Polynorbornene Copolymer with Side-Chain Iridium(III) Emitters and

Publication Date (Web): January 14, 2013 ... Synthesis of High-Triplet-Energy Host Polymer for Blue and White Electrophosphorescent Light-Emitting Dio...
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Polynorbornene Copolymer with Side-Chain Iridium(III) Emitters and Carbazole Hosts: A Single Emissive Layer Material for Highly Efficient Electrophosphorescent Devices Jun Ha Park,†,⊥ Tae-Wook Koh,‡,⊥ Jin Chung,‡ Sung Hoon Park,§ Maengsun Eo,† Youngkyu Do,†,* Seunghyup Yoo,‡,* and Min Hyung Lee§,* †

Department of Chemistry, KAIST, Daejeon 305−701, Republic of Korea Department of Electrical Engineering, KAIST, Daejeon 305−701, Republic of Korea § Department of Chemistry and EHSRC, University of Ulsan, Ulsan 680−749, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Vinyl addition copolymerization of norbornene monomers using a Pd(II) catalyst in combination with 1octene chain transfer agent efficiently produces well-defined soluble polynorbornene copolymers bearing side-chain (C∧N)2Ir(O∧O) emitters (C∧N = 2-(4,6-difluorophenyl)pyridine (M 3 ); 2-phenyl-pyridine (M 4 ); 2-(benzo[b]thiophen-2-yl)-pyridine (M5), O∧O = acetylacetonato) and 9,9′-(1,3-phenylene)bis-9H-carbazole (mCP) or 9,9′-(1,1′biphenyl)-4,4′-diylbis-9H-carbazole (CBP) host moieties (M1 and M2). The catalytic system provides high-molecular-weight copolymers (Mw = 151 000−457 000 g/mol) with a controlled incorporation of monomers. All copolymers possess high thermal stability with high decomposition (Td5 > 400 °C) and glass transition temperatures (Tg > 330 °C). Among the solutionprocessed devices fabricated based on a single emissive layer comprising the blue-, green-, and red-phosphorescent copolymers (PBn, PGn, and PRn, n = 1−4) with various concentrations of emitters (1.7−13.9 mol %-Ir), the devices based on PB4 (10.5 mol %-Ir), PG2 (5.3 mol %-Ir), and PR4 (13.9 mol %-Ir) display the best performances with maximum power efficiencies of 12.9, 25.6, and 3.3 lm/W and maximum external quantum efficiencies of 8.8, 13.3, and 5.1%, respectively, for each color. These results correspond to almost double the efficiencies of the corresponding doped polymer systems and are outstanding among the polymeric rivals reported thus far.

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INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs) based on heavy metal complexes have attracted great attention due to their excellent light-emitting properties, overcoming the efficiency bottleneck limited by the singlet-to-triplet ratio.1−5 Strong spin−orbit coupling induced by heavy metal atoms facilitates the efficient intersystem crossing from singlet to triplet excited state, allowing the complexes to fully utilize both singlet and triplet excitons and thereby theoretically achieve 100% of internal quantum efficiency.5,6 Among the complexes, cyclometalated iridium(III) complexes are considered the most promising electroactive materials because of excellent emitting properties, such as a relatively short phosphorescent lifetime, high photoluminescence efficiency, and facile color tunability.2,4,7,8 To prevent triplet−triplet annihilation and concentration quenching, which can severely compromise device performance particularly at mid to high brightness level,9 phosphorescent emitters in PhOLEDs are usually adopted as dopants dispersed into appropriate small molecular or polymeric host materials through a vacuum deposition or solution process, respectively.10 However, the emissive layers © 2013 American Chemical Society

prepared by doping phosphorescent emitters into polymeric hosts are prone to suffer from phase separation, i.e., aggregation of phosphorescent emitters, which often results in the significant reduction of device performance through the quenching processes mentioned above.11,12 A viable strategy to surmount this problem would be to fabricate PhOLED devices using a single emissive copolymer incorporating both host moiety and iridium(III) emitter. In this regard, many studies about conjugated copolymers in which iridium complexes are either covalently linked into13,14 or attached as the side chain of the host polymer backbone15,16 have been reported. In addition, side-chain copolymers, in which iridium complexes and host moieties are attached to the polymer backbone as pendant side groups, have been also investigated extensively by several groups.11,17−19 However, the performances of the resulting PhOLED devices based on such copolymers have lagged far behind those of vacuum deposited Received: November 13, 2012 Revised: December 14, 2012 Published: January 14, 2013 674

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Scheme 1. Synthesis of Monomers, M3−M5a

(i) NaH, THF, 0 °C. (ii) n-BuLi, THF, −20 °C. (iii) THF, 0 °C. (iv) [(4,6-dfppy)2Ir(μ-Cl)]2, Na2CO3, acetonitrile, reflux. (v) [(ppy)2Ir(μ-Cl)]2, Na2CO3, acetonitrile, reflux. (vi) [(btp)2Ir(μ-Cl)]2, Na2CO3, acetonitrile, reflux.

a

phase separation induced by aggregation. Hence, the incorporation of both CBP or mCP host moieties and iridium complexes into a single polynorbornene backbone as pendant side groups could be considered a promising approach to improve the device efficiency of PhOLEDs, by removing any phase separation or aggregation problem, and thereby providing a homogeneous emissive layer. In this study, we prepared vinyl-type polynorbornene copolymers containing side-chain CBP or mCP moieties with various iridium complexes by use of palladium(II) catalyst to provide a new type of highly efficient, single emissive layer materials for blue, green, and red PhOLEDs. Details of their synthesis and use as emissive materials in actual PhOLEDs are fully described to demonstrate that the proposed approach can provide high-performance RGB PhOLEDs based on a solution process.

