Efficient binary white light-emitting polymers grafted with iridium complexes as side groups

Abstract

Efficient binary white-light-emitting electrophosphorescent copolymers were designed and synthesized via Suzuki polymerization. These copolymers were constructed by grafting a small amount of fluorinated iridium complexes as the side chain of the poly(fluorene-co-2,3-bisphenyl-6-fluoroquinoxaline) backbone. Efficient white-light emission was obtained simultaneously from the fluorescent blue light-emitting backbone and the tethered phosphorescent yellow light-emitting iridium species. A peak luminous efficiency of 7.20 cd A⁻¹ with the Commission Internationale de l'Eclairage coordinates of (0.32, 0.38) were obtained based on copolymer PFQ-IrFppy. The white-light emission of devices from the copolymer is stable over the whole white-light region at various applied voltages, and the luminous efficiencies decline slightly with increasing current density. These observations highlighted that the strategy of utilizing both the blue fluorescence from the backbone and yellow phosphorescence from the side chains can be an encouraging approach to realize efficient binary white light-emission.

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Introduction

Polymer light-emitting diodes have attracted both academic and industrial interest in the past two decades, since their great advantages of cost-effective solution-processing techniques to fabricate the flexible and large-area devices for full-colour displays and backlight panels of liquid crystal displays.^1–6 In general, white light-emitting spectrum exhibits relatively broad full width at the half height, thus multi-species are typically involved in the emissive layer that can allow for the simultaneous emission to access the white light emission. In consideration of the potential energy transfer from the high energy blue light-emitting species to the low energy green and red light-emitting species, the balance of the component ratios should be carefully taken into account so that the quenching of high energy emission can be avoided.^7–9 Thus, the delicate control of the ratio of the low energy emitters is of particular importance, since the simultaneously light-emission from both high and low energy components can be accessed due to the incomplete energy transfer on the basis of the appropriate amount of the low energy emitters.^10–12

The spectra of white light-emitting polymers comprise either three primary colours of blue, green and red; or two complementary colours of blue and orange, or blue and yellow as long as the connection line of their Commission Internationale de l'Eclairage coordinates (CIE) coordinates lies across the white light region.^13–15 With respect to the white light-emission consisting of three primary components, the binary white light-emission bears specific advantages of easy synthesis and facially tuneable ratios of the low energy species.¹⁶ In order to achieve state-of-art binary white light emission with high efficiency, the coordinates of the incorporated yellow light emitting emitter should match well with that of the blue light emitting units, and both of monochromic blue and yellow species should exhibit relatively high efficiency. In this respect, despite blue light-emitting polyfluorenes have appropriate CIE coordinates, their relatively low luminous efficiency limit the overall performance of the white emission. To address this issue, Wang et al. developed an effective strategy of covalently incorporating high efficient fluorescent or phosphorescent blue species into the copolymer backbones or side chains, which gave a series of highly efficient single white light-emitting polymers.^17–19

In this manuscript, we developed a series of efficient white light-emitting polymers on the basis of an efficient blue light-emitting polyfluorene derivative that consisting of 5 mol% of the electron-deficient 2,3-bisphenyl-6-fluoroquinoxaline moiety in the main chain, where three yellow light-emitting iridium complexes were incorporated into the side chain in an appropriate molar ratio. It was noted that the blue-emission from the fluorescent poly(fluorene-co-2,3-bisphenyl-6-fluoroquinoxaline) backbone and the yellow emission from the phosphorescent iridium complexes can be simultaneously recognized, leading to binary single white light-emitting polymers with impressive light-emitting efficiency.

Results and discussion

Synthesis and characterization

The incorporation of difluorophenyl ligand as the light emitting center was motivated by the highly efficient bis[2-(4,6-difluorophenyl)p-C²,N](picolinato)iridium(III) (FIrpic),²⁰ which used fluorinated ligand and gave very highly efficient iridium complex. The preparation of monomers which grafting 3,6-dibromocarbazole was shown in Scheme 1. The ligands 5-(2,4-difluorophenyl)-2-phenylpyridine (Fppy), 2-(2,4-difluorophenyl)-benzo[d]thiazole (Fsn) and 2-(2,4-difluorophenyl)quinoline (Fpq) were prepared by Suzuki coupling reaction in good yields. The organometallic intermediates of [Ir(Fppy)₂Cl]₂, [Ir(Fsn)₂Cl]₂ and [Ir(Fpq)₂Cl]₂ were prepared by treating ligand Fppy, Fsn and Fpq with IrCl₃·3H₂O, which were used directly for the next step without further purification. The monomers of Br₂Cz-IrFppy, Br₂Cz-IrFsn and Br₂Cz-IrFpq could be prepared from the reaction of the bridge-linked intermediates with β-diketone of 3,6-dibromo-9-(12,14-pentadecyl)diketone under nitrogen by using 2-ethoxyethanol as the solvent. As shown in Scheme 2, all polymerizations were performed by Suzuki polymerization of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (1), 2,7-dibromo-9,9-dioctyl-fluorene (2), and 2,3-bis(4-bromophenyl)-6-fluoro-quinoxaline (3) and monomers of Br₂Cz-IrFppy, Br₂Cz-IrFsn and Br₂Cz-IrFpq. With the molar feed ratios of 0.25 mol%, 0.4 mol% and 0.25 mol% for the copolymerized iridium-monomers Br₂Cz-IrFppy, Br₂Cz-IrFsn and Br₂Cz-IrFpq, the resulted copolymers were named as PFQ-IrFppy, PFQ-IrFsn and PFQ-IrFpq, respectively.

