Tunable Emission Color of Iridium(III) Complexes with Phenylpyrazole Derivatives as the Main Ligands for Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.organomet.8b00491

Seven cyclometalated iridium(III) complexes Ir1-Ir7 based on phenylpyrazole derivatives as main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand were synthesized and fully characterized. The proligands of 1-[4-(trifluoromethyl)phenyl]-1H-pyrazole (cf₃ppz, 2a), substituted phenyl-2H-indazole {2-phenyl-2H-indazole (h-1-pidz, 2b), 2-(4-fluorophenyl)-2H-indazole (f-1-pidz, 2c), and 2-[4-(trifluoromethyl)phenyl]-2H-indazole (cf₃-1-pidz, 2d)}, and substituted phenyl-1H-indazole {1-phenyl-1H-indazole (h-7-pidz, 2e), 1-(4-fluorophenyl)-1H-indazole (f-7-pidz, 2f), and 1-[4-(trifluoromethyl)phenyl]-1H-indazole (cf₃-7-pidz, 2g)} were prepared with good yields. The emission maxima of all Ir(III) complexes can be tuned from 453 to 576 nm with different photoluminescence quantum yields (10.4-70.9%) by varying the type of substituent on the different main ligand frameworks. The organic light-emitting diodes using Ir(III) complexes as the emitters with blue, bluish-green, and yellow colors exhibit a maximum current efficiency and an external quantum efficiency of 39.2 cd A^-1 and 14.8%, respectively.

Full text of DOI:10.1021/acs.organomet.8b00491

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Tunable Emission Color of Iridium(III) Complexes with
Phenylpyrazole Derivatives as the Main Ligands for Organic Light-
Emitting Diodes
Zhi-Gang Niu,†,‡ Hua-Bo Han,† Min Li,‡ Zheng Zhao,‡ Guang-Ying Chen,‡ You-Xuan Zheng,*,†
Gao-Nan Li,*,‡ and Jing-Lin Zuo*,†
†State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Key
Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P.
R. China
‡College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, P. R. China
S
* Supporting Information
ABSTRACT: Seven cyclometalated iridium(III) complexes Ir1−Ir7 based on phenyl-
pyrazole derivatives as main ligands and tetraphenylimidodiphosphinate (tpip) as the
ancillary ligand were synthesized and fully characterized. The proligands of 1-[4-
(trifluoromethyl)phenyl]-1H-pyrazole (cf3ppz, 2a), substituted phenyl-2H-indazole {2-
phenyl-2H-indazole (h-1-pidz, 2b), 2-(4-fluorophenyl)-2H-indazole (f-1-pidz, 2c), and 2-
[4-(trifluoromethyl)phenyl]-2H-indazole (cf3-1-pidz, 2d)}, and substituted phenyl-1H-
indazole {1-phenyl-1H-indazole (h-7-pidz, 2e), 1-(4-fluorophenyl)-1H-indazole (f-7-pidz,
2f), and 1-[4-(trifluoromethyl)phenyl]-1H-indazole (cf3-7-pidz, 2g)} were prepared with
good yields. The emission maxima of all Ir(III) complexes can be tuned from 453 to 576
nm with different photoluminescence quantum yields (10.4−70.9%) by varying the type of
substituent on the different main ligand frameworks. The organic light-emitting diodes
using Ir(III) complexes as the emitters with blue, bluish-green, and yellow colors exhibit a
maximum current efficiency and an external quantum efficiency of 39.2 cd A−1 and 14.8%,
respectively.
properties but also decrease the rate of nonradioactive
deactivation and improve phosphorescence quantum yields.12
Among the most frequently used main ligands, phenyl-
pyrazole (ppz) is one typical ligand framework for constructing
Ir(III) complexes, and its derivatives can be used to fine-tune
the emission color of complexes by judicious modification.13
For instance, Thompson’s group first reported tris-cyclo-
metalated Ir(III) complexes with fluoro/trifluoromethyl-
substituted ppz ligands in 2003, and their emissions covered
a broad range from 390 to 440 nm.14 Subsequently, five ionic
Ir(III) complexes [Ir(R-ppz)2(bipy)][PF6] (R = H, Me, CF3,
NO2, or OMe) were synthesized and exhibited a wide range of
emission wavelengths (497−615 nm) with a change in the
substituents on the cyclometalated ligands.9c More recently, a
series of Ir(III) complexes based on ppz derivatives with
electron-withdrawing groups (-F, -CF3, -OCF3, and -SF5) were
prepared and revealed a broad, featureless emission peak that
ranged between 510 and 560 nm.15 However, to the best of
our knowledge, there have been no additional reports about
the tunability of colors achieved by different conjugation
positions on the ppz-based cyclometalated ligand.
INTRODUCTION
■
Iridium(III) complexes have attracted considerable attention
because of their remarkable photophysical properties, such as
their relatively short phosphorescence lifetimes, high quantum
efficiencies, and various emission colors.1 Because of these
unique photophysical features, Ir(III) complexes have been
applied in many fields, for example, organic photoredox
catalysis,2 organic photovoltaic cells,3 water photolysis,4 oxygen
sensors,5 and bioimaging.6 In particular, they are employed in
the fabrication of organic light-emitting diodes (OLEDs) and
full-color displays.7
To realize full-color displays, color tuning via the molecular
design of Ir(III) complexes has been studied extensively for
many years.8 Generally, one strategy is to alter the heterocycle,
the degree of conjugation in the C^N ligand, and/or the
ancillary ligand.9 Another strategy is to vary substituents on the
cyclometalated phenyl, the coordination heterocycle, or the
ancillary ligands.9c,10 Meanwhile, to obtain Ir(III) complexes
for efficient OLEDs, tetraphenylimidodiphosphinate (tpip)
derivatives with polar PO bonds and phenyl rings were used
as ancillary ligands in our group because they could increase
the electron mobility of the complexes and improve their
corresponding electroluminescent performances.11 Moreover,
the fluorine groups were also always introduced to modify the
ligands because they could not only modify the electronic
Received: July 14, 2018
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Routes of Main Ligands and Ir(III) Complexes Ir1−Ir7
Figure 1. ORTEP views of Ir(III) complexes Ir2 (CCDC 1578682), Ir3 (CCDC 1578683), Ir4 (CCDC 1578686), Ir5 (CCDC 1578684), Ir6
(CCDC 1578685), and Ir7 (CCDC 1578687) with the atom numbering scheme at the 50% probability level. Hydrogen atoms and solvent
molecules have been omitted for the sake of clarity.
