Synthesis and characterization of novel all-organic liquid crystalline radicals

Article abstract of DOI:10.1080/15421400701709785

We have synthesized a new series of all-organic liquid crystalline (LC) radical compounds (2), which are the structural isomers of the previously reported series (1) containing a chiral nitroxide unit in the mesogen core. Only difference between the two i

Full text of DOI:10.1080/15421400701709785

Mol. Cryst. Liq. Cryst., Vol. 479, pp. 213=[1251]–221=[1259], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701709785
Synthesis and Characterization of Novel All-Organic
Liquid Crystalline Radicals
Yoshiaki Uchida
Rui Tamura
Naohiko Ikuma
Graduate School of Human and Environmental Studies,
Kyoto University, Kyoto, Japan
Satoshi Shimono
Jun Yamauchi
Graduate School of Science, Kyoto University, Kyoto, Japan
Yoshio Aoki
Hiroyuki Nohira
Faculty of Engineering, Department of Applied Chemistry,
Saitama University, Saitama, Japan
We have synthesized a new series of all-organic liquid crystalline (LC) radical
compounds (2), which are the structural isomers of the previously reported series
(1) containing a chiral nitroxide unit in the mesogen core. Only difference between
the two isomers is the orientation in the ester functionality (ꢀOCOꢀ vs ꢀCOOꢀ).
The new LC compounds 2 with C4 to C8 alkoxy terminal chains showed the dis-
tinct odd-even effects of alkoxy terminal chains with respect to the thermal stability
of nematic phases, while such effects were not observed for the isomers 1.
Keywords: chiral nitroxide; nematic phase, nitroxyl radical; odd-even effects;
paramagnetic liquid crystals
INTRODUCTION
Paramagnetic calamitic liquid crystals have attracted great interest
due to the possibility of orientation control of LC molecules by weak
This work was supported by the Asahi Glass Foundation (R.T.) and the Research
Fellowships of the Japan Society for the Promotion of Science for Young Scientists (N.I.).
Address correspondence to Rui Tamura, Graduate School of Human and Environ-
mental Studies, Kyoto University, Yoshida-nihonmastsu-cho, Sakyo-ku, Kyoto 606-
8501, Japan. E-mail: tamura-r@mbox.kudpc.kyoto-u.ac.jp
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Y. Uchida et al.
magnetic fields [1,2] and the use as LC spin probes for electron para-
magnetic resonance (EPR) spectroscopic studies to observe the
dynamic behavior of diamagnetic liquid crystals. The paramagnetic
liquid crystals are classified into two categories; one is a metallomeso-
gen containing a paramagnetic metal ion and the other is an organic
radical mesogen. The former metallomesogens can have a large para-
magnetic susceptibility anisotropy (Dvpara) which originates from the
d- or f-metal ion and seems advantageous to the orientation control
of LC molecules by magnetic fields [3–6]. On the other hand, the latter
organic mesogens are appropriate for the LC spin probes to investigate
the orientation of the diamagnetic organic LCs, due to their low vis-
cosity and very small Dvpara which does not disturb the Dvdia-controlled
orientation of diamagnetic organic liquid crystals [7], if the spin source
moiety is fixed in the mesogen core so as to avoid the free rotation
inside the molecule.
We successfully synthesized the first all-organic paramagnetic LC
compounds 1 [8–10], which contain a chiral nitroxide unit in the
mesogen core. The racemic or enantiomerically-enriched 1 exhibited
N or Nꢁ, and=or SmC or SmCꢁ phases, respectively. Noteworthy is the
fact that the chiral SmCꢁ phase of 1 showed a ferroelectricity [9,10].
In order to investigate the influence of the direction and magnitude of
molecular electric dipole moments on the stability of LC phases, we have
modified the molecular structure of 1 and prepared a series of 4-alkoxy-
phenyl 4-[5-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxyl-2-yl]benzo-
ate (2) (Scheme 1). Here we report the preparation and characterization
of the new LC compounds (ꢂ)-2 with C4 to C8 alkoxy terminal chains.
EXPERIMENTAL
Unless otherwise noted, solvents and reagents were reagent grade
and used without further purification. Tetrahydrofuran (THF) which
is used for EPR spectral measurement or Grignard reactions was
distilled from sodium=benzophenone ketyl under argon. Dichloro-
methane (DCM) for esterification reaction was distilled from calcium
hydride under argon. Phase transition temperatures were determined
by differential scanning calorimetry (DSC) (SHIMADZU DSC-50) and
polarized optical microscopy (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 spec-
tra were recorded with a JEOL FE1XG. Magnetization was recorded
with QUANTUN DESIGN MPMS-2 and MPMS-5S. For variable-
temperature X-ray diffraction measurement, the data collections were
performed on a Rigaku RINT2200=PC-LH diffractometer.

