Highly efficient green phosphorescent organic light-emitting diodes with low efficiency roll-off based on iridium(III) complexes bearing oxadiazol-substituted amide ligands

Article abstract of DOI:10.1039/c6tc01041e

Two novel iridium(iii) complexes [Ir(ppy)₂(PhOXD)] (1, ppy = 2-phenylpyridine, PhOXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide) and [Ir(ppy)₂(POXD)] (2, POXD = N-(5-phenyl-1,3,4-oxadiazol-2-yl)-diphenylphosphinic amide) have been

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Highly Efficient Green Phosphorescent Organic Light-Emitting Diodes
with Small Efficiency Roll-Off Based on Iridium(III) Complexes Bearing
Oxadiazol-Substituted Amide Ligands
Fuli Zhang,a Weiling Li,b Yue Yu,c Yiming Jing,d Dongxin Ma,e Fuqiang Zhang,a Suzhi Li,a Guangxiu
Cao,a Zhongyi Li,a Ge Guo,a Bin Wei,*b Youxuan Zheng,d Depan Zhang,a Lian Duan,e Zhaoxin Wu,*c
Chunyang Li,a Yafei Feng,a and Bin Zhai*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two novel iridium(III) complexes [Ir(ppy)2(PhOXD)] (1, ppy = 2ꢀphenylpyridine, PhOXD = Nꢀ(5ꢀ
10 phenylꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)ꢀbenzamide) and [Ir(ppy)2(POXD)] (2, POXD = Nꢀ(5ꢀphenylꢀ1,3,4ꢀ
oxadiazolꢀ2ꢀyl)ꢀdiphenylphosphinic amide) have been designed and synthesized, and their
photoluminescence and electrochemistry properties were investigated. At room temperature, complexes 1
and 2 show green emissions at about 502 and 506 nm with photoluminescence quantum yields (PLQYs)
of 0.03 and 0.05 in CH2Cl2 solutions, respectively. In the 5 wt% doped poly(methyl methacrylate)
15 (PMMA) film, the PLQYs (0.42 for complex 1, 0.52 for complex 2, respectively) increase significantly.
The organic light emitting diodes (OLEDs) with the structure of ITO/HATꢀCN (Dipyrazino[2,3ꢀf:2',3'ꢀ
h]quinoxalineꢀ2,3,6,7,10,11ꢀhexacarbonitrile), 10 nm)/TAPC (1,1ꢀbis[4ꢀ(diꢀpꢀ
tolylamino)phenyl]cyclohexane, 40 nm)/Ir complexes (10 wt%): mCP (1,3ꢀbis(9Hꢀcarbazolꢀ9ꢀyl)benzene,
20 nm)/TPBi (1,3,5ꢀTris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) show
20 good performances. In particular, device G2 based on complex 2 showed superior performances with a
peak current efficiency (ηc) of 64.7 cd Aꢀ1 and a peak power efficiency (ηp) of 42.5 lm Wꢀ1 with low
electroluminescence efficiency rollꢀoff. The ηc data still remain over 60.6 cd Aꢀ1 even at the luminance of
10000 cd mꢀ2, which indicated that introducing electron transporting 1,3,4ꢀoxadiazole and diphenyl
phosphoryl groups into iridium complexes is an effective means of achieving efficient phosphors in
25 OLEDs.
significantly improved efficiency with peak external quantum
Introduction
efficiency of 12.6% corresponding to current efficiency of 44.3 cd
A–1. (ii) Adopting mixed host to balance charge carriers in EML
50 and broaden the recombination zone. Kim et al. achieved 10%
rollꢀoff of current efficiency at a luminance of 20000 cd m–2 in
Ir(ppy)3 doped in TCTA: TPBi mixed host with multilayer
device.6 Nevertheless, aforementioned complicated design
architecture of phosphorescent OLEDs is undesirable from the
55 manufacturing perspective. Recently, Zheng et al. developed a
series of iridium complexes using electron transporting
tetraphenylimidodiphosphinate (tpip) as ancillary ligand, which
can enhance the recombination probability of electrons and holes,
broaden the recombination zone, and reduce leakage current in
60 the devices.7 The devices based on these complexes exhibited low
rollꢀoff effect with high current efficiency and power efficiency.
These results showed that highly efficient electron mobility is
crucial to improve the efficiency of electrophosphorescent
OLEDs at high current density. As the first ligand of
65 europium(III) complex, oxadiazolꢀsubstituted amide has been
proved good electronꢀtransporting property, which make them
good candidates for photoelectric devices.8 However,
Since influential and pioneering work made by Forrest et al.,1
heavy transition metal complexes have been attracting increasing
attention due to their harvesting both singlet and triplet excitons
30 for phosphorescence emission to present internal quantum
efficiency of 100% in OLEDs,2 which are superior to fluorescent
emitters, for which only singlet excitons can be harvested, giving
an upper efficiency limit of 25%. However, these phosphorescent
organic lightꢀemitting diodes (PhOLEDs) always suffer from a
35 decrease of efficiency when the driving current density increases,
which is called efficiency rollꢀoff effect.3
The rollꢀoff phenomenon is mainly caused by triplet–triplet
annihilation (TTA)3a originating from the long lifetime of triplet
excited state and triplet–polaron quenching (TPQ),4 which limits
40 the current efficiency of PHOLEDs at high current density. To
address the quantum efficiency rollꢀoff issue, some solutions have
been employed: (i) Employing multilayer device structure to
reduced losses of triplet excitons at the interface adjacent to light
emitting layer (EML). Zhou et al. fabricated device with double
45 lightꢀemitting layers by doping both hole (TCTA) and electron
(Bphen) transport hosts with Ir(ppy)3.5 This PHOLEDs show
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DOI: 10.1039/C6TC01041E
phosphorescent heteroleptic Ir(III) complexes using oxadiazolꢀ
substituted amide derivatives as ligands have not been reported.
