Ferroelectric liquid-crystalline semiconductors based on a phenylterthiophene skeleton: Effect of the introduction of oligosiloxane moieties and photovoltaic effect

Article abstract of DOI:10.1039/c4tc01690d

Ferroelectric liquid-crystalline phenylterthiophene derivatives bearing a decenyl group, disiloxane chain, and cyclotetrasiloxane ring were synthesized. These compounds exhibited a chiral smectic C (SmC?) phase. The spontaneous polarizations of the compounds exceeded 50 nC cm^-2. The hole mobilities determined by the time-of-flight method were in the order of 1 × 10^-4 cm² V^-1 s^-1. The compound with a decenyl group exhibited an anomalous photovoltaic effect in the SmC? phase although the conversion efficiency was lower than 0.01%. Notably, even the compound bearing a bulky cyclotetrasiloxane ring exhibited an enantiotropic SmC? phase.

Full text of DOI:10.1039/c4tc01690d

Journal of
Materials Chemistry C
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PAPER
Ferroelectric liquid-crystalline semiconductors
based on a phenylterthiophene skeleton: effect of
the introduction of oligosiloxane moieties and
photovoltaic effect†
Cite this: J. Mater. Chem. C, 2015, 3,
1982
a
b
a
*
Yusuke Funatsu, Akinari Sonoda and Masahiro Funahashi
Ferroelectric liquid-crystalline phenylterthiophene derivatives bearing a decenyl group, disiloxane chain,
and cyclotetrasiloxane ring were synthesized. These compounds exhibited a chiral smectic C (SmC*)
phase. The spontaneous polarizations of the compounds exceeded 50 nC cmꢀ2. The hole mobilities
Received 31st July 2014
Accepted 14th December 2014
determined by the time-of-flight method were in the order of 1 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1. The compound
with a decenyl group exhibited an anomalous photovoltaic effect in the SmC* phase although the
DOI: 10.1039/c4tc01690d
conversion efficiency was lower than 0.01%. Notably, even the compound bearing
cyclotetrasiloxane ring exhibited an enantiotropic SmC* phase.
a bulky
www.rsc.org/MaterialsC
molecules tilt from the layer normal and form a helical
structure without an electric eld. Under a DC bias, the
helical structure disappears and the molecular dipoles orient
in the direction of the electric eld, resulting in spontaneous
polarization. By reversing the polarity of the electric
eld, the director of the LC molecules can be rapidly
changed.9c LC applications in displays have been studied
extensively.9d
LC semiconductors exhibiting a SmC* phase are expected to
facilitate novel electronic functions based on coupling between
electronic charge carrier transport and internal electric elds,
based on their ferroelectricity. The rst example of a ferroelec-
tric LC semiconductor was reported by Hanna and Funahashi.10
However, the spontaneous polarization was lower than 1 nC
cmꢀ2. No phenomena brought on by the synergism between
carrier transport and ferroelectricity have been observed in
ferroelectric LCs.
In this study, we synthesized LC phenylterthiophene deriv-
atives exhibiting SmC* phases, with spontaneous polarization
exceeding 100 nC cmꢀ2 and an effective hole mobility in the
order of 10ꢀ4 cm2 Vꢀ1 sꢀ1. An LC phenylterthiophene derivative
bearing a 1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane ring also
exhibited an enantiotropic SmC* phase. We observed an
anomalous photovoltaic effect based on the coupling of hole
transport and ferroelectricity in the SmC* phase.
Introduction
Liquid-crystalline (LC) semiconductors exhibit high carrier
mobilities exceeding 0.1 cm2 Vꢀ1 sꢀ1.1 The close and ordered
molecular packing structures in LC phases enhance the elec-
tronic charge carrier transport in columnar2 and smectic pha-
ses.3 Solution-processable LC oligothiophene and uorene
derivatives have been applied in eld-effect transistors (FETs)4
and polarized electroluminescence devices,5 respectively. Solar
cells based on LC phthalocyanine and LC diketopyrrolopyrrole
have also been reported.6 LC semiconductors are so and
consequently more suitable for exible devices than crystalline
semiconductors.4e
Moreover, the introduction of proper side chains and func-
tional groups into the p-conjugated cores of LC semiconductors
promotes nanosegregation.7a,b Three-dimensional network
structures are formed in the bicontinuous cubic phase of oli-
gothiophene, which has a polycatenar structure.7c LC phenyl-
terthiophene derivatives bearing imidazolium moieties exhibit
electrochromism in the smectic phase.7d,e Perylene tetra-
carboxylic bisimide derivatives bearing oligosiloxane moieties
exhibit nanosegregated columnar and lamellar phases at room
temperature.8
Some chiral LC molecules exhibit a ferroelectric chiral
smectic C (SmC*) phase.9a,b In the SmC* phase, the LC
Anomalous photovoltaic effects are exhibited by semi-
conductors showing ferroelectricity.11a In the presence of
internal electric elds, which are formed by spontaneous
polarization, excitons generated by illumination dissociate to
produce holes and electrons. They are transported to the elec-
trodes by the internal electric eld, resulting in photovoltaic
effects without the application of any external voltage.
aDepartment of Advanced Materials Science, Faculty of Engineering, Kagawa
University, 2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan. E-mail:
m-funa@eng.kagawa-u.ac.jp; Fax: +81-87-864-2411; Tel: +81-87-864-2411
bHealth Research Institute, National Institute of Advanced Industrial Science and
Technology, 2217-14 Hayashi-cho, Takamatsu, Kagawa 761-0395, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc01690d
1982 | J. Mater. Chem. C, 2015, 3, 1982–1993
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Anomalous photovoltaic effects have been observed in cells. Induced currents were recorded using a digital oscillo-
ferroelectric ceramics and their applications in solar cells have scope (Tektronics TDS 3044B) through a serial resistor or a
been proposed because solar cells can produce higher voltages serial capacitor.
than their band gaps.11a–c However, reports regarding these
effects in organic materials have been quite limited. Sasabe and
co-workers studied the photovoltaic effect of dye-doped poly-
vinylidene uoride, which is a typical ferroelectric polymer.11d
Synthesis of materials
All the 1H and 13C NMR spectra were recorded on a Varian
UNITY INOVA400NB spectrometer. FTIR measurements were
The density of the dye was very low and the resulting photo-
conducted on a JASCO FT/IR-660 Plus spectrometer. 4-Bromo-2-
current was very small.12a Tasaka and co-workers observed an
uorophenol and (S)-2-octanol were purchased from Tokyo
anomalous photovoltaic effect in the ferroelectric crystals of a
Chemical Industry. 5-Bromobithiophene was available from
triphenylene derivative.12b In this study, the sample was
Aldrich-Sigma
Inc.
1,1,1,3,3-Pentamethyl-1,3-disiloxane,
composed of vacuum-deposited crystalline thin lms.
