Synthesis of optically active regioregular polythiophenes and their self-organization at the air-water interface

Article abstract of DOI:10.1021/la4015527

Regioregular polythiophenes containing an optically active substituent in the third position of the thiophene ring, head-to-tail poly(3-[2-((S)-1- methyloctyloxy)ethyl]thiophene)s (HT-P(S)MOETs), were synthesized using highly reactive zinc. For comparison

Full text of DOI:10.1021/la4015527

Article
pubs.acs.org/Langmuir
Synthesis of Optically Active Regioregular Polythiophenes and Their
Self-Organization at the Air−Water Interface
Yuko Takeoka,* Fumihiko Saito, and Masahiro Rikukawa*
Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
S
* Supporting Information
ABSTRACT: Regioregular polythiophenes containing an
optically active substituent in the third position of the
thiophene ring, head-to-tail poly(3-[2-((S)-1-methyloctyloxy)-
ethyl]thiophene)s (HT-P(S)MOETs), were synthesized using
highly reactive zinc. For comparison, HT-P(R)MOET and
achiral HT-P( )MOET also were synthesized from R-type
monomers and racemic monomers, respectively. The HT-
PMOET possessed greater than 95% head-to-tail coupling with
a weight-average molecular weight (Mw) between 1.96 × 104
and 2.94 × 104. The polymers were characterized using 1H and
13C NMR, optical rotatory power measurements, circular dichroism (CD), and UV−vis spectroscopy. X-ray diffraction patterns
of the cast films demonstrated that regioregular HT-PMOET possessed a strong tendency to self-assemble into highly ordered,
crystalline structures. The HT-P(S)MOET and HT-P(R)MOET showed strong Cotton effects, while HT-P( )MOET showed
very weak Cotton effects. The presence of a circular dichroism effect indicated that the side chain chirality induced optical activity
in poly(thiophene) main chains. The monolayer formation of HT-PMOET spread on the water surface was characterized using a
pressure−area (π−A) isotherm. The molecular areas of HT-P(S)MOET and HT-P(R)MOET molecules on the water surface
were 33.5 and 32.9 Å2, respectively, at 10 °C, which were larger than that of HT-P( )MOET (27.9 Å2), suggesting that optically
active HT-PMOET expanded because of the chiral repulsion between side chains. Multilayer films of HT-PMOET were prepared
by repeating horizontal deposition of the monolayer on the water surface. The multilayer films of optically active HT-PMOET
obtained showed stronger Cotton effects than did the cast films. In addition, electrical conductivities of HT-PMOET multilayer
films were superior to those of spin-coated films. Head-to-tail poly(3-[2-((S)-1-methylpropyloxy)ethyl]thiophene) (HT-
P(S)MPET), which contained shorter side chain lengths compared to HT-P(S)MOET, also was synthesized. The CD intensities
of HT-P(S)MPET multilayer films were smaller than those of HT-P(S)MOET multilayer films, suggesting that the optically
active side-chain length is critically important to the optically active self-assembly.
regiochemical arrangements provide an opportunity to under-
stand the relationship between the structure and the properties
of conducting polymers with enhanced electrical and optical
properties. The electrical and optical properties of these
polymers, based on conjugated systems, strongly depend on
their nanostructural organization, which can be controlled by
the introduction of regioregularity. For example, the hole
mobility of regioregular poly(3-alkylthiophene) is greater than
that of the regiorandom polymer by 1 order of magnitude.20
The electrical and optical properties of conjugated polymers
can be determined by their solid-state morphology, which
depends on interchain conformation and the resulting
interpolymer interactions. Therefore, researchers have focused
on precise control of the molecular organization of
polythiophenes in the film state to improve their properties
and explore potential applications of these materials.21 In most
cases, conventional solution casting and spin-coating methods,
INTRODUCTION
■
Polythiophene and its derivatives have attracted considerable
interest as conducting polymers because of their excellent
electro-optical properties, good solubility, and environmental
stability.1,2 Their chemical versatility allows the use of various
monomer building blocks and has led to the development of
numerous polythiophene derivatives with transformable prop-
erties applicable to light-emitting diodes (LEDs),3−6 photo-
voltaic cells,7,8 organic field-effect transistors (OFETs),9−11 and
sensors.12 Modification of polythiophenes using side chain
functionalization provides a variety of nanostructures and
electronic properties. For example, polythiophene derivatives
containing optically active side chains exhibit the unique
behavior of macromolecular self-assembly into helical aggre-
gates that possess unique optical properties such as circular
dichroism (CD) and circular polarized luminescence in the
visible region.13−17 To evaluate and enhance these properties
for conducting polymers, the preparation of structurally
homogeneous materials is important. The recent development
of regioregular 3-substituted polythiophene offers a new
direction for molecular engineering research.18,19 These
Received: April 25, 2013
Revised: May 24, 2013
© XXXX American Chemical Society
A
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Article
1177 (m, νCOC), 1097 (m, νCOC), 914 (w, δC−H), 815 (m, νC−
H). 1H NMR (CDCl3): 7.79 (d, 2H), 7.32 (d, 2H), 4.60 (m, 1H), 2.44
(s, 1H), 1.38−1.63 (m, 2H), 1.11−1.27 (m, 15H), 0.87 (t, 3H). 13C
NMR (CDCl3): 144.36, 134.65, 129.69, 127.72, 80.73, 36.51, 31.68,
29.09, 29.06, 24.87, 22.74, 21.61, 20.87, 14.07. EIMS: m/z = 298 (M+),
126 (CH3CH2C7H15+). Anal. Calcd for C16H26O3S: C, 64.39; H, 8.78.
Found: C, 64.43; H, 8.51.
which offer very little control of the molecular organization of
the polymers, have been used to prepare polymer films.
Consequently, a disordered structure often is transferred to the
solid state during conventional film processing, making long-
range ordering difficult to achieve without specific postprocess-
ing.
The Langmuir−Blodgett (LB) technique offers a unique
approach for application of ordered ultrathin films with well-
defined architecture.22−27 Application of the LB technique to
conjugated polymers has produced various electrical and optical
ultrathin film devices including field-effect transistors and
LEDs. Polymeric materials suitable for the LB technique must
be made amphiphilic by introducing an ionic or polar side or
main chain to prepare a spreading monolayer at the air−water
interface. Therefore, fully hydrophobic hydrocarbon-based
polymers cannot form stable monolayers on water.28 For
polythiophene derivatives, regioregular poly(3-alkylthiophene)s
form stable monolayer films as a result of rigid polymer main
chains that direct spreading on the water surface.29 However,
the area per molecule of regioregular poly(3-alkylthiophene)
films is relatively small compared with the ideal values and
depends on preparation conditions. These results suggest that
regioregular poly(3-alkylthiophene)s tend to aggregate because
of strong self-organizing properties. To obtain ideal spread
monolayers of regioregular poly(3-alkylthiophene), various
supporting materials can be added to the polymer, such as
liquid crystal molecules.30 These LB films can be fabricated
from mixed monolayers containing stearic acid and regioregular
poly(3-alkylthiophene).31,32 This novel approach results in
well-defined and controllable molecular architecture with
desirable optical and electrical properties. Moreover, highly
ordered LB films of a regioregular chiral polythiophene, head-
to-tail poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene) (HT-
P(S)MBET), could be obtained without using stearic acid as an
amphiphilic molecule.33 Thus, LB manipulation of chiral
polythiophene having a longer side chain than HT-P(S)MBET
was demonstrated to be effective for monolayer formation.
