Highly luminescent platinum(II) complexes based on pyrazolo[1,5-f]phenanthridine-containing ligands

Article abstract of DOI:10.1016/j.orgel.2017.07.002

A series of highly luminescent [Pt(N?C?C?N)] emitters (ZPt1, ZPt2 and ZPt3) based on pyrazolo[1,5-f]phenanthridine-containing ligands were designed and synthesized. These Pt(II) complexes demonstrated extremely high thermal stabilities with the 5% weight-

Full text of DOI:10.1016/j.orgel.2017.07.002

Accepted Manuscript
Highly luminescent platinum(II) complexes based on pyrazolo[1,5-f]phenanthridine-
containing ligands
Danli Zhao, Xun Tang, Xiang-Yang Liu, Jian Fan, Liang-Sheng Liao
PII:
S1566-1199(17)30326-9
10.1016/j.orgel.2017.07.002
ORGELE 4194
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 14 May 2017
Revised Date: 15 June 2017
Accepted Date: 2 July 2017
Please cite this article as: D. Zhao, X. Tang, X.-Y. Liu, J. Fan, L.-S. Liao, Highly luminescent platinum(II)
complexes based on pyrazolo[1,5-f]phenanthridine-containing ligands, Organic Electronics (2017), doi:
10.1016/j.orgel.2017.07.002.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
luminescent platinum(II) complexes
pyrazolo[1,5-f]phenanthridine-containing ligands
Highly
based
on
Danli Zhao,†a Xun Tang,†a Xiang-Yang Liu,a Jian Fan,a,b* and Liang-Sheng Liaoa,b
aJiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute
of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou,
Jiangsu 215123, China
bInstitute of Organic Optoelectronics (IOO) of the Jiangsu Industrial Technology
Research Institute (JITRI), Suzhou, Jiangsu 215212, China
E-mail: jianfan@suda.edu.cn
†The two authors contribute equally to this paper
Abstract
A series of highly luminescent [Pt(N^C^C^N)] emitters (ZPt1, ZPt2 and ZPt3) based
on pyrazolo[1,5-f]phenanthridine-containing ligands were designed and synthesized.
These Pt(II) complexes demonstrated extremely high thermal stabilities with the 5%
weight-reduction temperatures over 450ºC due to the incorporation of the rigid
pyrazolo[1,5-f]phenanthridine motif and the robustness of the tetradentate
coordination framework. These Pt(II) complexes have the same coordination set
(pyridine^benzene^benzene^pyrazole) but with slightly different linkage between
coordination groups. Within ZPt1 and ZPt2, pyridine and the neighboring benzene
groups are separated by oxygen and aniline, respectively. In ZPt3, pyridine group is
rigidly grafted on the carbazole unit. The effect of the different linkages on the
frontier orbital energies, the photophysical and electroluminescent properties of
ZPt1-ZPt3 was investigated systematically. Organic light emitting diodes (OLEDs)
based on these Pt(II) complexes were fabricated with typical device structure. The
Pt(II) complexes displayed intense electroluminescence in the blue to yellowish green
spectral
region.
Among
the
three
Pt(II)
complexes,
the
3-(pyridin-2-yloxy)phenoxy-based ZPt1 compound showed the highest

ACCEPTED MANUSCRIPT
electroluminescence performance with the maximum CE, PE, and EQE of 58.0 cd A-1,
51.6 lm W-1, and 16.4%, respectively.
Introduction
Since the report on organic light-emitting diodes (OLEDs) in 1987 [1], there is a
continuous interest in developing highly emissive materials. During last decade, a
large number of transition metal complexes such as Ir(III), Pt(II), Au(III), Os(II) and
Cu(I) have been prepared as dopants in high efficiency OLEDs [2-15]. Among these
complexes, Ir(III) complexes have been found to be the most studied case due to their
high electroluminescence (EL) performance and high operational stability [16-19].
Recently, cyclometalated platinum(II) complexes have attracted much attention
because they exhibited remarkable electroluminescent properties comparable to Ir(III)
complexes [20-25]. For example, Fukagawa and coworkers reported red Pt(II) emitter
with the maximum PE and EQE of 30 lm W-1 and 19%, respectively. Moreover, the
half-life of this red OLED is estimated to be about 10000 hr at initial luminance of
1000 cd m-2 [26]. Che et al. reported that high PE of 126 lm W-1 was achieved with
green-emitting devices based on [Pt(O^N^C^N)] complex [27]. So far several
empirical strategies have been developed for the design of Pt(II) emitters [28]. One
2
2
practical strategy is to increase the energy level of the non-radiative 5dx −y orbital by
introducing strong-field ligands [29]. In addition, high thermal stabilities of emissive
materials are highly desirable to allow vacuum deposition for OLED fabrication. Che
et al. reported a series of [(C^N^N)PtX] complexes with decomposition temperature
over 400 ºC by using the extended π-conjugated cyclometalated ligands [30].
Typically, Pt(II) ion adopts a four-coordinated square planar geometry, and it can be
surrounded by bidentate, tridentate and tetradentate ligands. Although the bidentate
ligands are easily achievable as compared with tetradentate ligands, tetradentate
coordination pattern is getting more and more popular since it not only decreases the
non-radiative vibrational pathways by suppressing the vibration and rotation around
metal ion, but also minimizes the excited state distortion and ensures good color
purity [31-37]. In this contribution, we reported the design and synthesis of three
tetradentated
Pt(II)
complexes
(ZPt1-ZPt3)
based
on
pyrazolo[1,5-f]phenanthridine-containing ligands. Currently Pt(II) complexes based
on 1-phenylpyrazole [38-41] and 1,5-diarylpyrazole [42] have been well studied
relative
to
pyrazolo[1,5-f]phenanthridine-based
complexes
[43].

