Sky-blue-emitting cationic iridium complexes with oxadiazole/triazine-type counter-anions and their use for efficient solution-processed organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2020.108586

Counter-anion control has emerged as a facile approach to tune the properties of cationic iridium complexes for optoelectronic applications. Here we report sky-blue-emitting cationic iridium complexes with electron-deficient oxadiazole/triazine-type count

Full text of DOI:10.1016/j.dyepig.2020.108586

Journal Pre-proof
Sky-blue-emitting cationic iridium complexes with oxadiazole/triazine-type counter-
anions and their use for efficient solution-processed organic light-emitting diodes
Rubing Bai, Pei Wang, Xianwen Meng, Lei He
PII:
S0143-7208(20)30831-7
DOI:
https://doi.org/10.1016/j.dyepig.2020.108586
DYPI 108586
Reference:
To appear in: Dyes and Pigments
Received Date: 2 April 2020
Revised Date: 25 May 2020
Accepted Date: 25 May 2020
Please cite this article as: Bai R, Wang P, Meng X, He L, Sky-blue-emitting cationic iridium complexes
with oxadiazole/triazine-type counter-anions and their use for efficient solution-processed organic light-
emitting diodes, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108586.
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Author statement
Rubing Bai performed the measurements, analyzed the data and prepared the
manuscript draft, Pei Wang synthesized the compounds, Xianwen Meng fabricated
and characterized the devices, and Lei He conducted conceptualization, supervised the
project and revised the manuscript.

Sky-blue-emitting Cationic Iridium Complexes with
Oxadiazole/Triazine-type Counter-anions and Their Use for
Efficient Solution-processed Organic Light-emitting Diodes
Rubing Bai, Pei Wang, Xianwen Meng, and Lei He*
College of Chemistry, Key Laboratory of Pesticide and Chemical Biology of Ministry
of Education, Hubei International Scientific and Technological Cooperation Base of
Pesticide and Green Synthesis, Central China Normal University, Wuhan 430079,
People’s Republic of China.
E-mail: helei@mail.ccnu.edu.cn
Abstract
Counter-anion control has emerged as a facile approach to tune the properties of
cationic iridium complexes for optoelectronic applications. Here we report
sky-blue-emitting
cationic
iridium
complexes
with
electron-deficient
oxadiazole/triazine-type counter-anions and their use for highly efficient
solution-processed organic light-emitting diodes (OLEDs). A sky-blue-emitting
complex [Ir(meoppy)2(dmapzpy)]+ (meoppy is 4-methoxy-2-phenylpyridine and
dmapzpy is 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine) has been developed as the
−
phosphorescent
cation.
Hexafluorophosphate
(PF6 ),
−
3,5-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-7-SO3 ),
−
4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
(TRZ-p-SO3 )
and
−
3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-m-SO3 ) have been

developed as the counter-anions in complexes R, 1, 2 and 3, respectively. In doped
films, the complexes afford similar sky-blue emission peaked at around 472 nm with
high phosphorescent efficiencies of around 0.80. Solution-processed, double-layer
OLEDs using the complexes as dopants afford sky-blue electroluminescence peaked
at around 476 nm. The devices using complexes 1-3 show largely enhanced
performances compared to the device using the reference complex R, owing to the
improved carrier-recombination balance imparted by the electron-trapping effects of
the oxadiazole/triazine type counter-anions. The devices based on complexes 2 and 3
exhibit higher performances than that based on complex 1, because of the stronger
−
−
−
electron-trapping effects of TRZ-p-SO3 and TRZ-m-SO3 compared to OXD-7-SO3 .
The double-layer device using complex 3 shows a peak current efficiency of 29.4 cd
A−1 and an external quantum efficiency of 12.4%, which are the highest for blue
OLEDs using cationic iridium complexes reported so far.
Key words: iridium complex; cationic; counter anion; phosphorescence; organic light
emitting diode
1. Introduction
Phosphorescent cationic iridium complexes have been extensively researched in
the past decades, because of their attractive features such as easy preparation, high
phosphorescent efficiencies at room temperature, tunable emission color across the
entire visible spectrum, rich electrochemical properties, and good solubility in polar
solvents. They have been widely applied as functional agents in biological imaging or

sensing [1, 2], photo-catalysis [3], and light emitting devices [4-6]. In particular, they
have been demonstrated to be suitable dopants for solution-processed organic
light-emitting diodes (OLEDs) [4, 7-26], which, with high performances and low
fabrication cost, have emerged as promising devices for display or lighting
applications [27, 28].
Phosphorescent cationic iridium complexes have a general form of
[Ir(C˄N)2(N˄N)]+A−, with [Ir(C˄N)2(N˄N)]+ as the phosphorescent cation (C˄N is the
cyclometalating ligand and N˄N is the ancillary ligand) and A− as the counter-anion
−
(A− is usually PF6 ) [29]. In the past decade, huge efforts have been devoted to tuning
the emission properties of [Ir(C˄N)2(N˄N)]+ through judicious control of both C˄N
and N˄N ligands [4, 6-16, 18, 21-24, 26], but only limited attention has been paid to
−
A− [17, 19, 20, 25, 30-32]. The PF6 anion is electrochemically and thermally stable
but it does not afford any optoelectronic properties such as light-emitting or
carrier-transporting properties. Moreover, when the complexes are used as dopants in
−
OLEDs, the small PF6 anions migrate and redistribute within the emissive layer
under an external electrical field, which is undesired for the operation of OLED
device. Bulky anions, such as tetraphenylborate and its fluorine or
trifluoromethyl-substituted derivatives [5, 30], have been employed as counter-anions
in cationic iridium complexes, which successfully enhance the volatility of the
complexes for vacuum-evaporation devices. These bulky anions however are still
optoelectronically inactive, which cannot largely enhance the performances of the
complexes in OLEDs.

