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Synthesis, characterization, and photophysics of electroluminescent fluorene/dibenzothiophene- and fluorene/dibenzothiophene-S,S-dioxide-based main-chain copolymers bearing benzimidazole-based iridium complexes as backbones or dopants

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Synthesis, characterization, and photophysics of electroluminescent fluorene/

dibenzothiophene- and fluorene/dibenzothiophene-S,S-dioxide-based

main-chain copolymers bearing benzimidazole-based iridium complexes as

backbones or dopants

Wei-Sheng Huang

a

, Ying-Hsien Wu

b

, Ying-Chan Hsu

c

, Hong-Cheu Lin

a,*

, Jiann T. Lin

c,** aDepartment of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC

bElectro-Optical Engineering and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan, ROC cInstitute of Chemistry, Academia Sinica, Taipei, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 3 August 2009 Received in revised form 29 September 2009 Accepted 3 October 2009 Available online 22 October 2009 Keywords:

Phosphorescence Iridium complex Charge carrier mobilities

a b s t r a c t

A series of electroluminescent copolymers containing fluorene-2,8-disubstituted dibenzothiophene (PFD), fluorene-2,8-disubstituted dibenzothiophene-S,S-dioxide (PFDo) and phosphorescent benzimid-azole-based iridium (Ir) complexes in the backbones were synthesized by the Suzuki coupling reaction. The thermal stabilities, HOMO/LUMO levels and triplet energy gap (ET) values were enhanced with

increasing contents of dibenzothiophene (D) or dibenzothiophene-S,S-dioxide (Do) segments in the copolymers. The relative intensities of phosphorescence and fluorescence were affected by the energy transfer and back transfer efficiencies between the polymer backbones and iridium units as evidenced by solid state PL and EL spectra. PLED devices with a configuration of ITO/PEDOT:PSS (50 nm)/metal-free copolymers (P1–P5), Ir-copolymers (P7–P13) and Ir-doped copolymers (P3 doped with Ir-complexes 6 and 8) (60–80 nm)/TPBI (40 nm)/LiF (1 nm)/Al (120 nm) were fabricated, and the electroluminescence (EL) efficiencies depended on the chemical constituents and triplet energies of the copolymers. The space-charge-limited current (SCLC) flow technique was used to measure the charge carrier mobilities of these copolymers, where both hole and electron mobilities were in the following order: the metal-free copolymers (P2, P3 and P5) > Ir-doped copolymers (P3 þ 3 or 10 mol% Ir-complex 6) > Ir-copolymers (P7, P8, P12 and P13).

Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer light-emitting diodes (PLEDs) have been subjected to intensive studies for the past two decades because the spin-coating technique renders PLEDs with flexible substrates and/or large displays feasible[1]. In recent years, PLEDs based on phosphores-cent iridium complexes blended in a polymer matrix have also attracted attention and highly efficient PLEDs have been achieved

[2]. Nevertheless, phase separation and triplet energy confinement in such systems may lead to the aggregation of phosphors and thus induce phosphorescence quenching and reduction of the emission efficiency[3].

In order to solve those problems, PLEDs incorporating phos-phorescent moieties via covalent bonds into the polymer back-bones[4]or side chains [5]were developed by several research groups. In addition, it is also important to note that sufficiently large triplet energy (ET) values were needed to suppress back

energy transfer from the guest phosphors to the host polymers. For example, Park et al. designed a wide band-gap non-conjugated carbazole-based polymer (CP0) tethered with blue-emitting FIrpic

units via covalent bonds[6]. The high triplet energy (ET¼ 2.6 eV) of

CP0 ensures a high device efficiency, with luminescence and

emission efficiencies reaching 1450 cd/m2 and 2.23 cd/A, respec-tively. In comparison, conjugated polymers often exhibit relatively low ET values, such as polyfluorene (PF, ET¼ 2.10 eV) [7] and

polyfluorene-alt-carbazole (P(F-alt-C), ET¼ 2.18 eV)[8]. Therefore,

the covalently bonded emitters were usually used in conjunction with red-emitting iridium moieties to avoid energy back transfer

[3a,5c,7a,8,9]. For a fair comparison, Holdcroft et al. developed two different iridium-based main-chain conjugated polymers[10],

*Corresponding author. Fax: þ886 3 5724727. **Corresponding author. Fax: þ886 2 27831237.

E-mail addresses:linhc@cc.nctu.edu.tw (H.-C. Lin),jtlin@chem.sinica.edu.tw (J.T. Lin).

Contents lists available atScienceDirect

Polymer

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o l y m e r

0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.10.011

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fluorene-alt-pyridine (PFPy, ET¼ 2.13 eV) and

fluorene-alt-thiophene (PFT, ET¼ 2.88 eV). The latter was found to have better

photoluminescence (PL) and EL efficiencies because of its larger triplet energy, which could more effectively suppress the self-quenching of the phosphors.

Modification of carrier-transporting ability in polymers was also attempted. Compared with PF, dibenzothiophene-S,S-dioxide-fluorene co-oligomers [11] or co-polymers[12] were reported to improve electron affinity after incorporation of electron-deficient dibenzothiophene-S,S-dioxide segments. Accordingly, highly effi-cient blue-emitting PLEDs (external quantum efficiency (EQE) ¼ 5.5% at 69 mA/cm2) could be obtained by using dibenzothiophene-S,S-dioxide-fluorene co-polymers as an electron-transporting layer and poly-(9-vinylcarbazole) (PVK) as a hole-transporting layer [13]. Monkman et al. also demonstrated efficient single-layer light-emit-ting devices (EQE ¼ 1.3% at 100 cd/cm2) by using (9,9-dioctylfluorene-2,7-diyl)-dibenzothiophene-S,S-dioxide-3,7-diyl co-polymers[14].

Previously, a series of 2,8-disubstituted dibenzothiophene (D)

[15]and 2,8-disubstituted dibenzothiophene-S,S-dioxide (Do)[16]

derivatives containing peripheral diarylamines were developed and successfully used to fabricate efficient single-layer electrolu-minescent (EL) devices. Though dibenzothiophene-based[17]and dibenzothiophene-S,S-dioxide-based

p

-conjugating polymers[12– 14]have been widely studied in PLEDs, to our knowledge, there were no reports on PLEDs using phosphor emitters. We previously developed highly phosphorescent cyclometalated iridium complexes based on benzimidazole ligands (bi) that could be fabricated into high performance OLEDs[18]and dendrimer-type LEDs[19]. In this article, we developed solution-processable PLED using fluorene-2,8-disubstituted dibenzothiophene (PFD) and fluorene-2,8-disubstituted dibenzothiophene-S,S-dioxide (PFDo) copolymers as the host for iridium motifs. Copolymers incorpo-rating some phosphorescent iridium fragments into the backbones were also synthesized. Besides synthesis and characterization, the charge mobilities of the copolymers were measured by the space-charge limited current (SCLC) flow technique. PLED devices fabri-cated from metal-containing polymers as well as metal-free copolymers doped with standard phosphorescent iridium complexes will also be discussed.

2. Experimental

2.1. Characterization

1H NMR spectra were recorded on a Bruker AMX400

spec-trometer. FAB-mass spectra were collected on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan) with a resolution of 3000 for low resolution and 8000 for high resolution (5% valley definition). For FAB-mass spectra, the source accelerating voltage was operated at 10 kV with a Xe gun, using 3-nitrobenzyl alcohol as a matrix. The molecular weights of polymers were determined with a Viscotek TriSEC GPC in THF solvent. The number-average and weight-average molecular weights were estimated by using a cali-bration curve of polystyrene standards. Elemental analyses were performed on a Perkin–Elmer 2400 CHN analyzer. Cyclic voltam-metry (CV) experiments were performed with a CHI-621B elec-trochemical analyzer and carried out at room temperature under nitrogen at a scan rate of 100 mV/s with a conventional three-electrode configuration consisting of a platinum working three-electrode, an auxiliary electrode and a nonaqueous Ag/AgNO3 reference

electrode. The solvent used in all CV experiments was CH2Cl2and

the supporting electrolyte was 0.1 M tetrabutylammonium hexa-fluorophosphate (Bu4NPF6). Electronic absorption spectra were

obtained on a Cary 50 Probe UV–visible spectrometer. Emission spectra were recorded in deoxygenated solutions at 298 K by

a JASCO FP-6500 fluorescence spectrometer. The emission spectra in solution were collected on samples with O.D. w0.1 at the exci-tation wavelength, where emission maxima were reproducible within 2 nm. The fluorescent and phosphorescent quantum yields in solutions were calculated relative to a coumarin 1 standard (

F

em¼ 0.99 in ethyl acetate) [20] and Ir(ppy)3 (

F

em¼ 0.40 in

toluene)[21], respectively. The quantum yields in solid films were measured with an integrating sphere under an excitation wave-length of 350 nm on a quartz glass. Phosphorescence spectra of compounds (in toluene solutions and thin films) were measured by a HORIBA Jobin-Yvon FluoroMax-P spectrometer at 77 K using a 10-ms delay time between the excitation with a microsecond flash lamp and the measurement. Luminescence quantum yields were taken as the average of three separate determinations and were reproducible within 10%. Different scanning calorimetry (DSC) measurements were carried out using a Perkin–Elmer 7 series thermal analyzer at a heating rate of 10C/min from 30 to

300C. Thermogravimetric analysis (TGA) measurements were

performed on a Perkin Elmer Pyris 1 TGA at a heating rate of 10C/

min under nitrogen.

