Synthesis and physical properties of novel liquid crystals containing 2,3-difluorophenyl and 1,3-dioxane units

Article abstract of DOI:10.1039/b210765a

Three series of liquid crystalline materials have been synthesized: 2-(2,3-difluoro-4-alkoxyphenyl)-5-(4-alkylphenyl)-1,3-dioxanes (m,On-DFPPD), 2-(2,3-Difluoro-4-alkoxyphenyl)-5-(2-fluoro-4-alkylphenyl)-1,3-dioxanes (m,On-DFPFPD) and 2-(2,3-Difluoro-4-al

Full text of DOI:10.1039/b210765a

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Synthesis and physical properties of novel liquid crystals containing
2,3-difluorophenyl and 1,3-dioxane units
Gui-Xiang Sun,* Bing Chen, Hong Tang and Shou-Yi Xu
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.
E-mail: sgx01@mails.tsinghua.edu.cn; Fax: 186-10-6278-1697; Tel: 186-10-6278-2873
Received 1st November 2002, Accepted 18th February 2003
First published as an Advance Article on the web 28th February 2003
Three series of liquid crystalline materials have been synthesized: 2-(2,3-difluoro-4-alkoxyphenyl)-5-(4-
alkylphenyl)-1,3-dioxanes (m,On-DFPPD), 2-(2,3-Difluoro-4-alkoxyphenyl)-5-(2-fluoro-4-alkylphenyl)-1,3-
dioxanes (m,On-DFPFPD) and 2-(2,3-Difluoro-4-alkoxyphenyl)-5-(2,3-difluoro-4-alkylphenyl)-1,3-dioxanes
(m,On-DFPDFPD). Their mesmorphic properties were observed and measured by polarizing optical
microscopy and differential scanning calorimetry. The dielectric anisotropy (De) and birefringences (Dn) of some
target compounds were determined. Most of these compounds exhibit a wide temperature range smectic A
phase and/or a wide temperature range nematic phase with negative dielectric anisotropy (De) and low
birefringences (Dn).
microscopy using a polarizing microscope (Olympus PM-
10AD) fitted with a heating stage (Mettler FP-80) and control
Introduction
Liquid crystalline materials with high negative dielectric
anisotropy (De), low birefringences (Dn), low viscosities (g)
and low spontaneous polarizations (Ps) have shown great
promise for use in MVA-TFT (multi-domain vertical align-
ment thin film transistor) and FLC (ferroelectric liquid crystal)
displays.1–3 Hundreds of terphenyl liquid crystalline com-
pounds with two, three, or four lateral fluoro-subsitituents
have been reported, and many of them have proved to be
excellent host materials for ferroelectric mixtures.4–9 However,
their birefringence is relatively high (0.19).3,10 On account of
this, a variety of other related liquid crystals with negative
dielectric anisotropy have been designed and synthesized.10–13
However, the transition temperatures of the smectic C (SmC)
to the smectic A (SmA) phase or to the nematic phase of almost
all of these compounds are much lower than for the analogues
of terphenyl liquid crystals. Moreover, these materials do not
readily mix with conventional terphenyls.3
unit (FP-82), and by differential scanning calorimetry (Mettler
DTA TA-2010 DSC, heating and cooling rate was 10 uC min21).
Phase identification was made by comparing the observed
textures with those reported in the literature.15,16 Elemental
analysis for carbon and hydrogen were conducted on a Heraeus
CHN-O-Rapid elemental analyzer. The purity of each final
compounds (Tables 1, 3 and 4) was determined by HPLC
(Hypersil OD52 C18 Reverse Phase Column 250 6 4.6 mm)
and were found to be w99% pure.
The target compounds were prepared according to the
procedures detailed in Scheme 2. The compounds 4a–i, 6 and 7
were prepared by reported methods.2,5 Compounds 1a–e were
synthesized from 1-(4-alkylphenyl)ethanone by Willgerodt and
hydration reactions.17 All homologues were prepared using the
same methods and therefore the general experimental proce-
dures are illustrated using 185,5, 195,5 and 205,5 as examples.
One of the feasible ways to improve the temperature range of
the SmC phase, and at the same time reduce birefringence, is by
introducing heteroatoms into the alicyclic ring. Two series of
dioxanyl host materials that have structures based on
difluoroterphenyls have been reported.2 The results of studies
shows that most of these materials have reasonable smectic C
temperature ranges and low birefringences. Furthermore, some
chiral liquid crystals containing 2,3-difluorophenyl and 1,3-
dioxane units have been reported and most of them show wide
chiral smectic C temperature ranges.14 In order to meet the
higher quality requirements for materials and observe the
relationship between structures and properties, we synthesized
three novel types of compounds. The general structures of these
systems are shown in Scheme 1 (series A, B and C).
