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Synthesis and mesomorphic properties of super high
birefringence isothiocyanato bistolane liquid crystals
To cite this Article: Liao, Yung-Ming, Chen, Hsin-Lan, Hsu, Chain-Shu, Gauza,
Sebastian and Wu, Shin-Tson , 'Synthesis and mesomorphic properties of super
high birefringence isothiocyanato bistolane liquid crystals', Liquid Crystals, 34:4, 507
- 517
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Liquid Crystals, Vol. 34, No. 4, April 2007, 507–517
Synthesis and mesomorphic properties of super high birefringence
isothiocyanato bistolane liquid crystals
YUNG-MING LIAO{, HSIN-LAN CHEN{, CHAIN-SHU HSU*{, SEBASTIAN GAUZA{ and SHIN-TSON
WU{
{Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC
{College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA
(Received 12 June 2006; accepted 27 November 2006 )
Four series of high birefringence bistolane liquid crystals containing isothiocyanato termal
groups were synthesized and characterized. As well as the phenyl group, both biphenyl and
naphthyl moieties were introduced to enhance the birefringence. These bistolane compounds
exhibit reasonably low melting points and high birefringence of 0.5–0.8. A eutectic mixture
was formulated from these compounds exhibiting a wide nematic range, high figure-of-merit
and low viscosity.
1.
Introduction
High birefringence (Dn) liquid crystals (LCs) are useful
not only in conventional display devices such as STNLCDs, but also in scattering-type PDLCDs as a
reflective LCD, and in spatial light modulators. They
are also of interest as componens of LCDs; for example,
compensation films for improving the viewing angle,
reflectors and polarizers. A number of LCs have been
studied for these applications [1–3].
It is well known that high Dn values can be achieved
by increasing the molecular conjugation length [2]; a
considerable number of p-conjugated compounds have
been developed as high Dn LCs [4]. Molecules that
contain highly polarizable groups with high electron
densisity, such as benzene rings or acetylene linking
groups, will therefore have large optical anisotropies.
Tolane-based LCs exhibit reasonably high Dn, low
viscosity and good chemical, photo and thermal
stability [5]. Bistolane LCs show birefringences greater
than 0.3 in the visible spectrum and as a result have
attracted particular attention [6–16]. The same properties are also found for isothiocyanates (NCS) [17].
Therefore, coupling of the tolane and NCS groups
could lead to high optical and large dielectric anisotropies while preserving a relatively low viscosity [18].
Several molecular structures with high Dn values, e.g.
diphenyldiacetylene [19, 20], naphthalene tolanes [21]
and thiophenyldiacetylene [22–24] have been widely
studied. The Dn values of these compounds were
*Corresponding author. Email: cshsu@mail.nctu.edu.tw
reported in the range of 0.4–0.6; however, the diacetylene compounds have in adequate UV and thermal
stabilities, and thus their application is limited [25].
In this paper, we report the synthesis procedures and
physical properties of highly birefringent bistolane LCs
having a terminal isothiocyanato group. Phenyl, biphenyl and naphthyl moieties linked by an ethynyl unit
were applied as the core structure. Lateral methyl
substitution in the middle phenyl ring significantly
reduces the melting temperatures of bistolane LCs [14].
By introducing different laterally substituted short alkyl
chains and fluorine at various positions, the synthesized
compounds were characterized with low melting point,
relatively low viscosity, and high optical anisotropy,
ready for immediate practical applications.
2.
2.1.
Experimental
Characterization techniques
1
H NMR spectra were measured with a Varian
300 MHz spectrometer. Infrared spectra were obtained
using a Perkin-Elmer Spectrum One spectrophotometer
in the range of 400–4000 cm21. Differential scanning
calorimetry (DSC) was performed on a Perkin-Elmer
Pyris Diamond DSC instrument at a heating rate of
10uC min21. A Carl-Zeiss Axiophot polarizing microscope equipped with a Mettler FP 82 hot stage and a
Mettler FP 80 central processor was used to observe the
mesomorphic textures. The molecular mass and elemental analysis results were obtained from a T-200 GC/
MS spectrometer and Heraeus CHN-OS RAPID
instrument, respectively.
