Efficient sky-blue emitting Pt(II) complexes based on imidazo[1,2-: F] phenanthridine-containing tetradentate ligands

Article abstract of DOI:10.1039/c7tc02622f

Three blue-emitting Pt(ii) complexes (Pt1, Pt2 and Pt3) supported by imidazo[1,2-f]phenanthridine-containing tetradentate ligands have been designed and prepared. A fused ring system, imidazo[1,2-f]phenanthridine, was introduced into these complexes to mi

Full text of DOI:10.1039/c7tc02622f

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Efficient skyꢀblue emitting Pt(II) complexes based on
imidazo[1,2-f]phenanthridine-containing tetradentate ligands
Wenjuan Zhang,†a Jie Ma,†a GuoꢀJun Liu,a XiangꢀYang Liu,a Jian Fan,a,b* and
LiangꢀSheng Liaoa,b
aJiangsu Key Laboratory for CarbonꢀBased 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
*Eꢀmail: jianfan@suda.edu.cn
†The two authors contribute equally to this paper
Three blueꢀemitting Pt(II) complexes (Pt1, Pt2 and Pt3) supported by
imidazo[1,2ꢀf]phenanthridineꢀcontaining tetradentate ligands have been designed and
prepared. The fused ring system, imidazo[1,2ꢀf]phenanthridine), was introduced into
these complexes to minimize the vibration and rotation of the ligand due to its rigidity,
and the incorporation of this robust motif led to the high thermal stability of the
resulting complexes. In addition, this efficient bidentate possesses high triplet energy
(ET) to allow the preparation of blueꢀemitting complexes. Bulky mesityl group was
attached to this rigid unit to reduce the strong intermolecular interaction and suppress
the formation of excimer at solid state. Imidazo[1,2ꢀf]phenanthridine was connected
with three different pyridine^benzene bidentates to give the corresponding
tetradentate ligands (L1, L2 and L3). In L1, the pyridine group was directly bonded
to N atom of carbazole group; while in L2 and L3, the pyridine unit was linked to
benzene group via oxygen and spiro linkage, respectively. The introduction of
heteroatom linkage and spiro linkage into these tetradentate ligands would be
effective in breaking the conjugation, leading the wide bandgap and high ET. Despite

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the different linkages applied in the pyridine^benzene bidentates, the complexes
Pt1ꢀPt3 showed similar photoluminescence spetra with the intense emission peak at
460 nm. Organic lightꢀemitting diodes (OLEDs) based on these complexes were
fabricated by vacuum deposition with the typical device configuration. All these
complexes showed blue electroꢀphosphorescence at various doping ratios. Among
these Pt(II) complexes, Pt1 showed the highest OLED performance with the
maximum CE, PE, and EQE of 36.5 cd Aꢀ1, 33.1 lm Wꢀ1, and 16.2%, respectively,
with the CIE coordinates of (0.18,0.32).
Introduction
Since Tang and VanSlyke pioneered the electroluminescent device using organic
materials as the emitting elements in 1987,1 the last three decades have witness the
tremendous progress in organic optoelectronic field.2 Organic lightꢀemitting diodes
have shown successful commercialization in displays of mobile phones, televisions
and panel applications due to the development of material technologies and device
architectures. The multiꢀcolor emissive materials are considered to be important for
the fabrication of fullꢀcolor OLED display and white lighting. Since phosphorescent
metal complexes can harvest 100% of electroꢀgenerated singlet and triplet excitons
via the strong spinꢀorbit coupling of metal ions, they have been extensively studied
for decades.3 So far the external quantum efficiency (EQE), lifetime and driving
voltage of green and red phosphors have been enhanced greatly to a
commercialization level. The blue phosphorescent emitters demonstrated relatively
short lifetime although highly efficient blue phosphorescent OLEDs (PhOLEDs) with
the maximum EQE over 30% has been achieved.4 As a result, green and red
phosphorescent emitters, and blue fluorescent emitters were typically applied in
current OLED products.5 Traditional fluorescent emitters only exhibited 25% internal
quantum efficiency (IQE) and tripletꢀtriplet fusion process can result in 62.5% IQE.
Thermally activated delayed fluorescent (TADF)6 emitters can allow 100% IQE and
are considered as an alternative approach to achieve highꢀefficiency blue OLEDs.
Currently, the lifetime of blue TADF devices is short probably due to the synergetic
oxidation process.7
Although the fluorescent blue emitters have been successfully applied in OLED
product, their limited electroluminescence efficiency encouraged the exploration of
novel phosphorescent emitters. Ir(III)ꢀbased blue emitters have been widely