Herein, as shown in Scheme 1, on the basis of the precursor
ligand 1-[4-(trifluoromethyl)phenyl]-1H-pyrazole (2a), we
designed ppz derivatives (2b−2g) as the main ligands by
introducing a benzene group to increase the π system and a
fluorine group to enhance the photophysical quantum yields.
According to the different positions of the benzene group on
the main ligands, they can be divided into two series: ligands
2b−2d (looking like the number 1) and ligands 2e−2g
(looking like the number 7). Then we synthesized seven Ir(III)
complexes Ir1−Ir7 (Scheme 1) with ppz derivatives 2a−2g,
B
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respectively, as the main ligands and tpip as the ancillary
ligand. The emission maximum wavelengths of these
complexes can be tuned from 453 to 576 nm in a CH2Cl2
solution at room temperature. The monochrome phosphor-
escent OLED devices D1, D3, D4, D6, and D7 using Ir(III)
complexes Ir1, Ir3, Ir4, Ir6, and Ir7, respectively, as the
emitters emit blue, yellow, and bluish-green light.
metric analysis (TGA) under a nitrogen stream with a heating
rate of 10 °C min−1 (Figure 2). From the TG data in Table 1,
RESULTS AND DISCUSSION
■
Cyclometalated ligands 2a−2g and their corresponding Ir(III)
complexes, Ir1−Ir7, respectively, were prepared and are shown
in Scheme 1, and the details are provided in the Supporting
Information. Compounds 2a and 2e−2g were conveniently
synthesized via an efficient Ullmann reaction under CuI
catalysis, which used 1H-pyrazole or 1H-indazole and the
corresponding substituted iodobenzene as raw materials.
Compounds 2b−2d were prepared through a two-step
reaction. First, 2-nitrobenzaldehyde reacted with the sub-
stituted aniline by the condensation reaction in ethanol to give
1b−1d in good yields (over 95%). Subsequently, 2b−2d were
prepared by the cyclization reaction using P(OEt)3 as the
solvent and catalyst. Then Ir(III) complexes Ir1−Ir7 were
prepared by the reaction of chloride-bridged dimer [(C^N)2Ir-
(μ-Cl)]2 with Ktpip (2.5 equiv) in anhydrous 2-ethoxyetha-
nol.16 All the desired products were obtained in good yield and
Figure 2. TGA curves of Ir(III) complexes Ir1−Ir7.
it can be clearly seen that the decomposition temperatures are
345 °C for Ir1, 364 °C for Ir2, 374 °C for Ir3, 355 °C for Ir4,
373 °C for Ir5, 380 °C for Ir6, and 365 °C for Ir7. They all
possess decomposition temperatures of >300 °C, suggesting
that they have good thermal stability that improves their
application in OLEDs.
1
characterized using H nuclear magnetic resonance (NMR)
spectroscopy and mass spectrometry (see the Experimental
Section and Supporting Information).
Photophysical Properties. The ultraviolet−visible (UV−
vis) absorption spectra in CH2Cl2 solutions at room temper-
ature for complexes Ir1−Ir7 are depicted in Figure 3, and the
data are listed in Table 1. All complexes exhibit intense
absorption bands between 220 and 350 nm, mostly ascribed to
spin-allowed ligand-centered (LC) π−π* transitions involving
both the cyclometalated and ancillary ligands. The broad and
less intense bands at longer wavelengths [λ > 350 nm (Figure
3, inset)] are associated with metal-to-ligand charge transfer
(MLCT).22 In comparison with Ir1, the lowest-lying
absorption bands for complexes Ir2−Ir7 are remarkably red-
shifted, resulting from the different extended π conjugation of
the C^N ligands.23 Furthermore, the bathochromic shifts
observed in the series of Ir2−Ir4 are more pronounced than
those of analogues Ir5−Ir7, which is related to the different
positions of the phenyl ring on the indazole framework.
Specifically, the trend of the absorption onset in the first series
is Ir3 < Ir2 < Ir4, while the order of another series is Ir6 < Ir7
< Ir5. These results suggest that -F and -CF3 groups on the
cyclometalated ligands also could modulate the lower-lying
transitions, which will be proved by density functional theory
(DFT) calculations discussed below.
Crystal Structures. The ORTEP diagrams of six
representative complexes, Ir2−Ir7, are shown in Figure 1.
