Synthesis and structural, electronic, optical and FET properties of thiophene-pyrrole mixed hexamers end-capped with phenyl and pentafluorophenyl groups

Article abstract of DOI:10.1039/c1jm12966j

α-Linked thiophene-pyrrole mixed hexamers 3a-5a end-capped with two phenyl, two pentafluorophenyl and both phenyl and pentafluorophenyl units were newly synthesized, and their structural, electronic, optical and FET properties were investigated. Structura

Full text of DOI:10.1039/c1jm12966j

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PAPER
Synthesis and structural, electronic, optical and FET properties of
thiophene–pyrrole mixed hexamers end-capped with phenyl and
pentafluorophenyl groups†
Tohru Nishinaga, Toshihiko Miyata,a Masaki Tateno,a Masahide Koizumi,a Masayoshi Takase,a
a
*
a
b
Masahiko Iyoda, Norihito Kobayashi and Yoshihito Kunugi
b
*
Received 27th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1jm12966j
a-Linked thiophene–pyrrole mixed hexamers 3a–5a end-capped with two phenyl, two
pentafluorophenyl and both phenyl and pentafluorophenyl units were newly synthesized, and their
structural, electronic, optical and FET properties were investigated. Structural analyses by means of
X-ray crystallography and theoretical calculations showed that the p-system of the oligomer chains is
twisted due to the steric repulsion between the hexyl group of the pyrrole unit and the adjacent
thiophene moiety. From comparison of the absorption maxima and redox potentials, end-capping with
phenyl and pentafluorophenyl units did not affect the HOMO–LUMO gap for the thiophene–pyrrole
hexamer, and the multiple fluorine substituents slightly decreased both HOMO and LUMO levels by
the inductive effects of the electronegative fluorine atoms. Although the diphenyl derivative 3a with
a herringbone-type packing structure did not serve as an active layer for FET devices, a unique crystal-
to-crystal phase transition with a color change from yellow to red was observed upon heating. In
contrast to the herringbone-type packing structure of 3a, the unsymmetrical phenyl–pentafluorophenyl
derivative 5a showed a partially p-stacked structure, most likely due to dipole–dipole interactions. The
p-stacked structure of 5a caused a good hole mobility of 4.1 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 in a single crystal FET
device.
have also been synthesized, and some of them have been applied
Introduction
to electronic devices. Recently, we reported the synthesis, prop-
erties and FET performance of mixed oligomers composed of
thiophene–furan 19b,c and thiophene–pyrrole 2.11a They were
shown to serve as good p-type semiconductors comparable to
oligothiophenes.
Linear p-conjugated oligomers composed of five-membered
heteroaromatics have been extensively explored because of their
superior optical and charge transporting properties.1 In partic-
ular, oligothiophenes2,3 are representative p-conjugated oligo-
mers, partly because of their synthetic availability in contrast to
the limited synthetic access to other oligomers, such as oligo-
pyrrole4 and oligofuran.5 Thus, among the oligomers, oligo-
thiophenes have predominantly been utilized for organic
electronics2 such as organic light-emitting diodes6 and organic
field-effect transistors (FET).7
Concerning the performance of organic electronics, the
molecular packing structure is a critical issue.14 In most cases,
unsubstituted p-conjugated molecules crystallize into a layered
herringbone packing.15 However, a p-stacked structure is
considered advantageous in that it provides effective p–p inter-
actions that facilitate charge-carrier transport.15 To generate
such a p-stacked structure by crystal engineering, the introduc-
tion of phenyl and perfluorophenyl groups may be effective, as
p-stacked structures have been realized in various compounds
using phenyl–perfluorophenyl interactions.16,17 In addition, oli-
gothiophenes end-capped with pentafluorophenyl units at both
ends adopt a slipped, cofacial p-stacking motif.18 In this case, the
FET performance of the quaterthiophene was reported to be
better than other thiophene–benzene mixed oligomers with
herringbone packings.18b
As the related linear p-conjugated oligomers, oligothiophene-
based compounds mixed with other five-membered hetero-
aromatics, such as furan,8,9 pyrrole,10,11 silole12 and phosphole13
aDepartment of Chemistry, Graduate School of Science and Engineering,
Tokyo Metropolitan University, Hachioji, Tokyo, 192-0397, Japan.
E-mail: nishinaga-tohru@tmu.ac.jp; Fax: +81 42-677-2525
bDepartment of Applied Chemistry, Faculty of Engineering, Tokai
University, Hiratsuka, Kanagawa, 259-1292, Japan. E-mail: ykunugi@
keyaki.cc.u-tokai.ac.jp; Fax: +81 463-58-1211
† Electronic supplementary information (ESI) available: Fig. S1–S7.