counterparts in most cases, although Tokito and co-workers have shown high device performances based on random copolymers bearing blue (EQE = 3.5%), green (9%), or red (5.5%)-emitting iridium complexes and carbazole moieties as pendant side groups tethered to a polyethylene backbone.19 In order to achieve high device efficiency, it would thus be ideal to develop polymeric materials that contain both an established small-molecular host, such as 9,9′-(1,1′-biphenyl)4,4′-diylbis-9H-carbazole (CBP)4,20 (ET = 2.56 eV) or 9,9′(1,3-phenylene)bis-9H-carbazole (mCP)21 (ET = 2.90 eV) and a phosphorescent iridium complex such as (C∧N)2Ir(acac) (C∧N = cyclometalating ligand; acac = acetylacetonato)3,8,22 as pendant side groups within the polymer chain, so as to retain both the emitting properties of the iridium complex and the charge carrier transport properties of host moiety with its proper triplet energy (ET). Recently, we have demonstrated that vinyl-type polynorbornenes with various side groups that possess hole-transporting or host property can function as efficient polymeric materials for OLEDs applications.23−26 The successful demonstration was partly due to the excellent thermal and optical properties, as well as the amorphous nature of the parent vinyl-type polynorbornenes which have a saturated polymer backbone comprising bicyclic structural units.27 The polynorbornenes are also known to be efficiently produced by the transition metal catalysts such as Pd(II) and Ni(II) catalysts via the vinyl-addition polymerization of norbornene.28 It was revealed from our previous reports that polynorbornenes with CBP or mCP side groups as host material can show relatively high device performances in green (EQE = 7.2%) or blue (EQE = 4.3%) PhOLEDs.23,24 These results suggest that the electronic properties of host moieties are effectively retained in the polymer structure. Despite high device performances when compared to the existing polymeric host materials, however, the efficiency of the devices based on such polynorbornene hosts doped with phosphorescent emitters was found to be still lower than those of small molecular, vacuum-deposited counterparts, possibly due to the

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RESULTS AND DISCUSSION Synthesis and Characterization. The mCP- and CBPfunctionalized norbornene monomers M1 and M2 were prepared according to our reported procedures.23,24 For the blue-, green-, and red-emitting iridium(III) monomers M3−M5, the norbornene moiety was introduced into the O∧O ancillary ligand of heteroleptic cyclometalated iridium complexes of a general formula (C ∧ N) 2 Ir(O ∧ O) (C ∧ N = 2-(4,6difluorophenyl)pyridine (4,6-dfppy); 2-phenylpyridine (ppy); 2-(benzo[b]thiophen-2-yl)-pyridine (btp), O∧O = acetylacetonato) so as not to alter the photophysical properties of (C∧N)2Ir moieties (Scheme 1). 5-Norbornene-2-pentyl bromide (3) 29 was reacted with the bis-enolate of 2,4pentanedione obtained from a consecutive treatment with NaH followed by n-BuLi,16 producing the norbornenecontaining ancillary ligand, 1-(5-(bicyclo[2.2.1]hept-5-en-2yl)pentyl)pentane-2,4-dione (4) in moderate yield (55%). Subsequent reaction of 4 with chloro-bridged iridium(III) dimers led to M3, M4, and M5 monomers with 29%, 68%, and 67% yield, respectively, after purification by column chroma675

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Scheme 2. Synthesis of Copolymers, PBn, PGn, and PRna

a

[Pd(II)] = [(NHC)Pd(η3-allyl)][SbF6].

Table 1. Copolymerization Results of Host (M1−M2) and Emitter (M3−M5) Monomers with Pd(II) Catalysta copolymer a

PB1 PB2f PB3g PB4h PG1a PG2f PG3g PG4h PR1a PR2f PR3g PR4h

monomers

comonomer feed ratio (mol %)

convn (%)

comonomer composition (mol %)b

10−3Mwc

Mw/Mnc

Tg (°C)d

Td5 (°C)e

M1:M3 M1:M3 M1:M3 M1:M3 M2:M4 M2:M4 M2:M4 M2:M4 M2:M5 M2:M5 M2:M5 M2:M5

97:3 93.5:6.5 90:10 86:14 97:3 93.5:6.5 90:10 86:14 97:3 93.5:6.5 90:10 86:14

37 32 35 37 46 46 40 45 43 39 38 32

98.3:1.7 96.6:3.4 92.2:7.8 89.5:10.5 96.6:3.4 94.7:5.3 91.4:8.6 87.8:12.2 96.0:4.0 93.0:7.0 89.9:10.1 86.1:13.9

403 356 431 457 408 433 362 386 262 228 216 151

2.68 2.27 2.43 2.80 2.72 2.93 2.79 2.76 2.74 2.58 2.36 2.02

i i i i 339 367 362 339 367 372 333 365

433 427 416 413 439 431 421 401 439 437 413 413

Conditions: catalyst = [(NHC)Pd(η3-allyl)Cl]/AgSbF6; [Pd]/[AgSbF6] = 1/1.5; [Pd] = 1.43 μmol; [Mon.]/[Pd] = 1,000; solvent: 15 mL of chlorobenzene; Tp = 25 °C; tp = 20 h; chain transfer agent (1-octene) to monomer = 0.05 mol %. bDetermined by elemental analysis. cDetermined by GPC. dDetermined by DSC. eDetermined by TGA at 5% weight loss. f[Pd] = 1.49 μmol. g[Pd] = 1.55 μmol. h[Pd] = 1.62 μmol. iNot observed. a