Liquid Crystals
ISSN 0267-8292 print/ISSN 1366-5855 online # 2007 Taylor & Francis
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DOI: 10.1080/02678290701223954
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508
Y.-M. Liao et al.
For electro-optic measurements, homogenous cells
with cell gap d,8 mm were used. An a.c. voltage with
1 kHz square waves was used to drive the LC cell, whose
inner surfaces were coated with indium tin oxide (ITO)
electrodes. On top of the ITO, the substrate was covered
with a thin polyimide alignment film. The cell was
placed on a heating/cooling stage with a temperature
stability of 0.2uC. Commercial LCs ZLI-1565 and E-63
(from Merck) were used as the host. A conventional
guest–host method was applied to extrapolate the Dn
value at T,23.5uC.
2.2.
Synthesis
The compounds 1–5, 14–18, 29–30 and 39–40 were
prepared by reported methods [14, 26]. As mentioned
above, a large variety of high birefringence compound
Scheme 1.
structures has been investigated. However, in all cases the
synthesis of the final materials is greatly facilitated by the
use of a palladium-catalysed cross-coupling reaction [27–
33]; in the synthesis of some materials this synthesis
procedure is virtually essential. Scheme 1 shows the
synthesis of several isothiocyanato-substituted bistolanes
containing alkyl groups; scheme 2 shows the synthesis of
analogues containing alkoxy groups; scheme 3 shows the
synthesis of similar compounds containing the biphenyl
moiety; and scheme 4 shows the introduction of the
naphthyl moiety.
Despite the development of the palladium-catalysed
cross-coupling reaction to high levels, their exceptional
versatility and their extreme tolerance to a wide range of
functional groups, attempted couplings involving
isothiocyanato substituents resulted in failure [34]. The
Synthesis of compounds 24–28.
509
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Super high birefringence LCs
Scheme 2.
Synthesis of compounds 36–38.
use of thiophosgene and chloroform on an aromatic
amine in the presence of aqueous calcium carbonate is a
very useful and efficient method of introducing the
isothiocyanato group [34, 35] and this method has been
used in this research.
2.2.1. 1-(4-Pentylphenyl)-2-(4-amino-3-methylphenyl)acetylene, 9. Compound 4 (2.74 g, 10 mmol),
Pd(PPh3)2Cl2 (0.14 g, 0.2 mmol), triphenylphosphine
(0.2 g, 0.8 mmol), copper(I) iodide (0.08 g, 0.4 mmol)
and dry triethylamine (150 ml) were mixed and stirred
at room temperature for 30 min under nitrogen. A
solution of compound 5 (1.44 g, 11 mmol) dissolved in
75 ml of triethylamime was added dropwise and the
mixture stirred at 70uC for 24 h. After cooling to room
temperature, the mixture was filtered and filtrate
concentrated in vacuo to remove triethylamine. The
crude product was dissolved in diethyl ether and
extracted with aqueous ammonium chloride solution.
The organic phase was then washed with saturated
aqueous sodium chloride and dried over MgSO4. The
crude product was isolated by evaporating the solvent,
and purified by column chromatography using ethyl
acetate/n-hexane51/4 as eluant to give a deep yellow
solid; yield 2.0 g (72%). 1H NMR (d, CDCl3): 0.87–0.92
(t, 3H), 1.25–1.34 (m, 4H), 1.56–1.63 (m, 2H), 2.10 (s,
3H), 2.58–2.61 (t, 2H), 3.71 (s, 2H), 6.72–7.42 (m, 7H).