Scheme 1 Synthetic route of monomers.

Scheme 2 Synthetic route of copolymers.

In order to avoid the end-groups of the polymer main chain, the polymerization was end-capped by adding 2-(4,4,5,5-tetramethyl-1,3,2-dioxan-2-yl)fluorene and bromobenzene to remove the Br- and the boronic ester end groups, respectively. It was found that all these resulted polymers were soluble in organic solvents including toluene, xylenes, chloroform and tetrahydrofuran at room temperature. The number average molecular weight (M_n) lied in the range of 23.0–45.1 kDa with polydispersity index (PDI) of 1.92–2.30 (Table 1). The decomposition temperatures (T_d, corresponding to 5% weight loss) were higher than 425 °C (Table 1) as determined by thermogravimetric analysis (TGA), indicating the good thermal stability of these copolymers. The iridium contents of the copolymers were estimated by X-ray fluorescence spectrometry (XRF), illustrating that the actual iridium complex contents in the copolymers are substantially lower than the feed ratios of complex monomers. The fact was understandable since the small molecular weight fraction can be potentially removed during the workup.²¹ Detailed characterization data are summarized in Table 1.

Table 1 Molecular weight, composition, and thermal properties of polymers

Polymer	Ir content in (mol%)		Mn (kDa)	PDI	Td (°C)
	Feed ratio	Polymer^a
a Calculated from the content of C, H, N and Ir complex in copolymers.
PFQ-IrFppy	0.25	0.14	23.0	1.98	436
PFQ-IrFsn	0.4	0.25	39.0	1.92	429
PFQ-IrFpq	0.25	0.12	45.1	2.30	430

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Photophysical properties

Fig. 1 shows the UV-vis absorption spectra of Ir(Fppy)₂(acac), Ir(Fsn)₂(acac), Ir(Fpq)₂(acac) complexes and copolymer PFQ-Ir(Ligand), and the photoluminescent (PL) spectra of the host PFQ. It was found that all iridium complexes exhibited similar broad absorption profiles from 270 to 500 nm. The strong absorption before 350 nm could be attributed to the spin-allowed singlet state (¹π–*π) transition of cyclometalated ligands, the relatively weak absorption at 410 nm was ascribed to the spin-allowed metal to ligand charge transfer (¹MLCT).²² For the absorption peaks at 478, 452 and 455 nm for the iridium complexes Ir(Fppy)₂(acac), Ir(Fsn)₂(acac) and Ir(Fpq)₂(acac), the absorption could be attributed to the triplet metal to ligand charge transfer procedure (³MLCT).²³ The overlap between the fluorescent spectra of PFQ-backbone and the absorption spectra of iridium complexes indicated that the efficient Förster energy transfer can be expected. However, we failed to discern the absorption signal corresponding to the iridium complexes in the absorption profile of all polymers PFQ-Ir(Ligand), which may due to the exceptionally low molar ratios of the tethered iridium complexes in the resulted polymers.

Fig. 1 UV-Vis of Ir(Fppy)₂(acac), Ir(Fsn)₂(acac), Ir(Fpq)₂(acac) and PFQ-Ir(Ligand), PL spectrum of PFQ.

It was also worth noting that despite the electron-withdrawing difluoro-substitutions can lead to enlarged band gap of Fppy ligand, the peak emission of 550 nm of Ir(Fppy)₂(acac) showed the red shift of 68 nm with respect to the iridium(III) bis(2-phenylpridine) acetylacetonate complex [Ir(ppy)₂(acac)] of peak emission of 482 nm,²⁴ which can be attributed to the elongated conjugation of the Fppy ligand. The introduction of the electron-withdrawing difluoro-substitutions can lead to hypsochromic shift of the grafted iridium complexes based on Fsn or Fpq as the ligand,^25,26 which exhibited the emission peaked at 568 and 574 nm, respectively. These observations are understandable since the highest occupied molecular orbitals can be reduced that can lead to the enlarged band gaps of such iridium complexes. Nevertheless, when considering that the white light-emission generally need comparatively wide spectra (larger than 100 nm), the combination of such yellowish phosphorescence from the grafted iridium complexes and the blue fluorescence from the PFQ backbone can result in the efficient white light-emission in a single polymer.

