Strongly phosphorescent platinum(II) complexes supported by tetradentate benzazole-containing ligands

Article abstract of DOI:10.1039/c5tc01449b

A series of strongly phosphorescent Pt(ii) complexes (Pt1-Pt3) supported by rigid benzazole-containing tetradentate ligands (L1-L3) were designed and synthesized. The effect of heteroatoms (S, O and N) on the photophysical and electroluminescence properties of Pt(ii) complexes was studied systematically. Complex Pt1 based on the benzothiazole-containing ligand exhibited a red-shift in UV-vis and photoluminescence spectra in comparison with Pt2 and Pt3 due to the low electro-negativity and high polarizability of the S atom. Organic light-emitting diodes (OLEDs) with various doping levels of these Pt(ii) phosphors were fabricated. At low complex concentration, Pt1, Pt2 and Pt3 mainly showed high energy monomer emission. When the doping ratio was increased, low energy excimer emission was observed for both Pt1 and Pt2. On the contrary, Pt3 dominantly showed monomer emission even at a high doping level because the strong intermolecular interactions were greatly suppressed due to the presence of bulky alkyl chains as revealed by X-ray crystallographic analysis. OLEDs based on these complexes exhibited yellowish green to greenish yellow electro-phosphorescence with high efficiency. Notably, the device based on Pt3 at the doping level of 10 wt% achieved a maximum efficiency of 75.0 cd A^-1, 70.1 lm W^-1 and 21.4% with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.61).

Full text of DOI:10.1039/c5tc01449b

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Strongly phosphorescent platinum(II) complexes
supported by tetradentate benzazole-containing
ligands†
Cite this:DOI: 10.1039/c5tc01449b
Bin Wang,‡ Feng Liang,‡ Hailiang Hu, Yuan Liu, Zhenhui Kang, Liang-Sheng Liao
and Jian Fan*
A series of strongly phosphorescent Pt(II) complexes (Pt1–Pt3) supported by rigid benzazole-containing
tetradentate ligands (L1–L3) were designed and synthesized. The effect of heteroatoms (S, O and N) on the
photophysical and electroluminescence properties of Pt(II) complexes was studied systematically. Complex
Pt1 based on the benzothiazole-containing ligand exhibited a red-shift in UV-vis and photoluminescence
spectra in comparison with Pt2 and Pt3 due to the low electro-negativity and high polarizability of the S
atom. Organic light-emitting diodes (OLEDs) with various doping levels of these Pt(II) phosphors were
fabricated. At low complex concentration, Pt1, Pt2 and Pt3 mainly showed high energy monomer
emission. When the doping ratio was increased, low energy excimer emission was observed for both Pt1
and Pt2. On the contrary, Pt3 dominantly showed monomer emission even at a high doping level
because the strong intermolecular interactions were greatly suppressed due to the presence of bulky
alkyl chains as revealed by X-ray crystallographic analysis. OLEDs based on these complexes exhibited
yellowish green to greenish yellow electro-phosphorescence with high efficiency. Notably, the device
based on Pt3 at the doping level of 10 wt% achieved a maximum efficiency of 75.0 cd Aꢀ1, 70.1 lm Wꢀ1
and 21.4% with Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.61).