IR (KBr) nmax (cm21): 3392, 3180, 2960, 2923, 2860,
2220, 1640, 1462, 1380, 1247, 1070, 823. MS m/z 402
(M+), 345.
2.2.2. 1-(4-Pentylphenyl)-2-(4-iodo-3-methylphenyl)acetylene, 13. Compound 9 (1.95 g, 7 mmol) was
dissolved in THF (10 ml) and cooled to 0uC in an ice
Y.-M. Liao et al.
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510
Scheme 3.
Synthesis of compounds 47–50.
bath. Aqueous HCl (0.8 g, 21.0 mmol) was added and
the mixture stirred for 30 min. Sodium nitrite (0.63 g,
9.1 mmol) dissolved in 5 ml of water was slowly
added to the reaction mixture which was stirred for a
further 30 min. After checking that the pH of the
reaction mixture was acidic, potassium iodide
(1.71 g, 10.5 mmol) dissolved in 8 ml of water was
added, maintaining a temperature of 0–5uC. After
stirring for 3 h at 0–5uC the reaction mixture was
slowly heated to 45uC and held for 5 min. It was then
cooled, treated with aqueous sodium thiosulphate
solution and extracted with n-hexane. The organic
phase was washed with saturated aqueous sodium
chloride and dried over MgSO4. The crude product
was isolated by evaporating the solvent and purified
by column chromatography using ethyl acetate/nhexane51/10 as eluent to give a yellow liquid; yield
2.15 g (79%). 1H NMR (d, CDCl 3): 0.89–0.93 (t, 3H),
1.25–1.38 (m, 4H), 1.58–1.63 (m, 2H), 2.43 (s, 3H),
2.57–2.62 (t, 2H), 6.96–7.60 (m, 7H). IR (KBr) nmax
(cm21): 2980, 2935, 2870, 2200, 1640, 1450, 1366,
1254, 1113, 843. MS m/z 277 (M+), 220.
2.2.3. 2-Methyl-{4-[2-(4-pentylphenyl)-1-ethynyl]-1-ethyny}-3-fluoroaminobistolane, 22. By following the
procedure for compound 9 and using 13 and 15 as
starting materials, compound 22 was obtained as a deep
yellow solid; yield 1.18 g (60%). 1H NMR (d, CDCl3):
0.89–0.92 (t, 3H), 1.24–1.32 (m, 4H), 1.58–1.63 (m, 2H),
2.46 (s, 3H), 2.56–2.62 (t, 2H), 3.68 (s, 2H), 6.52–7.42 (m,
10H). IR (KBr) nmax (cm21): 3376, 3120, 2930, 2903,
2880, 2210, 1650, 1460, 1377, 1240, 1170, 833.
2.2.4. 2-Methyl-{4-[2-(4-pentylphenyl)-1-ethynyl]-1-ethyny}3-fluoroisothiocyanatobistolane, 27. Compound 22
(1.1 g, 2.5 mmol) was dissolved in 15 ml of chloroform
511
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Super high birefringence LCs
Scheme 4.
Synthesis of compounds 71–78.
and added to a stirred, cooled (0uC) mixture of water
(10 ml) calcium carbonate (0.39 g, 3.8 mmol),
chloroform (8 ml) and thiophosgene (0.38 g, 3.3 mmol);
stirring at 0uC was continued for 4 h. The stirred
mixture was allowed to come to room temperature,
heated to 45uC and held for 20 min, then poured into
water. The aqueous layer was washed with
dichloromethane; the combined organic extracts were
washed with 1% aqueous HCl, then washed with water,
brine and dried (MgSO4). The crude product was
isolated by evaporating the solvent, then purified by
column chromatography using ethyl acetate/nhexane51/10 as eluent to give a white solid; yield
0.81 g (80%). 1H NMR (d, CDCl3): 0.86–0.91 (t, 3H),
1.25–1.32 (m, 4H), 1.59–1.63 (m, 2H), 2.47 (s, 3H),
2.58–2.63 (t, 3H), 7.11–7.44 (m, 10H). 13C NMR (d,
CDCl3): 14.0, 20.5, 22.5, 30.9, 31.4, 35.9, 88.5, 90.9,
91.6, 92.7, 119.0, 119.2, 120.1, 120.3, 121.8, 123.4, 123.5,
124.0, 126.3, 128.0, 128.5, 128.8, 131.5, 131.9, 132.5,
140.3, 143.7, 156.4, 159.8. IR (KBr) nmax (cm21): 2950,
2926, 2855, 2212, 2046, 1659, 1510, 1411, 1271, 1035,
835. MS m/z 437 (M+), 380. Elemental analysis: calc. for
C29H24FNS, C 79.60, H 5.53, N 3.20; found, C 79.72, H
5.46, N 3.23%.
