Disilane- and siloxane-bridged biphenyl and bithiophene derivatives as electron-transporting materials in OLEDs

Article abstract of DOI:10.1016/j.jorganchem.2008.08.018

Optical, electrochemical, and electron-transporting properties of disilane- and siloxane-bridged biphenyl and bithiophene derivatives were investigated, in comparison with those of the monosilane-bridged analogues (siloles). The UV spectra and cyclic voltammograms indicated that elongation of the silicon bridge suppresses the π-conjugation, in accordance with the results of DFT calculations. The DFT calculations indicated also that the disilane-bridged biphenyl and siloxane-bridged bithiophene should have the low-lying HOMOs and LUMOs. The electron-transporting properties were evaluated by the performance of triple-layered OLEDs having vapor-deposited films of the Si-bridged compound, Alq₃, and TPD, as the electron-transport, emitter, and hole-transport, respectively. Of these, the device with a disilane-bridged biphenyl exhibited the high performance with the maximum current density of 590 mA/cm² at the applied electric field of 12 × 10⁷ V/m (applied bias voltage = 13 V) and the maximum luminance of 22 000 cd/m² at 13 × 10⁷ V/m.

Full text of DOI:10.1016/j.jorganchem.2008.08.018

Journal of Organometallic Chemistry 693 (2008) 3490–3494
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Disilane- and siloxane-bridged biphenyl and bithiophene derivatives as
electron-transporting materials in OLEDs
a
a
a
b
Hiroyuki Kai a, Joji Ohshita a, , Sayaka Ohara , Naohiro Nakayama , Atsutaka Kunai , In-Sook Lee ,
*
Young-Woo Kwak b
a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
b Department of Chemistry, Kyungpook National University, Daegu 702-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 April 2008
Received in revised form 5 August 2008
Accepted 18 August 2008
Available online 22 August 2008
Optical, electrochemical, and electron-transporting properties of disilane- and siloxane-bridged biphenyl
and bithiophene derivatives were investigated, in comparison with those of the monosilane-bridged ana-
logues (siloles). The UV spectra and cyclic voltammograms indicated that elongation of the silicon bridge
suppresses the
p-conjugation, in accordance with the results of DFT calculations. The DFT calculations
indicated also that the disilane-bridged biphenyl and siloxane-bridged bithiophene should have the
low-lying HOMOs and LUMOs. The electron-transporting properties were evaluated by the performance
of triple-layered OLEDs having vapor-deposited films of the Si-bridged compound, Alq3, and TPD, as the
electron-transport, emitter, and hole-transport, respectively. Of these, the device with a disilane-bridged
biphenyl exhibited the high performance with the maximum current density of 590 mA/cm2 at the
applied electric field of 12 ꢀ 107 V/m (applied bias voltage = 13 V) and the maximum luminance of
22000 cd/m2 at 13 ꢀ 107 V/m.
Keywords:
Silole
OLED
Electron-transport
Theoretical calculation
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
bithiophene (Chart 1, 6) and it has been demonstrated that intro-
ducing an oxygen atom into the disilane unit affects primarily
Much attention has been focused on the silole ring system, in
which the
*-orbital of the silicon atom effectively interacts with
the
*-orbital of the butadiene unit, leading to the low-lying LUMO
[1]. To date, many papers concerning the functionalities of silole-
based materials have been published [2], since a bipyridyl-substi-
tuted silole has been demonstrated to be usable as an efficient
electron-transporting material in multi-layered organic light-emit-
ting diodes (OLEDs) [3,4].
the HOMO energy level to lower it, with the LUMO level being al-
most untouched. This clearly indicates that the siloxane-bridged
bithiophene can be anticipated as efficient electron-transporting
materials with good hole-blocking properties [8]. However, the
electron-transporting properties have not yet been examined.
In this paper, we discuss the results of theoretical calculations
on simplified model compounds that have disilane- and siloxane-
bridged biphenyl and bithiophene units (Chart 1), and the optical
and electrochemical properties of these compounds, in comparison
with those of dibenzo- and dithienosiloles. Performance of triple-
layered OLEDs having vapor-deposited films of these compounds
as the electron-transporting materials is also described.