ACCEPTED MANUSCRIPT
Pyrazolo[1,5-f]phenanthridine and
1-phenylpyrazole are both
excellent
cyclometalated ligands for the planar platinum(II) complexes. Furthermore, the
introduce of this fused-ring system (pyrazolo[1,5-f]phenanthridine) could reduce the
structural vibration and enhance the thermal stabilities of the resulting complexes. The
four coordination groups of each Pt(II) complex are kept the same to investigate the
effect of linkages between coordination groups on the electroluminescent properties
of ZPt1-ZPt3. OLEDs based on these three complexes were fabricated, and ZPt1
compound showed the high electroluminescence performance with the maximum CE,
PE, and EQE of 58.0 cd A-1, 51.6 lm W-1, and 16.4%, respectively, with CIE
coordinates of (0.33, 0.57).
Experiment section
All chemicals were used as received from J&K Scientific, Alfa Aesar, Acros, and
Strem chemicals. Anhydrous solvents were obtained from an Innovative Technology
1
solvent purification system. H NMR spectra were recorded on a Bruker 400 MHz
NMR instrument and 13C NMR spectra were recorded on Agilent DD2-600 MHz
NMR instrument. Thermogravimetric analysis (TGA) was recorded on a TA SDT
2960 instrument at a heating rate of 10 oC min-1 under nitrogen. Matrix-Assisted Laser
Desorption/ Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS) was
measured with a BRUKER ultrafleXtreme MALDI-TOF/TOF. HOMO were
determined from Ultraviolet Photoemission Spectroscopy (Kratos Analytical
Shimadzu Group Company). UV-vis absorption spectra were measured on Cary 60
spectrometer (Agilent Technologies). PL spectra and phosphorescent spectra were
recorded on a Hitachi F-4600 fluorescence spectrophotometer. Different parameters
were used in the DFT simulations for structure optimizations and vibration analyses.
Both the ground states and triplet states of molecules were optimized in vacuum by
the restricted and unrestricted formalism of three-parameter hybrid exchange
functional and correlation functional (B3LYP)/6-31G(d) with no assistance of
experimental data.
Device Fabrication and Measurements
The OLEDs were fabricated through vacuum deposition at ca. 2×106 Torr onto
commercially ITO-coated glass substrates having a sheet resistance of ca. 30 ꢀ per
square. The ITO substrates were cleaned sequentially with acetone, ethanol, and

ACCEPTED MANUSCRIPT
deionized water in an ultrasonic bath, then dried in an oven, and finally exposed to
UV-ozone for 15 min. During the fabrication of the Pt(II) complexes OLEDs, the
Organic layers were deposited at a rate of 2-3 Å s-1. HAT-CN and Liq were deposited
at 0.2-0.5 Å s-1, and the Al electrode was deposited (at ca. 5 Å s-1) through a shadow
mask without breaking the vacuum. For all the devices, the emitting areas were
depended on the overlap of two electrodes as 0.09 cm2. The electroluminescence (EL)
performance of the devices was measured using a PHOTO RESEARCH Spectra Scan
PR 655 PHOTOMETER and a KEITHLEY 2400 Source Meter constant current
source at room temperature.
1-(2-bromo-5-methoxyphenyl)-5-(2-bromophenyl)-1H-pyrazole
(2-bromo-5-methoxyphenyl)hydrazine (6.5 mmol) was added to a solution of
(E)-1-(2-bromophenyl)-3-(dimethylamino)prop-2-en-1-one (5.9 mmol) and Na2CO3
(4.0 mmol) in MeOH (40 mL) and H2O (8 mL) under stirring at room temperature.
The resulting mixture was acidified with glacial acetic acid to PH = 4 and heated to
o
135 C for 2 hours. After cooling to room temperature, the suspension was diluted
with EtOAc (30 mL), the organic layer was washed with H2O (3×10 mL) and the
organic solvent was evaporated under reduce pressure. The residue was purified by
flash chromatography on silica gel with petroleum ether/EtOAc (12/1, v/v) as eluent.
The target product was obtained as a yellow solid (50%). 1H NMR (400 MHz, CDCl3)
δ (ppm): 7.80 (s, 1H), 7.59 (d, J = 7.6 Hz, 1H), 7.42 (d, J = 8.9 Hz, 1H), 7.24 – 7.12
(m, 3H), 6.94 (d, J = 2.6 Hz, 1H), 6.77 (dd, J = 8.9, 2.7 Hz, 1H), 6.58 (s, 1H), 3.71 (s,
3H). GC-MS: m/z calcd 408.09, found 407.53.
10-methoxypyrazolo[1,5-f]phenanthridine
Dry
1,4-dioxane
(15
mL)
was
added
to
a
mixture
of
1-(2-bromo-5-methoxyphenyl)-5-(2-bromophenyl)-1H-pyrazole (4 g, 9.80 mmol),
Pd(PPh3)4 (600 mg, 0.52 mmol) and Sn2(n-Bu)6 (14.2 g, 24.48 mmol) in a 30 mL
o
microwave tube under glovebox. Then the resulting solution was heated at 140 C
overnight. After the reaction solution cooling to room temperature, the solvent was
evaporated under reduce pressure. The crude product was purified by column
chromatography using petroleum ether/EtOAc (8/1, v/v) to afford the final product as
a white powder (1.6 g, 70%). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 7.81 (s, 1H),
7.68 (d, J = 7.6 Hz, 1H), 7.55 (d, J = 8.9 Hz, 1H), 7.37 – 7.24 (m, 3H), 7.04 (d, J =

