Efficient dinuclear Pt(ii) complexes based on the triphenylphosphine oxide scaffold for high performance solution-processed OLEDs

Article abstract of DOI:10.1039/d0tc05965j

Dinuclear Pt(ii) complexes have the potential to achieve high electroluminescence (EL) performance because of the enhanced phosphorescence emission induced by the extra metal center. However, to date, organic light-emitting devices (OLEDs) utilizing dinuc

Full text of DOI:10.1039/d0tc05965j

Volume 9
Number 16
28 April 2021
Pages 5291–5566
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
ISSN 2050-7526
PAPER
Guijiang Zhou, Zhao Chen, Xiaolong Yang et al.
Efficient dinuclear Pt(II) complexes based on the
triphenylphosphine oxide scaffold for high performance
solution-processed OLEDs

Journal of
Materials Chemistry C
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PAPER
Efficient dinuclear Pt(II) complexes based on the
triphenylphosphine oxide scaffold for high
performance solution-processed OLEDs†
Cite this: J. Mater. Chem. C, 2021,
, 5373
9
a
b
c
c
a
Yuanhui Sun, Chen Chen, Bochen Liu, Yue Guo, Zhao Feng,
a
c
a
Guijiang Zhou, * Zhao Chen * and Xiaolong Yang
*
Dinuclear Pt(II) complexes have the potential to achieve high electroluminescence (EL) performance
because of the enhanced phosphorescence emission induced by the extra metal center. However, to
date, organic light-emitting devices (OLEDs) utilizing dinuclear Pt(II) complexes as emitters usually show
low EL performance with external quantum efficiencies (EQE) less than 10%. In this work, with the
triphenylphosphine oxide group as the scaffold core and different N-heterocycles (pyridine, thiazole, and
quinoline) as the end-groups, dinuclear Pt(II) complexes PyPODPt, ThPODPt, and QuPODPt are
synthesized to show bright emissions peaking at 500, 543, and 586 nm in solutions. The
photoluminescence quantum yields are measured to be up to 0.96. More importantly, the solution-
processed orange-red device based on QuPODPt exhibits outstanding EL performance with the EQE
reaching 11.2%, which is among the highest EQEs reported for OLEDs employing dinuclear Pt(II)
complexes. The superior device performance demonstrates the promising potential of
triphenylphosphine oxide-based dinuclear Pt(II) complexes for OLED applications.
Received 21st December 2020,
Accepted 4th February 2021
DOI: 10.1039/d0tc05965j
rsc.li/materials-c
7
–9
form the corresponding dinuclear Pt(II) complex. Because of
the enhanced ISC induced by the second Pt(II) center, dinuclear
Introduction
Organic light-emitting diodes (OLEDs) have great application Pt(II) complexes could show higher photoluminescence yields
potential in the fields of high-resolution displays and solid- (PLQYs) than their corresponding mononuclear Pt(II) complexes,
1
,2
state lighting. The emission color and electroluminescence leading to significantly improved electroluminescence
1
0–12
efficiency of OLEDs greatly depend on the properties of organic efficiencies.
emitters.
However, compared with OLEDs based on other
3
,4
13–15
Due to the efficient singlet–triplet intersystem dinuclear organometallic complexes,
devices using dinuclear
crossing (ISC) promoted by heavy metal atoms, phosphorescent Pt(II) complexes as emitters usually exhibit relatively inferior perfor-
7,16–18
Pt(II) complexes can theoretically realize 100% internal quantum mance with external quantum efficiencies typically below 10%.
efficiency in OLEDs by utilizing all electrically generated In this contribution, we report the synthesis of three dinuc-
5
,6
singlet and triplet excitons. Therefore, the development of lear Pt(II) complexes based on the triphenylphosphine oxide
Pt(II) complexes is highly attractive. To date, the reported Pt(II) scaffold. The triphenylphosphine oxide group was adopted
complexes are usually composed of one metal atom, and thereby because its branched chemical structure is suitable to coordi-
the emission properties of these mononuclear Pt(II) complexes nate with more metal centers. Furthermore, the strong
are determined by organic ligands. Occasionally, to further electron-withdrawing property of the triphenylphosphine oxide
adjust the properties, an extra Pt(II) center is incorporated to group can benefit the electron-injection/transport process in
1
9,20
OLEDs.
