Deep Red Iridium(III) Complexes Based on Pyrene-Substituted Quinoxaline Ligands for Solution-Processed Phosphorescent Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.inorgchem.9b02477

In this paper, we systemically investigated the photoelectric properties of three new deep-red quinoxaline-based iridium(III) complexes: Ir-0, Ir-1, and Ir-2. (MPQ)₂Ir(dpm) (Ir-0) bore a 2-methyl-3-phenylquinoxaline cyclometalated ligand, while (c-PyMPQ)₂Ir(dpm) (Ir-1) and (t-PyMPQ)₂Ir(dpm) (Ir-2) possessed a 1-pyrene substituent that connected at the 6/7 position of the corresponding ligands. The configurations of the latter two complexes were well-confirmed by single-crystal X-ray diffraction, and both of them had large dihedral angles between the quinoxaline and pyrene units, preventing the emission peaks of the three complexes from being altered too much. Based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, we concluded that the emission of all complexes originated predominantly from the triplet metal-to-ligand/intraligand charge transfer (³MLCT/³ILCT) state of the non-pyrene-substituted counterpart Ir-0 core. Interestingly, we also obtained another type of pyrene-stacking characteristic crystal of Ir-1, which had an emission resembled the phosphorescence observed in thin film. The easily formed pyrene-stacking configuration would most probably limit their device performance at a higher concentration. Moreover, the fabricated organic light-emitting diodes (OLEDs) using these materials achieved considerable device performance at a low doping concentration of 0.5 wt %. This work provides an approach for reasonably designing large fused-ring-substituted quinoxaline ligands of iridium complexes.

Full text of DOI:10.1021/acs.inorgchem.9b02477

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Deep Red Iridium(III) Complexes Based on Pyrene-Substituted
Quinoxaline Ligands for Solution-Processed Phosphorescent
Organic Light-Emitting Diodes
Zhaoran Hao,‡ Kai Zhang,† Pu Wang,*,‡ Xumin Lu,‡ Zhiyun Lu,*,§ Weiguo Zhu,* and Yu Liu*
†School of Materials Science and Engineering, Jiangsu Collaboration Innovation Center of Photovoltaic Science and Engineering,
Jiangsu Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, National Experimental
Demonstration Center for Materials Science and Engineering, Changzhou University, Changzhou 213164, China
‡College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University,
Xiangtan 411105, China
§Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, and State Key Laboratory of
Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
S
* Supporting Information
ABSTRACT: In this paper, we systemically investigated the photoelectric
properties of three new deep-red quinoxaline-based iridium(III) complexes:
Ir-0, Ir-1, and Ir-2. (MPQ)2Ir(dpm) (Ir-0) bore a 2-methyl-3-phenyl-
quinoxaline cyclometalated ligand, while (c-PyMPQ)2Ir(dpm) (Ir-1) and (t-
PyMPQ)2Ir(dpm) (Ir-2) possessed a 1-pyrene substituent that connected at
the 6/7 position of the corresponding ligands. The configurations of the
latter two complexes were well-confirmed by single-crystal X-ray diffraction,
and both of them had large dihedral angles between the quinoxaline and
pyrene units, preventing the emission peaks of the three complexes from
being altered too much. Based on the density functional theory (DFT) and
time-dependent DFT (TD-DFT) calculations, we concluded that the
emission of all complexes originated predominantly from the triplet metal-
to-ligand/intraligand charge transfer (3MLCT/3ILCT) state of the non-
pyrene-substituted counterpart Ir-0 core. Interestingly, we also obtained
another type of pyrene-stacking characteristic crystal of Ir-1, which had an emission resembled the phosphorescence observed in
thin film. The easily formed pyrene-stacking configuration would most probably limit their device performance at a higher
concentration. Moreover, the fabricated organic light-emitting diodes (OLEDs) using these materials achieved considerable
device performance at a low doping concentration of 0.5 wt %. This work provides an approach for reasonably designing large
fused-ring-substituted quinoxaline ligands of iridium complexes.
infrared OLEDs based on iridium(III) complexes are still less
than satisfactory, compared with the high efficiencies of blue to
orange OLEDs. One of the most frequently mentioned limiting
factors is called “energy gap law”,4 which means a reduction of
luminescence quantum yields toward longer emission wave-
length. Therefore, it is still desirable and challenging to design
new deep-red/NIR iridium complexes.
To date, nitrogen-containing heterocycles (such as quino-
lines5/ isoquinolines,3b−d,6 quinoxalines7) within a donor−
acceptor ligand framework are used as one of the most
commonly employed approaches to obtain deep-red/NIR
phosphorescent iridium(III) complexes. Particularly, the nitro-
gen-rich phenyl quinoxalines are especially favored by
scientists. Since the pioneering work by Wang and co-workers,
where a model molecule of diphenyl-substituted quinoxaline
INTRODUCTION
■
As a promising candidate for display and solid-state lighting
source, organic light-emitting diodes (OLEDs) have drawn
continual academic attention in the past three decades,1 while
in the pursuit of high efficiency/long stable commercial
applicable devices, luminescent materials based on third-row
transition-metal complexes seem to be an especially long-
standing topic.2 For transition-metal complexes, especially
iridium(III) complexes, with their electronic states undergoing
strong spin−orbit coupling, formally spin-forbidden triplet
excitons can be harnessed through an efficient intersystem
crossing process, and they usually possess phosphorescence
with high quantum efficiency. So far, tremendous inves-
tigations based on iridium(III) complexes have been
conducted with tunable emission color from blue to near-
infrared in their high-efficiency devices.3 However, lower-
energy labeled emitters (deep-red/near-infrared (NIR)) are
still far too few, and the performances of deep-red/near-
Received: August 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02477
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a
Scheme 1. Synthetic Route of Complexes
a
Reagent and conditions: (i) ethanol, 80 °C; (ii) pyrene-1-boronic acid pinacol ester, Pd(PPh3)4, 2 M K2CO3 (aq), THF, 80 °C; and (iii) IrCl3·
H2O, 2-ethoxyethanol, THF, H2O, 120 °C; (iv) 2,2,6,6-tetramethyl-3,5-heptanedione, K2CO3, 2-ethoxyethanol, 90 °C.
iridium(III) complex exhibited a high external quantum
efficiency (EQE) of 5.5% with CIE coordinates of (0.71,
0.27),7a many groups had focused on more efficient or more
red-shifted quinoxaline iridium luminescent materials, either by
altering ancillary ligands7c−e or by increasing the π-conjugation
through adding7g/connecting7f aromatic rings. Among these
strategies, the easily bathochromic shift emission by simply π-
conjugate extension attracted our attention. However, research
has rarely been conducted on the effect of attached aromatic
rings in a quinoxaline iridium system, especially with different
substitution sites.
