Polynorbornene copolymer with side-chain triarylborane and iridium(III) groups: An emissive layer material with electron transporting properties for PhOLEDs

Polymer 66 (2015) 67e75

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Polynorbornene copolymer with side-chain triarylborane and iridium(III) groups: An emissive layer material with electron transporting properties for PhOLEDs Taewon Kim a, Sungoh Lim b, c, So-Ra Park b, Chul Jong Han b, **, Min Hyung Lee a, * a b c

Department of Chemistry and EHSRC, University of Ulsan, Ulsan, 680-749, Republic of Korea Display Convergence Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyunggi-do, Republic of Korea Department of Electronics and Electrical Engineering, Dankook University, Yongin-si, Gyeonggi-do, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 February 2015 Received in revised form 1 April 2015 Accepted 7 April 2015 Available online 14 April 2015

Vinyl-type polynorbornene copolymers (P1eP3) containing side-chain triarylborane and iridium moieties were prepared by palladium(II) catalyzed copolymerization of norbornene monomers bearing triarylborane (Mes2BPh, M1) and an iridium emitter ((ppy)2Ir(acac), ppy ¼ 2-phenyl-pyridinato-C2,N; acac ¼ acetylacetonato, M2). Copolymerization led to facile incorporation of the comonomers in a controlled manner depending on the feed ratio of the comonomers (4.0 mol%-Ir in P1; 6.7 mol%-Ir in P2; 12.4 mol%-Ir in P3). While solution PL spectra of the copolymers exhibited partial intra-chain energy transfer from the borane moiety to iridium, complete energy transfer occurred in the rigid matrix, such as at 77 K and in the film state. The LUMO level of the borane moiety (M1) was estimated to be 2.4 eV, which is lower than that of M2 and poly(N-vinylcarbazole) (PVK) (ca. 2.2 eV for both). Solution processed PhOLEDs (D1D3) incorporating P1eP3 as an emissive material in the PVK host displayed good performance in the low current density region, but D1D3 suffered severe efficiency roll-off with increasing current densities. Comparable performance of D2 with the device (D4) based on P2 emitter and PVK:PBD (2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) host suggested that a borane moiety in the emissive polymer chain plays an actual role in facilitating electron transport in the emissive layer. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Polynorborene Triarylborane Iridium

1. Introduction Phosphorescent heavy metal complexes have been widely used as an emissive material in high-efficiency phosphorescent organic light-emitting diodes (PhOLEDs) due to their excellent lightemitting properties and strong spineorbit coupling induced by the heavy metal atom [1e13]. In real applications, phosphorescent complexes are usually dispersed into small molecular or polymeric host materials through vacuum deposition or solution processes, respectively [14], to prevent tripletetriplet annihilation and concentration quenching [15e17]. However, doping phosphorescent emitters into polymeric hosts often causes phase separation or dopant aggregation, resulting in reduction of device performance [18,19]. To circumvent this problem, the use of a single emissive

* Corresponding author. Tel.: þ82 52 259 2335; fax: þ82 52 259 2348. ** Corresponding author. Tel.: þ82 31 789 7415; fax: þ82 31 789 7419. E-mail addresses: [email protected] (C.J. Han), [email protected] (M.H. Lee). http://dx.doi.org/10.1016/j.polymer.2015.04.020 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

material incorporating both host moiety and emitter in the polymer chain has been reported [20e38]. We have also recently demonstrated that vinyl-type polynorbornene copolymers containing side-chain host moieties, such as CBP (9,90 -(1,10 -biphenyl)4,40 -diylbis-9H-carbazole) or mCP (9,90 -(1,3-phenylene)bis-9Hcarbazole) and various cyclometalated iridium emitters can function as efficient single host-emitter layer materials for PhOLEDs to deliver much better device performance than doped systems (Chart 1, PNB IrCBP) [39]. This effect was attributed to not only the suppression of phase separation or dopant aggregation by the homogeneous emissive layer but also the excellent physical/mechanical properties inherited from the parent polynorbornene backbone and the retention of intrinsic properties from the attached functional side groups [40,41]. In an effort to extend this strategy to novel polymeric layer materials in PhOLEDs, we turned our attention to emissive material with charge carrier transporting properties. Due to its good holetransporting property, high triplet energy (Eg ¼ ca. 2.53.0 eV), and excellent film-forming property, poly(N-vinylcarbazole) (PVK)

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T. Kim et al. / Polymer 66 (2015) 67e75

2. Experimental y n

x

2.1. General considerations CH2

5

CH2

5

N

O N

N

Ir

O

N PNB IrCBP

N N O

B

N N PBD 3TPYMB N Chart 1. Examples of single host-emitter materials (PNB IrCBP) and electron transporting materials (PBD, 3TPYMB).

