Planar chirality: A fascinating symmetry breaking which leads to ferroelectricity in ferrocenyl liquid crystals

Article abstract of DOI:10.1039/b005640p

We describe the synthesis, characterization and properties of the first optically-active 1,3-unsymmetrically disubstituted ferrocene derivative which is to exhibit smectic C* and smectic A* phases.

Full text of DOI:10.1039/b005640p

Planar chirality: a fascinating symmetry breaking which leads to
ferroelectricity in ferrocenyl liquid crystals
Thierry Chuard,a Stephen J. Cowling,b Maria Fernandez-Ciurleo,a Isabelle Jauslin,a John W. Goodby*b and
Robert Deschenaux*a
a Institut de Chimie, Université de Neuchâtel, Avenue de Bellevaux 51, Case postale 2, CH-2007 Neuchâtel,
Switzerland. E-mail: robert.deschenaux@unine.ch
b Department of Chemistry, University of Hull, Hull, UK HU6 7RX. E-mail: j.w.goodby@chem.hull.ac.uk
Received (in Cambridge, UK) 12th July 2000, Accepted 9th August 2000
First published as an Advance Article on the web 11th October 2000
We describe the synthesis, characterization and properties of
the first optically-active 1,3-unsymmetrically disubstituted
ferrocene derivative which is to exhibit smectic C* and
smectic A* phases.
the molecules about their long axes. Within this packing
constraint, the V-shaped molecules will pack together to give a
non-symmetric arrangement thereby leading to the induction of
non-linear properties.
Ferroelectric liquid crystals are of considerable interest in
switchable, half-wave plate, bistable light-valves. For ferroce-
nyl liquid crystals, SmC* phases have been reported only for
two mono-substituted derivatives.7,8 Chirality was introduced
into the peripheral side-chain by means of asymmetric carbon
atoms, but for the two materials no ferroelectric behavior was
described.
In addition to the investigation of the dependency of
ferroelectricity on planar chirality, this study allowed us to
probe the influence of the chiral unit, which is embedded in the
central region of the molecular structure, on the self-organiza-
tion process and mesophase formation. In metallomesogens,
planar chirality was elegantly exploited by Malthête and
coworkers who reported optically-active butadiene–
tricarbonyliron liquid crystal complexes.9 The spontaneous
polarization of one of the complexes was determined, and a
value of 32 nC cm22 was obtained (response time: 9 ms).9b
We report, herein, the synthesis, mesomorphic behavior and
ferroelectric properties of the liquid-crystalline Fc derivative
(+)-1 (Scheme 1). The latter structure was selected because the
corresponding racemic analogue gave the broadest SmC range
among the family of homologues studied.5
The preparation of (+)-1 is shown in Scheme 1. Optical
resolution of acid (±)-2 with (+)-phenylethylamine [(+)-PEA]
gave (2)-2.† Treatment of (2)-2 with oxalyl chloride gave the
corresponding acid chloride derivative, which was condensed
with 4-hydroxyphenyl 4-(octadecyloxy)benzoate10 to furnish
(+)-1.‡
Symmetry breaking operations and chirality have held centre
stage in the fundamental investigations and applications of
liquid crystals.1,2 Furthermore, recent studies on mesogens with
banana-shaped molecular structures have shown that chiral
induction in self-organizing systems is possible even though the
materials are themselves achiral.3,4 The search for new chiral
effects in mesomorphic systems prompted us to synthesize
unsymmetrically 1,3-disubstituted ferrocene-containing liquid
crystals,5 where the different substituents at the 1- and
3-positions generate structures with planar chirality (Fig. 1). A
representative example is illustrated by compound 1 (see
Scheme 1); the two substituents are differentiated by the length
of the alkyl chains and the orientation of the outer ester groups.
Homologues of this material were prepared in racemic form
from (±)-25 (see Scheme 1). Over the entire range of the
compounds studied, smectic C (SmC) and smectic A (SmA)
phases were observed, and in some cases an additional nematic
phase was detected.
