Two blue iridium complexes for efficient electroluminescence with low efficiency roll-off

Article abstract of DOI:10.1039/c5ra14837e

Two bis-cyclometalated iridium complexes ((dfpypy)₂Ir(tpip) and (dfpypy)₂Ir(Ftpip)) with fluorinated substituted bipyridine (2′,6′-difluoro-2,3′-bipyridine, dfpypy) as the main ligand and tetraphenylimidodiphosphinate derivatives (tpip and Ftpip) as the ancillary ligands were prepared, and their X-ray crystallographic, photoluminescence and electrochemical properties were investigated. The (dfpypy)₂Ir(tpip) and (dfpypy)₂Ir(Ftpip) complexes showed blue emission at 457 nm with quantum efficiency yields of 7.0% and 7.1%, respectively. Organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 10 nm)/(dfpypy)₂Ir(tpip) or (dfpypy)₂Ir(Ftpip) (8 wt%): PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) (B2 and B3) exhibit performances with the maximum current efficiency (η_c) values of 22.83 and 20.79 cd A^-1, respectively, with low efficiency roll-off. For example, at 100 cd m^-2 display brightness, the current efficiencies of devices B2 and B3 are 19.78, 13.74 cd A^-1, respectively. At 1000 cd m^-2 light brightness, these values are still 20.39 and 20.75 cd A^-1, respectively. Even at the high luminance of 5000 cd m^-2, these data also remained at 19.95 and 20.08 cd A^-1, respectively.

Full text of DOI:10.1039/c5ra14837e

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Two blue iridium complexes for efficient
electroluminescence with low efficiency roll-off†
Cite this: RSC Adv., 2015, 5, 89218
Qiu-Lei Xu, Xiao Liang, Liang Jiang, Yue Zhao and You-Xuan Zheng*
2 2
Two bis-cyclometalated iridium complexes ((dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip)) with fluorinated
0
0
0
substituted bipyridine (2 ,6 -difluoro-2,3 -bipyridine, dfpypy) as the main ligand and tetraphenyl-
imidodiphosphinate derivatives (tpip and Ftpip) as the ancillary ligands were prepared, and their X-ray
crystallographic, photoluminescence and electrochemical properties were investigated. The
2 2
(dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) complexes showed blue emission at 457 nm with quantum
efficiency yields of 7.0% and 7.1%, respectively. Organic light emitting diodes (OLEDs) with the structure
of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)
benzene, 10 nm)/(dfpypy)
diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50
nm)/LiF (1 nm)/Al (100 nm) (B2 and B3) exhibit performances with the maximum current efficiency (h
2 2
Ir(tpip) or (dfpypy) Ir(Ftpip) (8 wt%): PPO21 (3-(diphenylphosphoryl)-9-(4-
(
c
)
Received 26th July 2015
Accepted 12th October 2015
ꢀ1
ꢀ2
values of 22.83 and 20.79 cd A , respectively, with low efficiency roll-off. For example, at 100 cd m
ꢀ1
display brightness, the current efficiencies of devices B2 and B3 are 19.78, 13.74 cd A , respectively. At
DOI: 10.1039/c5ra14837e
ꢀ
2
ꢀ
1
1
000 cd m light brightness, these values are still 20.39 and 20.75 cd A , respectively. Even at the high
ꢀ2
ꢀ1
www.rsc.org/advances
luminance of 5000 cd m , these data also remained at 19.95 and 20.08 cd A , respectively.
occupied molecular orbital (HOMO) and destabilizing the
lowest unoccupied molecular orbital (LUMO) level, since the
Introduction
Phosphorescent iridium complexes have been widely used as emission energies of Ir(III) complexes are mainly dominated by
dopants in organic light-emitting diodes (OLEDs) due to their the triplet energy of the C^N chelating ligand.
high quantum efficiency, broad emission range and short
Iridium(III)bis(4,6-diuorophenyl-pyridinato)picolinate (FIrpic)
1
triplet exited state lifetime. To achieve full color display, three was widely used as sky-blue dopant at 470 nm with two uorine
primary color (blue, green and red) emitters are needed. atoms in the phenyl unit to shi the HOMO level downward for
2
However, compared with green and red complexes, efficient high triplet energy. Furthermore, several deep blue dopants
blue phosphorescent materials are still limited for stable with additional electron withdrawing substituent were reported
OLEDs. Thus, it is still necessary to develop new efficient blue such as FCNIr(pic) (iridium(III)bis((3,5-diuoro-4-cyanophenyl)-
2
phosphorescent compounds to satisfy the high demand of the pyridine)picolinate), (HFP) Ir(pic) (iridium(III)(bis(2-(2,4-diuoro-
display and lighting industry.