Their crystallographic data and structure refinement details are
listed in Table S1; the selected bond lengths and bond angles
are listed in Table S2. As shown in Figure 1, the coordination
geometry of the iridium center in each complex is a distorted
octahedral geometry with cis-C,C, cis-O,O, and trans-N,N
chelate disposition.17 The lengths of Ir−C and Ir−N bonds are
in the range of 1.978(1)−2.010(1) and 1.938(1)−2.048(1) Å,
respectively, close to those in previously reported Ir(III)
complexes.18 It is noteworthy that the Ir−O bond lengths
[2.172(4)−2.227(3) Å] are slightly longer than the mean Ir−
O value of 2.088 Å reported in the Cambridge Crystallographic
Database, which may be attributed to the strong trans influence
of the carbon donors.19 In particular, the Ir−C bond lengths of
two series of Ir(III) complexes, Ir2−Ir4 and Ir5−Ir7, exhibit
the same trend: Ir3 < Ir4 < Ir2 and Ir6 < Ir7 < Ir5. Namely, a
stronger electron-withdrawing ability of substituted groups
would result in shorter the Ir−C bond lengths. In addition, the
C−Ir−C, O−Ir−O, C−Ir−O, and N−Ir−O angles increase in
the following order: Ir2−Ir4 < Ir5−Ir7. The order of C−Ir−N
and N−Ir−N angles is reversed. Among them, the O−Ir−O
bite angles are 87.1(3)−87.70(1)° for Ir2−Ir4 and 90.1(2)−
91.63(8)° for Ir5−Ir7. This may be consistent with the steric
hindrance, originating from different connection positions
between the indazole and phenyl group.20
Photoluminescence (PL) emission spectra of complexes
Ir1−Ir7 in degassed CH2Cl2 solutions at 298 and 77 K are
shown in Figure 3 and Figure S22, respectively. The
corresponding data are also summarized in Table 1. Complexes
Ir1−Ir7 are emissive in the blue-yellow spectral region upon
photoexcitation in CH2Cl2 solutions at 298 K, and each of the
emission profiles is a structured band, indicative of a
3
3
Thermal Properties. If an Ir(III) complex used as an
emitter is suitable for OLED application, it should have
sufficiently high melting points (Tm) and decomposition
temperatures (Td, 5% weight loss) to ensure that the material
could be deposited onto the solid face and survive long periods
of application without any decomposition.21 Therefore, the
thermal properties of these Ir(III) complexes are very
important for efficient OLEDs. The thermal properties of
investigated complexes were characterized by thermogravi-
pronounced MLCT character with a weaker LC contribu-
tion.25 As expected, emission maxima of complexes Ir2−Ir7
(494−576 nm) appear at higher energies with respect to that
of complex Ir1 (453 nm), and the series of Ir2−Ir4 complexes
are even more red-shifted than another series of Ir5−Ir7. This
observation is also in line with those of the lowest-energy
electronic transition in the UV−vis spectra mentioned above.
Upon cooling to 77 K (Figure S22), all the Ir(III) complexes
show well-resolved vibronic structures with a predominant
C
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Table 1. Photophysical and Thermal Stability Data of Complexes Ir1−Ir7
thermal properties
emission
τ (μs) kr (×105 s−1
T
m a
T
ΦPL
knr
d b
c
c
d
c
e
e
f
complex
(°C)
(°C)
absorption (nm)
298 K (nm)
(%)
)
(×105 s−1
)
77 K (nm)
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
Ir7
332
334
352
308
360
369
350
345
364
374
355
373
380
365
228, 253, 274, 282, 321, 354, 398
228, 252, 307, 318, 358, 396, 426, 465
230, 252, 304, 315, 352, 388, 414, 451
229, 250, 304, 317, 363, 398, 428, 462
230, 251, 311, 356, 393, 412
433, 453
566, 605
556, 594
576, 620
507, 528
494, 520
496, 521
10.4
48.7
70.9
50.5
23.2
30.9
36.3
2.15
2.80
2.75
2.72
2.63
2.75
2.96
0.48
1.74
2.58
1.86
0.88
1.12
1.23
4.17
1.83
1.06
1.82
2.92
2.51
2.15
440, 467
563, 607
555, 596
572, 622
524, 557, 607
488, 519, 558
485, 518, 557
231, 248, 309, 352, 387, 402
230, 251, 311, 354, 390, 409
a
b
c
Tm, melting temperature. Td, decomposition temperature corresponding to a 5% weight loss. Data were collected from degassed CH2Cl2
solutions at room temperature. fac-Ir(ppy)3 as the reference standard (0.4).24 The radiative decay rate (kr) and nonradiative decay rate (knr) were
estimated from the measured quantum yields and lifetimes. Data were collected from degassed CH2Cl2 solutions at 77 K.
d
e
f
Figure 3. (a) Absorption and (b) emission spectra of complexes Ir1−Ir7 in degassed CH2Cl2 solutions at 298 K.
contribution of ligand-centered 3π−π* transitions to the
excited state because LC excited states are almost unaffected
in frozen medium, because of their low dipole character.26 In
the meantime, it is found that the emission maxima further
undergo a hypsochromic shift of approximately 3−11 nm from
298 to 77 K (Table 1), due to the rigidochromic effect of the
frozen media.27
Ir1 has the highest knr value in all complexes. In comparison to
those of complexes Ir2−Ir4, complexes Ir5−Ir7 show much
higher knr values. Consequently, the largest kr/knr ratio is
observed for complex Ir3 and the smallest for complex Ir1, in
line with the highest and the lowest quantum yields,
respectively.
Electrochemical Properties and Theoretical Calcula-
tions. The electronic properties of Ir1−Ir7 were investigated
by cyclic voltammetry, and the electrochemical waves are
shown in Figure 4. On the basis of their oxidation potentials
and absorption spectra, the HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied molecular
orbital) energy levels are estimated and included in Table 2, as
well as DFT-calculated values for the purpose of comparison.
Phosphorescence relative quantum yields (Φem) of Ir1−Ir7
in dichloromethane solutions were measured to be 10.4−
70.9% (Table 1) at room temperature by using typical
phosphorescent fac-Ir(ppy)3 as a standard (Φem = 40%).
Compared with that of parent complex Ir1 (Φ = 10.4%), the
more extended π conjugation of the phenylpyrazole moiety in
other complexes led to the dramatic increase in the
phosphorescence quantum yield. As for the two sets of Ir(III)
complexes, fluorinated complexes Ir3 and Ir4 and fluorinated
complexes Ir6 and Ir7 always display a strongly increase
relative to fluorine-free analogues Ir2 and Ir5, respectively.
This is dependent on the fact that the presence of the C−F
bonds could lead to the reduced radiationless deactivation rate
in comparison with that seen with C−H bonds.28 Remarkably,
the emission quantum yield for Ir3 (Φem = 70.9%) is the
highest in the family of complexes, which would improve its
device performance. For all the investigated complexes, the
luminescence lifetimes (τ) are in the range of 2.15−2.96 μs,
indicating that their emitting excited states have triplet
character (Table 1).29 From the Φem and τ values, the
radiative decay rate (kr) and the nonradiative decay rate (knr)
were estimated. As one can see, the kr value of Ir1 is the lowest
among those of the investigated complexes. Complexes Ir5−
Ir7 all possess much smaller kr values in comparison to those
of another series of complexes (Ir2−Ir4). In contrast, complex
Figure 4. Cyclic voltammogram curves of CH2Cl2 solutions of Ir1−
Ir7 containing n-Bu4NClO4 (0.1 M) at a sweep rate of 100 mV s−1.
