Preparation and properties of C2-symmetric organic radical compounds showing ferroelectric liquid crystal properties

Article abstract of DOI:10.1080/15421400903065325

We have synthesized a new class of liquid crystalline (LC) organic radical compounds 2, which are the C2-symmetric derivatives of the previously reported compounds 1 containing a chiral nitroxide unit and an ester group in the mesogen core, and their LC p

Full text of DOI:10.1080/15421400903065325

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Preparation and Properties
of C 2-Symmetric Organic
Radical Compounds Showing
Ferroelectric Liquid Crystal
Properties
Naohiko Ikuma a , Yoshiaki Uchida a , Rui Tamura a ,
Katsuaki Suzuki a , Jun Yamauchi a , Yoshio Aoki b &
Hiroyuki Nohira b
a Graduate School of Human and Environmental
Studies , Kyoto University , Kyoto, Japan
b Graduate School of Science and Engineering ,
Saitama University , Saitama, Japan
Published online: 05 Oct 2009.
To cite this article: Naohiko Ikuma , Yoshiaki Uchida , Rui Tamura , Katsuaki Suzuki ,
Jun Yamauchi , Yoshio Aoki & Hiroyuki Nohira (2009) Preparation and Properties
of C 2-Symmetric Organic Radical Compounds Showing Ferroelectric Liquid Crystal
Properties, Molecular Crystals and Liquid Crystals, 509:1, 108/[850]-117/[859], DOI:
10.1080/15421400903065325
To link to this article: http://dx.doi.org/10.1080/15421400903065325
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 108=[850]–117=[859], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903065325
Preparation and Properties of C2-Symmetric Organic
Radical Compounds Showing Ferroelectric Liquid
Crystal Properties
Naohiko Ikuma1, Yoshiaki Uchida1, Rui Tamura1,
Katsuaki Suzuki1, Jun Yamauchi1, Yoshio Aoki2,
and Hiroyuki Nohira2
1Graduate School of Human and Environmental Studies, Kyoto
University, Kyoto, Japan
2Graduate School of Science and Engineering, Saitama University,
Saitama, Japan
We have synthesized a new class of liquid crystalline (LC) organic radical
compounds 2, which are the C2-symmetric derivatives of the previously reported
compounds 1 containing a chiral nitroxide unit and an ester group in the mesogen
core, and their LC properties have been fully characterized. The enantiomerically
enriched 2 showed Nꢀ and SmCꢀ phases. Their LC phase transition behavior is
similar to that of 1, whereas the ferroelectric properties of the SmCꢀ phase of
(2S,5S)-2 differed from those of (2S,5S)-1. We discuss the origin of this difference
in terms of their molecular structures optimized by molecular orbital calculations.
Keywords: chiral nitroxide; chiral smectic C phase; ferroelectric liquid crystals;
paramagnetic liquid crystals
INTRODUCTION
Endowing calamitic liquid crystals (LCs) with chirality results in the
generation of chiral LC phases with helical superstructure, such as
cholesteric (Nꢀ) and chiral SmC (SmCꢀ) phases. Further addition of
The present work is supported by the Grant-in-Aid for Scientific Research (No.
19350067) from Japan Society for the Promotion of Science and the Asahi Glass Founda-
tion (R.T.) and the Research Fellowships of the Japan Society for the Promotion of
Science for Young Scientists (N.I. and Y.U.).
Address correspondence to Rui Tamura, Graduate School of Human and Environ-
mental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto
606-8501, Japan. E-mail: tamura-r@mbox.kudpc.kyoto-u.ac.jp
108=[850]

