The synthesis of cyclometalated platinum(ii) complexes with benzoaryl-pyridines as C^N ligands for investigating their photophysical, electrochemical and electroluminescent properties

Article abstract of DOI:10.1039/d0dt02224a

A series of (C^N)Pt(acac)-type complexes has been successfully synthesized with a benzo[b]furan, benzo[b]thiophene, benzo[b]selenophene, or benzo[b]tellurophene group in the benzoaryl-pyridine ligand. Using X-ray crystallography, the chemical structures o

Full text of DOI:10.1039/d0dt02224a

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An international journal of inorganic chemistry
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H. yang, D. Zhong, B. Liu, X. Yang, L. Yue, G. Zhou, M. Ma and Z. Wu, Dalton Trans., 2020, DOI:
10.1039/D0DT02224A.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
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Synthesis of cyclometalated platinum(II) complexes with benzoayrDlO-pI: 1y0.r10i3d9/iDn0DeTs02224A
as C^N ligands for investigating their photophysical, electrochemical and
electroluminescent properties
Dezhi Wang,a,b Xi Chen,a Hua Yang,a Daokun Zhong,a Boao Liu,a Xiaolong Yang,a Ling Yue,a
Guijiang Zhou,*a Miaofeng Ma*b and Zhaoxin Wu*c
a
School of Chemistry, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of
Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong
University, Xi’an 710049, P. R. China.
E-mail: zhougj@mail.xjtu.edu.cn
b
Department of Applied Chemistry, College of Chemistry & Pharmacy, Northwest A&F
University, Yangling 712100, P. R. China.
E-mail: mmfyangling@126.com
c Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Faculty of
Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China.
E-mail: zhaoxinwu@mail.xjtu.edu.cn
† Electronic supplementary information (ESI) available: Synthetic procedure of the organic
compound and EL results for the devices except for the optimized ones.

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ABSTRACT
DOI: 10.1039/D0DT02224A
A series of (C^N)Pt(acac)-type complexes have been successfully synthesized with
benzo[b]furan, benzo[b]thiophene, benzo[b]selenophene and benzo[b]tellurophene groups in
their benzoayrl-pyridine ligands, respectively. With X-ray crystallography, the chemical
structures for the complexes with benzo[b]selenophene and benzo[b]tellurophene groups have
been clearly revealed. Photophysical, electrochemical and electroluminescent (EL) behaviors of
these (C^N)Pt(acac)-type complexes have been fully characterized. Furthermore, both
time-dependent functional theory (TD-DFT) and natural transition orbital (NTO) theoretical
results have been obtained to gain insight into their absorption and emission features. It has been
shown that both their absorption bands with the lowest energy and phosphorescent emission
behaviors are dominated by their benzoayrl-pyridine cyclometalating ligand. Importantly, the
effect of the atoms in group VIA on the properties of these (C^N)Pt(acac)-type complexes has
been revealed. Owing to the rareness of the (C^N)Pt(acac)-type complexes with
benzo[b]selenophene and benzo[b]tellurophene groups, their EL abilities have been
characterized by solution-processed organic light-emitting diodes (OLEDs). The optimized red
OLEDs with complex bearing benzo[b]selenophene unit can show maximum external quantum
efficiency (ηext) of 6.3%, current efficiency (ηL) of 10.5 cd A−1 and power efficiency (ηP) of 9.1
lm W−1, while the EL device with complex bearing benzo[b]tellurophene unit can give deep-red
emission at ca. 636 nm with ηext of 6.3%, ηL of 6.5 cd A−1 and ηP of 5.8 lm W−1. This research
not only provides novel (C^N)Pt(acac)-type complexes, but also furnishes critical information
for the photophysical and EL behavior of these new complexes.

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Introduction
DOI: 10.1039/D0DT02224A
Generally, Pt(II) complexes with C^N-type ligands can exhibit strong phosphorescent emission,1
which furnishes their crucial application in various fields, such as sensors,2 photosensitizers,3
photooxidants,4 bioimaging5 and electroluminescent (EL) emitters6-10 etc. It had been observed
that the nature of their phosphorescent emission can be affected by the cyclometalating
C^N-type ligand strongly through modifying their C^N-type ligand with electron-donating and
-withdrawing substituents.11 Hence, in the C^N-type ligands, more polarizable atoms such as
sulfur and nitrogen can significantly influence the features of phosphorescent emission from the
(C^N)Pt(acac)-type complexes.12 For the atoms in group VIA, oxygen and sulfur are frequently
incorporated into the C^N-type ligands to develop phosphorescent (C^N)Pt(acac)-type
complexes, showing high EL efficiencies.12 More importantly, sulfur-containing groups can be
converted into sulfonyl groups, which can offer electron injection/transporting (EI/ET) ability to
the phosphorescent (C^N)Pt(acac)-type complexes bearing these groups.12 Clearly, the atoms in
group VIA can show great potential in tuning and optimizing properties of the
(C^N)Pt(acac)-type complexes.
However, introducing oxygen and sulfur atoms to the C^N-type ligands is more prevail. For
14
16
18
20
example, benzo[b]furan,13,
dibenzo[b]furan,15,
thiophene,17,
benzo[b]thiophen,19,
dibenzo[b]thiophene,21 oxazole,22 benzo[b]oxazole,23 thiazole24, 25 and benzo[b]thiazole26, 27
moieties have been employed as building blocks for constructing the C^N-type ligands.
However, groups containing selenium and tellurium atoms have been rarely adopted to
synthesize phosphorescent emitters. One C^N-type Ir(III) complex with selenophene group had
been prepared.28 Recently, we have synthesized a series of unsymmetrical phosphorescent Ir(III)
complexes with selenophene as well as aryl selenide groups to show high EL efficiencies.29

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Arsenyan et al. reported two (C^N)Pt(acac)-type complexes with benzoselenophenylpyridine
DOI: 10.1039/D0DT02224A
ligands and characterized their photophysical and electroluminescent properties.30 In addition,
novel Ir(III) complexes with benzoselenophenylpyridine as C^N-type ligand have also been
synthesized with different auxiliary ligands by Arsenyan et al.31 These phosphorescent emitter
can furnish deep-red EL with nice efficiencies. All these encouraging results have indicated
research interest in exploring potential of the atoms in group VIA for offering unique
optoelectronic properties to the phosphorescent complexes. In view of the critical role played by
the atoms in group VIA, systematical investigation their effect on the properties of the
(C^N)Pt(acac)-type complexes should be very necessary. On this basis, a series of
(C^N)Pt(acac)-type complexes bearing the benzoayrl-pyridine ligand with benzo[b]furan,
benzo[b]thiophene, benzo[b]selenophene and benzo[b]tellurophene groups have been
synthesized, respectively. In addition, their photophysical, thermal, electrochemical and EL
properties have been studied in detail. Obviously, for the first time, the reported results should
reveal an overall picture for the effect of the atoms in group VIA on the properties of the
(C^N)Pt(acac)-type complexes.
Experimental
General information
Commercially available chemicals were used directly for synthesis without further purification.
All solvents for the reactions were dried and distilled in a proper way. The reactions were
monitored with thin layer chromatography (TLC) purchased from Merck & Co., Inc. Silica gel
used for flash column chromatography and preparative TLC plates were purchased from
Shenghai Qingdao (300-400 mesh).
Physical measurements