In this study, we report two novel green emitting iridium(III)
complexes, [Ir(ppy)2(PhOXD)] (1) and [Ir(ppy)2(POXD)] (2,
Good thermal stability of emitters is necessary to fabricate
efficient OLEDs. The decomposition temperatures of the
complexes were determined from the thermogravimetric analysis
(TGA) curves under a nitrogen atmosphere. From the TGA
5
Scheme 1), with 2ꢀphenylpyridine (ppy) as the cyclometalated 35 curves it can be observed that the decomposition temperature (5%
o
o
ligand
and
Nꢀ(5ꢀphenylꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)ꢀbenzamide
and Nꢀ(5ꢀphenylꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)ꢀ
loss of weight) are 178 C for 1 and 351 C for 2, respectively,
indicating that complex 2 is more stable than complex 1.
(HPhOXD)
diphenylphosphinic amide (HPOXD) as the ancillary ligands,
respectively. OLEDs based on these complexes showed highly
Photophysical properties
The absorption and photoluminescent (PL) spectra of complexes
40 1 and 2 in a dilute CH2Cl2 solution and thin film are shown in
Figure 2, and detailed photophysical characterizations are
summarized in Table 2. The strong absorption bands in the UV
10 efficient electroluminescence properties with small efficiency
rollꢀoff due to the electron transporting groups of 1,3,4ꢀ
oxadiazole and diphenyl phosphoryl in ancillary ligands.
1
region below about 360 nm are attributed to spinꢀallowed π−π*
Results and discussion
transitions of the ligands and the relatively weak absorption
45 bands extending to the visible region assigned to excitations to
1MLCT (metalꢀtoꢀligand chargeꢀtransfer), 1LLCT (ligandꢀtoꢀ
ligand chargeꢀtransfer), 3MLCT, 3LLCT, and ligandꢀcentered (LC)
3π−π* transitions.9
Preparation and characterization of compounds
15 Scheme 1 depicts the synthetic routes toward the ancillary ligands
and complexes 1 and 2. Two oxadiazolꢀsubstituted amide
ancillary ligands HPhOXD and HPOXD were readily synthesized
from acyl chloride and amine in pyridine.8 Complexes 1 and 2
were synthesized by reacting the dimeric iridium(III)
20 intermediate [Ir(ppy)2Cl]2 with potassium salt KPhOXD and
KPOXD of the ancillary ligands according to the literature
procedure.7a The ancillary ligand HPOXD and complexes 1 and 2
1
were fully structurally characterized by H NMR (Figure S1−S3
in the SI†), ESI (electrospray ionization)ꢀMS (Figure S4 in the
25 SI†), and elemental analysis.
50 Figure 2. Absorption and PL spectra of (a) complex 1 and (b) complex 2
O
in degassed CH2Cl2 and as thin film.
O
O
N
N
N
N
Cl
N
N
O
O
K
H
NH2
O
N
As depicted in Figure 2, complexes 1 and 2 show green
emission (peaks at 502 and 506 nm, respectively) in dilute
CH2Cl2 solutions at room temperature. When cooled to 77 K, 1
55 and 2 exhibit much more structured emission spectra with little
rigidochromic blue shift of the 0–0 transition compared to their
emission spectra at room temperature. This indicates that the
emissive excited states of complexes 1 and 2 have strong LC
3π−π* character.10 In degassed CH2Cl2 solutions, the
60 photoluminescent quantum yields (PLQYs, 0.03 for complex 1
and 0.05 for complex 2) are much less than those of other ppyꢀ
based iridium(III) complexes.11 The excitedꢀstate lifetimes (τ) of
1.79 ꢁs for 1 and 1.21 ꢁs for 2, respectively, which are almost
equal to those in previously reported iridium(III) complexes.7d In
KOH
KPhOXD
N
N
P
N
,
N
O
P
N
N
O
N
N
P
N
O
N
O
K
H
O
Cl
KPOXD
N
O
N
N
N
Ir
O
KPhOXD
2
EtOEtOH, N2, reflux for 12 h
complex 1
Cl
Ir
Ir
N
Cl
N
KPOXD
2
2
N
EtOEtOH, N2, reflux for 12 h
O
N
N
Ir
N
P
O
2
complex 2
65 the neat films, complexes 1 and 2 exhibit fairly broad
structureless emission band accompanied by large red shift (with
peaks at 520 and 524 nm, respectively), and unusually longer
lifetimes. These phenomena indicate that the broad emission band
in neat films may be attributed to the excimer emission.12
70 Complexes 1 and 2 also exhibit low PLQYs of 0.02 and 0.03,
respectively, because of the existing severe excitedꢀstate
quenching in the neat film.9a In the 5 wt% doped poly(methyl
methacrylate) (PMMA) film, the PLQYs (0.42 for complex 1,
0.52 for complex 2, respectively) increase significantly due to the
Scheme 1 Synthetic routes of ligands and complexes.