In cationic iridium complexes, [Ir(C˄N)2(N˄N)]+ and A− are closely bound
together by strong coulombic attraction between them. Therefore, they are in close
proximity yet can be independently tuned, which opens an avenue to tune the overall
optoelectronic property of the complex through counter-anion control [33, 34]. For
OLEDs, the performances are determined by carrier transport/recombination balance
in the emissive layers, which is largely affected by carrier-trapping/transporting on the
dopants [35-38]. Therefore, rendering the counter-anions of cationic iridium
complexes with carrier-trapping/transporting properties represents a facile approach to
tune the carrier transport/recombination balance in OLEDs using the complexes as
dopants [20, 25, 32]. To this end, suitable carrier trapping/transporting counter-anions
need to be developed. Because blue-emitting cationic iridium complexes have shown
much lower performances than green, yellow and orange-red-emitting ones in OLEDs
[4, 5], carrier-trapping/transporting counter-anions with high triplet energies (ET)
suitable for blue-emitting complexes are in demand.
Here we report electron-deficient oxadiazole/triazine-type counter-anions for
sky-blue-emitting cationic iridium complexes and the use of the complexes in
solution-processed OLEDs. A highly efficient, fluorine-free, sky-blue-emitting
cationic iridium complex, [Ir(meoppy)2(dmapzpy)]+, has been developed as the
phosphorescent cation (meoppy is 4-methoxy-2-phenylpyridine and dmapzpy is
4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine) (Chart 1). Previously reported
−
3,5-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-7-SO3 )
(ET = 2.7 eV) has been used as the counter-anion for complex 1 [20]. Two

triazine-type
anions,
4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
−
−
(TRZ-p-SO3 ) and 3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-m-SO3 )
(ET = 3.0 eV), have been developed as counter-anions for complexes 2 and 3,
respectively (Chart 1). The high triplet energies of these counter-anions prevent back
energy-transfer from the phosphorescent cation to the counter-anion, which helps to
maintain high luminescent efficiencies of the complexes. For comparison,
−
[Ir(meoppy)2(dmapzpy)]+PF6 (complex R) has also been prepared. In solution and
thin films, the complexes afford similar sky-blue emission peaked at 472 nm. Varying
the counter-anion does not alter the emission characteristics of
[Ir(meoppy)2(dmapzpy)]+. An efficient energy-transfer occurs from the
oxadiazole/triazine-type counter-anions to [Ir(meoppy)2(dmapzpy)]+ in doped films.
Electrochemical measurements have revealed that the two triazine-based anions show
−
stronger electron-accepting capability than OXD-7-SO3 . Solution-processed,
double-layer OLEDs using the complexes as dopants afford similar sky-blue
electroluminescence (EL) with an emission peak at 476 nm. Nevertheless, the devices
using complexes 1-3 as dopants show largely enhanced performances compared to the
reference device using complex R, owing to the electron-trapping effects of the
oxadiazole/triazine-type anions that improve the carrier-recombination balance. In
particular, the device using complex 3 with a triazine-based counter-anion shows the
highest performance, affording a peak current efficiency of 29.4 cd A−1 and an
external quantum efficiency (EQE) of 12.4%, which is the highest for blue OLEDs
(including solution-processed and vacuum-evaporated devices) based on cationic

iridium complexes reported so far [8, 16, 21, 23, 24, 26, 30, 39-44].
Chart 1. Chemical structures of the counter-anions and the complexes.
2. Results and Discussion
2.1. Synthesis and structural characterizations

Scheme 1. Synthesis of TRZ-p-SO3Na and TRZ-m-SO3Na.

Scheme 2. Synthesis of complexes R and 1-3.
Scheme 1 outlines the synthetic routes to TRZ-p-SO3Na and TRZ-m-SO3Na.
OXD-7-SO3Na was prepared according to
a
reported procedure [20].
(OXD-7) and
1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene
2,4,6-triphenyl-1,3,5-triazine (TRZ) were selected as the cores of the counter-anions
because of their electron-deficiency for trapping or transporting electrons and high
triplet energies (2.7 and 3.0 eV, respectively) [45-47]. The synthesis of TRZ-p-SO3Na
and TRZ-m-SO3Na started from bromine substituted benzenesulfonyl chloride, which,
upon esterification with neopentyl alcohol, palladium catalyzed boronation reaction
and palladium catalyzed C-C coupling reaction, leads to the triazine-based neopentyl
benzenesulfonate (TRZ-p-NeOSO2 and TRZ-m-NeOSO2). TRZ-p-SO3Na and
TRZ-m-SO3Na were then readily prepared by iodolysis of TRZ-p-NeOSO2 and

