New chiral side chains for ferro- and antiferro-electric liquid crystals derived from the preen-gland wax of the domestic goose

Article abstract of DOI:10.1039/a704503d

(2R,4R,6R,8R)-2,4,6,8-Tetramethyldecanoic acid and (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanol, as well as the (2R,4R,6R,8R) and the (2S,4R,6R,8R) diastereomers of 4,6,8-trimethyldecan-2-ol, have been obtained from the preen-gland wax of the domestic goose.

Full text of DOI:10.1039/a704503d

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New chiral side chains for ferro- and antiferro-electric liquid crystals derived from the
preen-gland wax of the domestic goose†
Gerd Heppke,*a Detlef Lo¨tzsch,a Michael Morrb and Ludger Ernstc
aT echnische Universitat Berlin, Sekr. ER11, Str. des 17. Juni 135, 10623 Berlin, Germany
¨
bGBF-Gesellschaft fur Biotechnologische Forschung mbH, Mascheroder Weg 1, 38124 Braunschweig, Germany
¨
cT echnische Universitat Braunschweig, NMR-L aboratorium der Chemischen Institute, Hagenring 30, 38106
¨
Braunschweig, Germany
(2R,4R,6R,8R)-2,4,6,8-Tetramethyldecanoic acid and (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanol, as well as the (2R,4R,6R,8R) and
the (2S,4R,6R,8R) diastereomers of 4,6,8-trimethyldecan-2-ol, have been obtained from the preen-gland wax of the domestic goose.
Starting from these alkanols and alkanoic acid, novel ferro- and antiferro-electric liquid crystals bearing four methyl branchings in
the chiral side chain have been synthesized and their mesomorphic and electro-optical properties have been investigated. The
results obtained are compared with the properties of the respective chiral (S)-2-methyldecanoic acid, (S)-decan-2-ol and (S)-2-
methyldecanol derivatives. The compounds with four methyl branchings in the chiral side chain are found to exhibit lower melting
points, broader SmC* phase ranges, higher values of spontaneous polarization and larger tilt angles in comparison to the
respective compound with only one methyl branching.
Many physical properties which are used in modern appli-
cations of liquid crystals depend entirely on the presence of
chiral molecules, e.g. the helical structure of cholesteric and
some smectic phases, the ferroelectricity of uniformly tilted
smectic phases and the antiferroelectricity of alternating tilted
smectic phases.1 Moreover, in certain systems high chirality
causes the induction of novel phases (Blue phases, Twist Grain
Boundary phases, Q phases etc.)1,2 Several chiral phases possess
frustrated structures displaying competition between chiral
forces and the tendency of the molecules to pack in a space-
filling arrangement. For a better understanding of the chiral
forces as well as for electro-optical applications the develop-
ment of new chiral liquid crystals plays an important role.
However, the design of novel structures is restricted by the
available chiral moieties, which can be obtained either by
enantioselective reactions or by using the natural chiral pool.
A novel natural source of chiral mono-, di-, tri- or tetra-
methyl branched alkanoic acids is the preen-gland wax of
poultry.3 For example, the wax of the domestic goose consists
of about 90% octadecyl (2R,4R,6R,8R)-2,4,6,8-tetramethyl-
decanoate,4,5 so that after transesterification and Spaltrohr
distillation large quantities of methyl (2R,4R,6R,8R)-2,4,6,8-
tetramethyldecanoate are obtained,6 which can be transformed
into the free acid by standard methods. As recently shown,
methyl (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoate can also
be transformed into (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanol
and further into (4R,6R,8R)-4,6,8-trimethyldecan-2-ol, from
which both the (2R,4R,6R,8R) and the (2S,4R,6R,8R) diastereo-
mers can be isolated by column chromatography.7 These
tetramethylalkanols, as well as (2R,4R,6R,8R)-2,4,6,8-tetra-
methyldecanoic acid, are promising chiral side chains for the
design of novel liquid crystals.
optical tilt angles and switching times) of the novel ferro- and
antiferro-electric liquid crystals are discussed.
