Highly conductive organosilica hybrid films prepared from a liquid-crystal perylene bisimide precursor

Article abstract of DOI:10.1002/adfm.201100444

Significant anisotropic electrical conduction in organosilica films is achieved by long-range orientation of electroactive perylene bisimide (PBI) moieties in the silica scaffold. A new PBI-based organosilane precursor is designed with lyotropic liquid-cr

Full text of DOI:10.1002/adfm.201100444

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Highly Conductive Organosilica Hybrid Films Prepared
from a Liquid-Crystal Perylene Bisimide Precursor
Norihiro Mizoshita, Takao Tani, and Shinji Inagaki*
devices because of the flexible design of
their functionalities by appropriate selec-
tion of the bridging organic group (R), and
also their high thermal and chemical sta-
Significant anisotropic electrical conduction in organosilica films is achieved
by long-range orientation of electroactive perylene bisimide (PBI) moieties in
the silica scaffold. A new PBI-based organosilane precursor is designed with
lyotropic liquid-crystalline properties. The PBI precursor with triethoxysilyl-
phenyl groups exhibits a hexagonal columnar phase in the presence of organic
solvents. The lyotropic liquid-crystalline behavior of the precursor enables
the preparation of dip-coated films consisting of uniaxially aligned columnar
aggregates of the PBI precursor on the centimeter scale. The oriented struc-
ture is successfully fixed by in situ polycondensation, which yields insoluble,
thermally stable PBI–silica hybrid films. The oriented organosilica films
doped with hydrazine exhibit high electrical conductivities on the order of
10−2 S cm−1 , which are at the highest level for organosilica materials, and are
comparable to those of all-organic PBI assemblies. Definite anisotropy of con-
ductivities is also found for these films. The present results suggest that the
induction of significant electrical properties in organic molecular assemblies is
compatible with the structural stabilization by inorganic–organic hybridization.
bility, which results from the formation of
covalent Si–O–Si bonds.[7–9] However, in
general it has been difficult to achieve sig-
nificant electrical conduction in organo-
silica materials because of the insulating
properties of the silica moieties and the
disorder in the organic moieties,[10] except
for an oligothiophene-based polysilsesqui-
oxane film that exhibits electrical conduc-
tivity on the order of 10−3 to 10−2 S cm−1
by doping with FeCl3.[11] Recently, highly
organized organosilicas with molecular-
scale periodicity in the organic groups
have been prepared by solid-state poly-
condensation or by the use of organosi-
lane precursors with R groups that exhibit
strong intermolecular interactions.[12–15]
Molecular-scale ordering of electroac-
1. Introduction
tive organic groups through strong interactions could lead to
high electrical conductivity in organosilica materials, despite
the presence of the insulating silica moiety. Lu and co-workers
reported the synthesis of organosilica microtubes and -flakes
with lamellar orientation of the organic groups from bridged
organosilane precursors containing π-conjugated perylene
bisimide (PBI) and porphyrin, respectively.[15] However, the
organosilica hybrids exhibited relatively low electrical conductiv-
ities (on the order of 10−7 to 10−4 S cm−1 ), probably because the
orientation of organic groups is limited within the micrometer-
sized tubes and flakes, and the conducting path is likely inter-
cepted at the grain boundary. Long-range orientation over
the entire centimeter-scale sample area responsible for the
current–voltage (I–V) properties is important for achieving
higher conductivity in organosilica hybrids.