In the present study, we systemically investigated the
photophysical properties of three deep-red iridium(III)
complexes bearing phenylquinoxaline-cyclometalated ligands
in the presence or absence of 1-pyrene substituted at the 6/7
position of a 3-methyl-2-phenylquinoxaline core. Pyrene was
chosen as the attached rigid fused rings considering its large
aromatic conjugation, which enables the intensive absorptivity
of complexes.8 Experimental data have shown that a pyrene
segment substituted at the 7-position exerted a more positive
effect on the effective conjugation extension of complex.
However, large dihedral angles existed between the quinoxaline
and pyrene units, preventing the emission peaks of the three
complexes from being altered too much. Compared with the
non-pyrene-substituted counterpart Ir-0, Ir-1 and Ir-2 exhibited
similar device performances in their solution-processed OLEDs
when at a low doping concentration, corresponding well with
their similar photoluminescent properties, which had con-
sistent emissive origins. However, pyrene-induced aggregation
characteristics limited their device performance at higher
doping concentrations, which was consistent with the photo-
physical performance of its face-on packing crystal powder and
solid thin film.
RESULT AND DISCUSSION
■
Synthesis and Characterization. Synthetic routes of the
iridium(III) complexes are depicted in Scheme 1. 4-Bromo-
benzene-1,2-diamine and 1-phenylpropane-1,2-dione were
used to afford the quinoxaline ring isomers (M1 and M2), of
which were formed in almost-equal yields. cPy-MPQ and t-
PyMPQ were synthesized through a common Suzuki coupling
reaction with pyrene-1-boronic acid pinacol ester, and act as
the starting materials for the synthesis of Ir-1 and Ir-2.
Complex Ir-0 was obtained by a conventional two-step
synthesis from the 2-methyl-3-phenylquinoxaline ligand
(MPQ) in high yield.8 As for the similar synthetic procedure,
a
complexes Ir-1 and Ir-2 suffered relatively lower yields, which
also occurred in the synthesis of complexes with aromatic ring
appendices at the peripheral side of phenylquinoxaline ligand.7f
All compounds were characterized by 1H NMR and 13C NMR
(see Figures S1−S12 in the Supporting Information) and mass
spectra. In the case of Ir-1 and Ir-2, X-ray diffraction (XRD)
was used to ensure their accurate structures. Both Ir-1 and Ir-2
were racemic mixtures, which can also be seen in many other
octahedral iridium complexes with stereoisomerism,9 and only
one type of configuration was performed here (Figure S13 in
the Supporting Information). Interestingly, the single-crystal
XRD showed that 6-bromo-3-methyl-2-phenylquinoxaline (c-
PyMPQ) and 6-bromo-2-methyl-3-phenylquinoxaline (t-
B
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PyMPQ) were also racemic mixtures that were labeled with
different orientations of phenyl segment induced by a methyl
steric hindrance (see Figure S14 in the Supporting
Information).
Single-Crystal Structures. Both functionalized Ir-1 and
Ir-2 molecular structures were determined by single-crystal
XRD, their packing mode and molecular structures are
depicted in Figure 1, as well as Figure S13, respectively. Single
Single-crystal structures showed that the fuchsia crystal
exhibited an intermolecular face-to-face packing mode based
on pyrene segments, while two red crystals set an edge-on
packing mode based on their quinoxaline/pyrene portion
(Figure 1). As shown in Figure S13, both complexes Ir-1 and
Ir-2 performed distorted octahedral coordination geometry.
The bond length of Ir−C, Ir−N, and Ir−O, as well as the bite
angles of the chelating ligands attached to central iridium,
were, respectively, of the same order of magnitude for the two
complexes, indicating a similar coordination ability of their
corresponding isomeric ligands. Besides, two C∧N ligands in
each complex were also isomers, and the dihedral angle of the
quinoxaline unit and the phenyl ring in each ligand was not
exactly the same, which could be attributed to the steric
hindrance of methyl and the chelating force in complexes. The
methyl effect was much more obvious in free ligands. For
instance, we also prepared the colorless needlelike crystal
(Figure S14) of the intermediate c-PyMPQ ligand through
solvent evaporation in petroleum ether. The single-crystal
structure showed that the two isomers were fixed at a ratio of
1:1, and an almost-mirror symmetry dihedral angle formed
between the quinoxaline and phenyl ring in each isomer was
46.73° and 132.79°, respectively. This finding has not been
mentioned by much previously reported literature that
focusing on phenylquinoxaline, and the sterically induced
isomerization of phenylquinoxaline may be a precursor factor
of many helical10 or hydrogenated11 chiral phenylquinoxaline
derivatives. These isomerization characteristics had little effect
on the basic photophysical properties of complexes. Moreover,
the dihedral angles between the quinoxaline unit and pyrene
unit in Ir-1 and Ir-2 were 63.19°/52.37° and 55.10°/46.45°,
respectively (each of the two isomeric ligand coordinated in
one complex had its own character, as mentioned above).
These large dihedral angles hindered the effect of pyrene unit
on the effective π-conjugation extension in complexes, while an
intramolecular charge transfer character should be formed in
such twisted ligands.
Figure 1. Single-crystal packing mode of Ir-1 and Ir-2 and their
crystalline color under 365 nm UV light. The inset shows the distance
between two center atoms of nearby pyrene segments.
crystals of Ir-1 and Ir-2 were grown from solvent diffusion in
their trichloromethane/acetonitrile (v/v, 1/2) solutions, and
both complexes exhibited black columnar crystals and showed
dark red/bright red color under ultraviolet radiation at a
wavelength of 365 nm. As for Ir-1, when the solvent diffusion
was conducted in dichloromethane/methanol (v/v, 1/1), the
complex showed fuchsia columnar crystal under UV light.
