Efficient organic light-emitting diodes based on iridium(iii) complexes containing indolo[3,2,1-jk]carbazole derivatives with narrow emission bandwidths and low efficiency roll-offs

Article abstract of DOI:10.1039/d1tc01449h

In this work, two cyclometalated ligands 2-(pyridin-2-yl)indolo[3,2,1-jk]carbazole (pyidcz) and 2-(4-(trifluoromethyl)pyridin-2-yl)indolo[3,2,1-jk]carbazole (tfpyidcz) containing an indolo[3,2,1-jk]carbazole unit were synthesized for achieving highly efficient iridium(iii) complexes (pyidcz)₂Ir(tmd) and (tfpyidcz)₂Ir(tmd) (tmd = 2,2,6,6-tetramethyl-3,5-heptanedione). The two Ir(iii) complexes exhibit emissions peaking at 499 and 554 nm, respectively, with narrow full width at half maximum (FWHM) values of 35 and 53 nm, respectively, short phosphorescence lifetimes less than 1 μs and high photoluminescence quantum yields of up to 90% in CH₂Cl₂solution. The organic light-emitting diodes (OLEDs) with (pyidcz)₂Ir(tmd) and (tfpyidcz)₂Ir(tmd) emitters show good performances. The (pyidcz)₂Ir(tmd)-based device shows a turn-on voltage (V_on) of 3.6 V with a maximum brightness (L_max) of 39123 cd m^?2, a maximum current efficiency (η_c,max) of 69.2 cd A^?1, a maximum power efficiency (η_p,max) of 36.8 lm W^?1and a maximum external quantum efficiency (EQE_max) of 21.2%. In particular, the FWHM bandwidth of the electroluminescence spectrum is only 33 nm, which was rarely reported for Ir(iii) complex-based OLEDs. The performance of the (tfpyidcz)₂Ir(tmd)-based device is slightly better with aV_onof 3.3 V, aL_maxof 31714 cd m^?2, aη_c,maxof 78.4 cd A^?1, aη_p,maxof 54.7 lm W^?1and an EQE_maxof 24.0%. Furthermore, the efficiency roll-offs of both devices are small, and theη_cvalues at 10?000 cd m^?2brightness are still maintained at 60.0 and 71.4 cd A^?1, respectively, which provide a powerful reference for developing indolo[3,2,1-jk]carbazole-based Ir(iii) complexes for efficient OLEDs.

Full text of DOI:10.1039/d1tc01449h

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Materials Chemistry C
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PAPER
Efficient organic light-emitting diodes based
on iridium(III) complexes containing indolo
[3,2,1-jk]carbazole derivatives with narrow
emission bandwidths and low efficiency roll-offs†
Cite this: J. Mater. Chem. C, 2021,
9, 8226
Xiang-Ji Liao,‡a Jin-Jun Zhu,‡a Li Yuan,a Zhi-Ping Yan,a Xu-Feng Luo,a
ab
Yi-Pin Zhang,a Jun-Jian Lua and You-Xuan Zheng
*
In this work, two cyclometalated ligands 2-(pyridin-2-yl)indolo[3,2,1-jk]carbazole (pyidcz) and 2-(4-(trifluoro-
methyl)pyridin-2-yl)indolo[3,2,1-jk]carbazole (tfpyidcz) containing an indolo[3,2,1-jk]carbazole unit were
synthesized for achieving highly efficient iridium(III) complexes (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) (tmd = 2,2,6,
6-tetramethyl-3,5-heptanedione). The two Ir(III) complexes exhibit emissions peaking at 499 and 554 nm,
respectively, with narrow full width at half maximum (FWHM) values of 35 and 53 nm, respectively, short
phosphorescence lifetimes less than 1 ms and high photoluminescence quantum yields of up to 90% in CH2Cl2
solution. The organic light-emitting diodes (OLEDs) with (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) emitters show
good performances. The (pyidcz)2Ir(tmd)-based device shows a turn-on voltage (Von) of 3.6 V with a maximum
brightness (Lmax) of 39123 cd mꢀ2, a maximum current efficiency (Zc,max) of 69.2 cd Aꢀ1, a maximum power
efficiency (Zp,max) of 36.8 lm Wꢀ1 and a maximum external quantum efficiency (EQEmax) of 21.2%. In particular,
the FWHM bandwidth of the electroluminescence spectrum is only 33 nm, which was rarely reported for Ir(III)
complex-based OLEDs. The performance of the (tfpyidcz)2Ir(tmd)-based device is slightly better with a Von of
