Oxidation of unsymmetrically substituted quaterthiophene with two terminal ferrocenyl groups

Article abstract of DOI:10.1246/cl.2009.924

An unsymmetrically substituted quaterthiophene with two terminal ferrocenyl groups was prepared as a long π-conjugated system. Electrochemical and spectroscopic studies were carried out to evaluate the electronic and/or electrostatic communication between

Full text of DOI:10.1246/cl.2009.924

924
Chemistry Letters Vol.38, No.9 (2009)
Oxidation of Unsymmetrically Substituted Quaterthiophene
with Two Terminal Ferrocenyl Groups
Masa-aki Satoꢀ and Hirokazu Kamine
Graduate School of Maritime Science, Kobe University,
5-1-1 Fukae-minami, Higashinada, Kobe 658-0022
(Received July 6, 2009; CL-090632; E-mail: msato@maritime.kobe-u.ac.jp)
An unsymmetrically substituted quaterthiophene with two
In the synthesis of the quaterthiophene derivatives 1–4, the
quaterthiophene moieties were prepared by the Migita–Kosuge–
Stille coupling of tributylstannyldimethoxyterthiophene or the
ꢁ-methyl derivative with 2-bromo-3-hexylthiophene or the ꢁ-
methyl derivative, and then the obtained quaterthiophenes were
iodized and finally coupled with ferrocenylzinc chloride. De-
tailed synthetic procedures are described in Supporting Informa-
tion.6
Oxidation of the quaterthiophene derivatives 1–4 was inves-
tigated by cyclic voltammetry (CV), square wave voltammetry
(SWV), and controlled potential coulometry (CPC). The electro-
chemical measurements were carried out in dichloromethane
solutions containing 2:0 ꢁ 10ꢂ3 M of each compound and
0.1 M of Bu4NClO4. The electrochemical measurements were
referenced against ferrocenium/ferrocene (Fcþ/Fc) as the inter-
nal standard. The oxidation potentials (Eox) and coulometric n-
values (na) of the quaterthiophene derivatives and ferrocene
were obtained by the CV measurements (Figures 1a and S1–
S4)6 and CPC measurements, and summarized in Table 1.
The Eox of ferrocene was 0.00 and 0.06 V lower than the first
of a dimethyl end-capped quaterthiophene derivative 2, indicat-
ing that in ferrocenyl quaterthiophene derivatives 1, 3, and 4 the
ferrocene moieties are more oxidizable than the quaterthiophene
terminal ferrocenyl groups was prepared as a long ꢀ-conjugated
system. Electrochemical and spectroscopic studies were carried
out to evaluate the electronic and/or electrostatic communica-
tion between the two terminals. The one-electron oxidation
would occur at one ferrocene moiety specified due to the unsym-
metry of the oligothiophene moiety. The one-electron oxidizing
species extended into the oligothiophene moiety apparently in-
teracts with the other terminal ferrocene moiety.
Long ꢀ-conjugated systems are of interest for their electron-
ic and redox properties. In order to clarify and modify the prop-
erties, the introduction of redox-active terminals into the systems
is effective. Furthermore, multiple introduction enables investi-
gation of electronic and/or electrostatic communication between
the terminals via the conjugated systems.1 Ferrocene is well
known as a highly stable compound in both the oxidized and
neutral states and thus appropriate for the terminals. Several re-
searchers have prepared diferrocenyl conjugated systems,2–5
which should be regarded as ꢀ-conjugated systems with two ter-
minals. Unfortunately, a considerable number of the compounds
showed no communication between the two terminals; for exam-
ple 1,4-diferrocenylbenzene3a and diferrocenyldihexylterthio-
phene.3b All the diferrocenyl compounds are symmetric in struc-
ture and the two terminals are equivalent entirely. Groups substi-
tuted on ꢀ-conjugated systems significantly affect the electronic
structures of the systems, suggesting that one can control the ox-
idation processes or the oxidation states of the conjugated sys-
tems. In this paper, we report an unsymmetrically substituted
quaterthiophene with two terminal ferrocenyl groups (1) and
with one or no terminal ferrocenyl group (2–4) (Chart 1). The
two terminal ferrocenyl groups of 1 are nonequivalent because
of linkage with the unsymmetrically substituted quaterthiophene
moiety.
µA
µA
30
20
10
0
1
3
4
(b)
(a)
20
10
0
-10
-20
-0.5
0
0.5
1
-0.2
-0.1
0
0.1
V(vs. Fc/Fc+)
V(vs. Fc/Fc+)
Figure 1. (a) Cyclic voltammogram of 1. (b) Square wave vol-
tammograms of 1, 3, and 4.
Hex
S
OMe MeO
Table 1. Electrochemical data of ferrocene and 1–4
S
S
S
S
S
S
S
Fe
Fe
1
1 a
ox
2 a
ox
3 a
ox
4 a
E
n1a b
E
n2a c
E
E
ox
Compound
/V
/F molꢂ1 /V /F molꢂ1 /V /V
Hex
S
OMe MeO
Me
Me
S
Ferrocene
0.00
ꢂ0:05
0.06
ꢂ0:05
ꢂ0:01
1.0
—
—
1.1
1.2
—
—
2.3
—
2.2
2.2
— —
0.27 0.59
—
0.54
0.47
2
d
1
2
3
4
0.05
0.38
0.14
0.14
Hex
S
OMe MeO
—
—
—
Me
Fe
S
Fe
3
Hex
S
MeO
OMe
aEonx are the n-th oxidation potentials. bn1a are coulometric n-val-
ues for the first oxidation. cna2 are coulometric n-values up to the
second oxidation. dThe value is uncertain on account of overlap-
ping of waves.
Me
S
S
S
4
Chart 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.9 (2009)
925
moieties. The dimethyl 2 had two Eoxs corresponding to the suc-
cessive generation of the radical cation and dication, as previ-
ously reported for long oligothiophenes.7 The cyclic voltammo-
gram of the diferrocenyl 1 revealed four reversible redox waves