Devices performances

To investigate the electroluminescence of the resulted polymers, polymer light emitting devices with structure of ITO/PEDOT: PSS (40 nm)/polymer + PBD (30 wt%) (80 nm)/Ba (4 nm)/Al (150 nm) was achieved. In considering that the doping of PBD could effectively improve the electron transport property of the emissive layer,²⁷ we herein introduced 30 wt% of PBD into emissive layer. From Fig. 2 one can see that the electroluminescent (EL) spectra of all copolymers exhibit white light-emission, which are quite close to pure white light with CIE of (0.33, 0.33). For the copolymer PFQ-IrFppy which containing 0.25 mol% of iridium complex Ir(Fppy)₂(acac), the independent two emission peaks at 425 nm for PFQ and 550 nm for Ir(Fppy)₂(acac) were shown in the electroluminescent spectrum, and the combination of the two peaks gives the CIE of (0.32, 0.38). With the feed ratio of the incorporated iridium complex Ir(Fsn)₂(acac) of 0.4 mol%, the resulted copolymer PFQ-IrFsn exhibit white light-emission with dual emission characteristics with the emission peaked at 425 nm corresponding to PFQ, and the emission peaked at 568 nm corresponding to the iridium complex of Ir(Fsn)₂(acac). The spectrum consisting of both blue and yellow emission with nearly equal intensity gave the CIE coordinate of (0.32, 0.34). For copolymer PFQ-IrFpq which contains 0.25 mol% of Ir(Fpq)₂(acac), despite the emission from PFQ at 425 nm was slightly higher than the emission at 574 nm for Ir(Fpq)₂(acac), the afforded CIE coordinates of (0.35, 0.25) also quite close to the standard white light-emission.

Fig. 2 Photoluminescent (PL) spectra (a) and electroluminescent (EL) spectra (b) of copolymers.

In comparison of the PL spectra and EL spectra (Fig. 2), it was found that the iridium complex emission from EL spectra was much higher than that of from PL spectra, indicating the different mechanism for the electroluminescence and photoluminescence.²⁸ In PL procedure, the energy transfer from polymer host to iridium complexes is mainly achieved by Förster energy transfer indicated from the overlap between PL spectra of polymer host and UV-Vis absorption spectra of grafted iridium complexes.²⁹ However, the charge trapping mechanism in EL procedures contributed much more pronounced than the corresponded Förster energy transfer procedure with respect to the PL procedure, indicating that the injected hole from anode and electron from cathode could be effectively trapped to form excitons for radiation.

Generally, to get highly efficient single-molecule white light-emission polymer, the highly efficient low energy emitting units and high energy emitting units of the wide band gap polymer host were both needed. In considering that the strong electron-withdrawing property of fluorinated quinoxaline (FQ) unit could effectively improve the electron transport property of the copolymers PFQ with respect to the PF, and the charge balance of the emissive layer could also be facilitated by the FQ unit,³⁰ thus the electroluminescent properties of the resulted copolymers could be potentially improved. The device performances of the resulted copolymers illustrated that the low threshold voltage of 4.3–4.9 V, and the highly efficient performances of all devices were given. The best device performance was achieved from the device with PFQ-IrFppy as the emissive layer, which gave the maximal external quantum efficiency (EQE) and luminous efficiency (LE) of 3.6% and 7.20 cd A⁻¹, respectively, and the power efficiency (PE) achieved 3.59 lm W⁻¹ with the maximal luminance of 6409 cd m⁻² with CIE of (0.32, 0.38). To our knowledge, this efficiency was among the most efficiency of the reported binary white PLEDs by simultaneously utilizing both singlet and triplet excitons. Since the iridium complex of Ir(Fsn)₂(acac) presents comparatively wide emission spectrum in the EL spectra of PFQ-IrFsn, thus the device based on PFQ-IrFsn as the emissive layer gives nearly state-of-art white light-emission with CIE coordinate of (0.32, 0.34), which is very close to the standard white-light emission of (0.33, 0.33). As summarized in Table 2, the maximal EQE and LE of PFQ-IrFsn based devices achieved 3.24% and 6.48 cd A⁻¹, with the maximal PE of 2.83 lm W⁻¹, along with the maximal luminance of 5680 cd m⁻². We note that these device performances in terms of luminance and efficiency are lower than those of devices based on vacuum evaporated organic light-emitting devices, or the devices based on solution processed small molecules blending with the phosphorescent dyes. The fact is understandable since the attached iridium complexes may suffer decomposition and then introduce a trace amount of β-diketone-trap centre in the side groups during the workup procedures, as revealed by XRF measurements. Nonetheless, the strategy of covalently attaching iridium complexes in the side chain can allow for the molecular level dispersion of the emissive centres that can effectively get rid of the unanticipated triplet–triplet annihilation due to the aggregation of small molecule phosphorescent dyes, and in turn lead to more stable white light emission.

