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TO DOWNLOAD A COPY OF THIS POSTER, VISIT WWW.WATERS.COM/POSTERS ©2012 Waters Corporation Rubrene is a compound of unique optical properties that is useful in OLED devices. Rubrene is particularly susceptible to degradation through photo-oxidation under ultraviolet light in the presence of air, forming a rubrene peroxide 4 (Figure 2). Since the resulting peroxide has different optical properties, it is highly desirable to quickly and accurately assess the impurity (rubrene peroxide) content and remove the impurity before use. Figure 4 shows the SFC UV chromatograms of (A) a commercial rubrene standard and (B) a solarized rubrene solution. The peaks were identified by their respective MS and UV spectra. Using a 2-EP column and a mixture of acetonitrile, MTBE, cyclohexane (1:1:1) as the co-solvent, rubrene was well separated from its peroxide and an unknown impurity present in the original commercial product in less than 8 min. The exotic solvent system also allowed for an easy scale up from analytical to preparative scale. Figure 5 shows a representative preparative SFC UV chromatogram of a rubrene/rubrene peroxide mixture. The shaded area indicates the collected fraction. Fractions were protected from light during collection and evaporation. The insert shows the SFC UV chromatogram of the collected rubrene fraction. Because of the use of CO 2 that is low in oxygen and the relatively short run time, there was no noticeable rubrene peroxide formation. INTRODUCTION There has been great interest in small molecules that can be used in fabricating organic light emitting diodes (OLEDs) with potential applications towards dynamic video displays 1 . A typical OLED consists of several molecules to fulfill different roles, including charge transport, hole transport, semiconductors and light emission through fluorescence or phosphorescence (Figure 1). The performance and lifetime of OLED devices highly depend on the purity of the involved material. As a result, there is great demand in developing analytical methodology capable of material characterization and purity assessment, and purification methodology for obtaining high purity material. Small molecules commonly used in OLEDs include poly- aromatic hydrocarbons (PAHs), aromatic amines and organometallic complexes. Due to their solubility, normal phase liquid chromatography (NPLC) has been the primary chromatographic technique for both analysis and purification of OLEDs. For high purity material purification, researchers often resort to sublimation and zone refining. However, the high temperatures used in both sublimation and zone refining can adversely perturb the desired chemical structures and/or lead to mixtures of isomers, thereby degrading the performance and shortening the lifetime of the final optical devices. To that end, supercritical fluid chromatography (SFC) readily lends itself as an alternative for the analysis and purification of OLED material. The low viscosity, high diffusivity and high solubilizing power of supercritical carbon dioxide, the main solvent used in SFC, enables fast separation with uncompromised efficiency. SFC also allows for a reduction in the consumption and disposal of toxic organic solvents typically used in NPLC. In this poster, we present several separations of the small molecules used in OLED devices by SFC, including both analytical and preparative scale. The retention mechanism and the advantages of SFC over competing technologies are discussed. APPLICABILITY OF SUPERCRITICAL FLUID CHROMATOGRAPHY (SFC) TO THE ANALYSIS AND PURIFICATION OF ORGANIC COMPOUNDS USED IN THE PRODUCTION OF ORGANIC LIGHT EMITTING DIODES (OLED) John P. McCauley Jr. 1 , Lakshmi Subbarao 1 , Peter Lee 2 , Timothy Jenkins 2 , Harbaksh Sidhu 3 , Rui Chen 1 1 Waters Corporation, New Castle, DE, USA, 2 Waters Corporation, Milford, MA, USA, 3 Waters Corporation, Pittsburgh, PA, USA REFERENCES 1. J.K. Borchardt, Materials Today, September 2004, pg 42-46; S. Kappaun, C. Slugovc, E. List, Int. J. Mol. Sci., 2008, 9, 1527-1547. 