Branched diphenylsilane derivatives containing electronically isolated indolyl moieties as host materials for blue organic light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2011.03.020

Branched diphenylsilane derivatives with pendent indolyl fragments were synthesized and characterized. The compounds show high thermal stability with thermal decomposition starting at temperatures above 367 °C and ability to form glasses with glass-transi

Full text of DOI:10.1016/j.dyepig.2011.03.020

Dyes and Pigments 91 (2011) 177e181
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Branched diphenylsilane derivatives containing electronically isolated indolyl
moieties as host materials for blue organic light emitting diodes
R. Zostautiene a, G. Krucaite a, J. Simokaitiene a, J.V. Grazulevicius a, Y.M. Lai b,
,
W.B. Wang b, J.H. Jou b, S. Grigalevicius a
*
a Department of Organic Technology, Kaunas University of Technology, Radvilenu Plentas 19, LT50254 Kaunas, Lithuania
b Department of Materials Science and Engineering, National Tsing-Hua University, No. 101, Kaungfu Rd., Hsin-Chu 30013, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Branched diphenylsilane derivatives with pendent indolyl fragments were synthesized and character-
ized. The compounds show high thermal stability with thermal decomposition starting at temperatures
above 367 ꢀC and ability to form glasses with glass-transition temperatures of 53e58 ꢀC. The electron
photoemission spectra of the layers of the synthesized compounds showed ionization potentials of ca.
5.85 eV. The derivatives were tested as host materials in phosphorescent OLEDs with iridium(III)[bis(4,6-
difluorophenyl)-pyridinato-N,C20]picolinate as the guest. The device with the host derivative containing
four electronically isolated indolyl fragments exhibited the best overall performance with maximum
current efficiency of about 12 cd/A.
Received 22 January 2011
Received in revised form
9 March 2011
Accepted 16 March 2011
Available online 23 March 2011
Keywords:
Indole
Oxetane
Ó 2011 Elsevier Ltd. All rights reserved.
Amorphous material
Ionization potential
Host
Light emitting diode
1. Introduction
2. Experimental
In phosphor-doped organic light emitting diodes (OLEDs), to
reduce quenching associated with relatively long excited-state
lifetimes of triplet emitters and tripletetriplet annihilation, triplet
emitters of heavy-metal complexes are normally used as emitting
guests in a host material, and thus suitable host materials are of
equal importance for the phosphorescent devices [1e3]. For elec-
trophosphorescence from triplet guests, it is essential that the
triplet level of the host be larger than that of the triplet emitter to
prevent reverse energy transfer from the guest back to the host and
to effectively confine triplet excitons on guest molecules [4e6]. It
was reported earlier that carbazole-, indole- and arylsilane-based
derivatives demonstrate rather large triplet energies and are
potential host materials for blue electrophosphorescent devices
[7e11]. Here, we report on new diphenylsilane-based host mate-
rials containing isolated indolyl fragments. These branched and
highly soluble compounds could be suitable for the preparation of
large area devices by spin coating.
2.1. Instrumentation
1H NMR spectra were recorded using a Varian Unity Inova
(300 MHz) instrument. Mass spectra were obtained on a Waters ZQ
2000 spectrometer. FT-IR spectra were recorded using a Perkin
Elmer FT-IR System. UV spectra were measured with a Spectronic
GenesysÔ 8 spectrometer. Fluorescence (FL) spectra were recorded
with a MPF-4 spectrometer.
Differential scanning calorimetry (DSC) measurements were
carried out using a Bruker Reflex II thermosystem. Thermogravi-
metric analysis (TGA) was performed on a Netzsch STA 409. The
TGA and DSC curves were recorded in a nitrogen atmosphere at
a heating rate of 10 ꢀC/min.
The ionization potentials of the layers of the compounds
synthesized were measured by the electron photoemission method
in air, which was described earlier [12,13]. The measurement
method was, in principle, similar to that described by Miyamoto
et al. [14].The standard error in the mean to 95% confidence for the
values of ionization potential was 0.04 eV [15].
