Analysis of Organic Light Emitting Diode Materials by …€¦ · Analysis of Organic Light Emitting Diode Materials by UltraPerformance Convergence Chromatography Coupled with Mass

1

Analysis of Organic Light Emitting Diode Materials by UltraPerformance Convergence Chromatography Coupled with Mass Spectrometry (UPC2/MS)Michael D. Jones, Andrew Aubin, Peter J. Lee, and Tim JenkinsWaters Corporation, Milford, MA, USA

IN T RO DU C T IO N

Organic light emitting diodes (OLEDs) are thin films that exhibit

electroluminescence when an electric current is applied. They are used

in a variety of everyday electronics such as televisions, mobile phones,

computer monitors, watches, and display screens. The continued development

and commercialization of OLEDs is of major interest to companies developing

next-generation display technology and their suppliers.

The raw materials used to construct OLED-based devices require high purity to

extend the life of the luminescence and quality of the final product, specifically

blue phosphorescent emitters. There is much intellectual property associated with

OLED materials, so manufacturing a high quality OLED raw material can also be

highly lucrative.

Intellectual property is a strong driver for the OLED chemical materials business,

which limits commoditization and drives high manufacturing costs for OLED-based

devices. Market analysis reports point to OLEDs as a key niche market over the

next five years, predicted to grow to a $5 billion market by 2016.1

Typically, OLED materials are analyzed by various microscopy techniques.

Interestingly, a limited amount of LC-based methods have been cited in

publications analyzing the stability of phosphorescent emitters used in OLED

devices.2,3,4 Many of the published methods require an analysis time greater

than 30 minutes.

In this application note, a rapid and selective method was developed utilizing

UltraPerformance Convergence Chromatography™ (UPC2TM) coupled with

photodiode array detection and mass spectrometry to analyze the purity of an

iridium complex dye, Ir(Fppy)3, shown in Figure 1. Ir(Fppy)3 is a phosphorescent

emitter material that is important for providing blue electroluminescence

longevity for use in OLED devices. The method specificity was also evaluated in

the presence of additional materials used in the construction of OLED devices.

The developed UPC2 method utilizes supercritical fluid chromatography (SFC)

carbon dioxide as the primary mobile phase, and methanol as a modifier

acting as the strong solvent to elute the planar organo-metallic based

constituents of interest.

WAT E R S SO LU T IO NS

Waters® ACQUITY UPC2TM System

ACQUITY UPC2 Column Kit

ACQUITY UPC2 PDA Detector

ACQUITY® SQD

Empower™ 3 CDS Software

K E Y W O R D S

ACQUITY UPC2, supercritical fluid

chromatography, fine chemical purity,

patent protection, stability indicating

methods, degradation profiling,

PhOLED, OLED, Ir(Fppy)3, impurities,

phosphorescent emitter

A P P L I C AT IO N B E N E F I T S■■ Provides an approach to improved profiling

of material purity

■■ 10X reduction in run time compared

to previously published liquid

chromatography techniques

■■ Significant cost reduction in mobile phase

consumption and waste

■■ Approximately $1.91 for HPLC

vs. $0.05 for UPC2

■■ Compatibility with normal phase diluents

and extraction solvents associated with

organic light emitting diode (OLED) analysis

2UPC2 Analysis of Ir(Fppy)3 Degradants in OLED Material

E X P E R IM E N TA L

Sample Description

Stock standards were prepared by diluting

Ir(Fppy)3 reference material in tetrahydrofuran

to a concentration of approximately 1.0 mg/mL.

Working standards were prepared by dilution of

the Ir(Fppy)3 stock standard with tetrahydrofuran

to a concentration of 0.1 mg/mL.