In addition to conventional ferroelectric LC phases consist-
ing of chiral rod-like molecules, achiral bent-cores13a–c and
columnar LC compounds13d–f exhibit ferroelectricity in so
mesophases. Ferroelectric switching currents based on polari-
zation inversion and second harmonic generation have been
observed. However, anomalous photovoltaic effects have not
been conrmed in ferroelectric LC phases.
1,3,3,5,5,7,7-heptamethyl-1,3,5,7-cyclotetrasiloxane, and the
Karstedt catalyst were purchased from Gelest Inc. Tetrahydro-
furan, toluene, and dimethoxyethane were obtained from Wako
Pure Chemical Industries and were used without purication.
Silica gel was purchased from Kanto Chemicals.
LC phenylterthiophene bearing linear alkyl chains exhibit
ordered smectic phases at room temperature. Homogenous
thin lms can be produced by spin-coating and they can be
applied in eld-effect transistors.4b TOF measurements revealed
band-like hole transport in the ordered smectic phase.3h
Ferroelectric phenylterthiophene derivative 1 bearing a
chiral alkyl chain was designed. A uorine atom substituted on
the phenyl ring increases spontaneous polarization. Another
functional group can be introduced via the double bond at the
terminus of the alkyl chain.
Compound 1 was synthesized as shown in Scheme 1.
Phenyl ether 4 was obtained from 4-bromo-2-uorophenol by
Mitsunobu reaction.15a Compound 4 was converted to phenyl
boric acid ether 5 using Grignard reagent. Boric acid 5 was
reacted with 5-bromobithiophene via Suzuki coupling15b to
produce the chiral phenylbithiophene derivative 6.
Compound 6 was brominated via the reaction with N-bromo-
succinimide to afford compound 7, which was coupled with a
Experimental section
Characterization of LC phases
The mesomorphic properties of the phenylterthiophene deriv-
atives were studied by differential scanning calorimetry (DSC),
polarizing optical microscopy, and X-ray diffraction (XRD). A
polarizing optical microscope (Olympus DP70) equipped with a
hand-made hot stage was used for the visual observation of
optical textures. DSC measurements were conducted on a
NETZSCH DSC 204 Phoenix. XRD measurements were carried
out on a Rigaku Rapid II diffractometer using Ni-ltered CuKa
radiation.
Characterization of carrier transport and spontaneous
polarization
Electron mobilities were measured by the time-of-ight (TOF)
method.14 A liquid crystal cell was fabricated by combining two
ITO-coated glass plates. The cell was placed on a hot stage and
heated at 125 ꢂC. LC samples were melted and lled into the cell
via a capillary. The cells were cooled at a rate of 0.1 degree
minꢀ1. The liquid crystal cell was placed on the hot stage of the
TOF setup, a DC voltage was applied to the cell using an elec-
trometer (ADC R8252) and a pulse laser was illuminated on the
cell. The excitation source was the third harmonic generation of
an Nd:YAG laser (Continuum MiniLite II, wavelength ¼ 356 nm,
pulse duration ¼ 2 ns) and the induced displacement currents
were recorded using a digital oscilloscope (Tektronics TDS
3044B) through a serial resistor. In order to obtain non-
dispersive transient photocurrent curves, the penetration depth
of the laser light has to be sufficiently thinner than the sample
thickness. The penetration depths for the three compounds
were estimated to be less than 500 nm from the absorption
spectra (Fig. S1 in ESI†).
Spontaneous polarization was measured by the triangular
wave and Sawyer Tower methods. Triangular waves generated
Scheme 1 Synthetic route of ferroelectric phenylterthiophene deriv-
by a function generator (NF WF1973) were applied to the LC ative 1.
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2-decenylthienyl boric acid derivative via Suzuki coupling
1H NMR (400 MHz, CDCl3): d ¼ 7.21 (dd, 1H, J ¼ 10.8, 2.4
catalyzed by a palladium(0) complex to afford the chiral phe- Hz), 7.14 (ddd, 1H, J ¼ 8.8, 2.4, 1.2 Hz), 6.82 (t, 1H, J ¼ 8.8 Hz),
nylterthiophene derivative 1.
4.28 (sext, 1H, J ¼ 6.0 Hz), 1.71–1.80 (m, 1H), 1.52–1.60 (m 1H),
Conventional liquid crystals bear alkyl side chains of which 1.45–1.21 (m, 8H), 1.27 (3H, d, J ¼ 6.0 Hz), 0.86 (t, 3H, J ¼ 7.2
thermal motion moderately weakens the strong intermolecular Hz); IR (ATR): n ¼ 2927, 2855, 1581, 1491, 1467, 1409, 1378,
interaction between the aromatic cores. A few liquid crystals 1302, 1263, 1206, 1130, 1115, 1035, 973, 939, 854, 800, 724, 638,
bearing oligosiloxane moieties have been investigated.16 Sil- 573, 452, 401 cmꢀ1
sesquioxane derivatives bearing rod-like mesogens exhibit
;
elemental analysis calcd (%) for
C
14H20BrFO: C, 55.46; H, 6.65; Br, 26.35; F, 6.27; O, 5.28; found:
smectic phases.16a,b Smectic liquid crystals bearing trisiloxane C, 55.7; H, 6.7. Exact mass: 302.07; molecular weight: 303.21, m/
and disiloxane chains at the ends of their alkyl side chains have z: 302.05, 304.05.
been synthesized.16c,d They exhibit smectic phases stabilized by
the nanosegregation of the oligosiloxane chains. This effect is
also seen in bent-core ferroelectric LCs.16e Recently, perylene
tetracarboxylic bisimide (PTCBI) derivatives bearing oligosilox-
ane chains that exhibit columnar phases have been reported.8
PTCBI derivatives bearing tetracyclosiloxane rings also exhibit
columnar phases. Nanosegregation of oligosiloxane moieties is
a signicant factor in the formation of columnar phases.8d
In addition to compound 1, we synthesized chiral phenyl-
terthiophene derivatives 2 and 3 bearing a 1,1,1,3,3-pentam-
4-(2-(R)-Octyloxy)-3-uorophenylboric acid 2,2-dimethyl-1,3-
propanediyl ester (5)
Turnings of magnesium (0.84 g, 35 mmol) were dispersed in
tetrahydrofuran (30 mL). Compound 4 (7.38 g, 24 mmol) was
added to the dispersion and the solvent was gently reuxed.
Aer the formation of the Grignard reagent, the reaction
mixture was cooled to ꢀ78 ꢂC and a solution (5 mL) of trimethyl
borate (2.7 g, 25 mmol) in tetrahydrofuran was added. Aer the
reaction mixture was stirred for 3 h, the temperature was
increased to room temperature. 2,2-Dimethylpropanediol (3.64
g, 35 mmol) was added to the mixture, which was stirred for 3 h.
Water was added to the mixture, and the product was extracted
with n-hexane. The extract was dried over sodium sulfate. Aer
the solvent was evaporated, the residual mixture was puried by
silica gel column chromatography (eluent: n-hexane–ethyl
acetate ¼ 10/1 v/v). Colorless crystals (3.77 g, 11 mmol) were
obtained in 45% yield.
ethyl-1,3-disiloxane chain and
a 1,3,3,5,5,7,7-heptamethyl-
1,3,5,7-cyclotetrasiloxane ring, respectively. The introduction of
the oligosiloxane moieties onto the termini of the alkyl side
chains was carried out by hydrosilylation using the Karstedt
catalyst,17 as shown in Scheme 2.