tography. The formation of M3−M5 was confirmed by 1H and 13 C NMR spectroscopy, mass spectrometry, and elemental analysis (Figures S2−S4 in the Supporting Information). Following our published polymerization procedure,26 the copolymerization reactions were investigated using a pair of host and emitter monomers that were chosen on the basis of their triplet energy, i.e., M1 (ET = 2.97 eV)23 with M3 (2.61 eV),30 and M2 (2.60 eV)24 with M4 (2.41 eV)8,31,32 or M5 (2.02 eV).32 The polymerization was carried out with cationic Pd(II) catalyst derived from the in situ reaction of N-heterocyclic carbene (NHC; N,N′-bis(2,6-diisopropylphenyl)-imidazol-2ylidene) complex, [(NHC)Pd(η3-allyl)Cl]33 with AgSbF6 in the presence of 0.05 mol % of 1-octene chain transfer agent (Scheme 2). Furthermore, a set of copolymerization reactions was performed by varying the feed ratio of the host monomer and iridium monomer from 97:3 to 86:14 mol % to optimize the emitter content in the copolymer (Table 1). As shown in

Table 1, all the copolymerization reactions produced the corresponding copolymers (PBn, PGn, PRn) with 30−50% conversion, which is similar to that observed for the homopolymerization of M1 and M2 monomers.23,24 This result indicates that the presence of (C∧N)2Ir functionality does not significantly affect the vinyl-addition polymerization by Pd(II) catalyst. It is notable that the M2/M4 copolymerization (PGn) showed an overall high level of conversion in comparison with those for the M 1 /M 3 and M 2 /M 5 copolymerizations. Interestingly, the conversion of M2/M5 copolymerization (PRn) apparently decreased with the increasing M5 feed amount, indicating the sulfur-containing monomer had an impact on the polymerization reactions. Characterization of the copolymers by elemental analysis revealed that the incorporation of iridium monomers into a polymer chain is quite facile and linearly proportional to the feed amount, leading to comonomer compositions ranging from 1.7 to 13.9 mol %. In 676

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Indeed, the intensity of those peaks gradually increases with increasing M4 feed ratio; i.e., it increases from PG1 to PG4 copolymer, as consistent with the M4 contents determined from elemental analyses. Thermal and Optical Properties. Thermal properties of copolymers were characterized using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). For example, TGA and DSC curves for PGn copolymers are shown in Figure 2 (see also Figures S9 and S10, Supporting

particular, the PRn copolymers possessed the highest amount of iridium monomer under the same polymerization conditions, while the PBn copolymers are lowest in content. This result might suggest the presence of a Lewis acid−base interaction between the sulfur atom of M5 monomer and a Pd(II) center inducing facile M5 incorporation while retarding M2 monomer insertions, i.e., the lowering of conversion upon increasing of the M5 feed amount. All copolymers are highly soluble in common organic solvents such as chloroform, dichloromethane, tetrahydrofuran, and chlorobenzene at room temperature. Gel-permeation chromatography (GPC) measurements indicated the formation of high-molecular-weight copolymers (Mw ≈ 151 000−457 000 g/mol) with a relatively narrow polydispersity index (PDI = 2.02−2.93) indicative of the involvement of single catalytic active species in the copolymerization. However, the PRn copolymers showed overall low molecular weight values which decrease with increasing M5 feed, supporting the presence of a sulfur−Pd(II) interaction. The chemical structures of copolymers were identified by 1H and 13C NMR spectroscopy, which confirmed the formation of vinyl-type polynorbornene with pendant iridium and carbazole moieties. For example, the 1H NMR spectra of PGn copolymers are shown in Figure 1. There is no vinyl proton

Figure 2. (a) TGA and (b) DSC curves of PG1−PG4.

Information). TGA measurements revealed that all PBn, PGn, and PRn copolymers possess high thermal stability with decomposition temperatures (Td5, temperature at 5% weight loss) above 413, 401, and 413 °C, respectively. In particular, the Td5s of copolymers gradually decrease with the increasing content of iridium moiety (M3−M5) in the copolymers probably due to the lower thermal stability of (C∧N)2Ir moieties (e.g., Td5 = 341 °C for (ppy)2Ir(acac)34 and 368 °C for (btp)2Ir(acac)35), than those of mCP- and CBPhomopolymers (Td5 = 410 and 451 °C).23,24 DSC analysis showed high glass transition temperatures (Tg) of 339−367 °C and 333−372 °C for PGn and PRn, respectively, with no other crystallization and melting peaks. The range of Tg values is also comparable to that of CBP-homopolymer (361 °C).24 For the PBn copolymers, no distinct glass transition was observed from DSC measurements. However, it can be envisaged that the glass transition of PBn will also occur at high temperature, judging by the high Tg (268 °C) of mCP-homopolymer.23 The high thermal stability and the amorphous nature of copolymers are highly desirable for OLED applications, particularly if fabricated by solution process. The UV/vis absorption and PL spectra of copolymers (PBn, PGn, and PRn) were measured in CH2Cl2 solution and are shown in Figure 3. The absorption spectra feature the major

Figure 1. 1H NMR spectra of PG1−PG4. The numbers in parentheses are the feed ratio of M2 and M4 monomers (∗ and † from residual CHCl3 and H2O, respectively, in CDCl3).

peak at δ 6.0 ppm and the peak broadening at the aliphatic region indicates a rigid polynorbornene backbone structure. The broad aromatic resonances for the CBP moiety of M2 monomer are identical to those found in the 1H NMR spectrum of the reported CBP-homopolymer.24 In particular, the small proton signals at around δ 5.0, 6.2, 6.5, and 8.4 ppm, which are assignable to the proton resonances for the iridium moiety of M4 monomer, are clearly observed and are different in intensity depending on the feed ratio of M4 monomer. 677

