Dibenzothiophene, dibenzofuran and pyridine substituted tetraphenyl silicon derivatives hosts for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2019.05.033

We report the application of four novel tetraphenyl silicon derivatives as host materials in phosphorescent organic light-emitting diodes (PHOLEDs). The novel derivatives are; 3-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)- phenyl)pyridine (DBTSiPy3), 4-(4- ((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenyl- silyl)phenyl)pyridine (DBTSiPy4), 3-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)- diphenylsilyl)phenyl)pyridine (DBFSiPy3) and 4-(4-((4-(dibenzo[b,d]furan-4- yl)phenyl)diphenylsilyl)phenyl)pyridine (DBFSiPy4). They were prepared by the introduction dibenzothiophene (DBT)/dibenzofuran (DBF) and pyridine units into bis(4-bromophenyl)diphenylsilane intermediate. The influences of DBT/DBF and the position of nitrogen atom within the pyridine unit (3- vs 4-position) were studied by theoretical calculations and experimental measurements. To evaluate the electroluminescent (EL) performance of these four materials, (2-phenylpyridine) iridium(III) (Ir(ppy)₃) based green PHOLEDs were fabricated using the common device structures. DBF substituted materials (DBFSiPy3 and DBFSiPy4) led to efficient green PHOLEDs with notable external quantum efficiencies (EQE) of 22.9% and 21.9% respectively.

Full text of DOI:10.1016/j.orgel.2019.05.033

Accepted Manuscript
Dibenzothiophene, Dibenzofuran and Pyridine Substituted Tetraphenyl Silicon
Derivatives Hosts for Green Phosphorescent Organic Light-Emitting Diodes
Lei Ding, Jiang-Nan Wang, Xiang-Yang Liu, Hua Chen, Femi Igbari, Liang-Sheng
Liao
PII:
S1566-1199(19)30261-7
10.1016/j.orgel.2019.05.033
ORGELE 5268
DOI:
Reference:
To appear in:
Organic Electronics
Received Date:
Accepted Date:
10 February 2019
18 May 2019
Please cite this article as: Lei Ding, Jiang-Nan Wang, Xiang-Yang Liu, Hua Chen, Femi Igbari,
Liang-Sheng Liao, Dibenzothiophene, Dibenzofuran and Pyridine Substituted Tetraphenyl Silicon
Derivatives Hosts for Green Phosphorescent Organic Light-Emitting Diodes, Organic Electronics
(2019), doi: 10.1016/j.orgel.2019.05.033
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ACCEPTED MANUSCRIPT
Dibenzothiophene, Dibenzofuran and Pyridine
Substituted Tetraphenyl Silicon Derivatives
Hosts for Green Phosphorescent Organic
Light-Emitting Diodes
Lei Ding,1,2,3* Jiang-Nan Wang, 2,3 Xiang-Yang Liu,1 Hua Chen,1 Femi Igbari,1
Liang-Sheng Liao1,3
1 Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices,
Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, P.R. China.
2 College of Electrical and Information Engineering, Shaanxi University of Science
and Technology, Xi'an 710021, P.R. China.
3 Jiangsu jitri org Optoelectronics Technology Co., Ltd, Suzhou, 215215, China.
* Corresponding authors.
E-mail addresses: dinglei@sust.edu.cn；
Tel: +86-029-86168320
Abstract
We report the application of four novel tetraphenyl silicon derivatives as host
materials in phosphorescent organic light-emitting diodes (PHOLEDs). The novel
derivatives
are;
3-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)-
phenyl)pyridine (DBTSiPy3), 4-(4- ((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenyl-

ACCEPTED MANUSCRIPT
silyl)phenyl)pyridine
(DBTSiPy4),
3-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)-
diphenylsilyl)phenyl)pyridine (DBFSiPy3) and 4-(4-((4-(dibenzo[b,d]furan-4-
yl)phenyl)diphenylsilyl)phenyl)pyridine (DBFSiPy4). They were prepared by the
introduction dibenzothiophene (DBT)/dibenzofuran (DBF) and pyridine units into
bis(4-bromophenyl)diphenylsilane intermediate. The influences of DBT/DBF and the
position of nitrogen atom within the pyridine unit (3- vs 4-position) were studied by
theoretical calculations and experimental measurements. To evaluate the
electroluminescent (EL) performance of these four materials, (2-phenylpyridine)
iridium(III) (Ir(ppy)3) based green PHOLEDs were fabricated using the common
device structures. DBF substituted materials (DBFSiPy3 and DBFSiPy4) led to
efficient green PHOLEDs with notable external quantum efficiencies (EQE) of
22.9 % and 21.9 % respectively.
Keywords:
Organic light-emitting diodes; Host materials; Tetraphenyl silicon; Triplet
energy.
Graphical