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investigated.3h,8 The typical strategies to achieve blueꢀemitting Ir(III) complexes are
as follows:1) adding electronꢀwithdrawing group such as F and CF3 on the aryl group
in the arylpyridine series to lower the energy of HOMO; 2) replacing the strong
electronꢀwithdrawing group pyridine with azole group or carbene to raise the energy
of LUMO and widen the LUMOꢀHOMO energy gap.5,9 For example,
bis((4,6ꢀdifluorophenyl)ꢀpyridine) iridium(III) picolinate (FIrpic), one of most studied
blue phosphors with the CIE coordinates of (0.16, 0.37),10 has two F atoms attached to
the benzene ring. However, it has been reported that F substituted emitters would
decompose during the evaporation process and the operation of OLEDs.11 Recently
much research has focused on developing Pt(II)ꢀbased blue emitters.12 For example,
Li and coworkers reported a new route for the development of stable and efficient
blueꢀemitting Pt(II) complexes by breaking conjugation with 6ꢀmembered chelate
rings.9a In addition, “pure” blue OLEDs based on tetradentated Pt(II) complex has
also been realized with a peak EQE of 24.8% and CIE coordinates of (0.148, 0.079).13
The tetradentate coordination model observed in Pt(II) complexes could suppress the
vibration and rotation around metal ion and hence reduce the nonꢀradiative decay.14
Furthermore, the great degree of rigidity induced by the tetradentate ligand led to a
less contribution of intraligand Franck−Condon vibrational modes in the longer
wavelength emission, thus ensuring good color purity.12f, 15
In this paper, three tetradentated platinum(II) complexes (Pt1, Pt2, and Pt3) have
been successfully prepared based on imidazo[1,2ꢀf]phenanthridine motif, which has
been widely used in the construction of fluorineꢀfree blue Ir(III)ꢀbased emitter.16 On
the contrary, the incorporation of this rigid unit into tetradentate ligands for the
preparation of blue Pt(II)ꢀbased emitters has been rarely reported.17 Herein, three
tetradentate ligands (L1ꢀL3) were prepared by linking imidazo[1,2ꢀf]phenanthridine
with
three
different
pyridine^benzene
bidentates,
namely
and
9ꢀ(pyridinꢀ2ꢀyl)ꢀ9Hꢀcarbazole,
2ꢀphenoxyꢀpyridine
2ꢀ(9ꢀphenylꢀ9Hꢀfluorenꢀ9ꢀyl)ꢀpyridine. Bulky mesityl group was attached to the
reactive 3ꢀposition of imidazo[1,2ꢀf]phenanthridine to reduce the side reaction under
electricꢀexcitation condition and increase the solubility of resulting complexes.
OLEDs based on Pt1ꢀPt3 were fabricated to investigate the effect of different
linkages applied in pyridine^benzene bidentates on the electroluminescence properties
of the three Pt(II) complexes. Among these three Pt(II) complexes, Pt1 showed the
highest OLED performance with the maximum CE, PE, and EQE of 36.5 cd Aꢀ1, 33.1

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lm Wꢀ1, and 16.2%, respectively, which was comparable to the best skyꢀblue
Pt(II)ꢀbased emitters.
Experimental
General Information
All chemicals were used as received from J&K Scientific, Alfa Aesar, Acros,
and Strem chemicals. Anhydrous solvents were obtained from an Innovative
Technology solvent purification system. 1H NMR spectra were recorded on a Bruker
400 MHz NMR instrument and 13C NMR spectra were recorded on Agilent DD2ꢀ600
MHz NMR instrument. Thermogravimetric analysis (TGA) was recorded on a TA
SDT 2960 instrument at a heating rate of 10 oC minꢀ1 under nitrogen. MatrixꢀAssisted
Laser Desorption/ Ionization Time of Flight Mass Spectrometry (MALDIꢀTOFꢀMS)
was measured with a BRUKER ultrafleXtreme MALDIꢀTOF spectrometer. The
HOMO was determined from Ultraviolet Photoemission Spectroscopy (Kratos
Analytical Shimadzu Group Company). UVꢀvis absorption spectra were measured on
Cary 60 spectrometer (Agilent Technologies). PL spectra and phosphorescent spectra
were recorded on a Hitachi Fꢀ4600 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 threeꢀparameter hybrid
exchange functional and correlation functional (B3LYP)/6ꢀ31G(d) with no assistance
of experimental data.
Synthesis of 2-bromo-5-methoxybenzonitrile
2ꢀBromoꢀ5ꢀmethoxybenzaldehyde (9 g, 41.85 mmol), hydroxylamine
hydrochloride (7.42 g, 106.7 mmol) were added into a mixture of ethanol (50 mL) and
pyridine (50 mL) under N2. The reaction system was heated to reflux overnight. After
cooling down to the room temperature, the mixture was extracted with DCM. After
the removal of organic solvent, the residue was dissolved in acetic anhydride (100 mL)
with trace amount of sodium acetate (0.236 g, 2.88 mmol), and this mixture was
o
refluxed at 140 C for 6 h. After cooling down to the room temperature, the solvent