Thermal stability
75 suppressed intramolecular rotations, vibrations and excitedꢀstate
13
quenching
in
doped
polymer
film.9f,
30 Figure 1. The TGA curves of complexes 1 and 2.
2
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Table 1. Photophysical characteristics of complexes 1 and 2.
Emission at
77 K [e]
Emission at room temperature
Absorption λ [nm]
[a]
Φem [τ[ꢁs]]
5 wt% in PMMA
Solutionλ[nm] [b]
Neat filmλ[nm] [c]
λ[nm] [d]
495, 528 sh
498, 532 sh
CH2Cl2 [b]
0.03[1.79]
NeatFilm [c]
0.02 [10.81]
1
2
262/334/435
256/327/440
502
506
520
524
0.42 [0.92]
0.05[1.21]
0.52 [1.08]
0.03[21.11]
[a] In CH2Cl2 solutions (1 × 10ꢀ5 M). [b] In degassed CH2Cl2 solutions. [c] Neat films were made on quartz substrates with thicknesses of about 100 nm.
[d] The symbol sh denotes the shoulder wavelength. [e] In CH2Cl2 glass at 77 K.
results of population calculation performed on the optimized S0
30 geometries in CH2Cl2 solution. Figure 4 displays the atomic
Electrochemical properties
5
Electrochemical properties of complexes 1 and 2 were
orbital composition of the HOMOs and LUMOs for complexes 1
and 2. As expected, the HOMO orbitals of all complexes were
composed of a mixture of Ir dπ orbitals and phenyl π orbitals
distributed among the cyclometalated ligands, which are similar
35 to those of other ppyꢀbased iridium complexes.7c The LUMO of
complex 1 reside almost completely on the ancillary ligand
PhOXD. But for complex 2, since the introduction of the
electronꢀdonating aryl phosphonate substituent on the ancillary
ligand POXD, the π*ꢀorbitals of the ancillary ligand are
40 drastically unstabilized, thus, their energy levels are higher than
that of the ppy π*ꢀorbitals, turning them into the LUMO for
complex 2.
investigated by cyclic voltammetry in THF solution. As shown in
Figure 3, except for complex 2, which exhibits a reversible
oxidation processes, other reduction and oxidation processes of
both complexes are irreversible. The oxidation potentials (vs.
10 Fc+/Fc, Fc is ferrocene) are 1.17 V for 1 and 1.02 V for 2,
respectively, which are almost identical to those other ppy based
9f, 13ꢀ14
iridium complexes.9b,
This clearly indicates a similar
HOMO localization on the phenyl ring of both the cyclometalated
ligands and iridium(III) ions. The oxidation potential of complex
15 2 is slightly shifted cathodically compared to that of complex 1,
indicating that the highest occupied molecular orbital (HOMO) of
complex 2 is unstabilized by introducing the electronꢀdonating P
atom on the ancillary ligand. The HOMO (− 5.97 eV for 1 and −
5.82 eV for 2) and lowest unoccupied molecular orbital (LUMO,
20 −2.47 eV for 1 and − 2.32 eV for 2) levels were calculated from
the oxidation and reduction potentials, respectively.
Figure 4 Molecular orbital surfaces of complexes 1 and 2: (a) LUMO
45 orbital of complex 1. (b) LUMO orbital of complex 2. (c) HOMO orbital
of complex 1. (d) HOMO orbital of complex 2. All of the molecular
orbital surfaces correspond to an isocontour value of |Ψ| = 0.025.
Figure 3. Cyclic voltammograms of complexes 1 and 2 in THF
solution.
Electron mobilities
25 Theoretical Calculations
The highly efficient electron mobility of the phosphorescent
50 emitters is crucial to facilitate the injection and transport of
DFT calculations at the B3LYP/6ꢀ31G ** + LANL2DZ level
were carried out to gain deeper insight into the photophysical and
electrochemical properties of complexes 1 and 2. We present the
electrons and balance the distribution of hole–electron, which
15
lead to suppressed TTA and TPQ effects,3a,
improved
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Characterization of OLEDs
recombination probability, high device efficiency, and low
efficiency We measured the transient
rollꢀoff.7e
electroluminescence of the device ITO/TAPC (50 nm)/ complex
1 or 2 (60 nm)/LiF (0.5 nm)/Al (100 nm) in order to obtain the
electron mobility of the two complexes. In this device, TAPC acts
as the holeꢀtransport layer, complexes 1 and 2 are used as both
the electronꢀtransport and emissive layer. As depicted in Figure 5,
the electron mobilities of complexes 1 and 2 in 60 nm layers are
between 6.12 – 7.30 ×10–6 cm2 V–1 s–1 and 7.41 – 8.57 ×10–6 cm2
5
10 V–1 s–1, respectively, under an electric field from 1100 (V cm–1)1/2
to 1300 (V cm–1)1/2, higher than that of the popular electron
transport material Alq3 under the same electric fields (4.74 – 4.86
–6
× 10
cm2 V–1 s–1).16 In addition, the electron mobility of
complex 2 is higher than that of complex 1 suggesting that the
15 incorporation of phosphine oxide group into the molecule may
further enhance the electron mobility of the complex due to the
two electron transport units of phosphine oxide and 1,3,4ꢀ
oxadiazole in complex 2.
Figure 6. Energy level diagrams of HOMO and LUMO levels (relative to
25 vacuum level) and molecular structures for the materials investigated.