D
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determined by comparison of the two series, complexes Ir2−
Ir4 are more easily oxidized than complexes Ir5−Ir7 as a result
of the different π conjugation positions. The energy levels of
the LUMO were obtained from the HOMO and Eopt,g values.
It can be observed that the LUMO energy levels and the
HOMO energy levels have the same tendency to vary based on
experimental and theoretical data. Remarkably, the HOMO/
LUMO energy levels inferred from the CV data are
systematically slightly lower than those from the DFT data,
as reported in ref 30 (Table 2).
Time-dependent DFT (TD-DFT) is an excellent tool for
accurately assigning the origin of each band in ultraviolet
absorption spectra of these complexes. The most representa-
tive molecular frontier orbital diagrams and the energy gap are
presented in Figure 5. The calculated spin-allowed electronic
transitions, oscillator strengths, and related molecular orbitals
are listed in Table 3 and compared with the experimental
absorption spectral data. The electron density distributions are
summarized in Table S3.
In all cases, the HOMOs are mainly dominated by iridium d
orbitals and π orbitals of cyclometalated ligands. Meanwhile,
the LUMOs and the LUMO+1s are mostly located on π*
orbitals of the two cyclometalated ligands. As demonstrated in
Table 3, the lowest-lying energy transitions of all Ir(III)
complexes arise from a HOMO → LUMO+1 orbital electronic
transition (except the HOMO → LUMO transition for Ir1).
Therefore, these transitions are assigned to MLCT in
combination with partial ligand-to-ligand charge transfer
(LLCT), in good agreement with the experimental absorption
spectra.
Table 2. Electrochemical and Theoretical Data of
Complexes Ir1−Ir7
Eox
(eV)
Eopt,g
HOMO/LUMO
HOMO/LUMO
a
b
c
complex
(eV)
(eV)
(eV)
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
Ir7
1.24
0.92
1.06
1.14
0.96
1.09
1.22
3.12
2.67
2.75
2.69
3.01
3.09
3.04
−6.04/−2.92
−5.72/−3.05
−5.86/−3.11
−5.94/−3.25
−5.76/−2.75
−5.89/−2.80
−6.02/−2.98
−5.46/−1.26
−5.23/−1.61
−5.37/−1.64
−5.48/−1.85
−5.14/−1.38
−5.26/−1.42
−5.40/−1.56
a
Calculated from the UV−vis absorption edges (Eopt,g = 1240/λonset).
b
Deduced from the equations HOMO = −(Eox + 4.8 eV) and LUMO
c
= HOMO + Eopt,g, respectively. Obtained from theoretical
calculations.
All complexes exhibit reversible oxidation peaks in the region
of 0.92−1.24 V, which are attributed to the Ir(III)/Ir(IV)
redox couples with contributions from the phenyl rings of
cyclometalated fragments, as evidenced by DFT calculations
(Figure 5). Compared to that of complex Ir1, the oxidation
With respect to pristine complex Ir1, the introduction of the
extended conjugation in complexes Ir2−Ir7 gives rise to the
destabilization of the HOMO levels and the stabilization of the
LUMO levels. The results lead to a shorter HOMO−LUMO
energy gap, thereby causing a red shift in the actual absorption
spectra (Table 3). In analogy to the rule, the red shift observed
in Ir2−Ir4 agrees well with the lower HOMO−LUMO gap
with regard to those in Ir5−Ir7. In the case of their HOMO
energy values, the orders are Ir2 > Ir3 > Ir4 and Ir5 > Ir6 >
Ir7, which are in accordance with the electron-withdrawing
ability of -H < -F < -CF3. Similar results have also been
obtained for their LUMO energy values. Different decreasing
trends of the HOMO/LUMO levels induce distinct HOMO−
LUMO energy gaps. For instance, the energy bandgaps of
fluorine-substituted complexes Ir3 and Ir6 are larger than
those of trifluoromethyl-substituted complexes Ir4 and Ir7,
respectively, which further proves that the nature of the
substituents of the cyclometalated ligands can also affect the
absorption spectra. Interestingly, the LUMOs have a topology
analogous to that of the LUMO+1s in complexes Ir2−Ir7
(Figure 5). Thus, it is not surprising that the LUMOs and
Figure 5. Frontier molecular orbital energy level diagrams of Ir1−Ir7
from DFT calculations.
waves for complexes Ir2−Ir7 are shifted to less negative
potentials. These results reflect the fact that the π conjugation
system on the cyclometalated ligand can enrich the electron
density around the iridium center and destabilize the
corresponding HOMO orbital (Table 2 and Figure 5).
Notably, the trends in Eox are Ir2 < Ir3 < Ir4 and Ir5 < Ir6
< Ir7, contrary to the order of the HOMO energy levels: Ir2 >
Ir3 > Ir4 and Ir5 > Ir6 > Ir7 (Table 2). These findings
indicate the electron-withdrawing fluorine atoms can lower the
HOMO energy levels, especially that of the CF3 group. As
Table 3. Main Experimental and Calculated Optical Transitions for Complexes Ir1−Ir7
complex
orbital excitations
nature of the transition
oscillation strength calcd (nm) exptl (nm)
Ir1
Ir2
Ir3
Ir4
Ir5
Ir6
Ir7
HOMO → LUMO
HOMO → LUMO+1
HOMO → LUMO+1
HOMO → LUMO+1
HOMO → LUMO+1
HOMO → LUMO+1
HOMO → LUMO (62%), HOMO → LUMO+1 (−34%)
Ir(dπ)/Lcf3ppz(π) → Lcf3ppz(π*)
Ir(dπ)/Lh‑1‑pidz(π) → Lh‑1‑pidz(π*)
Ir(dπ)/Lf‑1‑pidz(π) → Lf‑1‑pidz(π*)
Ir(dπ)/Lcf3‑1‑pidz(π) → Lcf3‑1‑pidz(π*)
Ir(dπ)/Lh‑7‑pidz(π) → Lh‑7‑pidz(π*)
Ir(dπ)/Lf‑7‑pidz(π) → Lf‑7‑pidz(π*)
Ir(dπ)/Lcf3‑7‑pidz(π) → Lcf3‑7‑pidz (π*)
0.0456
0.0808
0.0864
0.0698
0.0430
0.0496
0.0535
361
422
404
418
407
393
396
398
465
451
462
412
402
409
E
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Figure 6. Energy level diagram of phosphorescent OLEDs and the molecular structures of used materials.