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DOI: 10.1039/D0DT02224A
13
1H and C NMR spectra were measured in CDCl solvent with a Bruker AXS 400MHz NMR
3
spectrometer. Chemical shifts were quoted to the solvent residual peak at δ 7.26 ppm for 1H and
77.0 ppm for 13C NMR spectra, respectively. UV-vis spectra were recorded with a PerkinElmer
Lambda 950 spectrophotometer. The photoluminescent (PL) properties of these
(C^N)Pt(acac)-type complexes were measured with an Edinburgh Instruments FLS920
fluorescence spectrophotometer. The lifetimes for the excited states were measured by a single
photon counting spectrometer from Edinburgh Instruments FLS920 with a 360 nm picosecond
LED lamp as the excitation source. The phosphorescent quantum yields (ΦP) were determined in
CH2Cl2 solutions at 293 K against fac-[Ir(ppy)3] (ΦP = 97%).32 The absolute ΦP data for the
doped TCTA (4,4′,4′′-tri(9-carbazoyl)triphenylamine) films were measured with an integrated
sphere on Edinburgh Instruments FLS920 spectrometer. The differential scanning calorimetry
(DSC) and thermal gravimetric analysis (TGA) data were acquired on a NETZSCH DSC 200
PC unit and a NETZSCH STA 409C instrument, respectively. Cyclic voltammetry (CV) were
conducted with a Princeton Applied Research model 273A at a 100 mV s-1 scan rate. All the CV
experiments were conducted with a three-electrode compartment cell possessing a Pt-sheet
counter electrode, a glassy carbon working electrode and an Ag/AgCl reference electrode. The
supporting electrolyte was 0.1 M acetonitrile solution of [nBu4N]BF4 with ferrocene as internal
reference. The data of elemental analyses were obtained on a Flash EA 1112 elemental analyzer.
Mass spectrometry data were obtained on a Waters I-class Vion IMS QTof micro-spectrometer.
Computational details
DFT calculations were conducted with B3LYP method for all these (C^N)Pt(acac)-type
complexes. The 6-31G (d, p) basis set was applied for C, H, N, O, S, Se and Te atoms, while
effective core potentials employed for Pt atoms were a LanL2DZ basis set.33, 34 Excitation

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behaviors were acquired by the TD-DFT calculation on the basis of the optimized ground state
DOI: 10.1039/D0DT02224A
geometries. Additionally, the natural transition orbital (NTO) based on the first triplet state (T1)
geometries optimized by UB3LYP was analyzed for S0 → T1 excitation. All the calculations
were performed using the Gaussian 09 program.35
X-ray
crystallography.
Single
crystals
of
Platinum(II)
(2-2′-(4′,5′-benzo)selenophenyl)pyridinato-N,C3′) (2,4-pentanedionato-O,O) (Pt-Se) and
Platinum(II) (2-2′-(4′,5′-benzo)tellurophenyl)pyridinato-N,C3′) (2,4-pentanedionato-O,O) (Pt-Te)
that were suitable for X-Ray diffraction studies were successfully grown through slow diffusion
of hexane into their chloroform solutions. Single crystal data were collected on a Bruker
SMART CCD diffractometer (Mo Kα radiation and λ = 0.71073 Å) in Φ and ω scan modes at
293 K. The molecular structure was solved by direct methods followed by difference Fourier
syntheses and then refined by full-matrix least-squares techniques against F2 using SHELXL-97
program.36 The positions of hydrogen atoms were calculated and refined isotopically using a
riding model. All other non-hydrogen atoms were refined isotopically. Absorption corrections
were applied using SADABS.37
Synthesis
Synthetic
details
for
((2-bromophenyl)ethynyl)trimethylsilane
(1),
2-(trimethylsilylethynyl)pyridine (5), 2-ethynylpyridine (6) 2-(benzo[b]furan-2-yl)pyridine
(L-O) and 2-(benzo[b]thiophen-2-yl)pyridine (L-S) are provided in the Electronic
Supplementary Information (ESI).
(Z)-(1-bromo-2-(2-bromophenyl)vinyl)trimethylsilane (2). Diisobutylaluminum hydride
(DIBAL-H) (1.00 M in hexane, 13.0 mL, 13.0 mmol) was added dropwise with stirring to a
solution of ((2-bromophenyl)ethynyl)trimethylsilane (1) (3.00 g, 11.9 mmol) in hexane (40.0

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mL) under a nitrogen atmosphere at room temperature. After stirring for overnight, NBS (2.23 g,
DOI: 10.1039/D0DT02224A
12.5 mmol) was added slowly in an ice bath. The reaction mixture was reacted for 0.5 h and
then diluted with water (40.0 mL). The resulting hexane layer was separated, washed with
saturated aqueous NaCl, dried, and evaporated in vacuo. The residue was chromatographed on a
silica column using petroleum ether as the eluent to give 2 as yellow oil (3.12 g, 79%). 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.74 (dd, J = 7.8, 1.4 Hz, 1H), 7.59 (d, J = 8 Hz, 1H), 7.35-7.31 (m,
2H), 7.18 (td, J = 7.7, 1.2 Hz, 1H), 0.32 (s, 9H); 13C NMR (100 MHz, CDCl3): δ (ppm) 140.37,
139.01, 135.50, 134.60, 132.94, 131.48, 128.86, 125.57, 0.27; FAB-MS (m/z): 334 [M]+; Anal.
Calcd for C11H14Br2Si: C, 39.54; H, 4.22; found: C, 39.48; H, 3.98%.
2-(trimethylsilyl)benzo[b]selenophene
(3).
Under
a
N2
atmosphere,
(Z)-(1-bromo-2-(2-bromophenyl)vinyl)trimethylsilane (2) (3.00 g, 9.04 mmol) was dissolved in
t-Butyl methyl ether (30 mL), then n-BuLi (2.50 M in hexane, 9.00 mL, 21.7 mmol) was added
dropwise at 0 oC. The mixture was warned to room temperature and stirred for 2 h. Then, at 0 °C,
selenium powder (0.86 g, 10.8 mmol) was added slowly. The reaction mixture was warned to
room temperature and stirred for overnight. Water (10.0 mL) was added and extracted with
CH2Cl2 (3 × 20.0 mL). The organic layer was dried over anhydrous Na2SO4 and the solvent was
evaporated in vacuo. The residue was chromatographed on a silica column using petroleum
ether as the eluent to give 3 as oily liquid (2.07 g, 90%). 1H NMR (400 MHz, CDCl3): δ (ppm)
7.94 (d, J = 8.0 Hz, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.74 (s, 1H), 7.52 (td, J = 7.4, 1.2 Hz, 1H),
7.25 (td, J = 7.4, 1.2 Hz, 1H), 0.37 (s, 9H); 13C NMR (100 MHz, CDCl3): δ (ppm) 147.19,
144.50, 143.73, 134.51, 125.44, 125.22, 124.32, 124.29, 0.01; FAB-MS (m/z): 254 [M]+; Anal.
Calcd for C11H14SeSi: C, 52.16; H, 5.57; found: C, 52.07; H, 5.51%.
2-bromobenzo[b]selenophene (4). To a solution of 2-(trimethylsilyl)benzo[b]selenophene (3)