2.2. Light-emitting diode (LED) device fabrication

Prepatterned ITO substrates with an effective individual device area of 3.14 mm2 were cleaned via repeated ultrasonic washing with detergent, deionized water, ethanol and finally oxygen plasma treatment. A layer of poly(ethylenedioxythiophene):poly(styrene– sulfonic acid) (PEDOT:PSS) (Baytron AI4083) with a thickness of 50 nm was spin-coated on the pre-cleaned ITO glass substrates as a hole injection layer and then baked at 100C in air for 1 h. Then,

the polymers were dissolved in dichlorobenzene (concentration: 10 mg mL1 for the polymers) and filtered with a 0.2

m

m filter. A thin film of polymer was coated at a spin rate of 1500 rpm (revolution per min.). The film thickness of the polymer layer was around 60–80 nm, measured by a surface profilometer Dektak 3 (Veeco/Sloan Instrument Inc.). Afterward, a layer (with a thickness of 40 nm) of electron transporting 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBI) was deposited under vacuum. Finally, a layer of LiF/Al (1 nm/120 nm) was thermally evaporated as a cathode in a vacuum chamber (under a pressure of less than 2.5  105Torr). I–V curves were measured on a Keithley 2400

Source Meter in ambient environment, and the light intensity was measured with a Newport 1835 Optical Meter.

2.3. Hole-only and electron-only device fabrication

The hole-only and electron-only devices in this study consist of polymer films sandwiched between transparent ITO anodes and cathodes, where the device fabrication was the same as that for PLEDs. In the hole-only device, the modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene): polystyrenesulfonate (PEDOT:PSS) (w50 nm). After baking at 100C for 1 h, the substrates were then transferred into a

nitrogen-filled glove box. The active layer was spin-coated (spin rate ¼ 1000 rpm; spin time ¼ 40 s) on top of PEDOT:PSS and then dried in covered glass Petri dishes. Subsequently, 10 and 120 nm thicknesses of MoO3 and aluminum were thermally evaporated,

respectively, through a shadow mask. In the electron-only device, the PEDOT:PSS layer was replaced with Cs2CO3, which has been

used as an efficient electron injection layer. The modified ITO surface was obtained by spin-coating a layer of Cs2CO3(w2 nm).

The active layer was spin-coated (spin rate ¼ 1000 rpm; spin time ¼ 40 s) on top of the Cs2CO3and then dried in covered glass

Petri dishes. Consequently, 40 and 70 nm thicknesses of Ca and aluminum were thermally evaporated, respectively.

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2.4. Materials

Chemicals and solvents were reagent grades and purchased from Aldrich, Acros, TCI, and Lancaster Chemical Co. Solvents were dried by standard procedures. All reactions and manipulations were carried out under N2with the use of standard inert

atmo-sphere and Schlenk techniques. All column chromatography experiments were performed by using silica gel (230–400 mesh, Macherey-Nagel GmbH & Co.) as the stationary phase in a column which is 25–35 cm in length and 2.5 cm in diameter.

2.4.1. 2-(4-Bromophenyl)-1-phenyl-1H-benzimidazole (1) (pbi-Br) N-Phenyl-o-phenylenediamine (1 equiv) and 4-bromo-benzaldehyde (1 equiv.) were dissolved in 50 mL of 2-methox-yethanol. The mixture was refluxed for 48 h under nitrogen. The volatiles were removed under vacuum and the resulting solid was extracted by dichloromethane. The organic extract was washed with brine solution and dried over anhydrous MgSO4, and then it

was filtered and evaporated to dryness. The crude product was purified by column chromatography (silica gel) using a mixture of dichloromethane and hexanes (1:1 by volume) as the eluent. The pure compound was acquired as a white solid with a 60% yield.1H NMR (CDCl3, 400 MHz, ppm):

d

7.68 (d, J ¼ 8.0 Hz, 1H), 7.35–7.29

(m, 4H), 7.24–7.20 (m,1H), 7.15–7.02 (m, 7H). FABMS: m/z 348.9 (M)þ.

Anal. calcd. for C19H13BrN2: C, 65.35; H, 3.75; N, 8.02. Found: C,

65.22; H, 3.78; N, 8.01.

2.4.2. 2-(4-(9,9-Dihexyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-benzimidazole (2) (pbiF)

To a mixture of toluene and aqueous solution of K2CO3(1:1 v/v,

40 mL), compound 1 (1.39 g, 4.0 mmol), 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (1.73 g, 4.0 mmol), and tetrakis(triphenylphosphine)palladium (100 mg, 0.040 mmol) were added to react for 24 h. After cooling, the reaction was quenched with water and the mixture was extracted with dichloromethane. The combined extract was then washed with brine, dried over MgSO4, and evaporated to dryness. The crude

product was isolated by column chromatography on a silica gel column using a mixture of dichloromethane and hexanes (1:4 by volume) as the eluent. The pure compound was obtained as a bright yellow powder with a 40% yield (1.06 g).1H NMR (CDCl3,

400 MHz, ppm):

d

8.19 (s, 1H), 7.92 (d, J ¼ 8.0 Hz, 1H), 7.74 (d, J ¼ 8.0 Hz, 2H), 7.70 (d, J ¼ 8.4 Hz, 2H), 7.62 (d, J ¼ 8.4 Hz, 2H), 7.58– 7.53 (m, 4H), 7.41–7.27 (m, 8H), 2.01–1.97 (m, 4H), 1.13–1.03 (m, 20H), 0.75 (t, J ¼ 7.5 Hz, 6H), 0.66–0.63 (m, 4H). FABMS: m/z 659.2 (M þ H)þ. Anal. calcd. for C44H46N2: C, 87.66; H, 7.69; N, 4.65.

Found: C, 87.42; H, 7.78; N, 4.41.

2.4.3. 2-(4-(4-Bromobenzyloxy)phenyl)-1-phenyl-1H-benzimidazole (3) (pbiOPh-Br)

2-(Phenol-4yl)-1-phenyl-1H-benzimidazole [19] (1.66 g, 5.8 mmol), K2CO3(1.0 g, 5.88 mmol), and 4-bromobenzylbromide

(1.45 g, 5.88 mmol) were dissolved in 30 mL of N,N-dime-thylformamide (DMF). The mixture was heated to react at 100C

for 24 h. After cooling, the reaction was quenched with water and the mixture was extracted with dichloromethane. The combined extract was then washed with brine, dried over MgSO4, and

evap-orated to dryness. The crude product was isolated by column chromatography on a silica gel column using a mixture of dichloromethane and hexanes (1:4 by volume) as the eluent. The pure compound was a white solid with a 73% yield.1H NMR (CDCl3,

300 MHz, ppm):

d

7.88 (d, J ¼ 7.8 Hz, 1H), 7.54–7.46 (m, 7H), 7.35– 7.21 (m, 7H), 6.86 (d, J ¼ 7.8 Hz, 2H), 4.97 (s, 2H). FABMS: m/z 455.1 (M þ H)þ. Anal. calcd. for C26H19BrN2O: C, 68.58; H, 4,21; N, 6.15.

Found: C, 68.32; H, 4.18; N, 6.21.

2.4.4. 2-(4-(4-(9,9-Dihexyl-9H-fluoren-2-yl)benzyloxy)phenyl)-1-phenyl-1H-benzimidazole (4) (pbiOPhF)

Compound 4 was synthesized by the same procedure as illus-trated for compound 3 except that 4-bromobenzylbromide was used instead of 2-(4-(bromomethyl)phenyl)-9,9-dihexyl-9H-fluo-rene. White solid. Yield ¼ 56%.1H NMR (CDCl3, 400 MHz, ppm):

d

7.84 (d, J ¼ 8.0 Hz, 1H), 7.72 (d, J ¼ 8.0 Hz, 1H), 7.69 (d, J ¼ 7.2 Hz, 1H), 7.65 (d, J ¼ 8.0 Hz, 2H), 7.55–7.45 (m, 9H), 7.31–7.28 (m, 6H), 7.22–7.18 (m, 2H), 6.91 (d, J ¼ 8.8 Hz, 2H), 5.09 (s, 2H, OCH2), 1.99–

1.96 (m, 4H, CH2), 1.09–1.01 (m, 12H, CH2), 0.73 (t, J ¼ 6.8 Hz, 6H,

CH3), 0.66–0.64 (m, 4H, CH2). FABMS: m/z 709.5 (M þ H)þ. Anal.

Calcd. for C51H52N2O: C, 86.40; H, 7.39; N, 3.95. Found: C, 86.54; H,

7.40; N, 3.67.

2.4.5. (pbi-Br)2Ir(acac) (5)

To a flask containing IrCl3$nH2O (176 mg, 0.5 mmol) and

compound 1 (700 mg, 2.0 equiv.), a mixture of 2-methoxyethanol (S) and water (3:1 v/v, 25 mL) was added. The mixture was then refluxed to react for 48 h and cooled to room temperature. After cooling, the reaction was quenched with water, extracted with dichloromethane, and dried under vacuum. The solids yielded were collected by filtration and evaporation to give the crude product. The crude product, i.e.,

m

-chloro-bridged Ir(III) dimmer, was mixed with Na2CO3 (0.30 g, 3.0 mmol), 2,4-pentanedione (0.30 g, 3.0 mmol),

and 2-methoxyethanol (20 mL) in a flask. The mixture was heated to react for 24 h. After cooling, the reaction was quenched with water and the mixture was extracted with dichloromethane. The combined extracts were then washed with brine, dried over MgSO4,

and evaporated to dryness. The crude product was isolated by column chromatography on a silica gel column using a mixture of CH2Cl2and n-hexanes (1:1 by volume) as the eluent to afford the

pure compound as a yellow solid with a 65% yield.1H NMR (CDCl3,

400 MHz, ppm):

d

7.68–7.63 (m, 8H), 7.60–7.58 (m, 4H), 7.32–7.27 (m, 4H), 7.14–7.11 (m, 2H), 6.64 (dd, J ¼ 8.0 and J ¼ 2.0 Hz, 2H), 6.49 (d, J ¼ 2.0 Hz, 2H), 6.38 (d, J ¼ 8.4 Hz, 2H), 5.25 (s, 1H), 1.84 (s, 6H). FABMS: m/z 986.0 (M)þ. Anal. calcd. for C43H31Br2IrN4O2: C, 52.29; H,

3.16; N, 5.67. Found: C, 52.55; H, 3.26; N, 5.56.

