Synthesis and Characterization of a New Series of Paramagnetic Ferroelectric Liquid Crystalline Nitroxide Radicals

Article abstract of DOI:10.1080/15421406.2015.1066966

A new series of all-organic liquid-crystalline (LC) chiral radical compounds,trans-1-alkoxyphenyl-4-[(4-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)-benzylideneamino]benzenes (1) with various alkyl chains, containing a chiral nitroxide unit in the mesogen core were synthesized and their LC properties were fully characterized. The enantiomerically enriched compounds (2S,5S)-1 showed N, TGBA, SmA and SmC phases, while the racemates (±)-1 exhibited N, SmA and SmC phases. The phase transition behavior and the ferroelectric properties of the SmC phase of (2S,5S)-1 differed from those of previously reported analogues with an ester group as a substitute for the imino group of (2S,5S)-1. Some of the (2S,5S)-1 series showed a temperature-dependent spontaneous polarization P^S (θ) inversion. We discuss the origin of this difference in terms of their molecular structures optimized by molecular orbital calculations. The phase transition behavior supports the hypothesis that the large φ, which is defined as the angle between the molecular long axis (one of the principal axes of inertia) and the direction of the dipole moment in each molecule, results in the stabilization of the SmA (SmA) phase.

Full text of DOI:10.1080/15421406.2015.1066966

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Synthesis and Characterization of a New
Series of Paramagnetic Ferroelectric
Liquid Crystalline Nitroxide Radicals
ab
c
c
c
Yoshiaki Uchida , Rui Tamura , Katsuaki Suzuki , Hiroki Takahashi ,
d
d
Yoshio Aoki & Hiroyuki Nohira
a
Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka, Japan
b
PRESTO, Japan Science and Technology Agency (JST), Kawaguchi,
Saitama, Japan
c
Graduate School of Human and Environmental Studies, Kyoto
University, Kyoto, Japan
Click for updates
d
Faculty of Engineering, Department of Applied Chemistry, Saitama
University, Saitama, Japan
Published online: 21 Aug 2015.
To cite this article: Yoshiaki Uchida, Rui Tamura, Katsuaki Suzuki, Hiroki Takahashi, Yoshio Aoki &
Hiroyuki Nohira (2015) Synthesis and Characterization of a New Series of Paramagnetic Ferroelectric
Liquid Crystalline Nitroxide Radicals, Molecular Crystals and Liquid Crystals, 615:1, 89-106, DOI:
10.1080/15421406.2015.1066966
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Mol. Cryst. Liq. Cryst., Vol. 615: pp. 89–106, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1066966
Synthesis and Characterization of a New Series
of Paramagnetic Ferroelectric Liquid Crystalline
Nitroxide Radicals
YOSHIAKI UCHIDA,1 RUI TAMURA, KATSUAKI
,2,∗
3
3
3
4
SUZUKI, HIROKI TAKAHASHI, YOSHIO AOKI,
AND HIROYUKI NOHIRA
4
1Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka,
Japan
2
PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama,
Japan
3
Graduate School of Human and Environmental Studies, Kyoto University,
Kyoto, Japan
4
Faculty of Engineering, Department of Applied Chemistry, Saitama University,
Saitama, Japan
A new series of all-organic liquid-crystalline (LC) chiral radical compounds,
trans-1-alkoxyphenyl-4-[(4-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)-
benzylideneamino]benzenes (1) with various alkyl chains, containing a chiral
nitroxide unit in the mesogen core were synthesized and their LC properties were
∗
fully characterized. The enantiomerically enriched compounds (2S,5S)-1 showed N ,
∗
∗
∗
TGBA , SmA and SmC phases, while the racemates (± )-1 exhibited N, SmA and
SmC phases. The phase transition behavior and the ferroelectric properties of the
∗
SmC phase of (2S,5S)-1 differed from those of previously reported analogues with an
ester group as a substitute for the imino group of (2S,5S)-1. Some of the (2S,5S)-1
series showed a temperature-dependent spontaneous polarization P (θ) inversion. We
S
discuss the origin of this difference in terms of their molecular structures optimized by
molecular orbital calculations. The phase transition behavior supports the hypothesis
that the large ϕ, which is defined as the angle between the molecular long axis (one of
the principal axes of inertia) and the direction of the dipole moment in each molecule,
∗
results in the stabilization of the SmA (SmA) phase.
Keywords Paramagnetic chiral liquid crystals; smectic liquid crystals; ferroelectricity;
nitroxide radicals; spontaneous polarization inversion
Introduction
Nitroxides have been used as co-oxidants, [1] spin-labels, [2] spin-probes, [3] MRI contrast
agents, [4] antioxidants, [5] and double sensor molecules, [6] because of (i) the thermody-
namic stability of the free radical species originating from the delocalization of the unpaired
∗
Address correspondence to Yoshiaki Uchida, Graduate School of Engineering Science, Osaka
University, Toyonaka, Osaka 560-8531, Japan. E-mail: yuchida@cheng.es.osaka-u.ac.jp
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
89

