Novel light-emitting electrophosphorescent copolymers based on carbazole with an Ir complex on the backbone

Article abstract of DOI:10.1039/b618990c

Novel light-emitting phosphorescent polymers based on 3,6-carbazole with an iridium complex on the backbone were synthesized by Suzuki polycondensation of A-A and B-B type monomers. Three difluoro-substituted phenylpyridine iridium complexes with differen

Full text of DOI:10.1039/b618990c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel light-emitting electrophosphorescent copolymers based on carbazole
with an Ir complex on the backbone
Hongyu Zhen,ab Jie Luo,a Wei Yang,*a Qiliang Chen,a Lei Ying,a Zou Jianhua,a Hongbin Wua and
Yong Caoa
Received 2nd January 2007, Accepted 18th April 2007
First published as an Advance Article on the web 30th April 2007
DOI: 10.1039/b618990c
Novel light-emitting phosphorescent polymers based on 3,6-carbazole with an iridium
complex on the backbone were synthesized by Suzuki polycondensation of A-A and B-B
type monomers. Three difluoro-substituted phenylpyridine iridium complexes with different
ancillary ligands (FIrptz, FIrpic and FIrtmd) were introduced into the poly(3,6-carbazole)
(PCz) backbone, causing distinct differences in the photo- and electroluminescence properties.
The PL efficiency of PCz-FIrtmd5 is as high as 62% compared with the value of 9% of the
fluorene-alt-carbazole-based copolymer (PFCz-FIrtmd5), which indicated that the 3,6-carbazole
unit was suitable to serve as the host for a green electrophosphorescent polymer. The
ancillary ligand of bicycloiridium complexes also affects the device performance based on
chelating copolymers. The best device performances were obtained from copolymer PCz-FIrtmd5
with the maximal luminous efficiency of 4.4 cd A21 and a luminance of 453 mA cm22 at
a current density of 10.3 mA cm22 with the configuration: ITO/PEDOT/polymer +
PBD(30 wt%)/Ba/Al.
1.59% ph el21 with the peak emission at 610 nm was achieved.
Introduction
A device consisting of a fluorene-alt-carbazole copolymer
Phosphorescent organic light-emitting diodes (PHOLED)s
grafted with an Ir complex shows the highest external quantum
efficiency of 4.9% ph el21 and a peak emission at 610 nm.10
composed of phosphorescent iridium complexes have been
attracting much attention because of their relatively short
phosphorescent lifetime and high luminous efficiency.1–4
Recently, phosphorescent conjugated polymers that possess
M–C or p-bonds in the backbone have attracted substantial
Phosphorescent dye-doped polymer light-emitting devices
attention. A series of yellow and red conjugated well-defined
are extremely attractive due to their ease of fabricated by
oligo- and polyfluorenyl bis-cyclometalated Ir complexes
printing techniques and the potential applications in large-area
was synthesized by Sandee et al.with the maximal external
flat panel displays.5,6 Phosphorescent polymers dye-doped
quantum efficiencies of 0.12% ph el21 and 1.5% ph el21
chemically into the polymer chain can decrease phase
respectively.11 In our early work,12,13 yellow and red electro-
segregation, which leads to fast decay of efficiency with
phosphorescent copolymers with Ir complexes on the back-
increasing current density. Lee et al.7 synthesized a non-
bone of poly(fluorene-alt-carbazole) (PFCz) and polyfluorene
conjugated polyethylene main chain with phenylpyridine
(PFO) were synthesized by Suzuki polycondensation to
attached to the side chains as a ligand and with pendant
achieve maximal external quantum efficiencies of 4.6% and
N-vinylcarbazole as a host unit. A high external quantum
efficiency of 4.4% ph el21 (photons per electron) was achieved
7.0% ph el21 respectively. In this case, high-efficiency energy
transfer from the polymer host to the Ir complex in the
at a current density of 6.4 mA cm22. A similar approach of
polymer main chain via efficient intramolecular energy
using a non-conjugated main chain with a pendant diketone
transfer would be expected. Of the phosphorescent polymers,
was reported by Tokito et al.8 High external quantum
the reported blue and green light-emitting polymers are
efficiencies of 5.5%, 9% and 3.5% in red, green and blue
very rare especially for main chain-type conjugated polymers,
PLEDs respectively were achieved. Phosphorescent conjugated
as the complex in conjugation with a neighboring conjugated
polymers based on a polyfluorene backbone with a diketone
segment in the polymer backbone makes the luminous
pendant attached to the 9-carbon position of fluorene were
wavelength of copolymers red-shift. Iridium complexes with
reported by Chen et al.9 An external quantum efficiency of
fluoro-substituted phenylpyridine as a ligand were doped into
organic small molecules for greenish blue OLEDs.14–17
In this paper, 3,6-carbazole with a wide band-gap was
aInstitute of Polymer Optoelectronic Materials and Devices, Key Lab of
used as a host unit and three different ancillary ligand fluoro-
substituted phenylpyridine Ir complexes (FIrptz, FIrpic and
FIrtmd) were introduced into the polymer backbone by
bromo-substituted Ir complex monomers at the 5-position of
pyridine in the ligand.