2.4.6. (pbiF)2Ir(acac) (6)

Compound 6 was synthesized by the same procedure as illus-trated for compound 5 except that compound 2 was used instead of compound 1. The product was isolated as an orange solid with a 40% yield.1H NMR (CDCl 3, 400 MHz, ppm):

d

7.87 (s, 2H), 7.88 (dd, J ¼ 7.2 and 1.6 Hz, 2H), 7.69–7.55 (m, 10H), 7.50–7.48 (m, 2H), 7.46 (d, J ¼ 8.0 Hz, 2H), 7.37–7.34 (m, 4H), 7.24–7.15 (m, 8H), 7.01 (s, 2H), 6.82 (s, 2H), 6.80 (dd, J ¼ 8.0 and 1.6 Hz, 2H), 6.60 (d, J ¼ 8.0 Hz, 2H), 5.29 (s, 1H), 1.91 (s, 6H), 1.90–1.58 (m, 8H), 1.13–1.03 (m, 24H), 0.75 (t, J ¼ 7.6 Hz, 12H), 0.66–0.63 (m, 8H). FABMS: m/z 1495.8 (M)þ. Anal. calcd. for C93H97IrN4O2: C, 74.71; H, 6.54; N, 3.75. Found: C,

74.24; H, 6.75; N, 3.42.

2.4.7. (pbiOPh-Br)2Ir(acac) (7)

Compound 7 was synthesized by the same procedure as illustrated for compound 5 except that compound 3 was used instead of compound 1. The product was isolated as an orange solid with a 40% yield.1H NMR (CDCl

3, 400 MHz, ppm):

d

7.70–

7.60 (m, 8H), 7.46–7.41 (m, 2H), 7.27–7.18 (m, 10H), 7.10–7.05 (m, 4H), 6.86 (d, J ¼ 8.0 Hz, 2H), 6.46–6.44 (m, 2H), 6.12–6.02 (m, 4H), 5.21 (s, 1H), 4.57–4.54 (m, 4H), 1.83 (s, 6H). FABMS: m/z 1200 (M)þ. Anal. calcd. for C

57H43Br2IrN4O4: C, 74.71; H, 6.54; N, 3.75.

Found: C, 74.52; H, 6.35; N, 3.71. 2.4.8. (pbiOPhF)2Ir(acac) (8)

Compound 8 was synthesized by the same procedure as illus-trated for compound 5 except that compound 4 was used instead of

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compound 1. The product was isolated as an orange solid with a 42% yield.1H NMR (CDCl3, 400 MHz, ppm):

d

7.73–7.69 (m, 6H), 7.63–7.56 (m, 6H), 7.50–7.45 (m, 14H), 7.30–7.28 (m, 8H), 7.13 (d, J ¼ 7.6 Hz, 4H), 7.07 (d, J ¼ 7.6 Hz, 2H), 6.49 (d, J ¼ 8.8 Hz, 2H), 6.15 (dd, J ¼ 8.8 and 1.6 Hz, 2H), 6.12 (d, J ¼ 1.6 Hz, 2H), 5.23 (s, 1H), 4.65–4.62 (m, 4H), 1.96–1.90 (m, 8H), 1.84 (s, 6H), 1.17–1.01 (m, 24H), 0.74–0.70 (m, 12H), 0.70–0.63 (m, 8H). FABMS: m/z 1706.8 (M)þ. Anal. calcd. for C107H109IrN4O4: C, 75.28; H, 6.44; N, 3.28.

Found: C, 75.72; H, 6.57; N, 3.06.

2.4.9. General procedure of copolymerization by Suzuki cross-coupling method

The following generalized procedure was used for the prep-aration of all copolymers. To a 50 mL two-necked flask charged with a condenser, tricaprylymethylammonium chloride (Aliquat 336) (w20 wt% based on the monomer), dibromide (compounds 5 or 7, 9 or 10, and 11, 1 equiv.), diboronate (compound 12, 1 equiv.), and Pd(PPh3)4(0.005 equiv) were added. After the flask

was evacuated and refilled with nitrogen for three times, toluene N N Br (1) pbi-Br + H2N HN 2-methoxyethanol CHO Br + B(OH)2 C6H13 C6H13 N N C6H13 C6H13 (2) pbi-F Pd(PPh 3)4, K2CO3 (aq) toluene, Aliquat 336 N N Br + N N OH Br Br Br C6H13C6H13 or DMF, K2CO3 N N O Br N N O or C6H13 C6H13 (3) pbi-OPhBr (4) pbi-OPhF IrCl3. nH2O N N Ir Cl Cl 4 HCl + n H2O S / H2O (3:1) S= 2-methoxyethanol + O O Na2CO3, S O O

(5) (pbi-Br)2Ir(acac) from (1)

(6) (pbiF)2Ir(acac) from (2)

(7) (pbiOPhBr)2Ir(acac) from (3)

(8) (pbiOPhF)2Ir(acac) from (4)

G N N G Ir 2 2 N N Ir G 2 N N Ir Cl Cl G N N G Ir 2 2 (1-4) O O N N Ir Br 2 O O N N Ir 2 , C6H13 C6H13 O O Ir 2 C6H13 C6H13 N N o Br , O O Ir 2 N N o , (5) (6) (7) (8)

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(1.0 mL) was added. Once all the monomers were dissolved, an aqueous solution of K2CO3(2 M, 1.0 mL) was added. The mixture

was heated to react at 100C and stirred for 48 h under nitrogen.

2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-fluorene (100 mg) was added and stirred at the same tempera-ture for 12 h. Then, bromobenzene (0.5 mL) was added to the solution to react for another 12 h. The mixture was cooled and poured into a mixture of methanol and water (100 mL, 2:1 v/v). The crude copolymer was filtered and washed with excess methanol, water, and acetone, and then evaporated to dryness. The polymer was dissolved in dichloromethane and precipitated in methanol for two times. Except for P6, P14, and P15, the product was further purified by flash chromatography using silica gel and eluted with a mixture of dichloromethane and THF (3:1 v/v) as the eluent. A general nomenclature for the copolymers (P1–P15) with respect to abbreviations of their monomers and their mol% were adopted. For example, PF70D20(pbi)Ir10(P8) was

synthesized from the composition of the following monomers: 10 mol% of Ir-complex 5, 20 mol% of 2,8-dibromo-dibenzothio-phene (9) and 70 mol% of fluorenes (note: both 11 and 12 will contribute to the fluorene unit, F, in the polymer). In addition, PF77Do20(pbi)Ir3(P12) was synthesized from the composition of

the following monomers: 3 mol% Ir-complex 5, 20 mol% of 2,8-dibromo-dibenzothiophene-S,S-dioxide (10) and 77 mol% of F, respectively.

PF95D5 (P1): Light-green solid. Yield ¼ 60%. Anal. calcd for

(C29H40)95(C12H6S)5: C, 89.38; H, 10.20. Found: C, 88.78; H, 9.86.

Weight-average molecular weight (Mw): 20,700 Da. PDI ¼ 1.95.

PF80D20 (P2): Light-green solid. Yield ¼ 58%. Anal. calcd for

(C29H40)80(C12H6S)20: C, 88.53; H, 9.63. Found: C, 87.63; H, 9.04.

Weight-average molecular weight (Mw): 8000 Da. PDI ¼ 1.33.

PF50D50 (P3): Light-green solid. Yield ¼ 43%.1H NMR (CDCl3,

400 MHz, ppm):

d

8.54 (s, dibenzothiophene ring), 7.97 (d, J ¼ 8.8 Hz, dibenzothiophene ring), 7.90–7.77 (m, dibenzothiophene and fluo-rine ring), 7.69–7.60 (m, fluofluo-rine ring), 2.12 (br, –CH2), 1.15–1.00

(m, CH2), 0.90–0.75 (m, CH2 and CH3). Anal. calcd for

(C29H40)50(C12H6S)50: C, 86.27; H, 8.11. Found: C, 85.77; H, 7.98.

Weight-average molecular weight (Mw): 7380 Da. DPI ¼ 1.47.

PF95Do5 (P4): Gray solid. Yield ¼ 70%. Anal. calcd for

(C29H40)95(C12H6O2S)5: C, 89.00; H, 10.15. Found: C, 88.24; H, 9.73.

Weight-average molecular weight (Mw): 49,000 Da. PDI ¼ 1.85.

PF80Do20 (P5): Gray solid. Yield ¼ 55%. Anal. calcd for

(C29H40)80(C12H6O2S)20: C, 86.93; H, 9.45. Found: C, 86.34; H, 9.03.

Weight-average molecular weight (Mw): 19,600 Da. PDI ¼ 1.86.

PF50Do50 (P6): Gray solid. Yield ¼ 30%. 1H NMR (CDCl3,

400 MHz, ppm):

d

8.17–8.10 (m, dibenzothiophene-S,S-dioxide ring), 7.96–7.87 (m, dibenzothiophene-S,S-dioxide ring), 7.84–7.81 (m, dibenzothiophene-S,S-dioxide and fluorine ring), 7.69–7.60 (m,fluorine ring), 2.12 (br,

b

–CH2), 1.15–1.00 (m, CH2), 0.90–0.75 (m,

CH2and CH3). Anal. calcd for (C29H40)50(C12H6O2S)50: C, 81.69; H,

7.68. Found: C, 80.78; H, 7.56. Weight-average molecular weight (Mw): 5310 Da. PDI ¼ 1.32.

PF77D20(pbi)Ir3(P7): Brown solid. Yield ¼ 60%. Anal. calcd for

(C29H40)77 (C12H6S)20(C43H31IrN4O2)3: C, 86.69; H, 9.20; N, 0.47.