has been commonly adopted as a host material for phosphorescent emitters in solution processed devices [42,43]. However, the poor electron-transporting property of PVK results in unbalanced mobility between holes and electrons, leading to low device performance with high driving voltage, and thereby requiring a holeblocker and/or electron transporting materials (ETMs). To confine charge carriers within the emissive layer, small molecule ETMs are usually blended into the PVK host [14,44] or are introduced as a comonomer unit in the ambipolar host polymer [45,46]. Widely used ETMs are electron-deficient aromatic heterocycles such as oxadiazole, triazole, quinoline, and benzimidazole derivatives [47]. Among these heterocycles, oxadiazole derivatives such as 2-(4biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [48e52] and 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) [53,54] are the most well-known ETMs (Chart 1). In addition, group 13 triarylborane derivatives have also been successfully employed as ETMs with hole-blocking properties [55e57]. Triarylborane compounds have high electron affinity due to the presence of an empty pp orbital and possess relatively large HOMOLUMO energy gaps with deep HOMO levels, which are suitable for both electron-transporting and hole-blocking materials (HBMs). Since initial reports on bulky triarylboranes as an ETM [58,59] or HBM [60] by Shirota and coworkers, various types of functionalized triarylboranes, including the well-known tris-[3-(3pyridyl)mesityl]borane (3TPYMB) [61,62] have been reported (Chart 1) [63e73]. However, details about the use of polymeric triarylboranes as an ETM for OLEDs have not been disclosed except for several emissive material applications [74e76]. With incorporation of emissive moieties, polymeric triarylboranes may also constitute a homogeneous emissive layer with electrontransporting properties. This could be beneficial for improving device performance, as previously demonstrated by PNB IrCBP copolymer [39]. In this study, we prepared vinyl-type polynorbornene copolymers containing side-chain triarylborane and iridium moieties via palladium(II) catalysis to provide an emissive polymeric material with electron-transporting properties for PhOLEDs. Details of the synthesis and photophysical properties of the copolymers are described along with the electroluminescent properties of the PVK host based PhOLEDs.

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). HPLC grade tetrahydrofuran (THF, Merck) and acetonitrile (Merck) were used as received. Commercial reagents were used without any further purification after purchase from Aldrich (lithium tetrakis(pentafluorophenyl)borate ethyl etherate (Li [B(C6F5)4]$2.5Et2O), n-Bu4NF$3H2O (TBAF)) and Strem (allylchloro [N,N0 -bis(2,6-diisopropylphenyl)imidazol-2-ylidene]palladium(II) ([(NHC)Pd(h3-allyl)Cl])). (4-(5-(Bicyclo[2.2.1]hept-5-en-2-yl)pentyl)phenyl)dimesitylborane (M1) [77] and trans-bis(2phenylpyridinato-N,C2) (1-(5-(bicyclo[2.2.1]hept-5-en-2-yl)pentyl) pentane-2,4-dionato-O,O)iridium(III) (M2) [39] were analogously synthesized according to the published procedures. 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 were recorded on a Bruker 300 AM spectrometer (300.13 MHz for 1H and 75.48 MHz for 13C) at ambient temperature. Chemical shifts are given in ppm, and are referenced against external Me4Si (1H and 13C). Elemental analyses were performed on an EA1110 (FISONS Instruments) by the Environmental Analysis Laboratory at KAIST. UV/Vis absorption and PL spectra were recorded on a Varian Cary 100 and a HORIBA FluoroMax-4P spectrophotometer, respectively. Cyclic voltammetry experiment was performed using an AUTOLAB/PGSTAT101 system.