Figure 7. Characteristics of devices D1, D3, D4, D6, and D7: (a) normalized EL spectra at 8 V, (b) luminance−voltage−current density curves,
(c) current efficiency−luminance curves, and (d) external quantum efficiency−luminance curves.
LUMO+1s have very close energy values. The very small
orbital overlap between the HOMO and the LUMO/LUMO
+1 orbitals may explain the absence of the corresponding
absorptions.31
OLED Performance. To evaluate their electrolumines-
cence (EL) properties, monochrome devices D1, D3, D4, D6,
and D7 using complexes Ir1, Ir3, Ir4, Ir6, and Ir7,
respectively, as emitters with colors of blue, green, andyellow
were fabricated with a simple ITO/MoO3 (molybdenum
oxide, 5 nm)/TAPC (30 nm)/2,6-DCzPPy:Ir(III) complex (8
wt %, 10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm)
architecture. The schematic energy level diagrams of
phosphorescent OLEDs and the molecular structures of their
materials are shown in Figure 6. MoO3 and LiF act as a hole-
injecting layer (HIL) and an electron-injecting layer (EIL),
respectively. TAPC [4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-
tolylaniline)] acts as a hole-transporting layer (HTL), while
TmPyPB {1,3,5-tri[(3-pyridyl)phen-3-yl]benzene} acts as an
electron-transporting layer (ETL). Herein, 2,6-DCzPPy layers
doped with Ir(III) complexes act as the emissive layer.
Simultaneously, bipolar material 2,6-DCzPPy is used as the
buffer layer to achieve the cascade hole injection from HTL to
the emitting layer (EML) due to its nearly equal electron
mobility (μe) and hole mobility (μh) values (1−8 × 10−5 cm2
V−1 s−1 at an electric field between 6.0 × 105 and 1.0 × 106 V
cm−1), which improves the electron−hole balance in the
EML.32
F
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Table 4. EL Performance of Single-EML Devices D1, D3, D4, D6, and D7
a
b
c
c
c
d
e
device
emitter
Vturn‑on (V)
Lmax [cd m−2 (V)]
ηc (cd A−1
)
ηp (lm W−1
)
EQE (%)
EL (nm)
CIE (x, y)
D1
D3
D4
D6
D7
Ir1
Ir3
Ir4
Ir6
Ir7
4.0
3.9
3.9
3.8
3.9
3135 (12.6)
14127 (14.5)
9535 (11.5)
6689 (13.4)
8.6/8.0
3.4/3.0
2.6/2.4
9.0/8.0
10.7/9.6
14.8/9.9
10.3/8.3
450
(0.16, 0.17)
(0.47, 0.49)
(0.55, 0.44)
(0.21, 0.48)
(0.25, 0.53)
26.8/23.9
26.2/23.5
39.2/26.3
28.7/23.3
18.7/9.3
16.4/12.3
27.4/10.2
17.0/10.6
559, 595
577, 618
496
9568 (12.2)
491, 519
a
b
c
Turn-on voltage recorded at a luminance of 1 cd m−2. Maximum luminance. Data at maximum and 1000 cd m−2 for current efficiency (ηc),
power efficiency (ηp), and EQE, respectively. Values were collected at 8 V. CIE (Commission Internationale de l’Eclairage) coordinates (CIE) at
d
e
8 V.
The EL spectra and current density−luminance−voltage (J−
L−V), current efficiency−luminance (ηc−L), and external
quantum efficiency−luminance (EQE−L) characteristics of
devices are shown in Figure 7. The key EL data are
summarized in Table 4. It should be noted that all devices
emit blue, yellow, or bluish-green light with peaks at 450, 559,
577, 496, and 491 nm for devices D1, D3, D4, D6, and D7,
respectively. Their CIE coordinates are (0.16, 0.17), (0.47,
0.49), (0.55, 0.44), (0.21, 0.48), and (0.25, 0.53), respectively
(Table 4). The EL spectra match well with the PL spectra of
these complexes in solution, suggesting that the generated
excitons are mainly deactivated by phosphorescence emission
of Ir(III) complexes and the energy can be transferred from
2.6-DCzPPy to the emitters efficiently. To further investigate
the reason for the different PL and EL spectra, the PL
emissions of complexes Ir3 and Ir6 in the solid state were
measured and the spectra are shown in Figure S23. It is clear
that the PL emissions of complexes Ir3 and Ir6 in the solid
state are similar to the EL emissions of the devices, which
indicate that the difference is due to the solid effect.
Because of the similar molecular structures of the complexes,
their EL performances mainly depend on their photophysical
(PLQY especially) and electrochemical properties (energy
HOMO/LUMO levels especially). Because of the lowest
PLQY (10.4%) and blue emission (433 and 453 nm) of Ir1,
device D1 reveals the weakest performance with a maximum
luminance (Lmax) of 3135 cd m−2 at 12.6 V, a maximum
current efficiency (ηc,max) of 8.6 cd A−1 with an EQEmax of
2.6%, and a maximum power efficiency (ηp,max) of 3.4 lm W−1.
Compared with device D1, devices D3, D4, D6, and D7
practical luminance of 1000 cd m−2, the EQEs of devices D6
and D7 were reduced by ∼33.1 and ∼19.4%, respectively, but
the efficiencies of D3 and D4 decayed from their maximum
values by only 11.1 and 10.3%, respectively. At the same time,
the trends in efficiency roll-off are D3 > D4 (11.1% vs 10.3%)
and D6 > D7 (33.1% vs 19.4%) in the same types of
luminescent materials. These results suggest that the bulky
-CF3 series can affect the packing of molecules and the steric
protection around the metal can restrain the self-quenching of
luminescence to improve device performances.