Method Conditions

System: Waters ACQUITY UPC2

Column: ACQUITY UPC2 BEH 2-EP

3.0 x 100 mm, 1.7 µm

Mobile phase A: CO2

Mobile phase B: 2 g/L Ammonium formate

in methanol

Wash solvents: 70:30 Methanol/isopropanol

Separation mode: Gradient: 10% to

25% B over 5 minutes;

hold at 25% for 1 minute

Flow rate: 2.0 mL/min

CCM back pressure: 1885 psi

Column temp.: 60 °C

Sample temp.: Ambient

Injection volume: 2.0 µL

Run time: 5.0 minutes

Detection: ACQUITY UPC2 PDA

3D Channel:

200 to 410 nm; 20 Hz

2D Channel:

258 nm @ 4.8 nm

resolution (Compensated

500 to 600 nm)

Mass spectrometer: ACQUITY SQD

MS settings: 200 to 1200 Da;

10,000 Da/sec; ES PosNeg

Make-up flow: None

Data management: Empower 3 CDS Software

R E SU LT S A N D D IS C U S S IO N

Method development

Typical SFC system configurations utilize a secondary pump to provide post

column addition of proton-rich solvent, such as formic acid, to improve the

ionization efficiency for MS analysis. The lack of ionization effectiveness of

legacy SFC configurations can be attributed to the high flow of high-percentage

liquid CO2 that lacks the protons required for ionization. The need for make-up

flow was evaluated for use with UPC2.

The ACQUITY UPC2 System configuration and methodology used for this analysis

did not require a secondary pump for make-up flow to increase ionization. The

flow to the mass spectrometer was split post-UV detection with an Upchurch zero

volume PEEK tee. However, it was determined that using 30 cm of 0.0025-inch

I.D. PEEK tubing to the inlet of the MS probe provided enough back pressure after

the split to maintain a supercritical CO2 phase state.

A method development scheme was initially designed to investigate three

ACQUITY UPC2 Columns providing differences in chromatographic selectivity.

The columns utilized were UPC2 BEH, UPC2 BEH 2-EthylPyridine (2-EP), and

UPC2 CSH Fluoro-Phenyl stationary phases, specifically designed for use with

the UPC2 instrumentation. Injections of the Ir(Fppy)3 degraded sample were

performed using a rapid 5-minute gradient screening method.

Based on this method development scheme and system configuration, it was

observed that the ACQUITY UPC2 BEH 2-EP stationary phase provided the most

optimal selectivity potential and best peak shape attributes to separate seven

unknown impurities, as shown in Figures 2 and 3. The spectra inset in Figures 2

and 3 were used to determine relationship to the Ir(Fppy)3 phosphorescent emitter.

The MS data provided added sensitivity to detect trace-level impurities such

as the smaller peaks detected between 1 and 2 minutes, shown in Figure 3. By

detecting these trace impurities by MS, the spectral information facilitates the

determination of origin or possible relationship to the main species of interest.

N

FF

N

F

F

Ir

N

F

F

Figure 1. The chemical structure of iridium complex Tris[2-(2,4-difluorophenyl)pyridine]iridium(III), referred to as Ir(Fppy)3.


Impurity Identification

The peaks found in the UV and the MS data were integrated with spectral information extracted from the 3D

data channels in Empower 3 CDS Software. The software provided the ability to extract MS spectra from UV

peaks of interest within a single window for efficient data review.

A total of seven impurity peaks were found after subtraction of the blank injection peaks. Retention time,

UV spectra, and MS spectra are displayed in Table 1. Three of the seven impurities (RT 1.360 min,

RT 1.463 min, and RT 1.619 min) were observed to have similar UV spectral profiles to that of the Ir(Fppy)3

phosphorescent emitter. The MS spectral analysis of the seven peaks revealed a distinct relationship to the

Ir(Fppy)3 phosphorescent emitter.

The Ir(Fppy)3 MS spectra have a unique isotopic ratio, as seen in the spectra found in Figure 3. All seven

impurity peaks possessed isotopic ion ratios similar to the Ir(Fppy)3 phosphorescent emitter. Electrospray

negative ion results were observed for three of the four later-eluting peaks. The negative ion information will

help further investigations focused on impurity characterization by accurate mass measurements.