2.2. Polymerization procedure An activated catalyst solution (2.0 mM) was prepared in situ by the addition of chlorobenzene (2.8 mL) to the mixture of [(NHC) Pd(h3-allyl)Cl] (ca. 3.2 mg, 5.5 mmol) and 1.5 equiv of Li[B(C6F5)4], 2.5Et2O (ca. 7.2 mg) followed by stirring for 2 h at room temperature. The filtered activated catalyst solution (0.86e1.0 mmol Pd(II)) was introduced into the chlorobenzene solution containing a mixture of monomers (4.3e5.1 mmol, [M1 þ M2]/[Pd] ¼ 500) and 1-octene (0.1 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 mixed solvent of methanol and acetone (v/v ¼ 1/4, 200 mL) to precipitate the polymer. After stirring for 1 h, the precipitated polymer was collected by filtration and washed with acetone (2  10 mL). The polymer was purified by dissolving in chloroform and reprecipitating into acetone, which was repeated three times. The obtained copolymer was finally dried in a vacuum oven at 70  C to constant weight. P1. 1H NMR (CDCl3): d 0.5e1.4 (br), 1.4e1.7 (br), 1.93 (bs), 2.22 (bs), 2.50 (bs), 5.07 (bs), 6.20 (bs), 6.26 (bs), 6.73 (bs), 6.99 (bs), 7.33 (bs), 8.41 (bs). 13C NMR(CDCl3): d 21.3, 23.6, 28.5e37.1 (br), 128.3, 136.9, 138.4, 140.7, 141.9. 11B NMR signal was not observed. Anal. Found: C, 86.41; H, 9.04; N, 0.30. P2. 1H NMR (CDCl3): d 0.5e1.4 (br), 1.4e1.8 (br), 1.93 (bs), 2.22 (bs), 2.50 (bs), 5.07 (bs), 6.19 (bs), 6.27 (bs), 6.73 (bs), 6.99 (bs), 7.33 (bs), 7.47 (bs), 7.73 (bs), 8.42 (bs). 13C NMR(CDCl3): d 21.3, 23.6, 28.3e37.5(br), 128.2, 137.0, 138.4, 140.8, 141.9, 143.0, 147.3. 11B NMR signal was not observed. Anal. Found: C, 84.23; H, 8.83; N, 0.58. P3. 1H NMR (CDCl3): d 0.5e1.4 (br), 1.4e1.8 (br), 1.92 (bs), 2.21 (bs), 2.50 (bs), 5.07 (bs), 6.18 (bs), 6.26 (bs), 6.72 (bs), 6.8e7.1 (br), 7.1e7.4 (br), 7.48 (bs), 7.73 (bs), 8.40 (bs). 13C NMR(CDCl3): d 21.3, 23.6, 26.8e36.1(br), 54.5, 100.1, 121.4, 128.3, 136.9, 138.4, 140.8,

T. Kim et al. / Polymer 66 (2015) 67e75

141.9. 11B NMR signal was not observed. Anal. Found: C, 80.23; H, 8.22; N, 0.99. 2.3. Polymer analysis 1

H and 13C NMR spectra of the copolymers were recorded on a Bruker 300 AM spectrometer (300.13 MHz for 1H, 75.48 MHz for 13 C) in CDCl3 at ambient temperature. The monomer contents in the copolymers were determined from 1H NMR spectra. 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 10  C/min from 50 to 800  C. 2.4. UV/Vis absorption and photoluminescence (PL) measurements UV/Vis absorption and photoluminescence (PL) measurements were performed in THF with a 1 cm quartz cuvette. A THF solution of polymer (3.0 mL, ca. 3.0  105 M) was titrated with incremental amounts of fluoride anions by adding a TBAF solution (3.9  103 M). The absorption and PL spectra were recorded at every titration. The PL spectra of polymer films were obtained with 1 wt% solutions in CHCl3.

69

host is PVK (D1¡D3) or PVK:PBD (35 wt% PBD) (D4); PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); PVK is poly(N-vinylcarbazole); PBD is 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole; TPBi is 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene; and Bphen is 4,7-diphenyl-1,10phenanthroline. In the case of reference device (R1), the similar device configuration was used with the emissive layer containing PVK:PBD (35 wt%) as a host material and (ppy)2Ir(acac) (8 wt% to host) as an emitting material. Glass substrates precoated with ITO were treated with oxygen plasma using a plasma asher. Samples were then loaded into an N2-filled glovebox, and an aqueous dispersion of PEDOT:PSS was then spun (3000 rpm for 30 s) onto the substrates, and subsequently dried on a hot plate (180  C) for 30 min. The chlorobenzene solution of copolymer or (ppy)2Ir(acac) with host material (1 wt% in chlorobenzene) was spin-cast on the substrates at 2500 rpm for 30 s. Samples were then dried again on a hot plate for 30 min at 80  C. Film thicknesses of spin-coated layers were measured to be around 40 nm by Alpha-Step. After drying, TPBi (15 nm), Bphen (35 nm), LiF (5 nm), and Al (150 nm) were consecutively deposited using a vacuum evaporation method (~3  106 torr). EL spectra were obtained with a PR-650 spectrometer. Currentvoltage (JV) and luminancevoltage (LV) characteristics were recorded on a currentvoltage source (Keithley 237) and a luminescence detector (PR-650). All EL measurements were carried out in an N2-filled glovebox at room temperature.