90
Y. Uchida et al.
electron over the N–O bond and (ii) the unique redox properties of a nitroxyl group. An-
other important feature of the nitroxyl group is the possession of a large electric dipole
moment (∼3 Debye) along the N–O bond [7]. Focusing on such unique magnetic, elec-
tronic and electric characteristics of the nitroxide radical, we synthesized the all-organic
chiral LC radical compounds, which contain a chiral cyclic-nitroxide unit and an ester
group in the mesogen core, in both enatiomerically enriched and racemic forms [8–12].
∗
Furthermore, some of them showed a chiral smectic C (SmC ) phase in the bulk state and
a ferroelectricity in a thin sandwich cell; they are the first examples of magnetic ferroelec-
tric liquid crystals (FLCs) containing no paramagnetic metal ion [9, 10, 12]. Recently, we
have reported a new chiral paramagnetic liquid crystalline compound (2S,5S)-1i (Fig. 1),
trans-1-dodecyloxyphenyl-4-[(4-(4-dodecyloxyphenyl)-2,5- dimethylpyrrolidine-1-oxy-2-
yl)benzylideneamino]benzene, which has a cyclic nitroxide radical moiety and shows chi-
∗
∗
ral smectic A (SmA ) and SmC phases [13]. The compound shows pretransitional layer
∗
∗
contraction prior to the SmA -to-SmC phase transition in the cooling run, which has
not been observed for previously reported analogue compounds (2S,5S)-2 and (2S,5S)-3
(
Fig. 1).
To gain an insight into the origin of this difference in terms of their molecular structures,
we have synthesized a series of analogues racemic and (2S,5S)-1 with C4 to C13 alkyl chains
Fig. 1). Here we report the LC phase transition behavior of racemic and (2S,5S)-1 and the
(
ferroelectricity of (2S,5S)-1, and compare them with those of compounds 2 and 3.
Experimental
Unless otherwise noted, solvents and reagents were reagent grade and used with-
out further purification. Tetrahydrofuran (THF) which is used for electron paramag-
netic resonance (EPR) spectral measurement or Grignard reactions was distilled from
Figure 1. Molecular structures of (2S,5S)-1, (2S,5S)-2 and (2S,5S)-3.

Paramagnetic Ferroelectric LCs
91
sodium/benzophenone ketyl under argon. Phase transition temperatures were determined
by DSC (SHIMADZU DSC-50) and POM (Olympus BHSP). Hot-stage (JHC TH-600PH)
was used as the temperature control unit for the microscopy. IR spectra were recorded
with SHIMADZU IRPrestige-21. EPR spectra were recorded with a JEOL FE1XG. For
variable-temperature XRD measurement, the data collections were performed on a Rigaku
RINT2200/PC-LH diffractometer.
Synthesis of 1
The synthesis of racemic and (2S,5S)-1 is presented in Scheme 1. They were prepared from
racemic or (R)-4.
Scheme 1. Synthesis of compound 1.
Synthesis of 4-Alkoxy-1-bromobenzene used for the synthesis of 7
A mixture of 4-bromophenol (50 mmol), DMF (70 ml) and potassium carbonate (100 mmol)
◦
was stirred at 70 C for 0.5 h. To the suspension was added alkyl iodide (50 mmol) and
◦
the mixture was stirred at 70 C overnight. Water (200 ml) was added and the mixture
was extracted with hexane. The organic extracts were washed with brine (50 ml), 1N
NaOH solution (50 ml) and water (100 ml), dried over MgSO4, and evaporated to give
4-alkoxy-1-bromobenzene (> 90%).
Synthesis of Aryl magnesium bromides 5, 7 [8–19]
To magnesium turnings (0.243 g, 10 mmol) dried by heating at 3 mmHg were added THF
(10 ml), arylbromide (10 mmol), and a small amount of iodine under argon. The reaction
started by heating, and the reaction mixture was refluxed for 4 h to afford a solution of aryl
magnesium bromide 5 and 7 in THF.