Table 2 Device performances of polymers

Polymer	Vth (V)	EQE (%)	LE (cd A^−1)	PE (lm W^−1)	Lmax (cd m^−2)	CIE (x, y)
PFQ-IrFppy	4.3	3.60	7.20	3.59	6409	0.32, 0.38
PFQ-IrFsn	4.3	3.24	6.48	2.83	5680	0.32, 0.34
PFQ-IrFpq	4.9	1.42	2.84	1.24	4333	0.35, 0.25

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To evaluate the stability of these devices, the dependence of EL spectra on applied voltage was performed. As shown in Fig. 3a, for the polymer PFQ-IrFppy, the red emitting band slightly decreased with the increase of applied voltage, for which the CIE coordinates slightly changed from (0.32, 0.38) at 6 V to (0.31, 0.35) at 12 V. Similar trends were also realized for devices based on copolymers PFQ-IrFsn and PFQ-IrFpq as the emissive layer. The CIE coordinates changed from (0.32, 0.34) at 6 V to (0.31, 0.31) at 12 V for PFQ-IrFsn (Fig. 3b), and changed from (0.35, 0.26) at 6 V to (0.34, 0.25) at 10 V for PFQ-IrFpq (Fig. 3c). These results demonstrated the excellent EL spectral stability.

Fig. 3 EL spectra of device from the copolymers PFQ-IrFppy (a), PFQ-IrFsn (b) and PFQ-IrFpq (c), at various applied voltages with corresponding CIE coordinates.

The characteristics of luminous efficiency (LE) and luminance (L) as a function of current density (J) are shown in Fig. 4. The device based on copolymer PFQ-IrFppy achieved better EL efficiency than device based on the other two copolymers. In addition, unlike a physically blend systems comprises dye and polymer, the LE of these two devices exhibited slow roll-off regarding to current density. These observations indicated that the fabrication of electrophosphorescent polymers through grafting long alky side chain onto the copolymer can effectively decrease concentration quenching.³¹

Fig. 4 LE–J–L characteristics of copolymers.

Conclusions

In summary, a series of efficient binary white light-emitting copolymers that can simultaneously utilizing both singlet and triplet excitons were designed and synthesized via Suzuki polycondensation. It was realized that the combination of the fluorescent blue light-emitting poly(fluorene-co-2,3-bisphenyl-6-fluoroquinoxaline) backbone and phosphorescent yellow light-emitting iridium complex can form complementary white light-emission. The best device performance was achieved from the device with PFQ-IrFppy as emissive layer, which gave the maximal luminous efficiency of 7.20 cd A⁻¹ with maximal luminance of 6409 cd m⁻² and CIE coordinates of (0.32, 0.38). The white light-emitting spectra is quite stable over the whole visible light region at different applied voltages, and the electroluminescent efficiencies decline slightly with the increase of current density.

Experimental section

Materials

All manipulations involving air-sensitive reagents were performed under an atmosphere of dry argon. Toluene was purified by the standard procedure and distilled under dry argon before use. All reagents, unless otherwise specified, were obtained from Aldrich, Acros, and TCI Chemical Co. and used as received.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-fluorene (1), 2,7-dibromo-9,9-dioctylfluorene (2), 2,3-bis(4-bromophenyl)-6-fluoroquinoxaline(3), 3,6-dibromo-9-(12,14-penta-decyl-diketone) carbazole were synthesized according to reported procedures.^32–34

Synthesis of monomers

5-(2,4-Difluorophenyl)-2-phenylpyridine (Fppy). 5-Bromo-2-phenylpyridine (1.15 g, 4.9 mmol) was added into a solution of tertrakis(triphenylphosphine)palladium (0.17 g, 0.14 mmol) in 20 mL toluene. The mixture was stirred for 10 min under an argon atmosphere to give a yellow/green solution. 2-(2,4-Difluorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.77 g, 4.9 mmol) in 10 mL ethanol was added to the mixture to give a light red solution. The sodium carbonate solution (10 mmol, 2 M) was added to the mixture and refluxed for 24 h. Then the mixture was allowed to cool down to room temperature, and extracted by dichloromethane. The organic phase was washed by water and dried overnight with anhydrous MgSO₄. The solvent was removed by rotary evaporator, the crude products were purified by silica chromatography to give 1.02 g (yield, 78%) yellow needle product. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.83 (s, 1H), 8.05 (td, J = 1.56 Hz and 6.81 Hz, 2H), 7.92 (td, J = 2.07 Hz and 8.25 Hz, 1H), 7.81 (dd, J = 0.75 Hz and 7.53 Hz, 1H), 7.48 (m, 4H), 7.05–6.93 (m, 2H). Element anal. calcd (%) for C₁₇H₁₁F₂N: C 76.39, H 4.15, N 5.24. Found: C 76.31, H 4.09, N 5.20.