Specially Functional Materials, Ministry of Education, South China
University of Technology, Guangzhou, 510640, China.
E-mail: pswyang@scut.edu.cn; Fax: +86-20-87110606;
Tel: +86-20-87114346
bState Key Lab of Modern Optical Instrumentation, Zhejiang
University, Hangzhou, 310027, China
2824 | J. Mater. Chem., 2007, 17, 2824–2831
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obtained product was purified by column chromatography
(silica gel) (yield: 63%). 1H NMR (300 MHz; CDCl3) d (ppm):
8.82 (d, J = 2.07 Hz, 1H), 8.40 (d, J = 7.98 Hz, 1H), 8.20–8.12
(m, 2H), 8.02 (td, J = 7.73 Hz and 1.32 Hz, 1H), 7.90 (d, J =
8.79 Hz, 2H), 7.75 (d, J = 4.92 Hz, 1H), 7.49 (t, J = 5.85 Hz,
1H), 7.33 (d, J = 2.04 Hz, 1H), 6.55–6.37 (m, 2H), 5.82 (dd, J =
8.46 Hz and 2.22 Hz, 1H), 5.56 (dd, J = 8.37 Hz and 2.19 Hz,
1H). Elemental anal. Calcd. for C28H14Br2F4IrN3O2 (%): C,
39.45; H, 1.66; N, 4.93. Found (%): C, 39.62; H, 1.69; N, 4.78.
EIMS: m/z 853 (M + 1)+.
Experimental
Materials
All manipulations involving air-sensitive reagents were per-
formed in an atmosphere of dry argon. The solvents (THF,
toluene) were purified by routine procedures and distilled
in dry argon before being used. All reagents, unless otherwise
specified, were obtained from Aldrich, Acros and TCI
Chemical Co. and used as received.
Preparation of the monomers
Iridium(III) bis[5-bromo-2-(2,4-difluorophenyl)pyridine]-
2,2,6,6-tetramethyl-3,5-heptanedione (BrFIrtmd) (5). 5 was
synthesized according to the published procedure.19 1H
NMR (300 MHz; CDCl3) d (ppm): 8.39 (d, J = 2.22 Hz,
2H), 8.10 (dd, J = 8.88 Hz and 2.28 Hz, 2H), 7.88 (dd, J =
8.79 Hz and 2.13 Hz, 2H), 6.37 (td, J = 10.97 Hz and 2.31 Hz,
2H), 5.76 (dd, J = 8.67 Hz and 2.31 Hz, 2H), 5.59 (s, 1H), 0.95
(s, 18H). Elemental anal. Calcd C33H29Br2F4IrN2O2 (%): C,
43.38; H, 3.20; N, 3.07. Found (%): C, 43.52; H, 3.11; N, 2.97.
EIMS: m/z 914 (M + 1)+.