3-(peruoropropyl)phenyl)-4-methylpyridine))picolinate) and so
Typically, 2-phenylpyridine derivatives were widely used as on. Recently, uorinated dipyridine ligand, 2 ,6 -diuoro-2,3 -
3
0
0
0
the main ligands for blue Ir(III) emitters. It was easy to control bipyridine (dfpypy), has also attracted attention for deep blue Ir(III)
4
the emission spectra as the modication of phenyl unit with complexes. Compared with FIrpic, the (dfpypy)
Ir(pic) emission
2
electron-drawing group would lower the HOMO, while the was shied to blue region at 457 nm due to the electron deciency
modication of pyridine unit with electron-donating moiety can of pyridine ring.
improve the LUMO level of the Ir(III) complexes. Usually, there
In addition, good ancillary ligand will greatly enhance photo-
are two ways to develop blue Ir(III) complexes: (i) the adorption luminescent properties of Ir(III) complexes and benet their
of a main cyclometalated ligand with large triplet energy and (ii) OLEDs performances. Our group reported Htpip (tetraphenyli-
the inducement of a larger band gap by stabilizing the highest midodiphosphinate acid) derivatives as ancillary ligands, which
have two diphenyl phosphoryl (Ph P]O) groups, for efficient
2
5
devices. It is well known that Ph P]O unit has been widely
2
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of
used to construct electron transport and ambipolar host mate-
Advanced Microstructures, Collaborative Innovation Center of Chemistry for Life
rials due to its strongly electron-decient nature with improved
electron injecting and transporting property. When Ph
Sciences, School of Chemistry and Chemical Engineering, Nanjing University,
6
Nanjing 210093, P. R. China. E-mail: yxzheng@nju.edu.cn
2
P]O
†
Electronic supplementary information (ESI) available. CCDC 1062396 and unit was introduced in the ancillary ligand, not only the charge
1
062397. For ESI and crystallographic data in CIF or other electronic format see
transport ability of the complexes be improved, the four bulky
DOI: 10.1039/c5ra14837e
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Fig. 1 Oak Ridge thermal ellipsoidal plot (ORTEP) diagrams of the
complexes (dfpypy) Ir(tpip) (left, CCDC: 1062396) and (dfpypy) -
2 2
Ir(Ftpip) (right, CCDC: 1062397). Hydrogen atoms are omitted for
clarity. Ellipsoids are drawn at 30% probability level.
Scheme 1 Synthetic routes of ligands and complexes.
two complexes, the bond lengths of Ir–C (1.965(5)–1.982(4) A˚ )
˚
phenyl groups also lead to a larger spatial separation of the and Ir–N (2.030(4)–2.048(3) A) are within the range reported for
4a
neighboring molecules of the Ir(III) complexes to supress the related complexes. However, the bond lengths of Ir–O for
triplet–triplet annihilation (TTA) and triplet-polaron annihila- (dfpypy) Ir(tpip) are somewhat longer than that of (dfpypy) -
2
2
tion (TPA) effects. Therefore, the OLEDs with this kind of Ir(Ftpip), attributed to the inducement of withdrawing group
iridium emitters exhibited excellent high current efficiency and uorine in the ancillary ligand.
low efficiency roll-off. Herein, using dfpypy as the main ligand
and tpip derivatives as the ancillary ligands, two iridium
complexes ((dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) (Ftpip:
2 2
Thermal stability
The thermal stability of the emitters is important for the stable
OLEDs. In this cases, the thermal properties of the two iridium
complexes were characterized by differential scanning calo-
rimetry (DSC) and thermogravimetric (TG) measurements
under a nitrogen steam. From the DSC curves in Fig. 2 it can be
tetra(4-uorophenyl)imidodiphosphinate acid), Scheme 1) were
synthesized, and their photoluminescence and device perfor-
mances were studied.
