Photoluminescence and electroluminescence of iridium(iii) complexes with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 1,3,4-oxadiazole/1,3,4-thiadiazole derivative ligands

Article abstract of DOI:10.1039/c6dt03919g

Using 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine as a monoanionic cyclometalated ligand, 2-(5-(4-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)phenol and 2-(5-(4-(trifluoromethyl) phenyl)-1,3,4-thiadiazol-2-yl)phenol as ancillary ligands, two new heteroleptic iridium(iii) complexes (Ir1 and Ir2) were prepared and investigated. The ancillary ligand variations affected their emissions greatly, and the complexes Ir1 and Ir2 emitted green (503 nm) and orange (579 nm) lights, respectively. Moreover, the electron mobility of the two complexes is as high as that of the electron transport material Alq₃ (tris-(8-hydroxyquinoline)aluminium), which is useful for their performances in organic light-emitting diodes (OLEDs). The OLEDs with Ir1 as the emitter showed excellent performances with a maximum current efficiency of 74.8 cd A^?1, a maximum external quantum efficiency of 27.0%, a maximum power efficiency of 33.4 lm W^?1, and the efficiency roll-off is mild. These results suggest that complexes with 1,3,4-oxadiazole/1,3,4-thiadiazole derivatives have potential application as efficient emitters in OLEDs.

Full text of DOI:10.1039/c6dt03919g

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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ARTICLE
Photoluminescence and electroluminescence of
Cite this: DOI:
iridium(III)
bis(trifluoromethyl)-2,4'-bipyridine
complexes
with
and
2',6'-
1,3,4-
10.1039/x0xx00000x
oxadiazole/1,3,4-thiadiazole derivatives ligands
Received 00th January 2012,
Accepted 00th January 2012
YiꢀMing Jing, Yue Zhao, YouꢀXuan Zhenga,b*
DOI: 10.1039/x0xx00000x
Using 2',6'ꢀbis(trifluoromethyl)ꢀ2,4'ꢀbipyridine as a monoanionic cyclometalated ligand, 2ꢀ
(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)phenol or 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)
phenyl)ꢀ1,3,4ꢀthiadiazolꢀ2ꢀyl)phenol as ancillary ligands, two new heteroleptic iridium(III)
complexes (Ir1 and Ir2) were prepared and investigated. The ancillary ligand variations
affected their emissions greatly, and the complexes Ir1 and Ir2 emit green (503 nm) and
orange (579 nm) light, respectively. Moreover, the electron mobility of the two complexes
is as high as that of the electron transport material Alq3 (trisꢀ(8ꢀ
hydroxyquinoline)aluminium), which is useful for their performances in the organic lightꢀ
emitting diodes (OLEDs). The OLEDs with Ir1 as the emitter showed excellent
performances with a maximum current efficiency of 74.8 cd Aꢀ1, a maximum external
quantum efficiency of 27.0%, a maximum power efficiency of 33.4 lm Wꢀ1, and the
efficiency rollꢀoff is mild. These results suggest that the complexes with 1,3,4ꢀ
oxadiazole/1,3,4ꢀthiadiazole derivatives has potential application as efficient emitters in
OLEDs.
www.rsc.org/
not make contribution to the lowest excited state directly,
they indeed alter the energy levels of the excited states by
Introduction
Organic lightꢀemitting diodes (OLEDs) have been studied
intensely during the last decade due to their encouraging
application in highꢀresolution, fullꢀcolor, flatꢀpanel displays
and lighting sources.1 Especially, phosphorescent iridium(III)
complexes play an important part as the emitters in efficient
OLEDs due to the high quantum efficiency and short
lifetime of triplet excited states.2 The strong spinꢀorbit
coupling (SOC) introduced by the central heavy atom can
promote the triplet to singlet radiative transition, such
complexes may exhibit unusually high phosphorescence
quantum yields at room temperature.3 On the other hand,
since the phosphorescence of Ir(III) complexes primarily
originates from the metalꢀtoꢀligand charge transfers (MLCT)
and the ligandꢀcentered (LC) transitions,4 the energy level of
the excited state can be controlled by tuning the energy
levels of the ligands through substituent effects, which leads
to a wide flexible emission color range.5
modifying the electron density at the metal center. Thus, the
photophysical property and carrier mobility of iridium
complexes can be tuned trough functional substitutes on both
cyclometalated and ancillary ligands. However, for many
OLEDs with high efficiency based on Ir(III) complexes, the
device efficiency rollꢀoff ratios are serious, which can
mainly be attributed to the deterioration of charge carrier
balance and the increase of nonradioactive quenching
processes, including triplet–triplet annihilation (TTA), tripleꢀ
polaron annihilation (TPA), and electric field induced
dissociation of excitons at high current density.6 Therefore,
the balanced injection and transport of the electron–hole is a
crucial factor for high efficient OLEDs. Furthermore, as we
know, because the hole mobility of most hole transport
materials is roughly 2–3 orders of magnitude higher than the
electron mobility of the electron transport materials, the
efficiency and efficiency rollꢀoff of OLEDs rely on the
capability of electron transport. Thus, it is necessary to use
the ambipolar host materials and synthesize Ir(III) dopants
with outstanding electron mobility to obtain phosphorescent
OLEDs with low efficiency rollꢀoff.