3.3 V, a Lmax of 31714 cd mꢀ2, a Zc,max of 78.4 cd Aꢀ1, a Zp,max of 54.7 lm Wꢀ1 and an EQEmax of 24.0%.
Furthermore, the efficiency roll-offs of both devices are small, and the Zc values at 10 000 cd mꢀ2 brightness
are still maintained at 60.0 and 71.4 cd Aꢀ1, respectively, which provide a powerful reference for developing
indolo[3,2,1-jk]carbazole-based Ir(III) complexes for efficient OLEDs.
Received 30th March 2021,
Accepted 24th May 2021
DOI: 10.1039/d1tc01449h
rsc.li/materials-c
studied ones because the color and properties of Ir(III) complexes
can be adjusted by the combination of diversified cyclometalated
Introduction
The electro-optical conversion technology of organic light- main ligands and ancillary ligands.3 Nevertheless, for meeting the
emitting diodes (OLEDs) is widely used in flat panel display requirements of lighting and display applications, OLEDs posses-
and lighting applications.1 The light-emitting materials are the sing good color purity with narrow light bands, high electrolumi-
core part of OLEDs, especially for phosphorescent metal com- nescence efficiencies and low efficiency roll-offs are required.4
plexes, which always show high photoluminescence quantum
The carbazole group is an excellent chromophore with high
efficiencies (PLQYs) and good device performances.2 Among hole transport ability and triplet energy, and its derivatives have
them, trivalent iridium(III) (Ir(III)) complexes have some advan- been widely used in hole transport materials, host materials
tages such as easy modification of chemical structures, flexible and luminescent materials.5 There are several active reaction
control of photophysical properties including high luminous sites on carbazole derivatives, which can be easily modified by
efficiencies and relatively low phosphorescence lifetimes. combining with functional units to form luminescent materials
Notably, hetero-ligand-based Ir(III) complexes are the most and organic ligands for phosphorescent complexes. In addi-
tion, the electron-donating ability of different positions on the
a State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of
Advanced Organic Materials, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, P. R. China. E-mail: yxzheng@nju.edu.cn
b Shenzhen Research Institute of Nanjing University, Shenzhen 518057, P. R. China
† Electronic supplementary information (ESI) available. CCDC 2052136 and
2052163. For the ESI and crystallographic data in CIF or other electronic formats
see DOI: 10.1039/d1tc01449h
carbazole unit could influence the energy levels of the lumines-
cent materials, so the functionalization of the different sub-
stitution positions will effectively regulate the luminescence
color and efficiency of the complexes.5 Among them, Ir(III)
complexes based on ligands with carbazole units always show
a high hole transport ability5 and good device performances,6
which have attracted great attention in the past few years.
‡ The authors contributed equally to this paper.
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However, due to the high hole transporting ability of
Journal of Materials Chemistry C
carbazole-based Ir(III) complexes, designing bipolar ligands is
an effective way to achieve carrier equilibrium and improve the
device performance. In 2008, Wong et al. reported a series of
orange Ir(III) complexes with hetero-ligands containing carba-
zole units, which can enhance the HOMO energy level and the
hole injection and transport ability of the complexes.7 After
that, Bryce et al. reported five efficient Ir(III) complexes based on
2-(30-carbazolyl)pyridine ligands, and their OLEDs showed a
maximum external quantum efficiency (EQEmax) of 12%.8 In
2016, Ding et al. reported a blue Ir(III) complex with a bipolar
carbazole/triphenylphosphine oxide compound as the peri-
pheral dendrimer, which makes charge transfer more balanced,
and its undoped OLED showed excellent performance with
significantly reduced efficiency roll-off.9 This research shows
that the design and synthesis of bipolar phosphorescent den-
drimers is an effective strategy for the development of high- Scheme 1 Synthesis methods and routes for the ligands and Ir(III)
complexes.
performance OLEDs. However, in the above references, the free
relaxation of the groups connected to carbazole nitrogen is not
limited, which may increase the non-radiative transition of the
and reduce the efficiency roll-off.11,12 Furthermore, the Ir(III) complex
molecule, thus reducing the performance of the device.
based on the pyidcz ligand shows a narrower FWHM bandwidth of
Furthermore, for most reported Ir(III) complexes the emission
35 nm in solution. Furthermore, the OLEDs based on both com-
spectra are broad due to their complicated origins, such as
plexes exhibited good performances with low efficiency roll-off,
singlet/triplet metal-to-ligand charge transfer (1MLCT and
especially with a narrower FWHM bandwidth of 33 nm.