2.5. Cyclic voltammetry

3. Results and discussion

Cyclic voltammetry measurements were carried out with a three-electrode cell configuration consisting of platinum working and counter electrodes and an Ag/AgNO3 (0.01 M in CH3CN) reference electrode at room temperature. The coated P3 on the platinum wire was used as the sample and the solution of tetra-nbutylammonium hexafluorophosphate (n-Bu4NPF6, 0.1 M) in DMF was used as the supporting electrolyte. For M1 and M2, the measurements were carried out with 1 mM of monomer in CH3CN solution of n-Bu4NPF6 (0.1 M). The redox potentials were recorded at a scan rate of 200 mV/s and are reported with reference to the ferrocene/ferrocenium (Fc/Fcþ) redox couple.

3.1. Synthesis and characterization

2.6. Fabrication of electroluminescent (EL) devices The configuration of EL devices, D1¡D4 is as follows: ITO/ PEDOT:PSS (40 nm)/host:copolymer (8 wt% of M2 content to host, 40 nm)/TPBi (15 nm)/Bphen (35 nm)/LiF (5 nm)/Al (150 nm), where

The electron-transporting triarylborane and emitting iridium(III) functionalized norbornene monomers M1 and M2, respectively, were prepared according to reported procedures [39,77]. Following our published polymerization procedures [40,41,78,79], copolymerization reactions were carried out with a cationic Pd(II) catalyst derived from the in situ reaction of the N-heterocyclic carbene complex, [(NHC)Pd(h3-allyl)Cl] (NHC: N,N0 -bis(2,6diisopropylphenyl)-imidazol-2-ylidene) with Li[B(C6F5)4] cocatalyst in the presence of the 1-octene chain transfer agent (0.1 mol%) (Scheme 1). A set of copolymerization reactions was performed varying the feed ratio of M1 and M2 from 95:5 to 80:20 mol% to optimize the ratio of the iridium-to-triarylborane content in the copolymer (Table 1). As shown in Table 1, copolymerization reactions produced the copolymers with conversions of 30e60%, similar to the range

y n

x

N B

+

O Ir O

[Pd(II)]

N

chlorobenzene 25 oC 1-octene

N

O Ir O

B

N

M1 M2

P1 - P3 Scheme 1. Synthesis of copolymers P1eP3. [Pd(II)] ¼ [(NHC)Pd(h -allyl)][B(C6F5)4]. 3

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T. Kim et al. / Polymer 66 (2015) 67e75

Table 1 Copolymerization results of M1 and M2 monomers with Pd(II) catalyst. Polymera

Comonomer feed ratio (M1:M2) (mol%)

CTAb (mol%)

Conversion

Comonomer compositionc (M1:M2) (mol%)

Mwd (103 g/mol)

Mw/Mnd

Td5e ( C)

P1 P2 P3

95:5 90:10 80:20

0.10 0.10 0.10

30 47 60

96.0:4.0 93.3:6.7 87.6:12.4

113 109 108

2.2 3.3 2.3

307 294 289

a Conditions: catalyst ¼ [(NHC)Pd(h3-allyl)Cl]/Li[B(C6F5)4],2.5Et2O; [Pd]/[B(C6F5)e 4 ] ¼ 1/1.5; [Pd] ¼ 0.86e1.0 mmol; [monomer]/[Pd] ¼ 500; solvent: 5 mL of chlorobenzene; Tp ¼ 25  C; tp ¼ 20 h. b Molar ratio of chain transfer agent (1-octene) to monomer. c Determined by 1H NMR spectroscopy. d Determined by GPC. e Determined by TGA at 5% weight loss.

observed in the previous homo- and copolymerizations of M1 or M2 monomer and related functionalized norbornene monomers [39e41,77e79]. This result indicates that both M1 and M2 monomers are compatible in the Pd-catalyzed copolymerization. The resulting copolymers were readily soluble in common organic solvents, such as chloroform, dichloromethane, THF, and chlorobenzene at ambient temperature. Gel-permeation chromatography (GPC) measurements indicated the formation of high molecular weight copolymers (Mw z ca. 1.1  105 g/mol) with a relatively narrow polydispersity index (PDI ¼ 2.2e3.3), which is indicative of the involvement of a single catalytically active species in the copolymerization. Thermogravimetric analysis (TGA) measurements revealed that all of the copolymers possessed good thermal stability with decomposition temperatures (Td5, temperature at 5% weight loss) of ca. 290310  C. Note that these Td5s are all higher than that of the M1 homopolymer (240  C) [77]. The identity of the copolymers was characterized by NMR spectroscopy, and the 1H NMR spectra of P1eP3 are shown in Fig. 1. It can be seen that there are no vinyl proton peaks of the monomers at d 6.0 ppm for any of the copolymers. The broad peak in the aliphatic region indicates a rigid polynorbornene backbone