Methyl 4-pentylphenylacetate (2c)
Sulfuric acid (2 ml, 98%) was added dropwise to a solution of
1c (50 g, 0.24 mol) and methanol (150 ml). The reaction mixture
was heated under reflux for 16 h. The redundant methanol was
removed in vacuo and the residue poured into aqueous sodium
hydrogen carbonate solution (5%, 200 ml). The product was
extracted into dichloromethane (2 6 150 ml) and the combined
organic layers were washed with water (2 6 200 ml) and then
dried (Na2SO4). The solvent was removed in vacuo and the pure
Experimental
The structures of the final products and intermediates were
confirmed by infrared spectroscopy (Nicolet 560 spectro-
photometer), 1H NMR spectroscopy (Bruker 300 spectro-
meter, where the spectra, unless otherwise stated, were
recorded in CDCl3 using Me4Si as an internal standard; the
J values are given in Hz), and mass spectrometry (AEIMS-50
SD90 spectrometer). The phase transition temperatures of the
target compounds were determined by polarizing optical
Scheme 1 The structures of the target compounds.
742
J. Mater. Chem., 2003, 13, 742–748
This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b210765a

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Scheme 2 Synthetic pathway to the target materials.
product was isolated by distillation under reduced pressure.
Yield 48.5 g (91%); bp 112–114 uC/1.2 mmHg.
was taken up in ethyl acetate (200 ml), washed with water (2 6
150 ml) and dried (Na2SO4). The solvent was removed in vacuo
and the pure product was isolated by recrystallization from
hexane. Yield 16 g (33%), mp 71.7–73.5 uC.
2-(4-Pentylphenyl)-1,3-propanediol (3c)
dH: 0.902 (3H, t, J 6.5), 1.35 (4H, m, J 3.5), 1.60 (2H, m,
J 3.5), 2.20 (2H, s), 2.58 (2H, t, J 8.0), 3.10 (1H, m, J 6.5), 3.89–
4.02 (4H, m, J 8.0 and 6.0), 7.15 (4H, s). nmax (KBr disc)/cm21
3500–3050 (bonded O–H str.), 2929, 2858, 1513, 1461, 1421,
1122, 1062, 1047, 979, 819, 730. m/z: 222 (M1), 175, 161
(100%), 91.
Methyl formate (15 ml, 0.24 mol) was added dropwise to a
stirred warm (45 uC) mixture of sodium methoxide (23.8 g,
0.44 mol), 2c (48.5 g, 0.22 mol) and toluene (210 ml). The mix-
ture was stirred for 4 h at the same temperature. After cooling
to room temperature, the mixture was poured into stirred
aqueous sulfuric acid (20%, 170 ml), and the separated organic
layer was washed with water (2 6 200 ml) and dried (Na2SO4).
The solvent was removed in vacuo to yield a lavender oil. A
solution of the lavender oil in ethanol (30 ml) was added
dropwise to suspension of sodium borohydride (8 g, 0.21 mol)
in ethanol (220 ml) maintained at 15–30 uC by external cooling.
After the reaction mixture was heated to 55 uC for 4 h,
hydrochloric acid (10%, 120 ml) was added dropwise at a
temperature of less than 55 uC. Following addition of hydro-
chloric acid, the reaction mixture was heated at 55 uC for 1 h
and vacuum distilled to remove ethanol. The residual product
:
2,3-Difluoro-4-pentoxybenzaldehyde (5d)
n-Butyllithium (100 ml, 2.8 M in hexane, 0.28 mol) was added
dropwise to a stirred, cooled (278 uC) solution of compound
4d (50 g, 0.25 mol) in dry THF (400 ml) under an atmosphere of
dry nitrogen. The reaction mixture was stirred (1.5 h), then a
previously cooled (278 uC) solution of DMF (22 ml, 0.28 mol)
in dry THF (50 ml) was added dropwise at 278 uC. The reac-
tion mixture was allowed to warm to room temperature then
stirred (1 h) with hydrochloric acid (10%, 350 ml). The product
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was taken up in ether (2 6 150 ml), and the combined ether
extracts were washed with water (2 6 200 ml) and dried
(Na2SO4). The solvent was removed in vacuo and the pure
product was isolated by recrystallisation from hexane. Yield
33 g (58%), mp 39–41 uC.
