Highly Photoluminescent Nonconjugated Polymers for Single-Layer

Feb 23, 2017 - Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications. Zachariah A. Page , Benjapor...
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Highly Photoluminescent Nonconjugated Polymers for Single-Layer Light Emitting Diodes Zachariah A. Page,† Chien-Yang Chiu,† Benjaporn Narupai,†,‡ David S. Laitar,§ Sukrit Mukhopadhyay,§ Anatoliy Sokolov,§ Zachary M. Hudson,† Raghida Bou Zerdan,† Alaina J. McGrath,† John W. Kramer,§ Bryan E. Barton,§ and Craig J. Hawker*,†,‡ †

Materials Research Laboratory and ‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States § The Dow Chemical Company, Midland, Michigan 48674, United States S Supporting Information *

ABSTRACT: The design, synthesis, and characterization of solution-processable polymers for organic light emitting diode (OLED) applications are presented. Theoretical calculations were employed to identify a carbazole-pyrimidine based building block as an optimized host material for the emissive layer of an idealized OLED stack. Efficient, free radical homopolymerization and copolymerization with a novel methacrylate-based heteroleptic iridium(III) complex leads to a library of nonconjugated polymers with pendant semiconductors. Optoelectronic characterization reveals impressive photoluminescence quantum yield (PLQY) values exceeding 80% and single-layer OLEDs show optimal performance for copolymers containing 6 mol % of iridium comonomer dopant. KEYWORDS: polymer, photoluminescence quantum yield, phosphorescence, iridium, organic light emitting diode

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and triplet−triplet annihilation, these complexes are traditionally added as guests (dopants/emitters) into a host polymer matrix. However, physical blending of semiconducting polymer hosts with IrIII small molecules can lead to poor device lifetimes due to phase separation and subsequent aggregate-induced quenching.14 Alternatively, covalently binding IrIII to a polymer improves device longevity and provides a one-component emissive material that reduces the complexity associated with simultaneous materials processing, though at the expense of synthetic complexity and building block choice.15−24 In this report we describe the synthesis of readily available building blocks for high performance, single-layer OLEDs based on a simple comonomer strategy. Design principles for these nonconjugated OLED systems starts with an analysis of energy levels, since they dictate the mechanism by which energy transfer occurs along with emission color. Triplet energy back transfer (from emitter to host) via Dexter transfer has been identified as a common shortcoming among OLEDs. Utilizing a host with a large T1 (>T1 of the emitter)25 or spatially separating the emitter from the host through a nonconjugated spacer are two common methods to reduce energy back transfer.26 In addition to the relative T1 energy

ince the discovery of electroluminescence in polymeric systems by University of Cambridge researchers, significant efforts have been devoted to the design and synthesis of polymer semiconductors for organic light emitting diode (OLED) applications.1 This is driven by the identification of OLEDs as a promising solution to the challenge of energy efficient flat panel displays and solid state lighting.2−6 Although OLEDs constructed from vacuum-deposited small molecule semiconductors currently provide the most efficient devices, polymeric materials offer distinct advantages for commercial manufacturing. These advantages include improved device stability, flexibility, and solution processability that open up large-area and low-cost fabrication techniques (e.g., spincoating, inkjet printing, roll-to-roll, etc.). From a materials design and efficiency viewpoint, phosphorescent heavy metal complexes are traditionally added to these organic systems as they significantly increase the efficiency of singlet-to-triplet intersystem crossing (ISC) resulting in high emission efficiency from both excited states, which is critical given the generation of a 1:3 singlet:triplet under electrical bias.7 In selecting a phosphorescent building block, iridium(III) complexes outperform other heavy-metal phosphors due to their impressive photoluminescence quantum yield (PLQY), stability, short triplet state lifetimes, and spectral tunability from blue to near-infrared.8−13 To suppress concentration quenching © 2017 American Chemical Society

Received: December 15, 2016 Published: February 23, 2017 631

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levels, an idealized emissive layer includes a host with a deep lowest unoccupied molecular orbital (LUMO) for efficient injection from the cathode and its highest occupied molecular orbital (HOMO) energy level should be suitably shallow to accept holes from the anode (Figure 1).

Figure 1. Idealized energy level schematic for an OLED stack showing the anode (And.), host, emissive material (EM), and cathode (Cat.) with desired charge transport mechanisms for holes (h) and electrons (e).

Figure 2. Chemical structures for the six proposed host molecules (A) and their corresponding calculated energy levels relative to Ir(ppy)3 (B). T1, LUMO, and HOMO levels are given from top to bottom, respectively.

From these design guidelines, a select range of building blocks are common to many high performance OLED systems with carbazole and other N-heterocyclic aromatics being widely studied.27−32 Specifically, electron-rich carbazole units provide good hole transport mobility and thermal stability. In contrast, the electron-deficient pyrimidine moiety enables electron transport and lowers the LUMO energy level for improved electron injection. Recently, these two materials have been combined to provide small molecule hosts for efficient OLEDs, where proper energy level alignment can reduce the number of required materials and layers, significantly cutting the cost of device fabrication.33−36 Here the design, synthesis, and characterization of a novel carbazole-pyrimidine monomer and a library of corresponding copolymers with a novel Ir-based building block is presented as an inexpensive, solutionprocessable, one-component, emissive material for OLEDs. Photoluminescent quantum yield measurements highlight efficient charge recombination within thin films of these polymers with their utility as the emissive layer in OLEDs described.