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was removed under reduced pressure. The crude product was purified by column
chromatography (SiO2, DCM:PE, 2:3) to afford a white solid (7.8 g, 87.9%). 1H NMR
(400 MHz, CDCl3) δ 7.54 (d, J = 8.9 Hz, 1H), 7.15 (d, J = 2.8 Hz, 1H), 7.00 (dd, J =
8.9, 2.9 Hz, 1H), 3.82 (s, 3H). GCꢀMS: m/z calcd 210.96, found 210.94.
Synthesis of 8-methoxyphenanthridin-6-amine
An aqueous solution of Na2CO3 (2M, 20 mL) was added into a solution of
2ꢀbromoꢀ5ꢀmethoxybenzonitrile
(5.81
g,
27.40
mmol),
2ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)aniline (6 g, 27.40 mmol), and
(Ph3P)2PdCl2 (0.8 g, 1.14 mmol) in DMF (100 mL) under N2. The reaction mixture
was heated to 80 oC overnight before cooling down to the room temperature. After the
removal of solvent, the residue was extracted with DCM (3×100 mL) and washed
with water. The combined organic phase was dried over MgSO4. The crude product
was purified by column chromatography (SiO2, DCM:triethylamine, 50:1) to afford a
1
light yellow solid (5.1 g, 82.3%). H NMR (400 MHz, CDCl3) δ 8.43 (d, J = 9.0 Hz,
1H), 8.26 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.54 (t, J = 7.6 Hz, 1H), 7.48
(d, J = 8.9 Hz, 1H), 7.40 (t, J = 7.5 Hz, 1H), 7.35 (s, 1H), 3.99 (s, 3H). GCꢀMS: m/z
calcd 224.10, found 224.12.
Synthesis of 11-methoxyimidazo-[1,2-f]-phenanthridine
An aqueous solution of Na2CO3 (6.13 g, 57.87 mmol) was added into a solution
of 8ꢀmethoxyphenanthridinꢀ6ꢀamine (4.32 g, 19.29 mmol), chloroacetaldehyde (14.1
g, 57.87 mmol) in isopropanol (100 mL) under N2. The reaction mixture was heated
to 80 oC overnight before cooling down to the room temperature. After the removal of
solvent, the residue was extracted with DCM (3×100 mL) and washed with water. The
combined organic phase was dried over MgSO4. The crude product was purified by
column chromatography (SiO2, DCM:EA, 2:1) to afford a light yellow solid (4.0 g,
1
83.7%). H NMR (400 MHz, DMSO) δ 8.62 (dd, J = 19.7, 6.8 Hz, 3H), 8.31 (d, J =
8.2 Hz, 1H), 7.93 (d, J = 2.1 Hz, 1H), 7.66 (t, J = 7.6 Hz, 1H), 7.60 – 7.50 (m, 2H),
7.31 (dd, J = 8.9, 2.2 Hz, 1H), 3.96 (s, 3H). GCꢀMS: m/z calcd 248.10, found 248.19.
Synthesis of 3-bromo-11-methoxyimidazo-[1,2-f]-phenanthridine
11ꢀMethoxyimidazoꢀ[1,2ꢀf]ꢀphenanthridine (3.77 g, 15.20 mmol) and NBS (2.71
g, 15.20 mmol) were dissolved in DMF (100 mL) under N2. The reaction mixture was
stirred at the room temperature overnight. After the removal of solvent, the residue
was extracted with DCM (3×100 mL) and washed with water. The combined organic
phase was dried over MgSO4. The crude product was purified by column

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1
chromatography (SiO2, DCM:EA, 4:1) to afford a yellow solid (4.5 g, 90.7%). H
NMR (400 MHz, CDCl3) δ 9.25 (d, J = 8.0 Hz, 1H), 8.33 (d, J = 7.4 Hz, 1H), 8.20 (d,
J = 9.0 Hz, 1H), 7.98 (s, 1H), 7.57 – 7.46 (m, 3H), 7.21 (d, J = 8.8 Hz, 1H), 3.99 (s,
3H). GCꢀMS: m/z calcd 326.00, found 326.15.
Synthesis of 3-mesityl-11-methoxyimidazo-[1,2-f]-phenanthridine
An aqueous solution of K3PO4 (3 g, 10 mL) was added into a solution of
3ꢀbromoꢀ11ꢀmethoxyimidazoꢀ[1,2ꢀf]ꢀphenanthridine
(3.8
g,
11.66
mmol),
mesitylboronic acid (3.82 g, 23.32 mmol), Pd2(dba)3 (0.214 g, 0.233 mmol) and
P(tꢀBu)3HBF4 (0.272 g, 0.936 mmol) in toluene (100 mL) under N2. The reaction
mixture was heated to 85 oC overnight before cooling down to the room temperature.
The crude product was purified by column chromatography (SiO2, DCM:EA, 8:1) to
afford a white solid (2.2 g, 51.3%). 1H NMR (400 MHz, DMSO) δ 8.61 (d, J = 8.1 Hz,
1H), 8.56 (d, J = 9.0 Hz, 1H), 8.02 (d, J = 2.3 Hz, 1H), 7.45 (t, J = 7.2 Hz, 1H), 7.35 –
7.20 (m, 4H), 7.11 (s, 2H), 3.98 (s, 3H), 2.38 (s, 3H), 1.95 (s, 6H). GCꢀMS: m/z calcd
366.17, found 366.18.
Synthesis of 3-mesitylimidazo-[1,2-f]-phenanthridin-11-ol
The solution of 3ꢀmesitylꢀ11ꢀmethoxyimidazoꢀ[1,2ꢀf]ꢀphenanthridine (2 g, 5.46
mmol) in HBr (50 mL, 48%) was heated to 125 oC under N2 for 3 days before cooling
down to the room temperature. After the removal of solvent, the residue was extracted
by DCM (3×100 mL) and washed with water. The combined organic phase was dried
over MgSO4. The product was purified by column chromatography (SiO2, DCM:EA,
3:1) to afford a white solid (1.2 g, 62.1%). 1H NMR (400 MHz, DMSO) δ 8.64 (d, J =
9.2 Hz, 2H), 8.37 (s, 1H), 7.52 (d, J = 7.1 Hz, 1H), 7.47 (t, J = 7.5 Hz, 1H), 7.37 (s,
1H), 7.30 (t, J = 7.8 Hz, 1H), 7.23 (d, J = 8.6 Hz, 1H), 7.12 (s, 2H), 6.30 (s, 1H), 2.38
(s, 3H), 1.95 (s, 6H). GCꢀMS: m/z calcd 352.16, found 352.21.
Synthesis of ligand L1
K3PO4 (1.28 g, 6.04 mmol) was added into
2ꢀbromoꢀ9ꢀpyridinꢀ2ꢀylꢀ9Hꢀcarbazole (0.65 g,
a
solution of
mmol),
2.01
3ꢀmesitylimidazoꢀ[1,2ꢀf]ꢀphenanthridinꢀ11ꢀol (0.89 g, 2.52 mmol), CuI (72 mg, 0.38
mmol) and picolinic acid (93 mg, 0.76 mmol) in DMSO (100 mL) under N2. The
o
reaction mixture was heated to 120 C under microwave for 18 hours. Then the
DMSO was removed by distillation under reduced pressure. The residue was
extracted with DCM (3×100 mL) and washed with water. The combined organic
phase was dried over MgSO4. The crude product was purified by column