structure. The singlet methyl proton resonances for the Mes moiety of the M1 monomer are observed at d 1.9 and 2.2 ppm. The singlet peak at d 6.7 ppm and broad peaks at d 7.0 and 7.3 ppm are mainly assignable to the aromatic proton resonances for the Mes and Ph groups of the triarylborane moiety, respectively, in the M1 monomer. Although the proton resonances of the M2 monomer in the region of d 6.5e7.5 ppm are not well resolved due to overlap with large peaks from the triarylborane moiety, small peaks at d 5.1 ppm and at d 6.2 and 8.4 ppm, which are assignable to the proton resonances for the acac and ppy ligands of the iridium moiety, respectively, are clearly observed. The intensities of these peaks also gradually increased with increasing feed ratio of the M2 monomer. All of these results indicate the formation of a vinyl-type copolymer of M1 and M2 monomers. The comonomer content in the copolymers was determined from the proton peak intensities of the triarylborane and iridium moieties. Although incorporation of the M2 monomer was almost linearly proportional to the feed amount, the content of M2 monomer was lower than the initial feed ratio of M2 monomer, indicating a lower reactivity of the M2 monomer than M1 (Table 1). 3.2. Photophysical and electrochemical properties

Fig. 1. 1H NMR spectra of P1eP3 with peak assignments. The numbers in parentheses are the feed ratios of M1 and M2 monomers (* and y from residual CHCl3 and Me4Si, respectively, in CDCl3).

UV/Vis absorption and PL spectra of the copolymers (P1eP3) measured in THF are shown in Fig. 2. The absorption spectra exhibited major absorption bands at 310 nm (log ε ¼ 4.19e4.21) assignable to the dominant p(Mes)pp(B) transition in the triarylborane moiety [77]. The absorption features of the copolymers are almost identical with each other and with M1 except in the lower-energy region (ca. 350520 nm) (Fig. S1 in the Supplementary material). The weak absorption in the lower-energy region is a typical feature for the (ppy)2Ir(acac) complex [9,10], and thus can be assigned to the metal-to-ligand charge transfer (MLCT) transition in the iridium moiety. Indeed, the MLCT bands are well matched with those of the M2 monomer (Fig. S1) [39]. The intensity of the MLCT bands also increases with increasing content of iridium moiety in the copolymers (P1 < P2 < P3). The PL spectra of the copolymers displayed dual emissions centered at lPL ¼ 380 and 520 nm in THF. While the high-energy band is assignable to triarylborane fluorescence (Fig. S1) [77], the lower-energy band can be attributed to phosphorescence (3MLCT/3LC) of iridium moieties, as similarly observed for the (ppy)2Ir(acac) complex [9,10] and M2 monomer (Fig. S1) [39]. It is notable that whereas the relative intensity of the phosphorescence band to fluorescence increased with increasing content of iridium moiety in the copolymers (I520/I380 ¼ 0.42 to 0.97 to 3.3 for P1eP3), the changes in the intensity ratios displayed a larger difference from that of M2 content in the copolymers ([M2]/[M1] ¼ 0.04 to 0.07 to 0.14 for P1eP3). To elucidate possible energy transfer from the triarylborane moieties to iridium, the PL spectrum of a solution containing a mixture of M1 and M2 monomers in the same molar

T. Kim et al. / Polymer 66 (2015) 67e75

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Fig. 2. UV/Vis absorption and PL spectra of (a) P1, (b) P2, and (c) P3 in THF (ca. 3.0  105 M, lex ¼ 320 nm) at 298 K. Insets show the absorption band in the MLCT region.

ratio as for the P2 copolymer was obtained (M1:M2 ¼ 93.3:6.7 mol %). Different from the similar emission intensity of the two bands for P2, the PL spectrum of the monomer mixture exhibited dominant fluorescence with weak phosphorescence (Fig. 3). This result indicates the occurrence of intra-chain energy transfer from the triarylborane moieties to iridium in the copolymer. In fact, the emission energy of M1 (lPL ¼ 330500 nm) is well matched with the low-energy absorption wavelength of M2 (labs ¼ 350520 nm) €rster energy transfer (Fig. S1). Because the energy for efficient Fo transfer is highly sensitive to the distance between energy donor and acceptor, it hardly occurs in the dilute solution of a monomer mixture. To gain more insight into the energy transfer, we further carried out PL measurements with changes in the concentration of triarylborane moieties, as the phosphorescence band will also undergo a change in intensity. To this end, UV/Vis and PL titration experiments of P2 with fluoride anion were performed using a TBAF solution, since it is well known that triarylborane compounds readily bind fluoride due to the high Lewis acidity of the boron atom [55,77,80e86]. As shown in Fig. 4a, while the absorption band at 310 nm was gradually quenched upon addition of fluoride, the lower-energy MLCT band remained almost unchanged. This result is the consequence of fluoride binding to the triarylborane moieties, which disrupts the extended conjugation through the boron pp orbital. In contrast, both the fluorescence and phosphorescence bands of the PL spectrum of P2 were quenched with fluoride binding to the boron center (Fig. 4b). Interestingly, while the highenergy fluorescence band completely disappeared with the