Organic Liquid Crystalline Radicals
215=[1253]
SCHEME 1 Molecular structures of (ꢂ)-1 and (ꢂ)-2, and synthesis of (ꢂ)-2.
General Synthetic Procedure of (ꢂ)-2 (Scheme 1)
Aryl Magnesium Bromide (4, 6)
To magnesium turnings (0.243 g, 10 mmol) dried by heating at
3 mmHg was added THF (10 mL), arylbromide (10 mmol), and a small
amount of iodine under argon. The reaction started by heating, and

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Y. Uchida et al.
the reaction mixture was refluxed for 4 h to afford a solution of aryl
magnesium bromide in THF.
2,5-Diaryl-2,5-dimethylpyrrolidine-1-oxyl Radical (7) [8–11]
To a stirred solution of the freshly prepared Grignard reagent
4 (10 mmol) in THF (10 mL) was slowly added a solution of the nitrone
3 (5.0 mmol) in THF (10 mL) under argon at ꢀ78ꢃC. 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 DCM (2 ꢄ 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 cupper (II)
acetate monohydrate (0.160 g, 0.80 mmol). Oxygen was bubbled
through the yellow solution until a persistent deep blue color
developed. The solvent was then removed under reduced pressure
and the crude product was redissolved in DCM (50 mL). This solution
was washed with saturated aqueous NaHCO3 solution (50 mL), dried
over MgSO4, and evaporated. The water was removed by the azeo-
tropic method with benzene and the remaining trace of benzene was
removed in vacuo. The crude product 5 was dissolved in THF
(10 mL) and reacted with the freshly prepared Grignard reagent 6
(the same quantity and conditions as described above). After a similar
workup procedure, the crude product was obtained. It was again
oxidized by O2=cupper acetate. Purification by column chromato-
graphy (hexane=ether) on silica gel provided the nitroxyl radical 7.
2-(4-Alkyloxyphenyl)-5-(4-hydroxycarbonylphenyl)-2,5-
dimethylpyrrolidine-1-oxyl Radical (8)
To a solution of 7 (1.0 mmol) in THF (2 ml) was added aqueous
H2SO4 (5%; 0.50 mL) and the mixture was stirred for 8 h at room tem-
perature. Brine and DCM were then added. The organic phase was
separated, dried over MgSO4, and evaporated. To a vigorously stirred
suspension of Ag2O (463 mg, 2.0 mmol) in aqueous NaOH solution
(10%, 4 mL) was added the crude product in THF (1.5 mL) at 60ꢃC,
and the mixture was stirred under reflux for 40 min. After cooling,
the reaction mixture was acidified with aqueous H2SO4 (5%) to pH
2, and extracted with CHCl3 (3 ꢄ 10 mL). The organic phase was dried
over MgSO4 and evaporated. The residue was purified by flash column
chromatography (DCM=ether) to give carboxylic acid (ꢂ)-8.
(ꢂ)-8a: Mp 172.8–173.0ꢃC. EPR: g ¼ 2.0058, aN ¼ 1.33 mT. IR (DIF-
FUSE REFLECTION): n 2927, 2867, 1685, 1604, 1570, 1506, 1420,