Results and discussion
Preparation and characterization of compounds
observed the melting points of (dfpypy)
2
Ir(tpip) and (dfpypy)
2
-
ꢁ
Ir(Ftpip) are as high as 325 and 289 C, respectively. The TG
curves give the decomposition temperatures (5% loss of weight)
Scheme 1 shows the chemical structures and synthetic routes
for (dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) complexes. The
ꢁ
ꢁ
as 420 C for (dfpypy) Ir(tpip) and 414 C for (dfpypy) Ir(Ftpip),
2
2
2
2
respectively, indicating that the uorine atoms at the tpip
ligand will reduce the melting point effectively but will not
affect their decomposition temperature obviously. And the high
decomposition temperature of the complexes suggested they
are suitable for application in OLEDs.
reaction of 2,6-diuoropyridine with LDA (lithium diisopropy-
lamide), B(OPr-i) (isopropyl borate) gave 2,6-diuoropyridinyl-
3
5b,7
3-boronic acid.
The uorinated bipyridine ligand was
synthesized using a Suzuki coupling reaction from 2,6-
diuoropyridinyl-3-boronic acid and 2-bromopyridine. Htpip,

uorinated HFtpip and their potassium salts (Ktpip, KFtpip)
5
were prepared according to our previous publications.
dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) were puried by silica
Photophysical and electrochemical property
(
2
2
The UV-vis absorption and emission spectra of the (dfpypy)
Ir(tpip), (dfpypy) Ir(Ftpip) complexes are shown in Fig. 3 and
the photophysical data are collected in Table 1. The absorption
2
-
chromatography and vacuum sublimation. Both compounds
were fully characterized by H NMR, high resolution mass
spectrometry and X-ray crystallography.
2
1
X-ray crystallography
Single crystals grown from vacuum sublimation further
conrmed the molecular structures of (dfpypy)
dfpypy) Ir(Ftpip). Fig. 1 shows the Oak Ridge thermal ellip-
soidal plot (ORTEP) diagrams of (dfpypy)
dfpypy) Ir(Ftpip) given by X-ray analysis. Selected parameters
2
Ir(tpip) and
(
2
8
2
Ir(tpip) and
(
2
of the molecular structures and atomic coordinates were
collected in the Tables S1 and S2.† The iridium center adopts
a distorted octahedral coordination geometry with two C^N
Fig.
2
The DSC (a) and TG (b) curves of (dfpypy)
2
Ir(Ftpip) complexes.
2
Ir(tpip) and
cyclometalated ligands and one O^O ancillary ligand. For the (dfpypy)
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Fig.
3 The UV-vis absorption (a) and emission spectra (b) of
2 2 2 2
complexes (dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) in CH Cl at room
temperature.
Fig. 4 Cyclic voltammograms of complexes (dfpypy)
2
Ir(tpip) and
(
dfpypy) Ir(Ftpip).
2
spectra of these complexes show broad and intense bands
below 320 nm, assigned to the spin-allowed intraligand LC (p
1
/
p*) transition of cyclometalated main ligands and tpip type
ligands. The weak bands extend to 520 nm maybe assigned to
spin-allowed metal-ligand charge transfer band ( MLCT),
It is well known that the phosphorescence lifetime (s
Ir(III) complex is a crucial factor that determines the rate of
of the
forbidden MLCT transition bands caused by the large spin Ir(III) dopant usually causes serious TTA effect. The lifetimes of
orbital coupling (SOC) that was introduced by Ir(III) center and complexes (dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) are 1.83 ms and
an efficient spin–orbit coupling that is prerequisite for phos- 1.90 ms (Table 1, Fig. S1†), respectively, measured in degassed
p
) of the
1
partially overlapped by the broad LC absorption and spin triplet–triplet annihilation (TTA) in the OLEDs. The long s
p
3
11
2
2
ꢀ5
phorescent emission.
Both (dfpypy) Ir(tpip) and (dfpypy)
phosphorescence in CH Cl
under the irradiation of UV light for the excited states in each case. The (dfpypy)
dfpypy) Ir(Ftpip) quantum yields are 7.1% and 7.0%, respec-
CH
Ir(Ftpip) show sky-blue temperature, which are indicative of the phosphorescent origin
Ir(tpip) and
2
Cl
2
(2 ꢂ 10 M) with a excitation of 370 nm at room
2
2
2
2
2
(
at room temperature. Excited at 370 nm, (dfpypy)
2
Ir(tpip) and
2
(
dfpypy) Ir(Ftpip) show almost same emissions maxima at 457 tively, measured using an integration sphere at room tempera-
2
nm with shoulders of 485 nm at room temperature, indicating ture in degassed CH
Cl .
