(CH2)4 Si
O
Si
i-PrMgCl⋅LiCl
1 equiv
[NiCl2(dppe)]
1.0 mol%
NMgCl·LiCl
(CH2)4 Si
Br
O Si
1b
S
60 °C, 24 h
n
THF, rt, 3 h
6
59%
H
S
1a
THF, 60 °C, 1 h
M
n = 10500, PDI = 1.13
(CH2)4 Si
O Si
[NiCl2(PPh3)(IPr)]
1.0 mol%
Scheme 3. Nickel-catalyzed debrominative GRIM polymer-
ization of 1b.
S
n
60 °C, 24 h
6
93%, Mn = 12300, Mw/Mn = 1.81
Scheme 2. Nickel-catalyzed dehydrobrominative polymeriza-
tion of 1a with Knochel-Hauser base.
pentamethyldisiloxy group into the obtained 5a and 5b was
carried out with pentamethyldisiloxane by hydrosilylation with a
platinum catalyst leading to 1a and 1b in >99% and 93% yields,
respectively.
(a)
(b)
(c)
Figure 1. Attempted dissolution of 1 mg of polythiophene
derivatives in 1 mL of hexane. (a) Polythiophene 6 bearing
siloxane moiety, Mn = 12300, PDI = 1.81; (b) poly(3-hexyl-
thiophene), Mn = 12500, PDI = 1.35; (c) poly(3-dodecylthio-
phene), Mn = 12200, PDI = 1.72.
The key for successful preparation of brominated thiophene
derivatives bearing a pentamethyldisiloxane moiety is the use of
3-halomethylated thiophene, which allowed treatment of the
allyl Grignard reagent to afford 3-(3-buten-1-yl)halothiophenes;
otherwise, halogenation of the thiophene ring with NBS would
be unsuccessful for the thiophene derivatives bearing a carbon-
carbon double bond. Furthermore, the disiloxane moiety would
not be tolerable toward metalation-bromination with a strong
base and following treatment of Br+. In contrast, the synthetic
pathway shown in Scheme 1 proceeded to afford halothiophenes
in reasonable overall yields, thus providing practical synthesis
leading to the corresponding polythiophene.
The obtained monomer precursor 1a was then subjected to
polymerization by deprotonation with the Knochel-Hauser base9
(60 °C, 1 h), following cross-coupling polycondensation with a
nickel(II) catalyst.5 Use of the reaction condition that was
available for the synthesis of regioregular poly(3-hexylthio-
phene) was found to be successful in affording the correspond-
ing polythiophene bearing a disiloxane moiety. When 1.0 mol %
of [NiCl2(PPh3)(IPr)] was employed for the polymerization,
polymer 6 was obtained in 93% yield to exhibit Mn of 12300
(Mw/Mn = 1.81), as shown in Scheme 2. The HT-regioregularity
of 6 was estimated to be >98% by the measurement of 1H NMR
(see Supporting Information). It is remarkable that the color of
the reaction mixture remains clear reddish-orange throughout the
polymerization. This sharply contrasts with the preparation of
regioregular poly(3-hexylthiophene) (HT-P3HT), whose reaction
mixture gradually turns to heterogeneous dark purple suspension
with the progress of polymerization.5-8
(1 mg mL¹1) at room temperature, a clear deep orange solution
was immediately formed, as shown in Figure 1a, while other
regioregular polythiophene poly(3-hexylthiophene) (Mn =
12500, PDI = 1.35) was shown to be hardly soluble under
similar conditions, as observed in Figure 1b. Although poly-
thiophene bearing a longer alkyl chain poly(3-dodecylthio-
phene) (Mn = 12200, PDI = 1.72) slightly improved the solu-
bility (Figure 1c), it was found to be much inferior to that of 6.
The obtained polymer was used to measure spectroscopic
and physical properties. Polythiophene 6 was dissolved in
hexane and spin coated. The corresponding thin film of 53-nm
thickness was obtained after annealing at 110 °C for 10 min.
Measurement of UV-vis absorption spectrum exhibited the -max
value of 519 nm in the film. Comparison of the spectra in a
hexane solution (-max = 446 nm) and in the film state reveals
that the latter has a red-shifted absorption, as shown in
Figure 2a, indicating strong interchain interactions among the
thiophene chains in the film. The XRD analysis of polythio-
phene 6 was then performed to observe a peak at 2ª = 4.310°,
suggesting edge-on orientation and a layer distance of d =
20.48 ¡. Comparing with HT-P3HT (2ª = 5.325°, d = 16.60 ¡),
the layer distance of 6 was found to be slightly longer because
of the steric bulkiness of the siloxane moiety (Figure 2b). The
HOMO level of the thin-film of 6 bearing siloxane moiety is
estimated to be ¹4.96 eV by photoelectron yield spectroscopy,
which was found to be slightly lower than that of P3HT
(¹4.74 eV). This is because of better π stacking and improved
interchain coupling in the HT-P3HT film that enhances the donor
character. This tendency is in accordance with the literature that
shorter side chains in regioregular poly(3-substituted thiophene)
enhances the donor character.10 The observation of the AFM
image of the thin film of 6 is performed as shown in Figure 2c,
indicating a flat film within the region 5 ¯m © 5 ¯m, in which
the root mean square (RMS) roughness of the film was 1.45 nm.
Further studies on the use of the thin film of polythiophene 6
We next examined GRIM polymerization8 of 2,5-dibromo-
thiophene derivative bearing siloxane 1b. Treatment of 1b with
an equimolar amount of i-PrMgCl¢LiCl to form the correspond-
ing thienyl Grignard reagent and addition of nickel(II) catalyst
[NiCl2(dppe)] (1.0 mol %) induced the polymerization leading
to 6 (Scheme 3). The obtained polymer showed Mn of 10500
(Mw/Mn = 1.13) and the regioregularity was also found to be
excellent (>99:1).
It should be pointed out that the obtained polythiophene
bearing a pentamethyldisiloxane moiety 6 was found to be
soluble in hexanes. When polythiophene 6, whose Mn and PDI
were 12300 and 1.81, respectively, was dissolved in hexane
© 2014 The Chemical Society of Japan | 641
Mori, Atsunori