The emission colors of the resultant dinuclear Pt(II)
complexes could be tuned from green to orange-red by simply
using different N-heterocycles. Furthermore, these dinuclear
Pt(II) complexes showed impressively high PLQYs up to 0.96.
Most importantly, the solution-processed orange-red OLED
based on QuPODPt achieved the maximum EQE of 11.2%,
which is among the best reported for OLEDs based on dinuclear
Pt(II) complexes. This work sheds light on the potential of
developing efficient triphenylphosphine oxide-based dinuclear
Pt(II) complexes for high performance OLEDs.
a
School of Chemistry, Xi’an Key Laboratory of Sustainable Energy Material
Chemistry, Xi’an Jiaotong University, Xi’an 710049, P. R. China.
E-mail: zhougj@xjtu.edu.cn, xiaolongyang@xjtu.edu.cn
b
Science and Technology on Advanced Ceramic Fibers and Composites Laboratory,
College of Aerospace Science and Engineering, National University of Defense
Technology, Changsha 410073, P. R. China
c
School of Applied Physics and Materials, Wuyi University, Jiangmen 529020
P. R. China. E-mail: chenzhao2006@163.com
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0tc05965j
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Results and discussion
no contribution from metal–metal-to-ligand charge-transfer.
The molar extinction coefficients (e) for the low-energy absorptions
The synthetic routes of the target dinuclear Pt(II) complexes are of these dinuclear Pt(II) complexes were obviously larger than
2
2,23
illustrated in Scheme 1. The synthesis details are provided in those of conversational mononuclear Pt(II) complexes.
the ESI.† The key procedure was the synthesis of triphenylpho- For example, the log e of low-energy absorption for PyPODPt
sphine oxide-based ligands. The dibromo-substituted triphenyl- was 4.05, while that for the corresponding mononuclear Pt(II)
2
3
phosphine oxide moiety was firstly prepared and then reacted complex decreased to 3.55, which was in good accordance
with different N-heterocycles through Suzuki–Miyaura coupling with the conclusion that the incorporation of an extra metal
2
4
to form tetradentate ligands which could coordinate with centre could enhance the low-energy absorption. The larger e
two metal centres. The structures of the target dinuclear Pt(II) might suggest an enhanced MLCT process, which could
1
13
25
complexes were thoroughly characterized by H, C, and increase the phosphorescence emission ability. In addition,
3
1
22,23
P NMR and high resolution mass spectrometry analyses compared to conventional mononuclear Pt(II) complexes,
(see Fig. S1 and S2 in the ESI†).
these dinuclear Pt(II) complexes showed shorter lifetimes (0.52–
The thermal stability of these dinuclear Pt(II) complexes was 0.71 m). The shorter lifetime reflected the faster ISC process,
investigated by thermogravimetric analysis (TGA) under a N which would facilitate the triplet radiation. Therefore,
atmosphere. As revealed by the TGA curves (Fig. S3, ESI†), the PyPODPt, ThPODPt, and QuPODPt displayed impressively high
decomposition temperatures (T ) of PyPODPt, ThPODPt, and photoluminescence quantum yields (PLQYs) of up to 0.96
QuPODPt were ca. 361, 341, and 256 1C (Table 1), respectively, (Table 1), which was among the highest PLQYs reported for
2
d
7
,10,11,16
demonstrating the high thermal stability of these dinuclear dinuclear Pt(II) complexes.
By applying different
Pt(II) complexes. The high thermal stability would avoid the N-heterocycles, these dinuclear Pt(II) complexes exhibited low-
detrimental degradation during device fabrication and operation energy absorption peaks in the order of PyPODPt (378 nm) o
processes.