3MLCT) and triplet ligand-to-ligand charge transfer (3LLCT).
The broad emission spectrum would affect the device color
purity and efficiency for practical applications. To narrow the
emission spectra of Ir(III) complexes, one effective strategy is to
limit the free relaxation of the substitutes of the ligands by
introducing sterically hindered or rigid groups. For example,
Results and discussion
Synthesis and characterization
our group reported three Ir(III) complexes coordinated with Scheme 1 provides a brief overview of the synthesis routes for the
carbazole and pyridine, which are connected with carbazole main ligands, and (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) complexes
directly. The bulky CF3 and trimethylsilyl (TMS) substituents at from carbazole, o-fluorobromobenzene, 2-bromopyridine, 2-bromo-
the pyridine unit greatly improved the PLQYs and device 4-(trifluoromethyl)pyridine, 2,2,6,6-tetramethyl-3,5-heptanedione
performances of the complexes with a full width at half max- and other raw materials. Detailed NMR and high resolution mass
imum (FWHM) bandwidth of 49 nm, which provide a reference spectrometry (HRMS) spectra of the products are presented in the
for the design and development of efficient carbazole Ir(III) ESI† (Fig. S1–S5).
complexes with relatively narrow emission spectra.10 However,
The single crystal ORTEP diagrams of the two complexes are
to date, it is still difficult to find a strategy to design suitable shown in Fig. 1 and the crystal data are summarized in Tables
ligands for Ir(III) complexes with narrower emission band- S1–S3 (ESI†), which strongly prove the structures of the synthe-
widths than 40 nm.
sized complexes. The iridium atom is surrounded by C, N and O
Herein, we designed and synthesized two cyclometalated ligands atoms of the main ligand and the ancillary ligand tmd. The
2-(pyridin-2-yl)indolo[3,2,1-jk]carbazole (pyidcz) and 2-(4-(trifluoro- complexes adopt the distorted octahedral coordination geome-
methyl)pyridin-2-yl)indolo[3,2,1-jk]carbazole (tfpyidcz) composed of try, and due to the introduction of the CF3 group, the Ir–N bond
indolo[3,2,1-jk]carbazole and pyridine units for Ir(III) complexes length (2.011 Å) of (tfpyidcz)2Ir(tmd) is shorter than that of
(Scheme 1), which show good thermal stability (4400 1C), short (pyidcz)2Ir(tmd) (2.017 Å), and the bond angle (81.711)
phosphorescence lifetimes (o1 ms) and high PLQYs of up to 90%. of (tfpyidcz)2Ir(tmd) and [C–Ir–N] is greater than that of
From our previous study it was found that the bulky trifluoromethyl (pyidcz)2Ir(tmd) and [C–Ir–N] (81.01).
(CF3) substituent in the main ligand can cause a large steric
Furthermore, the high Td values of the emitters are
hindrance effect and greatly inhibit the molecular packing, which important for the stability of the practical production of
can increase the sublimation yield, inhibit the concentration OLEDs.1,2,3a–e,4 From the thermogravimetric analysis (TGA)
quenching and finally enhance the phosphorescence quantum curves (Fig. S6, ESI†) it can be observed that the decomposition
yield.10 Therefore, the large indolo[3,2,1-jk]carbazole and CF3 units temperatures (Td, 5% mass loss) of the (pyidcz)2Ir(tmd) and
not only help to reduce the molecular accumulation and inhibit the (tfpyidcz)2Ir(tmd) are 445 1C and 418 1C, respectively, which are
triplet–triplet annihilation (TTA) effect, but also improve the elec- very high and are possibly due to the rigid structure of the main
tron–hole recombination rate, ameliorate the device performance ligands.
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FWHM bandwidth of 53 nm, which indicates that the CF3 on the
main ligand can greatly affect the emission spectra of the Ir(III)
complexes. In solid state, the FWHM bandwidths of (pyidcz)2
Ir(tmd) and (tfpyidcz)2Ir(tmd) are 38 and 58 nm, respectively, which
are close to those in solution.