Furthermore, enhanced chiral, optical, and electrical properties
were characterized.
3-[2-((S)-(+)-1-Methyloctyloxy)ethyl]thiophene [(S)MOET]. 2-(3-
Thienyl)ethanol (1.80 g, 1.41 × 10−2 mol) and potassium hydroxide
(1.31 g, 2.34 × 10−2 mol) were dissolved in THF (10 mL) and
refluxed for 3 h. A THF solution (6 mL) of (S)MOT (4.01 g, 1.34 ×
10−2 mol) then was added dropwise over 30 min. The reaction mixture
was refluxed for 17 h, extracted with diethyl ether, and dried over
anhydrous MgSO4. After the solvent was evaporated, the residue was
purified using column chromatography on a Wakogel with hexane−
diethyl ether (10:1, v/v) as the eluent to give 1.80 g (52.8%) of
(S)MOET as pale yellow oil.34 [α]589: −3.07°. IR (KRS, cm−1): 2956
(s, νC−H), 2927 (s, νC−H), 2856 (s, νC−H), 1466 (m, δC−H),
1372 (w, δC−H), 1340 (w, νCC), 1121 (m, νCOC), 1095 (m,
1
νCOC), 855 (w, δC−H), 772 (m, νC−H). H NMR (CDCl3): 7.23
(d, 1H), 7.02 (s, 1H), 6.99 (d, 1H), 3.56−3.70 (m, 2H), 3.37 (m, 1H),
2.89 (t, 2H), 1.51 (m, 2H), 1.26−1.35 (m, 10H), 1.12 (d, 3H), 0.88 (t,
3H). 13C NMR (CDCl3): 139.57, 128.56, 125.05, 121.00, 75.69, 68.62,
36.69, 31.86, 31.20, 29.71, 29.32, 25.71, 22.68, 19.71, 14.12. EIMS: m/
z = 254 (M+), 126 (CH3CH2C7H15+), 111 (C6H7S+). Anal. Calcd for
C15H26OS: C, 70.81; H, 10.30. Found: C, 70.40; H, 10.43.
2,5-Dibromo-3-[2-((S)-(+)-1-methyloctyloxy)ethyl]thiophene
[DBr-(S)MOET]. Dibromination of (S)MOET was performed with N-
bromosuccinimide (NBS) in DMF. (S)MOET (3.00 g, 1.18 × 10−2
mol) was dissolved in DMF (10 mL) in an ice bath. A DMF solution
(50 mL) of NBS (4.61 g, 2.59 × 10−2 mol) was added dropwise into
the (S)MOET solution over 45 min. After the reaction mixture was
stirred for 24 h at 0 °C, the solution was extracted with diethyl ether (3
× 60 mL); the organic extracts were successively washed with water (2
× 60 mL), aq. NaHCO3 (3 × 60 mL), and aq. NaCl (2 × 60 mL). The
organic layer was dried over anhydrous MgSO4. After the diethyl ether
was evaporated, the residue was purified using column chromatog-
raphy on a Wakogel with hexane−diethyl ether (4:1, v/v) as the eluent
to give 4.85 g (99.7%) of DBr-(S)MOET as a yellow oil. [α]589
:
−3.94°. IR (KRS, cm−1): 2954 (s, νC−H), 2930 (s, νC−H), 2852 (s,
νC−H), 1538 (w, νCC), 1467 (m, δC−H), 1375 (w, δC−H), 1342
(w, νCC), 1124 (m, νCOC), 1098 (m, νCOC), 982 (w, νC−Br).
1H NMR (CDCl3): 6.88 (s, 1H), 3.47−3.63 (m, 2H), 3.38 (m, 1H),
2.79 (t, 2H), 1.55 (m, 2H), 1.26−1.39 (m, 10H), 1.12 (d, 3H), 0.88 (t,
3H). 13C NMR (CDCl3): 139.88, 131.59, 110.22, 108.87, 75.81, 67.00,
36.65, 31.86, 30.52, 29.70, 29.34, 25.55, 22.69, 19.66, 14.12. EIMS: m/
z = 412 (M+), 126 (CH3CH2C7H15+). Anal. Calcd for C15H26Br2OS:
C, 43.71; H, 5.87. Found: C, 43.97; H, 5.94.
EXPERIMENTAL SECTION
■
Materials. (S)-(+)-, (R)-(−)-2-Nonanol, (S)-(+)-butanol, iron(III)
chloride, tetrahydrofuran (THF, for organic synthesis), acetonitrile
(for organic synthesis), and 1,1,1,3,3,3-hexamethyldisilazane were
purchased from Wako Pure Chemicals and used without further
purification. Rieke Zn* (Zn*, Rieke Metals Inc., 5 wt % THF solution)
was used without further purification. Pyridine was purchased and
purified by distillation over potassium hydroxide. Hexane, N,N-
dimethylformamide (DMF), and chloroform were purchased and
purified by conventional methods. Diethyl ether was purchased and
purified by distillation, followed by drying with CaCl2 and Drynap. 2-
(3-Thienyl)ethanol was purified by vacuum distillation. p-Toluene-
sulfonyl chloride was purified by recrystallization from diethyl ether.
Monomer Syntheses. (S)-(+)-2−1-Methyloctyl p-Toluenesulfo-
nate [(S)MOT]. p-Toluenesulfonyl chloride (4.69 g, 2.46 × 10−2 mol)
was added slowly to a dry pyridine (9.00 g, 1.13 × 10−1 mol) solution
of (S)-(+)-2-nonanol (3.00 g, 2.09 × 10−2 mol) in an ice bath. The
reaction mixture was stirred for 20 h at 5 °C under nitrogen. The
solution was extracted with diethyl ether, and the organic layer washed
with 18 wt % hydrochloric acid and dried over anhydrous K2CO3/
MgSO4. After the solvent was evaporated, the residue was purified
using column chromatography on Wakogel with hexane−diethyl ether
(6:1, v/v) as the eluent to give 5.78 g (92.8%) of (S)MOT as colorless
oil.34 [α]589: +4.61°. IR (KRS, cm−1): 2929 (s, νC−H), 2858 (s, νC−
H), 1599 (w, νCC), 1462 (m, δC−H), 1364 (w, νC-SOO-OC),
(R)-(−)-2−1-Methyloctyl p-toluenesulfonate [(R)MOT]. (R)MOT
was prepared from (R)-(−)-2-nonanol in a manner identical to that for
1
(S)MOT. Yield: 79.5%. [α]589: −3.94°. H NMR (CDCl3): 7.78 (d,
2H), 7.32 (d, 2H), 4.60 (m, 1H), 2.44 (s, 1H), 1.38−1.64 (m, 2H),
1.11−1.26 (m, 15H), 0.87 (t, 3H). 13C NMR (CDCl3): 144.35,
134.66, 129.68, 127.73, 80.73, 36.51, 31.68, 29.09, 29.06, 24.87, 22.74,
21.62, 20.87, 14.07. EIMS: m/z = 298 (M+), 126 (CH3CH2C7H15+).
