Ferroelectric and antiferroelectric phases formed by mesogens with polyether terminal group

Article abstract of DOI:10.1039/b210345a

Two homologous series of 4-(4′-polyalkoxybiphenyl-4-yloxymethyl)benzoic acid derivatives were synthesized and their liquid crystalline properties were studied. The compounds substituted by n-alkyl esters reveal strong discrimination of the tilted phase fo

Full text of DOI:10.1039/b210345a

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Ferroelectric and antiferroelectric phases formed by mesogens with
polyether terminal group
Anna Kaminska,* Jozef Mieczkowski, Damian Pociecha, Jadwiga Szydlowska and
Ewa Gorecka
Warsaw University, Department of Chemistry, ul. Pasteura 1, 02-089 Warsaw, Poland.
E-mail: akamin@chem.uw.edu.pl
Received 22nd October 2002, Accepted 9th January 2003
First published as an Advance Article on the web 4th February 2003
Two homologous series of 4-(4’-polyalkoxybiphenyl-4-yloxymethyl)benzoic acid derivatives were synthesized
and their liquid crystalline properties were studied. The compounds substituted by n-alkyl esters reveal strong
discrimination of the tilted phase formation upon elongation of the alkyl group. In compounds with chiral,
2-(R)-alkyl terminal chain either ferroelectric or antiferroelectric phases were observed: however the type of
polar order is not in simple relation to the number of atoms in the chain. For compounds in which the length
of mesogenic core was extended by one phenyl ring both ferroelectric and antiferroelectric phases were detected
in a temperature phase sequence.
Inel CPS 120 curved positional sensitive counter. Due to the
problems with re-crystallization of samples only enantiotropic
phases could be studied by the X-ray method.
Introduction
So far, the majority of mesogenic compounds have had as
terminal groups alkyl chains attached to the rigid core through
an ether or ester linkage. In this paper, we describe the
synthesis and properties of materials with a polyalkoxy chain
as one of the terminal groups, and the mesogenic core formed
by a biphenyl–methoxy–phenyl moiety. The other terminal
group, linear or branched alkyl, was attached to the mesogenic
core with an ester joint. Materials with polyether tails have
already been reported and it has been shown that they have
lowered isotropization temperatures.1 The properties of the
studied compounds are compared to materials with the same
mesogenic core having alkyl terminal chains.2
In order to confirm the molecular structure of synthesised
compounds some analytical methods were applied. Infrared
(IR) spectra were obtained using a Nicolet Magna IR 500
spectrophotometer. NMR spectra were recorded on a Varian
1
Unity Plus Spectrometer operating at 500 MHz for H NMR
and at 125 MHz for 13C NMR. Tetramethylsilane was used as
an internal standard. Chemical shifts are reported in ppm. TLC
analyses were performed on a Merck 60 silica gel glass plate
and visualised using iodine vapour. The column chromato-
graphy was carried out at atmospheric pressure using silica gel
(100–200 mesh, Merck).
Experimental
Synthesis
The liquid crystalline phases were identified on the basis of
routine examinations, namely by observation of microscopic
textures.3 For the texture observations a Zeiss Jenapol-U
polarising microscope equipped with a Mettler FP82HT hot
stage was used. The calorimetric investigations were done with
a PerkinElmer DSC-7 set-up. Runs were performed mainly at a
scanning rate of 5 K min21. For the dielectric spectroscopy
and electrooptic studies the glass cells of various thicknesses
(10–25 mm) with ITO electrodes were used. The dielectric
permittivity was measured with a Wayne Kerr Precision
Component Analyzer 6425. For spontaneous polarization
(Ps) studies the switching current method was applied. The
molecular tilt was measured as an angle difference between
minimum transmission positions in a planar sample, which was
placed under a microscope between crossed polarizers and
subjected to opposite dc electric fields. The type of the polar
phase, ferro or antiferroelectric, was determined mainly by
observations of the Ps switching. The double current peak at
spontaneous polarisation reversal and tristable electrooptic
switching was detected via low frequency (y1 Hz) measure-
ments in antiferroelectric phases.4 The Ps measurements in
antiferroelectric phases were limited to a narrow temperature
range due to the high value of the threshold field. Additionally,
the ferroelectric–antiferroelectric phase transition was moni-
tored as a sudden change in relative dielectric permittivity.
The X-ray studies were carried out with powder samples in
Lindemann capillaries. The patterns were registered with an
The general procedure of the synthesis of studied compounds is
outlined in Scheme 1.
Methyl 4-(4’-(2-(2-butoxyethoxy)ethyl)biphenyl)-4-yloxymethyl)
benzoate (series 1, n = 1)
To 4,4’-dihydroxybiphenyl (20.1 g; 108 mmol) dissolved in
dimethylformamide (75 cm3), butoxyethoxyethyl iodide (19.7 g;
72 mmol) and anhydrous potassium carbonate (9.94 g; 72 mmol)
were added. The mixture was stirred and heated at 100 uC
for 6 h, cooled to room temperature and poured into water.