1.01

9

AU

0.00

0.03

0.06

0.09

0.12

Minutes

0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

1.019 Peak 1

225.4

258.4

373.4

AU

0.018

0.036

0.054

0.072

nm220.00 260.00 300.00 340.00 380.00

UV 258 nm

Ir(Fppy)3

Figure 2. UV 258 nm chromatogram (compensated 500 to 600 nm) of Ir(Fppy)3 and impurity peaks found in a degraded sample preparation. The integrated peak was determined as the Ir(Fppy)3 analyte. The inset figure is the UV spectal profile of Ir(Fppy)3 with an observed λmax of 258.4 nm.

Figure 3. MS ES+ total ion chromatogram (TIC) of Ir(Fppy)3 and impurity peaks found in a degraded sample preparation. The integrated peak was determined as the Ir(Fppy)3 analyte. The adjacent figure is the MS spectral isotopic profile of Ir(Fppy)3 with an observed m/z base peak of 763.9 Da.

1.00

3

Inte

nsity

1.8x107

3.6x107

5.4x107

7.2x107

9.0x107

Minutes

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

1.004 Peak 1 - SQ 1: MS Scan 1: 300.00-1000.00 ES+, Centroid, CV=Tune

761.95

763.92

Inte

nsity

0.0

7.0x106

1.4x107

2.1x107

2.8x107

m/z

744.80 748.60 752.40 756.20 760.00 763.80 767.60 771.40 775.20 779.00

Ir(Fppy)3

MS ES+


Unknown impurity characterization using nominal mass measurements is typically undesirable; however,

the impurities observed during this experiment resemble the same isotopic patterns as the parent Ir(Fppy)3

compound. Based on the MS results, it is hypothesized that the unknown peak at retention time 1.94 minutes

is an Ir(Fppy)3 isomer. The unknown peaks at retention times 1.330 min, 1.463 min, 1.619 min, and 2.75 min

each represents a reduction of a fluorine atom from one of the fluoro-phenyl rings. The unknown peak at

3.622 min represents a reduction of two fluorine atoms from one of the fluoro-phenyl rings. The peak eluting

at 4.12 min requires further investigation by MS/MS and accurate mass to better characterize a proposed

structure, as shown in Figure 4.

