Synthesis of star-shaped molecules with pyrene-containing π-conjugated units linked by an organosilicon core

Article abstract of DOI:10.1002/aoc.1551

Star-shaped molecules with pyrene-containing π-conjugated units linked by an organosilicon core (Py₃Si and Py₃C) were prepared and their applications to thin-film transistors (TFTs) and photovoltaic cells were studied. Bottom-contact type TFTs with spin-coated films of the star-shaped compounds as the active layerswere prepared and the field-effect mobility (μ_FET) and I_on/I_off ratios were determined to be approximately 10-5 cm² V^-1 s^-1 and 104, respectively. Photovoltaic properties of Py₃Si and Py₃C were studied in the cells, ITO-PEDOT-PSS-Py₃Si or Py ₃C-PCBM-LiF-Al. Although the power conversion efficiency (PCE) of the cells was only about 0.04%, they showed high open circuit voltages (V _oc) of 0.8-0.9 V, indicating the high potential of this type of compound as a hostmaterial. Copyright

Full text of DOI:10.1002/aoc.1551

Full Paper
Received: 15 June 2009
Revised: 25 August 2009
Accepted: 26 August 2009
Published online in Wiley Interscience: 14 October 2009
(www.interscience.com) DOI 10.1002/aoc.1551
Synthesis of star-shaped molecules
with pyrene-containing π-conjugated units
linked by an organosilicon core
Joji Ohshitaa∗, Yosuke Hatanakaa, Shigenori Matsuia, Yousuke Ooyamaa,
Yutaka Harimaa and Yoshihito Kunugib
Star-shaped molecules with pyrene-containing π-conjugated units linked by an organosilicon core (Py3Si and Py3C) were
prepared and their applications to thin-film transistors (TFTs) and photovoltaic cells were studied. Bottom-contact type TFTs
with spin-coated films of the star-shaped compounds as the active layers were prepared and the field-effect mobility (µFET) and
I
on/Ioff ratios were determined to be approximately 10−5 cm2 V−1 s−1 and 104, respectively. Photovoltaic properties of Py3Si
and Py3C were studied in the cells, ITO–PEDOT-PSS–Py3Si or Py3C-PCBM–LiF–Al. Although the power conversion efficiency
(PCE) of the cells was only about 0.04%, they showed high open circuit voltages (Voc) of 0.8–0.9 V, indicating the high potential
c
of this type of compound as a host material. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Keywords: π-conjugated compound; organosilicon compound; star-shaped molecule; organic transistor; photovoltaic cell
Introduction
Organic thin film transistors (TFTs) are of current interest because
of their utility as, for example, flexible flat-panel displays,
electronic paper and chemical sensors.[1] Organic TFTs of high
performance are generally achieved by using expanded π-
conjugated compounds as the semiconducting materials. On
the other hand, σ –π conjugated systems have been studied
as functional materials, such as hole-transports[2] and emitters
in organic light emitting diodes (OLEDs),[3] and light-harvesting
materials.[4] However, only a little is known about TFT activity
Scheme 1. Star-shaped oligothiophene derivatives.
also studied, and the pyrene units were anticipated to interact
with acceptor molecules (PCBM: phenyl C61 butyric acid methyl
ester) through π-stacking to facilitate the charge transfer.
of σ –π conjugated systems. Recently, Facchetti, Ratner, Marks
and coworkers demonstrated that silole-containing π-conjugated
polymers showed good p-type TFT activity in spin-coated films.[5]
We studied silicon-bridged oligothiophenes as TFT materials, in
the hope that the σ –π conjugation and/or strong donation
from the silicon unit will elevate the HOMO energy level
to enhance the hole affinity,[6,7] and we found that vapor-
deposited films of star-shaped compounds with oligothiophene
units linked by an organosilicon core exhibited good p-type
TFT activity (Scheme 1).[7] Preparation and functionalities of
some other conjugated star-shaped compounds with a silicon
core unit have been reported,[8,9] but no papers have been
published on their FET activity, except one concerning a
compound bearing oligothiophenes linked by a long siloxane
chain.[9] Interestingly, the TFT activity of nTSi was enhanced as
elongating the oligothiophene chain length from n = 3 to 5 and
tris[(quinquethiophenyl)dimethylsilyl]methylsilane(5TSi)showed
the best properties among them. The p-type field-effect mobility
and Ion/Ioff ratio of 5TSi were determined to be µ = 6.4 × 10−2
cm2 V−1 s−1 and 104, respectively, in the top-contact type device.