92
Y. Uchida et al.
General synthetic procedure for (± )-1 and (2S,5S)-1 [8–21]
To a solution of the nitrone 4 (5.0 mmol) in THF (10 ml) was slowly added a stirred
◦
solution of the freshly prepared Grignard reagent 5 (10 mmol) in THF under argon at –78 C
[
8–19]. The temperature was raised slowly to room temperature, and stirring was continued
overnight. The reaction mixture was poured into a saturated aqueous NH4Cl solution
50 ml), and then extracted with dichloromethane (DCM) (2 × 50 ml). The combined
(
DCM extract was dried over MgSO4 and evaporated. The residual oil was dissolved in
methanol (20 ml). To this solution were added aqueous NH3 solution (25 wt%, 1.4 ml)
and copper (II) acetate monohydrate (0.160 g, 0.80 mmol). Oxygen was bubbled through
the yellow solution until a persistent deep blue color was observed. The solvent was then
removed under reduced pressure and the crude product was dissolved in DCM (50 ml). This
solution was washed with saturated aqueous NaHCO3 (50 ml) and dried over MgSO4, and
the solvent removed under reduced pressure. The residue was dried by azeotropic removal
of water with benzene, followed by removal of the remaining benzene at 3 mmHg. The
crude acetal 6 was dissolved in THF (10 ml) and reacted with the freshly prepared Grignard
reagent 7 (the same quantity and conditions as described above). After a similar workup
procedure, the crude product was again oxidized by O2/copper acetate. The crude nitroxide
radical 8 was purified by column chromatography (hexane/ether = 17/3) on silica gel. To
a solution of 8 in THF (2 ml) was added aqueous H2SO4 (5%; 0.50 ml) and the mixture
◦
was stirred for 8 h at 25 C. Brine and DCM were then added. The organic phase was
separated and dried over MgSO4, and the solvent was evaporated under reduced pressure to
provide 2-(4-alkyloxyphenyl)-5-(4-formylphenyl)-2,5-dimethylpyrrolidine-1-oxy (9). To a
solution of 9 (0.31 mmol) in benzene (20 ml) was added 4-alkoxyaniline 10[20, 21] and
the mixture was refluxed by using Dean-Stark trap overnight. The reaction mixture was
evaporated and purified by column chromatography (hexane/ethyl acetate = 4/1) on silica
gel to afford the imine 1 (1∼10% yield from 4). The ee values of (2S,5S)-1 were determined
by HPLC analysis.
(
± )-trans-1-Butoxyphenyl-4-[(4-(4-butoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)
benzylideneamino]benzene (1a). Found: C, 77.38; H, 7.98; N, 5.62. Calc. for C33H41N2O3:
−1
C, 77.16; H, 8.04; N, 5.45%. νmax(KBr)/cm 2962, 2933, 2672, 1624, 1609, 1510, 1247,
and 837. EPR: g = 2.0057, aN = 1.33 mT. 3% yield from (± )-4.
2S,5S)-1a (85% ee). [α]14D –107.7 (c 0.075 in THF). 5% yield from (R)-4.
◦
(
(
2
± )-trans-1-Pentyloxyphenyl-4-[(4-(4-pentyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
-yl)benzylideneamino]benzene (1b). Found: C, 77.69; H, 8.24; N, 5.15. Calc. for
−1
C34H45N2O3: C, 77.60; H, 8.37; N, 5.17%. νmax(KBr)/cm 2949, 2932, 2864, 1622, 1609,
508, 1474, and 837. EPR: g = 2.0060, aN = 1.34 mT. 2% yield from (± )-4.
1
2S,5S)-1b (81% ee). [α]14D –97.4 (c 0.046 in THF). 5% yield from (R)-4.
◦
(
(
2
± )-trans-1-Hexyloxyphenyl-4-[(4-(4-hexyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
-yl)benzylideneamino]benzene (1c). Found: C, 77.94; H, 8.65; N, 4.78. Calc. for
−1
C37H49N2O3: C, 77.99; H, 8.67; N, 4.92%. νmax(KBr)/cm 2951, 2927, 2860, 1623, 1608,
508, 1470, 1247, and 839. EPR: g = 2.0060, aN = 1.34 mT. 3% yield from (± )-4.
1
2S,5S)-1c (86% ee). [α]14D –106.8 (c 0.198 in THF). 9% yield from (R)-4.
◦
(
(
2
± )-trans-1-Heptyloxyphenyl-4-[(4-(4-heptyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
-yl)benzylideneamino]benzene (1d). Found: C, 78.36; H, 8.92; N, 4.68. Calc. for

Paramagnetic Ferroelectric LCs
93
−1
C37H49N2O3: C, 78.35; H, 8.94; N, 4.69%. νmax(KBr)/cm 2956, 2926, 2856, 1623, 1606,
508, 1468, 1250, and 838. EPR: g = 2.0059, aN = 1.33 mT. 2% yield from (± )-4.
1
2S,5S)-1d (75% ee). [α]14D –76.4 (c 0.066 in THF). 3% yield from (R)-4.
◦
(
(
2
± )-trans-1-Octyloxyphenyl-4-[(4-(4-octyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
- yl)benzylideneamino]benzene (1e). Found: C, 78.48; H, 9.25; N, 4.34. Calc. for
−1
C37H49N2O3: C, 78.68; H, 9.18; N, 4.48%. νmax(KBr)/cm 2954, 2918, 2850, 1624, 1607,
509, 1467, 1247, and 837. EPR: g = 2.0061, aN = 1.33 mT. 4% yield from (± )-4.
1
2S,5S)-1e (91% ee). [α]14D –79.7 (c 0.050 in THF). 7% yield from (R)-4.
◦
(
(
2
± )-trans-1-Nonyloxyphenyl-4-[(4-(4-nonyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
-yl)benzylideneamino]benzene (1f). Found: C, 78.93; H, 9.46; N, 4.26. Calc. for
−1
C43H61N2O3: C, 78.97; H, 9.40; N, 4.28%. νmax(KBr)/cm 2955, 2923, 2854, 1624, 1606,
508, 1468, 1247, and 838. EPR: g = 2.0061, aN = 1.33 mT. 4% yield from (± )-4.
1
2S,5S)-1f (89% ee). [α]14D –94.1 (c 0.067 in THF). 4% yield from (R)-4.
◦
(
(
2
± )-trans-1-Decyloxyphenyl-4-[(4-(4-decyloxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-
-yl)benzylideneamino]benzene (1g). Found: C, 79.42; H, 9.70; N, 4.13. Calc. for
−1
C45H65N2O3: C, 79.25; H, 9.61; N, 4.11%. νmax(KBr)/cm 2954, 2921, 2852, 1621, 1607,
509, 1474, 1249, and 836. EPR: g = 2.0061, aN = 1.34 mT. 5% yield from (± )-4.
1
2S,5S)-1g (83% ee). [α]14D –85.4 (c 0.098 in THF). 2% yield from (R)-4.
◦
(
(
± )-trans-1-Undecyloxyphenyl-4-[(4-(4-undecyloxyphenyl)-2,5-dimethylpyrrolidine-1-
oxy-2-yl)benzylideneamino]benzene (1h). Found: C, 79.44; H, 10.04; N, 3.82. Calc. for
C47H69N2O3: C, 79.50; H, 9.79; N, 3.95%. νmax(KBr)/cm 2956, 2920, 2851, 1622,
−1
1
607, 1509, 1473, 1250, and 836. EPR: g = 2.0060, aN = 1.33 mT. 1% yield from
(
± )-4.
2S,5S)-1h (89% ee). [α]14D –91.4 (c 0.095 in THF). 16% yield from (R)-4.
◦
(
(
± )-trans-1-Dodecyloxyphenyl-4-[(4-(4-dodecyloxyphenyl)-2,5-dimethylpyrrolidine-1-
oxy-2-yl)benzylideneamino]benzene (1i). Found: C, 79.60; H, 9.76; N, 3.74. Calc. for
C49H73N2O3: C, 79.73; H, 9.97; N, 3.80%. νmax (KBr)/cm 2955, 2921, 2851, 1625,
−1
1
606, 1509, 1469, 1245, and 838. EPR: g = 2.0061, aN = 1.33 mT. 7% yield from
(
± )-4.
2S,5S)-1i (85% ee). [α]14D –50.6 (c 0.086 in THF). 6% yield from (R)-4.
◦
(
(
± )-trans-1-Tridecyloxyphenyl-4-[(4-(4-tridecyloxyphenyl)-2,5-dimethylpyrrolidine-1-
oxy-2-yl)benzylideneamino]benzene (1j). Found: C, 79.89; H, 10.27; N, 3.59. Calc. for
−1
C51H77N2O3: C, 79.95; H, 10.13; N, 3.66%. νmax(KBr)/cm 2955, 2919, 2851, 1625,
607, 1510, 1470, 1247, and 836. EPR: g = 2.0060, aN = 1.33 mT. 8% yield from (± )-4.
1
Measurements
High-performance liquid chromatographic (HPLC) analysis was performed using a chiral
stationary phase column (Daicel Chiralcel OD-H, 0.46 cm x 25 cm), a mixture of hexane
−
1
and 2-propanol (9:1) as the mobile phase at a flow rate of 1.0 mL min , and an UV-vis
◦
spectrometer (254 nm) as the detector. X-Band EPR spectra were measured in THF at 25 C