Organic Liquid Crystalline Radicals
217=[1255]
1279, 1242, 1180, 1067, 845, 779, 712 cmꢀ1. Anal. Calcd for C23H28NO4:
C, 72.23; H, 7.38; N, 3.66; Found: C, 72.05; H, 7.47; N, 3.49.
(ꢂ)-8b: Mp 170.5–171.5ꢃC. EPR: g ¼ 2.0056, aN ¼ 1.34 mT. IR (DIF-
FUSE REFLECTION): n 2937, 2872, 1689, 1607, 1570, 1510, 1424,
1289, 1247, 1185, 1051, 839, 780, 707 cmꢀ1. Anal. Calcd for
C24H30NO4: C, 72.70; H, 7.63; N, 3.53; Found: C, 72.89; H, 7.62; N, 3.48.
(ꢂ)-8c: Mp 168.4–169.3ꢃC. EPR: g ¼ 2.0059, aN ¼ 1.33 mT. IR (DIF-
FUSE REFLECTION): n 2949, 2859, 1684, 1606, 1571, 1505, 1424,
1290, 1247, 1182, 1075, 839, 780, 708 cmꢀ1. Anal. Calcd for
C25H32NO4: C, 73.14; H, 7.86; N, 3.41; Found: C, 73.04; H, 7.81; N, 3.31.
(ꢂ)-8d: Mp 168.3–169.2ꢃC. EPR: g ¼ 2.0055, aN ¼ 1.34 mT. IR (DIF-
FUSE REFLECTION): n 2919, 2853, 1685, 1607, 1570, 1507, 1424,
1285, 1247, 1185, 1074, 840, 780, 707 cmꢀ1. Anal. Calcd for
C26H34NO4: C, 73.55; H, 8.07; N, 3.30; Found: C, 73.69; H, 8.01;
N, 3.22.
General Procedure for Esterification to Give 2 [12]
Under argon, a 10 ml of oven-dried, round-bottomed flask contain-
ing anhydrous DCM (3 ml) is charged with the carboxylic acid (ꢂ)-8
(0.2 mmol), the phenol 9 (0.2 mmol) [13], dicyclohexylcarbodiimide
(DCC, 0.22 mmol), and 4-(dimethylamino)pyridine (DMAP, 0.04 mmol).
After the mixture was stirred for 12 h at room temperature, the
precipitated white solid was removed by filtration, and the filtrate
was evaporated. The residue was purified by flash column chromato-
graphy (hexane=ether) to afford the ester (ꢂ)-2.
(ꢂ)-2a: EPR: g ¼ 2.0058, aN ¼ 1.34 mT. IR (DIFFUSE REFLEC-
TION): n 2929, 2876, 1735, 1608, 1505, 1271, 1247, 1183, 1074, 839,
770, 703 cmꢀ1. Anal. Calcd for C33H40NO5: C, 74.69; H, 7.60; N, 2.64;
Found: C, 74.30; H, 7.69; N, 2.65.
(ꢂ)-2b: EPR: g ¼ 2.0056, aN ¼ 1.34 mT. IR (DIFFUSE REFLEC-
TION): n 2937, 2871, 1738, 1609, 1511, 1279, 1254, 1184, 1078, 830,
771, 705 cmꢀ1. Anal. Calcd for C35H44NO5: C, 75.24; H, 7.94; N, 2.51;
Found: C, 75.01; H, 8.01; N, 2.16.
(ꢂ)-2c: EPR: g ¼ 2.0058, aN ¼ 1.34 mT. IR (DIFFUSE REFLEC-
TION): n 2928, 2855, 1730, 1609, 1507, 1280, 1244, 1078, 834, 771,
705 cmꢀ1. Anal. Calcd for C37H48NO5: C, 75.73; H, 8.25; N, 2.39;
Found: C, 75.78; H, 8.30; N, 2.38.
(ꢂ)-2d: EPR: g ¼ 2.0059, aN ¼ 1.33 mT. IR (DIFFUSE REFLEC-
TION): n 2935, 2857, 1730, 1608, 1507, 1278, 1247, 1186, 1076, 833,
771, 705 cmꢀ1. Anal. Calcd for C39H52NO5: C, 76.19; H, 8.40; N, 2.28;
Found: C, 75.95; H, 8.40; N, 2.29.
(ꢂ)-2e: EPR: g ¼ 2.0060, aN ¼ 1.31 mT. IR (DIFFUSE REFLEC-
TION): n 2920, 2854, 1734, 1608, 1507, 1268, 1249, 1184, 1073, 858,