(5-(2,4-Difluorophenyl)-2-phenylpyridine)₂Ir(μ-Cl)₂Ir(5-(2,4-difluoro-phenyl)-2-phenylpyridine)₂ [Ir(Fppy)₂Cl]₂. With the use of a mixture of 30 mL of 2-ethoxyethanol and 10 mL of water as a solvent, 5-(2,4-difluorophenyl)-2-phenylpyridine (Fppy) (0.86 g, 3.6 mmol) and iridium chloride IrCl₃·3H₂O (0.32 g, 0.9 mmol) are mixed, and held at reflux at 120 °C for 24 hours in a nitrogen atmosphere to obtain a dinuclear complex [Ir(Fppy)₂Cl]₂ (red powder, yield: 35%). The crude product was used directly for the next step without further purification.

3,6-Dibromo-9-(iridium(III)bis(5-(2,4-difluorophenyl)-2-phenylpyridine-N,C^2′)-12,14-pentadecyl-diketone)carbazole (Br₂Cz-IrFppy). The chloride-bridged dimer [Ir(Fppy)₂Cl]₂ (0.44 g, 0.30 mmol), 3,6-dibromo-9-(12,14-pentadecyl-diketone)carbazole (0.43 g, 0.77 mmol), and sodium carbonate (0.10 g) were mixed and refluxed in 2-ethoxyethanol (30 mL) at 120 °C for 16 h under a nitrogen atmosphere. The solution was cooled to room temperature and filtered before washing with water and hexane. The crude products were purified by silica chromatography with a yield of 43%. ESI-MS: m/z, 1288.10, [m + 1]⁺. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.53 (s, 2H), 8.15 (d, 2H), 8.06 (d, 4H), 7.88 (s, 2H), 7.80 (s, 2H), 7.55 (d, 2H), 7.40 (m, 6H), 7.25 (s, 2H), 7.10 (t, 4H), 5.30 (s, 1H), 4.25 (t, 2H), 2.06 (m, 2H), 1.88 (m, 3H), 1.79 (m, 2H), 1.38–1.10 (s, 16H). Element anal. calcd (%) for C₆₁H₅₂Br₂F₄IrN₃O₂: C 56.92, H 4.07, N 3.26. Found: C 56.88, H 3.98, N 3.19.

2-(2,4-Difluorophenyl)benzo[d]thiazole (Fsn). This compound was synthesized according to the synthetic method of Fppy. Yield = 88%. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.42 (dd, J = 8.46 and 15.18 Hz, 1H), 8.09 (d, J = 8.10 Hz, 1H), 7.92 (td, J = 7.89 Hz, 1H), 7.52 (t, J = 7.23 Hz, 1H), 7.42 (d, J = 7.77 Hz, 1H), 7.07–6.93 (m, 2H). Element anal. calcd (%) for C₁₃H₇F₂NS: C 63.15, H 2.85, N 5.66. Found: C 63.10, H 2.78, N 5.58.

(2-(2,4-Difluorophenyl)benzo[d]thiazole)₂Ir(μ-Cl)₂Ir(2-(2,4-difluoro-phenyl)benzo[d]thiazole)₂ [Ir(Fsn)₂Cl]₂. This compound was synthesized according to the synthetic method of [Ir(Fppy)₂Cl]₂.

3,6-Dibromo-9-(iridium(III)bis((2,4-difluorophenyl)benzo[d]thiazole-N,C^2′)-12,14-pentadecyl-diketone)carbazole (Br₂Cz-IrFsn). This compound was synthesized according to the synthetic method of Br₂Cz-IrFppy. The crude products were purified by silica chromatography with a yield of 33%. ESI-MS: m/z, 1248.16, [m + 1]⁺. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.46 (d, 2H), 8.29 (d, 2H), 8.15 (d, 2H), 7.80 (s, 2H), 7.57 (d, 2H), 7.40 (s, 2H), 6.98 (m, 4H), 7.29 (s, 2H), 5.34 (s, 1H), 4.26 (t, 2H), 2.08 (m, 2H), 1.91 (m, 3H), 1.80 (m, 2H), 1.40–1.15 (s, 16H). Element anal. calcd (%) for C₅₃H₄₄Br₂F₄IrN₃O₂S₂: C 51.04, H 3.56, N 3.37. Found: C 50.07, H 3.48, N 3.31.