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Figure 3. UV/vis absorption and PL spectra of (a) PB1−PB4, (b) PG1−PG4, and (c) PR1−PR4 in CH2Cl2 (2.0 × 10−5 M) at 298 K. The inset shows the absorption band in the MLCT region of PB4, PG4, and PR4, respectively.

absorption bands at 293, 329, and 340 nm for PBn and at 295 and 325 nm for PGn and PRn assignable to the π−π* transitions in the carbazole groups (mCP and CBP, respectively), which is almost identical to those of the host monomers (M1 and M2) (Figure S11a, Supporting Information). The dominant absorption by the host moiety can be attributed to a much higher concentration of host moiety than iridium in the copolymers, as well as the spin-allowed nature of singlet transition (1π−π*). In addition, the very weak absorption bands assignable to the metal-to-ligand charge transfer (MLCT) transition in the iridium moieties are clearly observed in the low energy regions (ca. 370−550 nm) (Insets in Figure 3). The MLCT bands are well matched with those for the M3−M5 monomers (Figure S11b, Supporting Information) and the known (C∧N)2Ir(acac) complexes.3,8 These absorption features are essentially identical between each set of PBn, PGn, and PRn copolymers. Moreover, the intensity of the MLCT bands increases with the increasing content of iridium moiety in the copolymers. On the other hand, the PL spectra obtained in solution at room temperature displayed two main emission bands centered in the high and low energy regions (Figure 3). While the former band, which is identical to that of the M1 or M2 monomer, can be assigned to the fluorescence of host moieties, the band at the low energy region corresponds to phosphorescence (3MLCT/3LC) of iridium moieties (Figure S11, Supporting Information). This observation indicates that energy transfer from pendant host moieties to iridium within a polymer chain is inefficient in solution. It can also be noted that the intensity of the phosphorescence band increases with the increasing content of the iridium moiety (n = 1 → 4) and the relative intensity to the host emission is in the order of PBn > PGn > PRn copolymers, following the quantum efficiency of the respective (C∧N)2Ir moiety (Φ = 0.34 for (ppy)2Ir(acac);8 0.6 for (4,6-dfppy)2Ir(acac);22,30 0.21 for (btp)2Ir(acac)3). In sharp contrast, the PL spectra of thin films of copolymers exclusively featured the intense blue, green, and red emission originating from the iridium moieties of each copolymer although the host emission is concomitantly observed in the copolymers having the lowest amount of iridium moiety (PG1 and PR1) (Figure 4). This result indicates that the energy transfer from host moieties to iridium efficiently occurs in a rigid matrix like a thin film, rendering the copolymers feasible for use as a single emissive layer in PhOLED devices. Electroluminescent Properties. To examine the lightemitting properties of the copolymers, blue, green, and red PhOLED devices based on PBn, PGn, and PRn as a single

Figure 4. PL spectra of thin film of PB1−PB4, PG1−PG4, and PR1− PR4.

emissive layer were fabricated in the following configuration: ITO/PEDOT:PSS (40 nm)/polymeric emissive layer (40 nm)/TPBi (15 nm)/Bphen (35 nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS is poly(3,4ethylenedioxythiophene):poly(styrenesulfonate); TPBi is 2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole); Bphen is 4,7-diphenyl-1,10-phenanthroline. The emissive layers were constructed on top of PEDOT:PSS layers by spin-casting the polymers PBn, PGn, and PRn dissolved in chlorobenzene, respectively. As shown in Figure 5, devices D1−D12 exhibited blue, green, and red phosphorescence with the peak emission wavelength (λem) of 488, 522, and 619 nm, respectively,

Figure 5. EL spectra of devices (D1−D12) fabricated with PBn, PGn, and PRn as a single emissive layer. 678

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Figure 6. Current density−applied voltage−luminance (J−V−L) characteristics of devices with (a) PBn (D1−D4), (b) PGn (D5−D8), and (c) PRn (D9−D12) and external quantum efficiency−luminance (ηEQE−L) curves of devices with (d) PBn (D1−D4), (e) PGn (D5−D8), and (f) PRn (D9−D12).

(e.g., EQEmax = 3.5−4.3%,19,23 7.2−11.8%,18,24,36 and 5.5− 6.5%14,19 for blue, green, and red PhOLEDs, respectively). To further elucidate the high performance of the proposed copolymer system compared to the physical blend system, reference devices based on CBP-homopolymer doped with (ppy)2Ir(acac) emitter were also fabricated and compared with the devices based on green-phosphorescent copolymers. Among the devices (R1−R3) with a different doping ratio (Table 2 and Table S1, Supporting Information), the one having 6 wt % of emitter (R2) showed the highest efficiencies in terms of ηEQE (6.5%) and ηPE (11.5 lm/W) (see Figures S13−S15 (Supporting Information) for the EL characteristics of R1−R3), comparable to the values we reported previously with a CBP-homopolymer host and fac-Ir(ppy)3 dopant blend system.24 In comparison, the efficiencies of device D6, based on a PG2 copolymer system, are almost two times higher than those of device R2, clearly indicating the significant advantage of the copolymer system. A similar enhancement in performance is also found in the blue PhOLED devices (D4, EQE = 8.8%) in comparison with the devices based on mCPhomopolymer with Firpic (EQEmax = 4.3%).23 Therefore, these results suggest that device performances of PhOLEDs can be largely improved by using a single host-emitter layer material, presumably due to the suppression of phase separation or dopant aggregation that is known to have a negative effect on OLED performance in Ir-based PhOLEDs.37