ACCEPTED MANUSCRIPT
2.4 Hz, 1H), 6.98 – 6.90 (m, 1H), 6.64 (s, 1H), 3.71 (s, 3H). GC-MS: m/z calcd
248.29, found 248.10.
pyrazolo[1,5-f]phenanthridin-10-ol
10-methoxypyrazolo[1,5-f]phenanthridine (1.5 g, 6.41 mmol) and 47% aqueous
hydrobromic acid (25 mL) were mixed in a flask equipped with a magnetic stirrer and
o
heated at 123 C for 3 days under N2. After the reaction solution cooling to room
temperature, the precipitate was collected by filtration and washed with absolute ethyl
alcohol, and dichloromethane, and dried under vacuum to give the corresponding
1
alcohol as a white powder (1.05 g, 74%). H NMR (400 MHz, DMSO-d6) δ (ppm):
10.36 (s, 1H), 8.48 (d, J = 8.6 Hz, 2H), 8.25 (d, J = 7.7 Hz, 1H), 8.06 (s, 1H), 7.87 (d,
J = 2.2 Hz, 1H), 7.61 (dt, J = 26.8, 7.3 Hz, 2H), 7.29 (s, 1H), 7.02 (dd, J = 8.8, 2.2 Hz,
1H). GC-MS: m/z calcd 234.26, found 234.18.
10-(3-(pyridin-2-yloxy)phenoxy)pyrazolo[1,5-f]phenanthridine (Z1)
Dry DMF (15 mL) was added to a mixture of pyrazolo[1,5-f]phenanthridin-10-ol
(900 mg, 3.85 mmol), 2-(3-bromophenoxy)pyridine (1.44 g, 5.78 mmol), K3PO4 (1.63
g, 7.68 mmol), CuI (110 mg, 0.58 mmol) and picolinic acid (142 mg, 0.077 mmol) in
a 30 mL microwave tube under glovebox. Then the resulting solution was heated at
o
100 C for 40 hours. After the reaction solution cooling to room temperature, the
reaction mixture was poured into aqueous EDTA disodium solution (200 mL). The
aqueous phase was extracted with dichloromethane (3×100 mL), and the combined
organic layer was dried over MgSO4. The crude product was purified by column
chromatography using petroleum ether/EtOAc (4/1, v/v) to afford the final product as
1
a white powder (1.0 g, 65%). H NMR (400 MHz, DMSO-d6) δ (ppm): 8.71 (d, J =
8.9 Hz, 1H), 8.59 (d, J = 7.9 Hz, 1H), 8.30 (d, J = 7.9 Hz, 1H), 8.19 (d, J = 4.5 Hz,
1H), 8.07 (s, 1H), 7.98 (d, J = 2.4 Hz, 1H), 7.86 (t, J = 6.9 Hz, 1H), 7.73 – 7.63 (m,
2H), 7.52 (t, J = 8.3 Hz, 1H), 7.35 – 7.28 (m, 2H), 7.17 – 7.11 (m, 1H), 7.05 (dd, J =
16.1, 7.4 Hz, 4H). 13C NMR (151 MHz, CDCl3) δ (ppm): 163.26, 158.05, 157.54,
155.51, 147.78, 141.21, 139.52, 137.68, 135.16, 130.55, 128.33, 127.62, 126.68,
125.20, 124.41, 123.30, 122.41, 118.78, 116.86, 116.47 (d, J = 6.0 Hz), 115.36,
112.47, 111.75, 105.58, 99.31. GC-MS: m/z calcd 403.44, found 403.15.
Preparation of ZPt1

ACCEPTED MANUSCRIPT
Z1 (900 mg, 2.23 mmol), K2PtCl4 (973 mg, 2.34 mmol), CHCl3 (2 mL) and
HOAc (18 mL) were added to a 100 mL flask under N2. Then the mixture was heated
o
at 140 C for 5 days. After the reaction solution cooling to room temperature, the
reaction mixture was poured into aqueous sodium carbonate solution (200 mL). The
aqueous phase was extracted with dichloromethane (3×100 mL), and the combined
organic layer was dried over MgSO4. The crude product was purified by column
chromatography using petroleum ether/EtOAc (5/1, v/v) to afford the final product as
a greenish yellow powder (585 mg, 40%). 1H NMR (400 MHz, DMSO-d6) δ (ppm):
9.08 (d, J = 5.4 Hz, 1H), 8.72 (d, J = 8.1 Hz, 1H), 8.55 (d, J = 7.1 Hz, 2H), 8.40 (d, J
= 8.7 Hz, 1H), 8.26 (t, J = 7.4 Hz, 1H), 7.84 (t, J = 7.5 Hz, 1H), 7.75 (t, J = 7.4 Hz,
1H), 7.70 (d, J = 2.0 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.47 (t, J = 6.3 Hz, 1H), 7.37
(d, J = 8.7 Hz, 1H), 7.22 (t, J = 7.8 Hz, 1H), 7.15 (d, J = 7.6 Hz, 1H), 6.99 (d, J = 7.3
Hz, 1H). MALDI-TOF (m/z): calculated for: 596.081, found: 596.104. Anal. Calcd.
For C26H15N3O2Pt: C, 52.35; H, 2.53; N, 7.04; found: C, 52.46; H, 2.66; N, 7.16.
Preparation
of
N-(4-(tert-butyl)phenyl)-N-(3-(pyrazolo[1,5-f]phenanthridin-10-
yloxy)phenyl)pyridin-2-amine (Z2), ZPt2, 10-((9-(pyridin-2-yl)-9H-carbazol-2-
yl)oxy)pyrazolo[1,5-f]phenanthridine (Z3) and ZPt3
Z2, Z3, ZPt2, and ZPt3 were prepared by the procedures similar to those used for
Z1 and ZPt1, respectively. The final product Z2 was obtained as a white powder (800
mg, 52%). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.67 (d, J = 9.0 Hz, 1H), 8.57 (d,
J = 8.0 Hz, 1H), 8.30 (d, J = 7.4 Hz, 1H), 8.14 (d, J = 4.0 Hz, 1H), 8.10 (s, 1H), 7.94
(d, J = 2.3 Hz, 1H), 7.72 – 7.62 (m, 2H), 7.55 (t, J = 7.8 Hz, 1H), 7.42 (t, J = 8.1 Hz,
1H), 7.34 (d, J = 8.5 Hz, 3H), 7.29 – 7.24 (m, 1H), 7.10 (d, J = 8.4 Hz, 2H), 6.97 (t, J
= 7.6 Hz, 2H), 6.89 – 6.83 (m, 2H), 6.72 (d, J = 8.4 Hz, 1H), 1.20 (s, 9H). 13C NMR
(151 MHz, CDCl3) δ (ppm): 158.75, 158.47, 156.91, 148.17, 147.80 (d, J = 3.4 Hz),
142.88, 141.15, 137.67, 137.42, 135.09, 130.31, 128.29, 127.50, 126.72, 126.45,
126.10, 125.08, 124.38, 123.22, 122.35, 121.03, 116.86, 116.49, 116.37, 116.17,
114.83, 114.33, 104.94, 99.28, 34.39, 31.34. GC-MS: m/z calcd 534.66, found 534.26.
1
ZPt2 was obtained as a greenish yellow powder (600 mg, 61%). H NMR (400
MHz, CDCl3) δ (ppm): 9.09 (d, J = 5.2 Hz, 1H), 8.54 (d, J = 8.2 Hz, 1H), 8.26 (d, J =
7.9 Hz, 1H), 8.20 (d, J = 8.8 Hz, 1H), 8.08 (s, 1H), 7.75 (t, J = 7.5 Hz, 1H), 7.67 –
7.53 (m, 4H), 7.46 (d, J = 8.6 Hz, 1H), 7.37 (d, J = 8.3 Hz, 2H), 7.20 – 7.12 (m, 2H),
7.00 (t, J = 8.0 Hz, 1H), 6.85 (t, J = 8.0 Hz, 2H), 6.56 (d, J = 7.6 Hz, 1H), 1.39 (s, 9H).