2. E. Clar, Polycyclic Hydrocarbons (1964) Academic Press; G. Portella, J. Poater, M. Sola, J. Phys. Org. Chem. 2005, 18, 8, 785 3. C. Haigh, R. Mallion, Molecular Physics 1971, 22, 6, 945-953; K. Pawlewska, Z. Ruziewicz, H. Chojnacki, Chemical Physics 1992, 161, 3, 437-445. 4. C. Kloc, K. Tan, M. Toh, K. Zhang, Y. Xu, Appl. Phys. A, 2009, 95, 219-224.; T. Takahashi, Y. Harada, N. Sato, K. Seki, Bull. Chem. Soc. Jpn., 1979, 52, 2, 380-382. 5. A. Tamayo, B. Alleyne, P. Djurovich, S. Lamansky, I. Tsyba, N. Ho, R. Bau, M. Thompson, J. Am. Chem. Soc., 2003, 125, 24, 7377-7387 6. T. Karatsu, E. Ito, S. Yagai, A. Kitamura, Chemical Physics Letters, 2006, 424, 353-357. 7. E. Baranoff, S. Suarez, P. Bugnon, C. Barolo, R. Buscaino, R. Scopelliti, L. Zuppiroli, M. Graetzel, Md. K. Nazeeruddin, Inorg. Chem., 2008, 47, 6575-6577. Figure 3 shows SFC UV chromatograms of some PAHs. The peak identities were confirmed by injecting individual compound under the same conditions and mass spectrometry. Napthalene, anthracene and tetracene were separated readily on a 2-ethylpyridine column (2-EP) using a mixture of acetonitrile (ACN), MTBE and cyclohexane (1:1:1) as the co- solvent (Figure 3A). The same mixture solvent was also used as the sample diluents to enhance the solubility of some extremely non-polar compounds. Note that the polar component, ACN, is necessary in this exotic solvent to enable analyte elution in a reasonable time frame. Figure 3B and 3C shows the isomeric separations using the same co-solvent, but on a nitro column. Both 2-EP and nitro columns could induce interactions between the analytes and stationary phases. The elution order of the compounds seems to correlate well with their aromaticity, as described by Clar’s rule 2 . Compounds with high aromaticity are more retained on the column; thus, longer retention time. It is noteworthy that in Figure 3C, tetrahelicene was least retained despite its seemingly high resonance structure. Tetrahelicene is twisted out of plane due to steric repulsion; hence, it’s not completely planar and possess lower aromaticity 3 . CONCLUSION Supercritical fluid chromatography has a general applicability for the separations of the molecules used in OLED devices, including poly-aromatic hydrocarbons and organometallic complexes. Since molecules used in OLED devices often contain aromatic structures, stationary phases with embedded aromaticity, such as 2-EP and nitro, could promote interactions between the stationary phases and analytes. The elution order of the compounds correlates with their aromaticity. Diastereomeric separation of organometallic complexes was successfully demonstrated. Preparative chromatography of OLED associated compounds is possible. The main limitation, however, arises from the limited solubility of many of these compounds. An exotic solvent system, such as the one used in this study, is often required to improve the sample loading. For compounds that are oxygen sensitive, such as rubrene, the inert CO 2 used in SFC and shorter chromatography time, can alleviate or even eliminate the possible oxidation during the course of chromatography. For compounds susceptible to heat, such as the Iridium complex, the mild conditions used in SFC separation and post purification processing, are advantageous over sublimation and zone refining where high temperatures are often used. Traditional purification methods for such Iridium complexes involve sublimation processes at elevated temperatures. Isomerization and degradation can occur during the sublimation process 7 , leading to shortened device lifetime. Figure 6 shows the SFC UV chromatograms of the Mer- and Fac- isomers of the Tris[2-(4,6-difluorophenyl)pyridine]iridium, as well as a des-fluoro impurity. Figure 6 clearly confirms that under heat, the Mer- isomer converted to the more stable Fac- isomer. Thus, sublimation of such materials should be exercised with caution. Figure 1. Design of an OLED Device. http:// futuremediaroom.blogspot.com/2007/11/oled-future-display- technology.html Time 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 5.0e+1 1.0e+2 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 