Photoluminescence and electroluminescence of four platinum complexes with trifluoromethyl-substituted 2-phenylpyridine and tetraphenylimidodiphosphinate ligands

Article abstract of DOI:10.1016/j.dyepig.2017.04.016

Four cyclometalated platinum complexes with trifluoromethyl-substituted 2-phenylpyridine at different positions on its phenyl group as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand, Pt1-Pt4 (Pt1 is a trifluoromethyl-free compl

Full text of DOI:10.1016/j.dyepig.2017.04.016

Accepted Manuscript
Photoluminescence and electroluminescence of four platinum complexes with
trifluoromethyl-substituted 2-phenylpyridine and tetraphenylimidodiphosphinate
ligands
Guang-Zhao Lu, Hua-Bo Han, Yan Li, You-Xuan Zheng
PII:
S0143-7208(16)31398-5
10.1016/j.dyepig.2017.04.016
DYPI 5913
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 December 2016
Revised Date: 21 February 2017
Accepted Date: 12 April 2017
Please cite this article as: Lu G-Z, Han H-B, Li Y, Zheng Y-X, Photoluminescence and
electroluminescence of four platinum complexes with trifluoromethyl-substituted 2-phenylpyridine and
tetraphenylimidodiphosphinate ligands, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.04.016.
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ACCEPTED MANUSCRIPT
G1
G2
G3
G4
10
R
4
5
3
L1, Pt1: R = H
L2, Pt2: R2 = CF3
L3, Pt3: R3 = CF3
L4, Pt4: R4 = CF3
O P
O P
2
6
1
N
Pt
N
1
0
1000
2000
3000
4000
5000
-2
Lumminance (cd m )
Highly efficient green OLEDs based on four platinum complexes show a maximum
−
2
−1
luminance of 39196 cd m , a maximum current efficiency of 40.1 cd A and a
−
1
maximum power efficiency of 31.0 lm W , respectively, with low efficiency roll-off.

ACCEPTED MANUSCRIPT
Photoluminescence and electroluminescence of four platinum
complexes with trifluoromethyl-substituted 2-phenylpyridine
and tetraphenylimidodiphosphinate ligands
a
a
a
a,b
Guang-Zhao Lu , Hua-Bo Han , Yan Li , You-Xuan Zheng *
a
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures,
Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, P. R. China, e-mail: yxzheng@nju.edu.cn
b
MaAnShan High-Tech Research Institute of Nanjing University, MaAnShan, 238200, P. R. China
Abstract: Four cyclometalated platinum complexes with trifluoromethyl-substituted
2
-phenylpyridine at different positions on its phenyl group as the main ligands and
tetraphenylimidodiphosphinate as the ancillary ligand, Pt1-Pt4 (Pt1 is a trifluoromethyl-free
complex), were prepared, and their X-ray crystallography, photoluminescence, thermal stability,
electrochemical analyses, theoretical calculation and EL performances were investigated. The
positions of trifluoromethyl groups on the phenyl ring affected the emission spectra of platinum
complexes greatly, and their corresponding emission peaks at 504/534, 512/545, 495/526 and
5
10/542 nm were observed at room temperature, respectively. Constructed with complexes
Pt1-Pt4 as the emitters, respectively, the organic light-emitting diodes (OLEDs) with double light
emitting layers structure of ITO / TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm
/
Pt complexes (5 wt%) : TCTA (4,4’,4’’-tri(9-carbazoyl)-triphenylamine, 10 nm / Pt complexes
wt%): 2,6DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, 10 nm TmPyPB
1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm) / LiF (1 nm)/ Al (100 nm) showed good
(
5
/
(
performances. Particularly, device G2 based on 2-trifluoromethyl-substituted complex Pt2
−
2
−1
obtained a maximum luminance of 39196 cd m , a maximum current efficiency of 40.1 cd A
−
1
and a maximum power efficiency of 31.0 lm W , respectively, with low efficiency roll-off. The
results suggested that these complexes Pt1-Pt4 would have potential applications in efficient
OLEDs.
Keywords: Organic light-emitting diode; Platinum(II) complex; Tetraphenylimidodiphosphinate;
Trifluoromethyl; High efficiency; Efficiency roll-off