ACCEPTED MANUSCRIPT
MALDI-TOF (m/z): calculated for: 727.191, found: 727.240. Anal. Calcd. For
C36H28N4OPt: C, 59.42; H, 3.88; N, 7.70; found: C, 59.39; H, 3.98; N, 7.74.
1
Z3 was obtained as a white powder (700 mg, 62%). H NMR (400 MHz,
DMSO-d6) δ (ppm): 8.69 (d, J = 9.0 Hz, 2H), 8.58 (d, J = 8.0 Hz, 1H), 8.37 (d, J =
8.4 Hz, 1H), 8.29 (d, J = 4.4 Hz, 2H), 8.08 (t, J = 7.2 Hz, 1H), 7.97 (s, 1H), 7.89 (d, J
= 2.4 Hz, 1H), 7.82 (d, J = 8.7 Hz, 2H), 7.72 – 7.62 (m, 3H), 7.52 – 7.42 (m, 2H),
13
7.38 (t, J = 7.5 Hz, 1H), 7.31 (dd, J = 8.8, 2.1 Hz, 2H), 7.23 (d, J = 8.5 Hz, 1H). C
NMR (151 MHz, CDCl3) δ (ppm): 159.58, 154.85, 151.52, 149.64, 141.15, 140.61,
140.03, 138.55, 137.69, 135.16, 128.30, 127.43, 126.81, 125.82, 125.11, 124.40,
124.05, 123.18, 122.35, 121.26 (dd, J = 23.6, 7.5 Hz), 119.90, 118.94, 116.20, 115.67,
113.83, 111.10, 104.38, 103.40, 99.23. GC-MS: m/z calcd 476.54, found 476.15.
1
ZPt3 was obtained as a yellow powder (500 mg, 55%). H NMR (400 MHz,
DMSO-d6) δ (ppm): 9.53 (d, J = 5.2 Hz, 1H), 8.72 (dd, J = 14.2, 5.1 Hz, 2H), 8.57 (d,
J = 7.8 Hz, 1H), 8.43 (d, J = 8.8 Hz, 1H), 8.33 (d, J = 8.4 Hz, 1H), 8.28 – 8.17 (m,
2H), 8.12 (d, J = 8.1 Hz, 1H), 7.99 (d, J = 8.3 Hz, 1H), 7.84 (t, J = 7.6 Hz, 1H), 7.76
(dd, J = 8.6, 5.0 Hz, 2H), 7.52 – 7.37 (m, 5H). MALDI-TOF (m/z): calculated for:
669.113, found: 669.142. Anal. Calcd. For C32H18N4OPt: C, 57.40; H, 2.71; N, 8.37;
found: C, 57.57; H, 2.86; N, 8.39.
Results and discussion
The synthetic routes for ZPt1, ZPt2 and ZPt3 were illustrated in Scheme 1.
1,5-Diarylpyrazole was prepared by the tandem amine-exchange/heterocyclization of
(E)-1-(2-bromophenyl)-3-(dimethylamino)prop-2-en-1-one
[45].
Catalytic
stannylation [46] of the dibromide followed by intramolecular Stille coupling reaction
finished the synthesis of pyrazolo[1,5-f]phenanthridine. Cleavage of
10-methoxypyrazolo[1,5-f]phenanthridine by HBr afforded the free phenol derivative,
which was further connected with N^C bidentates via CuI/picolinic acid catalyzed
coupling reaction to give the corresponding tetradentate ligands Z1, Z2 and Z3. The
Pt(II) complexes were prepared by the reaction of the respective ligands and K2PtCl4
in the mixture of chloroform and acetic acid. ZPt1, ZPt2 and ZPt3 contain fused 6-6-5
membered metallacycle rings. Che and coworkers reported several different types of
Pt(II) complexes with 5-5-5 [47], 6-5-6 [48] and 6-5-5 [49] membered metallacycle
rings. Recently, Li et al. reported a new strategy for the development of deep blue
phosphorescent emitter by replacing the typical 5-membered metal chelation rings

ACCEPTED MANUSCRIPT
with 6-membered chelation [50].
ZPt1, ZPt2 and ZPt3 demonstrated extremely high thermal stability with the 5%
weight-reduction temperatures (∆T5%) of 450 ºC, 446 ºC and 492 ºC, respectively
(Figure 1). The highest occupied molecular orbital (HOMO) values of Pt(II)
complexes were determined by Ultra-Violet Photoemission Spectroscopy (UPS)
measurement (Figure S10). The HOMOs for these complexes are -5.21 eV, -5.39 eV
and -5.43 eV, respectively. The lowest unoccupied molecular orbital (LUMO) values
were estimated from the difference between the HOMO and the corresponding optical
1
band gap. ZPt1, ZPt2 and ZPt3 were fully characterized by H NMR, elemental
analysis and MALDI-TOF. Due to the poor solubility, their 13C NMR spectra were not
1
acquired. The H and 13C NMR spectra of ligands were given in the supporting
information (Figure S1-S9).
Photophysical properties
The UV-vis absorption and photoluminescence (PL) spectra at room temperature in
CH2Cl2 solution with a concentration of 10-5 M were depicted in Figure 2, and the
photophysical parameters are summarized in Table 1. The three Pt(II) complexes ZPt1,
ZPt2 and ZPt3 showed similar absorption properties with the absorption cutoff
wavelength around 400 nm. The intense absorption with wavelength below 300 nm
could be assigned to the intraligand π-π* transition and the weak absorption beyond
350 nm could be assigned to metal-to-ligand charge transfer (MLCT). Upon
photo-excitation at 370 nm in degassed dichloromethane (10-5 M) at room
temperature, ZPt1, ZPt2 and ZPt3 showed highly-structured emission spectra with
vibronic splitting of 1342 cm-1, 1307 cm-1 and 1425 cm-1, respectively, indicating that
PL could mainly arise from the ligand-centered (LC) excited states [51]. The Natural
Transition Orbital (NTO) analyses also supported the ligand-centered emission since
the NTO are mainly localized on the tetradentate ligands (Figure S11). The PL
quantum yields of the complexes are in the range of 0.24-0.70. The luminescent
properties of the neat films for ZPt1-ZPt3 have been studied, and the related PL
spectra were showed in Figure S12. The PL quantum yields of neat film for ZPt1,
ZPt2, and ZPt3 were 10.34%, 4.27% and 2.38%, respectively, much lower than those
measured in the solution, which could be due to strong intermolecular interaction in
the neat film. The luminescent properties of ZPt1, ZPt2, and ZPt3 in poly(methyl