2-Fluoro-4-pentylphenylacetonitrile (12)
A solution of compound 10 (22 g, 0.10 mol) in ethanol (15 ml)
was added dropwise to a stirred solution of sodium cyanide
(9.2 g, 0.18 mol) in water (10 ml) and heated under reflux. The
stirred mixture was heated under reflux for a further 4 h, and
then cooled. The product was taken up in toluene (3 6 100 ml),
and the combined toluene extracts were washed with water (3 6
200 ml) and dried (Na2SO4). The solvent was removed in vacuo
and the pure product was isolated by distillation under reduced
pressure. Yield 18 g (86%); bp 110–113 uC/0.6 mmHg.
nmax (film)/cm21: 2942, 2931, 2871, 2254 (CN), 1629, 1581,
1510, 1429, 1257, 1107, 869, 821, 742.
dH: 0.94 (3H, t, J 7.0), 1.47 (4H, m, J 6.6 and 4.2), 1.87 (2H,
m, J 7.0), 4.13 (2H, t, J 6.6), 6.84 (1H), 7.58 (1H), 10.19 (1H, s).
n
max (KBr disc)/cm21: 2943, 2937, 2853, 1689, 1623, 1510, 1471,
1303, 1080, 817, 777, 731.
2-Fluoro-4-pentylphenylmethanol (8)
n-Butyllithium (125 ml, 2.8 M in hexane, 0.35 mol) was added
dropwise to a stirred, cooled (2100 uC) solution of com-
pound 6 (50 g, 0.30 mol) and t-BuOK (40 g, 0.35 mol) in dry
THF (250 ml) and dry hexane (250 ml) under an atmosphere of
dry nitrogen. The reaction mixture was stirred (1.5 h), then
dried formaldehyde gas was introduced slowly with dry
nitrogen at 2100 uC for about 1 h (white solid appeared).
The reaction mixture was allowed to warm to room
temperature then stirred (1 h) with hydrochloric acid (10%,
400 ml). The product was taken up in ether (2 6 150 ml), and
the combined ether extracts were washed with water (2 6
200 ml) and dried (Na2SO4). The solvent was removed in vacuo
and the pure product was isolated by distillation under reduced
pressure. Yield 27 g (53%); bp 110–112 uC/0.5 mmHg.
2,3-Difluoro-4-pentylphenylacetonitrile (13)
The experimental procedure was the same as for the
preparation of compound 12. The following quantities were
used: compound 11 (26 g, 0.11 mol), ethanol (17 ml), sodium
cyanide (10 g, 0.2 mol) and water (11 ml). Yield 18 g (86%); bp
111–113 uC/0.7 mmHg.
nmax (film)/cm21: 2942, 2931, 2861, 2258 (CN), 1637, 1591,
1473, 1228, 1033, 913, 816.
Methyl 2-fluoro-4-pentylphenylacetate (14)
Compound 12 (18 g, 88 mmol) was added dropwise to a stirred
solution of sulfuric acid (10 ml, 98%) in methanol (25 ml) and
heated at 90 uC. The stirred mixture was heated at 95 uC for a
further 6 h and water (100 ml) was added. The product was
taken up in toluene (3 6 50 ml), and the combined toluene
extracts were washed with water (3 6 100 ml) and dried
(Na2SO4). The solvent was removed in vacuo and the pure
product was isolated by distillation under reduced pressure.
Yield 14.7 g (71%); bp 110–113 uC/0.4 mmHg.
n
max (film)/cm21: 361023100 (bonded O–H str.), 2943, 2929,
2858, 1627, 1577, 1508, 1425, 1251, 1114, 1012, 867, 823, 742.
2,3-Difluoro-4-pentylphenylmethanol (9)
n-Butyllithium (150 ml, 2.8 M in hexane, 0.42 mol) was added
dropwise to a stirred, cooled (278 uC) solution of compound 7
(56 g, 0.30 mol) in dry THF (400 ml) under an atmosphere of
dry nitrogen. The reaction mixture was stirred (1.5 h), then
dried formaldehyde gas was introduced slowly with dry
nitrogen at 278 uC for about 1 h (white solid appeared).
The reaction mixture was allowed to warm to room
temperature then stirred (1 h) with hydrochloric acid (10%,
450 ml). The product was taken up in ether (2 6 150 ml),
and the combined ether extracts were washed with water (2 6
200 ml) and dried (Na2SO4). The solvent was removed in vacuo
and the pure product was isolated by distillation under reduced
pressure. Yield 31 g (60%); bp 102–104 uC/0.5 mmHg.
nmax (film)/cm21: 3580–3100 (bonded O–H str.), 2943, 2931,
2861, 1637, 1471, 1286, 1006, 931, 819.
nmax (film)/cm21: 2942, 2931, 2858, 1747, 1629, 1579, 1512,
1434, 1342, 1255, 1110, 1016, 958, 868, 802, 739.
Methyl 2,3-difluoro-4-pentylphenylacetate (15)
The experimental procedure was the same as for the
preparation of compound 14. The following quantities were
used: compound 13 (18.5 g, 83 mmol), sulfuric acid (10 ml,
98%) and methanol (25 ml). Yield 14.7 g (71%); bp 115–117 uC/
0.5 mmHg.
nmax (film)/cm21: 2942, 2931, 2862, 1740, 1630, 1580, 1490,
1471, 1347, 1292, 1159, 1039, 938, 819, 770.
