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phosphorescence hybrid copolymers for white polymer light emitting devices
Tetrahedron xxx (2015) 1e7
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Synthesis and properties of hyperbranched fluorescence/ phosphorescence hybrid copolymers for white polymer light emitting devices Yuling Wu a, b, Jie Li a, b, Wenqing Liang a, b, Junli Yang a, b, Jing Sun a, b, Hua Wang a, b, Xuguang Liu c, Bingshe Xu a, b, * a b c
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, China Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 June 2015 Received in revised form 18 August 2015 Accepted 22 August 2015 Available online xxx
A series of hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) as the branches and the three-dimensional structured spiro[3,3]heptane-2,6-dispirofluorene (SDF, 10 mol %) as the core have been synthesized by adjusting the feeding ratios of Ir(piq)2acac (0.02e0.05 mol %). The copolymers showed good thermal and spectral stability, and amorphous film morphology because of the hyperbranched structures. The 2,7-substituted fluorenes of SDF were incorporated into the p-system of the €rster resonance energy transfer (FRET) efficiency from flupolyfluorene branches, and remained the Fo orene segment to Ir(piq)2acac unit. The copolymers exhibited efficient electroluminescent characters, and white-light emission was achieved in PFSDF-Ir4 (Ir(piq)2acac 0.04 mol %)-based single layer device with CIE coordinates at (0.30, 0.34), a maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A. Ó 2015 Published by Elsevier Ltd.
Keywords: Fluorescence/phosphorescence hybrid Hyperbranched copolymers Spiro[3,3]heptane-2,6-dispirofluorene White-light emission
1. Introduction White polymer light-emitting devices (WPLEDs) have received great attention towards their potential applications in large-area full-color and flexible displays combined with a color filter, backlights and solid lighting sources.1e4 Among the common approaches used to generate white light in WPLEDs, single whiteemitting polymer bearing either three primary colors (red (R), green (G) and blue (B)) or two complementary colors (generally blue (B) and orange (O)) emitting units incorporated into or attached to the backbone has advantages including relieved phase separation, controlled proportion of different emitting moieties, and easy fabrication process etc.5 The phosphorescent materials such as iridium complexes can realize theoretical 100% internal quantum efficiency due to the ability to harvest both singlet and triplet excitons to light.6,7 Therefore, phosphorescence molecules (usually as the narrowband-gap chromophores) embedded into the fluorescence polymer chains (usually as the wide-band-gap emitters) could
* Corresponding author. E-mail address: [email protected] (B. Xu).
effectively improve the electroluminescent (EL) performance of the WPLEDs.8e10 Furthermore, because the phosphorescence chromophores disperse into the polymer at the molecular level, the system can be regarded as homogeneous. The required content of the phosphorescence emitter was low (usually in the range of one ten thousandth to one thousandth (molar ratio)) because of the efficient energy transfer process.5 Thus, the tripletetriplet (TeT) annihilation of iridium complexes could be well-suppressed, and efficient EL performance could be expected. Polyfluorenes (PFs) are shown to be the most promising blue light-emitting materials because of high photoluminescence quantum efficiency, and relatively good chemical and thermal stabilities.11,12 When orange-light-emitting iridium complex is dispersed to PFs, white light emission could be obtained by in€rster resonance energy transfer (FRET) from the fluocomplete Fo rene segment to the Ir complex unit.13,14 However, linear copolymers suffer from intermolecular interactions, which reduce the efficiency of the device.15 On the other hand, hyperbranched polymers with three-dimensional structure can prevent the aggregation of polymer chains, make the material form amorphous films with good quality, and increase the glass transition temperature (Tg) of the polymers.16e21 Recently, a number of
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
hyperbranched electroluminescent polymers with the three principle colors (red,22 green23,24 and blue18) have been synthesized, and both emission efficiency and thermal stability were effectively improved with respect to their linear analogies.15,25e27 However, white-light-emitting hyperbranched polymers have rarely been reported. In this paper, hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) branches and spiro[3.3]heptane-2,6-dispirofluorene (SDF) core (10 mol %) were constructed. The three-dimensional-structured SDF exhibits great morphological stability and intense fluorescence,28 and furthermore, its steric hindrance can prevent rotation of the adjacent aryl groups, which reduces close packing and intermolecular interactions between the chromophores in the solid-state.29,30 In order to obtain white-light emission, the orange light-emitting unit Ir(piq)2acac was introduced with different contents from 0.02 mol % to 0.05 mol %. Such a highly branched framework may provide a highly efficient white-light electroluminescence. 2. Results and discussion 2.1. Synthesis and characterization The synthesis of Ir(Brpiq)2acac even reported used 1chloroisoquinoline, 4-bromophenylboronic acid and IrCl3$3H2O as the orange light-emitting unit. The hyperbranched copolymers with 9,9-dioctylfluorene and Ir(piq)2acac as the branches and SDF as the core were prepared by Suzuki polycondensation with yields ranging from 49% to 59% (Scheme 1). The feed molar ratios of the branching point SDF were 10% and Ir(piq)2acac were varied from 0.02 % to 0.05 % relative to the fluorenyl units, and the corresponding hyperbranched copolymers were named as PFSDF-Ir2 (Ir(piq)2acac 0.02 mol %), PFSDF-Ir3 (Ir(piq)2acac 0.03 mol %), PFSDF-Ir4 (Ir(piq)2acac 0.04 mol %) and PFSDF-Ir5 (Ir(piq)2acac 0.05 mol %), respectively. All of the copolymers are readily soluble at room temperature in common organic solvents such as CHCl3, THF and toluene. The synthetic and structural results of the copolymers were summarized in Table 1. The 1H NMR spectra of the copolymers were quite similar (Fig. S2, the proton signals of Ir(piq)2acac were not detected because of its low content), revealing the similar backbone structures of the copolymers. Taking PFSDF-Ir4 as an example (Fig. S3), the actual content of SDF was calculated by comparing the peak integral intensities of the proton signals of the spiro[3.3]heptane of SDF (d 3.0e3.5) and the aromatic ring of fluorene (d 7.4e8.0) of the copolymer (8.80:1), which was close to the feed ratio (8.25:1). The number-average molecular weights (Mns) of the copolymers were all around 13000 with the polydispersity indexes (PDIs) ranging from 1.56 to 3.35. All of the copolymers exhibit good thermal stability with the onset decomposition temperatures (Td, measured at a 5% weight loss) from 407 to 423 C. DSC data reveals the glass transition temperatures (Tgs) of copolymers were all around 155 C (Fig. S4, Supplementary data). 2.2. Photophysical properties Fig. 1 shows the UV-visible absorption of Ir(Brpiq)2acac and photoluminescence (PL) spectra of hyperbranched polyfluorenespiro[3.3]heptane-2,6-dispirofluorene (PFSDF, SDF 10 mol %) copolymer in CHCl3 solution at a concentration of 1.0105 mol/L. There are two strong absorption peaks seated at 243 nm and 297 nm, which are mainly attributed to the spin-allowed ligandcentered (1LC) state transitions. The weak absorption peaks seated at 345 nm are mainly assigned to the spin-allowed metal-to-ligand charge-transfer (1MLCT) state transitions. The unconspicuous