According to the density functional theory calculation, the
highest occupied molecular orbital (HOMO) is basically
centered on the Ir(III) metal while the lowest unoccupied
molecular orbital (LUMO) is generally localized on the
cyclometalated ligands. Although most ancillary ligands do
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Our group has reported high efficient phosphorescent Scheme 1 shows the chemical structures and synthetic protocols
OLEDs using Ir(III) complexes with 2',6'ꢀ for ligands and Ir(III) complexes. The main ligand 2',6'ꢀ
bis(trifluoromethyl)ꢀ2,4'ꢀbipyridine (BTBP) as the main bis(trifluoromethyl)ꢀ2,4'ꢀbipyridine was synthesized using
a
ligand and tetraphenylimidodiphosphinate (tpip) derivatives Suzuki coupling reaction from (2,6ꢀbis(trifluoromethyl)pyridinꢀ4ꢀ
as ancillary ligands.7 Trifluoromethyl unit can enhance the yl)boronic acid and 2ꢀbromopyridine. The 2ꢀ(5ꢀ(4ꢀ
electron mobility and result in a better balance of charge (trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)phenol,
2ꢀ(5ꢀ(4ꢀ
injection and transfer. Furthermore, the lower vibrational (trifluoromethyl)phenyl)ꢀ1,3,4ꢀthiadiazolꢀ2ꢀyl)phenol ancillary
frequency of the CꢀF bond also can reduce the rate of ligands and their potassium salt were prepared according to our
radiationless deactivation and the bulky CF3 substituents can previous publications.10 The Ir(III) complexes were obtained in
affect the molecular packing and the steric protection around two steps with popular methods via Ir(III) chloroꢀbridged dimer.
the metal can suppress the selfꢀquenching behavior.8 In Purification of the mixture by silica gel chromatography provided
addition, nitrogen heterocycle will increase the electron crude products, which were further purified by vacuum
affinity and a more negative framework of ligand C^N such sublimation. All the new compounds were fully characterized by
as bipyridine may improve the electron mobility of the 1H NMR and high resolution mass spectrometry (HR MS); the
complexes. Moreover, OLEDs based on Ir(III) complexes crystal structure obtained from vacuum sublimation further
with 1,3,4ꢀoxadiazole derivatives as ancillary ligands also confirmed the identity of Ir1 complex.
have good performances due to their high electron mobility,
The Oak Ridge Thermal Ellipsoidal plot (ORTEP) diagram of
high photoluminescence quantum yield and good the Ir1 complex is shown in Fig. 1, the corresponding
thermal/chemical stability.9,10 On this basis, as shown in crystallographic data are summarized in Table S1, and selected
Scheme 1, we synthesized two new heteroleptic Ir(III) bond lengths and angles are listed in Table S2. The central
complexes (Ir1 and Ir2) using 2',6'ꢀbis(trifluoromethyl)ꢀ2,4'ꢀ iridium atom is chelated by two anionic C^N main ligands and
bipyridine (BTBP) as the cyclometalated ligand and 2ꢀ(5ꢀ(4ꢀ one monoanionic bidentate N^O ancillary ligand. The
(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)phenol
coordination sphere presents a distorted octahedral geometry,
(HCF3POP) and 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀ with the cisꢀC,C and transꢀN,N in chelating disposition.
thiadiazolꢀ2ꢀyl)phenol (HCF3POPTDZ) as the ancillary Concerning the designed ancillary ligand with three rings, the
ligands. When the 1,3,4ꢀoxadiazole derivatives were phenol ring and the oxadiazole ring chelate with the iridium
replaced by the 1,3,4ꢀthiadiazole derivatives, the emission center via the O atom and a N atom, respectively, forming a
color of the complex is tuned from green to red. Here, we relatively rigid hexatomic coordination ring. The IrꢀC and IrꢀN
described the results of our investigation on the synthesis, bonds between iridium center and C^N main ligands are 2.0 Å
characterization,
electroluminescence properties of both Ir(III) complexes.
photoluminescence
and and the IrꢀO(1) bond (2.1 Å) is the longest among all the
coordination bonds. Furthermore, the C–C and C–N bond lengths
and angles are in agreement with the corresponding parameters
described in other similarly constituted complexes. The left 4ꢀ
(trifluoromethyl)phenyl ring presents a rather obvious dihedral
angle towards the other two rings, in order to minimize the steric
Results and discussion
Preparation and X-ray crystallography
CF3
N
F3C
Ir
F3C
N
Br
CF3 F3C
IrCl3•3H2O
CF3
Ir
CF3
F3C
CF3
N
N
effect.
N
N
Cl
Cl
LDA
B(OPr-i)3
F3C
Pd(PPh3)4
N
N
B
N
OH
HO
2
2
O
O
O
Cl
O O
NH
NH2
CF3
CF3
+
N
O
HN NH
O
O
CF3
3
OK
(S)
O
OH (S)
O
N
BBr3
KOH
CF3
CF3
N
N
O
N
N
S
BBr3
CF3
CF3
N
N
CF3
CF3
N
F3C
N
F3C
CF3
Ir
CF3
N
Cl
CF3
F3C
N
F3C
Ir
N
N
N
N
S
Cl
O
N
N
2
2
or
Ir
Ir
N
N
O
O
2
2
Fig. 1. Oak Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of the
complex Ir1 with the atomꢀnumbering schemes. Hydrogen atoms are
omitted for clarity. Ellipsoids are drawn at 30% probability level. Crystal
data: CCDC No. 1455234, C39H18F15IrN6O2, Mw = 1079.79, Monoclinic,
Ir2
Ir1
Scheme 1. Synthetic routes of ligands and complexes.
