New series of 4-(4'-octyloxybiphenyl-4-yloxymethyl)benzoic acid derivatives with mesogenic properties

Article abstract of DOI:10.1039/a805151h

Three homologous series of (4'-octyloxybiphenyl-4-yloxymethyl) benzoic acid derivatives were synthesised and studied. The compounds, substituted by n-alkyl esters, reveal strong discrimination of the tilted phases formation upon elongation of the alkyl gr

Full text of DOI:10.1039/a805151h

J O U R N A L O F
New series of 4-(4∞-octyloxybiphenyl-4-yloxymethyl)benzoic acid
derivatives with mesogenic properties
´
´
´
Jadwiga Szydłowska,*a Damian Pociecha,a Ewa Gorecka,a Dorota Kardas,b Jozef Mieczkowskib
and Jan Przedmojskic
C H E M I S T R Y
aLaboratory of Dielectrics and Magnetics, Department of Chemistry, Warsaw University, Al.
˙
Zwirki i Wigury 101, 02–089 Warsaw, Poland. E-mail: jadszyd@chem.uw.edu.pl
bLaboratory of Natural Products Analysis, Department of Chemistry, Warsaw University, ul.
Pasteura 1, 00–093 Warsaw, Poland
cDepartment of Physics, Warsaw Technical University, ul. Koszykowa 75, 00–662 Warsaw,
Poland
Received 3rd July 1998, Accepted 21st October 1998
Three homologous series of (4∞-octyloxybiphenyl-4-yloxymethyl)benzoic acid derivatives were synthesised and
studied. The compounds, substituted by n-alkyl esters, reveal strong discrimination of the tilted phases formation
upon elongation of the alkyl group. In compounds with 2 methylalkyl chains, tilted phases were exclusively
observed with both ferroelectric and antiferroelectric properties. It has been observed that for chiral derivatives an
elongation of the chiral chain leads to odd–even effect on the polar properties. Also elongation of non-chiral alkoxy
terminal chains affects the polar properties. For short homologues the antiferroelectric phases dominate, while for
long homologues only ferroelectric behaviour is observed. The influence of strongly polar lateral groups (Cl, Br,
NO ) attached to the biphenyl ring on LC phase properties was also determined. It was detected that mono-
2
substituted compounds have lower mesogenic stability then their disubtituted counterparts. The effect is probably
related to restricted rotation around the phenyl–O bond that leads to a more rigid core structure.
rates were modified and resultant temperatures were
recalculated for the standard rate.
1. Introduction
Synthesis and systematic studies of the liquid crystal properties
of several homologous series with (biphenyl-4-yloxymethyl)-
phenyl mesogenic cores, in which a biphenyl ring is joined to
a phenyl ring by a OCH linkage, were undertaken. In the
core structure there are moieties that exhibit competing,
favourable or discouraging influences on tilted phase forma-
tion. The electron donor alkoxy group and the dipolar acceptor
carboxy function (COO) strongly induce tilted phases, while
the polar central OCH linkage promotes orthogonal phases.
Surprisingly, so far this core structure has been relatively
rarely used to obtain mesogenic compounds.1–3 The most
comprehensive studies were performed for nitriles and nitro
derivatives.1
In the presented work the core is substituted by alkoxy and
ester groups (compounds 5–17 on Scheme 1). The terminal
ester moiety has been modified either to enhance or reduce
tilted phase formation. Three series 1–3 were studied in detail.
The influence of strongly polar lateral groups (Cl, Br, NO )
on the biphenyl ring on the LC phase properties was also
determined.
The selective reflection wavelength was measured in a
polarising microscope Nikon Optiphot2-Pol equipped with a
micro-spectrophotometry system working in the transmission
mode. The helical twist sense was determined in studies of
circularly polarised light transmission through a free sus-
pended film.5
The relative permittivity was measured at 120 Hz in 25 mm
thick cells, under a temperature sweep of 2 K min−1, using a
Wayne Kerr Precision Component Analyzer 6425. For spon-
taneous polarisation 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 polar-
izers and subjected to opposite dc electric fields. From the
direction of the induced tilt angle and the direction of the
2
2
applied field the spontaneous polarisation (P ) sign was deter-
s
mined. The vector P is defined as positive6 if directed along
s
a vector [c×z] for (n·z)>0, where n is the director, c is its
2
projection to the smectic layer and z is the layer normal. The
identification of the polar properties of the phases was based
mainly on observations of the apparent tilt and P electric
s
switching.7 The double current peak at spontaneous polaris-
2. Experimental
ation reversal and tristable electrooptic switching was detected
via low frequency (~1 Hz) measurements of antiferroelectric
phases. Additionally the ferroelectric–antiferroelectric phase
transition was monitored as a sudden change in the relative
permittivity. Molecular dimensions were estimated by
molecular modelling (HYPERCHEM 3.0).
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 Varian
Unity Plus Spectrometer operating at 500 MHz for 1H 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
The liquid crystalline phases were identified on the basis of
routine examinations, namely by observation of created micro-
scopic textures.4 For the texture observations a Zeiss Jenapol-
U polarising microscope equipped with a Mettler FP82HT
hot stage was used. The mesophase identification was
supported by X-ray studies that gave crystallographic
parameters. The measurements were taken for powder samples
with a DRON diffractometer equipped with a graphite
monochromator.
