Sublimation Point Depression of Tris(8-hydroxyquinoline)aluminum(III) (Alq3) by Crystal Engineering Tu Lee*,†,‡ and Ming Shiou Lin‡
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1803-1810
Department of Chemical & Materials Engineering, and Institute of Materials Science & Engineering, National Central UniVersity, 300 Jhong-Da Road, Jhong-Li City 320, Taiwan, R.O.C. ReceiVed March 8, 2007; ReVised Manuscript ReceiVed June 21, 2007
ABSTRACT: An initial solvent screening technique using 23 kinds of common organic solvents was utilized to engineer the polymorphism and crystallinity of the solids of organic electroluminescent molecules of tris(8-hydroxyquinoline) aluminum(III) (Alq3). Pure Alq3 recrystallized from N,N-dimethyl formamide (DMF) by temperature cooling from 60 to 25 °C gave rodlike, semicrystalline solid materials with traces of δ-form having a sublimation point of only around 200 ( 5 °C. The sublimation point depression of source materials could reduce the current evaporation temperature of organic light-emitting diodes (OLEDs) drastically. In addition, three new solvates of Alq3(C6H5NO2)1/5(H2O)6, Alq3(CHCl3)1/4, and Alq3(C4H8O2)1/2 were discovered from the initial solvent screening. Introduction The current way of mass producing organic light-emitting diodes (OLEDs) is by the method of organic vapor-phase deposition (OVPD). However, the high evaporation temperature is the key drawback of this technique.1 In OVPD, a hot inert carrier gas transports sublimed organic source materials through a hot-walled reactor to a cooled substrate, on which the organic molecules are preferentially physisorbed.2 Since organic molecules are heat sensitive and the work function of electrodes would change above 250 °C, any property alteration by heat damage could significantly reduce the OLEDs’ luminescence efficiency and lifetime.3 In addition to the development of high Tg and high-degradation-temperature electroluminescent (EL) materials by chemical approaches,4,5 there is also a demand to achieve the ideal evaporation temperature of no more than 200 °C in the future.6,7 Recently, we have found that the sublimation point of organic electroluminescent solids of tris(8-hydroxyquinoline) aluminum(III) (Alq3) could be depressed by crystal engineering. The enthalpy of sublimation, ∆Hs, is related to the difference of the intermolecular potential energy, ∆Einter, and the difference of the intramolecular potential energy, ∆Eintra, by8,9
∆Hs ) Hgas - Hsolid ) RT + ∆Einter + ∆Eintra
Figure 1. The molecular structure of tris(8-hydroxyquinoline)aluminum(III) (Alq3).
(1)
where Hgas and Hsolid are the enthalpy of gas and the enthalpy of solid, respectively; R is the ideal gas constant; and T is the temperature; thus, any alteration of the crystal lattice in Alq3 either by polymorphism or by crystallinity can cause changes in the sublimation point of the material. Therefore, the aim of this paper is to perform crystal engineering on the OLED organic luminescent and electron transport material, Alq3 (Figure 1) by the initial solvent screening method developed by our research group.10 The method provides four important pieces of information for process scale-up and for quality control of solid materials. They are (1) solubility of solid materials in different kinds of solvent * Corresponding author. Tel.: +886-3-422-7151 ext. 34204. Fax: +8863-425-2296. E-mail:
[email protected]. † Department of Chemical & Materials Engineering. ‡ Institute of Materials Science & Engineering.
Figure 2. The polymorphic transformations of Alq3.