CCDC reference numbers 795334 (3a) and 795335 (5a). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm12966j
In the present study, we designed and synthesized thiophene–
pyrrole mixed hexamers 3–5 end-capped with phenyl and pen-
tafluorophenyl groups. We chose the thiophene–pyrrole hexamer
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with N-hexyl groups for improving solubility as the core olig-
omer because hexyl end-capped derivative 2b11a showed the best
FET performance out of 1–2. During the course of this study, we
found an unexpected unique thermal phase transition in single
crystals of the diphenyl derivative 3a. Also the unsymmetrical
phenyl–pentafluorophenyl derivative 5a was found to show
a partially p-stacked structure, which gave good FET charac-
teristics in the crystalline state. Here we report these results along
with the results of synthesis and the basic structural and elec-
tronic properties of 3–5.
preparation of not only the bis-pentafluorophenyl derivative 4a
but also the unsymmetrical phenyl–pentafluorophenyl derivative
5a (Scheme 2). Thus, the diiodo derivative 8 was synthesized by
the reaction of dilithiated 6 with iodine. Compound 8 was
monolithiated with n-butyllithium, followed by the reaction with
hexafluorobenzene to give the monoiodo-pentafluorophenyl
derivative 9. The copper-mediated oxidative homocoupling of 9
gave 4a, while cross-coupling of 9 with stannylated 7 afforded 5a.
Structural and electronic properties
The basic structural and electronic properties of 3–5 were
assessed by absorption spectroscopy (Fig. S1†), cyclic voltam-
metry (Fig. S2†), and theoretical calculations. The results of the
longest absorption maxima in dichloromethane and redox
potentials of 3a–5a are shown in Table 1, and the calculated
HOMO and LUMO levels of 3–5 together with the energetics
and dipole moments at the B3LYP/6-31G(d) level are summa-
rized in Table 2.
As shown in Table 1, the absorption maxima of 3a–5a are
almost equal. In addition, the values (400–402 nm) are also
identical to the absorption maxima of 2b (405 nm),11a which has
hexyl substituents at both ends instead of the phenyl groups.
Thus, the HOMO–LUMO gap of the thiophene–pyrrole
hexamer is not affected by the phenyl and pentafluorophenyl
substituents irrespective of the p-extension with phenyl groups.
However, the oxidation potential of 3a–5a shows a slight positive
shift when the number of pentafluorophenyl units is increased,
indicating that the HOMO level is lowered by the inductive
electron-withdrawing effect of the fluoro-substituents. When
almost the same HOMO–LUMO gap for 3a–5a is taken into
consideration, it can be concluded that the fluoro-substituents
slightly decrease not only the HOMO but also the LUMO levels
to the same extent due to the cumulative electron-withdrawing
effect.
Results and discussion
Synthesis
For the synthesis of thiophene–pyrrole hexamer end-capped with
phenyl units at both ends, i.e. 3a, copper-mediated oxidative
homocoupling of the corresponding precursor 7 was conducted.
The precursor 7 was prepared by Stille coupling between
iodobenzene and the stannylated compound of mixed trimer 6
(R ¼ C6H13)11b as shown in Scheme 1.
As for the theoretical calculations, two all-transoid
conformers of 3b (Fig. 1) and 4b with C2 and Ci symmetry were
A few methods of introducing a pentafluorophenyl group to
the a-position of thiophene have been reported. One is nucleo-
philic aromatic substitution on hexafluorobenzene with a-lith-
iothiophene19 and the others are cross-couplings such as Stille18
and Suzuki–Miyaura20 couplings. Although the yield of the
nucleophilic aromatic substitution is usually moderate, we chose
the method because of the ability to use less expensive hexa-
fluorobenzene. In this route, we first introduced iodine at both
ends of 6 because the resulting monoiodo-derivative after the
introduction of a pentafluorophenyl group is useful for the
Scheme 1
Scheme 2
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Table 1 Observed longest-wavelength absorption and redox potential of
3a–5a
conformers are packed. One conformer (Fig. 2b) has a twisted
structure with all transoid connections, which resembles the
calculated Ci structure described above. The other (Fig. 2a) has
a nearly planar structure, which involves two energetically
unfavorable cisoid connections between the central thiophene
and the adjacent pyrrole rings. The all-transoid conformer is
stacked perpendicularly to the planar conformer, resulting in
a herringbone-type packing structure (Fig. 2c).