absorption peak around 472 nm is mainly belonged to the spininhibited metal-to-ligand charge-transfer (3MLCT) state and ligand-centered (3LC) state transitions.31 It is obvious that the emission of copolymer PFSDF and the absorption spectrum of the Ir complex show good spectra overlap. Therefore, efficient FRET from the PFSDF to the Ir complex, and white-light emission can be expected by the combination of blue light-emitting from PFSDF and orange light-emitting from Ir(piq)2acac through finely adjusting the content of Ir complex. The normalized UVevis absorption and PL spectra of the copolymers in CHCl3 solution (105 mol/L) and thin film states are shown in Fig 2. In dilute solution, all of the copolymers exhibit typical absorption and emission bands of PF with an absorption maximum around 373e379 nm, and emission peaks at 420, 440, and a shoulder peak at 474 nm. The presence of vibronic structure in the emission spectra indicates that the hyperbranched copolymers have rigid and well-defined backbone structures. The absorption and emission bands of Ir(piq)2acac were not detected due to its comparatively low content (0.02e0.05 mol %) in the copolymers. In dilute solution, the energy transfer was exclusively intrachain.32 In films, the copolymers exhibit UVevis absorption bands at around 375 nm similar to those in dilute solution. In the PL spectra, the maximum emission bands of the copolymers are at about 420 and 440 nm, showing no obvious bathochromic shift with respect to those in dilute solution. This result indicates that the hyperbranched molecular structure can prevent the aggregation and the interaction of the copolymer chains efficiently. The emission band of Ir(piq)2acac centered at 613 nm still can’t be observed due to the low content of Ir(piq)2acac, as a result of both intra- and interchain FRET from fluorene unit to Ir(piq)2acac.22,33 2.3. Film forming properties The morphology of the spin-coated films of the copolymers was estimated by atomic force microscopy (AFM) at a tapping mode, and the images are shown in Fig. 3. All the films show smooth surface with small root-mean-square (RMS) values of 1.295, 1.670, 2.316 and 2.884 nm for PFSDF-Ir2, PFSDF-Ir3, PFSDF-Ir4 and PFSDF-Ir5, respectively. The results indicate that the hyperbranched structure with three-dimensional-structured SDF branch point could provide a homogeneous morphology of spin-coating films, which could be favorable for the PLED fabrication. 2.4. Electroluminescence properties Using the copolymers as emitting materials, single-layer PLEDs were fabricated with the configuration of ITO/PEDOT:PSS (40 nm)/ Copolymers (50 nm)/TPBi (35 nm)/LiF (1 nm)/Al (150 nm). The electroluminescent spectra of the devices are shown in Fig. 4a and the characteristics were summarized in Table 2. Generally, all of the copolyemers exhibit much broader EL spectra than their PL counterparts. At the voltage of 16 V, PFSDF-Ir2 and PFSDF-Ir3 both exhibit blue-light emitting and the broad peaks locate at the 452 nm and 484 nm, respectively. The spectrum of PFSDF-Ir4 covers the visible light region with 400e700 nm and the main peaks locate at 425 and 548 nm, respectively. White-light emission is obtained with CIE coordinates located at (0.31, 0.35) (Fig. 4b). The emission peak of PFSDF-Ir5 mainly locates at 533 nm and yellow-light emission is achieved. For all of the copolymers, the emission band around 540 nm may be from the excimer of PFSDF formed under the electric field.5,22,34e36 As the electrons and holes can be trapped on the Ir complex,22 the formation of the excimer may be induced by Ir(piq)2acac under electric excitation.35 As a result, the excimer emission intensity increased with the Ir(piq)2acac contents. In PFSDF-Ir5, the emission from the PFSDF
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
Br
Br
Cl
Br
Pd(PPh3)4 NaCO3
N
+ B HO
IrCl3.3H2O
Toluene Methanol
3
N Br
N
N
Cl
Ir
Ir
Cl
N
OH
Br acetylacetone Br
N
Na2CO3
N
Br
Ir N
O O
Br
A
B
Ir(Brpiq)2acac Br
C8H17 C8H17
C8H17
C8H17
Br
Br
+
+
+
N Br
Ir N
O
Pd(PPh3)4 NaCO3
O
Aliquat336 72 h
Br
Br
M2
M1
Br
Br O B O
O B O
TBrSDF
Ir(Brpiq)2acac Br
Br
O
C8H17 N N
C8H17
Ir
O
C8H17
Br
C8H17
O
C8H17
Ir
O
N
n
n
C8H17 C H 8 17
m
n
C8H17
M3
C8H17
n
Br C8H17
C8H17
o
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
Br
C8H17
p
C8H17
N
C8H17
O B O
Br
N Ir
N
O O
Table 1 Polymerization results and characterizations of the copolymers
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
Ir(Brpiq)2acac (mol %) feed ratio
Yield (%)
0.02 0.03 0.04 0.05
54.2 59.0 48.7 50.4
GPC Mn
PDI
13,452 13,257 13,070 13,441
1.56 2.63 3.35 3.05
Tg ( C)
Td ( C)
154.3 163.6 162.8 159.8
407 417 413 423
segment was quenched completely when the Ir content was 0.05 mol %. For PFSDF-Ir4, the mechanism of white-light emission was investigated from the EL spectra under voltages varying from 12 V to 18 V (Fig. 5). In the EL spectra, the emission intensity of Ir(piq)2acac around 613 nm (Fig. S5) increased with the increasement of the voltage, leading to a gradually broader spectrum from 12 to 18 V. The difference in the PL and EL spectra imply that different mechanisms should be involved. Under photoexcitation, the singlet excited states are created on the PFSDF segment and subsequently transfer to the Ir complex by FRET. Under electrical excitation, electrons and holes are readily trapped on the Ir complex after the injection of the electrons from the cathode and the holes from anode in the device. In other words, the intra-, interchain FRET from PFSDF unit to Ir(piq)2acac and the charge trapping of Ir(piq)2acac exist simultaneously in the electroluminescence process,
1.0
Ir(Brpiq)2acac PFSDF
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
250
300
350
400
450
500
550
600
Normalized PL Intensity
Copolymer
Normalized Absorption Intensity
Scheme 1. Synthesis of monomer Ir(Brpiq)2acac and the hyperbranched copolymers.
0.0 650
Wavelength/nm Fig. 1. UVevis absorption of Ir(Brpiq)2acac and PL spectra of PFSDF in CHCl3 solution (105 mol/L).
and the charge trapping is more efficient under higher voltage. White-light emission was obtained at 16 V from the blue-light emitting of PFSDF segment, the green-light emitting of the excimer of PFSDF and the complementary orange-light emitting from Ir(piq)2acac through both incomplete intra-, interchain FRET and the charge trapping processes.
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Fig. 2. UVevis absorption and PL spectra of the copolymers: (a) in CHCl3 solution (105 M) and (b) in solid film.
Fig. 3. AFM images of the spin-coated films of the copolymers from CHCl3 solution (concentration¼105 M) on quartz substrates. (a) PFSDF-Ir2, (b) PFSDF-Ir3, (c) PFSDF-Ir4, (d) PFSDF-Ir5.
