Modular synthesis of polybenzo[b]silole compounds for hole-blocking material in phosphorescent organic light emitting diodes

Article abstract of DOI:10.1002/asia.201000112

A diversity-oriented synthetic strategy allowed us to design a series of conjugated molecules containing multiple benzosilole units that can be utilized as efficient hole-blocking materials for phosphorescent organic light emitting diodes (OLEDs). Some of

Full text of DOI:10.1002/asia.201000112

FULL PAPERS
DOI: 10.1002/asia.201000112
Modular Synthesis of Polybenzo[b]silole Compounds for Hole-Blocking
Material in Phosphorescent Organic Light Emitting Diodes
Laurean Ilies,[a] Yoshiharu Sato,*[b] Chikahiko Mitsui,[a] Hayato Tsuji,*[a] and
Eiichi Nakamura*[a, b]
Abstract: A diversity-oriented synthet-
ic strategy allowed us to design a series
of conjugated molecules containing
multiple benzosilole units that can be
utilized as efficient hole-blocking mate-
rials for phosphorescent organic light
emitting diodes (OLEDs). Some of
pounds were easily synthesized in a
modular fashion from a previously re-
ported 3-stannyl benzosilole building
unit. Studies on the properties of these
compounds in solution and in the solid
state indicate that they possess high
electron affinity, high ionization poten-
tial, and form stable amorphous films
that show high electron-drift mobility.
The correlation between their molecu-
lar properties and the efficiency of the
OLED device performance is also in-
vestigated.
Keywords: benzosilole
·
hole-
blocking material
·
materials
these compounds showed
a perfor-
science · phosphorescent OLED ·
synthesis design
mance surpassing that of the current
standard, bathocuproine. The new com-
Introduction
hole-blocking material (HBM), must possess high triplet
energy, high ionization potential, and low hole-drift mobility,
so that it can block the diffusion of triplet excitons and of
holes. Other requirements are high electron-drift mobility,
formation of amorphous films, and thermal, chemical, and
electrochemical stability. A widely used material as a HBM
is bathocuproine (BCP),[2] which does not necessarily satisfy
all of these requirements. Thus, we became interested in
compounds bearing multiple 2,3-disubstituted benzo[b]silole
units[3–5] (hereinafter benzosiloles) because of the high elec-
tron affinity of benzosilole[6] associated with its low-lying
LUMO level[7] and the ready structural modification achiev-
able by our diversity-oriented modular synthetic strategy.[8,9]
We report herein a detailed study of the synthesis, proper-
ties, and application of polybenzosiloles as an efficient
HBM for phosphorescent OLEDs. We also describe insights
into the relationship between the device efficiency and the
molecular properties.
Phosphorescent organic light emitting diodes (OLEDs)[1]
can serve as highly efficient light emitting devices, because
the phosphorescence emitters can harvest both the singlet
and triplet excitons and achieve high performance of the
device. This can be realized if the triplet excitons and posi-
tive charges (holes) can be confined into the emitting layer,
and therefore exciton–exciton annihilation and charge re-
combination outside the active layer can be effectively
avoided. Such confinement requires intercalation of an ap-
propriate buffer layer between the emission layer and the
electron-transporting layer. The buffer material, called a
[a] Dr. L. Ilies, C. Mitsui, Dr. H. Tsuji, Prof. Dr. E. Nakamura
Department of Chemistry
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5800-6889
E-mail: nakamura@chem.s.u-tokyo.ac.jp
tsuji@chem.s.u-tokyo.ac.jp
Results and Discussion
Modular Synthesis of Functionalized Benzosiloles 3–6
[b] Dr. Y. Sato, Prof. Dr. E. Nakamura
Nakamura Functional Carbon Cluster Project, ERATO
Japan Science and Technology Agency
For the synthesis of the polybenzosilole compounds, we
relied on our modular strategy (Scheme 1).[3] Treatment of
(2-alkynylphenyl)silane (1) with trimethylstannyllithium in
diethyl ether gave a stable 3-stannylbenzosilole (2) in excel-
lent yield, which after transmetallation to a more reactive
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: ysato@chem.s.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000112.