−5.3 eV and LUMO = −1.5 eV) as a host molecule (Figure 2B, see SI). Moreover, by tethering the M6 (host) to an Ir(ppy)3 (emitter) and computing frontier molecular orbitals, it was observed that the LUMO is located on the host and the HOMO and T1 on the emitter (Figure S1B). The hole and electron reorganization energies, which represent the associated penalty for charge transport, were also computed for the six carbazole-based molecules. It was determined that most of the host molecules have similar reorganization energy for electron transport (0.35 to 0.41 eV), apart from M3, which has a lower reorganization energy (0.22 eV; Table S1; see the SI for further computational discussion). However, we chose to investigate M6 given its deeper LUMO energy relative to M3, which can assist electron injection. Given the optimal energy levels calculated for M6, a synthetic strategy was developed for incorporation into a polymer backbone (Scheme 1). The synthesis of the M6 monomer began with a selective Suzuki-Miyaura coupling between commercially available phenyl carbazole boronate ester (1) and 5-bromo-2-iodopyrimidine (2) to yield the brominated carbazole-phenyl-pyrimidine adduct (3). Subsequent SuzukiMiyaura coupling of 3 with potassium vinyltrifluoroborate gave 4, which contains a polymerizable vinyl group. However, due to solubility limitations, this monomer could not to be used directly in a variety of polymerization reactions. To improve the solubility of the M6-monomer, a flexible spacer was then introduced between the building block and a reactive methacrylate unit. Starting from the vinyl derivative, this spacer could be incorporated in two efficient steps involving an initial Brown hydroboration/oxidation, which converted the alkene to a primary alcohol (anti-Markovnikov), 5, which was further reacted through acyl substitution of methacryloyl chloride to yield 6, denoted M6-MA. Recrystallization of M6-MA from cyclohexane provided the monomer in high purity (crystal structure shown in Scheme 1). Significantly, this four-step

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RESULTS AND DISCUSSION Computational studies were initially performed to identify a promising carbazole-containing37−39 host molecule. In order to design an OLED stack, HOMO, LUMO, and T1 energies of six carbazole-based molecules were computed with the molecular structures and the associated energies depicted in Figure 2. Idealized energy levels for the host molecules in an OLED stack are represented by the gray bands in Figure 2B; the LUMO of the chosen host should be deep for facile electron injection, and the HOMO of the host should be comparable to that of the anode in order to inject holes into the emissive layer. In addition, the T1 energy should be higher than that of the dopant (Ir(ppy)3) in order to prevent exciton migration. The T1 energies of all the prospective host molecules were calculated to be higher than Ir(ppy)3 (T1: 2.56 eV). Although these molecules have desirable T1 energies, M6 was determined to have the most desirable frontier orbital energies (HOMO = 632

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Scheme 1. M6-Methacrylate (6, M6-MA) Monomer Synthesis and Crystal Structure Inseta

a Reagents and conditions: (i) Pd(PPh3)4, K3PO4, AQ336, THF/H2O (3:1), 68%; (ii) potassium vinyltrifluoroborate, PdCl2, PPh3, Cs2CO3, THF/ H2O (1:1), 84%; (iii) 9-BBN, NaOH(aq), H2O2(aq), THF, 82%; (iv) methacryloyl chloride, TEA, CH2Cl2, 78%.

Scheme 2. Synthesis of Iridium Methacrylate Monomer (Ir-2C-MA)a

a

Reagents and conditions: (i) AgOTf, 2-(4-(pyridin-2-yl)phenyl)ethanol, DMA, 19%; (ii) methacryloyl chloride, TEA, CH2Cl2, 90%.

solubility of Ir-1C-MA did not allow for homogeneous polymerization at elevated Ir levels. Further studies were then focused on the use of Ir-2C-MA. Reaction of 7 with 2-(4(pyridin-2-yl)phenyl)ethanol led to compound 8, which underwent smooth esterification with methacryloyl chloride to yield the desired monomer Ir-2C-MA (compound 9) as a bright yellow solid, simply isolated through precipitation from hexanes, followed by filtration (Scheme 2).20,41,42 In this case, the solubility was significantly enhanced. Methacrylate-based copolymers could then be synthesized by free radical polymerization in anisole using azobis(isobutyronitrile) (AIBN) at an elevated temperature (70− 100 °C; Scheme 3). Polymerizations carried out at 100 °C were rapid (80%, which suggests that the reactivity of both monomers are similar under the given conditions. The suggested random copolymer architecture would ideally minimize aggregation and associated luminescence quenching and suppress triplet (Dexter) energy back transfer, while maintaining efficient charge injection at the electrodes, charge transport through the emissive layer and radiative charge recombination on the emitter.