Here we report the preparation of highly conductive PBI–silica
hybrid thin films with long-range orientation on the centimeter
scale. The PBI unit has been used as a key component in the
construction of electron-transport materials and self-assembled
nano-objects.[16] However, conventional PBI molecules tend to
form polycrystalline phases with small single-crystal domains
due to strong intermolecular interactions. Our strategy is to
prepare a liquid-crystal (LC) film from a PBI-based organosi-
lane precursor (PBI-Si, Figure 1) with long-range orientation,
and to subsequently fix the LC structure through polycondensa-
tion of the precursors. In order to develop solution-processable
LC PBI precursors, we designed PBI-Si with bulky and flexible
triethoxysilylphenyl substituents. As shown in Figure 1, PBI-Si
Electronic devices based on organic semiconductors have
attracted much attention due to numerous advantages such as
molecular-level tunability of structure–property relationships,
solution processability, light weight, and low-cost device fab-
rication.[1–3] Excellent electrical properties have been observed
for well-ordered assemblies of low-molecular-weight organic
compounds, such as vapor-deposited crystalline films,[4] self-
organized mesomorphic materials,[5] and large single crystals.[6]
However, their low chemical and mechanical stability is prob-
lematic for current applications. On the other hand, electroactive
polymers are chemically and mechanically stable and solution
processable, but they do not exhibit high electronic conduction
because of their less-ordered or amorphous structures.[2]
Organosilica hybrid materials, prepared by polycondensa-
tion of bridged organosilane precursors (R[Si(OR’)3]n; n ≥ 2),
have significant potential applications in optical and electrical
Dr. N. Mizoshita, Dr. T. Tani, Dr. S. Inagaki
Toyota Central R&D Laboratories, Inc.
Nagakute, Aichi 480-1192, Japan
E-mail: inagaki@mosk.tytlabs.co.jp
Dr. N. Mizoshita, Dr. T. Tani, Dr. S. Inagaki
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency (JST)
Kawaguchi, Saitama 332-0012, Japan
DOI: 10.1002/adfm.201100444
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exhibit the Col phase at room temperature.
UV–vis spectra of the LC mixtures showed
an absorption maximum (λmax) at 498 nm,
whereas the λmax of a dilute solution of PBI-Si
in DMF (2 × 10−6 M) was 526 nm (Figure 3).
The blueshift in λmax is due to face-to-face
aggregate formation of the PBI moieties
in the LC state, which indicates that the LC
structure is formed by a hexagonal arrange-
ment of columnar aggregates of PBI-Si.
2.4 nm
(Si to Si)
π－π
PBI-Si
stacking
Elimination of
ethoxy groups
2.2. Preparation of Oriented Organosilane
and Organosilica Films
Oriented films of PBI-Si were successfully
obtained by dip-coating quartz substrates
with PBI-Si/DMF solutions under optimal
Figure 1. Molecular structure of PBI-Si and space-filling models of the monomer and dimers.
has a dumbbell-like (rather than rod-like) molecular shape;
therefore the π-stacking of the PBI planes is expected to occur
with rotational displacement, leading to suppression of crystal-
lization and formation of mesomorphic structures. After hydro-
lytic treatment, the π–π stacking of PBI planes can be enhanced
by the elimination of ethoxy groups (Figure 1). A PBI precursor
with mesomorphic and self-assembling properties would thus
be applicable to the preparation of electroconductive organo-
silica films that maintain long-range homogeneous orientation
of the PBI units after in situ polycondensation.
conditions (concentration: 5 wt%, lift-off rate: 10 mm min−1 ).
The triethoxysilyl groups of PBI-Si were intact without conden-
sation in the dip-coated film (PBI-Si-F), because no water or
catalyst (acid or base) was added to the PBI-Si/DMF solutions.
Polarizing optical microscopy observations of PBI-Si-F indi-
cate long-range orientation of the optical axis over the entire
film (1.5 cm × 2.0 cm). Dark images were observed when the
polarization direction of the light was parallel or perpendicular
to the dip-coating direction, while the brightest images were
obtained when the polarization direction was 45° with respect
to the coating direction (Figure 4a). The molecular alignment of
PBI-Si in the film was shown by polarized UV–vis spectroscopy.