Compd lmaxa/nm E1/2ox1b/Vd E1/2ox2b/Vd Epcred1c/Vd Epcred2c/Vd
3a
4a
5a
402
400
402
0.13
0.20
0.16
0.21
0.30
0.25
—
ꢁ2.31
—
ꢁ2.68
—
—
a
In CH2Cl2. b In CH2Cl2. Supporting electrolyte: 0.1 M n-Bu4NPF6. c In
THF. Supporting electrolyte: 0.1
M n-Bu4NPF6. Cathodic peak
potentials of irreversible wave. d Versus Fc/Fc+.
first optimized. The calculations of the conformers including
cisoid connection(s) were not performed because the most stable
structure of 6 (R ¼ CH3) as well as bithiophene9a was found to
have transoid connections. In both the C2 and Ci conformers, the
thiophene and pyrrole rings are twisted due to the steric repulsion
between the N-methyl group of the pyrrole unit and the
b-hydrogen of the thiophene ring. For 5b, similar conformers in
which the thiophene–pyrrole core shows C2- and Ci-like
symmetry were also obtained as the optimized structures. In all
cases, the C2 (like) structures are slightly more stable than the Ci
(like) structure by ca. 0.1 kcal molꢁ1. However, the difference in
the structure does not affect the calculated HOMO and LUMO
levels. To estimate the effect of the alkyl chain length, the
calculations of 3a–5a with C2 (like) structures were also con-
ducted and the N-hexyl substituents were shown to slightly raise
both HOMO and LUMO levels by ca. 0.04 eV. As a result, the
calculated HOMO and LUMO levels both decrease when the
number of pentafluorophenyl units is increased, which is
consistent with the observed results.
Fig. 1 The top and side views of the calculated structures of 3b with (a)
C2 and (b) Ci symmetry.
When 3a was recrystallized from toluene, yellow microcrystals
and a very small amount of red crystals were obtained (Fig. S3†).
Interestingly, the yellow color of the microcrystals turned red at
Concerning the calculated dipole moment, the unsymmetri-
cally substituted compound 5a was shown to have a substantial
value with the parallel direction to the long axis of the oligomer.
The C2 structures of symmetrically substituted compounds 3a
and 4a were also predicted to have some dipole moment but the
direction was perpendicular to the oligomer chain. Thus, the
relatively large dipole moment in 5 may affect the packing
structure in the solid state as described below.
ꢂ
134 C. This color change is irreversible, and the red color lasts
after cooling. As shown in Fig. 3a, DSC analysis of the yellow
solid showed an endothermic peak at 131.1 ꢂC due to the melting
of the yellow solid and then an exothermic phase transition at
133.8 ꢂC. The next endothermic peak would be due to the melting
of the red crystals obtained by the phase transition as the red
crystals obtained from dichloromethane/methanol (see above)
showed an endothermic peak at the same temperature.
Furthermore, XRD analysis of the red solid obtained from the
annealing of the yellow microcrystals at 134 ꢂC (Fig. 3c) revealed
that the XRD pattern is quite similar to the calculated powder
pattern (Fig. 3d) obtained from X-ray crystallography of the red
X-Ray structures and thermal phase transition
A red single crystal of 3a suitable for X-ray structural analysis
was obtained by slow diffusion of methanol into a dichloro-
methane solution of 3a. In the crystal, two independent
Table 2 Calculated HOMO and LUMO levels, dipole moment and relative energy of 3–5
Compd
Point group
HOMO/eV
LUMO/eV
Dipole moment/D
Relative energy/kcal molꢁ1
3a
3b
3b
4a
4b
4b
5a
5b
5b
C2
C2
Ci
C2
C2
Ci
C1
C1
C1
ꢁ4.55
ꢁ4.59
ꢁ4.59
ꢁ4.78
ꢁ4.82
ꢁ4.82
ꢁ4.66
ꢁ4.70
ꢁ4.70
ꢁ1.66
ꢁ1.71
ꢁ1.72
ꢁ1.97
ꢁ2.01
ꢁ2.02
ꢁ1.86
ꢁ1.90
ꢁ1.90
0.72
1.09
0.0
1.34
0.60
0.0
2.95
3.08
3.12
—
0
0.12
—
0
0.09
—
0
a
a
b
0.07
Thiophene–pyrrole core takes C2-like conformation. b Thiophene–pyrrole core takes Ci-like conformation.
a
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As shown in Fig. 4, the absorption spectrum of the red crystals
of 3a in the solid state (red curve) is partly altered from that of
the yellow crystals (yellow curve), and a new shoulder band
around 520 nm appears. In the calculated absorption bands
using time-dependent (TD) DFT calculations at the B3LYP/6-
31G(d) level with the two conformers obtained from X-ray
crystallography, the longest absorption band (477 nm) of the
nearly planar conformer is 40 nm longer than that (438 nm) of
the twisted all-transoid conformer. These results suggest that the
yellow crystals consist only of a twisted conformer, and the
origin of the color change is considered to involve the confor-
mational change of 3a from a twisted conformer to the planar
structure. The packing structure of such a twisted conformer
would be less stable than that of the red crystal because the phase
transition is exothermic, even though the planar conformer is
considered to be energetically unfavorable in its single molecular
state.