2 | J. Name., 2012, 00, 1-3
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P21/c, a = 11.6755(5) Å, b = 13.7860(5) Å, c = 24.0468(9) Å, α = γ = 90°,
β = 93.2230(10) °, V = 3864.4(3) Å3, Z = 4, F(000) = 2088, GOF = 1.093,
R1 = 0.0306, wR2 = 0.0604.
The UVꢀvis absorption spectra of the Ir1 and Ir2 complexes in
degassed CH2Cl2 at 5 × 10ꢀ5 mol·Lꢀ1 are shown in Fig. 3(a) and
the photophysical data are listed in Table 1. The intense bands at
high energy (250ꢀ350 nm) are assigned to the spinꢀallowed
ligandꢀcentered 1LC (πꢀπ*) transition of the cyclometalated
primary ligand (BTBP) and ancillary ligands. The relatively weak
absorption bands at lower energies extending into the spectral
region (350ꢀ500 nm) are attributed to the mixing of the spinꢀ
allowed singlet metalꢀtoꢀligand chargeꢀtransfer (1MLCT) and
triplet metalꢀtoꢀligand chargeꢀtransfer (3MLCT) states, or LLCT
(ligandꢀtoꢀligand chargeꢀtransfer) transition through strong spin–
orbit coupling of iridium atoms.8,11 From Fig. 3(a) and Table 1 it
can be observed that the absorption bands of Ir2 with 2ꢀ(5ꢀ(4ꢀ
Thermal stability
2
100
80
60
40
20
0
100
80
60
40
20
0
TG
DSC
(a)
TG
DSC
(b)
1
1
0
0
-1
-1
200
400
600
800
150
300
450
600
750
Temperature (°C)
Temperature (°C)
(trifluoromethyl)phenyl)ꢀ1,3,4ꢀthiadiazolꢀ2ꢀyl)phenol
ancillary
ligand has an obvious red shift compared with that of Ir1 which
has the ancillary ligand of 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀ
oxadiazolꢀ2ꢀyl)phenol suggesting that the ancillary ligand
variation has effects on their excited state energy.
Fig. 2. TG and DSC thermograms of (a) Ir1 and (b) Ir2.
The thermal stability of the emitters is important for the stability
of OLEDs. The thermal properties of Ir1 and Ir2 were
characterized by thermogravimetric (TG) and different scanning
calorimetry (DSC) measurements under a nitrogen steam. From
the DSC curves in Fig. 2 it can be observed that the melting
points of Ir1 and Ir2 are as high as 345 and 359 °C, respectively.
The TG curves give the decomposition temperature (5% loss of
weight) of 382 °C for Ir1 and 378 °C for Ir2, respectively.
Furthermore, all of the two complexes can be vacuum evaporated
easily without decomposition and show good filmꢀforming ability,
which indicates that the complexes are potential emitting
materials for the fabrication of stable OLEDs.
Ir1
Ir2
(b)
Ir1, 298 K
Ir2, 298 K
Ir1, 77 K
Ir2, 77 K
(a)
250
300
350
Wavelength (nm)
400
450
500
550
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 3. Normalized (a) UVꢀvis absorption and (b) emission spectra of Ir1
and Ir2 in degassed CH2Cl2 solutions (5 × 10ꢀ5 mol·Lꢀ1) at room
temperature and 77 K.
Photophysical property
Table 1. Physical properties of Ir1 and Ir2.
a)
d)
Complex
Tm/Td
(°C)
Фp
HOMO/LUMO f)
(eV)
b)
c)
λabs
λem
(nm)
τ e)
(ꢀs)
(%)
(nm)
298 K
77 K
Ir1
Ir2
345/382
359/378
247, 263, 409
249,267, 456
503
579
495, 529
530, 565
93.04
24.32
2.28
2.21
ꢀ5.59/ꢀ2.84
ꢀ5.57/ꢀ3.09
Tm: melting temperature, Td: decomposition temperature; Measured in degassed CH2Cl2 solution at a concentration of 5 × 10ꢀ5 mol·Lꢀ1 at room
a)
b)
temperature; Measured in degassed CH2Cl2 solution at a concentration of 5 × 10ꢀ5 mol·Lꢀ1 at 298 and 77 K, respectively; Measured in degassed
c)
d)
CH2Cl2 solution at room temperature using facꢀIr(ppy)3 as the standard sample (Φ = 0.9). e) Measured in degassed CH2Cl2 solution at a concentration of
5 × 10ꢀ5 mol·Lꢀ1 at room temperature; From the onset of oxidation potentials of the cyclovoltammetry (CV) diagram using ferrocene as the internal
f)
standard and the optical band gap from the absorption spectra in degassed CH2Cl2.
Photoluminescence measurements were conducted in
deaerated CH2Cl2 solutions at room temperature (Fig. 3(b) and
Table 1) and 77 K (Fig. 3(b) and Table 1). Under the excitation
of 380 nm, the Ir1 complex emitted intense green
phosphorescence in CH2Cl2 at room temperature with the peak
maxima at 503 nm. It can be observed that when the 2ꢀ(5ꢀ(4ꢀ
(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)phenol derivative
was replaced by 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀ
thiadiazolꢀ2ꢀyl)phenol derivative, the emission of Ir2 is red shift
compared with that of Ir1. Under the excitation of 470 nm, the
Ir2 complex emitted intense orange phosphorescence peaked at
579 nm, which agrees with the absorption spectra. The result
suggests the introduction of 1,3,4ꢀthiadiazole into the ancillary
ligand can affect the luminescence properties of the Ir(III)
complexes. Moreover, the quantum yields of the two complexes
in solution are 93.04% and 24.32%, respectively.