2.0e+1 4.0e+1 6.0e+1 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 5.0e+1 1.0e+2 (A) (B) (C) Figure 3. SFC UV chromatograms of (A) a series of linear aro- matic compounds; (B) anthracene and phenanthrene; and (C) four isomers of aromatic compounds containing four rings. MATERIALS AND METHODS Materials: The chemicals and solvents were purchased from various commercial sources. Unless specified otherwise, the samples were used as received. Carbon dioxide was supplied by Keen Gases (Wilmington, DE). The SFC columns were obtained from Waters Corporation (Milford, MA); ES Industries (West Berlin, NJ) and Princeton Chromatography Inc. (Cranbury, NJ). All analytical columns are 4.6 150 mm packed with 5 μm particles. All preparative columns are 10 150 mm packed with 5 μm particles. Rubrene solutions were stored in amber vials in either dichloromethane (DCM) or chloroform (CHCl 3 ). Rubrene peroxide was formed by taking 50 mL of a 5 mg/ mL solution of rubrene solution in DCM in a clear glass vial and placed under direct sunlight in the presence of air for 3 hours (Figure 2). A gradual color change from deep orange to pale yellow was observed. The formation of rubrene peroxide was further confirmed by SFC MS. Ir(Flpic) 3 isomers were isomerized by placing 1 mg of solid in a glass vial covered with aluminum foil in a furnace maintained at 250 C for the specified time period. After being cooled to room temperature, the samples were dissolved in 1.5 mL of chloroform and subject to analysis. Instruments: All analytical experiments were performed on a Waters Method Station SFC MS System equipped with a 2998 photodiode array detector, a 2424 ELSD and a 3100 single quadrupole MS detector. The system is controlled by MassLynx™ software. Preparative experiments were performed on a Waters Investigator SFC System equipped with a 2998 photodiode array detector and a collector module with a make up pump. In addition, both systems consist of an Alias autosampler, a fluid delivery module (FDM), an automated backpressure regulator (ABPR), and an analytical-2-prep™ oven. Experimental: For analytical experiments, separations were run at a total flow rate of 3.0 or 3.5 mL/min. Backpressure was set at 120 bar and temperature was 40 C. Typical injection volumes were 10 uL. For preparative experiments, separations were run at 10 mL/ min. Backpressure was set at 120 bar and temperature was 40 C. Typical injection volume were 100 uL. Time 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 5.0e-1 1.0 1.5 2.0 2.5 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 1.0e-1 2.0e-1 3.0e-1 4.0e-1 5.0e-1 6.0e-1 O O (A) (B) Figure 4. SFC UV chromatograms of (A) rubrene and (B) a mixture of rubrene and rubrene peroxide. Figure 2. Rubrene photooxidation reaction with singlet oxygen. O O h , O 2 Figure 5. A representative preparative SFC UV chromatogram of a rubrene/rubrene peroxide mixture. Insert: SFC UV chro- matogram of the rubrene fraction. Another class of compounds we investigated was phosphorescent organometallic Iridium complexes. When these compounds are formed from three bidentate ligands, the resulting complexes can exist in one of two diastereomeric forms, termed meridional (Mer-) or facial (Fac-) isomers 5 . These isomers possess different physical properties, including the optical properties that enable the phosphorescence process used in OLED devices 6 . Figure 6. SFC UV chromatograms of an Iridium complex. (A) sample under 25 C; (B) Sample treated at 250 C for 4 hr; and (C) Sample treated at 250 C for 20 hr. Isopropanol was the co-solvent and the column was a nitro column. Time 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 1.0e+1 2.0e+1 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 1.0e+1 2.0e+1 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 1.0e+1 2.0e+1 3.0e+1 + + + Ir 3- N N N F F F F F F + + + Ir 3- N N F F F F N F F (A) 250 C, 20 hr (B) 250 C, 4 hr (C) 25 C RESULTS AND DISCUSSION Time 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 AU 0.0 2.5e-2 5.0e-2 7.5e-2 1.0e-1 1.25e-1 1.5e-1 1.75e-1 2.0e-1 2.25e-1 2.5e-1 2.75e-1 3.0e-1 3.25e-1