TRZ-m-NeOSO2 in acetone, respectively, in the presence of NaI. TRZ-p-SO3Na and
TRZ-m-SO3Na were isolated from the reaction mixture by filtration with high yields
(>90%) and high purity.
Scheme
2
outlines the synthetic routes to the complexes. Here
4-methoxyl-2-phenylpyridine (meoppy) was selected as the cyclometalating ligand,
because fluorine-free meoppy with a high-triplet energy is a suitable candidate for
constructing
blue-emitting
cationic
iridium
complexes
[24,
48];
4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy) was employed as the
ancillary ligand to ensure the generation of efficient blue emission from the
meoppy-centered triplet state [24, 49]. The [Ir(meoppy)2(dmapzpy)]+ cation was
readily prepared by cleaving the chloro-bridged iridium dimer [Ir(meoppy)2Cl]2 with
dmapzpy. Complexes R and 1-3 were then readily prepared by anion-exchange
reactions between [Ir(meoppy)2(dmapzpy)]+Cl− and NH4PF6, OXD-7-SO3Na,
TRZ-p-SO3Na or TRZ-m-SO3Na in CH3OH/H2O solution. The complexes were
purified by column chromatography or recrystallization and their structures were
characterized by NMR spectroscopy, high-resolution mass spectrometry and
elemental analysis.
2.2. Photophysical characterizations

Figure 1. Absorption and/or PL spectra of a) OXD-7-SO3Na, TRZ-p-SO3Na and
TRZ-m-SO3Na in CH3OH solution and b) complexes R and 1-3 in CH3CN solution.
The PL spectra of the complexes were measured under excitation at 380 nm.
TRZ-p-SO3Na and TRZ-m-SO3Na afforded no PL spectra because they were
non-emissive in the solution.
Table 1. Photophysical and electrochemical data of complexes R and 1-3.
Absorption
λ [nm]
PL in CH3CN b
PL in PMMA film c
Electrochemical
Data d
(ε [×104
λ
Φp
kr, knr
λ
Φp
kr, knr
Eox
[V]
Ered
[V]
M-1 cm-1]) a
[nm] (τ [μs]) [105 s−1]
[nm] (τ [μs]) [105 s−1]

R
1
260 (5.45), 472,
380 (0.53), 496
410 (0.27)
0.28
1.9, 4.8
2.0, 5.2
1.7, 5.2
470,
0.80
3.3, 0.8
3.3, 0.7
3.3, 0.9
0.75 qr −2.52 ir
(1.50)
497 (2.43)
269 (8.45), 473,
379 (0.54), 497
412 (0.26)
0.28
471,
0.82
0.72 qr −2.36, r
−2.55 ir
(1.39)
499 (2.51)
2
259 (7.16), 472,
380 (0.56), 496
411 (0.29)
0.25
471,
0.79
0.73 qr −2.09, r
−2.53 ir
(1.44)
499 (2.38)
3
264 (8.67), 472,
380 (0.55), 496
409 (0.30)
0.23
1.8, 6.2
471,
0.80
3.4, 0.8
0.72 qr −2.13,r
−2.57 ir
(1.25)
499 (2.38)
a
b
In CH3CN solution (10-5 M). ε denotes molar extinction coefficient. In degassed
c
d
CH3CN (10-5 M) solution. 2 wt.% doped PMMA films on quartz substrates.
Potentials were quoted versus Fc+/Fc (Fc is ferrocene); r, qr and ir denote reversible,
quasi-reversible and irreversible processes, respectively; for irreversible processes,
onset potentials were quoted.
Fig.1a depicts the absorption and/or PL spectra of TRZ-p-SO3Na and
TRZ-m-SO3Na in CH3OH solution. For comparison, those of OXD-7-SO3Na were
also shown [20]. TRZ-p-SO3Na and TRZ-m-SO3Na exhibit strong absorption bands in
the ultra-violet region with the absorption onsets at around 320 nm, which correspond
to the absorption from the 2,4,6-triphenyl-1,3,5-triazine core. From the absorption
onsets, the optical bandgaps of TRZ-p-SO3Na and TRZ-m-SO3Na were calculated at
3.9 eV, which are larger than that (3.7 eV) of OXD-7-SO3Na. While OXD-7-SO3Na
emits ultra-violet light centered at 360 nm [20], TRZ-p-SO3Na and TRZ-m-SO3Na are
almost non-emissive in CH3OH solution.
Fig. 1b shows the absorption and photoluminescence (PL) spectra of complexes R
and 1-3 in CH3CN solution and Table 1 compiles detailed photophysical

characteristics. Complex R exhibits strong absorption bands in the ultra-violet region
below 350 nm, which are ascribed to ligand-centered 1π-π* transitions in
[Ir(meoppy)2(dmapzpy)]+, and weak absorption bands that extend to the visible region,
which are assigned to metal-to-ligand charge-transfer (1MLCT), ligand-to-ligand
charge-transfer (1LLCT), 3MLCT, 3LLCT and ligand-centered 3π-π* transitions
[50-52]. The strong spin-orbital coupling on the heavy iridium atom makes the
spin-forbidden triplet transitions partially permitted [50, 51]. As shown in Fig. 1b,
complexes 1-3 show almost the same weak absorption bands above 350 nm, which
should come from the [Ir(meoppy)2(dmapzpy)]+ cation. Complexes 1-3 show much
stronger absorption bands than complex R below 350 nm in the ultra-violet region,
which should result from the superposition of absorption of [Ir(meoppy)2(dmapzpy)]+
and the counter-anion.
As shown in Fig. 1b, complexes R shows sky-blue emission peaked at 472 and
496 nm in degassed CH3CN solution. The structured emission spectrum indicates that
1
the emissive excited-state contains notable ligand-centered π-π* character. The
sky-blue emission from complex R is very similar to that afforded by an analogous
complex [Ir(buoppy)2(dmapzpy)]PF6 (buoppy is 4-butoxy-2-phenylpyridine) [24], of
3
which the emission has been assigned to the 3MLCT (Ir→buoppy)/ π-π*
(buoppy-centered) state. Because the only difference between complex R and
[Ir(buoppy)2(dmapzpy)]PF6 is the length of alkyl chain anchored to the
cyclometalating ligand, the emission of complex R should have similar excited-state
character to [Ir(buoppy)2(dmapzpy)]PF6, i.e., its emission should arise from the