2-(R)-(4-Bromo-2-uorophenyloxy)octane (4)
4-Bromo-2-uorophenol (5.83 g, 30 mmol), (S)-2-octanol (3.90 g,
30 mmol), and triphenylphosphine (7.88 g, 30 mmol) were
dissolved in toluene (50 mL). A solution of diethylazodicarbox-
ylate (2.2 mol Lꢀ1, 15 mL) in toluene was added to the mixture
for over 30 min at 0 ꢂC. The solution was stirred at room
temperature for 3 h. Then, n-hexane (50 mL) was added to the
reaction mixture, and the produced white precipitates were
ltered. The solvent was evaporated under reduced pressure,
and the residual oil was puried by silica gel column chroma-
tography (eluent: n-hexane). A pale yellow oil (8.32 g, 27 mmol)
was obtained in 90% yield.
1H NMR (400 MHz, CDCl3): d ¼ 7.47 (d, 1H, J ¼ 2.4 Hz), 7.44
(d, 1H, J ¼ 2.4 Hz), 6.91 (t, 1H, J ¼ 8 Hz), 4.38 (sext, 1H, J ¼ 6 Hz),
3.72 (s 4H), 1.72–1.81 (m, 1H), 1.52–1.61 (m, 1H), 1.35–1.48 (m,
2H), 1.24–1.31 (m, 6H), 1.31 (d, 3H, J ¼ 6.0 Hz), 0.99 (s, 6H), 0.86
(t, 3H, J ¼ 7.2 Hz); IR (ATR): n ¼ 2958, 2931, 2859, 1612, 1509,
1478, 1424, 1408, 1377, 1339, 1316, 1306, 1267, 1250, 1217,
1195, 1134, 1116, 1038, 991, 937, 892, 853, 813, 775, 748, 726,
703, 692, 675, 642, 564, 537, 496, 453, 401 cmꢀ1; elemental
analysis calcd (%) for C19H30BFO3: C, 67.87; H, 8.99; B, 3.22; F,
5.65; O, 14.27; found: C, 67.8; H, 9.1. Exact mass: 336.23;
molecular weight: 336.25, m/z: 336.23, 335.23, 337.23, 338.23.
5-{4-(2-(R)-Octyloxy)-3-uorophenyl}bithiophene (6)
Compound 5 (5.46 g, 16 mmol), 5-bromo-2,20-bithiophene (4.02
g, 16 mmol), and tetrakistriphenylphosphine palladium(0) (50
mg, 0.043 mmol) were dissolved in dimethoxyethane (150 mL).
An aqueous solution of sodium carbonate (10 wt%, 100 mL) was
added to the solution, and the resulting reaction mixture was
reuxed for 6 h. Dimethoxyethane was removed from the reac-
tion mixture under reduced pressure. The produced precipi-
tates were ltered and puried by silica gel column
chromatography (eluent: n-hexane–ethyl acetate ¼ 10/1 v/v).
Colorless crystals (5.75 g, 15 mmol) were obtained in 90% yield.
1H NMR (400 MHz, CDCl3): d ¼ 7.30 (dd, 1H, J ¼ 12.0, 2.4
Hz), 7.25 (ddd, 1H, J ¼ 8.8, 2.0, 1.2 Hz), 7.20 (dd, 1H, J ¼ 4.8, 0.8
Hz), 7.17 (dd, 1H, J ¼ 3.6, 1.2 Hz), 7.10 (d, 1H, J ¼ 3.6 Hz), 7.09
(d, 1H, J ¼ 3.6 Hz), 7.01 (dd, 1H, J ¼ 5.6, 3.6 Hz), 6.95 (t, 1H, J ¼
8.8 Hz), 4.36 (sextet, 1H, J ¼ 6.0 Hz), 1.83–1.73 (m 1H), 1.64–1.53
Scheme 2 Synthetic route of chiral phenylterthiophene derivatives
bearing oligosiloxane moieties.
1984 | J. Mater. Chem. C, 2015, 3, 1982–1993
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(m, 1H), 1.31 (d, 3H, J ¼ 6.0 Hz), 1.30–1.27 (m, 7H), 0.87 (t, 3H, J 696, 680, 658, 571, 497, 483 cmꢀ1; Elemental analysis calcd (%)
¼ 6.8 Hz); IR (ATR): n ¼ 2951, 2919, 2854, 1611, 1577, 1521, for C19H31BO2S: C, 68.26; H, 9.35; B, 3.23; O, 9.57; S, 9.59; found:
1504, 1466, 1428, 1375, 1300, 1266, 1245, 1226, 1205, 1123, C, 68.4; H, 9.4. Exact mass: 334.21; molecular weight: 334.32, m/
1064, 1028, 995, 967, 940, 888, 858, 848, 834, 800, 781, 729, 716, z: 334.21.
696, 647, 616, 534, 473, 448, 420 cmꢀ1; Elemental analysis calcd
5-(9-Decenyl)-500-{4-(2-(R)-octyloxy)}-3-uorophenyl}-2,20-50,200-
terthiophene (1)
(%) for C22H25FOS2: C, 68.00; H, 6.49; F, 4.89; O, 4.12; S, 16.50;
found: C, 68.5; H, 6.8. Exact mass: 388.13; molecular weight:
388.56, m/z: 388.13, 389.14, 390.13, 390.14.
Compound 7 (2.7 g, 5.8 mmol), compound 8 (2.89 g, 8.7 mmol),
and tetrakistriphenylphosphine palladium(0) (50 mg, 0.043
mmol) were dissolved in dimethoxyethane (150 mL). An
5-Bromo-50-{4-(2-(R)-octyloxy)-3-uorophenyl} bithiophene (7)
Compound 6 (5.75 g, 15 mmol) was dissolved in tetrahydro- aqueous solution of sodium carbonate (10 wt%, 100 mL) was
furan (200 mL). N-Bromosuccinimide (3.2 g, 18 mmol) was added, and the mixture was reuxed for 6 h. Dimethoxyethane
added to the solution over a period of 30 min. An aqueous was removed from the reaction mixture under reduced pres-
solution of sodium carbonate (10 wt%) was then added to the sure. The produced precipitate was ltered and puried by
reaction mixture, and the product was extracted with ethyl silica gel column chromatography (eluent: n-hexane–ethyl
acetate. The extract was dried over sodium carbonate and the acetate ¼ 10/1 v/v). Pale yellow crystals (1.56 g, 2.6 mmol) were
solvent was evaporated under reduced pressure. The residual obtained in 44% yield.
material was puried by silica gel column chromatography
1H NMR (400 MHz, CDCl3): d ¼ 7.29 (dd, 1H, J ¼ 12.0, 2.4
(eluent: n-hexane–ethyl acetate ¼ 10/1 v/v). The product was Hz), 7.25 (ddd, 1H, J ¼ 8.4, 2.0, 1.2 Hz), 7.09 (d, 1H, J ¼ 3.6 Hz),
recrystallized from n-hexane. Yellow crystals of compound 7 (5.3 7.07 (d, 1H, J ¼ 3.6 Hz), 7.04 (d, 1H, J ¼ 4.0 Hz), 6.98 (d, 1H, J ¼
g, 11 mmol) were obtained in 76% yield.