Experimental
Synthesis
Methyl
(2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoate
was
obtained by transesterification and Spaltrohr distillation of the
preen gland-wax of the domestic goose. According to the
reaction scheme shown in Scheme 1, the methyl ester was
transformed into the free acid 1f as well as into (2R,4R,6R,8R)-
2,4,6,8-tetramethyldecanol 1b, which was partially further
transformed into the (2R,4R,6R,8R) and the (2S,4R,6R,8R)
diastereomers 1d and 1e of 4,6,8-trimethyldecan-2-ol. All these
tetramethylalkanols, as well as (2R,4R,6R,8R)-2,4,6,8-tetra-
methyldecanoic acid, were obtained with a diastereomeric
excess of more than 99%.6,7 Compounds 1b, 1d, 1e and 1f, as
well as the commercially available compounds (purchased
from the Japan Energy Corporation) (S)-2-methyldecanol 1a,
(S)-decan-2-ol 1c and (S)-2-methyldecanoic acid 1g, were then
used as chiral starting materials for the synthesis of three series
of liquid crystalline products. These series differ by the linking
group between mesogenic core and chiral side chain. Within
each series, the number and the position of the chiral methyl
branchings are varied. As outlined in Scheme 2, compounds
2a–d were synthesized by esterification of 4-benzyloxybenzoic
acid with the chiral alkanols 1a–d, followed by hydrogenation
to remove the benzyloxy protecting group and finally esterifi-
cation of the obtained phenols with 4∞-octyloxybiphenyl-4-
carboxylic acid. The respective liquid crystalline ethers 3a–d
were obtained by a reaction between 4-hydroxyphenyl 4∞-
octyloxybiphenyl-4-carboxylate and the chiral alkanols 1a–c
and 1e in the presence of diethylazodicarboxylate (DEAD)
and triphenylphosphine8 (see Scheme 3). Compound 4c
was synthesized by esterification of the acid chloride of 1g
with 4-hydroxyphenyl 4∞-octyloxybiphenyl-4-carboxylate (see
Scheme 3), whereas compound 4d could not been obtained
optical pure in a similar way. Compound 4d was synthe-
sized by esterification of 1f with 4-hydroxyphenyl 4∞-
octyloxybiphenyl-4-carboxylate in the presence of dicyclo-
hexylcarbodiimide (DCC). All products were purified by
Here we present the first ferro- and antiferro-electric liquid
crystals having tetramethylalkyl groups in the chiral side chain.
In order to study the influence of the additional optically
active methyl branchings, the respective chiral 2-methylde-
canoic acid, decan-2-ol and 2-methyldecanol derivatives have
also been synthesized. Polymorphy, phase transition tempera-
tures and electro-optical properties (spontaneous polarization,
† Presented in part at the 5th International Conference on Liquid
Crystals, Cambridge, 1995.
J. Mater. Chem., 1997, 7(10), 1993–1999
1993

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Scheme 1 Reagents and conditions: i, LiAlH ; ii, PCC, CH Cl ; iii, 2,2∞-
4
2 2
bipyridyl–Cu complex, DABCO, ButOH, air; iv, methyloxazaborolid-
ine, BH –THF, THF; v, column chromatography [silica gel,
3
Scheme 3 Reagents and conditions: i, MeOH, H SO ; ii, K CO ,
CH Cl –Pri O (951)]; vi, NaOH, dioxane
2
4
2
3
2
2
2
C H Br, DMF; iii, KOH, EtOH; iv, HCl; v, SOCl ; vi, 4-
17
BnOC H OH, pyridine; vii, H , Pd; viii, ROH (1a–c,e), DEAD, PPh ,
8
2
6
4
2
3
THF; ix, 1f, DCC; x, RCOCl (from 1g and SOCl )
2
were then transferred to the remaining compounds while
considering the usual substituent effects on chemical shifts11
and attempting maximum internal consistency. The purity of
the products was checked by TLC and HPLC (pump: Knauer
HPLC pump 64, column: Nucleosil 120-5 C18, solvent: meth-
anol, detector: Severn Analytical SA 6503 at 303 nm) and their
optical purity was characterized by measuring the optical
rotation (Perkin-Elmer polarimeter 241). All compounds were
found to be of high purity (HPLC purity above 99%).
The synthesis of compound 2c has already been described;12
however no detailed information about the physical properties
of this compound were reported.
Synthesis of 2a–d
A
synthetic procedure for 2d is given as an example.
Dicyclohexylcarbodiimide (DCC) (1.24 g, 6 mmol) was added
at room temperature to a solution of 4∞-octyloxybiphenyl-
4-carboxylic acid (0.98 g, 3 mmol), (1R,3R,5R,7R)-1,3,5,7-
tetramethylnonyl 4-hydroxybenzoate (0.95 g, 3 mmol) and
4-dimethylaminopyridine (DMAP) (0.06 g, 0.5 mmol) in dry
dichloromethane (50 ml), and the mixture was stirred for 24 h.
