Conductive casting films based on an octasilicate-core dendrimer containing the mixed-valence state TCNQ on the periphery

Article abstract of DOI:10.1039/C6RA21636F

An octasilicate (OS)-core dendrimer terminated with imidazolium-7,7,8,8-tetracyanoquiodimethane (TCNQ) anion radicals (OS-mimTCNQ) was synthesized from an OS-core dendrimer having alkyl bromides as terminal groups (OS-mimBr) via anion exchange reaction wi

Full text of DOI:10.1039/C6RA21636F

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Conductive Casting Films Based on an Octasilicate-Core
Dendrimer Containing the Mixed-Valence State TCNQ on the
Periphery
Received 00th January 20xx,
Accepted 00th January 20xx
Yasuyuki Irie,a Li Lina,a Hiroaki Imoto,a Megumi Komada,b Takashi Nishino,b and Kensuke Naka*a
DOI: 10.1039/x0xx00000x
An octasilicate (OS)-core dendrimer terminated with imidazolium-7,7,8,8-tetracyanoquiodimethane (TCNQ) anion radicals
(OS-mimTCNQ) was synthesized from an OS-core dendrimer having alkyl bromides as terminal groups (OS-mimBr) via
anion exchange reaction with lithium TCNQ salt (LiTCNQ) in water. Conductive films were prepared by casting mixtures of
OS-mimTCNQ with the neutral TCNQ in acetonitrile on glass substrates. Although the casting film of OS-mimTCNQ showed
low conductivity (< 10-6 S·cm-1), the casting films showed relatively high electron conductivity. In particular, the casting film
with the same feed molar ratio between the neutral TCNQ and the terminated imidazolium moiety (OS-CT1.0) showed the
highest conductivity (5.4 × 10-2 S·cm-1). The stacking strucutre with the mixed-valence state of TCNQ in these casting films
was confirmed by UV-vis-NIR spectra. The grazing incidence X-ray diffraction (GI-XD) of OS-CT1.0 showed an anisotropic
pattern, indicating that the OS-core dendrimer arranged in the direction of the out-of-plane.
www.rsc.org/
electron conductivities (≤ 10-3 S·cm-1). Therefore, development
of new polymeric scaffolds to control the molecular
arrangement is desired to improve the material properties of
Introduction
7,7,8,8-Tetracyanoquinodimethane (TCNQ) has been utilized
as an electron acceptor, and it is well known that it forms
charge transfer (CT) complexes with electron donor molecules.
In the crystalline state, both the molecules form segregate
stacks with mixed-valence state to show high electron
conductivity.1 Thus, the CT complexes composed of TCNQ have
attracted much attention toward electronic applications, e.g.,
conductors, semiconductors, and superconductive materials.2
TCNQ can form anion radical species with various metal and
organic counter cations.3 The TCNQ anion radicals work as an
electron donor, and consequently the mixture of neutral TCNQ
and TCNQ anion radicals can construct conductive stacking
structures with mixed-valence state.4,5 On the other hand,
their brittleness derived from the crystalline character
prohibits their practical applications.6 To overcome this
problem, polymeric cations such as ionenes and poly(N-
methylvinylpyridinium bromide) are applied for the formation
of CT complexes.7 After the counter anions of these polymeric
cations are replaced by TCNQ anion radicals, conductive films
are produced by casting with the neutral TCNQ. However,
formation of continuous conducting pathway derived from the
TCNQ in a mixed-valence state is hardly achieved because of
their random coil structures, which lead to a decrease their
conductive CT materials.
Polyhedral oligomeric silsesquioxanes (POSSs) of the
formula R8Si8O12, and an octasilicate (OS) of the formula
(Si8O20)8- have attracted much attention because their
polyhedral inorganic cores can inhibit molecular motion of
their organic branches and arrange the branches in a radial
fashion, leading to form the globular architectures covered
with terminal groups.8 Thanks to their rigid inorganic cores,
POSS- and OS-core dendrimers contribute improvement of
mechanical and thermal properties. Because of these aspects,
the POSS- and OS-core dendrimers terminated with functional
units are ideal candidates for the construction of solid
materials which are well designed in nano-level.9 Recently, we
have reported single component free-standing films based on
the OS-core dendrimers bearing 16 aromatic groups as the
peripheral units.10 The OS-core dendrimers are suitable for the
study of solid state dendrimer materials due to their film-
formability.
