Unsymmetric Heteroleptic Ir(III) Complexes with 2-Phenylquinoline and Coumarin-Based Ligand Isomers for Tuning Character of Triplet Excited States and Achieving High Electroluminescent Efficiencies

Article abstract of DOI:10.1021/acs.inorgchem.0c01443

2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of synthesis have been selected to construct the unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six unsymmetric heteroleptic Ir(III) complexes have been obtained by employing four coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)] structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form C-Ir bond. Through adopting unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)] structure, these Ir(III) complexes can not only achieve impressive absolute quantum yield φp (ca. 0.5-1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4), but also realize a dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T1), showing both 3MLCT character and 3ILCT (intraligand charge transfer) character in their T1 states. AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir(III) center to ligand L-C5 or L-C7 and ILCT character in ligand L-C5 or L-C7 in their T1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T1 states. Moreover, the color-tuning mechanism and the lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the electroluminescent (EL) performance due to their high φP of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A-1, and a maximum power efficiency (ηP) of 60.7 lm W-1. All these impressive results can definitely demonstrate the effectiveness of our simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.

Full text of DOI:10.1021/acs.inorgchem.0c01443

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Article
Unsymmetric Heteroleptic Ir(III) Complexes with 2‑Phenylquinoline
and Coumarin-Based Ligand Isomers for Tuning Character of Triplet
Excited States and Achieving High Electroluminescent Efficiencies
Zhao Feng, Yue Yu, Xiaolong Yang, Yong Wu, Guijiang Zhou,* and Zhaoxin Wu*
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ABSTRACT: 2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of
∧
∧
synthesis have been selected to construct the unsymmetric heteroleptic [Ir(C1 N)(C2 N)-
(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six
unsymmetric heteroleptic Ir(III) complexes have been obtained by employing four
coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)]
structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form
∧
∧
C−Ir bond. Through adopting unsymmetric heteroleptic [Ir(C1 N)(C2 N)(acac)]
structure, these Ir(III) complexes can not only achieve impressive absolute quantum yield
Φp (ca. 0.5−1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4), but also realize a
dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red
(AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T1), showing
both 3MLCT character and 3ILCT (intraligand charge transfer) character in their T1 states.
AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir(III) center to ligand L-C5 or
L-C7 and ILCT character in ligand L-C5 or L-C7 in their T1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character
from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T1 states. Moreover, the color-tuning mechanism and the
lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the
electroluminescent (EL) performance due to their high ΦP of nearly up to unity. Optimal orange-emitting device B2 based on
AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A−1, and a
maximum power efficiency (ηP) of 60.7 lm W−1. All these impressive results can definitely demonstrate the effectiveness of our
simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.
harmful to the thermal stability of the concerned Ir(III)
complexes.22,23 Thereby, cyclometalated Ir(III) complexes
containing easily synthesized cyclometalating ligands are
much more preferred for practical application as high-
performance Ir-based phosphors in OLEDs.
INTRODUCTION
■
The strong spin−orbital coupling effect in C∧N cyclometalated
Ir(III) complexes gives rise to their intense room-temperature
phosphorescence and relatively short lifetimes of triplet excited
states.1−6 Beneficially, organic light-emitting diodes (OLEDs)
consisting of C∧N cyclometalated Ir(III) complexes generally
can harvest both electrically generated 25% singlet excitons
and 75% triplet excitons for light emission and realize a
theoretical 100% internal quantum efficiency (IQE).7−10
Thereby, C∧N cyclometalated Ir(III) complexes seem to be
innately ideal phosphors for OLEDs.11−16 Importantly, the
photophysical characteristics of C∧N cyclometalated Ir(III)
complexes are susceptible to and mainly dominated by the
electronic nature of the C∧N cyclometalating ligands.17−19
This provides a versatile platform to design Ir(III) phosphor-
escent emitters with expected performance. In the past
decades, great efforts have been devoted to synthesis of
novel and relatively complicated cyclometalating ligands or
functionalization of cyclometalating ligands for developing
highly efficient Ir-based phosphors.20−28 However, obtaining
these relatively complicated cyclometalating ligands suffers
tedious organic synthetical processes, which sometimes is
2-Phenylquinoline (PQ) can be readily prepared in high
yield.29 Meanwhile, orange-emitting [Ir(PQ)2(LX)]-type com-
plexes have been well-explored in OLEDs as well as
electrogenerated chemiluminescence, among others.30−34
However, [Ir(PQ)2(LX)]-type complexes suffer from low
photoluminescence quantum yields (Φp), which significantly
limits their optoelectronic applications.33,35 Therefore, it is
significant to improve the Φp of cyclometalated Ir(III)
complexes so as to extend their application potential. Recently,
∧
∧
we found that adopting an unsymmetric [Ir(C1 N)(C2 N)-
Received: May 17, 2020
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Scheme 1. Synthetic Scheme of These Unsymmetric Heteroleptic Ir(III) Complexes
(acac)] structure can not only effectively improve the Φp but
also tune the emission character.22,36,37 Coumarin derivatives
possess the characteristics of easy synthesis and modification,
typically showing high fluorescence quantum yields.38−40
Additionally, our recent report has demonstrated [Ir-
(C∧N)2(acac)]-type complexes containing 2-phenypyridine-
(ppy)-type coumarin-based ligands can show high Φp (ca.
0.4−1.0) and enhanced electron-injection/transporting (EI/
ET) properties. OLEDs based on these emitters can achieve
maximum EL efficiencies of ηext 22.7%, ηL 79.7 cd A−1, and ηP
58.2 lm W−1, indicating the great potential of coumarin-based
cyclometalating ligands in developing high-performance
B
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phosphors.41 Hence, a combination of PQ and coumarin-based
of 1.72−1.48 ppm can be attributed to two different methyl’s
protons of the coordinated acetylacetonate (Figure S1). These
outcomes are consistent with the reported unsymmetric
heteroleptic [Ir(N∧C1)(N∧C2)(acac)]-type complexes.37 The
chemical structures of coordination isomers for AIrC6out and
AIrC6in as well as AIrC7out and AIrC7in can be also
∧
cyclometalating ligand into the unsymmetric [Ir(C1 N)-
∧
(C2 N)(acac)] structure should be a feasible way to develop
high-performance Ir-based phosphors.
On the basis of all the aforementioned considerations, we
have obtained six unsymmetric heteroleptic Ir(III) complexes
by employing four coumarin-based ligand isomers (L-C5/L-
C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)] structure.
Fortunately, two coordination isomers were obtained in
synthesizing the corresponding unsymmetric heteroleptic
Ir(III) complexes using ligand L-C6/L-C7 due to two different
coordinating carbon atoms in ligand L-C6 or L-C7 to form C−
Ir bond. On account of the high rigidity of the coumarin block,
these unsymmetric heteroleptic Ir(III) complexes generally can
exhibit absolute quantum yield Φp (ca. 0.5−1.0), higher than
that of complex [Ir(PQ)2(acac)] (ca. 0.4). Through employing
coumarin-based ligand isomers in the [Ir(PQ)(C∧N)(acac)]
structure and changing the coordinating carbon atom in ligand
L-C6 or L-C7 to the Ir center, a dual modulation of both
emission color from orange (AIrC6out, λ = 578 nm) to red
(AIrC5, λ = 622 nm), and the features of their lowest triplet
excited states have been achieved for these unsymmetric
heteroleptic Ir(III) complexes. Encouragingly, we can achieve
maximum EL efficiencies of ηext 23.9%, ηL of 70.9 cd A−1, and
ηP of 60.7 lm W−1 based on orange-emitting device B2 with
AIrC8 as emitter. This work should not only provide some
new and important references to modulate the photophysical
behaviors of cyclometalated Ir(III) complexes but also open a
new outlet for developing high-performance Ir-based phos-
phorescent emitters.