In order to evaluate the EL properties of greenꢀemitting
complexes 1 and 2, we fabricated the devices (G1 for complex 1
and G2 for complex 2) and employed them as doped emitters by
thermal evaporation. The device structure is ITO/HATꢀCN (10
30 nm)/TAPC (40 nm)/1 (or 2) (10 wt %): mCP (20 nm)/TPBi (40
nm)/LiF (1 nm)/Al (100 nm) (ITO is indium tin oxide, HATꢀCN
is
dipyrazino[2,3ꢀf:2',3'ꢀh]quinoxaline
TAPC is
2,3,6,7,10,11ꢀ
1,1ꢀbis[4ꢀ(diꢀpꢀ
hexacarbonitrile,
tolylamino)phenyl]cyclohexane, mCP is 1,3ꢀbis(9Hꢀcarbazolꢀ9ꢀ
35 yl)benzene, TPBi is 1,3,5ꢀTris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀ
yl)benzene) (Figure 6). HATꢀCN and TAPC were employed as
the holeꢀinjecting and holeꢀtransporting layers, respectively. TPBi
acts as both an exciton blocker and electron transporter and
LiF/Al as the electronꢀinjection layer and cathode. The optimized
40 Ir(III) emitters with 10 wt% doped concentration in host material
mCP were used as the emitting layer.
20 Figure 5. Electric field dependence of charge electron mobility in the thin
films of complexes 1 and 2.
Table 2. EL performances of the devices G1, G2, G2a and G2b.
Vturnꢀon [d]
[V]
Lmax(voltage) [e]
[cd mꢀ2 (V)]
ηc,max(voltage) [f]
[cd Aꢀ1 (V)]
ηp,max(voltage) [g]
[lm Wꢀ1 (V)]
ηext
[% (V)]
ηc,L10000 [h]
[cd Aꢀ1]
CIE [i]
(x, y)
Device
G1 [a]
2.8
3.0
2.7
2.8
26286 (10.5)
57185 (11.6)
31212 (7.0)
26784 (8.5)
23.7 (4.8)
64.7 (7.8)
48.7 (3.1)
55.9 (4.4)
15.8 (4.6)
42.5 (4.6)
49.3 (3.1)
47.5 (3.5)
7.9 (4.9)
19.8 (4.8)
14.6 (3.1)
17.0 (4.4)
20.8
60.6
34.7
47.8
(0.25, 0.64)
(0.22, 0.60)
(0.24, 0.60)
(0.23, 0.61)
G2 [a]
G2a [b]
G2b [c]
[a] The emission layer thicknesses: 20 nm. [b] The emission layer thicknesses: 10 nm. [c] The emission layer thicknesses: 30 nm. [d] Vturnꢀon: Turnꢀon
voltage recorded at a luminance of 1 cd mꢀ2. [e] Lmax: maximum luminance. [f] ηc,max: maximum current efficiency. [g] ηp,max: maximum power efficiency.
45 [h] ηc,L10000: current efficiency at 10000 cd mꢀ2. [i] Taken at 6 V.
completely, indicating that the complete energy and/or charge
The electroluminescence (EL) spectra, potential (V)
ꢀ
55 transfer occurs from the host to the phosphors upon electrical
excitation. In addition, the emission wavelength and profile of EL
spectra of devices closely resemble those of PL spectra in
solutions or thin films, indicating that electroluminescence
originated from the triplet excited states of the phosphors 1 and 2.
luminance (L), current density (J) ꢀ current efficiency (ηc) and
current density (J) ꢀ power efficiency (ηp) of the OLEDs based on
complexes 1 and 2 are depicted in Figure 7, and detailed
50 electrical characteristics are summarized in Table 2. As illustrated
in Figure 7, devices G1 and G2 emit green light with CIE
coordinates of (0.25, 0.64) and (0.22, 0.60), respectively. In the
EL spectra, the emission from mCP (373 nm) disappears
4
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For
a better understanding of the correlation between
efficiency rollꢀoff and emission layer thickness, the devices based
on complex 2 with different emission layer thicknesses of 10 nm
(for device G2a) and 30 nm (for device G2b), respectively, were
40 fabricated and detailed electrical characteristics are summarized
in Table 2. The EL spectra of devices G2a and G2b, as in the
Figure S6 (SI†), show no residual peak from host materials even
at high luminance (Figure S7, SI†), indicating a complete energy
transfer from host to guest. Figure 8 depicts the current efficiency
45 – luminance – external quantum efficiency curves of devices G2a
and G2b. As depicted in Table 2 and Figure 8, the device G2a
with an emission layer thickness of 10 nm exhibits a heavier
efficiency rollꢀoff (28.7%) at the brightness of 10000 cd m−2 than
device G2b (14.5%) with an emission layer thickness of 30 nm,
50 indicating a more serious TTA within OLEDs with a 10 nm
emission layer thickness than that in 30 nm one.
Figure 7. Characteristics of devices of G1 and G2 with configuration
ITO/HATꢀCN (10 nm)/TAPC (40 nm)/1 (or 2) (10 wt %): mCP (20
nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm): (a) electroluminescence
spectra; (b) potential (V) ꢀ luminance (L) – current density (J) curves; (c)
luminance (L) ꢀ current efficiency (ηc) curves; (d) current density (J) ꢀ
power efficiency (ηp) curves.