The maximum absorption intensity was obtained when the
polarization direction of the incident light was perpendicular
to the coating direction (Figure 4b), which indicates that the
transition dipole moment of PBI-Si is distributed perpendicu-
larly to the coating direction. These results show that columnar
aggregates of π-stacked PBI-Si formed during evaporation of
DMF at the air–solution interface (Figure 2b) are laid onto the
substrate along the lift-off motion, as schematically illustrated
in Figure 4c. The λmax of PBI-Si-F was 497 nm (Figure 3),
which is almost the same as that of the lyotropic LC mixtures
(498 nm). The XRD pattern of PBI-Si-F showed an intense peak
at d = 2.70 nm, but very weak peaks at higher angles (Figure 2c),
which suggests that the hexagonal structure was distorted,
but that the columnar structure was preserved during solvent
evaporation.[17]
2. Results and Discussion
2.1. Lyotropic Liquid-Crystalline Behavior of PBI-Si
The PBI-Si precursor was successfully synthesized from
perylene-3,4:9,10-tetracarboxylic dianhydride in 84% overall
yield (two step, Scheme 1). In the first step, 4-iodophenyl-
substituted PBI was obtained in 96% yield by imidation using
4-iodoaniline. Rh-catalyzed silylation of the iodide gave PBI-Si
in 88% yield.
The lyotropic behavior of PBI-Si was examined in organic sol-
vents (Figure 2a and 2b). Polarizing optical microscopy observa-
tions show that concentrated solutions of PBI-Si in tetrahydro-
furan (THF) and dimethyl formamide (DMF) exhibit fan-like
optical textures, which are characteristic of
ordered mesophases (Figure 2a and S1 in the
SupportingInformation).TheX-raydiffraction
(XRD) pattern of a 60 wt% solution of PBI-Si
in DMF shows an intense peak at d = 2.83 nm
and small peaks at d = 1.58, 1.42, 1.20,
and 0.71 nm (Figure 2c). These peaks are
assignable to a hexagonal columnar (Col)
phase due to the d-spacing ratio of ca.
1:1/√3:1/2:1/√7:1/4. The phase-transition
temperatures of the PBI-Si/DMF mixtures
were plotted as a function of the concentration
of PBI-Si (Figure 2b). The PBI-Si/DMF
mixtures with more than 60 wt% PBI-Si
Scheme 1. Synthesis of PBI-Si.
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(c)
(a)
0.34 nm
0.34 nm
PBI-Si-F-AcOH
0.80 nm
2.41 nm
1.36 nm
0.83 nm
PBI-Si-F-NH3
PBI-Si-F
(b)
2.70 nm
bp of DMF
150
2.83 nm
100
50
0
Isotropic Solution
60% PBI-Si in DMF
Col
80
Iso.
+
Col
1.42 nm
0.71 nm
1.58 nm
evaporation
of DMF
1.20 nm
0
20
40
60
100
10
20
30
2θ / degree
Concentration of PBI-Si / wt%
Figure 2. a) Polarized photomicrograph of the optical texture developed in a 60 wt% solution of PBI-Si in DMF upon cooling; b) phase-transition
behavior of the PBI-Si/DMF mixture as a function of PBI-Si concentration upon heating; c) XRD patterns of organosilane and organosilica films. XRD
patterns were obtained for organosilica film samples peeled off substrates. The arrow in (b) indicates the change in the composition of the mixture
during the dip-coating process.
stable up to at least 350 °C.[18] The NH3- and AcOH-treated
The triethoxysilyl groups of PBI-Si were polycondensed by
exposure of PBI-Si-F to vapor from aqueous NH3 or acetic acid
solution. 29Si magic-angle spinning (MAS) NMR spectra showed
the disappearance of the T0 signal (R–Si(OEt)3) and the appear-
ance of new T1 and T2 signals (R–Si(OSi)n(OH)3–n; n = 1 and 2,
respectively) after NH3 and AcOH treatment, which indicated
that the polycondensation had proceeded (see Figure S2 in the
Supporting Information). The polycondensed organosilica films
were insoluble in common organic solvents and were thermally
films, denoted as PBI-Si-F-NH3 and PBI-Si-F-AcOH, respec-
tively, preserved the oriented optical textures of the original
noncondensed PBI-Si-F. The dichroism of the polarized UV–vis
spectra was maintained (PBI-Si-F-AcOH) or slightly enhanced
(PBI-Si-F-NH3) during polycondensation (Figure S3 in the Sup-
porting Information). On the other hand, the intense XRD peak
at d = 2.70 nm due to the columnar structure in PBI-Si-F disap-
peared, and a new peak at d = 0.34 nm—typical of a π-stacking
structure—appeared after polycondensation (Figure 2c). New
peaks also appeared at d = 1.36 and 0.83 nm for PBI-Si-F-NH3
and at 2.41 and 0.80 nm for PBI-Si-F-AcOH, which are due
to the packing structure of PBI units in the lateral direction.