2-(2,4-Difluorophenyl)quinolone (Fpq). This compound was synthesized according to the synthetic method of Fppy. Yield = 80%. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.22–8.10 (m, 3H), 7.85 (d, J = 8.28 Hz, 2H), 7.74 (t, J = 8.10 Hz, 1H), 7.55 (d, J = 7.74 Hz, 1H), 7.05 (dt, J = 2.22 and 8.55 Hz, 1H), 6.94 (dt, J = 2.37 Hz and 11.16 Hz, 1H). Element anal. calcd (%) for C₁₅H₉F₂N: C 81.07, H 4.08, N 6.30. Found: C 81.01, H 4.05, N 6.24.

(2-(2,4-Difluorophenyl)quinolone)₂Ir(μ-Cl)₂Ir(2-(2,4-difluoro- phenyl)quinolone)₂ [Ir(Fpq)₂Cl]₂. This compound was synthesized according to the synthetic method of [Ir(Fppy)₂Cl]₂ and used as obtained for the next step without further purification.

3,6-Dibromo-9-(iridium(III)bis(2,4-difluorophenyl)quinolone-N,C^2′)-12,14-penta-decyl-diketone)carbazole (Br₂Cz-IrFpq). This compound was synthesized according to the synthetic method of Br₂Cz-IrFppy. The crude products were purified by silica chromatography with a yield of 36%. ESI-MS: m/z, 1236.3, [m + 1]⁺. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.24 (d, 4H), 8.12 (d, 2H), 7.90 (d, 4H), 7.79 (s, 2H), 7.50 (d, 2H), 7.44 (d, 2H), 7.22 (s, 2H), 7.07 (s, 2H), 6.90 (s, 2H), 5.26 (s, 1H), 4.29 (t, 2H), 2.10 (m, 2H), 1.89 (m, 3H), 1.75 (m, 2H), 1.35–1.04 (s, 16H). Element anal. calcd (%) for C₅₇H₄₈Br₂F₄IrN₃O₂: C 55.43, H 3.92, N 3.40. Found: C 55.38, H 3.89, N 3.33.

Synthesis of copolymers

General procedures of Suzuki polycondensation of copolymers, taking PFQ-IrFppy as an example.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di-octylfluorene (1) (321.3 mg, 0.5 mmol), 2,7-dibromo-9,9-dioctylfluorene (2) (245.4 mg, 0.4475 mmol), 2,3-bis(4-bromo-phenyl)-6-fluoro-quinoxaline (3) (22.9 mg, 0.05 mmol), 3,6-dibromo-9-(iridium(III)bis(5-(2,4-difluorophenyl)-2-phenylpyridine-N,C^2′)-12,14-pentadecyl-diketone)carbazole (Br₂Cz-IrFppy) (4.1 mg, 0.0025 mmol), palladium(II) acetate (Pd(OAc)₂, 1.5 mol% equivalent) and tricyclohexylphosphine (P(Cyh)₃, 4 mol% equivalent) were dissolved in toluene (8 mL), after stirred for 0.5 h, deionized H₂O (2 mL) and Et₄NOH (35 wt%) aqueous solution (0.2 mL) were added. The mixture was heated to 90 °C and stirred for 48 hours under argon atmosphere. The reaction was then capped by adding phenyl boric acid (25 mg) and bromobenzene (1 mL) successively and stirring for another 12 h. The whole mixture was poured into methanol. The precipitated polymer was recovered by filtration and purified by silica column chromatography with toluene as eluent to remove small molecular fraction and catalyst residue (yield 68%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.52 (br, 2H), 7.87 (br, 2H), 7.79 (m, 2H), 7.75 (br, 2H), 7.55 (br, 2H), 7.25 (br, 2H), 4.44 (br, 2H), 2.17 (br, 2H), 1.97 (m, 4H), 1.48–1.12 (m, 20H), 1.03–0.77 (m, 23H).

PFQ-IrFsn. Monomer 1 (321.3 mg, 0.5 mmol), 2 (244.6 mg, 0.446 mmol), 3 (22.9 mg, 0.05 mmol) and Br₂Cz-IrFsn (5.0 mg, 0.004 mmol) were used for copolymerization. Yield: 86%. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.49 (br, 2H), 7.86 (br, 2H), 7.79 (m, 2H), 7.72 (br, 2H), 7.52 (br, 2H), 7.26 (br, 2H), 4.41 (br, 2H), 2.14 (br, 2H), 1.95 (m, 4H), 1.45–1.06 (m, 20H), 1.00–0.72 (m, 23H).