indicating that light emissions were only from phosphorescent iridium moieties by complete energy transfers from host to iridium moieties in the copolymer. Current density−voltage (J−V) and luminance−current density (L−J) characteristics for the devices (D1−D12) are displayed in Figure 6. According to external quantum efficiency (ηEQE) and power efficiency (ηPE) values (Figure 6, Figure S12 (Supporting Information), and Table 2), D4 (blue), D6 Table 2. Device Performance of PhOLEDs Incorporating a Single Polymeric Emissive Layer (PBn, PGn, and PRn) device

copolymer

D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 R2b

PB1 PB2 PB3 PB4 PG1 PG2 PG3 PG4 PR1 PR2 PR3 PR4

concn of Ir(III) (mol %)

Vturn‑on (V)a

Lmax (cd/m2)

ηEQEmax (%)

ηPEmax (lm/ W)

1.7 3.4 7.8 10.5 3.4 5.3 8.6 12.2 4.0 7.0 10.1 13.9 6.0 wt %

4.8 4.9 4.9 4.7 4.9 4.9 4.9 4.6 4.6 4.6 4.1 4.1 4.9

2740 4300 5200 6000 13 400 12 400 12 600 11 900 1300 1310 1250 1100 7100

2.8 5.4 8.0 8.8 9.8 13.3 11.0 10.1 3.4 3.5 3.7 5.1 6.5

3.3 7.2 11.6 12.9 16.5 25.6 20.5 20.4 1.6 1.9 2.3 3.3 11.5

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a

Applied voltage at the luminance of 1 cd/m2. bDevice based on CBPhomopolymer with (ppy)2Ir(acac) emitter.

EXPERIMENTAL SECTION

General Data. All operations were performed under an inert nitrogen atmosphere using standard Schlenk and glovebox techniques. Anhydrous grade solvents (Aldrich) were dried by passing them through an activated alumina column and stored over activated molecular sieves (5 Å). Spectrophotometric-grade dichloromethane was used as received from Aldrich. Commercial reagents were used without any further purification after purchasing from Aldrich (2,4pentanedione, sodium hydride (60% dispersion in mineral oil), sodium carbonate, n-butyllithium (2.5 M solution in hexanes)), and Strem (AgSbF6). Cyclometalated chloro-bridged Ir(III) dimers [(C∧N)2Ir(μ-

(green), and D12 (red) devices based on PB4, PG2, and PR4, respectively, exhibit the best performances among each set of the devices with max. power efficiencies (ηPEmax) of 12.9, 25.6, and 3.3 lm/W and max. external quantum efficiencies (ηEQEmax) of 8.8, 13.3, and 5.1%. These efficiencies are found to be the highest among the polymeric rivals reported so far including our previous reports on emitter-doped polynorbornene devices 679