In CH Cl
these dinuclear Pt(II) complexes showed strong absorption the emission peaks measured in CH
ThPODPt (384 nm) o QuPODPt (420 nm), suggesting the
ꢁ5
ꢁ1
2
2
solution (3 ꢀ 10 mol L ) at room temperature, gradually narrowed optical energy bandgaps (Egap). Consequently,
2
Cl at room temperature
2
bands in the region of 250–350 nm (Fig. 1b and Table 1), which were significantly red-shifted from 500 nm (PyPODPt) to 543 nm
were attributed to the spin-allowed singlet p–p* transitions of (ThPODPt) and 586 nm (QuPODPt), demonstrating the simplicity
the metal perturbed ligands. In addition to the strong high of largely adjusting the emission colors of dinuclear Pt(II)
energy absorptions, weak absorptions were observed in the complexes based on the triphenylphosphine oxide scaffold.
0
range from 370 to 470 nm, which could be assigned to the In 1 wt% doped 4,4 -bis(carbazol-9-yl)biphenyl (CBP) films,
charge transfer processes, including the intraligand charge PyPODPt and ThPODPt showed slightly red-shifted emissions,
transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and while QuPODPt displayed the blue-shifted emission (Fig. 1c),
2
1
metal-to-ligand charge transfer (MLCT) transitions. Theoretical indicating the relatively severe molecular aggregation behaviours
calculations revealed that the distances between the two Pt of PyPODPt and ThPODPt than QuPODPt even in such a low
26
centers in PyPODPt, ThPODPt, and QuPODPt were 10.668, doping level. The theoretical calculations revealed remarkably
0.686, and 10.686 Å, respectively, which were too far to form large dipole moments (m) of these dinuclear Pt(II) complexes
1
substantial Pt–Pt interactions. Therefore, the absorptions have (Table 1). With a larger m, an emitter tends to aggregate, which
Scheme 1 Synthetic routes of the dinuclear Pt(II) complexes.
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Table 1 Photophysical, thermal, and energy level data for PyPODPt, ThPODPt, and QuPODPt
a
a
b
c
d
e
l
abs (nm)
l
em (nm)
l
em (nm)
PLQY
T
d
(1C)
HOMO/LUMO (eV)
m (D)
PyPODPt
ThPODPt
QuPODPt
258 (4.80), 282 (4.74), 378 (4.05)
266 (4.47), 312 (4.39), 384 (3.95)
260 (4.51), 296 (4.51), 356 (4.13), 420 (3.82)
500 (0.52 ms)
543 (0.62 ms)
586 (0.71 ms)
502 (1.44 ms)
545 (1.22 ms)
580 (2.37 ms)
0.96/0.36
0.49/0.17
0.79/0.62
361
341
256
ꢁ5.56/ꢁ2.70
ꢁ5.67/ꢁ2.88
ꢁ5.59/ꢁ2.95
6.4
5.7
5.7
a
ꢁ5
ꢁ1
b
Measured in CH
2
Cl
Cl
Theoretically calculated dipole moment at the ground state.
2
(3 ꢀ 10 mol L ), log e values and the corresponding lifetimes are shown in parentheses. Measured in doped film.
c
d
Measured in CH
2
2
/1 wt% doped CBP film. Calculated according to the equations: EHOMO = ꢁ(Eox + 4.8) eV and ELUMO = EHOMO + Egap eV.
e
Fig. 1 (a) UV-vis absorption spectra in solutions. PL in (b) solutions and (c) doped films.
27
usually leads to concentration quenching. Therefore, these and ELUMO as well as the excitation energies were in good
dinuclear Pt(II) complexes showed reduced PLQYs in doped films. agreement with the experimental results. The theoretical
However, due to the larger molecular structure which could results exposed that S0 - S1 and S0 - T1 excitations were
suppress the aggregation to some extent, the QuPODPt doped mainly contributed by the HOMOꢁ1 - LUMO, HOMO -
film still showed an impressively high PLQY of 0.62, demonstrating LUMO, and HOMO
- LUMO+1 transitions (Table 2).
its great potential in fabricating efficient solution-processed The calculated UV-vis absorption spectra of these dinuclear
OLEDs.