5-Bromo-2-(2,4-difluorophenyl)pyridine (FppyBr) (1). 1 was
synthesized by the following procedure: 2,5-dibromopyridine
(10 mmol), 2,4-difluorophenylboronic acid (10 mmol) and
tetrakis(triphenylphosphine)palladium (0.1 mmol) were dis-
solved in toluene (15 ml) and ethanol (5 ml). Then an aqueous
solution of 2 M Na2CO3 (6 ml) was added to the mixture. The
resulting mixture was stirred at 100 uC for 24 h. The reaction
mixture was concentrated by evaporation of solvents and the
residue was dissolved in dichloromethane, washed with water
and dried under anhydrous sodium carbonate. After the
evaporation of solvent, the obtained product was purified by
column chromatography (silica gel) (yield: 75%). 1H NMR
(300 MHz; CDCl3) d (ppm): 8.76 (d, J = 2.28 Hz, 1H), 8.05–
7.97 (m, 1H), 7.88 (dd, J = 8.49 Hz and 2.40 Hz, 1H), 7.67
(dd, J = 8.49 Hz and 1.92 Hz, 1H), 7.01 (td, J = 8.30 Hz
and 2.49 Hz, 1H), 6.93 (td, J = 10.03 Hz and 2.49 Hz, 1H).
Elemental anal. Calcd for C11H6BrF2N (%): C, 48.97;
H,2.24; N, 5.19. Found (%): C, 49.03; H, 2.12; N, 5.04. GC-
MS (270, M+)
Iridium(III) bis[5-bromo-2-(2,4-difluorophenyl)pyridine]-
5-methyl-3-(pyridin-2-yl)-1,2,4-triazole (BrFIrptz) (6).
(FppyBr)4Ir2Cl2 (3) (0.078 mmol), Hptz (2) (0.2 mmol), and
100 mg of sodium carbonate were refluxed under inert gas
atmosphere in 2-ethoxyethanol for 16 h. After cooling to room
temperature, the colored precipitate was filtered off and was
washed with water, followed by 2 portions of ether and
hexane. The crude product was flash chromatographed using a
silica (dichloromethane : acetone = 1 : 1) column followed by
recrystallization with methanol to obtain the green powder
5-Methyl-3-(pyridin-2-yl)-4H-1,2,4-triazole (Hptz) (2)18
.
1
product after solvent evaporation and drying (yield: 67%). H
2-Cyanopyridine (50 mmol) and hydrazine monohydrate
(50 mmol) was mixed and 20 ml ethanol was added to obtain
a clear solution. After standing overnight at the room
temperature, the almost colorless crystals of 2-pyridine
carboxamidrazone could be filtered off. The product was
washed with diethyl ether and dried in air. The obtained solid
was added to a mixture of 30 ml acetic acid and acetic
anhydride (1 : 1) at 0 uC and the solution was stirred at
room temperature for 2 h. The solution was then concentrated
under vacuum and purified by repeated crystallization
from diisopropyl ether. Yield: 60%. 1H NMR (300 MHz;
CDCl3) d (ppm): 12.91 (s, 1H), 8.76 (d, J = 4.26 Hz, 1H), 8.20
(d, J = 7.92 Hz, 1H), 7.86 (td, J = 7.73 Hz and 1.65 Hz, 1H),
7.39 (t, J = 6.23 Hz, 1H), 2.54(s, 3H). Elemental anal. Calcd
for C8H8N4 (%): C, 60.00; H,5.03; N, 34.98. Found (%): C,
60.21; H, 4.89; N, 34.86. GC-MS (160, M+)
NMR (300 MHz; DMSO-d6) d (ppm): 8.23–8.12 (m, 4H), 8.08
(d, J = 4.26 Hz, 2H), 7.67 (d, J = 5.49 Hz, 1H), 7.48 (d,
J = 2.01 Hz, 1H), 7.43–7.38 (m, 2H), 6.95–6.80 (m, 2H), 5.81
(dd, J = 8.16 Hz and J = 2.31 Hz, 1H), 5.61 (dd, J= 8.64 Hz
and J = 2.34 Hz, 1H), 2.24 (s, 3H). Elemental anal. Calcd for
C30H17Br2F4IrN6 (%): C, 40.49; H,1.92; N, 9.45. Found (%):
C, 40.63; H, 1.80; N, 9.37. EIMS: m/z 890 (M + 1)+.