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1
. Introduction
A breakthrough came with the incorporation of transition metal complexes in 1998 which
enabled quantum yields beyond the fluorescence limit of 25% leading to the new class of
phosphorescent organic light-emitting devices (PhOLEDs) [1-2]. Therein, the heavy atom is
responsible for effective spin–orbit coupling (SOC) and triplet harvesting, leading to exceptional
photophysical properties and quantum yields as high as 100% [3]. Although different transition
metals were investigated as emitting compounds, neutral iridium(III) [4-24] and platinum(II)
[25-31] complexes with cyclometalated ligands showed the most promising results so far.
Prominent motifs in such Pt(II) complexes are bidentate cyclometalated aryl pyridine ligands
which have been tested together with acetylacetonate ancillary ligands and derivatives thereof
[32]. Based on this class of complexes, a lot of variations regarding substituents, size of the π
systems and ancillary ligands were investigated to tune their photophysical properties.
As far as we know, Ir(III) complexes carrying trifluoromethyl-substituted 2-phenylpyridine
(ppy) ligands have been reported with good performances [33-35], which suggested that the
introduction of electron-withdrawing CF groups in Pt(II) complexes may also improve the OLED
3
performances effectively because fluorination can enhance the electron mobility and the bulky
CF substituent can affect the molecular packing, providing steric protection around the metal,
3
which can suppress the self-quenching behavior [36-38]. Some groups have studied various
heteroleptic fluorinated iridium complexes, which suggested that the fluorine and trifluoromethyl
groups in different positions of the ppy ligands showed different effects on the emission properties
[39-40]. It may also work in the same way in Pt(II) complexes. However, comprehensive and
deeper studies are still needed.
Additionally, the emission energy of heteroleptic complexes [(C^N)Pt(LX)] can be fine-tuned
by a combination of main ligands (C^N) and LX types of ancillary ligands. Our group has
reported highly efficient phosphorescent OLEDs by introducing tetraphenylimidodiphosphinate
(tpip) derivatives as ancillary ligands [41-45]. Tpip derivatives have stronger polar P=O bonds,
which may improve the electron mobility of the Ir(III) complexes and benefit their OLED
performances [41-45]. And the bulky tpip derivatives can also introduce steric hinderance into the
structure, which will hinder the intermolecular interactions. So, they may be also a useful ancillary

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ligand for the phosphorescent Pt(II) complexes. To investigate the effect of the substituted
position of the trifluoromethyl group on the photophysical properties of the Pt(II) complexes for
their OLED performances and expand the application of tpip ligands, we synthesized a series of
Pt(II) complexes with tpip as the ancillary ligand and studied their OLED characteristics.
2
. Experimental Section
2
.1. General information
All reagents and chemicals were purchased from commercial sources and used without further
1
purification. H NMR spectra were measured on a Bruker AM 500 spectrometer. Mass
spectrometry (MS) spectra were obtained on an electrospray ionization (ESI) mass spectrometer
(
LCQ fleet, Thermo Fisher Scientific) for ligands and a matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectrometer (Bruker Daltonic Inc.) for complexes. Elemental
analyses for C, H, and N were performed on an Elementar Vario MICRO analyzer. TG-DSC
measurements were carried out on a DSC 823e analyzer (METTLER). Absorption and
photoluminescence spectra were measured on a UV-3100 spectrophotometer and a Hitachi F-4600
photoluminescence spectrophotometer, respectively. The single crystals of complexes were
carried out on a Bruker SMART CCD diffractometer using monochromated Mo Kα radiation (λ =
0
.71073 Å) at room temperature, which are analyzed by direct methods (SHELXS-97)
46-48].The decay lifetimes were measured with an Edinburgh Instruments FLS-920 fluorescence
spectrometer in degassed CH Cl solution at room temperature. Cyclic voltammetry
[
2
2
measurements were conducted on a MPI-A multifunctional electrochemical and chemiluminescent
system (Xi’an Remex Analytical Instrument Ltd. Co., China) at room temperature, with a polished
Pt plate as the working electrode, platinum thread as the counter electrode and Ag-AgNO (0.1 M)
3
in CH CN as the reference electrode, tetra-n-butylammonium perchlorate (0.1 M) was used as the
3
+
supporting electrolyte, using Fc /Fc as the internal standard, the scan rate was 0.1 V/s. The
luminescence quantum efficiencies were calculated by comparison of the emission intensities
(
integrated areas) of a standard sample (fac-Ir(ppy) ) and the unknown sample [49-50].
3
2
.2. OLED fabrication and measurement
All OLEDs were fabricated on the pre-patterned ITO-coated glass substrate with a sheet
resistance of 10 Ω/sq. The deposition rate for organic compounds is 1-2 Å/s. The phosphor and the