The large blueshift of λmax from 497 to 473 nm (Δ = –24 nm)
implied the promotion of strong π–π interactions between the
PBI units during polycondensation (Figure 3).
(E)
(D)
The nanostructure of the organosilica films was examined
by transmission electron microscopy (TEM). Figure 5a and 5b
show the TEM images of PBI-Si-F-NH3 and PBI-Si-F-AcOH,
respectively. Nanometer-scale periodic structures can be partially
observed in the organosilica films. For PBI-Si-F-AcOH, a striped
structure with ca. 2 nm periodicity can be seen (Figure 5b),
which is in agreement with the size of the columnar aggregates
of PBI and the appearance of the weak XRD peak at d = 2.41 nm.
On the other hand, the periodicity of the striped structure in
PBI-Si-F-NH3 was ca. 1 nm (Figure 5a), which is consistent
with the XRD peak at d = 1.34 nm. The 1 nm periodic structure
presumably corresponds to packing structure of the PBI–silica
hybrid in the short-axis direction of PBI units, considering the
(C)
(B)
(A)
400
500
600
700
Wavelength / nm
Figure 3. UV–vis spectra of (A) PBI-Si in a dilute DMF solution (2 ×
10−6 M), (B) lyotropic mixture of PBI-Si/DMF (60/40, w/w), (C) PBI-Si-F,
(D) PBI-Si-F-NH3, and (E) PBI-Si-F-AcOH.
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vapor according to a previously published
method.[19] The formation of PBI anionic radi-
cals through hydrazine doping was confirmed
by UV–vis spectroscopy.[20] Electrodes were
placed onto the films, as shown in Figure 6a ,
in order to compare the conductivities parallel
(σꢀ) and perpendicular (σ⊥) to the dip-coating
direction. In air (without hydrazine doping),
none of the films showed obvious electric cur-
rents upon application of voltages up to 50 V.
However, linear I–V profiles were obtained for
the films doped with hydrazine (Figure 6b).
The highest electric current was obtained
for PBI-Si-F in the coating direction, which
is parallel to the stacking direction of the PBI
moieties in the columnar aggregates. After
polycondensation, the slopes of the I–V pro-
files decreased to less than half. For these
oriented films, the electric currents along the
coating direction are larger than those in the
perpendicular direction; i.e., electrical conduc-
tion is anisotropic in the films. Table 1 gives
the electrical conductivities (σꢀ and σ⊥) and the
anisotropy (σꢀ/σ⊥) of conductivities. PBI-Si-F
exhibits high σꢀ, on the order of 10−1 S cm−1 ,
whereas σ⊥ is on the order of 10−3 S cm−1 .
The anisotropy is greater than 40. After poly-
condensation, the conductivities (σꢀ) of PBI-
Si-F-NH3 and PBI-Si-F-AcOH were still on
the order of 10−2 S cm−1 , which is the highest
(a)
(b)
Dip-coating
direction
A
P
50 μm
50 μm
50 μm
(c)
A
P
Dip-coating
direction
A
P
substrate
Figure 4. a) Polarized optical photomicrographs of PBI-Si-F (P: polarizer, A: analyzer); b) polar-
ized UV–vis spectra of PBI-Si-F; c) schematic illustration of the molecular arrangement in the
dip-coated film.
molecular size of the precursor (Figure 1). Based on these obser-
vations, polycondensation under acidic condi-
level for organosilica materials.[10,11,15] These values are also
tions seems to preserve the original columnar
(a)
(b)
structures with 2 nm periodicity, while poly-
condensation under basic conditions likely
results in rearrangement of the PBI columns
into a more closely packed structure with
edge-to-edge packing of PBIs in their short
axis direction.