The electrochemical properties were investigated in CH
Pt(II) complexes showed good agreement with the experimental
results as well (Fig. S4, ESI†), which indicated the mixed p–p*
2
Cl
2
solution using the cyclic voltammetry (CV) method. According (ca. 35–58%), ILCT (ca. 10–14%), LLCT (ca. 24–26%), and MLCT
to CV measurements, PyPODPt, ThPODPt, and QuPODPt (ca. 24–25%) features of the low-energy absorptions. For example,
showed the oxidation potential (E ) values of 0.76, 0.87, and the major assignment for the S - S excitation was HOMO -
ox
0
1
0.79 V, respectively. Thus, the highest occupied molecular LUMO+1 for PyPODPt. As shown in Fig. 2, the HOMO of PyPODPt
orbital (HOMO) levels (EHOMO) of PyPODPt, ThPODPt, and was mainly localized at the left arm (LA) with significant
QuPODPt were calculated to be ꢁ5.56, ꢁ5.67, and ꢁ5.59 eV,
respectively, according to the equation: EHOMO = ꢁ(Eox + 4.8) eV.
During the cathodic scanning, no obvious reduction waves were
recorded. Therefore, the lowest unoccupied molecular orbital
(LUMO) levels (ELUMO) were determined from EHOMO and
optical band gaps (Egap) according to the equation: ELUMO
=
2
8
EHOMO + Egap eV. The calculated ELUMO values for PyPODPt,
ThPODPt, and QuPODPt were ꢁ2.70, ꢁ2.88, and ꢁ2.95 eV,
respectively. Compared with the LUMO levels of other dinuclear
1
0,12,24,29
Pt(II) complexes showing similar emission colors,
the
deeper LUMO levels of PyPODPt, ThPODPt, and QuPODPt
would promote the electron injection process to benefit the
OLED performance.
Density functional theory (DFT) and time-dependent DFT
(TD-DFT) calculations were carried out to gain insights into the
aforementioned properties. The electron density distributions
of key molecular orbitals are depicted in Fig. 2. The calculated
energies and the corresponding main assignments of each
Fig. 2 Key molecular orbital distributions of PyPODPt, ThPODPt, and
transition are summarized in Table 2. The calculated EHOMO QuPODPt.
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Table 2 TD-DFT results for PyPODPt, ThPODPt, and QuPODPt based on optimized S
0
geometries
Main assignment for
0 1
Main assignment for S - T
a
a
a
MOs Contribution from Pt and ligands to MOs (%)
0 1
Energy level (eV) S - S excitation/Ecal /f excitation/Ecal
PyPODPt
ThPODPt
QuPODPt
Pt
L-PyPO
92.64
93.46
40.66
45.56
27.45
25.22
L-ThPO
93.67
94.39
38.92
54.35
32.92
29.45
L-QuPO
94.72
95.04
39.49
46.02
35.33
33.16
acac
1.30
1.20
27.04
21.78
35.87
38.86
acac
0.92
0.87
29.62
22.38
31.47
35.97
acac
0.85
0.84
28.77
21.87
27.81
30.70
L+1 6.06
ꢁ1.70
ꢁ1.80
ꢁ5.45
ꢁ5.54
ꢁ5.77
ꢁ5.88
H - L+1 (49.3%),
H - L (28.0%),
Hꢁ2 - L+1 (10.5%)
/3.05 eV
Hꢁ1 - L (38.6%),
Hꢁ3 - L (13.0%),
H - L (11.0%),
Hꢁ1 - L+1 (9.3%)
/2.58 eV
a
L
5.34
32.30
Hꢁ1 32.66
Hꢁ2 36.68
Hꢁ3 35.92
Pt
a
H
/0.0380
L+1 5.41
ꢁ1.87
ꢁ1.99
ꢁ5.57
ꢁ5.65
ꢁ5.77
ꢁ5.87
H - L (36.0%),
H - L+1 (30.8%),
Hꢁ2 - L+1 (12.3%),
Hꢁ2 - L (8.0%)
/2.99 eV
Hꢁ1 - L (27.4%),
Hꢁ3 - L (17.4%),
Hꢁ3 - L+1 (10.1%),
H - L (9.6%)
L
H
4.74
31.46
Hꢁ1 31.97
Hꢁ2 35.61
Hꢁ3 34.58
Pt
/2.39 eV
/0.0669
L+1 4.43
ꢁ2.06
ꢁ2.14
ꢁ5.49
ꢁ5.58
ꢁ5.72
ꢁ5.83
H - L+1 (50.8%),
H - L (23.7%),
Hꢁ2 - L+1 (12.5%)
/2.79 eV
Hꢁ2 - L+1 (29.6%),
Hꢁ2 - L (11.4%),
Hꢁ3 - L (9.8%),
Hꢁ1 - L (9.5%)
/2.33 eV
L
H
4.12
31.74
Hꢁ1 32.11
Hꢁ2 36.86
Hꢁ3 36.14
/0.0647
a
H and L stand for the HOMO and LUMO, respectively. Ecal and f are the calculated excitation energy and oscillator strength, respectively.