Received 21st May 2015,
Accepted 7th July 2015
DOI: 10.1039/c5tc01449b
www.rsc.org/MaterialsC
suppressing the vibration and rotation around metal ions, thus
leading to high luminescence efficiency.3 Meanwhile the chelated
Introduction
Organic light-emitting diodes (OLEDs) have attracted great Pt(II) complexes showed very high thermal stability. By employing
attention due to their applications in energy efficient displays, rigid tetradentate Pt(II) complexes, Li et al. demonstrated efficient
solid state lighting, and various electronic devices and accessories. pure blue OLEDs with an external quantum efficiency (EQE)
Phosphorescent Ir(III) and Pt(II) complexes have been widely of 22.7% at 100 cd mꢀ2 and Commission Internationale de
studied to achieve high efficiency OLEDs since their electro- L’Eclairage (CIE) coordinates of (0.148, 0.079).4 By using struc-
luminescence can be generated from both singlet and triplet turally robust phosphorescent [Pt(O4N4C4N)] emitters, Che and
excited states, and thus they can harvest 100% of electro- coworkers achieved a high power efficiency of up to 126 lm Wꢀ1
generated excitons.1 During the last decade Ir(III) complexes and an EQE over 20%.5
have been the mainstream light emitters in OLEDs.2 Recently
By now many platinum(II) complexes with symmetric/
platinum(II) complexes bearing rigid tetradentate ligands have asymmetric tetradentate ligands have been reported with their
emerged as an important class of phosphorescent materials emission energy ranging from blue to red.3–6 Generally the Pt(II)
because their rigid structure can reduce the probability of non- ion adopts a square planar coordination geometry, which is
emissive decay from ligand-centered excited states (3p–p*) and favorable for the mononuclear Pt(II) complex to form a tight
metal-to-ligand charge-transfer (MLCT) excited states through packing pattern via strong intermolecular interaction. The
electroluminescence properties of a Pt(II) complex could be
tuned significantly after aggregation of the planar molecules.
For example, a tetradentate Pt(II) complex Pt2O2 was used as a
single emissive material to fabricate efficient and stable excimer-
based white OLEDs (WOLEDs).7 Our interest in this area is to
develop robust phosphorescent Pt(II) complexes supported by
benzazole-containing ligands. Benzazoles are benzothiazole/
benzoxazole/benzimidazole-based derivatives (Scheme 1), a novel
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM), and Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University, Suzhou,
Jiangsu 215123, China. E-mail: jianfan@suda.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1061613. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5tc01449b
‡ These two authors contribute equally to this paper.
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stability of resultant Pt(II) complexes (Pt1–Pt3, Scheme 2), and
further to study the effect of heteroatoms on photolumines-
cence/electroluminescence properties of this series of Pt(II)
complexes systematically. OLEDs based on these Pt complexes
exhibited excellent device performance. Particularly, Pt3 achieved
a maximum efficiency of 75.0 cd Aꢀ1, 70.1 lm Wꢀ1 and 21.4% with
CIE coordinates of (0.33, 0.61).
Scheme 1 Molecular structures of benzazoles (left) and benzazole-
containing ligands L1–L3 (right).
Results and discussion
Preparation and characterization
class of N,O-chelate ligands, which could form sublimable Scheme 1 outlines the synthetic routes of tetradentate ligands L1–L3
luminescent materials as dopants in OLEDs.8 It was reported and Pt complexes Pt1–Pt3. L1, L2 and L3 were synthesized by Suzuki
that hetero-atoms of benzazole ligands played an important role coupling of 2-(3-(tert-butyl)-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
in tuning the luminescence properties of metal complexes.8a,b So 2-yl)phenyl)pyridine with 2-(4-bromo-benzothiazol-2-yl)phenol,10
far benzazole-based Ir(III) and Pt(II) complexes have been rarely 2-(4-bromo-benzoxazol-2-yl)-phenol11 and 2-[4-bromo-1-(2-ethylhexyl)-
reported,9 where benzazole derivatives serve merely as bidentate 1H-benzoimidazol-2-yl]phenol, respectively. Initially ligand L3
[N4O] ligands. On the other hand, the bidentate benzazole was designed and synthesized without the 2-ethylhexyl group
ligands were labile and could be detached from the metal attached to the N atom. However the desired compound showed
complex upon treatment with acid9c and addition of other metal very poor solubility in common organic solvents, which renders
ions.9e In this paper, we incorporated a benzazole motif into it intractable to purification. The target Pt(II) complexes Pt1,
tetradentate ligands L1, L2 and L3 (Scheme 1) to enhance the Pt2 and Pt3 were synthesized via one-pot reaction of K2PtCl4
Scheme 2 The synthetic routes to Pt(II) complexes Pt1, Pt2 and Pt3.