A schematic illustration for the structural
change in the dip-coated film before and after
polycondensation is presented in Figure 5c.
The columnar structure in PBI-Si-F is dis-
turbed by the anisotropic condensation of
(c)
PBI-Si, mainly in the lateral direction of the
2.7 nm
-Si(OEt)3
Siloxane bond
columns. However, condensation promotes
the close packing of PBI moieties through the
elimination of bulky ethoxy groups, which
results in the formation of strong π–π inter-
actions between PBI units with dense lateral
packing of the columnar aggregates.
Polycondensation
~ 0.34 nm
2.3. Measurements of Electrical Conductivity
The I–V characteristics of the thin films
before and after polycondensation were
measured using a two-probe method. The
Figure 5. TEM images of (a) PBI-Si-F-NH3 and (b) PBI-Si-F-AcOH. c) Schematic illustration of
arrangement of PBI columns before and after polycondensation. Black arrows indicate plau-
PBI-based films were doped with hydrazine sible charge conduction.
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conductivities. The conductivity of the present
organosilica film is at the highest level for
organosilica materials, and is comparable to
those reported for all-organic PBI assemblies.
The present results suggest that the induc-
tion of significant electrical properties in self-
assembled organic materials is compatible
with the structural stabilization by inorganic–
organic hybridization. The robustness and
chemical stability of organosilica hybrid films
are advantageous for the preparation of pat-
terned semiconductors and the lamination of
thin films using solution processes. The con-
cept for the construction of highly conductive
inorganic–organic hybrids is preferentially
applied in the design of various architectures,
including monolithic particles and mesostruc-
tured materials.
(a)
(b)
100
80
Dip-coating
direction
PBI-Si-F (||)
Direction of
measurement
(parallel, ||)
PBI-Si-F-NH3 (||)
60
Electrode
PBI-Si-F-AcOH (||)
40
PBI-Si-F (||,⊥)
non-doped (in air)
Electrode
20
70 μm
0
Electrode
Direction of
-20
-40
-60
-80
-100
PBI-Si-F-NH3 (⊥)
PBI-Si-F-AcOH (⊥)
measurement
(perpendicular,⊥ )
PBI-Si-F (⊥)
Dip-coating
direction
Electrode
70 μm
-40
-20
0
20
40
Voltage / V
4. Experimental Section
Figure 6. a) Polarized optical photomicrographs of PBI-Si-F-NH3 equipped with copper-foil
electrodes and b) I–V properties of the oriented organosilane and organosilica films measured
in saturated hydrazine vapor.
General: All reagents and solvents were
purchased from Aldrich and Tokyo Chemical
Industry and used without further purification.
comparable to those reported for all-organic PBI assemblies.[19,21]
The high conductivities of the present organosilica materials are
due to the long-range homogeneous orientation of the PBI moie-
ties between the electrodes, which had not been reported for pre-
vious conductive organosilica hybrids.[10,11,15] On the other hand,
the values of σ⊥ increased after polycondensation and the anisot-
ropy decreased from 40–50 to 2–8.
The conducting behavior of the films can be explained by
the change in the molecular arrangement (Figure 5c). Before
polycondensation, electron hopping occurs easily along the con-
ducting path consisting of the π-stacked columns of PBI-Si. In the
perpendicular direction, electron hopping is strongly suppressed
due to the weak interactions between PBI moieties and the pres-
ence of insulating layers consisting of triethoxysilyl groups. After
polycondensation, although the columnar order of the PBI aggre-
gates is disturbed, the PBI moieties are anisotropically aligned
with enhanced π -stacking in the silica scaffold, which leads to
the suppression of a large decrease in σꢀ. On the other hand,
σ⊥ increases because charge hopping is allowed, following the
elimination of the ethoxy groups and the partial overlap of PBI
moieties. Considering the different packing structure of the PBI–
silica hybrids (Figure 2c, 5a, and 5b), it is reasonable that the
anisotropy of conductivities of PBI-Si-F-NH3 with denser lateral
packing of PBIs is lower than that of PBI-Si-F-AcOH.