In this work, we synthesized an OS-core dendrimer having
imidazolium cations as terminal groups, of which counter
cations are TCNQ anion radicals (OS-mimTCNQ). The TCNQs in
a mixed-valence sate were prepared on the peripheries of the
OS-core dendrimer by the addition of the neutral TCNQ, and
conductive casting films were fabricated on substrates (Figure
1). The casting films with several tens of micron thickness
showed relatively high electron conductivity. We found that
the casting film with the same feed molar ratio between the
neutral TCNQ and the terminated imidazolium moiety showed
the highest conductivity.
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R
N
R
N
d = 13.4 Å
the mixture under reflux condition for 2 h, the residue was
washed with diethyl ether (5 ml × 4 times). The yield of LiTCNQ
was 0.498 g (96%). UV λmax (acetonitrile): 421, 744, 843 nm. FT-
IR (KBr): υ = 2208, 2194, 2180, 1509, 826 cm-1.
X-
X-
X-
N
N
R
X-
R
N
N
O
O
O
O
O
O
N
O
N
Si
Si
O
O
O
Si
Si
Si
Si
O
O
O
O
O
O
O
O
O
X-
X-
O
O
O
O
R
R
Si
Si
Si
Si
Si
Si
O
O
N
O
O
O
O
N
O
N
N
O
O
Si
O
O
O
O
O
Si
O
X-
N
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
R
X-
R
O
O
O
Si
Si
Si
O
O
O
Si
Si
Si
Si
Si
O
N
N
N
O
O
O
O
Si
O
Si
O
O
O
Si
Si
O
O
O
O
O
X-
O
O
Si
Si
O
O
O
O
O
Si
X-
N
O
Si
Si
Si
Si
O
O
Si
Si
O
Si
O
O
O
R
O
O
O
O
Si
O
O
O
N
O
R
O
N
O
Si
N
Si
Si
Si
Si
Si
Si
O
O
Si
O
O
O
O
O
O
O
Si
X-
O
O
O
O
O
O
N
X-
R
N
Si
Si
Si
Si
Si
O
Si
Si
O
R
N
O
O
O
O
O
O
O
N
O
Si
O
Si
Si
Si
Si
O
O
O
O
O
O
O
X-
N
O
O
X-
R
N
N
Bromoethyl-terminated OS-core dendrimer (OS-Br)
N
R
N
N
X-
X-
N
N
R
R
OS-SA (0.50 g, 0.23 mmol), 2-bromoethanol (0.14 ml, 1.98
mmol), and a catalytic amount of 4-dimethylaminopyridine
(DMAP) (25.0 mg, 0.21 mmol) were dissolved in 1,4-dioxane
(30 mL). The reaction was allowed to proceed for 12 h at 70 °C,
then concentrated by rotary evaporator at 40 °C and dried
under vacuum at ambient temperature. The residue was
further dissolved in dichloromethane (30 mL). To this solution,
2-bromoethanol (0.14 ml, 1.98 mmol), DMAP (30.0 mg, 0.27
mmol), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDAC) (0.81 g, 4.21 mmol) were added. After
stirring the mixture at room temperature under N2 for 16 h,
the reaction mixture was purified by separating funnel with
1M HCl, aqueous saturated NaHCO3, brine, then dried over
anhydrous MgSO4, and evaporated under vacuum. The residue
was dissolved in CHCl3 and purified by selective precipitation
NC
NC
CN
CN
d = 3.1 Å
X- =
•
Neutral TCNQ
Mixed valent state
TCNQ
TCNQ anion radical
Figure 1. Schematic image of the construction of the conductive pathway in OS-core
dendrimer network.
Experimental
Materials
All solvents and chemicals were of reagent-grade quality
and used without further purification. 1-Methyl-3-n-
PmimBr),11
12
propylimidazolium
bromide
(
and
octakis(propenylsuccinicanhydrido)octasilicate (OS-SA
)
were
prepared using the methods in the literatures
.