As shown in Fig. 3(b), the structured emission at 77 K reveals
3
that the mixing between the MLCT and the LLCT levels is so
effective that an almost ligandꢀcentered emission is observed
upon freezing of the matrix. The rigidity of the solvent
dramatically affects the stabilization of the chargeꢀtransfer
states, which shift to higher energy at low temperature, and the
electronic mixing of the two states decreases.
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The phosphorescence lifetime (τ) is the crucial factor that
determines the rate of tripletꢀtriplet annihilation in OLEDs.
Longer values usually cause greater tripletꢀtriplet
annihilation.12 The lifetimes of Ir1 and Ir2 are in the range of
microseconds (2.28 ꢁs and 2.21 ꢁs in CH2Cl2 solution
respectively) at room temperature (Table 1) and are indicative
of the phosphorescent origin for the excited states in each case.
for Ir atoms.16 The solvent effect of CH2Cl2 was taken into
consideration using conductorꢀlike polarizable continuum model
(CꢀPCM).17 All these calculations were performed with
Gaussian 09.18 QMForge program was used to give accurate
percentage data of FMOs. Contour plots of the frontier
molecular orbitals (FMOs) are shown in Fig. 4 (right). The
energies and % composition of ligand and metal orbitals are
shown in Table S4.The results are helpful for the assignment of
the electron transition characteristics and the discussion on the
photophysical variations. According to Fig. 4 (right) and Table
S4, it is obviously that the HOMOs correspond to a mixture of
the d orbitals of Ir (14.70ꢀ15.02%) and the π orbitals of the
phenyl ring of the ancillary ligand (79.65ꢀ79.95%) with minor
contribution from the BTBP ligand (5.23ꢀ5.36%). The oxidation
τ
Electrochemical properties and theoretical calculation
2
Ir1
Ir2
0
-2.20
-2.84
-2.36
-3.09
LUMO
-2
-4
-6
Ir1
Ir2
-5.54
-5.57
-5.57
-5.59
HOMO
processes occured with metal centered orbitals and
a
-2
-1
0
1
2
contribution from the phenyl ring of ancillary ligands.19 On the
other hand, the locations of the LUMOs are various in different
complexes. The LUMO level of Ir1 is mainly located on the
primary ligand (BTBP) with the composition of 69.39% and a
composition of 28.67% on the ancillary ligand. It is interesting
that for Ir2, the composition of LUMO on the primary ligand is
only as low as 9.23%, but the ratio on ancillary ligand reaches
89.81%, which is much higher than that of Ir1. The calculation
results indicated that the frontier orbitals and the electronic
properties of the complexes can be manipulated by introducing
1,3,4ꢀthiadiazole derivatives.
Voltage (V)
Fig. 4. Cyclic voltammogram curves (left) and contour plots of
HOMOs/LUMOs with theoretical (black) and experimental
(red) energy levels (right) of Ir1 and Ir2.
The redox properties and HOMO/LUMO energy levels of the
dopants are relative to the charge transport ability and the
OLED structure. In order to calculate the HOMO and LUMO
energy levels of the complexes, the electrochemical properties
of Ir1 and Ir2 were measured by cyclic voltammetry in
deaerated solution (CH2Cl2: CH3CN = 1: 1) (Fig. 4 (left)). The
HOMO levels were calculated from the oxidation peak
potentials (Eox) and the bandgaps (Eg) were calculated from the
UVꢀvis absorption edges.13 Then the LUMO levels were
determined according to the equation LUMO = HOMO + Eg.
The electrochemical data are collected in Table S3.The cyclic
voltammograms of the complexes in the positive range show
strong oxidation peaks, while the reduction peaks are not
obvious, demonstrating that the redox process of the complexes
is not reversible completely, which is also observed in related
Ir(III) complexes containing oxadiazole units.14 In the negative
potential scan rang all complexes exhibit quasiꢀreversible
oxidation and reduction process due to the 1,3,4ꢀ
oxadiazol/1,3,4ꢀ thiadiazole substituents suggesting the
electronꢀtransporting and electron trapping characteristics. From
Fig. 4 and Table S3 it can be observed that the HOMO levels of
Ir1 and Ir2 did not change significantly, which is ꢀ5.59 eV and ꢀ
5.57 eV, respectively. But the 1,3,4ꢀthiadiazole unit caused the
lower LUMO level (ꢀ3.09 eV) and the HOMOꢀLUMO gap (2.48
eV). The trend corresponds to the DFT calculation results very
well.
Electron Mobility
As the hole mobility is roughly 2–3 orders of magnitude higher
than the electron mobility in OLEDs,20 their excitation lifetime
relies on the electron transport capability. The good electron
mobility of the phosphorescent emitters would facilitate the
injection and transport of electrons, which will broaden the
recombination zone, balance the distribution of hole–electron
and reduce leakage current, leading to suppressed TTA and TPA
effects, improved recombination probability, high device
efficiency, and low efficiency rollꢀoff.6,21 As discussed before,
introducing nitrogen heterocycle with the strong electron
withdrawing trifluoromethyl group and 1,3,4ꢀoxadiazole
derivatives or 1,3,4ꢀthiadiazole derivatives may improve the
electron mobility of the complexes, which would suppress the
TTA and TPA effects effectively and obtain phosphorescent
OLEDs with low efficiency rollꢀoff.