Fig. 6a shows the current density-voltage-brightness (J-V-L) characteristics of the devices. The device performances are quite analogous with a maximum luminance around 6859 cd/m2 as a result of their homologous molecular structure of the copolymers. The PFSDF-Ir4 device displays white-light emitting, giving a maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A, respectively. As can be seen from Fig. 6b, the efficiencies decrease very slowly with increasing current density, suggesting that these hyperbranched copolymers and
their devices have good stabilities. Further investigations on the optimization of the device performance and the synthesis of copolymers with different branches are ongoing in our laboratory. 3. Conclusion In conclusion, a series of hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III)
Fig. 4. (a) EL spectra of the copolymer PLEDs at 16 V and (b) CIE coordinates of the copolymer PLEDs: (I) PFSDF-Ir2, (II) PFSDF-Ir3, (III) PFSDF-Ir4 and (IV) PFSDF-Ir5.
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5
Table 2 EL performances of the PLEDs Copolymer
Vona (V)
Lmaxb (cd/m2) (at the voltage (V))
CEmax (cd/A)
PEmax (lm/W)
CIEc (x,y)
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
7.06 7.24 7.05 7.04
6605.8 6859.0 6777.3 6620.0
3.82 3.86 4.0 3.91
0.95 0.88 0.97 0.97
(0.21,0.23) (0.24,0.30) (0.31,0.35) (0.36,0.47)
b c
Turn-on voltage (at 1 cd/m2). Maximum luminance at applied voltage. At the voltage of 16 V.
1.0 0.8
Intensity/a.u.
unit, and the charge trapping processes of Ir(piq)2acac. A maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A were obtained in the single-layer device. The hyperbranched copolymers and their devices have good stabilities, with slow efficiency decrease at high current density. The results indicate that the hyperbranched copolymers using SDF as the core and fluorene as the branches with Ir(piq)2acac as the complementary adjusted chromophore could be promising candidates as white-emitting materials with high efficiency.
12V(0.27,0.36) 14V(0.29,0.39) 16V(0.31,0.35) 18V(0.30,0.34)
0.6 0.4
4. Experimental
0.2
4.1. Materials 2,7-dibromo-9,9-dioctylfluorene (M1, 99.8%) and 9,9dioctylfluorene-2,7-bis(trimethyleneboronate) (M2, 99.5%) were purchased from Energy Chemical and Synwitech, respectively. THF and toluene were distilled using standard procedures. Other solvents were used without further purification unless otherwise specified. All reactions were carried out using Schlenk techniques under dry nitrogen atmosphere.
0.0 400
500
600
700
Wavelength/nm Fig. 5. Electroluminescence spectra of the copolymer PFSDF-Ir4.
500 400
1000
200
100
100 10
0 8
H NMR spectra were measured on a Bruker DRX 600 spectrometer, and chemical shifts were reported in parts per million using tetramethylsilane as an internal standard. Molecular weights and polydispersities of the copolymers were determined using gel permeation chromatography (GPC) on an HP1100 high performance liquid chromatograph (HPLC) system equipped with a 410 differential refractometer, and a refractive index (RI) detector, with polystyrenes as the standard and THF as the eluent at a flow rate of 1.0 mL/min at 30 C. The UV-visible absorption spectra were determined on a Hitachi U-3900 spectrophotometer and the PL emission spectra were obtained using a Horiba FluoroMax-4 spectrophotometer at room temperature. Thermogravimetric
4.0
300
6
1
b) 4.5
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
4
4.2. Characterization
10
12
Voltage (V)
14
16
18
Luminance (cd/m2)
a) Current density (mA/cm2)
(Ir(piq)2acac) as the branches and the three-dimensional structured spiro[3,3]heptane-2,6-dispirofluorene (SDF, 10 mol %) as the core have been prepared through Suzuki polycondesation. The hyperbranched structures suppress the interchain interactions efficiently; leading to no obvious bathochromic shifts in solid films with respect to those in dilute solution in the PL spectra, and helping the copolymers to form amorphous spin-coating films. As the introduction of SDF does not interrupt the conjugation of the €rster resonance energy transfer efficiency copolymer chains, the Fo from fluorene segment to Ir(piq)2acac unit is remained in the hyperbranched systems. White-light electroluminescence with CIE coordinates at (0.30, 0.34) was realized for PFSDF-Ir4 (Ir(piq)2acac €rster 0.04 mol %) through both intra-, interchain incomplete Fo resonance energy transfer from the blue-light emitting fluorene segment to the complementary orange-light emitting Ir(piq)2acac
Current Efficiency (cd/A)
a
(15.0) (17.1) (18.3) (17.7)
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
50 100 150 200 250 300 350 400 450 2 Current Density (mA/cm )
Fig. 6. (a) Current-voltage (left) and luminance-voltage (right) curves and (b) Current efficiency-current density characteristics of the copolymer PLEDs.
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
analysis (TGA) of the copolymers was conducted on a Setaram thermogravimetric analyzer at a heating rate of 10 C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were performed at both heating and cooling rates of 5 C/min under nitrogen atmosphere, using DSC Q100 V9.4 Build 287 apparatus. Atomic force microscopy (AFM) measurements were performed on an SPA-300HV from Digital Instruments Inc. (Santa Barbara, CA) at a tapping mode. 4.3. Device fabrication and characterization Patterned glass substrates coated with indium tin oxide (ITO) (20 U square1) were cleaned by a surfactant scrub, washed successively with deionized water, acetone and isopropanol in an ultrasonic bath, and then dried at 120 C in a heating chamber for 8 h. A 40-nm-thick poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) hole injection layer was spincoated on top of ITO and baked at 120 C for 20 min. Thin films (50 nm thick) of the copolymers as the emitting layer were deposited on top of the PEDOT:PSS layer by spin-coating the chlorobenzene solution of the copolymers, followed by thermal annealing at 110 C for 20 min. Then an electron-transporting layer of 1,3,5tris(N-phenylbenzimidazol-2-yl)benzene (TPBi, 35 nm) and LiF (1 nm) and Al (150 nm) as the cathode were deposited by vacuum evaporation under a base pressure of 5104 Pa. The devices were fabricated with the structure ITO/PEDOT:PSS (40 nm)/copolymers (50 nm)/LiF (1 nm)/Al (150 nm). The EL spectra and CIE coordinates were measured with a PR-655 spectra colorimeter. The currentvoltage-forward luminance curves were measured using a Keithley 2400 source meter and a calibrated silicon photodiode. 