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1
chromatography (SiO2, DCM:EA, 6:1) to afford a white solid (1.0 g, 66.6%). H
NMR (600 MHz, DMSO) δ 8.63 (d, J = 3.8 Hz, 1H), 8.56 (t, J = 8.9 Hz, 2H), 8.29 (d,
J = 8.4 Hz, 1H), 8.22 (d, J = 7.7 Hz, 1H), 8.04 – 7.98 (m, 1H), 7.95 (d, J = 2.1 Hz,
1H), 7.76 (d, J = 8.0 Hz, 2H), 7.65 (d, J = 1.5 Hz, 1H), 7.46 – 7.35 (m, 4H), 7.32 (t, J
= 7.5 Hz, 1H), 7.25 – 7.20 (m, 1H), 7.16 (ddd, J = 20.4, 12.0, 6.8 Hz, 3H), 7.02 (s,
2H), 2.29 (s, 3H), 1.86 (s, 6H). 13C NMR (151 MHz, DMSO) δ 158.90, 154.75,
150.89, 149.93, 139.90, 139.84, 138.59,131.66, 128.96, 126.53, 122.50, 122.31,
121.72, 120.70, 119.45, 114.29, 111.59, 109.86, 103.76, 21.28, 20.42. GCꢀMS: m/z
calcd 594.24, found 594.34.
Synthesis of Pt1
L1 (0.8 g, 1.35 mmol) and K2PtCl4 (0.62 g, 1.48 mmol) were added into a
mixture of CHCl3/AcOH (40 mL, v/v= 1:9) under N2. The reaction solution was
o
heated to 140 C for 5 days. After cooling down to the room temperature, the
precipitate was collected by filtration. The crude product was purified by column
1
chromatography (SiO2, DCM:PE, 6:1) to afford a yellow solid (0.9 g, 84.5%). H
NMR (400 MHz, DMSO) δ 9.61 (d, J = 5.3 Hz, 1H), 8.73 (d, J = 8.1 Hz, 1H), 8.32 (d,
J = 8.8 Hz, 2H), 8.26 (t, J = 7.5 Hz, 1H), 8.20 (d, J = 7.5 Hz, 1H), 8.12 (d, J = 8.1 Hz,
1H), 7.99 (t, J = 3.8 Hz, 2H), 7.61 (t, J = 7.5 Hz, 1H), 7.45 (dt, J = 14.9, 7.9 Hz, 5H),
7.34 (d, J = 5.0 Hz, 2H), 7.21 (s, 2H), 2.43 (s, 3H), 2.12 (s, 6H). 13C NMR (151 MHz,
CDCl3) δ 153.27, 151.86, 148.56, 142.55, 139.44, 132.37, 128.61, 127.21, 126.74,
124.89, 122.74, 119.68, 118.06, 115.90, 115.19, 114.44, 113.66, 93.98, 21.38, 20.51.
MALDIꢀTOF (m/z): calculated for: 787.191, found: 787.292. Anal. calcd for:
C41H28N4OPt :C, 62.51; H, 3.58; N, 7.11; found: C, 62.35; H, 3.82; N, 7.01%.
Synthesis of L2, Pt2, L3 and Pt3
L2, Pt2, L3 and Pt3 were prepared by the procedures similar to those used for L1 and
1
Pt1, respectively. L2 was obtained as a white solid (1.0 g, 56.2%). H NMR (600
MHz, DMSO) δ 8.67 (d, J = 16.3 Hz, 2H), 8.20 (dd, J = 4.7, 1.2 Hz, 1H), 8.05 (d, J =
2.4 Hz, 1H), 7.86 (s, 1H), 7.50 (dd, J = 18.1, 9.9 Hz, 3H), 7.31 (d, J = 8.7 Hz, 2H),
7.23 (d, J = 8.4 Hz, 1H), 7.14 (s, 1H), 7.11 (s, 2H), 7.08 – 7.04 (m, 2H), 7.01 (d, J =
13.6 Hz, 2H), 2.37 (s, 3H), 1.94 (s, 6H). 13C NMR (151 MHz, DMSO) δ 163.03,
157.55, 157.24, 155.74, 147.95, 142.17, 140.73, 139.31, 138.64, 132.95, 131.77,
131.38, 129.01, 128.93, 128.48, 127.51, 126.22, 125.76, 125.37, 125.01, 123.14,
121.82, 120.56, 119.81, 117.33, 115.78, 115.00, 112.96, 112.21, 110.80, 21.33, 20.47.
GCꢀMS: m/z calcd 521.21, found 521.36.