3.6 Hz), 6.97 (d, 1H, J ¼ 3.6 Hz), 6.94 (t, 1H, J ¼ 6.3 Hz), 6.67 (d,
1H NMR (400 MHz, CDCl3): d ¼ 7.28 (dd, 1H, J ¼ 12.0, 2.0 1H, J ¼ 3.6 Hz), 5.80 (ddt, 1H, J ¼ 16.8, 10.4, 6.8 Hz), 5.00 (ddd,
Hz), 7.23 (dd, 1H, J ¼ 8.0, 2.0 Hz), 7.07 (d, 1H, J ¼ 4.0 Hz), 7.03 1H, J ¼ 16.8, 3.6, 1.2 Hz), 4.92 (ddt, 1H, J ¼ 10.4, 3.6, 1.2 Hz),
(d, 1H, J ¼ 4.0 Hz), 6.96 (d, 1H, J ¼ 4.0 Hz), 6.94 (t, 1H, J ¼ 8.4 4.36 (sextet, 1H, J ¼ 6.0 Hz), 2.78 (2H, t ¼ J ¼ 7.6 Hz), 2.03
Hz), 6.90 (d, 1H, J ¼ 4.0 Hz), 4.36 (sextet, 1H, J ¼ 6.0 Hz), 1.73– (quintet, 2H, J ¼ 6.8 Hz), 1.82–1.73 (m, 1H), 1.67 (quintet, 2H, J
1.83 (m, 1H), 1.56–1.63 (m, 1H), 1.35–1.51 (m, 2H), 1.31, (d, 3H, J ¼ 7.2 Hz), 1.64–1.56 (m, 1H), 1.40–1.25 (m, 18H), 1.31 (d, 3H, J ¼
¼ 6.0 Hz), 1.23–1.35 (m, 6H), 0.86 (t, 3H, J ¼ 6.8 Hz); IR (ATR): n 6.4 Hz), 0.87 (t, 3H, J ¼ 6.4 Hz); 13C NMR (100 MHz CDCl3): d ¼
¼ 2927, 2855, 1615, 1575, 1556, 1524, 1498, 1460, 1427, 1377, 145.9, 139.4, 137.0, 136.4, 135.6, 134.6, 125.0, 124.5, 124.3,
1350, 1299, 1268, 1238, 1174, 1126, 1057, 999, 969, 937, 864, 123.7, 123.6, 121.6, 121.5, 118.0, 114.3, 114.0, 113.8, 113.5, 76.8,
788, 724, 688, 610, 577, 532, 477, 448 cmꢀ1; Elemental analysis 36.7, 34.0, 32.0, 31.8, 30.4, 29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 25.6,
calcd (%) for C22H24BrFOS2: C, 56.53; H, 5.17; Br, 17.09; F, 4.06; 22.8, 20.0, 14.3, 14.2; IR (ATR): n ¼ 3072, 2919, 2848, 1640, 1616,
O, 3.42; S, 13.72; found: C, 56.8; H, 5.1. Exact mass: 466.04; 1576, 1546, 1524, 1506, 1464, 1427, 1379, 1303, 1268, 1234,
molecular weight: 467.46, m/z: 468.04, 466.04, 469.04.
1209, 1123, 1069, 996, 942, 911, 894, 857, 810, 799, 790, 726, 661,
630, 594, 549, 474, 447 cmꢀ1; Elemental analysis calcd (%) for
C
36H45FOS3: C, 71.01; H, 7.45; F, 3.12; O, 2.63; S, 15.80; found: C,
2-(9-Decenyl)thienylboric acid 2,2-dimethyl-1,3-propanediyl
ester (8)
71.4; H, 7.6. Exact mass: 608.26; molecular weight: 608.94, m/z:
608.26, 609.27.
Reagent 2-(9-decenyl)thiophene (30.0 g, 0.13 mol) was dissolved
in tetrahydrofuran (20 mL). The solution was cooled to 0 ꢂC and
n-butyllithium (99.7 mL, 1.6 M/n-hexane solution) was added
over a 2 h period. The reaction mixture was cooled to ꢀ78 C
5-{10-(1,1,1,3,3-Pentamethyl-1,3-disiloxanyl)decan-1-yl}-500-{4-
(2-(R)-octyloxy)}-3-uorophenyl}-2,20-50,200-terthiophene (2)
ꢂ
and a solution (20 mL) of trimethyl borate (13.8 g, 0.13 mol) in Compound 1 (0.50 g, 0.82 mmol) and 1,1,1,3,3-pentamethyl-1,3-
tetrahydrofuran was added. While stirring for 6 h, the temper- disiloxane were dissolved in toluene (30 mL). The Karstedt
ature of the reaction mixture increased to room temperature. To catalyst (1,3-divinyl-1,1,3,3-tetramethyldisiloxane platinum(0),
the mixture, 2,2-dimethyl-1,3-propanediol (13.8 g, 0.13 mol) was 10 mL, 3.4 atom% in xylene) was added, and the solution was
added. Aer 4 h, water was added to the mixture. The product stirred for 12 h at room temperature. The solvent was evapo-
was extracted with ethyl acetate and the extract was dried over rated and the resulting crude product was puried by silica gel
sodium sulfate. The crude product was puried by silica gel column chromatography (eluent: n-hexane–ethyl acetate ¼ 10/1
chromatography (eluent: n-hexane). The white solid (20.4 g, 60 v/v). The yellow waxy product (0.21 g, 0.27 mmol) was obtained
mmol) was obtained in 45% yield.
in 34% yield.