3
Figure 2. Absorption and emission spectra of the complexes measured in dilute 2-MeTHF; inset shows the MLCT/3ILCT absorption portion.
C
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Figure 3. Emission spectra of the complexes measured in dilute 2-MeTHF, thin films, and crystal powders.
Table 1. Photophysical Properties of Complexes
λPL
Φ
τ
λPL
τ
a
a
pb
b
c
c
d
f
d
d
complex
Ir-0
λabs [nm] (ε [× 104 M−1 cm−1])
[nm]
[%]
[μs]
[nm]
[μs]
λPL [nm]
Φp [%]
τ [μs]
256 (2.90), 278 (2.85), 365 (1.64), 480 (0.38),
616 (0.10)
243 (6.41), 283 (4.35), 344 (3.66), 410 (1.67),
492 (0.47), 620 (0.09)
243 (4.89), 280 (3.68), 350 (2.87), 406 (1.00),
494 (0.31), 630 (0.11)
660
673
665
11.7
1.11
1.13
1.18
680
706
688
0.07
0.07
0.08
e
e
f
e
f
Ir-1
Ir-2
13.6
18.9
685, 706
1.60, 2.04
0.10, 0.10
677
1.48
0.08
a
b
Measured in 2-MeTHF solution to concentrations ∼10−5 at room temperature (rt). Quantum yields in degassed 2-MeTHF referenced to fac-
c
d
e
Ir(piq)2(acac) (Φp = 0.20 in degassed DCM); τ is the excited-state lifetime. Measured in thin films. Measured in crystal powders. Red crystal
powder. Fuchsia crystal powder.
f
Photophysical Properties. Ultraviolet-visible (UV-vis)
absorption and luminescence spectra of the three complexes in
dilute 2-MeTHF solutions are detailed in Figure 2. Ir-0 showed
absorption characteristics of the typical quinoxaline-based
iridium complex, with the intense absorption bands below 410
nm (extinction coefficient of 16 400−29 000 M−1 cm−1),
which should be assigned to spin-allowed intraligand π−π*
transitions, while the weak absorption at a longer wavelength
extending to 660 nm (extinction coefficient of 3800−1000
M−1 cm−1) corresponded to mixed transitions of singlet and
triplet MLCT.7 Ir-1 and Ir-2 exhibited similar absorption
characteristics to Ir-0, the molar absorptivity of Ir-1 (extinction
coefficient of 36 600−64 100 M−1 cm−1) and Ir-2 (extinction
coefficient of 28 700−48 900 M−1 cm−1) was increased,
compared with Ir-0 in the short wavelength absorption region,
which was caused by the extended conjugate molecular
skeleton. The extra absorption bands at 410 and 406 nm of
Ir-1 and Ir-2 were due to singlet intraligand charge transfer
(ILCT) from pyrene to quinoxaline, and were also presented
in the absorption spectra of the free ligands (see Figure S15 in
the Supporting Information). All complexes display similar
extinction coefficients at the low-energy 3MLCT/3ILCT
absorption bands (Figure 1, inset), illustrating that the same
degree of their spin−orbit coupling is observed.
three complexes was not altered too much, because of the
above-mentioned large dihedral angle between the pyrene and
quinoxaline. Besides, as the concentration was increased, all
complexes showed somewhat bathochromic shifts to similar
extents. Since the emissive state of Ir-0 have predominant
3MLCT/3ILCT characteristics,7 and considered the similar
phosphorescent lifetimes (τp solution/1.11, 1.13, 1.18 μs) (see
Figure S16 in the Supporting Information) and quantum yields
(Φp/11.7%, 13.6%, 18.9%), we concluded that the emissions of
Ir-1 and Ir-2 are also originated from the 3MLCT/3ILCT state
of Ir-0 core. The free ligands of c-PyMPQ and t-PyMPQ
exhibited typical ILCT character in the excited states; their
emission spectra presented remarkable bathochromic shifts
with the increased solvent polarity from nonpolar hexane
(422/430 nm) to polar acetonitrile (512/517 nm) solutions
(see Figures S17 and S18 in the Supporting Information),
endowing pyrene with a typical donation nature here.
However, as for complexes Ir-1 and Ir-2, there exist a large
energy separation between ILCT states (2.69/2.69 eV) and the
original emissive states of the complexes (1.83/1.84 eV) (see
Figure S15, as well as Figure 2), and the emissions of ligands
were completely quenched by self-absorption. Although
complexes Ir-1 and Ir-2, which incorporated pyrene, reflected
most of the photophysical characteristics of their parental
complex Ir-0 in dilute solutions; however, the emissive
behavior of the former was altered by the large planar
configuration of pyrene when in the solid state. The fabricated
thin films of Ir-1, which used either of two types of crystals,
performed the same emission feature. In thin films, all the
complexes showed a 20−33 nm bathochromic shift, compared
Complexes Ir-0−Ir-2 showed phosphorescence emission
peaks at 660, 673, and 665 nm in diluted 2-MeTHF solutions,
irrespective of excitation wavelength, respectively (Figure 2).
Therefore, we conclude that the effective conjugate extension
would be more preferred when the aromatic ring was attached
at the 7-position of MPQ. However, the emission color of the
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with the 2-MeTHF solutions (see Figure 3). The red crystal
powders of Ir-1 and Ir-2 showed a moderate 12 nm
bathochromic shift, while the fuchsia crystal powder of Ir-1
had a 33 nm red-shifted emission with a broad bandwidth of
the principal emission component resembling the phosphor-
escence observed in thin film, both with an emission peak at
706 nm. This finding might illustrate that Ir-1 had a tendency
to form a pyrene-stacking configuration in films and was easily
quenched by aggregation. This phenomenon might also occur
in Ir-2, which could be seen in electroluminescent parts.