The aboveꢀmentioned results suggest these iridium complexes
are good green phosphors and have potential applications in
OLEDs with high luminance and high current efficiency.
55 Ongoing novel iridium(III) complexes with oxadiazolꢀsubstituted
amide ligands and corresponding device studies are currently in
progress.
5
The OLEDs based on complexes 1 and 2 exhibit good EL
performances. Devices G1 and G2 display low turnꢀon voltages
10 (Vturnꢀon) (2.8 and 3.0 V for G1 and G2, respectively) and high
brightness (26286 cd m−2 at 10.5 V for G1 and 57185 cd m−2 at
11.6 V for G2, respectively). For device G1, a ηc,max of 23.7 cd
A−1 (4.8 V), a ηp,max of 15.8 lm Wꢀ1 (4.6 V) and a ηext of 7.9% (4.9
V) are achieved. For device G2, a ηc,max of 64.7 cd A−1 (7.8 V), a
15 ηp,max of 42.5 lm Wꢀ1 (4.6 V) and a ηext of 19.8% (4.8 V) are
achieved.
As depicted in Figure 7 and Figure S5 (SI†), the EL
efficiencies of G1 and G2 decrease gradually with a further
increase of current density and luminance. This phenomenon is
20 the soꢀcalled EL efficiency rollꢀoff and can be attributed to the
triplet–triplet annihilation (TTA)3a originating from the long
lifetime of triplet excited state and triplet–polaron quenching
(TPQ).4 Both devices keep high efficiency at relative high current
density and the rollꢀoff of EL efficiency is very low, thus
25 obtaining a high maximum brightness. For example, the current
efficiencies for devices G1 and G2 still remain as high as 20.8
and 60.6 cd A−1 even at the brightness of 10000 cd m−2 with the
decline of 12.5% and 0.7% compared with ηc,max (Figure S5 in the
Conclusions
In conclusion, two new greenꢀemitting iridium complexes,
60 [Ir(ppy)2(PhOXD)] (1) and [Ir(ppy)2(POXD)] (2), with
oxadiazolꢀsubstituted amide as the ancillary ligand, were
developed and fully characterized, which bear electronꢀ
transporting oxadiazole and/or P=O groups within the molecule.
The green PhOLEDs (ITO/HATꢀCN (10 nm)/TAPC (40 nm)/1
65 (or 2) (10 wt %): mCP (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100
nm) show superior performance with maximum luminance of
26286/57185 cd mꢀ2, maximum current efficiencies of 23.7/64.7
cd A−1 and maximum external quantum efficiencies of 7.9/19.8%
along with low efficiency rollꢀoff, respectively. The current
70 efficiencies for devices still remain as high as 20.8/60.6 cd A−1
even at the brightness of 10000 cd m−2 with the decline of
12.5/0.7 % compared with ηc,max. The research work presented
here paves a new way toward highly efficient phosphorescent
OLEDs.
SI†). The devices based on complex
2 exhibit better
75 Experimental
30 electroluminescent properties compared with those of complex 1.
This is mainly because of higher PLQYs, well thermal stability
and bettter electronꢀtransporting performance of P=O group in
complex 2.
Materials and measurements. Chemicals and reagents were
purchased from commercial sources and used without further
purification unless otherwise stated. 1H NMR spectra were
recorded on a JOEL JNMꢀECA600 NMR spectrometer with
80 tetramethylsilane as the internal standard. Elemental analysis was
determined on an Elementar Vario EL CHN elemental analyzer.
Mass spectrometry was performed with a Thermo Electron
Corporation Finnigan LTQ mass spectrometer. Absorption
spectra were recorded with a UV–vis spectrophotometer (Agilent
85 8453)
and
PL
spectra
were
recorded
with
a
fluorospectrophotometer (Jobin Yvon, FluoroMaxꢀ3). The excited
state lifetimes of the complexes were measured by the timeꢀ
correlated singleꢀphoton counting (TCSPC) upgrade on the
FluoroMaxꢀ4 spectrometer with a FluoroHub module. Absolute
90 photoluminescence quantum efficiency at room temperature were
35 Figure 8. ηc – L ꢀ ηext curves of devices G2a and G2b.
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Journal of Materials Chemistry C
Page 6 of 8
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DOI: 10.1039/C6TC01041E
measured with an absolute photoluminescence quantum yield
7.8 Hz, 1H), 7.44–7.38 (m, 6H), 7.28 (d, J = 7.8 Hz, 3H), 7.23 (t,
measurement system (Hamamatsu, C9920ꢀ02). Cyclic 60 J = 7.2 Hz, 1H), 7.07–7.05 (m, 3H), 6.91 (t, J = 7.8 Hz and 7.2 Hz,
voltammetry was performed on a CHI 820C electrochemical
station (Shanghai Chenhua Co., China) in THF solutions (1 ×
10ꢀ3 M) at a scan rate of 100 mV sꢀ1 with a platinum plate as the
working electrode, a silver wire as the pseudoꢀreference electrode,
1H), 6.84–6.79 (m, 2H), 6.66 (t, J = 7.2 Hz, 1H), 6.47 (t, J = 7.2
Hz and 6.0 Hz, 1H), 6.36 (d, J = 7.8 Hz, 1H), 6.16 (d, J = 7.8 Hz,
1H). MS (MALDIꢀTOF) [m/z]: 862.14 (M + 1)+. Anal. found: C
58.63, H 3.67, N 8.08. Anal. Calcd for C42H31N5O2PIr: C 58.59,
5
and a platinum wire as the counter electrode. The supporting 65 H 3.63, N 8.13.
electrolyte was nꢀBu4NPF6 (0.1 M) and ferrocene was selected as
Computational Methodology. Density functional theory
the internal standard. The solutions were bubbled with argon for
10 10 min before measurements.