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Pt2 was obtained as a yellow solid (0.71g, 68.9%). 1H NMR (600 MHz, DMSO)
δ 9.14 – 9.08 (m, 1H), 8.69 (d, J = 8.0 Hz, 1H), 8.27 (d, J = 8.7 Hz, 1H), 8.24 – 8.18
(m, 1H), 7.86 (s, 1H), 7.58 (t, J = 7.5 Hz, 1H), 7.52 (d, J = 8.3 Hz, 1H), 7.46 – 7.35
(m, 3H), 7.30 (d, J = 8.3 Hz, 1H), 7.19 (dd, J = 10.1, 5.3 Hz, 3H), 7.08 (dd, J = 7.9,
1.1 Hz, 1H), 6.95 (dd, J = 7.8, 1.1 Hz, 1H), 2.41 (s, 3H), 2.08 (s, 6H). 13C NMR (151
MHz, DMSO) δ 157.73, 154.61, 153.87, 151.12, 150.35, 141.65, 140.06, 139.42,
129.07, 128.27, 126.84, 126.39, 125.24, 124.41, 121.16, 119.43, 116.00, 115.62,
110.71, 97.96, 21.38, 20.63. MALDIꢀTOF (m/z), calculated for: 714.160, found:
714.232 . Anal. calcd for: C35H25N3O2Pt :C, 58.82; H, 3.53; N, 5.88; found: C, 58.81;
H, 3.73; N, 5.92%.
L3 was obtained as a white solid (0.95 g, 75.2%). 1H NMR (600 MHz, DMSO) δ 8.62
– 8.53 (m, 3H), 7.95 (d, J = 2.6 Hz, 1H), 7.88 (d, J = 7.5 Hz, 2H), 7.61 – 7.53 (m, 3H),
7.43 (t, J = 7.5 Hz, 1H), 7.36 (t, J = 7.4 Hz, 2H), 7.33 – 7.30 (m, 2H), 7.27 (dd, J =
14.7, 7.3 Hz, 4H), 7.21 (d, J = 8.3 Hz, 1H), 7.17 (dd, J = 7.0, 5.2 Hz, 1H), 7.08 (s,
2H), 7.04 – 6.99 (m, 2H), 6.78 (dd, J = 8.8, 5.0 Hz, 2H), 2.34 (s, 3H), 1.92 (s, 6H).
13C NMR (151 MHz, DMSO) δ 163.16, 157.56, 155.79, 149.93, 149.38, 148.79,
140.35, 139.31, 138.64, 137.22, 131.78, 130.48, 129.03, 128.51, 128.39, 128.33,
126.98, 126.09, 125.74, 124.95, 123.83, 122.53, 121.82, 121.49, 120.93, 120.27,
119.32, 117.78, 114.99, 110.24, 67.01, 21.33, 20.47. GCꢀMS: m/z calcd 669.28, found
669.45.
1
Pt3 was obtained as a yellow solid (0.22g, 17.1%). H NMR (600 MHz, DMSO) δ
9.50 (d, J = 5.4 Hz, 1H), 8.72 (d, J = 8.1 Hz, 1H), 8.62 (d, J = 7.8 Hz, 2H), 8.29 (d, J
= 8.6 Hz, 1H), 7.97 (d, J = 7.6 Hz, 2H), 7.86 (t, J = 7.8 Hz, 1H), 7.66 – 7.53 (m, 3H),
7.42 (dt, J = 12.9, 7.9 Hz, 4H), 7.31 (d, J = 8.4 Hz, 1H), 7.28 (t, J = 7.6 Hz, 2H), 7.18
(s, 2H), 7.07 (d, J = 7.8 Hz, 1H), 6.85 (d, J = 8.3 Hz, 1H), 6.76 (t, J = 7.8 Hz, 1H),
13
6.38 (d, J = 7.8 Hz, 1H), 2.41 (s, 3H), 2.11 (s, 6H). C NMR (151 MHz, DMSO) δ
160.28, 153.91, 153.12, 151.58, 140.10, 139.39, 139.07, 129.07, 132.83, 132.26,
129.07, 128.61, 128.36, 128.16, 127.55, 126.91, 126.39, 125.84, 125.24, 125.10,
124.63, 123.29, 121.15, 119.24, 118.10, 116.87, 115.68, 71.55, 21.38, 20.71.
MALDIꢀTOF (m/z): calculated for: 862.227, found: 862.201. Anal. calcd for:
C48H33N3OPt: C, 66.81; H, 3.85; N, 4.87; found: C, 66.67; H, 4.27; N, 4.83%.
Device Fabrication and Measurements
The OLED devices were fabricated through vacuum deposition on commercial

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ITOꢀcoated glass substrates having a sheet resistance of 15 ꢁ per square. The ITO
surface was ultrasonically cleaned with acetone, ethanol, and deionized water, then
dried in an oven at 120 °C 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 HATꢀCN, organic layers and Liq
were 0.4ꢀ0.8, 2ꢀ3 and 0.2ꢀ0.5 Å sꢀ1, respectively. Finally, the Al electrode was
evaporated (5 Å sꢀ1) through a shadow mask without breaking the vacuum.
The emitting area of each device was 3 × 3 mm2. Electroluminescence (EL) spectra,
CIE coordinates and currentꢀvoltage (I-V) characteristics were measured by a constant
current source (Keithley 2400s Source Meter) combined with a photometer (Photo
Research PR 655 spectrophotometer).
Results and Discussion
Preparation and Characterization
The synthetic route of complexes Pt1ꢀPt3 was illustrated in Scheme 1.
8ꢀmethoxyphenanthridinꢀ6ꢀamine was prepared by Suzuki Miyaura coupling of
aminoboronic ester and 2ꢀhalobenzonitrile followed by intramolecular nucleophilic
addition. Amineꢀaldehyde condensation and nucleophilic substitution afforded the
precursor imidazoꢀ[1,2ꢀf]ꢀphenanthridine. Subsequent selective bromination using
NBS, followed by Suzuki Miyaura coupling with mesitylboronic acid and hydrolysis
in the presence of HBr completed the synthesis of the intermediate
3ꢀmesitylimidazoꢀ[1,2ꢀf]ꢀphenanthridinꢀ11ꢀol. CuI/2ꢀpicolinic acid catalyzed coupling
reactions of phenol derivative and aryl bromides provided the target ligands L1ꢀL3.
Within L1 and L2, the O and N linkage applied in the pyridine^benzene bidentates
were both electronꢀdonating, which could enhance the energy level of HOMO of
corresponding Pt(II) complexes; while in L3 a spiroꢀlinkage was used, which could
sufficiently suppress the strong intermolecular interaction and reduce the formation of
excimer.14c Pt(II) complexes Pt1-Pt3 were prepared via oneꢀpot reaction of K2PtCl4
and the respective ligands in the mixture of chloroform and acetic acid. All Pt(II)
complexes were purified by column chromatography and sublimation, and their
structures were confirmed by 1H and 13C NMR, MALDIꢀTOF and elemental analysis.
The thermal stability of these complexes was investigated by thermal gravimetric
analysis under nitrogen. Pt1ꢀPt3 showed high thermal stability with the 5%