For calorimetric investigations a DSC7 Perkin-Elmer set-up
was used. Runs were performed mainly at a scanning rate of
5 K min−1. When necessary, to visualise some thermal effects
or in the case of thermally unstable compounds, the scanning
J. Mater. Chem., 1999, 9, 361–369
361

Scheme 1 Reagents and conditions: i, C H
Br, K CO , DMF; ii, 1, K CO , DMF; iii, NaOH, MeOH, reflux; iv, NaOH, BuOH, reflux;
m
2m+1
2
3
2
3
v, R1OH, H SO , toluene, reflux; vi, (COCl) , toluene, reflux; vii, R1OH, pyridine.
2
4
2
plate and visualised using iodine vapour. The column chroma-
tography was carried out at atmospheric pressure using silica
gel (100–200 mesh, Merck).
4-(4∞-Octyloxybiphenyl-4-yloxymethyl)benzonitrile 2 (m=8).
To 4,4∞-dihydroxybiphenyl (50 g, 270 mmol) in dimethylforma-
mide (400 ml), octyl bromide (58 g, 300 mmol) and anhydrous
potassium carbonate (42 g, 300 mmol) were added. The mix-
ture was stirred and heated at 100 °C during 4 h, cooled to
room temperature, and poured into water. The crude product
was crystallised from methanol. The monoalkylated (com-
pound 1 (m=8) (33 g, 41% yield) and dialkylated (25 g, 23%
2.1 Synthesis
Synthetic routes to the described compounds are outlined on
Scheme 1 and 2.
Scheme 2 Reagents and conditions: i, Cl , CCl ; ii, Br ; iii, HNO .
2
4
2
3
362
J. Mater. Chem., 1999, 9, 361–369

yield) products were obtained. Compound 1 (m=8) was
purified by single crystallisation from methanol and had
sufficient purity for the next step.
dissolved in dry toluene (10 ml), was added to a mixture of
the relevant alcohol (1 ml) in pyridine (2 ml). After evapor-
ation of the solvents the product was purified with column
chromatography on silica gel using toluene as eluent. Yields
varied from 50 to 70%.
To 4-hydroxy-4∞-octyloxybiphenyl 1 (m=8) (30 g, 100
mmol) in dimethylformamide (200 ml), anhydrous potassium
carbonate (21 g, 500 mmol) and 4-(bromomethyl)benzonitrile
(25 g, 130 mmol) were added, stirred for 2 h at room tempera-
ture, then heated at 100 °C during 8 h, cooled, poured into
water and extracted six times with toluene. The toluene extracts
were dried with anhydrous magnesium sulfate, and the product
was chromatographed on silica gel at 40 °C to avoid crystallis-
ation and eluted with toluene (23.7 g; 57%).
E.g.: Geranyl 4-(4∞-octyloxybiphenyl-4-yloxymethyl)benzo-
ate 14. Elemental analysis for C H O : Calc. C 80.28, H
38 48 4
8.45; Found C 80.15, H 8.51%; n (KBr)/cm−1 2990, 2850,
max
1730, 1620, 1500; d (CDCl ) 8.06 (d, 2H), 7.49 (d, 2H), 7.42
H
3
(dd, 4H), 6.95 (dd, 4H), 5.468 (t, 1H), 5.12 (s, 2H), 5.094 (t,
1H), 4.84 (d, 2H), 3.956 (t, 2H), 2.08–2.02 (m, 4H), 1.76 (s,
3H), 1.67 (s, 3H), 0.888 (s, 3H); d (CDCl ) 166.33, 158.30,
C
3
Elemental analysis for C H NO : Calc. C 81.35, H 7.50,
157.47, 142.35, 142.12, 134.01, 132.99, 131.80, 130.00, 129.87,
127.70, 127.64, 126.86, 123.72, 118.34, 115.05, 114.70, 69.36,
68.02, 61.89, 39.53, 31.82, 29.38, 29.30, 29.26, 26.28, 26.06,
25.68, 22.66, 16.55, 14.11.
28 31
2
N 3.38; Found C 81.45, H 7.65, N 3.25%; n (KBr)/cm−1
max
2940, 2830, 2210, 1600, 1500; d (CDCl ) 7.72–7.42 (m, 8H),
H
3
7.01–6.88 (t, 4H), 5.14 (s, 2H), 3.97 (t, 2H), 1.88–1.71 (dt,
2H), 1.56–1.22 (m, 12H), 0.889 (t, 3H); d (CDCl ) 158.39,
C
3
157.18, 142.54, 134.33, 132.86, 132.38, 127.80, 127.66, 127.52,
118.659, 115.00, 114.75, 111.65, 68.95, 68.0, 31.82, 29.37,
29.29, 29.24, 26.05, 22.05, 14.11.
N,N-Dioctyl-4-(4∞-octyloxybiphenyl-4-yloxymethyl)-
benzamide 27. To the sodium salt 4 (m=8) (1 g, 2.2 mmol) in
toluene (20 ml) was added oxalyl chloride (2 ml) and the
reaction mixture heated for 2 h at reflux. The precipitated
sodium chloride was filtered and the filtrate evaporated to
dryness. The acid chloride thus formed, dissolved in toluene,
was added to a solution of dioctylamine (2 g) in toluene
(20 ml). After evaporation of the solvents the reaction mixture
was chromatographed on silica gel using chloroform as eluent.
Compound 27 was obtained (1.05 g, 73%). Elemental analysis
for C H NO : Calc. C 80.61, H 9.92, N 2.13; Found C
4-(4∞-Octyloxybiphenyl-4-yloxymethyl)benzamide 3 (m=8).