r
p
Annulated silole derivatives, such as dibenzosiloles (Chart 1, 1),
have been also studied as OLED materials. Fascinating properties of
dibenzosilole–fluorene copolymers have been reported. For exam-
ple, incorporation of dibenzosilole units into the
p-conjugated
polyfluorene main chain leads to high-efficiency spectrum-stable
color-pure blue-emitting materials [5]. We have studied dithieno-
silole as a thiophene-annulated silole system (Chart 1, 4) and have
demonstrated that dithienosilole derivatives also exhibit highly
electron-transporting properties in their vapor-deposited films,
being applicable to OLEDs [6,7]. Disilane-bridged bithiophenes
(Chart 1, 5) again can be used as excellent electron-transports in
OLEDs, similar to dithienosiloles [7]. Recently we have studied
the optical, and electrochemical properties of a siloxane-bridged
2. Results and discussion
2.1. Theoretical calculations on Si-bridged biphenyls and bithiophenes
The relative HOMO and LUMO energy levels for the model com-
pounds 1a–5a derived from DFT [9,10] calculations are depicted in
Fig. 1. As can be seen in Fig. 1, the LUMO energy level of disilane-
bridged biphenyl 2a is comparable to that of dibenzosilole 1a,
while HOMO of 2a lies at lower level than that of 1a by 0.20 eV.
On the other hand, the LUMO level of 3a is considerably affected
by introducing an oxygen atom, elevating the level by 0.32 eV from
* Corresponding author.
E-mail address: jo@hiroshima-u.ac.jp (J. Ohshita).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.08.018

H. Kai et al. / Journal of Organometallic Chemistry 693 (2008) 3490–3494
3491
HOMOs at much lower levels than 5a. Disilane-bridged bithioph-
ene 5 has proven to work as an excellent electron-transport in
OLEDs [7], and hence we expected compounds 2 and 6 as elec-
tron-transports with better electron-blocking than 5. We also
examined dibenzosilole (1) as the electron-transport, since nothing
has been reported for electron-transporting properties of mono-
meric dibenzosiloles, to our knowledge.
X
X
Si
Si = SiR2 SiR2SiR2 SiR2OSiR2
X
= CH=CH
S
1
4
2
5
3
6
2.2. Synthesis of disilane- and siloxane-bridged biphenyls 2b, 2c, and
3b
Chart 1. Si-bridged biphenyl and bithiophene derivatives.
Si-bridged biphenyls 2b, 2c, and 3b were synthesized as shown
in Scheme 1. Compound 2b was obtained by an in-situ Grignard
reaction of dibromobiphenyl and the dichlorodimethyldiphenyldi-
silane with magnesium in 51% yield as a colorless solid composed
of 1:4 cis and trans isomers. A similar reaction with dichlorotetra-
ethyldisilane afforded compound 2c in 32% yield as a colorless oil.
Treatment of disilane-bridged biphenyl (2d) with trimethylamine
oxide produced siloxane 3b.
that of 2a. For the LUMOs, two factors seem to be operative in these
compounds, one of which is the twisting of two benzene rings, and
*
*
the other is the degree of
sesses a twisted biphenyl unit with the dihedral angel of C@C–
C@C = 42.1 deg, as shown in Fig. 1, suppressing the -conjugation
between the benzene units. This may be roughly balanced with
r –p conjugation. Compound 2a pos-
p
*
*
the enhanced
r –p conjugation in disilane 2a, resulting in the
LUMO energy level, similar to that of monosilane 1a. For 3a, the
*
*
r
–p
conjugation is interrupted by the oxygen atom and the
2.3. Optical and electrochemical properties of Si-bridged biphenyls and
bithiophenes
twisting is much more enhanced.
For bithiophene derivatives, the HOMO energy levels were cal-
culated to be higher than those of biphenyl derivatives, probably
due to the electron-donating properties of thiophene rings. Intro-
duction of an oxygen atom to 5a giving rise to 6a exerts little influ-
ence on the LUMO energy level, in contrast to that the LUMO of 3a
is at higher energy than that of 2a. In 6a, the electronegative oxy-
gen appears to lower the LUMO by its inductive effect, which may
Table 1 summarizes the UV absorption maxima and oxidation
peak potentials in the cyclic voltammograms (CVs) of Si-bridged
biphenyls and bithiophenes (see Scheme 1 and Chart 2). The UV
absorption maxima moved to longer wave length in the order of
3b < 2b,c < 1b < 6b < 5b in accordance with the results of DFT
calculations.
