Steric-Hindrance-Functionalized Polydiarylfluorenes - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Steric-Hindrance-Functionalized Polydiarylfluorenes: Conformational Behavior, Stabilized Blue Electroluminescence, and Efficient Amplified Spontaneous Emission Lubing Bai,†,⊥ Bin Liu,‡,⊥ Yamin Han,† Mengna Yu,‡ Jiong Wang,‡ Xinwen Zhang,‡ Changjin Ou,† Jinyi Lin,*,† Wensai Zhu,† Linghai Xie,*,‡ Chengrong Yin,† Jianfeng Zhao,† Jianpu Wang,† Donal D. C. Bradley,∥ and Wei Huang*,†,‡,§ †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ‡ Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China § Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an, Shaanxi 710072, China ∥ Departments of Engineering Science and Physics and Division of Mathematical, Physical and Life Sciences, Oxford University, 9 Parks Road, Oxford OX1 3PD, United Kingdom S Supporting Information *

ABSTRACT: Control of the hierarchical molecular organization of polydiarylfluorenes by synthetic strategies is significant for optimizing photophysical properties as well as the performance of light-emitting devices. Herein, for the suppression of molecular aggregation and enhancement of luminescence efficiency, a series of steric units were introduced into polydiarylfluorenes by copolymerization, with the aim of integrating the advantages of the steric-hindrance effect and of the β-phase. Optical and Raman spectroscopies revealed a βphase conformation for a polymer copolymerized with spiro[fluorene-9,9′-xanthene] (SFX), with photoluminescence (PL) peaks at 454, 482, and 517 nm. Moreover, the morphological stability and electroluminescence (EL) stability were also improved without compromising the performance of the polymer light-emitting diodes (PLEDs). Furthermore, three sterichindrance-functionalized copolymers showed significantly decreased thresholds for amplified spontaneous emission (EthASE) and enhanced stability following thermal annealing treatment. These results indicate that steric-hindrance functionalization is a superior approach to improve the overall stability and optoelectronic properties for blue-light-emitting π-conjugated polymers. KEYWORDS: polydiarylfluorene, spirofluorene, β-phase conformation, electroluminescence, amplified spontaneous emission

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INTRODUCTION π-conjugated polymers have attracted wide attention for application in flexible and large-area optoelectronic devices built using low-cost solution-processing techniques.1−3 For light-emitting π-conjugated polymers (LCPs), particular interest has been devoted to explore the behavior of the main chain in the solid state,4 where chain aggregates with diverse π−π interactions can influence both the charge transport properties and neutral excited-state species.5−7 Consequently, a wide range of photoelectric processes, such as self-absorption, excimer formation, charge separation and transport, and energy transfer, are sensitive to the morphology of solution-processed films. For luminescent materials, some of these interchain excited states lead to low photoluminescence (PL) efficiency, poor color purity, and poor spectral stability, which are © XXXX American Chemical Society

undesirable for applications in polymer light-emitting diodes (PLEDs) and polymer lasers.8−10 Therefore, rational molecular design of LCPs remains an essential strategy for the precise control of molecular aggregation and film morphology. In the design of high-performance LCPs, an effective method for suppressing chain aggregation in the solid state is to isolate the backbone chains.11−13 For red and green LCPs, sufficient progress has been made over the last few decades for applications in organic light-emitting diodes (OLEDs),14−16 polymer lasers,17,18 and thin-film transistors.19 However, blue LCPs that show both long-term stability and high efficiency Received: June 22, 2017 Accepted: October 9, 2017 Published: October 9, 2017 A

DOI: 10.1021/acsami.7b08980 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

tigation was carried out using UV−vis absorption spectroscopy, fluorescence spectroscopy, and Raman spectroscopy to understand the influence of steric hindrance on the behavior of the main chain conformation. Thermal treatment and PLEDs based on these copolymers were used to explore the morphology and electroluminescent spectral stability, respectively. We successfully obtained a novel copolymer P1 that, while being amorphous in the thin-film state, retains the capability of βphase formation. These copolymers, functionalized with steric hindrance, demonstrate a low threshold for amplified spontaneous emission (EthASE) showing great potential for application in optoelectronic devices.

