Novel phosphorescent polymers containing both ambipolar segments and functionalized IrIII phosphorescent moieties: Synthesis, photophysical, redox, and electrophosphorescence investigation

Article abstract of DOI:10.1039/c4tc01626b

Two series of novel phosphorescent polymers have been successfully synthesized by the Suzuki cross-coupling reaction among functionalized phosphorescent Ir^III ppy-type (ppy = 2-phenylpyridine anion) complex monomers with hole-injection/transpor

Full text of DOI:10.1039/c4tc01626b

Journal of
Materials Chemistry C
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PAPER
Novel phosphorescent polymers containing both
ambipolar segments and functionalized IrIII
phosphorescent moieties: synthesis,
photophysical, redox, and electrophosphorescence
investigation†
Cite this: J. Mater. Chem. C, 2014, 2,
9523
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b
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*
Meng Lian, Yue Yu, Jiang Zhao, Zuan Huang, Xiaolong Yang, Guijiang Zhou,
b
a
*
*
Zhaoxin Wu and Dongdong Wang
Two series of novel phosphorescent polymers have been successfully synthesized by the Suzuki cross-
coupling reaction among functionalized phosphorescent IrIII ppy-type (ppy ¼ 2-phenylpyridine anion)
complex monomers with hole-injection/transporting (HI/HT) properties, fluorene-based silane moieties
and triphenylamine-oxadiazole hybrid units showing ambipolar features. Their photo-physical,
electrochemical, and electroluminescent (EL) properties have been investigated. The concerned results
have indicated the efficient energy-transfer from the organic segments to the phosphorescent units in
the polymer backbone. The electrochemical characterization has shown that introducing the
triphenylamine-oxadiazole hybrid blocks can facilitate the injection processes for both kinds of charge
carriers and afford ambipolar features to the copolymers. All these features associated with these novel
phosphorescent polymers will guarantee their exceptional EL performances, which have been shown by
organic light-emitting devices (OLEDs). The OLEDs based on these phosphorescent polymers can
furnish a maximum current efficiency (hL) of 30.54 cd Aꢀ1, an external quantum efficiency (hext) of
12.93%, and a power efficiency (hP) of 10.12 lm Wꢀ1. So, these attractive results will offer a new outlet to
afford functionalized phosphorescent polymers for constructing highly efficient OLEDs with low-cost
device fabrication.
Received 24th July 2014
Accepted 13th September 2014
DOI: 10.1039/c4tc01626b
www.rsc.org/MaterialsC
which can adopt conjugated5,11–13 or non-conjugated14,16,17,26–28
structures. The phosphorescent chromophores adopted in
these polymers are typically ppy-type (ppy ¼ 2-phenylpyridine
Introduction
Phosphorescent polymers have received substantial research
interest for their application in organic light-emitting devices
(OLEDs).1–5 Compared with their small molecular phosphores-
cent counterparts, phosphorescent polymers can show the
advantages of low-cost fabrication, exibility, large area, easy
device construction, etc.6–10 In order to obtain phosphorescent
polymers, the phosphorescent chromophores can be either
attached5,11–19 or imbedded20–25 into the polymer backbones,
anion) IrIII complexes for their high phosphorescent quantum
yield (FP), tunable emission color, and relatively short lifetimes
for the triplet states (sP). According to the established theory of
the energy-level for single and triplet excited states in the
organic conjugated systems,29 the conjugated backbones of the
phosphorescent polymers typically possess nonemissive triplet
states with low energy-levels. This situation will increase the
chance of undesired back energy-transfer from phosphorescent
chromophores in the phosphorescent polymers to the non-
emissive triplet states associated with their conjugated back-
bones.3,22 Hence, the back energy-transfer process will hamper
the electroluminescent (EL) process of the concerned phos-
phorescent polymers. With the aim to deal with this problem,
phosphorescent polymers with nonconjugated backbones have
been developed through bonding the phosphorescent units to
side chains of the nonconjugated polymers.14,16,17,26–28 Unfortu-
nately, the nonconjugated backbones will do harm to the charge
carrier injection/transporting ability which is very critical to the
EL process. Additionally, the alkyl groups which afford
aMOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed
Matter, State Key Laboratory for Mechanical Behavior of Materials, Institute of
Chemistry for New Energy Materials, Department of Chemistry, Faculty of Science,
Xi'an Jiaotong University, Xi'an 710049, P. R. China. E-mail: zhougj@mail.xjtu.edu.
cn; Fax: +86-29-8266-3914
bKey Laboratory of Photonics Technology for Information School of Electronic and
Information Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China.
E-mail: zhaoxinwu@mail.xjtu.edu.cn; Fax: +86-29-82664867
† Electronic supplementary information (ESI) available: Synthetic details of some
organic compounds, PL spectra for the polymer solution, EL spectra for devices
G1–G4, and some electroluminescent data for the concerned devices. See DOI:
10.1039/c4tc01626b
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solubility to phosphorescent polymers will disfavor the charge In order to avoid the interference from the singlet emission
carrier injection/transporting process as well. So, functional signal, the 77 K PL spectrum for the pure organic sample was
groups have been introduced into either the backbones or side obtained with highly dilute solution. The lifetimes for the excited
chains of the phosphorescent polymers to improve their charge states were measured using a single photon counting spectrom-
carrier injection/transporting properties.12,14,16,17,26–28
eter from Edinburgh Instruments FLS920 with a 400 nm pico-
Recently, it has been shown that functionalizing should second LED lamp as the excitation source. The data analysis was
represent an important strategy for improving the EL perfor- conducted by the iterative convolution of the luminescence decay
mance of phosphorescent emitters through enhancing their prole with the instrument response function using the soware
charge carrier injection/transporting ability.30–37 So, functional package provided by Edinburgh Instruments. Differential scan-
groups with both hole injection/transporting (HI/HT) properties ning calorimetry (DSC) was performed with a NETZSCH DSC 200
ꢀ1
ꢁ
(triphenylamine, carbazole, etc.) and electron injection/trans- PC unit under a nitrogen ow at a heating rate of 10 C min
.
porting (EI/ET) characters (aromatic borane, diphenylphos- Thermogravimetric analysis (TGA) was conducted with
a
phoryl group, etc.) have been employed to develop highly NETZSCH STA 409C instrument under nitrogen with a heating
efficient phosphorescent emitters.30,37 Furthermore, ambipolar rate of 20 C minꢀ1. The molecular weights of the copolymers
ꢁ
host materials have also been developed to fulll balanced were determined by Waters 2695 GPC in THF. The number-
injection/transporting for both kinds of charge carriers and average and weight-average molecular weights were estimated by
hence increase their recombination efficiency.38–40 So, ambipo- using a calibration curve of polystyrene standards. Cyclic vol-
lar hosts can effectively enhance the EL efficiency of phospho- tammetry (CV) measurement for the sample solution was per-
rescent emitters as well. Obviously, these new strategies should formed on
a Princeton Applied Research model 2273A
provide valuable information for designing novel phosphores- potentiostat with a glassy carbon working electrode, a platinum
cent polymers which can cope with the problems associated counter electrode, and a platinum-wire reference electrode at a
with the traditional analogs. Bearing this in mind, we have scan rate of 100 mV sꢀ1. The solvent was deoxygenated
designed and prepared new phosphorescent polymers with dichloromethane, and the supporting electrolyte was 0.1 M
both triphenylamine functionalized phosphorescent chromo- [nBu4N][BF4]. Ferrocene (Fc) was added as an internal calibrant
phores and ambipolar triphenylamine-oxadiazole hybrid units. for the measurement, and all potentials reported were quoted
Furthermore, uorene-based silane blocks have also been with reference to the Fc–Fc+ couple. The oxidation (Eox) and
introduced to the backbones of the new polymers with the aim reduction (Ered) potentials were used to determine the HOMO
to increase their triplet energy and relieve the trouble of back and LUMO energy levels of the polymers by using the equations
energy-transfer and bring high EL efficiencies.