To measure the electron mobility of both complexes, we
conducted
the
transient
electroluminescence
(TEL)
measurement based on the device of ITO (indium tin oxide) /
TAPC (1,1ꢀbis[4ꢀ[N,Nꢀdi(pꢀtolyl)amino]phenyl]cyclohexane,
50 nm) / Ir complexes (60 nm) / LiF (1 nm)/ Al (100 nm). 22 The
TAPC is the holeꢀtransport layer, whereas the Ir(III) complexes
perform as both the emissive and electronꢀtransport layers. To
check the accuracy of our measurements, we also measured the
electron mobility of Alq3 (trisꢀ(8ꢀhydroxyquinoline)aluminum),
which is the typically wellꢀknown electron transport material,
For providing further study of the electronic structures of the
complexes, the theoretical calculation was performed on
optimized geometries in CH2Cl2. The calculations on the ground
electronic states of the complexes were carried out using density
functional theory (DFT) and timeꢀdependent DFT (TDꢀDFT) at
the B3LYP level.15 The basis set used for C, H, N, O, F and S
atoms was 6ꢀ31G(d, p) while the LanL2DZ basis set was used
4 | J. Name., 2012, 00, 1-3
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whose electron mobility has been reported in many references.23
The experimental results (Fig. 5) show that the electron mobility
in 60 nm Ir1 and Ir2 layers are between 3.69ꢀ4.39 × 10ꢀ6 cm2·Vꢀ
1·sꢀ1 and 4.33ꢀ4.74 × 10ꢀ6 cm2·Vꢀ1·sꢀ1, respectively, under an
electric field from 1150 (V·cmꢀ1)1/2 to 1300 (V·cmꢀ1)1/2, while
that of Alq3 is between 4.74ꢀ4.86 × 10ꢀ6 cm2·Vꢀ1·sꢀ1. From Fig.
5(c) it can also be observed that Ir1 and Ir2 complexes have
high electron mobility as that of Alq3. The good electron
transport ability of Ir1 and Ir2 will facilitate the injection and
transport of electrons, which broadens the recombination zone
and balances the distribution of holes and electrons, particularly
for high doping concentrations. Therefore, efficient OLEDs
with suppressed efficiency rollꢀoff are expected.
5.0x10-6
2.0
1.5
1.0
0.5
0.0
1.0
0.5
0.0
10.0 V
9.5 V
9.0 V
8.5 V
8.0 V
(c)
10.0 V
9.5 V
9.0 V
8.5 V
8.0 V
(a) Ir1
(b) Ir2
4.0x10-6
3.0x10-6
2.0x10-6
Pulse
(V)
10.0 0.76
t
Pulse
(V)
10.0 0.82
t
d
d
(ꢀs)
Ir1
Ir2
Alq3
(ꢀs)
9.5
9.0
8.5
8.0
0.84
0.90
0.98
1.04
9.5
9.0
8.5
8.0
0.98
1.10
1.16
1.22
1150
1200
1250
1300
0
2
4
6
8
10
0
2
4
6
s)
8
10
Electric Field (V cm-1)1/2
Time (
ꢀ
Time (ꢀs)
Fig. 5. The transient EL signals for the device structure of ITO/TAPC (50 nm)/Ir1 (a) and Ir2 (b) (60 nm) under different applied fields, and (c) the
electric field dependence of charge electron mobility in the thin films of Ir1, Ir2 and Alq3.
structure of ITO/TAPC (30 nm)/ mCP (N,N'ꢀdicarbazolylꢀ3,5ꢀ
benzene, 10 nm)/ Ir1 or Ir2 (8 wt%): PPO21 (3ꢀ
(diphenylphosphoryl)ꢀ9ꢀ(4ꢀ(diphenylphosphoryl)phenyl)ꢀ9Hꢀ
carbazole, 20 nm)/ TmPyPB (1,3,5ꢀtri(mꢀpyridꢀ3ꢀylꢀ
phenyl)benzene, 40 nm)/ LiF (1 nm)/ Al (100 nm). Fig. 6 shows
the energy level diagram of HOMO and LUMO levels (relative
to vacuum level) for materials investigated in this study and
their molecular structures. TAPC, PPO21 and TmPyPB were
employed as the hole transport, host and the electron transport
material, respectively. PPO21 was introduced as the host
material (HOMO = 6.21 eV), which has a similar transport
capability of electrons and holes. Furthermore, holes and
electrons will be well confined within the doped lightꢀemitting
layer. mCP was added as another hole transport material to
lower the HOMO energy barrier between TAPC and PPO21.24
Therefore, a good carrier trapping is expected in these devices,
which is the dominated EL mechanism.
OLEDs performance
Fig. 6. Energy level diagram of HOMO and LUMO levels
(relative to vacuum level) for materials investigated in this study
and their molecular structures.