However, all three complexes in pure films presented poor
quantum yields that are too low to be determined. Three
crystals of red Ir-1, fuchsia Ir-1, and Ir-2 show similar low Φp
values of 1.60%, 2.04%, and 1.48%, respectively. All complexes
both in films and in crystal powders show similar short
phosphorescent lifetimes of ∼0.1 μs (see Figures S19 and S20
in the Supporting Information), which indicating that a fast
nonradiative decay pathway caused by aggregation was formed
in the solid states of all three complexes. The corresponding
photophysical dates of these complexes are given in Table 1.
Electrochemical Properties and Thermal Analysis.
The electrochemical properties and energy levels of these
iridium complexes were evaluated by cyclic voltammetry (CV)
in CH3CN solution (Figure 4). The corresponding data are
addition, a similar reduction process occurred in Ir-1 and Ir-2
with a negative potential of −1.61/−1.61 V. The aromatic
pyrene segment attached to quinoxaline ring had a minor effect
on electron-withdrawing ability of complexes, which is unusual
in a donor−acceptor (D-A) system with an aromatic ring
bonded to the acceptor fragment,12 and it was mostly caused
by the large dihedral angle between the pyrene and quinoxaline
surfaces that inhibited the effective conjugation extend of the
acceptor segment. As a result, similar HOMO/LUMO energy
levels of complexes Ir-0, Ir-1, and Ir-2 were calculated to be
5.44/−3.14, 5.44/−3.19, and 5.43/−3.19 eV, respectively.
Therefore, all complexes exhibit a similar HOMO−LUMO
energy gap and were well-correlated with their similar
photophysical properties. Decomposition temperature (Td)
values of Ir-0, Ir-1, and Ir-2 observed at a 5% weight loss were
320, 353, and 350 °C, respectively (see Figure S21 in the
Supporting Information), indicating that grafting a pyrene unit
has a positive effect on increasing the thermal stability of its
corresponding complex.
Theoretical Calculations. DFT and TD-DFT calculations
were used to provide further support for the theory on
photophysical properties of complexes. Figure 5 presents the
HOMO and LUMO orbitals distribution of Ir-1 and Ir-2,
based on their optimized ground-state structures. Both
HOMO orbitals for the two complexes were mainly
contributed by the phenyl segment and the iridium ion,
while the LUMO orbitals were spread on the phenylquinoxa-
line moieties. Of the two complexes, the delocalized HOMO/
LUMO orbitals had almost no distributions on pyrene groups;
consequently, they exhibited almost consistent photophysical
properties to Ir-0. The calculated HOMO and LUMO values
for Ir-1 and Ir-2 were −5.06 eV/−2.29 eV and −5.10 eV/−
2.29 eV, respectively, and was consistent with their redox
behavior. Meanwhile, the nature of the calculated absorption
spectra (Figure S22 in the Supporting Information) matched
well with the experimentally observed data. The larger
oscillator strengths of Ir-1 to Ir-2 were attributed to the
more separated HOMO−LUMO distribution of the former,
and the less overlaps between the HOMO and LUMO orbitals
on pyrenyl-quinoxaline segment endowed Ir-1 with a more
distinct D−A characteristic and emitted to a longer wave-
length. Selecting Ir-2 for example (see Table 3), the calculated
low-energy absorption bands corresponding to states S1 and
S2 were mainly caused by 1MLCT transitions and had a certain
Figure 4. Cyclic voltammograms of complexes in CH3CN solutions.
summarized in Table 2. During the anodic scan, Ir-0 exhibited
continuous two-electron irreversible oxidation at a positive
potential of 0.64 V, and the oxidation process mainly occurred
at the iridium ion and the cyclometalated phenyl fragment. Ir-1
and Ir-2 had similar oxidation processes and possessed similar
positive potential of 0.64/0.63 V; a minor effect from pyrene
was observed in their oxidation procedure. While during the
cathodic scan, Ir-0 showed typical characteristics of quinoxa-
line complexes with an onset potential of −1.66 V, and it
underwent a two-electron reversible reduction process. In
1
degree of ILCT characteristic from the phenyl segment to
quinoxaline. The calculated absorption at ∼468 nm had
distinct features of interligand/intraligand charge transfer from
pyrene/dpm to quinoxaline, which was consistent with the
absorption band at 406 nm. To further investigate the emissive
excited states of complexes, we performed TD-DFT calcu-
lations based on the optimized triplet geometry of Ir-2 (see
Figure S23 in the Supporting Information). The calculated
states T1 (732 nm, 1.19 eV) and T2 (677 nm, 1.83 eV) in
Table 2. Electrochemical Properties of Complexes
a
a
b
c
d
complex
Eox (V)
Ered (V)
HOMO/LUMOexp (eV)
HOMO/LUMOcal (eV)
ΔEopt (eV)
Tg (°C)
Ir-0
Ir-1
Ir-2
0.64
0.64
0.63
−1.66
−1.61
−1.61
−5.44/−3.14
−5.44/−3.19
−5.43/−3.19
−
1.46
1.46
1.46
320
353
350
−5.06/−2.29
−5.10/−2.29
a
b
The data are versus Fc/Fc+ (Fc is ferrocene) estimated from the onset oxidation/reduction curve. Deduced from the equations EHOMO = −4.8 −
c
d
abs
Eox and ELUMO = −4.8 − Ered. From DFT calculations. Calculated from the optical edge λ
.
onset
E
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Figure 5. Calculated orbital distributions for the HOMO and LUMO levels of Ir-1 and Ir-2 at their optimized singlet state geometry.