EHOMO ¼ ꢀ(Eox + 4.8) eV and ELUMO ¼ ꢀ(Ered + 4.8) eV, which were
calculated by using the internal-standard ferrocene value of ꢀ4.8
eV with respect to the vacuum level.
Experimental
General information
Materials and synthesis
All reactions were carried out under a nitrogen atmosphere. The
solvents were puried by routine procedures and distilled under
dry nitrogen before use. All commercially available reagents
were used as received unless otherwise stated. The reactions
were monitored by thin-layer chromatography (TLC) with Merck
pre-coated aluminum plates. Flash column chromatography
and preparative TLC were carried out using silica gel. All Suzuki
copolymerization reactions were carried out with Schlenk
techniques in an inert atmosphere.
The compounds Tz-Am,41 Py-Am,30 BP,40 and 2,7-dibromo-9,9-
dibutyluorene42 were synthesized according to the published
procedures. Thallium(I)acetonacetate ([Tl(acac)]) was purchased
from Aldrich.
L1. To the solution of Tz-Am (0.38 g, 1.00 mmol) in CHCl3 (30
ꢁ
mL), NBS (0.185 g, 1.05 mmol) was added slowly at 0 C. Aer
addition, the reaction mixture was stirred overnight. Then, the
solution was washed with water (2 ꢂ 50 mL). The organic layer
was dried over anhydrous MgSO4. Then the solvent was
removed and the residue was puried by silica gel chromatog-
Physical measurements
raphy eluted with CH2Cl2–petroleum ether (b.p. 60–90 ꢁC) (1 : 1,
1H-NMR and 13C-NMR spectra were measured in CDCl3 or v/v) to get the product as a pale yellow solid (0.39 g, 86%). H
DMSO-d6 with a Bruker AXS 400 MHz spectrometer with the NMR (400 MHz, CDCl3, d, ppm): 8.02 (d, J ¼ 8.4 Hz, 1H), 7.93 (d,
chemical shis quoted relative to tetramethylsilane (TMS). Fast J ¼ 8.4 Hz, 2H), 7.88 (d, J ¼ 8 Hz, 1H), 7.47 (t, J ¼ 7.6 Hz, 1H),
atom bombardment (FAB) mass spectra were recorded on a 7.41–7.38 (m, 2H), 7.36–7.30 (m, 3H), 7.15–7.09 (m, 5H), 7.03 (d,
Finnigan MAT SSQ710 system. UV-vis spectra were recorded with J ¼ 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3, d, ppm): 167.60,
a Shimadzu UV-2250 spectrophotometer. The photoluminescent 154.24, 149.88, 146.50, 146.08, 134.82, 132.49, 129.65, 128.62,
(PL) properties of the copolymers were measured with an Edin- 127.18, 126.41, 126.23, 125.44, 124.82, 124.42, 122.82, 122.19,
burgh Instruments FLS920 uorescence spectrophotometer. 121.51, 116.36. FAB-MS (m/z): 456, 458 [M]+. Elemental analysis
With the aim to obtain the triplet energy-level, the 77 K PL spectra calcd. (%) for C25H17BrN2S: C 65.65, H 3.75, N 6.12; found: C
were obtained by dipping the degassed sample CH2Cl2 solution 65.39, H 3.85, N 6.09.
1
(ca. 0.02 mg mLꢀ1) in a thin quartz tube into a liquid nitrogen
Dewar and recorded the PL spectra aer 5 minutes of standing. (0.185 g, 1.05 mmol) in CHCl3 (30 mL), following the same
L2. It was prepared from Py-Am (0.32 g, 1.00 mmol) and NBS
9524 | J. Mater. Chem. C, 2014, 2, 9523–9535
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Journal of Materials Chemistry C
procedure as for L1. The product was obtained as a pale yellow FAB-MS (m/z): 866 [M]+. Elemental analysis calcd. (%) for
solid (0.35 g, 87%). 1H NMR (400 MHz, CDCl3, d, ppm): 8.65 (d, J
50H34Br2N4O2: C 69.29, H 3.95, N 6.46; found: C 69.21, H 4.15,
C
¼ 4.4 Hz, 1H), 7.87 (d, J ¼ 8.4 Hz, 2H), 7.75–7.70 (m, 1H), 7.67 (d, N 6.39.
J ¼ 8 Hz, 1H), 7.36 (d, J ¼ 8.4 Hz, 2H), 7.32 (d, J ¼ 16 Hz, 2H),
FlSi-Br. The solution of 2,7-dibromo-9,9-dibutyluorene
7.19 (t, J ¼ 6.4 Hz, 1H), 7.13 (t, J ¼ 8.4 Hz, 4H), 7.07 (t, J ¼ 7.2 Hz, (2.00 g, 4.60 mmol) in THF was treated with 1.66 mL of nBuLi
1H), 7.01 (d, J ¼ 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): d (4.15 mmol) slowly under an N2 atmosphere at ꢀ78 ꢁC. The
(ppm) 156.93, 149.64, 148.17, 147.04, 146.66, 136.71, 133.75, mixture was stirred for 30 min at ꢀ78 ꢁC. Then, dichlor-
132.28, 129.48, 127.89, 125.72, 124.87, 123.70, 123.58, 121.63, odiphenylsilane (0.44 mL, 2.08 mmol) was added and the
119.96, 115.40. FAB-MS (m/z): 400, 402 [M]+. Elemental analysis mixture was stirred for an additional 1.5 h at ꢀ78 ꢁC. Aer
calcd. (%) for C23H17BrN2: C 68.84, H 4.27, N 6.98; found: C warming to room temperature, the solution was stirred over-
68.93, H 4.32, N 6.76.
night. The reaction was quenched with water, extracted with
Ir-M1. Under an N2 atmosphere, the organic ligand L1 (0.46 g, CH2Cl2 and dried over anhydrous MgSO4. Then the solvent was
1.00 mmol) and IrCl3$nH2O (160 mg, 60 wt% Ir content) were removed and the residue was puried by silica gel chromatog-
heated to 110 ꢁC in a mixture of 2-ethoxyethanol and water (3 : 1, raphy eluted with petroleum ether (b.p. 60–90 ꢁC) and CH2Cl2–
v/v) for 16 h. Then the reaction mixture was cooled to room petroleum ether (2 : 1, v/v) to get the product as a white solid
temperature and water was added. The cyclometalated Ir(III) m- (1.20 g, 65%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.66–7.57 (m,
chloro-bridged dimer was formed as a precipitate which was 10H), 7.45 (t, J ¼ 8.8 Hz, 8H), 7.37 (t, J ¼ 7.2 Hz, 4H), 1.97–1.85
collected and dried under vacuum. The dimer and [Tl(acac)] (0.30 (m, 8H), 1.13–1.04 (m, 8H), 0.71 (t, J ¼ 7.2 Hz, 12H), 0.65–0.56
g, 1.00 mmol) were added to CH2Cl2 under an N2 atmosphere. (m, 8H). 13C NMR (100 MHz, CDCl3, d, ppm): 153.24, 149.41,
The reaction mixture was allowed to stir at room temperature for 141.55, 139.85, 136.28, 135.24, 134.73, 133.01, 130.79, 129.98,
12 h. Aer that, the solvent was removed under vacuum. The 129.63, 127.84, 126.19, 121.49, 121.38, 119.32, 55.28, 39.82,
residue was puried by column chromatography using CH2Cl2– 25.99, 22.94, 13.85. FAB-MS (m/z): 892 [M]+. Elemental analysis
petroleum ether (b.p. 60–90 ꢁC) (1 : 1, v/v) as the eluent to obtain calcd. (%) for C54H58Br2Si: C 72.47, H 6.53; found: C 72.29, H
the product as an orange solid (0.27 g, 45%). 1H NMR (400 MHz, 6.65.
