Article pubs.acs.org/cm
In Situ Observation of Degradation by Ligand Substitution in SmallMolecule Phosphorescent Organic Light-Emitting Diodes Matthew J. Jurow, Alberto Bossi, Peter I. Djurovich, and Mark E. Thompson* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: Solutions of facial-tris(1-phenylpyrazole)Ir(III) ( fac-Ir(ppz)3), when dissolved in either tert-butyl isocyanide or in solid films of 2naphthylisocyanide, undergo replacement of a ppz ligand by the isocyanide molecules after irradiation with UV light as demonstrated by liquid chromatograph mass spectrometer analysis. Similarly, solutions of Ir(ppz)3 and bathophenanthroline (BPhen) in CH2Cl2 or acetone-d6 form a brightly emissive species, [Ir(ppz)2(Bphen)]+ when irradiated with UV light as established by optical, mass, and 1H nuclear magnetic resonance spectroscopy. Electroluminescent data from blocked organic light-emitting diode (OLED) devices demonstrate that both mer- and fac-(Ir(ppz)3) dissociate a ligand and coordinate a neighboring BPhen molecule when the device is operated at moderate to high current levels. These experiments offer direct evidence of the dissociation of a metal−ligand bond and subsequent ligand substitution as a degradation pathway in active OLED devices during operation and provide a route to assay in situ the stability of future dopants.
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very low concentrations which make it difficult to identify the processes involved in device failure. Degraded devices and materials have been studied using laser desorption ionizationmass spectrometry (LDI-MS), and degradation products consisting of dopant molecules complexed with device components including bathophenanthroline (BPhen) have been reported.16,18,20−23 Interestingly, an earlier report of an OLED using a blue triscyclometalated iridium dopant showed a rapid red shift in electroluminescence during operation.24 The iridium complex used in this device had a meridional coordination geometry that is generally considered to be more labile than the facial isomer. The spectral shift was attributed to degradation of the iridium complex in its excited state followed by ligand substitution with one of the components in the emissive layer, likely the electron transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (Bu-PBD). To further investigate similar ligand substitution and degradation pathways of phosphorescent dopants, in this report we employ a combination of photoluminescence, electroluminescence, and other spectroscopic measurements to identify the degradation products of a cyclometalated Ir complex, and track their formation in real time. We have fabricated and tested a simple OLED device which allows for rapid in situ analysis of chemical degradation. By using meridional (mer) or facial ( fac) tris-Ir(1-phenylpyrazole)3 (Ir(ppz)3) (Figure 1) as a blocking layer between the common hole transporter N,N′-di[(1-naphthyl)-N,N′diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) and electron
ridium phosphors are well-established as electroluminescent materials for organic light-emitting diodes (OLEDs).1 While iridium-based phosphorescent OLEDs have been largely deployed in numerous commercial technologies for myriad applications, deep blue emitting phosphorescent OLED devices have thus far been unable to achieve the same device lifetimes and performance metrics seen for red and green emitting materials.2−5 Blue emitters require careful and laborious synthetic tuning of ligand energies to create a large energy gap that results in the emission of shorter wavelength photons. These high energies cause blue devices to suffer from short operational lifetimes and poor device stability. 3,4 The degradation mechanisms of phosphorescent OLED devices are poorly understood and are highly variable between different device architectures.6 There are numerous reports of photoinduced decomposition processes and photosubstitution reactions in analogous ruthenium-based systems.7−10 Device degradation can be caused by chemical breakdown of any of various component layers in the emissive stack, including dopant molecules whose long lifetimes in the excited state increase the likelihood of damaging side reactions.6,11−13 In general, cyclometalated Ir compounds emit from an admixture of triplet ligand centered (3LC) and metal-to-ligand charge-transfer (3MLCT) states. To induce blue phosphorescence, it is necessary to raise the energy of this state, which then allows for thermal access to higher triplet metal centered (3MC) states that increase nonradiative relaxation. Populating these antibonding orbitals can also result in the rupture of an Ir−N bond.14 Isomerization and photosubstitution may then occur through this pentacoordinate intermediate.15 The decomposition of phosphorescent dopants and analysis of their subsequent products have been described.12,16−19 These degradation products of OLED components appear at © 2014 American Chemical Society
Received: September 9, 2014 Revised: October 22, 2014 Published: October 23, 2014 6578