4.4. Synthesis The tetra-functional TBrSDF as the branching point was synthesized according to the literature.37,38 4.4.1. Preparation of the monomers 4.4.1.1. Synthesis of synthesis of 1-(4-bromophenyl)-isoquino line.6,7,39 To a solution of 1-chloroisoquinoline (0.98 g, 6 mmol), tetrakis (triphenylphosphine) palladium (Pd(PPh3)4) (0.35 g, 0.3 mmol), and 4-bromophenylboronic acid (1.00 g, 5 mmol) in toluene (40 mL) methanol (10 mL) was added 2M sodium carbonate solution. The mixture was stirred at 110 C for 24 h. After cooling to room temperature, diluted hydrochloric acid was added, and the precipitates were washed with water, filtered and added the sodium hydroxide to precipitate solid that was filtered to obtain the product 1-(4-bromophenyl)-isoquinoline (0.68 g, 40%) (A). 1H NMR (600 MHz, CDCl3) d (ppm): 8.61 (d, J¼5.4 Hz, 1H), 8.06 (d, J¼9.0 Hz, 1H), 7.90 (d, J¼7.8 Hz, 1H), 7.72 (t, J¼7.8 Hz, 1H), 7.68 (d, J¼8.4 Hz, 3H), 7.59 (d, J¼8.4 Hz, 2H), 7.57 (t, J¼7.8 Hz, 1H). 4.4.1.2. Synthesis of 1-(4-bromophenyl)-isoquinoline chlorobridge iridium dimmers [(A)2Ir(m-Cl)2Ir(A)2]. To a solution of trihydrate trichloride iridium (IrCl3$3H2O) (0.35 g, 1 mmol) in 2ethoxyethanol (24 mL) deionized water (8 mL) was added 1-(4bromophenyl)-isoquinoline (0.36 g, 1.25 mmol). The mixture was stirred at 110 C for 24 h. After cooling to room temperature, deionized water (200 mL) was poured into the reaction solution. The precipitates were filtered, washed with water and dried for 12 h at 45 C under vacuum to obtain 1-(4-bromophenyl)-isoquinoline chloro-bridge iridium dimmers (B) (1.81 g, 90%). 4.4.1.3. Synthesis of [1-(4-bromophenyl)-isoquinoline]2Ir(aceto acetone) Ir(Brpiq)2acac. To a solution of the chloro-bridged iridium dimmers (0.16 g, 0.1 mmol) in 2-ethoxyethanol (25 mL) were added
acetoacetone (acac) (0.5 mL) and anhydrous sodium carbonate (0.21 g, 2.0 mmol). The mixture was stirred at 105 C for 24 h. After cooling to room temperature, the mixture was poured into deionized water (200 mL). The precipitates were filtered, dried and further purified by column chromatography on silica gel with petroleum ether: CH2Cl2¼10:1 as eluent to give Ir(Brpiq)2acac as red acicular crystal (0.06 g, 72%). 1H NMR (600 MHz,CDCl3) d (ppm): 8.91 (d, J¼9.6 Hz, 2H), 8.37 (d, J¼6.0 Hz, 2H), 8.08 (d, J¼9 HZ, 2H), 7.96 (d, J1¼2.4 Hz, J2¼5.4 Hz, 2H), 7.76e7.74 (m, 4H), 7.52 (d, J¼6.6 Hz, 2H), 7.08 (dd, J1¼2.4 Hz, J2¼8.4 Hz, 2H), 6.47 (d, J¼1.8 Hz, 2H), 5.18 (s, 1H), 1.75 (s, 6H). 4.4.2. Preparation of the copolymers. To a solution of predetermined amount of monomers (M1, M2, TBrSDF and Ir(Brpiq)2acac) in toluene (20 mL) was added an aqueous solution (5 mL) of potassium carbonate (2 M) and a catalytic amount of Pd(PPh3)4 (2.0 mol %). Aliquat 336 (1 mL) in toluene (5 mL) was added as the phase transfer catalyst. The mixture was vigorously stirred at 90 C for 4 days. Phenylboronic acid was then added to the reaction mixture followed by stirring at 90 C for an additional 12 h. Finally, bromobenzene was added in the same way by heating for 12 h again. After cooling to room temperature, the mixture was washed with 2 M HCl solution and water. After separated and concentrated, the organic layer was added dropwise to excess methanol to precipitate the polymers. The precipitates were collected by filtration, and dried under vacuum. The solid was Soxhlet extracted with acetone for 72 h and then passed through a short chromatographic column using toluene as the eluent to afford the polymers. PFSDF-Ir2:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.08 mL, 2103 mol/L). Light yellow powder, yield: 54.2%. PFSDF-Ir3:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.12 mL, 2103 mol/L). Light yellow powder, yield: 59.0%. PFSDF-Ir4:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.16 mL, 2103 mol/L). Light yellow powder, yield: 48.7%. PFSDF-Ir5:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.2 mL, 2103 mol/L). Light yellow powder, yield: 50.4%. Acknowledgements This work was financially supported by the ‘Program for New Century Excellent Talents (NCET) in University’ (NCET-13-0927), the International Science & Technology Cooperation Program of China (2012DFR50460), the National Natural Science Foundation of China (21071108, 21101111, 61274056, 61205179, 61307030, 61307029), and the Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011). Supplementary data Supplementary data (1H NMR characterization and PL spectrum of Ir(Brpiq)2acac and the thermal performance of the copolymers. This material is available free of charge via the internet at http:// www.elsevier.com.) associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2015.08.053. References and notes 1. Gather, M. C.; Kohnen, A.; Meerholz, K. Adv. Mater 2011, 23, 233e248. 2. Liu, J.; Cheng, Y.; Xie, Z.; Geng, Y.; Wang, L.; Jing, X.; Wang, F. Adv. Mater. 2008, 20, 1357e1362. 3. Ying, L.; Ho, C. L.; Wu, H.; Cao, Y.; Wong, W. Y. Adv. Mater. 2014, 26, 2459e2473. 4. Liu, J.; Xie, Z. Y.; Cheng, Y. X.; Geng, Y. H.; Wang, L. X.; Jing, X. B.; Wang, F. S. Adv. Mater. 2007, 19, 531e535.
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7 5. Tang, C.; Liu, X. D.; Liu, F.; Wang, X. L.; Xu, H.; Huang, W. Macromol. Chem. Phys. 2013, 214, 314e342. 6. Li, Y.; Liu, Y.; Zhou, M. Dalton Trans. 2012, 41, 3807e3816. 7. Qiao, J.; Duan, L.; Tang, L.; He, L.; Wang, L.; Qiu, Y. J. Mater. Chem. 2009, 19, 6573e6580. 8. Chen, Q.; Liu, N.; Ying, L.; Yang, W.; Wu, H.; Xu, W.; Cao, Y. Polymer 2009, 50, 1430e1437. 9. Jiang, J. X.; Xu, Y. H.; Yang, W.; Guan, R.; Liu, Z. Q.; Zhen, H. Y.; Cao, Y. Adv. Mater. 2006, 18, 1769e1773. 10. Zhang, K.; Chen, Z.; Yang, C.; Tao, Y.; Zou, Y.; Qin, J.; Cao, Y. J. Mater. Chem. 2008, 18, 291e298. 11. Katsis, Y. H. G. D.; Ou, J. J.; Culligan, S. W.; Trajkovska, A.; Chen, S. H.; Rothberg, L. J. Chem. Mater. 2002, 14, 1332e1339. 12. Zeng, G.; Yu, W. L.; Chua, S. J.; Huang, W. Macromolecules 2002, 35, 6907e6914. 13. Guo, T.; Guan, R.; Zou, J.; Liu, J.; Ying, L.; Yang, W.; Wu, H.; Cao, Y. Polym. Chem. 2011, 2, 2193e2203. 14. Ma, Z.; Lu, S.; Fan, Q. L.; Qing, C. Y.; Wang, Y. Y.; Wang, P.; Huang, W. Polymer 2006, 47, 7382e7390. 15. Tsai, L. R.; Chen, Y. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4465e4476. 16. Konkolewicz, D.; Poon, C. K.; Weale, A. G.; Perrier, S. Chem. Commun. 2011, 239e241. 17. Tsai, L. R.; Chen, Y. Macromolecules 2008, 41, 5098e5106. 18. Wang, R.; Wang, W. Z.; Yang, G. Z.; Liu, T.; Yu, J.; Jiang, Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 790e802. 19. Shih, H. M.; Wu, R. C.; Shih, P. I.; Wang, C. L.; Hsu, C. S. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 696e710. € ller, A. H. E. Macromolecules 2011, 44, 1266e1272. 20. Pfaff, A.; Mu 21. Tsai, Y. T.; Lai, C. T.; Chien, R. H.; Hong, J. L.; Yeh, A. C. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 237e249.