(DFT) calculations using the B3LYP functional were performed.
“Doubleꢀξ” quality basis sets were employed for the C, H, N O
and P (6ꢀ31G**)17 and the Ir (LANL2DZ)18. An effective core
The amide ligand
Nꢀ(5ꢀphenylꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)ꢀ
benzamide (HPhOXD) were prepared according to the literature 70 potential (ECP) replaces the innerꢀcore electrons of Ir leaving the
procedure. 8
outer core 5s25p6 electrons and the 5d6 valence electrons of
Ir(III). The solvent effect of CH2Cl2 was taken into consideration
using a conductorꢀlike polarizable continuum model (CꢀPCM).
All of the calculations were performed with Gaussian 03 software
Synthesis
of
N-(5-phenyl-1,3,4-oxadiazol-2-yl)-
15 diphenylphosphinic amide (HPOXD). Diphenylphosphinyl
chloride (1.44 g, 6.08 mmol) in 5 mL pyridine was dropped
slowly into the stirred slurry of 2ꢀAminoꢀ5ꢀphenylꢀ1,3,4ꢀ 75 pakage19.
oxadiazole (0.64 g, 4.0 mmol) and 4ꢀdimethylaminopyridine
Fabrication and Characterization of OLEDs. Indiumꢀtinꢀ
(0.048 g, 0.40 mmol) in 10 mL pyridine. The mixture was stirred
20 at 60 °C for 16 h. After cooling, the solution was poured into
water, and the white precipitate was collected and dried under
vacuum. Pure product (1.16 g, 3.16 mmol) was obtained by reꢀ
crystallization from ethanol. Yield: 79%, white solid. H NMR
(600 MHz, CDCl3, δ): 7.94–7.89 (m, 6H), 7.53–7.40 (m, 10H).
25 MS (MALDIꢀTOF) [m/z]: 362.3 (M + H)+.
Synthesis of KPhOXD and KPOXD. The potassium salts
KPhOXD and KPOXD were synthesized according to similar
procedures.7a HPhOXD or HPOXD (2.0 mmol) was dissolved in
20 mL methanol solution and a solution of KOH (0.12 g, 2.0
30 mmol) in 1 mL methanol was added dropwise. After reacting for
two hours at room temperature, the solvent volume was
concentrated to 2 mL and diethyl ether was added (20 mL), a
white solid was obtained with a yields of 80% (0.49 g, 1.60 mmol)
for KPhOXD and 74% (0.59 g, 1.48 mmol) for KPOXD.
Synthesis of Ir(ppy)2(PhOXD) (1). The dichloroꢀbridged
diiridium complex [Ir(ppy)2Cl]2 (0.50 g, 0.47 mmol) and 2.2
equivalent KPhOXD (0.31 g, 1.02 mmol) were dissolved in 2ꢀ
methoxyethanol (10 mL). The mixture was then refluxed at
130 °C for 12 h under an argon atmosphere. The crude compound
oxide (ITO) substrates with sheet resistance of 15 Ω/sq were
sufficiently cleaned using chemical and UVꢀozone methods
before the deposition of the organic layers. The 10 nm holeꢀ
80 injecting material of HATꢀCN film was first deposited on the ITO
glass substrate. The transporting material of TAPC film (40 nm)
was then deposited on the holeꢀinjecting layer. The phosphor (10
wt%) and mCP host were coꢀevaporated to form 20 nm emitting
layer from two separate sources. Then, TPBi (40 nm), LiF (1
85 nm), and Al (100 nm) were evaporated, respectively. The vacuum
was less than 1 × 10–6 Pa during deposition of all materials. The
L–V characteristics and η–J curves of the devices were recorded
with Keithley 2400/2000 semiconductor characterization system.
The electroluminescent spectra were collected with a Photo
90 Research PR650 Spectrophotometer. The ηext of the device was
calculated based on the electroꢀoptic characteristics. The
procedure to calculate the ηext can be seen in Supporting
Information. All measurements of the devices were carried out in
ambient atmosphere without further encapsulations.
1
35
95 Acknowledgements
40 was filtered after the solution cooled to room temperature and
purified by column chromatography on silica gel (200–300 mesh)
with petroleum ether/ethyl acetate (1:2) as the eluent, yielding a
We would like to thank the National Natural Science Foundation
of China (Grant Nos. 21501117, 21371114, 21271125, 21201116,
21401126), the Program for Science and Technology Innovation
Talents in Universities of Henan Province (2012HASTIT031),
100 the Key Scientific Research Projects in University of Henan
Province (Grant Nos. 15A150023, 2010A150018) and Key
Teacher Project of Shangqiu Normal University (2012GGJS15)
for financial support. We also thank Prof. Haishun Wu of Shangxi
Normal University for the thoretical calculations.