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(1.00 g, 3.94 mmol) in chloroform (20.0 mL), NBS (0.700 g, 3.94 mmol) was aDdOdIe: 1d0.1a0t3V9ire/wDo0oADrmtiTc0le2O2n2l4inAe
temperature and stirred for 3 h. Water (20.0 mL) was added to the solution and the solution was
extracted with dichloromethane (3 × 10.0 mL). The combined organic layer was dried over
anhydrous Na2SO4. After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel with petroleum ether as the eluent to give 4 as
white solid (0.820 g, 80%). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.76 (d, J = 8 Hz, 1H), 7.69 (d,
J = 7.6 Hz, 1H), 7.53 (s, 1H), 7.34-7.24 (m, 2H); 13C NMR (100 MHz, CDCl3): δ(ppm) 141.0,
139.6, 126.6, 124.7, 124.5, 122.7, 121.6, 115.4; FAB-MS (m/z): 260, 262 [M]+; Anal. Calcd for
C8H5BrSe: C, 36.96; H, 1.94; found: C, 36.88; H, 1.89%.
2-(Benzo[b]selenophen-2-yl)pyridine
(L-Se).
Under
a
nitrogen
atmosphere,
2-bromobenzo[b]selenophene (4) (0.20 g, 0.77 mmol), 2-(tributylstannyl)pyridine (0.34 g, 0.92
mmol), and Pd(PPh3)4 (35 mg, 0.03 mmol) were added to degassed toluene (15 mL). The
reaction mixture was heated to 110 °C and stirred for 16 h. After cooling to room temperature,
the mixture was extracted with CH2Cl2 (3 × 30 mL). The organic layer was dried over
anhydrous Na2SO4 and evaporated under reduced pressure. Then, the residue was purified by
column chromatography over silica gel using ethyl acetate/petroleum ether (1:15, v/v) as the
eluent. The product was obtained as a white solid (0.17 g, 84%). 1H NMR (400 MHz, CDCl3): δ
(ppm) 8.59 (ddd, J = 4.9, 1.6, 1.0 Hz, 1H), 8.01 (s, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.82-7.80 (m,
2H), 7.72 (td, J = 7.4, 1.6 Hz, 1H), 7.36 (td, J = 7.5, 1.2 Hz, 1H), 7.28 (td, J = 7.6, 1.2 Hz, 1H),
13
7.20 (ddd, J = 7.4, 4.9, 1.1 Hz, 1H); C NMR (100 MHz, CDCl3): δ (ppm) 153.81, 149.66,
148.88, 143.23, 142.00, 136.52, 125.89, 125.70, 125.11, 124.72, 124.12, 122.58, 118.61.;
FAB-MS (m/z): 259 [M]+; Anal. Calcd for C13H9NSe: C, 60.48; H, 3.51; N, 5.43 found: C,
60.36; H, 3.43; N, 5.33%.

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2-((2-bromophenyl)ethynyl)pyridine (7). Under a N2 atmosphere, 1-bromo-2D-OioI:d10o.1b0e39n/zDe0DnTe02224A
(2.00 g, 7.07 mmol), 2-ethynylpyridine (6) (0.66 g, 6.36 mmol), Pd(PPh3)2Cl2 (50 mg, 0.07
mmol) and copper(I) iodide (27 mg, 0.14 mmol) were mixed in triethylamine (50 mL) and the
reaction mixture was left with stirring overnight at room temperature. Then the residue of the
solvent was filtered and the filtrate was removed under vacuum. The crude product was purified
by column chromatography using petroleum ether as the eluent to obtain the product as
1
pale-yellow oil (1.62 g, 99%). H NMR (400 MHz, CDCl3): δ (ppm) 8.63 (d, J = 4.4 Hz, 1H),
7.68 (td, J = 7.7, 1.5 Hz, 1H), 7.63-7.57 (m, 3H), 7.29 (td, J = 7.6, 1.0 Hz, 1H), 7.25-7.18 (m,
2H); 13C NMR (100 MHz, CDCl3): δ (ppm) 150.3, 143.3, 136.3, 133.9, 132.6, 130.2, 127.7,
127.2, 126.0, 124.6, 123.2, 92.9, 87.7; FAB-MS (m/z): 257, 259 [M]+; Anal. Calcd for
C13H8BrN: C, 60.49; H, 3.12; N, 5.43 found: C, 60.41; H, 3.01; N, 5.36%.
2-(Benzo[b]tellurophen-2-yl)pyridine (L-Te). Under a nitrogen atmosphere, tellurium
powder (0.30 g, 2.35 mmol) and NaBH4 (0.11 g, 2.82 mmol) were dissolved in degassed DMF
(15 mL), then the mixture was heated to 100 °C and stirred for 0.5 h approximately until no
more bubbles. At this point, the solution turned purple, indicating that NaTeH was generated.
Then 2-((2-bromophenyl)ethynyl)pyridine (7) (0.50 g, 1.95 mmol) was added and stirred for
overnight at 100 °C. After cooling to room temperature, the mixture was extracted with ethyl
acetate (3 × 10 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated under
reduced pressure. Then, the residue was purified by column chromatography over silica gel
using ethyl acetate/petroleum ether (1:15, v/v) as the eluent. The product was obtained as a
white solid (0.33 g, 55%).
1H NMR (400 MHz, CDCl3): δ (ppm) 8.49 (ddd, J = 4.9, 1.4, 1.0 Hz, 1H), 8.32 (s, 1H),
7.93 (d, J = 8 Hz, 1H), 7.84-7.79 (m, 2H), 7.69 (td, J = 7.6, 1.6 Hz, 1H), 7.36 (td, J = 7.6, 1.0 Hz,