contribution from the Pt centre (ca. 32.30%), the phenyl ring
The phosphorescence emission properties were investigated
ca. 33.94%) chelated to the Pt centre, and the acetylacetone by analyzing the natural transition orbitals (NTO) based on the
acac) ligand (ca. 27.04%), while the LUMO+1 was mainly optimized T geometries. As shown in Fig. 3, although
(
(
1
contributed from the L-PyPO ligand with ca. 71.71% electron ThPODPt possessed two pairs of degenerate hole - particle
density located on the left arm and ca. 18.93% electron density (H - P) transitions, the T states of all three complexes were
1
located on the right arm (RA). Therefore, the HOMO - substantially (around 99%) contributed by the H - P transitions.
LUMO+1 excitation consisted of p–p* (p orbital of the phenyl The hole orbitals were mainly concentrated on the Pt center
ring chelated to the Pt centre - p* orbital of the same phenyl (9–22%) and the segment (70–80%) chelated with this Pt center,
ring), ILCT (the phenyl ring chelated to the Pt centre - pyridyl while the particle orbitals were predominantly (92–94%) con-
rings), LLCT (acac - L-PyPO), and MLCT (the Pt centre - tributed by the segment chelated with this Pt center. Because
0 1 0 1
L-PyPO) transitions. Clearly, the S - S and S - T excitations the organic ligands made a high contribution to both hole and
of these dinuclear Pt(II) complexes all possessed similar properties. particle orbitals (Fig. 3 and Table S1, ESI†), the efficient
Different from the previous situations in which acac usually phosphorescence emissions mainly originated from the triplet
contributed very little or negligibly to the transitions of Pt(II) p–p* transitions mixed with notable MLCT characters.
10,18,23,30
complexes,
dinuclear Pt(II) complexes obviously participated in the transitions, broadened and structured emission spectra (Fig. 1).
indicating that the properties of these dinuclear Pt(II) complexes The EL properties were investigated by fabricating solution-
could be further adjusted by substituting or even changing the processed OLEDs with the simple configuration of ITO/
it is worth mentioning that acac in these Therefore, PyPODPt, ThPODPt, and QuPODPt displayed
2
2
acac ligands.
PEDOT:PSS (30 nm)/PVK (25 nm)/EML (20 nm)/TmPyPB
Fig. 3 NTO investigation of PyPODPt, ThPODPt, and QuPODPt.
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Fig. 4 (a) Device structure and energy level alignment. (b) EL spectra. (c) Curves of EQE vs current density. (d) Curves of CE and PE vs current density.