1H NMR (400 MHz, CDCl3): d ¼ 7.38 (d, 1H, J ¼ 4.0 Hz), 6.81
1H NMR (400 MHz, CDCl3): d ¼ 7.30 (dd, 1H, J ¼ 12.0, 2.0
(d, 1H, J ¼ 4.0 Hz), 5.77 (ddt, 1H, J ¼ 16.8, 10.4, 6.4 Hz), 4.96 (dd, Hz), 7.25 (dd, 1H, J ¼ 8.4, 2.0 Hz), 7.10 (d, 1H, J ¼ 4.0 Hz), 7.08
1H, J ¼ 16.8, 3.6 Hz), 4.88 (dd, 1H, J ¼ 10.4, 3.6 Hz), 3.73 (s, 4H), (d, 1H, J ¼ 4.0 Hz), 7.05 (d, 1H, J ¼ 4.0 Hz), 6.99 (d, 1H, J ¼ 3.6
2.80 (t, 2H, J ¼ 8.0 Hz), 2.02 (quartet, 2H, J ¼ 6.4 Hz), 1.68 Hz), 6.97 (d, 1H, J ¼ 3.6 Hz), 6.95 (t, 1H, J ¼ 6.3 Hz), 6.67 (d, 1H, J
(quintet, 2H, J ¼ 8.0 Hz), 1.24–1.39 (m, 10H), 1.00 (s, 6H); IR ¼ 3.6 Hz), 4.36 (sextet, 1H, J ¼ 6.0 Hz), 2.78 (t, 2H, J ¼ 3.6 Hz),
(ATR): n ¼ 2924, 2850, 1643, 1537, 1480, 1465, 1434, 1415, 1378, 1.84–1.75 (s, 1H), 1.69 (quintet, 2H, J ¼ 7.2 Hz), 1.65–1.57 (m,
1326, 1286, 1252, 1206, 1108, 1043, 993, 905, 848, 811, 779, 722, 1H), 1.42–1.25 (m, 18H), 1.32 (d, 3H, J ¼ 6.4 Hz), 0.87 (t, 3H, J ¼
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6.8 Hz), 0.49 (t, 2H, J ¼ 7.2 Hz), 0.05 (s, 9H), 0.02 (6H, s); 13C
NMR (100 MHz CDCl3): d ¼ 0.6, 2.2, 14.3, 18.6, 20.0, 22.8, 23.5,
25.6, 29.3, 29.4, 29.6, 29.8, 30.4, 31.8, 32.0, 33.6, 36.7, 76.8,
113.8, 114.0, 118.0, 121.5, 121.6, 122.5, 123.6, 123.7, 124.3,
124.5, 125.0, 134.6, 135.6, 136.5, 137.1, 145.9; IR (ATR): n ¼
2957, 2922, 2853, 1575, 1545, 1523, 1505, 1445, 1378, 1300,
1252, 1233, 1125, 1052, 942, 840, 789, 753, 625, 477, 447 cmꢀ1
;
Elemental analysis calcd (%) for C41H61FO2S3Si2: C, 65.03; H,
8.12; F, 2.51; O, 4.23; S, 12.70; Si, 7.42; found: C, 65.40; H, 8.1.
Exact mass: 756.34; molecular weight: 757.29, m/z: 756.34,
757.34, 758.33, 758.34, 759.33.
5-{10-(1,3,3,5,5,7,7-Heptamethyl-1,3,5,7-cyclotetrasiloxan-1-yl)
decan-1-yl}-500-{4-(2-(R)-octyloxy)}-3-uorophenyl}-2,20-50,200-
terthiophene (3)
Compound 1 (0.75 g, 1.24 mmol) and 1,3,3,5,5,7,7-heptamethyl-
1,3,5,7-cyclotetrasiloxane (0.52 g, 1.86 mmol) were dissolved in
toluene (30 mL). The Karstedt catalyst (1,3-divinyl-1,1,3,3-tet-
ramethyldisiloxane platinum(0), 10 mL, 3.4 atom% in xylene)
was added, and the solution was stirred for 12 h at room
temperature. The solvent was evaporated and the resulting
crude product was puried by silica gel column chromatog-
raphy (eluent: n-hexane–ethyl acetate ¼ 10/1 v/v). The yellow
waxy product (0.21 g, 0.27 mmol) was obtained in 34% yield.
1H NMR (400 MHz, CDCl3): d ¼ 7.31 (dd, 1H, J ¼ 12.4, 2.4
Hz), 7.26 (ddd, 1H, J ¼ 8.4, 2.4, 1.2 Hz), 7.10 (d, 1H, J ¼ 3.6 Hz),
7.09 (d, 1H, J ¼ 3.6 Hz), 7.06 (d, 1H, J ¼ 4.0 Hz), 7.00 (d, 1H, J ¼
4.0 Hz), 6.98 (d, 1H, J ¼ 4.0 Hz), 6.96 (t, 1H, J ¼ 8.4 Hz), 6.68 (d,
1H, J ¼ 4.0 Hz), 4.37 (sextet, 1H, J ¼ 6.0 Hz), 2.79 (t, 2H, J ¼ 3.6
Hz), 1.84–1.75 (m, 1H), 1.68 (quintet, 2H, J ¼ 7.2 Hz), 1.65–1.57
(m, 1H), 1.42–1.25 (m, 18H), 1.33 (d, 3H, J ¼ 6.8 Hz), 0.89 (t, 3H,
J ¼ 6.8 Hz), 0.50 (dd, 2H, J ¼ 9.2, 6.0 Hz), 0.09 (s, 18H), 0.07 (s,
3H); 13C NMR (100 MHz CDCl3): d ¼ 145.9, 136.4, 135.5, 135.4,
134.6, 125.0, 124.5, 124.3, 123.7, 123.6, 121.6, 121.5, 118.0,
117.9, 114.0, 113.8, 113.7, 76.7, 36.6, 33.3, 33.2, 31.9, 31.8, 30.4,
30.3, 29.8, 29.7, 29.5, 29.4, 29.3, 25.6, 25.5, 23.1, 22.8, 20.0, 17.3,
14.2, 0.96, 0.93; IR (ATR): n ¼ 2961, 2923, 2853, 1545, 1524, 1507,
1445, 1378, 1300, 1259, 1234, 1060, 941, 854, 789, 696, 553, 477
cmꢀ1; Elemental analysis calcd (%) for C43H67FO5S3Si4: C,
57.93; H, 7.57; F, 2.13; O, 8.97; S, 10.79; Si, 12.60; found: C, 57.8;
H, 7.6. Exact mass: 890.32; molecular weight: 891.52, m/z:
890.52, 891.52, 892.52.
Fig. 1 DSC thermograms of (a) compound 1, (b) compound 2, and (c)
compound 3. The heating and cooling rates are 10 K minꢀ1. The green
curves exhibit the thermograms of materials formed via recrystalliza-
tion from n-hexane.
ꢂ
129 C, respectively. The SmF phases were observed below the
SmC* phase. As reported previously for LC molecules bearing
disiloxane chains at the end of their alkyl side chain,16c–e the
temperature range of the SmC* phase for compound 2 was
slightly lower than that of compound 1.
Results and discussion
Characterization of mesophases of compounds 1–3
Compounds 1–3 exhibited an enantiotropic SmC* phase, as
well as more ordered smectic G (SmG) and smectic F (SmF)
phases. Fig. 1 shows the DSC thermograms of compounds 1–3.
Compound 1 formed crystals via recrystallization from n-
hexane. The crystals changed to the SmG phase at 58 ꢂC. Upon
cooling, they did not crystallize and the SmG phase was
retained. Compounds 2–3 formed waxy mesomorphic precipi-
tates during recrystallization from n-hexane and did not crys-
tallize upon cooling to ꢀ100 ꢂC. Compounds 1 and 2 exhibited
Surprisingly, compound 3 with a bulky cyclotetrasiloxane
ring also exhibited a SmC* phase between 110 and 122 ꢂC.