addition of two equiv of fluoride, the weak phosphorescence of the iridium moiety was still observed even after full titrations. These absorption and emission changes upon fluoride binding were similarly observed for the copolymers P1 and P3 (Fig. S2). Because there are no energy donors such as neutral triarylborane moieties in the polymer chain after fluoride binding, the residual weak phosphorescence can only be attributed to the remaining iridium moieties. This inference in turn indicates that the initial high intensity of the phosphorescence band in neutral P2 mainly originates from energy transfer from nearby triarylborane moieties to iridium. Although partial energy transfer from triarylborane moieties was observed in the solution state probably due to the flexible nature of the pendant groups, this finding implies that triarylborane moieties may act as hosts for emitting iridium moieties when the copolymers are incorporated into the emissive layer of PhOLEDs. This supposition led us to further investigate the emissive properties of the copolymers in a rigid matrix, such as at 77 K and in the film state. The PL spectra of all of the copolymers obtained at 77 K showed only phosphorescence centered at 509517 nm (Fig. 5a). Likewise, the thin film of copolymers also exhibited single phosphorescence (Fig. 5b). The observed single emission indicates complete energy transfer from the triarylborane moieties to iridium in a rigid matrix. These results suggest that the copolymers could act as an emissive layer material in PhOLEDs and that the iridium moieties may gain singlet energy from neighboring triarylborane moieties as well as external host materials for phosphorescent light emission.

Fig. 3. PL spectrum of a THF solution containing M1 (2.91  105 M) and M2 (0.21  105 M) at 298 K (lex ¼ 320 nm).

Fig. 4. Changes in the (a) UV/Vis absorption and (b) PL spectra of P2 in THF (3.1  105 M, lex ¼ 320 nm) upon incremental additions of TBAF (3.9  103 M) at 298 K.

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T. Kim et al. / Polymer 66 (2015) 67e75

alkyl norbornene linker in the monomer. Moreover, the redox potentials of M1 and M2 are similar to those observed for P3, reflecting the retention of intrinsic properties of the pendant groups in the side-chain polymers. From the electrochemical data and optical band gap of M1 (Eg ¼ 3.6 eV, Fig. S1) [77], the HOMO and LUMO levels of M1 were estimated to be 6.0 and 2.4 eV, respectively. Note that these energy levels are comparable to those of PBD [50,90e92] and other oxadiazole derivatives [93,94]. In the case of M2, the HOMO and LUMO levels and the Eg can be directly obtained from the above redox potentials, affording 5.2, 2.2, and 3.0 eV, respectively. The larger Eg of M1 compared with M2 also supports that the triarylborane moieties may act as hosts for iridium moieties, which is consistent with the PL spectra of the copolymers. 3.3. Electroluminescent properties Fig. 5. PL spectra of copolymers (a) at 77 K (ca. 3.0  105 M in THF) and (b) in film (1 wt% in CHCl3, 298 K) (lex ¼ 320 nm).

Next, the electrochemical properties of the copolymers and monomers were examined by cyclic voltammetry measurements (Fig. 6). Due to the low M2 content in the P1 and P2 copolymers, the electrochemical behavior of P3 was examined. While P3 showed reversible oxidation at 0.49 V assignable to the oxidation of dp orbitals of Ir, one broad reversible reduction was observed at 2.55 V (DEp ¼ 210 mV). The single reduction may indicate that the reductions at boron and Ir moieties are overlapped in the similar potential region. On the other hand, the monomers, M1 and M2 underwent reversible reduction at 2.44 and 2.59 V, respectively, similar to values reported for typical triarylborane compounds [60,87,88] and the (ppy)2Ir(acac) complex (2.60 V) [89]. M2 also displayed clean reversible oxidation at 0.41 V, which is identical to (ppy)2Ir(acac) (0.41 V) [89]. This result indicates that the electrochemical properties of the triarylborane and iridium functionalities were little affected by the

Fig. 6. Cyclic voltammograms of copolymer (P3, coated film in DMF) and monomer (M1 and M2, ~104 M in CH3CN) (scan rate ¼ 200 mV/s).