Table 3. Excited States of Ir-2 Calculated Using the TD-DFT Approach
state (E, λ)
dominant excitations
oscillator strength (f)
character
S1 (2.15 eV, 575 nm)
S2 (2.24 eV, 553 nm)
S3 (2.37 eV, 522 nm)
H → L (0.68)
0.0048 (singlet)
0.0456 (singlet)
0.0512 (singlet)
MLCT/ILCT
MLCT/ILCT
MLCT/ILCT
H → L + 1 (0.68)
H-3 → L (0.55)
H-1 → L (0.26)
H-2 → L (0.57)
H-10 → L (0.50)
H → L (0.92)
H → L + 1 (0.87)
H-3 → L + 2 (0.22)
H-2 → L (0.22)
H-2 → L + 2 (0.22)
H-1 → L + 3 (0.44)
S6 (2.65 eV, 468 nm)
S20 (3.41 eV, 364 nm)
T1 (1.19 eV, 732 nm)
T2 (1.83 eV, 677 nm)
T3 (1.96 eV, 633 nm)
0.1173 (singlet)
0.2374 (singlet)
0.0000 (triplet)
0.0000 (triplet)
0.0000 (triplet)
ILCT
ILCT
ILCT/MLCT
ILCT/MLCT
ILCT/MLCT
T4 (1.99 eV, 624 nm)
0.0000 (triplet)
LE
energy were close to the experimentally observed emission
band in 2-MeTHF solution at 669 nm, and these transitions
were all mainly contributed by HOMO → LUMO and
HOMO → LUMO+1. The experienced charge transfer was
mainly 3MLCT (from the Ir ion to phenylquinoxaline
segment) and a degree of 3ILCT (from phenyl to the
quinoxaline segment). Besides, no involvement of pyrene
fragment was found in the transitions. Apparently, the emission
characteristics of 1-pyrene substituted phenylquinoxaline
iridium(III) complexes originated from their homologous
complex Ir-0.
Electroluminescent Properties. To explore the electro-
luminescent (EL) properties of these iridium(III) complexes,
solution-processed OLEDs with a device configuration of
ITO/PEDOT:PSS (40 nm)/poly-TPD (30 nm)/[(PVK:
OXD-7) 7:3]: complex (x wt %, 60 nm)/DPEPO (10 nm)/
TmPyPB (40 nm)/CsF (1.2 nm)/Al (120 nm) were
fabricated, in which ITO/PEDOT:PSS acted as the anode
and poly-TPD was poly(N,N′-bis (4-butylphenyl)-N,N′-bis-
(phenyl)benzidine) with a HOMO level close to the work
function of the PEDOT:PSS anode and acted as a hole-
injecting layer (HIL) to facilitate hole injection. Poly-
(vinylcarbazole) (PVK) mixed with 1,3-bis[(4-tert-butylphen-
yl)-1,3,4-oxadiazolyl]phenylene (OXD-7) was selected as the
host materials of the emissive layer (EML), and DPEPO was
bis(2-(di(phenyl)phosphino)-phenyl)ether oxide and mainly
selected as a hole-blocking layer to confine triplet excitons in
EML. TmPyPB was 1,3,5-tri(pyrid-3-yl-phenyl)-benzene, and
it worked as an electron-transport layer (ETL), together with
DPEPO to form a balance with PEDOT:PSS and poly-TPD
upon exciton transport. Device structures and energy level
diagrams were performed in Figure S24 in the Supporting
Information. For the sake of convenience, devices based on Ir-
0, Ir-1, and Ir-2 were abbreviated as device 0, device 1, and
device 2, respectively. Figure 6 presents the overall EL spectra,
current density−voltage−radiant emittance (J−V−R) charac-
teristics, current efficiency (CE)−current density and EQE−
current density curves of three complexes with a doping
concentration of 0.5 wt %, while Figures S25−27 in the
Supporting Information show their concentration-dependent
EL behavior, and their corresponding data are listed in Table 4.
At a low doping concentration of 0.5%, all EL spectra of these
complexes bore a close resemble to their PL counterparts in
dilute 2-MeTHF and showed efficient energy transfer from
hosts to dopants, and almost no host emission (at ∼420 nm)
occurred, illustrating the strong electron trapping abilities of
these complexes. All devices showed moderate turn-on
voltages (Von) and brightness with values of 7.2 V, 870 cd/
m2; 7.6 V, 753 cd/m2; and 8.3 V, 585 cd/m2, respectively, all at
a doping ratio of 0.5 wt %. Device 0 exhibited the highest EQE
of 9.68% and CE of 2.53 cd/A at 0.86 mA/cm2 with an
emission peak at 660 nm at the lowest doping ratio of 0.5 wt %.
Devices 1 and 2 experienced similar EL characteristics, with
the highest EQEs of 11.43% and 10.48%, and CEs of 1.98 cd/
A and 2.24 cd/A at 0.30 mA/cm2 and 0.39 mA/cm2, with
emission peaks at 670 and 664 nm at the lowest doping ratio of
F
DOI: 10.1021/acs.inorgchem.9b02477
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Inorganic Chemistry
Article
Figure 6. (a) EL spectra, (b) current density−voltage−radiant emittance (J−V−R) characteristics, (c) current efficiency−current density (CE−J),
(d) external quantum efficiency−current density (EQE−J) curves of devices 0, 1, and 2 (based on Ir-0, Ir-1, and Ir-2, respectively) at
concentrations of 0.5 wt %.
Table 4. Electroluminescence Characteristics of the OLED Devices
a
b
c
d
e
f
g
dopant (wt %)
Von (V)
λEL (nm)
J (mA/cm2)
Lmax (cd/m2)
J (mA/cm2)
CEmax (cd/A)
EQEmax (%)
CIE (X, Y)
0−0.5
0−1
0−2
0−4
1−0.5
1−1
1−2
1−4
2−0.5
2−1
2−2
7.2
6.8
8.2
8.8
7.6
9.0
10.4
9.8
8.3
7.5
8.4
8.5
660
660
662
664
670
672
674
676
664
664
666
668
17.61
18.01
19.61
21.61
18.41
20.01
22.01
23.20
18.01
18.41
18.41
870
681
1321
951
753
626
629
684
585
1115
675
642
0.86
0.28
1.71
0.52
0.30
2.15
2.41
5.41
0.39
4.71
5.35
3.81
2.53
2.15
1.91
1.51
1.98
1.50
1.05
0.66
2.24
1.73
1.42
9.68
8.80
8.54
7.76
11.43
9.84
7.83
5.72
10.48
8.37
7.62
5.14
(0.65, 0.29)
(0.66, 0.28)
(0.68, 0.29)
(0.68, 0.28)
(0.63, 0.28)
(0.65, 0.28)
(0.66, 0.28)
(0.67, 0.28)
(0.65, 0.29)
(0.66, 0.29)
(0.67, 0.28)
2−4
21.21
b
0.88
(0.68, 0.28)
a
c
d
Devices with different doping concentrations. Turn-on voltage at a brightness of 1 cd/m2. Current density at a maximum radiant. The
e
f
g
maximum radiant intensity. Current density at maximum efficiencies. The maximum current efficiency. The maximum external quantum
efficiency.