CONCLUSION
■
Seven bis-cyclometalated phenylpyrazole-based Ir(III) com-
plexes Ir1−Ir7 with tpip as the ancillary ligand were
synthesized and investigated in detail. Via variation of the
main ligands, the emission colors can be adjusted from blue to
yellow in CH2Cl2 solutions. The extended π conjugation
within the C^N ligand can lead to a red shift, and then the
conjugated system introduced at difference positions in the
center unit can lead to different photoelectric properties. The
phosphorescent OLEDs comprising Ir1, Ir3, Ir4, Ir6, and Ir7
as blue, yellow, and bluish-green dopants realize device
performances with EQEmax values of 2.6, 9.0, 10.7, 14.8, and
10.3%, respectively. At a luminance of 1000 cd m−2, the EQE
values of devices D3, D4, D6, and D7 were reduced from their
maximum values by ∼11.1, ∼10.3, ∼33.1, and ∼19.4%,
respectively. Furthermore, the efficiency roll-off of D4 and
D7 is lower than that of D3 and D6, suggesting the bulky -CF3
series can improve device performance.
displayed much better performances. The EQEmax values (Lmax
,
EXPERIMENTAL SECTION
■
ηc,max, ηp,max) for devices D3, D4, D6, and D7 were measured
to be 9.0% (14127 cd m−2, 26.8 cd A−1, 18.7 lm W−1), 10.7%
(9535 cd m−2, 26.2 cd A−1, 16.4 lm W−1), 14.8% (6689 cd
m−2, 39.2 cd A−1, 27.4 lm W−1), and 10.3% (9568 cd m−2, 28.7
cd A−1, 17.0 lm W−1), respectively. From the results, it can also
be observed that the EL performance of device D6 is obviously
better than that of D7, though Ir7 has a photoluminescence
quantum efficiency that is higher than that of Ir6 (Ir6, 30.9%;
Ir7, 36.3%). Because the HOMO level of Ir6 (−5.89 eV) is
higher than that of Ir7 (−6.02 eV), the hole trapped by the
emitter from TAPC becomes easier, resulting in a lower turn-
on voltage and better usage of the carriers. Furthermore, the
shorter excited state lifetime of complex Ir6 (2.75 μs)
compared to that of Ir7 (2.96 μs) also contributes to
improving the device performances because the longer lifetime
will lead to exciton quenching through triplet−triplet
annihilation (TTA) and triplet−polaron annihilation (TPA)
effects, which would result in relatively poor EL performances.
It is noteworthy that devices D6 and D7 show efficiency roll-
off that is more severe than that of devices D3 and D4. At the
All reactions were performed under a nitrogen atmosphere, and all
reagents were purchased and used without further purification unless
otherwise stated. Potassium tetraphenylimidodiphosphinate (Ktpip)
was synthesized according to our previous work.33 1H NMR spectra
were recorded on a Bruker AM 400 MHz instrument, and chemical
shifts are reported in parts per million relative to Me4Si as an internal
standard. High-resolution mass spectrometry (HRMS) was performed
on an Agilent 6540 UHD Accurate-Mass Q-TOF LC/MS instrument.
UV−vis spectra were recorded on a Hitachi U3900/3900H
spectrophotometer. Photoluminescence spectra were recorded on a
Hitachi F7000 spectrophotometer as deaerated CH2Cl2 solutions at
room temperature and 77 K. Luminescence lifetime curves were
measured on an Edinburgh Instruments FLS920P fluorescence
spectrometer, and the data were subjected to one-order exponential
fitting using OriginPro 8 software. Thermogravimetric analysis (TGA)
was performed with a simultaneous NETZSCH STA 449C thermal
analyzer. Elemental analyses were performed on a Vario EL Cube
Analyzer system.
1-[4-(Trifluoromethyl)phenyl]-1H-pyrazole (cf3ppz, 2a). To a
mixture of 1H-pyrazole (1.0 g, 14.7 mmol), 1-iodo-4-
(trifluoromethyl)benzene (4.8 g, 14.6 mmol), N,N′-dimethylethyle-
nediamine (130 mg, 1.47 mmol), and K3PO4 (6.2 g, 29.4 mmol) in
G
DOI: 10.1021/acs.organomet.8b00491
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dry dimethylformamide (15 mL) was added CuI (140 mg, 0.73
mmol). The mixture was heated at 80 °C for 20 h, diluted with EtOAc
(50 mL), and filtered through a pad of Celite. The filtrate was washed
with H2O, and the aqueous layer was extracted with EtOAc. The
combined organic extract was concentrated, and the residue was
purified by column chromatography on silica gel and eluted with
EtOAc/hexanes [1:100 (v/v)] to give compound 2 (2.7 g, 87%) as a
white solid: 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 2.4 Hz, 1H),
7.83 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 1.2 Hz, 1H), 7.72 (d, J = 8.4 Hz,
2H), 6.52 (t, J = 2.0 Hz, 1H). Anal. Calcd for C10H7F3N2: C, 56.61;
H, 3.33; N, 13.20. Found: C, 56.60; H, 3.32; N, 13.22.
(m, 6H), 6.35 (d, J = 1.2 Hz, 2H), 6.30 (t, J = 2.8 Hz, 2H); HRMS
calcd for C44H32F6IrN5O2P2 1031.156 ([M + H]+), found 1032.156.
Anal. Calcd for C44H32F6IrN5O2P2: C, 51.26; H, 3.13; N, 6.79.
Found: C, 51.21; H, 3.12; N, 6.80.