ACCEPTED MANUSCRIPT
methacrylate) film (PMMA 15 wt%) have been studied (Figure S13), and the
emission peaks of ZPt1-ZPt3 doped in the PMMA film are obviously blue-shifted in
comparison with their emission peaks of the neat films, which is attributed to the
reduced intermolecular interaction in PMMA. Among these Pt(II) complexes, ZPt2
demonstrated the narrowest emission spectrum, which could be attributed to the high
energy level of LC states of the phenyl-pyridin-2-yl-amine unit. Li et al. reported the
significant spectral narrowing in the room temperature emission spectrum for one
tetradentated Pt(II) complex by introducing an electron-donating group (tert-butyl) to
the coordination backbone [41].
DFT Calculations
Density functional theory (DFT) calculations were carried out for these complexes
with Gaussian 09 to investigate the photophysical properties of these emitters. The
calculations were performed using a B3LYP method with a basis set of 6-31G(d) for
C, H, O and N atoms and a LANL2DZ basis set for Pt atom. The HOMOs and LUMO
distributions are shown in Figure 3.
For these three complexes, the HOMO distributions are mainly localized on
phenyl-oxygen-phenyl motifs. For ZPt3, the HOMO distributions also extend to the
whole carbazole unit. The phenylamine based complex (ZPt2) exhibited higher
HOMO energy levels than the carbazole substituted complex (ZPt3), which is
consistent with the better electron-donating ability of phenylamine [52]. The LUMO
distributions for ZPt1, ZPt2 and ZPt3 are largely confined to pyridine unit. Due to the
strong electron-donating effect of phenylamine, the electron-withdrawing ability of
pyridine in ZPt2 decrease and ZPt2 showed the highest LUMO among the three Pt(II)
complexes. For ZPt1- ZPt3, the HOMOs are in the range from -4.81 eV to -5.00 eV
and the LUMOs are in the range from -1.12 eV to -1.40 eV. ZPt2 exhibited the largest
bandgap (3.68 eV) than other two complexes (3.62 for ZPt1 and 3.49 for ZPt3 ). The
optimized structures of ZPt1, ZPt2 and ZPt3 indicated that the molecular frameworks
slightly deviated from square planar geometries (Figure S14). The dihedral angles
between pyridine and pyrazolo[1,5-f]phenanthridine in ZPt1-ZPt3 are 24.1o, 37.8o and
32.7o, respectively. The pending 4-(tert-butyl)phenyl group in ZPt2 was pointing away
from the coordination plane around Pt(II) ion, and the dihedral angle between this
benzene ring and pyrazolo[1,5-f]phenanthridine was 77.8o.

ACCEPTED MANUSCRIPT
Electroluminescent Properties
The OLEDs based on the three Pt(II) complexes were fabricated with the following
device configuration: ITO/HAT-CN (10 nm)/TAPC (45 nm)/TCTA (10 nm)/ CBP (or
26mCPy):Pt(II) complexes (x wt%, 20 nm)/TmPyPB (50 nm)/Liq (2 nm)/Al (110 nm).
HAT-CN (1, 4, 5, 8, 9, 11-hexaazatriphenylene-hexacarbonitrile) was used as hole
injection layer, and Liq (8-hydroxyquinolinolato lithium) as electron injection layer.
TAPC (1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane) was used as hole
transport layer, and TmPyPB (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene) as electron
transport layer. CBP (4,4′-bis(9-carbazolyl)biphenyl) was used as host material for
ZPt1 and ZPt3, and 26mCPy (2,6-bis(N-carbazolyl)pyridine) was used as host
material for ZPt2.
The current density-voltage-luminance (J-V-L) characteristics and electroluminescent
(EL) spectra of ZPt1 at various doping ratios are depicted in Figure 4. The shape and
the position of EL spectra are kept essentially constant at different doping levels,
indicating the negligible intermolecular interaction between dopants. Although there
is no bulky group attached to the coordination sphere of ZPt1, the considerable
distortion from planarity induced by the oxygen linking atoms as indicated by DFT
calculation could be responsible for the suppression of strong intermolecular
interaction even at high doping levels. Highly electroluminescent OLEDs based on
ZPt1 have been achieved with a maximum efficiency of 58.0 cd A-1, 51.6 lm W-1 and
16.4% at doping level of 30% with CIE coordinates of (0.33, 0.57), and the turn-on
voltages for these devices were found to be below 3.8 V (Table 2). OLEDs based on
ZPt2 exhibited blue electro-phosphorescence (Figure S15), and the peak PE, CE and
EQE were 16.5 cd A-1, 9.2 lm W-1 and 7.7%, respectively, with CIE coordinates of
(0.18, 0.30). OLEDs based on ZPt3 exhibited yellowish green
electro-phosphorescence as observed for ZPt1. As shown in Figure 5c, the emission
band of the host CBP was observed at 400 nm [53] at the doping ratio of 5%,
indicating the insufficient energy transfer from host to dopant, which accounted in
part for the low efficiency. As the doping ratio goes up to 20%, a maximum efficiency
of 52.5 cd A-1, 47.1 lm W-1 and 15.7% with CIE coordinates of (0.35, 0.55) was
achieved, and the CBP emission band disappeared in the EL spectrum.
Conclusion