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and tetradentate ligands L1, L2 and L3, respectively. All
platinum(II) complexes were purified by column chromatography
over silica gel and then subjected to sublimation (the subli-
mation temperature: o330 1C at 1 ꢁ 10ꢀ4 Pa and the yield of
sublimation: 490% for Pt1–Pt3) before they are subject to device
1
fabrication. They were fully characterized by H and 13C NMR,
MALDI-TOF mass spectrometry, and elemental analysis.
The thermal stability of Pt1, Pt2 and Pt3 was determined by
thermogravimetric analysis under N2. These Pt(II) complexes
exhibited excellent thermal stability with 5% weight-reduction
temperatures (DT5%) of 453 1C for Pt1, 420 1C for Pt2, and
407 1C for Pt3, which allows device fabrication by a vacuum
deposition method.
The electrochemical properties of these complexes were
examined by cyclic voltammetry. All these Pt(II) complexes
showed an irreversible oxidation process, which is typical for
tetradentate Pt(II) complexes.12 The HOMO values of Pt(II)
complexes were calculated from the oxidization potential onset,
and LUMO values were determined by the difference between
the HOMO and the corresponding optical band gap (Table 1).
The details of the compound’s synthesis, 1H and 13C NMR
spectra of ligands and complexes (Fig. S1–S12, ESI†), and thermo-
gravimetric analysis (TGA) curves (Fig. S13–S15) are given in the ESI.†
Fig. 1 Optical absorption and photoluminescence spectra of complexes
Pt1, Pt2 and Pt3 in dilute dichloromethane solution at room temperature.
peak at 499 nm, which was blue-shifted by 24 nm relative to
benzothiazole-based analogue Pt1. A red-shift by 8 nm was
observed for complex Pt3 relative to Pt2. Since the backbones of
Pt1–Pt3 are of very similar structural features, it can be deduced
that the heteroatoms (S, O, and N) in tetradentate ligands
L1–L3 influenced the electronic structures and photophysical
properties of the related Pt(II) complexes.
Photophysical properties
DFT calculations
The UV-vis and photoluminescence (PL) spectra of Pt1, Pt2 and
Pt3 are shown in Fig. 1. All these compounds displayed strong In order to understand the photophysical properties of the
absorption below 300 nm, which was assigned to spin-allowed emitters, density functional theory (DFT) calculations were
1p–p* transition of the cyclometalating ligands. Pt2 and Pt3 performed for complexes Pt1, Pt2, and Pt3 using Gaussian
exhibited moderate intense absorption bands in the 350–450 nm 09.15 The calculations were carried out using a B3LYP method
region, while Pt1 showed an absorption cut-off up to 470 nm. with a basis set of 6-31G(d) for C, H, O, N and S atoms and
Thus there is a red-shift of about 20 nm in the absorption a LANL2DZ basis set for a Pt atom. The rigid backbones in
spectrum of Pt1 relative to those of Pt2 and Pt3, which could Pt1–Pt3 are essentially coplanar, which allows the formation of
be ascribed to low electro-negativity and high polarizability of an excimer in the solid state.