1
yellowꢀgreen powder (0.45 g, 0.59 mmol). Yield: 63%. H NMR
(600 MHz, CDCl3, δ): 8.61 (d, J = 6.0 Hz, 1H), 8.39 (d, J = 6.0
45 Hz, 1H), 8.09 (d, J = 7.8 Hz, 2H), 7.88–7.84 (m, 2H), 7.79 (d, J =
7.8 Hz, 2H), 7.71–7.67 (m, 2H), 7.64 (d, J = 7.8 Hz, 1H), 7.59 (d,
J = 7.8 Hz, 1H), 7.43–7.34 (m, 4H), 7.29 (t, J = 7.8 Hz and 7.2 Hz,
2H), 7.06–7.01 (m, 2H), 6.93 (t, J = 7.8 Hz and 7.2 Hz, 1H), 6.89
(t, J = 7.2 Hz, 1H), 6.81 (t, J = 7.8 Hz and 7.2 Hz, 1H), 6.76 (t, J
50 = 7.8 Hz and 7.2 Hz, 1H), 6.40 (d, J = 7.8 Hz, 1H), 6.35 (d, J =
7.2 Hz, 1H). MS (MALDIꢀTOF) [m/z]: 766.19 (M + 1)+. Anal.
found: C 58.13, H 3.46, N 9.11. Anal. Calcd for C37H26N5O2Ir: C
58.10, H 3.43, N 9.16.
105 Notes and references
a College of Chemistry and Chemical Engineering, Shangqiu Normal
University, Shangqiu 476000, P. R. China. *Eꢀmail:
zhaibin_1978@163.com
b School of Materials Science and Engineering, Shanghai University,
110 Shanghai 200072, P. R. China. *Eꢀmail:
bwei@shu.edu.cn
Synthesis of Ir(ppy)2(POXD) (2). The synthesis of complex 2
55 is similar to that of complex 1, except that KPhOXD was
1
replaced with KPOXD. Yield: 76%. H NMR (600 MHz, CDCl3,
c Key Laboratory of Photonics Technology for Information, Xi’an
Jiaotong University, Xi’an 710049, P. R. China. *Eꢀmail:
zhaoxinwu@mail.xjtu.edu.cn
δ): 9.24 (d, J = 5.4 Hz, 1H), 8.24 (d, J = 6.0 Hz, 1H), 7.80–7.77
(m, 3H), 7.73 (d, J = 7.8 Hz, 1H), 7.65–7.59 (m, 4H), 7.51 (d, J =
6
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C6TC01041E
d State Key Laboratory of Coordination Chemistry, Collaborative
Innovation Center of Advanced Microstructures, Collaborative
Innovation Center of Chemistry for Life Sciences, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing 210093, P. R.
China.
Colombo and H. U. Guedel, Inorg. Chem., 1993, 32, 3081; (d)
M. G. Colombo, A. Hauser and H. U. Guedel,
Inorg. Chem., 1993, 32, 3088.
H. Z. Xie, M. W. Liu, O. Y. Wang, X. H. Zhang, C. S. Lee, L.
S. Hung, S. T. Lee, P. F. Teng, H. L. Kwong, H. Zheng and C.
11(a)
12
75
80
85
5
e Key Lab of Organic Optoelectronics and Molecular Engineering of
Ministry of Education, Department of Chemistry, Tsinghua University,
Beijing 100084, P. R. China.
M. Che, Adv. Mater., 2001, 13, 1245; (b)
Y. Liu, K. Ye, Y.
Fan, W. Song, Y. Wang and Z. Hou, Chem. Commun., 2009,
3699.
S. T. Hoffmann, P. Schrögel, M. Rothmann, R. Q.
Albuquerque, P. Strohriegl and A. Köhler, J. Phys. Chem. B,
2011, 115, 414.
L. He, L. Duan, J. Qiao, D. Zhang, L. Wang and Y. Qiu, Chem.
Commun., 2011, 47, 6467.
L. He, L. Duan, J. Qiao, R. Wang, P. Wei, L. Wang and Y. Qiu,
† Electronic Supplementary Information (ESI) available. 1H NMR spectra
10 of HPOXD, complexes 1 and 2, the ESIꢀMS spectra, the luminance (L) ꢀ
current efficiency (ηc) curves of devices G1 and G2, electroluminescence
spectra and potential (V) ꢀ luminance (L) – current density (J) curves of
devices G2a and G2b see DOI: 10.1039/b000000x/
13
14(a)
Adv. Funct. Mater., 2008, 18, 2123; (b)
F. Zhang, L.
References
Duan, J. Qiao, G. Dong, L. Wang and Y. Qiu, Org. Electron.,
2012, 13, 1277.
15
20
1
M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson
15
J. Kalinowski, W. Stampor, J. Mȩżyk, M. Cocchi, D. Virgili,
V. Fattori and P. Di Marco, Phys. Rev. B, 2002, 66, 235321.
M.ꢀY. Teng, S. Zhang, S.ꢀW. Jiang, X. Yang, C. Lin, Y.ꢀX.
Zheng, L. Wang, D. Wu, J.ꢀL. Zuo and X.ꢀZ. You, Appl. Phys.
Lett., 2012, 100, 073303.
M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S.
Gordon, D. J. DeFrees and J. A. Pople, J Chem Phys, 1982, 77,
3654.
2(a)
90 16
and S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4; (b)
C.
Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A.
Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett.,
2001, 79, 2082; (c) Y. Chi and P.ꢀT. Chou, Chem. Soc. Rev.,
2010, 39, 638; (d)
T. Hofbeck and T. Fischer, Coord. Chem. Rev., 2011, 255,
2622.