ACCEPTED MANUSCRIPT
Three Pt(II) complexes (ZPt1, ZPt2 and ZPt3) based on pyrazolo[1,5-f]phenanthridine
have been successfully prepared. Pyrazolo[1,5-f]phenanthridine showed strong
coordination capability with Pt(II) ion, and the resulting complexes demonstrated
extremely high thermal stabilities. These complexes have the same coordination
groups, which are linked in the same sequence. The different linkages didn’t affect the
absorption properties of these complexes significantly, but changed their PL and EL
properties remarkably. OLED based on the Pt(II) complexes displayed intense
electro-phosphorescence in the blue to yellowish green spectral region. The red shift
observed in the EL spectra of ZPt1 and ZPt3 relative to their PL spectra in solution
could be due to the strong intermolecular interaction at the solid state. For ZPt2, the
similarity observed in its EL spectra and PL spectra in solution indicated that the
pending 4-(tert-butyl)phenyl group could weaken the intermolecular interaction. ZPt1
showed high OLED performance (Ƞc.max = 58.0 cd A-1, Ƞp.max = 51.6 lm W-1, and
ȠEQE.max = 16.4%), indicating its potential utility as an OLED phosphor.
References
[1] C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51 (1987) 913.
[2] G. J. Zhou, Q. Wang, W. Y. Wong, D. G. Ma, L. X. Wang and Z. Y. Lin, J.
Mater. Chem., 19 (2009) 1872.
[3] Y.-C. Zhu, L. Zhou, H.-Y. Li, Q.-L. Xu, M.-Y. Teng, Y.-X. Zheng, J.-L. Zuo,
H.-J. Zhang and X.-Z. You, Adv. Mater., 23 (2011) 4041.
[4] C.-Y. Kuei, W.-L. Tsai, B. Tong, M. Jiao, W.-K. Lee, Y. Chi, C.-C. Wu, S.-H.
Liu, G.-H. Lee, and P-T. Chou, Adv. Mater., 28 (2016) 2795.
[5] X. Yang, G. Zhou and W.-Y. Wong, Chem. Soc. Rev., 44 (2015) 8484.
[6] G. Cheng, K. T. Chan, W.-P. To and C.-M. Che, Adv. Mater., 26 (2014) 2540.
[7] Y. Ma, H. Zhang, J. Shen and C. Che, Synth. Met., 94 (1998) 245.
[8] W. C. H. Choy, W. K. Chan and Y. Yuan, Adv. Mater., 26 (2014) 5368.
[9] C.-L. Ho, Z.-Q. Yu and W.-Y. Wong, Chem. Soc. Rev., 45 (2016) 5264.
[10]J. Zhang, C. Duan, C. Han, H. Yang, Y. Wei, and H. Xu, Adv. Mater., 28 (2016)
5975.
[11]D. Ma, L. Duan, Y. Wei, L. He, L. Wang and Y. Qiu, Chem. Commun., 50 (2014)
530.
[12]D. Xia, B. Wang, B. Chen, S. Wang, B. Zhang, J. Ding, L. Wang, X. Jing and F.
Wang, Angew. Chem. Int. Ed., 53 (2014) 1048.

ACCEPTED MANUSCRIPT
[13]M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson
and S. R. Forrest, Nature, 395 (1998) 151.
[14]W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 253 (2009) 1709.
[15]C. Shi, H. Sun, X. Tang, W. Lv, H. Yan, Q. Zhao, J. Wang and W. Huang,
Angew. Chem. Int. Ed., 52 (2013) 13434.
[16]X. Gong, J. C. Ostrowski, G. C. Bazan, D. Moses, A. J. Heeger, M. S. Liu and A.
K.-Y. Jen, Adv. Mater., 15 (2003) 45.
[17]H. Sasabe and J. Kido, Chem. Mater., 23 (2011) 621.
[18]X.-F. Wei, W.-Y. Tan, J.-H. Zou, Q.-X. Guo, D.-Y. Gao, D.-G. Ma, J. Peng, Y.
Cao and X.-H. Zhu, J. Mater. Chem. C, 5 (2017) 2329.
[19]M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest,
Appl. Phys. Lett., 75 (1999) 4.
[20]G.-Z. Lu, Y. Li, Y.-M. Jing and Y.-X. Zheng, Dalton Trans., 46 (2017) 150.
[21]H.-W. Fan, F.-Q. Bai, Z.-X. Zhang, Y. Wang, Z.-X. Qu, R.-L. Zhong and H.-X.
Zhang, RSC Adv., 7 (2017) 17368.
[22]W. Zhang, J. Wang, Y. Xu , W. Li and W. Shen, J. Organomet. Chem., 836-837
(2017) 26.
[23] J. Zhang, X. Zhu, A. Zhong, W. Jia, F. Wu, D. Li, H. Tong, C. Wu, W. Tang, P.
Zhang, L. Wang and D. Han, Org. Electron., 42 (2017) 153.
[24] G.-Z. Lu, H.-B. Han, Y. Li and Y.-X. Zheng, Dyes Pigments, 143 (2017) 33.
[25]B. Blondel, F. Delarue, M. Lopes, S. Ladeira-Mallet, F. Alary, C. Renaud and I.
Sasaki, Synth. Met., 227 (2017) 106.
[26]H. Fukagawa, T. Shimizu, H. Hanashima, Y. Osada, M. Suzuki and H. Fujikake,
Adv. Mater., 24 (2012) 5099.
[27] G. Cheng, S. C. F. Kui, W.-H. Ang, M.-Y. Ko, P.-K. Chow, C.-L. Kwong, C.-C.
Kwok, C. Ma, X. Guan, K.-H. Low, S.-J. Su and C.-M. Che, Chem. Sci., 5 (2014)
4819.
[28]K Li, G. S. M. Tong, Q. Wan, G. Cheng, W.-Y. Tong, W.-H. Ang, W.-L. Kwong
and C.-M. Che, Chem. Sci., 7 (2016) 1653.
[29]Y.-P. Li, X.-X. Fan, Y. Wu, X.-C. Zeng, J.-Y. Wang, Q.-H. Wei and Z.-N. Chen,
J. Mater. Chem. C, 5 (2017) 3072.
[30]S. C. F. Kui, I. H. T. Sham, C. C. C. Cheung, C.-W. Ma, B. Yan, N. Zhu, C.-M.
Che and W.-F. Fu, Chem.-Eur. J., 13 (2007) 417.
[31]E. Turner, N. Bakken and J. Li, Inorg. Chem., 52 (2013) 7344.