The crude product was chromatographed on silica gel using a
3% chloroform–tethrahydrofuran mixture as eluent. The
monoalkylated (compound A; 6.7 g; 19.5% yield) and dialky-
lated products were obtained.
To 4-hydroxy-4’-(2-(2-butoxyethoxy)ethyl)biphenyl A (6.7 g;
21 mmol) in dimethylformamide (60 cm3), anhydrous potas-
sium carbonate (4.5 g; 32 mmol) and methyl 4-(bromomethyl)-
benzoate (7.6 g; 32 mmol) were added and the mixture stirred
and heated at 100 uC for 16 h, cooled, poured into water and
filtered. The crude product was purified by double crystal-
lisation from methanol (4.7 g; 48%). Elemental analysis for
C29H34O6: calculated C 72.80, H 7.11; found C 72.64, H 7.15%;
nmax (KBr)/cm21 2950, 2930, 1730, 1600, 1500, 1480, 1060; dH
(CDCl3) 8.04 (d, 2H), 7.53–7.43 (m, 6H), 7.02 (d, 2H), 6.94 (d,
2H), 5.14 (s, 2H), 4.16 (t, 2H), 3.91 (s, 3H), 3.87 (t, 2H), 3.72 (t,
2H), 3.62 (t, 2H), 3.47 (t, 2H), 1.48–1.22 (m, 4H), 0.91 (t, 3H);
DOI: 10.1039/b210345a
J. Mater. Chem., 2003, 13, 475–478
This journal is # The Royal Society of Chemistry 2003
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R-2-Octyl 4-(4’-(2-(2-butoxyethoxy)ethyl)biphenyl)-4-
yloxymethyl) benzoate (series 2, n = 6)
To the acid chloride (0.65 g; 1.4 mmol) dissolved in THF
(10 cm3) was added a mixture of the relevant alcohol (2 cm3) in
TEA (2 cm3) and 50 mg of DMAP. The mixture was stirred and
heated under reflux for 10 h. After evaporation of the solvents
the reaction mixture was chromatographed on silica gel using a
2% mixture of isopropanol–chloroform as eluent. The product
was obtained with a yield of 66% (0.53 g). Elemental analysis
for C36H48O6: calculated C 75.0, H 8.33; found C 74.89, H
8.41%; nmax (KBr)/cm21 2950, 2930, 1730, 1610, 1500, 1480,
1060; dH (CDCl3) 8.04 (d, 2H), 7.52–7.43 (m, 6H), 7.01 (d, 2H),
6.93 (d, 2H), 5.14 (s, 2H), 4.20–3.38 (m, 10H), 1.72–1.48 (m,
1H), 1.55 (d, 3H), 1.48–1.18 (m, 14H), 0.91 (t, 3H), 0.87 (t, 3H);
dC (CDCl3) 165.92, 157.99, 157.57, 142.04, 133.93, 133.41,
133.46, 129.81, 127.75, 127.66, 126.89, 115.10, 114.90, 77.72,
77.08, 76.45, 71.79, 71.25, 70.89, 70.12, 69.77, 69.40, 67.52,
36.06, 31.73, 29.15, 25.40, 22.58, 20.07, 19.28, 14.06, 13.94.
Elemental analysis for C43H52O8 (compound 5): calculated C
74.14, H 7.47; found C 74.03, H 7.63 %; nmax (KBr)/cm21 2950,
2930, 1730, 1610, 1500, 1480, 1060; dH (CDCl3) 8.24 (d, 2H),
8.12 (d, 2H), 7.61–7.44 (m, 6H), 7.27 (d, 2H), 6.99–6.94 (m,
4H), 5.20 (s, 2H), 4.22–4.02 (m, 4H), 3.70 (t, 2H), 3.59 (t, 2H),
3.54–3.40 (t, 2H), 1.80–1.51 (m, 1H), 1.58–1.54 (d, 3H), 1.46–
1.28 (m, 14H), 0.91(t, 3H), 0.88 (t, 3H); dC (CDCl3) 166.45,
165.42, 159.05, 158.48, 155.42, 144.46, 135.12, 134.38, 132.17,
131.58, 129.62, 128.82, 128.70, 128.15, 122.68, 116.13, 115.94,
78.70, 78.06, 77.42, 72.97, 72.29, 71.91, 71.14, 70.79, 70.31,
68.55, 37.08, 32.75, 30.17, 26.42, 23.60, 21.10, 15.08.
