Two novel neutral and ionic Ir(iii) complexes based on the same bipolar main ligand: A comparative study of their photophysical properties and applications in solution-processed red organic light-emitting diodes

Article abstract of DOI:10.1039/d0nj02462g

Using 4-(5-(4-(dimesitylboranyl)phenyl)pyridin-2-yl)-N,N-diphenylaniline (BNpppy) as a cyclometalated ligand, 1,10-phenanthroline or 2,4-pentanedione (acac) as the ancillary ligand, respectively, two novel iridium(iii) complexes (Ir-1 and Ir-2) were succe

Full text of DOI:10.1039/d0nj02462g

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Two novel neutral and ionic Ir(III) complexes based
on the same bipolar main ligand: a comparative
study of their photophysical properties and
applications in solution-processed red organic
light-emitting diodes†
Cite this:DOI: 10.1039/d0nj02462g
ab
Rui Liao,a Ying Wang,a Jin Du,a Xiumei Wang,a Nana Wang,a Huibin Sun,
*
Jianpu Wanga and Wei Huangac
Using 4-(5-(4-(dimesitylboranyl)phenyl)pyridin-2-yl)-N,N-diphenylaniline (BNpppy) as a cyclometalated
ligand, 1,10-phenanthroline or 2,4-pentanedione (acac) as the ancillary ligand, respectively, two novel
iridium(III) complexes (Ir-1 and Ir-2) were successfully synthesized and their photoelectric properties have
been investigated in detail. The incorporation of electron-accepting dimesitylboron (Mes2B) moieties and
electron-donating triphenylamine moieties into iridium(III) complexes has been demonstrated with benign
photophysical and electronic properties. The solution-processed OLEDs were fabricated using these two
iridium(III) complexes, respectively. The devices of Ir-2 show good performances compared to those of
Ir-1 with a maximum luminance of 7916 cd mÀ2, a maximum external quantum efficiency (EQE) of
2.48% and a maximum current efficiency of 3.99 cd AÀ1. These results indicate that the introduction of a
donor–acceptor type of ligand into transition metal complexes has potential applications in constructing
an efficient emitter in OLEDs with excellent performances.
Received 15th May 2020,
Accepted 15th June 2020
DOI: 10.1039/d0nj02462g
rsc.li/njc
luminescence properties and the facile color tunability.9–12
Also, it is well known that the introduction of different
Introduction
Organic light-emitting diodes (OLEDs) are promising materials kinds of ligand into phosphorescent iridium(III) complexes
for novel flat-panel displays, and solid-state lighting sources.1–5 can drastically influence the electrochemical and photophysical
Transition metal complexes with effective intersystem crossing properties of iridium(III) complexes.13–17 Therefore, phospho-
have shown great potential for use in phosphorescent OLEDs rescent iridium(III) complexes possess the huge potentiality in
because they can fully harvest both the electro-generated developing energy-efficient and high-performing phosphores-
singlet and triplet excitons to achieve nearly 100% internal cent organic light-emitting diodes.
quantum efficiency.6–8 Among these transition metal complexes,
The charge-transport balance is significant for phos-
phosphorescent iridium(III) complexes have been regarded as phorescent OLEDs doped with iridium(III) complexes. Rational
the most popular organic electronic materials for the emission chemical tailoring of the cyclometalated ligand can facilitate
layer in the electroluminescent devices, owing to their efficient the hole and electron transport. As we know, the dimesityl-
boron (Mes2B) moieties have been widely used to construct
organic functional optoelectronic materials.18,19 And they also
have been introduced into ligands of transition metal com-
a China Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced
Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced
plexes as functional units to improve the corresponding device
Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road,
performance.20 For example, Wang et al. reported two novel
Nanjing 211816, P. R. China. E-mail: iamhbsun@njtech.edu.cn
b Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM),
Nanjing University of Posts and Telecommunications, 9 Wenyuan Road,
Nanjing 210023, China
c Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical
University (NPU), 127 West Youyi Road, Xi’an 710072, China
† Electronic supplementary information (ESI) available: Experimental details.