Found: C, 87.59; H, 9.13; N, 0.39. Weight-average molecular weight (Mw): 20,900 Da. PDI ¼ 1.83.

PF70D20(pbi)Ir10 (P8): Yellow-orange solid. Yield ¼ 55%. Anal.

calcd for (C29H40)70(C12H6S)20(C43H31IrN4O2)10: C, 82.88; H, 8.32; N,

1.43. Found: C, 82.59; H, 8.49 N, 1.59. A weight-average molecular weight (Mw): 9110 Da. PDI ¼ 1.61.

PF50D47(pbi)Ir3(P9): Orange solid. Yield ¼ 55%. Anal. calcd for

(C29H40)50(C12H6S)47(C43H31IrN4O2)3: C, 84.45; H, 7.85; N, 0.55. Br Br C8H17C8H17 S Br Br C8H17C8H17 B B O O O O (i) Pd(PPh3)4, K2CO3, toluene (ii) (iii) R R B O O Br x: y: z = 27:20:3 PF77D20(pbi)Ir3(P7) from 5 x: y: z = 20:20:10 PF70D20(pbi)Ir10 (P8) from 5 x: y: z = 0:47:3 PF50D47(pbi)Ir3(P9) from 5 x: y: z = 0:40:10 PF50D40(pbi)Ir10(P10) from 5 (5) or (7) x:y = 45:5 PF95Do5(P4) 30:20 PF80Do20 (P5) 0:50 PF50Do50(P6) (i) Pd(PPh3)4, K2CO3, toluene (ii) (iii) R R B O O Br S Br Br O O x:y = 45:5 PF95D5(P1) 30:20 PF80D20 (P2) 0:50 PF50D50(P3) S C8H17 C8H17 y n C8H17C8H17 x S C8H17 C8H17 n C8H17C8H17 x O O C8H17 C8H17 C8H17 C8H17 z S C8H17 C8H17 y n C8H17C8H17 x C8H17 C8H17 C8H17C8H17 N N Ir N N O O z S C8H17 C8H17 y n C8H17C8H17 x C8H17 C8H17 C8H17C8H17 N N Ir N N O O x: y: z = 27:20:3 PF77Do20(pbi)Ir3(P12) from 5 x: y: z = 20:20:10 PF70Do20(pbi)Ir10 (P13) from 5 x: y: z = 0:47:3 PF50Do47(pbi)Ir3(P14) from 5 x: y: z = 0:40:10 PF50Do40(pbi)Ir10(P15) from 5 O O (9) or (10) (11) (12) (9) or (10) (11) (12) S C8H17 C8H17 y C8H17C8H17 x C8H17 C8H17 N N O O O Ir N N O C8H17C8H17 n x: y: z = 0:40:10 PF50D40(pbi-OPh)Ir10(P11) from 7 y z

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Found: C, 83.24; H, 7.48; N, 0.43. Weight-average molecular weight (Mw): 20,900 Da. PDI ¼ 1.83.

PF50D40(pbi)Ir10 (P10): Orange solid. Yield ¼ 45%. 1H NMR

(CDCl3, 400 MHz, ppm):

d

8.54 (s, dibenzothiophene ring), 7.97

(d, J ¼ 8.8 Hz, dibenzothiophene ring), 7.90–7.77 (m, dibenzothio-phene and fluorine ring), 7.69–7.60 (m, fluorine ring), 2.12 (br,

b

–CH2), 1.15–1.00 (m, CH2), 0.90–0.75 (m, CH2and CH3). Anal. calcd

for (C29H40)50(C12H6S)40(C43H31IrN4O2)10: C, 80.99; H, 7.34; N, 1.60.

Found: C, 80.59; H, 7.49; N, 1.59. Weight-average molecular weight (Mw): 8000 Da. PDI ¼ 1.33.

PF50D40(pbiOPh)Ir10(P11): Yellow solid. Yield ¼ 55%.1H NMR

(CDCl3, 400 MHz, ppm):

d

8.54 (s, dibenzothiophene ring), 7.97

(d, J ¼ 8.8 Hz, dibenzothiophene ring), 7.90–7.77 (m, dibenzothio-phene and fluorine ring), 7.69–7.60 (m, fluorine ring), 2.12 (br,

b

–CH2), 1.15–1.00 (m, CH2), 0.90–0.75 (m, CH2and CH3). Anal. calcd

for (C29H40)50(C12H6S)40(C57H41IrN4O4)10: C, 80.93; H, 7.20; N, 3.46.

Found: C, 79.59; H, 7.49; N, 3.14. Weight-average molecular weight (Mw): 10,500 Da. PDI ¼ 1.42.

PF77Do20(pbi)Ir3(P12): Brown solid. Yield ¼ 55%. Anal. calcd for

(C29H40)77(C12H6SO2)20(C43H31IrN4O2)3: C, 85.17; H, 9.05; N, 0.46.

Found: C, 85.71; H, 8.60; N, 0.39. Weight-average molecular weight (Mw): 16,700 Da. PDI ¼ 1.63.

PF70Do20(pbi)Ir10 (P13): Brown solid. Yield ¼ 63%. Anal. calcd

for (C29H40)70(C12H6SO2)20(C43H31IrN4O2)10: C, 81.54; H, 8.19; N,

1.41. Found: C, 82.06; H, 8.16; N, 1.29. Weight-average molecular weight (Mw): 17400 Da. PDI ¼ 1.65.

PF50Do47(pbi)Ir3 (P14): Orange solid. Yield ¼ 45%. Anal. calcd

for (C29H40)50(C12H6SO2)47(C43H31IrN4O2)3: C, 80.48; H, 7.48; N,

0.53. Found: C, 79.77; H,7.24; N 0.34. Weight-average molecular weight (Mw): 5600 Da. PDI ¼ 1.51.

PF50Do40(pbi)Ir10 (P15): Orange solid. Yield ¼ 43%. 1H NMR

(CDCl3, 400 MHz, ppm):

d

8.17–8.10 (m,

dibenzothiophene-S,S-dioxide ring), 7.96–7.87 (m, dibenzothiophene-S,S-dibenzothiophene-S,S-dioxide ring), 7.84–7.81 (m, dibenzothiophene-S,S-dioxide and fluorine ring), 7.69–7.60 (m, fluorine ring), 2.12 (br,

b

–CH2), 1.15–1.00 (m,

CH2), 0.90–0.75 (m, CH2 and CH3). Anal. calcd for

(C29H40)50(C12H6SO2)40(C43H31IrN4O2)10: C, 78.13; H, 7.08; N, 1.54.

Found: C, 77.52; H, 6.78; N, 1.34. Weight-average molecular weight (Mw): 3690 Da. PDI ¼ 1.51.

3. Results and discussion 3.1. Synthesis and characterization

The synthetic routes and chemical structures of the copolymers are shown inSchemes 1 and 2. The benzimidazole ligands 1–4 and Ir-complexes 5–8 were synthesized following the same or similar literature procedures [18.19]. The 2,8-dibromo-dibenzothiophene (compound 9) and 2,8-dibromo-dibenzothiophene-S,S-dioxide (compound 10) were synthesized by following published methods

[22]. Copolymerization of dibromo-substituted Ir-complex 5 or 7, compound 9 or 10, and fluorenes (compounds 11 and 12) was achieved via the Suzuki coupling reaction. For systematic studies of the device performance and the optical properties of these copoly-mers, the feed ratios of iridium units were controlled at the levels of 3 and 10 mol%, and compound 9 (or 10) at the levels of 5, 20 and 50 mol%, respectively. Except for copolymers with high Do contents (P6, P14, and P15), all copolymers and Ir-complexes exhibit good solubilities in dichloromethane (CH2Cl2) and tetrahydrofuran (THF).

All copolymers were characterized by 1H NMR spectroscopy, gel permeation chromatography (GPC) and elemental analyses. The actual compositions of the copolymers and the feed ratios of the monomers are listed in Table 1. The weight-average molecular weights (Mn) of these copolymers lie in the range of 3690–26,400 g/

mol with polydispersity index (PDI) values ranging from 1.32 to 1.95.

3.2. Thermal analysis

The thermal properties of all copolymers were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) under nitrogen. Thermal analysis data for the copolymers and Ir-complexes are given inTable 2. All copolymers (P1–P15) exhibited good thermal stabilities with thermal decom-position temperatures (Tdat 5% weight loss) ranging from 357 to

442C. The T

dvalue decreases as the iridium feed ratio increases,

indicating that covalently bonded iridium segments in polymer backbones decrease the thermal stability of the polymers. The glass

Table 1

Molecular weight data of copolymers P1–P15. Copolymer Mna (g mol1) Mwa (g mol1) PDI (Mw/Mn)

F:D (or Do):Ir complexes (molar ratio)b Co-monomer in feed ratio Composition in copolymer P1 10600 20700 1.95 95:5:0 94:6:0 P2 16400 28000 1.70 80:20:0 83:17:0 P3 6030 8560 1.42 50:50:0 53:47:0 P4 26400 49000 1.85 95:5:0 97:3:0 P5 10500 19600 1.86 80:20:0 88:12:0 P6 4000 5310 1.32 50:50:0 51:49:0 P7 11400 20900 1.83 77:20:3 77.5:19.5:3 P8 5650 9110 1.61 70:20:10 72:18.7:9.3 P9 6000 8000 1.33 50:47:3 49.5:48.2:2.3 P10 5000 7380 1.47 50:40:10 52.2:38.5:9.3 P11 7400 10500 1.42 50:40:10 56.4:35:8.6 P12 10200 16700 1.63 77:20:3 80:18.2:1.8 P13 10500 17400 1.65 70:20:10 71.8:20:8.2 P14 3700 5600 1.51 50:47:3 51.5:46.5:2 P15 2440 3690 1.51 50:40:10 53:37.5:9.5

aMolecular weights were determined by GPC using polystyrene standards. b The iridium contents in copolymers were estimated by1H NMR.