In terms of structural characterization, the mol % of IrIII in the polymers was estimated using both 1H NMR and UV−vis absorption spectroscopy. For 1H NMR characterization, the four protons of M6-MA with chemical shifts (δ) ranging from 8.7 to 8.8 ppm and the 11 protons of Ir-2C-MA with δ ranging from 6.5 to 6.9 ppm are well resolved (Figure 3A) and can be used to determine the relative mol % of each comonomer (Figure S3 for copolymers with Ir-1C-MA). Similarly, UV−vis absorption spectroscopy (Figure 3B) was utilized to determine the Ir incorporation by calculating the absorption ratio between 420 nm (absorption only by the Ir-complex) and 350 nm (absorption by both M6 and Ir). Given predetermined molar extinction coefficients for both comonomers and similar UV− vis absorption profiles for the monomer (M6-MA) and homopolymer (poly(M6-MA)), the Beer−Lambert law could be applied to quantify the relative ratios of the two chromophores (Figures S4 and S5 and Table S3). To determine whether the energy levels of the molecules under study were in fact optimal, as suggested by theoretical calculations, they were characterized using a combination of cyclic voltammetry (CV), UV−vis absorption spectroscopy and time-resolved emission spectroscopy (TRES) (Figure 4). HOMO energy levels for poly(M6-MA) and poly(M6-MAco-Ir-2C-MA) containing 29 mol % IrIII were measured using CV (Figure 4A) in dichloromethane with ferrocene as an internal standard. In addition, saturated analogs of both the 634

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levels between −5.6 and −5.7 eV, which is deeper than the ideal range (−5.0 to −5.3 eV), potentially making it difficult to inject holes into the emissive material. On the other hand, the copolymer and Ir-isobutyrate had a HOMO energy level of −5.0 eV, similar to what was calculated. The T1 energies were determined in 2-methyltetrahydrofuran at 77 K using the emission onset (after a 350 μs delay for poly(M6-MA) to remove fluorescence), providing T1 values of 2.76 and 2.57 eV for poly(M6-MA) and poly(M6-MA-co-Ir-2C-MA), respectively, containing 29 mol % IrIII (Figures 4B and S7), which are comparable with the analogous small molecules, M6isobutyrate (2.84 eV) and Ir-isobutyrate (2.57 eV). The T1 energies were very similar to computed values (2.71 and 2.56 eV for M6 host and Ir(ppy)3 dopant, respectively) and are ideal given that the T1 energy of the host is greater than that of the dopant, supporting host-Ir charge transfer and mitigating triplet energy back-transfer. Finally, the energy gap (Eg) values were determined from the absorption onset found with UV−vis absorption spectroscopy on polymer thin films spun cast onto quartz (Figures 4C and S8). Subtracting the HOMO energy level from the optical Eg provided an estimate for the LUMO energy levels given in Figure 4C, finding that the LUMO energy of the host is higher than computed and may act as a barrier to electron injection in OLED devices. Photoluminescence quantum yield (PLQY) values for homopolymer and copolymer samples were measured in order to probe the efficiency of the synthesized polymers. PLQY calibration was accomplished using a thin film of europium doped barium magnesium aluminate in polyethylene.43 The polymer samples were prepared by spincoating thin films onto quartz substrates from anisole and excited with 365 (Figure 5A) or 340 nm (Figure 5B) UV-light, near the maximum absorption of M6-MA. Photoluminescence spectra are given in Figure 5B, where Ir incorporation is varied from 0 (homopolymer) to 29 mol %. The poly(M6-MA) homopolymer sample provided a high fluorescence quantum yield of 73%, which was further increased to 81% for copolymer samples containing 0.5 mol % of the Ir. The high PLQY values were obtained for copolymers containing small amounts of Ir (Figure 5B and Table S3), which steadily decreased upon increasing the concentration of Ir. For example, copolymers containing 3, 6, and 29 mol % Ir had PLQY values of 74, 65, and 23% respectively. Additionally, at very low Ir contents (≤1

Figure 4. Energy levels for poly(M6-MA) and poly(M6-MA-co-Ir-2CMA) with 29 mol % Ir-2C-MA. (A) Cyclic voltammetry used to measure HOMO energy from oxidation onsets, with current (I) vs electrode potential (EWE) normalized to an internal ferrocene reference, taken in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) solution in CH2Cl2. (B) Emission taken in 2-methyltetrahydrofuran at 77 K with an excitation pulse at 340 nm (350 μs delay for poly(M6-MA)). T1 energies determined from onset of emission. (C) Energy levels, where optical energy gap (Eg) was determined from UV−vis absorption and LUMO energies were calculated from Eg − HOMO.

host and Ir monomers (M6-isobutyrate and Ir-isobutyrate respectively, with chemical structures given in Figure S6) were measured for comparison. In all cases the M6 derivatives (small molecules and polymers) were shown to have HOMO energy

Figure 5. Photoluminescence of selected Poly(M6-MA-co-Ir-2C-MA) thin film samples with various Ir mol % incorporation. (A) Image of thin films on quartz discs excited with 365 nm light. (B) Photoluminescence quantum yield measurement showing Rayleigh scattering from excitation line and emission (at 30× intensity relative to excitation counts). (C) Chromaticity profile following 1931 CIE guidelines. 635