1H NMR and mass spectra. Extra absorption and emission spectra. See DOI:
10.1039/d0nj02462g
cyclometalated phosphorescent platinum(II) complexes function-
alized with dimesitylboryl and developed those complexes as
an anion-sensor for fluoride ions.21 Later on, the red emission
phosphorescent bis(5-(dimesitylboryl)-2-phenylpyridinato)iridium-
(acac) was reported by Zhou et al., which showed an excellent
electroluminescence (EL) performance.22,23 Besides, the electron
acceptor of the boron center can easily facilitate intramolecular
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mass spectrometer. Ultraviolet-Visible absorption spectra were
measured by using a Shimadzu Uv-3600 Uv-Vis-NIR spectro-
photometer. Emission spectra were recorded on a FL-400PC
Spectra spectrophotometer. Thermal gravimetric analysis (TGA)
was performed on a PerkinElmer TGA7 thermal analyzer at a
heating rate of 10 1C minÀ1 under a nitrogen atmosphere.
Time-resolved fluorescence spectra of the solutions in dichloro-
methane (DCM) were recorded using an Edinburgh Instru-
ments (FLS920) spectrometer. To evaluate the performance of
the devices, the characteristics of the devices were measured
using a Keithley 2400 source meter and a fiber integration
sphere (FOIS-1) coupled with a QE65 Pro spectrometer.
Synthesis and characterization
Scheme 1 and Scheme S1 in the ESI† show the synthetic routes
of BNpppy ligand and both iridium(III) complexes. All structures
of the intermediates and target products were characterized by
1H NMR and 13C NMR. Ir-1 and Ir-2 were prepared using a
typical two-step strategy by the cyclometalation of IrCl3Á3H2O
with the BNpppy ligand to initially form the chloride-bridged
Scheme 1 Synthetic routes of Ir(III) complexes.
charge transfer (CT) in the presence of an electron donor and dimers, followed by treatment with 1,10-phenanthroline or
provide the highly luminescent materials.24,25
2,4-pentanedione, respectively, to obtain the two iridium(III)
Herein, we designed and synthesized two novel iridium(III) complexes.26 Finally, Ir-1 and Ir-2 were obtained as red solids.
complexes (Ir-1 and Ir-2) with 4-(5-(4-(dimesitylboranyl)phenyl)- Synthesis of [(BNpppy)2Ir(l-Cl)2Ir(BNpppy)2]. mixture
A
pyridin-2-yl)-N,N-diphenylaniline (BNpppy) as the cyclometa- of IrCl3Á3H2O (70.00 mg, 0.15 mmol), BNpppy (194.00 mg,
lated ligand, which is composed of the triphenylamine group 0.3 mmol), ethoxyethanol (2.4 mL) and H2O (0.8 mL) was placed
and dimesitylboron (Mes2B) unit as the electron donor and in an evacuated round bottom flask and stirred for 24 h at
acceptor, and 1,10-phenanthroline or 2,4-pentanedione (acac) 110 1C under a nitrogen atmosphere. After cooling to room
as the ancillary ligand, respectively, as shown in Scheme 1. The temperature, the solution was treated with a large amount of
results showed that BNpppy exhibits apparent solvatochro- water and ethanol, and then the insoluble was dried to obtain a
mism behavior in different solvents. The incorporation of a reddish-brown solid (230.00 mg, yield: 88%).