Table 2

Electrochemical and thermal properties of Ir-complexes 5–8 and Ir-copolymers P1–P15. Copolymer or complexes Eox(V)a HOMO (eV)b LUMO (eV)c Eg(eV) Tgd(C) Tde(C) Ir-complex 5 0.43 5.27 2.25 3.02 na 363 Ir-complex 6 0.32 5.12 2.0 3.10 na 322 Ir-complex 7 0.30 5.10 2.0 3.10 na 300 Ir-complex 8 0.29 5.09 2.0 3.10 na 275 P1 0.98 5.78 2.84 2.94 81 410 P2 1.01 5.81 2.89 2.95 100 428 P3 1.07 5.89 2.71 3.18 130 442 P4 1.01 5.81 2.89 2.92 81 423 P5 1.03 5.83 2.89 2.94 96 430 P6 na na na 3.10 na 420 P7 0.91 5.71 2.76 2.95 96 420 P8 0.91 5.71 2.76 2.95 97 375 P9 1.12 5.92 2.74 3.18 na 415 P10 1.10 5.90 2.72 3.18 na 357 P11 1.02 5.82 2.64 3.18 96 360 P12 0.93 5.73 2.78 2.95 96 425 P13 0.93 5.73 2.78 2.95 98 362 P14 na na na 3.10 na 410 P15 na na na 3.10 na 365

aOxidation potential is adjusted by using ferrocene (E

1/2¼ 490 mV vs Ag/AgNO3)

as an internal reference. Conditions of cyclic voltammetruc measurements: Pt working electrode; Ag/AgNO3reference electrode. Scan rate: 100 mV/s. Electrolyte:

tetrabutylammonium hexafluorophosphate. na ¼ not detected.

b HOMO levels were calculated from CV potentials using ferrocene as a standard

[HOMO ¼ 4.8 þ (Eox EFc)].

c LUMO levels were derived via eq. E

g¼ HOMO–LUMO, where Egobtained from

the absorption spectra.

d Obtained from DSC measurements; na ¼ not detected. e Obtained from TGA measurements. T

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transition temperatures (Tg) of the copolymers are in the range of

81–130C (see Fig. 1). Incorporation of D or Do units into the

copolymer backbones results in higher Tg values than PF

(Tg¼ 75C)[7b]. Such an outcome may be due to the presence of

heavy atoms (sulfur)[23]and the absence of flexible hydrocarbon chains in D or Do units of the former.

3.3. Electrochemical properties

The electrochemical behavior of all Ir-complexes and Ir-copoly-mers were studied by cyclic voltammetric (CV) methods, and the relevant data are listed inTable 2. A reversible one-electron oxidation wave attributed to the oxidation of iridium(III) was detected at 0.43, 0.32, 0.30 and 0.29 V vs. Fc/Fcþ for Ir-complexes 5, 6, 7, and 8, respectively. Except P6, P14, and P15, which showed very poor solubility in dichloromethane solutions, all Ir-copolymers showed a quasi-reversible oxidation wave with an onset potential around 0.91–1.12 V vs. Fc/Fcþ reference electrode. The energies of the

highest occupied molecular orbitals (HOMOs) in all copolymers were calculated relative to ferrocene (Fc), which had a value of 4.8 eV with respect to the vacuum level[24]. The HOMO and LUMO (lowest unoccupied molecular orbital) data are also collected inTable 2. The HOMO energies in combination with the optical band gaps (Eg)

derived from the absorption band edges were used to calculate the LUMO energies of the polymers. In comparison with PF (HOMO -¼ 5.77 eV; band gap -¼ 2.76 eV)[3a], copolymers bearing D or Do units (with HOMO ¼ 5.73–5.92 eV and band gap ¼ 2.94–3.18 eV) have higher (or similar) HOMO energy levels and larger band gaps. There was no obvious variation of the HOMO level for copolymers upon incorporation of iridium segments, possibly due to the lower contents of the iridium units.

3.4. Optical properties

The photophysical properties of all compounds are presented in

Table 3, and some selected UV–vis absorption spectra are shown in

Table 3

Photophysical properties of Ir-complexes 5–8, copolymers (P1–P15), and Ir-doped copolymers (P3 doped with 10 mol% of Ir-complexes 6 and 8).

Compound/Copolymer Solution Film

labs,max(nm)a lPL max(nm) Ff(%) Fp(%) ET(eV)f labs,max(nm) lPL,max(nm)g Ff(%)h Fp(%)h ET(eV)f si

Ir-complex 5 303, 316, 388, 416, 450 518b 30d 2.39 539 3.6 2.30 Ir-complex 6 340, 400, 452, 472 566b 25d 2.19 578 2.5 2.14 Ir-complex 7 256, 304, 315, 372, 403, 430 510b 22d 2.43 523 2.3 2.37 Ir-complex 8 295, 317, 376, 400, 428 510b 40d 2.43 520 3.3 2.38 P1 375 418c 75e 2.25 380 439 30 2.15 0.22 ns P2 375 416c 45e 2.25 382 426 25 2.15 0.28 ns P3 336 384c 30e 2.39 357 410 12 2.28 0.51 ns P4 385 418c 70e 2.24 386 440 34 2.13 0.33 ns P5 373 417c 48e 2.26 384 440 28 2.15 0.49 ns P6 348 415c 25e 2.36 387 428 15 2.25 0.60 ns P7 370 418c 25e 576 2.3 0.32ms P8 374 418c 15e 595 1.5 0.08ms P9 335 384c 23e 567 4.8 0.45ms P10 336 400c 14e 570 3.2 0.30ms P11 334 387c 14e 418 0.5 0.12ms P12 370 422c 25e 435 3.5 0.30ms P13 373 424c 13e 593 2.0 0.10ms P14 350 420c 18e 430 5.0 0.31ms P15 342 425c 12e 575 3.8 0.39ms P3 þ 3 mol% 6 576 7.5 0.44ms P3 þ 3 mol% 8 410 1.8 0.12ms P3 þ 10 mol% 6 567 4.5 0.40ms P3 þ 10 mol% 8 410 1.1 0.07ms PVK þ 3 mol% 6 564 10.6 1.17ms PVK þ 3 mol% 8 568 9.7 1.04ms PVK D 10 mol% 6 511 30 0.94ms PVK D 10 mol% 8 515 20 1.17ms aMeasured in CH 2Cl2solutions at 298 K.

b Measured in toluene solutions. Excitation wavelength was 400 nm. c Measured in THF solutions. Excitation wavelength was 350 nm.

d The quantum yields were measured in degas toluene solutions relative to Ir(ppy)

3(FP¼ 0.4 in toluene)21. The excitation wavelength was 400 nm. e The quantum yields were measured in THF solutions in air relative to curmarin 1 (F

f¼ 0.99 in ethyl acetate)20as a reference. The excitation wavelength was 350 nm. f Measured at 77 K.

g The excitation was 350–400 nm.

hMeasured with an integrating sphere under an excitation wavelength of 350 nm on a quartz glass.

i Measured at 298 K. The excitation wavelength was 357 nm for all copolymers. The fluorescence lifetimes and phosphorescence lifetimes were monitored at 430 and

575 nm, respectively. 60 80 100 120 140 160 180 200 220 45 50 55 60 65 70 75 80 P8 P13 P3 Tg=130 °C Tg=97 Tg=98 Tg=96 Tg=96 Tg=81 Tg=81 Tg=100 Tg=96 P7 P12 P4 P1 P5 P2 ) g/ W( w olf t a e H Temperature (°C) °C °C °C °C °C °C °C

Fig. 1. DSC traces of copolymers measured under nitrogen at a heating rate of 10C/min.

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Fig. 2. InFig. 2(a), Ir-complexes 5–8 have strong absorption bands at 270–370 nm attributed to the

p

p

*transition of the

benzimida-zolyl ligands and relatively weaker absorption bands at 400– 500 nm attributed to singlet and triplet metal-to-ligand charge-transfer[18,19],1MLCT and3MLCT. The absorption characteristics of metal-free copolymers (P1–P6) are shown inFig. 2(b). The intense bands at 336–386 nm can be assigned to the

p

p

*transition of the

polymer backbones. The bands shifted to shorter wavelengths as the contents of D (or Do) units increased in the polymers, indicating that the D (or Do) linkages interrupted the delocalization of

p

-electrons along the polymer backbones. For example, the absorp-tion peak of P3 was blue shifted by 39 nm compared with that of P2 (336 nm vs. 375 nm), and the peak of P6 was blue shifted by 25 nm compared with that of P5 (348 nm vs. 373 nm). The absorption spectra of P8, P10, P13 and P15 (10 mol% iridium ratio) are shown in Fig. 2(c). Besides the

p

p

*transition band of the copolymer

backbones and the benzimidazolyl ligands, weak absorption bands due to the1MLCT and3MLCT transitions of the iridium moieties

were also noticeable in the range of w400–500 nm[4b,7b,8b,10]. Similar to P1–P6, polymers with higher fluorene contents (P8 and P13) had longer effective conjugation lengths of oligofluorene and exhibited longer absorption lengths. In comparison with the absorption spectra of P7, P9, P12 and P14 (3 mol% iridium ratio), the MLCT transitions were not clear and cannot be observed due to the lower ratio of iridium units.