1.360 Peak 2

213.7

221.9227.8239.5

262.0

317.8322.5

330.9

336.8

353.0360.2

367.4

381.8386.6392.6

AU

0.00024

0.00048

0.00072

0.00096

0.00120

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

1.463 Peak 3

213.7

228.9232.5

260.8

301.1

322.5

348.2351.8355.4362.6

377.0385.4

396.2

AU

0.00025

0.00050

0.00075

0.00100

0.00125

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

1.619 Peak 4

216.0

230.1233.6241.9

260.8

292.8

315.4322.5

330.9336.8348.2 360.2 375.8

393.8398.6

AU

0.00000

0.00025

0.00050

0.00075

0.00100

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

1.019 Peak 1

225.4

258.4

373.4

AU

0.018

0.036

0.054

0.072

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00


743.93

745.90

Inte

nsity

0.0

9.0x105

1.8x106

2.7x106

3.6x106

m/z734.40 737.10 739.80 742.50 745.20 747.90 750.60 753.30 756.00 758.70


743.97

745.91

Inte

nsity

0.0

1.2x106

2.4x106

3.6x106

4.8x106

m/z738.00 740.00 742.00 744.00 746.00 748.00 750.00 752.00 754.00


743.90

745.90

746.97

Inte

nsity

0

1x106

2x106

3x106

4x106

m/z739.20 741.30 743.40 745.50 747.60 749.70 751.80 753.90 756.00


761.95

763.92

765.01

Inte

nsity

0.0

7.0x106

1.4x107

2.1x107

2.8x107

m/z759.60 760.80 762.00 763.20 764.40 765.60 766.80 768.00 769.20

Retention Time UV Spectra MS ES+ Spectra MS ES- Spectra

N/A

N/A

N/A

N/A

Ir(Fppy)3 RT λmax ES+ m/z ES- m/z

1.11 min 258.4 nm 763.9 N/A

RT λmax ES+ m/z ES- m/z

1.36 min 262.0 nm 745.9 N/A


1.46 min 260.8 nm 745.9 N/A


1.62 min 260.8 nm 763.9 N/A

4.124 Peak 8

212.5221.9

238.4

264.3272.6

314.2320.1323.7332.1345.9

363.8367.4

379.4383.0386.6

AU

0.00075

0.00150

0.00225

0.00300

0.00375

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

3.622 Peak 7

212.5

237.2

275.0

341.3 354.2 368.6

383.0

AU

0.0006

0.0012

0.0018

0.0024

0.0030

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

2.746 Peak 6

236.0

273.8

345.9

371.0384.2

AU

0.0051

0.0102

0.0153

0.0204

0.0255

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00

1.935 Peak 5

234.8

270.3

343.6

371.0380.6

AU

0.013

0.026

0.039

0.052

nm220.00 240.00 260.00 280.00 300.00 320.00 340.00 360.00 380.00 400.00


761.97

763.93

765.08

Inte

nsity

0.0

7.0x106

1.4x107

2.1x107

2.8x107

m/z757.80 759.60 761.40 763.20 765.00 766.80 768.60 770.40 772.20


743.96

745.95

747.05

Inte

nsity

0.0

6.0x106

1.2x107

1.8x107

2.4x107

m/z738.00 740.00 742.00 744.00 746.00 748.00 750.00 752.00 754.00 756.00


725.97

727.94

729.01Inte

nsity

0.0

1.6x106

3.2x106

4.8x106

6.4x106

m/z722.40 723.80 725.20 726.60 728.00 729.40 730.80 732.20 733.60 735.00


804.01

805.99

806.96

Inte

nsity

0.0

1.2x106

2.4x106

3.6x106

4.8x106

m/z802.80 803.70 804.60 805.50 806.40 807.30 808.20 809.10 810.00

Retention Time UV Spectra MS ES+ Spectra MS ES- Spectra 1.920 Peak 1 - SQ 2: MS Scan 2: 300.00-1000.00 ES-, Centroid, CV=Tune

806.0

808.0

809.0

Inte

nsity

0.00

2.50x106

5.00x106

7.50x106

1.00x107

m/z798.60 800.80 803.00 805.20 807.40 809.60 811.80 814.00 816.20 818.40

2.730 Peak 2 - SQ 2: MS Scan 2: 300.00-1000.00 ES-, Centroid, CV=Tune

787.9

789.9

790.9

Inte

nsity

0.0

6.0x105

1.2x106

1.8x106

2.4x106

m/z785.40 786.50 787.60 788.70 789.80 790.90 792.00 793.10 794.20 795.30

4.103 Peak 3 - SQ 2: MS Scan 2: 300.00-1000.00 ES-, Centroid, CV=Tune

848.0

849.0

850.0

851.0Inte

nsity

0.0

5.0x105

1.0x106

1.5x106

2.0x106

m/z846.00 847.00 848.00 849.00 850.00 851.00 852.00 853.00 854.00 855.00

N/A


1.94 min 234.8 nm 763.9 808.0


2.75 min 236.0 nm 746.0 789.9


3.62 min 237.2 nm 727.9 N/A


4.12 min 238.4 nm 806.0 850.0

Table 1. Table of retention times, UV observed spectral profile, MS ES+ observed spectral profile, MS ES- observed spectral profile (if present). The MS spectral profiles verify similar isotopic patterns. Combined with the UV spectra, the unknown peaks were determined to be related to Ir(Fppy)3.