1
distinguished by H NMR spectra. For complexes AIrC6out
and AIrC6in with L-C6 as one ligand, when ligand L-C6 offers
the C7-position (atom numbering see Figure S2) to coordinate
to the Ir atom, the aromatic protons linking to the C5- and C8-
positions in ligand L-C6 should show singlet resonance peaks
due to the absence of H−H resonance coupling effect.
However, when ligand L-C6 offers the C5-position to
coordinate to the Ir atom, all the aromatic protons in ligands
PQ and L-C6 should show split multiple peaks because of the
presence of H−H resonance coupling effect. As mentioned
above, as shown from the 1H NMR spectrum of AIrC6in, two
singlet resonance peaks at 7.71 and 6.50 ppm have been
observed (Figure S1), indicating ligand L-C6 offered C7-
position in the coumarin block to coordinate to Ir atom. While
1
the H NMR spectrum of AIrC6out did not show any singlet
resonance peaks in the range of the aromatic proton resonance
signals, demonstrating ligand L-C6 offered the C5-position in
the coumarin block to coordinate to the Ir atom (Figure S1).
1
Likewise, we use the same method to analyze the H NMR
spectra of AIrC7in and AIrC7out. As for the 1H NMR
spectrum of AIrC7in, there were no singlet resonance signals
for its aromatic protons (Figure S1), suggesting ligand L-C7
offered the C8-position in the coumarin block to coordinate to
1
the Ir atom. As for AIrC7out, its H NMR spectrum showed
two singlet resonance peaks at 7.64 and 6.59 ppm, respectively
(Figure S1), reflecting ligand L-C7 offered the C6-position in
the coumarin block to coordinate to the Ir atom. In
combination with the information from their 13C NMR
spectra (Figure S3), the chemical structures of these
unsymmetric heteroleptic Ir(III) complexes can be correctly
confirmed.
Single-Crystal X-ray Crystallography. Good-quality
single crystals of AIrC8 were successfully grown by slowly
diffusing methanol into its chloroform solution. The structure
is well characterized by single-crystal X-ray crystallography.
The perspective drawing of AIrC8 is depicted in Figure 1. The
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The concrete synthetic
procedures of ligand L-C6 and the unsymmetric heteroleptic
∧
∧
[Ir(C1 N)(C2 N)(acac)]-type complexes are illustrated in
Schemes S1 and 1, respectively. Ligands L-C5/L-C7/L-C8
were prepared according to our previously reported proto-
cols.41 Br−C6 can be easily prepared through palladium-
catalyzed intermolecular annulation between 4-bromophenol
and methyl acrylate in a moderate yield.42 The target ligand L-
C6 was readily obtained by cross-coupling 2-(tributylstannyl)-
pyridine with Br−C6. The designed unsymmetric heteroleptic
Ir(III) complexes were prepared by two successive steps. First,
corresponding cyclometalating ligand L-C5/L-C6/L-C7/L-C8
and 2-phenylquinoline (PQ) were metalated with IrCl3·nH2O
to yield the corresponding Ir(III) μ-chloro-bridged dimers.
Second, the target unsymmetric heteroleptic Ir(III) complexes
were prepared by stirring a dry CH2Cl2 solution of the
concerned Ir(III) μ-chloro-bridged dimer and Tl(acac). These
air-stable colored unsymmetric heteroleptic Ir(III) complexes
were gained in moderate yields using preparative silica gel thin-
layer chromatography (TLC) plates with an appropriate eluent
system. As expected, two coordination isomers were obtained
in synthesizing the corresponding unsymmetric heteroleptic
Ir(III) complexes using ligand L-C6/L-C7, indicating that
ligands L-C6 and L-C7 can offer two different coordination
sites to form C−Ir bond, consistent with our previous report.41
Chemical structures of these final unsymmetric heteroleptic
Ir(III) complexes were properly identified by H and 13C
NMR spectroscopy. In each of their H NMR spectra, the
1
1
singlet resonance peak in the range of 4.77−4.98 ppm can be
safely ascribed to the proton of the coordinated acetylaceto-
nate, and meanwhile, two singlet resonance peaks in the range
Figure 1. Perspective drawing of AIrC8 using thermal ellipsoids at a
25% probability level. All the hydrogen atoms and most of the labels
are removed for clarity.
C
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specific crystallographic data and selected bond lengths and
angles are tabulated in Tables S1 and S2. In the single crystal,
the molecules spontaneously adopt the space group P121/a1
to form a monoclinic crystal system. The single-crystal
structure of AIrC8 shows three cyclometalating ligands with
an Ir atom as the center, affording a slightly distorted
octahedral chelating configuration which possesses the typical
chelating disposition of cis-C,C, trans-N,N, and cis-O,O. The
bond angles of C(2)−Ir(1)−O(3), N(5)−Ir(1)−N(6), and
C(14)−Ir(1)−O(7) are 174.34(12), 174.59(11), and
176.28(11)°, respectively. The bond lengths of Ir−O, Ir−N,
∧
and Ir−C nearly resemble those of the reported [Ir(C1 N)-
43
∧
(C2 N)(acac)]-type Ir(III) complexes.
Thermal and Photophysical Characterization. Thermal
stability of these unsymmetric heteroleptic Ir(III) complexes
has been evaluated with thermogravimetric analysis (TGA)
under a nitrogen flow. Their TGA traces suggested the good
thermal stability with decomposition temperature (Td) ranging
from 317 to 332 °C (Table 1 and Figure S4). Their high
thermal stability makes them applicable as phosphorescent
emitters to fabricate OLEDs.
The photophysical properties of all the final unsymmetric
heteroleptic Ir(III) complexes are characterized at a ca. 10−5
mol L−1 concentration in CH2Cl2 solution and the relevant
data are compiled in Table 1. As shown from their UV−vis
absorption spectra, two absorption bands can be clearly
observed (Figure 2a), correlated well with the reported results
of [Ir(N∧C1)(N∧C2)(acac)]-type complexes.37,43 The strong
absorption bands before ca. 400 nm can be firmly ascribed to
the spin-allowed π−π* transition of the two C∧N cyclo-
metalated ligands on account of the large extinction
coefficients (Table 1). On the basis of the calculation results
of time-dependent density functional theory (TD-DFT), the
character associated with the long-wavelength absorption
bands (after ca. 400 nm) for these unsymmetric heteroleptic
Ir(III) complexes can be interpreted by their HOMO →
LUMO (H → L) transitions (Table 2). In addition, their
HOMOs are mainly comprised of a mixture of the metal d-
and π-orbitals of the two phenyl rings coordinated to the Ir
center, while their LUMO distribution is noticeably different
(Figure 3). The LUMOs of AIrC6out, AIrC6in, and AIrC8
predominantly reside on the π*-orbitals of 2-phenylquinoline
(PQ), while those of AIrC5, AIrC7out, and AIrC7in
principally dwell on the π*-orbitals of the coumarin-based
ligand L-C5 or L-C7. Thus, on the basis of their molecular
orbital (MO) patterns in Figure 3, the low-energy absorption
bands of AIrC6out, AIrC6in, and AIrC8 can show metal to
ligand charge transfer (MLCT) character from the Ir(III)
center to the π*-orbitals of ligand PQ together with ligand to
ligand charge transfer (LLCT) character from the phenyl π-
orbitals of ligand L-C6 or L-C8 to the π*-orbitals of ligand PQ
and intraligand charge transfer (ILCT) character from the
phenyl π-orbitals to the quinoline π*-orbitals of ligand PQ,
while the low-energy absorption bands of AIrC5, AIrC7out,
and AIrC7in can exhibit MLCT character from the Ir(III)
center to the π*-orbitals of ligand L-C5 or L-C7 as well as a
LLCT feature from the phenyl π-orbitals of ligand PQ to the
π*-orbitals of ligand L-C5 or L-C7 and a ILCT feature from
the phenyl π-orbitals to the pyridine π*-orbitals in ligand L-C5
or L-C7.