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dissolved with acetonitrile. The resulting solution was analyzed by LCMS. Thin Film and Device Preparation. All organic materials were purified by gradient sublimation before use. Glass substrates coated with patterned indium tin oxide (ITO) (width of patterned stripes, 2 mm; thickness, 150 ± 10 nm; sheet resistance, 20 ± 5 Ω cm−2; transmission, 84% at 550 nm) (Thin Film Devices, Inc.) were rinsed with tergitol and deionized water and dried in a nitrogen stream. ITO substrates were exposed to an ozone atmosphere (UVOCS T10 × 10/ OES) for 10 min immediately before loading into the high-vacuum chamber. After organic depositions, masks with 2 mm stripe width were placed on substrates under N2, and LiF and Al electrodes were deposited. Devices were made in an Angstrom Engineering EvoVac 800 vacuum thermal evaporation (VTE) deposition system attached to a glovebox and Inficon SQS-242 deposition software was used to control deposited material thicknesses using a 6 MHz Inficon quartz monitor gold-coated crystal sensor. All films deposited in the VTE were performed at pressures ≤4 × 10−4 Pa and with deposition rates less than 1 Å/s. Organic films were stored under a nitrogen atmosphere and sealed with epoxy. An Inficon crystal sensor was calibrated via spectroscopic ellipsometry which was performed using a J.A. Woollam Co., Inc. VASE variable-angle ellipsometer with a VB-200 control module and a CVI instruments Digikrom 242 monochromator with a 75 W xenon light source to ensure accurate thickness of deposited films. OLED Testing. Current−luminescence−voltage (J−L−V) curves, under applied forward bias of 0−5.5 V, were measured using a Keithley power source meter model 2400, a Newport multifunction optical meter model 1835-C, a low-power Newport silicon photodiode sensor model 818-UV, and a fiber bundle (used to direct the light into the photodiode). The silicon diode was set to measure power/photons at an energy of 520 nm and was subsequently corrected during data processing to the average electroluminescence wavelength for each individual device. Electroluminescence of OLEDs was collected with the PMA from the above-described integrating sphere. Computational Methods. All calculations were performed using Jaguar 8.4 (release 17) software package on the Schrodinger Material Science Suite (v2014-2).27 Gas-phase geometry optimization was calculated using B3LYP functional with the LACVP** basis set as implemented in Jaguar.28−30
Figure 1. Structures of materials and relevant molecular geometries.
transport material BPhen, we observe the formation of an emissive species with an obvious spectral shift. The emission characteristics in the solid state and in experiments conducted in fluid solution indicate that the product is the substituted cation [Ir(ppz)2BPhen]+. These experiments suggest a new way to assay the stability of blue phosphorescent dopants by constructing an OLED in such a manner that diagnostic emissive degradation products are formed during operation of the device.
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EXPERIMENTAL SECTION
BPhen was purchased from Sigma-Aldrich. NPD was obtained from Universal Display Corporation. Fac- and mer-Ir(ppz)3 and fac-Ir(ppy)3 were prepared from literature procedures.25,26 All materials were purified by gradient sublimation before use. Physical Measurements. Photoluminescence spectra were measured using a QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorometer. Luminescent lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube instrument equipped with an LED excitation source. Quantum yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere, and model C10027 photonic multichannel analyzer (PMA). UV−vis spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. High-performance liquid chromatography (HPLC) analysis was performed on a Shimadzu Prominance-liquid chromatograph mass spectrometer (LCMS) 2020 equipped with a column oven (T = 40 °C), a PDA photodetector (200−800 nm), and an MS spectrometer (LCMS 2020; m/z range, 0−2000; ionization modes, electrospray ionization/atmospheric pressure chemical ionization (ESI/APCI). The ESI/APCI conditions were as follows: nitrogen gas pressure, 100 psi; nitrogen gas flow rate, 1.5 L/min; auxiliary nitrogen gas flow rates, 15 L/min; interface voltage, −3.5 kV; interface current, 0.1 uA; corona needle voltage, −3.5 kV; corona needle current, 0.1 uA; desolvation line (DL) voltage, 0 V; DL temperature, 250 °C; heat block temperature, 400 °C; Q-array radio frequency (RF) voltage, 14.6 V; detector voltage, 0.95 V; ion gauge (IG) vacuum, 7 × 10−4 Pa. HPLCs were performed using the Inertsil C18, 5 μm; 4.6 × 250 mm; 0.6 mL/ min gradient AcCN-H2O: 0−5′, 80%; 20′, 90%; 23′, 80%; 25′. Solutions of fac-Ir(ppz)3 in tert-butyl isocyanide were irradiated with 364 nm light and analyzed in regular time increments by LCMS. Films were spin-cast at 2000 rpm for 40 s from solutions of 1:10 (mol:mol) Ir(ppz)3:2-naphthylisocyaninde in CH2Cl2 (0.12 mM overall). Films were irradiated with 364 nm light for 6 h and then