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1H), 7.19-7.12 (m, 2H); 13C NMR (100 MHz, CDCl ): δ (ppm) 156.33, 149.74, 146.66, 136.31,
DOI: 10.1039/D0DT02224A
3
133.46, 132.13, 131.62, 128.32, 127.59, 125.25, 124.93, 122.59, 116.91; FAB-MS (m/z): 309
[M]+; Anal. Calcd for C13H9NTe: C, 50.89; H, 2.96; N, 4.57 found: C, 50.79; H, 2.90; N, 4.49%.
General synthetic procedure for the (C^N)Pt(acac)-type complexes
Under a nitrogen atmosphere, ligands (1.1 equiv) and K2PtCl4 (1.0 equiv) were added in a
mixture of 2-ethoxyethanol and water (3:1, v/v). The mixture was heated to 80-90 °C and stirred
for 16 h. Then the reaction mixture was cooled to room temperature, and water was added. The
colored precipitate of dimer complex was collected by filtration and dried under vacuum. After
that, t-BuOK (2.5 equiv) and acetylacetone (3.0 equiv) were added in dry CH2Cl2. The mixture
was stirred for 0.5 h under N2 at room temperature. Then, the dimer complex was added and
stirred for overnight. After evaporation of solvent, the crude product was chromatographed on a
silica column with mixture of petroleum ether and CH2Cl2 to give a pure colored product.
Platinum(II)
(2-2′-(4′,5′-benzo)furanyl)pyridinato-N,C3′)
(2,4-pentanedionato-O,O)
(Pt-O). CH2Cl2:PE = 1:1(v/v); Yield: 22%. 1H NMR (400 MHz, CDCl3) δ (ppm) 8.86 (d, J = 6.0
Hz, 1H), 8.07 (d, J = 7.6 Hz, 1H), 7.73 (td, J = 7.8, 1.6 Hz, 1H), 7.43 (d, J = 8.4 Hz, 1H),
7.36-7.30 (m, 2H), 7.23 (td, J = 7.6, 0.8 Hz, 1H), 6.96 (t, J = 6.8 Hz, 1H), 5.54 (s, 1H), 2.06 (s,
3H), 2.03 (s, 3H); 13C NMR (100 MHz, CDCl3): δ (ppm) 184.90, 183.51, 159.47, 156.55,
156.52, 147.77, 138.85, 133.28, 125.36, 123.72, 122.77, 119.04, 116.56, 116.29, 111.09, 102.46,
28.09, 26.34; MS (m/z): 488 [M]+; Anal. Calcd for C18H15NO3Pt: C, 44.27; H, 3.10; N, 2.87
found: C, 44.09; H, 2.93; N, 2.78%.
Platinum(II) (2-2′-(4′,5′-benzo)thiophenyl)pyridinato-N,C3′) (2,4-pentanedionato-O,O)
(Pt-S). CH2Cl2:PE = 1:1(v/v); Yield: 21% 1H NMR (400 MHz, CDCl3) δ (ppm) 8.92 (d, J = 5.6
Hz, 1H), 8.81-8.78 (m, 1H), 7.82-7.80 (m, 1H), 7.72 (t, J = 7.8 Hz, 1H), 7.36-7.30 (m, 3H), 6.97

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13
(t, J = 6.6 Hz, 1H), 5.56 (s, 1H), 2.10 (s, 3H), 2.03 (s, 3H); C NMR (100 MHz, CDCl ): δ
DOI: 10.1039/D0DT02224A
3
(ppm) 185.26, 183.42, 164.65, 146.94, 145.36, 142.28, 138.77, 138.69, 138.17, 126.65, 125.37,
124.14, 122.34, 118.84, 118.19, 102.57, 28.10, 26.42; MS (m/z): 504 [M]+; Anal. Calcd for
C18H15NO2PtS: C, 42.86; H, 3.00; N, 2.78 found: C, 42.76; H, 2.89; N, 2.71%.
Platinum(II) (2-2′-(4′,5′-benzo)selenophenyl)pyridinato-N,C3′) (2,4-pentanedionato-O,O)
(Pt-Se). CH2Cl2:PE = 1:1(v/v); Yield: 20%. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.97 (dd, J =
8.0, 0.8 Hz, 1H), 8.92 (dd, J = 6.0, 0.6 Hz, 1H), 7.86 (d, J = 7.7 Hz, 1H), 7.71 (td, J = 7.8, 1.5 Hz,
1H), 7.36 (td, J = 7.6, 1.1 Hz), 7.29-7.23 (m, 2H), 6.95 (t, J = 6.6 Hz, 1H), 5.56 (s, 1H), 2.09 (s,
3H), 2.03 (s, 3H); 13C NMR (100 MHz, CDCl3): δ (ppm) 185.27, 183.34, 166.52, 148.16,
146.44, 143.74, 142.43, 138.81, 137.63, 128.44, 125.51, 125.33, 124.58, 118.99, 118.59, 102.59,
28.08, 26.46; MS (m/z): 551 [M]+; Anal. Calcd for C18H15NO2PtSe: C, 39.21; H, 2.74; N, 2.54
found: C, 39.11; H, 2.64; N, 2.43%.
Platinum(II) (2-2′-(4′,5′-benzo)tellurophenyl)pyridinato-N,C3′) (2,4-pentanedionato-O,O)
1
(Pt-Te). CH2Cl2:PE = 1:1(v/v); Yield: 18%. H NMR (400 MHz, CDCl3) δ (ppm) 9.20 (d, J =
8.0 Hz, 1H), 8.88 (d, J = 5.6 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.64 (t, J = 7.6 Hz, 1H), 7.36 (t, J
= 7.6 Hz, 1H), 7.16-7.09 (m, 2H), 6.89 (t, J = 6.8 Hz, 1H), 5.54 (s, 1H), 2.07 (s, 3H), 2.00 (s,
13
3H); C NMR (100 MHz, CDCl3): δ (ppm) 185.28, 183.16, 170.51, 154.56, 151.96, 145.76,
138.77, 135.66, 131.70, 130.85, 127.57, 125.44, 125.25, 120.09, 118.14, 102.57, 28.04, 26.52;
MS (m/z): 601 [M]+; Anal. Calcd for C18H15NO2PtTe: C, 36.03; H, 2.52; N, 2.33 found: C,
35.91; H, 2.41; N, 2.43%.
Results and discussion
Synthesis and structural characterization
The synthetic pathways for the C^N-type ligands are shown in Scheme 1. Ligands

Dalton Transactions
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2-(benzo[b]furan-2-yl)pyridine (L-O) and 2-(benzo[b]thiophen-2-yl)pyridine (L-S) were VeieawsAirltiycle Online
DOI: 10.1039/D0DT02224A
prepared through Suzuki cross-coupling reaction between 2-bromopyridine and the
corresponding arylboronic acid in high yield of over 80% (ESI). However, the synthesis of
2-(benzo[b]selenophen-2-yl)pyridine (L-Se) was started with 1-bromo-2-iodobenzene, which
was converted into ((2-bromophenyl)ethynyl)trimethylsilane (1) by Sonogashira cross-coupling
reaction at room temperature. After reduction of (1) and followed by treating with NBS, the
compound of (Z)-(1-bromo-2-(2-bromophenyl)vinyl)trimethylsilane (2) could be obtained.
Through dilithiation in anhydrous t-Butyl methyl ether and subsequent addition of selenium
powder, we obtained the compound of 2-(trimethylsilyl)benzo[b]selenophene (3), which was
converted into 2-bromobenzo[b]selenophene (4) through treating with NBS. By Stille
cross-coupling between 2-bromobenzo[b]selenophene (4) and 2-(tributylstannyl)pyridine with
Pd(PPh3)4 as catalyst, ligand 2-(benzo[b]selenophen-2-yl)pyridine (L-Se) could be successfully
synthesized. With 2-bromopyridine as staring material, 2-ethynylpyridine (6) had been prepared
in high yield of ca. 90% by Sonogashira cross-coupling reaction followed by removing
trimethylsilyl group in methanol with presence of KOH. Again, with Sonogashira
cross-coupling reaction between 2-ethynylpyridine (6) and 1-bromo-2-iodobenzene, compound
2-((2-bromophenyl)ethynyl)pyridine (7) was synthesized with a very high yield of ca. 99%.
Finally, ligand 2-(benzo[b]tellurophen-2-yl)pyridine (L-Te) has been synthesized by the
treatment of compound 7 with NaTeH formed in suit by reduction tellurium powder with
NaBH4.