To sodium hydroxide (1 g) dissolved in methanol (100 ccm),
nitrile 2 (1 g, 2.4 mmol) was added and then refluxed for
30 min. The precipitated product was filtered and dried over
phosphorous anhydride. The amide 3 (m=8) was obtained
(0.81 g, 78%). Elemental analysis for C H NO : Calc. C
28 33
3
77.95, H 7.65, N 3.24; Found C 77.78, H 7.75, N 3.15%;
44 65
3
n
(KBr)/cm−1 3400, 3200, 2930, 2820, 1660, 1500, 1310,
80.49, H 10.05, N 2.16%; n (KBr)/cm−1 2940, 2830, 1670,
max
max
1300. NMR measurements were not performed since this
compound forms crystals which could not be dissolved in
standard solvents.
1500; d (CDCl ) 7.52–7.35 (m, 8H), 6.96 (dd, 4H), 5.11 (s,
H
3
2H), 3.98 (t, 2H), 3.47 (s, 2H), 3.19 (s, 2H), 1.79 (m, 2H).
Glycidyl 4-(4∞-octyloxybiphenyl-4-yloxymethyl)benzoate 18.
To the ester 12 (0.5 g, 0.1 mmol) dissolved in CH Cl (5 ml)
was added m-chloroperbenzoic acid (0.5 g, 3 mmol), and the
mixture was kept overnight at room temperature. The reaction
mixture was chromatographed on silica gel eluting with tolu-
ene. The epoxy ester 18 was obtained (0.21 g, 41%). Elemental
Sodium 4-(4∞-octyloxybiphenyl-4-yloxymethyl)benzoate
4
2
2
(m=8). Nitrile 2 (m=8) (20 g, 48.3 mmol) and sodium
hydroxide (5 g, 12.5 mmol) in butanol (300 ccm) were heated
at reflux for 4 days. The product was filtered, washed with
anhydrous diethyl ether and dried in a desiccator over phos-
phorous anhydride until the butanol smell disappeared. The
sodium salt 4 (m=8) was obtained (15.5 g, 80%). Elemental
analysis for C H O : Calc. C 76.22, H 7.37; Found C 76.08,
31 36 3
H 7.48%; n (KBr)/cm−1 2950, 2830, 1720, 1530; d (CDCl )
max
H
3
analysis for C H O Na: Calc. C 74.00, H 6.82; Found C
8.07 (d, 2H), 7.54 (d, 2H), 7.46 (t, 4H), 7.01 (d, 2H), 6.94
28 31 4
74.12, H 6.93%; n (KBr)/cm−1 2930, 2820, 1600, 1530, 1500,
(d, 2H), 5.17 (s, 2H), 4.67 (dd, 1H, J 3.0, J 12.0 Hz), 4.18
max
1
2
1420, 1250. NMR measurements were not performed since
(dd, 1H, J 6.0, J 12 Hz), 3.99 (t, 2H), 3.35 (m, 1H), 2.91
1
2
this compound forms crystals which could not be dissolved in
standard solvents.
(dd, 1H, J 5.0, J 4.5 Hz), 2.74 (dd, 1H, J 5.0, J 3.0 Hz),
1.79 (m, 2H), 1.52–1.22 (m, 10H), 0.89 (t, 3H).
1
2
1
2
4-(4∞-Octyloxybiphenyl-4-yloxymethyl)benzoate esters.
Unbranched alcohols (compounds 5 and 17). To the sodium
salt 4 (m=8) (1 g) a relevant alcohol (5 ml) in toluene (20 ml)
and concentrated sulfuric acid (6 drops) were added and
heated for 5 h under reflux. The reaction mixture was cooled,
washed with 10% aqueous sodium carbonate and dried with
magnesium sulfate. The product was chromatographed on
silica gel using toluene as eluent. Yields varied from 70 to 90%.
E.g.: n-Butyl 4-(4∞-octyloxybiphenyl-4-yloxymethyl)benzo-
Preparation of halogeno derivatives of methyl 4-(4∞-dodecyl-
oxybiphenyl-4-yloxymethyl)benzoate (20 and 21). To the
methyl ester 19 (1 g, 2.2 mmol) was added chlorine (or bro-
mine) (4.4 mmol) in carbon tetrachloride at 0 °C, and the
mixture was kept overnight at room temperature. Then the
solvent and the excess of chlorine (bromine) were evaporated
and the product was purified via chromatography on silica gel
using chloroform as eluent. The dichloro derivative 20 (or
dibromo derivative 21) was obtained.
ate 5 (n=4). Elemental analysis for C H O : Calc. C 78.68,
For dichloro ester 20 (0.36 g, 31%). Elemental analysis for
32 40 4
H 8.19; Found C 78.55, H 8.11%; n (KBr)/cm−1 2950, 2930,
max
C H O Cl : Calc. C 69.35, H 7.00; Found C 69.45, H 7.10%;
33 40 4
max
2
1720, 1600, 1500, 1480; d (CDCl ) 8.41 (d, 2H), 7.57–7.38
n
(KBr)/cm−1 2990, 2870, 1710, 1620, 1500; d (CDCl ) 8.07
H
3
H
3
(m, 6H), 6.98 (d, 2H), 6.92 (d, 2H), 5.12 (s, 2H), 4.32 (t,
2H), 3.96 (t, 2H), 1.81–1.65 (m, 3H), 1.56–1.22 (m, 14H),
0.97 (t, 3H), 0.89 (t, 3H); d (CDCl ) 166.36, 158.33, 157.49,
(d, 2H), 7.55 (d, 2H), 7.42 (m, 2H), 7.32 (m, 4H), 5.24 (s,
2H), 4.04 (t, 2H), 3.92 (s, 3H), 1.95 (t, 2H), 1.85–1.10 (m,
18H), 0.88 (t, 3H); d (CDCl ) 166.76, 153.75, 150.90, 141.34,
C
3
C
3
142.14, 134.02, 133.00, 130.02, 129.82, 127.71, 127.64, 126.90,
115.06, 114.72, 69.36, 68.04, 64.87, 31.82, 30.76, 29.38, 29.30,
29.25, 26.06, 22.67, 19.27, 14.11, 13.77.