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mol %) minor blue fluorescence is observed from 380 to 460 nm, which arises from recombination within the M6-host matrix. However, a slight increase in Ir concentration, >1 mol %, leads to green-only phosphorescence (Table S3). The small amount of Ir-based comonomer leads to minimal host emission, indicating efficient host-Ir charge transfer and radiative recombination. Continuing to increase the Ir concentration leads to a slight bathochromic shift in the emission profile, likely due to Ir aggregation, which also accounts for the reduced PLQY due to triplet−triplet annihilation. Similarly, copolymers synthesized from Ir-1CMA were also tested and found to possess comparable PLQYs (Table S3), which indicates that the nature of the linker does not impact performance with the host-dopant ratio providing a straightforward method to tune the emission color from deep blue (homopolymer) to a green-yellow color, as indicated by the 1931 Commission Internationale de L’Éclairage (CIE) coordinates (Figure 5C). Copolymer samples with varied amounts of M6 and Ir were then incorporated into simple OLED stacks, where both hole and electron transport layers were omitted to allow the relative performance of these novel materials as emissive layers in single-layer devices to be probed. Specifically, the architecture utilized was ITO/AQ1200/poly(M6MA-co-Ir-2C-MA)/LiQ/ Al, where ITO (indium tin oxide) is the anode and AQ1200 and LiQ (8-hydroxy quinolinato lithium)44 act as hole and electron injection layers, respectively (Figure 6). Copolymers Figure 7. Device plots for OLEDs with an architecture of ITO/ AQ1200/poly(M6-MA-co-Ir-2C-MA)/LiQ/Al. (A) Current density− voltage (filled symbols) and luminance−voltage (open symbols); (B) Power efficiency-luminance (filled symbols) and current efficiencyluminance (open symbols).

Although devices with the highest Ir content (13 mol %) were the brightest, plotting power and current efficiency (Figure 7B) with respect to luminance clearly indicates that devices containing copolymers with 6 mol % Ir outperform those with more (13 mol %) and less (1 mol %) Ir, where maximum current efficiencies were 3.6, 2.2, and 1.7 cd/A for 6, 13, and 1 mol % emissive copolymer layers, respectively (Table S4). Device optimization at 6 mol % dopant likely occurs as a result of a compromise between exciton generation (by either charge trapping or exciton hopping) that is efficient at low Ir doping (e.g., 1 mol %), and triplet−triplet annihilation, which occurs readily at higher Ir doping (e.g., 13 mol %).46−49 The discrepancy between optimal PLQY, which was highest for low Ir doping (∼1 mol %), and device performance (6 mol %) likely arises from barriers to charge injection/transport that is alleviated by additional Ir, as evidenced from a drop in turn-on voltage. The shallower HOMO energy level for Ir-2C-MA relative to M6-MA indicates that hole injection/transport improves for devices with more Ir (6 mol %), while too much Ir (13 mol %) leads to unwanted aggregate-induced quenching effects. The deep HOMO energy of M6 may act as a barrier to hole injection, while poor interchain and intrachain electronic coupling can result in low hole mobility.50,51 We are currently working on the preparation of second generation M6-based derivatives that simultaneously raise the HOMO energy level, while also extending the conjugation in order to improve hole injection and transport in single-layer OLED devices.

Figure 6. Electroluminescence spectra for OLEDs with an architecture of ITO/AQ1200/Poly(M6-MA-co-Ir-2C-MA)/LiQ/Al (device structure given as an inset).

containing 1, 6, and 13 mol % Ir-2C-MA repeat units were investigated and the electroluminescence spectra found to be similar to the photoluminescence spectra for the same polymer samples (Figure S9). Little-to-no blue fluorescence was observed from the M6-host matrix with a slight bathochromic shift in emission for devices containing copolymers at higher Ir loadings (Figure 6). Specifically, the full width at half max (fwhm) was 61, 66, and 70 nm for dopant concentrations of 1, 6, and 13 mol %, respectively, which is indicative of Iraggregation.45 The current density−voltage and brightness-voltage curves are plotted in Figure 7A for representative copolymers, and device metrics are provided in Table S4. Among these devices, the turn-on voltage decreases with increasing Ir dopant, from 4.7 V for 1 mol % Ir to 3.7 V for 13 mol %, suggesting improved charge injection and transport with higher Ir content. 636