ACCEPTED MANUSCRIPT
[32]Z. B. Wang, M. G. Helander, Z. M. Hudson, J. Qiu, S. Wang and Z. H. Lu, Appl.
Phys. Lett., 98 (2011) 213301.
[33]T. Fleetham, G. Li and J. Li, ACS Appl. Mater. Interfaces, 7 (2015) 16240.
[34]Z.-Q. Zhu, T. Fleetham, E. Turner and J. Li, Adv. Mater., 27 (2015) 2533.
[35]B. Wang, F. Liang, H. Hu, Y. Liu, Z. Kang, L.-S. Liao and J. Fan, J. Mater. Chem.
C, 3 (2015) 8212.
[36]X.-Q. Zhang, Y.-M. Xie, Y. Zheng, F. Liang, B. Wang, J. Fan and L.-S. Liao,
Org. Electron., 32 (2016) 120.
[37] G. Liu, F. Liang, Y. Zhao, H. Hu, J. Fan and L.-S. Liao, J. Mater. Chem. C, 5
(2017) 1944.
[38] J. Li and G. Li, U.S. Pat. Appl. Publ. (2016), US 20160359125 A1 20161208.
[39] J. Li and G. Li, U.S. Pat. Appl. Publ. (2016), US 20160133862 A1 20160512.
[40] X.-C. Hang, T. Fleetham, E. Turner, J. Brooks and J. Li, Angew. Chem., Int. Ed.,
52 (2013) 6753.
[41] T. Fleetham, G. Li , L. Wen and J. Li, Adv. Mater., 26 (2014) 7116.
[42] J. Li, G. Li and J. Brooks, U.S. Pat. Appl. Publ. (2015), US 20150105556 A1
20150416.
[43] G. Kottas, Z. Elshenawy, G. Szigethy and C. Xia, U.S. Pat. Appl. Publ. (2016),
US 20160285014 A1 20160929.
[44]H. Wölfle, H. Kopacka, K. Wurst, P. Preishuber-Pflügl and B. Bildstein. J.
Organomet. Chem., 694 (2009) 2493.
[45]S. Hernández, I. Moreno, R. SanMartin, G. Gómez, M. T. Herrero and E.
Domínguez. J. Org. Chem., 75 (2010) 434.
[46]K. Chen, P. He, S. Zhang and P. Li, Chem. Commun., 52 (2016) 9125.
[47]H.-F. Xiang, S.-C. Chan, K. K.-Y. Wu, C.-M. Che and P. T. Lai, Chem. Commun.,
2005, 1408.
[48]C.-M. Che, C.-C. Kwok, S.-W. Lai, A. F. Rausch, W. J. Finkenzeller, N. Zhu and
H. Yersin, Chem.-Eur. J., 16 (2010) 233.
[49]G. Cheng, P.-K. Chow, S. C. F. Kui, C.-C. Kwok and C.-M. Che, Adv. Mater., 25
(2013) 6765.
[50]T. B. Fleetham, L. Huang, K. Klimes, J. Brooks and J. Li, Chem. Mater., 28
(2016) 3276.
[51]J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau and M. E.
Thompson, Inorg. Chem., 41 (2002) 3055.

ACCEPTED MANUSCRIPT
[52]B. Huang, Z. Yin, X. Ban, W. Jiang, Y. Dai, J. Zhang, Y. Liu, Y. Yang, Y. Sun,
Dyes Pigments, 117 (2015), 141.
[53]M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest,
Appl. Phys. Lett., 75 (1999) 4.
Acknowledgements
We acknowledge financial support from the National Key R&D Program of China
(2016YFB0400703), the National Natural Science Foundation of China (21472135,
61307036) and Natural Science Foundation of Jiangsu Province of China
(BK20151216). This project is also funded by Collaborative Innovation Center of
Suzhou Nano Science and Technology (CIC-Nano), Soochow University and by the
Priority Academic Program Development of Jiangsu Higher Education Institutions
(PAPD).

ACCEPTED MANUSCRIPT
Scheme 1. The synthetic routes of ZPt1, ZPt2 and ZPt3.

ACCEPTED MANUSCRIPT
450 oC
446 oC
492 oC
100
95
90
85
80
ZPt1
ZPt2
ZPt3
100
90
80
70
60
200
300
400
500
600
700
Temperature ( C)
°
Figure 1. TGA curves of ZPt1, ZPt2 and ZPt3.

ACCEPTED MANUSCRIPT
1.0
0.8
0.6
0.4
0.2
0.0
ZPt1
ZPt2
ZPt3
UV-vis
PL
300 350 400 450 500 550 600 650
Wavelength (nm)
Figure 2. UV-vis absorption and PL spectra at room temperature in CH2Cl2 solution.

ACCEPTED MANUSCRIPT
Table 1. Photophysical and electrochemical data of platinum(II) complexes.
LUMO
[eV]c
HOMO
[eV]d
b
Compound λabs[nm]a λem [nm]a
Φp
Eg [eV]e
266, 282,
443, 471sh
0.70
-2.05
-5.43
3.16
ZPt1
ZPt2
ZPt3
306
273, 309 449, 477sh
0.24
0.34
-2.37
-2.40
-5.39
-5.21
3.02
3.03
270, 330 444, 474sh
a
Measured in CH2Cl2 at room temperature with a concentration of 10-5 M, sh =
shoulder. b Measured in degassed CH2Cl2 relative to fac-[Ir(ppy)3] (Φp = 0.4). c LUMO
= HOMO + Eg. d HOMO were calculated from UPS. e The optical band gap estimated
from the absorption spectra.