136.51, 132.32, 131.24, 129.97, 129.86, 129.82, 129.77, 128.78,
128.59, 127.27, 127.27, 127.23, 126.93, 126.67, 123.85, 114.08,
73.97, 70.21, 70.18, 52.18, 31.93.
For dibromo ester 21 (0.41 g, 36%). Elemental analysis for
Branched alcohols (compounds 6–16). To the sodium salt 4
(1 g, 2.2 mmol) suspended in dry toluene (20 ccm) was added
oxalyl chloride (2 ml), and then the mixture was refluxed for
2 h. The precipitated sodium chloride was filtered and the
filtrate evaporated to dryness. The acid chloride thus formed,
C H O Cl : Calc. C 60.00, H 6.06; Found C 60.05, H 6.18%;
33 40 4
max
2
n
(KBr)/cm−1 2990, 2820, 1730, 1610, 1510; d (CDCl ) 8.06
H
3
(d, 2H), 7.72 (dd, 2H), 7.55 (d, 2H), 7.35 (m, 2H), 6.90 (t,
2H), 5.21 (s, 2H), 4.03 (t, 2H), 3.95 (s, 3H), 1.83 (m, 2H),
1.49–1.11 (m, 18H), 0.88 (t, 3H); d (CDCl ) 166.77, 154.89,
C
3
J. Mater. Chem., 1999, 9, 361–369
363

Table 1 Phase transition temperatures and thermal effects for compounds 5
n
Mp/ °C (thermal effect/J g−1)
Phase sequence, T/ °C (thermal effect/J g−1)
1
2
3
4
5
6
7
8
144.4 (65.9)
113.7 (37.8)
88.7 (29.9)
80.9 (27.0)
69.6 (34.7)
65.1 (37.1)
70.5 (35.8)
83.1 (78.6)
Cry 169.9 (8.2) Cry 197.8 (8.6) S 208.6 (38.3) Iso
E
B
A
Cry 147.2 (7.0) Cry 178.2 (7.4) S 193.7 (36.1) Iso
E
B
A
Cry 136.7 (6.0) Hex 165.2 (7.7) S 167.4 (0.1) S 187.2 (33.5) Iso
K
I
C
A
Cry 134.0 (5.8) Hex 161.4 (7.0) S 173.1 (0.7) S 178.8 (31.9) Iso
K
I
C
A
A
Cry 128.6 (5.4) Hex 158.5 (6.4) S 171.3 (0.5) S 175.4 (31.1) Iso
K
I
C
Cry 127.3 (4.2) Hex 156.1 (5.6) S 170.7 (24.4) Iso
K
I
C
Cry 125.4 (5.3) Hex 155.0 (7.4) S 168.6 (32.0) Iso
K
I
C
Cry 126.3 (4.8) Hex 153.8 (6.8) S 165.7 (31.5) Iso
K
I
C
153.92, 141.54, 134.09, 123.91, 131.57, 134.40, 129.92, 129.73,
126.61, 126.52, 113.81, 113.21, 112.84, 112.64, 70.21, 69.32,
52.15, 31.91.
for C H NO : Calc. C 72.39, H 7.49, N 2.55; Found C
33 41
6
72.45, H 7.65, N 2.65%; n (KBr)/cm−1 2990, 2820, 1725,
max
1620, 1525; d (CDCl ) 8.85 (d, 2H), 7.79 (d, 1H), 7.52 (m,
H
3
4H), 7.31–7.41 (m, 2H), 7.21 (m, 2H), 5.22 (s, 2H), 4.12 (t,
2H), 3.93 (s, 3H), 1.85 (m, 2H), 1.61–1.2 (m, 18H), 0.88
(m, 3H).
Preparation of mono- and di-nitro derivatives of methyl 4-(4∞-
dodecyloxybiphenyl-4-yloxymethyl)benzoate (23 and 22). To
the methyl ester 19 (1 g, 2.2 mmol) was added concentrated
nitric acid (10 ml) at 0 °C and the mixture was stirred for 2 h
at room temperature. Then the mixture was diluted with water,
extracted with butyl acetate, washed with 10% aqueous potass-
ium hydrogen carbonate and finally the organic layer was
dried with magnesium sulfate. After evaporation of the organic
solvent the reaction mixture was chromatographed on silica
gel eluting with CH Cl . The mono- (23) and di-nitro
3. Results and discussion
The phase diagrams of the studied homologuous series 5–7
are shown in Fig. 1–4. The phase transition temperatures and
thermal effects are collected in Tables 1 and 2 for compounds
5 and 6, respectively, and for comparative compounds in
Table 3. Preliminary data for compounds 7 were presented
in ref. 8.
2
2
derivatives (22) were obtained.