1H and 13C NMR spectra were measured using a Jeol JNM-ECX400P
spectrometer. Mass spectra were recorded on a MICROMASS Q-TOF. IR
spectroscopy (Thermo Nicolet Avatar 360 FT-IR) was conducted using
an attenuated total reflection (ATR) attachment. XRD measurements
were performed using Cu Kα radiation (Rigaku RINT-TTR, 50 kV,
300 mA). UV–vis absorption spectra were measured using a Jasco
V-670 spectrometer. 29Si MAS NMR spectra were recorded on a Bruker
Avance 400 spectrometer at 79.49 MHz using 7 mm zirconia rotors and
a sample-spinning frequency of 4 kHz. The chemical shifts for all spectra
were referenced to tetramethylsilane at 0 ppm. Polarizing microscopy
was conducted with an Olympus BX51 equipped with a hot stage
(Mettler Toledo FP82HT). TEM was performed on a Jeol JEM-2010FEF
with an accelerating voltage of 200 kV. Thermogravimetric analysis was
performed using a Rigaku Thermo Plus TG 8120 at the heating rate of
10 °C min−1 under nitrogen gas flow (0.5 L min−1 ).
Synthesis of N,N′-Bis(4-iodophenyl)-3,4:9,10-tetracarboxylic acid bisimide:
A
mixture of perylene-3,4:9,10-tetracarboxylic dianhydride (2.35 g,
6.0 mmol), Zn(OAc)2 (0.85 g), and quinoline (45 mL) was stirred at 100 °C
for 0.5 h under an argon atmosphere. Then, 4-iodoaniline (6.57 g,
30.0 mmol) was added and the reaction mixture was stirred at 200 °C for
24 h. After cooling to room temperature, the reaction mixture was
poured into ethanol (350 mL) and the precipitate was collected by
suction filtration. The residue was stirred in a 5% K2CO3 aqueous
solution (500 mL) at 70 °C for 10 min then collected by suction filtration.
This procedure was repeated twice. The residue was then dispersed
in methanol (100 mL) and stirred at 70 °C for 30 min. The precipitate
was collected by suction filtration and dried under reduced pressure to
T a b le 1 . Electrical properties of dip-coated PBI films measured in satu-
rated hydrazine vapor.
3. Conclusions
Samplea)
σꢀ [S cm−1
]
σ⊥ [S cm−1
]
σꢀ/σ⊥
40–50
2–3
PBI-based organosilica hybrid thin films with long-range homo-
geneous orientation were successfully prepared by a dip-coating
method utilizing the lyotropic LC behavior of the PBI-Si pre-
cursor, followed by in situ polycondensation. The oriented
precursor and organosilica thin films doped with hydrazine
exhibited high electrical conductivities on the order of 10−1
and 10−2 S cm−1 , respectively, and definite anisotropy of the
(1.1 0.6) × 10−1
(4.4 2.2) × 10−2
(3.3 1.6) × 10−2
(2.7 1.3) × 10−3
(2.2 1.1) × 10−2
(6.3 3.1) × 10−3
PBI-Si-F
PBI-Si-F-NH3
PBI-Si-F-AcOH
5–8
a)Film thickness: 7–15 nm; channel width: 4–5 mm; channel length: 150–210 μm.
Measurements at nine points were averaged for each sample.
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afford an insoluble dark red solid (4.56 g, 96%). IR (ATR): ν = 3090–3030
(Ar–H), 1703 (C=O st), 1661, 1591, 1576, 1483, 1429, 1404, 1356, 1342,
1254, 1194, 1175, 1136, 1121, 1057 (Ar–I), 1009, 970, 955 cm−1 .
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Synthesis of N,N′-Bis[4-(triethoxysilyl)phenyl]-3,4:9,10-tetracarboxylic
acid bisimide (PBI-Si) : DMF (anhydrous; 40 mL) and dry trimethylamine
(anhydrous; 1.67 mL) were added to a mixture of N,N′-bis(4-iodophenyl)-
3,4:9,10-tetracarboxylic acid bisimide (1.59 g, 2.00 mmol) and Rh[(cod)
(CH3CN)2]BF4 (38.0 mg, 0.10 mmol) under argon atmosphere.