Photoluminescent spectra for these unsymmetric hetero-
leptic Ir(III) complexes are recorded in CBP films and CH2Cl2
solution at 293 and 77 K. Upon UV light irradiation at 400 nm,
D
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Figure 2. (a) UV−vis absorption spectra and (b) photoluminescence (PL) spectra of the obtained unsymmetric heteroleptic Ir(III) complexes
recorded in CH2Cl2 solution at 293 K.
Table 2. TD-DFT Results of These Unsymmetric Heteroleptic Ir(III) Complexes Based on Their Optimized S0 Geometries
contribution of metal dπ-orbitals and
ligand π-orbitals to MOs (%)
c
c
complex MO
Ir
PQ
L-C5
acac
main configuration of S0→S1 excitation/Ecal/λcal/f
H→L (93.27%)
main configuration of S0→T1 excitation/Ecal/λcal
L
AIrC5
3.37
46.88
12.58
19.41
83.08
29.09
0.97
4.62
PQ
H→L (85.33%)
2.033 eV/609.8 nm
H
2.438 eV/508.6 nm/0.0108
a
Ir
L-C6C5
acac
L
H
3.97
48.50
Ir
91.91
23.47
PQ
2.95
1.17
4.59
acac
H→L (97.57%)
2.503 eV/495.3 nm/0.0197
H→L (83.87%)
2.260 eV/548.7 nm
AIrC6out
AIrC6in
AIrC7out
AIrC7in
AIrC8
23.44
a
L-C6C7
L
H
3.70
47.66
Ir
92.08
22.46
PQ
3.11
24.90
L-C7C6
1.11
4.98
acac
H→L (96.88%)
2.553 eV/485.7 nm/0.0217
H→L (73.05%)
2.294 eV/540.5 nm
b
L
H
2.27
48.19
Ir
12.74
19.86
PQ
84.31
26.99
0.68
4.96
acac
H→L (87.20%)
2.407 eV/515.0 nm/0.0063
H→L (81.33%)
2.055 eV/603.4 nm
b
L-C7C8
L
H
3.88
49.68
Ir
1.59
26.97
PQ
93.55
18.46
L-C8
0.98
4.89
acac
H→L (96.12%)
2.421 eV/512.1 nm/0.0150
H→L (90.44%)
2.136 eV/580.4 nm
L
H
3.93
47.68
93.44
21.46
1.55
25.72
1.08
5.14
H→L (97.20%)
2.505 eV/495.0 nm/0.0251
H→L (78.43%)
2.264 eV/547.5 nm
a
L-C6C5 indicates ligand L-C6 offers the C5-position of the coumarin block to coordinate to the Ir atom, while L-C6C7 means ligand L-C6 provides
b
the C7-position of the coumarin block to coordinate to the Ir atom. L-C7C6 indicates ligand L-C7 offers the C6-position of the coumarin block to
coordinate to the Ir atom, while L-C7C8 means ligand L-C7 provides the C8-position of the coumarin block to coordinate to the Ir atom. H → L
c
indicates transition from HOMO to LUMO. Ecal, λcal, and f are the calculated excitation energy, calculated absorption wavelength, and calculated
oscillator strength, respectively. The calculated oscillator strength of S0 → T1 should be zero on account of the spin-forbidden character of singlet−
triplet transition because TD-DFT calculation through the Gaussian program does not consider the spin−orbital coupling effect.
all the obtained unsymmetric heteroleptic Ir(III) complexes
luminesce from orange (λ = 578 nm) to red (λ = 622 nm)
phosphorescent bands in CH2Cl2 solution (Figure 2b and
Table 1). The emission peak wavelengths of these unsym-
metric heteroleptic Ir(III) complexes almost show a
hypsochromic effect in both CH2Cl2 solution at 77 K and
the CBP films in comparison with those in CH2Cl2 solution at
293 K, maybe due to some restrained molecular vibrations in
the rigid matrix (Figures 2b, S5, and S6, Table 1). The room-
temperature luminescent lifetimes of these unsymmetric
heteroleptic Ir(III) complexes in CH2Cl2 solution at 293 K
and CBP films span from 1.26 to 2.36 μs (Table 1),
demonstrating their emission from triplet excited states.
These unsymmetric hetroleptic Ir(III) complexes can uni-
formly show high ΦP values of 0.09 for AIrC5, 0.17 for
AIrC6out and AIrC6in, 0.10 for AIrC7out, 0.13 for AIrC7in,
and 0.18 for AIrC8 in degassed CH2Cl2 solution relative to
[Ir(PQ)2 (acac)] (ΦP = 0.1) (Table 1), respectively.34 Their
absolute quantum yields (ΦP) in the 8 wt % CBP films span
from 0.46 to 0.99 (Table 1), higher than that of complex
[Ir(PQ)2(acac)] (ΦP = 0.41). These results suggest adoption
of unsymmetric heteroleptic [Ir(C1 N)(C2 N)(acac)] struc-
ture together with introduction of coumarin-based ligands can
effectively enhance the ΦP values of PQ-based Ir(III)
complexes, which are helpful to obtain high EL performance,
so adopting unsymmetric structures should represent an
effective way to optimize optoelectronic properties of the
PQ-based Ir(III) complexes.
∧
∧
With aim to deepen the understanding of the phosphor-
escent character (T1 state character) of these unsymmetric
heteroleptic Ir(III) complexes, we performed natural transition
orbital (NTO) analysis for S0 → T1 excitations based on the
E
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Figure 4. Distribution patterns of natural transition orbital
(isocontour value = 0.030) for S0 → T1 excitation of these
unsymmetric heteroleptic Ir(III) complexes based on their optimized
T1 geometries. (a) AIrC5, AIrC6out, and AIrC6in; (b) AIrC7out,
AIrC7in, and AIrC8.
Figure 3. Molecular orbital (MO) patterns (isocontour value =
0.030) of these unsymmetric heteroleptic Ir(III) complexes based on
their optimized S0 geometries. (a) AIrC5, AIrC6out, and AIrC6in;
(b) AIrC7out, AIrC7in, and AIrC8.