the S atom in Pt1. A similar red-shift has been observed in the
The orbital densities of the highest occupied molecular
absorption spectrum of benzothiazole-based Ir(III) complexes in orbitals (HOMOs) and lowest unoccupied molecular orbitals
comparison with that of benzoxazole-based analogues.9e The (LUMOs) are shown in Fig. S16 (ESI†). The HOMOs of Pt1–Pt3
absorption bands with the wavelength over 400 nm of Pt1–Pt3 are in the range from ꢀ5.05 to ꢀ4.82 eV, and LUMOs are in the
could be attributed to the metal-to-ligand charge-transfer and range from ꢀ1.52 to ꢀ1.70 eV. The similar composition and
spin–orbit coupling promoted 3p–p* states.13,14
bonding pattern in these complexes help us to explain the small
Upon photoexcitation at 420 nm, Pt1, Pt2 and Pt3 showed range of variation within the HOMO and LUMO energy levels. As
intense emission with vibronic progressions of 1199 cmꢀ1
estimated from the composition of the frontier orbitals, platinum
,
1208 cmꢀ1 and 1171 cmꢀ1, respectively. Considering that there made about 12–15% contribution to the HOMOs of Pt1–Pt3. The
is a red-shift in the absorption spectrum of Pt1, it is not surprising HOMOs of all these platinum(II) complexes primarily resided
that Pt1 showed a red-shift in phosphorescence in comparison on the benzazole part, and obviously the major contributor was
with Pt2 and Pt3. For example, the phosphorescence of Pt2 has a the phenoxide group. In Pt1 the sulfur atom makes 3.13%
Table 1 Photophysical and electrochemical data obtained experimentally for the platinum(II) complexes
HOMOc [eV]
LUMOd [eV]
HOMOe [eV]
Eg [eV]
a
a
b
f
Compound
labs [nm]
lem [nm]
Fp
Pt1
Pt2
Pt3
253, 297, 391, 428, 445
247, 286, 370, 401, 421
248, 289, 334, 372, 411
523, 558sh
499, 531sh
507, 539sh
0.26
0.25
0.41
ꢀ4.89
ꢀ4.83
ꢀ5.07
ꢀ2.26
ꢀ2.02
ꢀ2.35
ꢀ5.12
ꢀ5.08
ꢀ5.32
2.63
2.81
2.72
a
b
Measured in CH2Cl2 at 293 K with a concentration of 10ꢀ5 M, sh = shoulder. Measured in degassed CH2Cl2 relative to fac-[Ir(ppy)3] (Fp = 0.4).
c
d
e
Estimated from onset potentials using FeCp02/+ values of 4.8 eV below the vacuum level. LUMO = HOMO + Eg. HOMO values were calculated
f
from UPS. The optical band gap was estimated from the absorption spectra.
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contribution to the HOMO, higher than oxygen (1.10%) in Pt2 sandwiched structure. 1,4,5,8,9,11-Hexaazatriphenylene-hexacarbo-
and nitrogen (2.07%) in Pt3, which could be correlated with the nitrile (HAT-CN) and 8-hydroxyquinolinolato lithium (Liq) were
high polarizability of the S atom. The LUMOs were mainly used as hole and electron injection layers, respectively.
localized in the pyridine group and the phenyl group that was 1,1-Bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC) and
connected to the pyridyl ring. Due to the presence of doubly 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were utilized as
bonded N atoms in the ring structure, the pyridine group made hole and electron transport layers, respectively. 4,40,400-Tris(carbazol-
a significant contribution to the LUMOs of Pt1–Pt3.
9-yl)-triphenylamine (TCTA) was applied as the host material. The
device configuration was ITO/HAT-CN (10 nm)/TAPC (40 nm)/
TCTA:dopant (x wt%, 15 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al
(120 nm) (dopant = Pt1, Pt2, and Pt3; x = 1, 3, 5, 10, and 15). The
device data are summarized in Table 2.
X-ray crystal structure
A single crystal of Pt3 was obtained by the slow evaporation
method (Table S1, ESI†). The Pt atom in Pt3 took a typical square
planar coordination geometry with metal–ligand bond lengths in
the range of 1.96–2.06 Å and the bond angles around the Pt atom
in the range of 81.3–95.01 (Fig. 2). Within Pt3, the pyridyl group,
the benzimidazole motif and the benzene ring that contained C19
were essentially coplanar (this plane was defined as plane A). The
phenoxide group was tilted by 21.11 with respect to plane A. The
benzene ring of the phenoxide group and the bulky alkyl chain,
2-ethylhexyl, were located at each side of plane A, leading to the
loose packing of these mononuclear motifs. In the solid state,
Pt3 molecules appeared in pairs with an intermolecular pꢂꢂꢂp
distance of about 3.6 Å and a PtꢂꢂꢂPt distance of about 6.99 Å,
which excluded any strong intermolecular PtꢂꢂꢂPt interaction.