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Chem. Asian J. 2010, 5, 1376 – 1381

pound 4 in excellent yield by the coupling of 2 with dichlor-
opyrazine (entry 2). This compound consists of two rotation-
al isomers of 4 (NMR) because the methyl substituents on
the pyrazine linker hamper the rotation of the benzosilole
moieties.[11] In light of the popularity of the bipyridyl motif
for the design of n-type organic semiconductors,[12] we pre-
pared the bipyridyl-core compound 5 in good yield by the
coupling of 2 with dibromobipyridyl (entry 3). Triple cou-
pling of 2 with 2,4,6-trichlorotriazine (cyanuric chloride)
proceeded smoothly, and the triazine-linked[13] benzosilole 6
was obtained in good yield (entry 4).
Scheme 1. Modular synthesis of functionalized benzosiloles.
zinc intermediate was subjected to Pd0-catalyzed cross-cou-
pling to achieve functionalization at the C-3 position.
The compounds thus prepared are illustrated in Table 1.
Phenylene-linked benzosilole 3 was synthesized from 1,4-
diiodobenzene in good yield (entry 1). Focusing on the abili-
ty of nitrogen-containing heterocycles to increase the elec-
tron affinity (EA),[10] we synthesized the pyrazine-core com-
Properties
Studies on compounds 3–6 in solution and in the solid state
(Tables 2 and 3) showed that these compounds possess high
EA and IP, and form stable amorphous films showing high
electron-drift mobility.
As shown in Table 2, the ab-
sorption maxima of compounds
Table 1. Preparation of conjugated molecules containing multiple benzosilole units.[a]
3–6 were in the range of 288–
325 nm, with an optical gap of
3.10–3.26 eV, which is rather
small because of the limited
conjugation in these nonplanar
compounds. The fluorescence
maximum of phenylene-linked
compound 3 was approximately
390 nm, while introduction of
nitrogen heterocycles in 4–6
red-shifted the emission to
460 nm. These 2,3-disubstituted
benzosiloles showed very low
emission quantum yields in so-
lution, as expected from previ-
ous reports on polysubstituted
Entry
Electrophile
Product
Yield [%][b]
1
3
4
84
2
3
96
64
5
6
siloles.[14]
All
compounds
showed irreversible reduction
waves in THF at a scan rate of
100 mVsÀ1, the first reduction
potential being in the range of
À2.21 to À2.62 V versus a fer-
rocene standard. The introduc-
tion of nitrogen-containing het-
erocycles caused an increase in
the electron affinity of the com-
pounds, but this effect was
4
71
[a] For the reaction conditions, see Scheme 1 and the Experimental Part. [b] Yield of isolated product.
rather small, probably because of the nonplanarity of the
compounds (Figure 1). The triazine-linked 6, however, ex-
hibited a significantly higher reduction potential (entry 6),
probably because of the high electron affinity of the triazine
unit.[13]
Abstract in Japanese:
Compounds 3–6 form stable amorphous films possessing
high electron-drift mobility (Table 3). Fast cooling of a melt
of compounds 3–6 gave an amorphous solid, and its thermal
behavior was evaluated by differential scanning calorimetry
(DSC). The stability of the amorphous state increases as the
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1377

Y. Sato, H. Tsuji, E. Nakamura et al.
FULL PAPERS
Table 2. Optical and electrochemical properties in solution for com-
pounds 3–6.
electron mobility, presumably because of the excessively
high electron affinity of the triazine moiety.[13] The triplet
energy was calculated by TD-DFT, and was found to
depend by a small degree on the nature of the linker.