Below 110 ꢂC, a SmF phase appeared and a glass transition
associated with the thermal motion of the cyclotetrasiloxane
ꢂ
moiety was observed at ꢀ70 C.
Fig. 2(a) shows the XRD patterns of compound 1. In the high-
temperature phase, peaks derived from the (001), (002), and
(003) diffraction planes were observed at 2q ¼ 2.97ꢂ, 6.35ꢂ, and
9.47ꢂ in the low-angle region. A broad halo associated with the
ꢂ
an SmC* phase between 124 and 140 C and between 116 and
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dimeric aggregation in the low-temperature phase, and these
peaks were indexed to the (002), (003), (004), and (007)
diffraction planes. The diffraction peak derived from the (001)
plane was not observed because it was shielded by a stopper.
In the wide-angle region, two halos around 12ꢂ and 18ꢂ were
attributed to the liquid-like orders of disiloxane and the alkyl
chains, respectively. The lattice constant derived from the
˚
(002) diffraction was 33.5 A, which was shorter than the fully
˚
extended length of compound 2 (42 A), indicating that the LC
molecules tilt from the normal of the layers or the disiloxane
chains interdigitate. Thus, the high-temperature phase was
identied as the SmC* phase. In the low-temperature phase, a
broad peak around 18ꢂ was observed in addition to the low-
angle peaks associated with the lamellar organization of the
LC molecules. This broad peak suggested a hexatic bond order
within the layers. The low-temperature phase should be a SmF
phase. From the low-angle peaks, the dimeric structure was
retained in the low-temperature phase. The interaction
between the disiloxane chains promoted the formation of the
dimeric layer structure.
Fig. 2(c) shows the XRD patterns of compound 3 bearing a
bulky cyclotetrasiloxane ring. In the high-temperature phase,
three low angle peaks derived from the (001), (002), (003), and
(004) diffraction planes were observed. In the high-angle
region, two halos appeared. The halo around 18ꢂ was attrib-
uted to the liquid-like packing of alkyl chains, and the other
halo around 12ꢂ was ascribed to the disordered aggregation of
the cyclotetrasiloxane rings. No diffraction peaks indicating
the long-range order of the molecular position within the
˚
smectic layers were observed. The layer spacing of 36.2 A was
shorter than the extended molecular length, indicating the
tilted orientation of the molecules in the smectic layers.
Therefore, the high-temperature phase was assigned to the
SmC* phase.
The XRD patterns revealed that the low-temperature phase
should be an SmF phase. The low-angle peaks were assigned
to the (001), (002), (003), and (004) diffraction planes. Similar
to compound 2, the positional order within the smectic layers
was ambiguous in the low-temperature phase. The layer
spacing was also shorter than the extended molecular length.
Two broad halos were observed around 12ꢂ and 18ꢂ, which
were derived from the liquid-like aggregation of the cyclo-
Fig.
2 X-ray diffraction patterns in the smectic phases of (a)
compound 1, (b) compound 2, and (c) compound 3.
liquid-like packing of the alkyl chains appeared around 2q ¼ tetrasiloxane rings and alkyl chains, respectively. Compared
16ꢂ. The layer spacing estimated from the (001) diffraction was to the SmG phase of compound 1, the diffraction peaks
˚
29.6 A, which was shorter than the extended molecular length of associated with the positional order within the smectic layers
˚
37 A. Therefore, this phase was assigned to the SmC* phase. In were broad, indicating a more disordered structure in the
the low-temperature phase, wide-angle peaks derived from the SmF phase18 for compound 3.
(200), (010), (110), and (210) diffraction planes were observed,
In contrast to compounds 1 and 3, compound 2 exhibited
indicating the long-range rectangular order of the molecular a dimeric SmC* phase. In compound 2, the interaction
position within the layers. The layer spacing was shorter than between the disiloxane chains and their interdigitation
the extended molecular length, indicating that the low- should promote the formation of a dimeric layer structure. In
temperature phase was an SmG phase.18
compound 3, the same interaction between the cyclo-
Fig. 2(b) displays the XRD patterns of compound 2. In the tetrasiloxane rings should be plausible. However, the inter-
high-temperature phase, the ratio of the lattice constants digitation between the cyclotetrasiloxane rings of compound
determined by the low angle peaks at 2q ¼ 2.61ꢂ, 4.04ꢂ, 4.83ꢂ, 3 is difficult because of their bulkiness as compared to the
and 9.23ꢂ was 1/2 : 1/3 : 1/4 : 1/7. This result suggested disiloxane rings of compound 2. Therefore, a monomeric
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Characterization of the spontaneous polarization in the SmC*
phases of compounds 1–3
Spontaneous polarizations in the SmC* phases of compounds
1–3 were determined by the triangular wave method. Fig. 5(a)
shows the polarization inversion current curves in the SmC*
phases of compounds 1–3 when triangular voltages were
applied to the LC cells. As observed in the SmC* phase of
conventional ferroelectric LCs, current peaks caused by polari-
zation inversion were obtained in the current response curves.
In the SmC* phase of compound 1, the polarization inver-
sion occurred in several microseconds. This rate was slower
than that of conventional ferroelectric LCs. The higher viscosity
attributed to stronger p–p interactions in the SmC* phase of
compound 1 should result in this slow inversion. The sponta-
neous polarization was determined to be 151 nC cmꢀ2. This
value was considerably higher than those of classical ferro-
electric LCs such as DOBANBC9b and ferroelectric phenyl-
naphthalene derivatives.10
Fig. 3 Polarizing optical micrographs without the application of a DC
bias in the SmC* phase of (a) compound 1 at 130 ꢂC, (b) compound 2 at
125 ꢂC, and (c) compound 3 at 120 ꢂC.
layer structure should be favorable in compound 3 in spite of
the presence of oligosiloxane moieties.
Polarizing optical micrographic textures of compounds 1–3
In the SmC* phases of compounds 1 and 3, broken fan-like
domains with stripe patterns were observed without an electric
eld under a polarizing optical microscope (Fig. 3). The stripe
patterns were derived from the helical structures of the SmC*
phases of the compounds. The helical pitches were about 2 mm
for compound 1, 3 mm for compound 2, and 10 mm for
compound 3.
In the SmC* phase of compound 2, the spontaneous polar-
ization was 120 nC cmꢀ2. The bulky disiloxane moiety should
slightly inhibit close molecular aggregation in the SmC* phase
As observed in conventional ferroelectric SmC* phases, the
stripe patterns disappeared when a DC voltage was applied to
the LC cells. As shown in Fig. 4, the transmittances of the
domains changed with the crossed polarizers when the polarity
of the DC voltage was inverted. This behavior is typical of a
ferroelectric SmC* phase. The transmittance switching of the
optical textures was based on the change in the molecular
orientation caused by the inversion of the spontaneous
polarization.
Fig. 5 (a) Polarization inversion current measured by the triangular
Fig.