Ferroelectric Radical Liquid Crystals
109/[851]
paramagnetism to the chiral LCs is expected to provide novel
advanced soft materials that can combine the optical and electrical
properties of conventional chiral LCs with the magnetic properties of
paramagnetic compounds [1–4]. For example, it is quite interesting
to investigate whether magneto-electric effects are observable in the
ferroelectric LC (FLC) state of paramagnetic materials. It is known
that the magneto-electric effects originate from the interactions
between magnetic-dipole and electric-dipole moments and are detect-
able as the influence of a magnetic (electric) field on the polarization
(magnetization) of a material and vice versa [5].
In this context, we could obtain a series of prototypic all-organic
chiral LC radical compounds 1 which have a nitroxyl group with a
large electric dipole moment (3 Debye) as well as a polar ester group
(1.8 Debye) used very often for the preparation of conventional FLCs
(Fig. 1) [6]. Indeed, (2S,5S)-1 (n ¼ 11 ꢀ 15) showing an SmCꢁ phase
exhibited ferroelectricity in a thin sandwich cell (4 mm thickness);
the highest Ps value of 24 nC cmꢂ2 was recorded for (2S,5S)-1
(n ¼ 13) [7,8]. Furthermore, it has been found that both enantiomeri-
cally enriched and racemic samples of 1 exhibit unusual short-range
intermolecular ferromagnetic-like interactions under weak magnetic
fields in their LC phases [4,9,10].
To observe the magneto-electric effects, it would be essential to
develop paramagnetic compounds showing a high Ps value and mag-
netic interactions in the FLC state. Here we report the synthesis of
enantiomerically enriched C2-symmetric (2S,5S)-4-[5-(4-(4-alkoxyphenyl)
FIGURE 1 Molecular structures of 1 and 2.

110/[852]
N. Ikuma et al.
calbonyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl]phenyl 4-alkoxy-
benzoates (2), which have an additional polar ester group to improve
the Ps value compared to (2S,5S)-1, and discuss their ferroelectric
properties (Fig. 1).
EXPERIMENTAL
General
Transition temperatures refer to the peak top of each transition curve
by differential scanning calorimetry (DSC), which was performed at a
scanning rate of 5ꢀC=min. Enantiomeric excess (ee) was determined by
HPLC analysis using chiral stationary phase column (Daicel Chiralcel
OD-H, 0.46 Â 25 cm), a mixture of hexane and 2-PrOH (9:1) as the
mobile phase at a flow rate of 1.0 mL=min, and a UV-vis spectrometer
(254 nm) as the detector. Ps was measured by the triangular wave
method at a frequency of 20 Hz in a 4 mm sandwich cell with ITO
electrodes coated with polyimide in an electrical field of 10 V peak-to-
peak. Tilt angles were measured by polarized optical microscopy
(POM). Their g values and hyperfine coupling constants (aN) were
determined by electron paramagnetic resonance (EPR) spectroscopy
in THF at 25ꢀC. The variable temperature X-ray diffraction (XRD)
patterns were recorded at a continuous scaÀn1ning rate of 2ꢀ 2h minÀ1
at a heating and cooling rate of 4ꢀC min
using CuKa radiation
(40 kV, 20 mA), with the intensity of the diffracted X-rays being
collected at intervals of 0.02ꢀ 2h.
General Synthetic Procedure of (2S,5S)-2 (Scheme 1)
Enantiomerically enriched nitrone (R)-3 was prepared according to
the published procedure [11]. The nitrone (R)-3 (1.81 g, 16 mmol)
was reacted with the Grignard reagent 4 (32 mmol) in THF (55 mL)
at À78ꢀC. The reaction temperature was slowly raised to room
temperature. After being stirred overnight, the reaction mixture was
poured into aqueous saturated NH4Cl solution (100 mL), followed by
extraction with CH2Cl2 and concentration in vacuo. The residue was
dissolved in MeOH (30 mL) and oxidized by treatment with Cu(OAc)2
(0.26 g), conc. NH3 solution (2.1 mL), and O2 bubbling until the solu-
tion color became dark blue. The solution was evapolated in vacuo,
and the residue extracted by CHCl3 (100 mL). The organic phase
was washed with saturated aqueous NaHCO3 solution (50 mL), dried
over MgSO4, and concentrated in vacuo to give the crude nitrone
(S)-5. The crude (S)-5 was reacted with 4 and the resulting crude