17
H. Yersin, A. F. Rausch, R. Czerwieniec,
95
18
19
P. J. Hay and W. R. Wadt, J Chem Phys, 1985, 82, 299.
Gaussian 03, Revision B.05, Gaussian, Inc., Wallingford, CT,
2004.
25 3(a)
M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B, 2000,
62, 10967; (b)
Rev. B, 2008, 77, 235215; (c)
N. C. Giebink and S. R. Forrest, Phys.
W. S. Jeon, T. J. Park, S. Y.
Kim, R. Pode, J. Jang and J. H. Kwon, Appl. Phys. Lett., 2008,
93, 063303.
30
35
40
45
50
4
5
S. Reineke, K. Walzer and K. Leo, Phys Rev B, 2007, 75,
125328.
X. Zhou, D. S. Qin, M. Pfeiffer, J. BlochwitzꢀNimoth, A.
Werner, J. Drechsel, B. Maennig, K. Leo, M. Bold, P. Erk and
H. Hartmann, Appl. Phys. Lett., 2002, 81, 4070.
6
S. H. Kim, J. Jang, K. S. Yook and J. Y. Lee, Appl. Phys. Lett.,
2008, 92, 023513.
7(a)
Y.ꢀC. Zhu, L. Zhou, H.ꢀY. Li, Q.ꢀL. Xu, M.ꢀY. Teng, Y.ꢀX.
Zheng, J.ꢀL. Zuo, H.ꢀJ. Zhang and X.ꢀZ. You, Adv. Mater.,
2011, 23, 4041; (b) C.ꢀC. Wang, Y.ꢀM. Jing, T.ꢀY. Li, Q.ꢀL.
Xu, S. Zhang, W.ꢀN. Li, Y.ꢀX. Zheng, J.ꢀL. Zuo, X.ꢀZ. You and
X.ꢀQ. Wang, Eur. J. Inorg. Chem., 2013, 2013, 5683; (c)
Q.ꢀL. Xu, C.ꢀC. Wang, T.ꢀY. Li, M.ꢀY. Teng, S.
Zhang, Y.ꢀM. Jing, X. Yang, W.ꢀN. Li, C. Lin, Y.ꢀX. Zheng, J.ꢀ
L. Zuo and X.ꢀZ. You, Inorg. Chem., 2013, 52, 4916; (d)
H.ꢀY. Li, L. Zhou, M.ꢀY. Teng, Q.ꢀL. Xu, C. Lin,
Y.ꢀX. Zheng, J.ꢀL. Zuo, H.ꢀJ. Zhang and X.ꢀZ. You, J. Mater.
Chem. C, 2013, 1, 560; (e)
Q.ꢀL. Xu, X. Liang, S.
Zhang, Y.ꢀM. Jing, X. Liu, G.ꢀZ. Lu, Y.ꢀX. Zheng and J.ꢀL.
Zuo, J. Mater. Chem. C, 2015, 3, 3694; (f) J. Wang, J. Liu, S.
Huang, X. Wu, X. Shi, C. Chen, Z. Ye, J. Lu, Y. Su, G. He and
Y. Zheng, Org. Electron., 2013, 14, 2854; (g)
J.
Wang, J. Liu, S. Huang, X. Wu, X. Shi, G. He and Y. Zheng,
Org. Electron., 2013, 14, 2682.
8
F. Zhang, Y. Hou, C. Du and Y. Wu, Dalton T., 2009, 7359.
L. He, L. Duan, J. Qiao, R. Wang, P. Wei, L. Wang and Y. Qiu,
55 9(a)
Adv Funct Mater, 2008, 18, 2123; (b)
Duan, G. Dong, D. Zhang, L. Wang and Y. Qiu, Adv. Funct.
Mater., 2009, 19, 2950; (c) M. Mydlak, C. Bizzarri, D.
Hartmann, W. Sarfert, G. Schmid and L. De Cola, Adv. Funct.
Mater., 2010, 20, 1812; (d) C.ꢀH. Yang, J. Beltran, V.
L. He, J. Qiao, L.
60
Lemaur, J. Cornil, D. Hartmann, W. Sarfert, R. Fröhlich, C.
Bizzarri and L. De Cola, Inorg. Chem., 2010, 49, 9891; (e)
L. He, L. Duan, J. Qiao, G. Dong, L. Wang and Y.
Qiu, Chem. Mater., 2010, 22, 3535; (f)
F. Zhang, D. Ma,
65
L. Duan, J. Qiao, G. Dong, L. Wang and Y. Qiu, Inorg. Chem.,
2014, 53, 6596.
10(a)
A. B. Tamayo, S. Garon, T. Sajoto, P. I. Djurovich, I. M.
Tsyba, R. Bau and M. E. Thompson, Inorg. Chem., 2005, 44,
8723; (b) M. S. Lowry, W. R. Hudson, R. A. Pascal and S.
70
Bernhard, J. Am. Chem. Soc., 2004, 126, 14129; (c)
M. G.
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DOI: 10.1039/C6TC01041E
Two new iridium(III) complexes using electron transporting oxadiazolꢀsubstituted
amide as ancillary ligand were synthesized and highly efficient OLEDs (maximum
luminance of 26286/57185 cd mꢀ2, maximum current efficiencies of 23.7/64.7 cd A−1
and maximum external quantum efficiencies of 7.9/19.8% for complexes 1 and 2,
respectively) were fabricated with low efficiency rollꢀoff.