Page 13 of 36
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DOI: 10.1039/D0DT02224A
OH
B
OH
N
N
Br
N
O
S
O
L-O
Pd(PPh3)4
2M Na2CO3
OH
B
OH
S
L-S
Si
Si
Si
I
DIBAL-H
NBS
Br
Pd(PPh3)2Cl2
CuI, Et3N
Br
(2)
Br
Br
(1)
nBuLi
Selenium powder
SnBu3
N
N
NBS
Br
Si
Pd(PPh3)4
Se
(4)
Se
Se
(3)
L-Se
Si
Si
Br
KOH
N
MeOH
N
Pd(PPh3)2Cl2
CuI, Et3N
N
(5)
(6)
I
Pd(PPh3)2Cl2
CuI, Et3N
Br
Br
N
N
NaBH4
Tellurium powder
Te
L-Te
(7)
Scheme 1. Synthetic scheme for the organic ligands.
After obtaining all the four C^N-type ligands, the (C^N)Pt(acac)-type complexes had been
synthesized by the typical two-step procedure. Firstly, metalation of the corresponding
cyclometalating ligand (L-O/L-S/L-Se/L-Te) with K2PtCl4 gave the cyclometalated PtII
μ-chloro-bridged dimers, which were then converted into the target (C^N)Pt(acac)-type
complexes by reaction with potassium(I) acetylacetonate formed in-suit by treating

Dalton Transactions
Page 14 of 36
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1
acetylacetone with t-BuOK. In the H NMR spectra of the (C^N)Pt(acac)-type complexes, the
DOI: 10.1039/D0DT02224A
resonance signal peaked at ca. 9.0 ppm can be assigned to the proton attached to the aromatic
carbon atom adjacent to the nitrogen atoms in pyridyl ring of the C^N-type ligands. The protons
on the two methyl groups in the acetylacetone auxiliary ligands split into two sets of a single
resonance peak with the chemical shifts at ca. 2.0 and 2.1 ppm, respectively.
N
N
N
N
S
O
Se
Te
L-O
L-S
L-Se
L-Te
i) K2PtCl4
ii) Acetylacetone, tBuOK
N
N
N
N
O
O
O
O
O
O
O
O
Pt
Pt
Pt
Pt
O
S
Se
Te
Pt-O
Pt-S
Pt-Se
Pt-Te
Scheme 2. Synthetic scheme for the (C^N)Pt(acac)-type phosphorescent complexes.
X-ray crystallography. Structures of Pt-Se and Pt-Te have been investigated by single crystal
X-ray crystallography. The ORTEP drawings have been shown in Figure 1 and the detailed
structural data are summarized in Table 1 and 2. The molecules of both Pt-Se and Pt-Te pack in
the space to form the crystal system of monoclinic with space group of P2(1)/c. The complex
molecules adopt a slightly distorted square planar geometry with cis-O^O and cis-N^C chelating
disposition (Figure 1).

Page 15 of 36
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DOI: 10.1039/D0DT02224A
(a)
(b)
Fig. 1 ORTEP drawings of (a) Pt-Se and (b) Pt-Te with thermal ellipsoids drawn at the 10%
probability level. The H atoms are omitted for clarity.
Table 1. Crystal and data parameters for structures Pt-Se and Pt-Te.
compound
CCDC No.
formula
Pt-Se
Pt-Te
1918815
C18H15NO2PtSe
551.36
1918816
C18H15NO2PtTe
600.00
Formula weight
Crystal system
Space group
a (Å)
monoclinic
P2(1)/c
19.364 (4)
11.864 (3)
14.588 (3)
90
monoclinic
P2(1)/c
19.507 (5)
12.005 (3)
14.592 (4)
90
b (Å)
c (Å)
 (deg)
104.857 (3)
90
103.877 (3)
90
 (deg)
 (deg)
V (Å3)
3239.3 (12)
8
3317.4 (15)
8
Z
Dcalcd (g cm-3)
2.261
2.403
F (000)
2064
2208
 (mm-1)
10.919
10.191
4.352-50
22371
4.018-51.23
19311
 range (deg)
diffrn reflns number
reflns number total
no. of parameter
R1, wR2 [I > 2.0 (I)]a
R1, wR2 (all data)
GOF on F2 b
5636
6187
419
419
0.0247, 0.0559
0.0343, 0.0604
0.0314, 0.0750
0.0428, 0.0809
1.033
1.036
2
2 2
a R1 = ||F0|-|Fc||/|F0|. wR2 = {[w(F0 -Fc2)2]/[w(F0 ) ]}1/2. b GOF = [(w|F0|-|Fc|)2/(Nobs-Nparam)]1/2
.

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DOI: 10.1039/D0DT02224A
In both Pt-Se and Pt-Te, the bond length of Pt1-O1 is longer than that of Pt1-O2 due to the
stronger trans-effect of the coordinated carbon atom than that of nitrogen atom (Table 2). The
bond angle of O-Pt-C in Pt-Se and Pt-Te is 173.73(18)o and 171.1(2)o, respectively, which that
for O-Pt-N is 176.60(17)o for Pt-Se and 178.4(2)o for Pt-Te (Table 2). The O-Pt-O bond angle
in Pt-Se and Pt-Te is ca. 91o, while that of N-Pt-C is about 82o (Table 2). The two C-Se bonds
in benzo[b]selenophene possess unit bond length of 1.892(6) and 1.898(6) Å, respectively,
which are shorter than that of the typical C-Se bond (ca. 1.90-1.93 Å).29 Similarly, the two C-Te
bonds (2.088(6) and 2.084(6) Å, respectively) in benzo[b]tellurophene unit are shorter than the
typical C-Te bond (ca. 2.1-2.2 Å) as well.38 These results should indicate the conjugation effect
between Se/Te atom with the two bonded sp2 carbon atoms to form fused aromatic systems
Table 2. Selected structural parameters of Pt-Se and Pt-Te.
compound
Pt-Se
Pt-Te
Pt1-O1
Pt1-O2
2.079(4)
2.000(3)
2.002(4)
1.980(6)
1.892(6)
1.898(6)
91.16(15)
92.12(17)
173.73(18)
176.60(17)
95.09(18)
81.6(2)
Pt1-O1
2.071(5)
2.018(4)
1.993(5)
2.004(7)
2.088(6)
2.084(6)
91.31(19)
90.3(2)
Pt1-O2
bond lengths (Å) Pt1-N1
Pt1-C12
Pt1-N1
Pt1-C7
Se1-C11
Te1-C6
Se1-C18
Te1-C13
O1-Pt1-O2
O1-Pt1-N1
O1-Pt1-C7
O2-Pt1-N1
O2-Pt1-C7
N1-Pt1-C7
O1-Pt1-O2
O1-Pt1-N1
O1-Pt1-C12
O2-Pt1-N1
171.1(2)
178.4(2)
96.6(2)
angles (o)
O2-Pt1-C12
N1-Pt1-C12
C11-Se1-C18
81.8(2)
85.5(3)
C6-Te1-C13
80.1(3)
Thermal and photophysical properties. Under a nitrogen flow, both thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) had been employed to characterize