Ferroelectric Radical Liquid Crystals
111/[853]
SCHEME 1 Synthesis of (2S,5S)-2.
addition product was oxidized by the same procedure. The crude
nitroxide product was purified by column chromatography on silica
gel (hexane=ether 99=1) to give pure (2S,5S)-6 as yellow solid in 9%
yield from (R)-3.
To (2S,5S)-6 (0.4 mmol) in THF (10 mL) was added a THF solution
of tetrabutylammonium fluoride (1 M solution, 1.0 mL) at 0ꢁC. After
being stirred at 0ꢁC for 20 min, a solution of Et3N (2.4 mL) and the
p-alkoxybenzoyl chloride 7 (2.4 mmol) in THF (4 mL) was added. The
reaction mixture was slowly warmed to 25ꢁC and stirred for 40 h. Then
the mixture was poured into aqueous NaHCO3 solution and the
aqueous mixture was extracted with ether (3 ꢂ 50 mL). The combined
organic phase was dried over MgSO4 and evaporated in vacuo. Column
chromatography on silica gel (hexane=ether 9=1) of the residue gave
pure (2S,5S)-2 as yellow solid in 70–90% yield from (2S,5S)-6.
26
(2S,5S)-2a (93.5% ee): ½aꢃD ꢄ75.66 (c ¼ 0.987, THF). EPR (THF):
g ¼ 2.0059, aN ¼ 1.33 mT. IR (KBr): n 2928, 1735, 1603, 1507, 1448,
1369, 1248, 1206, 1166, 1076, 844, 762 cmꢄ1. Anal. Calcd for
C48H60NO7: C, 75.56; H, 7.93; N, 1.84; Found: C, 75.47; H, 7.98;
N, 1.75.
24
(2S,5S)-2b (83.9% ee): ½aꢃD ꢄ62.77 (c ¼ 0.971, THF). EPR (THF):
g ¼ 2.0059, aN ¼ 1.33 mT. IR (KBr): n 2922, 1735, 1604, 1507, 1449,
1374, 1255, 1200, 1162, 1076, 846, 764 cmꢄ1. Anal. Calcd for
C56H76NO7: C, 76.85; H, 8.75; N, 1.60; Found: C, 76.69; H, 8.75; N, 1.58.
24
(2S,5S)-2c (90.6% ee): ½aꢃD ꢄ59.42 (c ¼ 0.990, THF). EPR (THF):
g ¼ 2.0059, aN ¼ 1.33 mT. IR (KBr): n 2912, 1735, 1603, 1507, 1374,
1250, 1199, 1160, 1052, 845, 766 cmꢄ1. Anal. Calcd for C58H80NO7:
C, 77.12; H, 8.93; N, 1.55; Found: C, 76.87; H, 8.89; N, 1.53.