To explore the possibility of using the copolymers as an emissive layer material in solution processed PhOLEDs, devices D1¡D3 based on emissive layers containing PVK host doped with P1eP3 as an emitter were first fabricated in the following configuration: ITO/ PEDOT:PSS (40 nm)/PVK:emitter (P1eP3, 8 wt% of M2 content to host) (40 nm)/TPBi (15 nm)/Bphen (35 nm)/LiF (5 nm)/Al (150 nm). Note that due to the identical iridium content (M2 content) in the emissive layer for D1¡D3, the amount of borane moiety (M1) is in the order D1 > D2 > D3. A reference device R1 with PVK:PBD (35 wt %) host in the emissive layer was also fabricated using the small molecular (ppy)2Ir(acac) emitter in an identical device structure. The EL spectra and device characteristics of D1¡D3 and R1 are summarized in Fig. 7 and Table 2. As shown in Fig. 7a, D1¡D3 emitted green phosphorescent light (lEL ¼ ca. 520 nm) similar to the PL spectra of thin films of P1eP3. This finding indicates that energy transfer occurs completely from host (PVK) and borane moieties to iridium. Among D1¡D3, all of the device performances in terms of brightness (L), current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) were highest for the D2 device, while D2 and D3 were similar in performance. Although the maximum performances of D1¡D3 are lower than those of the reported high-efficiency OLEDs employing polymeric EML with side-chain Ir-moieties [22,29,35,38,39,95], the performances are much better than those of OLEDs with conjugated polymeric EML [20,21,26,30]. In particular, D1¡D3 all showed higher maximum performances than the R1 device except for luminance. For example, D2 exhibited good performance with a maximum CE of 12.3 cd/A, maximum PE of 8.6 lm/W, and maximum EQE of 3.6%. We attribute these results to an enhanced balance between holes and electrons in the emissive layer due to the presence of the electrontransporting borane moiety. We note that the LUMO level of M1 (2.4 eV) is lower in energy than the LUMOs of M2 and PVK host (ca. 2.2 eV for both). Thus, charge carrier (electron) transport could be facilitated in the emissive layer of D1¡D3. To elucidate the electron-transporting property of borane moiety, the D4 device with P2 emitter and PVK:PBD (35 wt%) host was fabricated and the performance of D4 was compared with that of D2 lacking PBD (Fig. 7 and Table 2). The result showed that the overall maximum efficiencies of D4 are almost comparable to those of D2, which in turn indicates that a borane moiety in the copolymer itself possesses electron-transporting property. However, as shown in Fig. 7bed, significant efficiency “roll-off” in the mid to high current density region was observed for the copolymer-based devices D1¡D3 and to a lesser extent, D4, while R1 exhibited moderate roll-off over a wide range of current density. Indeed, the overall performance of R1 became superior to D1¡D4 in the region above ca. 20 mA/cm2. The observed efficiency roll-off of D1¡D4 also

T. Kim et al. / Polymer 66 (2015) 67e75

73

Fig. 7. (Top) Energy level diagram of devices and (bottom) (a) EL spectra and (bed) characteristics of devices (D1eD4 and R1) fabricated with P1eP3 and (ppy)2Ir(acac) as an emitter. The inset shows a photograph of the working device D1.

reflects strong involvement of a borane moiety in electron transport and is associated with instability of the borane moiety (Mes2BPh) under high electrical stress. To overcome this problem, it may be required to fully protect the boron center of the borane moiety with ortho-alkyl substituents, as has been realized with the

well-known electron-transporting 3TPYMB compound [61,62]. Nevertheless, the results of this study suggest that incorporation of a borane moiety into the emissive polymer chain could be beneficial for improving device performance when compared to emitter only doped systems.

Table 2 Device performance of PhOLEDs. Devicea

Emitter

Host

lEL/nm

Von (V)b

L (cd/m2)c

CEmax (cd/A)d

PEmax (lm/W)e

EQEmax (%)f

D1 D2 D3 D4 R1

P1 P2 P3 P2 (ppy)2Ir(acac)

PVK PVK PVK PVK:PBDg PVK:PBDg

518 519 522 522 524

4.5 5.0 6.0 4.5 4.0

250 558 283 929 2172

7.4 12.3 7.3 12.6 6.1

5.8 8.6 5.1 8.3 4.0

2.3 3.6 2.2 3.4 1.6

a b c d e f g

ITO/PEDOT:PSS (40 nm)/host:emitter (8 wt% of M2 content or (ppy)2Ir(acac) to host (40 nm)/TPBi (15 nm)/Bphen (35 nm)/LiF (5 nm)/Al (150 nm). Applied voltage at the luminance of 1 cd/m2. Maximum luminance. Maximum current efficiency. Maximum power efficiency. Maximum external quantum efficiency. PVK:PBD ¼ 100:35 in wt%.