The CVs of the present Si-bridged biphenyls and bithiophenes
were measured in acetonitrile. These compounds underwent irre-
versible oxidation in CVs without reductive counter waves, indicat-
ing some decompositions such as polymerization occurred during
the measurements. The oxidation peak potentials were noted as
listed in Table 1, which moved to lower potentials, in the order
of 3b > 1b > 2b,c, 6b > 5b. This is in accordance with the results
of DFT calculations, which indicate the HOMO energy levels are
elevated in the same order, except for 1b. The reason for higher
oxidation potential for 1b than that expected from the calculations
is not clear yet. However, the irreversible CV profiles for these com-
compensate the reduced r*
–
p* conjugation. Thiophene ring sys-
tem, which is essentially electron-donating, would be more af-
fected by the inductive effect of the oxygen inserted, as
compared with benzene system. Similarly, the HOMO of 6a is
much more influenced by the oxygen to lower the energy level sig-
nificantly, as compared with 3a. The smaller twisting of the bithi-
ophene unit in 6a (C@C–C@C = 38.7 deg) than that in 3a (C@C–
C@C = 54.1 deg) seems to be primarily responsible for smaller
HOMO–LUMO energy gap for 6a.
It is noteworthy that the LUMOs of 2a, and 6a lay at the ener-
gies, approximately as low as that of 5a, while they have the
eV
- 0.5
eV
- 4
LUMO
- 4.5
- 5
- 1
- 1.5
- 2
- 5.5
- 6
- 2.5
- 3
HOMO
- 6.5
S
S
S
S
Si
H
1a
H Si SiH
2
2
H Si
2
SiH
2
H Si SiH
2
H Si
2
SiH
6a
2
2
O
2
O
2a
3a
54.1
5a
dihedral angle
C=C-C=C/deg
42.1
38.7
17.6
Fig. 1. Relative HOMO and LUMO energy levels of Si-bridged biphenyl and bithiophene derivatives based on DFT calculations at the level of B3LYP/6-31G(d,p).

3492
H. Kai et al. / Journal of Organometallic Chemistry 693 (2008) 3490–3494
(R1R2SiCl)2
Mg
condensed films, stacking of the molecules would considerably af-
fect the electronic states. In fact, the UV spectrum of a vapor-
deposited film of compound 2b showed shoulders around 300
and 320 nm, together with the absorption maximum at almost
the same energy as that in a solution (280 nm). Moreover, emission
spectrum of 2b in THF showed a band at 355 nm, but the band dis-
appeared for its vapor-deposited film. These results are indicative
of intermolecular interaction between chromophores in the con-
densed phase. We could not estimate the LUMO level of 2b, due
to its broad oxidation peak.
Br
R2R1Si SiR1R2
2b R1 = Ph, R2 = Me
2c R1 = R2 = Et
Br
Me3NO
(Me2SiCl)2
Mg
Me2Si
SiMe2
Me2Si SiMe2
O
3b
2d
By applying the electric field, these devices emitted green light
arising from Alq3 emission. Fig. 2 depicts the current density–elec-
tric field (I–V) characteristics of the devices, indicating that device
A with 2b exhibited the best I–V characteristics among the devices
examined. Device A showed the maximum current density of
590 mA/cm2 at the electric field of 12 ꢀ 107 V/m (applied bias volt-
age = 13 V). That all three triple-layered devices A–C showed high-
er current density in whole region of the electric field than device
D indicates the electron-transporting properties of the vapor-
deposited films of 1b, 2b, and 6b. The luminance–electric field
(L–V) characteristics of the devices are presented in Fig. 3. The
maximum luminance from device A was as high as 22000 cd/m2
at 13 ꢀ 107 V/m. Despite our expectation based on the DFT calcula-
tions, luminance of device B with 6b was much lower than that of
devices C and D in the whole region of the applied electric field,
with the maximum luminance of 1530 cd/m2 at 11 ꢀ 107 V/m.
Since device B showed the I–V characteristics, a little inferior but
comparable to those of A, the low luminance seems to be due to
the insufficient hole-blocking properties of the layer of 6b.
In conclusion, we studied novel Si-bridged biphenyl and bithi-
ophene derivatives that were predicted to possess low-lying
HOMO and LUMO by DFT calculations. OLEDs having vapor-depos-
ited films of these compounds as the electron-transports, were fab-
ricated and their performance was examined. Excellent device
performance was demonstrated for the OLED with disilane-bridged
biphenyl 2b. It was also found that the siloxane-bridged bithioph-
ene was potentially useful as an electron-transporting material.
Scheme 1.
Table 1
Properties of Si-bridged biphenyls and bithiophenes
Compound
Absorptiona kmax/nm
CVb Epa/Vc
1b
2b
2c
3b
5bd
6be
320
290
281
243
350
324
1.4
1.1
1.0
1.9
0.8
1.0
a
In THF or CHCl3.
b
In acetonitrile containing 100 mM LiClO4 as the supporting electrolyte using Pt
plates as the working and counter electrodes.