(Figure 1a). The two lower waves were observed opposite each
other at 0.00 V, the Eox of ferrocene. In addition, the na up to the
second oxidation was 2.3 F molꢂ1 (two-electron oxidation).
These results indicate that the two lower redox waves at
Eoxs = ꢂ0:05 and 0.05 V and the two higher waves at
Eoxs = 0.26 and 0.58 V are assigned to the oxidation of the
two ferrocene moieties and the two stepwise oxidation of the
quaterthiophene moiety, respectively. For the monoferrocenyl
derivatives 3 and 4, three reversible redox waves were detected
(Figures S3 and S4)6 and the nas up to the first were 1.1–
1.2 F molꢂ1 (one-electron oxidation). These results suggest that
1.25
1.00
0.75
0.50
0.25
1.25
1.00
0.75
0.50
0.25
(a)
(b)
0
2
3
0
1
1
3
2
Energy/eV
Energy/eV
Figure 2. Electronic absorption of (a) 1 and (b) 2: neutral (solid
line) and one-electron oxidized (dashed line).
crease of the ꢀ–ꢀꢀ transition band, accompanied by the appear-
ance of two strong bands at 1.6 and 0.7 eV. This spectral change
during one-electron oxidation resembles those of 2 and previous-
ly reported oligothiophene derivatives,7 being interpreted as the
generation of a cation radical (polaron). These results evidenced
the presence of an oxidizing species such as a cation radical in
the quaterthiophene moiety of one-electron oxidized 1. There-
fore, the one-electron oxidizing species does not remain in the
one ferrocene moiety of 1, but spreads over the ferrocene and
the quaterthiophene moieties, and then would coulombically in-
teract with the other ferrocene moiety.
In summary, an unsymmetrically substituted quaterthio-
phene with two terminal ferrocene groups, 1, was prepared as
a long ꢀ-conjugated system with two redox active terminals.
The electrochemical and optical properties were compared with
those of a series of the quaterthiophene derivatives, 2–4. The re-
sults show that the one-electron oxidation of 1 would occur at the
ferrocene moiety attached to the end methoxythiophene ring and
the resulting oxidizing species extends to the neighboring qua-
terthiophene moiety. The extended oxidizing species would
electrostatically interact with the other terminal ferrocene moie-
ty. The present results demonstrate the effectiveness of substitu-
ents on the quaterthiophene moiety for controlling the oxidation
process in long conjugated systems having terminals.
the lowest redox waves (at Eox ¼ ꢂ0:05 V for
3 and
Eox ¼ ꢂ0:01 V for 4) and the two higher waves (at Eoxs =
0.14 and 0.54 V for 3, and Eoxs = 0.14 and 0.47 V for 4) are at-
tributed to the oxidation of the ferrocene moieties and the two
stepwise oxidation of the quaterthiophene moieties, respectively.
Figure 1b displays the square wave voltammograms of 1, 3,
and 4 in the voltage range from ꢂ0:20 to 0.15 V. The peak po-
tentials of the waves agreed within limits of error with the cor-
responding Eoxs obtained by CV. The waves attributable to the
oxidation of the ferrocene moieties in 3 and 4 were observed
at ꢂ0:05 and ꢂ0:01 V, respectively. Both the peak potentials
are shifted from the Eox of ferrocene, 0.00 V, and the degrees
of the shifts are apparently different. These results demonstrate
that the intensity of the interaction between the two neighboring
moieties, the ferrocene moiety and the quaterthiophene moiety,
would depend on the substituted position of the ferrocenyl group
on the unsymmetric quaterthiophene moiety; the ꢁ-position of
the end methoxythiophene ring for 3 and the ꢁ-position of the
end hexylthiophene ring for 4. The voltammogram of 1 shows
two waves at ꢂ0:05 and 0.05 V assignable to the oxidation of
the two ferrocene moieties. The peak potential of the one wave
(at ꢂ0:05 V) agrees with that of the wave due to the ferrocene
moiety in 3, however, the peak potential of the other (at
0.05 V) is clearly higher than that of the wave due to the ferro-
cene moiety in 4 (at ꢂ0:01 V). That is, the two peak potentials,
the Eoxs, of the ferrocene moieties in 1 are not completely con-
sistent with the combination of the peak potentials, the Eoxs, of
the ferrocene moieties in 3 and 4. The inconsistence of the poten-
tials leads us to infer that the two terminal ferrocenyl groups in 1
interact with each other. Further, the agreement of the first Eox of
the ferrocene moiety in 1 and the Eox of the ferrocene moiety in 3
implies that the one-electron oxidation of 1 occurs at the ferro-
cene moiety that is attached to ꢁ-position of the end methoxy-
thiophene ring and the interaction between the ferrocene moie-
ties in 1 does not take place in the neutral state but in one-elec-
tron oxidized state.
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4
5
To elucidate the oxidized state of 1, a stoichiometric amount
of FeCl3 was added to a dichloromethane solution containing
2:0 ꢁ 10ꢂ3 M of each 1 and 2 and then the chemically oxidized
solutions were subjected to electronic absorption spectral analy-
sis. Figure 2 shows the spectra of 1 and 2 in neutral and one-elec-
tron oxidized states. The spectrum of neutral 1 has a ꢀ–ꢀꢀ tran-
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6
7
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The one-electron oxidation of 1 resulted in a remarkable de-
Published on the web (Advance View) August 22, 2009; doi:10.1246/cl.2009.924