-plane XRD results also indicated no evident change of the films as the coverage increased. The XRD results reflect that the 3PT films at the early stages present structure similar to the bulk phase. Figure 7a show the morphologies of ZnPc grown on 3PT monolayer film at the substrate temperature of 190 °C. In Figure 7a, we can observe the needlelike ZnPc crystals present two in-plane orientations and form many parallelograms in a single domain. Meanwhile, the height data show that the ZnPc crystals are about 2.5 nm, which is approximately equal to the double molecule dimension, for each molecule layer of ZnPc films is about 1.3 nm, just about half of crystal cell parameter a (2.598 nm) with R form.23 The experiments of SAED were performed

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Figure 7. AFM topography (4 µm × 4 µm) of 2-nm (a) ZnPc and (c) H2Pc grown on 3PT monolayer film at the substrate temperature of 190 °C. The height data of films shown under each figure are corresponding to the black lines indicated in the figures. Electron micrograph morphology and corresponding electron diffraction pattern of 2-nm (b) ZnPc and (d) H2Pc on 3PT monolayer film, respectively. The in-plane orientated axes are denoted. The length scale bar is 1 µm and height scale bars is 1 nm.

to study the epitaxial relations between ZnPc and the 3PT. Figure 7b is the electron micrograph morphology and corresponding SAED pattern of 2-nm ZnPc grown on 3PT monolayer film. The indexed ED pattern shows it is consisted of one [001] zone of 3PT and two [100] zones of ZnPc. Two sets of inplane orientations of incommensurate epitaxy were revealed by the indexed ED pattern. There are two symmetrical oriented relations with an angle of about (10° between the c axis of ZnPc and the a axis of 3PT, which correspond to the angle of two oriented direction in AFM topography (Figure 7a). There is only orientated relationship but no lattice matching between the ZnPc and 3PT, which indicated such epitaxial relation is incommensurate. Unlike the morphology of ZnPc, stripelike H2Pc crystal with only single orientation and long-range order on 3PT monolayer film as shown in Figure 7c. The oriented stripelike H2Pc crystals along the direction of 3PT crack, namely, b direction of 3PT film. Each molecule layer H2Pc crystal is also about 1.3 nm, just about half of crystal cell parameter a (2.614 nm) with R form.24 The height data of the H2Pc crystals indicate that most H2Pc crystal is near to the double molecule dimension, while some third layer of H2Pc films have emerged. Figure 7d is the electron micrograph and corresponding SAED pattern of 2 nm H2Pc grown on 3PT monolayer film. The indexed ED pattern shows it is consisted of one [001] zone of 3PT and one [100] zone of H2Pc. The epitaxial relation between H2Pc and the 3PT substrate shows as follows: (100)H2Pc//(001)3PT, [010]H2Pc// [010]3PT, [001]H2Pc//[100]3PT. The lattice mismatching between the (100)H2Pc and (001)3PT planes are calculated and shows as follows: along the [100] direction of 3PT, mismatching % ) (d(002)H2Pc - 3 × d(200)3PT)/(3 × d(200)3PT) ) 5.11%; along the [010] direction of 3PT, mismatching % ) (3 × d(010)H2Pc - 2 × d(010) 3PT)/(2 × d(010)3PT) ) -0.68%, for 3PT, using the data obtained by calculating from SAED pattern, d(200) ) 3.80 Å and d(010) ) 5.76 Å, for H2Pc, using the R-form crystal

Figure 8. AFM topography of 2-nm ZnPc grown on 1.3 ML 3PT film at the substrate temperature of 190 °C. (c and d) Zoomed images of the areas shown in (a). All the length scale bars are 1 µm.

parameters d(002) ) 11.983 Å and d(010) ) 3.814 Å.18 This almost perfect epitaxial relationship with good orientation and lattice matching corresponds to commensurate epitaxy. To investigate the influence of inducing layer on the WEG growth behavior, we compared the morphologies of ZnPc and H2Pc epitaxial film between using 3PT and p-6P film as inducing layer. Figure 8 is the AFM height images of 2 nm ZnPc grown on 1.3 ML 3PT film at the substrate temperature of 190 °C. The similar needlelike ZnPc crystals with two consistent inplane orientation present on 3PT monolayer and double-layers as shown in parts c and d of Figure 8. It indicated that the

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Figure 9. AFM topography images of (a) 2-nm H2Pc grown on 1.0 ML 3PT film, (b) 2-nm H2Pc grown on 0.9 ML p-6P film, (c) 2-nm ZnPc grown on 0.9 ML 3PT film, and (d) 0.5-nm ZnPc grown on 1.2 ML p-6P film. Dashed ellipses in each image involve the representative morphology of domain boundaries. All the length scale bars are 5 µm.