Scheme 1 Reagents and conditions: (i) C4H9OCH2CH2OCH2CH2I,
K2CO3, DMF; (ii) K2CO3, DMF; (iii) KOH, EtOH, reflux; (iv)
(COCl)2, toluene, reflux; (v) ROH, TEA, THF, DMAP.
dC (CDCl3) 166.83, 158.01, 157.56, 142.29, 133.99, 133.41,
129.89, 129.67, 127.78, 127.68, 126.96, 115.10, 114.91, 77.68,
77.04, 76.41, 71.27, 70.89, 70.12, 69.77, 69.40, 67.53, 52.13,
31.71, 19.28, 13.93.
Results and discussion
The phase transition temperatures and their thermal effects are
collected in Tables 1–3 for compounds studied. Additionally
for homologous series 1 and 2 they are presented on the phase
diagrams (Figs. 1 and 2, respectively).
Potassium 4-(4’-(2-(2-butoxyethoxy)ethyl)biphenyl)-4-
yloxymethyl) benzoate B
To the ester described above (series 1, n ~ 1) (1.6 g; 3.4 mmol)
dissolved in ethanol (20 cm3), potassium hydroxide (1.34 g;
23.9 mmol) was added and the mixture was heated with reflux
for 18 h. The product was filtered and dried in a desiccator over
phosphorus anhydride. The potassium salt B was obtained
(1.5 g; 91%).
To the potassium salt B (1.5 g; 3.1 mmol) in toluene (25 cm3)
was added oxalyl chloride (2 cm3) and the reaction mixture
heated for 5 h at reflux. The precipitated potassium chloride
was filtered and the filtrate evaporated to dryness. The acid
chloride was obtained (1.4 g; 96%).
Non-chiral compounds—series 1
Compounds of homologous series 1 have the n-alkyl chain
attached to the carboxy moiety. They reveal unusually strong
discrimination of the tilted phases stability upon elongation of
the n-alkyl R group (Fig. 1).
Short chain homologues (n = 1, 2) exhibit exclusively
orthogonal SmA, SmBHex and CrE phases. Extending the
terminal chain, n w 3, destabilises completely the orthogonal
phases, resulting in the sequence of tilted phases SmC, SmF,
Table 1 The phase transition temperatures and enthalpy changes (in J g21) for compounds of series 1
n
mp
CrE
CrG
SmF
SmBHex
SmC
SmA
Iso
a
1
2
3
4
5
6
7
8
110.0 (40.60)
75.5 (38.23)
66.9 (34.02)
61.2 (15.22)
57.5 (18.11)
71.5 (28.74)
75.5 (26.95)
55.6 (31.99)
$
$
155.3 (5.62)
130.9 (6.25)
$
$
167.4a
152.4 (4.0)
$
$
$
172.8 (22.22)
155.7 (34.07)
144.9 (25.14)
$
$
$
$
$
$
$
$
$
$
$
$
$
$
116.0 (6.19)
112.3 (6.43)
108.0 (6.40)
106.7 (6.37)
104.6 (6.18)
103.4 (6.52)
$
$
$
$
$
$
132.0 (5.76)
124.1 (3.72)
120.6 (3.78)
117.9 (3.17)
115.2 (3.50)
113.9 (3.13)
$
$
$
$
$
$
134.8 (0.07)
135.4 (25.04)
132.9 (25.89)
128.2 (25.23)
126.7 (25.91)
125.1 (26.08)
aTotal enthalpy change for SmBHex–SmA and SmA–Iso phase transitions.
Table 2 The phase transition temperatures and enthalpy changes (in J g21) for compounds of series 2
SmF*
107.5 (1.22)
SmCA
SmC*
SmA
Iso
*
n
mp
CrH
CrG
2
3
5
6
85.4 (47.23)
91.4 (49.68)
90.9 (44.76)
90.3 (61.03)
$
$
$
$
102.6 (5.82)
98.1 (10.54)
88.0 (9.70)
87.6 (11.47)
$
$
$
117.6a
$
118.7 (20.91)a
$
$
$
$
$
$
108.1 (21.11)
94.9 (19.44)
$
$
57.9 (6.10)
68.4 (9.20)
94.8 (16.39)
aTotal enthalpy change for SmC*–SmA and SmA–Iso phase transitions.
476
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Table 3 The phase transition temperatures and enthalpy changes (in J g21) for compounds 3–5
Compound mp
Iso
3
4
5
CrG 63.7 (8.64)
SmC
SmC* 108.7 (17.70)
SmFA 122.8 (3.79) SmCA 164.4 (0.11) SmC* 168.8 (0.65)
89.0 (1.56)
SmA
91.2 (12.28)
$
$
$
86.2 (67.37)
114.3 (42.53)
SmF*
87.8 (0.12)
*
*
SmA 176.7 (15.39)
Fig. 3 Spontaneous electric polarization versus temperature.