Single crystals of phenylene-linked benzosilole 3 suitable
for crystallographic analysis were obtained by recrystalliza-
tion from hexane/chloroform, the X-ray crystal structure
being shown in Figure 1. While the benzosilole framework is
planar, the 2-substituent is tilted out of the benzosilole
plane at a dihedral angle of 44.41(17)8. The similar dihedral
angles reported by Yamaguchi for 1,4-bis(1,1-dimethylben-
zosilol-2-yl)benzene are smaller (average 278),[5d] suggesting
that the 3-substituent in 3 forces the 2-substituent out of the
benzosilole plane. The 3-substituent is also tilted out at
72.25(19)8, explaining the small effect of this substituent on
the LUMO level of the molecule (Table 2). The crystal
packing of 3 (Figure 2) suggests the lack of strong intermo-
lecular interactions, explaining the facile formation of the
amorphous state.
[a]
[a,b]
[c]
[d]
Cmpd.
labs
Optical
gap [eV]
lem
FF
Eredn
[V]
IP[e]
[eV]
EA[f]
[eV]
[nm]
[nm]
3
4
5
6
325
296
322
288
3.26
3.26
3.26
3.10
392
462
460
–
0.02
0.01
0.01
<0.01
À2.62
À2.50
À2.57
À2.21
5.44
5.56
5.49
5.69
2.18
2.30
2.23
2.59
[a] In dichloromethane. [b] Irradiated at 320–340 nm. [c] Quinine sulfate
in aqueous 0.5m sulfuric acid was used as a reference. [d] First reduction
potential determined by CV for a THF solution (0.5 mmolLÀ1), using
TBAP (0.1 molLÀ1) as an electrolyte. The scan rate was 100 mVsÀ1, three
cycles per measurement. Glassy carbon was used as the working elec-
trode, platinum wire as the counter electrode and Ag+/Ag as a standard
electrode. Eredn refers to the cathode potential for the irreversible reduc-
tion wave, and its value was determined by DPV and corrected from a
ferrocene standard. [e] Inferred from the optical gap and EA. [f] EA=
4.8+Eredn
.
Table 3. Solid-state properties and triplet energy for compounds 3–6.
Cmpd.
Tg [8C][a]
Tc [8C][a]
Tm [8C][a]
me [cm2 Vs][b]
ET [eV][c]
3
4
5
6
66
83
103
112
125
138
148
149
234
225
300
261
6ꢁ10À4
7ꢁ10À4
2ꢁ10À3
7ꢁ10À6
2.38
2.43
2.39
2.32
[a] Determined by differential scanning calorimetry. Tg, Tc, and Tm refer
to the glass transition temperature, crystallization temperature and melt-
ing temperature, respectively. [b] Electron-drift mobility for a thick amor-
phous film, determined by the time-of-flight technique, at an applied
electric field of 5ꢁ105 VcmÀ1. [c] The triplet energy level was estimated
by a TD-DFT calculation.
molecular size increased, and tris-benzosilole 6 showed a
glass transition temperature as high as 1128C. The electron-
drift mobility of a thick film of compounds 3–6 was mea-
sured by the time-of-flight technique.[15] Except for the
poorly conducting triazine-linked 6, all benzosiloles showed
high electron-drift mobility in the 10À4–10À3 cm2 Vs range, at
an applied electric field of 5ꢁ105 VcmÀ1. The bipyridyl-
linked 5 showed the highest value in the series, probably be-
cause of a synergetic effect between the benzosilole unit and
the bipyridyl moiety.[12] The triazine-linked 6 showed low
Figure 2. The crystal packing of phenylene-linked benzosilole 3.