oriented effect of monolayer and double-layer 3PT films on ZnPc epitaxial films are identical. According to our foregoing study,8,9 there must exist similar geometrical channels between the prominent H-atoms of the (001) plane of 3PT film. In the same way, the nucleation of initial molecules tends to happen along the channels in the nucleation stage. Furthermore the angle of orientation is related to the lattice constant, the angle of herringbone packing, and the angle of rotation from the direction of channel by small angle to ensure the energy lowest. This is the reason for that the WEG behavior of ZnPc and H2Pc on 3PT monolayer are similar but slightly different in comparison with those on p-6P monolayer. Here, we look back the WEG behavior of ZnPc on p-6P films. The representative AFM image of ZnPc grown on p-6P monolayer and double-layer films is shown in Figure 9d. The needlelike ZnPc crystals exhibit mainly three kinds of in-planed orientations and short-range order on p-6P monolayer film. Whereas, the needlelike ZnPc crystals with only one kind of in-planed orientation and long-range order are observed on p-6P double-layer film. The different in-plane orientations on p-6P monolayer and double-layer films result from the change of in-plane lattice structure and the dissimilar phase states. The p-6P monolayer and double-layer films correspond to the highly ordered film phase of liquidlike crystal and approximate β crystal phase, respectively.25 On the contrary, the similar ZnPc crystals on the 3PT monolayer and doublelayer films in Figure 8 imply that the 3PT monolayer and double-layer films are supposed to the identical lattice structure and phase state. Then both the 3PT monolayer and double-layer films are thinfilm phase or crystal phase that is worth inquiring. First of all, we compared the morphologies of H2Pc grown on 3PT film and on p-6P film. As shown in parts a and b of Figure 9, the discrepancy of H2Pc film at the joint of 3PT and p-6P monolayer domains was observed. At the joint of adjacent 3PTh domains, differently oriented H2Pc crystals grown in accordance with their original direction of growth, and no visible coalescence phenomenon. However, the different oriented H2Pc crystals present such excellent coalescence at the joint of adjacent p-6P domains. It is difficult to find the domain boundaries, which

Wang et al. results from the liquidlike crystal characteristics of p-6P monolayer film. In other words, the p-6P domains are not rigid collision and joint, but the p-6P molecules around the domain boundaries can adjust arrangement to coalesce in the way of adapting to each other. So the epitaxial H2Pc on p-6P monolayer film is also expressed as the film phase. In contrast, the 3PT domains are rigid collision and joint simply, and the epitaxial H2Pc films are almost no coalescence on 3PT monolayer consequently. Therefore, it can be concluded that the 3PT monolayer film exhibits characteristics of crystal. The morphology discrepancy of ZnPc films grown on the joint area of 3PT monolayer domains and p-6P monolayer domains also reflects the different characteristics of the 3PT monolayer from p-6P monolayer. As shown in parts c and d of Figure 9, different oriented ZnPc crystals on neighboring domains grown in accordance with their original direction of growth on the joint of 3PT monolayer domains. Two kinds of oriented intertexture-shape joint together, which make the borderline of neighboring domains clear and reveal the grain boundary in 3PT monolayer. However, the ZnPc crystals in various oriented directions grown at random in the region of adjacent p-6P domains (Figure 9d). Consequently, the domain boundaries are almost invisible. Besides, it is noticed that the double-layer island tend to emerge on borderline of neighboring domains (parts a and b of Figure 8), which result from more defects and traps existing in the joint of 3PT monolayer domains. Because of the binding energy between the organic molecules and defects often bigger than flat surfaces of the substrates, deposited molecules are prone to aggregate into nucleus in the defects in early nucleation stage. This further prove that 3PT monolayer film have characteristics of crystal. 4. Conclusions We introduce a new inducing layer molecular 3PT to study WEG behavior. Not only the scope of films systems prepared by WEG method has been expanded but also the high oriented phthalocyanine films were obtained. It is indicated that the 3PT monolayer and double-layer films have identical in-plane and out-of-plane structures by XRD and SAED measurements. In addition, we discuss the phase behavior shown in different films systems during the epitaxial growth and give a reasonable explanation. WEG behavior of H2Pc and ZnPc on 3PT monolayer and double-layer films were investigated by AFM and SAED. Unlike utilization of p-6P as inducing layer, the ZnPc films grown on 3PT monolayer and double-layer films have similar orientation or intertexture-shape for the reason of identical oriented effect of 3PT on epitaxial films. At the joint of adjacent 3PT domains, differently oriented H2Pc and ZnPc crystals on neighboring domains grow in accordance with their original direction of growth on the 3PT monolayer domains, which reveal the grain boundary in 3PT monolayer evidently. In addition, the double-layer films nucleate more easily at the joint of adjacent domains on 3PT monolayer film as a result of a mass of boundaries or defects in existence. From comparison between p-6P and 3PT as inducing layer, we disclose the monolayer of 3PT have characteristics of crystal. Meanwhile the distinguishing effect of inducing layer phase state on morphology of epitaxial films were shown, respectively. These results will help to better understand the epitaxy growth behavior of organic molecules and select the inducing layer materials appropriately. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (50773079

Weak Epitaxy Growth of Phthalocyanine and 50803063) and The National Basic Research Program (2009CB939702). The authors thank beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. References and Notes (1) Horowitz, G. AdV. Mater. 1998, 10, 365. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (3) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. AdV. Mater. 1997, 9, 42. (4) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 11. (5) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (6) Haemori, M.; Yamaguchi, J.; Yaginuma, S.; Itaka, K.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 44, 3740. (7) Wang, H. B.; Zhu, F.; Yang, J. L.; Geng, Y. H.; Yan, D. H. AdV. Mater. 2007, 19, 2168. (8) Yang, J. L.; Wang, T.; Wang, H. B.; Zhu, F.; Li, G.; Yan, D. H. J. Phys. Chem. B 2008, 112, 3132. (9) Wang, T.; Ebeling, D.; Yang, J. L.; Du, C. A.; Chi, L. F.; Fuchs, H.; Yan, D. H. J. Phys. Chem. B 2009, 113, 2333. (10) Huang, L.; Zhu, F.; Liu, C.; Wang, H.; Geng, Y.; Yan, D. Org. Electron. 2010, 11, 195. (11) Yang, J.; Yan, D. Chem. Soc. ReV. 2009, 38, 2634.

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