112/[854]
N. Ikuma et al.
24
(2S,5S)-2d (90.5% ee): ½aꢃD ꢄ56.85 (c ¼ 0.986, THF). EPR (THF):
g ¼ 2.0059, aN ¼ 1.32 mT. IR (KBr): n 2923, 1734, 1605, 1507, 1374,
1254, 1203, 1163, 1074, 846, 764 cmꢄ1. Anal. Calcd for C60H84NO7:
C, 77.38; H, 9.09; N, 1.50; Found: C, 77.08; H, 9.20; N, 1.49.
24
(2S,5S)-2e (88.0% ee): ½aꢃD ꢄ56.52 (c ¼ 1.009, THF). EPR (THF):
g ¼ 2.0059, aN ¼ 1.32 mT. IR (KBr): n 2915, 1734, 1603, 1507, 1369,
1248, 1201, 1161, 1050, 843, 762 cmꢄ1. Anal. Calcd for C62H88NO7:
C, 77.62; H, 9.25; N, 1.46; Found: C, 77.34; H, 9.13; N, 1.49.
RESULTS AND DISCUSSION
Characterization of LC Phases
The phase transition behaviour of (2S,5S)-2 was characterized by
DSC, POM, and XRD analysis, which were measured below 160ꢁC
because (2S,5S)-2 instantly decompose in the isotropic phase above
160ꢁC (Table 1, Figs. 2 and 3).
Similarly to the case of 1, (2S,5S)-2a and (2S,5S)-2b with
respective C8 and C12 chains exhibited only an Nꢀ phase, while
(2S,5S)-2c–2e with longer alkyl chains showed Nꢀ and SmCꢀ phases,
with oily-streak and broken fan-shaped textures observed, respec-
tively, on the cooling run (Table 1, and Figs. 2 and 3). Variable
temperature XRD analysis verifies the existence of the Nꢀ and SmCꢀ
phases (Table 2 and Fig. 4).
To estimate the tilt angles of the SmCꢀ phases, each molecular con-
formation with two structurally fixed linear anti-conformational alkyl
chains was optimized by the PM3 semi-empirical calculation using PC
Spartan’02 [12]. The optimized molecular geometries have a distorted
zigzag structure that is advantageous for the appearance of an SmCꢀ
phase (Fig. 4b) [6,13]. The geometrical tilt angles were estimated to be
close to 50ꢁ (Table 2).
TABLE 1 Phase Transition Temperature (ꢁC) and DH (in parentheses,
kJ=mol) of (2S,5S)-2 on the First Heating Run
Cr
SmCꢀ
Nꢀ
I
(S,S)-2a (n ¼ 8) (93.5% ee)
(S,S)-2b (n ¼ 12) (83.9% ee)
(S,S)-2c (n ¼ 13) (90.6% ee)
(S,S)-2d (n ¼ 14) (90.5% ee)
(S,S)-2e (n ¼ 15) (88.0% ee)
.
.
.
.
.
124.1 (36.7)
101.7 (32.8)
97.6 (38.2)
99.0 (30.0)
93.4 (35.7)
.
.
.
.
.
193.7 (1.5)
165.3 (2.4)
169.7 (3.5)
169.8 (2.1)
163.0 (1.2)
.
.
.
.
.
.
.
.
128.6 (1.1)
141.3 (1.3)
143.9 (2.1)

Ferroelectric Radical Liquid Crystals
113/[855]
FIGURE 2 LC behavior of (2S,5S)-2.
FIGURE 3 Polarized optical micrographs of (2S,5S)-2d (90.5% ee). (a)
oily-streaks texture at 150ꢁC under homeotropic boundary conditions and (b)
broken fan-shaped texture in a thin sandwich cell (4 mm) under homogenous
planar boundary conditions at 130ꢁC.
TABLE 2 Ferroelectric Properties Measured in the SmCꢀ Phases of (2S,5S)-2c-
e, Layer Distances, Calculated Molecular Lengths, and Geometrical Tilt Angles.
cosꢄ1(d=l)
[deg]f
ꢁ
PS(ꢄ10
)
h (ꢄ10ꢁ)
[nC ꢅ cmꢄ2]a,b
[deg]b,c
d [A]
d
e
˚
˚
l [A]
(S,S)-2c (n ¼ 13)
(90.6% ee)
(S,S)-2d (n ¼ 14)
(90.5% ee)
(S,S)-2e (n ¼ 15)
(88.0% ee)
36 (116)g
42 (116)g
43 (129)g
44 (129)g
38.6 (115)g
40.9 (130)g
41.2 (110)g
57.5
59.8
62.3
48
47
49
28 (129)g
19 (129)g
b
aSpontaneous polarization. Measured at a temperature 10ꢁC below the phase transi-
c
tion from the Nꢀ phase to the SmCꢀ phase during the cooling process. Tilt angle mea-
sured by hot-stage POM. dLayer distance from XRD data. eMolecular length
f
determined by a PM3 semiempirical calculation (PC Spartan’02). Calculated geometri-
g
cal title angle from d and l values. Temperature (ꢁC) in parentheses.