For di-nitro ester 22 (0.25 g, 21%). Elemental analysis for
C H N O : Calc. C 66.88, H 6.78, N 4.72; Found C 66.75,
3.1 Non-chiral compounds 5
33 40 2 8
H 6.84, N 4.61%; n (KBr)/cm−1 2990, 2820, 1720, 1620,
max
1520; d (CDCl ) 8.10–8.02 (m, 2H), 7.98 (d, 2H), 7.67 (dd,
Phase sequence. Molecules of homologous series 5 have the
n-alkyl chain attached to the carboxy moiety. The compounds
of the series (Fig. 1) reveal unusually strong discrimination of
the tilted phases stability upon elongation of the n-alkyl R1
group. Short chain homologues (n=1, 2) exhibit exclusively
orthogonal phases S , Cry and Cry . Extending the terminal
H
3
2H, J 2.4, J 8.8 Hz), 7.55 (d, 2H, J 8.2 Hz), 7.16 (dd, 2H,
8.6, J 8.8 Hz), 5.32 (s, 2H), 4.13 (t, 2H), 3.92 (s, 3H),
1.85 (m, 2H), 1.60–1.40 (m, 18H), 0.87 (t, 3H); d (CDCl )
166.59, 152.18, 151.05, 140.30, 140.20, 140.00, 131.91, 131.82,
131.60, 130.27, 130.03, 129.99, 126.53, 123.68, 123.47, 115.58,
70.56, 69.90, 52.14, 31.86.
1
2
J
1
2
C
3
A
B
E
chain by just one carbon atom destabilises completely the
Cry and Cry phases, resulting in the sequence of tilted
For mononitro ester 23 (0.051 g, 4.7%). Elemental analysis
B
E
Table 2 Phase transition temperatures and thermal effects for compounds 6
Phase sequence, T/ °C (thermal effect/J g−1)
m
Cry
Cry
Hex
S
S
Iso
J
I
CA
C
7
8
9
10
11
•
•
•
•
•
75.8 (50.9)
53.1 (13.4)
67.6 (23.8)
67.9 (36.0)
78.6 (52.7)
•
•
•
•
•
120.2 (5.4)
104.6 (3.4)
91.3 (3.1)
88.3 (6.6)
91.34
•a
•a
•a
•a
•a
127.5 (3.1)
119.7 (3.2)
112.5 (3.5)
109.7 (2.8)
106.6 (2.6)
•
•
•
•
•
134.2 (0.2)
132.0 (0.1)
125.3 (0.1)
127.2
•
•
•
•
•
138.1 (24.4)
134.3 (24.6)
129.1 (24.0)
128.2 (24.3)
125.4 (23.5)
•
•
•
•
•
117.8 (0.1)
(8.3)
12
13
14
16
•
•
•
•
81.2 (52.0)
88.9 (74.3)
88.5 (79.6)
91.1 (83.0)
•
•
•
•
89.5 (8.1)
88.5 (8.0)
86.6 (8.1)
83.0 (4.8)
•a
•
•
104.0 (3.4)
101.3 (3.5)
99.2 (2.5)
94.5 (2.4)
•
119.3 (0.1)
•
•
•
•
123.6 (22.6)
121.5 (22.3)
120.0 (21.8)
116.8 (19.7)
•
•
•
•
aHex phase.
IA
364
J. Mater. Chem., 1999, 9, 361–369

Table 3 Phase transition temperatures and thermal effects for compounds 8–27
R1
R2
M / °C (thermal effect/J g−1) Phase sequence, T/ °C (thermal effect/J g−1)
p
8
9
CH(C H
Bui
)
—
—
<room temp.
<50
S 38.2 (0.16) S 62.1 (2.4) Iso
Cry 120.8 (4.7) Hex 143.0 (5.2) S 167.8(1.5) S 177.7
8
17 2
C
A
J
I
C
A
(27.8) Iso
Cry 121.0 (5.0) Hex 147.8 (5.1) S 169.7(3.1) Iso
10 (CH ) CHMe
11 CH CHMeBu
—
—
82 (19.1)
<room temp.
2 2
2
2
J
J
I
C
Cry 90.4 (7.6) Hex 95.0 (0.45 jump) S 129.2 (5.2) S 132.4
I
C
A
(15.9) Iso
Cry 142.5 (6.4) Hex 167.3 (8.4) S 186.7 (34.3) Iso
12 CH CHLCH
—
—
—
—
—
87.4 (32.7)
53.0 (14)
64.2 (31.0)
111.3 (96.8)
107, broad peak
2
2
E
I
A
13 [(E )KCH CMeLCHCH ] H
Hex 108.6 (3.8) S 118.4 (27.1) Iso
2
2 3
2 3
I
C
14 [(E )KCH CHLCMeCH ] H
Cry 112.6 (4.0) Hex 126.6 (2.9) S 136.2 (27.2) Iso
2
15 CH CMCH
2
J
I
C
Cry 137.9 (6.4) Hex 163.0 (7.7) S 176.5 (32.5) Iso
E
B
A
16 cyclohexyl
Cry 113.5 (3.9) Hex 151.6 (6.9) S 168.3 (1.0) S 169.4
J
I
C
A
(28.5) Iso
Cry 108.2 (5.2) Hex 121.0 (3.6) S (150.4 (2.0) S 156.3
17 CH CCl
—
—
129.1 (55.4)
104.8 (15.5)
2
3
J
J
C
A
(18.8) Iso
Cry 140.0 (6.5) Hex 152.6 (0.1) Hex 162.4 (9.9) S 177.6
18 glycidyl
J
I
B
A
(29.8) Iso
19
H
20 Cl
H
133.2 (93.9)
80
65.5 (64.5)
122 (76.7)
77 (53)
Cry 168.4 (6.9) Cry 193.1 (5.7) S 201.6 (40.0) Iso
E
B
A
Cl
Br
NO
H
—
21 Br
22 NO
23 NO
S
83.9 (0.4) N 88.5 (2.4) Iso
142.9 (15.6) Iso
A
S
2
2
2
A
—
24 CH
25 Pri
H
Br
91.6 (31.8)
70.4 (27.1)
S
A
S
A
123.1 (10.0) Iso
80.7 (9.3) Iso
3
26 NH
—
258 (54.8)
S
263 (1.6) Iso
2
A
27 N(C H
)
<room temp. S 62.1 (0.4) N 66.6 (3.6) Iso
8
17 2
A
phases S , Hex and Cry preceded by a short temperature
range where an S phase is observed. The inclination of the
heat capacity anomalies. The lack of any peak broadening at
C
I
K
the S –Cry phase transition confirms its first order nature.9
A
A
B
phase transition line Cry –Hex that divides orthogonal and
This is in agreement with the identification of the smectic B
phase as a crystal-like phase not a hexatic one. For the long
chain homologues, the second order S –S phase transition
B
I
tilted phase regions |dT/dn| is about 60 K. Details of the phase
diagram topology were determined by studying binary mix-
tures of homologues with terminal chain length n=2 and 3
(Fig. 2). The phase sequence moving from the crystalline to
hexatic smectic phases Cry –Hex observed on decreasing
temperature suggests reentrant succession of liquid-like trans-
lational order. The attempts to detect the exact position of the
Cry –Cry phase transition line by studies of the binary
mixtures failed, since neither enthalpy changes nor clear differ-
ences in the observed textures were visible at the transition
point. A phase diagram with a positive slope (dT/dn>0) for
the Cry –Cry phase transition line was proposed (Fig. 1),
which does not suggest any reentrant phenomenon related to
the Cry –Cry phase transition.