0.5 wt %. The deepest color of device 1 could be ascribed to its
more-distinct D−A characteristics at phenylquinoxaline
portion judged by the DFT calculation. All devices experienced
similar moderate efficiency rolloffs at high currents, and when
at current density of 100 mA/cm2, the EQEs of three devices
remained above 3.2%, which could be attributed to their
similar short phosphorescent emission lifetimes. With the
doping ratio increasing from 0.5 wt % to 4 wt %, all EL spectra
showed slight bathochromic shift of ∼4−6 nm and
accompanied decreased efficiencies, which was attributed to
molecular aggregation. Device efficiencies based on Ir-1 and Ir-
2 were more sharply decreased with the increase doping
concentration. When the doping concentration reached 4
wt %, the efficiencies of devices 1 and 2 were even inferior to
device 0. Considering the equally smooth surface features of
the doped EML of the three complexes in a surface area of 5
μm × 5 μm with average root-mean-square (RMS) roughness
values of 0.58, 0.61, and 0.57 nm, respectively. (See Figure 7.)
It could preclude the differences in charge-transfer properties
that caused by morphological changes of films. The most
possible mechanism underlying such a large efficiency
dropping at higher concentrations of devices 1 and 2 could
be attributed to their easier aggregate characteristics induced
by pyrene, as mentioned above. Therefore, the pyrene-induced
quenching should not be ignored when fabricating concen-
tration-dependent devices.
G
DOI: 10.1021/acs.inorgchem.9b02477
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Inorganic Chemistry
Article
= 0.20 in dichloroethane solution) by using the following
equation:14
2
i
j
y
z
η A I
r s
s
j
j
j
z
z
z
Φ = Φ
s
r
2
j
j
z
z
η AsIr
r
k
{
Solid-state samples were measured through absolute measure-
ment and recorded on a Horiba Jobin Yvon Fluorolog-3
fluorescence spectrometer that was equipped with an
integrating sphere. Time-resolved spectroscopy was performed
using a DeltaHub 405 and a Spectra 370 laser source on a
Horiba Jobin Yvon Fluorolog-3 fluorescence spectrometer
through time-corrected single-photon-counting (TCSPC)
measurement. Photoluminescence (PL) spectra at 77 K
measured in 2-MeTHF matrix were also obtained on a Horiba
Jobin Yvon Fluorolog-3 fluorescence spectrometer. Crystal
powder was grinded onto a clean glass during PL measure-
ment; after that, tested samples were dissolved in DCM and
spin-coated to conducted thin films PL measurement. Cyclic
voltammetry measurement was performed using a Model
CHI620 voltammetric analyzer equipped with a three-
electrode configuration (platinum disk working electrode,
platinum plate counter electrode, and Ag/AgCl reference
electrode). During the scan, a 0.1 M n-Bu4NPF6 was used as
the supporting electrolyte, and finally, ferrocene/ferrocenium
(Fc/Fc+) was used as the external standard. The measurement
was performed in a degassed acetonitrile solution with a scan
rate of 0.1 V/s. Thermogravimetric analysis (TGA) was
performed by using a Netzsch Model STA449 instrument with
a heating rate of 20 °C min−1 under a nitrogen atmosphere.
Atomic force microscopy (AFM) was conducted on a Veeco,
DI multimode NS-3D apparatus in a tapping mode under
normal air conditions at RT with a 5 μm scanner.
Theoretical Calculations. In this work, all of the
calculations were performed using the Gaussian 09 program
package.15 The ground and excited electronic states of Ir-1 and
Ir-2 were calculated using density functional theory (DFT) and
time-dependent density functional theory (TD-DFT). The
B3LYP functional and 6-31G* basis set were used for C, H, N,
O and LANL2DZ basis set for Ir.16 The lowest 50 singlet-to-
singlet excitation energies are calculated in the TDDFT
calculation.
Fabrication and Characterization of OLEDs. As a
common fabrication process, a 40-nm poly(3,4-ethylenediox-
ythiophene) (PEDOT):poly(styrenesulfonate) (PSS) layer
was spin-coated onto a precleaned ITO substrate and dried
under vacuum overnight. A 30-nm hole-injection layer of poly-
TPD then was spin-coated on top of the PEDOT:PSS layer in
its chlorobenzene solution with thermal annealing to remove
the solvent residue. The 60-nm emitting layer was spin-coated
on top of the HIL from its prepared chlorobenzene solution,
followed by an annealing treatment at 120 °C for 20 min. The
10-nm hole blocking layer DPEPO and the 40-nm electron
transporting layer TmPyPB was then evaporated orderly on
top of the emitting layer at a deposition rate of 0.5−1.0 nm s−1
under a pressure of 10−5 mbar. Lastly, the cathode CsF (1.2
nm)/Al (120 nm) layer was evaporated with a shadow mask at
a pressure of 3 × 10−4 Pa. The current−voltage−luminance
(J−V−L) characteristics were measured with a Keithley 236
source measurement and a Lambertian distribution. The EL
spectra were obtained using a Photo Research PR705 optical
analyzer. The EQEs are calculated on the basis of the existing
methods.17
Figure 7. Atomic force microscopy (AFM) measurement of the three
complexes based on emissive blends at concentrations of 4 wt %.
Height images (top) and phase images (bottom).