The devices were fabricated on glass substrates and consisted of
multiple organic layers sandwiched between the bottom indium tin
* Corresponding author.
E-mail address: saulius.grigalevicius@ktu.lt (S. Grigalevicius).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.020

178
R. Zostautiene et al. / Dyes and Pigments 91 (2011) 177e181
oxide (ITO) anode and the top metal cathode (Al). The device
structure was ITO/poly(3,4-ethylenedioxythiophene): poly(-
a catalytic amount of TBAS were added to the mixture, and it was
heated under reflux for 24 h. Then the cooled mixture was dec-
anted and the new portion of inorganic materials was added to the
solution, and the mixture was again heated under reflux for 24 h.
After TLC control the mixture was filtered, the solvent was evapo-
rated and the product was purified by column chromatography
with silica gel using hexane/ethyl acetate (vol. ratio 5:1) as an
eluent. After crystallization from methanol yield of the product was
53% (4.8 g). Mp: 120e122 ꢀC.
ꢀ
styrenesulfonate) {PEDOT:PSS} (ca. 300 A)/DInSi or TInSi doped
ꢀ
ꢀ
ꢀ
with FIrpic (250 A)/TPBi/LiF (5 A)/Al (1500 A), where the con-
ducting polymer (PEDOT:PSS) was used as the hole-injection layer
[16], DInSi or TInSi doped with the blue phosphorescent iridiu-
m(III)[bis(4,6-difluorophenyl)-pyridinato-N,C20]picolinate (FIrpic)
was used as the emitting layer and LiF was used as the electron-
injection layer [17]. 1,3,5-Tris(2-N-phenylbenzimidazolyl)benzene
(TPBi) was used as an electron-transporting layer for the devices.
The luminance and CIE chromatic coordinates of the resulting
OLEDs were measured using a Minolta CS-100 luminance meter.
A Keithley 2400 electrometer was used to measure the cur-
rentevoltage (IeV) characteristics. All the devices were character-
ized without encapsulation and all the measurements were carried
out under ambient conditions [18]. The emission area of all the
devices was 25 mm2 and only the luminance in the forward
direction was measured.
IR (KBr, cmꢁ1): 3119, 3101, 3058 (CeH, Ar); 2957, 2919, 2875
(CeH); 1609, 1574 (C]C, Ar); 1308 (CeN); 1270, 1242, 1090
(CeOeC); 766, 739, 724 (CH]CH).
1H NMR (300 MHz, CDCl3,
d, ppm): 7.66e7.61 (m, 2H, Ar),
7.19e7.07 (m, 6H, Ar), 6.95 (d, 2H, J ¼ 3.3 Hz, Ar), 6.57 (d, 2H,
J ¼ 3.3 Hz Ar), 4.59 (s, 4H, eNeCH2e), 4.33 (s, 4H, eCH2eOe).
MS (APCIþ, 20 V), m/z: 317.4 ([M þ H]þ, 100%), 132.1 (10%).
2.2.1. Di[(1-(2-phenylindol-1-yl)-2-chloromethyl-2-methyl)
propoxy]diphenylsilane (DInSi)
3-(2-Phenylindol-1-ylmethyl)-3-methyloxetane (1,1 g, 3.6 mmol)
and TBAB (0.06 g, 0.18 mmol) of were dissolved in THF (3 mL), and
dichlorodiphenylsilane (5, 0.42 g,1.63 mmol) was added. The reaction
mixture was stirred at room temperature for 30 min. Then the
temperature was raised to 50 ꢀC and the mixture was stirred for 48 h.
The reaction was stopped after TLC control. The product was sepa-
rated by column chromatography with silica gel using hexane/ethyl
acetate (vol. ratio 2:1) as an eluent. Yield of the amorphous product
was 0.8 g (60%).
2.2. Materials
3-(2-Phenylindol-1-ylmethyl)-3-methyloxetane (1) was prepared
by the reaction of 2-pheny-1H-indole with an excess of 3-bromo-
methyl-3-methyloxetane under basic conditions in the presence of
a phase transfer catalyst. The procedure was described earlier [19].