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Cl)]2 (C∧N = 2-(4,6-difluorophenyl)pyridine (4,6-dfppy); 2-phenylpyridine (ppy); 2-(benzo[b]thiophen-2-yl)-pyridine (btp)),8 9-(4′carbazol-9-yl-biphenyl-4-yl)-3-(5-(bicyclo[2.2.1]hept-5-en-2-yl)pentyl)-9H-carbazole (M1),23 9-(3-(9H-carbazol-9-yl)phenyl)-3-(5(bicyclo[2.2.1]hept-5-en-2-yl)pentyl)-9H-carbazole (M2),24 5-norbornene-2-pentyl bromide (endo/exo = 2:1) (3),29 and allylchloro[N,N′bis(2,6-diisopropylphenyl)imidazol-2-ylidene]palladium(II) ([(NHC)Pd(η3-allyl)Cl])33 were analogously synthesized according to the published procedures. Sublimed grade (ppy)2Ir(acac) emitter was obtained from Lumtec. 1-Octene (Aldrich) was purified by passing it through an activated alumina column and stored over activated molecular sieves (5 Å). CDCl3 from Cambridge Isotope Laboratories was used as received. NMR spectra of compounds were recorded on a Bruker Avance 400 spectrometer (400.13 MHz for 1H, 100.62 MHz for 13C) at ambient temperature. Chemical shifts are given in ppm, and are referenced against external Me4Si (1H, 13C). HR EI−MS measurement (JEOL JMS700) was carried out at Korea Basic Science Institute (Daegu). Elemental analyses were performed on an EA1110 (FISONS Instruments) by the Environmental Analysis Laboratory at KAIST. UV/vis absorption and photoluminescence spectra were recorded on a Jasco V-530 and a Spex Fluorog-3 Luminescence spectrophotometer, respectively, in CH2Cl2 with a 1-cm quartz cuvette at ambient temperature. Synthesis of 1-(5-(Bicyclo[2.2.1]hept-5-en-2-yl)pentyl)pentane-2,4-dione (4). A THF solution (10 mL) of 2,4pentanedione (2.15 g, 21.5 mmol) was added dropwise to the THF (50 mL) slurry containing NaH (0.86 g, 21.5 mmol) at 0 °C. The reaction mixture was stirred for 30 min at 0 °C and 1 equiv of n-BuLi (8.6 mL) was added dropwise to the reaction mixture at −20 °C. The reaction mixture was stirred for 30 min and then allowed to warm to room temperature. A THF solution (10 mL) of 5-norbornene-2-pentyl bromide (5.23 g, 21.5 mmol) was added dropwise to the reaction mixture at 0 °C. The reaction mixture was stirred for 3 h at 0 °C and then for 3 h at room temperature. After the addition of a saturated aqueous solution of NH4Cl (20 mL), the organic portion was separated, and the aqueous layer was extracted with ether (2 × 30 mL). The combined organic portions were dried over MgSO4, filtered, and evaporated to dryness, affording an oily residue. The crude product was purified by column chromatography on silica (eluent: ether/n-hexane = 1/3) followed by removal of 2,4-pentanedione by distillation, affording 4 as a pale yellow oil. Yield = 3.10 g (55%). 1H NMR (CDCl3): δ 0.45−0.50 (m), 0.95−2.00 (m) (15H), 2.06 (s, 2.4H), 2.22−2.30 (m, 2.2H), 2.45−2.52 (m, 0.4H), 2.72−2.79 (m, 2H), 3.57 (s, 0.4H), 5.49 (s, 0.8H), 5.90 (dd, J = 5.7/2.9 Hz, Hendo), 6.01 (dd, J = 5.7/2.9 Hz, Hexo), 6.06−6.08 (m, Hexo), 6.10 (dd, J = 5.7/ 3.0 Hz, Hendo) (2H). 13C NMR (CDCl3): δ 23.37, 24.99, 25.69, 28.43, 28.65, 29.22, 29.57, 30.88, 32.40, 33.06, 34.71, 36.50, 38.25, 38.70, 41.84, 42.49, 43.83, 45.19, 45.38, 46.31, 49.54, 57.93, 99.73, 132.37, 136.15, 136.87, 191.43, 194.28. EI−MS: m/z calcd for C17H26O2, 262; found, 262. Anal. Calcd for C17H26O2: C, 77.82; H, 9.99. Found: C, 78.10; H, 9.65. General Procedure for the Synthesis of M3−M5. A mixture of 4 (3.8 mmol), [(C∧N)2Ir(μ-Cl)]2 (1.5 mmol), and Na2CO3 (15.0 mmol) in MeCN (50 mL) was heated to reflux and stirred for 48 h. After cooling to room temperature, water (30 mL) was added to the reaction mixture. The organic portion was separated, and the aqueous layer was extracted with ethyl acetate (2 × 30 mL). The combined organic portions were dried over MgSO4, filtered, and evaporated to dryness, affording an oily residue. The crude product was purified by column chromatography. trans-Bis(2-(4,6-difluorophenyl)-pyridinato-N,C 2 )(1-(5(bicyclo[2.2.1]hept-5-en-2-yl)pentyl)pentane-2,4-dionatoO,O)iridium(III) (M3). M3 was prepared from 4 (1.00 g, 3.8 mmol), [(4,6-dfppy)2Ir(μ-Cl)]2 (1.82 g, 1.5 mmol), and Na2CO3 (1.59 g, 15 mmol). Purification by column chromatography on silica gel (eluent: EtOAc/n-hexane = 1/5) gave M3 as a yellow solid. Yield = 0.73 g (29%, mixture of endo and exo isomers (2/1)). 1H NMR (CDCl3): δ 0.42−0.48 (m), 0.85−1.42 (m) (13H), 1.77 (d, J = 3.7 Hz), 1.80 (s), 1.83 (d, J = 3.7 Hz), 1.85−1.95 (m) (5H), 1.98 (t, J = 7.4 Hz, 2H), 2.46 (s), 2.70−2.78 (m) (2H), 5.21 (s, 1H), 5.63 (dd, J = 7.9/3.0 Hz,