The photoluminescence (PL) properties of Ir-complexes 5–8, copolymers P1–P15 and Ir-doped copolymer P3 doped with 10 mol% of Ir-complexes 6 and 8 are demonstrated inTable 3. In

Fig. 3(a), Ir-complexes 5, 7 and 8 emitted in green region (

l

max¼ 510–518 nm), which were inherited from their precedent

complex, (pbi)2Ir(acac), whose chemical structure is shown in

Fig. 3(a) [18]. It is important to note that the PL emission of Ir-complex 6 emitted in the orange-yellow range (

l

max¼ 566 nm),

which was attributed to the extension of the ligand conjugation with fluorene units in Ir-complex 6. The PL spectra of selected copolymers in THF solutions and all copolymers (P1–P15) in solid films are shown in Fig. 3(b) and (c)–(e), respectively, while Ir-doped copolymers (P3 Ir-doped with 3 or 10 mol% Ir-complexes 6 and 8 in solid films) are also shown inFig. 3(f). Consistent with the absorption spectra, fluorescence wavelengths increased with increasing effective conjugation lengths (decreasing D or Do units) of the polymer both in solution and in the film state, i.e., P1 > P2 > P3, P4 ¼ P5 > P6, P7 > P9 and P12 > P14. Similar to P1– P6, Ir-copolymers P7–P15 and Ir-doped copolymer P3 doped with 3 or 10 mol% Ir-complexes 6 and 8 emitted only characteristic violet-blue light in dilute THF solutions due to the

p

p

*transition

of the polymer backbones (Fig. 3(b)), indicating that energy transfer from the polymer backbones to the iridium units was very inefficient in solution. In comparison, the phosphorescence emissions of Ir-copolymers P7–P15 and Ir-doped copolymers (3 and 10 mol% of iridium units) were more obvious in solid films (Fig. 3(d)–(f)), especially for the higher concentration (i.e., 10 mol%) of iridium units (seeFig. 3(e)), indicating the presence of energy transfer from the

p

p

* transition to MLCT bands. The

efficiency of the energy transfer appeared to be higher as the D or Do ratios in the polymer backbones increased, i.e. Ir-copolymers P9 and P14 were more efficient than P7 and P12 (seeFig. 3(d)). In comparison with Ir-copolymers, Ir-doped copolymers in solid films had similar PL spectra as the corresponding analogues with equivalent concentrations of iridium units as shown inFig. 3(f). It is important to note that the PL spectra of P11 tethered with a emitting congener of 8 and Ir-doped P3 with green-emitting Ir-complex 8 still exhibited strong blue emission from the polymer backbones in addition to the weak green emission from iridium units; that is, they had less efficient energy transfer compared to other Ir-copolymers tethered with a yellow-orange emitter (vide infra). It is believed that energy back transfer from the Ir-center to the polymer backbone also plays an important role in phosphorescence efficiency. In contrast to the long phospho-rescence lifetimes of Ir-doped PVK (

s

¼ 0.72–1.04

m

s), the faster phosphorescence decay in P7–P15 (

s

¼ 0.07–0.45

m

s) and Ir-doped P3 (

s

¼ 0.07–0.44

m

s) implied that there was more facile quench-ing of the triplet state of the iridium complex in the latter due to triplet energy back transfers from iridium complexes to the polymer backbones. Energy back transfer was even more serious for green-emitting Ir-copolymer P11 (

s

¼ 0.12

m

s) and Ir-doped P3 containing 3 or 10 mol% of Ir-complex 8 (

s

¼ 0.07–0.12

m

s), which exhibited shorter phosphorescence lifetimes than PVK doped with Ir-complex 8.

The fluorescence/phosphorescence quantum yield (

F

f/

F

p)

values of Ir-complexes 5–8, copolymers P1–P15 and Ir-doped copolymer P3 doped with 3 or 10 mol% of Ir-complexes 6 and 8 are also listed inTable 3. The

F

p values of Ir-complexes 5–8 were

300 400 450 500 550 600 0.0 0.2 0.4 0.6 0.8 1.0

P8

P10

P13

P15

350 300 350 400 450 500 550 600 300 350 400 450 500 550 600

Wavelength (nm)

c

0.0 0.2 0.4 0.6 0.8 1.0

b

P1

P2

P3

P4

P5

P6

).

u.

a(

n

oit

pr

o

s

b

A

0.0 0.2 0.4 0.6 0.8 1.0

Ir-complex 5

Ir-complex 6

Ir-complex 7

Ir-complex 8

a

Fig. 2. UV–vis absorption spectra in CH2Cl2 solutions of (a) Ir-complexes 5–8,

(b) metal-free copolymers P1–P6 and (c) selected Ir-copolymers (P8, P10, P13 and P15).

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22–40% in degassed toluene solutions and 2.3–3.6% in solid films. The

F

fvalues of metal-free copolymers P1–P6 ranged from 30 to

75% in THF solutions and from 12 to 34% in solid films. As the contents of D or Do units increased, the

F

f values of P1–P6

decreased in both solutions and solid films, possibly due to the presence of the heavy atom of sulfur[25]. Compared with the solid films of metal-free copolymers (

F

fvalues ¼ 12–34%), the solid films

of Ir-copolymers P7–P15 and Ir-doped copolymer P3 doped with 10 mol% of Ir-complexes 6 and 8 were found to have lower

F

p

values, with ranges of 0.5–5.0% and 1.1–7.5%, respectively. In general, the PL efficiency (

F

fand

F

p) decreased as the contents of

iridium units increased[2d,3a,3b,4b]. This can be rationalized by the greater tendency of triplet–triplet annihilation at higher iridium concentrations. However, the

F

pvalues of Ir-copolymers

were enhanced as the D (Do) ratio in the polymer backbone increased, i.e. P9, P10, P14 and P15 were higher than P7, P8, P12 and P13, respectively. It is believed that the larger triplet energy (vide infra) for the polymer with higher D or Do contents (w50 mol%) can more effectively suppress the energy back transfer from the phosphor to the polymer backbone (vide infra). The quenching of phosphorescence via energy back transfer of the phosphor excited state to the polymer triplet excited state has been well demonstrated for the polymer with lower triplet energy[10]. Phosphorescence spectra of P1–P6 and iridium complexes 6 and 8 in solid films were measured at 77 K (liquid nitrogen tempera-ture) using a 5-ms delay time between the excitation with

a microsecond flash lamp and the measurement. Representative phosphorescence spectra of selected copolymers (P2, P3 and P5) are shown inFig. 4. The triplet energies of the compounds were then determined from the peak maximum of the shortest emission wavelength in the phosphorescence spectra. The ETvalues of P1–P6

450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Ir-complex 5 Ir-complex 6 Ir-complex 7 Ir-complex 8 (pbi)2Ir(acac)

a

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0

b

P2 P3 P5 P6 P10 P15 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0

c

P1P2 P3 P4 P5 P6

).

u.

a(

yti

s

n

et

ni

L

P

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 P7 P9 P12 P14

d

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 P8 P10 P13 P15 P11

e

Wavelength (nm)

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 P3 + 3 mol% 6 P3 + 3 mol% 8 P3 + 10 mol% 6 P3 + 10 mol% 8

f

Fig. 3. PL spectra of (a) Ir-complexes 5–8 and (pbi)2Ir(acac) in toluene solutions, (b) selected copolymers in THF solutions, (c) metal-free copolymers P1–P6 in solid films,

(d) Ir-copolymers P7, P9, P12 and P14 in solid films (3 mol% of iridium units), (e) Ir-copolymers P8, P10, P11, P13 and P15 in solid films (10 mol% of iridium units) and (f) selected Ir-doped copolymers in solid films.

500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 P2 P3 P5

).

u.

a(

yti

s

n

et

nI

e

c

n

e

c

s

er

o

h

p

s

o

h

P

Wavelength (nm)

2.28 eV 2.15 eV

Fig. 4. Phosphorescence spectra (measured at 77 K) of some selected metal-free copolymers (P2, P3 and P5) in solid films.

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in solid films were measured to be 2.15, 2.15, 2.28, 2.15, 2.15 and 2.25 eV, and the ETvalues of Ir-complexes 6 and 8 in solid films

were 2.14 and 2.38 eV (from the phosphorescent emission wave-length), respectively. The ETincreased with increasing number of D

units, for example, P3 > P1 and P2. Illustration of relative energies of states for P3 and Ir-complexes 6 and 8 as well as the transitions among different states are shown inFig. 5. It is believed that the higher

F

pvalues in solid films of Ir-doped copolymers (P3 doped

with 3 or 10 mol% of Ir-complexes 6 and 8) compared with those of Ir-copolymers (P9–P11) partially benefited from the less efficient energy back transfer in the former[10]. The energy transfer effi-ciency depended on the distance, orientation and overlapped areas of absorption-PL spectra between the host and guest. The slightly lower PL efficiencies of Ir-copolymers may be explained by the constrained orientation of iridium complexes covalently bonded to the polymer backbones as well as the larger

p

p

interactions induced by the polymer main chains, which diminished the mobility of the phosphorescent iridium moieties and thus hampered the energy transfer.