CDCl3, d, ppm): 7.87–7.85 (m, 2H), 7.75–7.73 (m, 2H), 7.37–7.35
FlSi-B. The mixture of FlSi-Br (0.40 g, 0.45 mmol), bis(pina-
(m, 6H), 7.08 (t, J ¼ 7.6 Hz, 4H), 7.01 (d, J ¼ 8.9 Hz, 4H), 6.97–6.91 colato)diborate (0.34 g, 1.34 mmol), KOAc (0.26 g, 2.60 mmol),
(m, 6H), 6.69 (d, J ¼ 8.8 Hz, 4H), 6.47–6.45 (m, 2H), 5.78 (d, J ¼ 2.0 Pd(dppf)2Cl2 (0.036 g, 0.038 mmol), and dry 1,4-dioxane (15 mL)
Hz, 2H), 5.14 (s, 1H), 1.78 (s, 6H). FAB-MS (m/z): 1202 [M]+. was heated at 100 C for 16 h. Aer cooling to room tempera-
ꢁ
Elemental analysis calcd. (%) for C55H39Br2IrN4O2S2: C 54.86, H ture, the mixture was ltered and washed with ethyl acetate (3 ꢂ
3.26, N 4.65; found: C 54.69, H 3.48, N 4.39.
20 mL). The organic phase was dried over MgSO4. Then the
Ir-M2. It was prepared from L2 (0.40 g, 1.00 mmol), solvent was removed and the residue was puried by silica gel
IrCl3$nH2O (160 mg, 60 wt% Ir content) and [Tl(acac)] (0.30 g, chromatography eluted with CH2Cl2–petroleum ether (b.p. 60–
1.00 mmol), following the same procedure as for Ir-M1. The 90 ꢁC) (2 : 1, v/v) to get the product as a white solid (0.27 g, 60%).
1
product was obtained as a yellow solid (0.22 g, 40%). H NMR 1H NMR (400 MHz, CDCl3, d, ppm): 7.81 (d, J ¼ 8.0 Hz, 2H), 7.75
(400 MHz, CDCl3, d, ppm): 8.25 (d, J ¼ 4.4 Hz, 2H), 7.46 (d, J ¼ (s, 2H), 7.71 (t, J ¼ 7.6 Hz, 4H), 7.66 (s, 2H), 7.61 (d, J ¼ 6.8 Hz,
4.0 Hz, 4H), 7.31 (d, J ¼ 8.4 Hz, 2H), 7.24–7.14 (m, 8H), 6.98–6.92 4H), 7.47 (d, J ¼ 7.6 Hz, 2H), 7.43 (d, J ¼ 7.6 Hz, 2H), 7.36 (t, J ¼
(m, 6H), 6.86–6.84 (m, 2H), 6.77–6.70 (m, 4H), 6.51–6.48 7.2 Hz, 4H), 1.98–1.92 (m, 8H), 1.39 (s, 24H), 1.09–1.02 (m, 8H),
(m, 2H), 5.65 (d, J ¼ 2.4 Hz, 2H), 5.25 (s, 1H), 1.79 (s, 6H). 13C 0.68 (t, J ¼ 7.2 Hz, 12H), 0.64–0.58 (m, 8H). 13C NMR (100 MHz,
NMR (100 MHz, CDCl3, d, ppm): 184.53, 167.51, 147.93, 147.79, CDCl3, d, ppm): 150.35, 150.19, 143.86, 142.40, 136.32, 136.60,
147.11, 146.80, 146.64, 138.83, 136.16, 131.74, 128.99, 126.67, 135.02, 134.99, 133.69, 133.05, 130.94, 129.52, 128.88, 127.79,
125.75, 125.07, 124.22, 123.06, 120.27, 117.91, 114.95, 114.18, 119.66, 119.31, 83.73, 54.98, 39.82, 26.02, 24.93, 23.00, 13.89.
100.53, 28.87. FAB-MS (m/z): 1090 [M]+. Elemental analysis FAB-MS (m/z): 988 [M]+. Elemental analysis calcd. (%) for
calcd. (%) for C51H39Br2IrN4O2: C 56.10, H 3.60, N 5.13; found:
C 55.93, H 3.39, N 5.09.
C
66H82B2O4Si: C 80.15, H 8.36; found: C 80.03, H 8.69.
P-O-1. Under an N2 atmosphere, Ir-M1 (2.0 mg, 0.0017
BP-Br. NBS (0.26 g, 1.46 mmol) was added slowly into a mmol), BP-Br (42.3 mg, 0.0489 mmol), FlSi-B (50 mg, 0.0506
solution of BP (0.50 g, 0.71 mmol) in CHCl3 (100 mL), and then mmol), Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg,
stirred at 0 ꢁC for 24 h. The solution was washed with water (2 ꢂ 0.008 mmol) were mixed in a solvent mixture of degassed
100 mL), and the organic layer was dried over anhydrous toluene/THF/H2O (5.0 mL/5.0 mL/1.25 mL). The mixture was
ꢁ
MgSO4. The solvent was removed under reduced pressure and vigorously stirred for 48 h at 100 C. The reaction mixture was
the residue was puried by silica gel chromatography eluted then stirred at 100 ꢁC for 12 h aer adding phenylboronic acid
with CH2Cl2 to get the title compound as a pale yellow solid (3.0 mg, 0.025 mmol), and then stirred at 100 ꢁC for a further 12
(0.58 g, 95%). 1H NMR (400 MHz, CDCl3, d, ppm): 8.21 (d, J ¼ 8.4 h aer adding bromobenzene (4.0 mg, 0.025 mmol). Aer
Hz, 4H), 7.74 (d, J ¼ 8.4 Hz, 4H), 7.55 (d, J ¼ 8.8 Hz, 4H), 7.30 (t, J adding water (20 mL), the mixture was extracted with CH2Cl2 (3
¼ 8 Hz, 4H), 7.15 (t, J ¼ 8.4 Hz, 8H), 7.09 (t, J ¼ 7.2 Hz, 2H), 7.02 ꢂ 25 mL) and the organic phase was dried over anhydrous
(d, J ¼ 8.8 Hz, 4H). 13C NMR (100 MHz, CDCl3, d, ppm): 164.51, Na2SO4. Then the solvent was removed and the residue was
147.60, 147.00, 146.61, 143.72, 133.81, 132.35, 129.56, 127.98, redissolved in CH2Cl2 and puried with a 0.45 mm PTFE syringe
127.44, 127.11, 125.74, 124.90, 123.82, 123.78, 122.24, 115.52. lter. Aer concentration, the copolymer was puried by
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precipitation twice in methanol and washed with acetone in a Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008
Soxhlet apparatus for 72 h, and dried under vacuum. It was mmol), following the same procedure as for P-O-1. The copol-
1
obtained as a light orange solid (yield: 64%). 1H NMR (400 MHz, ymer was obtained as a yellow solid (yield: 71%). H NMR (400
CDCl3, d, ppm): 8.20 (d), 7.77–7.01 (m), 1.98–1.95 (m), 1.15–1.06 MHz, CDCl3, d, ppm): 8.21 (d), 7.79–7.08 (m), 5.24 (s), 1.98 (t),
(m), 0.71–0.67 (m); gel permeation chromatography (GPC): 1.82 (s), 1.10–1.06 (m), 0.72–0.69 (m); GPC: Mn ¼ 1.7 ꢂ 104 g
number-average molecular weight (Mn) ¼ 3.5 ꢂ 104 g molꢀ1 and molꢀ1 and PDI ¼ 2.3 (against polystyrene standards).
polydispersity index (PDI) ¼ 2.1 (against polystyrene standards).