The presence of the SmC phase in this family of compounds
provided a unique challenge to make the materials optically
active (SmC*) and thereby ferroelectric. Of course symmet-
rically 1,3-disubstituted ferrocene derivatives have the potential
to exhibit ferro- or antiferro-electricity if the packing constraints
of the molecules within the layers induce a restricted rotation of
Derivatization of the acid (2)-2 with (+)-PEA gave the amide
3a,§ which served for the determination of the enantiomeric
1
excess (ee) of (2)-2 by H NMR and HPLC techniques. We
Fig. 1 Planar chirality in unsymmetrically 1,3-disubstituted Fc derivatives.
Planar chirality: chirality resulting from the arrangement of out-of-plane
groups with respect to a reference plane, called the ‘chiral plane’.6
assumed that the reactions used to prepare 3a and (+)-1 did not
alter the optical purity of (2)-2. Therefore, the determination of
the diastereomeric excess (de) of 3a is a measure of the ee of
(2)-2 and, consequently, of (+)-1. The 1+1 diastereomeric
mixture of 3a+3b synthesized from (±)-2 and (+)-PEA was used
to fix the analytical conditions. The 1H NMR and HPLC
characteristics of diastereoisomer 3b [from (+)-2 and (+)-PEA]
were deduced by comparing the data of the 1+1 diastereomeric
mixture with that of 3a.
Scheme 1 Reagents and conditions: i, (+)-phenylethylamine [(+)-PEA],
CH2Cl2; ii, (a) oxalyl chloride, pyridine, CH2Cl2, reflux, 3 h; (b)
4-hydroxyphenyl 4-(octadecyloxy)benzoate,10 triethylamine, CH2Cl2, re-
flux, 3 h.
In the 1H NMR spectrum of the 1+1 diastereomeric mixture,
each diastereoisomer gave well separated signals for the five
protons of the unsubstituted cyclopentadienyl (Cp) ring (3a:
DOI: 10.1039/b005640p
Chem. Commun., 2000, 2109–2110
This journal is © The Royal Society of Chemistry 2000
2109

1.16 mmol) in dry CH2Cl2 (15 ml). The solution was allowed to stand
overnight at room temperature. The salt was recovered by filtration, dried
(743 mg), and crystallized in a 1+1 mixture of dry CH2Cl2–acetone (75 ml);
yield: 554 mg. A mixture of the salt (554 mg), CH2Cl2 (40 ml) and 5 M HCl
(40 ml) was vigorously shaken and the layers were separated. The organic
layer was washed (water), dried (MgSO4), and evaporated to dryness.
Crystallization (CH2Cl2–hexane) of the solid residue gave (2)-2 (327 mg,
45% with respect to the desired enantiomer) of ee = 98%. Mp 170 °C. [a]D
218 (c 0.45, CH2Cl2). dH(200 MHz, acetone-d6) 8.27 (d, 2 H, arom.), 7.52
(d, 2 H, arom.), 7.22 (d, 2 H, arom.), 7.01 (d, 2 H, arom.), 5.52 (t, 1 H, Cp),
5.19 (dd, 1 H, Cp), 5.14 (dd, 1 H, Cp), 4.44 (s, 5 H, Cp), 4.03 (t, 2 H, OCH2),
1.80 (m, 2 H, OCH2CH2), 1.60–1.20 [m, 14 H, O(CH2)2(CH2)7], 0.89 (t, 3
H, CH3). Anal. Calc. for C35H38O7Fe (626.53): C, 67.10; H, 6.11. Found: C,
67.08; H, 6.27%.
‡ Synthesis of (+)-1: first step: a mixture of (2)-2 (250 mg, 0.441 mmol),
oxalyl chloride (670 mg, 5.29 mmol), pyridine (5 drops) and dry CH2Cl2 (15
ml) was heated under reflux for 3 h, and evaporated to dryness. The solid
residue was extracted with hot light petroleum (bp 60–90 °C) until the
extracts remained colorless. Evaporation of the solvent gave the acid
chloride derivative (273 mg, 96%), which was used in the next step without
further purification. dH(400 MHz, CDCl3) 8.29 (d, 2 H, arom.), 7.33 (d, 2 H,
arom.), 7.13 (d, 2 H, arom.), 6.94 (d, 2 H, arom.), 5.74 (t, 1 H, Cp), 5.34 (dd,
1 H, Cp), 5.21 (dd, 1 H, Cp), 4.50 (s, 5 H, Cp), 3.97 (t, 2 H, OCH2), 1.79 (m,
2 H, OCH2CH2), 1.60–1.20 [m, 14 H, O(CH2)2(CH2)7], 0.89 (t, 3 H, CH3).
Second step: a solution of the above acid chloride derivative (250 mg, 0.39
mmol), 4-hydroxyphenyl 4-(octadecyloxy)benzoate10 (188 mg, 0.39
mmol), triethylamine (40 mg, 0.39 mmol) in dry CH2Cl2 (15 ml) was heated
under reflux for 3 h, cooled to room temperature, and evaporated to dryness.