1
from methanol. The yield of pure OS-Br was 0.50 g (53%). H-
Measurement
NMR (CDCl3): δ 0.055 (s, 6H, -SiCH3), 0.54 (m, 2H, -SiCH2-), 1.29
(quint, 2H, -SiCH2CH2-), 1.49 (m, 1H, -SiCH2CH2CH-), 1.63 (sext,
1H, -SiCH2CH2CH-), 2.44-2.68 (m, 2H, -OCOCH2-), 2.81 (m, 1H, -
OCOCH-), 3.45 (m, 4H, -COOCH2-), 4.31 (m, 4H, -COOCH2CH2-)
ppm. 13C-NMR (CDCl3): δ 0.0005, 17.73, 20.77, 29.03, 29.18,
35.68, 35.94, 41.12, 64.31, 171.62, 174.49 ppm. 29Si-NMR
(CDCl3): δ 12.74 (SiCH2-), -107.49 (-OSiO-) ppm. MALDI-TOF MS
(m/z): calcd for C104H184Br16O52Si16K [M+K]+: 4033.5; found:
4034.0.
1H (400 MHz), 13C (100 MHz) and 29Si (80 MHz) nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker
DPX-400 spectrometer (Bruker Biospin GmbH, Rheinstetten,
Germany). Ultraviolet-visible (UV-vis) spectra were recorded
on a Jasco spectrophotometer V-670 KKN (Jasco, Tokyo,
Japan). Fourier transform infrared spectra (FT-IR) were
obtained on a JASCO FT/IR-4100 spectrometer (JASCO, Tokyo,
Japan) using KBr pellets. Matrix assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF-MS)
was conducted on a Bruker Autoflex II instrument (Bruker
Daltonics, Billerica, MA, USA) using dithranol matrix. The
morphologies of the casting films were observed using an EV-
8800 scanning electron microscope (SEM) (KEYENCE, Osaka,
Japan) and a JSM-7600F field emission SEM (FESEM) (JEOL Ltd,
Tokyo, Japan) with X-ray spectrometry (EDX) of X-max
(OXFORD Instruments, Oxfordshire, UK). Electrical conductivity
Imidazolium bromide terminated OS-core dendrimer (OS-mimBr)
OS-Br (1.00 g, 0.25 mmol) was dissolved in 1-
methylimidazole (1.60 ml, 20.0 mmol). After stirring the
reaction mixture at room temperature for 24 h, the mixture
was poured into excess ethyl acetate. The resulting precipitate
was dissolved in methanol and purified by selective
precipitation from ethyl acetate. The yield of pure OS-mimBr
1
of the casting samples was measured by
a four-probe
was 0.98 g (74%). H-NMR(DMSO-d6): δ 0.090 (s, 6H, -SiCH3 ),
technique at room temperature using Loresta-EP MCP-T360
(Mitsubishi Chemical Co.). Powder X-ray diffractometry (XRD)
0.51 (m, 2H, -SiCH2-), 1.22 (m, 2H, -SiCH2CH2-), 1.42 (m, 1H, -
SiCH2CH2CH2-), 1.50 (m, 1H, -Si CH2CH2CH2-), 2.51 (m, 2H, -
OCOCH2-), 2.73 (m, 1H, -OCOCH-), 3.93 (s, 6H, -NCH3), 4.30-
4.51 (m, 8H, -COOCH2CH2-), 7.83 (m, 4H, -NCH=CHN-), 9.41 (s,
2H, -NCH =N-) ppm.13C-NMR (DMSO-d6): δ -0.0749, 17.23,
20.27, 31.18, 35.34, 36.37, 60.23, 62.77, 64.43, 123.06, 124.04,
137.62, 171.76, 174.32 ppm. 29Si-NMR (DMSO-d6): δ 14.18, -
108.48 ppm.
studies were performed on
diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2
mode at room temperature. The 2 scan data were collected
at 0.01˚ intervals and the scan speed was 5˚ (2 )/min. The
a Rigaku Smartlab X-ray
θ/θ
θ
θ
grazing incidence X-ray diffraction (GI-XD) was carried out on
RIGAKU X-ray Diffractometer SmartLab with in-plane and out-
of-plane geometries (CuKα, 40 kV/30 mA, incidence angle α:
0.15°, 2θ: 3° to 40°).