3.5. Electroluminescent properties

Except for the poor solubilities of P6, P14 and P15 in chloroben-zene due to their higher Do contents in the polymer backbones, all copolymers can be fabricated into PLED devices by using the spin-coating technique. PLED devices were fabricated with a configuration of ITO/poly(ethylenedioxythiophene):poly(styrene–sulfonic acid) (PEDOT:PSS, 50 nm)/metal-free copolymers (P1–P5), Ir-copolymers (P7–P13) or Ir-doped copolymers (P3 or PVK doped with 3 or 10 mol% of Ir-complexes 6 and 8) (60–80 nm)/1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBI) (40 nm)/LiF (1 nm)/Al (120 nm); where vacuum-deposited TPBI was used as an electron-transporting and hole-blocking layer. The configuration of PLED devices, the chemical structures of PEDOT:PSS and TPBI and the triplet energy level diagrams of the hosts and guests are all illustrated inFig. 6. The EL spectra and the performance data of the PLED devices are shown in

Fig. 7andTable 4, respectively. The EL curves of current–voltage– brightness (I–V–L) characteristics along with the external quantum efficiency and power efficiency vs. current density for selected

P3 Ir-complex 6 S1 T1 S0 T1 2.14 eV S0 2.28 eV S1 ISC 3.18 eV

(a) energy transfer

2.80 eV non radiative decay P3 Ir-complex 8 S1 S0 T1 S0 2.28 eV S1 ISC 3.18 eV (a) 2.80 eV

(b) energy back transfer

2.38 eV

(a)

T1

(b)

Fig. 5. Energy level diagrams (eV) of P3 and Ir-complexes 6 and 8.

N N N N N N Al (120 nm) TPBI (40 nm) LiF (1nm) TPBI S O O n m PEDOT : PSS SO3 -+ PEDOT: PSS ITO (50 nm) Glass metal-free copolymers ( P1-P6 ) Ir-copolymers (P7-P13), and Ir-doped copolymers (P3 with 3, or 10 mol Ir-complex 6 or 8) (80 nm) 2.7 5.2 TPBI 5.71-5.92 6.2 h+ e -5.08-5.27

energy levels of iridium complexes Al 4.2 LiF ITO/ PEDOT:PSS polymers 2.64-2.89 2.2-2.0

a

b

Fig. 6. (a) The configurations of PLED devices and the molecular structures of PEDOT and TPBI used in the device, and (b) the relative energy levels of the compounds utilized in the PLED devices.

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devices are shown inFigs. 8 and 9, respectively. The PLED devices without TPBI were excluded due to their extremely low efficiencies, which dropped at least one order of magnitude. Copolymer P3 was chosen as the doped-host, because its higher ETmay increase the EL

efficiencies of the PLED devices.

3.5.1. Metal-free copolymers

InFig. 7(a) and (b), the EL spectra of metal-free copolymers P1– P5 were similar to their PL spectra in solid films. The EL emission peaks of these copolymers ranged from 400 to 500 nm, which were in the deep blue region based on Commision Internationale de I’Eclarage (CIE) 1931 color coordinates. Among the metal-free copolymers (P1–P5) inTable 4, the best fluorescence PLED devices with the highest

h

ext,maxvalues were found to be 1.77%, 0.71 lm/W

and 1726 cd/m2at 14 V for P2 and 1.53%, 0.95 lm/W and 2330 cd/ m2at 14 V for P5. It is worth noting that the P3-based PLED device had the lowest EL efficiency in fluorescence due to its low

F

fvalue

in the solid film.

3.5.2. Ir-copolymers

Similar to PL spectra in solid films, the EL spectra of PLED devices from Ir-copolymers (P7–P13) shown inFig. 7(c) and (d)had both contributions from the fluorescence of the polymer backbones and iridium moieties. However, the relative emission intensities of phosphorescence vs. fluorescence in EL spectra was higher than those in PL spectra, implying that charge trapping played an

important role in the PL and EL emission colors. If the host does not have sufficiently high ET values, energy back transfer can occur

readily, which will counteract the contribution from phosphor moieties (with the maximum value of 75% theoretically). Based on the results of PL studies, the triplet energy back transfers from Ir-copolymers P7, P8, P12 and P13 had a greater tendency than those from Ir-copolymers P9 and P10. It can be thought that copolymers with less fluorene units (e.g., P9 and P10 with the repeating unit of x ¼ 0 and higher repeating unit of y, i.e., higher molar ratios of D or Do units) and higher iridium ratios (e.g., P8, P10 and P13 with the repeating unit of z ¼ 10 and higher molar ratios of D or Do units) have higher

h

ext,maxand

h

p,maxvalues. The EL efficiencies of Ir-copolymers

(P7–P13) were in the following order: P9 (

h

ext,max¼ 0.90%,

h

p,max¼ 0.73 lm/W) > P7 (

h

ext,max¼ 0.80%,

h

p,max¼ 0.48 lm/W), P10

(

h

ext,max¼ 0.94%,

h

p,max¼ 0.94 lm/W) > P8 (

h

ext,max¼ 0.60%,

h

p,max¼ 0.64 lm/W), P10 (

h

ext,max¼ 0.94%,

h

p,max¼ 0.94 lm/

W) > P11 (

h

ext,max¼ 0.15%,

h

p,max¼ 0.17 lm/W), and P13

(

h

ext,max¼ 0.80%,

h

p,max¼ 0.79 lm/W) > P12 (

h

ext,max¼ 0.45%,

h

p,max¼ 0.21 lm/W). The EL efficiencies of Ir-copolymers

corre-sponded with the

F

pvalues in solid film in our observation. Being

tethered with a green emitting Ir-unit possessing a higher ETvalue,

the P11-based device had much smaller

h

ext,maxand

h

p,maxvalues

than P10 tethered with a yellow-emitting unit because of the higher tendency of energy back transfer in the former. There was prominent emission from the polymer backbone in the PLED device of P11, as shown inFig. 7(d). 400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0

a

P1

P2

P3

400 450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0

b

P4

P5

400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0

c

P7

P9

P12

).

u.

a(

yti

s

n

et

nI

L

E

400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0

d

P8 P10 P13 P11 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0

e

P3+ 3 mol% 6 P3+ 3 mol% 8 P3+ 10 mol% 6 P3+ 10 mol% 8

Wavelength (nm)

400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0

f

PVK + 3 mol% 6 PVK + 3 mol% 8 PVK + 10 mol% 6 PVK + 10 mol% 8

Fig. 7. EL spectra (at 9 V) of various PLED devices containing (a) and (b) metal-free copolymers, (c) Ir-copolymers (3 mol% of iridium units), (d) Ir-copolymers (10 mol% of iridium units), (e) Ir-doped copolymer P3 with 3 or 10 mol% of Ir-complexes 6 and 8 and (f) PVK with 3 or 10 mol% of Ir-complexes 6 and 8.

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Table 4

EL properties of PLED devices containing metal-free copolymers (P1–P5), Ir-copolymers (P7–P13), and Ir-doped copolymers (P3 and PVK doped with 3 or 10 mol% of Ir-complexes 6 and 8)

Polymer VON(V) Lmax(cd/m2) (at V) hext,max(%) hc,max(cd/A) hp,max(lm/W) lem,max(nm) CIE (x,y)

Metal-free copolymers P1 6.0 1995 (14.5) 1.46 1.23 0.64 436 0.16, 0.10 P2 6.0 1726 (14.0) 1.77 1.46 0.71 440 0.15, 0.10 P3 5.5 390 (12.0) 0.85 0.62 0.23 428 0.16, 0.10 P4 5.5 694 (14.0) 0.97 0.98 0.41 444 0.15, 0.12 P5 5.0 2330 (14.0) 1.53 1.66 0.95 440 0.17, 0.14 Ir-copolymers P7 5.0 827 (18.0) 0.80 1.59 0.48 578 0.40, 0.33 P8 5.5 1449 (17.5) 0.60 1.42 0.64 588 0.41, 0.30 P9 6.0 1505 (13.0) 0.90 1.98 0.73 592 0.50, 0.40 P10 4.0 3517 (16.0) 0.94 2.09 0.94 584 0.50, 0.44 P11 5.0 336 (15.0) 0.15 0.44 0.17 538 0.34, 0.50 P12 5.5 499 (18.5) 0.45 0.93 0.21 590 0.49, 0.38 P13 5.0 2274 (17.0) 0.80 1.88 0.79 580 0.49, 0.42 Ir-doped copolymers P3 D 3 mol% 6 4.0 2595 (19.0) 2.09 5.45 2.63 574 0.54, 0.44 P3 D 10 mol% 6 5.0 3697 (15.0) 2.32 6.20 2.16 572 0.47, 0.43 PVK D 3 mol% 6 4.0 3980 (14.5) 3.98 9.68 3.80 576 0.51, 0.44 PVK D 10 mol% 6 4.5 5146 (17.0) 2.90 7.51 2.79 578 0.53, 0.45 P3 D 3 mol% 8 4.0 149 (18.0) 0.21 0.18 0.05 394 0.21, 0.18 P3 D 10 mol% 8 6.0 491 (17.0) 0.23 0.68 0.27 526 0.33, 0.51 PVK D 3 mol% 8 5.0 3980 (14.5) 5.36 18.24 6.37 514 0.28, 0.61 PVK D 10 mol% 8 5.0 3356 (18.0) 4.58 15.58 4.89 528 0.34, 0.59

Von, turn-on voltage; L, luminance; V, voltage;hext, external quantum efficiency;hc, current efficiency;hp, power efficiency.

0 2 4 6 8 10 12 14 16 18 20 0 500 1000 1500 2000 2500 3000 3500 4000 m/ d c( e c n e c e s ni m u L 2 ) Voltage (V) P2 P5 P9 P10 P3+ 3 mol% 6 P3+ 10 mol% 6

a

b

0 2 4 6 8 10 12 14 16 18 20 0 100 200 300 400 500 600 700 800 m c/ A m( yti s n e d t n er r u C 2 ) Voltage (V) P2 P5 P9 P10 P3+ 3 mol% 6 P3+ 10 mol% 6

Fig. 8. The selected EL characteristic curves for PLED devices containing metal-free copolymer, Ir-copolymers and Ir-doped copolymers: (a) luminance vs. voltage and (b) current density vs. voltage.