2 2
that the introducing of uorine in tpip have no obvious effect on
The redox properties and highest occupied molecular orbital
the absorption and emission properties of the two complexes. (HOMO), lowest unoccupied molecular orbital (LUMO) energy
And at 77 K, the two major peaks become sharper and the third levels of the dopants are relative to the charge transport ability
4e
shoulder peak is in evidence. According to literature, the and design of the OLED structure. To calculate the HOMO and
emissions of our complexes are similar to that of (dfpypy)
2
-
LUMO energy levels of the complexes, cyclic voltammetry exper-
iments were carried out using ferrocene as the internal standard
Ir(acac). The maximum peaks generated from the electronic 0–
0
transition between the lowest triplet excited state to the (Fig. 4). During the anodic scan in CH
Cl , (dfpypy) Ir(tpip) and
2 2 2
ground state. The emission at lower energy range might stem (dfpypy)
from overlapping vibrational satellites. In general, the emis- potential in the range of 1.39–1.46 V, attributed to the metal-
sion bands from MLCT states are broad and featureless, centered Ir(III)/Ir(IV) redox couple. Complex (dfpypy) Ir(Ftpip)
2
Ir(Ftpip) exhibit a reversible redox with the oxidation
2
9
12
whereas a highly structured emission band mainly originates shows lower oxidation potential (1.39 V) than that of (dfpypy)
from the p–p* state. Accordingly, all of the complexes emit Ir(tpip) (Table 1), indicating the uorine atoms in the tpip
2
-
3
from a mixture of MLCT states and the dominant ligand-based ancillary ligand make the complex easier be oxidated. The HOMO
3
levels of the (dfpypy) Ir(tpip) and (dfpypy) Ir(Ftpip) complexes
p–p* state. This indicates that the MLCT characters involved
2
2
in the emitting T states of different complexes are various, but are ꢀ6.00 and ꢀ5.93 eV, respectively, calculated from the oxida-
1
signicant, since a dominant MLCT character in T1 usually tion potentials. And the LUMO levels of them are ꢀ3.20 and
leads to large inhomogeneity and low-energy lying metal-ligand ꢀ3.15 eV, respectively, obtained from the HOMOs and band gap
13
vibrational satellites, smearing out the spectrum below the obtained from UV-vis absorption spectra (Table 1). The F atoms
10
electronic original emission.
in the tpip increased the HOMO/LUMOs of the complexes.
Table 1 Photophysical data of (dfpypy)
2
Ir(tpip) and (dfpypy)
2
Ir(Ftpip)
a
d
ꢁ
b
b
b
c
p
d
e
Complex
T
m
/T
[ C]
l
Abs [nm]
l
em,RT [nm]
s [ms]
F
[%]
E
T
Eox [V]
HOMO/LUMO [eV]
(
(
dfpypy)
dfpypy)
2
Ir(tpip)
Ir(Ftpip)
325/420
289/414
252/373
253/373
457/484
457/484
1.83
1.90
7.1
7.0
2.69
2.72
1.46
1.39
ꢀ6.00/ꢀ3.20
ꢀ5.93/ꢀ3.15
2
a
b
m
T : melting temperature, T
d
: decomposed temperature. Absorption, emission spectra and lifetime were taken in degassed CH
2
Cl
2
at room
c
d
temperature. F: emission quantum yields were measured using the integrating-sphere system. Triplet energy level was calculated from the
frozen phosphorescent spectra measured at 77 K. From the onset of oxidation potentials of the cyclovoltammetry (CV) diagram using
e
2 2
ferrocene as the internal standard and the optical band gap from the absorption spectra in degassed CH Cl .
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2
Fig. 5 MO patterns and energy diagram for (dfpypy) Ir(tpip) and
(
2
dfpypy) Ir(Ftpip).
Scheme 2 The molecular structures of the materials and energy level
diagrams of HOMO/LUMO levels of devices.
Theoretical calculation
materials. The device characteristics are shown in Fig. 6
and S3,† and the key EL data are summarized in Table 2.