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Dibenzothiophene, Dibenzofuran and Pyridine Substituted
Tetraphenyl Silicon Derivatives Hosts for Green Phosphorescent
Organic Light-Emitting Diodes
Lei Ding,a,b,c* Jiang-Nan Wang,a Xiang-Yang Liu,a Hua Chen,a Femi Igbari,a
Zuo-Quan Jianga, Liang-Sheng Liaoa,c
a) Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices,
Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, P.R. China.
b) College of Electrical and Information Engineering, Shaanxi University of Science
and Technology, Xi'an 710021, P.R. China.
c) Jiangsu jitri org Optoelectronics Technology Co., Ltd, Suzhou, 215215, China.
* Corresponding authors.
E-mail addresses: dinglei@sust.edu.cn (L. Ding)
Tel.: +86-029-86168320; fax: +86-029-86168709;
Abstract: We reported the application of four novel tetraphenyl silicon derivatives as
host materials in phosphorescent organic light-emitting diodes (PHOLEDs). The
novel derivatives are 3-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)-
phenyl)pyridine (DBTSiPy3), 4-(4- ((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenyl-

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silyl)phenyl)pyridine
(DBTSiPy4),
3-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)-
diphenylsilyl)phenyl)pyridine (DBFSiPy3) and 4-(4-((4-(dibenzo[b,d]furan-4-
yl)phenyl)diphenylsilyl)phenyl)pyridine (DBFSiPy4). They were prepared by the
introduction of dibenzothiophene (DBT)/dibenzofuran (DBF) and pyridine units into
bis(4-bromophenyl)diphenylsilane intermediate. The influences of DBT/DBF and the
position of nitrogen atom within the pyridine unit (3- vs 4-position) were studied by
theoretical calculations and experimental measurements. To evaluate the
electroluminescent (EL) performance of these four materials, (2-phenylpyridine)
iridium(III) (Ir(ppy)3) based green PHOLEDs were fabricated using the common
device structures. DBF substituted materials (DBFSiPy3 and DBFSiPy4) led to
efficient green PHOLEDs with notable external quantum efficiencies (EQEs) of
22.9% and 21.9%, respectively.
Keywords: Dibenzothiophene, Dibenzofuran, Pyridine, Organosilicon derivatives,
Host material, Green phosphorescent organic light-emitting diode
1. Introduction
Following the pioneering report on organic light-emitting diodes (OLEDs) in
1987[1], enormous efforts have been made in the development of stable devices which
can generate light with high electrical to optical conversion efficiencies[2-11]. So far,
the realization of high efficiency and long lifetime in OLEDs is still a challenge[12-14]
.
A traditional fluorescent OLED operates by the injection of holes and electrons into
the organic materials where they bind to form excitons. According to the
spin-statistics, the ratio of singlet and triplet excitons is approximately 1: 3.
Consequently, roughly 75% energy is lost due to the nonradiative triplet excitons in
fluorescent emitters[15-20]. To overcome this spin restriction, phosphorescent emitters

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were introduced in 1998[15,16]. These materials owe their properties to the possession
of heavy transition metals (e.g. Pt and Ir) which are capable of achieving strong
intramolecular spin-orbit coupling. Thus, harvesting singlet and triplet excitons can
yield phosphorescent organic light-emitting diodes (PHOLEDs) with almost 100%
internal quantum efficiency (IQE) [12-14]
.
Usually, phosphorescent emitters of doped into suitable host materials to prevent
self-aggregation quenching and triplet-involved quenching effects. Such effects
include triplet-triplet annihilation (TTA) and triplet exciton-polaron quenching
(TPQ)[18,21-23]. Therefore, the development of highly efficient host materials, requires
basic qualities such as good thermal stability, good charge transporting ability, high
triplet energy and suitable highest occupied molecular orbital (HOMO)/lowest
unoccupied molecular orbital (LUMO) level[12-14]. To realize these qualities, measures
such as limited π-conjugation (to realized high triplet energy) and the introduction of
electron donor and acceptor (to realize more balanced carrier transport) have been
taken[24-26]. In this regard, tetraphenyl silyl is a good building block to break the
intramolecular π-conjugation as several organosilicon compounds have been reported
with ultrahigh triplet energies[27-31]. For instance, a series of tetraaryl silicon
compounds possessing ultrahigh triplet energies (~3.5 eV) and serving as host
materials were reported by Thompson et al[27]. Yang et al. also reported a series of
organosilicon derivatives with high triplet energies (~2.93 eV)[30]. However, their
ultrawide energy bandgaps generally aggravated the charge hopping between the
adjacent phosphorescent emitters resulting in a relatively higher driving voltage[26, 32]
.