0.1 1 10 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ) %( y c n ei cif f e m ut n a u q l a nr et x E

Current density (mA/cm2)

P2 P5 P9 P10 P3+ 3 mol% 6 P3+ 10 mol% 6

a

b

0.1 1 10 100 0 1 2 3 4 5 ) W/ ml( y c n ei cif f e r e w o P

Current density (mA/cm2)

P2 P5 P9 P10 P3+ 3 mol% 6 P3+ 10 mol% 6

Fig. 9. The selected EL characteristic curves for PLED devices containing metal-free copolymer, Ir-copolymers and Ir-doped copolymers: (a) external quantum efficiency vs. current density and (b) power efficiency vs. current density.

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3.5.3. Ir-doped copolymer

Similar to Ir-copolymers EL spectra, the EL spectra Ir-doped copolymers (P3 or PVK doped with 3 or 10 mol% of Ir-complexes 6 and 8) are shown inFig. 7(e) and (f), which also had both contribu-tions from the polymer backbones and iridium moieties. Charge trapping also played an important role here. In this study, the ET

values of P3, PVK and Ir-complexes 6 and 8 were 2.28, 2.50[2a,26], 2.14 and 2.38 eV, respectively (vide supra). Therefore, the energy back transfer from Ir-complex 8 to P3 had a greater tendency than that from Ir-complex 6 to P3, and the relative phosphorescence intensity of Ir-complex 8 to P3 was smaller than that of Ir-complex 8 to PVK in the PLED devices as shown inFig. 7(e) and (f). The trends of Ir-doped copolymers are listed as follows: (copolymer P3 doped with 3 or 10 mol% of complexes 6 and 8) P3 doped with 10 mol% of Ir-complex 6 (

h

ext,max¼ 2.32%,

h

p,max¼ 2.16 lm/W) > P3 doped with

10 mol% of Ir-complex 8 (

h

ext,max¼ 0.23%,

h

p,max¼ 0.27 lm/W) and

P3 doped with 3 mol% of Ir-complex 6 (

h

ext,max¼ 2.09%,

h

p,max¼ 2.63 lm/W) > P3 doped with 3 mol% of Ir-complex 8

(

h

ext,max¼ 0.21%,

h

p,max¼ 0.05 lm/W). The low EL performances of P3

doped with Ir-complex 8 (in different molar ratios, i.e., 3 and 10 mol%) can be attributed to the more facile back energy transfers from 8 to P3. When copolymer P3 was replaced by PVK, the PLED performances improved significantly, especially for Ir-complex 8-doping devices, possibly because the large triplet energy of PVK greatly suppressed the back energy transfer from Ir-complex 8. The device efficiency of PVK doped with 10 mol% of Ir-complex 6 (2.90%, 7.51 cd/A and 2.79 lm/W) was not greatly enhanced compared to PLED of P3 (2.32%, 6.20 cd/A and 2.16 lm/W) exhibiting the best performance. Hence, copolymer P3 was more appropriate as the host for a yellow-orange triplet emitter than for a green triplet emitter. According to our observations, the EL efficiencies of Ir-doped copolymers also corresponded with the

F

pvalues in solid film.

3.6. Hole and electron mobility properties

The space-charge limited current (SCLC) flow technique was used to measure the mobility of the charge carrier in a film[27]. The charge mobility in the hole-only or electron-only devices can be determined precisely by fitting the current vs. voltage (J–V) curve to the SCLC model for a single carrier device [28,29]. The current density is given by J ¼ 9

3

0

3

r

m

V2/8L3[30], where

3

0

3

ris the

permit-tivity of the polymer,

3

0¼ 8.85  1012F/m,

3

r is the dielectric

constant of the polymer,

m

is the carrier mobility, and L is the film thickness. The characteristic data of the SCLC measurements are listed inTable 5.

From the capacitance–voltage measurements, the relative dielectric constants

3

rof 5.40, 6.48, 5.61, 4.32, 4.03, 10.3, 10.8, 9.22

and 7.56 were obtained for the solid films of P2, P3, P5, 3 mol% Ir-complex 6 þ P3, 10 mol% Ir-complex 6 þ P3, P9, P11, P12 and P13, respectively. Therefore, their hole mobilities were calculated to be 2.10  106, 7.67  106, 8.51 106, 1.91 106, 1.58  106, 5.38  107, 4.42  107, 1.34  106 and 7.05  107cm2/V s, respectively. The hole mobilities decreased with increasing iridium contents in the devices based on either Ir-copolymers or Ir-doped copolymers. This may be explained by the lower contents of iridium units in the system enlarging the distance between the hole-hopping sites of the polymer backbones due to the mismatch of the HOMO levels between the polymer backbones and the iridium units. Hole trapping by the iridium units might also play a role because of the higher HOMO levels of Ir-complexes 5–8 than those of the metal-free polymers. Possibly the presence of iridium complexes in the polymer backbones increased the spacing of hopping sites in the polymers.

Their electron mobilities were also measured and calculated to be 3.10  107, 1.46  107, 2.09  106, 1.58  107, 2.89  107,

8.28  108, 4.75  108, 2.86  107 and 1.48  107cm2/V s,

respectively. Similar to hole mobilities, the electron mobilities of iridium-containing polymers were almost one order of magnitude lower than those of metal-free polymers. The lower electron mobilities in both Ir-copolymers and Ir-doped copolymers (doped with iridium complexes) were likely due to the enlarged distance between the electron-hopping sites. In contrast to the aforemen-tioned hole trapping, the iridium units have higher LUMO levels than the polymer backbones and were not likely to form electron traps.

In general, the electron mobilities were nearly one order lower than the hole mobilities for all copolymers. The faster hole mobilities of copolymers may cause hole decays in the cathode and inefficiency of the PLED devices. This might be the reason that the electron-transporting and hole-blocking layer, TPBI (mobility w105cm2/V s)

[31], was needed for better performance of the PLED device. Though both electron and hole mobilities in Ir-doped copolymers were higher than those Ir-copolymers, (i.e. 3 mol% Ir-complex 6 in P3 vs. P9), there was still an imbalance in the electron and hole mobilities in both PLED devices because the electron mobility of TPBI was w1 or 2 orders of magnitude higher than the hole mobilities of both PLED devices. However, the insertion of a TPBI layer into the PLED devices could retard the hole mobility, which increases the proba-bility of recombination and enhances the exciton recombination region near the interface between the emitting and TPBI layers, thus enhancing the PLED performance. The better hole and electron mobilities of metal-free copolymers and Ir-doped copolymers may improve the PLED performances.

4. Conclusion

In conclusion, we have successfully synthesized a series of 2,8-disubstituted fluorene-dibenzothiophene (PFD) and 2,8-disubsti-tuted fluorene-dibenzothiophene-S,S-dioxide (PFDo) copolymers. Copolymers with 3 and 10 mol% of covalently-bonded iridium segments in the backbones were also synthesized. The thermal stabilities of copolymers were enhanced as D (or Do) segments increased and deteriorated with increasing contents of iridium segments. Incorporation of D or Do units into the polymer back-bones increased the HOMO–LUMO gaps and the triplet energy levels. As the contents of D or Do units in the copolymers increased, more efficient energy transfers were induced from the polymer

Table 5

Characteristic data of space-charge limited current (SCLC) measurements.

Polymer J/V2 3 r L (nm) m(cm2/V s) Hole-only device P2 3.34 5.40 150 2.10  106 P3 14.6 6.48 150 7.67  106 P5 21.7 5.61 130 8.51  106 P3 D 3 mol% 6 2.43 4.32 150 1.91  106 P3 D 10 mol% 6 2.31 4.03 140 1.58  106 P7 1.63 10.3 150 5.38  107 P8 1.41 10.8 150 4.42  107 P12 4.49 9.22 140 1.34  106 P13 1.93 7.56 140 7.05  107 Electron-only device P2 1.11 3.60 100 3.10  107 P3 0.62 4.32 100 1.46  107 P5 8.63 4.14 100 2.09  106 P3 D 3 mol% 6 1.26 1.73 60 1.58  107 P3 D 10 mol% 6 1.02 2.59 90 2.89  107 P7 1.15 4.79 70 8.28  108 P8 0.69 5.04 70 4.75  108 P12 3.82 4.61 70 2.86  107 P13 1.63 3.78 70 1.48  107

J, electric current; V, voltage; 3r, dielectric constant of the polymer; m, carrier

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backbones to the iridium units, and less efficient energy back transfers from the iridium units to the polymer backbones occurred due to the enlarged triplet energy levels of the latter. Both Ir-copolymers and Ir-doped copolymers were used as an emitting layer of phosphorescent PLEDs. Copolymers with larger triplet energies in the polymer backbones had better performance due to more efficient suppression of energy back transfer. Less energy back transfer also led to better PLED performance for the Ir-doped copolymers compared with the Ir-copolymers. SCLC measurements carried out on both Ir-copolymers and Ir-doped copolymers confirmed that the iridium units form traps for holes and led to lower hole mobilities for both systems.

Acknowledgments

We thank the Academic Sinica, National Chiao Tung University, National Taiwan University, and National Science Council for sup-porting this work.

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數據

Table 3 , and some selected UV–vis absorption spectra are shown in
Fig. 2 . In Fig. 2 (a), Ir-complexes 5–8 have strong absorption bands at 270–370 nm attributed to the p – p * transition of the
Fig. 3. PL spectra of (a) Ir-complexes 5–8 and (pbi) 2 Ir(acac) in toluene solutions, (b) selected copolymers in THF solutions, (c) metal-free copolymers P1–P6 in solid films,
Fig. 7 and Table 4 , respectively. The EL curves of current–voltage– brightness (I–V–L) characteristics along with the external quantum efficiency and power efficiency vs
+3

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