3,6-Dibromo-9-ethylhexylcarbazole (7), 3,6-bis(4,4,5,5-tetra-
(8)20
methyl-1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole
and
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (9)21,22 were prepared according to the pub-
lished methods.
Preparation of the polymers
General procedures of Suzuki polycondensation, taking PCz-
FIrpic1 as an example. 3,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9-ethylhexylcarbazole (265.5 mg, 0.5 mmol),
3,6-dibromo-9-ethylhexylcarbazole (214 mg, 0.49 mmol),
BrIrpic (6.6 mg, 0.01 mmol) and palladium(II) acetate (2 mg)
were dissolved in a mixed solution of toluene–THF (1 : 1,
10 ml), stirred for 0.5 h, and then Et4NOH (20%) aqueous
solution (4 ml) was added. The mixture was heated to
100 uC and stirred for 42 h under argon atmosphere. Then
the polymer was capped by adding 3-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (45 mg) by
Chloride-bridged dimer complex (FppyBr)4Ir2Cl2 (3) was
synthesized by the procedure reported.19
Iridium(III) bis[5-bromo-2-(2,4-difluorophenyl)pyridine]picoli-
nate (BrFIrpic) (4). (FppyBr)4Ir2Cl2 (3) (0.2 mmol) and
picolinic acid (0.5 mmol) were refluxed under an inert gas
atmosphere in dichloromethane for 24 h. After cooling to
room temperature, the reaction mixture was dissolved in
dichloromethane, washed with water and dried under anhy-
drous sodium carbonate. After the evaporation of solvent, the
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continuous stirring for 12 h, and then bromobenzene (0.25 ml)
followed by continuously reacting for another 12 h. The whole
mixture was poured into methanol. The precipitated polymer
was recovered by filtration and purified by silica column
chromatography with toluene to removethe small molecular
complex and catalyst residue (yield: 50%). Elemental anal.
Calcd. (%): C, 85.80; H, 8.16; N, 5.08. Found (%): C, 86.51; H,
7.84; N, 4.89.
Measurements
1H NMR spectra were recorded on a Bruker 300 MHz
Spectrometer operating at 300 MHz in deuterated chloroform
(CDCl3) or dimethylsulfoxide-d6 (DMSO-d6) solution with
tetramethylsilane as a reference. Mass spectra (MS) of the
ligands were obtained by a CE Instruments Trace 2000 Series
GC/MS, and the MS of the Ir complexes were recorded on a
LCQ DECA XP Liquid Chromatograph–Mass Spectrometer
(Thermo Group). The molecular weight of the polymers was
determined by using a Waters GPC 2410 in tetrahydrofuran
(THF). Number-average (Mn) and weight-average (Mw)
molecular weights were estimated by using a calibration curve
of polystyrene standards. Elemental analyses were performed
on a Vario EL Elemental Analysis Instrument (Elementar
Co.). The iridium content analyses were determined by using a
Philips (Magix PRO) sequential X-ray fluorescence spectro-
metry (XRF), with a rhodium tube operated at 60 kV
and 50 mA, a LiF 200 crystal and a scintillation counter.
Tris(acetylacetonate)iridium(III) (Ir content of 38%, from
Alfa Aesar Co.) was used as an internal reference specimen.
Samples and specimens were pressed into homogeneous tablets
(W = 30 mm) of compressed (375 MPa) powder of the
copolymers. UV-vis absorption spectra were recorded on a HP
8453 spectrophotometer. The PL quantum yields were
determined in an Integrating Sphere IS080 (LabSphere) with
325 nm excitation of a HeCd laser (Melles Griot). PL spectra of
the copolymer were obtained on a Fluorolog-3 Spectrometer
(Jobin-Yvon) with 90u angle detection.
PCz-FIrpic3. Monomer feed ratio: 3,6-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (0.5 mmol), 3,6-
dibromo-9-ethylhexylcarbazole (0.47 mmol) and BrIrpic
(0.03 mmol) (yield: 50%). Elemental anal. Calcd. (%): C, 84.19;
H,7.88; N, 5.14. Found (%): C, 85.07; H, 7.12; N, 4.97.