A
C
reveals a characteristic heat capacity jump without a transition
enthalpy peak. For the S –Hex phase transition significant
C
I
heat capacity anomalies were observed, typical for the creation
of long-range bond orientational order. The enthalpy changes
at this transition were found to be nearly constant along the
S –Hex line, similar to the enthalpy changes along the
B
I
E
K
C
I
Hex –Cry borderline, since Hex phase stability does not
I
K
I
depend on the terminal chain length.10
X-Ray studies. Crystallographic parameters obtained in the
X-ray investigation of compounds 5 are presented in Table 4.
E
K
As expected, the S phase without long range positional order
E
K
C
On DSC thermograms, phase transition S –Cry –Cry
enthalpy peaks are sharp without noticeable pretransitional
within the smectic layer displays a diffuse signal related to the
A
B
E
averaged intermolecular spacing. This signal becomes narrower
J. Mater. Chem., 1999, 9, 361–369
365

found for the Cry and Cry smectic phases point to periodic
E
K
long-range molecular arrangement. In the Cry phase, the
E
positions of the measured peaks confirm an orthogonal rec-
tangular crystalline lattice substantially distorted from a reg-
ular hexagon (b5a ratio in Table 4). For the tilted crystalline
phase Cry the obtained X-ray results fit rather well to the
K
monoclinic unit cell with the tilt being towards the longer
edge. Attempts to find reasonable parameters for monoclinic
cells with the tilt directed to the shorter edge were not
successful. Thus the sequence of tilted phases in compounds 5
is S –Hex –Cry , which conserves the azimuthal direction of
C
I
K
the tilt angle with respect to the local crystallographic axes,
rather than S –Hex –Cry . The presence in the Cry and Cry
C
F
H
K
E
phases of a few peaks (hkl) with non-zero l indices indicates
a high degree of interlayer molecular correlation. Additionally
the appearance of the (210) signal, which is detected in both
the crystalline Cry and Cry phases, confirms that the molecu-
E
K
lar rotation in these phases is strongly restricted.12 For the
orthogonal Cry , Cry phases the interlayer distance is nearly
B
E
equal to the molecular length (d/L~1) with faint growth (about
˚
0.1 A) in the Cry phase.
E
3.2 Chiral compounds 6 and 7
Phase sequence. Compounds 6 and 7 contain in their terminal
chains an asymmetric carbon atom placed next to the carboxy
group. Similarly to compounds 5, substances with long ester
terminal chain form exclusively tilted phases (Fig. 3 and 4):
the liquid-like smectic C, hexatic smectic I and crystalline
smectic J phases. Assignment of the hexatic phases as a smectic
I phase followed identification based on X-ray studies per-
formed for aligned samples of 6/7 (m=8, n=6).13† The melting
and clearing temperatures are visibly depressed with respect
to those observed for compounds 5 due to steric hindrance
exerted by the methyl group substituted at the chiral carbon
atom. Both presented series of compounds 6 and 7 reveal
tilted phases with ferroelectric or antiferroelectric7 properties.
Strong discrimination of polar properties takes place upon
changing both the chiral and achiral terminal chains. In
compounds 6, in which the achiral tail is changed, homologues
with mÁ13 form exclusively ferroelectric phases, while in
homologues with m<13 antiferroelectric properties dominate
(Fig. 3). In compounds 7, in which the chiral chain is modified,
the odd–even effect was detected upon its elongation (Fig. 4).
Compounds with odd n number of carbon atoms in chiral
chain form only ferroelectric phases, while for even n antiferro-
electric phases appear.
Fig. 1 Phase diagram for compounds 5.