In addition, the complexes (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd)
show high absolute PLQYs of 75% and 90% in dilute DCM (10ꢀ5 M,
Fig. S8, ESI†), respectively, which are the key characteristics of the
OLEDs. Besides the PLQYs, the phosphorescence lifetime (tp) of the
complexes is also a key factor for OLED performances. Generally
Fig. 1 The single crystal ORTEP diagrams of (pyidcz)2Ir(tmd) (left, speaking, a longer phosphorescence lifetime would lead to more
serious TTA13 and triplet–polaron annihilation (TPA) effects.14 The
measured phosphorescence lifetime curves of the complexes are
CCDC: 2052163) and (tfpyidcz)2Ir(tmd) (right, CCDC: 2052136). Hydrogen
atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
shown in Fig S9 (ESI†), which show that both complexes have short
values of 0.39 and 0.64 ms for (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd),
Photophysical properties
respectively. Therefore, the high absolute PLQYs and short phos-
The measured UV-vis absorption and emission spectra in dilute
phorescence lifetimes of the two complexes are beneficial to obtain-
dichloromethane (DCM, 10ꢀ5 M) are shown in Fig. 2, and their
ing sufficient electroluminescence efficiency with low efficiency
corresponding data are summarized in Table 1. From the
roll-off.
absorption spectra, it can be observed that the two complexes
show intense bands before 300 nm, which are due to the spin- Electrochemical properties and DFT calculations
allowed ligand-to-ligand charge transfer (1LLCT) and ligand-
In order to understand the electron cloud distributions of the
frontier orbitals of two Ir(III) complexes, the time-dependent
centered 1LC (p - p*) transitions of cyclometalated and
ancillary ligands, while the forbidden metal-to-ligand charge
density functional theory (TD-DFT) calculations with Gaussian
3
transfer (1MLCT and MLCT) absorptions are distinguished at
09 software were applied.15 After the calculations, the electron
approximately 402–407 nm with lower extinction coefficients.
cloud distributions of the frontier molecular orbitals of
The (pyidcz)2Ir(tmd) shows a green emission peaking at 499 nm
(pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) complexes were
with the CIE coordinates of (0.16, 0.60), which belongs to the
obtained using QMForge (Fig. 3), and the theoretical calcula-
tion data are presented in Table S4 (ESI†). It can be seen that
1MLCT. The maximum emission peak of (tfpyidcz)2Ir(tmd) is
554 nm with the CIE coordinates of (0.43, 0.56), indicating a
the HOMO electron clouds of (pyidcz)2Ir(tmd) and (tfpyidc-
yellow–green emission. Due to the stable rigid structure of
z)2Ir(tmd) are mainly distributed on the iridium atom
indolo[3,2,1-jk]carbazole in the main ligand, the (pyidcz)2
(36.36% and 31.44%) and the indolezo[3,2,1-jk]carbazole of
Ir(tmd) complex has a very narrow FWHM bandwidth of
the main ligand (60.12% and 65.20%), while the LUMO elec-
3
3
35 nm, and the MLCT and LC transitions (Fig. S7, ESI†) are
not obvious at room temperature, which is seldom found for
Ir(III) complexes and beneficial to improving the monochromaticity
of its OLED. Compared with (pyidcz)2Ir(tmd), the (tfpyidcz)2Ir(tmd)
displays an obvious redshift emission spectrum with a broad
tron clouds are basically distributed on the pyridine of the
main ligand (93.45% and 94.52%), respectively. Meanwhile,
the electron cloud distributions of the LUMO and HOMO on
the (tfpyidcz)2Ir(tmd) main ligand are higher than those of
(pyidcz)2Ir(tmd), and the Eg of (tfpyidcz)2Ir(tmd) is also low-
ered, which redshifts the emission wavelength. Furthermore,
the introduction of CF3 into the main ligand also lowers the
LUMO level of the complex, and the (tfpyidcz)2Ir(tmd) exhibits
better electron transport properties than (pyidcz)2Ir(tmd). The
results suggest that the photophysical properties of the Ir(III)
complexes are mainly manipulated by the main ligands.