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Therefore, an optimization of carrier transport and recombination in the emitting layer
is prerequisite to achieving high device performance. In general, electron transport
ability is much lower than hole transport ability for most of the host materials. This
could be attributed to the presence of strong electron donors such as aromatic amine
derivatives (carbazole and triphenylamine) as the main constituents of such
materials[33-37]. Hence, achieving highly efficient host materials with improved
electron transport capability requires a coupling of electron acceptors such as
benzimidazole[38-41]
,
triazine[42-44]
,
oxadiazole[45]
,
phosphine oxide[4,46,47] and
pyridine[24-26] with the electron donor building blocks.
In this article, four novel host materials, DBTSiPy3, DBTSiPy4, DBFSiPy3 and
DBFSiPy4 were synthesized with tetraphenyl silicon as π-conjugation interrupter
between dibenzofuran (DBF)/dibenzothiophene (DBT) and pyridine (C3- and
C4-position) blocks. The silicon bridge suppressed the intramolecular charge transfer
from the electron donor to the electron acceptor, yielding molecules with high triplet
energies. All of these host materials present high efficiencies in green PHOLEDs with
(2-phenylpyridine) iridium(III) (Ir(ppy)3) as an emitter. Maximum external quantum
efficiencies (EQE) of 22.9 % and 21.9 % and maximum device luminance of 18500
cd/m2 and 21000 cd/m2 were achieved for devices fabricated with DBFSiPy3 and
DBFSiPy4, respectively.
2. Experimental Section
All chemicals and reagents were commercially sourced and used as received with
no further purification. PURE SOLV (Innovative Technology) purification system

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was used to purify THF 1H NMR and 13C NMR spectra were obtained from a Bruker
400 spectrometer at room temperature. Using a direct exposure probe, mass spectra
were recorded on a Thermo ISQ mass spectrometer. UV-vis absorption analyses were
conducted by using a Perkin Elmer Lambda 750 spectrophotometer. PL and
phosphorescent spectra were recorded on a Hitachi F-4600 fluorescence
spectrophotometer. Differential scanning calorimetry (DSC) was carried out at a
heating rate of 10 oC/min under nitrogen, using a TA DSC 2010 unit. Glass transition
temperatures (Tg) were determined from the second heating scan. Thermogravimetric
analysis (TGA) was performed at a heating rate of 10 oC/min under nitrogen, using a
TA SDT 2960 instrument. The temperature at 5% weight loss served as the
decomposition temperature (Td). Ultra-Violet Photoemission Spectroscopy (UPS) was
conducted, using an unfiltered He I (21.22 eV) gas discharge lamp and a
hemispherical analyzer made by KRATOS ANALYTICAL SHIMADZU GROUP
COMPANY. DFT calculations were performed using B3LYP/6-31 G(d) basis set on
Gaussian 09.
Commercially
1,1-bis[4-[N',N'-di(p-tolyl)amino]-phenyl]cyclohexane
pyridyl)phen-3-yl]benzene (TmPyPB),
available
(2-Phenylpyridine)
iridium
(III)
(Ir(ppy)3),
(TAPC),
1,3,5-tri[(3-
dipyrazino[2,3-f:2',3'-h]-
quinoxaline-2,3,6,7,10,11-hexacarbo-nitrile (HAT-CN) and 8-hydroxyquinolinolato-
lithium (Liq) were used. Indium-tin oxide (ITO) coated transparent glass substrates
were used for the OLEDs fabrication. The conductive ITO layer had a thickness, ca.
100 nm and a sheet resistance, ca. 30 Ω per square. The substrates were cleaned with

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deionized water, acetone and ethanol in succession. They were dried in an oven for 1
hour and subsequently subjected to UV ozone treatment for 30 min. All organic and
metal layers were deposited under a vacuum, ca. 10-6 Torr. Four identical OLED
devices, each having an emission area of 0.09 cm2, were formed on each substrates.
The EL performances of the blue and white devices were evaluated at
room-temperature, using
a
PHOTO RESEARCH SpectraScan PR 655
PHOTOMETER and a KEITHLEY 2400 SourceMeter constant current source.
The synthetic routes of 3-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)-
phenyl)pyridine (DBTSiPy3), 4-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenyl-
silyl)phenyl)pyridine (DBTSiPy4), 3-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)diphenyl-
silyl)phenyl)pyridine (DBFSiPy3) and 4-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)-
diphenylsilyl)phenyl)pyridine (DBFSiPy4) are outlined in Scheme 1. We used
1,4-dibromobenzene and dichlorodiphenylsilane as the starting materials to obtain the
key intermediate of bis(4-bromophenyl)diphenylsilane in moderate yield.
Consequently, good yields of the target products can be obtained via two-step
Pd-catalyzed Suzuki−Miyaura coupling reaction of the intermediate with the
corresponding boronic acid of dibenzothiophene (DBT), dibenzofuran (DBF) and
pyridine (3- and 4-position).
3. Results and Discussion
3.1 Synthesis and Characterization
PURE SOLV (Innovative Technology) purification system was used to purify THF.
Commercially
available
1,4-dibromobenzene,
dichlorodiphenylsilane,