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host TCTA or 2,6DCzPPy were co-evaporated to form emitting layer from two separate sources.
The cathode consisting of LiF / Al was deposited by evaporation of LiF with a deposition rate of
0
.1 Å/s and then by evaporation of Al metal with a rate of 3 Å/s. The characteristic curves of the
devices were measured with a computer which controlled KEITHLEY 2400 source meter with a
calibrated silicon diode in air without device encapsulation. On the basis of the uncorrected PL
and EL spectra, the Commission Internationale de l’Eclairage (CIE) coordinates were calculated
using a test program of the Spectra scan PR650 spectrophotometer.
2
.3. Syntheses procedure
The synthesis procedures of ligands are listed in Scheme 1.
2
.3.1. General synthesis of L2-L4
Tetraphenylimidodiphosphinate acid (Htpip) and potassium tetraphenylimidodiphosphinate
(
Ktpip) were prepared according to our previous literatures [41-44]. All CF -substituted ligands
3
L2−L4 with a ppy core were synthesized by the reaction of the corresponding 2-bromopyridine
(
0)
(
21.1 mmol) and arylboronic acids (25.5 mmol) using tetrakis(triphenylphosphine)-palladium
0.63 mmol) as the catalyst in 50 mL of THF. After 30 mL of aqueous 2 N Na CO was delivered,
(
2
3
the reaction mixture was heated at 70 °C for 1 day. The cooled mixture was poured into water and
extracted with CH Cl (50 mL × 3). Finally, silica column purification (n-hexane : EtOAc = 10 : 1
2
2
as the eluant) gave white solid products.
-[2-(Trifluoromethyl)phenyl]pyridine (L2). Yield: 80%. H NMR (CDCl , 500 MHz): δ
1
2
3
7
.2−7.6 (m, 4H), 7.49 (d, J = 7.2 Hz, 1H), 7.55 (d, J = 7.3 Hz, 1H), 7.65−7.8 (m, 1H), 8.60 (m,
+
1
H). ESI-MS (m/z): calcd for C H F N [M+H] 224, found 224.
1
2
8 3
1
2
-[3-(Trifluoromethyl)phenyl]pyridine (L3). Yield: 75%. H NMR (CDCl , 500 MHz): δ 7.1
3
(m, 1H), 7.44 (t, J = 7.8 Hz, 1H), 7.59−7.71 (m, 3H), 8.06 (d, J = 7.8 Hz, 1H), 8.3 (s, 1H), 8.62
+
(
dd, J = 4.7, 0.9 Hz, 1H). ESI-MS (m/z): calcd for C H F N [M+H] 224, found 224.
1
2
8 3
1
2
-[4-(Trifluoromethyl)phenyl]pyridine (L4). Yield: 76%. H NMR (CDCl , 500 MHz,): δ 7.1
3
(ddd, J = 8.6, 4.7 and 2.1 Hz, 1H), 7.55 (d, J = 7.5 Hz, 4H), 7.93 (d, J = 8 Hz, 2H), 8.56 (d, J = 4.4
+
Hz, 1H). ESI-MS (m/z): calcd for C H F N [M+H] 224, found 224.
1
2
8 3

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2
.3.2. General synthesis of Pt1-Pt4
The K PtCl (0.415 g, 1 mmol) and 1.5 equivalent of C^N ligand (1.5 mmol) were mixed in a
2
4
o
3
:1 mixture of 2-ethoxyethanol and water. The mixture was heated at 80 C for 16 h. The Pt-dimer
was precipitated in water. The resulting yellow-greenish powder was filtered and was
subsequently reacted with Ktpip without further purification. A stirred mixture of Pt-dimer (0.30
o
g, 0.33 mmol) and Ktpip (0.99 mmol, 0.45 g) in 2-ethoxyethanol (25 mL) was heated at 80 C for
1
6h. After cooling the mixture to room temperature, the solution was concentrated under reduced
pressure. The resulting residue was subjected to column chromatography (silica gel, CH Cl /
2
2
petroleum ether 1:2 (v/v)) to give product as a yellow solid, which was further purified by
sublimation in vacuum.
1
Pt1. Yield: 30.0%. H NMR (500 MHz, CDCl ) (Fig. S7) δ 7.75 (d, J = 5.45 HZ, 1H),
3
6
.64-6.55 (m, 8H), 6.45 (t, J = 8.53 Hz, 1H), 6.34 (d, J = 7.09 Hz, 1H), 6.22 (d, J = 7.45 Hz, 1H),
.09-6.04 (m, 13H), 5.79 (dd, J = 15.86, 23.24 Hz, 3H). MALDI-TOF (Fig. S2), m/z: calcd for
6
C H N O P Pt, 765.127 [M]; found 766.620 [M+1]. Anal. Calcd for C H N O P Pt: C, 54.91;
3
5
28
2
2
2
35 28
2
2 2
H, 3.69; N, 3.66. Found: C, 54.75; H, 3.59; N, 3.65%.
1
Pt2. Yield: 45.0%. H NMR (500 MHz, CDCl ) (Fig. S8) δ 9.27 (m, 1H), 8.0 (dd, J = 8.01,
3
1
2
3.74 Hz, 2H), 7.91-7.84 (m, 9H), 7.47 (d, J = 7.41 Hz, 1H), 7.36-7.32 (m, 12H), 7.11-7.18 (m,
H). MALDI-TOF (Fig. S3), m/z: calcd for C H F N O P Pt, 833.115 [M]; found 834.189
3
6
27
3
2
2 2
[
M+1]. Anal. Calcd for C H F N O P Pt: C, 51.87; H, 3.26; N, 3.36. Found: C, 51.68; H, 3.47;
36 27 3 2 2 2
N, 3.34%.
1
Pt3. Yield: 25.0%. H NMR (500 MHz, CDCl ) (Fig. S9) δ 9.09 (d, J = 5.13 Hz, 1H), 7.84
3
(ddd, J = 7.89, 14.19, 23.58 Hz, 13H), 7.61-7.56 (m, 2H), 7.78-7.34 (m, 10H), 7.13 (t, J = 6.08
Hz, 1H). MALDI-TOF (Fig. S4), m/z: calcd for C H F N O P Pt, 833.115 [M]; found 833.366
3
6
27
3
2
2 2
[
M]. Anal. Calcd for C H F N O P Pt: C, 51.87; H, 3.26; N, 3.36. Found: C, 51.71; H, 3.40; N,
36 27 3 2 2 2
3
.31%.
1
Pt4. Yield: 50.0% H NMR (500 MHz, CDCl ) (Fig. S10) δ 9.08 (d, J = 5.13 Hz, 1H),
3
7
.92-7.77 (m, 10H), 7.56 (d, J = 8.02 Hz, 1H), 7.38-7.31 (m, 14H), 7.11 (t, J = 6.63 Hz, 1H).
MALDI-TOF (Fig. S5), m/z: calcd for C H F N O P Pt, 833.115 [M]; found 835.015 [M+2].
3
6
27
3
2
2 2
Anal. Calcd for C H F N O P Pt: C, 51.87; H, 3.26; N, 3.36. Found: C, 51.83; H, 3.53; N,
3
6
27
3
2
2 2
3
.28%.