Investigation of Specificity

The method was investigated in terms of specificity using a mixture of other main ingredients used in the

construction of OLED devices. The constituents within the mixture represent the phosphorescent emitter

[Ir(Fppy)3], the hole transport material (α-NHP), the host material (TCTA), and the hole blocking material/

electron transport material Alq3. The mixture was injected, and the assay method was determined to be

specific to analyze all three doped compounds without interference with the Ir[Fppy]3 phosphorescent emmitter.

System precision was determined over n=5 injections to assess method reproducibility, displayed in Figure 5.

1.01

9

1.36

01.

463

1.61

9

1.93

5

2.74

6

3.62

2

4.12

4

AU

0.00

0.03

0.06

0.09

0.12

Mi t

0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

Ir (Fppy )3

N

FF

N

F

F

Ir

N

F

F

N

F

F

Ir

N

F

F

N

F

F

Ir (Fppy )3Isomer

N

F

Ir

N

F

F

N

F

F

Ir (Fppy )3 –FIsomers

N

F

Ir

N

F

N

F

F

Ir (Fppy

Relatedunknown

)3 -2FIr (Fppy )3 -F

Minutes

Figure 4. Proposed structure assignments for the unknown impurities found in the Ir(Fppy)3 phosphorescent emitter sample.

1.0

09

1.4

21

1.9

20

2.0

28

2.4

71

AU

0.000

0.025

0.050

0.075

0.100

Minutes0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

Ir(Fppy)3Area = 0.4%Rt = 0.1%

TCTAArea = 1.1%Rt = 0.1% Alq3

Area = 1.1%Rt = 0.1%

α-NHPArea = 0.4%Rt = 0.1%

Figure 5. Method tested for specificity and system precision. Overlay of a mixture of commonly used components in OLED production (n=5). %RSD of retention time and area are reported above each major compound in the mixture. The additional impurity peaks are those impurities related to Ir(Fppy)3 .

Waters Corporation34 Maple Street Milford, MA 01757 U.S.A. T: 1 508 478 2000 F: 1 508 872 1990 www.waters.com

References

1. Bcc Research Market Report SMC069B, “Organic Light Emitting Diodes (OLEDs): Technologies and Global Markets” July 2011

2. Sivasubramaniam et al, “Investigation of FIrpic in PhOLEDs via LC/MS Technique”, Cent. Eur. J. Chemistry, 7(4), (2009), 836-845

3. Baranoff et al, “Sublimation Not an Innocent Technique: A Case of Bis-Cyclometalated Iridium Emoitter for OLED”, Inorg. Chem, 47, (2008), 6575-6577

4. Kondakov, D., Lenhart, W., and Nichols, W., “Operational degradation of organic light-emitting diodes: Mechanism and identification of chemical products”, J. Appl. Physics, 101, 024512, (2007)

CO N C LU S IO NS

A rapid 5-minute UPC2/MS method was developed to analyze

Ir(Fppy)3 phosphorescent emitter purity, and an approach to monitor

long-term stability. The MS data provided by the ACQUITY SQD

facilitated characterization of three of the unknown impurity peaks.

The UPC2 approach resulted in up to 10X improvements in run time,

solvent consumption, and considerably reduced amounts of solvent

waste when compared with previously reported LC/MS techniques

that used 100% organic solvents during the analysis. The highly

selective and unmatched specificity of this UPC2 approach will help

chemical material manufacturers better understand and control the

performance of these unstable blue phospho-organic light emitting

diodes, ultimately leading to better quality OLED-based products

and a means to improve protection of intellectual property.

Waters and ACQUITY are registered trademarks of Waters Corporation. UltraPerformance Convergence Chromatography, UPC2, ACQUITY UPC2, Empower, and T he Science of What’s Possible are trademarks of Waters Corporation. All other trademarks are the property of their respective owners.

©2012 Waters Corporation. Produced in the U.S.A.April 2012 720004305EN AG-PDF

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Analysis of Organic Light Emitting Diode Materials by …€¦ · Analysis of Organic Light Emitting Diode Materials by UltraPerformance Convergence Chromatography Coupled with Mass