3
3MLCT (Ir → meoppy)/ π-π* (meoppy-centered) state. The use of highly
electron-rich dmapzpy ancillary ligand ensures the generation of highly efficient
3
sky-blue emission from the cyclometalating ligand with a high π-π* triplet energy
such as buoppy and meoppy [24, 48]. In degassed CH3CN solution, complex R
affords a luminescent efficiency (Φ) of 0.28 and an excited-state lifetime (τ) of 1.50
μs, from which the radiative (kr) and non-radiative (knr) decay rates were calculated at
1.9×105 s−1 and 4.8×105 s−1, respectively.
In degassed CH3CN solution, complexes 1-3 show almost identical PL spectra to
complex R, when only the [Ir(meoppy)2(dmapzpy)]+ cation is excited at the excitation
wavelength of 380 nm. Moreover, complexes 1-3 also afford similar experimental Φ
(0.23−0.28) and τ (1.25−1.44 μs) and calculated kr (1.7−2.0×105 s−1) and knr
(5.2−6.2×105 s−1) values to complex R (Table 1). These results indicate that the
oxadiazole/triazine type counter anions do not largely affect the emission of
[Ir(meoppy)2(dmapzpy)]+ in solution. At the excitation of 300 nm where both the
[Ir(meoppy)2(dmapzpy)]+ cation and the counter-anion were excited, complex 1
−
exhibits ultraviolet emission peaked at 352 nm from the OXD-7-SO3 anion and
sky-blue emission from the [Ir(meoppy)2(dmapzpy)]+ cation (Fig. S1a), which
−
indicates
an
incomplete
energy
transfer
from
OXD-7-SO3
to
[Ir(meoppy)2(dmapzpy)]+, because of the fluctuation of distance between them in
fluid solution. At the excitation of 300 nm, complexes 2 and 3 only show sky-blue
emission from [Ir(meoppy)2(dmapzpy)]+ (Fig. S1b and c), because the triazine-based
counter-anions are nearly non-emissive.

The emission properties of complexes R and 1-3 were also measured in the 2 wt.%
doped poly(methylmethacrylate) PMMA films. Fig. S2 depicts the PL spectra in the
doped films and detailed emission characteristics were summarized in Table 1. In the
doped film, complex R affords sky-blue emission peaked at 472 and 496 nm (Fig.
S2a), with a high Φ value of 0.80 and a τ value of 2.43 μs at the excitation of 380 nm.
The kr and knr values were calculated at 3.3×105 and 8×104 s−1, respectively. The
remarkable increase of Φ in going from solution to doped film largely results from the
suppressed non-radiative deactivation in the rigid solid film. In the doped films,
complexes 1-3 show almost identical sky-blue emission to complex R (Fig. S2b-d),
with emission peaks at around 472 and 496 nm at the excitation of 380 nm. Moreover,
complexes 1-3 show very similar experimental Φ (0.79−0.82) and τ (2.38−2.51 μs)
and calculated kr (3.3−3.4×105 s−1) and knr (7−9×104 s−1) values to complex R (Table
1). It is thus further confirmed that the electron-deficient oxadiazole/triazine type
counter-anions do not largely disturb the emission of [Ir(meoppy)2(dmapzpy)]+. The
similar photophysical properties of complexes 2 and 3 indicate that varying the
position (para- or meta-) of the sulfonate group on the 2,4,6-triphenyl-1,3,5-triazine
core does not largely affect the interaction between the phosphorescent cation and the
counter-anion. The high Φ values indicate that the counter-anions do not quench the
phosphorescence from the [Ir(meoppy)2(dmapzpy)]+ cation, because of their high
triplet energies that prevent back energy-transfer [53].
At the excitation of 300 nm where both cation and counter-anion are excited,
complex 1 exhibits predominant sky-blue emission from the [Ir(meoppy)2(dmapzpy)]+

−
cation and negligible emission from the OXD-7-SO3 anion (Fig. S2b), which
−
indicates an almost complete energy-transfer from OXD-7-SO3
to
[Ir(meoppy)2(dmapzpy)]+ within the single complex in the highly diluted film. The
−
efficient energy-transfer indicates that OXD-7-SO3 and [Ir(meoppy)2(dmapzpy)]+ are
located quite close to each other in the rigid film owing to the strong coulombic
attraction between them. The close proximity between the cation and
electron-trapping/transporting counter-anion benefits the use of the complexes in
light-emitting devices. At the excitation of 300 nm, complexes 2 and 3 also show
dominant sky-blue emission from the [Ir(meoppy)2(dmapzpy)]+ cation. The emission
from the triazine-based counter-anions were not observed because of their
non-emissive characteristics.
2.3. Electrochemical measurements