1
Ir2 (104 mg, 49% yield): yellow powder; H NMR (400 MHz,
CDCl3) δ 8.34 (s, 2H), 8.17 (d, J = 8.8 Hz, 2H), 7.77−7.82 (m, 4H),
7.45 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 7.6 Hz, 2H), 7.28−7.32 (m,
6H), 7.10−7.15 (m, 4H), 6.98 (t, J = 7.6 Hz, 2H), 6.83−6.89 (m,
4H), 6.64−6.70 (m, 6H), 6.54 (t, J = 7.2 Hz, 2H), 6.08 (d, J = 7.2 Hz,
2H); HRMS calcd for C50H38IrN5O2P2 995.213 ([M + H]+), found
996.225. Anal. Calcd for C50H38IrN5O2P2: C, 60.35; H, 3.85; N, 7.04.
Found: C, 60.28; H, 3.82; N, 7.11.
2-Phenyl-2H-indazole (h-1-pidz, 2b). A solution of phenyl-
amine (1.0 g, 10.7 mmol) and 2-nitrobenzaldehyde (1.6 g, 10.7
mmol) in EtOH (20 mL) was refluxed for 8 h. After removal of most
of the solvent, the yellow solid was collected by filtration to afforded
compound 1b, which was used in the next step without further
purification. Compound 1b (1.2 g, 5.3 mmol) was dissolved in
P(OEt)3 (15 mL) and heated at 110 °C for 10 h. The solvent was
removed, and the resulting residue was purified by column
chromatography on silica gel and eluted with EtOAc/hexanes [1:50
1
Ir3 (104 mg, 66% yield): orange powder; H NMR (400 MHz,
CDCl3) δ 8.29 (2s, 2H), 8.10 (d, J = 8.8 Hz, 2H), 7.78−7.82 (m,
4H), 7.44 (d, J = 7.2 Hz, 2H), 7.32−7.39 (m, 8H), 7.08−7.13 (m,
4H), 6.97 (t, J = 7.6 Hz, 2H), 6.88 (t, J = 7.6 Hz, 2H), 6.57−6.69 (m,
8H), 5.68−5.71 (m, 2H); HRMS calcd for C50H36F2IrN5O2P2
1031.194 ([M + H]+), found 1032.223. Anal. Calcd for
C50H36F2IrN5O2P2: C, 58.25; H, 3.52; N, 6.79. Found: C, 58.22; H,
3.48; N, 6.83.
1
1
Ir4 (138 mg, 61% yield): orange powder; H NMR (400 MHz,
(v/v)] to give compound 2b (760 mg, 86%) as a white solid: H
CDCl3) δ 8.30 (s, 2H), 8.09 (d, J = 8.8 Hz, 2H), 7.77−7.82 (m, 4H),
7.45 (d, J = 8.4 Hz, 2H), 7.31−7.39 (m, 8H), 7.07−7.12 (m, 4H),
6.99 (t, J = 8.0 Hz, 2H), 6.86−6.90 (m, 2H), 6.65−6.77 (m, 8H),
5.82 (d, J = 1.2 Hz, 2H); HRMS calcd for C52H36F6IrN5O2P2
1131.188 ([M + H]+), found 1164.186 ([MS + H + MeOH]+).
Anal. Calcd for C52H36F6IrN5O2P2: C, 55.22; H, 3.21; N, 6.19.
Found: C, 55.17; H, 3.32; N, 6.25.
NMR (400 MHz, CDCl3) δ 8.40 (s, 1H), 7.90 (d, J = 7.6 Hz, 2H),
7.81 (d, J = 8.8 Hz, 1H), 7.70 (d, J = 8.8 Hz, 1H), 7.52 (t, J = 7.6 Hz,
2H), 7.31−7.41 (m, 2H), 7.12 (dd, J1 = 7.2 Hz, J2 = 8.0 Hz, 1H).
Anal. Calcd for C13H10N2: C, 80.39; H, 5.19; N, 14.42. Found: C,
80.40; H, 5.18; N, 14.43.
Compounds 2c and 2d (2e−2g) were obtained in good to
excellent yields by following the procedure described for 2b (2a).
2-(4-Fluorophenyl)-2H-indazole (f-1-pidz, 2c). White solid:
1
Ir5 (128 mg, 57% yield): yellow powder; H NMR (400 MHz,
1
CDCl3) δ 8.17 (s, 2H), 8.12 (d, J = 8.8 Hz, 2H), 7.77−7.83 (m, 4H),
7.68 (d, J = 7.2 Hz, 2H), 7.48−7.53 (m, 4H), 7.28−7.40 (m, 10H),
7.21−7.25 (m, 2H), 6.92 (t, J = 7.2 Hz, 2H), 6.76 (t, J = 7.6 Hz, 2H),
6.65−6.69 (m, 4H), 6.54 (t, J = 7.2 Hz, 2H), 6.13 (dd, J1 = 1.6 Hz, J2
= 7.2 Hz, 2H); HRMS calcd for C50H38IrN5O2P2 995.213 ([M +
H]+), found 996.219. Anal. Calcd for C50H38IrN5O2P2: C, 60.35; H,
3.85; N, 7.04. Found: C, 60.40; H, 3.79; N, 7.07.
750 mg (83% yield); H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H),
7.84−7.86 (m, 2H), 7.78 (d, J = 8.8 Hz, 1H), 7.69 (d, J = 8.4 Hz,
1H), 7.31−7.35 (m, 1H), 7.19−7.23 (m, 2H), 7.12 (dd, J1 = 6.8 Hz,
J2 = 8.0 Hz, 1H). Anal. Calcd for C13H10FN2: C, 73.57; H, 4.27; N,
13.20. Found: C, 73.55; H, 4.26; N, 13.23.
2-[4-(Trifluoromethyl)phenyl]-2H-indazole (cf3-1-pidz, 2d).
1
White solid: 820 mg (89% yield); H NMR (400 MHz, CDCl3) δ
1
Ir6 (146 mg, 63% yield): yellow powder; H NMR (400 MHz,
8.45 (s, 1H), 8.05 (d, J = 8.8 Hz, 2H), 7.77−7.79 (m, 3H), 7.70 (d, J
= 8.4 Hz, 1H), 7.33−7.37 (m, 1H), 7.11−7.15 (m, 1H). Anal. Calcd
for C13H10F3N2: C, 64.12; H, 3.46; N, 10.68. Found: C, 64.14; H,
3.45; N, 10.69.