Fig. S17 (ESI†) shows the EL spectra of complex Pt1 at
various doping levels. At the doping level of 1 wt%, Pt1 emitted
yellowish green light with a peak wavelength at 524 nm and a
shoulder at 560 nm. Obviously, the EL spectra of Pt1 were
dependent on the doping ratios. As the doping ratios of Pt1
were increased from 1 wt% to 15 wt%, a new emission band
with the wavelength 4600 nm appeared gradually and the
CIE coordinates were shifted from (0.34, 0.62) to (0.44, 0.54).
Thus at the high doping level Pt1 showed the excimer-based
emission, which was typically red shifted and broader than the
monomer-based emission.6a,g Excimer based emission provides
a powerful approach to achieve multiple-wavelength emission
and white OLEDs with a single emissive material.7,16–18 Pt2 also
showed excimer-based emission at a high doping level. Pt2
emitted yellowish green light with a peak wavelength at 504 nm
and a shoulder at 536 nm with CIE coordinates of (0.28, 0.60) at
the doping level of 1 wt%, and emitted greenish yellow light
with CIE coordinates of (0.45, 0.51) at the doping level of
15 wt% (Fig. S20, ESI†). The EL spectra of Pt3 with different
doping ratios in TCTA are shown in Fig. 3. Pt3, unlike Pt1 and
Pt2, didn’t develop any new emission band and the CIE
coordinates were shifted slightly as the doping content was
increased, which implied that the excimer-based emission was
somehow suppressed greatly. Miskowski et al. suggested that
strong Ptꢂ ꢂ ꢂPt interaction and/or pꢂ ꢂ ꢂp interaction with an inter-
planar distance o3.5 Å would give rise to the low energy
excimer emission.19 As revealed by X-ray crystallographic analysis,
the introduction of the 2-ethylhexyl group into Pt3 eliminated the
intimate metalꢂꢂꢂmetal interaction and strong pꢂꢂꢂp interaction
within the dimeric structure, thus leading to the observation of
the lack of excimer-based emission even at a high doping ratio. As
expected from the observation of a red-shift in phosphorescence
of Pt1, at the same doping level the CIE coordinates of Pt1 were
red shifted in comparison with those of Pt2 and Pt3 (Table 2). So
heteroatoms (S, N and O) also influenced the electroluminescence
properties of this series of Pt complexes.
Electroluminescence properties
To investigate the electroluminescence properties of Pt1–Pt3,
OLEDs based on these dopants were fabricated with a normal
Highly electroluminescent OLEDs based on Pt1–Pt3 have
been achieved, and the turn-on voltages for these devices were
found to be below 3.5 ꢀV1. For Pt1 and Pt2, the maximum device
efficiencies (73.5 cd A , 66.8 lm Wꢀ1 and 19.8% for Pt1, and
62.0 cd Aꢀ1, 56.2 lm Wꢀ1 and 19.0% for Pt2) were obtained at
the same doping level (5 wt%). For Pt3, the best device
performance data were achieved at the doping level of 10 wt%
with a peak Zc of 75.0 cd Aꢀ1, Zp of 70.1 lm Wꢀ1 and ZEQE of 21.4%.
These efficiencies were among the highest for green/yellow
Fig. 2 Perspective view (top) and a dimeric structure (bottom) of Pt3
(all hydrogen atoms are omitted for clarity).
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Table 2 EL performance of the Pt(II) complexes-doped OLEDs
Zc.max.