In order to evaluate the electroluminescence (EL) performances
of two Ir(III) complexes, the devices D1 and D2 using Ir1 and
Ir2 as the emitters, respectively, were fabricated with the
Table 2. EL performances of the devices D1 and D2.
a)
b)
e)
g)
j)
CIE(x,y)k)
c)
i)
Device
Emitter
Vturnꢀon
(V)
Lmax
ηc,L1000
ηc,L10000
λmax
ηc,max
(cd Aꢀ1)
ηp,max
(cd mꢀ2)(V)
(cd Aꢀ1)
(EQEL1000
(cd Aꢀ1)
(EQEL10000 )
(nm)
(lm Wꢀ1)
(V)
f)
h)
d)
)
(EQEmax
)
D1
D2
Ir1
Ir2
3.9
4.0
37270 (13.9)
18146 (12.4)
74.8 (27.0%)
41.0 (13.9%)
71.6 (25.9%)
37.1 (12.6%)
46.2 (16.7%)
15.7 (5.30%)
33.4 (6.9)
18.7 (6.5)
497
579
0.18, 0.55
0.52, 0.47
a)
b)
c)
d)
Vturnꢀon: turnꢀon voltage recorded at a luminance of 1 cd mꢀ2, Lmax: maximum luminance, ηc,max: maximum current efficiency, EQE: maximum
external quantum efficiency; e) ηc,L1000: current efficiency at 1000 cd mꢀ2, f) EQEL1000: EQE at 1000 cd mꢀ2, g) ηc,L10000: current efficiency at 10000 cd mꢀ2,
h)
i)
j)
k)
EQEL10000: EQE at 10000 cd mꢀ2, ηp,max: maximum power efficiency, values were collected at 8 V, CIE(x, y): Commission Internationale de
l'Eclairage coordinates at 8V.
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250
200
150
100
50
(b)
104
(a)
D1
D1
D2
D2
103
102
101
100
0
300
400
500
600
700
0
4
8
12
16
Voltage (V)
Wavelength (nm)
100
100
10
1
(d)
D1
D2
(c)
D1
D2
10
1
0.1
0
2000
4000
6000
8000
10000
0
2000
4000
6000
8000
10000
Luminance (cd m-2)
Luminance (cd m-2)
Fig. 7. Characteristics of devices (D1 and D2) with configuration ITO/ TAPC (30 nm)/ mCP (10 nm)/Ir1 or Ir2 (8 wt%) : PPO21 (20 nm)/ TmPyPB
(40 nm)/ LiF (1 nm)/ Al (100 nm): (a) Normalized EL spectra at 8 V, (b) current densityꢀvoltageꢀluminance (JꢀVꢀL), (c) current efficiencyꢀluminance
(ηcꢀL), and (d) power efficiencyꢀluminance (ηpꢀL) curves.
The EL spectra, current densityꢀvoltageꢀluminance (JꢀVꢀL),
current efficiencyꢀluminance (ηcꢀL) and power efficiencyꢀ
luminance (ηpꢀL) characteristics of each device are shown in
Fig. 7. The key EL data are summarized in Table 2. As shown in
Fig. 7(a), the device D1 emits green light with the EL emission
peak at 497 nm, while the device D2 emits orange light with the
EL emission peak at 579 nm, respectively. Both of them are
close to the PL spectra of the Ir1 and Ir2 complexes in CH2CL2
solution, indicating that the EL emission of the devices originate
from the triplet excited states of the phosphors. The EL
emission spectra shapes are almost invariant of the current
density and also do not show any concentration dependent. No
emission from TAPC, mCP and TmPyPB suggests that the
exciton was only formed in the emissive layers. The absence of
the PPO21 emission demonstrates that the energy and charge
transfer from the host excitons to the phosphors is complete
upon electrical excitation. The CIE (Commission Internationale
de l’Eclairage) color coordination are x = 0.18, y = 0.55 for D1,
and x = 0.52, y = 0.47 for D2, corresponding to the green and
orange region, respectively.
It can also be observed that the turn on voltages of the two
devices are high (3.9 V for D1 and 4.0 V for D2) though the
dopants have good electron mobility, and the charge transport of
TAPC/TmPyPb are also high. One reason is that the carrier
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mobility of the host material PPO21 is very low (ꢁh = 9 × 10ꢀ6
cm2 Vꢀ1 sꢀ1 and ꢁe = 3.0 × 10ꢀ6 cm2 Vꢀ1 sꢀ1).6 Furthermore, the
insertion of the mCP also would cause energy loss during the
current flow. These two factors may lead to the high turnꢀon
voltages of the devices.
shifts the peaks of PL and EL spectra to the orange area, all of
these have an effect on the OLED performances.
The notable EL properties and EL efficiency rollꢀoff effects
of device D1 should be due to the application of 2ꢀ(5ꢀ(4ꢀ
(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)phenol as the
ancillary ligand and 2',6'ꢀbis(trifluoromethyl)ꢀ2,4'ꢀbipyridine as
the cyclometalated ligand. The dopant acts as the hole and
elctron traps to retard the motion of both types of carriers. The
lower LUMO level of the dopant is particularly important for
the reason that the hole mobility of TAPC is higher than the
electron mobility of TmPyPB in OLEDs,20(a) the excition
accumulation is expected in the hole blocking layer (TmPyPB)
near the interface of the emitting layer (Ir1 : PPO21/ TmPyPB)
due to the high energy barrier between TmPyPB and PPO21.25
The accumulation of exciton will cause the serious TTA, TPA
of the Ir(III) complexes, and high efficiency rollꢀoff
consequently. In this case, the bipyridine and the 1,3,4ꢀ
oxadiazole units will benefit the electron transport properties.