After filtration the solvent was evaporated and the resulting
product was purified by chromatography over silica gel using
Scheme 2 Reagents and conditions: i, MeOH, H SO ; ii, K CO ,
2
4
2
3
C H Br, DMF; iii, KOH, EtOH; iv, HCl; v, SOCl ; vi, ROH (1a–d);
17
vii, H , Pd; viii, DCC, CH Cl
8
2
2
2 2
chromatography, followed by recrystallization until the trans-
ition temperatures remained constant. The structure of the
products was confirmed by 1H and 13C NMR experiments
(Bruker ARX-400, AM-400 and DPX-300; 9.4 and 7.0 T,
respectively) and, in case of compounds 2d, 3d and 4d, addition-
ally by IR (Perkin-Elmer PE 257) and mass (Varian MAT
44F) spectroscopy. Signal assignments in the 13C NMR spectra
were achieved by DEPT-135 experiments,9 by two-dimensional
13C, 1H COSY10 and COLOC10 experiments for 2a, 3b and 4d
and by comparison with literature data.6,7 These assignments
dichloromethane as the eluent (R 0.70), followed by re-
crystallization from ethanol until the transition temperature
f
remained constant. Yield: 1.34 g (71%); n (CCl )/cm−1 2955,
4
2925, 2871, 2854, 1735, 1715, 1604, 1504; m/z 628.5 (M+); 1H
and 13C NMR data are given in Tables 1 and 2; [a]20 −0.30
D
(c 5, CHCl ) (2b, −2.36; 2c, +18.29; 2d, −18.68).
3
1994
J. Mater. Chem., 1997, 7(10), 1993–1999

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Table 1 1H NMR data for compounds 2a–d, 3a–d and 4c–d (400 or 300 MHz; CDCl ; Me Si)
3
4
chemical shift (multiplicity, coupling constant)a,b
2d 3a 3b 3c
4.01 4.01
proton
O1
2a
2b
2c
4.01
(t,6.6)
1.81
(qi,7.0)
0.89†
(t,6.9)
7.69
(8.4)
8.23
7.59
(8.7)
7.00
7.30
(8.6)
8.12
1.34
3d
4.01
4c
4d
4.01
(t,6.6)
1.81
(qi,7.0)
0.89†
(t,6.9)
7.69
4.01
4.01
4.01
4.01
(t,6.6)
4.01
(t,6.6)
1.80
(t,6.6)
1.82
(qi,7.0)
(t,6.6)
(t,6.6)
1.81
(t,6.6)
1.88
(t,6.5)
1.81
(t,6.6)
O2
(qi,7.0)
0.89†
(t,6.9)
7.67
(8.3)
8.22
7.59
(8.7)
7.00
7.12
(9.0)
6.93
3.82
(dd,8.9,5.8)
3.72
(qi,7.0)
0.89
(t,ca. 7)
7.67
(8.4)
8.22
7.58
(8.7)
6.99
7.12
(9.0)
6.93
3.84
(dd,8.9,5.0)
3.70
(qi,7.0)
0.89†
(t,6.9)
7.67
(8.3)
8.21
7.58
(8.6)
6.99
7.11
(8.9)
6.91
1.30
(qi,7.0)
0.89†
(t,6.9)
7.68
(8.4)
8.22
7.59
(8.7)
7.00
7.24‡
(9.0)
7.13‡
O8
0.89
(t,6.9)
7.68
(8.4)
8.22
7.58
(8.7)
7.00
7.24†
(9.0)
7.13†
B2
7.70
(8.3)
8.23
7.59
(8.7)
7.00
7.32
(8.6)
7.69
(8.4)
8.22
7.59
(8.7)
7.00
7.31
(8.7)
8.12
1.35
(d,6.1)
7.68
(8.5)
8.22
7.58
(8.8)
7.00
7.12
(9.0)
6.92
1.30
(8.4)
8.23
7.59
(8.7)
7.00
7.31
(8.6)
8.13
B3
B2∞
B3∞
P2
P3
D1
8.13
4.25
4.22
(dd,10.7,5.8)
4.12
(dd,10.7,5.0)
4.09
(d,6.3)
(d,ca. 6)
(d,ca. 6)
(dd,10.7,6.7)
1.93
(oct,6.4)
0.88†
(t,6.9)
(dd,10.7,6.8)
2.06
(oct,6.5)
(dd,8.9,6.8)
1.93
(oct,6.4)
0.88†
(t,6.9)
1.02
(dd,8.9,6.8)
2.04
(oct,6.5)
0.85
(t,7.3)
1.04
(d,6.8)
D2
5.16
5.28
4.32
4.43
(m)
2.69
2.82