D–A (donor–acceptor) type N^C chelate ligand into iridium(III)
metal complexes can facilitate metal-to-ligand charge transfer
Synthesis of [(BNpppy)2Ir(1,10-phenanthroline)]+(PF6À) (Ir-1).
mixture of [(BNpppy)2Ir(m-Cl)2Ir(BNpppy)2] (182.50 mg,
A
(MLCT) transitions, which should be expected to achieve highly 0.06 mmol), 1,10-phenanthroline (29.70 mg, 0.15 mmol),
efficient phosphorescent iridium(III) complexes. Furthermore, methanol (2.0 mL) and dichloromethane (4.0 mL) was placed
solution-processed OLEDs based on iridium(III) complexes in an evacuated round bottom flask and stirred for 4 h at 45 1C
with 2,4-pentanedione as the ancillary ligand also gave a good under a nitrogen atmosphere. Then after cooling to room
performance due to their excellent solubility and solid-state temperature, KPF6 was added in five portions, and continued
stability. In this paper, the synthesis, photophysical properties to stir for 2 h at room temperature. The solution was evaporated
and electroluminescence properties in OLEDs of both the to dryness with a rotary evaporator, the crude material was
iridium(III) complexes were investigated.
purified by column chromatography over silica gel (PE : EA =
15 : 1) to obtain a reddish brown solid (197.00 mg, yield: 93%).
1H NMR (300 MHz, DMSO-D6): d = 8.90 (d, J = 8.0 Hz, 2H), 8.46
(d, J = 4.2 Hz, 2H), 8.33 (s, 2H), 8.21–8.13 (m, 2H), 7.91 (s, 4H),
7.77 (d, J = 8.6 Hz, 2H), 7.30 (d, J = 8.1 Hz, 6H), 7.05 (dd, J = 18.5,
7.6 Hz, 20H), 6.83 (s, 12H), 6.59 (d, J = 9.0 Hz, 2H), 5.93 (s, 2H),
Experimental
General information
4-Bromo-N,N-diphenylaniline, 5-bromo-2-iodopyridine, 1,4-dibromo- 2.26 (s, 12H), 1.91 (s, 24H); 13C NMR(126 MHz, DMSO-D6):
benzene, IrCl3Á3H2O, 1,10-phenanthroline, 2,4-pentanedioneand d = 166.26, 151.97, 149.49, 147.07,146.85, 146.38, 145.52,
and all other chemicals were purchased from Shanghai J&K 144.96, 144.53, 141.46, 140.36, 139.33, 138.87, 138.07, 136.80,
company and used without any further purification. Nuclear 136.19, 136.05, 132.12, 131.60, 129.57, 128.64, 127.55, 126.79,
magnetic resonance (NMR) (300 MHz, 500 MHz) spectra were 126.16, 125.49, 124.48, 122.08, 119.27, 114.09, 23.16, 21.09;
recorded at room temperature on a Bruker Ultra shield Plus 300 IR (KBr) n 3059, 3023, 2954, 2919, 2855, 2729, 1723, 1605,
and 500 Hz instrument, which adopts CDCl3 or (CD3)2SO as 1573, 1574, 1508, 1490, 1475, 1454, 1426, 1374, 1316, 1284,
the solvent, and TMS as the internal standard. Infrared spectra 1236, 1220, 1173, 1152, 1069, 1027, 1014, 1002, 960, 908, 873,
were recorded on a Bruker Tensor 27 spectrometer. Mass 840, 754, 724, 696, 662, 643, 584, 556, 523, 510, 471, 432 cmÀ1
spectra were obtained on a Bruker autoflex MALDI-TOF/TOF MALDITOF-MS m/z: calculated: 1664.723, found: 1663.916;
;
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anal. calcd. for C106H92B2IrN6: C 76.52, H 5.57, N 5.05. Found:
C 76.72, H 5.58, N 5.03.
Synthesis of [(BNpppy)2Ir(acac)] (Ir-2).
A mixture of
[(BNpppy)2Ir(m-Cl)2Ir(BNpppy)2] (577.90 mg, 0.19 mmol),
2,4-pentanedione (50.10 mg, 0.5 mmol), Na2CO3 (219.40 mg,
2.07 mmol) and ethoxyethanol (12.0 mL) was placed in an
evacuated round bottom flask and stirred for 12 h at 85 1C under
a nitrogen atmosphere. After cooling to room temperature, the
solution was washed with methanol and evaporated to dryness
using a rotary evaporator. Then the crude material was purified
by column chromatography over silica gel (PE : DCM = 4 : 1) to
1
obtain a red solid (500.00 mg, yield: 80%). H NMR (500 MHz,
Fig. 1 Normalized emission spectra of BNpppy in various solvents (10À5 M).