Anal. Calcd for C16H26O3S: C, 64.39; H, 8.78. Found: C, 64.56; H,
8.85.
3-[2-((R)-(−)-1-Methyloctyloxy)ethyl]thiophene [(R)MOET].
(R)MOET was prepared from (R)MOT in a manner identical to
1
that for (S)MOET. Yield: 61.6%. [α]589: +3.56°. H NMR (CDCl3):
7.24 (d, 1 H), 7.03 (s, 1H), 6.99 (d, 1H), 3.56−3.70 (m, 2H), 3.37 (m,
1H), 2.89 (t, 2H), 1.51 (m, 2H), 1.26−1.35 (m, 10H), 1.12 (d, 3H),
0.87 (t, 3H). 13C NMR (CDCl3): 139.58, 128.57, 125.05, 121.00,
75.70, 68.63, 36.70, 31.86, 31.21, 29.72, 29.32, 25.71, 22.69, 19.72,
14.12. EIMS: m/z = 254 (M+), 126 (CH3CH2C7H15+), 111 (C6H7S+).
Anal. Calcd for C15H26OS: C, 70.81; H, 10.30. Found: C, 70.97; H,
10.16.
2,5-Dibromo-3-[2-((R)-(-)-1-methyloctyloxy)ethyl]thiophene
[DBr-(R)MOET]. DBr-(R)MOET was prepared from (R)MOET in a
manner identical to that for DBr-(S)MOET. Yield: 82.3%. [α]589
:
B
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Scheme 1. Syntheses of HT-PMPET and HT-PMOET
+2.63°. 1H NMR (CDCl3): 6.90 (s, 1H), 3.47−3.63 (m, 2H), 3.38 (m,
1H), 2.79 (t, 2H), 1.54 (m, 2H), 1.26−1.41 (m, 10H), 1.11 (d, 3H),
0.88 (t, 3H). 13C NMR (CDCl3): 139.88, 131.59, 110.22, 108.87,
75.81, 67.00, 36.65, 31.86, 30.52, 29.70, 29.34, 25.55, 22.69, 19.65,
14.12. EIMS: m/z = 412 (M+), 126 (CH3CH2C7H15+). Anal. Calcd for
C15H26Br2OS: C, 43.71; H, 5.87. Found: C, 43.99; H, 5.77.
Head-to-Tail Poly(3-[2-((R)-(−)-1-methyloctyloxy)ethyl]-
thiophene) [HT-P(R)MOET]. HT-P(R)MOET was obtained from
DBr-(R)MOET using a method similar to that described for HT-
P(S)MOET. Yield: 10.2%. IR (ZnSe, cm−1): 2958 (s, νC−H), 2925 (s,
νC−H), 2854 (s, νC−H), 1516 (w, νCC), 1466 (m, δC−H), 1372
(w, δC−H), 1341 (w, νCC), 1138 (m, νCOC), 1091 (m, νCOC).
1H NMR (CDCl3): 7.09 (s, 1H), 3.64−3.74 (m, 2H), 3.44 (m, 1H),
3.06 (t, 2H), 1.57 (m, 2H), 1.24−1.41 (m, 10H), 1.12 (d, 3H), 0.85 (t,
3H). 13C NMR (CDCl3): 136.27, 133.50, 131.72, 129.23, 68.04, 36.70,
31.89, 30.41, 29.79, 29.34, 25.63, 22.67, 19.79, 14.11. Anal. Calcd for
(C15H24OS)n: C, 71.37; H, 9.58; S, 12.70. Found: C, 70.60; H, 10.15;
S, 11.86.
(S)-(+)-2-1-Methylpropyl p-Toluenesulfonate ((S)MPT). (S)MPT
was synthesized from (S)-(+)-2-butanol using a procedure similar to
that for (S)MOT. (S)MPT was obtained as colorless oil. Yield: 84.5%.
1
[α]589: +10.25°. H NMR (CDCl3): 7.80 (d, 2H), 7.32 (d, 2H), 4.56
(m, 1H), 2.45 (s, 1H), 1.53−1.65 (m, 2H), 1.25 (m, 2H), 0.82 (t, 3H).
EIMS: m/z = 228(M+). Anal. Calcd for C11H16O3S: C, 57.87; H, 7.06.
Found: C, 57.81; H, 7.21.
Head-to-Tail Poly(3-[2-(( )-1-methyloctyloxy)ethyl]thiophene)
(HT-P( )MOET). HT-P( )MOET was obtained using a method
similar to that for HT-P(S)MOET, beginning with a mixture of
DBr-(S)MOET and DBr-(R)MOET (DBr-(S)MOET/DBr-
(R)MOET = 1.0:1.0 w/w) as starting materials. Yield: 12.5%. IR
(ZnSe, cm−1): 2958 (s, νC−H), 2924 (s, νC−H), 2854 (s, νC−H),
1510 (w, νCC), 1456 (m, δC−H), 1373 (w, δC−H), 1340 (w,
3-[2-((S)-(+)-1-Methylpropyloxy)ethyl]thiophene [(S)MPET].
(S)MPET was synthesized from (S)-MPT using a method similar to
that for (S)MOET. (S)MPET was obtained as pale yellow color oil.
Yield: 42.2%. [α]589: −15.60°. 1H NMR (CDCl3): 7.24 (d, 1 H), 7.03
(s, 1 H), 6.99 (d, 1 H), 3.56−3.71 (m, 2 H), 3.34 (m, 1 H), 2.90 (t, 2
H), 1.40−1.58 (m, 2 H), 1.12 (d, 3 H), 0.88 (t, 3 H). EIMS: m/z =
184 (M+). Anal. Calcd for C10H11OS: C, 65.17; H, 8.75. Found: C,
65.11; H, 8.70.
2,5-Dibromo-3-[2-((S)-(−)-1-methylpropyroxy)ethyl]thiophene
(DBr-(S)MPET). DBr-(S)MPET was prepared from (S)MPET in a
manner identical to that for DBr-(S)MOET. DBr-(S)MPET was
obtained as yellow oil. Yield: 91.5%. [α]589: −8.00°. 1H NMR
(CDCl3): 6.88 (s, 1 H), 3.50−3.71 (m, 2 H), 3.30−3.34 (m, 1 H),
2.77 (t, 2 H), 1.40−1.58 (m, 2 H), 1.12 (d, 3 H), 0.88 (t, 3 H). EIMS:
m/z = 184 (M+). Anal. Calcd for C10H11Br2OS: C, 35.11; H, 4.12.