PCz-FIrpic5. Monomer feed ratio: 3,6-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (0.5 mmol), 3,6-
dibromo-9-ethylhexylcarbazole (0.45 mmol) and BrIrpic
(0.05 mmol) (yield: 46%). 1H NMR (300 MHz, CDCl3) d
(ppm): 8.92 (s, Ar–H), 8.56 (s, 2H), 7.96 (s, Ar–H), 7.91 (d,
2H), 7.76–7.62 (m, Ar–H), 7.54–7.36 (m, 2H), 4.29–4.08 (m,
2H), 2.10 (m, 1H), 1.50–1.18 (m, 8H), 0.96–0.78 (m, 6H).
Elemental anal. Calcd. (%): C, 82.66; H,7.61; N, 5.20.
Found (%): C, 83.51; H, 7.34; N, 4.84.
PCz-FIrtmd3. Monomer feed ratio: 3,6-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (0.5 mmol), 3,6-
dibromo-9-ethylhexylcarbazole (0.47 mmol) and BrIrtmd
(0.03 mmol) (yield: 50%). Elemental anal. Calcd. (%): C,
82.66; H,7.83; N, 4.87. Found (%): C, 83.47; H, 7.54; N,
4.31.
LED fabrication and characterization
Copolymers were dissolved in p-xylene and filtered with a
0.45 mm filter. Patterned ITO coated glass substrates were
cleaned with acetone, detergent, distilled water and isopropanol,
subsequently in an ultrasonic bath. After treatment with oxygen
plasma, 150 nm of poly(3,4-ethylenedioxythiophene) (PEDOT)
doped with poly(styrenesulfonic acid) (PSS) (Batron-P 4083,
Bayer AG) was spin-coated onto the ITO substrate followed
by drying in a vacuum oven at 80 uC for 8 h. A thin film of
copolymer was coated onto the anode by spin casting inside a
dry box. The film thickness of the active layers was around 75–
80 nm, measured with an Alfa Step 500 surface profiler
(Tencor). A thin layer of Ba (4–5 nm) and subsequently a
200 nm layer of Al were vacuum-evaporated on the top of an EL
polymer layer in a vacuum of 1 6 1024 Pa. Device perfor-
mances were measured inside a dry box. Current–voltage (I–V)
characteristics were recorded with a Keithley 236 source meter.
EL spectra were collected by a PR 705 photometer (Photo
Research). Luminance was measured by a calibrated silicon
diode and calibrated by a PR 705 photometer. The external
quantum efficiencies were determined by a Si photodiode with
calibration in an integrating sphere (IS080, Labsphere).
PCz-FIrtmd5. Monomer feed ratio: 3,6-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (0.5 mmol), 3,6-
dibromo-9-ethylhexylcarbazole (0.45 mmol) and BrIrtmd
(0.05 mmol) (yield: 50%). 1H NMR (300 MHz, CDCl3) d
(ppm): 8.89 (s, Ar–H), 8.54 (s, 2H), 7.93 (s, Ar–H), 7.89 (d,
2H), 7.78–7.65 (m, Ar–H), 7.52–7.41 (m, 2H), 4.30–4.06 (m,
2H), 2.08 (m, 1H), 1.49–1.28 (m, 8H), 1.18 (s, tmd-CH3),
0.92–0.75 (m, 6H). Elemental anal. Calcd. (%): C, 80.28;
H,7.55; N, 4.76. Found (%): C, 81.06; H, 7.14; N, 4.35.
PFCz-FIrtmd5. Monomer feed ratio: 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (341 mg,
0.5 mmol), 3,6-dibromo-9-ethylhexylcarbazole (0.45 mmol)
1
and BrIrtmd (0.05 mmol) (yield: 50%). H NMR (300 MHz,
CDCl3) d (ppm): 8.74 (s, Ar–H), 8.52 (s, 2H), 7.85–7.72 (m,
6H), 7.53 (d, 2H), 7.24 (d, 2H), 4.28–4.05 (m, 2H), 2.16 (m,
1H), 1.95 (m, 4H), 1.58–1.03 (m, 32H), 0.93–0.74 (m, 12H).