remain a challenge due to the high energy levels and wide bandgaps.20,21 It has been demonstrated that polyfluorenes (PFs) are promising candidates for blue-light-emitting materials, with the advantages of easy modification, deep blue emission, and high quantum efficiency.16,22,23 However, for PFs in the solid state, a green band emission due to chain aggregation or the ketone at the C-9 position limits commercial application.24−26 Many approaches have been developed to decrease the tendency of aggregation and to avoid this detrimental excimer emission, including substitution of C-9 with bulky groups,27−30 inducing quasi-planar conformation,31−33 copolymerization with steric hindrance,34 and use of configurations of starburst oligomers.35−37 The planar conformation of PFs (β-phase) can also enhance the charge carrier mobility, spectral purity of the blue emission, and efficiency of the amplified spontaneous emission (ASE).33,38 Compared to the quasi-planar conformation of chains in the robust spiro-bridged ladder-type poly(pphenylene),33 the β-phase in PFs stems from an intriguing “planar zigzag” (21 helix) chain conformation stabilized by the balance between interchain steric interaction and molecular aggregation in the solid state in thin films, which can induce an enhanced effective conjugation length, close molecular chain packing, and a larger Förster radius for excitation transfer. In our previous work, the β-phase in a bulky polydiarylfluorene was obtained by regulating the interplay between bulky groups using steric hindrance and between alkoxy side chains using van der Waals forces.27 Recently, a series of blue-light-emitting materials equipped with spirofluorenes were developed, whose orthogonal configuration with large steric hindrance endowed the materials with enhanced thermal stability and spectral stability.23,39−41 To combine the advantages of spirofluorenes and the β-phase, we designed and compared three kinds of polydiarylfluorenes by copolymerization with spirofluorene derivatives [spiro[fluorene-9,9′-xanthene] (SFX), 6,6-diphenyl-6H-benzo[c]chromene (SFSO), and 6,6-diphenyl-6H-benzo[c]chromene (DPOF)] (Scheme 1).31,42 A systematic inves-

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RESULTS AND DISCUSSION

Synthesis and Structural Characterization of Four Polymers. All polymers were prepared via Yamamoto-type polymerization following our previous work (Scheme 1).27,43 The SFX/SFSO steric groups were synthesized according to previous reports.40,43 The DPOF unit was prepared by a BF3· Et2O-catalyzed Friedel−Crafts reaction; the synthetic steps are detailed in our previously reported work (Figure S1).27 1H NMR spectra were used here to calculate the SFX/SFSO/ DPOF fraction linked to the copolymer backbone; the copolymerization ratio was calculated to be approximately 20% for all polymers (Figure S2). The number-average molecular weight (Mn) and polydispersity index (PDI) of each polymer were determined by gel permeation chromatography (GPC) to be approximately 24−45 kDa and 1.7−2.2, respectively (Table 1 and Figure S3). Thermal analyses, i.e., differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), revealed an inconspicuous glass transition temperature (Tg) for P1 and P2 (Figure S4); all the polymers showed a high decomposition temperature (Td) of greater than 400 °C, corresponding to a 5% weight loss (Figure S5). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated using the energy level of a ferrocene redox couple (4.8 eV below vacuum) as reference: HOMO/LUMO = −[Eonset − E(Fc/Fc+) + 4.8] eV (Figure S12). All the results are listed in Table 1. Conformational Behavior of Four Polymers. Optical spectroscopy was used to investigate the photophysical properties of the four polymers in various states. The formation of the β-phase can strongly depend on the solvents used and postdeposition treatment.44 In dilute toluene solution, a weak shoulder peak was observed at 443 nm for P1 and P4 (10−5 mg/mL), indicating the formation of the β-phase (Figure S6). In thin films spin-coated from toluene solutions, no characteristic β-phase absorption peak was identified at 443 nm, with the emission peaks at 435, 462, and 500 nm all attributed to emission from the amorphous state (Figure S7). We infer that the balancing of intermolecular forces (van der Waals forces) and steric interaction is destroyed in the polymer films because

Scheme 1. Scheme Illustration and Chemical Structures of the Steric-Hindrance-Functionalized Polydiarylfluorenes

Table 1. Physical, Optical, and Electrochemical Properties of the Polymers

a

polymer

Mn

P1 P2 P3 P4

× × × ×

2.85 2.41 3.04 4.54

PDI 4

10 104 104 104

1.76 2.23 2.06 1.77

Tg (°C)

Td (°C)

λabsa (nm)

200 210

412 410 414 417

396, 443 396 396 400, 412, 443

λema (nm) 437, 440, 440, 454,

HOMOb/LUMOb (eV)

454, 482, 517 466 466 485, 518

−5.85/−2.15 −5.70/−2.31 −5.77/−2.31 −5.73/−2.14

Films were drop-cast from toluene solutions of 1 mg/mL. bEstimated from the onset of oxidation and reduction potentials. B


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Figure 1. Optical and Raman spectra. (a) UV−vis and PL spectra of the polymer films drop-cast from toluene solutions (1 mg/mL). (b) Comparison of the P1 PL spectra obtained for a drop-cast film with the spectra from the individual β-phase and amorphous states. (c) Raman spectra of the four polymer films prepared by spin-coating (blue lines) and drop-casting (orange lines) from toluene solutions.