commonly used in manufacturing, (2) polymorphism, (3) crystallinity, and (4) crystal habit of a relatively small amount of solid materials recrystallized in good solvents by temperature cooling from 60 to 25 °C, which can be readily extended to
10.1021/cg070226e CCC: $37.00 © 2007 American Chemical Society Published on Web 08/08/2007
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Table 1. Solvent Miscibility Table (gray boxes), Single Solvent Systems (diagonal boxes), Cosolvent Systems (blue boxes), and Antisolvent Systems (green boxes) of Alq3
large-scale operations. Although Alq3 can occur in two different stereoisomers: meridional (mer) and facial (fac),11 it is generally believed that the Alq3 mer-isomer is predominant in both amorphous films and crystals.12 Alq3 has also been studied and characterized extensively for its luminescence,13 thermal properties,14 polymorphism (R, β, γ, δ, and ),15-17 and crystallization behavior18 in the literature. The inter-relationships and the operating conditions of the polymorphs are summarized in Figure 2. By far, Alq3 has a better stability with a glass temperature at 170 °C and an efficiency of 1.4 × 10-6 (cm2 V-1 s-1) than other organic emitting materials such as poly(2methoxy-5-(2′-ethyl)-hexoxy-1,4-phenylene vinylene) (MEH-
PPV) and meta-methyl-tris(diphenyl amino)triphenylamines (m-MTDATA).19 Materials and Methods OLED Organic Material. Yellowish brown Alq3 (C27H18AlN3O3, MW: 459.44, purity of 98%) solids were purchased from SigmaAldrich (USA). Ground Alq3 powders were prepared by grinding about 0.5 g of the purchased Alq3 solids with about 0.1 mL of reversible osmosis (RO) water for an hour by an agate mortar and pestle. Solvents. Acetone (CH3COCH3, HPLC/spectro grade, 99.5%, bp: 56 °C, MW: 58.08, Lot 411050), acetonitrile (CH3CN, ACS grade, 99.96%, bp: 81.6 °C, MW: 41.05, Lot 812045), benzene (C6H6, ACS
Sublimation Point Depression of Alq3
Figure 3. (a-c) Solubility curves of Alq3 in nine different kinds of good solvent.
grade, 99%, bp: 80 °C, MW: 78.11, Lot 310008), benzyl alcohol (C6H5CH2OH, certified grade, 99.9%, bp: 205 °C, MW: 108.14, Lot 809010), n-butyl alcohol (CH3(CH2)3OH, ACS grade, 99.4%, bp: 117.7°C, MW: 74.12, Lot 205027), chloroform (CHCl3, HPLC/spectro grade, 99.9%, bp: 60.5-61.5 °C, MW: 119.38, Lot 401059), dimethyl sulfoxide (DMSO) ((CH3)2SO, HPLC/spectro grade, 99.8%, bp: 189°C,
Crystal Growth & Design, Vol. 7, No. 9, 2007 1805 MW: 78.13, Lot 202060), N,N-dimethylformamide (DMF) (HCON(CH3)2, ACS grade, 99.8%, bp: 153°C, MW: 73.10, Lot 020505), n-heptane (CH3(CH2)5CH3, HPLC/spectro grade, 99.4%, bp: 98 °C, MW: 100.21, Lot 111110), isopropyl alcohol (IPA) ((CH3)2CHOH, HPLC/spectro grade, 99.8%, bp: 82.4 °C, MW: 60.1, Lot 310067), methanol (CH3OH, HPLC/spectro grade, 99.9%, bp: 64.7 °C, MW: 32.04, Lot 411070), methyl tert-butyl ether (MTBE) ((CH3)3COCH3, certified grade, 99.9%, bp: 55.2 °C, MW: 88.15, Lot 712032), methyl ethyl ketone (MEK) (C2H5COCH3, ACS grade, 99.6%, bp: 81.6 °C, MW: 72.11, Lot 201021), tetrahydrofuran (THF) (C4H8O, HPLC/ spectro grade, 99%, bp: 65-67 °C, MW.: 72.11, Lot 411013), toluene (C6H5CH3, HPLC/spectro grade, 99.8%, bp: 110.6 °C, MW: 92.14, Lot 406050), and xylene (C6H4(CH3)2, ACS grade, 98.5%, bp: 137144 °C, MW: 106.17, Lot 305065) were all obtained from TEDIA company (Fairfield, USA). N,N-Dimethylaniline (DMA) (C6H5N(CH3)2, ACS grade, 99%, bp: 193 °C, MW: 121.18, Lot A0213203001), nitrobenzene (C6H5NO2, ACS grade, 99%, bp: 210-211 °C, MW: 123.11, Lot A019568301), and p-xylene (C6H4(CH3)2, ACS grade, 99%, bp: 138 °C, MW: 106.17 °C, Lot 48754/2) were obtained from Acros Organics company (New Jersey). 