P-OSi. Under an N2 atmosphere, Ir-M1 (12.0 mg, 0.01 mmol),
P-O-2. It was prepared from Ir-M1 (4.0 mg, 0.0033 mmol), FlSi-Br (64.0 mg, 0.0717 mmol), FlSi-B (80.8 mg, 0.0817 mmol),
BP-Br (40.9 mg, 0.0473 mmol), FlSi-B (50 mg, 0.0506 mmol), Na2CO3 (0.78 g, 7.38 mmol), and Pd(PPh3)4 (55.44 mg, 0.056
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008 mmol) were mixed in a solvent mixture of degassed toluene/
mmol), following the same procedure as for P-O-1. The copol- THF/H2O (10.0 mL/10.0 mL/2.5 mL). The mixture was vigorously
ymer was obtained as an orange solid (yield: 76%). 1H NMR (400 stirred for 48 h at 100 ꢁC. The reaction mixture was then stirred
MHz, CDCl3, d, ppm): 8.20 (d), 7.77–6.99 (m), 5.11 (s), 2.03–1.91 at 100 ꢁC for 12 h aer adding phenylboronic acid (18.0 mg, 0.15
(m), 1.77 (s), 1.13–1.06 (m), 0.72–0.66 (m); GPC: Mn ¼ 3.3 ꢂ 104 mmol), and then stirred at 100 ꢁC for a further 12 h aer adding
g molꢀ1 and PDI ¼ 2.2 (against polystyrene standards).
bromobenzene (24.0 mg, 0.15 mmol). Aer adding water (40
P-O-3. It was prepared from Ir-M1 (6.0 mg, 0.0050 mmol), mL), the mixture was extracted with CH2Cl2 (3 ꢂ 30 mL) and the
BP-Br (39.4 mg, 0.0456 mmol), FlSi-B (50 mg, 0.0506 mmol), organic phase was dried over anhydrous Na2SO4. Then the
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008 solvent was removed and the residue was redissolved in CH2Cl2
mmol), following the same procedure as for P-O-1. The copol- and puried using a 0.45 mm PTFE syringe lter. Aer concen-
ymer was obtained as an orange solid (yield: 86%). 1H NMR (400 tration, the copolymer was puried by precipitation twice in
MHz, CDCl3, d, ppm): 8.20 (d), 7.75–6.99 (m), 5.15 (s), 2.00–1.92 methanol and washed with acetone in a Soxhlet apparatus for
(m), 1.76 (s), 1.11–1.08 (m), 0.75–0.67 (m); GPC: Mn ¼ 3.1 ꢂ 104 72 h, and dried under vacuum. It was obtained as a light orange
g molꢀ1 and PDI ¼ 2.5 (against polystyrene standards).
solid (yield: 82%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.82–7.62
P-O-4. It was prepared from Ir-M1 (8.0 mg, 0.0067 mmol), (m), 7.53–7.41 (m), 7.39–7.36 (m), 5.13 (s), 2.01–1.86 (m), 1.76
BP-Br (37.9 mg, 0.0439 mmol), FlSi-B (50 mg, 0.0506 mmol), (s), 1.13–1.04 (m), 0.75–0.57 (m); gel permeation chromatog-
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008 raphy (GPC): number-average molecular weight (Mn) ¼ 2.9 ꢂ
mmol), following the same procedure as for P-O-1. The copol- 104 g molꢀ1 and polydispersity index (PDI) ¼ 2.4 (against
ymer was obtained as an orange solid (yield: 81%). 1H NMR (400 polystyrene standards).
MHz, CDCl3, d, ppm): 8.20 (d), 7.77–6.91 (m), 5.14 (s), 1.98–1.91
P-Org. Under an N2 atmosphere, BP-Br (43.7 mg, 0.0506
(m), 1.76 (s), 1.14–1.08 (m), 0.74–0.67 (m); GPC: Mn ¼ 2.4 ꢂ 104 mmol), FlSi-B (50 mg, 0.0506 mmol), Na2CO3 (0.13 g, 1.23
g molꢀ1 and PDI ¼ 2.2 (against polystyrene standards).
mmol), and Pd(PPh3)4 (9.24 mg, 0.008 mmol) were mixed in a
P-G-1. It was prepared from Ir-M2 (2.0 mg, 0.0018 mmol), solvent mixture of degassed toluene/THF/H2O (5.0 mL/5.0 mL/
BP-Br (43.1 mg, 0.0487 mmol), FlSi-B (50 mg, 0.0506 mmol), 1.25 mL). The mixture was vigorously stirred for 48 h at 100 ꢁC.
ꢁ
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008 The reaction mixture was then stirred at 100 C for 12 h aer
mmol), following the same procedure as for P-O-1. The copol- adding phenylboronic acid (3.0 mg, 0.025 mmol), and then
1
ymer was obtained as a yellow solid (yield: 62%). H NMR (400 stirred at 100 ꢁC for a further 12 h aer adding bromobenzene
MHz, CDCl3, d, ppm): 8.22 (d), 7.79–7.08 (m), 5.12 (s). 1.98 (t), (4.0 mg, 0.025 mmol). Aer adding water (20 mL), the mixture
1.70 (s), 1.12–1.07 (m), 0.73–0.62 (m); GPC: Mn ¼ 2.1 ꢂ 104 g was extracted with CH2Cl2 (3 ꢂ 25 mL) and the organic phase
molꢀ1 and PDI ¼ 2.0 (against polystyrene standards).
was dried over anhydrous Na2SO4. Then the solvent was
P-G-2. It was prepared from Ir-M2 (4.0 mg, 0.0037 mmol), removed and the residue was redissolved in CH2Cl2 and puri-
BP-Br (40.4 mg, 0.0468 mmol), FlSi-B (50 mg, 0.0506 mmol), ed using a 0.45 mm PTFE syringe lter. Aer concentration, the
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008 copolymer was puried by precipitation twice in methanol and
mmol), following the same procedure as for P-O-1. The copol- washed with acetone in a Soxhlet apparatus for 72 h, and dried
1
ymer was obtained as a yellow solid (yield: 82%). H NMR (400 under vacuum. It was obtained as a pale yellow solid (yield:
1
MHz, CDCl3, d, ppm): 8.20 (d), 7.76–7.05 (m), 5.11 (s), 1.98 (t), 87%). H NMR (400 MHz, CDCl3, d, ppm): 8.21 (d), 7.76–7.01
1.68 (s), 1.09–1.07 (m), 0.72–0.66 (m); GPC: Mn ¼ 2.4 ꢂ 104 g (m), 1.98–1.94 (m), 1.11–1.09 (m), 0.75–0.69 (m); gel permeation
molꢀ1 and PDI ¼ 1.9 (against polystyrene standards).
chromatography (GPC): number-average molecular weight (Mn)
P-G-3. It was prepared from Ir-M2 (6.0 mg, 0.0055 mmol), ¼ 3.2 ꢂ 104 g molꢀ1 and polydispersity index (PDI) ¼ 2.3
BP-Br (38.9 mg, 0.0450 mmol), FlSi-B (50 mg, 0.0506 mmol), (against polystyrene standards).
Na2CO3 (0.13 g, 1.23 mmol), and Pd(PPh3)4 (9.24 mg, 0.008
mmol), following the same procedure as for P-O-1. The copol-
ymer was obtained as a yellow solid (yield: 83%). H NMR (400
1
OLED fabrication and measurements
MHz, CDCl3, d, ppm): 8.20 (d), 7.77–6.99 (m), 5.20 (s), 1.98 (t), The pre-cleaned ITO glass substrates were treated with ozone
1.81 (s), 1.09–1.06 (m), 0.72–0.67 (m); GPC: Mn ¼ 2.1 ꢂ 104 g for 20 min. Then, the PEDOT:PSS was deposited on the surface
molꢀ1 and PDI ¼ 2.0 (against polystyrene standards).