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Characterization. Nuclear magnetic resonance spectra were recorded on a Varian 400, 500 or 600 MHz spectrometer. Size exclusion chromatography (SEC) for molecular weight analysis, relative to linear polystyrene standards, was performed on a Waters 2690 separation module equipped with Waters 2414 refractive index and 2996 photodiode array detectors using CHCl3 containing 0.25% triethylamine as eluent at a flow rate of 1 mL/min. Mass spectrometry was performed on a Waters GCT Premier time-of-flight mass spectrometer (EI and FD). UV−visible absorption spectra were recorded on a Shimadzu UV3600 spectrometer. Steady state and time-resolved emission spectra were collected on a QuantaMaster 40 PTI photoluminescence spectrometer. Excitation was provided by either a continuous or pulsed Xe arc lamp and the desired excitation wavelength was achieved using a pair of monochromators. Samples were prepared by dissolving the analyte in 2methyltetrahydrofuran (1 mg/mL) and filtering the solution through a 0.2 μm PTFE filter. Low temperature spectra were obtained by placing the sample into a quartz dewar and cooling to 77 K with liquid nitrogen. Time-resolved emission spectra were obtained by delaying data collection by at least 100 μs after excitation. Emission spectra and photoluminescence quantum yield measurements on spin-cast films were obtained on a Hamamatsu Quantaurus-QY absolute PL quantum yield spectrometer. Cyclic voltammograms were performed using a platinum disc, Ag/AgCl, and platinum wire as the working, reference, and auxiliary electrodes, respectively, and oxidation onsets were utilized to calculate the HOMO energy levels (4.8 eV − EoxFerrocene + Eox). Single crystal X-ray diffraction was performed on a Kappa APEX II diffractometer. Synthesis. 9-(4-(5-Bromopyrimidin-2-yl)phenyl)-9H-carbazole (3). To a two-neck, 250 mL round-bottom flask equipped with a magnetic stir bar, inlet adapter, condenser, and septum was added THF (60 mL), 2 M tripotassium phosphate (20 mL, aqueous), and Aliquat 336 (Starks’ catalyst, 0.5 mL). The reaction mixture was degassed with argon for 5 min prior to adding the carbazole-phenyl-boronic ester, 1 (5.0 g, 13.5 mmol), halo-pyrimidine, 2 (3.86 g, 13.5 mmol), and Pd(PPh3)4 (0.78 g, 0.7 mmol). After addition, the mixture was further degassed with argon for 5 min then heated to reflux for 16 h with vigorous stirring. The reaction was then concentrated to remove THF, extracted with CH2Cl2 (3 × 50 mL), washed with H2O (3 × 100 mL), dried with MgSO4 (anh), filtered, and concentrated to obtain a yellow solid. The crude product was purified by column chromatography on silica-gel, eluting with a gradient solvent system: starting at 7:3 Hex/CH2Cl2 and ending at 1:1 Hex/CH2Cl2. The solvent was removed by rotary evaporation, and the resulting solid was crystallized from hot ethanol to provide the desired product after filtration as white crystals (3.69 g, 68% yield). 1H NMR (600 MHz, CDCl3) δ 8.88 (s, 2H), 8.66 (d, J = 8.5 Hz, 2H), 8.16 (d, J = 7.8 Hz, 2H), 7.73 (d, J = 8.5 Hz, 2H), 7.52 (d, J = 8.2 Hz, 2H), 7.46−7.42 (m, 2H), 7.34−7.30 (m, 2H). 13C NMR (151 MHz, CDCl3) δ 162.25, 158.11, 140.62, 140.51, 135.39, 129.92, 127.00, 126.22, 123.80, 120.52, 120.42, 118.65, 110.02. LRMS (FD+) Calcd for C22H14BrN3, 399.04; Found, 399.02. 9-(4-(5-Vinylpyrimidin-2-yl)phenyl)-9H-carbazole (4). To a two-neck, 250 mL round-bottom flask equipped with a magnetic stir bar, inlet adapter, condenser, and septum was added THF and deionized H2O (67 mL of each) and cesium carbonate (16.4 g, 50.2 mmol). The reaction mixture was degassed with argon for 5 min prior to adding 3 (6.7 g, 16.7 mmol), potassium vinyltrifluoroborate (2.69 g, 20.1 mmol),

CONCLUSIONS The design, synthesis, and purification of a novel fluorescent host monomer, M6-MA and two phosphorescent Ir III monomers, Ir-1C-MA and Ir-2C-MA, was accomplished. Free radical copolymerization of these monomers was utilized to provide a series of single-component host−guest materials with varying degrees of polymerization and monomer ratio. 1H NMR and UV−vis absorption spectroscopies were effectively utilized to determine the mol % incorporation of Ir comonomer within the polymers. Photoluminescence quantum yield measurements were performed on all samples, providing impressive values (∼70−80%) for polymer films containing dopant concentrations ≤6 mol %. Utilization of these polymers as one-component emissive materials in simple PhOLEDs led to effective turn-on.

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EXPERIMENTAL SECTION

Materials. Dichlorotetrakis[2-(2-pyridyl)phenyl]diiridium(III) (Ir(C^N)2-μ-Cl]2 (7) was synthesized following the Nonoyama route.40 Tripotassium phosphate (≥98%), hydrogen peroxide solution (30 wt % in H2O), methacryloyl chloride (≥97%, contains 0.02% 2,6-di-tert-butyl-4-methylphenol as stabilizer), triphenylphosphine (99%), cesium carbonate (cabot high-purity grade), 2,2′-azobis(isobutyronitrile) (98%), sodium borohydride (98%), N,N-dimethylacetamide (anhydrous), Aliquat 336, AQ1200, 8-hydroxy quinolinato lithium (LiQ), 2-ethoxyethanol (99%), acetonitrile, toluene (anhydrous), 2-methyltetrahydofuran, and anisole (anhydrous) were purchased from Sigma-Aldrich. 9-[4-(4,4,5,5-Tetramethyl[1,3,2]dioxaborolan-2-yl)phenyl]-9H-carbazole (>95%) and 4(pyridin-2-yl)benzaldehyde (98%) were purchased from AK Scientific. 5-Bromo-2-iodopyrimidine (98%) was obtained from Oakwood Chemical. Tetrakis(triphenylphosphine)palladium(0) (99%), palladium(II) chloride (99.9%), and iridium(III) chloride hydrate (99.9%) were purchased from STREM Chemicals. Potassium vinyltrifluoroborate (98%) and 2-phenylpyridine (97%) were purchased from Combi-Blocks. 9Borabicyclo[3.3.1]nonane (0.5 M solution in THF) and dichloromethane (99.8%, extra dry) were purchased from ACROS Organics. Sodium hydroxide (≥97%), magnesium sulfate (anhydrous), tetrahydrofuran, ethyl acetate, hexane, isopropanol, and acetone were purchased from Fisher Scientific. Ethyl alcohol (200 proof) was purchased from Gold Shield. Triethylamine (99%) was purchased from EMD Millipore Corporation. 2-(Tributylstannyl)pyridine (92%) was purchased from Synthonix. 2-(4-Bromophenyl)ethanol (97%) was obtained from Accela ChemBio Inc. Silver trifluoromethanesulfonate (99%) was purchased from Matrix Scientific. Pixelated anode ITO substrates for OLEDs were purchased from Ossila. Methods. Computation. The ground state (S0) and first excited triplet state (T1) configurations were computed using Density Functional Theory (DFT) with hybrid functional (B3LYP) and 6-31g* basis set. For the Ir atom, modified LANL2DZ basis set was chosen for the core electrons and effective core potential (lanl2dz) was used to describe the valence electrons. The energies of the HOMO and LUMO were obtained from the S0 geometry. The energies of the T1 state were computed as the difference in energy between the minima of S0 and T1 potential energy surfaces (PES). The vibrational analysis on these geometries was performed and the lack of imaginary frequencies helped to ascertain the minima in the PES. 637