PFQ-IrFpq. Monomer 1 (321.3 mg, 0.5 mmol), 2 (245.4 mg, 0.4475 mmol), 3 (22.9 mg, 0.05 mmol) and Br₂Cz-IrFpq (3.09 mg, 0.0025 mmol) were used for copolymerization. Yield: 80%. ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.55 (br, 2H), 7.87 (br, 2H), 7.77 (m, 2H), 7.72 (br, 2H), 7.57 (br, 2H), 7.22 (br, 2H), 4.45 (br, 2H), 2.17 (br, 2H), 1.98 (m, 4H), 1.49–1.16 (m, 20H), 1.08–0.81 (m, 23H).

Fabrication of polymer light emitting devices

The PLEDs were fabricated on ITO-covered glass substrates. Patterned indium tin oxide (ITO)-coated glass substrates were cleaned with acetone, detergent, distilled water, and isopropyl alcohol, subsequently in an ultrasonic bath. After treatment with oxygen plasma, 50 nm of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS; Batron-P4083, Bayer AG) was spin-coated onto the ITO substrate followed by drying in a vacuum oven at 80 °C for 8 h. The polymers were dissolved in toluene and filtered through a 0.45 mm filter. A thin film of polymer was coated onto the anode by spin-casting inside a dry box. The film thickness of the active layers was around 80 nm, as measured with an Alfa Step 500 surface profiler (Tencor). A thin layer Ba (4 nm), and a layer of Al (120 nm) were subsequently vacuum-evaporated onto the top of the emissive layer under vacuum of 1 × 10⁻⁴ Pa. Device performances were measured inside a dry box. Current–voltage (I–V) characteristics were recorded by a Keithley 236 source meter. Electroluminescent spectra were recorded by Oriel Instaspec IV CCD spectrograph. Luminance was measured by a PR 705 photometer (PhotoResearch). The external quantum efficiencies were determined by a Si photodiode with calibration in an integrating sphere (IS080, Labsphere).

Measurements

The ¹H nuclear magnetic resonance spectra were recorded on a Bruker DRX 500 in deuterated chloroform solution. Mass spectrometric detection was performed using Shimadzu LCMS-2010A quadrupole mass spectrometer with electrospray ionization (ESI) interface. The number-average molecular weights (M_n) were determined by a Waters GPC 2410 in tetrahydrofuran (THF) with a calibration curve of polystyrene standards. The element analyses were performed on a Vario EL element analysis instrument (Elementar Co.). The iridium content analyses were determined by using a Philips (Magix PRO) sequential X-ray fluorescence spectrometry (XRF), with a rhodium tube operated at 60 kV and 50 mA, a LiF 200 crystal and a scintillation counter. Tris(acetylacetonate) iridium(III) (Ir content of 38%, from Alfa Aesar Co.) was used as an internal reference specimen. Samples and specimens were pressed into homogeneous tablets (Φ = 30 mm) of compressed (375 MPa) powder of the copolymers. UV-Vis absorption spectra were recorded on an HP 8453 UV-Vis spectrophotometer. The thermogravimetric analysis (TGA) of the polymers was performed at a heating rate of 20 °C min⁻¹ with a NETZSCH TGA-209 thermal analyzer under nitrogen atmosphere. PL and EL spectra were recorded on an Instaspec IV CCD spectrophotometer (Oriel Co.).

Acknowledgements

The authors are grateful for financial support from the Ministry of Science and Technology-China (2015AA033402 and 2015CB655004), and the National Natural Science Foundation of China (Grants 61177022, 51473054 and 51273069).