Figure 2. Cyclic voltammograms of a) OXD-7-SO3Na, TRZ-p-SO3Na and
TRZ-m-SO3Na and b) complexes R and 1-3 in N,N-dimethylformamide (DMF)
solution.
The electrochemical properties of the counter-anions and the complexes were
characterized by cyclic voltammetry in solution. Fig. 2a shows the cyclic
voltammograms of TRZ-p-SO3Na and TRZ-m-SO3Na. For comparison, the cyclic
voltammogram of OXD-7-SO3Na was also shown [20]. In DMF solution,
TRZ-p-SO3Na and TRZ-m-SO3Na both show reversible reduction processes (Fig. 2a),
with the reduction potentials at −2.05 and −2.09 V, respectively. The reversible
−
−
reductions indicate that TRZ-p-SO3 and TRZ-m-SO3 have good electrochemical
stability for accepting electrons. The reduction potentials of TRZ-p-SO3Na and
TRZ-m-SO3Na are anodically shifted compared to that (–2.15 V) of OXD-7-SO3Na,
which indicates that the triazine-type counter-anions are more electron-deficient than
the oxadiazole-type counter-anion. From the reduction potentials, the LUMO levels of

−
−
TRZ-p-SO3 and TRZ-m-SO3 were calculated at around −2.8 eV, which lie lower
−
−
−
than that (−2.7 eV) of OXD-7-SO3 , indicating that TRZ-p-SO3 and TRZ-m-SO3
−
have stronger electron-trapping capability than OXD-7-SO3 .
Fig. 2b shows the cyclic voltammograms of the complexes. In DMF solution,
complex R exhibits a quasi-reversible oxidation, with the oxidation potential at
around 0.72 V, and an irreversible reduction with the onset reduction potential at
around −2.52 V (Fig. 2b). Complexes 1-3 show similar quasi-reversible oxidations
and irreversible reductions to complex R (Fig. 2b), which occur on the
[Ir(meoppy)2(dmapzpy)]+ cation. As shown in Fig. 2b, complexes 1-3 also show
reversible reductions that correspond to the reductions of counter-anions.
2.4. Solution-processed OLEDs
Figure 3. EL spectra of OLEDs based complexes R and 1-3. Inset shows the device
structure.
Solution-processed OLEDs with complexes R and 1-3 as dopants were fabricated.

The device structure is indium-tin-oxide (ITO) /PEDOT: PSS (40 nm)/26DCzPPy: x %
complex (50 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) (Inset in Fig. 3), with
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the anode
buffer layer, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) as the host for the
complex in the emissive layer and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)
as the electron-transporting/hole-blocking layer. The emissive layer was deposited
from a solution-process. The doping concentrations of complexes R and 1-3 in the
emissive layers were set at 10 wt.%, 13 wt.%, 13 wt.% and 15 wt.%, respectively. The
doping concentrations of [Ir(meoppy)2(dmapzpy)]+ in the emissive layers were all at
8.5 wt.%.
Fig. 3 depicts the EL spectra of OLEDs. The devices all showed sky-blue
emission with the emission peak at around 476 nm and Commission Internationale de
L’Eclairage (CIE) coordinates at around (0.18, 0.42). The EL arose from the
[Ir(meoppy)2(dmapzpy)]+ cation and the counter-anions and the host did not
contribute to the emission. The PL spectra of the emissive layers show dominant
emission from the [Ir(meoppy)2(dmapzpy)]+ cation and very weak ultra-violet
emission from the counter-anions or the host (Fig. S3), which indicates almost
complete energy-transfer from the counter-anions and the host to the
[Ir(meoppy)2(dmapzpy)]+ cation.

Figure 4. Current-density and brightness versus voltage curves for OLEDs based on
complexes R and 1-3.
Table 2. Summary of performances for OLEDs based on complexes R and 1-3.
Von
(V) a
4.0
ηc,max
(cd A‒1) b
24.0
EQEmax
(%) c
9.9
Bmax
(cd m‒2) d
5400
EL λ
(nm)
CIE
(x,y)
R
1
2
3
476, 506
476, 504
476, 504
476, 504
c
(0.18, 0.42)
(0.18, 0.42)
(0.17, 0.42)
(0.17, 0.42)
4.1
26.2
11.0
5600
4.2
29.2
12.3
7100
4.2
29.4
12.4
8500
a
b
Voltage to reach 1 cd m2. Maximum current efficiency. Maximum external
quantum efficiency. d Maximum brightness.
Fig. 4 depicts the current-density and brightness versus voltage curves of the
OLEDs and Table 2 summarizes the performances of OLEDs. The devices based on
complexes 1-3 showed lower current-density than the devices based on complex R,
which implies that complexes 1-3 behave as stronger charge-trappers than complex R
[35]. From the energy-level diagrams of the OLEDs (Fig. S4), the
[Ir(meoppy)2(dmapzpy)]+ cation behaves as a hole trapper in the emissive layer
because of its shallower highest occupied molecular orbital (HOMO) level than that