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Synthesis and properties of hyperbranched fluorescence/ phosphorescence hybrid copolymers for white polymer light emitting devices Yuling Wu a, b, Jie Li a, b, Wenqing Liang a, b, Junli Yang a, b, Jing Sun a, b, Hua Wang a, b, Xuguang Liu c, Bingshe Xu a, b, * a b c
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, China Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 June 2015 Received in revised form 18 August 2015 Accepted 22 August 2015 Available online xxx
A series of hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) as the branches and the three-dimensional structured spiro[3,3]heptane-2,6-dispirofluorene (SDF, 10 mol %) as the core have been synthesized by adjusting the feeding ratios of Ir(piq)2acac (0.02e0.05 mol %). The copolymers showed good thermal and spectral stability, and amorphous film morphology because of the hyperbranched structures. The 2,7-substituted fluorenes of SDF were incorporated into the p-system of the €rster resonance energy transfer (FRET) efficiency from flupolyfluorene branches, and remained the Fo orene segment to Ir(piq)2acac unit. The copolymers exhibited efficient electroluminescent characters, and white-light emission was achieved in PFSDF-Ir4 (Ir(piq)2acac 0.04 mol %)-based single layer device with CIE coordinates at (0.30, 0.34), a maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A. Ó 2015 Published by Elsevier Ltd.
Keywords: Fluorescence/phosphorescence hybrid Hyperbranched copolymers Spiro[3,3]heptane-2,6-dispirofluorene White-light emission
1. Introduction White polymer light-emitting devices (WPLEDs) have received great attention towards their potential applications in large-area full-color and flexible displays combined with a color filter, backlights and solid lighting sources.1e4 Among the common approaches used to generate white light in WPLEDs, single whiteemitting polymer bearing either three primary colors (red (R), green (G) and blue (B)) or two complementary colors (generally blue (B) and orange (O)) emitting units incorporated into or attached to the backbone has advantages including relieved phase separation, controlled proportion of different emitting moieties, and easy fabrication process etc.5 The phosphorescent materials such as iridium complexes can realize theoretical 100% internal quantum efficiency due to the ability to harvest both singlet and triplet excitons to light.6,7 Therefore, phosphorescence molecules (usually as the narrowband-gap chromophores) embedded into the fluorescence polymer chains (usually as the wide-band-gap emitters) could
* Corresponding author. E-mail address: [email protected] (B. Xu).
effectively improve the electroluminescent (EL) performance of the WPLEDs.8e10 Furthermore, because the phosphorescence chromophores disperse into the polymer at the molecular level, the system can be regarded as homogeneous. The required content of the phosphorescence emitter was low (usually in the range of one ten thousandth to one thousandth (molar ratio)) because of the efficient energy transfer process.5 Thus, the tripletetriplet (TeT) annihilation of iridium complexes could be well-suppressed, and efficient EL performance could be expected. Polyfluorenes (PFs) are shown to be the most promising blue light-emitting materials because of high photoluminescence quantum efficiency, and relatively good chemical and thermal stabilities.11,12 When orange-light-emitting iridium complex is dispersed to PFs, white light emission could be obtained by in€rster resonance energy transfer (FRET) from the fluocomplete Fo rene segment to the Ir complex unit.13,14 However, linear copolymers suffer from intermolecular interactions, which reduce the efficiency of the device.15 On the other hand, hyperbranched polymers with three-dimensional structure can prevent the aggregation of polymer chains, make the material form amorphous films with good quality, and increase the glass transition temperature (Tg) of the polymers.16e21 Recently, a number of
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
hyperbranched electroluminescent polymers with the three principle colors (red,22 green23,24 and blue18) have been synthesized, and both emission efficiency and thermal stability were effectively improved with respect to their linear analogies.15,25e27 However, white-light-emitting hyperbranched polymers have rarely been reported. In this paper, hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) branches and spiro[3.3]heptane-2,6-dispirofluorene (SDF) core (10 mol %) were constructed. The three-dimensional-structured SDF exhibits great morphological stability and intense fluorescence,28 and furthermore, its steric hindrance can prevent rotation of the adjacent aryl groups, which reduces close packing and intermolecular interactions between the chromophores in the solid-state.29,30 In order to obtain white-light emission, the orange light-emitting unit Ir(piq)2acac was introduced with different contents from 0.02 mol % to 0.05 mol %. Such a highly branched framework may provide a highly efficient white-light electroluminescence. 2. Results and discussion 2.1. Synthesis and characterization The synthesis of Ir(Brpiq)2acac even reported used 1chloroisoquinoline, 4-bromophenylboronic acid and IrCl3$3H2O as the orange light-emitting unit. The hyperbranched copolymers with 9,9-dioctylfluorene and Ir(piq)2acac as the branches and SDF as the core were prepared by Suzuki polycondensation with yields ranging from 49% to 59% (Scheme 1). The feed molar ratios of the branching point SDF were 10% and Ir(piq)2acac were varied from 0.02 % to 0.05 % relative to the fluorenyl units, and the corresponding hyperbranched copolymers were named as PFSDF-Ir2 (Ir(piq)2acac 0.02 mol %), PFSDF-Ir3 (Ir(piq)2acac 0.03 mol %), PFSDF-Ir4 (Ir(piq)2acac 0.04 mol %) and PFSDF-Ir5 (Ir(piq)2acac 0.05 mol %), respectively. All of the copolymers are readily soluble at room temperature in common organic solvents such as CHCl3, THF and toluene. The synthetic and structural results of the copolymers were summarized in Table 1. The 1H NMR spectra of the copolymers were quite similar (Fig. S2, the proton signals of Ir(piq)2acac were not detected because of its low content), revealing the similar backbone structures of the copolymers. Taking PFSDF-Ir4 as an example (Fig. S3), the actual content of SDF was calculated by comparing the peak integral intensities of the proton signals of the spiro[3.3]heptane of SDF (d 3.0e3.5) and the aromatic ring of fluorene (d 7.4e8.0) of the copolymer (8.80:1), which was close to the feed ratio (8.25:1). The number-average molecular weights (Mns) of the copolymers were all around 13000 with the polydispersity indexes (PDIs) ranging from 1.56 to 3.35. All of the copolymers exhibit good thermal stability with the onset decomposition temperatures (Td, measured at a 5% weight loss) from 407 to 423 C. DSC data reveals the glass transition temperatures (Tgs) of copolymers were all around 155 C (Fig. S4, Supplementary data). 2.2. Photophysical properties Fig. 1 shows the UV-visible absorption of Ir(Brpiq)2acac and photoluminescence (PL) spectra of hyperbranched polyfluorenespiro[3.3]heptane-2,6-dispirofluorene (PFSDF, SDF 10 mol %) copolymer in CHCl3 solution at a concentration of 1.0105 mol/L. There are two strong absorption peaks seated at 243 nm and 297 nm, which are mainly attributed to the spin-allowed ligandcentered (1LC) state transitions. The weak absorption peaks seated at 345 nm are mainly assigned to the spin-allowed metal-to-ligand charge-transfer (1MLCT) state transitions. The unconspicuous