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white needles (1.58 g, 78% yield). 1H NMR (400 MHz, CDCl3) δ 8.75 (s, 2H), 8.67 (d, J = 8.5 Hz, 2H), 8.16 (d, J = 7.7 Hz, 2H), 7.72 (d, J = 8.6 Hz, 2H), 7.44 (ddd, J = 8.3, 7.1, 1.2 Hz, 2H), 7.31 (ddd, J = 8.0, 7.1, 1.1 Hz, 2H), 6.13 (t, J = 1.2 Hz, 1H), 5.61 (t, J = 1.6 Hz, 1H), 4.44 (t, J = 6.4 Hz, 2H), 3.06 (t, J = 6.4 Hz, 2H), 1.96 (t, J = 1.3 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 167.21, 162.82, 157.74, 140.70, 140.00, 136.40, 136.02, 129.75, 129.30, 126.97, 126.34, 126.17, 123.73, 120.49, 120.31, 110.02, 64.06, 29.89, 18.45. LRMS (FD+) Calcd for C28H23N3O2, 433.18; Found, 433.17. 2-(4-(Pyridin-2-yl)phenyl)ethanol (Ancillary Ligand for 8). To a dry, 25 mL, two-neck, round-bottom flask equipped with a magnetic stir bar, condenser, inlet adapter, and septum was added 2-(4-bromophenyl)ethanol (2.0 g, 10 mmol) and toluene (10 mL, anh). The mixture was degassed with argon for 5 min prior to adding Pd(PPh3)4 (0.57 g, 0.5 mmol) under a stream of argon, followed by 2-(tributylstannyl)pyridine (4.03 g, 10.9 mmol). The mixture was degassed with argon for 5 min and heated to reflux for 12 h. The reaction was cooled to room temperature, quenched with H2O (50 mL), extracted with EtOAc (3 × 50 mL), dried with MgSO4 (anh), filtered, and concentrated under reduced pressure. The crude oil was added to a silica gel column, wet packed with 7:3:0.01 Hex/EtOAc/ TEA. The product was eluted with a gradient solvent system: starting with 7:3:0.01 and ending at 7:7:0.01 Hex/EtOAc/TEA, and concentrated under reduced pressure to yield the desired product as a yellow oil (1.09 g, 55% yield). 1H NMR (400 MHz, acetone-d6) δ 8.65 (d, J = 4.7 Hz, 1H), 8.03 (d, J = 8.2 Hz, 2H), 7.88 (d, J = 8.0 Hz, 1H), 7.81 (td, J = 7.7, 1.8 Hz, 1H), 7.36 (d, J = 8.2 Hz, 2H), 7.27 (ddd, J = 7.3, 4.8, 1.2 Hz, 1H), 3.80 (t, J = 6.8 Hz, 2H), 2.87 (t, J = 7.0 Hz, 2H). 13C NMR (101 MHz, acetone-d6) δ 157.60, 150.36, 141.56, 137.89, 137.63, 130.14, 127.34, 122.85, 120.61, 63.55, 39.93. Bis[[2-(2-pyridyl)phenyl]4-(pyridin-2-yl)ethanol]-iridium(III) (8). To a two-neck, 100 mL, round-bottom flask equipped with a magnetic stir bar, inlet adapter, condenser and septum was added iridium-dimer, 7 (1.0 g, 0.93 mmol), AgOTf (0.96 g, 3.73 mmol), and dimethylacetamide (15 mL, anh). The mixture was degassed with argon for 10 min, heated to 100 °C for 30 min, followed by the addition of 2-(4-(pyridin-2yl)phenyl)ethanol (0.46 g, 2.33 mmol) under a stream of argon. The reaction was then heated to 130 °C for 5 h, cooled to room temperature, and filtered and washed with acetonitrile to remove silver salts. The filtrate was added to deionized H2O (160 mL) and the resulting precipitate was filtered, then dissolved in CH2Cl2 (50 mL) and washed with deionized H2O (3 × 50 mL). The organic layer was dried over MgSO4 (anh), filtered and concentrated under reduced pressure to yield a yellow-brown solid. The crude product was purified by column chromatography on silica gel, eluting with a gradient starting at 1:0 and ending at 0.9:0.1 CH2Cl2/EtOAc. The solvent was removed by rotary evaporation and the resulting solid was precipitated from CH2Cl2 into hot hexanes to provide the desired product after filtration as a bright yellow powder (248 mg, 19% yield). 1H NMR (400 MHz, DMSO-d6) δ 8.18−8.02 (m, 3H), 7.83−7.59 (m, 6H), 7.50−7.37 (m, 3H), 7.08 (dq, J = 13.9, 6.4, 4.9 Hz, 3H), 6.80 (td, J = 8.2, 7.6, 3.3 Hz, 2H), 6.69 (d, J = 4.3 Hz, 5H), 6.51 (s, 1H), 4.49 (s, 1H), 3.35 (t, J = 7.6 Hz, 2H), 2.39 (t, J = 7.6 Hz, 2H). 13C NMR (101 MHz, DMSO-d6) δ 165.61, 160.91, 160.86, 160.76, 146.78, 146.72, 146.67, 143.78, 141.83, 139.63, 136.85, 136.79, 136.74, 136.35, 136.32, 129.16, 129.13, 124.16, 124.06, 122.80, 122.37, 120.42,