of the host. The three electron-deficient counter-anions, with deeper lowest
unoccupied molecular orbital (LUMO) levels than that of the host, behave as strong
electron-trappers, when the counter-anions cannot form electron-transporting channels
within the emissive layers at low doping levels (4.5−6.5 wt.%) [7, 35]. The lowered
current densities for the devices based on complexes 1-3 compared to the device
based on complex R agree with the electron-trapping effects of the oxadiazole/triazine
type counter-anions. The slightly higher turn-on voltages for the devices based on
complexes 1-3 compared to the device based on complex R also consist with the
electron-trapping effects of the oxadiazole/triazine type counter-anions in the emissive
layers [7]. It is noted that the device based on complex 2 showed lower current
−
density then the device based on complex 3, which implies that TRZ-p-SO3 has a
−
stronger electron-trapping effect than TRZ-m-SO3 . It is thus suggested that changing
the anchoring position of sulfonate at the phenyl ring (meta- or para-position with
respect to the triazine) tunes the electron-trapping capability of the
2,4,6-triphenyl-1,3,5-triazine core. Substitution of sulfonate at the para position exerts
a stronger electron-withdrawing inductive effect to the triazine as compared to
substitution at the meta position, which partially explains the stronger
−
electron-trapping effect of TRZ-p-SO3 .
As shown in Table 2, the device based on complex R afforded a peak current
efficiency of 24 cd A−1 and a peak EQE of 9.9%. The devices based on complexes 1-3
showed largely enhanced performances, with peak current efficiency/EQE of 26.2 cd
A−1/11%, 29.2 cd A−1/12% and 29.4 cd A−1/12.4%, respectively. The higher device

performances for the devices based on complexes 1-3 indicate improved carrier
recombination balance in the emissive layers, which can be ascribed to more balanced
electron/hole transport and facilitated direct electron-hole recombination on the
complexes imparted by the electron-trapping effects of the counter-anions in
complexes 1-3. Similar effects have been observed in neutral iridium complexes
carrying electron-trapping oxadiazole or triazole type units [54, 55]. The devices
based on complexes 2 and 3 showed higher performances than complex 1, which
indicates that the triazine type counter-anions can improve the device performances to
a larger extent owing their stronger electron-trapping effects than the oxadiazole type
counter-anion. It is worth noting that the device based complex 3 is the most efficient
blue OLED using cationic iridium complexes as dopants reported so far (Table S1) [8,
16, 21, 23, 24, 26, 30, 39-44]. The devices showed decreased efficiencies at high
brightness (Fig. S5), owing the triplet-triplet or triplet-polaron quenching at high
current densities [56, 57]. The devices based on complexes 2 and 3 still showed high
current efficiencies of 27.0 and 27.2 cd A−1, respectively, at a practically high
brightness of 1000 cd m−2.
3. Conclusion
Sky-blue-emitting cationic iridium complexes with electron-deficient
−
oxadiazole/triazine type counter-anions, [Ir(meoppy)2(dmapzpy)]+[OXD-7-SO3 ] (1),
−
[Ir(meoppy)2(dmapzpy)]+[TRZ-p-SO3 ]
(2)
and
−
[Ir(meoppy)2(dmapzpy)]+[TRZ-m-SO3 ] (3), have been designed and synthesized.

The counter-anions were found not to affect the emission of the phosphorescent
[Ir(meoppy)2(dmapzpy)]+ cation and in films, they transferred efficiently the energy to
the [Ir(meoppy)2(dmapzpy)]+ cation. When the complexes were used as dopants in
solution-processed OLEDs, the counter-anions behaved as electron-trappers which
improved the carrier-recombination balance in the emissive layers, leading to largely
−
enhanced device performances compared to the reference complex with a PF6
counter-anion. The sky-blue device using complex 3 showed a peak EQE of 12.4%,
which is the highest for blue OLEDs using cationic iridium complexes as dopants
reported so far. The work has revealed that the oxadiazole/triazine type anions with
high triplet energy and electron-trapping effects are promising counter-anions for
blue-emitting cationic iridium complexes for advanced optoelectronic applications. In
particular, the triazine-type anions with high-triplet energy of 3.0 eV can be suitable
counter-anions for deep blue-emitting cationic iridium complexes.
4. Experimental Section
General information. High-resolution mass spectrometry, NMR spectra and
elemental analysis were measured on a BRUKER 400/500 NMR spectrometer, a
LTQ-ORBITRAP-ETD mass spectrometer and an EA3000 elemental analyzer,
respectively. PL and absorption spectra were recorded on a Macy UV-1800 UV-vis
spectrophotometer and a Gangdong F-320 fluorospectrophotometer, respectively. PL
transient lifetimes and photoluminescent efficiencies in thin films were measured with
a transient spectrofluorometer equipped with an integrating sphere (Edinburgh

Instruments, FLS1000). The photoluminescent efficiencies in degassed solution were
measured versus quinine sulfate (Φp = 0.545 in 1 M H2SO4). Cyclic voltammetry was
performed in degassed solution (10-3 M) at a scan rate of 100 mV s−1 with a LK1100
voltammetric analyzer.
A
glass-carbon working electrode,
a
silver-wire
pseudo-reference electrode and a platinum-wire counter-electrode were used for the
measurements. The supporting electrolyte and the internal standard were
tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene, respectively.
Materials. All reactants and solvents were purchased from commercial sources and
used
as
received.
[Ir(meoppy)2Cl]2
was
synthesized
by
reacting
4-methoxyl-2-phenylpyridine (meoppy) with IrCl3.nH2O in refluxed 2-ethoxyethanol:
H2O (3: 1) under nitrogen atmosphere according to reported procedures [58, 59].
4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy) was prepared according to a
reported procedure [24].
Synthesis
of
neopentyl
4-bromobenzenesulfonate
(Br-Ph-p-NeOSO2).
4-bromobenzene-1-sulfonyl chloride (5.1 g, 20 mmol) was dissolved in
1,2-dichloroethane (15 mL) and the solution was cooled in an ice bath. To the solution,
pyridine (3.1 g, 40 mmol) and 2,2-dimethyl-1-propanol (2.2 g, 25 mmol) dissolved in
1,2-dichloroethane (15 mL) were added dropwise under a nitrogen atmosphere. The
mixture was stirred at room temperature for 12 h and then diethyl ether was added.
The mixture was filtered. The filtrate was washed with HCl solution and water and
dried over anhydrous Na2SO4. The solvent was removed under vacuum and the solid
was recrystallized from ethanol, affording a white solid (3.8 g, 12.4 mmol). Yield:

1
62%. H NMR (500 MHz, CDCl3, δ[ppm]): 7.77 (d, J = 8.0 Hz, 2H), 7.71 (d, J = 8.0
Hz, 2H), 3.69 (s, 2H), 0.91 (s, 9H). HRMS (ESI, m/z): 307.0007 [M+H]+ (calc.
306.9998).
Synthesis
of
neopentyl
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
benzenesulfonate (PinB-Ph-p-NeOSO2). Br-Ph-p-NeOSO2 (1.5 g, 5 mmol),
bis(pinacolato)diboron (1.9 g, 7.5 mmol), [1,1'-bis(diphenylphosphino)ferrocene]
dichloropalladium(II) (184 mg, 0.25 mmol) and CH3COOK (1.5 g, 15 mmol) were
suspended in dioxane (15 mL). The mixture was stirred at 100
under a nitrogen
atmosphere for 12 h. After being cooled to room temperature, the mixture was
extracted with ethyl acetate. The organic phase was washed with water and then dried
over anhydrous Na2SO4. The solvent was removed under vacuum and the residual
was purified by column chromatography on silica gel with petroleum ether/CH2Cl2
(10: 1) as the eluent, affording a white solid (1.4 g, 3.9 mmol). Yield: 78%. 1H NMR
(400 MHz, CDCl3, δ[ppm]): 7.98 (d, J = 8.0 Hz, 2H), 7.89 (d, J = 8.0 Hz, 2H), 3.67 (s,
2H), 1.37 (s, 12H), 0.90 (s, 9H). HRMS (ESI, m/z): 355.1756 [M+H]+ (calc.
355.1745).
Synthesis
of
neopentyl
4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
(2.5 g, 7.1 mmol),
(TRZ-p-NeOSO2).
PinB-Ph-p-NeOSO2
2-chloro-4,6-diphenyl-1,3,5-triazine (2.1 g, 7.9 mmol), Pd(PPh3)4 (413 mg, 0.36
mmol) and K2CO3 (5 g, 36 mmol) were suspended in toluene: ethanol: water
(40/20/40 mL). The mixture was stirred at 80 under a nitrogen atmosphere for 24 h.
After being cooled to room temperature, the mixture was extracted with CH2Cl2 (50

mL×2). The organic phase was washed with water and then dried by anhydrous
Na2SO4. The solvent was removed under vacuum and the residual was purified by
column chromatography on silica gel with petroleum ether/CH2Cl2 (10: 1) as the
1
eluent, affording a white solid (2.6 g, 5.7 mmol). Yield: 80%. H NMR (500 MHz,
CDCl3, δ[ppm]): 8.95 (d, J = 8.4 Hz, 2H), 8.78 (d, J = 7.2 Hz, 4H), 8.12 (d, J = 8.4 Hz,
2H), 7.63 (m, 6H), 3.76 (s, 2H), 0.93 (s, 9H). HRMS (ESI, m/z): 460.1701 [M+H]+
(calc. 460.1689).
Synthesis
(TRZ-p-SO3Na). TRZ-p-NeOSO2 (1.0 g, 2.2 mmol) and NaI (3.3 g, 21.8 mmol) were
dissolved in acetone (40 mL). The mixture was stirred at 80 under a nitrogen
of
sodium
4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
atmosphere in a sealed pressure tube for 12 h. The mixture was cooled to room
temperature and then filtered. The solid was washed with acetone (10 mL), affording
1
a white solid (0.85 g, 2.0 mmol). Yield: 91%. H NMR (400 MHz, DMSO-d6,
δ[ppm]): 8.81-8.70 (m, 6H), 7.88 (d, J = 8.2 Hz, 2H), 7.78-7.64 (m, 6H). HRMS (ESI,
m/z): 388.0763 [M−Na]− (calc. 388.0761).
Synthesis of neopentyl 3-bromobenzenesulfonate (Br-Ph-m-NeOSO2). The synthesis
of Br-Ph-m-NeOSO2 was similar to that of Br-Ph-p-NeOSO2, except that
3-bromobenzene-1-sulfonyl chloride replaced 4-bromobenzene-1-sulfonyl chloride.
1
Yield: 60%. H NMR (400 MHz, CDCl3, δ[ppm]) 8.06 (s, 1H), 7.84 (d, J = 7.9 Hz,
1H), 7.79 (d, J = 8.0 Hz, 1H), 7.45 (t, J = 7.9 Hz, 1H), 3.71 (s, 2H), 0.92 (s, 9H).
HRMS (ESI, m/z): 307.0010 [M+H]+ (calc. 306.9998).
Synthesis
of
neopentyl
3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)