1,4-Dioxane (C4H8O2, ACS grade, 98%, bp: 100-102°C, MW: 88.11, Lot sp-3432R) was obtained from Showa Chemical Co., Ltd. (Tokyo, Japan). Ethanol (CH3CH2OH, HPLC/spectro grade, 99.5%, bp: 78 °C, MW: 46.7) was obtained from Echo Chemical Co. Ltd. (Taipei, Taiwan). Ethyl acetate (CH3COOC2H5, ACS grade, 99.5%, bp: 76.5-77.5 °C, MW: 88.11, Lot G43342) was obtained from Grand Chemical Co. Ltd. (Daejeon, South Korea). Reversible osmosis (RO) water was clarified by a water purification system (model: Milli-RO Plus) bought from Millipore (Billerica, USA). Solvent Screening. Under the initial solvent screening, a total of 23 solvents for scale-up listed in Materials and Methods were chosen.20 About 5 mg of Alq3 powders were weighed in a 25 mL scintillating vial. Drops of a given solvent were titrated carefully by a micropipette into the vial with 1-2 min of intermittent shaking until all Alq3 solids were just dissolved at 25 °C. The solubility of Alq3 in that particular solvent was approximated first as the weight of Alq3 in the vial divided by the total volume of the solvent added to the vial (gravimetric method). A good solvent was defined as a solvent that gave a solubility of g1 mg/mL at 25 °C. The solubility of Alq3 in the good solvent at 15°, 25°, 40°, and 60 °C was then measured by the gravimetric method assuming the volumes of the solvents were the volumes of solution and the volumes of solvents did not change significantly with temperature. Although the gravimetric method appeared to be rough, its advantages were its robustness and simplicity, without the need to perform any calibration and without the concern of solvate formation. However, due to the inherent inaccuracy of the method,10 solubility values measured gravimetrically in good solvents which did not produce Alq3 solvates were verified again and corrected by the more accurate weighing method. An excess amount of Alq3 was suspended and stirred in a good solvent in a 25 mL scintillation vial at a given temperature for 24 h to reach a dynamic equilibrium between the solid and the solution phase. All temperatures were maintained and controlled by a water bath. At equilibrium, a few drops of supernatant were then withdrawn and filtered. A known volume of filtrate was then introduced in an empty 25 mL scintillation vial and oven dried at 40 °C overnight. The oven dried solids were weighed and divided by the volume of filtrate to obtain the solubility. Solid generation of Alq3 in each good solvent was achieved by cooling the saturated Alq3 solution from 60 to 25 °C with intermittent shaking. All solids were characterized for their purity, polymorphism, solvates, crystallinity, and crystal habit by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), powder X-ray diffraction (PXRD), and optical microscopy (OM). Instrumentations. Transmission FTIR spectroscopy was utilized to measure purity, detect bond formation, and verify chemical identity. Transmission FTIR spectra were recorded on a Perkin-Elmer Spectrum One spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT). The KBr sample disk was scanned with a scan number of 8 from 450 to 4000 cm-1 having a resolution of 2 cm-1. Thermal Gravimetric Analysis (TGA). TGA analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10 °C/min ranging from 50 to 450 °C. Weight loss was usually associated with solvent evaporation close to the boiling point of a solvent as in the case of solvates or sample decomposition. The open platinum pan and
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Figure 4. (a-c) The van’t Hoff plots of the solubility values of Alq3 in nine different kinds of good solvent. stirrup were washed by ethanol and burned by a spirit lamp to remove all impurities. All samples were heated under nitrogen atmosphere to avoid oxidization. About 3 mg of sample was placed on the open platinum pan suspended in a heating furnace. Powder X-ray Diffraction (PXRD). A PXRD diffractograph at 25 °C provided another piece of information for the polymorphism and crystallinity of Alq3 solids. PXRD diffractographs were detected by Bruker D8 Advance (Germany). The source of PXRD was Cu KR
(1.542 Å), and the diffractometer was operated at 40 kV and 41 mA. The X-ray was passed through a 1 mm slit and the signal a 1 mm slit, a nickel filter, and another 0.1 mm slit. The detector type was a scintillation counter. The scanning rate was set at 0.05° 2θ/s ranging from 5° to 35°. The quantity of sample used was around 20-30 mg. Optical Microscopy (OM). An optical microscope (SZII; Olympus, Tokyo, Japan) equipped with a CCD camera (SSC-DC50A; SONY, Tokyo, Japan) was used to take images of crystal habit.
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Figure 5. Optical micrographs of Alq3 solids generated from (a) DMF, (b) nitrobenzene, (c) chloroform, and (d) 1,4-dioxane (scale bar ) 100 µm).