(acac)] structure through employing coumarin-based ligand
isomer (L-C5/L-C6/L-C7/L-C8) as well as alternating the
coordinating site of coumarin-based ligand L-C6/L-C7. As
optimized T1 geometries. As shown from the distribution
patterns of the NTO hole and particle orbitals for these
unsymmetric heteroleptic Ir(III) complexes (Figure 4 and
3
3
aforementioned, AIrC5 and AIrC7out show both MLCT
Table 3), we can see clearly that they all display both MLCT
3
3
character and ILCT character in their T1 states, and their
character and ILCT (intraligand charge transfer) character in
emission characters are dominated by coumarin-based ligand
L-C5/L-C7. Therefore, AIrC5 (λ = 622 nm) and AIrC7out (λ
= 612 nm) emit at relatively longer emission wavelengths.
Furthermore, our previous photophysical investigation on
coumarin-based cyclometalated Ir(III) complexes has demon-
strated that the heterocycle of the coumarin block can accept
electrons in the MLCT transition process.41 Thereby, the
electron-accepting coumarin heterocycles in AIrC5 and
AIrC7out will obviously facilitate both MLCT transition
processes and thus form more stable T1 excited states.
Accordingly, AIrC5 and AIrC7out present a red-shifted effect
in their phosphorescent emission wavelengths with respect to
their symmetric counterparts (IrC5 λ = 608 nm, IrC7 λ = 604
nm, [Ir(PQ)2(acac)] λ = 589 nm).41,32 However, AIrC6out,
AIrC6in, and AIrC8 display both 3MLCT character and
3ILCT character in their T1 states, and their emission
characters are governed by ligand PQ; hence, AIrC6out,
AIrC6in, and AIrC8 emit at relatively shorter emission
wavelengths (AIrC6out λ = 578 nm, AIrC6in λ = 582 nm,
AIrC8 λ = 584 nm). Additionally, the existence of electron-
accepting coumarin heterocycles in AIrC6out, AIrC6in, and
AIrC8 will definitely restrain charge transfer from the dπ-
orbitals of the Ir centers to the π*-orbitals of ligand PQ to
some extent, which leads to a higher energy level of the MLCT
states in AIrC6out, AIrC6in, and AIrC8 than that formed in
their T1 states. However, the symmetric coumarin-based Ir(III)
complexes mainly show MLCT character for IrC5, IrC7 and
3
IrC7-A as well as ligand-centered character for IrC8 in their T1
state.41 For AIrC5, AIrC7out, and AIrC7in, the hole orbitals
dwell mainly on the dπ-orbitals of the Ir centers and coumarin-
based ligand L-C5 or L-C7, while their particle orbitals reside
strongly on the coumarin-based ligand L-C5 or L-C7. This
result implies that AIrC5, AIrC7out, and AIrC7in show
MLCT feature from Ir(III) center to ligand L-C5 or L-C7 and
ILCT character in ligand L-C5 or L-C7 in their T1 states.
Hence, ligand L-C5 or L-C7 dominates the emission character,
rather than ligand PQ. This can be attributed to the fact that
the triplet energy levels of L-C5 and L-C7 are lower than that
of ligand PQ. However, as for AIrC6out, AIrC6in, and AIrC8,
the hole orbitals principally reside on the dπ-orbitals of the Ir
centers and ligand PQ, while their particle orbitals are
prevailingly located on ligand PQ. This outcome means that
AIrC6out, AIrC6in, and AIrC8 exhibit MLCT character from
the Ir(III) center to ligand PQ and ILCT character in ligand
PQ in their T1 states. Thus, their emission character is
governed by ligand PQ, instead of ligand L-C6 or L-C8.
Likewise, this can be ascribed to the fact that ligands L-C6 and
L-C8 have higher triplet energy levels than that of ligand PQ.
Interestingly, a fine color-tuning from orange (λ = 578 nm) to
red (λ = 622 nm) has been realized based on [Ir(PQ)(C∧N)-
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Table 3. NTO Results for These Unsymmetric Heteroleptic
Ir(III) Complexes Based on Optimized T1 Geometries
contribution percentages of metal dπ-orbitals and
π-orbitals of ligand to NTOs (%)
a
complex
NTO
Ir
PQ
L-C5
acac
H
P
NTO
33.34
4.56
Ir
6.48
1.30
PQ
56.94
93.13
L-C6C5
3.24
1.01
acac
AIrC5
a
b
complex
H
P
NTO
31.49
6.54
Ir
59.09
90.24
PQ
7.54
1.60
L-C6C7
1.88
1.62
acac
AIrC6out
a
b
complex
H
P
NTO
29.57
6.32
Ir
62.49
90.93
PQ
4.93
1.32
L-C7C6
3.01
1.43
acac
AIrC6in
Figure 5. Molecular orbital (MO) patterns for the organic ligands (a)
L-C5, (b) L-C6, (c) L-C7, and (d) L-C8. Green circles are used to
highlight the electron density on the chelating carbon atoms.
a
c
c
complex
H
P
NTO
31.44
2.83
Ir
6.43
0.89
PQ
59.10
95.59
L-C7C8
3.03
0.69
acac
AIrC7out
a
complex
612 nm). As for AIrC7in, the much lower contribution of
electron density to HOMO in ligand L-C7C8 (0.36%) will
significantly reduce the electron density on the Ir(III) center,
thus restraining the MLCT process to give higher energy-level
of the 3MLCT state in AIrC7in. As a result, AIrC7in (λ = 586
nm) exhibits shorter phosphorescent wavelength than
AIrC7out (λ = 612 nm). As aforementioned, ligand PQ
governs the emission characters of AIrC6out, AIrC6in, and
AIrC8 and its triplet energy is lower than those of ligands L-
C6 and L-C8. Furthermore, the existence of electron-accepting
coumarin heterocycles in AIrC6out (λ = 578 nm), AIrC6in (λ
= 582 nm), and AIrC8 (λ = 584 nm) causes a hypsochromic
emission relative to [Ir(PQ)2(acac)] (λ = 589 nm). In
addition, the contribution of electron density on the chelating
carbon atom to HOMO in a ligand also affects their emission
character. A slightly higher contribution of electron density on
the chelating carbon atom to HOMO in ligand L-C8 (2.85%)
in AIrC8 can be observed in comparison with that of ligand L-
C6C5 (2.57%) in AIrC6out (Figure 5 and Table S3). Hence,
the emission wavelength of AIrC8 (λ = 584 nm) is slightly
longer than that of AIrC6out (λ = 578 nm). Interestingly,
despite of markedly higher contribution of electron density on
the chelating carbon atom to the HOMO in ligand L-C6C7
(7.55%) than that in ligands L-C6C5 (2.57%) and L-C8
(2.85%), the emission wavelength of AIrC6in (λ = 582 nm)
falls in between AIrC6out and AIrC8, rather than longer than
AIrC6out and AIrC8. This is due to the fact that the
contribution of the Ir center in AIrC6in (29.57%) to hole
orbitals is less than those of AIrC6out (31.49%) and AIrC8
(31.18%) (Table 3). Moreover, the reduced electron density
on the Ir center definitely restrains the MLCT transition
H
P
NTO
43.73
6.19
Ir
11.18
1.22
PQ
40.84
91.16
L-C8
4.25
1.43
acac
AIrC7in
a
complex
H
P
31.18
6.43
59.90
90.77
5.65
1.35
3.27
1.45
AIrC8
a
b
H denotes NTO hole orbital and P denotes particle orbital. L-C6C5
indicates that ligand L-C6 offers the C5-position of the coumarin
block to coordinate to the Ir atom, while L-C6C7 means that the
ligand L-C6 provides the C7-position of the coumarin block to
c
coordinate to the Ir atom. L-C7C6 indicates that the ligand L-C7
offers the C6-position of the coumarin block to coordinate to the Ir
atom, while L-C7C8 means that the ligand L-C7 provides the C8-
position of the coumarin block to coordinate to the Ir atom.