The Use of Benzosiloles as HBM for Phosphorescent
OLEDs
Next, we investigated the behavior of benzosiloles 3–6 as
HBM for phosphorescent OLEDs. The devices were fabri-
cated employing the standard materials[1] (Figure 3): trans-
parent glass coated with indium-tin oxide (ITO) was used as
an anode, poly(ethylenedioxy)thiophene:polystyrene sulfo-
nate (PEDOT:PSS, 30 nm) was used as a hole-injection ma-
terial, N,N’-di-[(1-naphthyl)-N,N’-diphenyl]-1,1’-biphenyl)-
4,4’-diamine (a-NPD, 45 nm) was used as a hole-transport-
ing material, ambipolar 4,4’-bis(carbazol-9-yl)biphenyl
(CBP, 30 nm) doped with 4 wt% green phosphorescent fac-
IrACTHNUGTRENNG(U ppy)3 was used as an emission layer, benzosilole 3–6 or
BCP (10 nm) was used as a HBM, tris(8-hydroxyquinolina-
to)aluminum (Alq3, 20 nm) was used as an electron-trans-
porting material, and aluminum (80 nm) coated with 8-hy-
droxyquinolinato lithium (Liq, 1 nm) was used as a cathode.
The characteristics of the device are summarized in Fig-
ures 4 and 5 and Table 4. Compounds 3–5 showed an overall
performance comparable with that of BCP, and the pyra-
zine-linked compound 4 showed the highest performance,
surpassing BCP for the driving voltage, luminance efficiency,
current efficiency, and maximum luminance (Table 4). The
Figure 1. ORTEP drawing of phenylene-linked benzosilole 3 (50% prob-
ability for thermal ellipsoids). Hydrogen atoms are omitted for clarity.
Selected bond angles: Si1-C1-C2-C3=À6.73(14)8; Si1-C1-C11-C12=
44.41(17)8; C1-C2-C17-C19=72.25(19)8.
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Chem. Asian J. 2010, 5, 1376 – 1381

Modular Synthesis of Polybenzo[b]silole Compounds
the aluminium cathode, while
higher IP and ET efficiently
confine the holes, and the trip-
let excitons, respectively, into
the emission layer. These corre-
lations suggest that for a series
of structurally related com-
pounds, rational design of effi-
cient HBMs could be achieved
by evaluating their molecular
properties.
Figure 3. The structure of phosphorescent OLED and the materials utilized.
Figure 5. EQE characteristics of a phosphorescent OLED utilizing benzo-
silole 3–6 or BCP as a HBM.
Figure 4. L–V characteristics of a phosphorescent OLED utilizing benzo-
silole 3–6 or BCP as a HBM.
Table 4. Summary of the characteristics of a green phosphorescence
OLED utilizing compounds 3–6 or BCP as HBM.[a]
[b]
[c]
[d]
[e]
[f]
HBM
V1000
[V]
h1000
[lm/W]
L/J1000
[cdA]À1
EQE1000
[%]
Lmax
[cdmÀ2
]
device using triazine-linked compound 6 showed a lower
performance.
G
E
U
3
4
5
6
6.5
6.9
7.9
9.5
7.4
6.5
7.6
5.7
2.9
6.5
13.5
17.0
14.6
8.7
4.3
5.8
4.8
3.3
5.2
55530
59400
66690
34950
49940
To gain insight into the factors controlling the efficiency
of HBMs, we plotted the external quantum efficiency
(EQE) recorded at a luminance of 1000 cdmÀ2 versus EA
(Figure 6), IP (Figure 7), ET (Figure 8), and electron mobili-
ty (me) (Figure 9). As shown in Figure 9, for compounds 3–5,
which possess electron-drift mobility higher than Alq3 (~1ꢁ
10À5 cm2 Vs),[16] factors other than the mobility control their
performance. However, the poorly semiconducting 6, which
possesses a lower electron-drift mobility than Alq3, showed
poor performance, regardless of its other properties. The
performance of the high mobility compounds 3–5 showed
almost linear dependence on their EA, IP, and ET. We
assume that higher EA favors injection of electrons from
BCP
15.5
[a] All OLED performance data were collected at
a luminance of
1000 cdmÀ2. [b] Driving voltage. [c] Luminance efficiency. [d] Current ef-
ficiency. [e] External quantum efficiency. [f] Maximum luminance.