To explore further the scope of the Si-linked star-shaped system,
we prepared pyrene-containing star-shaped compounds with
an organosilicon core and investigated their TFT activity. Their
applications as the donor materials for photovoltaic cells were
Results and Discussion
Previously, we demonstrated that there are two possible stable
geometries for star-shaped oligothiophenes nTSi, and the fork-
shaped one is responsible for the high field effect mobility of their
vapor-deposited films (Scheme 2).[7,10] Thus, for the fork-shaped
structure, hole-transport by a hopping process is conducted not
only by intermolecular but also by intramolecular π-stacking.
Prior to the synthetic studies of pyrene-containing star-
shaped compounds, we carried out DFT calculations on a model
∗
Correspondence to: Joji Ohshita, Department of Applied Chemistry, Graduate
School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527,
Japan. E-mail: jo@hiroshima-u.ac.jp
a
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, Higashi-Hiroshima 739-8527, Japan
b
Department of Applied Chemistry, Faculty of Engineering, Tokai University,
1117 Kitakaname, Hiratsuka 259-1292, Japan
c
Appl. Organometal. Chem. 2010, 24, 540–544
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

Star-shaped organosilicon compounds
Bottom-contact type TFTs were prepared using spin-coated
films of PySi and PyC. Attempted preparation of their films by
vapor-deposition was unsuccessful, because these compounds
decomposed to give black solids on heating under vacuum, before
reaching the sublimation temperature. This is probably ascribed
to their decomposition concerning acetylene units. In fact, heating
these compounds to 230 ◦C led to the formation of dark yellow
solids whose IR spectra showed decreased intensities of the C
C
Scheme 2. Two possible stable geometries of nTSi.
stretching bands. The performance of the TFTs with PySi and
PyC is summarized in Table 1 and the transistor characteristics
are presented in Fig. 3 for the TFT with PyC as an example. They
showed clear TFT response with Ion/Ioff ratios of 104. However, the
field effect mobility (µ) was determined to be 1.1 × 10−5 cm2 V−1
s−1 for both of the devices, which is much smaller than that of
the vapor-deposited film of 5TSi. The rather low mobility of PySi
and PyC can probably be ascribed to low crystallinity of the films
of PySi and PyC. In fact, XRD analysis of the films revealed no
signals, indicating that these films were amorphous, in contrast to
the vapor-deposited film of 5TSi whose XRD analysis suggested
that high crystallinity of the film with the molecules standing in
a perpendicular fashion on the substrate to form a conduction
path horizontal to the substrate surface through π-stacking.[7]
Annealing the devices with PySi and PyC at 60 ◦C resulted in a
decrease in the TFT mobility to µ = 8.8 × 10−6 and 9.7 × 10−6
cm2 V−1 s−1 for the devices with PySi and PyC, respectively.
Next, we examined photovoltaic properties of PySi and PyC.
As shown in Fig. 4, when C60 was added into the solution
of PySi, the emission intensity markedly decreased, indicating
that energy or electron transfer occurred from photoexcited
PySi to C60, efficiently. Photovoltaic cells with the structure
of ITO–PEDOT-PSS–PySi or PyC:PCBM (1 : 4)–LiF–Al [ITO =
indiumtinoxide,PEDOT-PSS=poly(3,4-ethylenedioxythiophene)-
poly(styrenesulfonate), PCBM = phenyl C61 butyric acid methyl
ester] were fabricated. As shown in Fig. 5, the cells showed
incident photon-to-current efficiency (IPCE) maxima around
410 nm, corresponding to the absorption maxima of PySi and
PyC. The shoulders at about 580 nm may be ascribed to the
interaction of PySi and PyC with PCBM. Although we do not have
any direct evidence for this, the UV spectra of spin-coated films
of PySi and PyC with PCBM (1 : 4) show weak shoulders in the
same region (Fig. 5a). Open circuit voltages (Voc) were adequately
high in the range of 0.8–0.9 V, whereas short circuit photocurrents
(Isc) were rather low (∼0.1 mA cm−2) and consequently power
conversion efficiencies (PCEs) of the cells were only about 0.04%.
In conclusion, we prepared star-shaped compounds with
pyrene-containing π-conjugated units linked by an organosilicon
core. Their spin-coated films showed TFT activity, in spite of which
they are amorphous without highly ordered molecular packing.