From the cyclic voltammetry (CV) curves of the two com-
plexes (Fig. S10, ESI†) it can be seen that the oxidation peaks of
the two complexes are relatively obvious, but their reduction
peaks are not obvious, which may be due to the introduction of
an indolezo[3,2,1-jk]carbazole heterocyclic ring on the main
ligand. The HOMO/LUMO energy gaps (Eg) of 3.47 and
2.70 eV were calculated from the UV-vis absorption spectra of
the (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) complexes. There-
fore, the HOMO/LUMO energy levels of (pyidcz)2Ir(tmd) and
(tfpyidcz)2Ir(tmd) are ꢀ5.14/ꢀ2.81 and ꢀ5.28/ꢀ2.96 eV, respec-
tively, obtained from the CV curves and Eg values. It can be seen
that the introduction of CF3 into the main ligands can lower the
Fig. 2 Normalized absorption and emission spectra of the Ir(III) complexes
in dilute dichloromethane (DCM, 10ꢀ5 M) and solid state at room
temperature.
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Table 1 Summary of the physical, photophysical and electrochemical properties of the Ir(III) complexes
Td (1C) labs (nm) lPL (nm) FWHMc (DCM/solid, nm) FPL (%) tp (ms) CIE (x, y)
Eg [eV] HOMO/LUMOg (eV)
f
a
b
b
d
e
Complex
(pyidcz)2Ir(tmd)
(tfpyidcz)2Ir(tmd) 418
445
343, 407
355, 403
499
554
35/38
53/58
76
90
0.39
0.64
(0.18, 0.62) 3.47
(0.43, 0.53) 2.70
ꢀ5.14/ꢀ2.81
ꢀ5.28/ꢀ2.96
a
b
Decomposition temperature determined from TGA curves. Absorption and phosphorescence peaks measured in dilute DCM (10ꢀ5 M) at room
temperature. Full width at half maximum. Absolute PLQYs measured in DCM (10ꢀ5 M) in a N2 atmosphere. Phosphorescence lifetime
c
d
e
measured in dilute DCM (10ꢀ5 M) at room temperature. Estimated from the onset of the absorption spectrum. HOMO level measured from the
oxidation potential in acetonitrile solution (5 ꢁ 10ꢀ4 M) by cyclic voltammetry curves (EHOMO = ꢀ(Eox + 4.8)), and the LUMO level calculated as
ELUMO = EHOMO ꢀ Eg.
f
g
respectively. The device structure and material structure are
shown in Scheme 2, the device characteristics are displayed in
Fig. 4, and the key data are presented in Table 2.
As shown in Fig. 4, the turn-on voltage (Von) values of devices
S1 and S2 are 3.6 and 3.3 V with electroluminescence peaks at
502 and 558 nm, respectively. The CIE color coordinates of
devices S1 and S2 are (0.18, 0.62) and (0.43, 0.53), showing
green and yellow green light, respectively. Notably, due to the
stable and rigid structure of the main ligands containing an
indolo[3,2,1-jk]carbazole unit, the FWHM bandwidths of
S1 and S2 are about 33 and 48 nm, respectively, which are
much narrower than those of most reported Ir(III) complex-
based OLEDs.
Furthermore, both devices show good device perfor-
mances. Device S1 exhibited a maximum brightness (Lmax) of
39123 cd mꢀ2, a maximum current efficiency (Zc,max) of
69.2 cd Aꢀ1, a maximum power efficiency (Zp,max) of 36.8 lm Wꢀ1
,
and an EQEmax of 21.2%. Device S2 shows a better performance
with a Lmax of 31714 cd mꢀ2, a Zc,max of 78.4 cd Aꢀ1, a Zp,max of
54.7 lm Wꢀ1 and an EQEmax of 24.0%. In addition, both devices
show low efficiency roll-offs; when the luminance reaches
10 000 cd mꢀ2, the Zc values of S1 and S2 still remain as high
as 60.0 and 71.4 cd Aꢀ1, respectively. By comparison, the device
performance of S2 is basically better than that of S1, mainly due
to the introduction of CF3 into the main ligand, which
improved the PLQY of the (tfpyidcz)2Ir(tmd) complex and can
also reduce the TTA and TPA effects of the device.
Fig. 3 Optimal HOMO and LUMO diagrams and distributions of
(pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) complexes calculated using theory
and cyclic voltammetry curves.