Results and Discussion Out of the 23 solvents, Alq3 dissolved well in 9 good solvents (yellow boxes) and only slightly dissolved in 14 bad solvents (red boxes) in the “form space”10 (Table 1). The good solvents for Alq3 were dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), benzyl alcohol, nitrobenzene, 1,4-dioxane, acetone, N,N-dimethylaniline (DMA), tetrahydrofuran (THF), and chloroform. The bad solvents for Alq3 were water, methanol, ethanol, acetonitrile, isopropyl alcohol (IPA), n-butyl alcohol, methyl ethyl ketone (MEK), benzene, methyl t-butyl ether (MTBE), toluene, ethyl acetate, p-xylene, xylene, and n-heptane. Of course, there were other solvent pairs such as cosolvents (i.e., good solvent + good solvent) represented by the 72/2 ) 36 blue boxes and antisolvents (i.e., bad solvent + good solvents) symbolized by the 234/2 ) 117 green boxes, which might be worth trying in the future for the recrystallization of Alq3. The number of boxes was divided by 2 because of the symmetry of the “form space”. However in this paper, we were only interested in the single solvent systems (i.e., the diagonal boxes). Consequently, only nine solubility curves for Alq3 in nine different good solvents at 15, 25, 40, and 60 °C (55 °C was used instead if the solvent’s boiling point is close to 60 °C) were constructed and grouped by their similar solubility ranges for ease of comparison (Figure 3). Those solubility curves were essential for crystal yield calculations upon temperature cooling. The authenticity of the solubility values and the close-to-ideality nature of the solutions were verified by the van’t Hoff equation:10
ln x ) -
∆Hd ∆Sd + RT R
(2)
in which x is the mole fraction of Alq3 in the solution; T is the solution temperature; R is the gas constant; ∆Hd is the enthalpy
of the dissolution; and ∆Sd is the entropy of dissolution. When ln x was plotted against 1/T, linear fits with high correlation coefficients of 0.93 or more resulted (Figure 4). If desired, ∆Hd and ∆Sd could be deduced from the slope and the y-intercept of the straight line. Out of the nine good solvents, Alq3 solids were generated in only four of them. Optical micrographs (Figure 5) showed that DMF, nitrobenzene, chloroform, and 1,4-dioxane produced Alq3 crystals with shapes of rod, prism, needle, and plate, respectively. These four solvents all had a narrow range of Hansen dispersion parameter, δd, from 17.8 to 20 MPa1/2.21 Apparently, the dispersion forces of the solvent played a more dominant role than either the polar forces or the hydrogen bonding forces in producing the Alq3 solids. The Alq3 solids that grown from chloroform with a relatively low polar and hydrogen-bonding characters appeared as long fragile needles and were thought to be exposing mostly nonpolar faces. TGA results (Figure 6) indicated that Alq3 solids grown from DMF contained no traces of residual solvent, but the ones grown from nitrobenzene, chloroform, and 1,4-dioxane gave three newly discovered solvates with calculated empirical formulas of Alq3(C6H5NO2)1/5(H2O)6, Alq3(CHCl3)1/4, and Alq3(C4H8O2)1/2, respectively. Although other solvated phases of Alq3 such as Alq3(C7H8)1/2, Alq3(C2H 5OH), Alq3(MeOH), and Alq3(C6H5Cl)1/2 were reported in the literature,15,22 they were not observed in our experiments because solvents that gave an Alq3 solubility of less than 1 mg/mL at 25° C were not considered for recrystallization at all. The thermal behavior of the Alq3 solids grown from DMF was further compared with the ones of the purchased Alq3 powders and the ground Alq3 solids by TGA (Figure 7). The onset temperatures of the weight loss of the purchased Alq3 powders, the ground Alq3 solids, and the Alq3 solids grown from DMF were around 300°, 250°, and 150 °C, respectively.
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Figure 7. TGA scan of (a) purchased Alq3 solids, (b) ground Alq3 powders, and (c) Alq3 solids grown in DMF.
Figure 8. FTIR spectrum of the Alq3 crystals grown in DMF.
Figure 6. TGA scans of Alq3 solids generated from (a) DMF, (b) nitrobenzene, (c) chloroform, and (d) 1,4-dioxane.