Although the PCEs in the photovoltaic cells are not as high as
we expected, Vocs were adequate, indicating high potential of the
star-shaped compounds as the photovoltaic materials. Tuning of
the molecular structure, such as by introducing these units into
polymeric systems and expansion of the π-conjugated units, is in
progress.
compound, shown in Scheme 3, and found that the fork- and
fan-shaped molecules showed almost the same thermal stability
and the difference of their heats of formation was within 1 kcal
mol−1. As the more condensed fork-shaped molecule is likely to
be preferred in the solid state than in the gas phase, these pyrene-
containing star-shaped compounds would possess highly stacked
fork-shaped geometry in the films to facilitate the carrier transport
by intra- and intermolecular hopping, as in the case of nTSi.
Pyrene-containing star-shaped compounds (PySi and PyC)
were prepared by Sonogashira coupling of ethynylpyrene and
tris[(bromobithiophenyl)dimethylsilyl]methylsilaneand-methane
in 41 and 39% yields, respectively, as shown in equation (1).
They were yellow solids and could be readily isolated by column
chromatography. They are soluble in common organic solvents,
such as toluene, chloroform, and THF. The flexible organosilicon
core,aswellasthestar-shapedstructuresthatseemtosuppressthe
intermolecular π-stacking, presumably provide good solubility.
The UV spectra of PySi and PyC are quite similar, indicating
that the center element little affects the electronic states of the
compounds (Fig. 1). The absorption maxima in THF were at 407
and404 nmforPySiandPyC, respectively, withshouldersatabout
440 nm. They were slightly red-shifted when measured in the spin-
coated films, presumably arising from the π-stacking in the solid
state. We also prepared (hexylbithiophenyl)pyrenylacetylene (Py)
to determine the bridging effects in PySi and PyC, as shown in
equation (2). Compound Py showed its absorption maximum at
405 nm, indicatingthenegligiblebridgingeffectsontheelectronic
states of the compounds with respect to the absorption.
Experimental
Emission spectra of compounds PySi, PyC and Py are shown in
Fig. 2. These compounds showed emission maxima at almost the
same energy. Star-shaped compounds PySi and PyC, however,
showed broad shoulders at 500–650 nm, presumably due to the
formation of intramolecular π-stack in solution phase, agreeing
with the results of the theoretical calculations.
All reactions were carried out in dry nitrogen. Ether and THF
weredriedoversodium–benzophenoneanddistilledimmediately
before use. Triethylamine was distilled from KOH and stored
over activated molecular sieves before use. Tris[(5ꢁ-bromo-2,2ꢁ-
bithiophenyl)dimethylsilyl]methylsilane was prepared as reported
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Appl. Organometal. Chem. 2010, 24, 540–544
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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J. Oshita et al.
Scheme 3. Structure of the model compound and the optimized geometries at the level of B3LYP/6-31G. (left) Fork-shaped and (right) fan-shaped
geometries.
Figure 1. UV spectra of (- - -) PySi in THF, ( –– ) PySi in film, (
. . . .
) PyC
in THF, and (
) PyC in film.
Figure 3. Drain current (Id)-drain voltage (Vd) curves with the function of
gate voltage (Vg) for the TFT with PyC.
Figure 2. Emission spectra of PySi, PyC, and Py in chloroform.
Table 1. Performance of TFTs and photovoltaic cells with PySi and
PyC
TFT
Photovoltaic cell
Comp µ (cm2 V−1 s−1)Ion/Ioff Isc (mA cm−2
)
Voc (V) FF PCE (%)
PySi
PyC
1.1 × 10−5
1.1 × 10−5
104
104
0.18
0.18
0.82 0.25 0.037
0.89 0.24 0.038
Figure 4. Emission spectra of PySi in the presence and absence of C60
.
Preparation of tris[(5ꢀ-bromo-2,2ꢀ-bithiophenyl)di-
methylsilyl]methane
in the literature.[7] The usual work-up described below involves
hydrolysis of the reaction mixture with water, separation of the
organic layer, extraction of the aqueous layer with chloroform,
drying the combined organic layer and extracts over anhydrous
magnesium sulfate, evaporation of the solvent and subjecting the
residue to silica gel column chromatography, in that order.