absorption peak around 472 nm is mainly belonged to the spininhibited metal-to-ligand charge-transfer (3MLCT) state and ligand-centered (3LC) state transitions.31 It is obvious that the emission of copolymer PFSDF and the absorption spectrum of the Ir complex show good spectra overlap. Therefore, efficient FRET from the PFSDF to the Ir complex, and white-light emission can be expected by the combination of blue light-emitting from PFSDF and orange light-emitting from Ir(piq)2acac through finely adjusting the content of Ir complex. The normalized UVevis absorption and PL spectra of the copolymers in CHCl3 solution (105 mol/L) and thin film states are shown in Fig 2. In dilute solution, all of the copolymers exhibit typical absorption and emission bands of PF with an absorption maximum around 373e379 nm, and emission peaks at 420, 440, and a shoulder peak at 474 nm. The presence of vibronic structure in the emission spectra indicates that the hyperbranched copolymers have rigid and well-defined backbone structures. The absorption and emission bands of Ir(piq)2acac were not detected due to its comparatively low content (0.02e0.05 mol %) in the copolymers. In dilute solution, the energy transfer was exclusively intrachain.32 In films, the copolymers exhibit UVevis absorption bands at around 375 nm similar to those in dilute solution. In the PL spectra, the maximum emission bands of the copolymers are at about 420 and 440 nm, showing no obvious bathochromic shift with respect to those in dilute solution. This result indicates that the hyperbranched molecular structure can prevent the aggregation and the interaction of the copolymer chains efficiently. The emission band of Ir(piq)2acac centered at 613 nm still can’t be observed due to the low content of Ir(piq)2acac, as a result of both intra- and interchain FRET from fluorene unit to Ir(piq)2acac.22,33 2.3. Film forming properties The morphology of the spin-coated films of the copolymers was estimated by atomic force microscopy (AFM) at a tapping mode, and the images are shown in Fig. 3. All the films show smooth surface with small root-mean-square (RMS) values of 1.295, 1.670, 2.316 and 2.884 nm for PFSDF-Ir2, PFSDF-Ir3, PFSDF-Ir4 and PFSDF-Ir5, respectively. The results indicate that the hyperbranched structure with three-dimensional-structured SDF branch point could provide a homogeneous morphology of spin-coating films, which could be favorable for the PLED fabrication. 2.4. Electroluminescence properties Using the copolymers as emitting materials, single-layer PLEDs were fabricated with the configuration of ITO/PEDOT:PSS (40 nm)/ Copolymers (50 nm)/TPBi (35 nm)/LiF (1 nm)/Al (150 nm). The electroluminescent spectra of the devices are shown in Fig. 4a and the characteristics were summarized in Table 2. Generally, all of the copolyemers exhibit much broader EL spectra than their PL counterparts. At the voltage of 16 V, PFSDF-Ir2 and PFSDF-Ir3 both exhibit blue-light emitting and the broad peaks locate at the 452 nm and 484 nm, respectively. The spectrum of PFSDF-Ir4 covers the visible light region with 400e700 nm and the main peaks locate at 425 and 548 nm, respectively. White-light emission is obtained with CIE coordinates located at (0.31, 0.35) (Fig. 4b). The emission peak of PFSDF-Ir5 mainly locates at 533 nm and yellow-light emission is achieved. For all of the copolymers, the emission band around 540 nm may be from the excimer of PFSDF formed under the electric field.5,22,34e36 As the electrons and holes can be trapped on the Ir complex,22 the formation of the excimer may be induced by Ir(piq)2acac under electric excitation.35 As a result, the excimer emission intensity increased with the Ir(piq)2acac contents. In PFSDF-Ir5, the emission from the PFSDF
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
Br
Br
Cl
Br
Pd(PPh3)4 NaCO3
N
+ B HO
IrCl3.3H2O
Toluene Methanol
3
N Br
N
N
Cl
Ir
Ir
Cl
N
OH
Br acetylacetone Br
N
Na2CO3
N
Br
Ir N
O O
Br
A
B
Ir(Brpiq)2acac Br
C8H17 C8H17
C8H17
C8H17
Br
Br
+
+
+
N Br
Ir N
O
Pd(PPh3)4 NaCO3
O
Aliquat336 72 h
Br
Br
M2
M1
Br
Br O B O
O B O
TBrSDF
Ir(Brpiq)2acac Br
Br
O
C8H17 N N
C8H17
Ir
O
C8H17
Br
C8H17
O
C8H17
Ir
O
N
n
n
C8H17 C H 8 17
m
n
C8H17
M3
C8H17
n
Br C8H17
C8H17
o
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
Br
C8H17
p
C8H17
N
C8H17
O B O
Br
N Ir
N
O O
Table 1 Polymerization results and characterizations of the copolymers
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
Ir(Brpiq)2acac (mol %) feed ratio
Yield (%)
0.02 0.03 0.04 0.05
54.2 59.0 48.7 50.4
GPC Mn
PDI
13,452 13,257 13,070 13,441
1.56 2.63 3.35 3.05
Tg ( C)
Td ( C)
154.3 163.6 162.8 159.8
407 417 413 423
segment was quenched completely when the Ir content was 0.05 mol %. For PFSDF-Ir4, the mechanism of white-light emission was investigated from the EL spectra under voltages varying from 12 V to 18 V (Fig. 5). In the EL spectra, the emission intensity of Ir(piq)2acac around 613 nm (Fig. S5) increased with the increasement of the voltage, leading to a gradually broader spectrum from 12 to 18 V. The difference in the PL and EL spectra imply that different mechanisms should be involved. Under photoexcitation, the singlet excited states are created on the PFSDF segment and subsequently transfer to the Ir complex by FRET. Under electrical excitation, electrons and holes are readily trapped on the Ir complex after the injection of the electrons from the cathode and the holes from anode in the device. In other words, the intra-, interchain FRET from PFSDF unit to Ir(piq)2acac and the charge trapping of Ir(piq)2acac exist simultaneously in the electroluminescence process,
1.0
Ir(Brpiq)2acac PFSDF
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
250
300
350
400
450
500
550
600
Normalized PL Intensity
Copolymer
Normalized Absorption Intensity
Scheme 1. Synthesis of monomer Ir(Brpiq)2acac and the hyperbranched copolymers.