LUMO energy levels of the (tfpyidcz)2Ir(tmd) complex, which is
conducive to the electron injection/transport and reduction of
the device driving voltage.
OLED performance
The good device performances based on two Ir(III) complexes
can be attributed to the cyclometalated main ligands composed
of indole[3,2,1-jk]carbazole and pyridine units. The indole
Based on their good photophysical properties, (pyidcz)2Ir(tmd) and
(tfpyidcz)2Ir(tmd) were used as emitters to prepare single emissive
layer-doped devices. After optimizing the doping concentration of
the guest and host materials, the optimal device structure was
obtained as ITO/HAT-CN (hexaazaztriphenylene-hexacabonitrile,
5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)phenyl] cyclohexane, 50 nm)/
Ir(III) complex (5 wt%): 2, 6DCzPPy (2,6-bis(3-(9H-carbazol-9-yl)
phenyl)pyridine, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)ben-
zene, 50 nm)/LiF (1 nm)/Al (100 nm). HAT-CN and LiF serve as the
hole- and electron-injection materials, respectively. TAPC with a high
hole mobility (1 ꢁ 10ꢀ2 cmꢀ2 Vꢀ1 sꢀ1) and a high-lying LUMO level
(ꢀ1.80 eV) serves as the hole transport/electron blocking layer,16
while TmPyPB acts as an electron transport/hole blocking material,
which has a high electron mobility (1 ꢁ 10ꢀ3 cmꢀ2 Vꢀ1 sꢀ1) and a
low-lying HOMO level (ꢀ6.70 eV).17 The bipolar 2,6-DCzPPy material
was chosen as the host, the electron mobility of which is close to the
hole mobility (1 ꢁ 10ꢀ5–8 ꢁ 10ꢀ5 cmꢀ2 Vꢀ1 sꢀ1).18 The devices based
on (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) were named S1 and S2,
Scheme 2 Energy level diagrams of OLEDs and molecular structures of
the used materials.
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Fig. 4 Device characteristics of S1 and S2: (a) electroluminescence spectra at 8 V; (b) luminance–voltage curves; (c) current efficiency–luminance
curves; and (d) external quantum efficiency–luminance curves.
Table 2 Key EL data of devices S1 and S2
Device Von (V) lEL (nm) FWHMc (nm) CIEd (x, y) Lmax (cd mꢀ2
)
Zc,max (cd Aꢀ1
)
Zc,L10 000 (cd Aꢀ1
)
EQEmax (%) Zp,max (lm Wꢀ1
)
a
b
e
f
g
h
i
S1
S2
3.6
3.3
502
558
33
46
(0.18, 0.62) 39123
(0.43, 0.53) 31714
69.2
78.4
60.0
71.4
21.2
24.0
36.8
54.7
a
b
c
d
Turn-on voltage, the driving voltage at 1 cd mꢀ2
.
Electroluminescence peak spectrum at 8 V. Full width at half maximum. CIE color
coordinates at an applied voltage of 8 eV. Maximum luminance. Current efficiency. Current efficiency at 1000 cd mꢀ2 brightness. External
quantum efficiency calculated within the visible spectral region. Power efficiency.
e
f
g
h
i
[3,2,1-jk]carbazole unit is beneficial for hole transport and the short phosphorescence lifetimes less than 1 ms and high PLQYs of
rigid structure of the main ligands is helpful to improve the up to 90%. Notably, the (pyidcz)2Ir(tmd) complex without a CF3
electron mobility of the emitter, realizing balanced carrier injec- unit shows a narrow FWHM bandwidth of about 35 nm in solution
tion and transport. Besides, the high PLQYs of the Ir(III) complexes and 33 nm in OLEDs, which were rarely reported for Ir(III)
are also favorable for the improvement of the device perfor- complexes and their OLEDs. Furthermore, both devices based on
mances. Furthermore, the bulk indole[3,2,1-jk]carbazole unit (pyidcz)2Ir(tmd) and (tfpyidcz)2Ir(tmd) showed good performances
and CF3 substituent of the main ligands are helpful to reduce with a Lmax of 39123 cd mꢀ2, a Zc,max of 78.4 cd Aꢀ1, an EQEmax of
molecular accumulation and inhibit the TTA/TPA effect, giving 24.0% and low efficiency roll-off. These results are mainly attri-
rise to improved efficiency and low efficiency roll-off.
buted to the rigid structure of the main ligands, and this work
provides an effective strategy to improve the performances of Ir(III)
complexes and their OLEDs.