Raman spectroscopy was also used as an effective tool to probe the conformational transition and molecular rotation/ vibration. To some degree, the β-phase of PFs is a type of quasi-planar conformation resulting from the conformational change of the polymer chain, characterized by high intrachain order, and with a fixed rotational angle of 165° between the monomer units.45 According to our previous report,31 the planar conformation can be detected by measurement of the relative Raman intensity at 1304 cm−1 and the frequency shift at 1600 cm−1. As shown in Figure 1c, the main Raman peaks were observed in the range 1200−1700 cm−1, correlated to the stretching models of carbon bonds in conjugated structures. The data show a distinct enhancement in the Raman intensity of the drop-cast P1 and P4 films in the spectral region from 1290 to 1305 cm−1, which is assigned to the planarization of the polymer backbone. In addition, approximately 3 and 6 cm−1 frequency shifts are observed at approximately 1600 cm−1 for the P1 and P4 polymer films, respectively, attributed to the enhanced planarity of the backbone that increases the delocalization of π-electrons. However, no frequency shift was observed at 1600 cm−1 for the P2 and P3 polymer films. Consequently, we propose that the strong molecular interactions deriving from the sulfur and oxygen atoms in P2 destroy the formation of the β-phase, as does the asymmetric steric hindrance of the DPOF unit in P3. The Raman results are consistent with the optical spectroscopy measurements. Therefore, we consider that only by copolymerization with appropriate conjugated steric units can one generate planar segments in polymer films. Morphological and Spectral Stability of the Four Polymers. For investigation of the effect of steric hindrance on the thermal stability of LCPs, atomic force microscopy (AFM) was used to study variation in the detailed film morphology before and after the annealing treatment (Figure 2). Uniform and smooth pristine films were deposited by spin-coating from toluene solutions; the polymer films showed a root-meansquare (RMS) roughness of 0.51, 0.52, 0.43, and 0.90 nm for P1−P4, respectively. No distinct granular domains appeared in the copolymer films after annealing treatment at 250 °C, indicating good thermal stability of the copolymer morphology. In contrast, P4 showed a relatively large increase in the RMS roughness (1.70 nm) and the formation of granular domains,

of the centripetal force and fast solvent evaporation experienced during film formation.27 Typically, the β-phase can be induced by exposing the thin film to solvents with moderate solubility or slowing down the kinetic process of film formation during solution processing. Therefore, drop-casting can be used to promote polymer chain rearrangement and further facilitate the efficient formation of the β-phase conformation. As shown in Figure 1a, distinct shoulder absorption peaks are observed at 443 nm for the drop-cast films of P1 and P4, but these peaks are absent for P2 and P3 (Figure 1a and Table 1). This observation indicates the formation of the β-phase conformation in the thin films, consistent with measurements of the excitation PL spectra (Figure S8). The β-phase was further confirmed by identification of the emission peak features at 454, 482, and 517 nm, which are assigned to the 0−0, 0−1, and 0−2 vibrational transitions of polyfluorene in a planar conformation (Figure 1a). Moreover, the characteristic emission peaks at 454, 482, and 517 nm become much more distinguishable for P1 when the excitation wavelength is tuned from 390 to 440 nm (Figure S9). By comparison with the PL spectrum obtained in the amorphous state, we speculate that the spectral response contains contributions from a mix of two types of chromophores present in the amorphous state and the β-phase. We deducted the amorphous emission spectrum from the mixed spectrum of P1 to obtain the individual β-phase emission (Figure 1b). Through analysis of the optical spectra of the three copolymers, we found that the typical β-phase only exists in the P1 polymer film. In addition, the PL quantum yields (ΦPLQY’s) of P1, P2, and P3 films are approximately 55%, 50%, and 47%, respectively, which are all higher than that of P4 (ΦPLQY = 43%). That is because the steric hindrance in the copolymers could effectively reduce the aggregation, and additionally the stronger crystallization of P4 was unfavorable for the luminescence efficiency (Figure S11) so that the ΦPLQY values of these copolymers were higher than that of P4. The supramolecular effects of the sulfone units in P2 made the chain interaction stronger than that of P1, so that the ΦPLQY of P2 was lower than that of P1. As for P3, the copolymerized moiety was biphenyl-embedded by oxygen atoms, and the effective conjugation length was shorter than those of fluorine monomer, so that the ΦPLQY of P3 was lower than those of P1 and P2. C


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Figure 2. AFM images of the P1 (a), P2 (b), P3 (c), and P4 (d) pristine films (top) and films annealed at 250 °C in a nitrogen atmosphere (bottom). Time-dependent PL spectra of the P1 (e), P2 (f), P3 (g), and P4 (h) films annealed at 200 °C in air.