A red, very thin needle of 4a was obtained from a mixed
solution of hexane and carbon disulfide at 5 ꢂC, but X-ray
structural analysis of the crystal was not successful due to the too
thin shape. On the other hand, an orange single crystal of 5a
suitable for X-ray structural analysis was obtained by slow
diffusion of hexane into a carbon disulfide solution of 5a. In the
crystal, the phenyl and pentafluorophenyl rings were disordered,
and the occupancy rates of each fluorine atom shown in Fig. 5
were 0.5, respectively. In the oligomer chain, the phenyl rings
were parallel to the next thiophene ring, while the thiophene and
pyrrole rings were twisted with transoid connections. As a result,
the torsion angle between the mean planes of the phenyl and
central thiophene rings was 66ꢂ.
In spite of the twisted oligomer chain of 5a, a partially
p-stacked structure is formed in which p-contacts are observed
between the central thiophene rings, with the shortest carbon–
Fig. 2 ORTEP drawings (50% probability) showing top and side views
of X-ray structures of 3a with (a) a planar and (b) a twisted structure, and
(c) a space filling drawing of their packing structure.
ꢀ
carbon length being 3.38 A, and between one of two phenyl rings
and the outer thiophene ring, with the shortest length being
ꢀ
3.52 A. This packing manner is in contrast to the herringbone-
type packing of 3a, and the introduction of a pentafluorophenyl
ring instead of a phenyl ring causes the stacking structure. The
stacking is not necessarily derived from the interaction between
electron-deficient pentafluorophenyl rings and electron-rich
thiophene rings, because the phenyl and pentafluorophenyl rings
are disordered. As shown in Table 2, 5a, in which the phenyl and
pentafluorophenyl rings are unsymmetrically arranged at both
ends of the thiophene–pyrrole hexamer, has a fairly large dipole
Fig. 3 (a) The DSC trace upon heating the yellow crystal of 3a, and the
XRD patterns and optical micrographs (inset) of 3a; (b) yellow micro-
crystals, (c) the red crystals after annealing of the yellow crystals and (d)
the calculated powder pattern using the data from X-ray crystallography
of 3a.
Fig. 4 Normalized UV–vis absorption spectra of the red crystals (red
curve) and the yellow crystals (yellow curve) of 3a in the solid state. The
results of TD-DFT calculations using the planar (red bars) and twisted
(yellow bars) structures are also depicted. The heights of the calculated
spectra reflect the calculated oscillator strengths.
single crystal of 3a, whereas the pattern is totally different from
that of the yellow solid (Fig. 3b). Thus, the color change is
ascribed to a unique single-crystal to single-crystal phase
transition.
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Fig. 6 Transfer (left) and output (right) characteristics of the 5a-based
single crystal FET device.
thin single crystals suitable for the FET devices were only
obtained for 4a and 5a. The source and drain electrodes were
painted on the crystal with carbon-paste. For 4a (Fig. S6†), the
single-crystal device showed a mobility (4.2 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1
)
one order of magnitude higher than the thin film device.
However, the mobility is still moderate for a single-crystal device.
These results suggest that the packing structure of 4a is not the
best for carrier transport among 3a–5a, even though the X-ray
structure of 4a could not be determined. Among the devices
fabricated in the present study, 5a showed the highest mobility
(4.1 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1) (Fig. 6). As described above, the central
thiophene rings of 5a, despite the twisted conformation, stack in
the single crystal principally due to dipole–dipole interactions. In
addition, the stacking direction is along the a-axis which corre-
sponds to the direction of the long axis of the single crystal
(Fig. S7†). Thus, the stacking direction is consistent with the
direction of the source–drain current and therefore the p-stack-
ing structure is considered to cause good carrier mobility in
a single crystal of 5a.
Fig. 5 (a) ORTEP drawings (50% probability) showing the X-ray
structures of 5a. Occupancy rates for disordered fluorine atoms are in
parentheses. (b) The packing structure of 5a.
moment with parallel direction to the long axis of the oligomer.
Thus, a dipole–dipole interaction is considered to play an
important role in the formation of the p-stacked structure, which
would cause an attractive interaction more effectively when the
phenyl and pentafluorophenyl rings are alternately arranged.
FET properties
Thin-film FET devices of 3a–5a were fabricated with a bottom-
contact configuration on Si/SiO2 substrates and the source and
drain electrodes were constructed with gold. The vacuum depo-
sition of these materials was difficult and the active layers were
prepared by spin-coating 0.4 wt% of chloroform solution.21
Among these thin films, 3a and 5a showed amorphous structures
while 4a formed a layer-by-layer structure with a tilt angle of ca.