(dqd,9.8,6.9,4.9)
0.85
(t,ca. 7)
1.31
(d,6.9)
(sext,6.3) (m)
0.88†
(t,6.9)
(sext,6.0)
0.88†
(t,6.9)
(sext,7.0)
0.88†
(t,6.9)
1.30
D10
D2-Me 1.02
(d,6.7)
1.04
(d,6.6)
(d,6.7)
(d,6.9)
aCoupling constants (J) are given in Hz; qi=quintet, sext=sextet, oct=octet. Footnote symbols (†,‡) indicate interchangeable assignments. For
the B and P protons, which are parts of AA∞XX∞ spin systems, the N values (J +J ) are given in parentheses. bFurther signals: 2a: d 1.52–1.27
(m); 2b: d 1.71–0.81 (m); 2c: d 1.73–1.26 (m); 2d: d 1.90–0.80 (m); 3a: d 1.54–1.21 (m); 3b: d 1.68–0.82 (m); shifts assigned by 2D experiments: d
AX AX∞
1.64 (m, D4), 1.60 (m, D6), 1.47 (m, D3), 1.46 (m, O3), 1.43 (m, D8), 1.38 (m, D9), 1.36, 1.32 (m, O4, O5), 1.30 (m, O7), 1.28 (m, O6), 1.22 (m,
D5, D7), 1.05 (m, D9), 0.98 (m, D3), 0.90, 0.86 (m, D5, D7), 0.90 (d, 6.6, D4-Me), 0.85 (d, 6.6, D8-Me), 0.83 (d, 6.5, D6-Me); 3c: d 1.73–1.28 (m);
3d: d 1.87–0.83 (m); 4c: d 1.86–1.28 (m); 4d: d 1.96–0.83 (m); shifts assigned by 2D experiments: d 1.90 (ddd, 13.9, 9.7, 4.3, D3), 1.66 (m, D4), 1.63
(m, D6), 1.46 (m, O3)1.43 (m, D8), 1.36 (m, D9), 1.36, 1.32 (m, O4, O5), 1.30 (m, O7), 1.28 (m, O6), 1.23 (m, D5), 1.20 (m, D7), 1.16 (m, D3),
1.08 (m, D9), 0.97 (m, D5), 0.96 9d, 6.5, D4-Me), 0.90 (m, D7), 0.85 (d, ca. 7, D6-Me, D8-Me).
Synthesis of 3a–d
13C NMR data are given in Tables 1 and 2; [a]20 +3.05 (c 5,
D
CHCl ) (3b, −2.02; 3c, −2.11; 3d, −3.44).
3
A synthetic procedure for 3d is given as an example. 4-
Hydroxyphenyl 4∞-octyloxybiphenyl-4-carboxylate (1.26 g,
Synthesis of 4c
3 mmol),
triphenylphosphine
(0.79 g,
3 mmol)
and
(2S,4R,6R,8R)-4,6,8-trimethyldecan-2-ol 1e (0.4 g, 2 mmol)
were dissolved in dry tetrahydrofuran (50 ml). While the
temperature was kept at 0 °C, diethyl azodicarboxylate
(DEAD) (0.52 g, 3 mmol) was added dropwise under a nitrogen
atmosphere and stirring was continued for 36 h at room
temperature. Afterwards the solvent was removed and the
resulting product was purified by chromatography over silica
4-Hydroxyphenyl 4∞-octyloxybiphenyl-4-carboxylate (1.26 g,
3 mmol) was dissolved in dry pyridine (50 ml) and (S)-2-
methyldecanoyl chloride (0.71 g, 3 mmol) was added while the
temperature was kept at 0 °C. Stirring was continued for
16 h at room temperature. After hydrolysis in an excess of
diluted HCl, the product was extracted into dichloromethane
(3×150 ml). The combined dichloromethane solutions were
gel using dichloromethane as the eluent (R 0.90), followed by
recrystallization from ethanol until the transition temperature
washed with water (200 ml) and dried (MgSO ). The solvent
f
4
was removed and the resulting product purified by chromato-
graphy over silica gel using dichloromethane as the eluent