(Inset: Photograph of BNpppy in various solvents under a 365 nm UV lamp.)
DMSO-D6): d = 8.53 (s, 2H), 8.03 (d, J = 9.6 Hz, 2H), 7.83 (d, J =
8.6 Hz, 2H), 7.65 (d, J = 8.0 Hz, 4H), 7.58 (d, J = 8.0 Hz, 4H),
7.53 (d, J = 8.5 Hz, 2H), 7.09 (t, J = 7.7 Hz, 8H), 6.89 (s, 20H), 6.32
(d, J = 8.4 Hz, 2H), 5.73 (s, 12H), 5.28 (s, 1H), 2.30 (s, 12H), 2.03 peak with a shoulder in the region of 400–500 nm can be assigned
(s, 24H), 1.76 (s, 6H). 13C NMR (126 MHz, CDCl3): d = 184.72, to the mixture of metal-to-ligand charge transfer (1MLCT, 3MLCT)
166.85, 148.52, 147.73, 147.25, 145.77, 141.68, 140.84, 139.81, and intramolecular charge transfer (ICT) transitions.28 The
138.78, 137.77, 137.20, 134.32, 131.93, 128.77, 128.26, 125.92, absorption bands of Ir-1 with 1,10-phenanthroline as the ancillary
125.64, 125.41, 124.56, 122.87, 117.47, 113.97, 28.74, 23.49, ligand have a noticeable difference compared with those of Ir-2
21.21. IR (KBr) n 3022, 2952, 2916, 2854, 2729, 1597, 1574, that has the ancillary ligand acetylacetonate (acac) (Fig. 2). It
1546, 1510, 1490, 1476, 1449, 1401, 1375, 1313, 1291, 1273, suggests that the ancillary ligand variation affects their excited
1257, 1238, 1218, 1173, 1152, 1138, 1065, 1027, 1015, 958, 872, state energy. The emission spectra of Ir-1 and Ir-2 exhibit intense
846, 823, 804, 752, 724, 695, 668, 635, 585, 574, 560, 511, 470, red phosphorescence at 625 nm and 601 nm in CH2Cl2 at room
420 cmÀ1; MALDITOF-MS m/z: calculated: 1583.699, found: temperature, respectively. To understand these differences
1583.342; anal. calcd. for C99H91B2IrN4O2: C 75.13, H 5.80, in luminescence properties, the theoretical calculations were
N 3.54. Found: C 74.91, H 5.82, N 3.55.
carried out and the frontier orbital distribution was obtained.
As shown in Fig. 3, the lowest unoccupied molecular orbitals
(LUMOs) of Ir-1 and Ir-2 were primarily dominated by BNpppy
ligands. The highest occupied molecular orbitals (HOMOs) of
Ir-1 were mainly located over the auxiliary ligand, but the
HOMOs of Ir-2 were mainly located on the main ligands.
Furthermore, the spectra of the two complexes were almost the
same in both the solution state and the film state, which is
attributed to the steric hindrance of the dimesitylboron (Mes2B)
units suppressing the structural relaxation of the iridium(III)
complexes in the solid state. The photoluminescence (PL) quan-
tum yields (FP) of Ir-1 and Ir-2 were calculated to be 5.8% and
42.9%, respectively, which were measured in degassed CH2Cl2
in a N2 atmosphere using Ir(ppy)3 (FP = 40%) as a standard.29
The phosphorescence lifetimes (t) for both the iridium(III) com-
plexes are in the range of microseconds, with observed times of
0.185 ms and 1.065 ms in CH2Cl2 solution for Ir-1 and Ir-2,
respectively (Table 1 and Fig. S21 in the ESI†). This proves that
Ir-2 exhibits at higher FP (42.9%) and for longer t (1.065 ms), in
OLEDs which indicates their excellent performance.