55ꢂ, as revealed by XRD analysis (Fig. S4†). As a result, the 3a-
film did not show any FET characteristics and the 5a-film
exhibited a very poor mobility (Table 3, Fig. S5†). In contrast,
the 4a-film served as the active layer of the FET devices.
However, unlike oligothiophenes capped with pentafluorophenyl
units, which act as n-type FET semiconductors,18 4a showed only
p-type characteristics (Fig. S5†), and the hole mobility was
moderate (1.3 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1). The p-type characteristics are
due to the higher donor ability of pyrrole rings which raise the
HOMO level of the oligomer.
Conclusions
In this study, we designed and synthesized thiophene–pyrrole
mixed hexamers end-capped with phenyl and pentafluorophenyl
groups, i.e. 3a–5a. By comparing the structural and electronic
properties of 3a–5a, the introduction of phenyl and penta-
fluorophenyl rings was shown not to affect the single-molecular
properties of the oligomers, but the rings apparently altered the
oligomers’ packing structures in the crystal. As a result, the
diphenyl derivative 3a showed a unique thermal crystal-to-
crystal phase transition with a color change from yellow to red,
while the unsymmetrically substituted derivative 5a showed
a partially p-stacked structure in contrast to the herringbone-
type packing structure of 3a. This p-stacked structure of 5a
caused a good hole mobility up to 4.1 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 in
a single-crystal FET device which was higher than that of 4a.
Thus, it is expected that the unsymmetrical substitution of phenyl
and pentafluorophenyl groups, which induces a fairly large
dipole moment, causes a p-stacked structure even in a long
molecule such as oligothiophene. This strategy might open a new
way for the design of efficient organic semiconductors.
To shed light on the packing structure–FET properties rela-
tionship, single-crystal FET devices22,23 were also fabricated.
In the device structure, a very thin single crystal is ideal and such
Table 3 Field-effect mobility (m), on/off ratio, and threshold voltage
(Vth) for transistors of 3a–5a in thin film and single crystal
Compd
Active layer
m/cm2 Vꢁ1 sꢁ1
On/off ratio
Vth/V
Experimental
3a
4a
Thin film
Thin film
Single crystal
Thin film
Single crystal
Not active
1.3 ꢀ 10ꢁ4
4.2 ꢀ 10ꢁ3
<10ꢁ7
4.1 ꢀ 10ꢁ2
—
104
10
—
50
—
ꢁ2.8
General
18
—
5a
1H and 13C NMR spectra were recorded on a JEOL L-500
instrument. Chemical shifts are reported in ppm with reference to
31
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tetramethylsilane. Preparative gel-permeation chromatography
(GPC) was performed with a JAI LC-08 chromatograph equip-
ped with JAIGEL 1H and 2H columns. Electronic spectra were
recorded on a SHIMADZU UV–Vis–NIR scanning spectro-
photometer (Model UV-3101-PC). Cyclic voltammetry (CV) was
performed on a BAS-ALS620B electrochemical analyzer using
a standard three-electrode cell consisting of a glassy-carbon
working electrode, a Pt-wire counter electrode, and a Ag/AgNO3
reference electrode under argon atmosphere. The potentials were
calibrated with ferrocene as an external standard. Elemental
analyses were performed at the microanalysis laboratory of
Tokyo Metropolitan University.
6.99 (d, 2H), 6.39 (s, 4H), 4.24 (t, 4H), 1.70–1.62 (m, 4H), 1.24–
1.15 (m, 12H), 0.81 (t, 6H); 13C NMR (CDCl3) d 143.7, 136.5,
134.1, 134.1, 133.7, 128.8, 128.8, 128.3, 127.4, 126.5, 126.0, 125.5,
123.6, 123.2, 111.0, 110.9, 45.5, 31.2, 26.2, 22.6, 14.1. Anal calcd
for C48H48N2S4: C, 73.80, H, 6.19, N, 3.59, S, 16.42; found C,
73.75, H, 6.38, N, 3.63.
N-Hexyl-2,5-bis(5-iodo-2-thienyl)pyrrole (8). To a solution of
N-hexyl-2,5-di(2-thienyl)pyrrole (6 (R ¼ C6H13)) (5.07 g, 16.1
mmol) and TMEDA (5.27 mL, 35.4 mmol) in THF (50 mL),
n-BuLi in hexane (21.4 mL, 35.4 mmol) was added dropwise at
ꢁ78 ꢂC under N2. After stirring for 1 h, iodine (8.99 g,
ꢂ
Commercially available reagents were used as received.
Solvents were distilled from relevant drying agents prior to use.
N-Hexyl-2,5-di(2-thienyl)pyrrole (6 (R ¼ C6H13))10b,11b was
prepared according to the literature procedures.