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3
. Results and Discussion
3
.1. Crystal structure
Single crystals of Pt2-Pt4 complexes were obtained from the mixture of CH OH and CH Cl
2
3
2
solutions. Fig. 1 shows the Oak Ridge thermal ellipsoidal plot (ORETP) diagrams of Pt2-Pt4
given by X-ray analysis. Selected parameters of the molecular structures and tables of atomic
coordinates were collected in the Table S1 and Table S2. As shown in Fig. 1, for the complexes
Pt2-Pt4, the Pt-C bond lengths are 1.940-1.947 Å, which are slightly shorter than the Pt-N bond
lengths (1.967-1.980 Å). The Pt-O bond lengths are all over 2 Å. The average dihedral angles of
planes [C1-Pt2-N1], [N1-Pt2-O2] and [O2-Pt2-O1] is 178.00° in Pt2, that is 179.33° of planes
[
C1-Pt3-N1], [N1-Pt3-O2] and [O2-Pt3-O1] in Pt3 and that is 177.60° of planes [C1-Pt4-N1],
N1-Pt4-O2] and [O2-Pt4-O1] in Pt4, which suggests that they maintain a good square-planar
[
coordination geometry with the two O atoms opposite to the C, N atoms of the -CF substituted
3
ppy respectively. Seen from the crystals of Pt2-Pt4, the bulky CF group will cause distortion to
3
the phenyl pyridine chelate more or less. For complex 2, greater bowing of the cyclometalated ppy
chelate was observed. This is revealed by the bowling angle of 10.00°, which is defined as a
dihedral angle between the mean planes of the phenyl and pyridine moieties of the ppy chelate.
This value was then reduced to 1.60° and 2.00° for complexes 3 and 4, respectively, which
suggests that the different positions of CF on the phenyl rings of ppy will cause distortion to the
3
structure of phenyl pyridine in different degrees. The bond lengths and angles around the central
platinum atom found for these new complexes are in good agreement with the few previously
published structures of this type [51].
3
.2. Thermal property
The thermal stability of the emitters is important for the stable OLEDs. In this cases, the
thermal properties of the four platinum complexes were characterized by differential scanning
calorimetry (DSC) and thermogravimetric (TG) measurements under a nitrogen steam. From the
DSC curves in Fig. S6, it can be observed the melting points of Pt1-Pt4 are as high as 187, 164,
o
2
54 and 262 C, respectively. The TG curves in Fig. 2 give the decomposition temperatures (5%
o
loss of weight) as 299, 306, 268 and 325 C for Pt1-Pt4, respectively, indicating that the
introduction of the unit of trifluoromethyl group and the different positions on the phenyl rings of