3,3-Bis(chloromethyl)oxetane (2), 1H-indole (3), potassium
carbonate, tetrabutylammonium hydrogen sulphate (TBAS), tetra-
butylammonium bromide (TBAB) and dichlorodiphenylsilane (5)
were purchased from Aldrich and used as received.
3,3-Bis(indol-1-ylmethyl)oxetane (4) was prepared by the
reaction of 3,3-bis(chloromethyl)oxetane (2) with an excess of 1H-
indole (3) under basic conditions in the presence of a phase transfer
catalyst e TBAS. 1H-indole (3, 10 g, 85 mmol) and 3,3-bis(chlor-
omethyl)oxetane (4.41 g, 28 mmol) were heated to reflux in ethyl
methyl ketone (100 mL). Then powdered potassium carbonate
(11.8 g, 85 mmol), potassium hydroxide (14.3 g, 256 mmol) and
IR (KBr, cmꢁ1): 3419 (OeH); 3052, 3028 (CeH, Ar); 2951, 2926,
2872 (CeH); 1605, 1592 (C]C, Ar); 1347, 1316 (CeN); 1127, 1117,
1075 (CeO); 767, 750, 741, 719, 670 (CH]CH).
1H NMR (300 MHz, CDCl3,
d, ppm): 7.62e7.26 (m, 24H, Ar),
7.16e7.06 (m, 4H, Ar), 6.5 (s, 2H, Ar), 4.54e4.32 (m, 4H, CleCH2e), 3.36
(s, 4H, eNeCH2e), 3.16e3.02 (m, 4H, CH2eOe), 0.51 (s, 6H, eCH3).
MS (APCIþ, 20 V), m/z: 807,5 ([M]þ, 100%), 810,5 ([M þ 3]þ, 50%),
811,5 ([M þ 4]þ, 30%).
O
5
Cl
Si
Cl
Cl
O
Si
O
Cl
N
N
N
TBAB
1
THF
50 o
C
DInSi
O
2
Cl
Cl
TBAS
H
N
ethyl methyl ketone
reflux
3
5
Cl
Si
Cl
N
N
O
Si O
Cl
Cl
O
TBAB
THF
N
N
50 oC
N
N
4
TInSi
Scheme 1. Synthesis of DInSi and TInSi.

R. Zostautiene et al. / Dyes and Pigments 91 (2011) 177e181
179
Elemental analysis for C50H48Cl2N2O2Si % Calc.: C 74.33, H 5.99,
N 3.47; % Found: C 74.25, H 5.84, N 3.39.
2.2.2. Di[(1-chloro-2,2-bis(indol-1-ylmethyl))propoxy]
diphenylsilane (TInSi)
DInSi
TInSi
3,3-bis(indol-1-ylmethyl)oxetane (4, 1 g, 3.2 mmol) and TBAB
(0.05 g, 0.16 mmol) were dissolved in THF (5 mL) and dichlor-
odiphenylsilane (5, 0.36 g, 1.4 mmol) was added. The reaction
mixture was stirred at room temperature for 30 min. Then the
temperature was raised to 50 ꢀC and the mixture was stirred for
24 h. The reaction was stopped after TLC control. The product was
separated by column chromatography with silica gel using hexane/
ethyl acetate (vol. ratio 5:1) as an eluent and crystallized from the
eluent mixture of solvents. Yield of the product was 0.4 g (32%).
IR (KBr, cmꢁ1): 3102, 3072, 3051 (CeH, Ar); 2946, 2929, 2887
(CeH); 1611, 1592 (C]C, Ar); 1337, 1311 (CeN); 1131, 1118, 1090,
1052 (CeO); 788, 767, 740, 699 (CH]CH).
225
250
275
300
325
350
375
Wavelength, nm
1H NMR (300 MHz, CDCl3,
d, ppm): 7.58e7.48 (m, 10H, Ar), 7.43
Fig. 2. UV absorption spectra of dilute THF solutions of DInSi and TInSi.