35.4 mmol) was added to the mixture at ꢁ78 C. The reaction
mixture was allowed to warm to room temperature, stirred for 1
day and quenched with aqueous Na2S2O3. After extraction with
CHCl3, the organic layer was washed with brine, dried over
Na2SO4, and concentrated. The residue was purified by column
chromatography (SiO2, hexane–CH2Cl2) to give 8 (7.75 g, 14.4
mmol) in 89% yield as pale yellow crystals. Mp 57.9–60.0 ꢂC; 1H
NMR (CDCl3) d 7.21 (d, 2H), 6.73 (d, 2H), 6.28 (s, 2H), 4.05
(t, 2H), 1.52–1.52 (m, 2H), 1.20–1.10 (m, 6H), 0.82 (t, 3H); 13C
NMR (CDCl3) d 140.5, 137.0, 127.4, 127.2, 111.0, 72.6, 45.1,
31.0, 31.0, 26.0, 22.3, 13.9. Anal calcd for C18H19I2NS2: C, 38.11,
H, 3.38, I, 44.74, N, 2.47, S, 11.30; found C, 38.34, H, 3.39,
N, 2.39.
Synthesis
N-Hexyl-2-(5-phenyl-2-thienyl)-5-(2-thienyl)pyrrole (7). To
a solution of N-hexyl-2,5-di(2-thienyl)pyrrole (6 (R ¼ C6H13))
(4.00 g, 12.7 mmol) and TMEDA (2.3 mL, 15 mmol) in THF
(70 mL), n-BuLi in hexane (9.5 mL, 15 mmol) was added drop-
wise at ꢁ45 ꢂC under N2. After stirring for 1 h, trimethyltin
chloride in THF (15.2 mL, 15.2 mmol) was added to the mixture
ꢂ
at ꢁ60 C. The reaction mixture was allowed to warm to room
temperature, stirred for 1 day and quenched with water. After
extraction with CH2Cl2, the organic layer was washed with brine,
dried over Na2SO4 and concentrated. To the mixture of the
resultant stannane, Pd(PPh3)4 (0.618 g, 0.535 mmol) and iodo-
benzene (1.6 mL, 14 mmol), DMF (70 mL) was added under N2,
and the mixture was stirred for 2 days at 100 ꢂC. After filtration
through Celite, the reaction mixture was extracted with CH2Cl2,
and the organic layer was washed with brine, dried over Na2SO4
and concentrated. The residue was purified by column chroma-
tography (Al2O3, CHCl3) and recrystallization from CH2Cl2–
ethanol to give 7 (2.62 g, 6.69 mmol) in 53% yield as pale yellow
needles. 1H NMR (CDCl3) d 7.62 (m, 2H), 7.39 (m, 2H), 7.32 (m,
1H), 7.29 (m, 2H), 7.10–7.07 (m, 2H), 7.04 (m, 1H), 6.38 (m, 1H),
6.34 (m, 1H), 4.18 (t, 2H), 1.64–1.57 (m, 2H), 1.18–1.15 (m, 6H),
0.80 (t, 3H); 13C NMR (CDCl3) d 134.8, 134.3, 134.1, 128.8,
128.5, 128.3, 127.3, 127.1, 126.4, 125.8, 125.5, 125.1, 123.1, 110.8,
110.6, 45.3, 31.2, 31.2, 26.2, 22.5, 14.0. Anal calcd for
C24H25NS2: C, 73.61, H, 6.43, N, 3.58, S, 16.38; found C, 73.64,
H, 6.47, N, 3.65.
N-Hexyl-2-(5-iodo-2-thienyl)-5-(5-pentafluorophenyl-2-thienyl)
pyrrole (9). To a solution of 8 (3.48 g, 6.14 mmol) in THF
(60 mL), n-BuLiꢂ in hexane (3.7 mL, 6.1 mmol) was added
dropwise at ꢁ78 C under N2. After stirring for 10 min, hexa-
fluorobenzene (3.5 mL, 31 mmol) was added to the mixture at
ꢁ78 ꢂC. The reaction mixture was allowed to warm to room
temperature, stirred overnight and concentrated. The residue
was purified by column chromatography (SiO2, hexane) and
preparative GPC to give 9 (1.11 g, 1.82 mmol) in 30% yield as
pale yellow crystals. Mp 49.9–51.3 ꢂC; 1H NMR (CDCl3) d 7.51
(d, 1H), 7.23 (d, 1H), 7.11 (d, 1H), 6.75 (d, 1H), 6.41 (d, 1H), 6.32
(d, 1H), 4.14 (t, 2H), 1.60–1.57 (m, 2H), 1.21–1.17 (m, 6H), 0.81
(t, 3H); 13C NMR (CDCl3) d (except for C6F5) 140.7, 137.2,
130.4, 128.1, 127.7, 127.5, 125.4, 111.5, 111.4, 72.9, 45.4, 31.3,
31.2, 26.1, 22.5, 14.0. Anal calcd for C24H19F5INS2: C, 47.45, H,
3.15, F, 15.64, I, 20.89, N, 2.31, S, 10.56; found C, 47.67, H, 3.13,
N, 2.32.