ACCEPTED MANUSCRIPT
ppy ligand will affect the melting point effectively and also their decomposition temperature
obviously. And the high decomposition temperature of the complexes suggested they are suitable
for application in OLEDs.
3
.3. Electrochemical property and theoretical calculations
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) energy levels of the dopants are important for the design of the OLEDs. In order to
determine the HOMO/LUMO values of Pt1-Pt4, the electrochemical properties were investigated
by cyclic voltammetry in deaerated CH Cl (Fig. 3). The cyclic voltammograms of the complexes
2
2
Pt1-Pt4 in the positive range show strong oxidation peaks and only an irreversible oxidation
potential (E ) is observed at 0.734 V for Pt1, 0.928 V for Pt2, 0.832 V for Pt3 and 0.738 V for
ox
+
Pt4 vs Fc/Fc , while the reduction peaks are not obvious, demonstrating that the redox process of
the complexes is not reversible completely. According to the equation of EHOMO = − (Eox + 4.8)
eV, the HOMO energy levels (EHOMO) of Pt1-Pt4 are calculated to be -5.38 eV, -5.58 eV, -5.44 eV
and -5.39 eV, respectively. Based on the EHOMO and optical band gap, the lowest unoccupied
molecular orbital (LUMO) energy levels are evaluated to be -2.50 eV and -2.80 eV, -2.56 eV and
-2.55 eV for Pt1-Pt4. Additionally, from the Table 1, it is observed that the difference of the
HOMO/LUMO values of the four complexes is considerable. This means that the introduction of
trifluoromethyl group and the different positions on ppy ligand have obvious effect on
electrochemical properties of Pt(II) complexes. Especially, Pt2 has the highest redox voltage
(0.928 V) and lowest HOMO/LUMO energy levels (-5.58/-2.80 eV).
The density functional theory (DFT) calculations for Pt(II) complexes are conducted to gain
insights into the electronic states and the orbital distribution employing Gaussian09 software with
B3LYP functional [52]. The basis set used for C, H, N, O, F and P atoms was 6-31G(d, p) while
the LanL2DZ basis set were employed for Pt atoms [53-56]. The solvent effect of CH Cl was
2
2
taken into consideration using conductor like polarizable continuum model (C-PCM) [57]. The
optimized HOMO/LUMO structures of both complexes on the ground state are clearly showed in
Fig. 4. Concerning the energy levels and compositions, electronic structures of HOMOs/LUMOs
are similar with each other in the four complexes. For these complexes, the HOMOs correspond to
a mixture of tfmppy (36.34-40.71%) and Pt d orbitals (51.22-53.85%) with minor contribution

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from the tpip ligand (8.07-9.81%), while the LUMOs are mainly localized on the main ligands
(91.90-93.11%) with minor contribution from Pt d orbitals (5.70-6.46%) and tpip ligand
(1.18-1.97%). Therefore, modification of the phenyl ring changes both the HOMO and LUMO
energy. The trends of the calculated HOMO and LUMO energies of 1-4 are basically in agreement
with the trends of the measured oxidation and reduction potentials: the lower (more negative)
EHOMO and ELUMO energies correspond to the higher (more positive) E and respectively.
ox
3
.4. Photophysical property
The UV−vis absorption spectra and emission spectra of complexes Pt1-Pt4 in degassed CH Cl2
2
−
5
at 5 × 10 M are shown in Fig. 5, and the related data are listed in Table 1. The absorption spectra
of these complexes shows intense bands below 320 nm, which were assigned to the spin-allowed
1
intraligand LC (π→π*) transition of cyclometalated ppy derivatives and tpip ligands. The bands
1
around 370 nm can be assigned to spin allowed metal−ligand charge-transfer ( MLCT) bands. The
3
spin-forbidden MLCT transition bands around 440 nm indicate an efficient spin−orbit coupling
that is prerequisite for phosphorescent emission [58]. Compared with the complex Pt1 in CH Cl2
2
with a emission band at 504/534 nm, complexes Pt2 and Pt4 show obvious red-shifted emissions
at 512/545 and 510/542 nm, respectively, whereas Pt3 is blue-shifted to 495/526 nm (Fig. 5b).
These results suggest that the position of the trifluoromethyl group on the phenyl ring affects the
emitting properties of Pt(II) complexes. The trifluoromethyl group on position 3 (Scheme 1) leads
to a hypsochromic shift of the emission of the complex Pt3, while the trifluoromethyl group on
positions 2 and 4 (Scheme 1) causes a bathochromic shift of the emission of the complexes Pt2
and Pt4, respectively. To better understand the emission properties of four complexes, the
radiative (K ) and non-radiative (K ) constants were also computed and summed up in Table 1.
r
nr
5
-1
5
-1
Both the K and K values are all around 10 s . And the K value for Pt1, Pt3 of 1.92 × 10 s ,
r
nr
r
5
-1
5
-1
5
-1
1
.77 × 10 s , respectively, is smaller than that of Pt2 and Pt4 of 3.94 × 10 s and 5.28 × 10 s .
5
-1
5 -1
On the contrary, The K nr value for Pt2, Pt4 of 1.06× 10 s , 1.38 × 10 s , respectively, is smaller
5
-1
5
-1
than that of Pt1 and Pt3 of 3.34 × 10 s and 4.48 × 10 s . Furthermore, among the four
complexes, Pt3 exhibits the weakest PL intensity and lowest quantum yield (28.4%), but Pt2 and
Pt4 have almost the same quantum yield as high as 78.9% and 79.2%, respectively, indicating that
they are potential emitters for efficient OLEDs. Perhaps there are two reasons for the higher
emission quantum yields of Pt2 and Pt4 than that of Pt1 and Pt3. Firstly, it may be attributed to