Imidazolium-TCNQ anion radical terminated OS-core dendrimer
(OS-mimTCNQ)
Lithium TCNQ salt (LiTCNQ)
OS-mimBr (0.300 g, 5.65 × 10-2 mmol) was dissolved in
water. An aqueous solution of LiTCNQ (0.286 g, 1.37 mmol)
was added to the dendrimer aqueous solution. The deep blue
dendrimer precipitated out of solution. The precipitate was
washed several times with water, until no residual LiTCNQ was
detected with the FT-IR spectrum. The yield of OS-mimTCNQ
By using the method of the literature,13 lithium TCNQ salt
(LiTCNQ) was prepared by reaction of TCNQ with lithium
iodide. TCNQ (0.504 g, 2.47 mmol) was dissolved in acetonitrile
(50 ml) at 40 °C under N2. To this solution, lithium iodide (1.00
g, 7.50 mmol) in acetonitrile (3.0 ml) was added. After stirring
2 | J. Name., 2012, 00, 1-3
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was 0.29 g (70%). UV λmax (acetonitrile): 421, 744, 843 nm. FT- Finally, the imidazolium-TCNQ anion radical terminated OS-
IR (KBr): υ = 2180, 2155, 1733, 1576, 1505, 1353, 1171, 1079 core dendrimer OS-mimTCNQ was prepared by anion
cm-1. Anal. Calcd. for C380N96O52H344Si16: C, 59.26; H, 4.75; N, exchange reaction of OS-mimBr with lithium TCNQ salt
(
)
18.43%. Found: C, 59.50; H, 4.06; N, 20.18%.
(LiTCNQ) in water. After a LiTCNQ aqueous solution was added
to the aqueous dendrimer solution, precipitates were isolated
by suction filtration and washed with water for several times.
1-Methyl-3-n-propylimidazolium TCNQ anion radical (PmimTCNQ)
PmimTCNQ was obtained using PmimBr and LiTCNQ by The FT-IR spectrum of the precipitates detected no band at
following the similar method for OS-mimTCNQ. The yield of 2208 cm-1 derived from the CN stretching band in LiTCNQ
,
pure PmimTCNQ was 69%. UV λmax (acetonitrile): 421, 744, suggesting no residual LiTCNQ was included in the product.
843 nm. FT-IR (KBr): υ = 2191, 2175, 2152, 1587, 1509, 1362, The FT-IR spectrum of OS-mimTCNQ showed a new CN
1182, and 1164 cm-1. Anal. Calcd. for C19N6H17: C, 69.28; H, stretching band at 2180 cm-1, suggesting that the cation
5.20; N, 25.51%. Found: C, 69.26; H, 5.22; N, 25.47%.
exchanging to the TCNQ anion radical (Figure 2).3b,c,14 Other
major bands were also observed at 1730 and 1073 cm-1 due to
the ester and O–Si–O bonds, respectively. In addition, no
Preparation of the calibration curve for the TCNQ anion radical
The calibration curve for the determination of the degree of desymmetrization and split of the stretching band of O–Si–O
introduction of the TCNQ anion radical in OS-mimTCNQ were were detected, indicating that there was no decomposition of
prepared by using the absorbance of PmimTCNQ at 843 nm in the OS core.
acetonitrile solutions with concentrations of 5.95, 11.9, and
5.95 × 10-7 M. The calibration equation was Y = 48556 C and
Br
Br
Br
correlation coefficient (R2) of 1.
O
O
O
Br
O
O
O
O
O
O
O
O
O
O
O
Preparation of cast films
Br
O
Si
O
Si
O
1) 2-Bromoethanol
DMAP
1,4-dioxane
12 h, 80 °C
Si
Si
Br
Si
Si
O
O
O
O
O
O
O
O
Br
O
O
O
Si
O
Si
Si
O
Si
O
Si
Si
O
Si
The acetonitrile solution of neutral TCNQ was added to the
acetonitrile solution of OS-mimTCNQ at 50 °C. The mixture
solutions were casted on glass substrates and subsequent
drying at 40 °C for 3 h.
O
O
O
O
O
O
Br
O
O
O
O
O
Si
Si
O
O
O
O
O
O
O
O
O
O
O
O
Si
O
O
Si
Si
O
O
Si
Si
O
O
2) 2-Bromoethanol
DMAP, EDAC
CH2Cl2
O
O
O
O
O
Si
O
Si
O
O
O
O
O
O
O
Br
Si
Br
O
O
Si
Si
Si
O
O
16 h, r.t.