(R 0.71), followed by recrystallization from ethanol until the
remained constant. Yield: 0.64 g (53%); n(CCl )/cm−1 2954,
4
2926, 2870, 2854, 1733, 1606, 1504; m/z 600.5 (M+); 1H and
f
J. Mater. Chem., 1997, 7(10), 1993–1999
1995

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Table 2 13C NMR data for compounds 2a–d, 3a–d and 4c–d (101 or 75 MHz; CDCl )
3
chemical shifta
carbon
2a
2b
2c
2d
3a
3b
3c
3d
4c
4d
O1
O2
68.2
29.3
68.2
29.3
68.3
29.3
68.2
29.3
68.2
29.3
68.2
29.3
68.2
29.3
68.2
29.3
68.2
29.3
68.2
29.3
O3
O4
O5
O6
O7
O8
26.1
26.1
26.1
26.1
26.1
26.1
26.1
26.1
26.1
26.1
29.3†
29.4†
31.9‡
22.7
29.3†
29.4†
31.8
22.7
14.1
29.3†
29.4†
31.9
22.7
14.1
29.3†
29.4†
31.9
22.7
14.1
29.3†
29.4†
31.9‡
22.7
29.3†
29.4†
31.9
22.7
14.1
29.3†
29.4†
31.9
22.7
14.1
29.3†
29.4†
31.9
22.7
14.1
29.3†
29.4†
31.9‡
22.7
29.3†
29.4†
31.9
22.7
14.1
14.1
14.1
14.1
B1
B2
B3
B4
B4-CO
B1∞
B2∞
B3∞
B4∞
P1
P2
P3
146.3
126.7
130.8
127.1
164.6
131.9
128.4
115.1
159.7
154.7
121.8
131.2
128.2
166.0
70.1
32.8
33.5
26.9
29.9†
29.6†
29.3†
31.8‡
22.7
146.3
126.6
130.8
127.0
164.6
131.8
128.4
115.0
159.7
154.7
121.8
131.2
128.1
165.9
69.8
30.2
41.4
27.6‡
44.7
27.5‡
45.7
31.5
28.8
11.2
146.3
126.7
130.8
127.1
164.7
131.9
128.4
115.1
159.7
154.6
121.7
131.2
128.6
165.5
20.1
72.0
36.1
25.5
29.5†
29.5†
29.5†
31.9
22.7
14.1
146.3
126.7
130.8
127.1
164.7
131.9
128.4
115.1
159.7
154.6
121.8
131.1
128.5
165.5
20.6‡
70.0
43.1
26.7
45.0
27.4
45.5
31.6
29.1
11.2
145.9
126.6
130.7
127.7
165.5
132.1
128.4
115.0§
159.6
144.4
122.4
115.2§
157.1
145.9
126.6
130.7
127.7
165.5
132.0
128.4
115.0
159.6
144.3
122.4
115.1
157.1
145.9
126.6
130.7
127.7
165.5
132.1
128.4
115.0
159.6
144.3
122.5
116.6
156.0
145.9
126.6
130.7
127.7
165.5
132.0
128.4
115.0
159.6
144.3
122.5
116.5
156.2
146.1
126.6
130.7
127.3
165.0
131.9
128.4
115.0
159.6
148.3
122.6§
122.5§
148.3
146.1
126.6
130.8
127.4
165.0
132.0
128.4
115.1
159.7
148.4
122.6‡
122.5‡
148.4
P4
P4-CO
D1
D2
D3
D4
D5
D6
D7
D8
73.7
33.2
33.6
27.0
30.0†
29.6†
29.4†
32.0‡
22.7
73.5
30.7
41.3
27.7
44.7‡
27.5
45.6‡
31.6
28.9
11.2
18.3
21.1
21.0
20.0
19.8
74.6
36.6
25.6
29.7†
29.6†
29.3†
31.9
22.7
14.1
20.4‡
72.4
44.1
26.6
44.9
27.4
45.7
31.6
29.0
11.2
175.3
39.7
33.8
175.3
37.7
41.3
28.5
45.6
27.4
45.0
31.6
29.1
11.2
18.3
20.5
20.8
20.0
27.3
29.6†
29.5†
29.3†
31.8‡
22.7
D9
D10
Me-D2
Me-D4
Me-D6
Me-D8
14.1
17.1
14.1
17.1
14.1
17.0
18.3
20.9
20.9
20.0
20.6‡
21.0‡
19.9
20.7‡
20.7‡
20.0
aFootnote symbols (†,‡,§) indicate interchangeable assignments.
transition temperature remained constant. Yield: 1.18 g (67%);
1H and 13C NMR data are given in Tables 1 and 2; [a]20 +8.60
(c 5, CHCl ).
heptyloxycarbonyl)phenyl 4∞-octyloxybiphenyl-4-carboxylate
(MHPOBC)13 have been used as reference compounds.