2-(2-Fluoro-4-pentylphenyl)-1,3-propanediol (16)
The experimental procedure was the same as for the
preparation of compound 3c. The following quantities were
used: methyl formate (3 ml, 48 mmol), sodium methoxide (4.8 g,
88 mmol), compound 14 (9 g, 38 mmol), toluene (50 ml),
ethanol (110 ml) and sodium borohydride (2.7 g, 71 mmol).
Yield 2.5 g (28%), mp 51–53 uC.
2-Fluoro-4-pentylphenylchloromethane (10)
A mixture of 8 (26 g, 0.13 mol) and zinc chloride (0.5 g,
3.7 mmol) in hydrochloric acid (37%, 60 ml) was heated under
reflux for 4 h. After cooling to room temperature, the product
was taken up in ether (2 6 150 ml), and the combined ether
extracts were washed with water (2 6 200 ml) and dried
(Na2SO4). The solvent was removed in vacuo to yield a pale-
yellow oil which was purified by column chromatography on
silica gel using light petroleum (bp 60–90 uC) as eluent. A
colourless oil was obtained. Yield 22 g (79%).
dH: 0.89 (3H, t, J 6.8), 1.32 (4H, m), 1.59 (2H, m, J 7.5), 2.09
(2H, s), 2.56 (2H, t, J 7.8), 3.40 (1H, m, J 6.4), 3.95 (4H, m,
J 10.7 and 5.5), 6.90 (2H), 7.15 (1H). nmax (KBr disc)/cm21
:
3500–3100 (bonded O–H str.), 2952, 2928, 2858, 1627, 1573,
1510, 1425, 1369, 1346, 1252, 1117, 1067, 1020, 983, 946, 866,
825, 719.
nmax (film)/cm21: 2942, 2931, 2858, 1627, 1581, 1510, 1427,
1267, 1108, 954, 871, 740, 677.
2-(2,3-Difluoro-4-pentylphenyl)-1,3-propanediol (17)
The experimental procedure was the same as for the
preparation of compound 3c. The following quantities were
used: methyl formate (5.3 ml, 84 mmol), sodium methoxide
(8.5 g, 157 mmol), compound 15 (17 g, 67 mmol), toluene
(90 ml), ethanol (200 ml) and sodium borohydride (5.4 g,
140 mmol). Yield 4.5 g (17%), mp 44–46 uC.
2,3-Difluoro-4-pentylphenylchloromethane (11)
The experimental procedure was the same as for the pre-
paration of compound 10. The following quantities were used:
compound 9 (31 g, 0.15 mol), zinc chloride (0.5 g, 3.7 mmol)
and hydrochloric acid (37%, 70 ml). Yield 26 g (77%). nmax
(film)/cm21: 2942, 2931, 2861, 1637, 1475, 1290, 1034, 819, 700.
dH: 0.89 (3H, t, J 6.8), 1.34 (4H, m, J 3.6), 1.59 (2H, m, J 7.5),
744
J. Mater. Chem., 2003, 13, 742–748

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2.63 (2H, t, J 7.0), 3.42 (1H, m), 3.70 (2H, s), 4.03 (4H, m, J
10.7 and 5.5), 6.92 (2H). nmax (KBr disc)/cm21: 3500–3100
(bonded O–H str.), 2954, 2931, 2863, 1635, 1579, 1498, 1351,
1252, 1113, 1071, 1033, 974, 945, 926, 851, 825, 783.
here all have an unusual core structure: a 1,3-dioxane ring and
two substituted phenyl rings at the 2 and 5 positions.
The transition temperatures for compounds of series A are
given in Table 1. Compounds 183,2, 183,3, 183,4 and 183,5 with
the shortest alkyl chains (m ~ 3) are only nematic (N) and they
are all monotropic except for 183,4. On heating, two peaks
occur before the appearance of the liquid crystal phase in the
DSC trace of compounds 183,3, 183,4 and some other final
compounds reported here. The first peak should be the result of
transition from one crystal (Cr1) to another crystal form (Cr)
because its enthalpy is smaller than that of the second peak and
only one peak appears on cooling from the previous liquid
crystal phase. Moreover, no liquid crystal texture is observed
under a polarizing microscope between these two peaks on
heating. It is interesting that the shortest compound 183,2 (with
the shortest alkyl and alkoxy chains) has the highest melting
point in series A and shows the highest transition temperatures
between the isotropic liquid (Iso) and the nematic phase.