of ITO glass by the spin-coating method to form a 40 nm-thick
P-G-4. It was prepared from Ir-M2 (8.0 mg, 0.0073 mmol), hole-injection layer aer being cured at 120 ꢁC for 30 min in the
BP-Br (37.3 mg, 0.0432 mmol), FlSi-B (50 mg, 0.0506 mmol), air. The emitting layer (40 nm) was obtained by spin-coating a
9526 | J. Mater. Chem. C, 2014, 2, 9523–9535
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chlorobenzene solution of each corresponding phosphorescent character to them. The chemical structures of the complexes
polymer. The ITO glass was dried in a vacuum oven at 50 ꢁC for have been veried by the NMR, mass spectra, etc. The ambipolar
15 min and it was transferred to the deposition system for unit BP-Br has been prepared by two step reactions: the Suzuki
organic and metal deposition. TPBi (40 nm), LiF (1 nm) and Al cross-coupling between 2,5-bis(4-bromophenyl)-1,3,4-oxadi-
cathode (100 nm) were successively evaporated at a base pres- azole and 4-(diphenylamino)phenylboronic acid will get the pre-
sure less than 10ꢀ6 Torr. The EL spectra and CIE coordinates compound BP (Scheme S1 in the ESI†), which was bromized
were measured with a PR650 spectra colorimeter. The J–V–L with NBS to obtain BP-Br. In this reaction, there is a possibility
curves of the devices were recorded with a Keithley 2400/2000 to obtain the isomer with dibromo-substitution in the triphe-
source meter and the luminance was measured using a PR650 nylamine moiety on one side of BP. Clearly, the undesired
SpectraScan spectrometer. All the experiments and measure- isomer should show much more carbon signals (ca. 24 reso-
ments were carried out under ambient conditions.
nance peaks) in its 13C NMR spectrum. However, there are
seventeen resonance peaks for the carbon atoms in the 13C
NMR spectrum of the obtained product, which is well consis-
tent with the chemical structure of BP-Br shown in Scheme 1.
So, it can be safely concluded that the obtained product is a
pure compound in the form of BP-Br rather than a mixture with
the undesired isomer aforementioned. This result can be
explained as follows: the bromination of BP with NBS is an
electrophilic substitution reaction. Hence, introducing one
bromine group showing electron-withdrawing ability to the
triphenylamine moiety will denitely disfavor the substitution
of the second one. Furthermore, the reaction is carried out with
a weak brominating reagent under very mild conditions to
guarantee the good selectivity of the reaction. As a result,
pure BP-Br with the desired structure can be properly
obtained. The ambipolar property of BP-Br is afforded by both
Results and discussion
Synthesis and structural characterization
In order to obtain the phosphorescent polymers, three kinds of
monomers should be prepared. For the phosphorescent chro-
mophores, they were prepared according to the established two-
step strategy by the cyclometalation of IrCl3$nH2O with the
corresponding organic ligands prepared by mono-bromination
of the concerned triphenylamine derivatives to initially form the
m-chloro-bridged dimers, followed by coordination of the ace-
tylacetone (acac) anion in the presence of [Tl(acac)] under very
mild conditions to get the target phosphorescent IrIII complexes
Ir-M1 and Ir-M2 (Scheme 1). In Ir-M1 and Ir-M2, the functional
group of triphenylamine was introduced to afford an HI/HT
Scheme 1 Synthetic scheme for the monomers required for the synthesis of phosphorescent polymers.
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the 1,3,4-oxadiazole ring with the EI/ET property and the tri- to relieve the triplet–triplet (T–T) annihilation effect due to long-
phenylamine moiety with the HI/HT feature. The bromine lived emissive triplet excited states. So, the weight percentage
substituents in Ir-M1, Ir-M2 and BP-Br can act as the reaction for Ir-M1 and Ir-M2 in the nal polymer is controlled less than
sites in polymer synthesis. Aromatic silane blocks have been 10 wt%. Based on the character associated with the copoly-
employed to construct host materials for phosphorescent merization, the phosphorescent blocks Ir-M1 and Ir-M2 can
emitters due to their high triplet energy.43 Hence, uorene- only link with FlSi-B, which will make them well scattered along
based silane block FlSi-B has been designed with the aim to the copolymer backbone to reduce T–T annihilation. This
block the undesired back energy transfer process aforemen- should benet the EL performance of the phosphorescent
tioned. The FlSi-B was synthesized by the cross-coupling polymers. Aer polymerization, the crude polymer solution was
between FlSi-Br and bis(pinacolato)diborate under base condi- ltered with a 0.45 mm PTFE syringe lter to remove the parti-
tions with Pd(dppf)Cl2 as the catalyst. The two pinacol ester cles resulting from the decomposition of the Pd(PPh3)4 catalyst.
groups can afford FlSi-B the ability of copolymerization with Aer concentration, the ltered solution was added to meth-
Ir-M1/Ir-M2 and BP-Br to form the designed phosphorescent anol to precipitate the copolymers as colored powders.
polymers.
The feed ratio of the orange phosphorescent monomer Ir-M1
The synthetic procedure for the phosphorescent polymers is was set at 1.58 to 6.35 mol% and the corresponding weight
shown in Scheme 2. The concerned copolymers were prepared percentage ca. 2.0, 4.0, 6.0, and 8.0 wt%. According to the feed
using the Suzuki cross-coupling reaction. According to the ratios of Ir-M1, FlSi-B, and BP-Br, the obtained copolymers are
inherent character of the phosphorescent system, Ir-M1 and named P-O-1 (1.58 50 48.42), P-O-2 (3.27 50 46.73), P-O-3
Ir-M2 should contribute a small content in the nal copolymers (4.95 50 45.05), and P-O-4 (6.53 50 43.47), respectively (Scheme 2).
Scheme 2 Synthetic sketch for the phosphorescent polymers.
9528 | J. Mater. Chem. C, 2014, 2, 9523–9535
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The synthesis of green phosphorescent copolymers has differential scanning calorimetry (DSC) under a nitrogen
employed the feed ratio of the Ir-M2 to retain its weight atmosphere. The TGA results indicate their thermal stability
percentage similar to that of Ir-M1 with respect to other with the 5% weight-reduction temperature (DT5%) ranging from
monomers FlSi-B and BP-Br (Scheme 2). The resulting copoly- 412 to 398 ꢁC (Table 1). The DSC traces of the copolymers have
mers are coded as P-G-1 (1.78 50 48.22), P-G-2 (3.66 50 46.34), revealed their high glass-transition temperatures (Tg) in the
P-G-3 (5.45 50 44.55), and P-G-4 (7.23 50 42.77). In the 1H-NMR range from 109 to 120 ꢁC. The good thermal properties of these
spectra of the polymers, the resonance peaks with d at ca. 1.98, copolymers will benet their application in the eld of OLEDs.
1.10–1.05, and 0.70–0.60 ppm come from the butyl group of the
From Fig. 1, it can be clearly seen that the main absorption
uorene-based block. The proton signal with d at 8.20 ppm bands for the two series copolymers are located before ca. 450
should be induced by the ambipolar segment, comparing with nm. It can be safely ascribed to the p–p* transition from both
that of BP-Br. Owing to the low content of the phosphorescent the aromatic silane and ambipolar units. The contribution from
moieties, the associated 1H-NMR signals cannot be observed the p–p* transition of organic ligands from IrIII phosphores-
obviously. However, a very weak resonance signal with d at ca. cent chromophores should be inessential due to their low
5.20 and 1.80 ppm can still be detected. This is the character- content in the copolymers. However, for the copolymers with
istic proton signal of the acac anion in the IrIII phosphorescent higher IrIII phosphorescent chromophore content, such as
blocks. All these spectral data have clearly indicated that all the P-O-3, P-O-4, and P-G-4, inconspicuous absorption bands with
building blocks have been copolymerized together successfully. very low intensity can be observed in the range from ca. 450 nm
These phosphorescent polymers are readily soluble in common to 500 nm, which can be assigned to the triplet metal-to-ligand
organic solvents, such as CHCl3, CH2Cl2, THF, etc. Taking charge transfer (3MLCT) states (Fig. 1 and Table 1). The other
polystyrene as standards, the number average molecular copolymers cannot display detectable MLCT absorption bands
weights (Mn) of the copolymers range from 1.7 ꢂ 104 to 3.5 ꢂ due the low level of phosphorescent units (Fig. 1 and Table 1).