Commercially available test cells (E.H.C.) with a layer spacing
of 10 mm were used for the electro-optical investigations. The
spontaneous polarization was measured by the triangular wave
method. Optical tilt angles were obtained by an extrapolation
of the switching angles to zero field. Switching times, defined
as the rise time from 10 to 90% transmission, were determined
by measuring the optical response to an applied electric field
(rectangular wave) at a field strength of 10 V mm−1 (com-
pounds 2a–d, 3a–d and 4c) or 20 V mm−1 (compound 4d).
D
3
Synthesis of 4d
Dicyclohexylcarbodiimide (DCC) (0.70 g, 3.4 mmol) was
added at room temperature to a solution of 4-hydroxyphenyl
4∞-octyloxybiphenyl-4-carboxylate
(0.71 g,
1.7 mmol),
(2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoic acid 1f (0.82 g,
3.4 mmol) and 4-dimethylaminopyridine (DMAP) (0.27 g,
2.2 mmol) in dry dichloromethane (50 ml), and the mixture
was stirred for 24 h. After filtration, the reaction mixture was
washed twice with a solution of 5% citric acid in water (50 ml)
and once with water (50 ml). The organic phase was dried
(Na SO ), the solvent removed and the resulting product
Results
Liquid crystalline properties
2
4
purified by chromatography over silica gel using dichloro-
methane as the eluent (R 0.81), followed by recrystallization
Textural observations and miscibility studies have been carried
out in order to determine the liquid crystalline properties of
the compounds of series 2–4. On cooling from the isotropic
phase for all compounds the SmA phase appears showing
characteristic textures (planar oriented region: focal-conic fan
texture, homeotropic oriented regions: no texture). On further
cooling a phase transition into the SmC phase occurs: in the
planar oriented region the focal-conic fan texture of the SmA
phase transforms into a broken focal-conic fan texture and in
the homeotropic regions a schlieren texture appears. Below
the SmC phase, most of the compounds show a direct transition
into a higher ordered smectic phase (denoted as SmIII phase
in the following text, probably SmI) which exhibits either a
broken focal-conic fan texture with thin round coloured bands
f
from ethanol until the transition temperature remained con-
stant. Yield: 0.42 g (39%); n(CCl )/cm−1 2954, 2925, 2870,
4
2854, 1756, 1732, 1604, 1504; m/z 628.5 (M+); 1H and 13C NMR
data are given in Tables 1 and 2; [a]20 −10.53 (c 5, CHCl ).
D
3
Equipment and methods
Phase transition temperatures were determined optically by
observing the textural changes with a polarizing microscope.
Transition enthalpies were measured by differential scanning
calorimetry (DSC) using a Perkin-Elmer DSC 7. Miscibility
studies were carried out by the contact method and,
in one case, in addition, by choosing specific concentrations.
For these investigations both enantiomers of 4-(1-methyl-
or a schlieren texture. In the other two compounds, a SmC
A
1996
J. Mater. Chem., 1997, 7(10), 1993–1999

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antiferroelectric phases. For both antiferroelectric phases
(SmC and SmI ) of compounds 2c and 4d, a ‘tristate’ switch-
A
A
ing has been observed, confirming the antiferroelectric nature
of these phases. In order to allow a comparison to the electro-
optical properties of the SmC phases of the other compounds,
in the temperature range of the antiferroelectric SmC phases
polarization and optical tilt angle of the field induced ferroelec-
A
tric states, as well as the rise time from 10 to 90% transmission
for the direct switching between the two ferroelectric switching
states, have been determined. Since no switching was observed
for the SmC phase of compound 4d at E 10 V mm−1, the
switching times of this compound were measured at E 20 V
A
mm−1.