Elemental anal. Calcd. (%): C, 82.84; H,8.73; N, 2.11. Found
(%): C, 83.21; H, 8.27; N, 1.86.
PCz-FIrptz3. Monomer feed ratio: 3,6-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9-ethylhexylcarbazole (0.5 mmol), 3,6-
dibromo-9-ethylhexylcarbazole (0.47 mmol) and BrIrptz
(0.03 mmol) (yield: 50%). 1H NMR (300 MHz, CDCl3) d
(ppm): 8.92 (s, Ar–H), 8.57 (s, 2H), 8.14 (m, Ar–H), 7.91 (d,
2H), 7.76–7.62 (m, Ar–H), 7.49 (m, 2H), 4.25 (m, 2H), 2.11
(m, 2H), 1.52–1.22 (m, 8H), 0.97–0.78 (m, 6H).
Results and discussion
Synthesis and characterization
The synthetic routes to the monomers are depicted in Scheme 1.
5-Bromo-2-(2,4-difluorophenyl)pyridine was synthesized via
a
Suzuki coupling reaction of 2,5-dibromopyridine and
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Scheme 1 Synthetic route to monomers.
2,4-difluorophenylboronic acid in high yield. The iridium
complexes were synthesized as chloride-bridged iridium dimers
with picolinate (Hpic), 2,2,6,6-tetramethyl-3,5-heptanedione
(Htmd) and 5-methyl-3-(pyridin-2-yl)-4H-1,2,4-triazole (Hptz)
ligands. The copolymers with different complexes were pre-
pared from 3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-9-ethylhexylcarbazole, 3,6-dibromo-9-ethylhexylcarbazole
and the complexes by Suzuki polycondensation (SPC;
Scheme 2). The feed ratios of complexes in the polycondensa-
tion were 1 mol%, 3 mol% and 5 mol%, and the corresponding
copolymers were named PCz-FIrpic1, PCz-FIrpic3, PCz-
FIrpic5, PCz-FIrtmd3, PCz-FIrtmd5 and PCz-FIrptz3, respec-
tively. In order to investigate the effect of the host segment on
the luminous performances of the copolymers, the copolymer
Scheme 2 Synthetic route to copolymers.
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Table 1 Molecular weights and compositions of the polymers
Complex content (mol%)
Polymer
Mn (6103) PDI in feed ratio in copolymera
PCz-FIrpic1
PCz-FIrpic3
PCz-FIrpic5
PCz-FIrptz3
PCz-FIrtmd3
PCz-FIrtmd5
8.17
8.01
5.06
6.01
4.53
6.07
1.60
1.52
2.19
1.31
1.81
1.44
1.67
1
3
5
3
3
5
5
0.4
1.3
2.6
2.7
1.6
3.2
3.5
PFCz-FIrtmd5 11.20
a
Calculated from the contents of carbon, hydrogen, nitrogen and
iridium complex in copolymers.
PFCz-FIrtmd5 with a fluorene-alt-carbazole unit as a host was
prepared to make a comparison. The iridium contents in the
copolymers were estimated by X-ray fluorescence spectrometry
(XRF), and the molar ratios of Ir complexes incorporated into
the copolymers were calculated by combining XRF data with
the elemental analyses of carbon, hydrogen and nitrogen
(Table 1). The results indicate that the actual Ir complex
contents in the copolymers are substantially lower than the
feed ratios of complex monomers, and increase with the
increasing feed ratios. Meanwhile, the Ir complex contents in
these copolymers are also different even at the same feed ratios
of complex monomers. The FIrptz complex content is
2.7 mol% in the PCz-FIrptz3 polymer, while the FIrtmd
complex content is 1.6 mol% in the PCz-FIrtmd3 polymer at
the same feed ratio of 3 mol%, which implies that BrFIrptz
complex monomer has a higher reaction activity compared
with BrFIrtmd.
Fig. 1 UV-visible absorption and PL spectra of the complexes in
CH2Cl2 solution (0.01 M).