Purification of the solid residue twice by column chromatography (CC)
[first CC+silica gel, CH2Cl2–AcOEt (50+1); second CC: silica gel, CH2Cl2–
AcOEt (100+1)], and crystallization (CH2Cl2–EtOH) gave (+)-1 (232 mg,
54%) of ee = 98%. [a]D +3 (c 0.42, CH2Cl2). dH(200 MHz, CDCl3) 8.29
(d, 2 H, arom.), 8.15 (d, 2 H, arom.), 7.35 (d, 2 H, arom.), 7.27 (d, 4 H,
arom.), 7.12 (d, 2 H, arom.), 6.98 (d, 2 H, arom.), 6.94 (d, 2 H, arom.), 5.79
(t, 1 H, Cp), 5.27 (br s, 2 H, Cp), 4.47 (s, 5 H, Cp), 4.06 (t, 2 H, OCH2), 3.97
(t, 2 H, OCH2), 1.80 (m, 4 H, OCH2CH2), 1.60–1.20 (m, 44 H, aliph.), 0.89
(2 3 t, 6 H, CH3). Anal. Calc. for C66H82O10Fe (1091.21): C, 72.65; H,
7.57. Found: C, 72.79; H, 7.77%.
4.28 ppm; 3b: 4.33 ppm) and the two Cp protons ortho to the
amide function (3a: 5.70 and 5.19 ppm; 3b: 5.65 and 5.24 ppm).
For these signals, the five Cp protons gave two sharp singlets
with baseline resolution, and appeared to be suitable for the
determination of the de of 3a. HPLC analysis (column Nucleosil
120 RP-C18 (250 3 4 mm) equipped with a precolumn
Nucleosil 120 RP—C18 (30 3 4 mm); solvent, acetonitrile–
water (80+20); elution rate, 1.5 ml min21) of the 1+1
diastereomeric mixture furnished two peaks (3a: 40.2 min; 3b:
38.0 min) with almost baseline resolution. Both techniques led
to reliable and reproducible results.
Optical resolution of (±)-2 by (+)-PEA proved to be efficient:
1H NMR and HPLC methods gave a de of 98% for 3a, leading
to an ee value of 98% for (+)-1.
The absolute configuration of the Fc in (+)-1, (2)-2 and 3a is
shown arbitrarily. So far, crystallization of the salt (+)-PEA/
(2)-2 and amide 3a failed to give crystals suitable for X-ray
analysis.
The liquid-crystalline properties were investigated by differ-
ential scanning calorimetry (DSC, 10 °C min21, under N2) and
polarized optical microscopy.¶ Ferrocene (+)-1 gave enantio-
tropic SmC* and SmA* phases (Cr ? SmC*: 170 °C; SmC* ?
SmA*: 198 °C; SmA* ? I: 202 °C; onset point, first heating
run). The SmC* to SmA* phase transition was not detected by
DSC (second order transition). No decomposition or racemiza-
tion was observed for (+)-1. Similar transition temperatures
were obtained for racemic (±)-1 (Cr ? SmC: 166 °C; SmC ?
SmA: 198 °C; SmA ? I: 200 °C).5 No mesomorphic behavior
was observed for the acid (2)-2 and amide 3a.
Fig. 2 shows the value of the spontaneous polarization as a
function of temperature.∑ At the Curie point, the spontaneous
polarization increased rapidly to a value of ca. 1.36 nC cm22, at
which point it increased almost linearly as the temperature was
lowered. At a temperature of 30 °C below the Curie point, a
value of only 2.8 nC cm22 was reached (response time: ca. 200
ms).