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RESULTS AND DISCUSSION At 77 K, Ir(ppz)3 is a phosphorescent complex that emits blue light (E0−0 = 415 nm) from a 3LC-MLCT state.25 At room temperature, neither the fac-isomer nor mer-isomer is emissive since the excited state can nonradiatively relax through a thermally accessible 3MC state.25 Furthermore, the mer isomer can be rapidly and irreversibly converted to the thermodynamically favored fac isomer by heating, or by irradiation in a coordinating solvent.25,31 This isomerization must proceed by dissociation and reformation of a ligand−metal bond. Some bond ruptures result in reformation of the original compound, indicating that the actual rate of Ir−N bond dissociation is faster than the rate of isomerization.32 Ligand Substitution Using Isocyanides. To study the process of metal−ligand bond rupture in the excited state, facIr(ppz)3 was dissolved in tert-butyl isocyanide (t-BuNC), irradiated with 364 nm light and analyzed using LCMS. Aliquots of the reaction mixture were analyzed after 10, 60, 90, and 120 min of irradiation. After as little as 10 min of irradiation, a substituted complex with m/z = 706.2 elutes. This mass is assigned to Ir(ppz)2(1η-ppz)(t-BuNC), where an Ir−N bond has ruptured and been replaced by a tert-butyl isocyanide (calculated m/z + H = 706.23). Addition of the t-BuNC ligand likely occurs onto a pentacoordinate intermediate proposed to 6579
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form after Ir−N bond homolysis.25 Continued irradiation yields a new species [Ir(ppz)2(t-BuNC)2]+ (m/z calculated, 645.23; found, 645.3) caused by detachment of the ppz ligand. After 90 min of irradiation, a third product, [Ir(ppz)2(t-BuNC)3]+ (m/z calculated, 728.31; found, 728.4) is observed. This species is likely formed after a second Ir−N bond rupture creates a new pentacoordinate intermediate that undergoes addition of a third solvent (t-BuNC) molecule. No further substitution was detected after 120 min. To explore bond labilization in the solid state, thin films were cast by spin coating from a mixture of fac-Ir(ppz)3 and 10 equiv of 2-naphthylisocyanide in CH2Cl2 (at a concentration of 0.67 mg/mL of Ir(ppz)3) on clean glass slides. Samples of the films were irradiated for 6 h using 364 nm light under a nitrogen atmosphere. The films were subsequently dissolved in acetonitrile and the solutions analyzed using LCMS. Chromatographic peaks with m/z = 776.2 corresponding to Ir(ppz)2(1ηppz)(2-napthylisocyanide) (calculated m/z + H = 776.2) eluted at 19 min. No further substituted products were observed under prolonged irradiation, likely due to kinetic inhibition of substitution reactions in the solid state. Moreover, no ligand substitution was observed for films that were left standing for 120 min without UV irradiation. Ligand Substitution Using BPhen. To assess the degree to which ligand substitution is possible with molecules relevant to OLED technologies, we performed a series of experiments with solutions of Ir(ppz)3 and a common electron-transporting material BPhen. Solutions of either mer- or fac-Ir(ppz)3 in CH2Cl2 with 2 equiv of BPhen at a total concentration of 5 mg/mL were thoroughly degassed with nitrogen. UV−visible absorption and photoluminescence spectra were recorded before and after irradiation with 364 nm light and spectra were recorded at fixed time intervals. The absorption spectra of mixtures before irradiation are similar to a simple summation of spectra from the two component molecules, showing a strong band (λmax = 275 nm, shoulder at 308 nm) arising from allowed ligand-centered π−π* transitions on both species and weaker bands (λ = 350−400 nm) assigned to MLCT transitions on the metal complex. After irradiation, the primary absorption peak broadens and red shifts (centered at 285 nm) with the weaker transitions extending out to visible wavelengths. Emission spectra from solutions of Ir(ppz)3:BPhen before irradiation display a band at ca. 425 nm and have a photoluminescent (PL) quantum yield (Φ < 1%) and lifetime (τ = 1.2 ns). After irradiation, the solutions luminescence orange from a broad featureless emission band centered at 575 nm that dominates the spectrum (Figure 2). Emission lifetime data from the photolyzed solution could be fit to a biexponential decay with a majority component indicative of phosphorescence (τ = 1.6 μs using the mer-isomer as a starting complex). On the basis of the previous results using isocyanides, we hypothesized that the emissive photoproduct is formed by detachment of a ppz ligand followed by substitution of a BPhen molecule. We thus synthesized the proposed emissive product, [Ir(ppz)2BPhen]+, directly from the μ-dichloro bridged [Ir(ppz)2Cl]2 dimer.33 The 1H NMR of the product (see Supporting Information) shows 10 resonances for the ppz and BPhen ligands (excluding BPhen phenyl protons), indicating the presence of a C2 axis with ppz nitrogens trans across the Ir.34 The complex has a quantum yield of 71% in degassed CH2Cl2 at room temperature and a lifetime of 1.5 μs.