(t, 6H, J ¼ 7,5 Hz, Ar), 7.3 (t, 2H, J ¼ 7,5 Hz, Ar), 7.04e6.9 (m, 10H, Ar),
6.44e6.35 (m, 6H, Ar), 4.18 (s, 8H, eNeCH2e), 3.6 (s, 4H, CleCH2e),
3.35 (s, 4H, eCH2eOe).
were soluble in common organic solvents, such as acetone, chlo-
roform or THF at room temperature. Transparent thin films of these
materials were prepared by spin coating from solutions.
MS (APCIþ, 20 V), m/z: 887,4 ([M þ H]þ, 90%), 888,3 ([M þ 2]þ,
50%).
Elemental analysis for C54H50Cl2N4O2Si % Calc.: C 73.20, H 5.69,
N 6.32; % Found: C 73.31, H 5.62, N 6.43.
The behavior under heating of DInSi and TInSi was studied by
DSC and TGA. Both these derivatives demonstrate high thermal
stability. The temperatures at which initial loss of mass was
observed (T5%) are 367 ꢀC for DInSi and 371 ꢀC for TInSi. DInSi was
amorphous in nature. When the sample of DInSi was heated, Tg
was observed at 53 ꢀC and no peaks due to crystallization and
melting appeared. Cooling down and repeated heating revealed
only the glass transition again. TInSi was obtained as crystalline
material by re-crystallization from solution, however it readily
formed glass when the molten sample was cooled on standing in air
or with liquid nitrogen. The DSC thermograms of TInSi are shown
in Fig. 1. When the crystalline sample was heated, the endothermic
peak due to melting was observed at 189 ꢀC. When the molten
sample of TInSi was cooled down and heated again, the glass-
transition phenomenon was observed at 58 ꢀC and on further
heating no peaks due to crystallization and melting appeared.
UV absorption spectra of dilute THF solutions of DInSi and TInSi
are presented in Fig. 2. The comparison of UV spectra of these
derivatives with that of other compounds containing electronically
isolated indolyl or 2-phenylindolyl moieties [19] shows that the
spectra are very similar. This observation confirms that an inter-
action between chromophores is not observed in dilute solutions of
3. Results and discussion
The synthesis of derivatives containing isolated indolyl and
diphenylsilane fragments was carried out by the synthetic route
shown in Scheme 1. Oxetane 1 was prepared by the reaction of 2-
pheny-1H-indole with an excess of 3-bromomethyl-3-methyl-oxe-
tane under basic conditions as described earlier [19]. The derivative 1
was treated with dichlorodiphenylsilane (5) to yield the diphenylsi-
lane containing two electronically isolated 2-phenylindolyl moieties
(DInSi). 3,3-Bis(indol-1-ylmethyl)oxetane (4) was prepared by the
reaction of 3,3-bis(chloromethyl)oxetane (2) with an excess of 1H-
indole (3) under basic conditions in the presence of a phase transfer
catalyst TBAS. Oxetane 4 was treated with dichlorodiphenylsilane (5)
in the presence of TBAB to yield the branched derivative TInSi con-
taining four electronically isolated indolyl fragments.
The newly synthesized derivatives were identified by IR and 1H
NMR spectroscopy and mass spectrometry. The data were found to
be in good agreement with the proposed structures. The derivatives
2nd heating
TInSi
DInSi
Tg = 58 oC
Cooling
1st heating
Tm = 189 oC
20
40
60
80 100 120 140 160 180
Temperature, oC
5,6
5,8
6,0
hν (eV)
6,2
6,4
Fig. 1. DSC curves of TInSi.
Fig. 3. Electron photoemission spectra of layers of DInSi and TInSi.

180
R. Zostautiene et al. / Dyes and Pigments 91 (2011) 177e181
Fig. 4. OLED characteristics of devices containing DInSi as the host: luminanceevoltage characteristics and current efficiencies versus current density.
12
2
10
Firpic (wt%)
10
14
18
22
1
10
8
6
4
2
0
0
10
Firpic (wt%)
14
18
22
-1
10
-2
-1
0
1
5
6
7
8
9
10
10
10 10
Current Density (mA/cm2)
10
Voltage (V)
Fig. 5. OLED characteristics of devices containing TInSi as the host: luminanceevoltage characteristics and current efficiencies versus current density.