Found: C, 35.24; H, 3.99.
1
νCC), 1138 (m, νCOC), 1093 (m, νCOC). H NMR (CDCl3):
7.10 (s, 1H), 3.63−3.80 (m, 2H), 3.44 (m, 1H), 3.07 (t, 2H), 1.57 (m,
2H), 1.17−1.41 (m, 10H), 1.12 (d, 3H), 0.85 (t, 3H). 13C NMR
(CDCl3): 136.28, 133.51, 131.73, 129.23, 68.05, 36.71, 31.90, 30.41,
29.79, 29.36, 25.64, 22.69, 19.79, 14.12. Anal. Calcd for (C15H24OS)n:
C, 71.37; H, 9.58; S, 12.70. Found: C, 71.67; H, 9.37; S, 12.36.
Head-to-Tail Poly(3-[2-((S)-(+)-1-methylpropyloxy)ethyl]-
thiophene) [HT-P(S)MPET]. HT-P(R)MPET was obtained by a
method similar to that used for HT-P(S)MOET beginning with
DBr-(R)MPET as a starting material. Yield: 29.8%. IR (ZnSe, cm−1):
2965 (s, νC−H), 2923 (s, νC−H), 2870 (s, νC−H), 1508 (w, νC
C), 1451 (m, δC−H), 1373 (w, δC−H), 1340 (w, νCC), 1135 (m,
Polymer Syntheses. Head-to-Tail Poly(3-[2-((S)-(+)-1-
methyloctyloxy)ethyl]thiophene) (HT-P(S)MOET). HT-P(S)MOET
was prepared using highly reactive Zn*18 and [1,2-bis-
(diphenylphosphino)ethane]dichloronickel(II) (Ni(dppe)Cl2), as de-
scribed below. DBr-(S)MOET (1.65 g, 4.00 mmol) was added to a
THF solution of Zn* (7.30 mL) in an argon-filled drybox. The
solution was kept in an ultrasonic bath at 30−35 °C for 1.5 h. Then,
Ni(dppe)Cl2 (1.10 × 10−2 g, 2.08 × 10−3 mmol) was transferred in
THF (12 mL) via cannula to the reaction mixture. The mixture was
refluxed for 4.5 h under argon. The polymer was precipitated using a
solution of methanol and 2 M hydrochloric acid and then filtered,
followed by Soxhlet purification with methanol and hexane for 3 days
each. The purified polymer was dissolved in chloroform and cast on a
Teflon sheet to obtain free-standing films under nitrogen. Dark purple
films with a metallic luster were obtained (1.64 × 10−1 g, 16.3%). IR
(ZnSe, cm−1): 2958 (s, νC−H), 2920 (s, νC−H), 2851 (s, νC−H),
1516 (w, νCC), 1466 (m, δC−H), 1372 (w, δC−H), 1341 (w,
1
νCOC), 1084 (m, νCOC). H NMR (CDCl3): 6.88 (s, 1H), 3.50−
3.71 (m, 2H), 3.30−3.34 (m, 1H), 2.77 (t, 2H), 1.40−1.58 (m, 2H),
1.12 (d, 3H), 0.88 (t, 3H). 13C NMR (CDCl3): 136.30, 133.51,
131.74, 129.26, 68.04, 30.40, 29.21, 19.730, 9.86. Anal. Calcd for
(C10H14OS)n: C, 65.89; H, 7.74; S, 17.59. Found: C, 65.54; H, 7.33; S,
16.47.
Preparation of Polymer Spin-Coated Films. Chloroform
solutions (5 mg mL−1) of polymers were prepared and filtered
through a 0.5 μm filter prior to use. Spin-coated films were fabricated
on quartz substrates using a MIKASA 1H-D7 spin coater at 1500 rpm
for 5 s and 3000 rpm for 10 s.
Preparation of LB Films. Hydrophobic glass slides were prepared
by treatment with 1,1,1,3,3,3-hexamethyl-disilazane. Chloroform
solutions (0.05 mg mL−1) of polymers were spread on a water
subphase with a specific resistance over 17.6 Ωcm, and purified with a
TORAYPURE purification system. The subphase temperature was
maintained at 10 °C. The surface pressure−area (π−A) isotherm was
measured on a Lauda film balance. LB films of polymers were prepared
using the following procedure. The floating monolayer on pure water
surface was compressed continuously up to 20 mN m−1, allowing 60
min to establish the monolayer equilibrium. The monolayer on the
1
νCC), 1138 (m, νCOC), 1091 (m, νCOC). H NMR (CDCl3):
7.09 (s, 1H), 3.64−3.76 (m, 2H), 3.44 (m, 1H), 3.07 (t, 2H), 1.55 (m,
2H), 1.25−1.41 (m, 10H), 1.12 (d, 3H), 0.85 (t, 3H). 13C NMR
(CDCl3): 136.28, 133.51, 131.73, 129.24, 68.05, 36.70, 31.90, 30.41,
29.79, 29.34, 25.63, 22.68, 19.79, 14.11. Anal. Calcd for (C15H24OS)n:
C, 71.37; H, 9.58; S, 12.70. Found: C, 70.54; H, 10.51; S, 12.02.
C
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Table 1. Molecular Weights and HT Ratios of HT-PMOETs and HT-P(S)MPET
band gap (Eg) /eV
a
a
a
b
c
d
polymer
Mn
Mw
Mw/Mn
HT %
solution
solid
HT-P(S)MOET
HT-P(R)MOET
HT-P( )MOET
HT-P(S)MPET
29 400
28 600
26 300
39 000
35 200
33 400
29 400
1.31
1.23
1.27
1.20
95.2
95.4
95.2
92.3
2.20
2.21
2.21
2.22
1.83
1.83
1.78
1.83
19 600
a
b
c
1
Determined by GPC (eluent: THF). Head-to-tail coupling ratio determined by H NMR. Determined by band edge wavelengths of polymer
d
chloroform solutions. Determined by band edge wavelengths observed for polymer spin-coated films.
water surface then was transferred on a hydrophobic glass slide using a
horizontal deposition method.
poly(3-[2-(1-methyloctyloxy)ethyl]thiophene), HT-P( )-
MOET, also was synthesized from achiral 2,5-dibromo-3-[2-
(1-methyloctyloxy)ethyl] thiophene. To investigate the effect
of chain length on the optical properties, monolayer formation
properties, and self-organized structure, HT-P(S)MPET, which
has a shorter chain length than HT-P(S)MOET, was
synthesized.