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the enhanced metal-ligand interaction of Pt2 and Pt4, which hence suppress the nonradiative
decay rate constant, giving rise to the intense phosphorescence. Secondly, compared to Pt2 and
Pt4, Pt1 and Pt3 may have enhanced solvent affinity and thus increase the efficiency quenching in
the solvents. In addition, the lifetimes for complexes Pt1-Pt4 are all in the range of microseconds
(1.5 - 2.0 µs) (Table 1, Fig. S1) and indicative of the phosphorescent origin from the excited
states. The short lifetimes would improve the spin-state mixing and suppress the excitons
annihilation.
3
.5. OLEDs Performance
As schematically shown in Scheme 2, the optimized OLEDs (named G1-G4, respectively) of
ITO / TAPC (1,1-bis-(di-4-tolylaminophenyl)cyclohexane, 40 nm) / Pt1-Pt4: TCTA
(
4,4’,4”-tris(carbazol-9-yl)triphenylamine, 10 nm,
2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine, 10
5
wt%)
nm,
/
Pt1-Pt4: 2,6DCzPPy
wt%)
(
5
/
1
,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB, 40 nm) / LiF (1 nm) / Al (100 nm) were used
to test the device performances of these complexes. To obtain good device performances, double
gradient emitting layers were introduced with hole transport material TCTA and bipolar material
2
,6DCzPPy as the hosts. The device structure may balance charge transporting for several reasons.
Effective electron charging could be realized because of little LUMO energy barrier between
TmPyPB, 2,6DCzPPy and TCTA. And TAPC possesses an effectively electron blocking
properties with LUMO level 0.6 eV higher than that of TCTA. While the HOMO level of TCTA
is very close to that of TAPC, benefiting the hole transporting from the HTL to the EML. The low
HOMO energy level of TmPyPB (-6.7 eV) also assures hole confinement in the emitting layers.
The EL spectra, luminance (L) versus voltage (V), current efficiency (η ) versus luminance,
c
power efficiency (η ) versus luminance characteristics of each device are shown in Fig. 6. The key
p
EL data are summarized in Table 2. All devices show typical emission maxima the same as the
corresponding complexes Pt1-Pt4 (504, 512, 495, and 510 nm, respectively) in CH Cl solutions,
2
2
suggesting that the energy can be transferred from TCTA and 2.6DCzPPy to the emitters.
However, it is noted that there is a little residual emission from the host 2,6DCzPPy for device
G1, which means that the energy and/or charge transfer from the host to the phosphor is not
complete upon electrical excitation.

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All devices display good performances with low turn-on voltages (Vturn‑on) as 3.2-3.3 V.
−
1
Respectively, the maximum current efficiency and power efficiency for device G1 are 33.1 cd A
−
1
−2
and 28.8 lm W with a maximum luminance of over 25000 cd m at a driving voltage of 7.1 V.
−
2
The driving voltages to reach the practical brightness of 100 and 1000 cd m are 3.5 V and 3.9 V,
−
2
respectively. For G3 device, the maximum luminance of 24151 cd m is achieved at a driving
−
1
voltage of 8.1 V. The maximum current efficiency and power efficiency are 32.6 cd A and 24.4
−
1
−2
lm W , respectively. The driving voltages to get the practical brightness of 100 and 1000 cd m
are 3.6 V and 4.1 V, respectively. For device G4, The maximum current efficiency and power
−
1
−1
efficiency are 40.6 cd A and 30.1 lm W , respectively, with a maximum luminance of 25493 cd
−
2
m at a driving voltage of 7.7 V. Among all devices, G2 fabricated with Pt2 as the guest material
−
2
displays the best performances. The maximum luminance for device G2 is over 39000 cd m .
−
1
−1
The maximum current efficiency and power efficiency are 41.0 cd A and 31.0 lm W ,
respectively. Additionally, all devices also show low efficiency roll-off. For example, the current
-1
-2
efficiency of 37.8 cd A is still maintained for device G2 as the luminance reaches to 5000 cd m .
Seen from above, it can be found that the EL performances of four devices differ from each
other and the complexes with CF units show better results. Compared with G1, the maximum
3
current efficiencies of G2 and G4 are increased by more than 20%, which suggests that the OLED
performances can be really enhanced by the incorporation of bulky CF substituent in Pt(II)
3
complexes. Furthermore, similarly with the PL properties, the complexes with trifluoromethyl
groups at different positions on phenyl ring also show different device performances and Pt2
based device exhibits the best values, indicating that the positions of trifluoromethyl groups at the
phenyl ring of ppy greatly affect the EL properties.
The good EL properties should be due to the application of tetraphenylimidodiphosphinate as
the ancillary ligand to improve the electron mobility of the complexes. The dopants act as the hole
and electron traps to retard the motion of both types of carriers. In our devices, due to the higher
-2
2
-1 -1
-3
hole mobility of the TAPC (1×10 cm V s ) than the electron mobility of the TmPyPB (1×10
2
-1
-1
cm V s ), and the high energy barrier between TmPyPB and 2,6DCzPPy, the exciton
accumulation is expected near the interface between hole blocking layer (TmPyPB) and the
emitting layer (Pt complexes (5 wt%) : 2.6DCzPPy) [59-60], which will cause the serious
triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) of the platinum