0.0 650
Wavelength/nm Fig. 1. UVevis absorption of Ir(Brpiq)2acac and PL spectra of PFSDF in CHCl3 solution (105 mol/L).
and the charge trapping is more efficient under higher voltage. White-light emission was obtained at 16 V from the blue-light emitting of PFSDF segment, the green-light emitting of the excimer of PFSDF and the complementary orange-light emitting from Ir(piq)2acac through both incomplete intra-, interchain FRET and the charge trapping processes.
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Fig. 2. UVevis absorption and PL spectra of the copolymers: (a) in CHCl3 solution (105 M) and (b) in solid film.
Fig. 3. AFM images of the spin-coated films of the copolymers from CHCl3 solution (concentration¼105 M) on quartz substrates. (a) PFSDF-Ir2, (b) PFSDF-Ir3, (c) PFSDF-Ir4, (d) PFSDF-Ir5.
Fig. 6a shows the current density-voltage-brightness (J-V-L) characteristics of the devices. The device performances are quite analogous with a maximum luminance around 6859 cd/m2 as a result of their homologous molecular structure of the copolymers. The PFSDF-Ir4 device displays white-light emitting, giving a maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A, respectively. As can be seen from Fig. 6b, the efficiencies decrease very slowly with increasing current density, suggesting that these hyperbranched copolymers and
their devices have good stabilities. Further investigations on the optimization of the device performance and the synthesis of copolymers with different branches are ongoing in our laboratory. 3. Conclusion In conclusion, a series of hyperbranched fluorescence/phosphorescence hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III)
Fig. 4. (a) EL spectra of the copolymer PLEDs at 16 V and (b) CIE coordinates of the copolymer PLEDs: (I) PFSDF-Ir2, (II) PFSDF-Ir3, (III) PFSDF-Ir4 and (IV) PFSDF-Ir5.
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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
5
Table 2 EL performances of the PLEDs Copolymer
Vona (V)
Lmaxb (cd/m2) (at the voltage (V))
CEmax (cd/A)
PEmax (lm/W)
CIEc (x,y)
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
7.06 7.24 7.05 7.04
6605.8 6859.0 6777.3 6620.0
3.82 3.86 4.0 3.91
0.95 0.88 0.97 0.97
(0.21,0.23) (0.24,0.30) (0.31,0.35) (0.36,0.47)
b c
Turn-on voltage (at 1 cd/m2). Maximum luminance at applied voltage. At the voltage of 16 V.
1.0 0.8
Intensity/a.u.
unit, and the charge trapping processes of Ir(piq)2acac. A maximum luminance of 6777.3 cd/m2 (at 18.3 V), and a maximum current efficiency of 4.0 cd/A were obtained in the single-layer device. The hyperbranched copolymers and their devices have good stabilities, with slow efficiency decrease at high current density. The results indicate that the hyperbranched copolymers using SDF as the core and fluorene as the branches with Ir(piq)2acac as the complementary adjusted chromophore could be promising candidates as white-emitting materials with high efficiency.
12V(0.27,0.36) 14V(0.29,0.39) 16V(0.31,0.35) 18V(0.30,0.34)
0.6 0.4
4. Experimental
0.2
4.1. Materials 2,7-dibromo-9,9-dioctylfluorene (M1, 99.8%) and 9,9dioctylfluorene-2,7-bis(trimethyleneboronate) (M2, 99.5%) were purchased from Energy Chemical and Synwitech, respectively. THF and toluene were distilled using standard procedures. Other solvents were used without further purification unless otherwise specified. All reactions were carried out using Schlenk techniques under dry nitrogen atmosphere.
0.0 400
500
600
700
Wavelength/nm Fig. 5. Electroluminescence spectra of the copolymer PFSDF-Ir4.
500 400
1000
200
100
100 10
0 8
H NMR spectra were measured on a Bruker DRX 600 spectrometer, and chemical shifts were reported in parts per million using tetramethylsilane as an internal standard. Molecular weights and polydispersities of the copolymers were determined using gel permeation chromatography (GPC) on an HP1100 high performance liquid chromatograph (HPLC) system equipped with a 410 differential refractometer, and a refractive index (RI) detector, with polystyrenes as the standard and THF as the eluent at a flow rate of 1.0 mL/min at 30 C. The UV-visible absorption spectra were determined on a Hitachi U-3900 spectrophotometer and the PL emission spectra were obtained using a Horiba FluoroMax-4 spectrophotometer at room temperature. Thermogravimetric
4.0
300
6
1
b) 4.5
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
4
4.2. Characterization
10
12
Voltage (V)
14
16
18
Luminance (cd/m2)
a) Current density (mA/cm2)
(Ir(piq)2acac) as the branches and the three-dimensional structured spiro[3,3]heptane-2,6-dispirofluorene (SDF, 10 mol %) as the core have been prepared through Suzuki polycondesation. The hyperbranched structures suppress the interchain interactions efficiently; leading to no obvious bathochromic shifts in solid films with respect to those in dilute solution in the PL spectra, and helping the copolymers to form amorphous spin-coating films. As the introduction of SDF does not interrupt the conjugation of the €rster resonance energy transfer efficiency copolymer chains, the Fo from fluorene segment to Ir(piq)2acac unit is remained in the hyperbranched systems. White-light electroluminescence with CIE coordinates at (0.30, 0.34) was realized for PFSDF-Ir4 (Ir(piq)2acac €rster 0.04 mol %) through both intra-, interchain incomplete Fo resonance energy transfer from the blue-light emitting fluorene segment to the complementary orange-light emitting Ir(piq)2acac
Current Efficiency (cd/A)
a
(15.0) (17.1) (18.3) (17.7)
PFSDF-Ir2 PFSDF-Ir3 PFSDF-Ir4 PFSDF-Ir5
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
50 100 150 200 250 300 350 400 450 2 Current Density (mA/cm )
Fig. 6. (a) Current-voltage (left) and luminance-voltage (right) curves and (b) Current efficiency-current density characteristics of the copolymer PLEDs.