DInSi and TInSi. The lmax of DInSi is red shifted in comparison with
that of TInSi due to the phenyl ring attached to the 2nd position of
indole core.
Ionization potentials (Ip) of thin amorphous layers of the
derivatives were determined from electron photoemission spectra
of the layers (Fig. 3). The Ip values of the layers of DInSi and TInSi
are close to 5.85 eV. As it could be expected from the UV absorption
spectroscopy data, the Ip values for films of the synthesized
derivatives are similar to that of other derivatives containing elec-
tronically isolated indole rings [19].
To evaluate the performance of derivatives DInSi and TInSi as
hosts, phosphorescent OLEDs were fabricated by using the blue
emitter FIrpic as the guest. The structure of the multilayer devices
is described in the experimental section. In both devices of DInSi
and TInSi, the electroluminescence was found to originate only
from FIrpic at different bias voltages. No host and doped transport
molecular emission was visible from the devices, indicating an
energy transfer or charge transfer from the hosts to the guest as
well as the sufficient injection of both holes and electrons into the
emitting layer.
The host DInSi was used in concentration-dependent experi-
ments with the dopant amount ranging from 14 to 26 wt%. Fig. 4
shows the luminanceevoltage characteristics and curves of
current efficiency versus current density for the devices. These
OLEDs exhibit turn-on voltages of 5.5e7.2 V and current efficiencies
of 1.9e4.2 lm/W. The device containing 22 wt% of FIrpic as the
guest exhibits the highest current efficiency of 4.2 lm/W.
The host TInSi was also used in concentration-dependent
experiments with the dopant amount ranging from 14 to 22 wt%.
Fig. 5 shows the luminanceevoltage characteristics and current
efficiencies versus voltage for the devices. Up to 18 wt%, as the
FIrpic guest concentration increases, the efficiency of the device is
gradually increased. Device with an 18 wt% doping ratio exhibits
a maximum current efficiency of w12 cd/A. The maximum lumi-
nance of the device exceeds 90 cd/m2. When the doping ratio
reaches 22 wt%, the maximum current efficiency is decreased due
to concentration quenching and tripletetriplet annihilation. The
characteristics of the devices containing TInSi host are promising
also in comparison to the well known host: 4,40-bis(9-carbazolyl)-
2,20-biphenyl (CBP)-based devices [4]. It should be pointed out that
these characteristics were obtained in non-optimized test devices
under ordinary laboratory conditions. The device performance may
be further improved by an optimization of the layer thicknesses and
processing conditions.
In conclusion, branched derivatives containing a diphenylsilane
core and pendent indolyl moieties were synthesized from indole-
based oxetanes and dichlorodiphenylsilane. The derivatives show
sufficient thermal stability and form homogeneous amorphous
films with glass-transition temperatures of 53e58 ꢀC. The
compounds were tested as host materials in phosphorescent OLEDs
with iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C20]picoli-
nate as the guest. The device with the host derivative containing
four electronically isolated indolyl fragments exhibited the best
overall performance with maximum current efficiency of about
12 cd/A.
Acknowledgements
This research was funded by a grant No. TAP-02/2010 from the
Research Council of Lithuania and by National Science Council of
Taiwan, ROC. GK acknowledges Student Research Fellowship Award

R. Zostautiene et al. / Dyes and Pigments 91 (2011) 177e181
181
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Diphenylsilanes containing electronically isolated carbazolyl fragments as
host materials for light emitting diodes. Synth Met 2011;161:92.
from the Lithuanian Science Council. Habil. dr. V. Gaidelis is
thanked for the help in ionization potential measurements.
[10] Lengvinaite S, Grazulevicius JV, Grigalevicius S, Lai YM, Wang WB, Jou JH.
Polyethers containing 2-phenylindol-1-yl moieties as host materials for light
emitting diodes. Synth Met 2010;160:1793.
[11] Chou HH, Cheng CH. A highly efficient universal bipolar host for blue, green,
and red phosphorescent OLEDs. Adv Mater 2010;22:2468.
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