CONCLUSION
■
In summary, we explored novel iridium(III) complexes with 1-
pyrene-substituted 3-methyl-2-phenylquinoxaline ligands. Ar-
omatic rings substituted at the 7-position of 3-methyl-2-
phenylquinoxaline exerted a more positive effect on the
effective conjugate extension of complexes. As for the
iridium(III) complexes in the present work, large dihedral
angles existed between the quinoxaline and pyrene units, which
prevented the emission color of the three complexes from
being altered too much. Three complexes exhibited similar
device performances in their solution-processed OLEDs when
at low doping concentrations, and corresponded well with their
similar photoluminescent properties in dilute solutions, which
had consistent emissive origins. However, pyrene-induced
aggregation characteristics limited their device performance at
higher concentrations. Collectively, our findings provided an
approach for reasonably designing substituted quinoxaline
iridium complexes.
EXPERIMENTAL SECTION
■
General Information. Chemicals and reagents were
purchased from Energy Chemical and Aladdin. Reactions
were monitored by thin-layer chromatography (TLC) on silica
plates with fluorescence indicators under 254/365 nm UV
irradiation. Purification of complexes were performed via
1
column chromatography, using 200−300 mesh silica gel. H
and 13C NMR spectra were recorded on a Bruker Avance 400
spectrometer at 400/100 Hz (spectrum of Ir-0 was recorded
on Avance-400(III) with the upgrading of equipment), CDCl3
and tetramethylsilane (TMS) were used as the solvent and the
internal standard, respectively. Mass spectrometry (MS) results
were obtained on a Bruker Autoflex MALDI-TOF instrument.
The intermediate 6-bromo-3-methyl-2-phenyl-quinoxaline
(CCDC 1890304),13 Ir-1 (CCDC 1890344, 1941812) and
Ir-2 (CCDC 1890345) were also determined by a Bruker
SMART Apex CCD single-crystal X-ray diffractometer that
was equipped with monochromatized graphite (Mo Kα).
Spectroscopic and CV Measurements. Ultraviolet−
visible (UV-vis) absorption and photoluminescent (PL)
spectra were recorded with a Varian Cary 50 UV-vis
spectrophotometer and PerkinElmer Ls 50 B luminescence
spectrometer, respectively. The photoluminescence quantum
yields (Φp) values of complexes in degassed 2-MeTHF
solutions were measured referenced to fac-Ir(piq)2(acac) (Φp
H
DOI: 10.1021/acs.inorgchem.9b02477
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of 6-Bromo-3-methyl-2-phenylquinoxaline
(M1) and 6-Bromo-2-methyl-3-phenylquinoxaline (M2).
A mixture of 4-bromo-benzene-1,2-diamine (2.5 g, 17.2 mmol)
and 1-phenylpropane-1,2-dione (3.2 g, 17.2 mmol) was
dissolved in ethanol (40 mL) and heated to reflux overnight.
After removal of solvents, the crude product was purified via
column chromatography using petroleum ether (PE)/tetrahy-
drofuran (THF) (V/V, 20/1) as the eluent to afford a colorless
crystal solid M1 (2.2 g, 38.6%): 1H NMR (400 MHz, CDCl3)
δ 8.25 (s, 1H), 7.98 (d, J = 8.9 Hz, 1H), 7.80 (d, J = 8.7 Hz,
1H), 7.66 (d, J = 6.4 Hz, 2H), 7.60−7.49 (m, 3H), 2.78 (s,
3H). Matrix-assisted laser desorption ionization−time of flight
(MALDI-TOF) MS (m/z) for C15H11BrN12. Calcd: 298.011;
Found: 300.097. Also formed was a colorless structure isomer
crystal solid M2 (2.9 g, 50.9%): 1H NMR (400 MHz, CDCl3)
δ 8.29 (s, 1H), 7.92 (d, J = 8.8 Hz, 1H), 7.81 (d, J = 8.7 Hz,
1H), 7.65 (d, J = 6.6 Hz, 2H), 7.53 (d, J = 6.8 Hz, 3H), 2.77
(s, 3H). MALDI-TOF MS (m/z) for C15H11BrN12. Calcd:
298.011; Found: 300.104.
column using PE-DCM-THF (V/V/V, 50/10/1) as the eluent
to afford a purplish red solid (57 mg, 87.4%): H NMR (400
1
MHz, CDCl3) δ 8.26 (d, J = 8.2 Hz, 2H), 8.09 (d, J = 8.3 Hz,
2H), 8.00 (d, J = 7.5 Hz, 2H), 7.63 (t, J = 7.6 Hz, 2H), 7.33 (t,
J = 7.8 Hz, 2H), 7.07 (t, J = 7.6 Hz, 2H), 6.71 (t, J = 7.3 Hz,
2H), 6.64 (d, J = 7.6 Hz, 2H), 4.84 (s, 1H), 3.35 (s, 6H),0.65
(s, 18H). 13C NMR (126 MHz, CDCl3) δ 194.64, 164.61,
155.27, 151.54, 145.94, 141.30, 139.46, 137.33, 129.78, 128.99,
128.62, 127.73, 126.35, 121.09, 100.80, 88.84, 40.41, 27.70,
26.97. MALDI-TOF MS (m/z) for C41H41IrN4O2. Calcd:
814.286; Found: 815.495.
Synthesis of Ir-1. The purplish red solid (yield 24.7%) was
obtained from c-PyMPQ on the basis of the similar procedure
1
for Ir-0: H NMR (400 MHz, CDCl3) δ 8.28 (d, J = 9.7 Hz,
9H), 8.25−8.18 (m, 6H), 8.13 (s, 5H), 8.05 (d, J = 7.3 Hz,
4H), 7.65 (d, J = 7.9 Hz, 2H), 7.12 (s, 2H), 6.81 (s, 4H), 5.06
(s, 1H), 3.39 (s, 6H), 0.76 (s, 18H). 13C NMR (101 MHz,
CDCl3) δ 194.99, 164.67, 155.39, 152.22, 146.00, 142.00,
140.77, 139.68, 137.47, 135.99, 132.94, 131.50, 131.09, 130.96,
129.23, 128.93, 128.87, 128.43, 127.97, 127.86, 127.81, 127.43,
126.18, 125.46, 125.12, 124.89, 121.31, 89.22, 40.66, 27.92,
27.15. MALDI-TOF MS (m/z) for C73H57IrN4O2. Calcd:
1214.411; Found: 1215.723.