For all R enantiomers of compounds 6 and 7 in the S *
C
phase, a left-handed twist of the helix was found, as determined
from left-handed circular light reflection.5 Taking into account
the spatial configuration (R) and the parity of the asymmetric
centre (odd) as well as the inductive effect of the substituted
methyl group (+I), the obtained helical handedness fully
complies with theoretical predictions.6,14
Substances with exclusively ferroelectric phases were found
to reveal a drastically different temperature dependence of the
helical pitch when compared to materials forming both ferro-
electric and antiferroelectric phases. For a compound with
only ferroelectric phases the helical pitch was in the visible
light wavelength range, and it became shorter as the tempera-
ture is reduced (Fig. 5). This decreasing of the pitch seems to
be mainly connected with specific behaviour of the elastic
coefficients, 15 as the ratio of spontaneous polarisation to the
Fig. 2 Phase diagram for binary mixtures of 5 (n=2) and 5 (n=3).
tilt angle P /h is approximately temperature independent. In
and more intense in the hexatic Hex smectic phase, showing
s
I
compounds that exhibit a S * phase below the S * phase, in
the ferroelectric phase the helical pitch increases on approach-
ing the S *–S * phase transition. In the antiferroelectric phase
an increase in the positional order to a range of hundreds of
CA
C
angstroms, which is for hexatic phases. 11 Moreover, in the
Hex phase the molecules become more closely packed and the
C
CA
I
X-ray reflection is slightly shifted toward a higher diffraction
angle. A similar shift was also observed at the transition from
the S to the crystalline Cry phase. A few rather sharp signals
†Compound 6 (m=8)MCompound 7 (m=8, n=6), i.e. this compound
is common to both series of homologues.
A
B
366
J. Mater. Chem., 1999, 9, 361–369

Table 4 X-Ray diffraction signals for compounds 5, and corresponding crystallographic distances obtained from diffractograms and (in
parentheses) calculated from assumed crystallographic cell
˚
Crystallographic distances/A
(11−1)
n=2, L=30.5
(110)
(111)
(200)
4.18
(201)
(202)
(210)
(001)
30.45
(002)
Cell dimensions
˚
Cry
4.46a
15.2
(15.2)
a=8.93 A
B
˚
b=5.15 A
˚
c=30.45 A
a=b=c=90°
˚
Cry
4.56
4.45
(4.51)
4.13
(4.15)
4.07
(4.04)
3.30
(3.31)
3.50
15.3
(15.25)
a=8.36 A
E
˚
b=5.43 A
˚
c=30.50 A
a=b=c=90°
b/a=0.649
n=6, L=34.9
S
4.5a
32.11
33.61
33.61
16.1
(16.05)
C
Hex
4.52a
4.54
17.10
(16.85)
I
˚
Cry
4.67
4.35
4.21
4.02
(4.08)
3.35
(3.32)
17.09
(16.85)
a=8.42 A
K
˚
b=5.69 A
˚
c=35.49 A
a=108.7°
b=c=90°
b/a=0.676
aBroad signal.
Fig. 3 Phase diagram for compounds 6.
Fig. 4 Phase diagram for compounds 7.
temperature range the helix is unwound (Fig. 5). Helix
unwinding allows the appearance of the in-plane director
modulations in the S * and Hex * phases, detected as stripe
and square textures in thick (above ~10 mm) free suspended
films.16
of layer spacing vs. temperature for compounds 6 with m=12
and m=13 is presented at Fig. 6. The phase transition into the
hexatic (Hex * or Hex *) phase was observed as a slight
increase of the layer distance, which is probably related to the
growth of the orientational order of long molecular axes in
the hexatic phase.
CA
IA
IA
I
The ferro–antiferro phase transition was detected during
routine DSC scans as small enthalpy (ca. 0.2 J g−1 K−1) peaks.
In the X-ray studies no detectable changes in the smectic layer
thickness at the S –S phase transition were found. The plot
The studied compounds have a mesogenic core structure
similar to MHPOBC. However, in comparison to MHPOBC
C
CA
J. Mater. Chem., 1999, 9, 361–369
367

Fig. 7 Temperature dependence of spontaneous polarisation P for 6
s
(m=14) and 6 (m=9). Arrows indicate the phase transition tempera-
tures. Inset: P switching curves in ferroelectric or antiferroelectric
s
smectic C phases measured 3 K above the transition into the hexatic
phase.
smectic C phase a switching onset field is observed17 (inset in
Fig. 7). Using sufficiently low frequency (below 1 Hz) electric
field, tristable optical switching and double current peak were
resolved in the antiferroelectric phases. For the R enantiomers
the negative sign of the spontaneous polarisation was found
in both ferro- and antiferro-electric phases. This result suggests
that the alkyl chain at the asymmetric atom remains predomi-
nantly in the trans conformation with respect to the O–C
bond of the carbonyl group and implies that the molecular
core tilt is stronger than the tilt of the substituted terminal
chain.6 The high value (about 200 nC cm−2) of the spon-
taneous polarisation shows that, apart from the dipole moment
of the methyl group, the dipole moment of the carboxy group
Fig. 5 Temperature dependence of selective reflection wavelength l
for compounds ($) 6 (m=12) and (#) 6 (m=13). Inset: tilt angle h
vs. temperature for 6 (m=13). Arrows indicate the Hex *–S *phase
I
C
transition.
also contributes significantly to P . This is due to steric
s
repulsion between the carbonyl oxygen and the methyl group
that hinders rotation of the carbonyl joint. As a result, the
energetically favourable conformation with the carbonyl group
out the mesogenic core plane is adopted. Thus the transverse
components of the dipole moments of the carbonyl oxygen
group and the methyl group are summed.
The dielectric properties were also studied (Fig. 8) for
compounds with antiferroelectric and ferroelectric phases. In
the ferroelectric S * phase a high value for the relative
C
permittivity (e=400), which is usual for this phase, is observed.
A two-order lower relative permittivity in the antiferroelectric
phases shows that the contribution of the Goldstone mode is
eliminated. A slightly higher relative permittivity in the Hex *
IA
than in the S * smectic phase probably results from a new
CA
unknown mode inherent to the hexatic phases coming into
play.18 Due to tight molecular packing and the inability to
trace an alternation of the measuring field, the Cry * phase
J
exhibits the smallest relative permittivity.