114/[856]
N. Ikuma et al.
FIGURE 4 XRD patterns of (a) (2S,5S)-2c and (b) molecular structure of 2c
simulated by a PM3 geometry optimization.
Ferroelectric Properties
The magnitude of PS and the optical tilt angles (h) evaluated by
oscilloscopic traces (Fig. 5a and b) and POM (Fig. 5c-f) of (2S,5S)-
2c–2e are shown in Table 2 and Figure 6. On the cooling run from
the Nꢀ-to-SmCꢀ transition temperature, the PS values rapidly
increased (Fig. 6). The maximum PS(ꢄ10ꢁ) value (36 nC cmꢄ2
)
observed for (2S,5S)-2c is larger than that (24 nC cmꢄ2) of (2S,5S)-1
(n ¼ 13) [7,8]. This increase in Ps is consistent with the optimized
molecular structure, in which two ester carbonyl groups direct to the
same direction, resulting in the increase in the negative dielectric
anisotropy. The experimental h(ꢄ10ꢁ) values of (2S,5S)-2 were
between 42ꢁ and 44ꢁ, which were smaller by 4 ꢆ 6ꢁ than those esti-
mated by the VT-XRD analysis and the PM3 calculation (Table 2).
The tilt angles were so large that we could not measure the optical
response times, because we observed only a bright-dark-bright or
"
FIGURE 5 Ferroelectric switching modes of (2S,5S)-2 in a sandwich cell
(4 mm) under homogeneous planar boundary conditions between crossed
polarizers. (a) Polarization reversal current of (2S,5S)-2d measured by the tri-
angular wave method at a frequency of 15 Hz in an electric field of 10 V
peak-to-peak at 130ꢁC. (b) Optical response of (2S,5S)-2c measured by the rec-
tangular wave method at a frequency of 20 Hz in an electric field of 10 V
peak-to-peak at 120ꢁC: Transmitted light response (lower) and applied rectan-
gular wave (upper). Applied DC voltage dependence of texture change of
(2S,5S)-2c in the same area at 116ꢁC: (c) þ12 V, (d) 0 V, (e) ꢄ12 V, and (f) 0 V.

Ferroelectric Radical Liquid Crystals
115/[857]

116/[858]
N. Ikuma et al.
FIGURE 6 Temperature dependence of the Ps values in the SmCꢀ phases of
(2S,5S)-2c–e.
dark-bright-dark texture change upon switching the polarity of the
applied electric field (Fig. 5b).
Moreover, in contrast to the case of (2S,5S)-1, we observed the
relaxation of the ferroelectric state of (2S,5S)-2 in the same type of
FLC cell, i.e., POM texture change by turning off the electric fields
(Fig. 5c-f). At the same time, the textures in the two field-free states
considerably differed from each other, suggesting the short-pitch
SmCꢀ nature and the incomplete formation of a helical superstructure
in the FLC cell (Fig. 5d and f).
CONCLUSION
Thus, we could obtain a new class of chiral organic radical compounds
showing FLC properties. One of them, (2S,5S)-2c, showed the highest
Ps value of 36 nC cmꢄ2, which was larger than that (24 nC cmꢄ2) of
(2S,5S)-1 (n ¼ 13) [7,8]. Their unique C2-symmetric molecular struc-
ture possessing one nitroxyl and two ester groups seems responsible
for the resulting large tilt angles (42 ꢆ 44ꢁ) in their SmCꢀ phase.
(2S,5S)-2c will be utilized as a soft magneto-electric material as well

Ferroelectric Radical Liquid Crystals
117/[859]
as a real paramagnetic chiral LC probe or dopant to investigate
the dynamic behavior of a given diamagnetic LC materials by EPR
spectroscopy.
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