104 with polydispersity indices (PDIs) between 1.9 and 2.5.
The photoluminescent (PL) spectra for the phosphorescent
With the aim to investigate the photophysical and electro- copolymers are also investigated in both solution and lm
chemical properties of the concerned phosphorescent poly- states (Fig. S1 in the ESI† and Fig. 2). In CH2Cl2 solution, all the
mers, the model polymers P-Org and P-OSi are also obtained copolymers exhibit an emission band at ca. 490 nm (Fig. S1†
(Scheme 3). The pure organic polymer P-Org was synthesized by and Table 1). Based on their nanosecond lifetime together with
the Suzuki cross-coupling between BP-Br and FlSi-B, while P-OSi the PL spectra of the copolymer P-Org (Table 1 and Fig. S1†),
was prepared by the cross-coupling reaction among FlSi-B, FlSi-Br, these emission bands should be induced by the pure organic
and Ir-M1 which will correspond for ca. 8.0 wt% in P-OSi.
segments in the phosphorescent copolymers. With respect to
that in the lm state (ca. 480 nm, Table 1), the slight bath-
ochromic effect of the high-energy emission band in solution
should be induced by the solvent effect of CH2Cl2. Differently,
these copolymers can show obvious phosphorescent bands in
Thermal and photophysical properties
The thermal properties of the phosphorescent copolymers have
been investigated by thermogravimetric analysis (TGA) and
Scheme 3 Synthetic scheme for the polymer of P-Org and P-OSi.
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Table 1 Photophysical and thermal-stability data for the concerned polymers
sc solution/lm
DT5%/Tg (ꢁC)
a
Polymers
Absorption labs (nm) 298 K
Emission lemb (nm) 298 K solution/lm
P-O-1
P-O-2
P-O-3
P-O-4
P-G-1
P-G-2
P-G-3
P-G-4
P-Org
292, 305, 318, 356, 382
292, 304, 317, 356, 383
292, 303, 318, 355, 382, 483
292, 305, 318, 356, 382, 483
296, 326, 358, 380
394, 319, 356, 380
300, 312, 325, 375
297, 325, 358, 380, 458
292, 316, 356, 380
493/478, 575, 625
488/478, 575, 625
496/577, 626
490/577, 625
498/478, 526
499/480, 530, 574
493/480, 533, 573
499/478, 533, 573
490/480
2.45 ns/0.88 ms
2.34 ns/0.80 ms
2.51 ns/0.78 ms
2.40 ns/0.74 ms
2.55 ns/0.19 ms
2.51 ns/0.15 ms
2.42 ns/0.20 ms
2.55 ns/0.16 ms
2.51 ns/—
411/115
410/111
408/110
398/110
412/109
410/120
405/120
403/113
415/118
a
b
c
Measured in CH2Cl2 at a concentration of 0.02 mg mLꢀ1
.
Measured in CH2Cl2 at a concentration of 0.02 mg mLꢀ1
.
For the solution samples, the
lifetime (s) was referred to the emission band at ca. 490 nm. For the lm samples, the s was assigned to the phosphorescent band. The excitation
wavelength for the measure was set at 400 nm.
the lm state. The phosphorescent emission bands of the the phosphorescent units. For P-O-1–P-O-4, the high-energy
copolymer lms generally are enhanced by increasing the emission band almost disappears (Fig. 2a) due to the efficient
content level of phosphorescent units (Fig. 2). However, the energy-transfer from the pure organic segments to the phos-
phosphorescent intensity shows a slight decrease at high phorescent units, which is typically shown in phosphorescent
contents of the IrIII complex blocks (P-O-4 and P-G-4) (Fig. 2), polymers. Different from P-O-1–P-O-4, the copolymers P-G-1–P-G-4
which should be induced by the weak quenching effect among can still show obvious high-energy emission bands at ca.
480 nm besides the phosphorescent ones (Fig. 2b), indicating
Fig. 1 UV-vis spectra for the phosphorescent polymers. (a) P-O-1–P- Fig. 2 The PL spectra for the film of the phosphorescent copolymers
O-4 and (b) P-G-1–P-G-4.
at 298 K.
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the inefficient energy-transfer process. In order to explain this
issue, the UV-vis spectra for the organic copolymer P-Org and the
IrIII monomers (Ir-M1 and Ir-M2) have been obtained (Fig. 3).
Under the excitation wavelength of 390 nm, the organic ligands of
the IrIII monomers can be excited based on their absorption
spectra (Fig. 3). The photophysical properties of the organic
segments formed between BP-Br and FlSi-B in the polymer back-
bones should be similar to that of P-Org due to the low content of
the IrIII monomers. The ratiocination has been conrmed by the
great resemblance between their UV-vis, PL, and lifetime (Table 1,
Fig. 1, 3 and S1†). Hence, the photophysical property of P-Org can
be safely employed to represent those of the organic segments in
the polymer backbones. According to the UV-vis spectrum of
P-Org in Fig. 3, the 390 nm light can induce the excitation of
organic segments in the phosphorescent polymers as well. For
P-O-1–P-O-4, the energy-level for the rst singlet states (S10 ) of their
organic segments is higher than that for the rst singlet states (SO1 )
of the organic ligands in Ir-M1 (Fig. 3). There should be a cascade
Fig.
4
energy-transfer process involved in the phosphorescent
polymers. (a) P-O-1–P-O-4 and (b) P-G-1–P-G-4.
0
1
energy-transfer process from S1 to SO1 , then from S1O to MLCTO
(singlet states of MLCT in Ir-M1), which is converted via inter-
system crossing (ISC) into emissive 3MLCTO (triplet states of
MLCT in Ir-M1) (Fig. 4a) to induce phosphorescence signals in the
polymers of P-O-1–P-O-4 (Fig. 2a). According to the energy-level
involved, the energy-transfer process in P-O-1–P-O-4 is an energy-
favored down-hill procedure to guarantee its high efficiency
(Fig. 4a). Additionally, there is good over-lap between the
absorption spectrum of Ir-M1 and the PL spectrum of P-Org
(Fig. 3), indicating the facilitation of the energy-transfer from the
organic segments to phosphorescent units in P-O-1–P-O-4. Hence,
the highly efficient cascade energy-transfer process should greatly
reduce the chance for the decay of S10 to the ground state (So).
Accordingly, there is almost no high-energy uorescent emission
bands associated with the transition of S10 to So in the lm of
P-O-1–P-O-4 (Fig. 2a). However, polymers P-G-1–P-G-4 should show
a different situation. According to Fig. 3, the S10 states show almost
the same energy level to that of the rst singlet states (SG1 ) of the
organic ligands in Ir-M2. So, there is resonance energy-transfer
between the S10 and SG1 states (Fig. 4b). Obviously, the resonance
procedure will not only increase the chance for the decay of the
radiative S10 to So to induce high-energy uorescence bands, but
also render ineffectiveness to the cascade energy-transfer process
from S10 to SG1 , then from SG1 to 1MLCTG (singlet states of MLCT in
3
Ir-M2). As a result, the emission from MLCTG (triplet states of
MLCT in Ir-M2) will decrease, which will weaken the phospho-
rescent signal in P-G-1–P-G-4. Furthermore, the over-lap between
the absorption spectrum of Ir-M2 and the PL spectrum of P-Org is
poor (Fig. 3), disfavoring the energy-transfer from the organic
segments to phosphorescent units in P-G-1–P-G-4. As a result,
both uorescence and phosphorescence bands can be observed in
the lm of P-G-1–P-G-4 (Fig. 2b). Different from electrical excita-
tion, photoexcitation typically generates singlet states. Further-
more, with excitation at 390 nm, both the 3MLCT of
phosphorescent units and the triplet states of the organic
segments in the polymers cannot be formed directly through the
photoexcitation. Hence, the triplet energy-transfer among the
triplet states might be ruled out in the polymer lms under
photoexcitation.