Page 17 of 36
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the thermal properties of these (C^N)Pt(acac)-type complexes. All four complexes show good
DOI: 10.1039/D0DT02224A
thermal stability with their 5% weight-reduction temperature (ΔT5%) ranging from 250 to 296 °C
(Table 3). Clearly, Pt-Te possesses the lowest ΔT5%, which might be assigned to the weak C–Te
bonds due to the large radius of tellurium atom. The DSC traces reveal a high glass transition
temperature (Tg) > 120 °C for these (C^N)Pt(acac)-type complexes, which should be attributed
to the rigid configuration of their coordination skeleton. The thermal properties of these
(C^N)Pt(acac)-type complexes are sufficient for the fabrication of OLEDs with a
solution-process technique.
Table 3. Photophysical and thermal data of these (C^N)Pt(acac)-type complexes.
a
Emission λem
e
Absorption (293K) a
λabs (nm)
ΦP (%)
ΔT5%
τ d (s)
293 K/77 K
f
(nm)
solution/film b
Tg
solution/film c
(oC)
234(3.21), 259(3.29),
497, 616, 672
/616, 674
4.6 ns(497), 0.46 s(616)
/2.06 s(625)
Pt-O 322(3.25), 429(2.82),
441(2.81)
7.3/11.1
9.1/13.6
8.3/12.1
260
126
131
133
235(3.39), 267(3.29),
Pt-S 320(3.28), 428(2.81),
445(2.81)
508, 612, 659
/611, 664
3.5 ns(508), 0.34 s(612)
/3.38 s(622)
281
296
232(3.42), 271(3.24),
Pt-Se 323(3.28), 352(3.08),
431(2.80), 446(2.81)
236(3.51), 258(3.31),
503, 620, 671
/621, 676
3.1 ns(503), 0.40 s(620)
/2.21 s(631)
279(3.22), 330(3.24),
Pt-Te
514, 634, 687
/634, 690
6.6 ns(514), 0.40 s(634)
/0.97 s(651)
5.1/7.4
250
122
364(3.00), 383(2.96),
437(2.77), 457(2.81)
a
b
c
Measured in CH2Cl2 at 10-5 M and log ε values are shown in parentheses. Measured in 6 wt% doped PMMA film. Measured in a
degassed CH2Cl2 solution relative to fac-[Ir(ppy)3] (ΦP = 97%) for the solution and the 8 wt% doped film of TCTA with an integrated
sphere. d Recorded in a degassed CH2Cl2 solution with a 360 nm light source and the wavelengths in parentheses. e ΔT5% is measured under
N2 flow with a heating rate of 20 K min-1. f Glass transition temperature (Tg) is measured under N2 flow with a heating rate of 10 K min-1.

Dalton Transactions
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DOI: 10.1039/D0DT02224A
3000
2500
2000
1500
1000
500
Pt-O
Pt-S
Pt-Se
Pt-Te

0
250
300
450
500
550
W3a5v0elen4g0t0h (nm)
Fig. 2 UV−vis absorption spectra for these (C^N)Pt(acac)-type complexes in CH2Cl2 at 293 K.
Generally, two kinds of absorption bands have been detected in the in the UV−vis absorption
spectra of these (C^N)Pt(acac)-type complexes (Figure 2). The intense absorption bands with
high energy <400 nm can be safely assigned to the spin-allowed π → π* transitions of the
aromatic segments in the cyclometalating ligands.11 Clearly, From Pt-O to Pt-Te, all the intense
absorption bands exhibit noticeable bathochromic effect (Figure 2 and Table 3). The much
weaker absorption bands >400 nm were tentatively assigned to the excitation to both singlet and
triplet charge transfer (CT) states.11
In order to get a deep insight into the features of the absorption behaviors of these
(C^N)Pt(acac)-type complexes, the simulation of their UV−vis spectra (Figure 3) with discrete
vertical vibronic transitions and the time-dependent density functional theory (TD-DFT)
calculations have been carried out (Figure 4 and Table 4). Based on their optimized S0
geometries, the distribution patterns of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) mainly corresponding the key S0 → S1 transition
have been shown in Figure 4. As shown in Figure 3, the good resemblance between simulated

Page 19 of 36
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and experimental UV−vis spectra in terms of both absorption wavelength and spectra profile has
DOI: 10.1039/D0DT02224A
indicated the he validity of the calculation method. Clearly, from oxygen to tellurium,
extra-nuclear electrons possess higher energy level to facilitate their contribution to the HOMOs
(Figure 4). Hence, the contribution from the group VIA atom in the C^N-type ligand increases
from oxygen to tellurium (4.4% for Pt-O, 14.2% for Pt-S, 18.6% for Pt-Se and 31.5% for Pt-Te)
(Table 4). Based on their chemical properties, electron-donating ability will enhance from
oxygen to tellurium. Obviously, the enhanced electron-donating ability should facilitate electron
transition process in the concerned aromatic segments. Hence, the high-energy bands >400 nm
corresponding to the electron transitions in the aromatic segments in the C^N-type ligands can
show bathochromic effect from L-O to L-Te (Figure 2). Based on the calculation results in
Figure 4, the low-energy absorption bands of these (C^N)Pt(acac)-type complexes should be
induced mainly by the HOMO → LUMO (H → L) transition. According to the molecular
orbital (MO) patterns in Figure 4, the low-energy absorption bands (> 400 nm) of these
(C^N)Pt(acac)-type complexes should mainly show intraligand charge transfer (ILCT) feature
from π orbitals of the fused-ring units to π* orbitals of the pyridyl group in the C^N-type ligands.
In addition, based on the contribution from the dπ orbitals to the front MOs, metal-to-ligand
charge transfer (MLCT) can also account for the feature for the low-energy absorption bands of
these (C^N)Pt(acac)-type complexes.

Dalton Transactions
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DOI: 10.1039/D0DT02224A
Fig. 3 Both simulated (black) and experimental (red) UV−vis absorption spectra with discrete
vertical vibronic transitions for these (C^N)Pt(acac)-type complexes (H → L: HOMO → LUMO
transition).
Fig. 4 Molecular orbital (MO) patterns for these (C^N)Pt(acac)-type complexes based on their
optimized S0 geometries.