Results and discussion
The solvatochromism effect of BNpppy
At room temperature, we measured the Uv-Vis absorption
spectrum and the fluorescence emission spectrum of BNpppy,
which was dissolved in CH2Cl2 and the solution concentration
was 10À5 M. As shown in Fig. S1 in the ESI,† its absorption
spectrum shows a sharp peak at 300–400 nm, which is mainly
on account of the p–p* transition and charge transfer transition
in BNpppy. In the emission spectrum, the emission peak
wavelength is located at 522 nm and the solution reveals
a strong yellow-green emission. At the same time, we also
measured the emission spectra of BNpppy in different solvents
(cyclohexane, ethyl acetate, tetrahydrofuran, methylene chloride,
and acetonitrile), and found that the highest emission peak was
located at different wavelengths in different solvents. This
means that BNpppy has a distinct behavior of solvatochromism,
as shown in Fig. 1, which proves the existence of the intra-
molecular charge transfer process.
Electrochemical and thermal properties of Ir-1 and Ir-2
The electrochemical properties of Ir-1 and Ir-2 were investigated
using cyclic voltammetry (CV) using ferrocene as an internal
Photophysical properties of Ir-1 and Ir-2
The photophysical properties of the iridium(III) complexes in standard. Both the complexes exhibit a reversible oxidation
CH2Cl2 and thin films were investigated at 298 K, and the progress in CH2Cl2 (Fig. 4). According to the oxidation poten-
photophysical data are listed in Table 1. In the absorption tials, the HOMOs were calculated to be À5.60 eV (Ir-1) and
spectra, both the complexes show a full vibronic-structured À5.45 eV (Ir-2). Correspondingly, the LUMOs were calculated
absorption band at 250–350 nm, which was assigned to the to be À3.12 eV (Ir-1) and À3.11 eV (Ir-2), according to their
spin-allowed ligand-central p–p* transition.27 The low-lying HOMOs and absorption spectra.30 In Fig. 4, Ir-2 shows a good
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Table 1 Physical properties of Ir-1 and Ir-2
labs (nm)
lem (nm)
Fp (%)
Sol.a
Filmb
Sol.a
Filmb
Sol.a
Filmb
tc (ms)
HOMOd (eV)
LUMOd (eV)
Td (1C)
e
Complex
Ir-1
Ir-2
278, 451
310, 421, 480
325, 450
317, 428, 489
625
601
605
610
5.8
42.9
5.96
5.69
0.185
1.065
À5.60
À5.45
À3.12
À3.11
213
189
a
b
c
d
In dilute DCM (10À5 M). Films at a quartz substrate. Recorded in the degassed DCM solution with a 400 nm light source. Determined by
e
cyclic voltammetry and the optical band gap from the absorption spectra. Decomposition temperature corresponding to 5% weight loss
measured by TGA.
Fig. 4 Cyclic voltammograms in degassed CH2Cl2 (scan rate = 100 mV sÀ1).
Fig. 2 The normalized absorbance and emission spectra: (a) Ir-1 solution
(in CH2Cl2, 10À5 M), (b) Ir-1 thin film, (c) Ir-2 solution (in CH2Cl2, 10À5 M),
and (d) Ir-2 thin film.
OLEDs have been fabricated with the configuration of indium
tin oxide (ITO)/polyethylenimine ethoxylated (PEIE)-modified
zinc oxide (ZnO, around 20 nm)/CBP: x wt% Ir complex (around
57 nm)/bis(dimethyl aniline) phenyl cyclohexane (TAPC,
40 nm)/molybdenum oxide (MoOx, 7 nm)/Al (100 nm), in which
PEIE-modified ZnO and TAPC layers were used as the electron
transport layer and hole transport layer, respectively. The
MoOx/Al bilayers and ITO were selected as the top electrodes
and bottom electrodes in the sequence. Fig. 5 shows the energy
level diagram of the HOMO and LUMO levels for the materials
Fig. 3 The HOMO and LUMO distributions of the Ir-1 and Ir-2 complexes.
redox reversibility, which gives advantages for the complexes
applied in electroluminescent devices. As shown in Fig. S22 in the
ESI,† the thermal gravimetric analysis (TGA) curves show that the
two compounds exhibited good thermal stability. A weight loss of
5% was found at temperatures of 213 1C for Ir-1 and 189 1C for Ir-2.