Fig. 6 Layer thickness d vs. temperature for 6 (m=12) and 6 (m=
13). Arrows indicate the phase transition temperatures.
our related compound [6/7 (m=8, n=6)] does not create a
S
phase and reveals a much broader tilted phase temperature
A
range (ca. 80 °C, for MHPOBC, ca. 57 °C).7 Moreover, no
intermediate S * and ferrielectric S * phases were observed.
Ca
Cc
Polar properties. In the studied chiral compounds a high
value of the spontaneous polarisation P is detected in the
s
smectic C phase, which increases slightly in the hexatic phase
(Fig. 7). In the ferroelectric smectic C phase a non-threshold
tilt and P switching is observed, while in the antiferroelectric
Fig. 8 Relative permittivity vs. temperature for 6 (m=8).
s
368
J. Mater. Chem., 1999, 9, 361–369

3.3 Comparative compounds
terminal chains with non-conjugated double bonds does not
affect the appearance of the tilted phases, whereas the introduc-
tion of the triple bond destroys the tilted phases. The branching
of the terminal chains has no significant influence on the
sample’s ability to form the tilted phases. The replacement of
In order to discuss the influence of different substituents on
mesogenicity we synthesised some compounds with modified
ester terminal groups (Table 3). The replacement of the n-
alkyl ester chain with a branched chain (compounds 8–11)
lowers the tendency to form strongly crystalline smectic phases,
and a sequence of S –Hex –Cry smectic phases without the
the ester group by an amide (KCONL) group introduce an S
A
phase. This confirms the known ability of amides substituted
as the terminal outboard group to favour orthogonal phases.6
The destabilisation of the liquid crystal properties by the
lateral moieties that were introduced into the core structure is
plausibly due to the large dimensions of the substituents,
which reduce the molecular shape anisotropy. The better meso-
stability of the disubstituted compounds over their monosubsti-
tuted counterparts seems to come from changing a potential
barrier responsible for rotation about the inner phenyl–oxygen
bond of the core linkage. Any moiety attached to this phenyl
ring at an ortho position would predominantly stay in an s-
trans conformation to the methylene fragment of the central
linkage. Molecular modelling showed that the energy necessary
for achieving full rotation about this bond is highest for
bromine and lowest for chlorine. The potential barrier is
probably high enough to restrict the rotation in bromine- and
nitro-substituted molecules. It gives rise to a more rigid
mesogenic core and causes better molecular alignment in the
created mesophases.
C
I
J
Cry phase is observed, similar to compounds 6 and 7.
K
Moreover, as usual, the branching decreases the clearing and
melting temperatures. The lowering of these temperatures is
stronger for compounds 8 and 11 in which the tertiary carbon
atom is closer to the mesogenic core. The branched isopropyl
moiety ending the longer alkyl chains (compounds 9 and 10)
influences the transition temperatures only weakly.
Insertion of a double or triple bond at the end of the short
terminal alkyl chain (compounds 12 and 15) makes the mol-
ecules more rigid and promotes orthogonal smectic phases
(CryE, HexB). However, the presence of a few double bonds
of E configuration and lateral methyl groups (compounds 13
and 14) preserves the tilted phases. It should be pointed out
that substitution of the methyl groups in either the odd or the
even positions is not equivalent. When the methyl group is
placed closer to the mesogenic core (compound 13) the meso-
phase stability is depressed and polymorphism is less complex.
Modification of the terminal chain by attachment of a
cyclohexyl ring (compound 16) instead of an alkyl chain does
not change markedly the liquid crystalline properties. In
comparison to the homologue 5 (n=4) with comparable
molecular length there is observed a similar phase sequence of
orthogonal and tilted phases with slightly reduced phase
transition temperatures.
We thank the Chemistry Department, Warsaw University, for
financial support in the form of Grant BW-1343/21/96.
Financial support from the Foundation for Polish Science
Moltek 95 is also acknowledged.
References
The epoxy substituent (compound 18) shows an interesting
hexatic phase sequence Hex –Hex , in which the tilt angle is a
B
I
1
2
3
4
5
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B
I
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structure. Bulky bromine and chlorine atoms or a nitro group
were substituted onto one or two of the biphenyl rings at
lateral positions. Strongly polar substituents Cl, Br, NO
(compounds 20–25) substantially destabilise the liquid crystal-
2
line phases. The substituted compounds reveal much lower
melting and clearing temperatures than non-substituted sub-
6
stance 19. However, some tendency to create the S and N
A
phases remains for the bromine (21, 24 and 25) and nitro (22)
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8
9
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pletely. It is also worth noting that the dinitro-substituted
material 22 exhibits mesophases while its mononitro
counterpart 23 does not.
Replacement of the ester terminal chain with a primary
amide group (compound 26) supports a crystal phase that is
stabilised by hydrogen bonding. This results in very high
melting and clearing temperatures with a rather narrow smectic
A phase. Distinctly lower melting and clearing temperatures
were obtained by application of a branched tertiary amide
(compound 27). This compound exhibits mesogenic properties
at room temperature.
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4. Conclusions
The competing tendency of the functional terminal groups
(–O–, –COO–) and the linkage group (–OCH –) to form tilted
or orthogonal phases, respectively,6 is reflected in the observed
phase sequences for compounds 5–7 and comparative com-
pounds. For the long terminal chains with single C–C bonds
the carboxy and alkoxy groups appear more influential, pro-
moting the tilted phases. Shortening of the terminal chains
gives the orthogonal phases exclusively. The use of long
2
Paper 8/05151H
J. Mater. Chem., 1999, 9, 361–369
369