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dibenzothiophene-4-boronic acid, n-butyl lithium dibenzofuran-4-boronic acid,
pyridine-3-boronic and pyridine-4-boronic and other reactants or reagents were used
as received.
Preparation of bis(4-bromophenyl)diphenylsilane
1,4-Dibromobenzene (9.91 g, 42 mmol) was placed in a 500 mL Schlenk tube. It was
o
then dissolved in with 150 mL THF under argon. The solution was cooled to -78 C.
n-butyl lithium (17.5 mL, 42 mmol, 2.4 M) was added, dropwise, to the solution via a
o
syringe. After 1 hour of reaction at -78 C, dichlorodiphenylsilane (5.06 g, 20 mmol)
in 20 mL THF was added over a period of 30 min. The reaction mixture was allowed
to stir for additional 1 hour at -78 oC. The mixture was gradually warmed up to room
temperature overnight. Eventually, the reaction was quenched by adding water (5 mL).
The resulting mixture was placed into 50 mL of water, and the desired product
extracted with EtOAc (3×50 mL). The organic top layer was separated, and dried over
sodium sulfate (Na2SO4). It was then filtered and evaporated under reduced pressure
to give a crude product. Using petroleum ether/dichloromethane (5/1, v/v), the crude
product was further purified by column chromatography. The final product obtained
1
had a white-powdery appearance (7.15 g, 72.3%). H NMR (400 MHz, CDCl3): δ =
7.53-7.50 (m, 8H, Ar-H), 7.47-7.44 (m, 2H, Ar-H ), 7.40-7.37 (m, 8H, Ar-H ) ppm.
13C NMR (100 MHz, CDCl3): δ = 137.82, 136.23, 133.01, 132.66, 131.22, 130.01,
128.11, 124.97 ppm. HRMS (EI): m/z calcd for: 491.9495, found: 491.9849. Anal.
Calcd for C24H18Br2Si (%): C 58.32, H 3.67; found: C 58.23, H 3.61.

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Preparation of (4-bromophenyl)(4-(dibenzo[b,d]furan-4-yl)phenyl)diphenylsilane
Bis(4-bromophenyl)diphenylsilane (1.98 g, 4 mmol), dibenzo[b,d]furan-4-ylboronic
acid (0.85 g, 4 mmol) and Pd(PPh3)4 (0.23 g, 0.2 mmol) were dissolved with 50 mL
THF in under argon. 2 M K2CO3 (THF/H2O = 4/1, v/v) was then added. The resulting
o
solution was heated at 70 C overnight. After cooling to room temperature, the
solution was placed into 200 mL of water and the desired product extracted with
dichloromethane 3 times. The organic top layer was collected and evaporated. The
obtained crude product was purified by column chromatography using petroleum
ether/dichloromethane (3/1, v/v) to give the final product as a white powder (1.67 g,
1
71.6%). H NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 8.0 Hz, 1H, Ar-H), 7.96-7.94
13
(m, 3H, Ar-H), 7.71 (d, J = 8.0 Hz, 2H, Ar-H), 7.64-7.31 (m, 19H, Ar-H) ppm. C
NMR (100 MHz, CDCl3): δ = 156.14, 153.39, 137.99, 136.65, 136.46, 136.37, 133.57,
133.24, 133.04, 131.15, 129.86, 128.23, 128.04, 127.29, 126.83, 125.42, 125.02,
124.81, 124.13, 123.27, 122.83, 120.68, 119.98, 111.84 ppm. HRMS (EI): m/z calcd
for: 580.0858; found: 580.0850. Anal. Calcd for C36H25BrOSi (%): C 74.35, H 4.33;
found: C 74.45, H 4.34.
Preparation of (4-bromophenyl)(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane
(4-Bromophenyl)(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane was prepared
in a similar manner with (4-bromophenyl)(4-(dibenzo[b,d]furan-4-yl)phenyl)di-
1
phenylsilane. The final product was obtained as a white powder (72.4%). H NMR
(400 MHz, CDCl3): δ = 8.18-8.13 (m, 2H, Ar-H), 7.82-7.80 (m, 1H, Ar-H), 7.76 (d, J