4
Polarizing optical micrographs of the SmC* phases for wave method in the SmC* phase of compound 1 at 130 ꢂC, compound
compound 1 under the application of a (a) positive and (b) negative bias 2 at 125 ꢂC, and compound 3 at 120 ꢂC. The sample thickness was 2
(ꢃ1.5 V for 4 mm thick sample) as well as for compound 2 ((c) positive mm. (b) Hysteresis loops in the SmC* phase of compounds 1 (130 ꢂC), 2
and (d) negative bias (ꢃ5 V for 9 mm thick sample)) and compound 3 (125 ꢂC), and 3 (120 ꢂC). The frequency was 100 Hz and the capaci-
((e) positive and (f) negative bias (ꢃ5 V for 9 mm thick sample)).
tance of serial capacitor was 33 nF.
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of compound 2 and decrease the macroscopic polarization. The
more disordered structure of the SmC* phase of compound 2
than that of compound 1 should decrease the viscosity of the
SmC* phase of compound 2. The polarization inversion was
more rapid than that of compound 1.
In the SmC* phase of compound 3, the disorder of the
molecular aggregation structure is more remarkable, compared
to compounds 1 and 2. The measurement revealed a sponta-
neous polarization of 62 nC cmꢀ2 in the SmC* phase of
compound 3. This value was lower than that of compound 1 by a
factor of 2.5. Compound 3 bears a very bulky cyclotetrasiloxane
ring, which perturbs close molecular aggregation in its SmC*
phase. This disordered structure inhibits macroscopic ordering
of molecular dipole moments. The response time in the SmC*
phase of compound 3 was two times slower than that of
compound 1. This slower response in the polarization current
for compound 3 should also be attributed to the higher viscosity
in its SmC* phase. The temperature range of the SmC* phase of
compound 3 was 10 degrees lower than that of compound 1 and
thermal uctuation was smaller in the SmC* phase of
compound 3 than the case of compound 1. The larger molecular
weight of compound 3 than that of compound 1 should also
contribute to the higher viscosity in the SmC* phase of
compound 3.
Fig. 5(b) exhibits hysteresis D–V loops in the SmC* phase of
compounds 1–3, which were measured by the Sawyer–Tower
method. For the SmC* phase of all the compounds, ferroelectric
hysteresis loops were obtained. The values of the obtained
spontaneous polarizations for compounds 1–3 were 83 nC
cmꢀ2, 58 nC cmꢀ2, and 38 nC cmꢀ2, which are about half of
those determined by the triangular method. The hysteresis loop
indicates that the coercive voltage and electric eld in the SmC*
phase of compound 1 are around 1 V and 5 ꢁ 103 V cmꢀ1
,
respectively.
Carrier transport properties in the SmC* phase of compounds
1–3
The carrier mobilities in the SmC* phases of compounds 1–3
were determined by the TOF method. In the SmC* phases of
compounds 1–3, non-dispersive transient photocurrent signals
originating from hole transport were observed. The transient
photocurrent signals for electrons were too weak for the deter-
mination of electron mobilities.
Fig. 6(a) shows the transient photocurrent curves for holes in
the SmC* phase of compound 1 at 130 ꢂC. Non-dispersive
transient photocurrent curves with initial spikes were obtained
and transit times were determined by the linear plots of the
curves. The hole mobility calculated from the transit times was
3 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1, and was independent of the electric eld
and temperature.
Fig. 6(b) shows the transient photocurrent curves for holes in
the SmC* phase of compound 2 at 125 ꢂC. Non-dispersive
transient curves were obtained and the hole mobility was
determined to be 2 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1. The hole mobility was
independent of the electric eld and temperature in the SmC*
phase.
Fig. 6 Transient photocurrent curves for holes in the SmC* phases of
(a) compound 1 at 130 ꢂC (sample thickness ¼ 15 mm), (b) compound 2
at 125 ꢂC (sample thickness ¼ 9 mm), and (c) compound 3 at 120 ꢂC
(sample thickness ¼ 9 mm).
Fig. 6(c) shows the transient photocurrent curves for the
holes in the SmC* phase of compound 3 at 120 ꢂC. Non-
dispersive transient photocurrent curves with delays in the rise
of the curves were observed. The transit times were determined
from the linear plots of the transient photocurrent curves. The
hole mobility was 1 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1. The hole mobility was
also independent of the electric eld and temperature.
In the smectic phases of LC semiconductors above room
temperature, temperature- and eld-independent mobilities are
typically observed.3a,b These temperature- and eld-
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independent mobilities are explained based on Gaussian
disorder19 or small polaron hopping models.20 In contrast to
amorphous organic semiconductors in which carrier mobilities
are strongly dependent on the electric eld and the tempera-
ture, smaller energetic and positional disorders result in a
smaller dependency of the carrier mobilities in the liquid crystal
phases.2g,3g In addition, the positive temperature-dependence of
the carrier mobility is cancelled by the negative temperature-
dependence, which is derived from the thermal uctuation of
the mesomorphic structures.8e
In amorphous organic semiconductors, the dipole
moments of semiconductor molecules increase the energetic
disorder because local electric elds produced by randomly
oriented molecular dipoles increase the dispersion width of
the energy levels of semiconductor molecules.21 In the
columnar phase of a nitrotriphenylene derivative, increased
energetic disorder was reported.21c However, molecular
dipoles are oriented in one direction in the SmC* phase under
the application of electric elds. Therefore, the temperature-
and eld-dependencies in the SmC* phases of compounds 1–3
should not be remarkable.
Anomalous photovoltaic effect in the SmC* phase of
compounds 1 and 3
Conventional photovoltaic effects are caused by an internal
electric eld, which is produced at the p–n junction or Schottky
barrier formed at the interface between a semiconductor layer
and an electrode. In contrast, anomalous photovoltaic effects
are observed in ferroelectrics and they originate from the elec-
tric eld generated by spontaneous polarization. The polarity of
the photovoltaic effect is determined by the polarity of the
internal electric eld. Aer the application of a DC voltage
pulse, light illumination under zero bias produces a photocur-
rent with a reversed polarity to the DC bias prior to the light
illumination. The inversion of the DC bias direction changes
the polarity of the photovoltaic effect under zero bias.12b
Fig. 7(a) shows the steady state photocurrent response under
zero bias. In the initial state, in which the sample was cooled
from the isotropic phase to the SmC* phase, the photocurrent
response was quite ambiguous. In this state, the dipole
moments of the molecules did not orient macroscopically
because of the helical structure of the SmC* phase. However,
aer the application of the DC voltage, a clear photocurrent
response was observed. It should be emphasized that the
polarities of the photocurrents were opposite to those of the DC
biases applied prior to light illumination. In these states aer
the application of the positive and negative biases, the helical
structures disappeared and macroscopic spontaneous polari-
zations and internal electric elds were produced. The internal
elds are opposite to the polarities of the DC biases applied
prior to the light illumination.