For device B1, the optimal device performances are achieved
at the doping level of 8 wt% with blue electroluminescence (EL)
at 458 nm. As shown in Fig. 6, the EL spectrum is very close to
The density functional theory (DFT) calculations for Ir(III)
complexes were conducted to gain insights into the electronic
states and the orbital distribution employing Gaussian09 so-
14
15
ware with B3LYP functional. The basis set used for C, H, N,
O, F and P atoms was 6-31G(d,p) while the LanL2DZ basis set
16
2 2 2
the PL spectrum of the (dfpypy) Ir(tpip) in CH Cl solution
was employed for Ir atoms. The CH
2 2
Cl solvent effect was
indicating that the EL originates from the triplet excited states
of the Ir(III) complex. No emission from the host material
suggests that the energy and/or charge transfer from the host
exciton to the phosphor is complete upon electrical excitation.
The turn-on voltage of B1 is 3.4 V with a maximum current
taken into consideration using conductor like polarizable
17
continuum model (C-PCM). The relative energies of the
HOMO/LUMO for complexes are shown in Fig. 5 and the orbital
distributions are summarized in Table S3.† The results are
helpful for the assignment of the electron transition charac-
teristics and the discussion on the photophysical variations. For
these complexes, the HOMOs correspond to a mixture of dfpypy
group (35.36–37.36%) and Ir d orbitals (56.11–56.22%) with
minor contributions from the tpip, Ftpip ligands (6.41–8.52%).
While the LUMOs are mainly localized on the dfpypy (90.77–
ꢀ
1
efficiency (hc,max) of 5.74 cd A
and a maximum external
quantum efficiency (EQE) of 3.7% at 6.2 V, a maximum power
ꢀ1
efficiency (hp,max) of 3.19 lm W at 5.0 V and a maximum
ꢀ
2
luminance (Lmax) of 4805 cd m
.
The device efficiency and luminance are lower compared
with the reported device results based on Ir(dfpypy)3,
91.53%) with minor contributions from Ir d orbitals (5.02%)
4
(dfpypy)
2
Ir(pic) and (dfpypy)
2
Ir(acac). For device B1, although
and tpip, Ftpip ligands (3.45–4.01%). Compared with
2
the energy levels of host and (dfpypy) Ir(tpip) are match, the
(
(
dfpypy)
dfpypy)
2
2
Ir(tpip), the orbital distributions of HOMO/LUMO for
Ir(Ftpip) have less contributions from Ir d orbitals and
energy barrier from the hole transport layer TAPC to the host
material PPO21 is too large (DE ¼ 0.31 eV), which make against
the hole injection and connement in the emissive layer,
leading to low efficiency and luminance. Therefore, to lower the
HOMO energy barrier between TAPC/PPO21 and improve the
device performances, a 10 nm mCP was inserted as a “ladder”
between the TAPC and PPO21 layers to decrease the energy
barrier due to its suitable HOMO level (ꢀ5.90 eV). The devices
with the structure of ITO/TAPC (40 nm)/mCP (10 nm)/
dfpypy ligand, more contributions from the Ftpip ligand, which
indicated that the F atom affect the orbital distributions in
some degree.
OLEDs performance
Since the HOMO levels of the two complexes are low (ꢀ6.00 and
ꢀ
5.93 eV, respectively), a bipolar host material of PPO21 (3-
diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carba-
(
2 2
(dfpypy) Ir(tpip) or (dfpypy) Ir(Ftpip) (8 wt%): PPO21 (25 nm)/
zole) with low HOMO level (ꢀ6.21 eV) was used for complete
TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) were named B2 and
B3, respectively. The low-lying HOMO level (ꢀ6.70 eV) and high
triplet energy level (2.78 eV) of TmPyPB will block the hole and
achieve a well connement of hole within the emissive layer.
Moreover, the small energy barrier (0.02 eV) between TmPyPB
and PPO21 will make it easy for electron transport to the
emissive layer.
energy transfer from the host to the dopant and good device
18
performances. The device named as B1 using (dfpypy)
as the emitter as the example has a structure of ITO/TAPC (1,1-
bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm)/(dfpypy)
2
Ir(tpip)
2
-
Ir(tpip) (8 wt%): PPO21 (25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-
phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm). TAPC and
19
TmPyPB act as hole and electron transport materials, respec-
tively. Scheme 2 shows the energy level diagrams of HOMO/
LUMO levels (relative to vacuum level) for devices and
Compared with device B1, device B2 with same emitter
displays much higher performances. The maximum luminance
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2 2
Fig. 6 Characteristics of devices of B1, B2 and B3 with configuration ITO/TAPC (40 nm)/mCP (10 nm)/(dfpypy) Ir(tpip) or (dfpypy) Ir(Ftpip) (8
wt%): PPO21 (25 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm): (a) electroluminescence spectra; (b) luminance–voltage (L–V) curves; (c) current
efficiency–luminance (h –L) curves; (d) external quantum efficiency–luminance (EQE–L) curves.