Figure 2. Photoluminescence (PL) spectra of mer (○) and fac (□) Ir(ppz)3:BPhen solutions after irradiation in CH2Cl2 compared with PL spectrum of thermally synthesized Ir(ppz)2BPhen]+PF6− (∇).
Solution-phase experiments were repeated in an NMR tube to directly track formation of product. mer-Ir(ppz)3 and BPhen (1:1.7 mol/mol) were dissolved in acetone-d6 along with excess KPF6. 1H NMR spectra were recorded at time of mixing and after the tube was irradiated for 20 h. The initial solution was clear and colorless with no visible PL, but it took on an orange color after photolysis and displayed orange PL when irradiated with a hand lamp. A diagnostic region of the 1H NMR spectrum for the mixture of mer-Ir(ppz)3 and BPhen before irradiation is shown in spectrum 1, Figure 3. The doublet at δ = 9.25 ppm is assigned
Figure 3. 1H NMR spectra of (1) a mixture of mer-Ir(ppz)3 and BPhen in acetone-d6; (2) the same mixture after 20 h irradiation with 364 nm light; and (3) [Ir(ppz)2(BPhen)]+[PF6]−.
to the BPhen, and the four resonances between δ = 8.40−8.50 ppm are characteristic of the C1 symmetric meridional isomer. After irradiation (spectrum 2), growth of new products are observed as well as conversion to the symmetric facial isomer with its lone doublet at δ = 8.45 ppm. The new resonances observed at δ = 8.74, 8.67, 8.29, and 8.03 ppm are assigned to [Ir(ppz)2(BPhen)]+ by comparison to the thermally prepared sample shown in spectrum 3. The irradiated solution was further analyzed by LDI-MS after evaporation of the solvent. m/z peaks corresponding to [Ir(ppz)2BPhen]+ were observed along with signals from the starting materials. The same experiment was repeated with the facial isomer and similar results were obtained, although the ratios of product peak heights and integrations relative to 6580
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starting materials were diminished. The decreased amount of photoproducts from fac-Ir(ppz)3 indicates a slower rate of reaction, and is consistent with the greater stability of the facial isomer over the meridional analog. To confirm the structure of the photoproduct, we also scaled up the initial experiment, irradiating a solution of 1:2 facIr(ppz)3:BPhen (mol/mol) in CH2Cl2 at 364 nm. The photolyzed mixture was stirred with KPF6 to facilitate isolation of the resultant emissive molecule. After stirring, the mixture was purified by column chromatography and the emissive fraction recrystallized from chloroform by slow vapor diffusion in ether. Unfortunately, the resultant crystals were badly twinned and could not be analyzed by X-ray diffraction. However, the identity of the photoproduct was confirmed by 1 H NMR spectroscopy as the same [Ir(ppz)2BPhen]+ C2 isomer (trans ppz nitrogens) as the thermally prepared product. Calculated Energies for Isomers of Ir(ppz)2(BPhen). Density functional theory calculations were carried out to evaluate the relative energies for possible isomers of [Ir(ppz)2BPhen]+ (Scheme 1). The calculations show the lowest
Figure 4. Electroluminescent (EL) spectra of ITO/NPD/Bphen/LiF/ Al device at 1 mA constant current (time in minutes). (Inset) Normalized EL spectra at turn-on and after operation (times in minutes).