Bis-pentafluorophenyl derivative (4a). To a solution of 8 (1.13 g,
1.86 mmol) in THF (10 mL), n-BuLi in hexane (1.1 mL,
1.9 mmol) was added dropwise at ꢁ78 ꢂC. After stirring for
15 min, CuCl2 (0.838 g, 6.28 mmol) was added to the mixture at
ꢁ78 ꢂC. The reaction mixture was allowed to warm to room
temperature. After stirring for 1 day, water and hydrazine
monohydrate were added and the mixture was extracted with
CH2Cl2. The organic layer was washed with brine, dried over
Na2SO4, and concentrated. The residue was purified by column
chromatography (Al2O3, hexane–CH2Cl2), preparative GPC and
recrystallization from CS2–hexane to give 4a (0.170 g, 0.177
mmol) in 19% yield as red crystals. Mp 148.2–152.3 ꢂC; 1H NMR
(CDCl3) d 7.53 (d, 2H), 7.17 (d, 2H), 7.14 (d, 2H), 7.00 (d, 2H),
6.44 (d, 2H), 6.40 (d, 2H), 4.24 (t, 4H), 1.66–1.64 (m, 4H), 1.19–
1.17 (m, 12H), 0.81 (t, 6H); 13C NMR (CDCl3) d 144.0 (dm),
Diphenyl derivative (3a). To a solution of 7 (0.787 g, 2.01
mmol) and TMEDA (0.33 mL, 2.2 mmol) in THF (10 mL),
n-BuLi in hexane (1.4 mL, 2.2 mmol) was added dropwise at 0 ꢂC
under N2. After stirring for 1 h, CuCl2 (0.325 g, 2.42 mmol) was
added to the mixture at ꢁ50 ꢂC. The reaction mixture was
allowed to warm to room temperature, stirred for 1 day and
quenched with aqueous NaHCO3. After extraction with CH2Cl2,
the organic layer was washed with brine, dried over Na2SO4, and
concentrated. The residue was purified by column chromatog-
raphy (Al2O3, hexane–CH2Cl2), preparative GPC and recrys-
tallization from CH2Cl2–ethanol to give 3a (0.368 g, 0.470 mmol)
in 47% yield as pale yellow needles. 1H NMR (CDCl3) d 7.63 (m,
4H), 7.40 (m, 4H), 7.30 (m, 4H), 7.16 (d 2H), 7.04 (d, 2H),
14964 | J. Mater. Chem., 2011, 21, 14959–14966
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139.9 (dm), 138.0 (m), 137.9 (dm), 136.8, 133.6, 130.6 (m), 129.1,
127.8, 126.4, 125.6 (m), 125.4, 123.8, 111.4, 111.1, 109.9 (m),
45.5, 31.1, 31.1, 26.1, 22.4, 13.9. Anal calcd for C48H38F10N2S4:
C, 59.99, H, 3.99, F, 19.77, N, 2.91, S, 13.35; found C, 59.97, H,
3.90, N, 3.01.
Crystal data for 3a. C48H48N2S4, M ¼ 781.12; triclinic, space
ꢁ
group P1; a ¼ 9.7580(12), b ¼ 12.2984(13), c ¼ 18.563(2),
3
ꢀ
a ¼ 99.104(3), b ¼ 103.488(2), g ¼ 102.840(2), V ¼ 2059.5(4) A ;
Dc (Z ¼ 2) ¼ 1.260 g cmꢁ3; T ¼ 293 K, 9385 collected reflections,
5894 reflections with I > 2s(I), R1 ¼ 0.055, wR2 ¼ 0.118.
Phenyl–pentafluorophenyl derivative (5a). To a solution of 7
(0.103 g, 0.264 mmol) and TMEDA (0.04 mL, 0.3 mmol) in
THF (3 mL), n-BuLi in hexane (0.2 mL, 0.3 mmol) was added
dropwise at ꢁ78 ꢂC under N2. After stirring for 1 h, trime-
thyltin chloride in THF (15.2 mL, 15.2 mmol) was added to the
Crystal data for 5a. C48H43F5N2S4, M ¼ 871.08; triclinic,
ꢁ
space group P1; a 5.5928(11), b ¼ 10.505(2), c ¼ 18.392(4),
3
ꢀ
a ¼ 104.347(4), b ¼ 93.216(4), g ¼ 90.447(4), V ¼ 1045.0(4) A ;
Dc (Z ¼ 1) ¼ 1.384 g cmꢁ3; T ¼ 293 K, 3852 collected reflections,
2855 reflections with I > 2s(I), R1 ¼ 0.046, wR2 ¼ 0.153.