2-(2,3-Difluoro-4-pentoxyphenyl)-5-(4-pentylphenyl)-1,3-
dioxane (185,5
)
A mixture of compound 3c (1.00 g, 4.5 mmol), 5d (1.03 g,
4.5 mmol), dry toluene (50 ml) and p-toluenesulfonic acid
(10 mg) was heated under refux using a Dean and Stark trap to
remove water. After cooling to room temperature, the mixture
was poured into aqueous sodium hydrogen carbonate (10%,
50 ml). The separated organic layer was washed with water (2 6
50 ml) and dried (Na2SO4). The solvent was removed in vacuo
and the residue purified by column chromatography on silica
gel using light petroleum (bp 60–90 uC)–ethyl acetate (10 : 1) as
the eluent, to give pale yellow crystals which were recrystallized
from light petroleum (bp 60–90 uC) twice to yield white needle
crystals of compound 185,5. Yield 0.8 g (41%); purity (HPLC):
99.4%. Elemental analysis (%): calc. (found) for C26H34F2O3: C
72.20 (72.21); H 7.92 (7.94); mp 69.1 uC.
dH: 0.91 (6H, m, J 7.0 and 4.0), 1.32 (4H, m), 1.57 (6H, m),
1.82 (2H, t, J 7.2), 2.58 (2H, t, J 7.5), 3.65 (1H, m), 4.03 (4H, m,
J 8.5 and 3.5), 4.31 (2H, t, J 6.9), 5.80 (1H, s), 6.79 (1H), 7.15
(4H), 7.33 (1H). nmax (KBr disc)/cm21: 2973, 2931, 2872, 2860,
1626, 1517, 1487, 1394, 1317, 1296, 1277, 1146, 1083, 1020, 798.
m/z: 432 (M1), 375, 284, 174, 117 (100%), 91.
Unlike other homologues, compounds 184,2 and 184,3
,
unexpectedly, do not exhibit any observable liquid crystalline
behaviour. When the carbon numbers of both the terminal
alkyl and alkoxy chains (m and n) increase to four, the smectic
A phase begins to appear, whereas compounds 184,4, 184,5 and
185,2 are only monotropic because of their shorter terminal
chains. A typical heating thermogram is shown in Fig. 1 for
compound 185,4. The base line is relatively flat, and three peaks
are observed on both heating and cooling. On cooling, the
nematic phase separated from the isotropic liquid in the form
of oily streaks. Further cooling gives a transition to a smectic A
phase. The focal-conic fan texture of the smectic A phase is
observed for compounds 185,4 under a polarizing microscope
(see Fig. 2). This type of texture is also observed, on cooling,
for all the other compounds having smectic A phases (in
Table 1) under a polarizing light microscope, and the values for
2-(2,3-Difluoro-4-pentoxyphenyl)-5-(2-fluoro-4-pentylphenyl)-
1,3-dioxane (195,5
)
The experimental procedure was the same as for the pre-
paration of compound 185,5. The following quantities were
used: compound 16 (0.30 g, 1.3 mmol), 5d (0.29 g, 1.3 mmol),
dry toluene (50 ml) and p-toluenesulfonic acid (5 mg). Yield
0.2 g (36%); purity (HPLC): 99.2%. Elemental analysis (%):
calc. (found) for C26H33F3O3: C 69.31 (69.12); H 7.38 (7.41);
mp 66.7 uC.
Table 1 Transition temperatures (uC) of series A
dH: 0.92 (6H, m, J 7.0 and 4.2), 1.32 (4H, m), 1.57 (6H, m),
1.82 (2H, t, J 7.2), 2.58 (2H, t, J 7.5), 3.66 (1H, m), 4.11 (4H, m,
J 8.5 and 3.5), 4.32 (2H, t, J 6.5), 5.82 (1H, s), 6.78 (1H), 6.93
(2H), 7.10 (1H), 7.37 (1H). nmax (KBr disc)/cm21: 2971, 2931,
2871, 2860, 1632, 1518, 1485, 1469, 1391, 1325, 1301, 1277,
1148, 1086, 1031, 974, 920, 798. m/z: 450 (M1), 220, 192
(100%), 164, 135, 109.
m
n
Cr1
Cr
SmA
N
Iso
3
3
3
3
4
4
4
4
5
5
5
5
5
5
5
5
5
7
7
7
7
7
7
9
9
9
9
9
9
2
3
4
5
2
3
4
5
2
3
4
5
6
7
8
9
10
5
6
7
8
9
10
5
6
7
$ 97.4
$ 79.4
$ 69.3
$ 79.7
$ 89.3
$ 80.3
$ 78.9
$ 68.6
$ 92.5
$ 59.2
$ 64.2
$ 69.1
$ 63.9
$ 65.7
$ 63.2
$ 67.8
$ 64.0
$ 58.5
$ 56.7
$ 65.9
$ 66.3
$ 69.7
$ 67.1
$ 59.8
$ 54.9
$ 57.1
$ 63.6
$ 70.5
$ 70.5
($ 92.2)
($ 75.0)
$ 87.1
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$ 69.9
$ 59.2
2-(2,3-Difluoro-4-pentoxyphenyl)-5-(2,3-difluoro-4-
)
($ 80.1)
$ 84.2
$ 72.5
$ 57.5
$ 56.0
pentylphenyl)-1,3-dioxane (205,5
The experimental procedure was the same as for the pre-
paration of compound 185,5. The following quantities were
used: compound 17 (0.33 g, 1.3 mmol), 5d (0.29 g, 1.3 mmol),
dry toluene (50 ml) and p-toluenesulfonic acid (5 mg). Yield
0.24 g (39%); purity (HPLC): 99.1%. Elemental analysis (%):
calc. (found) for C26H32F4O3: C 66.65 (66.46); H 6.88 (6.91);
mp 103.5 uC.