ACCEPTED MANUSCRIPT
complexes, and high efficiency roll-off consequently. Therefore, the improved electron mobility
of the dopants is particularly useful for enhanced OLED performances. Furthermore, the four
bulky phenyl and CF groups also lead to a larger spatial separation of the neighboring molecules
3
to suppress TTA and TPA effects. Therefore, the OLEDs with this kind of platinum emitters
exhibited high current efficiency and lower efficiency roll-off. Additionally, double light-emitting
layers are adopted in the devices, which can broaden the recombination zone, balance the
distribution of holes and electrons and reduce leakage current, improved recombination
probability, high device efficiency and low efficiency roll-off [61-62].
4
. Conclusion
In conclusion, four bis-cyclometalated platinum complexes with trifluoromethyl substituents at
different positions on the phenyl rings of ppy as main ligand and tpip containing P=O unit as the
ancillary ligand have been successfully applied in OLEDs as emitters. The positions of CF3
substituents on the phenyl rings of ppy can affect the emission color and intensity of these
complexes obviously. The devices ITO / TAPC (40 nm) / Pt complexes (5 wt %): TCTA (10 nm) /
Pt complexes (5 wt%): 2,6DCzPPy (10 nm) / TmPyPB (40 nm) / LiF (1 nm) / Al (100 nm) show
good performances. Especially, device G2 with Pt2 complex as emitter show a maximum
−
2
luminance of over 39000 cd m and a maximum current efficiency and a power efficiency of 41.0
−
1
−1
cd A and 31.0 lm W , respectively, with low efficiency roll-off. And a current efficiency of
-1
-2
3
7.8 cd A is still maintained even at the high luminance of 5000 cd m . The results suggest that
all of these complexes with -CF have potential applications in OLEDs.
3
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21371093,
9
1433113), the Major State Basic Research Development Program (2013CB922101) and the
Natural Science Foundation of Jiangsu Province (BK20116075-02).
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Table captions
Table 1. Photophyscial date of platinum complexes.
Table 2. EL performances of the devices G1-G4.
Table 1. Photophyscial date of platinum complexes.
a)
b)
b)
b)
c)
d)
-1 e)
-1 e)
Compound
Td
℃)
λab / (nm)
Emission
τ298 K
(µs)
ΦP
Eox
HOMO/LUMO
(eV)
kr (s )
knr (s )
5
-1
-1
(
( ε / 10 M cm )
λ
max (nm), 298 K
(%)
(V)
5
5
5
5
5
5
5
5
Pt1
299
306
268
325
279(0.16), 316(0.06),
504/534
1.9
36.5
0.734
0.928
0.832
0.738
-5.38/-2.50
-5.58/-2.80
-5.44/-2.56
-5.39/-2.55
1.92 × 10
3.94 × 10
1.77 × 10
5.28 × 10
3.34 × 10
1.06 × 10
4.48 × 10
1.38 × 10
3
61(0.05)
Pt2
Pt3
Pt4
262(0.24), 324(0.06),
74(0.05)
512/545
495/526
510/542
2.0
1.6
1.5
78.9
28.4
79.2
3
263(0.29), 317(0.05),
64(0.05)
3
278(0.19), 323(0.05),
69(0.05)
3
a)
b)
c)
Td: decomposition temperature.
Absorption, emission spectra and lifetime were taken in degassed CH Cl .
2
2
Φ: emission quantum yields were measured relative to fac-Ir(ppy) (Φ = 0.4) in degassed CH Cl
2
3
2
solution.
d)
From the onset of oxidation potentials of the cyclovoltammety (CV) diagram using ferrocene as the
internal standard and the optical band gap from the absorption spectra in degassed CH Cl . HOMO
2
2
(
eV) =-(E -E1/2,Fc)-4.8, LUMO (eV) =HOMO+Ebandgap
.
ox
e)
k r and k nr were calculated according to the equations, k =
Φ/τ and k = (1/τ) - kr.
nr
r