O
O
O
O
Br
Br
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Br
OS-SA
Br
Br
N
Br
Br
OS-Br (53%)
N
Br
N
N
Br
Results and discussion,
N
Br
N
N
N
O
O
N
O
O
O
O
Synthesis of imidazolium-TCNQ anion radical terminated OS-core
dendrimer (OS-mimTCNQ)
O
Br
N
O
Br
N
N
O
Si
Si
Si
O
O
O
O
N
N
O
O
O
Si
O
O
O
Br
Si
Si
O
Si
The OS-core dendrimer with imidazolium-TCNQ anion
radicals (OS-mimTCNQ) was synthesized from octakis(propenyl
succinicanhydrido)OS (OS-SA) by the three step reactions
(Scheme 1). First, the bromoethyl-terminated OS-core
Br
O
N
O
O
N
O
O
Si
1-Methylimidazole
24 h, r.t.
O
O
O
O
O
Si
O
O
O
Si
O
O
Si
N
O
O
N
Br
Br
O
O
O
N
N
Si
Si
O
O
O
O
N
N
Br
O
N
N
O
Br
O
O
dendrimer (OS-Br
)
was synthesized via the ring-opening
O
O
O
O
Br
Br
N
N
N
N
reaction and subsequent condensation reaction with 2-
1
N
N
N
bromoethanol. OS-Br was identified based on H, 13C, and 29Si-
Br
Br
N
OS-mimBr (74%)
CN
NC
NC
CN
NMR, and MALDI TOF-MS data (Figures S1-4). MALDI TOF-MS
spectrum of OS-Br showed the presence of the peak at 4034.0
m/z, which agreed with the molecular mass of the fully
functionalized dendrimer including potassium (4033.5 g·mol-1).
CN
NC
NC
CN
N
NC
NC
CN
N
NC
CN
CN
NC
N
N
NC
N
N
CN
N
NC
NC
O
O
N
O
O
O
O
CN
CN
O
1
NC
N
N
CN
In the H-NMR spectrum for OS-Br, however, the introduction
O
CN
N
NC
NC
O
Si
N
ratio of the ester groups was estimated to be 84% by
comparing the integration ratio of the –CH2CH2Br with that of
the –SiCH3. The next step involved the ionization of the
terminal group of OS-Br using 1-methylimidazole to obtain the
Si
O
O
CN
O
CN
CN
O
N
N
O
O
O
Si
CN
NC
O
Si
Si
O
O
Si
O
N
O
O
N
O
O
Si
O
NC
O
LiTCNQ
H2
O
O
O
O
Si
O
O
O
Si
Si
O
O
O
Si
N
O
O
N
CN
CN
NC
O
O
O
N
N
NC
NC
Si
NC
Si
O
O
CN
CN
O
O
N
N
imidazolium-bromide terminated OS-core dendrimer
(OS-
CN
CN
O
N
N
O
CN
NC
NC
mimBr), which was characterized based on 1H, 13C, and 29Si-
NMR spectra (Figures S5-7). The 29Si-NMR spectrum of OS-
mimBr showed two peaks corresponding to the silicate core
unit and the dimethylsilyl unit (Figure S7), and no other peaks
were detected, indicating no decomposition of the OS-core.
O
O
NC
O
O
O
O
CN
CN
CN
N
N
N
CN
N
NC
N
NC
N
NC
NC
CN
CN
N
N
CN
CN
CN
NC
NC
CN
CN
CN
OS-mimTCNQ (70%)
Scheme 1 Synthesis of OS-mimTCNQ.
1
The H-NMR spectrum of OS-mimBr showed that integration
ratio of the protons on the N-methyl groups was 84%
The UV-vis spectrum of a acetonitrile solution of OS-
mimTCNQ shows three characteristic peaks at 421, 744, and
compared with integrations from the –SiCH3 (Figure S5).
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843 nm, which are assigned to the TCNQ anion radical (Figure molar ratios). The mixture solutions were casted on glass
3).3c The degree of the introduction of the TCNQ anion radical substrates and subsequently drying at 40 °C for 3 h. The film
in OS-mimTCNQ against the N-methyl groups was 95% thickness of the casting films was around 40 ꢀm as estimated
estimated by the calibration curve method using the by a micrometer caliper. The mixture solution for OS-CT1.0 was
absorbance of 1-methyl-3-n-propylimidazolium TCNQ anion spin-coated on a polyvinyl chloride film and found that the film
radical (PmimTCNQ) at 843 nm in the UV-vis spectra (Figure showed no cracking after bending the film (Figure S10).