dH: 0.93 (6H, m, J 7.2 and 4.5), 1.36 (4H, m), 1.59 (6H, m),
1.84 (2H, t, J 7.1), 2.65 (2H, t, J 7.5), 3.69 (1H, m), 4.09 (4H, m,
J 10.2 and 5.5), 4.36 (2H, t, J 6.5), 5.83 (1H, s), 6.82 (1H), 6.94
(2H), 7.10 (1H), 7.36 (1H). nmax (KBr disc)/cm21: 2953, 2931,
2887, 2865, 1641, 1517, 1487, 1470, 1389, 1317, 1301, 1277,
1156, 1084, 1031, 974, 806. m/z: 468 (M1), 238, 210, 153, 43
(100%).
($ 65.2)
($ 65.6)
($ 62.7)
$ 74.5
$ 83.4
$ 84.2
$ 84.5
$ 85.1
$ 84.7
$ 83.9
$ 83.2
$ 91.6
$ 91.3
$ 92.1
$ 91.8
$ 91.2
$ 90.2
$ 93.4
$ 95.6
$ 94.8
$ 93.7
$ 93.2
$ 92.5
($ 73.9)
($ 69.4)
($ 92.1)
$ 82.5
$ 88.5
$ 88.5
$ 47.6
$ 35.9
$ 45.8
$ 44.0
$ 87.0
$ 85.0
$ 85.2
$ 56.7
$ 60.0
$ 43.0
Results and discussion
8
9
10
Some very interesting and unexpected results have been
achieved through this fundamental research work. Unlike
more conventional liquid crystals, the final materials reported
$ 42.1
$ 41.8
J. Mater. Chem., 2003, 13, 742–748
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Fig. 3 Transition temperature vs. n for compounds 185,n (m ~ 5).
group. In addition to the phase behaviors of these two
interesting compounds mentioned above, the transition
temperatures of the other seven compounds also provide
interesting results to discuss. With the increase of the terminal
alkoxy chain length (n), the melting points of the compounds
varied little except that an obvious odd–even effect was
observed. The transition temperatures between the smectic A
and the nematic phase (or isotropic liquid) varied little with
change in the value of n and so the temperature ranges of
the smectic A phase varied little (about 19 uC and 15 uC,
alternately) except for an obvious odd–even phenomenon.
Increasing the value of n gives rise to little change in the
clearing points of these compounds although the compounds
185,5 and 185,7 do not show any nematic phase.
Fig. 1 Differential scanning thermogram as a function of texture for
the first heating (upper trace) and cooling (lower trace) cycles for 2-(2,3-
difluoro-4-butoxyphenyl)-5-(4-pentylphenyl)-1,3-dioxane (185,4); scan
rate 10 uC min21
.
Fig. 4 shows plots of the transition temperatures as a
function of the alkoxy chain lengths (n) of compounds 187,n and
189,n (m ~ 7 and 9, respectively). It is interesting that there are
several similarities of the phase behavior between the
compounds 187,n and 189,n. All these compounds only show
the smectic A phase. Increasing the length of the terminal
alkoxy chain (n) across each group, the transition temperatures
between the smectic A and the nematic phase varied little (90.2–
95.6 uC). With the increasing of the terminal alkoxy chain
length (n), the melting points of the compounds 187,n increase
steadily and exhibit an obvious odd–even effect, whereas the
melting points of the compounds 189,n first decrease, and then
increase. It is very interesting that the compound 189,6 has the
lowest melting point and the highest transition temperature
between the smectic A phase and the isotropic liquid in series A.
As far as the transition temperatures of all series A
compounds are concerned, the lengths of alkyl chains (m)
and alkoxy chains (n) have a great effect on the transition
behaviors of the liquid crystals. Generally, compounds with
short alkyl chains (m ~ 3, 4) exhibit wider nematic temperature
ranges than compounds with longer alkyl chains (m ~ 5, 7, 9).