[Ir(PQ)2(acac)]. Consequently, their emission wavelengths
present a blue-shifted effect relative to that of [Ir(PQ)2(acac)]
(AIrC6out λ = 578 nm, AIrC6in λ = 582 nm, AIrC8 λ = 584
nm, [Ir(PQ)2(acac)] λ = 589 nm).
As is well-known, the photophysical feature of the
cyclometalated Ir(III) complexes is susceptible to and
governed by the electronic nature of the cyclometalating
ligands.17−19 Thereby, with aim to better understand this fine
color-tuning behavior, we conducted the density functional
theory (DFT) calculations for ligands L-C5, L-C6, L-C7, and
L-C8 to check their MO distribution patterns. According to
the DFT results, noticeable differences can be seen in
contribution of the electron density on the chelating carbon
atoms to HOMOs in these ligands (Figure 5 and Table S3).
Our early reports have indicated that the higher the
contribution of electron density on the chelating carbon
atom to HOMO ligand, the stronger the electron-donating
ability it will show. Therefore, the stronger electron-donating
ability obviously raise the electron density on the Ir(III) center
of the corresponding cyclometalated Ir(III) complexes. As a
result, it will facilitate the MLCT transition process and lead to
a more stable MLCT state, producing a red-shifted effect in the
emission wavelength with a MLCT transition feature.44,45 As
aforementioned, coumarin-based ligand L-C5 or L-C7
dominates the emission characters of AIrC5, AIrC7out and
AIrC7in, and the triplet energies of L-C5 and L-C7 are lower
than that of ligand PQ. Furthermore, ligand L-C5 (17.63%)
has a higher contribution of the electron density on the
chelating carbon atom to HOMO than that of L-C7C6
(11.97%) (Figure 5 and Table S3). Hence, AIrC5 (λ = 622
nm) emits at a longer wavelength relative to AIrC7out (λ =
3
process to furnish higher energy-level of the MLCT state.
Accordingly, the net result should be that AIrC6in (λ = 582
nm) gives a longer emission with respect to AIrC6out (λ =
578 nm) and a shorter emission relative to AIrC8 (λ = 584
nm). More interestingly, the emission wavelength of AIrC6in
(λ = 582 nm) is red-shifted by 4 nm in comparison with that of
its coordination isomer AIrC6out (λ = 578 nm), While a
larger bathochromic emission (26 nm) is observed for the two
coordination isomers between AIrC7out (λ = 612 nm) and
AIrC7in (λ = 586 nm). As seen from the DFT results (Figure
5 and Table S3), the contribution of electron density on two
respective coordination carbon atoms in ligand L-C7 to
HOMO obviously shows a bigger difference than that of ligand
L-C6, which should originate from the higher unsymmetry of
L-C7.46−48 Thereby, we presume the higher the unsymmetry
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of cyclometalting ligand with two possible coordination carbon
atoms possesses, the larger the difference in emission character
the corresponding cyclometalated Ir(III) complexes may show.
Clearly, all these results will provide a novel and promising
pathway to tune the character of the triplet excited states and
the emission color of the concerned Ir(III) complexes.
Electrochemical Properties. Cyclic voltammetry (CV)
measurements have been conducted to investigate the
electrochemical properties of these unsymmetric heteroleptic
Ir(III) complexes in acetonitrile solution under a nitrogen
atmosphere calibrated with ferrocene (Fc). The HOMO and
LUMO levels are calculated on the basis of the CV results, and
the data are compiled in Table 4. In the anodic scan, these
V, which originate from the oxidation of the Ir(III) center as
well as the chelated phenyl rings (Table 4 and Figure S7).
Hence, their corresponding HOMO levels are calculated to
range from −5.29 to −5.41 eV. We can observe two
irreversible reduction waves in the cathodic scan. The first
reduction wave can be assigned to the reduction of the
heterocycle in the coumarin block due to the stronger electron-
accepting ability relative to the pyridyl units in the organic
ligands, while another reduction wave can be attributed to the
reduction of the pyridyl rings of the organic ligands. On the
basis of their first reduction wave, the LUMO levels are
calculated to be in the range of −2.52 to −2.68 eV, which are
relatively lower than the typically reported cyclometalated
Ir(III) complexes.49−51 The low LUMO levels may arise from
the introduction of the coumarin-based ligand.41,52 More
importantly, the relatively lower LUMO levels can strengthen
the EI/ET properties of these unsymmetric heteroleptic Ir(III)
complexes, thereby improving the EL performance of the
related OLEDs.
Electroluminescent Properties. Considering the crucial
role of ΦP in determining the device performance, we select
AIrC6in and AIrC8 as emitters to evaluate the electro-
luminescent (EL) performance due to their high ΦP of nearly
up to unity. In light of good thermal stability of AIrC6in and
AIrC8 indicated by the TGA data (Table 1), their vacuum-
deposited OLEDs were fabricated using a general device
configuration ITO/MoO3 (3 nm)/NPB (40 nm)/Ir x wt
%:CBP (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)
(Figure 6). As shown in Figure 6, we adopt vacuum-deposited
MoO3 on ITO glass substrates to obtain a hole injection layer
(HIL). The 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl
(NPB) is used to produce a hole-transporting and electron-
blocking layer, and the well-known 4,4′-N,N′-dicarbazole-
Table 4. Redox Properties of These Unsymmetric
Heteroleptic Ir(III) Complexes
Epa
E
HOMdO
ELUMO
e
a
b
c
complex
(V)
Epc (V)
Egcv
(eV)
(eV)
AIrC5
0.50
0.61
0.57
0.49
0.56
0.59
−2.12, −2.45
−2.22, −2.50
−2.28, −2.54
−2.15, −2.47
−2.23, −2.54
−2.22, −2.51
2.62
2.83
2.85
2.64
2.79
2.81
−5.30
−5.41
−5.37
−5.29
−5.36
−5.39
−2.68
−2.58
−2.52
−2.65
−2.57
−2.58
AIrC6out
AIrC6in
AIrC7out
AIrC7in
AIrC8
a
b
Reversible. E1/2 was used to calculate the values. Irreversible or
quasi-reversible. The values were set as the cathodic peak potential.
c
d
CV energy gap Egcv = LUMO − HOMO. HOMO levels are
e
calculated based on the equation HOMO = −(4.8 + Epa) eV. Peak
potential of the first reduction wave of the CV data was used to
calculate LUMO levels, LUMO = −(4.8 + Epc) eV.
unsymmetric heteroleptic Ir(III) complexes generally can
exhibit a reversible oxidation wave in the range of 0.49−0.61
Figure 6. Device structure of the vacuum-deposited OLEDs for AIrC6in and AIrC8. Chemical structures and energy level diagram of the adopted
materials in the devices.