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= 8.0 Hz, 2H, Ar-H), 7.69 (d, J = 7.6 Hz, 2H, Ar-H), 7.61 (d, J = 8.0 Hz, 4H, Ar-H),
13
7.56-7.39 (m, 14H, Ar-H) ppm. C NMR (100 MHz, CDCl3): δ = 141.87, 139.49,
138.43, 137.96, 136.75, 136.59, 136.35, 136.31, 135.70, 133.47, 133.40, 133.17,
131.14, 129.89, 128.06, 127.73, 126.94, 126.84, 125.13, 124.84, 124.40, 122.58,
121.72, 120.67 ppm. HRMS (EI): m/z calcd for: 596.0630; found: 596.0626. Anal.
Calcd for C36H25BrSSi (%): C 72.35, H 4.22; found: C 72.23, H 4.31.
Preparation
of
3-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)diphenylsilyl)phenyl)-
pyridine (DBFSiPy3)
(4-Bromophenyl)(4-(dibenzo[b,d]furan-4-yl)phenyl)diphenylsilane (2.33 g, 4 mmol),
pyridin-3-ylboronic acid (0.54 g, 4.4 mmol) and Pd(PPh3)4 (0.23 g, 0.2 mmol) were
dissolved in THF under argon, 2 M K2CO3 (THF/H2O = 4/1, v/v) was then added.
The resulting solution was heated at 70 oC overnight and was allowed to cool to room
temperature. The solution was thereafter placed into 200 mL of water and the desired
product extracted by adding dichloromethane. This process was repeated 3 times. The
organic layer was separated and evaporated. Using petroleum ether/dichloromethane
(2/1, v/v), the obtained crude product was purified via column chromatography. The
1
final product had a white powdery appearance (2.18 g, 94.0%). H NMR (400 MHz,
CDCl3): δ = 8.90 (d, J = 2.0 Hz, 1H, Ar-H), 8.61 (dd, J = 4.8, 1.5 Hz, 1H, Ar-H),
8.05-8.78 (m, 5H, Ar-H), 7.76 (dt, J = 14.1, 7.1 Hz, 4H, Ar-H), 7.72-7.56 (m, 8H,
Ar-H), 7.53-7.32 (m, 10H, Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 156.17,

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153.43, 148.73, 148.42, 138.93, 137.71, 137.23, 136.75, 136.48, 136.42, 134.43,
134.30, 133.91, 133.41, 129.84, 128.25, 128.06, 127.32, 126.88, 126.62, 125.51,
125.04, 124.17, 123.65, 123.31, 122.85, 120.72, 119.99, 111.87 ppm. HRMS (EI):
m/z calcd for: 579.2018; found: 579.2010. Anal. Calcd for C41H29NOSi (%): C 84.94,
H 5.04, N 2.02; found: C 84.78, H 4.98, N 2.12.
Preparation of 3-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)phenyl)-
pyridine (DBTSiPy3), 4-(4-((4-(dibenzo[b,d]furan-4-yl)phenyl)diphenylsilyl)phenyl)-
pyridine (DBFSiPy4), 4-(4-((4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilyl)-
phenyl)pyridine (DBTSiPy4)
DBTSiPy3, DBFSiPy4 and DBTSiPy4 were prepared in a similar manner with
DBFSiPy3. The final products were obtained as white powders (yield 94.5%, 92.3%,
and 91.5% for DBTSiPy3, DBFSiPy4 and DBTSiPy4, respectively).
DBTSiPy3: 1H NMR (400 MHz, CDCl3): δ = 8.90 (d, J = 2.1 Hz, 1H, Ar-H), 8.61 (dd,
J = 4.8, 1.6 Hz, 1H, Ar-H), 8.26-8.11 (m, 2H, Ar-H), 7.92 (ddd, J = 9.4, 7.9, 5.2 Hz,
1H, Ar-H), 7.87-7.72 (m, 7H, Ar-H), 7.71-7.61 (m, 6H, Ar-H), 7.61-7.40 (m, 10H,
Ar-H), 7.38 (ddd, J = 7.9, 4.8, 0.7 Hz, 1H, Ar-H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 148.76, 148.44, 141.84, 139.54, 138.98, 138.48, 137.21, 136.86, 136.70, 136.47,
136.40, 136.35, 135.75, 134.42, 134.20, 133.82, 133.77, 129.88, 128.09, 127.75,
127.00, 126.89, 126.64, 125.19, 124.45, 123.64, 122.63, 121.77, 120.70 ppm. HRMS