For the compounds of series 2 the results of the electro-
optical investigations are shown in Figs. 2–4. Both tetramethyl
derivatives (2b, 2d) are found to exhibit two to three times
higher values of spontaneous polarization in comparison to
their respective reference compound (2a, 2c) with only one
chiral methyl branching. Additionally, an increase of the optical
tilt angle by about a factor of 1.5 is observed; e.g. 50 K below
Fig. 1 Phase diagram between (R)-MHPOBC and 4d
phase occurs between the SmC and the higher ordered smectic
phase SmI . At the transition from the SmC to the SmC
A
A
phase the number of chirality lines in the broken focal-conic
fan texture strongly decreases and the number of chevron
defects strongly increases. The SmC–SmC phase transition is
A
also indicated by an inversion of the helical twist sense which
can be observed in the schlieren texture. Moreover, in the
supercooled region of three compounds a transition into a
high ordered smectic phase SmIV can clearly be observed by
the formation of a mosaic texture.
The classification of the SmA, SmC, SmC and SmI phases
A
A
was confirmed by miscibility studies. As an example, the phase
diagram between the compounds 4d and (R)-MHPOBC, which
has been investigated in more detail, is shown in Fig. 1. The
SmA, SmC, SmC and SmI phases of (R)-MHPOBC
Fig. 2 Temperature dependence of spontaneous polarization of com-
pounds 2; (#) 2a, ($) 2b, (%) 2c and (&) 2d
A
A
are uninterruptedly miscible, with the respective smectic
modifications of 4d confirming thereby the phase sequence
SmI –SmC –SmC–SmA for compound 4d.
A
A
Polymorphy, phase transition temperatures and transition
enthalpies of the liquid crystalline products are shown in
Table 3 [NB: compounds with different linking groups X are
distinguished by different numbers (2–4), whereas the letters
(a–d) label the kind of the chiral side chain]. As can be seen,
the introduction of three additional methyl branchings leads
to a decrease of the melting points of about 10 K for series 2
and 4 and 25 K for series 3, as well as to about 30 K lower
clearing temperatures, resulting in a smaller liquid crystalline
phase range for the tetramethyl derivatives. However, the SmC
temperature range increases slightly in the case of series 2 and
4 and by more than a factor of three in case of series 3. The
largest SmC phase ranges are observed in series 2, ranging
from 47 to 62 K.
Fig. 3 Temperature dependence of the tilt angle of compounds 2;
(#) 2a, ($) 2b, (%) 2c and (&) 2d
In the two compounds 2c and 4d, an alternating tilted SmC
phase occurs in a broad temperature range. With respect to
A
the appearance of this SmC phase the influence of the three
additional methyl branchings is puzzling. Whereas in series 2
A
the SmC phase of the 2-methyldecanol derivative 2c is
replaced by a SmC phase in the respective tetramethyl
derivative 2d, the opposite effect is observed in series 4.
A
Electro-optical properties
All compounds of series 2–4 (see Table 3) show ferroelectric
switching in the SmC and SmIII phases. To characterize the
ferroelectric properties the temperature dependence of spon-
taneous polarization, optical tilt angle and switching time
(t
measured. In two of the compounds (2c, 4d) the ferroelectric
smectic modifications (SmC, SmIII) are almost replaced by
at E 10 V mm−1) of the SmC phases have been
10–90
Fig. 4 Temperature dependence of the switching time of compounds 2;
(#) 2a, ($) 2b, (%) 2c and (&) 2d
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1997

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Table 3 Polymorphy, phase transition temperatures and transition enthalpies of the liquid crystalline products
compound
X
R
transition temperatures/°C [enthalpies/kJ mol−1]
2a
CO
Cr 66.4
[30.6]
(SmIII 54.5)
[1.02]
SmC 108.0 SmA 164.1 I
[0.00] [6.24]
2
2b
2c
2d
3a
CO
CO
CO
O
Cr 53.9
[22.9]
SmIII 54.0
[0.80]
SmC 115.7 SmA 134.3 I
[0.00] [7.08]
2
2
2
Cr 62.8 (SmI 62.5) SmC 109.3
[1.50]
SmC 113.4 SmA 140.4 I
[0.00] [5.83]
A
A
[29.4]
[0.021]
Cr 53.5
[19.9]
(SmIII 38.0)
[0.43]
SmC 94.8 SmA 102.1 I
[0.50] [1.98]
Cr 84.4 (SmIV 56.6) SmIII 101.6
[31.2] [1.96] [2.96]
SmC 107.9 SmA 164.4 I
[0.00] [7.04]