Interestingly, those thermal behaviors remained unchanged if the purified Alq3 solids produced by subliming twice the purchased Alq3 powders were utilized instead as a starting material. This suggested that the amount of impurities originally present in the purchased Alq3, if there was any, did not play a significant role in the evaporation temperature. Since Alq3 did not decompose until 430 °C,12 the onset temperatures of the weight loss in Figure 7 must correspond to the sublimation points. When these samples were heated in vacuumed test tubes under 2 × 10-2 Pa, they began to sublime at 250 ( 5 °C, 220 ( 5 °C, and 200 ( 5 °C, respectively. To explain the cause for the various degrees of sublimation point depression, all three samples were subjected to the analyses of FTIR and XRD. The purity of all three samples was checked by FTIR whose detection limit was 0.1%.23 Apparently, all IR bands were identical, and no detectable impurity was observed. A typical IR spectrum was illustrated in Figure 8, and assignments for R-form Alq3 were made. The Al-N stretching was found at 405 cm-1, 418 cm-1 and Al-O stretching was found at 522, 542, and 549 cm-1.24
However, there were some subtle differences among the three samples which could not be distinguished by FTIR that were detected by XRD. The purchased Alq3 powders appeared to be R-form crystals with high X-ray diffraction intensities (Figure 9a) indicating a relatively good crystallinity. The low X-ray diffraction intensities and the disappearance of the three diffraction peaks ranging from 2θ ) 6° to 8° indicated that the crystal lattices of ground Alq3 solids were distorted and lose the R-form character (Figure 9b) by grinding. The Alq3 solids grown from DMF also exhibited low X-ray diffraction intensities indicating that the solids were partially amorphous. Traces of δ-form with the characteristic peaks at 2θ ) 15° and 23.5° were also observed in Figure 9c.11 The presence of the δ-form12 that had a reduced overlap of the π-orbitals and the low crystallinity in the Alq3 solids grown from DMF had lowered its sublimation point significantly. Therefore, we recommend the use of semicrystalline δ-form or even δ-form Alq3 for sublimation. Conclusions The success of a large-scale preparation of organic electroluminescent molecules and the production of OLEDs depends on the knowledge of solubility, polymorphism, crystal habit, and crystallinity of the solid phase. The solvent screening strategy,
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Acknowledgment. This work was supported by a research grant from the National Science Council of Taiwan, R. O. C. (NSC 95-2113-M-008-012-MY2). Suggestions from Ms. JuiMei Huang in TGA and Ms. Shew-Jen Weng in XRD at National Central University Precision Instrument Center and High Valued Center are gratefully acknowledged. References
Figure 9. XRD diffractogram of (a) purchased Alq3 solids, (b) ground Alq3 powders, and (c) Alq3 solids grown in DMF.
coupled with crystallization by temperature cooling in 25 mL scintillation vials and solid characterizations by TGA, FTIR, PXRD, and OM, has provided a technology enabling platform for (1) using miniaturized tools on a routine basis to study luminescent materials’ physical, thermal, and structural properties, (2) maximizing the screens from a minimum quantity of the luminescent materials, and (3) providing a data bank of very insightful and informative material properties for process chemistry and process development work. The strategy closely simulates the scale-up conditions and is easy to be implemented in the laboratory.