Electrochemical characterization
Under a nitrogen atmosphere, the electrochemical behavior of
these phosphorescent polymers was characterized by the cyclic
voltammetry (CV) measurements calibrated with ferrocene (Fc)
as the internal standard. In order to evaluate the effect of the
ambipolar moiety on the charge carrier injection ability of the
copolymers, the CV characterization for P-Org and P-OSi was
also conducted. The polymers P-O-1–P-O-4 and P-G-1–P-G-4 all
show an reversible oxidation process at ca. 0.5 V ranging from
0.49 V to 0.55 V (Table 2). Similarly, these polymers also show
alike reduction potential at ca. ꢀ2.5 V (Table 2). With the aim to
assign the oxidation and reduction processes, these data are
compared with those of P-Org and P-OSi. It can be clearly seen
ox
red
that the CV data of P-Org with E at ca. 0.5 V and E at ca.
1/2
1/2
Fig. 3 UV-vis spectra for P-Org, Ir-M1 and Ir-M2 together with the PL
spectrum of P-Org in CH2Cl2 at 298 K.
ꢀ2.50 V are very similar to those of the phosphorescent
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Table 2 Redox properties of the polymers
ox
1/2
reda
1/2
Copolymers
E
(V)
E
(V)
EHOMO (eV)
ELUMO (eV)
P-O-1
P-O-2
P-O-3
P-O-4
P-G-1
P-G-2
P-G-3
P-G-4
P-Org
P-OSi
0.53
0.55
0.52
0.55
0.49
0.50
0.53
0.49
0.47
1.05
ꢀ2.48
ꢀ2.43
ꢀ2.45
ꢀ2.45
ꢀ2.51
ꢀ2.55
ꢀ2.47
ꢀ2.49
ꢀ2.50
—
ꢀ5.33
ꢀ5.35
ꢀ5.32
ꢀ5.35
ꢀ5.29
ꢀ5.30
ꢀ5.33
ꢀ5.29
ꢀ5.27
ꢀ5.85
ꢀ2.32
ꢀ2.37
ꢀ2.35
ꢀ2.35
ꢀ2.29
ꢀ2.25
ꢀ2.33
ꢀ2.31
ꢀ2.30
—
a
Irreversible.
polymers. So, it can be concluded that the oxidation and
reduction processes in the phosphorescent polymers are
induced by their organic segments, composed of aromatic
silane and the ambipolar moieties. It indicates that the CV
signal associated with the IrIII phosphorescent units cannot be
properly detected, which should be due to their low content in
the corresponding polymers. Without the ambipolar moiety,
Fig. 5 The configuration of the PHOLEDs made from the phospho-
rescent polymers and the chemical structures for the involved func-
tional materials.
P-OSi shows a noticeably different CV behavior from the other
chlorobenzene solution of the concerned phosphorescent
polymer. Similar to their analogs, the organic segments in the
polymer backbone will act as the host material for the phos-
phorescent IrIII complex moieties. The 1,3,5-tris(1-phenyl-1H-
benzo[d]imidazol-2-yl)benzene (TPBi) layer plays the role of
both hole-blocking and electron-transporting, while LiF acts as
an electron-injection layer.
When the proper voltage was applied to the PHOLEDs,
intense electrophosphorescence can be observed. For devices
O1–O4, they show orange EL representing line-shapes similar to
that of the corresponding polymers in the lm state, indicating
that the origin of EL is from the phosphorescent units (Fig. 6).
There is only a very weak EL band from organic segments in the
polymers for devices O1 and O2 due to the low level of the
phosphorescent units which cannot take in all the energy
ox
polymers with much higher E at ca. 1.05 V and no detectable
1/2
reduction process in the scan potential range (Table 2). Taking
all the results into consideration, the oxidation and reduction
processes of the concerned phosphorescent copolymers should
come from the ambipolar moieties in their backbone. The
electron-rich triphenylamine group in the ambipolar moiety
induces the oxidation process of the phosphorescent polymers
and the electron-decient oxadiazole unit will be responsible
for their reduction process. Comparing the CV behaviors
between P-OSi and the concerned phosphorescent polymers, it
can be clearly seen that introducing ambipolar moieties can
effectively improve the injection ability for both kinds of charge
carriers of the concerned phosphorescent polymers. This
should benet the EL performance of the investigated
copolymers.
Electroluminescent properties
Inspired by their charge carrier injection/transporting proper-
ties, these polymers were employed as emitters to construct
phosphorescent OLEDs (PHOLEDs) to evaluate their electrolu-
minescent properties. These phosphorescent polymers are very
soluble in common organic solvents such as chloroform, THF,
toluene, chlorobenzene, etc. and thus the concerned PHOLEDs
are fabricated with a solution-processing approach. The device
conguration was ITO/PEDOT:PSS (40 nm)/emission layer, EML
(40 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). Fig. 5 depicts the
structure of the multi-layer PHOLEDs and the chemicals
involved in the fabrication of the devices. The PEDOT:PSS layer
was rstly deposited by spin-coating on the pre-cleaned ITO
surface to form a hole-injection layer (HIL) for its matching
HOMO level with that of ITO. The emission layer (EML) is
constructed on the PEDOT:PSS layer by spin-coating the Fig. 6 The EL spectra for the devices O1–O4 at ca. 12 V.
9532 | J. Mater. Chem. C, 2014, 2, 9523–9535
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The current–density–voltage–luminance (J–V–L) curves for
the PHOLEDs are shown in Fig. 7 and S3,† and the corre-
sponding EL data are summarized in Table 3. Compared with
the other devices emitting orange color, device O3 can show the
maximum EL abilities with a peak luminance (Lmax) of 30 532 cd
mꢀ2 at 18.4 V, a current efficiency (hL) of 30.54 cd Aꢀ1, an
external quantum efficiency (hext) of 12.93% and a power effi-
ciency (hP) of 10.12 lm Wꢀ1. Besides device O3, device O2 can
also furnish attractive EL performance with an Lmax of 18 148 cd
mꢀ2 at 16.9 V, 27.46 cd Aꢀ1, 11.91% and 9.82 lm Wꢀ1, respec-
tively, (Table 3 and Fig. S4 in the ESI†). However, compared with
the orange-emitting analogs, the green-emitting PHOLEDs
show noticeably poorer EL abilities. For the optimized device
G3, it exhibits peak EL performance with an Lmax of 8305 cd mꢀ2
at 15.4 V, a hL of 13.25 cd Aꢀ1, a hext of 5.07% and a hP of 5.19 lm
Wꢀ1. The proper explanation for the difference between the two
series of PHOLEDs can be as follows: clearly, the energy-transfer
from the organic segments (host) to the phosphorescent units
(guest) is much more efficient in the EL process associated with
devices O1–O4 compared with that involved in devices G1–G4,
indicated by the obvious EL band in the short wavelength region
for the EL spectra of devices G1–G4 (Fig. S2 in the ESI†). The EL
band from the organic segments will indicate the formation of
the nonemissive triplet excitons, which may give no contribu-
tion to the EL of the devices G1–G4 and will lower their EL
efficiencies accordingly. Furthermore, the low-temperature PL
spectrum for P-Org has indicated that the triplet energy-level for
the organic segment is ca. 2.23 eV, which is lower than the
triplet energy-level of ca. 2.33 eV for the green phosphorescent
units in P-G-1–P-G-4 (Fig. S5†). It means that there will be
Fig. 7 The J–V–L curves for the optimized devices O3 and G3.
transferred from the organic segments in the polymers.
Importantly, the undesired EL band vanishes in devices O3 and
O4 made from polymers (P-O-3 and P-O-4, respectively) with the
higher level of the phosphorescent units, showing the complete
energy-transfer from the host (organic segments) to the guests
(phosphorescent units). Differently, for devices G1–G4, there is
an obvious EL band from organic segments even for the device
G4 made from the polymer P-G-4, a high level of the phospho-
rescent units besides the green EL band from iridium complex
moieties (Fig. S2 in the ESI†). With respect to that of devices
O1–O4, devices G1–G4 show the much inefficient host–guest
energy-transfer process, which can be ascribed to the resonance
energy-transfer between S10 and S1G states as shown before
(Fig. 4b).