Anodic peak potential versus Ag/Ag+.
c
d
Data taken from Ref. [7].
Data taken from Ref. [8].
e
Me3Si
SiMe3
Me3Si
S
S
SiMe3
S
S
Si
Si Si
Me
Me
Me2Si
SiMe2
O
Me Ph
Me
Me
5b
6b
1b
Chart 2.
3. Experimental
pounds would not allow us to discuss the electronic states of the
compounds in detail on the basis of the oxidation peak potentials.
3.1. General
All reactions were carried out under an inert atmosphere. NMR
spectra were recorded on JEOL Model JNM-EX 270 and JEOL Model
JNM-LA 400 spectrometers. Mass spectra were measured with a
Hitachi M80B spectrometer. UV spectra were measured on a Hit-
achi U-3210 spectrophotometer. The usual workup mentioned in
the following part involves separating the organic layer, extracting
the aqueous layer, drying the combined organic layer and the ex-
tracts, and evaporating the solvents, in that order.
2.4. OLED performance of the devices having 2b and 6b as the
electron-transport
To evaluate electron-transporting properties of the present Si-
bridged biphenyl and bithiophene, we fabricated triple-layered
OLEDs having a vapor-deposited layer of 2b or 6b as the elec-
tron-transport. The structure of the devices and the thickness of
the layers were ITO/TPD (40 nm)/Alq3 (50 nm)/2b or 6b (20 nm)/
Mg–Ag (device A for 2b and device B for 6b) where Alq3 and TPD
were tris(8-quinolinolato)aluminium (III) and N,N0-diphenyl-N,N0-
di-m-tolylbiphenyl-4,40-diamine, respectively. In these devices,
TPD, 2b or 6b, and Alq3 layers were the hole- and electron-trans-
ports, and the emitter, and ITO and Mg–Ag were the anode and
cathode, respectively. We also examined device C having a layer
of 1b as the electron-transport in place of 2b/6b (ITO/TPD
(40 nm)/Alq3 (50 nm)/1b (20 nm)/Mg–Ag), and a typical double-
layered device D without the electron-transport layer (ITO/TPD
(40 nm)/Alq3 (50 nm)/Mg–Ag). One might consider that the LUMOs
of 2b and 6b would be at higher level than Alq3 (ꢁ2.59 eV [11]). For
example, the LUMO energy level for 6b from the onset of CV oxida-
tion and UV absorption edge was estimated to be ꢁ2.2 eV [12].
However, it is not unusual to assume that the electronic states
may strongly depend on the conditions, in solutions or films. In
3.2. Materials
THF and toluene were dried over sodium and distilled just be-
fore use. Acetonitrile was distilled from P2O5 and stored in dark un-
der an argon atmosphere at 4 °C before use.
3.3. Preparation of 2b,c
In a 30 mL three-necked flask fitted with a dropping funnel
were placed 0.17 g (6.9 mol) of magnesium, 1,2-dichloro-1,2-di-
methyl-1,2-diphenyldisilane (1.0 g 3.2 mol), and 5 mL of THF. To
this was added a solution of 2,20-dibromobiphenyl (1.0 g 3.2 mol)
in 20 mL of THF at room temperature. The resulting mixture was
refluxed for 30 min. The mixture was then cooled to room temper-

H. Kai et al. / Journal of Organometallic Chemistry 693 (2008) 3490–3494
3493
700
600
500
400
300
200
100
0
6.0
8.0
10.0
12.0
14.0
16.0
Electric Field (107V/m)
Fig. 2. Plots of operating electric field vs current density of (d) device A, (N) device B, (h) device C (ꢀ) device D.
25000
20000
15000
10000
5000
0
6
8
10
12
14
16
Electric Field (107V/m)
Fig. 3. Plots of operating electric field vs luminance of (d) device A, (N) device B, (h) device C (ꢀ) device D.
ature and was hydrolyzed with water. After the usual workup, the
residue was chromatographed on a silica gel column with hexane/
ethyl acetate (10/1 vol%) as eluent to give 2b as a 1:4 cis/trans iso-
meric mixture (yield 51%), which was recrystallized from ethanol
to give 2b as a colorless solid (yield 9%): m.p. 84.5–89.0 °C; MS
m/z 392 (M+); 1H NMR (d in CDCl3) 0.45 (s, 6H, Me), 7.07–7.28
(m, 14H, ring protons), 7.37 (t, 2H, J = 7.5 Hz, ring protons), 7.45
(d, 2H, J = 7.5 Hz, ring protons). Anal. Calc. for C26H24Si32: C,
79.53; H, 6.16. Found: C, 79.33; H, 6.19%.