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analysis (TGA) of the copolymers was conducted on a Setaram thermogravimetric analyzer at a heating rate of 10 C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were performed at both heating and cooling rates of 5 C/min under nitrogen atmosphere, using DSC Q100 V9.4 Build 287 apparatus. Atomic force microscopy (AFM) measurements were performed on an SPA-300HV from Digital Instruments Inc. (Santa Barbara, CA) at a tapping mode. 4.3. Device fabrication and characterization Patterned glass substrates coated with indium tin oxide (ITO) (20 U square1) were cleaned by a surfactant scrub, washed successively with deionized water, acetone and isopropanol in an ultrasonic bath, and then dried at 120 C in a heating chamber for 8 h. A 40-nm-thick poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) hole injection layer was spincoated on top of ITO and baked at 120 C for 20 min. Thin films (50 nm thick) of the copolymers as the emitting layer were deposited on top of the PEDOT:PSS layer by spin-coating the chlorobenzene solution of the copolymers, followed by thermal annealing at 110 C for 20 min. Then an electron-transporting layer of 1,3,5tris(N-phenylbenzimidazol-2-yl)benzene (TPBi, 35 nm) and LiF (1 nm) and Al (150 nm) as the cathode were deposited by vacuum evaporation under a base pressure of 5104 Pa. The devices were fabricated with the structure ITO/PEDOT:PSS (40 nm)/copolymers (50 nm)/LiF (1 nm)/Al (150 nm). The EL spectra and CIE coordinates were measured with a PR-655 spectra colorimeter. The currentvoltage-forward luminance curves were measured using a Keithley 2400 source meter and a calibrated silicon photodiode. 4.4. Synthesis The tetra-functional TBrSDF as the branching point was synthesized according to the literature.37,38 4.4.1. Preparation of the monomers 4.4.1.1. Synthesis of synthesis of 1-(4-bromophenyl)-isoquino line.6,7,39 To a solution of 1-chloroisoquinoline (0.98 g, 6 mmol), tetrakis (triphenylphosphine) palladium (Pd(PPh3)4) (0.35 g, 0.3 mmol), and 4-bromophenylboronic acid (1.00 g, 5 mmol) in toluene (40 mL) methanol (10 mL) was added 2M sodium carbonate solution. The mixture was stirred at 110 C for 24 h. After cooling to room temperature, diluted hydrochloric acid was added, and the precipitates were washed with water, filtered and added the sodium hydroxide to precipitate solid that was filtered to obtain the product 1-(4-bromophenyl)-isoquinoline (0.68 g, 40%) (A). 1H NMR (600 MHz, CDCl3) d (ppm): 8.61 (d, J¼5.4 Hz, 1H), 8.06 (d, J¼9.0 Hz, 1H), 7.90 (d, J¼7.8 Hz, 1H), 7.72 (t, J¼7.8 Hz, 1H), 7.68 (d, J¼8.4 Hz, 3H), 7.59 (d, J¼8.4 Hz, 2H), 7.57 (t, J¼7.8 Hz, 1H). 4.4.1.2. Synthesis of 1-(4-bromophenyl)-isoquinoline chlorobridge iridium dimmers [(A)2Ir(m-Cl)2Ir(A)2]. To a solution of trihydrate trichloride iridium (IrCl3$3H2O) (0.35 g, 1 mmol) in 2ethoxyethanol (24 mL) deionized water (8 mL) was added 1-(4bromophenyl)-isoquinoline (0.36 g, 1.25 mmol). The mixture was stirred at 110 C for 24 h. After cooling to room temperature, deionized water (200 mL) was poured into the reaction solution. The precipitates were filtered, washed with water and dried for 12 h at 45 C under vacuum to obtain 1-(4-bromophenyl)-isoquinoline chloro-bridge iridium dimmers (B) (1.81 g, 90%). 4.4.1.3. Synthesis of [1-(4-bromophenyl)-isoquinoline]2Ir(aceto acetone) Ir(Brpiq)2acac. To a solution of the chloro-bridged iridium dimmers (0.16 g, 0.1 mmol) in 2-ethoxyethanol (25 mL) were added
acetoacetone (acac) (0.5 mL) and anhydrous sodium carbonate (0.21 g, 2.0 mmol). The mixture was stirred at 105 C for 24 h. After cooling to room temperature, the mixture was poured into deionized water (200 mL). The precipitates were filtered, dried and further purified by column chromatography on silica gel with petroleum ether: CH2Cl2¼10:1 as eluent to give Ir(Brpiq)2acac as red acicular crystal (0.06 g, 72%). 1H NMR (600 MHz,CDCl3) d (ppm): 8.91 (d, J¼9.6 Hz, 2H), 8.37 (d, J¼6.0 Hz, 2H), 8.08 (d, J¼9 HZ, 2H), 7.96 (d, J1¼2.4 Hz, J2¼5.4 Hz, 2H), 7.76e7.74 (m, 4H), 7.52 (d, J¼6.6 Hz, 2H), 7.08 (dd, J1¼2.4 Hz, J2¼8.4 Hz, 2H), 6.47 (d, J¼1.8 Hz, 2H), 5.18 (s, 1H), 1.75 (s, 6H). 4.4.2. Preparation of the copolymers. To a solution of predetermined amount of monomers (M1, M2, TBrSDF and Ir(Brpiq)2acac) in toluene (20 mL) was added an aqueous solution (5 mL) of potassium carbonate (2 M) and a catalytic amount of Pd(PPh3)4 (2.0 mol %). Aliquat 336 (1 mL) in toluene (5 mL) was added as the phase transfer catalyst. The mixture was vigorously stirred at 90 C for 4 days. Phenylboronic acid was then added to the reaction mixture followed by stirring at 90 C for an additional 12 h. Finally, bromobenzene was added in the same way by heating for 12 h again. After cooling to room temperature, the mixture was washed with 2 M HCl solution and water. After separated and concentrated, the organic layer was added dropwise to excess methanol to precipitate the polymers. The precipitates were collected by filtration, and dried under vacuum. The solid was Soxhlet extracted with acetone for 72 h and then passed through a short chromatographic column using toluene as the eluent to afford the polymers. PFSDF-Ir2:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.08 mL, 2103 mol/L). Light yellow powder, yield: 54.2%. PFSDF-Ir3:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.12 mL, 2103 mol/L). Light yellow powder, yield: 59.0%. PFSDF-Ir4:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.16 mL, 2103 mol/L). Light yellow powder, yield: 48.7%. PFSDF-Ir5:M1 (0.19 g, 0.35 mmol), M2 (0.35 g, 0.55 mmol), M3 (0.07 g, 0.1 mmol) and M4 (0.2 mL, 2103 mol/L). Light yellow powder, yield: 50.4%. Acknowledgements This work was financially supported by the ‘Program for New Century Excellent Talents (NCET) in University’ (NCET-13-0927), the International Science & Technology Cooperation Program of China (2012DFR50460), the National Natural Science Foundation of China (21071108, 21101111, 61274056, 61205179, 61307030, 61307029), and the Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011). Supplementary data Supplementary data (1H NMR characterization and PL spectrum of Ir(Brpiq)2acac and the thermal performance of the copolymers. This material is available free of charge via the internet at http:// www.elsevier.com.) associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2015.08.053. References and notes 1. Gather, M. C.; Kohnen, A.; Meerholz, K. Adv. Mater 2011, 23, 233e248. 2. Liu, J.; Cheng, Y.; Xie, Z.; Geng, Y.; Wang, L.; Jing, X.; Wang, F. Adv. Mater. 2008, 20, 1357e1362. 3. Ying, L.; Ho, C. L.; Wu, H.; Cao, Y.; Wong, W. Y. Adv. Mater. 2014, 26, 2459e2473. 4. Liu, J.; Xie, Z. Y.; Cheng, Y. X.; Geng, Y. H.; Wang, L. X.; Jing, X. B.; Wang, F. S. Adv. Mater. 2007, 19, 531e535.
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