§ 3a: prepared analogously to the synthesis of (+)-1, from (2)-2 (3.00 mg)
and (+)-PEA. Purification by column chromatography [silica gel, CH2Cl2–
AcOEt (98+2)] gave the desired product of de = 98%. dH(200 MHz,
acetone-d6) 8.26 (d, 2 H, arom.), 7.74 (d, 1 H, NH), 7.56–7.27 (m, 5 H,
arom.), 7.48 (d, 2 H, arom.), 7.21 (d, 2 H, arom.), 7.01 (d, 2 H, arom.), 5.70
(t, 1 H, Cp), 5.28 (qut., 1 H, CHCH3), 5.19 (dd, 1 H, Cp), 5.10 (dd, 1 H, Cp),
4.28 (s, 5 H, Cp), 4.03 (t, 2 H, OCH2), 1.84-1.73 (m, 2 H, OCH2CH2), 1.57
(d, 3 H, CHCH3), 1.55–1.25 [m, 14 H, O(CH2)3(CH2)7], 0.89 (t, 3 H, CH3).
Anal. Calc. for C43H47NO6Fe (729.69): C, 70.78; H, 6.49, N, 1.92. Found:
C, 70.60; H, 6.61, N, 1.90%.
¶ For instrumentation, see: B. Dardel, R. Deschenaux, M. Even and E.
Serrano, Macromolecules, 1999, 32, 5193.
∑ Electro-optic studies on the ferroelectric properties were carried out using
homogeneously aligned cells (Linkam) which were constructed from ITO
coated glass, treated with antiparallel-buffed polyimide (PI) coated layers so
as to give sites for planar, homogeneous growth of the liquid-crystalline
state. The cell gap (ca. 5 mm) maintained by glass spacers was verified by
UV–VIS interferometry. The effective electrode areas of the cells used were
0.9 cm2. The cells were filled by capillary action at atmospheric pressure
with (+)-1 in the isotropic phase. Good alignment was achieved by cooling
slowly ( < 0.1 °C min21) from the isotropic liquid into the liquid-crystalline
state. The spontaneous polarization was determined using the triangular
wave method.
Fig. 2 The spontaneous polarization measured as a function of temperature
from the Curie point for (+)-1.
An appreciable scatter in the data was observed because of
the weak ferroelectric properties. The results obtained indicate
firstly that planar chirality can induce ferroelectric properties,
and secondly the induced effects are weak, revealing a poor
coupling between the chirality associated with the Fc moiety at
the centre of the molecular structure, the strongly polar
functional groups in the material, and the liquid-crystalline
environment. The weak ferroelectric properties are probably
linked to the fact that the two arms attached to the Fc unit are not
greatly different in length or polarity.
1 J. W. Goodby, J. Mater. Chem., 1991, 1, 307.
2 J. W. Goodby, R. Blinc, N. A. Clark, S. T. Lagerwall, M. A. Osipov,
S. A. Pikin, T. Sakurai, K. Yoshino and B. Zeks, in Ferroelectric Liquid
Crystals—Principles, Properties and Applications, Gordon and Breach,
Philadelphia and Reading, 1991.
3 R. H. Tredgold, J. Phys. D: Appl. Phys., 1990, 2, 119.
4 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe, J. Mater.
Chem., 1996, 6, 1231.
5 R. Deschenaux and J. Santiago, Tetrahedron Lett., 1994, 35, 2169.
6 E. L. Elliel and S. H. Wilen, in Stereochemistry of Organic Compounds,
Wiley, New York, 1994.
7 C. Imrie and C. Loubser, J. Chem. Soc., Chem. Commun., 1994,
2159.
R. D. acknowledges the Swiss National Science Foundation
for financial support.
8 T. Seshadri and H.-J. Haupt, Chem. Commun., 1998, 735.
9 (a) L. Ziminsky and J. Malthête, J. Chem. Soc., Chem. Commun., 1990,
1495; (b) P. Jacq and J. Malthête, Liq. Cryst., 1996, 21, 291.
10 R. Deschenaux, J.-L. Marendaz and J. Santiago, Helv. Chim. Acta, 1993,
76, 865.
Notes and references
† Optical resolution of (±)-2: to a warm solution of (±)-25 (1.45 g, 2.31
mmol) in dry CH2Cl2 (130 ml), was added a solution of (+)-PEA (140 mg,
2110
Chem. Commun., 2000, 2109–2110