We then constructed devices where a 10 nm layer of either mer- or fac-Ir(ppz)3 was deposited on top of the NPD, creating an emissive stack of NPD/Ir(ppz)3/BPhen. The device architecture, relevant energetics, and EL spectra are depicted in Figures 5 and 6.44,45 This configuration was chosen to create
Scheme 1. Possible Geometric Isomers of Substituted Product, [Ir(ppz)2BPhen]+
energy isomer of this species has the pyrazolyl nitrogens trans and phenyl carbons cis in the Ir coordination shell (structure A in Scheme 1), whereas structures B (pyrazolyls and phenyls, both cis) and C (pyrazolyls cis and phenyls trans) are 0.6 and 9.4 kcal/mol higher, respectively. The energetic preference for structure A conforms to the fact that this isomer is the only one observed and isolated from the photolyzed solutions. Simple replacement of the ppz ligand trans to both phenyls in merIr(ppz)3 would be expected to yield this conformer because the phenyl group is a stronger trans director than pyrazolyl.26 However, ligand substitution in fac-Ir(ppz)3 would be expected to yield structure B whereas structure C is thermodynamically disfavored. Apparently, isomer B rearranges, either during photolysis or thermally during workup, to isomer A under our reaction conditions. Ligand Substitution in OLED Devices. To determine if the same bond dissociation process observed in solution is active in OLEDs, several devices were fabricated and tested. First, a simple NPD/Bphen device was prepared as a control. Clean ITO was coated with 30 nm of NPD, on top of which was deposited 30 nm of BPhen. The device was capped with 1 nm of LiF and 700 nm of Al cathode. Electroluminescence (EL) from this bilayer device is centered at 450 nm, and is likely emission from the NPD layer (Figure 4).35−37 Some contribution from an exciplex state involving the BPhen (LUMO = −1.8 eV) and the NPD (HOMO = −5.1 eV) is also plausible.38−42 In this simple bilayer device the EL spectra remains constant over time even as the EL intensity decreases by other degradation mechanisms.43 The identity of the material(s) responsible for luminescence does not change over the lifetime of the device.
Figure 5. Architecture of blocked OLED devices (top left); HOMO/ LUMO energy levels of relevant materials determined from electrochemical redox potentials44,45 (top right); EL spectra of an ITO/ NPD/mer-Ir(ppz)3/BPhen/LiF/Al device operating at 0.5 mA constant current (times in minutes, bottom). Inset: EL intensity at 653 nm versus time of the same device.
an “electrically blocked” device where charge carriers are preferentially confined to an interfacial region as opposed to being distributed throughout the emissive layer. During operation, holes are readily transported from the NPD layer to Ir(ppz)3 where they accumulate at the Ir(ppz)3/BPhen interface due to the deep HOMO level of BPhen (HOMO = −6.4 eV). Similarly, electrons are blocked in the BPhen layer due to the high LUMO level of Ir(ppz)3 (LUMO = −0.6 eV). The holes and electrons trapped in these layers can then be expected to undergo recombination via exciplex emission between the Ir(ppz)3 (HOMO = −5.1 eV) and BPhen (LUMO 6581
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visual examination, EL from this device changes gradually from pale blue to yellow-orange. The broad exciplex emission between the chemically stable facial isomer and the BPhen ETL is relatively persistent, and conceals the contribution of the new emission feature. Normalized EL spectra (Figure 6, top inset) more clearly demonstrate luminescence from the new product. The rate of the spectral shift for both isomers of Ir(ppz)3 was observed to increase when operated at larger fixed currents (1 mA). Figure 6 (bottom) shows the rates of decrease of initial luminance assigned to the exciplex. The decrease occurs at comparable rates for both mer and fac species, indicating that the materials degrade by similar processes despite the higher lability of the meridional species relative to the facial isomer.31,46 The current−voltage behavior of the OLEDs show decreases in performance during operation that is characteristic of the generation of charge-trapping sites (Figure 7). Turn-on
Figure 7. Luminance−voltage and current−voltage curves for facIr(ppz)3 (□), mer-Ir(ppz)3 (○), and bilayer control (∇) devices at time of turn-on (filled) and after degradation (open).
Figure 6. Top: EL spectra of ITO/NPD/fac-Ir(ppz)3/BPhen/LiF/Al device running under 1 mA constant current (time in minutes). Inset: normalized EL spectra highlighting peak changes over time. Bottom: relative changes in EL intensity versus time for devices using fac- and mer-Ir(ppz)3 monitored at 528 and 554 nm, respectively, and facIr(ppy)3 monitored at 526 nm.
voltages and resistivity increase as expected when the proposed cationic product forms at the Ir(ppz)3/BPhen interface, trapping electrons and inhibiting the flow of holes in the recombination zone. To probe the role of intrinsic lability of the cyclometalating ligand, OLEDs were made with a 10 nm layer of neat fac-Ir(2phenylpyridine)3 (Ir(ppy)3) in place of Ir(ppz)3. Ir(ppy)3 is known for its high quantum efficiency and robust photostability.1,47 Previously, Rabelo de Moraes et al. observed the formation of trace amounts of [Ir(ppy)2(BPhen)]+ by LDI-MS in a severely degraded OLED (7% initial luminance) when using Ir(ppy)3 as an emissive dopant.16 As indicated in Figure 6, devices made with Ir(ppy)3 are considerably more stable. When the devices are driven at 1 mA for 12 h, luminance can be made to decrease to