1
Measurements. H and 13C NMR spectra were recorded on a
JEOL lambda500 spectrometer. All NMR spectra were recorded in
CDCl3 using tetramethylsilane (TMS) as an internal standard. FT-IR
spectra were recorded using a Nicolet Magna-IR Spectrometer System
750 equipped with a liquid nitrogen cooled mercury−cadmium−
telluride (MCT-A) detector. FT-IR spectra were collected for 64 scans
at a resolution of 2 cm−1. Molecular weights of the polymers were
determined by gel permeation chromatography (GPC) using a
Shimadzu LC-10AD chromatosystem equipped with Shodex KF804L
columns. The eluent used was THF, which was maintained at 40 °C. A
molecular weight calibration curve was obtained with standard
polystyrenes. X-ray diffraction measurements of the polymer films
were obtained using a Rigaku X-ray diffractometer RINT 2100 with
Ni-filtered Cu−Kα radiation at 40 kV and 30 mA. The UV−Vis
absorption spectra were measured on either of HT-PMOET and HT-
PMPET solutions in chloroform or thin films on quartz substrates
using a Shimadzu UV-3100 spectrophotometer. Specific optical
rotations [α] at 589 nm were obtained using a Horiba SEPA-300
high-sensitive polarimeter at 20 °C. CD spectra of the polymer spin-
coated and LS films were measured using a JASCO J-800
spectropolarimeter. In-plane conductivities were measured using the
van der Pauw method at room temperature with a Keithley 2400
source meter and a Keithley 60517A electrometer.
The Rieke zinc is very sensitive to moisture in the air;
therefore, all procedures were conducted under an argon
atmosphere. Special effort was made to keep the apparatus dry
and maintain the purity of the starting dibromo monomers,
DBr-MOET and DBr-MPET. Rieke zinc chemoselectively
underwent direct oxidative addition with the dibromo
monomers to form a 2-bromo-3-substituted-5-(bromozincio)-
thiophene predominantly. The regioselectivity of this oxidative
addition is a unique advantage of Zn*, providing a convenient
route for the specific synthesis of regioregular organometallic
reagents. Although the reaction with Zn* was indispensable to
the regiospecific polymerization, the yield of 2-bromo-3-
substituted-5-(bromozincio)thiophene could be low because
of heterogeneous reactions. A metal surface usually is not pure.
A solid coating of oxides, hydroxides, and carbonates
constitutes a “passivation” layer that can prevent reagents in
solution from reaching the metal. Ultrasonic irradiation has
been used to facilitate heterogeneous reactions by removing
surface absorbed contaminants.35,36 Using an ultrasonication
treatment was expected to activate the surface of Zn* and
increase its catalytic activity. Therefore, ultrasonication treat-
ment was used to activate the zinc.
After precipitation from the reaction solution using
methanol, HT-PMOET was Soxhlet-extracted with hexane,
followed by extraction with chloroform. The GPC results
indicated that the HT-PMOET extracted with hexane had a
lower molecular weight than that extracted with chloroform,
therefore chloroform-extracted HT-PMOET was used for the
following experiments. The HT-PMOET and HT-PMPET
obtained were reddish brown with a metallic luster, and were
readily soluble in CHCl3 and THF.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The syntheses of
monomers and polymers are outlined in Scheme 1. Since 3-
substituted thiophene is not a symmetrical molecule, three
relative orientations are available when thiophene rings are
coupled between the 2- and 5-positions. The first is 2,5′ or
head-to-tail coupling (HT), the second is 2,2′ or head-to-head
coupling (HH), and the third is 5,5′ or tail-to-tail coupling
(TT). This leads to a mixture of four chemically distinct triad
regioisomers in the polymer chain when 3-substituted
thiophene monomers are employed, that is, HT-HT, HT-HH,
TT-HT, and TT-HH triads. These structurally irregular
polymers are denoted as irregular or regiorandom. In particular,
HH coupling is sterically unfavorable for the coplanarity of
polythiophene backbones. This causes a significant loss of
conjugation, which causes a loss in the desired physical
properties of the material. The synthesis of regioregular head-to-
tail poly(3-alkylthiophene)s has been accomplished using the
McCullogh and Rieke method. The Rieke method has been
applied for the synthesis of several 3-substituted polythiophenes
because it can be conducted under mild reaction conditions at
approximately 40 °C. To elucidate the structural disorder
induced by the mixed regioisomers, optically active S-type and
R-type poly(3-[2-(1-methyloctyloxy)ethyl]thiophene)s, HT-P-
(S)MOET and HT-P(R)MOET, were synthesized from 2,5-
dibromo-3-[2-((S)-(+)-1-methyloctyloxy)ethyl] thiophene and
2,5-dibromo-3-[2-((R)-(−)-1-methyloctyloxy)ethyl] thiophene,
respectively, using the Rieke method. For comparison, achiral
Table 1 shows the number-average molecular weight Mn,
weight-average molecular weight Mw, and polydispersity index
ĐM = Mw/Mn of the polymers synthesized in this study. The
molecular weight of HT-PMOET was 26.3−29.4 kg mol−1 with
a narrow weight distribution of 1.23−1.31. The molecular
weight of HT-P(S)MPET was 19.6 kg mol−1, which was lower
than that of HT-PMOET. The number of monomer units per
single chain for HT-PMOET was in the range of 104−116, and
that for HT-P(S)MPET was 107. Note that the numbers of
monomer units of the polymers were nearly equal, allowing
comparison of their properties. Regioregularity of the polymers
1
1
was estimated by H NMR in CDCl3. The H NMR spectrum
of poly(3-substituted thiophene) provided sensitive probes for
the substitution pattern in the polymer backbone. The α-
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methylene protons of the thiophene ring produced two signals
because of the regioisomers. In the H NMR spectrum, the
alkylthiophene)s, which produced a significant red shift
between solution and solid film.18 Racemic polymer, HT-
P( )MOET, showed relatively sharp peaks compared with
other HT-PMOETs, suggesting that HT-P( )MOET formed
stable π−π stacking. This is probably due to the loss of
homochiral interactions. The chiral discrimination phenomen-
on arises from a nonequivalence in the interaction potential
between two molecules of the same chirality (S:S) versus that
between two molecules of opposite chirality (S:R). If the S:S
and R:R interactions are more favorable than the S:R
interaction, homochiral discrimination is indicated, while a
more favorable S:R interaction indicates heterochiral discrim-
ination. These results indicate that the π−π stacking was
affected by chiral interactions and exhibited heterochiral
behavior. For similar reasons, the band gap value of racemic
HT-P( )MOET was less than that of chiral HT-PMOET, as
shown in Table 1. The HT-PMOET showed fluorescence in
CHCl3 and spin-coated films. In CHCl3 solutions, similar
fluorescence spectra were observed for all HT-PMOETs.