Notes and references

1. L. Ying, C. L. Ho, H. B. Wu, Y. Cao and W. Y. Wong, Adv. Mater., 2014, 26, 2459.
2. H. B. Wu, L. Ying, W. Yang and Y. Cao, Chem. Soc. Rev., 2009, 38, 3391.
3. L. X. Wang, X. B. Jing and F. S. Wang, Acta Polym. Sin., 2009, 10, 980.
4. G. M. Farinola and R. Ragni, Chem. Soc. Rev., 2011, 40, 3467.
5. C. Tang, X. D. Liu, F. Liu, X. L. Wang, H. Xu and W. Huang, Macromol. Chem. Phys., 2013, 214, 314.
6. H. Sasabe and J. Kido, J. Mater. Chem. C., 2013, 1, 1699.
7. S. Y. Shao, J. Q. Ding, T. L. Ye, Z. Y. Xie, L. X. Wang and F. S. Wang, Adv. Mater., 2011, 23, 3570.
8. L. Yu, J. Liu, S. J. Hu, R. F. He, W. Yang, H. B. Wu, J. B. Peng, R. D. Xia and D. D. C. Bradley, Adv. Funct. Mater., 2013, 23, 4366.
9. T. Guo, L. Yu, Y. Yang, Y. H. Li, Y. Tao, Q. Hou, L. Ying, W. Yang, H. B. Wu and Y. Cao, J. Lumin., 2015, 167, 179.
10. L. Ying, Y. H. Li, C. H. Wei, M. Q. Wang, W. Yang, H. B. Wu and Y. Cao, Chin. J. Polym. Sci., 2013, 31, 88.
11. L. Chen, P. C. Li, H. Tong, Z. Y. Xie, L. X. Wang, X. B. Jing and F. S. Wang, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 2854.
12. L. J. Zhang, S. J. Hu, J. W. Chen, Z. H. Chen, H. B. Wu, J. B. Peng and Y. Cao, Adv. Funct. Mater., 2011, 21, 3760.
13. C. Tang, X. D. Liu, F. Liu, X. L. Wang, H. Xu and W. Huang, Macromol. Chem. Phys., 2013, 214, 314.
14. H. Sasabe and J. Kido, J. Mater. Chem. C, 2013, 1, 1699.
15. C. Y. Mei, J. Q. Ding and B. Yao, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1746.
16. B. H. Zhang, G. P. Tan, C. S. Lam, B. Yao, C. L. Ho, L. H. Liu, Z. Y. Xie, W. Y. Wong, J. Q. Ding and L. X. Wang, Adv. Mater., 2012, 24, 1873.
17. J. Liu, Z. Y. Xie, Y. X. Cheng, Y. H. Geng, L. X. Wang, X. B. Jing and F. S. Wang, Adv. Mater., 2007, 19, 531.
18. J. Liu, B. X. Gao, Y. X. Cheng, Z. Y. Xie, Y. H. Geng, L. X. Wang, X. B. Jing and F. S. Wang, Macromolecules, 2008, 41, 1162.
19. J. Liu, Y. X. Cheng, Z. Y. Xie, Y. H. Geng, L. X. Wang, X. B. Jing and F. S. Wang, Adv. Mater., 2008, 20, 1357.
20. Y. P. Jeon, K. S. Kim, K. K. Lee, I. K. Moon, D. C. Choo, J. Y. Lee and T. W. Kim, J. Mater. Chem. C., 2015, 3, 6192.
21. T. Guo, R. Guan, J. H. Zou, J. Liu, L. Ying, W. Yang, H. B. Wu and Y. Cao, Polym. Chem., 2011, 2, 2193.
22. C. Fan and C. L. Yang, Chem. Soc. Rev., 2014, 43, 6439.
23. S. Y. Shao, J. Q. Ding, L. X. Wang, X. B. Jing and F. S. Wang, J. Am. Chem. Soc., 2012, 134, 15189.
24. T. Kim, S. Lim, S. R. Park, C. J. Han, J. Chul and H. Min, Polymer, 2015, 66, 67.
25. F. Yan, G. C. Chen, R. Chen, H. V. Demir, H. D. Sun, T. C. Sum and X. W. Sun, Appl. Phys. Lett., 2015, 106, 023302.
26. Y. Y. Zhou, W. F. Li, L. P. Yu, Y. Yu, X. M. Wang and M. Zhou, Dalton Trans., 2015, 44, 1858.
27. L. Ying, J. H. Zou, W. Yang, H. B. Wu, A. Q. Zhang, Z. L. Wu and Y. Cao, Macromol. Chem. Phys., 2009, 210, 457.
28. S. Y. Shao, J. Q. Ding, L. X. Wang, X. B. Jing and F. S. Wang, J. Am. Chem. Soc., 2012, 134, 20290.
29. C. Tang, R. Bi, X. D. Cao, C. Fan, Y. T. Tao, S. F. Wang, H. M. Zhang and W. Huang, RSC Adv., 2015, 5, 65481.
30. L. Ying, J. H. Zou, W. Yang, A. Q. Zhang, Z. L. Wu, W. Zhao and Y. Cao, Dyes Pigm., 2009, 82, 251.
31. S. L. Gong, C. L. Yang and J. G. Qin, Chem. Soc. Rev., 2012, 41, 4797.
32. Y. Yang, L. Yu, Y. Xue, Q. H. Zou, B. Zhang, L. Ying, W. Yang and J. B. Peng, Polymer, 2014, 55, 1698.
33. L. Ying, Y. H. Xu, W. Yang, L. Wang, H. B. Wu and Y. Cao, Org. Electron., 2009, 10, 42.
34. T. Guo, L. Yu, B. F. Zhao, L. Ying, H. B. Wu, W. Yang and Y. Cao, J. Polym. Sci., Part A: Polym. Chem., 2015, 53, 1043.

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