94
Y. Uchida et al.
◦
Table 1. Phase transition temperatures ( C) and transition enthalpies (in parentheses,
kJ/mol) of racemic 1a-1j
◦
Compounds
Phase transition temperature ( C) (transition enthalpy (kJ/mol))
a
a
(
(
(
(
(
(
(
(
(
(
± )-1a (n = 4)
± )-1b (n = 5)
± )-1c (n = 6)
± )-1d (n = 7)
± )-1e (n = 8)
± )-1f (n = 9)
± )-1g (n = 10)
± )-1h (n = 11)
± )-1i (n = 12)
± )-1j (n = 13)
Cr 133 (-) Cr 137 (-) N 165 (2.3) Iso : heating
b
Iso 163 (6.3) N [< 25] Cr : cooling
Cr 129 (43.8) N 148 (1.4) Iso : heating
Iso 146 (1.0) N 53 (7.6) Cr : cooling
Cr 109 (27.8) N 134 (1.7) Iso: heating
Iso 130 (1.4) N 67 (21.9) Cr: cooling
Cr 109 (31.2) N 130 (1.6) Iso : heating
Iso 126 (1.5) N 73 (19.9) Cr : cooling
Cr 101 (27.1) N 127 (2.2) Iso : heating
Iso 125 (2.3) N 79 (24.9) Cr : cooling
Cr 96 (22.4) SmA 109 (0.50) N 118 (1.8) Iso: heating
Iso 115 (2.0) N 106 (0.28) SmA 89 (-) SmC 70 (19.7) Cr: cooling
Cr 98 (28.1) SmA 119 (5.3) Iso: heating
Iso 115 (2.8) SmA 97 (-) SmC 75 (18.0) Cr: cooling
Cr 96 (27.6) SmA 113 (7.3) Iso: heating
c
c
c
Iso 110 (5.6) SmA 88 (-) SmC 67 (21.5) Cr: cooling
Cr 93 (26.2) SmC 97 (-) SmA 118 (7.9) Iso: heating
Iso 114 (6.7) SmA 94 (-) SmC 72 (24.0) Cr: cooling
Cr 91 (29.0) SmC 97 (-) SmA 116 (9.0) Iso: heating
c
c
c
c
Iso 113 (8.5) SmA 95 (-) SmC 72 (27.9) Cr: cooling
aTwo peaks are overlapping so that the transition enthalpies are united.
b
The transition from the nematic phase to the crystalline phase occurs below room temperature.
The phase transition is not observed by DSC but by POM.
c
under an applied magnetic field of 0.34 T. The variable temperature XRD patterns were
◦
−1
◦
recorded at a continuous scanning rate of 2 2θ min and at a heating or cooling rate of 4 C
−1
min using Cu Kα radiation (40 kV, 40 mA), with the intensity of diffracted X-rays being
◦
◦
−1
collected at intervals of 0.02 2θ. DSC was performed at a scanning rate of 5 C min .
The magnitude of spontaneous polarization, PS, was determined by using 4 μm cells with
indium tin oxide (ITO) electrodes coated with polyimide in an electric field of 20 V (5.0 V
−1
μm ) at a frequency of 20 Hz for (2S,5S)-1. The tilt angle (θ) was measured as a function
of the temperature between crossed Nicol polarisers under an applied direct-current (DC)
−1
electric field of 2.5 V μm , and was recorded as half the rotation angle between the two
extinction positions associated with the oppositely directed polarization. For the X-ray
crystallographic analysis of (± )-1a, the single crystal was mounted in a sealed capillary.
The single crystal of (± )-1a appropriate for X-ray analysis was obtained by crystallization
from hexane/DCM. The data collections were performed at 173 K on a Rigaku RAXIS
◦
RAPID diffractometer with graphite monochromated Mo-Kα radiation to 2θmax of 55.0 .
All of the crystallographic calculations were performed by using the CrystalStructure 3.6.0
of Rigaku and Rigaku/MSC. The crystal structure was solved by direct methods and refined
by using full-matrix least squares. All non-hydrogen atoms were refined anisotropically.
The X-ray crystal structure of (± )-1a is shown in Fig. A1 in the Appendix. Crystallographic
data (excluding structure factors) for the structure have been deposited with the Cambridge
Crystallographic Data Center (CCDC-685828). Copies of the data can be obtained, free