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weightꢀreduction temperatures (∆T5%) over 430 ºC, which could be attributed in part
to the introduce of extending πꢀconjugation system into the tetradentate ligands.18 The
HOMOs of Pt(II) complexes were determined by UltraꢀViolet Photoemission
Spectroscopy (UPS) measurement. As expected, the HOMOs of Pt1 and Pt2 were
slightly highly than that of Pt3. The LUMOs were calculated from the difference
1
between the HOMO and the corresponding optical band gap. The H, 13C NMR
spectra (Figure S1ꢀFigure S18), thermal gravimetric analysis (TGA) curves (Figure
S19) and UPS spectra (Figure S20) of Pt1ꢀPt3 are given in the supporting
information.
Photophysical Properties
The absorption and emission spectra of complexes Pt1ꢀPt3 are depicted in Figure 1,
and the photophysical data are summarized in Table 1. All the complexes showed
strong absorption below 350 nm, which could mainly be assigned to phenylꢀOꢀphenyl
→ pyridine intraligand (IL) πꢀπ* transition. There is a redꢀshift of about 15 nm in the
absorption spectrum of Pt2 relative to that of Pt3, which could be attributed to the
oxygen lone pair delocalization onto the aryl ring. A 7 nm redꢀshift has been observed
in the absorption spectrum of Pt1 in comparison with that of Pt2, which could be due
to the presence of large conjugation system (carbazole) in Pt1. The absorption bands
with the wavelength over 350 nm of Pt1ꢀPt3 could be assigned to the metalꢀtoꢀligand
chargeꢀtransfer (MLCT). Upon photoexcitation at 340 nm, Pt1ꢀPt3 exhibited intense
emission bands at 460 nm with wellꢀresolved vibronic features, indicating the
photoluminescence could mainly arise from the LC excited states, which was also
supported by the analyses of Natural Transition Orbital (NTO). The NTO was mostly
localized on the tetradentate ligands (Figure S21). The triplet lifetimes of Pt(II)
complexes in both solution and solid state at room temperature have been determined
in the range of 8.33ꢀ10.0 ꢂs (Table 1).
DFT Calculations
Density functional theory (DFT) calculations were carried out for complexes Pt1ꢀPt3
with Gaussian 09. The calculations were performed using a B3LYP method with a
basis set of 6ꢀ31G(d) for C, H, O and N atoms and a LANL2DZ basis set for Pt atom.
The orbital densities of HOMOs and LUMOs were shown in Figure 2, and the
calculation result was listed in Table S1.

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Since pyridine unit is the most electronꢀwithdrawing group used in these ligands, the
LUMOs of Pt1ꢀPt3 mainly resided on pyridine part. As estimated from the
composition of the frontier orbitals, the pyridine group made about 83.83%
contribution to the LUMO of Pt1, 87.07% for Pt2 and 84.98% for Pt3. Due to the
weak electronꢀdonating capability of spiro linkage, the LUMO of Pt3 (ꢀ1.30 eV) was
slightly lower than those of Pt1 (ꢀ1.28 eV) and Pt2 (ꢀ1.02 eV). The HOMOs are
primarily localized on the phenylꢀOꢀphenyl motif, and platinum atom made about
18.65%, 20.78% and 22.06% contribution to the HOMOs of these complexes. The
optimized structures of Pt1ꢀPt3 showed that the mesityl group is essentially
perpendicular to the imidazoꢀ[1,2ꢀf]ꢀphenanthridine with the dihedral angle of 89.97º.
The molecular structures of Pt1ꢀPt3 significantly deviated from square planar
geometries due to the steric hindrance between imidazoꢀ[1,2ꢀf]ꢀphenanthridine and
pyridine
group.
The
dihedral
angles
between
pyridine
and
imidazoꢀ[1,2ꢀf]ꢀphenanthridine are 41.89º, 20.46º and 31.69º for Pt1, Pt2 and Pt3,
respectively.
Electroluminescent Properties
In order to study the electroluminescent properties of Pt1ꢀPt3, OLEDs based on these
dopants were fabricated with a normal sandwiched structure: ITO/HATꢀCN (10
nm)/TAPC (40 nm)/TCTA(10 nm)/26mCPy:dopant (x wt%, 20 nm)/TmPyPB (45
nm)/Liq (2 nm)/Al (120 nm) (dopant = Pt1, Pt2, Pt3; x = 2.5, 5, 7.5, 10). 1, 4, 5, 8, 9,
11ꢀhexaazatriphenyleneꢀhexacarbonitrile (HATꢀCN) and 8ꢀhydroxyquinolinolato
lithium (Liq) were used as hole and electron injection layers, respectively.
1,1ꢀbis[4ꢀ[N,Nꢀdi(pꢀtolyl)ꢀamino]phenyl]cyclohexane
(TAPC)
and
1,3,5ꢀtri[(3ꢀpyridyl)ꢀphenꢀ3ꢀyl]benzene (TmPyPB) were utilized as hole and electron
transport layers, respectively. 2,6ꢀbis(Nꢀcarbazolyl)pyridine (26mCPy) with high ET
of 2.8 eV was used as bipolar host material.19
To optimize the device efficiency, OLEDs were fabricated with Pt1ꢀPt3 at different
doping ratios. The EL spectra of Pt1ꢀPt3 were depicted in Figure 3, 5 and 7,
respectively. All the complexes showed intense EL emission band at around 460 nm
with the shoulder at around 490 nm, matching well with their PL spectra, which could
be due to the efficient triplet confinement of the excitons blocking layers.20 The CIE
coordinates (Table 2) for Pt(II) complexes were in the skyꢀblue region, and they were
shifted slightly as the doping level was increased from 2.5% to 10%, suggesting that