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(EI): m/z calcd for: 595.1790; found: 595.1779. Anal. Calcd for C41H29NSSi (%): C
82.65, H 4.91, N 2.35; found: C 86.11, H 4.93, N 2.50.
1
DBFSiPy4: H NMR (400 MHz, CDCl3): δ = 8.68 (dd, J = 4.6, 1.6 Hz, 2H, Ar-H),
8.03-7.92 (m, 4H, Ar-H), 7.77 (d, J = 7.6 Hz, 4H, Ar-H), 7.72-7.62 (m, 7H, Ar-H),
7.62-7.52 (m, 3H, Ar-H), 7.52-7.40 (m, 8H, Ar-H), 7.36 (td, J = 7.6, 0.8 Hz, 1H, Ar-H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 156.17, 153.42, 150.34, 148.15, 139.18,
137.76, 137.24, 136.74, 136.47, 135.66, 133.77, 133.26, 129.88, 128.26, 128.08,
127.33, 126.87, 126.44, 125.47, 125.05, 124.16, 123.31, 122.87, 121.68, 120.73,
120.01, 111.86 ppm. HRMS (EI): m/z calcd for: 579.2018; found: 579.2023. Anal.
Calcd for C41H29NOSi (%): C 84.94, H 5.04, N 2.02; found: C 84.83, H 5.00, N 2.04.
1
DBTSiPy4: H NMR (400 MHz, CDCl3): δ = 8.68 (dd, J = 4.5, 1.6 Hz, 2H, Ar-H),
8.25-8.09 (m, 2H, Ar-H), 7.88-7.72 (m, 7H, Ar-H), 7.71-7.63 (m, 6H, Ar-H),
13
7.61-7.39 (m, 12H, Ar-H) ppm. C NMR (100 MHz, CDCl3): δ = 150.34, 148.06,
141.85, 139.49, 139.20, 138.43, 137.19, 136.82, 136.63, 136.42, 136.32, 135.71,
135.51, 133.66, 133.59, 129.88, 128.07, 127.73, 126.96, 126.86, 126.43, 125.16,
124.43, 122.59, 121.74, 121.63, 120.68 ppm. HRMS (EI): m/z calcd for: 595.1790;
found: 595.1779. Anal. Calcd for C41H29NSSi (%): C 82.65, H 4.91, N 2.35; found: C
82.66, H 4.93, N 2.45.
3.2 Thermal Analyses
The thermal properties of DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 were

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studied by differential scanning calorimetry (DSC) and thermogravimetric analysis
(TGA). As shown in Figure 1, these materials exhibited good thermal stability. Glass
o
transition temperatures (Tg) of 88 °C and 93 C were obtained for DBTSiPy3 and
DBTSiPy4 respectively. However, DBFSiPy3 and DBFSiPy4 yielded slightly lower
o
Tg of 86 °C and 88 C respectively. The negligible difference observed in the Tg of
these materials could be attributed to their similar molecular structures and weights.
The thermal decomposition temperatures (Td, corresponding to 5% weight loss) of
o
DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 are 363, 417, 338 and 400 C
respectively. In addition, it is worth noting that 4-Py based materials DBTSiPy4 and
DBFSiPy4 show relatively higher thermal stability compared to 3-Py based materials
DBTSiPy3 and DBFSiPy3.
3.3 Photophysical Properties
Figure 2 shows the UV-Vis absorption and photoluminescence (PL) spectra of
DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 in dichloromethane at room
temperature and the phosphorescence (Phos) spectra measured in
2-methyltetrahydrofuran (2-MeTHF) at 77 K. The detailed data are summarized in
Table 1. The absorption spectra of DBT- and DBF-based materials show similar
absorption bands. However, the absorption onset and the maximum absorption
wavelengths (Abs λmax) of DBT-based materials are red-shifted by 16 nm relative to
the DBF-based materials. The optical band gaps (Eg) of DBTSiPy3 and DBTSiPy4
(3.59 eV) are therefore smaller than DBFSiPy3 and DBFSiPy4 (3.77 eV). On the
other hand, they possess different PL features. The well-defined PL spectra of

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DBF-based materials and the structureless PL spectra of DBT-based materials
indicate a difference in their excited states. The peaks at 464 and 460 nm represent the
highest vibronic bands of phosphorescence spectra at 77 K for DBT- and DBF-based
materials respectively. Therefore, the triplet energies (ET), 2.67 and 2.70 eV were
extracted for DBT- and DBF-based materials respectively. Furthermore, the
photoluminescence quantum yield (PLQY) of Ir(ppy)3 (9 wt%) in the DBTSiPy3,
DBTSiPy4, DBFSiPy3 and DBFSiPy4 thin films were measured in an integrating
sphere to be 0.78, 0.76, 0.85 and 0.80, respectively, which are comparable to the
reported results[48,49]. The relative high PLQY of DBF based materials would lead to a
better device performance.
3.4 DFT Simulation.
In order to have a better understanding of the electronic distributions of DBTSiPy3,
DBTSiPy4, DBFSiPy3 and DBFSiPy4, their frontier molecular orbital (FMO) spatial
distributions were simulated by density function theory (DFT) calculations at a
B3LYP/6-31G(d) level. As Figure 3 illustrates, all of these materials have similar
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) distributions. The HOMOs are localized at the DBT and DBF units with
Si-phenyl ring acting as a separation group while the LUMOs are distributed on the
pyridine part and the phenyl separation group. Therefore, all of these four Si-aryl
compounds have similar HOMO/LUMO energy levels, which are -5.72/-1.16 eV,
-5.74/-1.32 eV, -5.89/-1.23 eV and -5.92/-1.31 eV for DBTSiPy3, DBTSiPy4,
DBFSiPy3 and DBFSiPy4, respectively. In addition, the lowest triplet energies (ETs)