Paramagnetic Ferroelectric LCs
95
◦
Table 2. Phase transition temperatures ( C) and transition enthalpies (in parentheses,
kJ/mol) of enantiomerically enriched 1a–1j
◦
Phase transition temperature ( C)
Compounds
ee (%)
(transition enthalpy (kJ/mol))
∗
∗
(
(
(
(
2S,5S)-1a (n = 4)
2S,5S)-1b (n = 5)
2S,5S)-1c (n = 6)
2S,5S)-1d (n = 7)
85
Cr 112 (22.7) N 159 (2.1) Iso: heating
a
Iso 151 (0.1) N [< 25] Cr: cooling
∗
81
86
75
Cr 105 (3.7) N 132 (1.9) Iso: heating
∗
a
Iso 128 (2.0) N [< 25] Cr: cooling
∗
Cr 64 (18.0) N 128 (1.7) Iso: heating
∗
a
Iso 126 (1.7) N [< 25] Cr: cooling
b
b
∗
Cr 89 (-) Cr 105 (-) N 132 (1.8) Iso:
heating
∗
a
Iso 129 (1.4) N [< 25] Cr: cooling
∗
(
(
2S,5S)-1e (n = 8)
2S,5S)-1f (n = 9)
91
89
Cr 87 (22.5) N 131 (1.7) Iso: heating
∗
Iso 127 (1.5) N 40 (5.7) Cr: cooling
∗
∗
c
∗
∗
Cr 81 (49.0) SmC 90 (-) SmA 108
c,d
c,d
(
0.67) TGBA 109 (-) N 118
2.4) Iso: heating
(
∗
c,d
∗
Iso 115 (2.6) N 109 (0.45) TGBA
c,d
∗
c
∗
1
08 (-) SmA 83 (-) SmC 46 (24.1)
Cr: cooling
Cr 94 (56.0) SmA 118 (6.73) TGBA
∗
d
∗e
(
2S,5S)-1g (n = 10)
83
c,d
∗
c,d
1
18 (-) N 120 (-) Iso: heating
d
∗
c,d
∗e
Iso 117 (6.26) N 114 (-) TGBA
c,d
∗
c
∗
1
14 (-) SmA 84 (-) SmC 42 (31.8)
Cr: cooling
Cr 95 (43.0) SmA 118 (6.0) Iso: heating
∗
∗
(
(
(
2S,5S)-1h (n = 11)
2S,5S)-1i (n = 12)
2S,5S)-1j (n = 13)
89
85
86
c
∗
Iso 115 (5.1) SmA 85 (-) SmC 48
35.6) Cr: cooling
Cr 89 (35.7) SmA 118 (5.6) Iso: heating
Iso 115 (4.7) SmA 89 (-) SmC 46
22.4) Cr: cooling
Cr 84 (40.5) SmC 96 (-) SmA 116
8.7) Iso: heating
Iso 113 (7.8) SmA 94 (-) SmC 49
34.9) Cr: cooling
(
∗
∗
c
∗
(
∗
c
∗
(
∗
c
∗
(
aTwo peaks are overlapping so that the transition enthalpies are united.
b
The transition from the nematic phase to the crystalline phase occurs below room temperature.
The phase transition is not observed by DSC but by POM (Fig. S2).
c
d
e
Transition enthalpies were unable to be determined.
This phase was observed only between the oily-streak texture of the N phase and the fan-shaped
∗
∗
of the SmA phase by POM.
of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (Fax: +44-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk). The summary of the
fundamental crystal data and experimental parameters for the structure solution is given in
Table A1 in the Appendix.