Z
/
Zp.max./
Z
Dopant
Dopant level [wt%]
lem
[nm] [nm]
lsh
a
b
c
c.500/Zc.1000 [cd Aꢀ1
]
p.500/Zp.1000 [lm Wꢀ1
]
ZEQE.max./Z500/Z1000 [%]
CIE (x, y)d Vturn-on [V]
Pt1
1
3
5
10
15
58.5
71.0
73.5
58.5
37.7
51.6
67.7
72.1
58.3
37.4
46.2
45.4
57.3
66.8
53.4
32.7
35.8
57.1
61.2
47.7
29.6
27.6
15.3
18.3
19.8
14.6
18.2
13.5
17.8
19.4
17.8
14.5
12.0
18.2
18.4
17.4
14.2
524, 560
528 560
528 560
532 564
532 568
(0.34, 0.62) 3.4
(0.35, 0.62) 3.3
(0.36, 0.61) 3.3
(0.39, 0.58) 2.6
(0.44, 0.54) 3.2
68.7
69.7
57.0
37.4
53.3
55.0
42.9
26.7
Pt2
1
3
5
10
15
45.5
51.6
62
51
43.4
38.7
51.5
59.5
50.8
43.3
32.5
44.5
56.3
49.4
42.8
42.8
46.6
56.2
43.9
36.5
32.1
43.8
49.4
37.7
31.6
23.8
34.0
43.1
34.0
28.5
13.8
15.7
19.0
17.9
17.6
11.8
15.7
18.2
17.5
17.3
9.9
13.5
17.3
16.9
16.8
504 536
504 536
504 536
540 508
(0.28, 0.60) 2.9
(0.29, 0.60) 2.8
(0.32, 0.59) 2.8
(0.39, 0.55) 3.1
588 544, 508 (0.45, 0.51) 2.7
Pt3
1
3
5
10
15
58.0
60.4
66.0
75.0
70.8
54.0
59.3
63.8
74.3
70.0
46.9
56.6
63.0
71.9
70.0
52.6
50.9
54.1
70.1
65.1
44.4
49.5
54.1
64.5
60.8
35.4
43.4
49.3
57.5
55.7
16.3
16.9
18.5
21.4
20.5
15.1
16.6
17.9
21.1
20.4
13.3
15.8
17.6
20.3
20.2
512 544
512 544
512 544
512 544
516 548
(0.31, 0.61) 2.9
(0.31, 0.61) 3.0
(0.31, 0.61) 2.9
(0.33, 0.61) 2.7
(0.34, 0.61) 2.7
a
The maximum current efficiency (Zc.max.), and current efficiencies (Zc.500 and Zc.1000) measured at a brightness of 500 cd mꢀ2 and 1000 cd mꢀ2
,
respectively. The maximum power efficiency (Zp.max.), and power efficiencies (Zp.500, Zp.1000) measured at a brightness of 500 cd mꢀ2 and 1000 cd mꢀ2
,
b
respectively. The maximum external quantum efficiency (ZEQE.max.), and EQEs (ZEQE.500, ZEQE.1000) measured at 500 cd mꢀ2 and 1000 cd mꢀ2, respectively.
c
d
Commission Internationale de L’Eclairage coordinates measured at 5 mA cmꢀ2
.
Fig. 3 The EL spectra of complex Pt3 with a current density at 5 mA cmꢀ2
.
Fig. 4 The current efficiency–current density curves for Pt3 at different
doping ratios.
emitting OLEDs based on tetradentate Pt complexes,3e,5,6a
and also comparable with those for the yellow emitting
OLEDs based on Ir(III) complexes.2h The current efficiency–
current density curves for Pt3 are depicted in Fig. 4. As
the current density was increased, the current efficiency
decreased gradually, which was ascribed to triplet–triplet
annihilation and field-induced quenching effects.20 As shown
in Fig. 5, Pt3 exhibited a low efficiency roll-off at a brightness
of 1000 cd mꢀ2 with an external quantum efficiency ZEQE of
20.3% (5.1% roll-off). While the EQE roll-off ratio from the
peak value to that at a brightness of 1000 cd mꢀ2 was 7.1% for
Pt1, and 8.9% for Pt2. So the introduction of the 2-ethylhexyl
group into Pt3 enhanced the emission quantum yield and
diminished the efficiency roll-off ratios because the bulky
chain discouraged the strong intermolecular interaction in
the solid state.