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T. Kim et al. / Polymer 66 (2015) 67e75

4. Conclusion We have demonstrated that vinyl-type polynorbornene copolymers containing side-chain triarylborane and iridium moieties can be efficiently produced by palladium(II) catalysis with a controlled amount of comonomers. The PL study showed that intrachain energy transfer from the borane moiety to iridium occurs partially in solution, but completely in a rigid matrix. The electrochemically determined LUMO level of the borane moiety was well suited for an electron transporting material. While solution processed PhOLEDs incorporating the copolymers as emissive material in the PVK host displayed good performance in the low current density region, severe efficiency roll-off was observed under high electrical stress. The results in this study suggest that incorporation of a stable triarylborane moiety into the emissive polymer chain could be potentially useful for improving device performance in comparison to emitter only doped PhOLEDs. Acknowledgment This work was supported by the Basic Science Research Program (NRF-2012R1A1A2039773 and NRF-2014R1A1A2056364 for M.H. Lee) and Priority Research Centers Program (No. 2009-0093818 for M.H. Lee) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. C.J. Han thanks to the support from the Industrial Strategic Technology Development Program (10042590, Meterials Development for 50-Inches UD OLED TV Using Super Hybrid Process) funded by the Ministry of Trade, Industry and Energy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2015.04.020. References [1] Zhou G, Wong W-Y, Yang X. Chem Asian J 2011;6(7):1706e27. €tzel M, Nazeeruddin MK. Dalton [2] Baranoff E, Jung I, Scopelliti R, Solari E, Gra Trans 2011;40(26):6860e7. [3] Chi Y, Chou P-T. Chem Soc Rev 2010;39(2):638e55. [4] You Y, Park SY. Dalton Trans 2009;(8):1267e82. [5] Chou P-T, Chi Y. Chem Eur J 2007;13(2):380e95. [6] Lowry MS, Bernhard S. Chem Eur J 2006;12(31):7970e7. [7] Nazeeruddin MK, Humphry-Baker R, Berner D, Rivier S, Zuppiroli L, Graetzel M. J Am Chem Soc 2003;125(29):8790e7. €hler A, Wilson JS, Friend RH. Adv Mater 2002;14(10):701e7. [8] Ko [9] Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Lee HE, Adachi C, et al. J Am Chem Soc 2001;123(18):4304e12. [10] Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Kwong R, Tsyba I, et al. Inorg Chem 2001;40(7):1704e11. [11] Baldo MA, Thompson ME, Forrest SR. Nature 2000;403(6771):750e3. [12] Baldo MA, Lamansky S, Burrows PE, Thompson ME, Forrest SR. Appl Phys Lett 1999;75(1):4e6. [13] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Nature 1998;395(6698):151e4. [14] Tao Y, Yang C, Qin J. Chem Soc Rev 2011;40(5):2943e70. [15] Reineke S, Walzer K, Leo K. Phys Rev B 2007;75(12):125328. [16] Kawamura Y, Brooks J, Brown JJ, Sasabe H, Adachi C. Phys Rev Lett 2006;96(1): 017404. [17] Baldo MA, Adachi C, Forrest SR. Phys Rev B 2000;62(16):10967e77. [18] You Y, Kim SH, Jung HK, Park SY. Macromolecules 2006;39(1):349e56. [19] Noh Y-Y, Lee C-L, Kim J-J, Yase K. J Chem Phys 2003;118(6):2853e64. [20] Ma Z, Ding J, Cheng Y, Xie Z, Wang L, Jing X, et al. Polymer 2011;52(10): 2189e97. [21] Fei T, Cheng G, Hu D, Dong W, Lu P, Ma Y. J Polym Sci Part A Polym Chem 2010;48(9):1859e65. [22] Thesen MW, Krueger H, Janietz S, Wedel A, Graf M. J Polym Sci Part A Polym Chem 2010;48(2):389e402. [23] Ying L, Zou J, Zhang A, Chen B, Yang W, Cao Y. J Org Chem 2009;694(17): 2727e34. [24] Niedermair F, Sandholzer M, Kremser G, Slugovc C. Organometallics 2009;28(9):2888e96.

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