PdCl2 (59 mg, 0.3 mmol), and PPh3 (263 mg, 1 mmol). After addition, the mixture was further degassed with argon for 5 min then heated to reflux for 6 h with vigorous stirring. The reaction was concentrated to remove THF, extracted with CH2Cl2 (3 × 50 mL), washed with H2O (3 × 100 mL), dried with MgSO4 (anh), filtered, and concentrated to obtain a yellow−brown solid. The crude product was purified by column chromatography on silica gel, eluting with a gradient solvent system: starting at 7:3 Hex/CH2Cl2 and ending at 1:1 Hex/CH2Cl2. The solvent was removed by rotary evaporation and the resulting solid was crystallized from hot ethanol to provide the desired product after filtration as white crystals (4.87 g, 84% yield). 1H NMR (400 MHz, CDCl3) δ 8.87 (s, 2H), 8.69 (d, J = 8.4 Hz, 2H), 8.17 (d, J = 7.8 Hz, 2H), 7.73 (d, J = 8.4 Hz, 2H), 7.53 (d, J = 8.2 Hz, 2H), 7.44 (t, J = 7.6 Hz, 2H), 7.32 (t, J = 7.4 Hz, 2H), 6.73 (dd, J = 17.7, 11.1 Hz, 1H), 5.99 (d, J = 17.8 Hz, 1H), 5.53 (d, J = 11.1 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 162.96, 154.88, 140.64, 140.04, 136.26, 130.39, 129.78, 128.70, 126.96, 126.17, 123.71, 120.48, 120.31, 117.91, 110.03. LRMS (FD+) Calcd for C24H17N3, 347.14; Found, 347.13. 2-(2-(4-(9H-Carbazol-9-yl)phenyl)pyrimidin-5-yl)ethanol (5). To a dry two-neck, 500 mL round-bottom flask equipped with a magnetic stir bar, inlet adapter, and septum was added 4 (4.8 g, 13.8 mmol) and THF (145 mL, anh). The mixture was degassed with argon for 10 min followed by the addition of a 0.5 M 9-BBN solution in THF (55.3 mL, 27.6 mmol). The reaction mixture was stirred at room temperature for 12 h, then cooled in an ice bath prior to adding 50 wt % NaOH(aq) (22 mL) followed by 30 wt % H2O2(aq) (47 mL). The reaction was warmed to room temperature, stirred for 2 h, concentrated, partitioned between CH2Cl2 and H2O (50 mL each), extracted with CH2Cl2 (3 × 50 mL), washed with water (2 × 100 mL), dried with MgSO4 (anh), filtered, and concentrated to obtain a yellow oil. The crude product was purified by column chromatography on silica gel, eluting with a gradient solvent system: starting at 1:0 CH2Cl2/EtOAc and ending at 4:1 CH2Cl2/EtOAc. The solvent was removed by rotary evaporation and the resulting solid was crystallized from hot hexanes to provide the desired product as white crystals (4.14 g, 82% yield). 1H NMR (600 MHz, CD2Cl2) δ 8.73 (s, 2H), 8.68 (d, J = 8.5 Hz, 2H), 8.17 (d, J = 7.8 Hz, 2H), 7.72 (d, J = 8.5 Hz, 2H), 7.54 (d, J = 8.2 Hz, 2H), 7.45 (t, J = 8.2 Hz, 2H), 7.32 (t, J = 7.7 Hz, 2H), 3.91 (t, J = 6.3 Hz, 2H), 2.87 (t, J = 6.3 Hz, 2H). 13 C NMR (151 MHz, CD2Cl2) δ 162.61, 158.25, 141.14, 140.16, 137.02, 130.86, 130.01, 127.26, 126.60, 124.06, 120.79, 120.69, 110.47, 62.95, 33.96. LRMS (FD+) Calcd for C24H19N3O, 365.15; Found, 365.15. 2-(2-(4-(9H-Carbazol-9-yl)phenyl)pyrimidin-5-yl)ethyl Methacrylate (6, M6-MA). To a dry two-neck, 100 mL roundbottom flask equipped with a magnetic stir bar, inlet adapter, and septum was added 5 (1.7 g, 4.7 mmol), CH2Cl2 (34 mL, anh), and TEA (2.6 mL, 18.6 mmol). The mixture was degassed with argon for 10 min and cooled to 0 °C, followed by dropwise addition of methacryloyl chloride (0.9 mL, 9.3 mmol). The reaction mixture was stirred at 0 °C for 10 min under argon then warmed to room temperature and stirred for an additional hour. The reaction mixture was concentrated and added to a silica gel column, wet packed with 1:0.99:0.01 Hex/ CH2Cl2/TEA. The product was eluted with a gradient solvent system: starting at 1:1 and ending at 1:3 Hex/CH2Cl2, concentrated under reduced pressure then crystallized from cyclohexane to provide the desired product after filtration as 638