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of these four DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 were also evaluated
to be 2.87 eV, 2.89 eV, 2.91 eV and 2.97 eV, respectively. The narrow ET range of
DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 could be attributed to their
similar molecular configuration.
3.5 FMO Energy Levels
From the ultraviolet photoemission spectroscopy (UPS) data presented in Figure 4,
the HOMO levels of DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4 are
estimated as -6.24, -6.38, -6.36 and -6.58 eV respectively. The LUMOs can be
calculated as following equation：ELUMO = Eg + EHOMO. As a result, the LUMO levels
are determined as -2.65, -2.79, -2.59 and -2.81 eV for DBTSiPy3, DBTSiPy4,
DBFSiPy3 and DBFSiPy4, respectively.
3.6 Electroluminescent Performances
To investigate the electroluminescent (EL) properties of DBTSiPy3, DBTSiPy4,
DBFSiPy3 and DBFSiPy4 host materials, (2-phenylpyridine) iridium (III) (Ir(ppy)3)
based green phosphorescent OLEDs were fabricated using the following device
structure: ITO/HAT-CN (10 nm)/TAPC (55 nm)/Host: 9% Ir(ppy)3 (20 nm)/TmPyPB
(35 nm)/Liq (2 nm)/Al (120 nm). DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4
were doped with 9 % Ir(ppy)3 to form the emitting layer (EML). To confine the triplet
excitons in the EML, 1,1-bis[4-[N',N'-di(p- tolyl)amino]-phenyl]cyclohexane (TAPC)
and 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB) were used as hole transporting
layer (HTL)/electron blocking layer (EBL) and electron transporting layer (ETL)
/hole blocking layer (HBL), respectively. Furthermore, to achieve efficient carrier

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injection,
dipyrazino[2,3-f:2',3'-h]-quinoxaline-2,3,6,7,10,11-hexacarbo-nitrile
(HAT-CN) and 8-hydroxyquinolinolato- lithium (Liq) were used as hole injection
layer (HIL) and electron injection layer (EIL) respectively. The energy diagram and
molecular structures can be seen in Figure 5. The current density-voltage-luminance
(J-V-L) characteristics;, current efficiency (CE), power efficiency (PE) and EQE as a
function as luminance; and the EL spectra are presented in Figure 6 (a), 6 (b) and 6
(c), respectively. The detailed EL performance data are summarized in Table 2.
As shown in Figure 6(a), by using 4-Py based materials (DBTSiPy4 and
DBFSiPy4) as host, the OLEDs show relatively lowered driving voltage of 3.8 and
4.1 V for Ir(ppy)3 based devices at 1000 cd/m2 compared to the devices hosted by
3-Py based materials (DBTSiPy3 and DBFSiPy3). In addition, all of these devices
exhibit a maximum luminance of 17500 cd/m2. The devices hosted by DBFSiPy4
achieve a maximum luminance above 21000 cd/m2. Figure 6(b) illustrates the
efficiencies of the Ir(ppy)3-based green devices. High efficiencies of these green
devices are achieved. Maximum current efficiencies (CE) of 77.5 cd/A and 74.5 cd/A,
power efficiencies (PE) of 65.0 lm/W and 67.4 lm/W and external quantum
efficiencies (EQE) of 22.9 % and 21.9 % are respectively reached for DBFSiPy3 and
DBFSiPy4 based devices. The devices hosted by DBTSiPy3 and DBTSiPy4 showed
relative lower; CE of 58.0 cd/A and 57.5 cd/A, PE of 49.9 lm/W and 56.7 lm/W and
EQE of 16.2% and 16.0%, respectively. Moreover, these devices also show relatively
flat EQE roll-off, especially for DBFSiPy4 based device. At 1000 cd/m2, the EQE
reduce from the maximum value of 21.9% to 21.1 %. At a high brightness of 5000