H
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biphenyl (CBP) was selected as the host material. The 1,3,5-
tris[N-(phenyl)-benzimidazole]-benzene (TPBi) layer serves
as both electron-transporting and hole-blocking function in the
OLEDs. The electron-blocking function of the NPB layer and
the hole-blocking function of the TPBi layer can enable good
exciton confinement within the CBP EML layer (Figure 6),
which is good for the improvement of EL efficiency.
Additionally, LiF functions as an electron-injection layer. We
simply optimized the doping level in the host materials to
explore the EL potential of AIrC6in and AIrC8 in OLEDs.
After applying a proper driven voltage, the OLEDs using
AIrC6in and AIrC8 as emitter can emit an intense orange
phosphorescence at the peak wavelengths of 580 and 584 nm,
respectively (Figure 7 and Table 5). Their EL spectra profiles
from the lowest triplet exited states of AIrC6in and AIrC8
(Figures 7 and 2b).
The EL data of these vacuum-deposition OLEDs are
tabulated in Table 5. Current density−voltage−luminance
(J−V−L) curves and EL efficiency−luminance characteristics
for these vacuum-deposited OLEDs are drawn in Figures 8 and
Figure 8. Current−density−voltage−luminance (J−V−L) character-
istic curves for the optimized OLEDs of AIrC6in and AIrC8.
9 as well as in Figures S8 and S9. All these devices can show a
very low turn-on voltage in the span of 3.1−3.4 V, probably
due to the enhanced EI/ET properties conferred by the
coumarin-based ligands. As expected, these vacuum-deposited
OLEDs can give an extraordinarily high luminescence above
60 000 cd m−2 at a less than 20 V driving voltage, benefiting
from their high power efficiency. Encouragingly, excellent EL
performance has been obtained for all these fabricated OLEDs
of AIrC6in and AIrC8. Meanwhile, the best EL device data are
obtained at an 8 wt % doping level of the emission layer.
Optimized device A2 based on AIrC6in can achieve a
maximum external quantum efficiency (ηext) = 22.2%, a
maximum current efficiency (ηL) = 66.8 cd A−1, and a
maximum power efficiency (ηP) = 57.9 lm W−1, while optimal
Figure 7. EL spectra of the optimized OLEDs of AIrC6in and AIrC8
at ca. 10 V.
correlate well with those observed in the CH2Cl2 solution,
implying the electrophosphorescent emission truly originates
Table 5. EL Data of the Electrophosphorescent OLEDs
a
d
device
dopant (amt (wt %))
Vturn‑on (V)
luminescence Lmax (cd m−2
)
ηext (%)
ηL (cd A−1
)
ηP (lm W−1
)
λmax (nm)
a
18.6 (4.7)
56.0 (4.7)
56.0
53.4
43.6 (3.7)
37.1
29.1
b
A1
AIrC6in (6%)
3.4
63568 (14.9)
18.6
580 (0.55, 0.45)
580 (0.55, 0.45)
580 (0.55, 0.45)
584 (0.55, 0.44)
584 (0.55, 0.44)
584 (0.55, 0.44)
c
17.8
22.2 (4.7)
21.8
22.2
17.3 (5.1)
16.9
17.3
21.9 (4.7)
21.9
21.1
23.9 (4.4)
23.8
22.2
66.8 (4.7)
65.7
66.7
52.1 (5.1)
51.0
52.1
64.8 (4.7)
64.8
62.5
70.9 (4.4)
70.4
65.7
57.9 (3.1)
50.8
41.2
47.0 (3.1)
39.4
32.2
52.4 (3.4)
42.9
34.1
60.7 (3.4)
46.6
35.8
A2
A3
B1
B2
B3
AIrC6in (8%)
AIrC6in (10%)
AIrC8 (6%)
3.1
3.1
3.4
3.4
3.4
77473 (14.6)
82425 (14.9)
66914 (15.3)
67655 (14.6)
75626 (14.6)
AIrC8 (8%)
22.2 (4.7)
22.2
65.9 (4.7)
65.9
50.1 (3.7)
43.6
AIrC8 (10%)
21.2
62.9
34.3
a
b
Maximums obtained in the devices. The voltages corresponding to the achieved maximums are given in the parentheses Values were achieved at
c
d
100 cd m−2. Values were obtained at 1000 cd m−2. Values were gained at ca. 10 V with CIE coordinates (x, y) presented in parentheses.
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Figure 9. Relationships between EL efficiencies and luminance for the optimized devices of AIrC6in and AIrC8. (a) Device A2, (b) device B2.
device B2 based on AIrC8 can achieve the highest EL
efficiencies of ηext = 23.9%, ηL = 70.9 cd A−1, and ηP = 60.7 lm
W−1, respectively. Even at other doping levels, the devices
based on AIrC6in and AIrC8 can also exhibit very impressive
EL performance. With AIrC6in as emitter, we at least can get
EL efficiencies of ηext = 17.3%, ηL = 52.1 cd A−1, and ηP of 43.6
lm W−1. As for AIrC8 as emitter, all the devices show a
maximum external quantum efficiency over 21.9%, and we at
least can get EL efficiencies of ηext = 21.9%, ηL = 64.8 cd A−1,
and ηP = 50.1 lm W−1. Besides these impressive EL efficiencies,
these devices can exhibit a very low efficiency roll-off blow 8%
even at a 1000 cd m−2 high luminance (Table 5). Except for
the extremely high ΦP of AIrC6in as well as AIrC8 and the
effective exciton confinement within the EML layer endowed
by NPB and TPBi, these impressive EL results may originate
from the following reasons as well: On the one hand, as shown
in the energy level diagram of the emission layer in the OLEDs
Figure 6, AIrC6in and AIrC8 have much higher HOMO levels
(ca. 5.40 eV) than those of CBP (ca. 5.90 eV) and also possess
relatively lower LUMO levels than those of CBP (ca. 2.50 eV),
which leads to effective energy transfer from the host CBP to
the emitters;53,54 on the other hand, CV results have
demonstrated that AIrC6in and AIrC8 are endowed with
enhanced EI/ET ability, resulting in more balanced charge
transporting properties. These competitive EL performance
can definitely demonstrate the effectiveness of our simple
approach for high-performance Ir-based phosphors in OLEDs.
All these encouraging results can definitely demonstrate the
effectiveness of our simple approach for tuning the feature of
the triplet excited states and achieving high-performance Ir-
based phosphors in OLEDs.
EXPERIMENTAL SECTION
■
General Information. Ligands L-C5/L-C7/L-C8 were prepared
according to our previously reported protocols.41 The synthesis of
ligand L-C6 can be found in the Supporting Information. The details
of the instrumentation used, X-ray crystallography, theoretical
computation, and OLED fabrication and measurements can be
found in the Supporting Information as well.
General Synthetic Procedure of the Unsymmetric Hetero-
leptic Ir(III) Complexes. IrCl3·nH2O (1.0 equiv, 60 wt % Ir content)
and L-C5/L-C6/L-C7/L-C8 (1.1 equiv) were added to a solvent
system of 2-ethoxyethanol and H2O (3/1, v/v) under a nitrogen
atmosphere. The reaction was allowed to proceeded by magnetically
stirring at 80 °C for 2 h. Then, PQ (1.1 equiv) was added to the
mixture with a nitrogen flow at room temperature. The reaction was
proceeded for another 12 h at 110 °C. After reaction completion, we
poured the reaction mixture into saturated salt water, collected the
precipitated Ir(III) μ-chloro-bridged solids through filtration, and
dried them under vacuum. Thereafter, the Ir(III) μ-chloro-bridged
solids (1.0 equiv) and thallium(I) acetylacetonate [Tl(acac)] (2.2
equiv) were dissolved into an dry CH2Cl2 solution and stirred at room
temperature for 6 h. Centrifugation was conducted to remove the
inorganic salt, and the solvent was removed under reduced pressure.