To a solution of 3.1 g (9.6 mmol) of 5,5ꢁ-dibromo-2,2ꢁ-bithiophene
in 50 ml of ether was added 6.2 ml (9.7 moL) of a 1.57 M n-
butyllithium solution in hexane at −80 ◦C and the mixture
was stirred at room temperature for 4 h. The resulting solution
of bromobithiophenyllithium was cooled at −80 ◦C and 0.95 g
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 540–544

Star-shaped organosilicon compounds
A
yellow solid of PyC was obtained in
a
man-
ner similar to that of PySi using tris[(5ꢁ-bromo-2,2ꢁ-
bithiophenyl)dimethylsilyl]methane instead of tris[(5ꢁ-bromo-
2,2ꢁ-bithiophenyl)dimethylsilyl]methylsilane in 39% yield: m.p.
119–121 ◦C; FAB-MS m/z 1354 (M+); 1H NMR (δ in CDCl3) 0.39
(s, 1H, HC), 0.46 (s, 18H, CH3Si), 7.02 (d, 3H, J = 3.6 Hz, thiophene
ring H), 7.10 (d, 3H, J = 4.0 Hz, thiophene ring H), 7.19 (d, 3H,
J = 3.6 Hz, thiophene ring H), 7.32 (d, 3H, J = 3.6 Hz, thiophene
ring H), 7.75–8.10 (m, 24H, pyrene ring H), 8.43 (d, 3H, J = 9.2
Hz, pyrene ring H); 13C NMR (δ in CDCl3) 2.61 (CH3Si), 6.44 (HC),
· · ·
88.42, 93.78 (C C), 117.20, 122.21, 123.65, 123.97, 124.09, 124.23,
125.02, 125.19, 125.35, 125.45, 125.96, 126.94, 127.91, 128.12,
129.01, 130.82, 130.97, 131.00, 131.38, 132.80, 135.04, 139.07,
141.61, 142.19 (ring carbons). Exact-MS (FAB) calcd for C85H58S6Si3
(M+): 1354.1271. Found: 1354.2164.
Preparation of Py
A mixture of 27.4 mg (0.10 mmol) of bromohexylbithiophene,
29.8 mg (0.10 mmol) of trimethylsilylethynylpyrene, 2.5 mg of
Pd(PPh3)4, 0.25 mg of CuI, 1.0 ml of triethylamine, and 1.0 ml
of water was heated at the reflux temperature for 48 h. After the
usual work-up, 7.5 mg (0.016 mmol, 16% yield) of Py as a yellow
solid: m.p. 83–85 ◦C; MS m/z 474 (M+); 1H NMR (δ in CDCl3) 0.90 (t,
3H, J = 6.7 Hz, CH3), 1.34 (m, 6H, CH2), 1.70 (t, 2H, J = 7.3 Hz, CH2),
2.82 (t, 2H. J = 7.8 Hz, CH2), 6.72 (d, 1H, J = 3.9 Hz, thienyl ring
H), 7.07 (d, 2H, J = 3.0 Hz, thienyl ring H), 7.33 (d, 1H, J = 3.9 Hz,
thienyl ring H), 8.00–8.26 (m, 8H, pyrene ring H), 8.6 (d, 1H, J = 9.8
Hz, pyrene ring H); 13C NMR (δ in CDCl3) 14.10, 22.58, 28.76, 29.70,
Figure 5. (a) UV spectra of spin-coated films of PySi and PyC with PCBM.
(b) IPCE spectra of the cells with PySi and PyC.
· · ·
30.21, 31.57 (hexyl carbons), 88.48, 93.33 (C C), 117.41, 121.30,
122.90, 123.99, 124.31, 124.49, 124.57, 124.98, 125.47, 125.64,
125.70, 126.28, 127.25, 128.24, 128.43, 129.33, 131.09, 131.26,
131.32, 131.68, 132.88, 134.14, 139.80, 146.32 (ring carbons).
(3.2 mmol) of tris(chlorodimethylsilyl)methylmethane was added
to the mixture, then the resulting mixture was stirred for 24 h at
room temperature. After the usual work-up, 0.67 g (0.73 mmol;
22% yield) of the title compound was obtained as a pale green
solid: FAB-MS m/z 920 (M+); 1H NMR (δ in CDCl3) 0.31 (s, 1H, HC),
0.37 (s, 18H, CH3Si), 6.66 (d, 3H, J = 3.4 Hz, ring H), 6.93 (2d, 6H,
J = 3.4 Hz, ring H), 7.02 (d, 3H, J = 3.4 Hz, ring H); 13C NMR (δ
in CDCl3) 2.52 (CH3Si), 6.04 (HC), 110.90, 123.70, 124.83, 130.54,
134.90, 138.84, 141.29, 141.52 (ring carbons). The rather low yield
was ascribed to the difficult separation of the compound from
by-products including partially substituted compounds.