The good electron mobility of the phosphorescent emitter would
facilitate the injection and transport of electrons, which broaden
the recombination zone, balance the distribution of holes and
electrons, particularly at the high doping concentration, leading
to the suppressed the TTA, TPA effects and efficiency rollꢀoff.21
Both devices show exciting performances at 8 wt% doped
concentration. Respectively, the maximum current efficiency
(ηc,max) and external quantum efficiency (EQEmax) of device D1
are 74.8 cd Aꢀ1 and 27.0 %, respectively, obtained at 7.2 V, the
maximum power efficiency (ηp,max) is 33.4 lm Wꢀ1 (6.9 V), and
the maximum luminance (Lmax) is 37270 cd mꢀ2 at 13.9 V.
Device D2 has a maximum luminance of 18146 cd mꢀ2 at 12.4 V
and displays a maximum current efficiency of 41.0 cd Aꢀ1 (6.9
V) with a maximum EQE of 13.9% and a maximum power
efficiency of 18.7 lm Wꢀ1 (6.5 V). Furthermore, the device D1
keeps high efficiency at relative high luminance and the rollꢀoff
of the efficiency is low. The current efficiencies and EQE for
device D1 at the brightness of 1000 cd mꢀ2 and 10000 cd mꢀ2 are
71.6 cd Aꢀ1 (25.9%) and 46.2 cd Aꢀ1 (16.7%), which indicates
the complex Ir1 has encouraging application potential in
OLEDs.
Compared with the complexes with the same cyclometalated
ligand 2',6'ꢀbis(trifluoromethyl)ꢀ2,4'ꢀbipyridine (BTBP) main
ligand and tetraphenylimidodiphosphinate (tpip) ancillary ligand
in our former work (Lmax: 52515 cd mꢀ2, ηc,max: 99.97 cd Aꢀ1,
ηp,max: 43.60 lm Wꢀ1, EQEmax: 30.5%), 7 the device using Ir1 as
the emitter performed lower performances because tpip resulted
higher electron mobility (5.11–5.29 × 10ꢀ6 cm2·Vꢀ1·sꢀ under an
electric field from 1150 (V·cmꢀ1)1/2 to 1300 (V·cmꢀ1)1/2) than
that of 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀ
yl)phenol (HCF3POP).7 But the device performances are better
Conclusions
In conclusion, two new heteroleptic iridium(III) complexes
using 2',6'ꢀbis(trifluoromethyl)ꢀ2,4'ꢀbipyridine as the main
ligand and 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀ
yl)phenol or 2ꢀ(5ꢀ(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,4ꢀthiadiazolꢀ
2ꢀyl)phenol as ancillary ligands were synthesized. Both
complexes have a high electron mobility as that of a wellꢀknown
electron transport material Alq3. The OLEDs based on Ir1 and
Ir2 with the structure ITO/TAPC (30 nm)/ mCP (10 nm)/ Ir
complex (8 wt%): PPO21 (20 nm)/ TmPyPB (40 nm)/ LiF (1
nm)/ Al (100 nm) showed good performances. Especially, the
device D1 with Ir1 as the emitter shows a maximum current
efficiency of 74.8 cd Aꢀ1, a maximum external quantum
efficiency of 27.0%, a maximum power efficiency of 33.4 lm
Wꢀ1. Furthermore, the efficiency rollꢀoff of D1 from the peak
value to that at the practical luminance is mild, which is helpful
to obtain high efficiency at relatively high current density and
high luminance. The study suggests that the 1,3,4ꢀthiadiazol
group would affect the PL spectra, the electron mobility, and the
OLED performances of iridium(III) complexes.
than that with
(trifluoromethyl)phenyl)pyridine
Ir(III) complex containing 2ꢀ(4ꢀ
(4ꢀtfmppy) or 2ꢀ(2ꢀ
trifluoromethyl)pyrimidineꢀpyridine (TPP) main ligands and
HCF3POP ancillary ligand (41.08 cd Aꢀ1),10 suggesting BTBP
ligand benefits better device performances than 4ꢀtfmppy and
TPP. However, the replacement of HCF3POP by 2ꢀ(5ꢀ(4ꢀ
(trifluoromethyl)phenyl)ꢀ1,3,4ꢀthiadiazolꢀ2ꢀyl)phenol
(HCF3POPTDZ) shifted the emission from green to orange area,
which indicates that the variety of ancillary ligands would help
to find more novel materials and adjust the device color.
The OLED using Ir1 as the emitter exhibits better EL
performance than that using Ir2 as the emitter, the performances
of OLEDs are attributed to not only the electron mobility of the
iridium complexes but also the quantum efficiency. Although
the electron mobility of the complex Ir2 is better than that of
Ir1, the quantum efficiency of Ir2 is poorer than that of Ir1,
which may cause the poor EL performance. The results show
that the substituents of the ancillary ligand can influence the EL
performances of the Ir(III) complexes. The introduction of S
atom in the ancillary ligand can increase the electron mobility of
the Ir(III) complexes, but it also decreases the PL yield and
Experimental section
Materials and measurements
All chemicals were commercial purchased without further
purification. All the reactions were carried out under nitrogen
1
atmosphere. H NMR spectra were measured on a Bruker AM
500 spectrometer. Electrospray ionization mass spectra (ESIꢀ
MS) were obtained with ESIꢀMS (LCQ Fleet, Thermo Fisher
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Scientific). The high resolution mass spectra (HR EIꢀMS) were
recorded on an Agilent 6540 UHD AccurateꢀMass QꢀTOF
LC/MS. UVꢀvis absorption and photoluminescence (PL) spectra
were measured on a Shimadzu UVꢀ3100 and a Hitachi Fꢀ4600
spectrophotometer at room temperature, respectively. The
The chloroꢀbridged dimer ([(BTBP)2Ir(ꢁꢀCl)]2, 0.2 mmol) and
the ancillary potassium salt (KCF3POP or KCF3TDZ, 0.5 mmol)
in 2ꢀethoxyethanol (20 mL) was refluxed for 24 h. After the
mixture was cooled, the solvent was evaporated at low pressure.