Fig. 7 (a) Steady state photocurrent response in the SmC* phase of
compound 1 at 127 ꢂC under zero bias. The sample thickness was 2 mm
and the electrode area was 0.16 cm2. (b) Current–voltage character-
istics in the same sample. The wavelength and intensity of the illumi-
nated light were 360 nm and 3 mW cmꢀ2, respectively.
holes are generated and efficiently transported in the SmC*
phase as veried in the TOF experiments.
Fig. 7(b) exhibits the current–voltage characteristic of the
same sample of compound 1. The open circuit voltage was 0.35
V and the short circuit current was 200 nA. In the SmC* phase of
compound 1, the coercive electric eld and voltage are 1 ꢁ 104 V
cmꢀ1 and 2 V at 100 Hz, respectively, as shown in Fig. 5(b). This
result indicates that the spontaneous polarization is retained
for several ms. However, the spontaneous polarization should
be partially relaxed on the time scale (several seconds) of the
experiment of the steady-state photocurrent measurement.
The transient photovoltaic effect was also studied. Fig. 8(a)
shows the photocurrents under zero bias aer pulse laser illu-
mination in the SmC* phase of compound 1 at 130 ꢂC. Prior to
the photocurrent measurement, DC biases of 0 V, +20 V, and
ꢀ20 V were applied to the sample for 1 min. When the DC bias
was 0 V prior to illumination, the photocurrent under zero bias
was almost zero. Immediately aer the light illumination, a
non-zero photocurrent was generated, but it was very low. A
strong spike should be caused by the local electric eld formed
on the electrode surface, due to local molecular alignment.
When the DC bias was +20 V prior to the light illumination, a
photocurrent of approximately 10 nA was generated. The
When the illuminated electrode was negatively biased, the
photocurrent response under zero bias was stronger than that
observed aer the application of the positive DC voltage. In the
case of the stronger photocurrent response, the photocurrent
was attributed to the hole transport. Compared to electrons, the
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measurements were smaller than the coercive electric eld
determined at 100 Hz by the Sawyer–Tower method, as shown in
Fig. 5(b). The partial relaxation of the spontaneous polarization
should decrease the internal electric eld.
The value of spontaneous polarization (145 nC cmꢀ2) should
lead to the production of an electric eld in the order of 5 ꢁ 105
V cmꢀ1. In this experiment, the induced internal eld was in the
order of 103 V cmꢀ1, which was considerably smaller than the
theoretically expected value. In this case, the sample thickness
was 2 mm. Near the electrode–LC interface, ferroelectric polar-
ization should be retained although it should be relaxed in the
bulk under the zero-eld because of the reconstruction of the
helical structure.
This zero-eld photocurrent was not observed in the
isotropic phase without spontaneous polarization. In the SmG
phase, the inversion of the sign of the zero-eld photocurrent
did not occur when the polarity of the DC voltage was changed
prior to the laser illumination.
For compounds 2 and 3, zero-eld photocurrents were
observed in the SmC* phase and the polarities were changed
according to the sign of the DC bias prior to laser illumination.
However, the value of the photocurrent was weaker than that of
compound 1. Such photovoltaic effects can be caused by the
formation of an electrical double layer at the interface between
the electrode and the LC layer. In the samples containing ionic
impurities such as sodium and halogen ions, which are
contaminated in the synthetic processes, additional peaks other
than the peak originating from the ferroelectric polarization
inversion are observed in the triangular wave measurements.
Fig. 8 (a) Photocurrent under zero bias after laser pulse (wavelength ¼
356 nm, pulse duration ¼ 2 ns) illumination in the SmC* phase of
compound 1 at 130 ꢂC. Before the laser illumination, DC voltages of 0,
20, and ꢀ20 V were applied to the sample (thickness of 2 mm) for 1 min. However, no additional peaks associated with ionic transport
(b) Transient photocurrent curves under DC biases of 0.5 and ꢀ0.5 V in
the SmC* phase of compound 1.
were observed in the SmC* or isotropic phases. The detection
limit of the current peaks in the triangular wave experiment was
about 1 nC cmꢀ2. The charge formed by ionic impurities on the
electrodes should be lower than 1 nC cmꢀ2, which is two orders
of magnitude lower than the spontaneous polarization in the
SmC* phase of compound 1.
The anomalous photovoltaic effect can produce a higher
voltage than the band gap of the semiconductors. The produc-
tion of thin lms of the ferroelectric phenylterthiophene
derivatives and measurements of the photovoltaic effect in the
thin lm states are in progress.
polarity of the photocurrent was reversed to that of the DC bias
prior to the illumination. In the case of the DC bias of ꢀ20 V
before the illumination, a photocurrent of over 10 nA was
generated and the polarity was reversed. In the isotropic phase,
the generated photocurrents under zero bias were considerably
smaller than those in the SmC* phase. In the SmG phase, a non-
zero photocurrent was observed, but such a change in the
photocurrent polarity did not occur.
Fig. 8(b) shows the photocurrent curves in the SmC* phase of
Conclusions
ꢂ
compound 1 at 130 C under the DC bias of +0.5 and ꢀ0.5 V,
which corresponds to the electric eld of ꢃ2.5 ꢁ 103 V cmꢀ1. In
the electric eld region, the diffusion of the charge carrier was
dominant over the dri movement of the carriers resulting in a
featureless current decay without clear kink points.
Phenylterthiophene derivatives bearing a decenyl group, dis-
iloxane chain, and cyclotetrasiloxane ring were synthesized.
They exhibited hole mobilities in the order of 10ꢀ4 cm2 Vꢀ1 sꢀ1
in the SmC* phase. Even the compound bearing a bulky cyclo-
tetrasiloxane ring exhibited an enantiotropic SmC* phase. The
spontaneous polarization in the SmC* phases exceed 100 nC
cmꢀ2. Compound 1 exhibited a photovoltaic effect based on the
internal electric eld produced by spontaneous polarization.
The strengths and shapes of the transient photocurrents
were similar to those of the zero-eld photocurrents aer the
DC bias application shown in Fig. 8(a). This indicated that an
internal electric eld in the order of 103 V cmꢀ1 was formed by
spontaneous polarization in the ferroelectric SmC* phase.
The estimated value of the internal electric eld was in the
same order of that determined by the steady-state photocurrent
measurement as shown in Fig. 7(b). These internal electric eld
values determined by the steady and transient photocurrent
Acknowledgements
This study was nancially supported by the Japan Security
Scholarship Foundation, the Ogasawara Foundation, a Grant-
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in-Aid for Scientic Research on Innovative Areas (Coordination
Programming, no. 24108729 and Element-Block Polymers, no.
25102533) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), a Grant-in-Aid for Scientic
Research (B) (no. 22350080) from the Japan Society for the
Promotion of Science (JSPS), the Iwatani Naoji Foundation, the
Asahi Glass Foundation, and the Murata Science Foundation.
The authors thank T. Kusunose at Kagawa University for help
with the DSC measurements. We also thank Prof. N. Tamaoki of
Hokkaido University, Prof. T. Kato of the University of Tokyo,
and Dr Emi Uchida of the National Institute of Advanced
Materials Science and Technology for fruitful discussions.
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