However, the fluorescence spectra of HT-PMOET in the
solid state were affected by the presence of chirality. HT-
P(S)MOET, HT-P(R)MOET, and their mixture HT-P(S
+R)MOET spin-coated films showed a fluorescence peak at
740 nm, and HT-P( )MOET showed a fluorescence peak at
635 nm, with shoulders at 696 and 808 nm.
1
downfield signal was assigned to a head-to-tail coupled dyad (δ
= 3.07) and the upfield signal was assigned to a head-to-head
coupled dyad (δ = 2.83). The head-to-tail ratio of HT-PMOET
and HT-P(S)MPET, as estimated from the α-methylene signal
(Figure S1, Supporting Information), indicated greater than
92% head-to-tail, which was nearly equal to the ratios found for
conventional poly(3-alkylthiophene)s obtained by the Rieke
method.32
Optical Properties and Structural Analyses of Poly-
mers. A qualitative measure of π-orbital overlap in solution can
be obtained using UV−vis absorption spectroscopy. In
conjugated polymers, the extent of conjugation length directly
affects the observed energy of the absorption. Therefore, UV−
vis spectroscopy can be used to observe the conformational
state and structure of π-conjugated polymers. An increase in the
maximum absorption wavelength (λmax) is evidence for
increased coplanarity and longer π-conjugation length. Figure
1 shows the UV−vis absorption spectra of chiral HT-PMOET
HT-P(S)MPET with a shorter chain length than HT-
P(S)MOET showed an absorption peak at 443 nm owing to
the π−π* transition, and a fluorescence peak at 567 nm in
CHCl3 solution (Figure S2, Supporting Information). These
spectra were similar to those of HT-P(S)MOET. For a spin-
coated film, absorption peaks were observed at 524 and 555
nm, with a shoulder at 613 nm, and sharp fluorescence peaks
were observed at 648 and 714 nm. These results indicate that
the optical properties were independent of chain length. The
fluorescence intensity of HT-P(S)MPET was greater than that
of HT-P(S)MOET.
Figure 1. UV−vis absorption (solid line) and fluorescence (dotted
line) spectra of (a) CHCl3 solutions and (b) spin-coated films of HT-
PMOETs. The excitation wavelength was equal to the absorption
maximum (λmax) of each polymer.
The self-organization properties of the chiral polythiophenes
were examined using X-ray diffraction. Figure 2 shows X-ray
in CHCl3 and spin-coated films. For comparison, optical
absorption spectra of racemic polymer HT-P( )MOET and a
mixture of HT-P(S)MOET and HT-P(R)MOET (50:50 w/w;
HT-P(S+R)MOET) are displayed in Figure 1. All of the
polymer solutions produced similar absorption spectra with a
maximum absorption peak at approximately 446 nm, assigned
to the π−π* transition of the conjugated polymer backbone.
The side-chain chirality did not cause any significant changes in
the UV−Vis absorption spectra in the solution state. From the
onset of absorption observed near 600 nm, the band gap (Eg) in
solution was estimated to be approximately 2.2 eV for all
polymers, as shown in Table 1. The HT-PMOET spin-coated
films prepared from a chloroform solution exhibited distinct
three absorption peaks; the absorption maximum at 577 nm,
the lower-intensity peak appear at 608 nm, and a shoulder at
529 nm. The lowest energy peak at 608 nm was attributed to
the exciton band, and the higher energy peak was assigned to
vinronic sidebands.37 The absorption edge of the film was
approximately 780 nm (1.59 eV), which was about 200 nm red-
shifted from that of the chloroform solution. This indicates that
HT-PMOET possesses a more planar conformation in the solid
film compared with that in solution. These results were
consistent with the optical properties of typical poly(3-
Figure 2. X-ray diffraction patterns of HT-PMOETs and HT-PMPET
cast films.
diffraction patterns of the polymer cast films. The Bragg
reflection was observed at a 2θ angle of 3.3° for HT-PMOET
and at 5.3° for HT-P(S)MPET. These peaks corresponded to
the interlayer d-spacings of 26.7 Å for HT-PMOET and 16.7 Å
for HT-PMPET, which are the stacking distances between two
polymer main chains. These results showed that the
regioregular HT-PMOET and HT-PMPET possessed self-
assembling tendencies to form a highly ordered and crystalline
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structure. Peak intensity and sharpness of HT-P(S)MPET were
greater than those of HT-P(S)MOET, suggesting that the
crystallinity of HT-P(S)MPET was greater than that of HT-
P(S)MOET. The X-ray diffraction peak observed for racemic
HT-P( )MOET was sharper than that of other HT-PMOETs,
suggesting that HT-P( )MOET has a more crystalline
structure. The chiral substituents twist the polymer main
chains; thus, the interaction between the polymer main chains
of chiral polythiophenes was weakened compared to that of
achiral polymers. For HT-P(R+S)MOET, because the homo-
chiral organization was preferred, phenomena similar to those
exhibited by HT-P(S)MOET and HT-P(R)MOET were
observed.
HT-P(S)MOET showed clear solvatochromism upon adding
methanol as a poor solvent (Figure S2, Supporting
Information). The maximum absorption peak of HT-P(S)-
MOET red-shifted upon varying the mixture ratio of CHCl3
and MeOH. The absorption spectra for a 1:4 (v/v) CHCl3/
MeOH solution was similar to that of the HT-P(S)MOET spin-
coated films, indicating that the polymer chains are self-
organized to form an aggregated phase. While no CD effect was
observed in CHCl3, bisignate Cotton effects were clearly
observed for 2:3 and 1:4 (v/v) chloroform/methanol solutions,
as shown in Figure SI1 (Supporting Information). Strong
positive Cotton effects for HT-P(S)MOET were observed at
about 570 and 610 nm, and negative Cotton effects were
observed at about 490 nm in 1:4 (v/v) CHCl3/MeOH
solution. The presence of a CD effect in the π−π* transition
region shows that the side chain chirality induced optical
activity in the poly(thiophene) main chains. Consequently, the
bisignate Cotton effects suggested that the conjugated unit of
HT-P(S)MOET was self-organized to array clockwise along the
main chain, or that their main chains stacked in a twisted form,
according to exciton chirality theory.38
π−π* absorption region of the conjugated main chains were
observed for the chiral polymers in the solid state. For HT-
P(S)MOET, strong positive Cotton effects were observed at
about 570 and 610 nm and negative Cotton effects at about 490
nm, which were consistent with the 1:4 (v/v) CHCl3/MeOH
solution, as shown in Figure 3b. This result indicates that the
chiral polymer forms the same aggregated structure in mixed
solution and in film. For HT-P(R)MOET, inversed CD spectra
for HT-P(S)MOET were observed, indicating that HT-
P(R)MOET forms a counterclockwise array. HT-P(S)MPET
also showed a strong CD effect in the solid state (Figure 3b).
For HT-P(S)MPET, negative Cotton effects were observed at
about 575 and 615 nm and negative Cotton effects at about 469
nm. Shortening the side chain length of the chiral side chain
inversed the helicity and dramatically increased CD intensity.