c
Table 2 EL performances of the devices B1–B3
b
c
f
L
max
h
c,max
h
p,max
a
e
V
turn-on
(voltage)
[cd m (V)]
(voltage)
EQEmax
[%]
(voltage)
ꢀ
2
ꢀ1
d
d
ꢀ1
Device
Emitter
(V)
[cd A (V)]
h
c,L1000
h
c,L5000
[lm W (V)]
CIE (x, y)
B1
B2
B3
(dfpypy)
(dfpypy)
(dfpypy)
2
2
2
Ir(tpip)
Ir(tpip)
Ir(Ftpip)
3.4
3.8
3.7
4805(10.8)
10 182(11.5)
11 406(10.8)
5.74(6.2)
22.83(5.2)
20.79(6.1)
5.34
20.39
20.75
—
19.95
20.08
3.7
14.3
13.0
3.19(5.0)
13.87(5.2)
10.70(6.1)
0.16, 0.21
0.19, 0.28
0.17, 0.22
a
ꢀ2
b
c
d
V
turn-on: turn-on voltage recorded at a luminance of 1 cd m
.
L
max: maximum luminance.
h
c,max: maximum current efficiency.
h
c,L1000
,
:
ꢀ
2
e
f
h
c,L5000: the current efficiency obtained at 1000 and 5000 cd m respectively. EQEmax: maximum external quantum efficiency.
h
p,max
maximum power efficiency.
ꢀ
2
was improved to 10 182 cd m , the current efficiency was efficiencies are comparative to that using other dfpypy based
ꢀ
1
4
increased to 22.83 cd A together with a maximum EQE of complexes. However, from the current efficiency–luminance
1
W
4.3%, and the maximum power efficiency reached to 13.7 lm (h
c
–L) and external quantum efficiency–luminance (EQE–L)
ꢀ
1
at 5.2 V. The results suggest that the optimization of the curves it can be observed that the efficiency roll-off ratios of
device structure leads to a more effectively triplet exciton devices B2 and B3 are mild, which are useful for practical
ꢀ2
connement and charge balance in the emitting layer. Device application. For example, at 100 cd m brightness for display,
B3 using (dfpypy)
a maximum current efficiency of 20.79 cd A , a maximum EQE
of 13.0% and a maximum power efficiency of 10.76 lm W . The values are still 20.39 and 20.75 cd A , respectively. Even at the
introducing of uorine in the ancillary ligand tpip have no high luminance of 5000 cd m , these data are also kept at 19.95
2
Ir(Ftpip) also show similar performances with the current efficiencies of devices B2 and B3 are 19.78, 13.74 cd
ꢀ
1
ꢀ1
ꢀ2
A
, respectively. At 1000 cd m brightness for light, these
ꢀ1
ꢀ1
ꢀ
2
ꢀ
1
much effect on the characters of the OLED. The device and 20.08 cd A , respectively.
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2
22
full-matrix least-squares on F using the program SHELXS-97.
The positions of metal atoms and their rst coordination
Conclusions
Two blue-emitting iridium complexes, (dfpypy)
2
Ir(tpip) and spheres were located from direct-methods E-maps; other non-
(dfpypy)
2
Ir(Ftpip), containing uorinated substituted bipyridine hydrogen atoms were found in alternating difference Fourier
as the main ligand and tetraphenylimidodiphosphinate deriva- syntheses and least-squares renement cycles and during the
tives as the ancillary ligands were prepared. PHOLEDs fabricated nal cycles rened anisotropically. Hydrogen atoms were
with the structure of ITO/1,1-bis[4-(di-p-tolylamino)phenyl]cyclo- placed in calculated position and rened as riding atoms with
hexane (TAPC, 40 nm)/1,3-bis(9H-carbazol-9-yl)benzene (mCP, a uniform value of Uiso.