S8). The EDX pattern of OS-mimTCNQ includes no peaks due to
The conductivity of the casting films was evaluated with a
Br (around 1.5 keV), indicating the absence of the unreacted four-probe technique as the average of the ten distinct points
terminal groups and the residual bromide anion species on the films. The conductivities of the casting films are
(Figure S9).
summarized in Table 1. Although the casting film of OS-
mimTCNQ showed low conductivity (< 10-6 S·cm-1), the casting
films with the neutral TCNQ showed the relatively high
electron conductivity. The casting film with the same feed
molar ratio between the neutral TCNQ and the terminated
imidazolium moiety
conductivity.
(
OS-CT1.0
)
showed the highest
Figure 2. (left) IR spectra of OS-mimTCNQ (a) and LiTCNQ (b), and (right) expanded
spectra around the CN stretching band
Figure 3. UV-vis spectrum of OS-mimTCNQ in CHCl3.
Table 1. Conductivity and thickness of OS-CTx.
Figure 4. UV-vis-NIR spectra of, OS-mimTCNQ (a), OS-CT0.8 (b), OS-CT1.0 (c), and OS-CT1.2
(d).
Casting
film
[TCNQ] /
Thickness
Surface
conductivity,
×105 Ω/-
0.32
Conductivity,
S/cm
Stacking structures with the mixed-valence state in the
casting films were confirmed by the absorption band around
2000 nm in the UV-vis-NIR absorption spectra (Figure 4).5,13 In
solid salts of TCNQ anion radical such as LiTCNQ, TCNQ dimer
gives absorption bands near 360 nm and 640 nm, and that an
intermolecular CT band is observed near 1100 nm.3c,12 The UV-
vis NIR spectrum of the casting film of OS-mimTCNQ showed
characteristic peaks at 380, 680, and 1100 nm assignable to
the TCNQ dimer and the intermolecular charge transfer
transition, respectively, while no absorption band of the
mixed-valence state stacking was detected (Figure 4a). In the
UV-vis-NIR spectra of OS-mimTCNQ with the neutral TCNQ,
the broad absorption peak in the region of 2000 nm was
observed, indicating the existence of the mixed-valence state
stacking in all the samples with the neutral TCNQ. The relative
intensities of the absorption band of the TCNQ dimer
decreased with increasing the X values from 0.8 to 1.0. This
result indicates that the growth of the mixed valence state
stacking from the charge transfer transition state against the
TCNQ dimer proceeded by increasing the feed ratio of the
[Imidazolium]a
, ꢀmb
OS-CT0.7
OS-CT0.8
OS-CT0.9
OS-CT1.0
OS-CT1.1
OS-CT1.2
OS-CT1.5
0.7
0.8
0.9
1.0
1.1
1.1
1.5
53
36
29
32
44
32
37
5.8×10-3
4.6×10-3
1.2×10-2
5.4×10-2
9.4×10-3
1.4×10-2
9.3×10-3
0.60
0.28
0.058
0.24
0.22
0.29
aThe feed molar ratios of the neutral TCNQ against the terminated imidazolium moiety
in OS-mimTCNQ. bEstimated by a micrometer caliper.
Preparation of the conductive casting films of OS-mimTCNQ with
neutral TCNQ
An acetonitrile solution of the neutral TCNQ was added to
an acetonitrile solution of OS-mimTCNQ at 50 °C. The feed
molar ratios of the neutral TCNQ against the terminated
imidazolium moiety in OS-mimTCNQ ([TCNQ]/[Imidazolium])
were used the value of from 0.7 to 1.5 (OS-CTx; X = the feed
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neutral TCNQ. On the other hand, in the case of the excess SEM analysis (Figure 6). The SEM image of the casting film of
feed ratio of the neutral TCNQ, the absorption bands of the OS-CT1.0 showed a smooth surface. On the other hand, in the
TCNQ dimer around 680 nm again appeared, suggesting the SEM image of OS-CT0.7, a rough surface was observed in
mixed valence state stacking from the charge transfer comparison to that of OS-CT1.0. In the case of OS-CT1.5, some
transition state against the TCNQ dimer is inhibited. These crystals were observed on its surface, suggesting the
results support that the casting film with the same feed molar crystallization of the excess neutral TCNQ. These observations
ratio between the neutral TCNQ and the terminated clearly show that the OS-core dendrimer can suppress the
imidazolium moiety
conductivity.