96
Y. Uchida et al.
∗
Table 3. Layer distances d from XRD of (± )-1f-j ((2S,5S)-1f-j) in the SmA (SmA ) and
∗
a
SmC (SmC ) phases
Compound [ee (%)]
XRD d (Å)
∗
∗
SmA or SmA phases
SmC or SmC phases
◦
◦
◦
(
(
(
(
(
(
(
(
(
(
± )-1f (n = 9)
2S,5S)-1f [89]
± )-1g (n = 10)
2S,5S)-1g [83]
± )-1h (n = 11)
2S,5S)-1h [89]
± )-1i (n = 12)
2S,5S)-1i [85]
± )-1j (n = 13)
2S,5S)-1j [86]
38.9 (100 C)
38.3 (80 C)
◦
37.1 (100 C)
35.7 (75 C)
◦
◦
40.7 (100 C)
39.8 (85 C)
◦
◦
38.5 (110 C)
36.2 (50 C)
◦
◦
42.8 (95 C)
42.8 (85 C)
◦
◦
40.5 (110 C)
39.2 (70 C)
◦
◦
44.1 (104 C)
43.5 (85 C)
◦
◦
41.6 (100 C)
39.8 (70 C)
◦
◦
46.5 (105 C)
46.0 (85 C)
◦
◦
44.6 (96 C)
45.0 (70 C)
a
On the cooling run.
Results and Discussion
The phase transition behaviors of compounds 1 were characterized by differential scanning
calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD)
analysis (Table 1–3, Fig. 2–4).
Generally, (± )-1 showed higher crystal-to-LC transition and clearing temperatures
than those of (± )-2 with the same alkyl chains, most likely due to their higher stability
in both the crystalline and LC states of (± )-1 (Fig. 2a). Racemic 1a–1e with C4–C8
Figure 2. Liquid crystalline behaviour of (a) (± )-1 and (b) (2S,5S)-1: transition temperatures deter-
mined by DSC and POM on the heating (upper bar) and cooing (lower bar) runs.

Paramagnetic Ferroelectric LCs
97
Figure 3. Polarized optical microphotographs of (± )-1 on the cooling run: (a) a thread-like texture for
◦
◦
the N phase of (± )-1c at 113.3 C; (b) a fan-shaped texture for the SmA phase of (± )-1j at 104.9 C;
◦
(
c) a broken fan-shaped texture for SmC phase of (± )-1j at 85.0 C. All textures were observed
under homogeneous planar boundary conditions in a thin-sandwich LC cell (25 μm thickness and
0 μm thickness for (± )-1c and (± )-1j, respectively) consisting of two glass plates covered with
antiparallel-rubbed polyimide films.
5

98
Y. Uchida et al.
∗
Figure 4. Polarized optical microphotographs of (2S,5S)-1: (a) a fan-like texture for the N phase
◦
∗
of (2S,5S)-1f at 110.0 C on the cooling run; (b) a fan-shaped texture for SmA phase of (2S,5S)-1j
◦
∗
at 110.0 C on the cooling run; (c) a broken fan-shaped texture for the SmC phase of (2S,5S)-1j at
5.0 C on the cooling run; (d) a blurred fan-shaped texture for the TGBA phase of (2S,5S)-1f at
08.7 C on the heating run. Texture (a) was observed under random conditions, while other textures
◦
∗
7
◦
1
were observed under homogeneous planar boundary conditions in a thin-sandwich LC cell (4 μm
thickness and 50 μm thickness for (2S,5S)-1f and (2S,5S)-1j, respectively) consisting of two glass
plates covered with antiparallel-rubbed polyimide films.
chains showed only an enantiotropic N phase, while no odd-even effect of alkyl chains was
noted. Racemic 1f, 1g, and 1h with C9–C11 chains showed SmA and monotropic SmC
phases, while enantiotropic SmA and SmC phases were observed for (± )-1i and (± )-1j
with C12 and C13 chains. The N—SmA transition for (± )-1f was accompanied by very
small enthalpy changes (0.50 and 0.28 kJ/mol on the heating and cooling runs, respectively)
(Table 1). A typical thread-like texture, a fan-shaped texture and a broken fan-shaped one
were observed for these N, SmA and SmC phases by POM, respectively (Fig. 3). Variable
temperature XRD analysis verifies the existence of these smectic phases (Table 3).
The phase transition behavior of (2S,5S)-1 was quite different from that of the corre-
sponding (± )-1 (Fig. 2b). Namely, (2S,5S)-1 exhibited a wider temperature range of the
∗
SmC phase than that of the SmC phase of (± )-1. As to the optical texture by POM,
∗
(
2S,5S)-1a–1f with C4–C9 chains exhibited a fan-like texture which is typical of N phase,
respectively, on the cooling run (Fig. 4a), while (2S,5S)-1f–1j with C9–C13 chains did
∗
∗
fan-shaped and broken fan-shaped textures which are typical of SmA and SmC phases,
respectively, on the cooling run (Fig. 4b, c). Interestingly, (2S,5S)-1f with C9 chains showed
∗
∗
∗
∗
not only N , SmA and SmC phases but also a TGBA phase with a blurred fan-shaped
◦
∗
or Granjean texture in a very narrow temperature range (0.3∼0.4 ) between the N and
∗
∗
∗
SmA phases on the cooling run (Fig. 4d). The N -to-TGBA transition for (2S,5S)-1f was
accompanied by very small enthalpy changes (less than 0.45 kJ/mol) (Table 2). (2S,5S)-1g
∗
with C10 chains also showed a TGBA phase; this unstable phase was observed between