1H), 5.71 (dd. J = 7.9/3.0 Hz, 1H), 5.87 (dd, J = 5.5/2.9 Hz, Hendo), 6.00 (dd, J = 5.5/2.7 Hz, Hexo), 6.05−6.07 (m, Hexo), 6.10 (dd, J = 5.6/ 2.9 Hz, Hendo) (2H), 6.28−6.35 (m, 2H), 7.10−7.18 (m, 2H), 7.72− 7.80 (m, 2H), 8.22 (t, J = 9.5 Hz, 2H), 8.38−8.42 (m, 2H). 13C NMR (CDCl3): δ 27.05, 28.45, 28.67, 28.73, 29.67, 32.41, 33.07, 34.76, 36.55, 38.69, 41.51, 41.84, 42.50, 45.19, 45.39, 46.32, 49.54, 96.93, 97.20, 97.47, 100.28, 114.96, 115.16, 115.34, 121.39, 121.48, 122.26, 122.45, 122.65, 128.57, 132.34, 136.17, 136.87, 137.71, 137.77, 148.03, 148.12, 151.58, 151.66, 159.63, 161.20, 163.89, 165.24, 165.32, 165.39, 184.99, 188.69. EI−MS: m/z calcd for C39H37F4IrN2O2, 834; found, 834. Anal. Calcd for C39H37F4IrN2O2: C, 56.17; H, 4.47; N, 3.36. Found: C, 56.07; H, 4.67; N, 3.26. trans-Bis(2-phenylpyridinato-N,C2)(1-(5-(bicyclo[2.2.1]hept5-en-2-yl)pentyl)pentane-2,4-dionato-O,O)iridium(III) (M4). M4 was prepared from 4 (1.00 g, 3.8 mmol), [(ppy)2Ir(μ-Cl)]2 (1.61 g, 1.5 mmol), and Na2CO3 (1.59 g, 15 mmol). Purification by column chromatography on silica gel (eluent: EtOAc/n-hexane = 1/3) gave M4 as a yellow solid. Yield = 1.55 g (68%, mixture of endo and exo isomers (2/1)). 1H NMR (CDCl3): δ 0.43−0.49 (m), 0.88−1.42 (m) (13H), 1.78 (s, 3H), 1.80−2.02 (m, 4H), 2.47 (s), 2.70−2.80 (m) (2H), 5.19 (s, 1H), 5.89 (dd, J = 5.5/2.7 Hz, Hendo), 6.02 (dd, J = 5.5/ 2.7 Hz, Hexo), 6.09−6.11 (m, Hexo), 6.12 (dd, J = 5.9/2.8 Hz, Hendo) (2H), 6.25 (d, J = 7.7 Hz, 1H), 6.33 (d, J = 7.6 Hz, 1H), 6.63−6.72 (m, 2H), 6.75−6.83 (m, 2H), 7.03−7.13 (m, 2H), 7.53 (t, J = 8.2 Hz, 2H), 7.63−7.73 (m, 2H), 7.82 (t, J = 8.3 Hz, 2H), 8.50 (d, J = 5.3 Hz, 2H). 13C NMR (CDCl3): δ 27.13, 28.47, 28.69, 28.82, 28.88, 29.71, 32.41, 33.07, 34.76, 36.56, 38.69, 41.65, 41.83, 42.48, 45.37, 46.30, 49.53, 99.91, 118.06, 118.27, 120.52, 121.12, 123.60, 123.77, 128.91, 129.03, 132.37, 133.00, 133.26, 136.15, 136.60, 136.66, 136.83, 144.65, 144.68, 148.00, 148.09, 148.12, 148.23, 168.57, 168.65, 184.58, 188.21. EI−MS: m/z calcd for C39H41IrN2O2, 762; found, 762. Anal. Calcd for C39H41IrN2O2: C, 61.47; H, 5.42; N, 3.68. Found: C, 61.96; H, 5.59; N, 3.68. trans-Bis(2-(benzo[b]thiophen-2-yl)-pyridinato-N,C3)(1-(5(bicyclo[2.2.1]hept-5-en-2-yl)pentyl)pentane-2,4-dionatoO,O)iridium(III) (M5). M5 was prepared from 4 (1.00 g, 3.8 mmol), [(btp)2Ir(μ-Cl)]2 (1.94 g, 1.5 mmol), and Na2CO3 (1.59 g, 15 mmol). Purification by column chromatography on silica gel (eluent: EtOAc/ n-hexane = 1/4) gave M5 as a red solid. Yield = 1.76 g (67%, mixture of endo and exo isomers (2/1)). 1H NMR (CDCl3): δ 0.45−0.51 (m), 0.83−1.43 (m) (13H), 1.78 (s, 3H), 1.80−2.03 (m, 4H), 2.49 (s), 2.72−2.81 (m) (2H), 5.23 (s, 1H), 5.89 (dd, J = 5.4/2.7 Hz, Hendo), 6.00 (dd, J = 5.6/2.8 Hz, Hexo), 6.08−6.09 (m, Hexo), 6.10 (dd, J = 5.6/ 2.7 Hz, Hendo) (2H), 6.20 (d, J = 7.9 Hz, 1H), 6.27 (d, J = 7.9 Hz, 1H), 6.77−6.83 (m, 2H), 6.93−7.00 (m, 2H), 7.05 (t, J = 7.7 Hz, 2H), 7.59 (t, J = 6.2 Hz, 2H), 7.63 (d, J = 8.1 Hz, 2H), 7.68−7.75 (m, 2H), 8.39−8.43 (m, 2H). 13C NMR (CDCl3): δ 27.06, 28.46, 28.51, 28.68, 28.76, 29.71, 32.43, 33.09, 34.78, 36.58, 38.70, 41.36, 41.84, 42.50, 45.21, 45.39, 46.32, 49.55, 100.17, 117.99, 118.12, 118.38, 118.55, 122.62, 123.50, 124.73, 124.79, 125.61, 132.38, 134.93, 135.01, 136.17, 137.83, 142.16, 142.33, 146.45, 146.82, 149.11, 149.22, 165.95, 166.10, 184.57, 188.28. EI−MS: m/z calcd for C43H41IrN2O2S2, 874; found, 874. Anal. Calcd for C43H41IrN2O2S2: C, 59.08; H, 4.73; N, 3.20; S, 7.34. Found: C, 59.53; H, 4.72; N, 3.29; S, 7.72. Polymerization Procedure. An activated catalyst solution (2.0 mM) was prepared in situ by the addition of chlorobenzene (6.0 mL) to the mixture of [(NHC)Pd(η3-allyl)Cl] and 1.5 equiv of AgSbF6 followed by stirring for 2 h at room temperature. The filtered solution of the activated catalyst (1.43−1.62 μmol of Pd(II)) was introduced into the chlorobenzene solution containing a mixture of monomers, M1 with M3, or M2 with M4 or M5 ([total monomer]/[Pd] = 1000) and 1-octene (0.05 mol % to total monomer) to initiate polymerization. The polymerization was carried out at 25 °C for 20 h. The mixture was poured into the large volume of acidified methanol (5% v/v, 300 mL) to precipitate the polymer. After stirring for 1 h, the precipitated polymer was collected by filtration and washed with methanol (3 × 50 mL). The copolymer was purified by dissolving in chloroform and reprecipitating into methanol/acetone (v/v = 4/1), which was repeated three times. The obtained copolymer was finally dried in a vacuum oven at 70 °C to constant weight. 680