ACCEPTED MANUSCRIPT
Figure 3. Density functional theory calculation of orbit density for the HOMOs and
LUMOs of ZPt1, ZPt2 and ZPt3.

ACCEPTED MANUSCRIPT
0.05
0.04
0.03
0.02
0.01
0.00
100
10
1
10k
1k
(c)
ZPt1:CBP 15%
ZPt1:CBP 20%
ZPt1:CBP 25%
ZPt1:CBP 30%
(a)
40
30
20
10
0
ZPt1:CBP 15%
ZPt1:CBP 20%
ZPt1:CBP 25%
ZPt1:CBP 30%
0.1
10
1
-2
@ 5 mA cm
0.1
10
ZPt1:CBP 15%
ZPt1:CBP 20%
ZPt1:CBP 25%
ZPt1:CBP 30%
100
1
(b)
0.1
300
400
500
600
700
800
0
2000
4000
6000
-2
8000
3
4
5
6
7
8
Voltage (V)
Wavelength (nm)
Luminance (cd m
)
Figure 4. (a) Current density-voltage-luminance (J-V-L) characteristics, (b) current efficiency (CE)-, power efficiency (PE)- and external
quantum efficiency (EQE)-luminance, and (c) electroluminescent (EL) spectra curves of ZPt1 at different doping ratios at 5 mA cm-2.

ACCEPTED MANUSCRIPT
100
10
1
(c)
ZPt3:CBP 5%
ZPt3:CBP 10%
ZPt3:CBP 15%
ZPt3:CBP 20%
(a)
40
30
20
10
0
0.03
0.02
0.01
0.00
ZPt3:CBP 5%
ZPt3:CBP 10%
ZPt3:CBP 15%
ZPt3:CBP 20%
1k
0.1
10
1
-2
@ 5 mA cm
100
10
0.1
10
ZPt3:CBP 5%
ZPt3:CBP 10%
ZPt3:CBP 15%
ZPt3:CBP 20%
1
0.1
(b)
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
Voltage (V)
300
400
500
600
700
800
0
1000 2000 3000 4000 5000 6000 7000
-2
Wavelength (nm)
Luminance
(cd m
)
Figure 5. (a) Current density-voltage-luminance (J-V-L) characteristics, (b) current efficiency (CE)-, power efficiency (PE)- and external
quantum efficiency (EQE)-luminance, and (c) electroluminescent (EL) spectra curves of ZPt3 at different doping ratios at 5 mA cm-2.

ACCEPTED MANUSCRIPT
Table 2. EL performance of the Pt(II) complexes-doped OLEDs.
Dopant ratio
[wt %]
Ƞc.max./ Ƞc.500/Ƞc.1000
Ƞp.max./ Ƞp.500/Ƞp.1000
ȠEQE.max./ Ƞ500/Ƞ1000
λem
λsh
Vturn-on
[V]
Dopant
CIE (x,y) (d)
[cd A-1] (a)
[lm W-1] (b)
[%] (c)
[nm]
[nm]
15
20
25
30
3
45.4
49.9
56.4
58.0
2.5
34.1
40.2
49.2
54.7
1.5
25.9
32.4
42.4
50.1
-
40.4
44.0
49.8
51.6
2.1
25.3
30.0
37.1
42.7
0.6
17.0
21.6
28.8
35.8
-
12.9
14.1
16.0
16.4
1.3
9.6
7.3
9.1
542
540
540
540
456
456
456
456
533
534
538
541
-
(0.34, 0.57)
(0.34, 0.57)
(0.33, 0.57)
(0.33, 0.57)
(0.25, 0.27)
(0.19, 0.27)
(0.19, 0.27)
(0.18, 0.30)
(0.38, 0.44)
(0.31, 0.53)
(0.33, 0.55)
(0.35, 0.55)
3.80
3.77
3.71
3.65
5.06
4.14
3.90
3.57
5.03
4.05
3.80
3.64
11.3
-
ZPt1
14.0 12.0
15.5 14.1
-
-
0.8
2.3
2.9
6.0
-
-
486
486
486
487
497
503
503
-
10
15
20
5
7.1
4.3
3.4
4.4
9.8
-
6.2
2.4
1.6
2.0
5.7
-
3.7
1.8
2.2
4.6
-
ZPt2
10
6.0
9.0
3.6
4.9
16.5
8.5
12.9
-
15.4
7.3
9.2
7.7
-
3.1
10
15
20
28.0
45.5
52.5
12.6
29.8
47.5
7.7
21.3
40.6
24.6
39.9
47.1
8.4
4.5
13.9
29.0
8.7
4.0
10.3
2.5
6.5
ZPt3
21.9
37.1
13.8
15.7
14.2 12.1
a)
Maximum current efficiency (Ƞc.max), and current efficiency (Ƞc.500, Ƞc.1000) measured at a brightness of 500 cd m-2 and 1000 cd m-2,
respectively; b) Maximum power efficiency (Ƞp.max), and power efficiency (Ƞp.500, Ƞp.1000) measured at a brightness of 500 cd m-2 and 1000 cd m-2,
c)
respectively; Maximum external quantum efficiency (ȠEQE.max.), and EQE (ȠEQE.500, ȠEQE.1000) measured at 500 cd m-2 and 1000 cd m-2,
respectively; d) Commission Internationale de L’Eclairage coordinates measured at 5 mA cm-2.

ACCEPTED MANUSCRIPT
Table of Contents
N
N
N
N
N
N
N
N
N
Pt
N
Pt
O
N
Pt
O
O
O
ZPt1
ZPt2
ZPt3
A
series
of
platinum(II)
complexes
based
on
pyrazolo[1,5-f]phenanthridine-containing ligands have been prepared for highly
efficient phosphorescent OLEDs.

ACCEPTED MANUSCRIPT
Highlights
Three
highly
luminescent
Pt(II)
emitters
based
on
pyrazolo[1,5-f]phenanthridine-containing ligands were designed and synthesized.
The rigid and conjugated motif led to the extremely high thermal stabilities of Pt(II)
complexes.
ZPt1 showed high OLED performance, indicating its potential utility as an OLED
phosphor.