Fabrication of TFTs
Heavily doped n+-Si (100) wafers with a thermally grown
insulating SiO2 layer (210 nm thick) were used as the substrates.
A gold electrode (50 nm thick) was deposited on the Si side of
the substrate as a gate contact, while gold drain and source
electrodes (50 nm thick) were patterned on the SiO2 side using a
photolithography technique with the drain-source channel length
(L) and width (W) as 10 µm and 2.0 cm, respectively. On the SiO2
surface with drain and sourse electrodes, a thin film of PySi or
PyC was prepared by spin-coating of its 0.4 wt% chloroform
solution at 2000 rpm. Field-effect mobility (µFET) was calculated in
the saturated regime of the drain current (Id) using the following
equation:
Preparation of PySi and PyC
A
mixture of 0.48 g (0.5 mmol) of tris[(5ꢁ-bromo-2,2ꢁ-
bithiophenyl)dimethylsilyl]methylsilane, 0.34 g (1.5 mmol) of
ethynylpyrene, 29 mg (0.025 mmol) of Pd(PPh3)4, 5 mg
(0.03 mmol) of CuI, 5 ml of triethylamine and 5 ml of THF was
heated at the reflux temperature for 24 h. After the usual work-
up, 0.30 g (0.21 mmol, 41% yield) of PySi as a yellow solid: m.p.
123–126 ◦C; FAB-MS m/z 1384 (M+); 1H NMR (δ in CDCl3) 0.41 (s,
3H, CH3Si), 0.46 (s, 18H, CH3Si), 6.97 (d, 3H, J = 3.6 Hz, thiophene
ring H), 7.05 (d, 3H, J = 3.9 Hz, thiophene ring H), 7.21 (d, 3H,
J = 3.3 Hz, thiophene ring H), 7.30 (d, 3H, J = 3.6 Hz, thiophene
ring H), 7.75–8.20 (m, 24H, pyrene ring H), 8.40 (d, 3H, J = 9.2
Hz, pyrene ring H); 13C NMR (δ in CDCl3) −11.77, −0.35 (CH3Si),
Id = (WCi/2L)µFET(Vg − Vth)2
where Ci is capacitance of the SiO2 insulator and Vd and Vth are
the gate and threshold voltages, respectively. Current on/off ratio
(Ion/Ioff) was determined from the maximum (Ion) and minimum
(Ioff) value of the Id.
Fabrication of photovoltaic cells
Glass slides patterned with ITO were cleaned by sonication
sequentially in detergent, water, acetone and ethanol. The ITO
glass substrates were spin coated at 500 rpm for 60 s and then
6000 rpm for 10 s with an aqueous solution of PEDOT-PSS (Baytron
PAl4083). The resulting spin-coated films were dried in a vacuum
oven for 5 h. The active layer was prepared by spin-coating of
· · ·
88.42, 93.80 (C C), 117.18, 122.18, 123.65, 123.93, 124.03, 124.18,
125.16, 125.32, 125.41, 125.92, 126.90, 127.87, 128.07, 128.97,
130.78, 130.92, 130.96, 131.34, 132.83, 134.99, 139.02, 139.62,
142.22 (ring carbons). Exact-MS (FAB) calcd for C85H60S6Si4 (M+):
1384.2096. Found: 1384.2078.
c
Appl. Organometal. Chem. 2010, 24, 540–544
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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J. Oshita et al.
an o-dichlorobenzene solution (0.3 ml) of the active components
of PySi or PyC: PCBM = 1 : 4 (wt), at 1500 rpm for 60 s and then
6000 rpm for 10 s on the PEDOT-PSS-coated substrate. A thin layer
ofLiF(4 Å)wasfirstdepositedontheactivelayerpriortodeposition
of the Al top electrode (400 Å). The active pixel area was 0.25 cm2
(0.5 × 0.5 cm). The photocurrent–voltage characteristics (I–V)
of the photovoltaic devices were measured using a potentiostat
(Hokuto Denko HAB-151) under a simulated solar light (AM 1.5,
100 mW cm−2).
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