Table 3 The EL performance of the PHOLEDs
Luminance Lmaxa (cd mꢀ2
)
hext (%)
hL (cd Aꢀ1
)
hp (lm Wꢀ1
)
lmax (nm)
d
Device
Polymers
Vturn-on (V)
O1
P-O-1
5.1
12 695 (13.9)
8.10 (8.1)a
6.74b
19.35 (7.8)
16.17
9.94
7.82 (7.8)
6.27
2.89
572
(0.52, 0.44)
4.15c
O2
O3
O4
G1
G2
G3
G4
P-O-2
P-O-3
P-O-4
P-G-1
P-G-2
P-G-3
P-G-4
5.7
6.7
5.4
7.0
6.4
5.7
7.4
18 148 (16.9)
30 532 (18.4)
13 812 (16.6)
2516 (19.1)
8824 (15.2)
8305 (15.4)
7184 (18.5)
11.91 (8.8)
11.27
5.45
12.93 (10.6)
12.68
9.71
9.52 (8.1)
8.75
4.50
1.36 (8.6)
0.95
0.61
1.29 (11.5)
1.13
1.27
5.07 (8.6)
4.95
2.49
27.46 (8.8)
26.03
12.05
30.54 (10.6)
30.06
23.0
23.11 (8.1)
21.22
10.83
2.68 (8.6)
1.87
1.20
3.40 (11.5)
2.98
3.37
13.25 (8.6)
12.88
6.50
9.82 (8.8)
8.64
2.95
10.12 (8.6)
8.34
5.03
9.79 (7.1)
7.05
2.65
1.04 (6.3)
0.41
0.23
1.86 (9.5)
1.88
1.74
5.19 (7.6)
4.35
1.70
572
(0.53, 0.44)
576
(0.54, 0.44)
572
(0.54, 0.44)
488, 532
(0.32, 0.41)
488, 536
(0.38, 0.49)
488, 536
(0.44, 0.50)
3.97 (8.0)
2.72
8.98 (8.0)
6.16
3.56 (7.7)
1.61
488, 536
(0.44, 0.51)
1.87
4.24
0.88
a
c
Maximum values of the devices. Values in parentheses are the voltage at which they were obtained. b Values were collected at 20 mA cmꢀ2
.
Values
collected at 100 cd mꢀ2
.
The CIE coordinates (x, y) are shown in parentheses.
d
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Journal of Materials Chemistry C
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undesired back energy-transfer from the emissive triplet states
of the green phosphorescent units to the nonemissive triplet
states of the organic segments in the EL process of the devices
G1–G4, which will denitely disfavor the EL performance of the
concerned devices. So, taking all of these into consideration, it
is will be easy to interpret the inferior EL performance of devices
G1–G4. Despite these disadvantages, device G3 can still show
decent EL efficiencies for polymer PHOLEDs, which might be
ascribed to both the ambipolar feature and functionalizing of
the phosphorescent units of the polymers. However, the situa-
tion in the devices O1–O4 should be different. Only a very weak
EL band from organic segments can be observed in the devices
O1 and O2, indicating much less nonemissive triplet excitons of
the organic segments formed in the devices O1–O4. Critically,
the energy-level for these undesired triplet excitons (ca. 2.23 eV)
is higher than that of the orange phosphorescent units in
P-O-1–P-O-4 (ca. 2.15 eV), which are derived from their low
temperature PL spectra (Fig. S5†). Hence, despite that these
nonemissive triplet excitons are formed, they will be harnessed
easily by the emissive triplet excitons of the orange phospho-
rescent units in P-O-1–P-O-4 through cascade energy-transfer to
contribute the EL of the devices O1–O4. As a result, devices
O1–O4 can outperform their analogous devices G1–G4 (Table 3).
Generally, most of the phosphorescent polymers show rela-
tively low EL efficiency with hL less than 10 cd Aꢀ1. Encourag-
ingly, some advanced phosphorescent polymers can also bring
high EL efficiencies. Some functionalized blue-emitting phos-
phorescent polymers with a phosphine oxide ether backbone
can show a hL of 19.4 cd Aꢀ1
.
The phosphorescent polymers
27
Fig. 8 The dependence of EL efficiencies on the current density for (a)
device O3 and (b) device G3.
with nonconjugated backbones attached with blue-emitting IrIII
dendrons can achieve a hL of 28.1 cd Aꢀ1
.
Attaching green-
17
emitting IrIII complexes to the carbazole-based backbone can
also bring highly efficient phosphorescent polymers which can
be employed to make PHOLEDs with an excellent hL of 33.9 cd
conguration associated with both uorene-based silane units and
phosphorescent moieties, which will afford twisted backbones to
the concerned polymers. The twisted polymer backbones will make
the phosphorescent moieties keep away from each other to relieve
the T–T annihilation effect. All these encouraging EL data have
obviously shown the importance of the involved molecular design
conception, which will construct a new platform for the design and
synthesis of high-performance phosphorescent polymers.
Aꢀ1 15 Novel polyuorene-based phosphorescent polymers have
.
also been developed with a red-emitting OsII complex as the
phosphorescent unit, triphenylamine as the pendent hole
transporting group, and oxadiazole as the electron transporting
pendent moiety. These highly efficient red phosphorescent
polymers can achieve a high hL of 26.3 cd Aꢀ1 21 Besides all these
.
high performance phosphorescent polymers, the carbazole-
based silane moiety has also been employed to construct a
polymer backbone graed with a blue-emitting IrIII complex
Conclusions
(FIrpic).19 The concerned phosphorescent polymers can furnish Novel phosphorescent polymers have been successfully devel-
a hL of 2.3 cd Aꢀ1. To compare with these attractive EL effi- oped by Suzuki cross-coupling among the functionalized mono-
ciencies reported, the optimized device O3 made from P-O-3 can mers with the aim to deal with the drawbacks associated with the
achieve a competitive hL of 30.54 cd Aꢀ1. Even for the device G3 traditional analogues. Both functionalized phosphorescent IrIII
bothered by the undesired back energy-transfer, it can also ppy-type complex units and ambipolar blocks together with
maintain a maximum hL of 13.25 cd Aꢀ1, representing the state- ourene-based silane moieties have been employed to optimize
of-the-art EL efficiencies ever achieved by phosphorescent the opt-electronic characters of the corresponding polymers. The
polymers. These EL data have clearly indicated the great great potential of this strategy has been indicated by the high EL
potential of these novel phosphorescent polymers in PHOLEDs. efficiencies of the OLEDs made from these phosphorescent
The EL efficiency–current density curves for the PHOLEDs are polymers, showing a maximum current efficiency (hL) of 30.54 cd
given in Fig. 8 and S4.† From these gures, it can clearly be seen Aꢀ1, an external quantum efficiency (hext) of 12.93% and a power
that the undesired EL efficiency roll-off effect is unserious for the efficiency (hP) of 10.12 lm Wꢀ1. Clearly, the results will reveal
concerned PHOLEDs, indicating very slight T–T annihilation important information for the design and synthesis of new highly
involved. This result might be ascribed to the contorted efficient phosphorescent polymers.
9534 | J. Mater. Chem. C, 2014, 2, 9523–9535
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Paper
Journal of Materials Chemistry C
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Acknowledgements
This work was nancially supported by Tengfei Project from
Xi'an Jiaotong University, the Fundamental Research Funds for
the Central Universities, the Program for New Century Excellent
Talents in University, theMinistry of Education of China (NECT-
09-0651), the Key Creative Scientic Research Team in Shaanxi
Province (2013KCT-05), the China Postdoctoral Science Foun-
dation (Grant no. 20130201110034), and the National Natural
Science Foundation of China (no. 20902072).
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