These results indicate that HT-P(S)MPET had a favorable side
chain length for forming chiral morphology.
Monolayer Formation of Polymers at the Air−Water
Interface and Fabrication of Ultrathin Multilayers.
Ultrathin films have attracted attention because of their
potential as a novel class of materials. To realize the maximum
potential of polymeric materials, the structure−property
relationships must be well understood. Ultrathin films are
useful not only for technological applications but also for
scientific investigations. The LB technique is considered the
most suitable for ultrathin film fabrication with specific
structures planned at the molecular level. Interest has increased
recently in the preparation of conducting polymer multilayer
films from functional organic compounds. A previous study
reported that chiral poly(3-substituted thiophene), head-to-tail
poly(3-{2-{(S)-2-methylbutoxy}ethyl}thiophene, HT-P(S)-
MBET, with ether groups in the side chains, can form a stable
monolayer at the air−water interface without the need for
supporting materials such as stearic acid.33 Because HT-
PMOET has a longer side chain, which is effective for
monolayer formation, monolayer stability and film forming
ability were investigated.
Figure 3 shows CD spectra of the HT-PMOET and HT-
P(S)MPET spin-coated films. Racemic HT-P( )MOET and
the mixture HT-P(R+S)MOET films showed no circular
dichroism. Strong positive or negative Cotton effects in the
Chloroform solutions of the polythiophene derivatives were
spread on the water surface of a Lauda trough. Figure 4 shows
Figure 4. π−A isotherms of HT-PMOETs, HT-P(S)MPET, and HT-
PHT monolayers on pure water at 10 °C.
the pressure−area (π−A) isotherms of HT-P(S)MOET, HT-
P(R)MOET, HT-P(S+R)MOET, HT-P( )MOET, and HT-
P(S)MPET for the compression process at a subphase
temperature of 10 °C. The horizontal axis indicates the
molecular area per thiophene unit. For comparison, the π-A
isotherm of head-to-tail poly(hexythiophene) (HT-PHT),
which is a hydrophobic polymer, is shown. For nonamphiphilic
HT-PHT, the surface pressure began to increase at 9.3 Å2, and
increased up to 20 mN m−1, indicating that an ideal spread
Figure 3. CD spectra of (a) HT-PMOET and (b) HT-P(S)MPET
spin-coated films.
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monolayer did not form, because the molecular area was
considerably smaller than the estimated value. In contrast, π−A
isotherms of HT-PMOET and HT-P(S)MPET showed a
relatively smooth rise in pressure at a large molecular area.
This is because of the existence of hydrophilic ether groups in
the side chains. By compressing the HT-PMOET and HT-
P(S)MPET monolayers on the water surface, a phase transition
was observed at a specific surface pressure that depended on
the polymer. After the pressure reached the transition point, the
surface pressure remained constant as the area decreased. The
pressure rose again when the area per molecule reached that of
a pure condensed polythiophene monolayer. Table 2 shows the
limiting area per repeat unit of HT-PMOET and HT-
P(S)MPET before and after phase transition. Here the limiting
area per molecule was determined by extrapolating the area
from the isotherm to pressure π = 0.
The effect of chiral interactions on monolayer formation of
HT-PMOET was studied. For HT-PMOET, the limiting area
per molecule before phase transition was 33−34 Å2 molecule−1
for HT-P(S)MOET, HT-P(R)MOET, and HT-P(S+R)MOET.
According to the Corey−Pauling−Koltun (CPK) molecular
models, these values correspond to the cross section of the
thiophene ring and alkoxy side chains, as shown in Figure 5.
For HT-P( )MOET, the limiting area per molecule was 28 Å2,
which was smaller than that of other HT-PMOETs. While
chiral polythiophene expanded on the water surface because of
chiral repulsion and steric distortion, racemic polythiophene
showed a stronger packing nature because of the achiral side
chains. As the monolayer was compressed, a phase transition
was observed at 7.1 mN m−1 for HT-P(S)MOET, HT-
P(R)MOET, and HT-P( )MOET, and at 15.0 mN m−1 for
HT-P(S+R)MOET. The surface pressure remained constant
with decreasing area at the transition point, and then the
pressure began to rise as the limiting area reached
approximately 18 Å2 molecule−1, as shown in Table 2.
According to the CPK molecular models, these values
correspond to the cross section of the thiophene ring of HT-
PMOET. The most probable molecular orientation involves
Table 2. Area per molecule of HT-PMOETs and HT-
P(S)MPET monolayers at the air−water interface before and
after transition pressure at 10 °C
area per molecule/Å2 molecule−1
polymer
before transition
after transition
HT-P(S)MOET
HT-P(R)MOET
HT-P( )MOET
HT-P(S+R)MOET
HT P(S)MPET
HT P(S)MBET
34
33
28
34
19
18
18
20
18
16
a
a
17
14
a
Reference 33.
Figure 5. CPK models of HT-PMOETs and HT-P(S)MPET at the air−water interface before and after transition.
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coplanar polythiophene backbones that float on the water
surface and side chains on one side of the zigzag formation
orient toward the water phase.
Next, the effect of side chain length on monolayer formation
was investigated. The monolayer of HT-P(S)MPET showed a
steep rise at 19 Å2, which is smaller than that of HT-
P(S)MOET, with a phase transition at 25 mN m−1, which is
higher than that of HT-P(S)MOET. According to the CPK
molecular models, this molecular area corresponds to the cross
section of the thiophene ring of HT-PMOET. Because the
length of the hydrophobic portion of the side chain of HT-
P(S)MPET was shorter than that of HT-P(S)MOET, the
amphiphilic balance in the polythiophene, which affects
monolayer spread behavior, was affected. Compared with chiral
polythiophene, HT-P(S)MEBT and HT-P(S)MOET form
stable monolayers on the water surface.
Figure 7. X-ray diffraction patterns of HT-PMOETs and HT-
P(S)MPET LB films with 50 layers.
Multilayer Formation of HT-PMOET and HT-P(S)MPET.
Layer-by-layer multilayer deposition was attempted by repeated
horizontal deposition procedures at a pressure 20 mN m−1 and
temperature of 10 °C. UV−vis absorption spectra of the layer-
by-layer deposited films of HT-P(S)MOET are shown in Figure
6. HT-P(S)MOET LB films showed absorption peaks at 564
Figure 8. CD spectra of HT-PMOETs and HT-P(S)MPET LB films
with 50 layers.
and HT-P(R)MOET LB films showed strong positive and
negative Cotton effects, respectively, in the π−π* absorption
region of the conjugated main chains, while HT-P( )MOET
LB films did not produce a CD signal. The CD intensities of
HT-P(S)MOET and HT-P(R)MOET LB films were much
greater than those of their spin-coated films. The presence of
stronger CD signals show that the highly organized polymer
structure formed by spreading on the water surface enhanced
the chirality of the poly(thiophene) main chains. In addition,
CD signals of the LB films were inverse compared to those of
the spin-coated films. Using the horizontal dipping method,
polymer films were transferred upside down on the substrate;
the LB films obtained showed CD signals with a reverse sign.