218=[1256]
Y. Uchida et al.
770, 707 cmꢀ1. Anal. Calcd for C40H54NO5: C, 76.40; H, 8.66; N, 2.23;
Found: C, 76.31; H, 8.51; N, 2.26.
RESULTS AND DISCUSSION
The magnetic properties of (ꢂ)-2 are summarized in Table 1. The EPR
spectra were measured in THF at a field of 0.34 T at 25ꢃC, displaying
an intense 1:1:1 triplet. The magnetic susceptibility was measured in a
quartz tube (3.5 / ꢄ 40 mm) on a SQUID magnetometer at a field of
0.5 T in the temperature range 2–300 K, showing the existence of very
weak antiferromagnetic intermolecular interactions (h < 0). The Curie
constants indicate that the radical purity of (ꢂ)-2 is very high.
The phase transition behavior of (ꢂ)-2 is characterized by DSC
analysis and polarized optical microscopy (Table 1 and Fig. 1). All of
(ꢂ)-2 showed a Schlieren or thread-like texture typical of a nematic
phase under planar boundary conditions by hot-stage polarized
microscopy (Fig. 2) [14]. These nematic phases were observed over a
wide temperature range during the cooling process, and the phases
of (ꢂ)-2b were monotropic. Variable temperature X-ray diffraction
analyses of (ꢂ)-2 verified the existence of nematic phases, showing
only a halo.
Compounds (ꢂ)-2 with C4 to C8 alkoxy terminal chains exhibited the
distinct odd-even effects of alkoxy terminal chains with respect to the
thermal stability of nematic phases; (ꢂ)-2a and (ꢂ)-2c with respective
C4 and C6 alkoxy terminal chains showed the higher clearing points
and the wider temperature ranges of the nematic phases than (ꢂ)-2b
and (ꢂ)-2d with respective C5 and C7 terminal chains (Fig. 1). Further-
more, when one of the two C7 terminal chains of (ꢂ)-2dwas lengthened
by one carbon, the resultant (ꢂ)-2e with C7 and C8 terminal chains-
showed a higher clearing point and a wider temperature range of the
nematic phase than (ꢂ)-2d. These phenomena were often observed
for the LC compounds showing a nematic phase [15].
Such odd-even effects of alkoxy terminal chains were not observed
for (ꢂ)-1, the structural isomers of (ꢂ)-2. This result indicates that
the origin of the observed odd-even effects seems to be related to the
thermal stability of nematic phases (Fig. 1). As a preliminary study,
it has been found that the molecular electric dipole moments of (ꢂ)-2,
which have been calculated by the PM3 method after optimization of
the molecular structures, are smaller by ca. 1 Debye than those of
(ꢂ)-1. Therefore, it is conceivable that the weak intermolecular
electric dipole-dipole interactions in the nematic phases of (ꢂ)-2 allow
the appearance of the odd-even effects which are sensitive to the
surroundings.

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Y. Uchida et al.
FIGURE 1 Comparison of LC behavior of (Æ)-2 with that of (Æ)-1. Transition
temperatures determined by DSC analysis at a scanning rate of 5ꢀC minÀ1
upon the heating (h) and cooling (c) processes. At this scanning rate, hysteresis
is observed for the N-I or C-N phase transition between the heating and cool-
ing runs, as is likely since thermal equilibrium is not reached during the
measurement.
FIGURE 2 Polarized optical micrographs showing (a) a Schlieren texture at
104.0ꢀC and (b) a thread-like texture in a sandwich cell (50 mm)at 100.0ꢀC,
for (Æ)-2c under planar boundary conditions.

Organic Liquid Crystalline Radicals
221=[1259]
CONCLUSION
We have synthesized the racemates of novel mesogenic nitroxyl
radicals 2 which contain a chiral nitroxide unit in the mesogen core.
The racemic 2 bearing C4 to C8 alkoxy terminal chains showed a
nematic phase over a wide temperature range during the cooling
process and the odd-even effects of alkoxy terminal chains with
respect to the clearing temperature and the temperature range of
the nematic phase.
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