1
0 nm)/(dfpypy) Ir(tpip) or (dfpypy) Ir(Ftpip) (8 wt%): 3-(diphe-
2 2
nylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole
PPO21, 25 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB,
OLEDs fabrication and measurement
(
All OLEDs were fabricated on the pre-patterned ITO-coated
glass substrate with a sheet resistance of 15 U sq . All chem-
5
0 nm)/LiF (1 nm)/Al (100 nm) showed maximum current effi-
ꢀ1
ꢀ1
ciency (h
c
) values as 22.83 and 20.79 cd A , maximum external
ꢀ4
icals used for EL devices were sublimed in vacuum (2.2 ꢂ 10
quantum efficiency as 14.3% and 13.0% with low efficiency roll-
off, respectively, which proves that these complexes have
potential application as the efficient blue emitters in OLEDs.
Pa) prior to use. The deposition rate for organic compounds is
ꢀ1
1
–2 A˚ s . The phosphors and PPO21 host were co-evaporated to
form the 25 nm emitting layer from two separate sources. The
cathode consisting of LiF/Al was deposited by the evaporation of
ꢀ
1
˚
ꢀ
LiF with a deposition rate of 0.1 A s and then by evaporation
Experimental section
Materials and measurements
1
˚
of Al metal with a rate of 3 A s . The characteristics of the
devices were measured with a computer controlled KEITHLEY
Htpip, HFtpip and their potassium salts (Ktpip, KFtpip) were
2400 source meter with a calibrated silicon diode in air without
5
synthesized according our former publications. All reagents
device encapsulation. On the basis of the uncorrected PL and EL
spectra, the CIE coordinates were calculated using a test
program of the spectra scan PR650 spectrophotometer.
and chemicals were purchased from commercial sources and
1
used without further purication. H NMR spectra were
measured on a Bruker AM 500 spectrometer. Mass spectra (MS)
were obtained with an ESI-MS (LCQ Fleet, Thermo Fisher
Scientic). High resolution mass spectra (HR EI-MS) were
recorded from Agilent 6540 UHD Accurate-Mass Q-TOF LC/MS.
Absorption and photoluminescence spectra were measured on
a UV-3100 spectrophotometer and a Hitachi F-4600 photo-
luminescence spectrophotometer, respectively. The decay life-
times and absolute photoluminescent quantum yields were
measured with an Edinburgh Instrument FLS-920 uorescence
spectrometer equipped with an integrating sphere in degassed
CH Cl solution at room temperature. Cyclic voltammetry
General syntheses of ligands
A stirred solution of LDA (lithium diisopropylamide, 2 M, 5.17
ꢁ
mL, 10.35 mmol) in diethyl ether (30 mL) was cooled to ꢀ60 C.
2,6-Diuoropyridine (1 g, 8.6 mmol) was injected dropwise into
the cool solution over 15 min. One hour later, B(OPr-i) (2.38
mL, 10.35 mmol) was added and the mixture was kept at ꢀ60 C
3
ꢁ
for one hour. Aer the reaction mixture was warmed to room
temperature slowly and concentrated under reduced pressure,
the pH was adjusted to 10 by the slow addition of aqueous
NaOH solution (1 M, 20 mL). The aqueous layer was separated
and the organic phase was extracted with one portion water. The
combined water phase was acidied to pH ¼ 4 by the dropwise
addition of 2 N HCl. Extraction with ethyl acetate and evapo-
ration of the solvent gave the crude (2,6-diuoropyridin-3-yl)
boronic acid. 2-Bromopyridine (1 mL, 10 mmol), tetrakis
2
2
measurements were conducted on a MPI-A multifunctional
electro-chemical and chemiluminescent system (Xi'an Remex
Analytical Instrument Ltd Co., China) at room temperature with
a polished Pt plate as the working electrode, platinum thread as
3 3
the counter electrode and Ag–AgNO (0.1 M) in CH CN as the
reference electrode, tetra-n-butylammonium perchlorate (0.1 M)
0
+
(triphenylphosphine)palladium (0.34 g, 0.3 mmol) and the
boronic acids (10 mmol) were added in 50 mL THF. Aer 20 mL
was used as the supporting electrolyte, using Fc /Fc as the
ꢀ
1
internal standard, the scan rate was 0.1 V s
.
of aqueous 2 N K
heated at 70 C for 1 day under nitrogen. The mixture was
2
CO
3
was delivered, the reaction mixture was
ꢁ
X-ray crystallography
poured into water and extracted with CH Cl (10 mL ꢂ 3 times).