Conclusions
A series of conjugated molecules containing multiple benzo-
silole units linked by an aromatic core have been synthe-
sized. They showed high electron affinity and readily
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1379

Y. Sato, H. Tsuji, E. Nakamura et al.
FULL PAPERS
formed stable amorphous films that possess a high electron-
drift mobility. The functionalized benzosiloles were then uti-
lized as HBMs for phosphorescent OLEDs, and they exhib-
ited a good overall performance that surpasses the current
standard material, BCP. Correlation of the device efficiency
with the molecular properties of the benzosiloles suggested
that for the HBM materials that possess higher electron-
drift mobility than the electron-transporting material (e.g.,
Alq3), the device performance increases almost linearly with
the electron affinity, the ionization potential, and the triplet
energy of the HBM.
Figure 6. Correlation of EQE with EA.
Experimental Section
Compounds 3–6 were prepared according to our previously reported pro-
cedure.[3]
Data for the new compounds:
2,5-Bis(1,1-dimethyl-2-phenylbenzo[b]silol-3-yl)-3,6-dimethylpyrazine (4):
1
white solid. M.p.: 224–2258C; H NMR (500 MHz, C2D2Cl4): two isomers,
~6:1 ratio. d=0.40+0.41 (s, 6H, SiACTHNUTRGENN(UG CH3)2), 0.65+0.66 (d, 6H, SiACHTUNGTRENNUNG(CH3)2),
2.15+2.16 (s, 6H, PyrazineCH3), 6.62–6.81 (m, 2H), 7.00–7.06 (m, 4H),
7.18–7.30 (m, 10H), 7.64–7.66 ppm (m, 2H). 13C NMR (125 MHz,
CD2Cl2): mixture of two isomers. d=À4.2, À2.6, 21.15, 21.24, 123.7,
123.9, 126.9, 127.0, 127.37, 127.39, 128.2, 128.4, 128.6, 128.8, 130.2, 132.2,
138.3, 139.8, 145.7, 149.3, 149.4, 149.6, 149.9, 150.2 ppm. TOF MS
(APCI+): 577 [M+1]; elemental analysis: calcd (%) for C38H36N2Si2:
C 79.12, H 6.29, N 4.86; found: C 78.84, H 6.33, N 4.57.
Figure 7. Correlation of EQE with IP.
6,6’-Bis(1,1-dimethyl-2-phenylbenzo[b]silol-3-yl)-2,2’-bipyridine (5): white
solid. M.p.: 294–2958C. 1H NMR (500 MHz, C2D2Cl4): d=0.52 (s, 12H,
SiACHTNUGRTNEUNG(CH3)2), 7.04–7.37 (m, 18H), 7.68 (d, J=8.0 Hz, 4H), 8.07 ppm (d, J=
8.0 Hz, 2H); 13C NMR (100 MHz, C2D2Cl4, 708C): d=À3.4, 120.0, 124.4,
125.4, 126.0, 126.8, 128.2, 128.5, 129.8, 131.8, 136.9, 138.4, 140.0, 145.5,
149.9, 151.8, 156.4, 156.7 ppm; TOF MS (APCI+): 625 [M+1]; elemental
analysis: calcd (%) for C42H36N2Si2: C 80.72, H 5.81, N 4.48; found:
C 80.66, H 5.83, N 4.44.
1,3,5-tris(1,1-dimethyl-2-phenylbenzosilol-3-yl)triazine (6): white solid.
M.p.: 256–2578C; 1H NMR (500 MHz, CDCl3): d=0.38 (s, 18H, Si-
AHCTUNGTRENNG(NU CH3)2), 6.26 (d, J=7.5 Hz, 3H), 6.94–6.96 (m, 6H), 7.07–7.10 (m, 3H),
7.18 (t, J=7.0 Hz, 3H), 7.20–7.24 (m, 9H), 7.51 ppm (d, J=7.0 Hz, 3H);
13C NMR (125 MHz, CDCl3): d=À4.1, 123.1, 126.3, 126.8, 128.0, 128.4,
130.1, 131.7, 137.5, 139.1, 147.98, 148.0, 148.6, 174.9 ppm; TOF MS
(APCI+): 784 [M+1]; elemental analysis: calcd (%) for C51H45N3Si3:
C 78.11, H 5.78, N 5.36; found: C 77.94, H 5.93, N 5.25.