The crude product was washed by water, and the column
chromatography using CH2Cl2 as the eluent gave Ir(III)
complexes which were further purified again by sublimation in
vacuum.
luminescence quantum efficiencies were calculated by
a
comparison of the emission intensities (integrated areas) of a
standard sample facꢀIr(ppy)3 and the unknown samples in
deaerated CH2Cl2 solutions of 5 × 10ꢀ5 mol/L.26
1
Ir1. (yield: 48%): H NMR (300 MHz, CDCl3) δ 8.69 (d, J =
5.7 Hz, 1H), 8.15 (d, J = 7.8 Hz, 1H), 8.09 (d, J = 5.4 Hz, 2H),
8.04 (s, 1H), 7.93 – 7.79 (m, 5H), 7.75 (d, J = 8.4 Hz, 2H), 7.59
(dd, J = 8.2, 1.7 Hz, 1H), 7.20 (t, J = 6.8 Hz, 2H), 7.01 (t, J =
6.1 Hz, 1H), 6.64 (d, J = 8.4 Hz, 1H), 6.53 (t, J = 7.4 Hz, 1H).
HR EIꢀMS m/z calcd for C39H18F15IrN6O2: 1080.0959, found:
1081.0956 [M+H]+. Anal. Calcd for C39H18F15N6O2Ir: C, 43.38;
H, 1.68; N, 7.78. Found: C, 43.40; H, 1.71; N, 7.75.
X-ray crystallography
Xꢀray crystallographic measurements of the single crystals were
carried out on a Bruker SMART CCD diffractometer (Bruker
Daltonic Inc.) using monochromated Mo Kα radiation (λ =
0.71073 Å) at room temperature. Cell parameters were retrieved
27
using SMART software and refined using SAINT program in
order to reduce the highly redundant data sets. Data were
collected using a narrowꢀframe method with scan width of
0.30°in ω and an exposure time of 5 s/frame. Absorption
1
Ir2. (yield: 53%): H NMR (300 MHz, CDCl3) δ 8.76 (d, J =
5.8 Hz, 1H), 8.13 (d, J = 8.2 Hz, 1H), 8.08 (d, J = 7.1 Hz, 2H),
8.05 (s, 1H), 7.91 – 7.78 (m, 3H), 7.66 (d, J = 8.4 Hz, 2H), 7.47
(d, J = 8.3 Hz, 2H), 7.23 – 7.11 (m, 3H), 6.96 (dd, J = 11.4, 5.4
Hz, 1H), 6.61 (d, J = 8.6 Hz, 1H), 6.52 – 6.40 (m, 1H). HR EIꢀ
MS: m/z calcd for C39H18F15IrN6OS: 1096.0731, found:
1097.0727 [M+H]+. Anal. Calcd for C39H18F15N6OSIr: C, 42.75;
H, 1.66; N, 7.67. Found: C, 42.79; H, 1.62; N, 7.71.
28
corrections were applied using SADABS supplied by Bruker.
The structures were solved by Patterson methods and refined by
fullꢀmatrix leastꢀsquares on F2 using the program SHELXSꢀ97.29
The positions of metal atoms and their first coordination spheres
were located from directꢀmethods Eꢀmaps; other nonꢀhydrogen
atoms were found in alternating difference.
OLEDs fabrication and measurement
Acknowledgements
All OLEDs with the emission area of 0.1 cm2 were fabricated on
the preꢀpatterned ITOꢀcoated glass substrate with a sheet
resistance of 15 ꢂ/sq. All chemicals used for devices were
sublimed in vacuum (2.0 × 10ꢀ4 Pa) prior to use. The deposition
rate for organic compounds is 1ꢀ2 Å/s. The phosphor and host
were coꢀevaporated from two separate sources. The cathode
consisting of LiF/Al was deposited by evaporation of LiF with a
deposition rate of 0.1 Å/s and then by evaporation of Al metal
with a rate of 3 Å/s. The effective area of the emitting diode is
0.1 cm2. The characteristics of the devices were measured with a
computer controlled KEITHLEY 2400 source meter with a
calibrated silicon diode in air without 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.
This work was supported by the National Natural Science
Foundation of China (21371093, 91433113), the Major State
Basic Research Development Program (2013CB922101) and the
Natural Science Foundation of Jiangsu Province (BK20130054).
Notes and references
a
State Key Laboratory of Coordination Chemistry, Collaborative
Innovation Center of Advanced Microstructures, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing 210093, P. R.
China, eꢀmail: yxzheng@nju.edu.cn
b
MaAnShan HighꢀTech Research Institute of Nanjing University,
MaAnShan, 238200, P. R. China
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8 | J. Name., 2012, 00, 1-3
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Graphical abstract
100
D1
D2
10
1
0
2000
4000
6000
8000
10000
Luminance (cd m-2
)
Highly efficient OLEDs based on green and orange iridium(III) complex
based on 2',6'-bis(trifluoromethyl)-2,4'-bipyridine and
2-(5-(4-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)phenol show peak
current efficiencies of 74.8 and 41.0 cd⋅A-1, respectively, with low
efficiency roll-off.