To conveniently design the structure of the electroluminescent
device, all these data are summarized in Table 1.
Solution-processed OLEDs
To investigate the applicability of these bipolar complexes in
electroluminescent devices, a series of solution-processed
Fig. 5 Energy level diagram of the HOMO and LUMO levels for materials
used in OLEDs and their molecular structures.
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efficiencies of Ir-2 doped devices reach a maximum at a
14% doping-level. Device B5 showed luminance (Lmax) of
7916 cd mÀ2 at 16V, current efficiency (ZL) of 3.99 cd AÀ1, and
external quantum efficiency (EQE) of 2.48% (Fig. 7 and Table S1
in the ESI†). As shown in Fig. 7, the devices A1–A5 for Ir-1
turned on at a voltage of 6 V, while the optimized device A5
(25 wt%) can only emit a maximum luminance of 983 cd mÀ2
at 13 V, and has the highest ZL of 0.50 cd AÀ1, and EQE of 0.28%,
which is much poorer in performance than the highly red
phosphorescent emitter Ir-2. These results can reveal that the
OLED using Ir-2 as the emitter exhibits a much better electro-
luminescence performance than the one using Ir-1 as the emitter,
which can be attributed to the higher quantum efficiency.
Conclusions
Fig. 6 Characteristics of devices (A1–A5): (a) current density–external
quantum efficiency (J–EQE), (b) normalized EL spectra, and (c) voltage–
current density (V–J) and (d) voltage–luminance (V–L) curves.
In summary, we have achieved the design and synthesis of a
novel bipolar ligand and its corresponding complexes Ir-1
and Ir-2. The photophysical and electrochemical properties of
all these compounds were investigated. The solution-processed
OLEDs based on the two iridium(III) complexes were then
fabricated by the optimization of the doping concentrations
at the emissive layer, and gave a good performance based on
the Ir-2 complex at a 14% doping-level with the maximum EQE,
peak luminance, and the highest current efficiency of 2.48%,
7916 cd mÀ2, and 3.99 cd AÀ1, respectively. The good performance
can be attributed to the incorporation of donor and acceptor
functional groups in the cyclometalated ligand, and these results
can provide a deeper insight into the significant improvements in
the OLED performance based on these bipolar ligands.
investigated in this study as well as their molecular structures.
To optimize the electroluminescence efficiency, the emissive
layer was prepared by spin-coating a solution of the com-
plex:CBP blend at different concentrations in chloroform. For
the OLED fabrication, the details of PEIE-modified ZnO spin-
coated onto ITO glass can be found elsewhere.31 The emissive
layer was formed by mixing the Ir-1/Ir-2 with CBP prepared in
chloroform via spin-coating onto the ZnO films and then
annealing at 60 1C for 20 min. Finally, the TAPC layers and
MoOx/Al electrode were fabricated via vacuum evaporation
deposition. The current density–external quantum efficiency
( J–EQE), EL spectra, voltage–current density (V–J), and voltage–
luminance (V–L) curves of each device are showed in Fig. 6
and 7. Furthermore, the critical electroluminescence data are
summarized in Table S1 in the ESI.†
Conflicts of interest
There are no conflicts to declare.
Doping Ir-2 in CBP, with the doping level increasing from
device B1 (4 wt%) to B5 (14 wt%), the electroluminescence
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21504040), the Natural Science Founda-
tion of Jiangsu Higher Education Institutions (15KJB150012),
and the National Key Basic Research Program of China (973
Program, 2015CB932200)
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