Surprisingly, HT-P(S+R)MOET produced CD signals that
were not identical to those of chiral HT-P(S)MOET and HT-
P(R)MOET. HT-P(S)MPET produced no CD signal for two
main reasons: the strong packing interactions between main
chains owing to small repulsions between side chains, and the
lower transfer pressure compared to the transition pressure of
HT-P(S)MPET that optimized chiral conformation.
Electrical conductivity of the polymer spin-coated and LB
films was measured using the van der Pauw method.39
Chemical doping of polymers was performed by dipping the
polymer films for a specified time into acetonitrile containing
FeCl3 as a dopant. Doping was accompanied by an obvious
color change from red to bluish purple. The UV−vis absorption
spectra provided valuable information about the level of
oxidation achieved by various doping processes. After doping,
absorption bands for the polythiophene conjugated backbone
disappeared at about 530 nm and new lower energy bands
centered at about 860 and 2500 nm appeared. These bands are
characteristic of a highly oxidized conjugated backbone, that is,
supporting localized defect states in the form of bipolarons.
Figure 6. UV−vis absorption spectra of HT-P(S)MOET multilayer
films transferred on a glass substrate. The transfer was performed at 20
mN m−1 by repeating the horizontal deposition sequence for up to 50
cycles.
and 614 nm, with shoulders at 480 and 528 nm, increasing the
deposition number. The maximum absorption peak of the LB
films showed a lower π−π* transition energy than that of the
spin-coated films, while the intensity of the shoulder peak near
610 nm was relatively stronger than that of the spin-coated film.
These results suggest that the HT-P(S)MOET molecules in the
LB films tend to form a more planar conformation with longer
conjugation length compared to those in spin-coated films.
Similar results were observed for other HT-PMOETs and HT-
P(S)MPETs. Figure 7 shows X-ray diffraction patterns of the
50-layer multilayer films of HT-PMOET and HT-P(S)MPET.
Diffraction patterns were observed for all LB films. More
intense and sharper peaks were observed for HT-PMOET LB
film than for HT-PMOET spin-coated films. Thus, the LB
technique provides molecular control over regioregular
polymers for forming more organized structures that exhibit
more planar conformations. In contrast, the X-ray diffraction
peak of the HT-P(S)MOET LB films was broad and weak
compared with spin-coated films. The short side chains of HT-
P(S)MOET are not favorable for constructing organized
structures using the LB method.
Chiroptical and Electrical Properties of LB Films.
Figure 8 shows the CD spectra of the 50-layer multilayer films
of HT-PMOET and HT-P(S)MPET. Chiral HT-P(S)MOET
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Significant reductions in the interband π−π* transition region
were observed for all polymer spin-coated and LB films. Table
3 shows the in-plane conductivity of the polymer spin-coated
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, UV−vis absorption spectra, CD spectra of polymers;
UV−vis-NIR spectra of polymer spin-coated films after FeCl3
doping. This material is available free of charge via the Internet
at http://pubs.acs.org
Table 3. Conductivity of Prepared and Doped Polymer
Films As Measured by the van der Pauw Method
conductivity/S cm−1
preparation
AUTHOR INFORMATION
a
■
polymer
method
as prepared
doped
Corresponding Author
*E-mail: y-tabuch@sophia.ac.jp (Y.T.); m-rikuka@sophia.ac.jp
(M.R.).
HT-P(S)MOET
spin-coat
LS
6.6 × 10−5
2.9 × 10−4
8.8 × 10−5
2.8 × 10−4
6.6 × 10−5
2.9 × 10−4
4.0 × 10−5
1.8 × 10−4
1.1 × 10−5
1.2 × 10−4
1.9 × 10−1
2.1
1.7 × 10−1
1.3
2.2 × 10−1
1.6
2.2 × 10−1
5.0× 10−1
5.0 × 10−1
6.0
HT-P(R)MOET
HT-P( )MOET
HT-P(S+R)MOET
HT P(S)MPET
spin-coat
LS
Notes
spin-coat
LS
The authors declare no competing financial interest.
spin-coat
LS
REFERENCES
■
(1) Perepichka, I. F., Perepichka, D. F., Eds. Handbook of Thiophene-
based Materials; Wiley: Chichester, 2009.
(2) Angelopoulus, M. In Handbook of Conducting Polymers, 2nd ed.;
Sktheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel
Dekker: New York, 1998; Chapter 32, pp 921−944.
spin-coat
LS
a
Chemical doping was performed by dipping polymer films in 2 mg
mL−1 FeCl3 acetonitrile solution for 10 min.
(3) Andersson, M. R.; Inganas, O.; Gustafsson, G.; Gustafsson-
Carlberg, J. C.; Selse, D.; Hjertberg, T.; Wennerstrom, O.
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and LB films before and after doping with FeCl3. The in-plane
conductivities of the neutral HT-PMOET and HT-PMPET
spin-coated films were on the order of 10−5 Scm−1, whereas
those of the neutral LB films with 20 layers were on the order
of 10−4 Scm−1. After being doped with FeCl3, the in-plane
conductivities of both spin-coated and LB films were enhanced
dramatically. The in-plane conductivities of the doped HT-
PMOET and HT-PMPET spin-coated films were on the order
of 10−1 Scm−1, whereas the conductivities of the HT-PMOET
and HT-PMPET LB films were 0.5−6 S cm−1. This suggests
that the LB manipulation induced self-organized properties in
HT-PMOET and HT-P(S)MPET molecules and extended their
conjugation length. While differences between S, R, and
racemic HT-PMOET were not observed, HT-P(S+R)MOET
LB films showed relatively low electrical conductivity.
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CONCLUSIONS
■
The synthesis and LB film formation of regioregular chiral
polythiophenes were investigated, and the effects of chirality
and side chain structure on the electrical and optical properties
of polythiophenes were determined. Regioregular chiral (S- and
R-) and achiral HT-PMOET and chiral HT-PMPET (S-type)
with greater than 92% head-to-tail coupling were synthesized
using the Rieke method. X-ray diffraction patterns of the cast
films indicated that HT-PMOET and HT-PMPET possessed a
well-organized lamellar structure, with a decrease in crystallinity
upon introduction of chirality and side chain extension. Chiral
HT-PMOET spin-coated films and aggregated state in solution
exhibited circular dichroism in the π−π* transition region. The
presence of a CD effect indicates that side chain chirality
induced optical activity in poly(thiophene) main chains. HT-
PMOET and HT-PMPET formed stable monolayers on the
water surface to produce highly electrically conductive and
chiral LB films. The introduction of suitable functional groups
and application of LB method can improve molecular
organization, which can also provide materials with excellent
optical and electrical properties.
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