Electroluminescent Properties. As discussed above, the four polymers show a deep blue emission, high fluorescence quantum yield, and excellent stability. For investigation of the influence of steric hindrance on the electroluminescence (EL) properties, particularly the color stability under electrical bias, PLEDs with a simple device configuration of ITO/ PEDOT:PSS (40 nm)/emissive layer (50 nm)/TPBi (20 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated using the four polymers (Figure 3a). At low applied voltage, the EL spectra (Figure 3b) of all the devices were similar to the corresponding PL spectra (Figure 2) and consisted of three emission peak features at 430, 460, and 485 nm. Upon an increase of the applied voltage from 5 (>40 mA cm−2) to 9 (>500 mA cm−2) V, the EL spectra of the P1 devices showed little change, with a turn-on voltage (Von) of 3.8 V. Moreover, the value for the Commission Internationale de L’Eclairage (CIE) coordinate showed negligible variation (Table 2), suggesting a voltage-

which can be derived from chain aggregation-induced crystalline structures.31 By thermal annealing at 200 °C in air, no spectral variation was observed, as shown in Figure 2d, which demonstrates the excellent spectral stability for the four polymers. As expected, a small amount of β-phase was found in the annealed P4 film, but none for the copolymers. The PL spectra of all polymer films were stable after thermal annealing in air for 10 min at various temperatures (Figure S10). Moreover, no diffraction peaks were observed in the copolymer films as measured by one-dimensional X-ray diffraction (1DXRD) (Figure S11). Therefore, the crystallization induced by chain aggregation was effectively suppressed in P1, P2, and P3 because of copolymerization with high-rigidity spirofluorene derivatives. These results demonstrate that the incorporation of a moiety, with appropriate steric hindrance, can improve morphological stability as well as suppress crystallization induced by chain aggregation. D


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Figure 3. (a) Energy level diagram of the PLEDs with the structure of ITO/PEDOT:PSS (40 nm)/emissive layer (50 nm)/TPBi (20 nm)/LiF (0.8 nm)/Al (100 nm). [ITO, indium tin oxide; PEDOT:PSS, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; TPBi, 2,2′,2″(1,3,5benzinetriyl)tris(1-phenyl-1-H-benzimidazole).] (b) Electroluminescence (EL) spectra of the P1−P4 devices. (c) Current density−voltage− luminance (J−V−L) characteristics and (d) current efficiency versus current density curves of the devices.

Table 2. Summary of the Device Performances and ASE Characteristics ASE threshold polymer

Vona (V)

Lmaxb (cd/m2)

ηc‑maxc (cd/A)

P1 P2 P3 P4

3.75 3.57 3.46 3.58

1856 1780 1548 1899

0.70 0.75 0.64 0.95

CIE (x, y)d (0.15, (0.15, (0.16, (0.16,

0.10) 0.12) 0.13) 0.14)

λASE (nm)

μJ/cm2

nJ/pulse

thicknesse (nm)

459 461 462 461

1.02 3.24 5.13 8.10

12.3 38.3 61.5 94.4

155 160 150 148

a Turn-on voltage at a brightness of 1 cd/m2. bMaximum luminance. cMaximum current efficiency. dMeasured at 7 V. eSpin-coated films from toluene solution (10 mg/mL).

absorption maximum. As shown in Figure 4a, upon optical pumping, all samples show prominent ASE from the 0−1

independent EL for the P1 devices. However, the 0−1 emission in the EL spectra of the three other polymer devices (especially P4) showed a slight enhancement with increased voltage, consistent with our previous work.31 Furthermore, no green band emission (due to energy transfer or aggregation) was observed in these three copolymers with film thicknesses of approximately 50 nm. Therefore, use of the steric-hindrancefunctionalized polymers to suppress polymer chain aggregation in the solid state leads to negligible low-energy band emission during device operation. Figure 3c,d shows the current density−voltage−luminance (J−V−L) characteristics and the current efficiency versus current density curves of the devices, respectively. The P1 and P4 polymers show the best device performances with a maximum luminous efficiency of 1856 cd/ m2 (0.70 cd/A) and 1899 cd/m2 (0.95 cd/A), respectively (Table 2). The steric-hindrance-functionalized polydiarylfluorenes show pure blue emission with superior EL stability and high luminance with comparable current efficiency, and thus have great potential for application in light-emitting devices. ASE Properties. High PLQY and good thermal and spectral stability are key factors for LCPs utilized as the gain medium in an organic solid-state laser. The polymer chemical structure can significantly affect the optical gain properties.18,46,47 The influence of the polymer steric groups on the ASE characteristics was studied using a Q-switched Nd3+:YAG laser operating as the pump source at a wavelength near the polymer