Paramagnetic Ferroelectric LCs
99
Figure 5. The molecular conformations of (a) 1j, (b) 2 (n = 13) and (c) 3 (n = 13) optimized by
the unrestricted AM1 method (PC Spartan’02), and (d) the schematic representation of the angle φ
between the molecular long axis and the direction of the dipole moment. Arrows show the direction
of each dipole moment.
∗
∗
the oily-streak texture of the N phase and the fan-shaped texture of the SmA phase only
by POM. Variable temperature XRD analysis verifies the existence of these smectic phases
(
Table 3).
In order to shed light on the difference in the phase transition behavior of 1, 2 and 3,
each of their molecular dipole moments was calculated for (2S,5S)-1j, (2S,5S)-2 (n = 13)
and (2S,5S)-3 (n = 13) with the same C13 alkyl chains. Each molecular conformation with
two structurally fixed linear anti-conformational alkyl chains was optimized by the Monte
Carlo method using the Merck Molecular Force Field (MMFF), followed by the AM1
semi-empirical calculation using PC Spartan’02.[ The optimized molecular geometries
22]
∗
have a distorted zigzag structure that is advantageous for the appearance of an SmC phase
(Fig. 5a–c).[23] The angle φ, which is defined as the angle between the molecular long
axis (one of the principal axes of inertia) and the direction of the dipole moment in each
molecule (Fig. 5d), was 69.2 (73.7), 39.6 (33.9) and 18.8 (19.5) degrees for 1j, 2 (n = 13)
and 3 (n = 13), respectively (Fig. 5a-c).
Although further investigation is necessary to know the whole aspect, the difference in
the direction of dipole moment between 1j, 2 (n = 13) and 3 (n = 13) must be responsible for
the difference in the phase transition behavior; the large φ may stabilize the interlayer dipole
interaction to direct the molecular dipole moment perpendicular to the molecular long axis

100
Y. Uchida et al.
Table 4. Basic magnetic properties of 1
EPR
SQUID (racemate)
SQUID (2S,5S-isomer)
aN
(mT)
C (emu K
mol )
θω
ee
(%)
C (emu K
mol )
θ ω
−
1
−1
Compound
g
(K)
(K)
1a (n = 4)
1b (n = 5)
1c (n = 6)
1d (n = 7)
1e (n = 8)
1f (n = 9)
1g (n = 10)
1h (n = 11)
1i (n = 12)
1j (n = 13)
2.0057
2.0060
2.0060
2.0059
2.0061
2.0061
2.0061
2.0060
2.0061
2.0060
1.33
1.34
1.34
1.33
1.33
1.33
1.34
1.33
1.33
1.33
0.398
0.374
0.381
0.331
0.384
0.380
0.349
0.362
0.362
0.362
–0.16
–0.40
–3.24
–18.15
–1.88
–0.30
–0.62
–0.36
–0.36
+0.34
85
81
86
75
91
89
83
89
85
86
0.380
0.378
0.381
0.380
0.375
0.374
0.376
0.377
0.377
0.373
–0.52
–0.15
–0.19
–0.54
–0.47
–0.36
+0.11
–0.47
–0.56
–0.05
∗
and parallel to the smectic layer plane. Therefore, the stabilization of SmA (SmA ) phases
for 1 and 2 can be rationalized in analogy to H-type aggregation in π-organogels.[24] For
∗
∗
the same reason, the small φ for 3 favors N (N ) and SmC (SmC ) phases.
The basic magnetic properties of 1 are summarized in Table 4. The EPR spectra were
◦
measured in THF at a field of 0.34 T at 25 C, displaying a characteristic intense 1:1:1
triplet. The magnetic susceptibility was measured in a quartz tube (3.5 φ × 40 mm) on
a SQUID magnetometer at a field of 0.5 T in the temperature range of 2-300 K. The
Curie constants (C) thus obtained indicate that the radical purity of 1 is very high. Almost
all of 1 showed very weak intermolecular antiferromagnetic interactions (Weiss constant:
θW < 0), except that only (± )-1d showed relatively strong antiferromagnetic interactions
(
θW = –18 K). Although the crystal structure of (± )-1d has not been obtained yet due to
the poor crystallinity, it is conceivable that the SOMO-SOMO overlapping due to a close
contact between the neighboring nitroxyl groups is responsible for such relatively strong
antiferromagnetic interactions.[25]
Table 5. Ferroelectric properties of (2S,5S)-1.
◦
a
◦ c
◦ d
θ (–10 )
(deg)
Compound
PS(–10 )
τ10-90
γ (–10 )
−
2
◦ b
[ee (%)]
(nC cm )
(–10 ) (ms)
(m Pa s)
(2S,5S)-1f [89]
(2S,5S)-1g [83]
(2S,5S)-1h [89]
(2S,5S)-1i [85]
(2S,5S)-1j [86]
6.9
1.9
6.2
5.8
3.7
0.465
0.650
0.750
0.480
0.465
91.6
35.2
133
79.5
49.2
17
18
17
17
19
aSpontaneous polarization.
b
c
Optical response time required to attain 10–90% of the optical transmittance after field reversal.
Viscosity calculated using γ = |Ps|Eτ10-90/1.75 (ref 8).
d
Optical tilt angle measured by POM.