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DOI: 10.1039/D0DT02224A
Table 4. TD-DFT results of these (C^N)Pt(acac)-type complexes on the basis of their optimized
S0 geometries.
Main configuration of S0→S1
excitation/Ecal/cal/f a
Main configuration of S0→T1
Contribution of metal d orbitals and 
a
Complexes
MO
orbitals of ligand to MOs (%)
excitation/Ecal/cal
Pt
LO
acac
O
H→L (93.2%)
2.945 eV
421 nm
H→L (91.9%)
2.126 eV
L
7.4
91.6 1.0
1.6
Pt-O
583 nm
H
17.7
78.5 3.8
4.4
0.170
Pt
LS
acac
S
H→L (94.0%)
2.931 eV
423 nm
H→L (90.1%)
2.134 eV
Pt-S
Pt-Se
Pt-Te
L
7.6
91.4 1.0
2.9
581 nm
H
17.1
79.0 3.8
14.2
0.139
Pt
LSe acac Se
H→L (94.2%)
2.917 eV
425 nm
H→L (89.1%)
2.108 eV
L
7.6
91.5 0.9
2.7
588 nm
H
16.3
80.1 3.6
18.6
0.140
Pt
LTe acac Te
H→L (95.5%)
2.844 eV
436 nm
H→L (85.1%)
2.073 eV
L
7.6
91.4 0.9
83.5 2.8
2.8
598 nm
H
13.7
31.5
0.120
a
H→L denotes transition from HOMO to LUMO. Ecal, cal and f denote calculated excitation energy, calculated emission
wavelength and oscillator strength, respectively. The oscillator strength of S0→T1 is zero owing to the spin-forbidden character
of singlet-triplet transition under TD-DFT calculation in the Gaussian program with no consideration of spin-orbital coupling.
Photoluminescence (PL) spectra of these (C^N)Pt(acac)-type complexes at 293 K have been
collected under excitation at 400 nm. All these complexes can obviously show two PL bands
(Figure 5a). The high-energy PL bands centered at ca. 490 nm should be fluorescent signals
indicated by their short lifetime in the order of nano-second (4.6 ns for Pt-O, 3.5 ns for Pt-S, 3.1
ns for Pt-Se and 6.6 ns for Pt-Te) (Table 3). This observation can be ascribed to the small
contribution of the Pt(II) centers to the key front molecular orbitals of these (C^N)Pt(acac)-type
complexes due to electron-rich and extended π-conjugation of the benzoaryl groups in the

Dalton Transactions
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C^N-type ligands.39, 40 Accordingly, weak spin-orbital coupling (SOC) effect will DbOeI:i1n0.d10u3Vc9iee/wDd0ADrttiToc0le2O2n2l4inAe
bring too slow efficient inter-system crossing (ISC) process to complete eliminate fluorescent
signals. On the contrary, long wavelength emissions in red-color region can be ascribed to the
phosphorescent signals, substantiated by their much longer micro-second lifetimes (τp) which
can be even prolonged at 77 K (Table 3). At both 293 K and 77 K, all four (C^N)Pt(acac)-type
complexes show well-structured spectra profile for the phosphorescent bands (Figure 5).
(a)
Pt-O
Pt-S
Pt-Se
Pt-Te
1.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
700
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
Pt-O
Pt-S
Pt-Se
Pt-Te
550
600
650
700
750
Wavelength (nm)
Fig. 5 PL spectra of these (C^N)Pt(acac)-type complexes at 293 K (a) and 77 K (b) in CH2Cl2.
With the aim to clearly elucidate the PL behaviors of these (C^N)Pt(acac)-type complexes,

Page 23 of 36
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the natural transition orbitals (NTOs) have been obtained based on the optimized T geometries
DOI: 10.1039/D0DT02224A
1
for their S0 → T1 transition (Figure 6). Furthermore, contribution from different segments of
these (C^N)Pt(acac)-type complexes to both NTO hole (H) and particle (P) orbitals have been
list in Table 5 as well. The NTO patterns have indicated that both H and P orbitals are mainly
located on the C^N-type ligands of these (C^N)Pt(acac)-type complexes with the contribution >
89% (Table 5). Accordingly, large percentage of H → P transition over 90% should indicate
their ligand-centered S0 → T1 transition with the feature of ILCT in the C^N-type ligand (Figure
6). Hence, the phosphorescent emission in these (C^N)Pt(acac)-type complexes should
dominantly exhibit ligand-centered character, which has been indicated by their structured
phosphorescent band at both 293 K and 77 K (Figure 5). Owing to their phosphorescent feature
of ILCT from the π orbitals of the fused-ring unit to π* orbitals of the pyridyl group in the
C^N-type ligands (Figure 6), the phosphorescent wavelengths of these (C^N)Pt(acac)-type
complexes from Pt-O to Pt-Te should show bathochromic effect due to the fact that the
enhanced electron-donating ability from oxygen to tellurium will facilitate the ILCT process to
lower its energy level.

Dalton Transactions
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DOI: 10.1039/D0DT02224A
Fig. 6 Natural transition orbital (NTO) patterns for S0 → T1 excitation for these
(C^N)Pt(acac)-type complexes based on their optimized T1 geometries.
Table 5. NTO results for these (C^N)Pt(acac)-type complexes based on their optimized T1
geometries.
NTO
Contribution percentages of metal d orbitals and  orbitals of ligand to NTOs
Complexes
a
(%)
Pt
9.3
6.9
Pt
LO
89.2
92.4
LS
acac
1.4
Pt-O
H
P
0.7
acac
1.4
Pt-S
Pt-Se
Pt-Te
H
P
9.1
7.1
Pt
89.5
92.1
LSe
90.2
92.2
LTe
91.3
92.2
0.8
acac
1.3
H
P
8.6
7.0
Pt
0.7
acac
1.1
H
P
7.6
7.1
0.7
a H and P denote NTO hole and particle orbital, respectively.
Out of expectation, the complex Pt-S does not follow this order, even showing hypochromic
effect compared with Pt-O (612 nm for Pt-S vs. 616 nm for Pt-O) (Table 3 and Figure 5). In
addition, this unexpected result has been predicted by the TD-DFT results as well (Table 4).
From the UV−vis absorption spectra of these (C^N)Pt(acac)-type complexes (Figure 2), it seems

Page 25 of 36
Dalton Transactions
that Pt-S shows slight hypochromic effect in its absorption onset of the long-wavDeOleI:n10g.1t0h3V9Iie/LwD0CADrtTiTc0le2O2n2l4inAe
absorption band with respect to that of other complexes. Reasonably, Pt-S will show
phosphorescent emission with the shortest wavelength among all these (C^N)Pt(acac)-type
complexes. This result can be interpreted as follows: As indicated by the NTO results, the
phosphorescence of these (C^N)Pt(acac)-type complexes show ILCT feature of from the π
orbitals of the fused-ring unit to π* orbitals of the pyridyl group in the C^N-type ligands.
Clearly, among furan, thiophene, selenophene and tellurophene, thiophene shows the highest
aromaticity to furnish its higher stability and resistance to oxidation. Hence, benzo[b]thiophene
unit should be more reluctant to be excited to transfer electrons to the pyridyl group in Pt-S.
Accordingly, it will furnish ILCT states in slightly higher energy-level. As a result, Pt-S can
show the shortest phosphorescent wavelength. For Pt-O, Pt-Se and Pt-Te, all the fused ring
units show weaker aromaticity and energy-level of the ILCT process is mainly up to the
electron-donating ability of the group VIA atoms. So, from Pt-O (ca. 616 nm) to Pt-Se (ca. 620
nm) and then to Pt-Te (ca. 634 nm), bathochromic effect can be clearly seen in their
phosphorescent wavelength.
These (C^N)Pt(acac)-type complexes exhibit relatively low phosphorescent quantum yield
(ΦP) in dilute solution (Table 3). In order to clarify this result, radiative constant (kr) for each of
these (C^N)Pt(acac)-type complexes has been obtained by theoretical calculation (Figure 7).
Their krs are in the order of ca. 104, which is much lower than that associated with analogs.
Hence, these (C^N)Pt(acac)-type complexes possess low ΦP < 10% (Table3). In Figure 7, it can
be seen clearly that the order of krs of these (C^N)Pt(acac)-type complexes follows Pt-S > Pt-Se
> Pt-O > Pt-Te, which is the same to the order of their ΦPs. The experimental krs of these
(C^N)Pt(acac)-type complexes have been obtained by following the equations of 1/τp = (kr + knr)