Figure 4. (a) Normalized PL and ASE spectra (excited at 380 nm) of the spin-coated films, and temperature dependence of the ASE threshold (inset). (b) ASE output intensity versus pump energy density for P1−P4; inset shows the same data near the threshold of ASE.

vibration transition, with the peak wavelength centered at approximately 460 nm. Moreover, the ASE threshold of sterichindrance-functionalized polydiarylfluorenes was found to be lower than the threshold of P4 under the same experimental conditions (Figure 4b), which suggests superior optical amplifying properties for the copolymers with steric groups. Notably, the P1 sample showed an ultralow ASE threshold of E


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ACS Applied Materials & Interfaces 12.3 nJ/pulse (1.02 μJ/cm2) compared to the other three polymers, which is among the best reported for a fluorine-based polymer or macromolecule (similar ASE peaks). The ASE characteristics were measured as a function of gradient annealing temperature to investigate the thermal stability of the gain property. The film samples were annealed for 10 min in a N2-filled glovebox at each temperature gradient (Figure 4a, inset). Only a nominal change in the threshold for all samples is observed for increasing the annealing temperature from room temperature to 150 °C, indicating no drastic change in the material morphology or chemical structure. However, a further increase in the temperature up to 220 °C led to a significant increase in the ASE threshold for P4 due to the presence of crystalline aggregates with a rough surface (Figure 2d). Consequently, P4 shows lower ASE stability compared to the copolymers. From these control experiments using reference polymers, we speculate that the introduction of steric spirofluorenes can effectively suppress the energy loss caused by molecular vibration and promote higher radiation transition efficiency, while the orthogonal bulky structure can reduce aggregation leading to superior ASE performance. The results show that copolymerization with appropriate steric-hindrance units can be used to realize a significantly reduced EthASE.

ORCID

Linghai Xie: 0000-0001-6294-5833 Wei Huang: 0000-0001-7004-6408 Author Contributions ⊥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Key Basic Research Program of China (973) (2015CB932200), National Natural Science Foundation of China (21504041, 21502091, 21404057), Synergetic Innovation Center for Organic Electronics and Information Displays, Priority Academic Program Development of Jiangsu Higher Education Institutions, PAPD (YX03001), Natural Science Foundation of Jiangsu Province for Young Scholars (BK20140948), and Open Project from State Key Laboratory of Supramolecular Structure and Materials at Jilin University (sklssm201710). Dr Jinyi Lin also gratefully acknowledges SICAM Fellowship to support work at the Plastic Electronics Centre, physic department of Imperial College London (IC).

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CONCLUSION In this work, we designed and synthesized three sterichindrance-functionalized polydiarylfluorenes with varying degrees of steric effect and systemically investigated their conformational behavior and optoelectronic properties. The β-phase conformation-forming ability of copolymer P1 was verified by UV−vis absorbance and fluorescence spectroscopies, with a typical absorption peak at 443 nm and red-shifted emission curve. The Raman spectra showed enhanced intensity at 1304 cm−1 and a soft shift of approximately 3 cm−1 at 1600 cm−1, which further confirmed the quasi-planar conformation of the P1 chain in the drop-cast film. The reduced roughness transformation and spectral change following thermal annealing treatment revealed the enhanced morphological stability for the copolymers. Furthermore, the EL spectral stability over an applied voltage range from 5 to 9 V was more stable for the copolymers than for P4. Finally, the EthASE of the steric copolymers was demonstrated to be lower than that of P4. P1 showed the lowest EthASE among all reported fluorene-based materials. Therefore, considering the morphological, EL, and ASE spectral stability of the thin films, copolymerization with steric-hindrance groups is an effective strategy to improve the properties of LCPs for use in optoelectronic devices.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08980. Experimental conditions and methods, materials characteristics, 1H NMR, DSC, TGA, CV data, and additional data (PDF)

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Lubing Bai and Bin Liu equally contributed to this work.

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. F


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