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Macromolecules

Article

PB1−PB4. 1H NMR (CDCl3): δ 0.3−2.2 (br), 2.3−3.3 (br), 5.0 (bs), 5.6 (bs), 6.20 (bs), 7.2 (bs), 8.0 (bs), 8.3 (bs). 13C NMR (CDCl3): δ 26.3−49.5 (br), 109.3, 109.6, 119.6, 120.2, 123.4, 124.8, 125.3, 126.0, 126.7, 130.8, 134.6, 138.8, 139.0, 140.3. Anal. Found for PB1: C, 87.58; H, 7.42; N, 4.62. PB2: C, 86.79; H, 7.35; N, 4.63. PB3: C, 84.82; H, 6.79; N, 4.47. PB4: C, 83.69; H, 6.52; N, 4.47. PG1−PG4. 1H NMR (CDCl3): δ 0.3−2.2 (br), 2.3−3.1 (br), 5.0 (bs), 6.2 (bs), 6.5 (bs), 7.3 (bs), 7.8 (bs), 8.1 (bs), 8.3 (bs). 13C NMR (CDCl3): δ 26.5−49.7 (br), 109.7, 120.0, 120.3, 123.4, 125.9, 126.9, 127.2, 128.1, 134.6, 137.0, 137.1, 138.6, 138.9, 140.6. Anal. Found for PG1: C, 88.04; H, 6.50; N, 4.31. PG2: C, 87.43; H, 7.76; N, 4.41. PG3: C, 86.37; H, 5.90; N, 4.28. PG4: C, 85.24; H, 6.70; N, 4.26. PR1−PR4. 1H NMR (CDCl3): δ 0.3−2.2 (br), 2.3−3.1 (br), 5.0 (bs), 6.1 (bs), 6.6 (bs), 7.3 (bs), 7.8 (bs), 8.1 (bs), 8.3 (bs). 13C NMR (CDCl3): δ 26.5−49.7 (br), 109.7, 120.0, 120.3, 123.4, 125.9, 126.9, 127.1, 128.1, 134.5, 137.0, 137.1, 138.6, 138.9, 140.6. Anal. Found for PR1: C, 87.54; H, 5.73; N, 4.27. PR2: C, 86.36; H, 5.24; N, 4.24. PR3: C, 85.17; H, 5.94; N, 4.10. PR4: C, 83.74; H, 5.53; N, 4.17. Polymer Analysis. 1H and 13C NMR spectra of the copolymers were recorded on a Bruker Avance 400 spectrometer in CDCl3 at ambient temperature. The monomer contents in the copolymers were determined from elemental analyses (carbon content). The molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the copolymers were analyzed by gel-permeation chromatography (GPC) on a Viscotek T60A equipped with UV and RI detectors using THF as an eluent at 35 °C and calibrated with narrow polystyrene standards as a reference. Thermogravimetric analysis (TGA) was performed under N2 atmosphere using a TA Instrument Q500 at a heating rate of 20 °C/min from 50 to 800 °C. Differential scanning calorimetry (DSC) measurement was performed on a TA Instrument Q100. Any thermal history in the copolymers was eliminated by the first heating the samples at 20 °C/min, and then recording the second DSC scan at 10 °C/min to the decomposition temperature. Device Fabrication. Glass substrates precoated with 150-nm-thick ITO were cleaned using soapy water, deionized water, acetone, and isopropyl alcohol in sequence in a heated ultrasonic bath, and treated by air plasma using a plasma cleaner (PDC-32G, Harrick Plasma). An aqueous dispersion of PEDOT:PSS (Baytron AI4083, H.C. Starck) was then spun (2500 rpm for 30 s) onto the substrates, and subsequently dried on a hot plate (100 °C) for 10 min. Samples were then loaded into an N2-filled glovebox, and emissive copolymers (PBn, PGn, and PRn) were spin-cast on the substrates from a chlorobenzene solution (0.9 wt %) at 1,500 rpm. Samples were then dried again on hot plate for 10 min at 80 °C. Film thicknesses of spin-coated layers were measured by atomic force microscopy to be around 40 nm. After the drying process, samples with the copolymers were brought into a thermal evaporation chamber (HS-1100, Digital Optics & Vacuum), which is enclosed by the glovebox. Using shadow masks, TPBi (15 nm), Bphen (35 nm), LiF (1 nm), and Al (100 nm) were consecutively deposited. Vacuum deposition was done under high vacuum ( 400 °C) and glass transition temperatures (Tg > 330 °C). The solutionprocessed PhOLED devices which were fabricated based on a single emissive layer comprising the blue-, green-, and redphosphorescent copolymers (PBn, PGn, and PRn) exhibited excellent performances, which were almost double the efficiencies of the corresponding doped polymer systems, and are outstanding among the polymeric rivals reported thus far.

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ASSOCIATED CONTENT

S Supporting Information *

Analysis data of polymers and EL characteristics of reference devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.D.) [email protected]; (S.Y.) [email protected]; (M.H.L.) [email protected]. Author Contributions ⊥

These authors made an equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the National Research Foundation (NRF) of Korea (No. 2010-0008264 for Y.D., No. 2012039773 for M.H.L., and No. 20120000815 for S.Y.), and the Priority Research Centers Program of the NRF (No. 2009-0093818 for M.H.L.) funded by the Korea Ministry of Education, Science and Technology are gratefully acknowledged. Authors are also grateful to the support from Samsung Display Company (SDC) through the SDC KAIST OLED Research Center Program.

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REFERENCES

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CONCLUSION We have demonstrated that polynorbornene copolymers with side-chain iridium(III) emitters and CBP or mCP host moieties can be efficiently produced by vinyl addition copolymerization of norbornene monomers, using Pd(II) catalyst in combination with 1-octene chain transfer agent. All copolymers possessed 681

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dx.doi.org/10.1021/ma302342p | Macromolecules 2013, 46, 674−682