PCz-FIrpic3 and PCz-FIrpic5 are red-shifted by 20 nm, and
the new absorption peak at 420 nm is due to the triplet metal-
to-ligand charge-transfer (3MLCT) transition of the complex
unit. In Fig. 2b, the spectra of PCz-FIrtmd3 and PCz-FIrtmd5
are similar and the spectrum of PFCz-FIrtmd5 is red-shifted
owing to the longer conjugated length of the backbone. In
Fig. 2c, there is an obvious absorption peak at 420 nm due to
The number-average molecule weights (Mn) of carbazole-
based copolymers are less than 10 000 with the polydispersity
index (PDI) from 1.31 to 2.19 (Table 1). These results are
comparable with those of the other copolymers synthesized by
Suzuki polycondensation between diborate and two different
kinds of dibrominated monomers.12,13
Photophysical properties
UV-vis absorption and PL spectra of the Ir complexes
BrFIrpic, BrFIrtmd and BrFIrptz in dichloromethane solution
are shown in Fig. 1. The intense absorption peaks below
1
300 nm are attributed to the spin-allowed singlet state (p–p*)
transition of cyclometalated ligands and the weak absorption
peaks around 320 nm can be assigned to the spin-allowed
singlet metal-to-ligand charge-transfer (1MLCT) transition.23
The absorption peaks around 390–400 nm are due to the
triplet metal-to-ligand charge-transfer (3MLCT) transition for
the three complexes. In the PL spectra, the maximum
wavelengths of BrFIrpic, BrFIrptz and BrFIrtmd are at
480 nm, 475 nm and 500 nm respectively. The UV-vis
absorption and PL spectra are almost red-shifted by 10 nm
compared with the pristine complexes without bromo-sub-
stitution in the ligands.14–16
Fig. 2a shows the UV-vis absorption spectra of the
copolymers PCz-FIrpic. The absorption spectrum of PCz-
FIrpic1 is almost as the same as that of PCz, with two peaks
at 260 nm and 310 nm,23 owing to the low content of
the complex. Compared with PCz-FIrpic1, the spectra of
Fig. 2 UV-visible absorption spectra of the copolymers in film form.
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Fig. 3 PL spectra of the copolymers in film form.
Fig. 4 EL spectra of the copolymers.
the 3MLCT transition of the complex unit in PCz-FIrptz3. All
absorptions of the complexes units in copolymers are red-
shifted, which indicates that the complexes have been
introduced into the conjugated polymer backbone. PL spectra
of the copolymers are shown in Fig. 3. The energy transfer
from the host segment to the Ir complex is efficient for Ir
complex content as low as 1 mol% (Fig. 3a). The PL spectrum
of PFCz-FIrtmd5 is red-shifted by 30 nm (Fig. 3b), because the
extended ligand of the Ir complex unit in the PFCz-based
copolymer has a longer conjugation length.12,13 The optical
properties of the polymers are listed in Table 2. The PL
efficiency of PCz-FIrtmd5 is as high as 62%, whereas that of
PFCz-FIrtmd5 is as low as 9%, which indicates that 3,6-
carbazole is suitable for a host unit as a greenish blue Ir
complex guest.
Table 2 Optical properties of the polymers
Photoluminescence
Polymer
labs max/nm
lPL/nm
QPL (%)
PCz-FIrpic1
PCz-FIrpic3
PCz-FIrpic5
PCz-FIrptz3
PCz-FIrtmd3
PCz-FIrtmd5
PFCz-FIrtmd5
260, 310
532, 574
532, 574
532, 574
531, 569
531, 567
531, 568
560, 601
26
20
25
27
34
62
9
270, 320, 420
270, 320, 420
260, 330, 420
260, 310, 430
260, 310, 430
350, 450
Fig. 5 CIE chromaticity diagram of the devices fabricated from the
copolymers.