Compound 2c was prepared in a fashion similar to 2b, using 1,2-
dichloro-1,1,2,2-tetraethyldisilane instead of 1,2-dichloro-1,2-di-
methyl-1,2-diphenyldisilane. Data for 2c: 32% yield; colorless oil;
MS m/z 324 (M+); 1H NMR (d in CDCl3) 0.81 (q, 8H, J = 7.0 Hz,
CH2CH3), 0.92 (t, 12H, J = 7.0 Hz, CH2CH3), 7.23–7.28 (m, 2H, phe-
nyl ring protons), 7.40–7.42 (m, 4H, ring protons), 7.45 (dd, 2H,
J = 7.01 Hz, J = 1.09 Hz, ring protons); 13C NMR (d in CDCl3) 3.18,
8.73 (Et), 126.33, 129.76, 129.93, 133.68, 134.05, 148.10 (ring car-
bons). Anal. Calc. for C20H28Si2: C, 74.00, H, 8.96. Found: C, 74.02;
H, 6.76%.
lane instead of 1,2-dichloro-1,2-dimethyl-1,2-diphenyldisilane,
and 0.17 g (2.24 mol) of trimethylamine oxide in 30 mL of dry tol-
uene. After being stirred at room temperature for 2 day, the mix-
ture was hydrolyzed with water. After the usual workup, the
residue was chromatographed on a GPC column with toluene as
eluent to give 3b (yield 42%) as a colorless solid: mp 74.9 °C
(DSC); MS m/z 284 (M+); 1H NMR (d in CDCl3) ꢁ0.28 (s, 6H, MeSi),
0.55 (s, 6H, MeSi), 7.38–7.56 (m, 6H, ring protons), 7.63 (d, 2H,
J = 7.6 Hz, ring protons); 13C NMR (d in CDCl3) ꢁ0.64, 0.13 (Me),
126.47, 130.06, 133.03, 137.99, 149.44 (ring carbons). Anal. Calc.
for C16H20Si2O: C, 67.55; H, 7.09. Found: C, 67.09; H, 7.29%.
3.5. DFT calculations
The density functional theory (DFT) calculations were carried
out using the Gaussian03 program package [10]. The Becke-
three-parameter-Lee–Yang–Parr hybrid functional [14–17] was
employed with the 6-31G(d,p) basis set.
3.6. CV measurements
3.4. Preparation of 3b [13]
CV measurements were carried out using a three-electrode sys-
tem in acetonitrile solutions containing 100 mM of lithium per-
chlorate as the supporting electrolyte and 2 mM of the substrate.
Pt wires were used as the working and counter electrodes, and
In a 50 mL three-necked flask fitted with a reflux condenser was
placed a mixture of 0.20 g (0.75 mmol) of 2d that was prepared in a
fashion similar to 2b, using 1,2-dichloro-1,1,2,2-tetramethyldisi-

3494
H. Kai et al. / Journal of Organometallic Chemistry 693 (2008) 3490–3494
Ag/Ag+ was used as the reference electrode. The current-electric
field curves were recorded at room temperature on a Hokuto Den-
ko HAB-151 potentiostat/galvanostat.
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3.7. Preparation of OLEDs
[6] (a) J. Ohshita, H. Kai, A. Takata, T. Iida, A. Kunai, N. Ohta, K. Komaguchi, M.
Shiotani, A. Adachi, K. Sakamaki, K. Okita, Organometallics 20 (2001) 4800;
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[9] Results of DFT-calculations, and electrochemical and optical properties of
disilane- and siloxane-bridged bithiophene at a different level of the theory
have been already reported, see Ref. [8].
[10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
V.G. Zaikrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich,
J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V.
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Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox,
T. Keith, M.A. Al-Lalam, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M.
Head-Gordon, E.S. Replogle, J.A. Pople, Program Packages from GAUSSIAN, Inc.,
Pittsburgh, PA.
Each layer of the OLEDs was prepared by vacuum deposition at
1 ꢀ 10ꢁ5 torr in the order of TPD, Alq3, and the present compound,
on ITO coated on a glass substrate with a sheet resistance of 15
X/
cm (Nippon Sheet Glass Company). Finally a layer of magnesium–
silver alloy with an atomic ratio of 10:1 was vacuum deposited as
the top electrode. The emitting area was 5 ꢀ 5 mm2.
Acknowledgments
This work was supported by a Grant-in-Aid for Science Research
(No. 20350092) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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