Conclusions
In summary, a bulk and rigid indolo[3,2,1-jk]carbazole unit
was introduced into the main ligands (2-(pyridin-2-yl)indolo
Experimental section
[3,2,1-jk]carbazole and 2-(4-(trifluoromethyl)pyridin-2-yl)indolo[3,2, All reagents were commercially purchased and used without
1-jk]carbazole) for Ir(III) complexes ((pyidcz)2Ir(tmd) and (tfpyidcz)2 further treatment. The synthesis details of Scheme 1 are
Ir(tmd)), which show excellent thermal stability higher than 400 1C, described in the following.
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1
Synthesis of the main ligands pyidcz and tfpyidcz
Pyidcz (54% yield): H NMR (400 MHz, CDCl3) d 8.79–8.74
(m, 1H), 8.63 (s, 2H), 8.14–8.08 (m, 2H), 7.90–7.85 (m, 1H),
7.84–7.75 (m, 3H), 7.56–7.47 (m, 2H), 7.36–7.29 (m, 2H),
7.26–7.20 (m, 1H).
For the synthesis of compound 1, 5.00 g (30 mmol) of carbazole,
4 mL (36 mmol) of o-fluorobromobenzene, 11.70 g (36 mmol) of
cesium carbonate and 50 mL of DMF were taken in a 500 mL
flask, and the solution was stirred overnight at 160 1C in a N2
atmosphere. Then the reaction was cooled to room tempera-
ture, 40 mL of water was added and the solution was stirred for
1 hour. After filtration, the obtained solid was washed twice
with water, ethanol and petroleum ether, respectively, and
8.10 g of compound 1 was obtained as a white solid in
84% yield. 1H NMR (400 MHz, CDCl3) d 8.21–8.14 (m, 2H),
7.87 (dd, J = 8.0, 1.4 Hz, 1H), 7.59–7.47 (m, 2H), 7.46–7.38
(m, 3H), 7.35–7.28 (m, 2H), 7.12–7.05 (m, 2H).
For the synthesis of compound 2, 6.44 g (20 mmol) of
compound 1, 0.22 g (1 mmol) of Pd(OAc)2, 5.52 g (40 mmol)
of anhydrous K2CO3, and 100 mL of DMAC were taken in a
250 mL flask, and the solution was refluxed at 170 1C overnight
in a N2 atmosphere. The mixture was cooled to room tempera-
ture and water (50 mL) was added to it. The solution was stirred
for 0.5 hours and extracted with DCM (3 ꢁ 20 mL). The
combined organic extracts were concentrated and the obtained
solid was purified by flash chromatography (DCM/PE = 1/2) to
obtain 4.20 g of compound 2 as a white solid in 82% yield.
1H NMR (400 MHz, CDCl3) d 8.16–8.09 (m, 2H), 8.03 (d, J = 7.4
Hz, 2H), 7.88–7.82 (m, 2H), 7.63–7.51 (m, 3H), 7.40–7.32
(m, 2H).
Tfpyidcz (62% yield): 1H NMR (400 MHz, CDCl3) d 8.92
(d, J = 5.0 Hz, 1H), 8.64 (s, 2H), 8.15–8.06 (m, 3H), 7.83
(d, J = 8.1 Hz, 2H), 7.58–7.51 (m, 2H), 7.49–7.43 (m, 1H),
7.36 (dd, J = 7.6, 1.0 Hz, 2H). 19F NMR (376 MHz, CDCl3)
d ꢀ64.66.
Synthesis of Ir(III) complexes
A mixed solution of ethylene glycol diethyl ether (20 mL) and
water (5 mL) containing IrCl3 (1 mmol) and pyidcz or tfpyidcz
(2.2 mmol) was heated at 110 1C under a N2 atmosphere for
12 hours. After cooling to room temperature, the corresponding
chlorine bridge was obtained by filtration. Then, the chlorine
bridge (0.5 mmol) and tma-Na (1.25 mmol) were refluxed in
ethylene glycol diethyl ether (20 mL) for another 12 hours.
Then, the (pyidcz)2Ir(tmd) or (tfpyidcz)2Ir(tmd) complex was
filtered and separated by flash chromatography (EA/PE = 1/3)
and further purified by vacuum sublimation.