3b
3c
O
O
Cr 55.1 (SmIV 44.7) SmIII 80.6
[2.40]
SmC 104.6 SmA 133.6 I
[0.00] [4.80]
[18.8]
[1.84]
Cr 65.2
[23.5]
SmIII 81.3
[2.16]
SmC 94.5 SmA 144.2 I
[0.00] [6.07]
3d
4c
4d
O
Cr 41.6
[22.7]
SmIII 49.5
[1.04]
SmC 90.4 SmA 116.3 I
[0.00] [4.22]
O C
Cr 62.6 (SmIV 58.5) SmIII 91.7
[26.1] [1.66] [2.72]
SmC 136.1 SmA 157.5 I
[0.00] [5.48]
2
O C
Cr 56.3 SmI 59.5 SmC 108.5
[23.2]
SmC 109.4 SmA 119.2 I
[0.28] [2.68]
2
A
A
[0.12]
[1.32]
Table 4 Spontaneous polarization, optical tilt angles and switching
times of the compounds of series 3 at 5, 10, 20, 30 and 40 K below
the SmA–SmC transition temperature
Table 5 Spontaneous polarization, optical tilt angles and switching
times of the compounds of series 4 at 5, 10, 20, 30 and 40 K below
the SmA–SmC transition temperature
compound
T−T /K
P /nC cm−2
h(°)
t/ms
compound
T−T /K
P /nC cm−2
h(°)
t/ms
c
s
c
s
3a
3b
3b
3b
3c
3c
3d
3d
3d
3d
3d
−5
−5
−10
−20
−5
−10
−5
−10
−20
−30
−40
1.8
5.1
6.8
8.8
24.4
32.6
49.2
63.6
82.8
93.3
97.9
9.1
17.7
20.4
22.5
11.1
13.5
19.0
23.0
26.3
27.5
27.2
22.3
48.6
70.0
90.0
9.6
4c
4c
4c
4c
4c
4d
4d
4d
4d
4d
−5
−10
−20
−30
−40
−5
−10
−20
−30
−40
8.0
10.7
13.9
16.2
18.0
25.6
29.6
35.7
41.0
44.3
18.0
21.5
24.5
25.9
26.5
27.1
29.6
32.2
33.5
33.8
17.6
19.1
21.3
22.9
26.4
12.4a
14.4a
20.2a
32.9a
62.1a
9.9
12.3
13.8
18.4
25.5
45.8
aMeasured at a field strength of 20 V mm−1.
the SmA–SmC transition temperatures, optical tilt angles of
about 30° for the tetramethyl derivatives are measured, com-
pared to about 20° for the reference compounds. Thus the
strong increase of the spontaneous polarization can not only
be attributed to an increase of the polarization–tilt angle
coupling constant (as expected for the introduction of three
additional chiral centres), but is also caused by the remarkable
increase of the tilt angle. Although the spontaneous polariz-
ation of the tetramethyl derivatives are much higher, their
switching times are of the same order. In the vicinity of the
higher ordered smectic phase (probably SmI), an exponential
increase of the switching times is observed for the SmC phases
of compounds 2a, 2b and 2d, whereas the switching time of
the SmC phase of compound 2c remains almost constant
close above the transition to the SmI phase.
For the electro-optical properties of the compounds of series
3, similar results are obtained (see Table 4). Again, the tetra-
methyl derivatives are found to exhibit nearly two times larger
optical tilt angles and two to three times higher values of
spontaneous polarization in comparison to their respective
reference compounds.
In series 4 only two compounds with the (first) chiral centre
in the a-position have been synthesized. The electro-optical
properties of compounds 4c and 4d are summarized in Table 5.
As in series 2 and 3, the introduction of the additional methyl
branchings leads to a remarkable increase of the optical tilt
A
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J. Mater. Chem., 1997, 7(10), 1993–1999

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angle and of the spontaneous polarization. In comparison to
the respective compounds of series 2, whose molecular struc-
tures differ only by the direction of the ester group between
the mesogenic core and the chiral side chain, the spontaneous
polarization of compounds 4c and 4d is reduced by about a
factor of five.
References
1
2
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Starting from the natural source of the preen-gland wax of
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for use in broad range SmC room temperature mixtures.
According to the electro-optical investigations, the introduc-
tion of the additional methyl branchings leads to an increase
of the spontaneous polarization and the optical tilt angle. The
use of new chiral tetramethylalkyl side chains seems to be
promising, especially for the development of new materials for
device applications where switching angles of 45° are required
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The authors thank the Deutsche Forschungsgemeinschaft (Sfb
335) for financial support.
Paper 7/04503D; Received 26th June, 1997
J. Mater. Chem., 1997, 7(10), 1993–1999
1999