(
OS-CT1.0
)
showed the highest crystallization of the mixed-valence state TCNQ. However, the
addition of the excess neutral TCNQ is unfavorable because of
the formation of the insulating crystal.
To evaluate formation of nanostructures of the mixed-
valence state staking in the casting films, GI-XD was carried out
(Figure 7). The XRD pattern of OS-minTCNQ shows a broad
peak (Figure S12), suggesting an amorphous character. On the
other hand, the GI-XD profiles of all samples showed
characteristic peaks of 2θ at 10.8˚ (d-spacing = 8.2 Å) and 28.6˚
(d-spacing = 3.1 Å), corresponding to the plane spacing of
TCNQ stacking and interplanar distance of TCNQ species,
respectively.16 The diffraction peak of the octa-substituted
caged silsesquioxanes shows a characteristic peak of 2θ at
Figure 5. FT-IR spectrum of OS-CT1.0
.
around 6˚ to 9˚ dependent on sizes of the substituents.17 In the
present case, the diffraction peak at 6.6˚ showed an
anisotropic pattern, indicating that the OS-core dendrimer
arranged in the direction of the out-of-plane at 13.4 Å (Figure
In the FT-IR spectrum of OS-CT1.0, the CN stretching band
appeared as a broad peak in the region from 2120 to 2230 cm-
1
(Figure 5). In addition to the CN stretching bands of the
neutral TCNQ at 2225 cm-1 and the OS-mimTCNQ at 2180 cm-1
and 2155 cm-1 due to B1u and B2u modes, respectively,14 the
peak in the CN stretching region of OS-CT1.0 at 2193 cm-1 was
assignable, which may be assignable to partially charge
transfer TCNQ.15 In the case of other casting films of OS-
mimTCNQ with the neutral TCNQ, similar CN stretching bands
were observed and no obvious difference was observed in the
FT-IR analysis (Figure S11).
1). The diffraction peaks of 2θ higher than 10˚ show no
obvious difference, suggesting no anisotropic orientation for
the crystalline domains derived from neutral TCNQ and the
terminated TCNQ anion radical in OS-minTCNQ
.
Figure 7. GI-XD profiles of OS-CT0.7 (a), OS-CT1.0 (b), and OS-CT1.5 (c); (A) in-plane and (B)
out-of-plane.
Conclusions
We synthesized the imidazolium-TCNQ anion radical
terminated OS-core dendrimer (OS-mimTCNQ) and prepared
the conductive films by casting the mixture of OS-mimTCNQ
and the various amounts of the neutral TCNQ in acetonitrile on
the substrates. The casting films with several tens of micron
thickness showed the relatively high electron conductivity and
the construction of the mixed-valence state stacking in these
casting films was confirmed by UV-vis-NIR spectra. In
particular, the casting film with the same feed molar ratio
Figure 6. SEM images of OS-CT0.7 (a), OS-CT1.0 (b), and OS-CT1.5 (c).
The surface morphology of the mixed-valence TCNQ
constructed in the OS-core dendrimer films is observed by an
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17 D. B. Cordes, P. D. Lickiss and F. Rataboul, Chem. Rev., 2010,
110, 2081-2173.
between the neutral TCNQ and the terminated imidazolium
moiety (OS-CT1.0) showed the highest conductivity (5.4 × 10-2
S·
cm-1) in the present samples and good film formability. This
study suggests that the OS-core dendrimers are ideal
candidates as a scaffold for the construction of the continuous
conductive pathway. Thus, the OS-core dendrimer solid
materials have potential to be a platform for new series of
nanomaterials.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (No.2401)” (JSPS KAKENHI Grant Number
JP24102003) of The Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We thank Prof. Tsuyoshi
Kawai of Nara Institute of Science and Technology for
measuring MALDI-TOF-MS supported by Nanotechnology
Platform.
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