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cd/m2, the EQE is still as high as 19.6 %. At 10000 cd/m2, the EQE remains above the
maximum EQE, with 17.8 % for DBTSiPy3 and DBTSiPy4 based devices. The green
devices show CIE coordinates of (0.28, 0.64) at 5 mA/cm2 as expected for a typical
Ir(ppy)3 based device. There are many factors of host materials responsible for OLED
performance, such as high ET, suitable HOMO/LUMO energy level, good and
balanced charge transport ability, as well as good thermal property. In this article,
taking the very close ETs, HOMO/LUMO energy levels and thermal properties of four
materials into consideration, we ascribe the relative better performance of DBFSiPy3
and DBFSiPy4 hosted device to their good and balanced charge transport properties
(Figure 7). Moreover, the relative high PLQY of the dopant in host films of
DBFSiPy3 and DBFSiPy4 also partly account for their better device performance.
4. Conclusion
In conclusion, the design, synthesis and full characterization of four novel
organosilicon derivatives, DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4
consisting of DBT, DBF and pyridine units were achieved. Due to suitable linking
strategy, these materials exhibit high triplet energy and good thermal stability They
were used as host materials in the fabrication of green PHOLEDs with Ir(ppy)3 as
emitter. Green PHOLEDs based on DBF substituted material DBFSiPy3 and
DBFSiPy4 achieve maximum EQE of 22.9% and 21.9% respectively. This notable
device performance demonstrates the great potential of tetraphenyl silicon derivatives
in constructing bipolar host materials for PHOLEDs.

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Acknowledgement
We appreciate the financial support from the National Natural Science Foundation of
China under Grant No 61076066, 21572152, 61575136 and the Innovation Project of
Science and Technology Plan Projects of Shaanxi Province under Grant No
2011KTCQ01-09. Key research and development plan of Shaanxi Province
(2017ZDCXL-GY-06-03), Special scientific research plan of Shaanxi Provincial
Education Department (17JK0095), Jiangsu Youth Foundation Project (BK20180288),
Sixty-second grants from China Post Doctoral Science Fund(7131700918).
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Pd(PPh3)4, K2CO3,
THF/H2O
Br
Br
Si
Si
HO
Br
Br
O
B
OH
O
n-BuLi
Br
Si
Br
Cl Cl
Si
Pd(PPh3)4, K2CO3,
THF/H2O
HO
S
B
OH
S
Pd(PPh3)4, K2CO3,
THF/H2O
Si
Si
N
N
OH
B
S
O
N
OH
DBFSiPy3
DBTSiPy3
Pd(PPh3)4, K2CO3,
THF/H2O
N
Si
N
Si
OH
N
B
S
O
OH
DBFSiPy4
DBTSiPy4
Scheme 1. Synthetic routes to DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4.
Figure 1. Differential scanning calorimetry (DSC, left) and thermogravimetric analysis (TGA,
right) curves of DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4.

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Figure 2. UV-Vis absorption, PL and Phos spectra of DBTSiPy3, DBTSiPy4, DBFSiPy3 and
DBFSiPy4.
Si
N
Si
Si
N
Si
N
N
S
S
O
O
DBTSiPy3
DBTSiPy4
DBFSiPy3
DBFSiPy4
LUMO
-1.16 eV
ET = 2.87 eV
-1.23 eV
-1.32 eV
-1.31 eV
ET = 2.89 eV
-5.74 eV
ET = 2.91 eV
-5.89 eV
ET = 2.97 eV
-5.92 eV
-5.72 eV
HOMO
Figure 3. HOMO/LUMO distributions and molecular structure of DBTSiPy3, DBTSiPy4,
DBFSiPy3 and DBFSiPy4.

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Figure 4. UPS spectra of DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4.
Table 1. Physical Properties of DBTSiPy3, DBTSiPy4, DBFSiPy3 and DBFSiPy4.
a
a
b
c
d
e
Abs λmax
PL λmax
Tg
Td
Eg
ET
Host
HOMOf /eV LUMOg /eV
/nm
/nm
/oC /oC
/eV
/eV
2.67
2.67
2.67
2.70
DBTSiPy3
DBTSiPy4
DBFSiPy3
278
356
88 363 3.59
93 417 3.59
-6.24
-6.38
-6.36
-6.58
-2.65
-2.79
-2.59
-2.81
266
356
254
333, 346 86 338 3.77
333, 346 88 400 3.77
DBFSiPy4
254
a
b
c
Measured in toluene solution at room temperature. Tg: Glass transition temperature. Td:
d
Decomposition temperature. Eg: Band gaps calculated from the corresponding absorption
e
f
onset. ET: Measured in 2-MeTHF glass matrix at 77 K. HOMO levels calculated from UPS
data. g LUMO levels calculated from the HOMO and Eg.

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N
NC
N
CN
N
N
N
N
N
CN
NC
N
N
CN
NC
N
HAT-CN
N
TAPC
TmPyPB
Ir
N
Si
N
S
N
Si
3
Ir(ppy)3
DBTSiPy3
S
DBTSiPy4
Si
N
Si
N
O
O
DBFSiPy3
DBFSiPy4
Figure 5. Molecular structures and representative energy level diagram of the materials.