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the bulky mesityl group could make the molecule less prone to excimer formation.
The EQEꢀluminanceꢀcurrent efficiency curves of Pt1ꢀPt3 were showed in Figure 4, 6
and 8, respectively. The best performance was achieved for Pt1 with the doping ratio
at 10%, which reached maximum EQE, current efficiency, and power efficiency of
16.2%, 36.5 cd Aꢀ1, and 33.1 lm Wꢀ1, respectively. For Pt1, the EQE of 14.1% and
12.7% were achieved at the brightness of 100 cd mꢀ2 and 1000 cd mꢀ2, respectively.
Although the maximum EQE (16.2%) for Pt1 is not very high, compared to the
stateꢀofꢀtheꢀart Ir(III)ꢀbased blue OLEDs,4,8g Pt1 showed comparable device
performance with the best skyꢀblue Pt(II)ꢀbased emitters. For example, Li et al.
demonstrated efficient skyꢀblue OLEDs employing a tetradentate platinum complex
with EQE of 14.6% at 1000 cd mꢀ2,12c and blue OLEDs based on PtNON with EQE of
11.5% at 1000 cd mꢀ2 and CIE coordinates of (0.18, 0.39).9a
Among Pt1ꢀPt3, Pt1 demonstrated the best electroꢀluminescence properties. The
device based on Pt2 at the doping level of 10 wt% achieved a maximum efficiency of
18.5 cd Aꢀ1, 15.7 lm Wꢀ1 and 7.9%, and 12.0 cd Aꢀ1, 11.3 lm Wꢀ1 and 5.9% for Pt3.
Considering that these complexes showed similar PL and EL spectra, the relatively
poor EL properties of Pt2 and Pt3 could be attributed to the unbalanced charge
transport in the emitting layer, which could arise from the different linkage applied in
the pyridine^benzene bidentates.
Conclusions
In summary, three blueꢀemitting tetradentated Pt(II) complexes Pt1ꢀPt3 have been
successfully prepared, where the rigid imidazo[1,2ꢀf]phenanthridine motif with high
ET was used as efficient bidentate. In addition, breaking the conjugation via the
introduce of heteroatoms and spiro linkage accounted in part for the high ET of Pt(II)
complexes. The pending mesityl group was used to block the active site and to
suppress the formation of excimer, which led to the small variation in CIE coordinates
at different doping levels. The emission lifetime of Pt1 is slightly shorter than those of
Pt2 and Pt3, which could account in part for the higher efficiency observed in
Pt1ꢀbased device. Although different linkages were applied in the pyridine^benzene
bidentates, these Pt(II) complexes showed similar PL and EL spectra. However, they
demonstrated different EL properties, which could be due to different
chargeꢀtransporting properties of these pyridine^benzene bidentates. Notably, OLEDs
based on Pt1 showed high device performance with the maximum Ƞc of 36.5 cd Aꢀ1,

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Ƞp of 33.1 lm Wꢀ1 and ȠEQE of 16.2%.
Acknowledgements
We thank Mr. Yun Hu for the UPS measurement. 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 the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and
Technology (CICꢀNano), Soochow University and by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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20120518; (e) M. Mishima, Jpn. Kokai Tokkyo Koho 2011, JP 2011054748 A
20110317; (f) H. Konno, J. Taniuchi, R. Harada, T. Shigetomi, and Y. Masahiro, PCT
Int. Appl. 2016, WO 2016111256 A1 20160714; (g) D. B. Knowles, C. Lin, P. B.
MacKenzie, J.ꢀY. Tsai, R. W. Walters, S. Beers, C. S. Brown, W. Yeager, and E.
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Journal of Materials Chemistry C
Scheme 1 The synthetic routes of Pt(II) complexes Pt1ꢀPt3

Journal of Materials Chemistry C
Page 18 of 24
View Article Online
DOI: 10.1039/C7TC02622F
1.0
0.8
0.6
0.4
0.2
0.0
Pt1
Pt2
Pt3
PL
UV-vis
300
400
500
600
700
Wavelength (nm)
Figure 1 Optical absorption and photoluminescence spectra of complexes Pt1ꢀPt3 in
dilute dichloromethane solution at room temperature.