Synthesis of Ir-2. The purplish red solid was obtained
from 2-methyl-3-phenyl-6-(pyren-1-yl)quinoxaline on the basis
of the similar procedure for Ir-0 (29.5%): 1H NMR (400 MHz,
CDCl3) δ 8.40 (s, 2H), 8.32 (d, J = 6.7 Hz, 2H), 8.16 (dd, J =
13.2, 7.4 Hz, 4H), 8.11−7.99 (m, 14H), 7.92 (d, J = 7.9 Hz,
2H), 7.83 (d, J = 5.7 Hz, 2H), 7.71 (s, 2H), 7.04 (s, 2H), 6.92
(s, 2H), 4.57 (s, 1H), 3.27 (s, 6H), 1.58 (s, 18H). 13C NMR
(101 MHz, CDCl3) δ 195.04, 164.82, 156.69, 151.63, 146.45,
142.66, 138.54, 137.50, 136.08, 132.14, 131.37, 130.93, 130.66,
129.24, 128.72, 128.22, 127.70, 127.48, 127.33, 125.87, 124.99,
124.81, 124.78, 124.72, 124.60, 121.40, 88.91, 39.76, 26.91,
26.78. MALDI-TOF MS (m/z) for C73H57IrN4O2. Calcd:
1214.411; Found: 1215.718.
Synthesis of 3-Methyl-2-phenyl-6-(pyren-1-yl)-
quinoxaline (c-PyMPQ). A mixture of M1 (0.54 g, 1.81
mmol), pyrene-1-boronic acid pinacol ester (0.59 g, 1.81
mmol), aqueous K2CO3 (2 M, 5 mL), and Pd(PPh3)4 (62 mg,
0.054 mmol) was added to 15 mL of THF, stirring at reflux
temperature for 24 h under a N2 atmosphere. The clear yellow
solution changed to a kelly green color; after being cooled to
RT, the mixture was extracted with CH2Cl2 and the combined
organic layer was dried over MgSO4. The solvent was removed
by rotary evaporation and the residue was passed through a
flash silica gel column using PE-THF (V/V, 25/1) as the
1
eluent to afford a kelly green solid (0.48 g, 63.1%): H NMR
(400 MHz, CDCl3) δ 8.37−8.19 (m, 3H), 8.16−8.01 (m, 3H),
7.73 (d, J = 5.2 Hz, 1H), 7.57 (d, J = 7.2 Hz, 1.5H), 2.85 (s,
1.5H). 13C NMR (101 MHz, CDCl3) δ 155.01, 153.15,
142.91, 141.22, 140.27, 139.03, 136.23, 132.44, 131.44, 130.98,
129.55, 129.10, 129.01, 128.65, 128.48, 128.01, 127.82, 127.39,
126.16, 125.40, 124.99, 124.86, 124.82, 24.53. MALDI-TOF
MS (m/z) for C31H20N2. Calcd: 420.163; Found: 422.272.
Synthesis of 2-Methyl-3-phenyl-6-(pyren-1-yl)-
quinoxaline (t-PyMPQ). The white solid was obtained
(0.45 g, 58.9%) on the basis of the similar procedure for c-
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02477.
1
PyMPQ: H NMR (400 MHz, CDCl3) δ 8.42 (s, 1H), 8.26
1H and 1C NMR spectra, absorption and emission
spectra of free ligands, single-crystal structures, polarity-
dependent emission character, emission decay curves,
TD-DFT calculations of Ir-2, and device energy diagram
(PDF)
(dd, J = 18.1, 6.6 Hz, 5H), 8.15−8.04 (m, 6H), 7.72 (s, 2H),
7.55 (d, J = 7.3 Hz, 3H), 2.87 (s, 3H). 13C NMR (101 MHz,
CDCl3) δ 155.43, 152.69, 142.39, 140.97, 140.48, 139.02,
136.20, 132.90, 131.44, 131.02, 130.92, 130.43, 129.09, 128.99,
128.61, 128.47, 128.13, 127.97, 127.76, 127.37, 126.13, 125.37,
125.07, 124.99, 24.48. MALDI-TOF MS (m/z) for C31H20N2.
Calcd: 420.163; Found: 422.272.
Synthesis of Ir-0. IrCl3·H2O (24 mg, 0.08 mmol) and H2O
(1 mL) were added to a solution of 2-methyl-3-phenyl-
quinoxaline (53 mg, 0.24 mmol) in 2-ethoxyethanol (3 mL)
and THF (1 mL). The mixture was refluxed for 24 h under N2
atmosphere. After cooling to RT, the mixture was poured into
water and filtered, then the precipitate was washed with water
and a small amount of methanol and dried under high vacuum
to yield a crude dimer. Without further purification, the dimer
(75 mg, 0.08 mmol) and 2,2,6,6-tetramethyl-3,5-heptanedione
(177 mg, 0.96 mmol) with K2CO3 (200 mg, 1.24 mmol) in 2-
ethoxyethanol (5 mL) and were refluxed at 90 °C for 24 h
under N2 atmosphere. The reaction was quenched with water
and the mixture was extracted with CH2Cl2. The product was
obtained by column chromatography through a flash silica gel
Accession Codes
CCDC 1890304, 1890344, 1890345, and 1941812 contain 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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: 947041300@qq.com (P. Wang).
*E-mail: luzhiyun@scu.edu.cn (Z. Lu).
*E-mail: zhuwg18@126.com (G. Zhu).
*E-mail: liuyu03b@126.com (Y. Liu).
I
DOI: 10.1021/acs.inorgchem.9b02477
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Inorganic Chemistry
Article
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ORCID
Zhiyun Lu: 0000-0002-1733-4061
Weiguo Zhu: 0000-0002-4244-2638
Yu Liu: 0000-0002-4533-9118
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are thankful for the financial support from the
National Natural Science Foundation of China (Nos.
U1663229, 51473140, and 51573154); Hunan Postgraduate
Science Foundation for Innovation (No. CX2016-B261);
Open Project for the National Key Laboratory of Luminescent
Materials and Devices (No. 2017-skllmd-12); Brand Specialty
& Preponderant Discipline Construction Projects of Jiangsu
Higher Education Institutions.
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