Fig. 5 The EQE–luminance–current efficiency curves for Pt3 at different
doping ratios.
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Conclusions
layers and Liq were 0.4–0.8, 2–3 and 0.2–0.5 Å sꢀ1, respectively.
Finally, the Al electrode was evaporated (5 Å sꢀ1) through a
shadow mask without breaking the vacuum.
In summary, three Pt complexes (Pt1–Pt3) based on benzazole-
containing tetradentate ligands were successfully synthesized.
The heteroatoms within the rigid ligands influenced the electro-
nic structures and photophysical/electroluminescence properties
of these Pt complexes greatly. High device performance was
achieved by using Pt1–Pt3 as emitters, and notably Pt3 exhibited
The emitting area of each device was 3 ꢁ 3 mm2. Electro-
luminescence (EL) spectra, CIE coordinates and current–voltage
(I–V) characteristics were measured using a constant current
source (Keithley 2400s Source Meter) combined with a photo-
meter (Photo Research PR 655 spectrophotometer).
an excellent efficiency with the maximum Zc of 75.0 cd Aꢀ1
,
Zp of 70.1 lm Wꢀ1 and ZEQE of 21.4%. The bulky alkyl chain in Pt3
suppressed the intermolecular interaction and eliminated the
excimer-based emission, and thus was responsible for its high
efficiency and stability.
Acknowledgements
We thank Prof. Hongtao Sun for his helpful discussion.
We acknowledge financial support from the National Natural
Science Foundation of China (21472135, 61307036). This pro-
ject is also funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (PAPD).
Experimental
General information
1H NMR spectra were recorded on a Bruker 400 MHz NMR
instrument and 13C NMR spectra were recorded on an Agilent
DD2-600 MHz NMR instrument. Mass spectra were recorded on
a Thermo ISQ mass spectrometer using a direct exposure probe
and MALDI-TOF mass spectra were recorded on a Bruker ultra-
flextreme MALDI-TOF spectrometer. UV-vis absorption spectra
were recorded on a Cary 60 spectrometer (Agilent Technologies).
PL spectra were recorded on a HORIBA Scientific FluoroMax-4
spectrofluorometer. Thermogravimetric analysis (TGA) was per-
formed on a TA SDT 2960 instrument at a heating rate of
10 1C minꢀ1 under nitrogen. HOMO values were determined
by Ultra-violet photoemission spectroscopy (Kratos Analytical
Shimadzu Group Company). Cyclic voltammograms of these
platinum complexes were recorded on a CHI600 voltammetric
analyzer at room temperature in a 0.1 M tetrabutylammonium
hexafluorophosphate solution under nitrogen gas protection.
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X-ray crystallographic study
Crystal data for compound Pt3 were collected on a Bruker APEX-
II CCD diffractometer with Mo Ka radiation (l = 0.71073 Å) at
296(2) K. The structure was solved by direct methods and refined
by full-matrix least-squares on F2 using the SHELXTL crystallo-
graphic software package.21 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms of the molecule were generated
theoretically onto the specific atoms and refined isotropically with
fixed thermal factors. A summary of the crystallographic data and
structure refinement details is given in Table S1 (ESI†). The CCDC
number is 1061613 for Pt3.
Device fabrication and measurements
OLED devices were fabricated through vacuum deposition
on commercial ITO-coated glass substrates having a sheet
resistance of 15 O per square. The ITO surface was ultrasonically
cleaned with acetone, ethanol, and deionized water, then dried
in an oven at 120 1C and treated by UV ozone for 15 min.
All layers were deposited under a base pressure of 2 ꢁ
10ꢀ6 Torr. The deposition rates and doping concentration of
the films were monitored in situ by the calibrated thickness
monitors. The typical evaporation rate for HAT-CN, organic
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