(
40 nm)/LiF (1 nm)/Al (100 nm) (Fig. 4a). Polymers PEDOT:PSS ThPODPt, device B displayed inferior performance with EQE, CE,
ꢁ
1
ꢁ1
[
[
poly(ethylenedioxythiophene):poly(styrenesulfonate)] and PVK and PE of 4.52%, 10.3 cd A , and 6.20 lm W , respectively.
poly(9-vinylcarbazole)] were used as hole-injection and transport Their relatively low efficiencies were related to the low PLQY of
materials, respectively. The emissive layer (EML) was prepared the ThPODPt doped film. Furthermore, the inefficient energy
by doping the dinuclear Pt(II) complex into CBP at a very low transfer from the host to ThPODPt, supported by the relatively
concentration of 1 wt%. Due to its relatively high electron strong emission from CBP in device B (Fig. 4b), was also
mobility and significantly deep HOMO level, TmPyPB [1,3,5- responsible for the unsatisfactory performance of device B.
tri(m-pyridin-3-ylphenyl)benzene] was chosen to act as both Fortunately, device C achieved the highest EQE, CE, and PE of
ꢁ
1
ꢁ1
electron transport and hole blocking layers at the same time. up to 11.2%, 21.3 cd A , and 11.7 lm W , respectively (Fig. 4c
The device performance is presented in Fig. 4, and the key EL and d). To the best of our knowledge, these are among the
data are summarized in Table 3. These devices showed typical highest efficiencies reported for OLEDs based on dinuclear Pt(II)
7
,10–12,16,18,35
turn-on voltages in the range of 5.0–5.4 V for solution-processed complexes (Table S2, ESI†).
OLEDs.
In addition to the high
As shown in Fig. 4b, the EL spectra of these devices PLQY of the QuPODPt doped film, the excellent EL performance
were similar to the PL spectra of doped films, except that weak of device C was also inseparable from the relatively more
3
1–33
3
4
emissions around 400 nm from the CBP host were observed.
efficient energy transfer, as supported by the weakest emission
Device A based on PyPODPt showed the peak EQE, current from the CBP host among these devices. It is also worth
ꢁ
ꢁ1
1
efficiency (CE), and power efficiency (PE) of 6.85%, 21.3 cd A
,
,
,
mentioning that device C showed orange-red emission with a
remarkably large full width at half maximum over 105 nm, which
indicated the great potential of QuPODPt in fabricating OLEDs
ꢁ1
and 11.7 lm W , respectively. At a luminance of 100 cd m
this device still gave decent efficiencies of 6.82%, 21.1 cd A
ꢁ1
ꢁ
1
and 11.0 lm W , indicating a low efficiency roll-off. Based on that can emit high-quality white light.
Table 3 Key EL performance of OLEDs based on PyPODPt, ThPODPt,
and QuPODPt
Conclusion
a
b
b
ꢁ1
b
ꢁ1
To conclude, three efficient dinuclear Pt(II) complexes were
developed based on the triphenylphosphine oxide group with
different N-heterocycles. These thermally stable complexes
showed green to orange-red emissions with impressively high
PLQYs among all the reported dinuclear Pt(II) complexes to
date. Accordingly, even at a low doping level of 1.0 wt%,
Devices lEL (nm) Vturn-on (V) EQE (%) CE (cd A ) PE (lm W
)
A
B
C
502, 538 5.1
546, 590 5.0
582, 608 5.4
6.85/6.82 21.3/21.1
4.52/3.19 10.3/6.85
11.2/8.80 21.7/16.8
11.7/11.0
6.20/3.26
11.8/8.28
a
ꢁ2
b
Driving voltage at ca. 1.0 cd m
peak value/at a luminance of 100 cd m
.
Efficiencies in the order of the
2
ꢁ
.
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solution-processed OLEDs based on these complexes could 12 Z. Hao, K. Zhang, K. Chen, P. Wang, Z. Lu, W. Zhu and
show high EL efficiencies. Particularly, the orange-red emitting
Y. Liu, Dalton Trans., 2020, 49, 8722–8733.
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ꢁ1
ꢁ1
PE of 11.2%, 21.3 cd A , and 11.7 lm W , respectively, which
is among the most efficient OLEDs based on dinuclear Pt(II)
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There are no conflicts to declare.
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