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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 peroxide4 (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 CO2 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 displays1. 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 Subbarao1, Peter Lee2, Timothy Jenkins2, Harbaksh Sidhu3, Rui Chen1 1Waters Corporation, New Castle, DE, USA, 2Waters Corporation, Milford, MA, USA, 3Waters 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 rule2. 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 aromaticity3.
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 CO2 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 process7, 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
Time1.00 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 (CHCl3). 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.
Time1.00 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 Oh , O2
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-) isomers5.
These isomers possess different physical properties, including
the optical properties that enable the phosphorescence process
used in OLED devices6.
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.
Time1.00 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
+
+
+
Ir3-
N
N
N
F
F
F
F
F
F
+
++
Ir3-
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
Time1.00 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