Fig. 1 The phase diagram for homologous series 1. Filled squares
represent melting points.
relevant compounds of series 1 (Fig. 2) due to the chain
branching.
Chiral compounds form easily tilted phases, some of them
show also orthogonal SmA phase. Tilted phases formed by
enantiomeric materials have polar properties. For compound 4
only ferroelectric phases were detected. In series 2 either
ferroelectric (n = 2, 5) or antiferroelectric phases (n = 3, 6) were
observed: however, there was no simple relation, e.g., odd–even
effect, between the terminal chain length and the type of polar
order.7 For compound 5, in which the lengh of mesogenic core
was extended by one phenyl ring, both ferroelectric and
antiferroelectric phases were observed.
Compounds of series 2 exhibit a high value (about 200
nC cm22) of spontaneous electric polarisation (Fig. 3). The high
value of the spontaneous electric polarisation shows that apart
from the dipole moment of the methyl group at the chiral
centre, the dipole moment of the carboxy group also
contributes significantly to Ps.8,9 Due to steric interactions
with the methyl group the rotation of the carbonyl group is
strongly hindered and the transverse components of the dipole
moments of both groups are summed up. The significantly
lower Ps value for compound 5 seems to be connected with the
lower tilt angle (Figs. 3 and 4). For lactate acid derivative 4 the
Fig. 2 The phase diagram for homologous series 2. Filled squares
represent melting points.
CrG. For homologue n = 3 the sequence SmA, SmC, SmF,
CrG was observed.
On DSC thermograms the peak broadening at the SmA–
SmBHex phase transition was observed that is characteristic of
the hexatic B phase formation.5,6 A similar shape of the DSC
signal, typical for the creation of long-range bond orientational
order, was also observed at the SmC–SmF phase transition.
Chiral compounds—series 2 and compounds 3, 4, 5
Compounds of series 2 as well as compounds 4 and 5 contain in
their terminal chains an asymmetric carbon atom, placed next
to the carboxy group. In the compound 3 the position of the
chiral carbon atom was moved away, by one –CH2– group,
from the carbonyl group (this material was synthesised only in
racemic version). The melting and clearing temperatures for
those materials are visibly depressed in comparison with the
Fig. 4 The tilt angle versus temperature.
J. Mater. Chem., 2003, 13, 475–478
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mesogenic core, to form tilted or orthogonal phases,11 is
reflected in the observed phase sequences for all synthesised
compounds. For short groups homologous orthogonal phases
dominate, while for long groups homologous tilted phases were
exclusively observed.
Until now mainly mesogenic compounds having alkoxy or
alkyl terminal groups were studied. The comparative com-
pounds with a biphenyl/phenyl mesogenic core and a chiral and
non-chiral ester tail were described in detail in ref. 2. They have
much higher (about 30–40 degrees) clearing temperatures and
most of them lower melting temperatures. Thus, apparently
replacing the alkoxy group by the polyalkoxy group destabi-
lizes the liquid crystalline properties. Despite lower stability,
the phase sequence observed for polyalkoxy materials is similar
to that found in reference compounds: liquid like SmC-hexatic
SmF–soft crystal CrG. Also, the polarity of chiral tilted phases
is only weakly influenced by the terminal chains; in both
homologue series, with polyether or alkoxy terminal groups,
ferro- as well as antiferro-phases were observed.
Fig. 5 The X-ray diffraction signal for homologue n = 6 of series 2
obtained in CrG phase (at 90 uC). In the inset the high angle part of the
diffractogram in SmCA and CrG phases is shown.
value of Ps is comparable to that observed for compounds of
series 2, which suggests that the second carbonyl group in the
terminal chain rotates nearly freely.
The dielectric properties were also studied for compounds
with polar phases. In the ferroelectric SmC* phase a Goldstone
mode was observed with the relaxation frequency in kHz range,
which softened at the transition to hexatic phase.10 No
collective mode was observed in antiferroelectric phases in
the studied frequency window.
References
1
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liquid crystalline phases (Fig. 5). In the not very organized
*
SmCA phase, in high angle range, the diffused single was
detected. At the transition to CrG phase the signals narrowed
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6
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9
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The indexing of these four signals fits the CrG phase better
than the CrJ phase. In this phase a weak and diffused peak at
the position related to (210) signal points to the fact that there
is only short range herring-bone order of molecules in the
smectic layer.
ˇ
10 I. Rychetsky´, D. Pociecha, V. Dvora´k, J. Mieczkowski, E. Go´recka
and M. Glogarova´, J. Chem. Phys., 1999, 111(4), 1541–1550.
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et al., Gordon and Breach Science Publications, Philadelphia,
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Conclusions
The competing influences of the functional terminal groups
(–O–, –COO–) and the linkage group (–OCH2–) in the
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