benzenesulfonate (PinB-Ph-m-NeOSO2). The synthesis of PinB-Ph-m-NeOSO2 was
similar to that of PinB-Ph-p-NeOSO2, except that Br-Ph-m-NeOSO2 replaced
1
Br-Ph-p-NeOSO2. Yield: 70%. H NMR (500 MHz, CDCl3, δ[ppm]): 8.34 (s, 1H),
8.05 (d, J = 7.4 Hz, 1H), 7.97 (d, J = 7.7 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 3.69 (s, 2H),
1.36 (s, 12H), 0.91(s, 9H). HRMS (ESI, m/z): 355.1754 [M+H]+ (calc. 355.1745).
Synthesis
of
neopentyl
3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
(TRZ-m-NeOSO2). The synthesis of TRZ-m-NeOSO2 was similar to that of
TRZ-p-NeOSO2, except that PinB-Ph-m-NeOSO2 replaced PinB-Ph-p-NeOSO2.
Yield: 85%.1H NMR (500 MHz, CDCl3, δ[ppm]): 9.28 (s, 1H), 9.06 (d, J = 7.8 Hz,
1H), 8.78 (d, J = 7.8 Hz, 4H), 8.14 (d, J = 7.8 Hz, 1H), 7.80 (t, J = 8.0 Hz, 1H), 7.69 –
7.56 (m, 6H), 3.80 (s, 2H), 0.95 (s, 9H). HRMS (ESI, m/z): 460.1699 [M+H]+ (calc.
460.1689).
Synthesis
of
sodium
3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate
(TRZ-m-SO3Na). The synthesis of TRZ-m-SO3Na was similar to that of
TRZ-p-SO3Na, except that TRZ-m-NeOSO2 replaced TRZ-p-NeOSO2. Yield: 93%.
1H NMR (500 MHz, DMSO-d6, δ[ppm]): 8.96 (s, 1H), 8.75 (d, J = 7.5 Hz, 4H), 8.71
(d, J = 8.0 Hz, 1H), 7.94 (d, J = 7.5 Hz, 1H), 7.77 – 7.67 (m, 6H), 7.64 (t, J = 7.5 Hz,
1H). HRMS (ESI, m/z): 388.0760 [M−Na]− (calc. 388.0761).
Synthesis of [Ir(meoppy)2(dmapzpy)]+Cl−.
[Ir(meoppy)2Cl]2 (1.8 g, 1.5 mmol) and dmapzpy (0.56 g, 3.0 mmol) were dissolved
in ethanol/1,2-dichloroetahne (30: 30 mL). The mixture was refluxed under a nitrogen
atmosphere for 12 h. The solvent was removed under vacuum and the residual was

purified by column chromatography on silica gel with CH2Cl2/CH3OH (60: 1) as the
eluent, affording a yellow solid (1.9 g, 2.4 mmol). Yield: 80%.
−
Synthesis of [Ir(meoppy)2(dmapzpy)]+PF6 (complex R).
[Ir(meoppy)2(dmapzpy)]+Cl− (0.50 g, 0.64 mmol) were dissolved in CH3OH/H2O
(30/30 mL). To the solution, NH4PF6 (0.65 g, 4.0 mmol) were added under stirring.
The mixture was stirred at room temperature for 1 h. The precipitate was collected
and purified by column chromatography on silica gel with CH2Cl2 as the eluent,
affording a yellow solid (0.47 g, 0.53 mmol). Yield: 83%. 1H NMR (400 MHz, CDCl3,
δ[ppm]): 8.81(d, J=2.8Hz, 1H), 7.60–7.53 (m, 3H), 7.42 (d, J= 6.7 Hz, 1H), 7.33 (t, J=
3.4 Hz, 2H), 7.20–7.16 (m, 2H), 7.11 (d, J= 2.0 Hz, 1H), 7.00–6.93 (m, 2H), 6.88–
6.84(m, 2H), 6.66–6.61 (m, 3H), 6.38 (t, J = 6.8 Hz, 2H), 6.32 (dd, J= 6.8,2.5 Hz, 1H),
3.97-3.94 (m, 6H), 3.16 (s, 6H). HRMS (ESI, m/z): 749.2234 [M−PF6]+ (calc.
749.2211). Anal. found: C, 45.78; H, 3.76; N, 9.33. Anal. calcd for C34H32F6IrN6O2P:
C, 45.69; H, 3.61; N, 9.40.
Synthesis of [Ir(meoppy)2(dmapzpy)]+[OXD-7-SO3 ] (complex 1).
−
[Ir(meoppy)2(dmapzpy)]+Cl− (235 mg, 0.3 mmol) was dissolved in methanol/H2O
(45/15 mL). To the solution, OXD-7-SO3Na (174 mg, 0.3 mmol) dissolved in
methanol/H2O (15/5 mL) was added dropwise under stirring. The mixture was stirred
at room temperature for 1 h and then concentrated. The precipitate was collected and
recrystallized from CH3OH/H2O, affording a yellow solid (282 mg, 0.22 mmol).
Yield: 73%. 1H NMR (500 MHz, DMSO-d6, δ[ppm]): 9.35 (s, 1H), 8.72 (s, 1H), 8.51
(s, 2H), 8.12 (d, J = 8.1 Hz, 4H), 7.91 (t, J = 6.7 Hz, 2H), 7.76 (s, 2H), 7.68 (d, J = 8.1