ꢂ
mixture at ꢁ78 C. The reaction mixture was allowed to warm
to room temperature, stirred for 1 day and quenched with
water. After extraction with CHCl3, the organic layer was
washed with brine, dried over Na2SO4, and concentrated.
Preparative GPC separation gave crude stannane (87 mg, 0.016
mmol). To the mixture of the resultant stannane, 9 (79 mg,
0.17 mmol) and Pd(PPh3)4 (10 mg, 0.01 mmol), DMF (3 mL)
was added under N2. The mixture was stirred at 90 ꢂC over-
night and then water and hydrazine monohydrate were added.
After extraction with CH2Cl2, the organic layer was washed
with brine, dried over Na2SO4 and concentrated. The residue
was purified by short column chromatography (Al2O3, CHCl3),
preparative GPC and recrystallization from CH2Cl2–hexane to
give 5a (65 mg, 0.074 mmol) in 57% yield as orange crystals.
FET device preparation
Fabrication of thin-film FET devices. Heavily doped n+-Si (100)
wafers with a thermally grown insulating SiO2 layer (210 nm
thick) were used as 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 drain–
source channel length (L) and width (W) of 10 mm and 2.0 cm,
respectively. On the SiO2 surface with drain and source elec-
trodes, a thin film of 3a–5a was prepared by spin-coating of its
0.4 wt% chloroform solution at 2000 rpm.21
ꢂ
1
Fabrication of single-crystal FET devices. The top-contact-type
single-crystal FET devices were prepared by the crystal lamina-
tion technique.22,23 As substrates for the crystal lamination, we
used n+-doped Si wafer with 210 nm thick thermally grown SiO2,
which was covered with a thin film (ca. 300 nm) of polystyrene
(PS). Shiny, thin and defect-free 4a or 5a single crystals were
carefully selected and laminated on top of the substrates. Source
and drain electrodes were painted with a water-based solution of
colloidal graphite.
Mp 108.2–110.5 C; H NMR (CDCl3) d 7.63 (d, 2H), 7.52 (d,
1H), 7.40 (m, 2H), 7.30 (m, 2H), 7.17 (m, 2H), 7.14 (d, 1H),
7.05 (d, 1H), 7.00 (m, 2H), 6.42 (m, 4H), 4.24 (t, 4H), 1.65 (m,
4H), 1.18 (m, 12H), 0.81 (t, 6H); 13C NMR (CDCl3) d 143.9
(dm), 143.8, 139.9 (dm), 138.0 (m), 137.9 (dm), 136.9, 136.5,
134.2, 134.1, 134.0, 133.5, 130.5 (m), 129.1, 129.0, 128.3, 127.8,
127.5, 126.6, 126.4, 126.2, 125.6, 125.4, 123.8, 123.7, 123.3,
111.6, 111.3, 111.1, 111.0, 45.5, 45.4, 31.1, 31.0, 26.1, 26.0,
22.4, 22.4, 13.9, 13.9. Anal calcd for C48H43F5N2S4: C, 66.18,
H, 4.98, F, 10.90, N, 3.22, S, 14.72; found C, 66.22, H, 4.88,
N, 3.23.
The electrical properties of the FET devices were measured
using an Agilent 4155C semiconductor parameter analyzer under
vacuum at room temperature. The field-effect mobility (mFET
)
was calculated in the saturated regime of the drain current (Id)
Theoretical calculations
using the following equation,
The GAUSSIAN 0324 series of programs was used for all
calculations. All molecules were fully optimized using the hybrid
density functional at the B3LYP level of theory with the 6-31G
(d) basis set. At first, a preliminary conformer search for
compounds 3b–5b was performed without phenyl groups and
conformers with C2 symmetry were found to have a lower total
energy than that of Ci conformers. The entire molecular struc-
tures of 3b–5b were then optimized starting from the Ci and C2
conformers. The geometry optimization of 3a–5a was started
based on the optimized structure of 3b–5b.
Id ¼ (WCi/2L)mFET(Vg ꢁ Vth)2
where Ci is the capacitance of the insulator and Vg and Vth are the
gate and threshold voltages, respectively. The current on/off
ratio (Ion/Ioff) was determined from the maximum (Ion) and
minimum (Ioff) value of the Id at maximum Vg.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
X-Ray crystallography
X-Ray data were taken on a Bruker Smart APEX diffractometer
equipped with a CCD area detector with graphite-mono-
Notes and references
ꢀ
chromated MoKa radiation (l ¼ 0.71073 A). The structure was
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hydrogen atoms were refined anisotropically, and hydrogen
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