This phenomenon is similar to the analogues with the 2,3-
difluorophenyl unit.2,18 Compounds with short alkyl and
alkoxy chains barely exhibit a smectic phase. However, all
the compounds with relatively long chains show wide
smectic temperature ranges. This is due to the larger lateral
Fig. 2 The focal-conic fan texture exhibited by the SmA phase of
compound 185,4 (75 uC, on cooling), 6100.
the transition temperatures obtained by polarizing microscopy
observations agree well (about 1–2 uC) with those values
determined by differential scanning calorimetry for all of the
compounds prepared.
The transition temperatures of compounds 185,n (m ~ 5) are
represented graphically in Fig. 3. The data clearly shows that
all these compounds exhibit a smectic A phase, and most of
them show a nematic phase. The phase behaviours of
compounds 185,2 and 185,3 differ considerably from other
compounds in this group. Compound 185,2 has the highest
melting point, and it is solely monotropic. However, a wide-
range nematic phase (29.4 uC) is shown by compound 185,2 on
cooling, and this stability of the nematic phase is the highest of
all compounds in series A. Its neighboring compound 185,3, on
the contrary, has the lowest melting point in this group, which
is 33.3 uC lower than the melting point of compound 185,2
.
Moreover, its clearing point is the lowest (82.5 uC) in this
Fig. 4 Transition temperature vs. n for compounds 187,n and 189,n (m ~ 7 and 9, respectively).
J. Mater. Chem., 2003, 13, 742–748
746

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Table 4 Transition temperatures (uC) of series C
n
Cr
Iso
2
3
4
5
6
7
8
9
10
$
$
$
$
$
$
$
$
$
115.8
106.7
103.5
104.5
99.4
98.9
96.3
94.8
96.1
$
$
$
$
$
$
$
$
$
Scheme 3 Structures of previously studied materials used in compara-
tive thermal studies.
intermolecular attractive force and smaller terminal intermo-
lecular cohesions.
We have compared the transition temperatures of series A
compounds with two types of known materials of related
structure (Scheme 3, series D and E).2,4,19 Table 2 shows the
transition temperatures for some compounds of series D and E.
In all cases, materials with equivalent alkyl/alkoxy chains were
compared. It is easy to see that the clearing points (Sm/N to
Iso) of series A compounds are much lower than these
previously studied materials (Table 2). The melting point of
series A is much lower than the analogous difluoroterphenyl
derivatives (D), but similar to series E where the two aryl rings
are adjacent. The nematic phase stability of series A is reduced
markedly, but the smectic phase stability is virtually identical to
series E. Moreover, comparing the 5,6 homologues (185,6 and
E5,6), the series A material actually has a higher smectic phase
stability than the series E material. It is very surprising that no
smectic C phase is observed for any of the series A compounds.
In conclusion, the mesophase stability of series A is not as good
as series D and E because of the adverse effect on liquid crystal
phase stability from the breaking up of the aromatic rings by an
unconjugated 1,3-dioxane unit.
Table 2 Transition temperatures (uC) of the previously studied
materials used in comparative thermal studies
Material
m
n
Transition temperatures/uC
D
D
D
D
E
E
E
5
5
5
7
5
5
7
2
6
8
8
6
8
8
Cr 105 SmC 135N 185 I
Cr 97.5 SmC 145.5 SmA 166.0 I
Cr 93.5 SmC 144.0 SmA 148.0 N 159.0 I
Cr 89.5 SmC 148.0 SmA 151.5 N 154.0 I
Cr 62 SmC 81.1 N 139.9 I
Cr 62 SmC 85.8 N 133.9 I
Cr 58 SmC 108.2 N 133.4 I
Table 3 Transition temperatures (uC) of series B
Moving on to the series B and C, the phase behaviours are
much more simple than series A. The transition temperatures
for compounds of series B and C are given in Tables 3 and 4,
respectively, and are represented graphically in Fig. 5. Com-
pounds of these two series were prepared because increasing
one or two fluoro subsitituents in another phenyl ring was
expected to generate very low melting points, almost certainly
lower than for the analogous compounds shown in Table 1;
hence it is very surprising that the melting points of compounds
n
Cr
N
Iso
2
3
4
5
6
7
8
9
10
$ 91.7
$ 79.4
$ 72.4
$ 66.7
$ 65.4
$ 55.9
$ 53.7
$ 56.8
$ 58.9
$
$
$
$
$
$
$
$
$
19m,n and 20m,n are similar to or considerably higher than 18m,n
.
As far as the influence of the terminal alkoxy chain length is
concerned, as the value of n increases, the melting points of the
compounds 205,n decrease steadily and exhibit an obvious odd–
even effect, whereas the melting points of the compounds 195,n
first decrease significantly, and then increase slightly. Unfortu-
nately, these two series compounds either do not show a
($ 51.3)
$ 55.3
($ 52.6)
($ 55.1)
Fig. 5 Transition temperature vs. n for series B and C.