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Table 2. EL performances of the devices G1-G4.
a)
b)
c)
d)
e)
f)
Device Emitter
Vturn-on
V)
Lmax(Voltage)
ηc (voltage)
ηc, L5000
EQEmax
ηp (voltage)
CIE
-2
-1
-1
-1(
(
(cd m (V))
(cd A (V))
(cd A )
(%)
(lm W V))
(x,y)
G1
G2
G3
G4
Pt1
Pt2
Pt3
Pt4
3.2
3.2
3.2
3.3
25203(7.1)
39196(9.0)
24151(8.1)
25544(7.7)
33.1(3.7)
41.0(4.2)
32.6(4.3)
40.6(4.1)
27.0
37.8
27.3
27.1
9.8
28.8(3.8)
31.0(4.0)
24.4(3.8)
30.1(4.0)
(0.28,0.60)
(0.34,0.61)
(0.25,0.60)
(0.27,0.62)
10.8
10.1
12.0
a)
b)
c)
-2
Vturn-on: Turn-on voltage recorded at a luminance of 1 cd m .
Lmax: maximum luminance.
ηc,max: maximum current efficiency.
d)
-2
ηc,5000: current efficiency at 5000 cd m .
EQEmax: maximum external quantum efficiency.
ηp,max: maximum power efficiency.
e)
f)

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Scheme and figure captions:
Scheme 1. Synthetic route of Pt1-Pt4.
Scheme 2. Device structure and energy level diagram of HOMO and LUMO levels
(relative to vacuum level) for materials investigated in this work and their molecular
structures.
Fig. 1. Oka Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of Pt2 (CCDC No.
1
509842), Pt3 (CCDC No. 1509841) and Pt4 (CCDC No. 1509840) complexes with the
atom-numbering schemes. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn
at 50% probability level.
Fig. 2. TGA spectra of Pt1-Pt4.
Fig. 3. The cyclic voltammograms spectra of Pt1-Pt4.
Fig. 4. HOMO/LUMO electron-state-density distributions of Pt1-Pt4.
-5
Fig. 5. UV/vis absorption (a) and PL spectra (b) of Pt1-Pt4 in degassed CH Cl (5 10
×
2
2
M) at room temperature
Fig. 6. Characteristics of devices with configuration ITO / TAPC (40 nm) / Pt1-Pt4:
TCTA (10 nm, 5 wt%) / Pt1-Pt4: 2,6DCzPPy (10 nm, 5 wt%)/ TmPyPB (40 nm) / LiF
(1 nm) / Al (100 nm): (a) EL spectra , (b) luminance(L) versus voltage (V), (c)
current efficiency (η ) as a function of luminance (L) and (d) power efficiency (η ) as
c
p
a function of luminance (L) for G1-G4.
1

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Scheme 1. Synthetic route of Pt1-Pt4.
2

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Scheme 2. Device structure and energy level diagram of HOMO and LUMO levels
(relative to vacuum level) for materials investigated in this work and their molecular
structures.
3

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Pt2
Pt3
Pt4
Fig. 1. Oka Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of Pt2 (CCDC No.
509842), Pt3 (CCDC No. 1509841) and Pt4 (CCDC No. 1509840) complexes with the
1
atom-numbering schemes. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn
at 50% probability level.
4

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1
00
Pt1
Pt2
Pt3
Pt4
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
100
200
300
400
500
600
700
800
Temperature (°C)
Fig. 2. TGA spectra of Pt1-Pt4.
5

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5
4
3
2
1
0
1
2
Pt1
Pt2
Pt3
Pt4
-
-
0.0
0.5
1.0
1.5
Potential (V)
Fig. 3. The cyclic voltammograms spectra of Pt1-Pt4.
6

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Fig. 4. HOMO/LUMO electron-state-density distributions of Pt1-Pt4.
7

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0.4
(a)
Pt1
Pt2
Pt3
Pt4
(b)
Pt1
Pt2
Pt3
Pt4
4000
3000
2000
1000
0
0
.3
0.2
0.1
0.0
250
300
350
400
450
500
550
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
-5
Fig. 5. UV/vis absorption (a) and PL spectra (b) of Pt1-Pt4 in degassed CH Cl (5 10
×
2
2
M) at room temperature.
8