Page 19 of 24
Journal of Materials Chemistry C
Table 1 Photophysical and electrochemical data obtained experimentally for the platinum(II) complexes.
b
b
λabs[nm]a
λem [nm]a
Φp
Φfilm
LUMO [eV]d HOMO [eV]e Eg [eV]f
c
c
Compound
τ[µs]so
τ[µs]film
272, 295, 326, 372 460, 487sh
256, 286, 316, 352 460, 489sh
0.36
0.29
0.63
0.43
0.34
8.33
8.98
8.76
8.71
10.0
9.22
ꢀ2.29
ꢀ2.28
ꢀ5.27
ꢀ5.31
2.98
3.03
Pt1
Pt2
Pt3
243, 267, 289, 330 460, 490sh
0.33
ꢀ2.17
ꢀ5.32
3.15
a Measured in CH2Cl2 at 293 K with a concentration of 10ꢀ5 M, sh = shoulder.
b Measured in degassed CH2Cl2 relative to facꢀ[Ir(ppy)3] (Φp = 0.4) and film doped in PMMA.
c The extracting triplet lifetimes (τ) in degassed CH2Cl2 and doped in PMMA at room temperature.
d LUMO= HOMO + Eg.
eHOMO were calculated from UPS.
f The optical band gap estimated from the absorption spectra.

Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02622F
Pt1
Pt2
Pt3
Figure 2 Density functional theory calculation of orbit density for the HOMOs
(bottom) and LUMOs (top) of Pt1ꢀPt3.

Page 21 of 24
Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02622F
26mCP:Pt1 (2.5%)
26mCP:Pt1 (5%
26mCP:Pt1 (7.5%
26mCP:Pt1 (10%)
)
)
400
500
600
700
800
Wavelength
(nm)
Figure 3. The EL spectra of complex Pt1 with different doping ratios at 5 mA/cm2.
100
10
26mCP:Pt1 (2.5%)
26mCP:Pt1 (5%)
26mCP:Pt1 (7.5%)
26mCP:Pt1 (10%)
1
10
1
0
1000 2000 3000 4000 5000 6000 7000
Luminance (cd/m2)
Figure 4. The EQEꢀluminanceꢀcurrent efficiency curves for Pt1 at different doping
ratios.

Journal of Materials Chemistry C
Page 22 of 24
Table 2. EL performance of the Pt(II) complexesꢀdoped OLEDs.
(e)
Dopant level
[wt %]
Ƞ
c.max./ Ƞc.500/Ƞc.1000
[cd Aꢀ1] (a)
Ƞp.max./ Ƞp.500/Ƞp.1000
[lm Wꢀ1] (b)
ȠEQE.max./ Ƞ500/Ƞ1000
[%] (c)
λem
λsh
Vturnꢀon
Dopant
CIE (x,y) (d)
[nm] [nm]
[V]
2.5
5
14.0
23.5
30.5
36.5
6.0
10.3
18.6
26.5
30.6
4.6
8.6
16.1
23.0
27.5
3.8
6.2
9.9
12.5
ꢀ
12.1
6.8
13.2
19.6
23.9
2.6
5.1
7.1
5.3
9.6
12.6
14.1
2.4
3.6
5.7
6.5
1.5
2.5
3.9
4.3
4.4
460
460
460
460
460
460
460
460
464
464
464
464
492
492
492
492
492
492
492
492
494
494
494
494
(0.17,0.28)
(0.17,0.28)
(0.17,0.30)
(0.18,0.32)
(0.18,0.27)
(0.21,0.32)
(0.19,0.30)
(0.20,0.31)
(0.23,0.30)
(0.19,0.29)
(0.18,0.29)
(0.17,0.29)
4.2
4.0
3.8
3.7
4.8
4.0
4.3
4.3
4.3
4.0
3.9
3.7
20.5
27.3
33.1
4.9
10.2
15.6
19.8
1.9
3.8
5.7
7.4
ꢀ
11.7
14.2
16.2
3.2
8.0
11.1
12.7
2.0
2.9
4.8
5.8
ꢀ
Pt1
7.5
10
2.5
5
13.0
16.0
18.5
4.0
7.7
11.8
13.5
15.7
3.6
5.4
5.5
Pt2
Pt3
7.5
10
2.5
5
11.8
14.3
2.8
7.6
7.3
9.3
7.9
1.8
1.8
7.0
4.9
4.0
6.3
7.4
6.3
3.4
2.5
4.2
5.1
3.5
2.1
3.1
3.7
7.5
11.0
7.7
10.1
5.7
5.5
10
12.0
8.7
11.3
6.6
5.9
a)
Maximum current efficiency (Ƞc.max), and current efficiency (Ƞc.500, Ƞc.1000) measured at a brightness of 500 cd mꢀ2 and 1000 cd mꢀ2,
respectively; b) Maximum power efficiency (Ƞp.max), and power efficiency (Ƞp.500, Ƞp.1000) measured at a brightness of 500 cd mꢀ2 and 1000 cd mꢀ2,
c)
respectively; Maximum external quantum efficiency (ȠEQE.max.), and EQE (ȠEQE.500, ȠEQE.1000) measued at 500 cd mꢀ2 and 1000 cd mꢀ2,
respectively; d) Commission Internationale de L’Eclairage coordinates measured at 5 mA cmꢀ2; e)Turnꢀon voltage at 200 cd mꢀ2 for Pt1ꢀPt3.

Page 23 of 24
Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02622F

Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02622F
Table of Contents
N
N
N
N
Pt
O
Three blue-emitting platinum(II) complexes based on imidazo[1,2-f]phenanthridine-
containing tetradentate ligands have been prepared as efficient dopants for phosphorescent
OLEDs.