(1) Baldo, M.; Deutsch, M.; Burrows, P.; Gossenberger, H.; Gerstenberg, M.; Ban, V.; Forrest, S. Organic Vapor Phase Deposition. AdV. Mater. 1998, 10, 1505-1514. (2) Yang, F.; Shtein, M.; Forrest, S. R. Morphology Control and Material Mixing by High-Temperature Organic Vapor-Phase Deposition and Its Application to Thin-Film Solar Cells. J. Appl. Phys. 2005, 98, 014906-1-014906-10. (3) Jonda, Ch.; Mayer, A. B. R.; Stolz, U.; Elschner, A.; Karbach, A. Surface Roughness Effects and their Influence on the Degradation of Organic Light Emitting Devices. J. Mater. Sci. 2000, 35, 56455651. (4) Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J.; Winnacker, A.; Spreitzer, H.; Weisso¨rtel, F.; Salbeck, J. White Light Emission from Organic LEDs Utilizing Spiro Compounds with High-Temperature Stability. AdV. Mater. 2000, 12, 130-133. (5) Huang, C.-H.; Yang, S.-H.; Chen, K.-B.; Hsu, C.-S. Synthesis and Light Emitting Properties of Polyacetylenes Having Pendent Fluorene Groups. J. Polym. Sci. Part A. Polym. Chem. 2006, 44, 519531. (6) Zhang, K.; Zhu, F.; Huan, C. H. A.; Wee, A. T. S. Indium Tin Oxide Films Prepared by Radio Frequency Magnetron Sputtering Method at a Low Processing Temperature. Thin Solid Films 2000, 376, 255263. (7) Sugimoto, A.; Ochi, H.; Fujimura, S.; Yoshiyuki, A.; Miyadera, T.; Tsuchida, M. Flexible OLED Displays Using Plastic Substrates. IEEE J. Sel. Top. Quantum Electron. 2004, 10, 107-114. (8) Bedrov, D.; Smith, G. D.; Sewell, T. D. Molecular Dynamics Simulations of HMX Crystal Polymorphs Using a Flexible Molecule Force Field. J. Comput.-Aided Mater. Des. 2001, 8, 7785. (9) Foti, G. Silicon Carbide: From Amorphous to Crystalline Material. Appl. Surf. Sci. 2001, 184, 20-26. (10) Lee, T.; Kuo, C. S.; Chen, Y. H. Solubility, Polymorphism, Crystallinity and Crystal Habit of Acetaminophen and Ibuprofen by Solvent Screening. Pharm. Technol. 2006, 30, 72-92. (11) Braun, M.; Gmeiner, J.; Tzolov, M.; Coelle, M.; Meyer, F. D.; Milius, W.; Hillebrecht, H.; Wendland, O.; von Schu¨tz, J. U.; Bru¨tting, W. A New Crystalline Phase of the Electroluminescent Material Tris(8-hydroxyquinoline) Aluminum Exhibiting Blueshifted Fluorescence. J. Chem. Phys. 2001, 114, 9625-9632. (12) Co¨lle, M.; Gmeiner, J.; Milius, W.; Hillebrecht, H.; Bru¨tting, W. Preparation and Characterization of Blue-Luminescent Tris(8-hydroxyquinoline)aluminum (Alq3). AdV. Funct. Mater. 2003, 13, 108112. (13) Hing, L. S.; Chen, C. H. Recent Progress of Molecular Organic Electroluminescent Materials and Devices. Mat. Sci. Eng. 2002, R39, 143-222. (14) Tang, C. W.; Van Slyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (15) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. Correlation between Molecular Packing and Optical Properties in Different Crystalline Polymorphs and Amorphous Thin Films of mer-Tris(8-hydroxyquinoline)aluminum(III). J. Am. Chem. Soc. 2000, 122, 5147-5157. (16) Rajeswaran, M.; Blanton, T. N.; Klubek, K. P. Refinement of the Crystal Structure of the δ-Al(C9H6NO)3, the blue luminescent Alq3.” Z. Kristallogr. New Cryst. Struct. 2003, 218, 439-440. (17) Rajeswaran, M.; Blanton, T. N. Single-crystal Structure Determination of a New Polymorph (-Alq3) of the electroluminescence OLED (Organic Light-Emitting Diode) Material, Tris(8-hydroxyquinoline)aluminum (Alq3). J. Chem. Cryst. 2005, 35, 71-76. (18) Papadimitrakopoulos, F.; Zhang, X. M.; Higginson, K. A. Chemical and Morphology Stability of Aluminum Tris(8-hydroxyquinoline)(Alq3): Effects in Light-Emitting Devices. IEEE J. Top. Quantum Electron. 1998, 4, 49-57. (19) Shinar, J.; Savvateev, V. Introduction to Organic Light-Emitting DeVices; Shinar, J., Ed.; Springer-Verlag, New York; Chapter 1, p 15.
1810 Crystal Growth & Design, Vol. 7, No. 9, 2007 (20) Anderson, N. G. SolVent Selection; Academic Press: New York, 2000; Chapter 4, pp 81-111. (21) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed.; CRC Press: Boston, 1991. (22) Kim, T. S.; Kim, D. H.; Im, H. J.; Shimada, K.; Kawajiri, R.; Okubo, T.; Murata, H.; Mitani, T. Improved Lifetime of an OLED using Aluminum(III) Tris(8-hydroxyquinolate). Sci. Tech. AdV. Mater. 2004, 5, 331-337.
Lee and Lin (23) Skoog, D. A.; Holler, F. J.; Nieman, T. A. Applications of Infrared Spectrometry, 5th ed.; Brooks Cole Thomson Learning: United States, 1998; Chapter 17, p 422. (24) Co¨lle, M.; Bru¨tting, W. Thermal, Structural and Photophysical Properties of the Organic Semiconductor Alq3. Phys. Status Solidi A 2004, 201, 1095-1115.
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