Paramagnetic Ferroelectric LCs
101
S
Figure 6. (a) θ and (b) P of (2S,5S)-1f-j.
The magnitude of the spontaneous polarization (PS) and the tilt angle (θ) of (2S,5S)-
1
f–1j evaluated by POM are shown in Table 5 and Fig. 6. A temperature-dependent PS
∗
(
(
θ) inversion [26] was observed by measurement of θ in the SmC phase of (2S,5S)-1f–1j
Fig. 6a). On the cooling run from the SmA -to-SmC transition temperature, the positive θ
∗
∗
◦
gradually increased, the positive-to-negative θ inversion occurred between 45 and 60 C and
then the absolute value of negative θ increased (Fig. 6a). Around the transition temperature
θ was unable to be measured with accuracy, because interconversion between two types of

102
Y. Uchida et al.
domains with opposite signs of θ occurred. In the case of (2S,5S)-1f, only positive θ was
observed.
The positive PS value of (2S,5S)-1f, (2S,5S)-1g, (2S,5S)-1h and (2S,5S)-1j initially
increased, reached a plateau and gradually decreased in the proximity of the θ inversion
temperature on the cooling run (Fig. 6b). The peak of PS, however, was too broad to measure
below the θ inversion temperature (Fig. 6b). In the case of (2S,5S)-1f, the positive PS value
initially increased, reached a plateau, gradually decreased and finally was unable to be
measured. We previously reported that (2S,5S)-1i showed an analogous PS (θ) inversion
and estimated that their inversion resulted from the temperature dependent interconversion
between different conformations of the chiral mesogen core [13]. This estimation would
be applicable to the other cases of (2S,5S)-1 that exhibited a positive or a negative θ above
◦
or below 45∼60 C, respectively. This phenomenon is of little relevance to the alkyl chain
length.
Conclusion
We have reported the synthesis, characterisation and ferroelectric properties of a new series
of all-organic radical liquid crystals 1. The enantiomerically enriched or racemic 1 showed
∗
∗
∗
∗
N , TGBA , SmA and SmC phases, or N, SmA and SmC phases, respectively. The phase
transition behaviour supports the hypothesis that the large φ results in the stabilization
∗
∗
of the SmA (SmA) phase. The stabilization of SmA (SmA ) phases for 1 and 2 can be
rationalized in analogy to H-type aggregation in π-organogels. Additionally, we observed
a temperature-dependent θ inversion for (2S,5S)-1f–1j. Although the origin of this PS
(
θ) inversion cannot be fully explained at present, an EPR spectroscopic study of these
compounds will provide further important information on the conformational change in the
LC molecules.
Acknowledgment
This work was supported by the Grant-in Aid for Scientific Research (No. 19350067)
from Japan Society for the Promotion of Science, the Asahi Glass Foundation (R.T.) and
the Research Fellowships of the Japan Society for the Promotion of Science for Young
Scientists (Y.U. & N.I.).
References
[
1] Formaggio, F., Bonchio, M., Crisma, M., Peggion, C., Mezzato, S., Polese, A., Barazza, A.,
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[
[
[
7] Aurich, H. G. (1989). In: Nitrones, Nitrorates and Nitroxides, Patai, S. & Rappoport, Z. (Eds.),
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[
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15] Kalai, T., Jeko, J., Hubbell, W. L., & Hideg, K. (2003). Synthesis, 2084.
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[
17] Ikuma, N., Tamura, R., Shimono, S., Kawame, N., Tamada, O., Sakai, N., Yamamoto, Y., &
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104
Y. Uchida et al.
Appendix: Crystal structure of (± )-1a (Fig. A1), DSC curves (Fig. A2) and crystallo-
graphic data for (± )-1a (Table A1).
Appendix A. Table A1. Crystallographic Data for (± )-1a
structure formula
formula weight
crystal dimensions (mm)
crystal system
space group
a (Å)
C33H41O3N2
513.70
0.30 × 0.30 × 0.20
Monoclinic
C2/c
24.660(2)
b (Å)
11.8753(8)
c (Å)
β ( )
vol (Å )
20.4163(14)
107.214(3)
5710.9(7)
◦
3
Z
8
3
ρcalc (g/cm )
1.197
55.0
◦
2θmax ( )
radiation
MoKα
wavelength (Å)
0.71075
scan mode
ω
temp (K)
173
number of measured reflections
number of independent reflections
number of reflections included in the refinement
σ limits
27222
6551
6996
I > 2.00σ(I)
−1
μ (cm )
0.759
method of structure solution
method of refinement
number of parameters
Goodness-of-fit on F2
R1
Direct Method (SIR97)
Full-matrix least-square on F
2
373
0.982
0.096
0.239
1.16
wR2
−
−
3
maximum peak in final diff. map (e /Å )
minimum peak in final diff. map (e /Å )
3
1.00

Paramagnetic Ferroelectric LCs
105
Figure A1. Crystal structure of (± )-1a: (a) a heterochiral dimer on the bc plane containing two
kinds of CH/O interactions; C—O distance, (i) 3.55 Å and (ii) 3.33 Å. (b) a homochiral 1D chain
along the a axis containing CH/π interactions [C—phenyl centroid distance, (iii) 3.55 Å] and CH/N
interactions [C—N distance, (iv) 3.58 Å]. Carbon, oxygen and nitrogen atoms are denoted by white,
gray, and black circles, respectively.

106
Y. Uchida et al.
Figure A2. DSC curves of (a) (2S,5S)-1f and (b) (2S,5S)-1g.