Dalton Transactions
Page 26 of 36
and Φ = k /(k + k ), in which τ is phosphorescent lifetime at 293 K, k is radiative constaVnietwaAnrtidcle Online
DOI: 10.1039/D0DT02224A
p
r
r
nr
p
r
knr is nonradiative constant. Despite higher than the theoretical data, the experimental krs still
fell in the same order of Pt-S (ca. 2.7 × 105 s-1) > Pt-Se (ca. 2.1 × 105 s-1) > Pt-O (ca. 1.6 × 105
s-1) > Pt-Te (ca. 1.3 × 105 s-1). So, there is a good consistency between experimental and
theoretical results, indicating the validity of the theoretical calculations.
10
1.2x104
1.0x104
8

8.0x103
6
6.0x103
Pt-O
Pt-Se
Pt-Te
Pt-S
Fig. 7 Calculated radiation constants (krs) and phosphorescent quantum yields (ΦPs) for these
(C^N)Pt(acac)-type complexes.
Electrochemical Properties. The electrochemical properties of these (C^N)Pt(acac)-type
complexes had been characterized by cyclic voltammetry (CV) with ferrocene as the internal
standard. In the anodic scan, all these (C^N)Pt(acac)-type complexes exhibit two irreversible
oxidation processes (Table 6 and Figure S1). The first oxidation process should be induced by
the oxidation of the electron-rich benzoayrl groups in the C^N-type ligands. Clearly, Pt-S can
possess the highest first Epa ca. 0.30 V (Table 6), which can be ascribed to the enhanced
aromaticity associated with thiophene unit to restrain its oxidation. In addition, the first Epa (ca.
0.16 V) for Pt-Te is lowest due to the easiness of oxidation of tellurium atom. The second Epas
(ca. 0.5 V) of these (C^N)Pt(acac)-type complexes can be assigned to the oxidation of the Pt(II)
centers in these complexes.12 One reversible reduction process had been observed at ca. -2.0 V

Page 27 of 36
Dalton Transactions
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for these (C^N)Pt(acac)-type complexes, which can be ascribed to the reduction oDfOtIh: 1e0.p10y39r/iDd0iDnTe02224A
rings in the C^N-type ligands (Table 6 and Figure S1).
Table 6. Redox properties of these (C^N)Pt(acac)-type complexes.
Complex
Epa (V)
Epc (V)
Eg (eV)
EHOMO (eV) e
ELUMO (eV) f
cv d
Egopt c/Eg
2.61/2.64
2.63/2.64
2.61/2.68
2.56/2.48
Pt-O
Pt-S
0.27 a, 0.45a
0.30 a, 0.46a
0.28 a, 0.57a
0.16 a, 0.53a
-2.19b
-2.19b
-2.11b
-1.95b
-5.25
-5.26
-5.37
-5.33
-2.61
-2.61
-2.69
-2.85
Pt-Se
Pt-Te
a Irreversible. The value was derived from the anodic peak potential. b Reversible. The value was
set as E1/2. cOptical energy gap was calculated from the absorption onset (λonset, the wavelength at
the cross-point between absorption zero line and tangent line of the lowest-energy absorption
opt
cv
band) of the UV-vis absorption spectra, Eg = 1240/λonset eV. d CV energy gap Eg = LUMO –
f
HOMO. e HOMO levels are calculated according to the equation HOMO = –(4.8 + Epa). Data
are obtained from the onset potential of the first reduction wave of the CV data.
Electrophosphorescent OLEDs. For the complexes Pt-Se and Pt-Te, they should represent the
unprecedented phosphorescent emitters with benzo[b]selenophene and benzo[b]tellurophene
units. Hence, it should be very necessary to characterize their EL ability. With Pt-Se and Pt-Te
as emitters, solution-processed OLEDs with the configuration of ITO/PEDOT:PSS (45 nm)/Pt
x%:TCTA (ca. 40 nm)/TPBi (45 nm)/LiF(1 nm)/Al (100 nm) (Figure 8) had been constructed.
The compound of TCTA (4,4′,4′′-Tri(9-carbazoyl)triphenylamine) was selected as the host
material.
The
water
solution
of
PEDOT:PSS
(poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) was used to deposit hole injection
layer, while a layer of TPBi (1,3,5-tris[N-(phenyl)-benzimidazole]benzene) served the purpose
of electron-transporting as well as hole-blocking. The thin layer of LiF could play the role of
electron-injection.

Dalton Transactions
Page 28 of 36
View Article Online
DOI: 10.1039/D0DT02224A
Fig. 8 Energy diagram of the solution-processed OLEDs together with chemical structures of
the materials employed in the fabricated OLEDs.
In order to optimize EL performance, solution-processed OLEDs with different doping levels
of Pt-Se or Pt-Te in emission layer had been fabricated. The doping-levels were set at 6.0 wt%,
8.0 wt% and 10.0wt% and the concerned devices were named as A1, A2 and A3 with Pt-Se as
emitter together with B1, B2 and B3 with Pt-Te as emitter (Figure 8). Both Pt-Se and Pt-Te can
furnish the optimized EL performances with a doping level of 8.0 wt%, corresponding to device
A2 and B2, respectively (Table 7). When a proper voltage applied, device A2 and B2 can emit
stable red electrophosphorescence at ca. 620 nm for device A2 and 636 nm for device B2
(Figure 9). The EL spectra of optimized OLEDs (Figure 9) are similar to the PL spectra of
corresponding complexes Pt-Se and Pt-Te in solution (Figure 5a), indicating efficient energy
transfer from the TCTA host to the phosphorescent emitters Pt-Se and Pt-Te.

Page 29 of 36
Dalton Transactions
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DOI: 10.1039/D0DT02224A
Device A2
Device B2
1.0
0.8
0.6
0.4
0.2
0.0
500
600
700
800
Wavelength (nm)
Fig. 9 EL spectra for the optimized OLEDs at ca. 10V.
Current density-voltage-luminance (J-V-L) characteristics and EL efficiency-luminance
curves for the solution-processed OLEDs are shown in Figure 10, 11 and ESI (Figure S2 and
S3). The corresponding EL data have been tabulated in Table 7. All these devices show a low
turn-on voltage ca. 4.0 V. For the optimized device A2 with Pt-Se as emitter, EL performances
with maximum external quantum efficiency (ηext) of 6.3%, current efficiency (ηL) of 10.5 cd A−1,
and power efficiency (ηP) of 9.1 lm W−1 have been achieved for the red-emitting device,
showing the highest luminance ca. 6423 cd m-2 at 16.7 V. Doped with Pt-Te, deep-red device
B2 can show ηext of 6.3 %, ηL of 6.5 cd A−1 and ηP of 5.8 lm W−1. Despite of their inferior EL
performance compared with the state-of-the-art EL data from the analogs, the EL efficiencies
achieved by Pt-Se and Pt-Te can still show their potential as red phosphorescent emitters for
OLEDs. Critically, to the best of our knowledge, the EL results for Pt-Te can represent the first
EL performances ever achieved by the phosphorescent emitters with benzo[b]tellurophene
moiety.