The residue was purified by preparative thin-layer chromatography
(TLC) made of silica gel with proper eluent to give the corresponding
unsymmetric heteroleptic Ir(III) complex. Caution: The toxic
compound thallium(I) acetylacetonate (Tl(acac)) must be handled with
care.
CONCLUSION
■
∧
∧
1
In this work, six unsymmetric heteroleptic [Ir(C1 N)(C2 N)-
(acac)]-type complexes with easily synthesized 2-phenylquino-
line (PQ) and four coumarin-based ligand isomers as ligands
have been prepared in pursuit of tuning character of triplet
excited states and high-performance Ir-based phosphors for
OLEDs. Fortunately, all these unsymmetric heteroleptic Ir(III)
complexes can exhibit absolute quantum yield Φp (ca. 0.5−
1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4),
exhibiting the advantage of unsymmetric structure in
optimizing optoelectronic properties of PQ-based Ir(III)
complexes. A dual modulation of both emission color from
orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm)
and the features of their lowest triplet excited states has been
achieved for these unsymmetric heteroleptic Ir(III) complexes.
We selected AIrC6in and AIrC8 as emitters to evaluate the
electroluminescent (EL) performance due to their high ΦP of
nearly up to unity. Optimal orange-emitting device B2 based
AIrC5. Yield: 33.4%. H NMR (400 MHz, CDCl3, δ): 8.62 (d, J =
10.0 Hz, 1H), 8.59 (m, 1H), 8.48 (dd, J = 6.0, 1.2 Hz, 1H), 8.18 (d, J
= 8.8 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 8.02 (d, J = 8.4 Hz, 1H),
7.89−7.83 (m, 2H), 7.75 (d, J = 7.6 Hz, 1H), 7.56−7.52 (m, 2H),
7.22 (td, J = 6.6, 1.2 Hz, 1H), 6.94 (td, J = 7.2, 1.2 Hz, 1H), 6.77 (d, J
= 8.4 Hz, 1H), 6.71 (td, J = 7.6, 1.2 Hz, 1H), 6.65 (d, J = 8.4 Hz, 1H),
6.45 (d, J = 10.0 Hz, 1H), 6.05 (d, J = 7.6 Hz, 1H), 4.95 (s, 1H), 1.71
(s, 3H), 1.59 (s, 3H). 13C NMR (100 MHz, CDCl3, δ): 185.68,
185.13, 169.48, 167.83, 160.73, 151.60, 150.20, 149.85, 149.18,
146.65, 146.54, 140.74, 139.45, 139.42, 137.95, 137.37, 132.95,
130.66, 128.91, 128.01, 127.18, 126.16, 126.12, 125.84, 122.28,
122.12, 121.52, 117.16, 116.79,116.76, 115.63, 100.49, 28.55, 28.27.
FAB-MS (m/z): 718 [M]+. Anal. Calcd for C34H25IrN2O4: C, 56.89;
H, 3.51; N, 3.90. Found: C, 56.81; H, 3.43; N. 3.85%.
1
AIrC6out. Yield: 18.5%. H NMR (400 MHz, CDCl3, δ): 8.23−
8.20 (m, 2 H), 8.18 (d, J = 8.8 Hz, 1H), 8.08 (d, J = 8.8 Hz, 1H), 7.93
(d, J = 8.4 Hz, 1H), 7.88 (d, J = 8.8 Hz, 1H), 7.85−7.79 (m, 3H),
7.50 (td, J = 7.2, 1.2 Hz, 1H), 7.45 (td, J = 7.8, 1.6 Hz, 1H), 7.07 (td,
J = 6.6, 1.2 Hz, 1H), 7.04−7.00 (m, 2H), 6.77 (td, J = 7.2, 1.2 Hz,
1H), 6.63 (d, J = 9.6 Hz, 1H), 6.34 (d, J = 7.2 Hz, 1H), 5.37 (d, J =
9.6 Hz, 1H), 4.77 (s, 1H), 1.60 (s, 3H), 1.48 (s, 3H). 13C NMR (100
on AIrC8 can achieve the highest EL efficiencies of ηext
=
23.9%, ηL = 70.9 cd A−1, and ηP = 60.7 lm W−1, respectively.
J
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MHz, CDCl3, δ): 185.71, 184.75, 169.27, 167.73, 161.74, 155.24,
153.55, 149.48, 149.20, 148.20, 147.86, 146.56, 143.50, 138.12,
137.69, 134.56, 130.70, 128.88, 127.95, 127.48, 126.55, 126.45,
126.37, 126.25, 126.11, 121.90, 121.10, 118.39, 117.09, 112.75,
110.74, 100.08, 28.46, 27.93. FAB-MS (m/z): 718 [M]+. Anal. Calcd
for C34H25IrN2O4: C, 56.89; H, 3.51; N, 3.90. Found: C, 56.83; H,
3.48; N. 3.82%.
General experimental information; selected X-ray
crystallographic data for AIrC8; atom numbering used
in describing coordinating atoms of the coumarin block;
CV and TGA traces of these unsymmetric heteroleptic
Ir(III) complexes; PL spectra at 77 K in CH2Cl2
solution and in CBP films for these unsymmetric
heteroleptic Ir(III) complexes; contribution of electron
density on the chelating carbon atom to the HOMOs of
the coumarin-based ligand isomers; characterization of
EL properties of the devices except the optimized ones
(PDF)
AIrC6in. Yield: 19.4%. 1H NMR (400 MHz, CDCl3, δ): 8.69−8.66
(m, 1H), 8.38 (d, J = 4.8 Hz, 1H), 8.20 (d, J = 8.8 Hz, 1H), 8.02 (d, J
= 8.8 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.87−7.83 (m, 2H), 7.75 (d,
J = 7.2 Hz, 1H), 7.71 (s, 1H), 7.59 (d, J = 9.6 Hz, 1H), 7.57−7.53 (m,
2H), 7.22 (td, J = 6.6, 1.2 Hz, 1H), 6.94 (td, J = 7.2, 1.2 Hz, 1H), 6.72
(td, J = 7.2, 1.2 Hz, 1H), 6.50 (s, 1H), 6.18 (d, J = 7.2 Hz, 1H), 6.12
(d, J = 9.6 Hz, 1H), 4.98 (s, 1H), 1.72 (s, 3H), 1.61 (s, 3H). 13C
NMR (100 MHz, CDCl3, δ): 185.78, 185.00, 169.52, 166.86, 161.52,
157.13, 154.11, 149.32, 149.19, 148.82, 146.55, 144.38, 143.07,
138.19, 137.55, 132.63, 130.88, 128.90, 128.10, 127.36, 126.28,
126.18, 125.83, 123.15, 122.34, 122.20, 121.77, 118.51, 116.71,
113.19, 111.85, 100.60, 28.56, 28.31. FAB-MS (m/z): 718 [M]+. Anal.