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Table 3 Device performances of the polymers
Maximum efficiency
Polymer
Bias/V
J/mA cm22
L/cd m22
QEext (%)
LE/cd A21
CIEb coordinates (x, y)
PCz-FIrpic1
PCz-FIrpic3
PCz-FIrpic5
PCz-FIrptz3
PCz-FIrtmd3
PCz-FIrtmd5
PFCz-FIrtmd5
a
9.8
9.0
7.8
8.8
6.5
7.8
7.5
13.5
11.0
38.5
10.5
36.9
10.3
33.1
106
69
0.40
0.32
0.71
0.58
0.97
2.22
0.63
0.79
0.63
1.43
1.14
1.56
4.40
0.72
0.37, 0.60
0.36, 0.61
0.36, 0.61
0.38, 0.59
0.37, 0.60
0.37, 0.60
0.48, 0.52
552
119
575
453
237
b
Device structure: ITO/PEDOT/polymer + PBD(30 wt%)/Ba/Al. Obtained at the current density of 12 mA cm22
.
Electrophosphorescent properties
The single-layer devices were fabricated in the configuration:
ITO/PEDOT/polymer + PBD(30 wt%)/Ba/Al. EL spectra of
the copolymers PCz-FIrpic are shown in Fig. 4a, which shows
that the spectra of the copolymers are identical and the energy
transfer from the host segments to the Ir complex is efficient.
The EL spectra of copolymers PCz-FIrtmd5 and PFCz-
FIrtmd5 are shown in Fig. 4b; as in the PL emission, the EL
spectrum of PFCz-FIrtmd5 is also red-shifted by 30 nm.
Typical CIE coordinates of the devices based on PCz-FIrpic3
(0.36, 0.61), PCz-FIrptz3 (0.38, 0.59) and PCz-FIrtmd5
(0.37,0.60), along with Ir(ppy)3 (0.29, 0.61) at a current density
of 12 mA cm22, are presented in Fig. 5. Compared with
Ir(ppy)3 (yellowish green), the EL emissions of the copolymers
are in the region of yellowish green or green yellow. The device
performances of the copolymers are shown in Table 3; the best
performance was observed for the device based on PCz-
FIrtmd5, and a maximal luminous efficiency of 4.4 cd A21
with a luminance of 453 mA cm22 was achieved at a current
density of 10.3 mA cm22. Compared with PCz-FIrtmd5, the
device performances based on the PFCz-FIrtmd5 copolymer
are much poorer. For the three Ir complexes with different
ancillary ligands, the copolymers with the FIrtmd complex
have better luminous performances, which indicates that
Fig. 6 Current density vs. efficiency (a) and luminance (b) charac-
teristics of the PCz-FIrtmd5 based devices: ITO/PEDOT/polymer/Ba/
Al (1), ITO/PEDOT/PVK/polymer/Ba/Al (2) and ITO/PEDOT/poly-
mer + PBD(30 wt%)/Ba/Al (3).
the ancillary ligand also has a great effect on the device
performance of chelating electrophosphorescent polymers.
In order to investigate the characteristics of this kind of
chelating polymer, three device structures were used for
comparison: ITO/PEDOT/polymer/Ba/Al (1), ITO/PEDOT/
poly(N-vinylketone) (PVK)/polymer/Ba/Al (2) and ITO/
PEDOT/polymer + 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole (PBD) (30 wt%)/Ba/Al (3). The external quantum
efficiency and light intensity versus current density of the devices
based on PCz-FIrtmd5 are shown, as an example, in Fig. 6. It is
found that device 3 has a higher efficiency and light intensity
than the others. The device performances are poor with PVK as
a inserting layer, but the blended copolymers are better with
PBD as an emission layer, which indicates that the poor
electron-transporting capability of the copolymers is not
equivalent to the good hole-transporting capability from PCz
and PBD needs to be used for balance.24
3,6-carbazole as a host unit and greenish blue light-emitting
iridium complexes as guests. The ancillary ligand of the iridium
complex has a great effect on the device performance. The best
device performances were achieved from PCz-FIrtmd5 with the
maximal luminous efficiency of 4.4 cd A21 and a luminance
of 453 mA cm22 at a current density of 10.3 mA cm22
.
Acknowledgements
The authors are grateful to the Ministry of Science and
Technology of China (No. 2002CB613403) and the National
Natural Science Foundation of China (Nos. 20574021,
50433030 and U0634003) for their financial support.
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2830 | J. Mater. Chem., 2007, 17, 2824–2831
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