DOI: 10.1021/acsphotonics.6b00994 ACS Photonics 2017, 4, 631−641

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119.58, 119.55, 119.07, 119.04, 118.77. LRMS (FD+) Calcd for C35H28IrN3O, 699.19; Found, 699.14. Bis[[2-(2-pyridyl)phenyl]4-(pyridin-2-yl)oxyethyl methacrylate]-iridium(III) (9, Ir-2C-MA). The methacylate derivative, 9, was prepared following the same general acylation procedure used to synthesize compound 6 (bright yellow powder, 90% yield). 1H NMR (400 MHz, CDCl3) δ 7.89−7.73 (m, 3H), 7.67−7.42 (m, 9H), 6.95−6.67 (m, 11H), 6.02 (s, 1H), 5.48 (s, 1H), 4.20 (t, J = 7.1 Hz, 2H), 2.74 (t, J = 7.0 Hz, 2H), 1.87 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 167.58, 166.75, 166.72, 166.55, 161.59, 161.34, 161.17, 147.15, 147.07, 147.05, 143.76, 143.72, 142.32, 139.29, 137.89, 137.27, 137.23, 136.43, 136.02, 135.99, 129.93, 129.90, 125.54, 123.98, 123.96, 123.94, 122.03, 122.01, 121.80, 120.63, 119.87, 119.84, 118.83, 118.81, 118.69, 65.71, 35.26, 18.53. LRMS (FD+) Calcd for C39H32IrN3O2, 767.21; Found, 767.18. General Free Radical Polymerization Procedure (3 mol % IrIII). To a 4 mL scintillation vial equipped with a magnetic stir bar and septum was added M6-MA (95 mg, 0.22 mmol), Ir2C-MA (5.2 mg, 0.007 mmol), anisole (240 μL, anh), and AIBN (46 μL of a 4 mg/mL solution in anisole). The vial was degassed with argon for 5 min prior to heating to 100 °C for 1 h. The viscous solution was cooled in an ice bath, diluted with CH2Cl2 (1 mL), precipitated into acetone, filtered and washed with acetone to provide the polymer as a light yellow solid (typical yields were 70−80%, see Table S3 for further characterization details). OLED Device Fabrication and Testing. Pixelated ITO substrates were precleaned at room temperature by sequential sonication in Hellmanex detergent (10 wt % in deionized water) for 10 min, deionized water, acetone and isopropanol for 5 min each. The substrates were dried under a stream of nitrogen, treated with oxygen plasma for 10 min at an RF power of 100 MHz and an O2 pressure of approximately 200 mTorr. After plasma treatment, the substrates were brought into a N2 glovebox for device fabrication. The hole injection layer, AQ1200 (Plexcore), was filtered through a 0.45 μM PVDF syringe filter prior to use. The AQ1200 was spin-cast by dropping 35 μL onto substrates at a spin speed of 5000 rpm, allowing the device to spin to dryness over the course of 20 s, and annealing at 150 °C for 20−30 min. To prepare the OLEDs, the emissive polymers were dissolved at approximately 2 wt % concentrations in anisole. The solutions were filtered through a 0.2 μm PTFE filter and subsequently spin-coated directly onto the AQ1200 layer at varying spin speeds in order to achieve a thickness of approximately 30 nm. The thickness optimization was carried out on silicon wafers and measured with ellipsometry to ensure consistent thicknesses prior to device manufacturing. The OLED devices were completed by thermally evaporating lithium quinolate (LiQ; 2 nm) as an electron injection layer and aluminum (100 nm) as the cathode through a shadow mask. The layer thickness and the deposition rate of materials were checked in situ by an oscillating quartz thickness monitor. The luminance−voltage−current (JVL) characteristics were measured using a custom built JVL instrument from Ossila Inc.

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Further synthetic details, computational studies, and characterization (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Craig J. Hawker: 0000-0001-9951-851X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the MRSEC program of the National Science Foundation (DMR 1121053) and The Dow Chemical Company through the Dow Materials Institute at UCSB for financial support. We thank Dr. Hongjun Zhou for help with NMR, Dr. Guang Wu for help with X-ray crystallography, and Dr. James Pavlovich for help with mass spectrometry. We thank Drs. David Devore (Dow) and Bryan Barton (Dow) for insightful discussions.

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REFERENCES

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