Triethoxysilane (1.48 mL, 1.31 g, 8.00 mmol) was added at 0 °C and the
mixture was stirred at 80 °C for 4 h. After removal of the solvents by a
rotary evaporator, chloroform (150 mL) was added to the residue. The
mixture was filtered using Celite and activated carbon, and the filtrate was
washed quickly with water. The organic phase was dried with MgSO4 and
concentrated under reduced pressure. After adding hexane (300 mL),
the resulting precipitate was collected by centrifugation (3500 rpm,
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5 min) to afford the product as a dark red solid (1.53 g, 88%). H NMR
(400 MHz, CDCl3, δ): 1.30 (t, J = 7.0 Hz, 18H), 3.95 (q, J = 7.0 Hz, 12H),
7.39 (d, J = 8.4 Hz, 4H), 7.89 (d, J = 8.4 Hz, 4H), 8.67 (d, J = 8.5 Hz, 4H),
8.75 ppm(d, J = 8.5 Hz, 4H); 13C NMR (100 MHz, CDCl3, δ): 18.2, 58.8,
123.0, 123.2, 125.7, 128.0, 129.0, 131.1, 132.0, 134.0, 135.8, 136.6,
162.9 ppm; IR (ATR): ν = 3090–3030 (Ar–H), 2974, 2926, 2889, 1705
(C=O st), 1662, 1591, 1578, 1506, 1483, 1431, 1404, 1356, 1344,
1254, 1165, 1149, 1065, 955 cm−1 ; MS (APCI) m/z: [M]+ calcd for
C48H46N2O10Si2, 866.27; found: 866.29.
Preparation and Polycondensation of Dip-Coated Films: Thin films of
PBI-Si were prepared by dipping quartz substrates (1.5 cm × 2.5 cm) in
DMF solutions of PBI-Si (0.1–10 wt%) at a lift-off rate of 1–40 mm min−1
under constant temperature and humidity of 20–22 °C and 55–58%,
respectively. In situ polycondensation of the dip-coated films was conducted
by exposure to the vapor from a 50% aqueous solution of AcOH at 75 °C for
12 h or a 10% aqueous solution of NH3 at 60 °C for 6 h in a sealed vessel.
Conductivity Measurement: Current–voltage (I–V) properties of dip-
coated films were measured using a two-probe method with a Keithley
2636A System SourceMeter. The sample cells were prepared by placing
two copper foil electrodes (3M CU-35C, width: 4–5 mm, gap: 150–210 μm)
onto the films as shown in Figure 6a. In order to obtain I–V properties
of doped films, the sample cells were fixed onto sealed glass vessels
containing a small amount of hydrazine (monohydrate) and exposed to the
saturated hydrazine vapor. Conductivity measurements were carried out
after standing for more than 3 min to stabilize the I–V properties in the
hydrazine vapor. The conductivity (σ) extracted from the linear region of
I–V profiles was calculated using the equation σ = I d/ (V S), where d is the
distance between electrodes, and S is the cross section of the samples.
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Supporting Information
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[17] The PBI-Si fi lms for XRD measurements were prepared using a
10 wt% solution of PBI-Si in DMF in order to increase the film
thickness.
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank Dr. Yasutomo Goto for the TEM measurements.
[18] The thermal stability of the oriented films up to 350 °C was con-
firmed by polarizing optical microscopy. The high thermal stability
of the PBI–silica hybrids was also confirmed by thermogravimetric
analysis. Bulk decomposition of the PBI–silica hybrids occurred at
550–650 °C. See Figure S5 in the Supporting Information.
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Soc. 2007, 129, 6354–6355.
[20] Doping of the PBI materials with hydrazine resulted in a PBI ani-
onic radical that exhibited absorption bands at 600–1000 nm. See
Figure S4 in the Supporting Information.
Received: February 26, 2011
Published online: July 19, 2011
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2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2011, 21, 3291–3296