J. Mater. Chem., 2003, 13, 742–748
747

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Table 5 Dielectric anisotropy (De) and birefringences (Dn) of some
compounds of series A and B
Acknowledgements
We express our thanks to Mrs S. Dong, Mrs L. Shi and Mr
Z. Ding for the provision of analytical services.
Material
m
n
Dea
Dnb
A
A
A
A
A
A
B
B
5
5
7
7
9
9
5
5
5
8
5
8
5
8
8
9
21.351
21.333
21.326
21.340
21.335
21.330
20.098
20.103
0.102
0.104
0.106
0.103
0.104
0.107
0.106
0.108
References
1
2
P. Kirsch, M. Heckmeier and K. Tarumi, Liq. Cryst., 1999, 26, 449.
C. C. Dong, P. Styring, J. W. Goodby and L. K. M. Chan,
J. Mater. Chem., 1999, 9, 1669.
3
J. W. Goodby, K. J. Toyne, M. Hird, P. A. Beer, C. C. Dong,
M. E. Glendenning, J. C. Jones, K. P. Lymer, A. J. Slaney,
V. Minter and L. K. M. Chan, Proc. SPIE Int. Soc. Opt. Eng.,
2000, 3955, 2.
aExtrapolated values from 5% w/w solution in the mixture M at
20 uC. bExtrapolated values from 5% w/w solution in the mixture M
at 20 uC and l ~ 589 nm; The mixture M was composed of equal
weight percentages (25%) of 4-pentyloxyphenyl trans-4-ethylcyclohexyl-
carboxylate, 4-propyloxyphenyl trans-4-ethylcyclohexylcarboxylate,
4-pentyloxyphenyl trans-4-propylcyclohexylcarboxylate and 4-propyl-
oxyphenyl trans-4-propylcyclohexylcarboxylate.
4
5
6
G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, J. Chem. Soc.,
Perkin Trans., 1989, 2, 2041.
G. W. Gray, M. Hird and K. J. Toyne, Mol. Cryst. Liq. Cryst.,
1991, 204, 43.
C. C. Dong, M. Hird, J. W. Goodby, S. Peter and K. J. Toyne,
Ferroelectrics, 1996, 180, 245.
mesomorphic phase or show a nematic phase with only a very
narrow temperature range.
7
8
M. Hird, K. J. Toyne and J. W. Goodby, Liq. Cryst., 1996, 18, 1.
M. E. Glendenning, J. W. Goodby, M. Hird and K. J. Toyne,
J. Chem. Soc., Perkin Trans., 1999, 2, 481.
M. E. Glendenning, J. W. Goodby, M. Hird and K. J. Toyne,
J. Chem. Soc., Perkin Trans., 2000, 2, 27.
In order to obtain other properties of these compounds, we
have measured the dielectric anisotropy (De) and birefringences
(Dn) of some compounds. The results of these experiments are
shown in Table 5. As can be seen from the table, the
birefringences of these compounds are low, and the dielectric
anisotropy of all these compounds are negative. Further studies
of the properties of these compounds are currently in progress.
9
10 M. Hird, G. W. Gray and K. J. Toyne, Liq. Cryst., 1992, 4, 531.
11 M. Hird and K. J. Toyne, Liq. Cryst., 1994, 16, 625.
12 M. Hird, K. J. Toyne, A. J. Slaney and J. W. Goodby, J. Mater.
Chem., 1995, 5, 423.
13 M. Klasen, M. Bremer and K. Tarumi, Jpn. J. Appl. Phys., 2000,
39, 1180.
14 R. Asep, Y. Aoki, T. Hirose and H. Nohira, Liq. Cryst., 2001, 28,
785.
15 D. Demus and L. Richter, in Textures of Liquid Crystals,
V. Chemie, Weinheim, New York, 1978.
16 G. W. Gray and J. W. Goodby, in Smectic Liquid Crystals:
Textures and Structures, ed. L. Hill, Heyden, PA, 1984.
17 E. Schwenk and E. Bloch, J. Am. Chem. Soc., 1942, 64, 3051.
18 M. Hird, J. W. Goodby and K. J. Toyne, Proc. SPIE Int. Soc. Opt.
Eng., 2000, 3955, 15.
Conclusions
Forty-seven novel dioxanyl-based compounds were prepared.
Although some of them do not show a mesomorphic phase, the
synthesis and properties of these new compounds give us a
better understanding of the structure–property relationships in
liquid crystals. Many of them exhibit wide temperature range
smectic A and/or nematic phases with negative dielectric
anisotropy and low birefringences. These materials could be
used as suitable low birefringence hosts in reflective displays
and actively addressed devices.
19 V. Reiffenrath, J. Krause, H. J. Plach and G. Weber, Liq. Cryst.,
1989, 5, 159.
748
J. Mater. Chem., 2003, 13, 742–748