Calcd for C34H25IrN2O4: C, 56.89; H, 3.51; N, 3.90. Found: C, 56.79;
H, 3.44; N. 3.87%.
Accession Codes
CCDC 1915117 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AIrC7out. Yield: 18.7%.1H NMR (400 MHz, CDCl3, δ): 8.68−
8.66 (m, 1H), 8.40 (d, J = 5.2 Hz, 1H), 8.19 (d, J = 8.8 Hz, 1H), 8.03
(d, J = 8.8 Hz, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.89 (dd, J = 7.2, 1.6 Hz,
1H), 7.88−7.84 (m, 1H), 7.76 (d, J = 7.6 Hz, 1H), 7.64 (s, 1H),
7.57−7.52 (m, 2H), 7.24 (td, J = 7.2, 1.2 Hz, 1H), 7.18 (d, J = 9.6 Hz,
1H), 6.94 (td, J = 7.6, 0.8 Hz, 1H), 6.73 (td, J = 7.2, 1.2 Hz, 1H), 6.59
(s, 1H), 6.21 (d, J = 7.2 Hz, 1H), 6.13 (d, J = 9.6 Hz, 1H), 4.95 (s,
1H), 1.70 (s, 3H), 1.58 (s, 3H). 13C NMR (100 MHz, CDCl3, δ):
185.57, 185.12, 169.80, 167.14, 161.49, 150.56, 150.45, 149.44,
149.36, 149.03, 146.52, 143.74, 139.80, 137.87, 137.60, 133.74,
132.98, 130.64, 128.97, 128.00, 127.17, 126.42, 126.08, 125.81,
123.09, 121.46, 119.73, 119.60, 116.76, 116.00, 111.55, 100.50, 28.56,
28.28. FAB-MS (m/z): 718 [M]+. Anal. Calcd for C34H25IrN2O4: C,
56.89; H, 3.51; N, 3.90. Found: C, 56.87; H, 3.41; N. 3.89%.
AUTHOR INFORMATION
■
Corresponding Authors
Guijiang Zhou − MOE Key Laboratory for Nonequilibrium
Synthesis and Modulation of Condensed Matter, School of
Chemistry, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, PR China;
orcid.org/0000-0002-5863-3551; Email: zhougj@
mail.xjtu.edu.cn; Fax: +86-029-82663914
Zhaoxin Wu − Key Laboratory of Photonics Technology for
Information, School of Electronic and Information Engineering,
Xi’an Jiaotong University, Xi’an 710049, PR China;
Collaborative Innovation Center of Extreme Optics, Shanxi
University, Taiyuan 030006, PR China; orcid.org/0000-
0001-9362-2236; Email: zhaoxinwu@mail.xjtu.edu.cn
1
AIrC7in. Yield: 17.9%. H NMR (400 MHz, CDCl3, δ): 8.53 (d, J
= 8.4 Hz, 1H), 8.40 (d, J = 4.8 Hz, 1H), 8.22 (d, J = 8.4 Hz, 1H), 8.03
(d, J = 8.8 Hz, 1H), 8.02 (d, J = 7.6 Hz, 1H), 7.87−7.83 (m, 2H),
7.76 (d, J = 7.6 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.49−7.41 (m,
2H), 7.33 (d, J = 9.2 Hz, 1H), 7.20 (td, J = 6.6, 1.2 Hz, 1H), 7.00 (d, J
= 8.0 Hz, 1H), 6.93 (t, J = 7.2 Hz, 1H), 6.69 (td, J = 7.2, 0.8 Hz, 1H),
6.16 (d, J = 7.2 Hz, 1H), 5.81 (d, J = 9.2 Hz, 1H), 4.96 (s, 1H), 1.70
(s, 3H), 1.58 (s, 3H). 13C NMR (100 MHz, CDCl3, δ):185.14,
185.12, 168.29, 168.02, 161.08, 160.89, 150.90, 150.25, 148.85,
147.65, 147.56, 143.03, 137.77, 137.36, 134.69, 132.87, 129.53,
128.16, 127.58, 127.57, 127.14, 125.51, 125.36, 122.38, 121.40,
121.19, 120.17, 119.35, 118.71, 116.57, 116.48, 100.54, 28.58, 28.26.
FAB-MS (m/z): 718 [M]+. Anal. Calcd for C34H25IrN2O4: C, 56.89;
H, 3.51; N, 3.90. Found: C, 56.86; H, 3.46; N. 3.84%.
Authors
Zhao Feng − MOE Key Laboratory for Nonequilibrium
Synthesis and Modulation of Condensed Matter, School of
Chemistry, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
Yue Yu − MOE Key Laboratory for Nonequilibrium Synthesis
and Modulation of Condensed Matter, School of Chemistry,
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, PR China; School of
Physics and Optoelectronic Engineering, Xidian University, Xi’an
710071, PR China; orcid.org/0000-0002-4758-8492
Xiaolong Yang − MOE Key Laboratory for Nonequilibrium
Synthesis and Modulation of Condensed Matter, School of
Chemistry, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, PR China;
orcid.org/0000-0002-0243-6698
1
AIrC8. Yield: 33.7%. H NMR (400 MHz, CDCl3, δ): 9.17 (d, J =
8.4 Hz, 1H), 8.66−8.63 (m, 1H), 8.41 (d, J = 4.8 Hz, 1H), 8.19 (d, J
= 8.8 Hz, 1H), 8.02 (d, J = 8.8 Hz, 1H), 7.93 (td, J = 8.0, 1.6 Hz, 1H),
7.86−7.84 (m, 1H), 7.75 (d, J = 7.6 Hz, 1H), 7.56−7.53 (m, 3H),
7.22 (td, J = 6.6, 1.2 Hz, 1H), 6.95 (t, J = 7.2 Hz, 1H), 6.75 (t, J = 7.6
Hz, 1H), 6.62 (q, J = 7.4 Hz, 2H), 6.23 (d, J = 9.6 Hz, 1H), 6.20 (d, J
= 7.6 Hz, 1H), 4.94 (s, 1H), 1.70 (s, 3H), 1.58 (s, 3H). 13C NMR
(100 MHz, CDCl3, δ): 185.76, 185.04, 169.48, 165.80, 161.60,
161.20, 152.14, 149.69, 149.22, 148.55, 146.40, 145.53, 138.16,
137.95, 133.05, 132.74, 132.59, 130.58, 129.01, 127.97, 127.53,
127.18, 126.39, 126.10, 125.75, 124.37, 122.10, 121.74, 116.83,
113.22, 110.70, 100.46, 28.55, 28.66. FAB-MS (m/z): 718 [M]+. Anal.
Calcd for C34H25IrN2O4: C, 56.89; H, 3.51; N, 3.90. Found: C, 56.84;
H, 3.50; N. 3.88%.
Yong Wu − MOE Key Laboratory for Nonequilibrium Synthesis
and Modulation of Condensed Matter, School of Chemistry,
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, PR China;
orcid.org/0000-0002-9460-3579
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01443
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01443.
Notes
The authors declare no competing financial interest.
K
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (21572176,
21875197, and 21602170), the Natural Science Foundation of
Shaanxi Province (2019JZ-29 and 2019JQ-188), and Key
Laboratory Construction Program of Xi’an Municipal Bureau
of Science and Technology (201805056ZD7CG40). The
characterization assistance from the Instrument Analysis
Center of Xi’an Jiaotong University is also acknowledged.
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