1-Phenyl-1H-indazole (h-7-pidz, 2e). White solid: 980 mg
(92% yield); 1H NMR (400 MHz, CDCl3) δ 8.23 (s, 1H), 7.74−7.83
(m, 4H), 7.55 (t, J = 7.6 Hz, 2H), 7.42−7.46 (m, 2H), 7.35−7.39 (m,
1H), 7.22−7.25 (m, 2H). Anal. Calcd for C13H10N2: C, 80.39; H,
5.19; N, 14.42. Found: C, 80.41; H, 5.19; N, 14.41.
CDCl3) δ 8.13 (s, 2H), 8.05 (d, J = 8.8 Hz, 2H), 7.76−7.81 (m, 4H),
7.62 (dd, J1 = 4.8 Hz, J2 = 8.4 Hz, 2H), 7.48−7.54 (m, 4H), 7.32−
7.38 (m, 6H), 7.27−7.29 (m, 2H), 7.22−7.25 (m, 4H), 6.76 (t, J =
7.6 Hz, 2H), 6.62−6.69 (m, 6H), 5.78 (dd, J1 = 2.8 Hz, J2 = 9.6 Hz,
2H); HRMS calcd for C50H36F2IrN5O2P2 1031.194 ([M + H]+),
found 1032.201. Anal. Calcd for C50H36F2IrN5O2P2: C, 58.25; H,
3.52; N, 6.79. Found: C, 58.20; H, 3.47; N, 6.86.
1
Ir7 (113 mg, 49% yield): yellow powder; H NMR (400 MHz,
CDCl3) δ 8.19 (s, 2H), 8.11 (d, J = 8.4 Hz, 2H), 7.72−7.81 (m, 6H),
7.57 (t, J = 8.0 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.34−7.41 (m, 6H),
7.28 (t, J = 7.6 Hz, 2H), 7.21−7.25 (m, 6H), 6.76 (t, J = 7.2 Hz, 2H),
6.62−6.67 (m, 4H), 6.36 (d, J = 1.6 Hz, 2H); HRMS calcd for
C52H36F6IrN5O2P2 1131.188 ([M + H]+), found 1132.196. Anal.
Calcd for C52H36F6IrN5O2P2: C, 55.22; H, 3.21; N, 6.19. Found: C,
55.27; H, 3.20; N, 6.21.
1-(4-Fluorophenyl)-1H-indazole (f-7-pidz, 2f). White solid:
1.05 g (90% yield); 1H NMR (400 MHz, CDCl3) δ 8.10 (s, 1H), 7.71
(d, J = 8.0 Hz, 1H), 7.56−7.60 (m, 3H), 7.31−7.35 (m, 1H), 7.11−
7.16 (m, 3H). Anal. Calcd for C13H10FN2: C, 73.57; H, 4.27; N,
13.20. Found: C, 73.57; H, 4.28; N, 13.21.
1-[4-(Trifluoromethyl)phenyl]-1H-indazole (cf3-7-pidz, 2g).
White solid: 1.14 g (88% yield); 1H NMR (400 MHz, CDCl3) δ 8.25
(s, 1H), 7.91 (d, J = 8.0 Hz, 2H), 7.82 (t, J = 8.4 Hz, 4H), 7.49 (t, J =
8.0 Hz, 1H), 7.28−7.32 (m, 1H). Anal. Calcd for C13H10F3N2: C,
64.12; H, 3.46; N, 10.68. Found: C, 64.13; H, 3.47; N, 10.70.
General Synthetic Procedure for Ir(III) Complexes Ir1−Ir7.
The cyclometalated Ir(III) dichloro-bridged dimers, [(C^N)2Ir(μ-
Cl)]2, were synthesized by IrCl3 hydrate with 2.2 equiv of 2a−2g in a
3:1 mixture of 2-ethoxyethanol and deionized water according to a
method similar to that reported by Nonoyama.34 Then Ir(III)
complexes Ir1−Ir7 were prepared by the reaction of Ir(III) dichloro-
bridged dimers with 2.2 equiv of Ktpip in 2-ethoxyethanol at 120 °C.
After the reaction was completed, the reaction mixture was diluted
with water and extracted multiple times with dichloromethane. The
combined organic phase was concentrated, and the residue was
purified by column chromatography to provide the crude product,
which was further purified by vacuum sublimation.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00491.
1
OLED fabrication and measurement, H NMR spectra
and mass spectrometry of main ligands 2a−2g and
iridium complexes Ir1−Ir7, emission spectra of iridium-
(III) complexes Ir1−Ir7 in degassed CH2Cl2 solutions
at 77 K, PL spectra of Ir3 and Ir6 in solution and solid
state, EL spectra of D3 and D6, crystallographic data
and selected bonds and angles of complexes Ir2−Ir7,
and frontier orbital energy and electron density
distributions of complexes Ir1−Ir7 (PDF)
1
Ir1 (124 mg, 53% yield): yellow powder; H NMR (400 MHz,
CDCl3) δ 7.88 (d, J = 2.8 Hz, 2H), 7.67−7.72 (m, 4H), 7.61 (d, J =
2.0 Hz, 2H), 7.27−7.36 (m, 10H), 7.15−7.23 (m, 4H), 7.06−7.11
H
DOI: 10.1021/acs.organomet.8b00491
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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Accession Codes
CCDC 1578682−1578687 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: yxzheng@nju.edu.cn.
*E-mail: zuojl@nju.edu.cn.
*E-mail: ligaonan2008@163.com.
■
ORCID
Zhi-Gang Niu: 0000-0003-4842-1464
You-Xuan Zheng: 0000-0002-1795-2492
Jing-Lin Zuo: 0000-0003-1219-8926
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (51773088 and 21501037), the Natural
Science Foundation of Jiangsu Province (BY2016075-02), the
Natural Science Foundation of Hainan Province (218QN236),
and the Program for Innovative Research Team in University
(IRT-16R19).
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