(pyidcz)2Ir(tmd) (yellow solid, yield: 81%): 1H NMR
(400 MHz, CDCl3) d 8.74 (s, 2H), 8.41–8.34 (m, 2H), 8.19–8.15
(m, 2H), 8.14–8.10 (m, 2H), 7.86–7.76 (m, 4H), 7.67–7.62 (m,
2H), 7.47–7.41 (m, 2H), 7.35–7.28 (m, 2H), 7.16–7.10 (m, 2H),
6.84–6.77 (m, 2H), 6.59–6.51 (m, 2H), 6.45–6.38 (m, 2H), 5.31
(s, 1H), 0.79 (s, 18H). HRMS (ESI) m/z calcd for C57H45IrN4O2
[M + H]+: 1011.3245, found: 1011.3249.
For the synthesis of compound 3, 2.41 g (10 mmol)
of compound 2 was dissolved in 100 mL of DCM, then
1.78 g (10 mmol) of NBS was slowly added under illumination
using a 26 W lamp, and the solution was stirred at room
temperature for 12 hours. Then the mixture was concentrated
and the obtained solid was washed with ethanol to obtain
2.88 g of compound 3 as a white solid in 90% yield. H NMR
(400 MHz, CDCl3) d 8.11 (s, 2H), 8.07–8.00 (m, 2H), 7.88–7.80
(m, 2H), 7.60–7.52 (m, 2H), 7.39–7.32 (m, 2H).
(tfpyidcz)2Ir(tmd) (orange solid, yield: 81%): 1H NMR
(400 MHz, CDCl3) d 8.80 (s, 2H), 8.59–8.52 (m, 2H), 8.28–8.23
(m, 2H), 8.23–8.17 (m, 2H), 7.82–7.76 (m, 2H), 7.72–7.64 (m,
2H), 7.51–7.43 (m, 2H), 7.39–7.30 (m, 2H), 7.23–7.14 (m, 2H),
7.00 (dd, J = 6.1, 1.8 Hz, 2H), 6.64–6.54 (m, 2H), 6.36–6.30 (m,
2H), 5.36 (s, 1H), 0.80 (s, 18H).19F NMR (376 MHz, CDCl3) d
ꢀ64.73. HRMS (ESI) m/z calcd for C59H43F6IrN4O2 [M + H]+:
1147.2992, found: 1147.2992.
1
In a N2 atmosphere, 1.60 g (50 mmol) of compound 3, 3.14 g
(55 mmol) of pinacol diboride, 1.68
g (2.5 mmol) of
Pd(dppf)Cl2, 10.00 g (100 mmol) of KOAc and 300 mL of
dioxane were reacted in a 500 mL flask overnight at 100 1C.
The mixture was quenched with water (200 mL) and extracted
with EA (3 ꢁ 50 mL). The combined extracts were concentrated
under vacuum, and the obtained solid was purified by flash
chromatography (DCM/PE = 1/1) to obtain 12.80 g of compound
4 as a white solid in 70% yield. 1H NMR (400 MHz, CDCl3)
d 8.58 (s, 2H), 8.16–8.12 (m, 2H), 7.93–7.87 (m, 2H), 7.59–7.52
(m, 2H), 7.40–7.34 (m, 2H), 1.47 (s, 12H).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Guangdong Basic and
Applied Basic Research Foundation (2020B1515120030) and
the National Natural Science Foundation of China (51773088
and 21975119).
For the synthesis of the main ligands, 1.83 g (5 mmol)
of compound 4, 0.87 g (5.5 mmol) of 2-bromopyridine or 1.24 g
(5.5 mmol) of 2-bromo-4-(trifluoromethyl) pyridine, 0.18
g
Notes and references
(0.25 mmol) of Pd(dppf)Cl2, 4.89 g (15 mmol) of Cs2CO3, 5 mL of
water and 50 mL of dioxane were taken in a 200 mL flask and
refluxed overnight under N2 protection. After being filtered at room
temperature, DCM (100 mL) was added and then concentrated in
vacuum. Finally, 0.86 g pyidcz or 1.20 g tfpyidcz was obtained by
flash chromatography (DCM/PE = 2/1) as a white solid.
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This journal is © The Royal Society of Chemistry 2021
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