2
2
X-ray crystallography of the single crystals of the complexes Finally silica column purication (n-hexane : EtOAc ¼ 7 : 1 as
0
0
0
were carried out on a Bruker SMART CCD diffractometer using eluant) gave colorless solid 2 ,6 -diuoro-2,3 -bipyridine
1
˚
monochromated Mo Ka radiation (l ¼ 0.71073 A) at room (dfpypy) with a yield of 60%. H NMR (500 MHz, CDCl
3
): d 8.57
temperature. Cell parameters were retrieved using SMART (m, 2H), 7.69 (m, 2H), 7.18 (m, 1H), 6.87 (m, 1H). MS(ESI): calcd:
20
+
+
soware and rened using SAINT on all observed reections. m/z 192.05 for [M] (C10
H
6
F
2
N
2
), found: m/z 293.25 [M + H] .
General syntheses of iridium complexes. A mixture of
$nH O (0.78 g, 4.00 mmol) and dfpypy (0.57 g, 1.62 mmol)
Data were collected using a narrow-frame method with scan
widths of 0.301 in o and an exposure time of 10 s per frame. The IrCl
3
2
highly redundant data sets were reduced using SAINT and in 2-ethoxyethanol/water (10 mL, 1 : 1, v/v) was reuxed for 24 h.
corrected for Lorentz and polarization effects. Absorption Aer cooling, the yellow solid precipitate was ltered to give the
21
corrections were applied using SADABS supplied by Bruker. crude cyclometalated Ir(III) chloro-bridged dimer (0.55 g, 0.4
The structures were solved by direct methods and rened by mmol). Then the slurry of crude chloro-bridged dimer (0.4
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mmol) and Ktpip or KFtpip (0.8 mmol) in 2-ethoxyethanol (20
mL) was reuxed for 24 h. The solvent was evaporated at low
pressure and the crude product was washed by water, and then
chromatographed using CH Cl as eluant gave the corre-
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2
2
sponding complexes, which were further puried by sublima-
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J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
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tion in vacuum.
1
(
dfpypy)
2
Ir(tpip). Yield: 40%. H NMR (500 MHz, CDCl
3
) d 8.99
(
d, J ¼ 5.6 Hz, 2H), 8.08 (d, J ¼ 8.2 Hz, 2H), 7.79 (dd, J ¼ 12.4, 7.8
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Hz, 4H), 7.58 (t, J ¼ 7.9 Hz, 2H), 7.50–7.25 (m, 11H), 7.18 (t, J ¼ 7.4
Hz, 2H), 7.01 (dd, J ¼ 10.5, 4.4 Hz, 4H), 6.75 (dd, J ¼ 7.2, 6.0 Hz,
+
2
9
H), 5.55 (s, 2H). ESI-MS: 992.33 [M + H] . HR EI-MS calcd: m/z
+
91.1440 for [M] (C H F IrN O P ), found: m/z 992.1517
44
30
4
5 2 2
+
[M + H] .
1
(
2 3
dfpypy) Ir(Ftpip). Yield: 36%. H NMR (500 MHz, CDCl ) d 8.90
(
d, J ¼ 5.7 Hz, 2H), 8.12 (d, J ¼ 8.2 Hz, 2H), 7.74 (ddd, J ¼ 43.5,
1
8
2
3.9, 7.1 Hz, 6H), 7.27 (ddd, J ¼ 12.0, 6.8, 4.0 Hz, 4H), 7.10 (t, J ¼
.5 Hz, 4H), 6.83 (t, J ¼ 6.4 Hz, 2H), 6.71 (t, J ¼ 8.6 Hz, 4H), 5.53 (s,
+
H). ESI-MS: 1064.08 [M + H] . HR EI-MS calcd: m/z 1063.1063 for
+
+
[M] (C H F IrN O P ), found: m/z 1064.1138 [M + H] .
44 26 8 5 2 2
Acknowledgements
14321; (h) J.-B. Kim, S.-H. Han, K. Yang, S.-K. Kwon,
This work was supported by the National Natural Science
Foundation of China (21371093, 91433113, 21401099), the
Major State Basic Research Development Program
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Foundation of Jiangsu Province (BK20130054).
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