UV/Vis absorption spectra were recorded with a JASCO V-570 spectrom-
eter at a resolution of 0.5 nm. Spectroscopy-grade dichloromethane was
used as the solvent. Ca. 10À5 molLÀ1 sample solutions in a 1 cm square
quartz cell were used for the measurements. All measurements were per-
formed at room temperature.
Figure 8. Correlation of EQE with ET.
Fluorescence spectra were recorded with a HITACHI F-4500 spectrome-
ter. Degassed spectroscopy-grade dichloromethane was used as the sol-
vent. Quantum yields were determined by comparison with the quantum
yield (0.546) of a ca. 10À5 molLÀ1 solution of quinine sulfate in 0.5m sul-
furic acid. The excitation was performed at 320–340 nm, on the flat
region of the absorption spectrum. All measurements were performed at
room temperature.
Cyclic voltammetry was performed using a HOKUTO DENKO HZ-5000
voltammetric analyzer. A glassy carbon electrode was used as the work-
ing electrode, a platinum coil as the counter electrode and Ag+/Ag as
the reference electrode, at a scan rate of 100 mVsÀ1, three cycles per
measurement. The compounds were dissolved in degassed, dry THF at a
concentration of ca. 5ꢁ10À3 molLÀ1, and tetrabutylammonium perchlo-
rate at a concentration of 0.1 molLÀ1 was added as an electrolyte. The re-
duction potentials were determined by differential pulse voltammetry
(DPV), and were corrected from a ferrocene standard.
Figure 9. Correlation of EQE with the electron-drift mobility, and com-
parison with the electron-drift mobility of Alq3.
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Chem. Asian J. 2010, 5, 1376 – 1381

Modular Synthesis of Polybenzo[b]silole Compounds
The single crystal X-ray diffraction study was carried out on a Rigaku
MERCURY CCD system. Single crystals were obtained by slowly cool-
ing a solution of 3 in hexane/chloroform (9:1).
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cooling of the melt of benzosiloles 3–6, and the obtained sample was
heated at
a a flow rate of
rate of 10 KminÀ1 under N2 gas, at
18 mLminÀ1
.
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The time-of-flight measurements were performed at Sumitomo Heavy In-
dustries Advanced Machinery, TOF-401. Films were deposited on ITO-
coated (145 nm) glass substrates. The vacuum deposition was performed
at ULVAC KIKO (VPC-260). The films were deposited by vacuum subli-
mation at 1ꢁ10À3 Pa, with an average deposition rate of 20–30 nmsÀ1
.
The ITO-coated glass substrate was spaced at 100 mm from the sample
and was kept at 258C. The thickness of the films obtained was 3–5 mm.
All calculations were performed using the Gaussian 03 program.[17] The
geometries of benzosiloles were optimized at the B3LYP/6-31G* level.
The triplet energy levels were determined by TD-DFT calculations at the
B3LYP/6-31G* level, utilizing the optimized geometries.
For the device fabrication and evaluation, compounds 3–6 of analytical
purity were further purified by train sublimation. All of the other materi-
als were commercially available and were used as purchased. An ITO-
coated glass substrate treated by O3-plasma was used as the anode. A
PEDOT:PSS water dispersion was spin-coated into this substrate, it was
dried at 1208C, then it was annealed at 1808C under nitrogen. All layers
were sequentially vacuum deposited into this substrate at a pressure of
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Received: February 8, 2010
Published online: April 15, 2010
Chem. Asian J. 2010, 5, 1376 – 1381
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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