2.2.5. 1-(4-Pentylbiphenyl)-2-(4-amino-3-ethylphenyl)acetylene, 41. By following the procedure for compound
9 and using 39 and 40 as starting materials, compound
41 was obtained as a deep yellow solid; yield 2.6 g
(70%). 1H NMR (d, CDCl3): 0.91–0.93 (m, 6H), 1.26–
1.37 (m, 4H), 1.63–1.65 (m, 2H), 2.62–2.68 (t, 2H), 2.84–
2.89 (q, 2H), 3.82 (s, 2H), 7.13–7.75 (m, 11H). IR (KBr)
nmax (cm21): 3363, 3150, 2959, 2927, 2869, 2199, 1661,
1619, 1502, 1463, 1312, 1240, 1004, 891.
2.2.6. 1-(4-Pentylbiphenyl)-2-(4-iodo-3-ethylphenyl)acetylene, 42. By following the procedure for
compound 13 and using 41 as starting material,
compound 42 was obtained as a yellow solid; yield
1.57g (82%). 1H NMR (d, CDCl3): 0.87–0.91 (t, 3H),
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512
Y.-M. Liao et al.
1.19–1.24 (t, 3H), 1.28–1.35 (m, 4H), 1.61–1.66 (m, 2H),
2.61–2.66 (t, 2H), 2.70–2.75 (q, 2H), 7.01–7.79 (m, 7H).
IR (KBr) nmax (cm21): 2955, 2925, 2853, 2210, 1632,
1514, 1495, 1462, 1388, 1260, 1225, 1134, 887.
2.2.7. 2-Ethyl-{4-[2-(4-pentylbiphenyl)-1-ethynyl]-1-ethyny}-2methylaminobistolane, 46. By following the procedure
for compound 9 and using 42 and 18 as starting
materials, compound 46 was obtained as a yellow solid;
yield 1.0 g (63%). 1H NMR (d, CDCl3): 0.88–0.93 (t,
3H), 1.25–1.37 (m, 7H), 1.63–1.68 (m, 2H), 2.17 (s, 3H),
2.62–2.67 (q, 2H), 2.84–2.92 (t, 2H), 3.79 (s, 2H), 7.01–
7.79 (m, 14H). IR (KBr) nmax (cm21): 3357, 2923, 2851,
2213, 1631, 1467, 1355, 1052, 892. MS m/z 481 (M+),
466.
2.2.8. 2-Ethyl-{4-[2-(4-pentylbiphenyl)-1-ethynyl]-1-ethyny}2-methylisothiocyanatobistolane, 50. By following the
procedure for compound 27 and using 46 as starting
material, compound 50 was obtained as a white solid;
yield 0.71 g (81%). 1H NMR (d, CDCl3): 0.86–0.93 (t,
3H), 1.25–1.37 (m, 4H), 1.63–1.68 (m, 2H), 2.17 (s, 3H),
2.62–2.67 (t, 2H), 2.84–2.92 (q, 2H) 7.01–7.79 (m, 14H).
13
C NMR (d, CDCl3): 14.0, 14.6, 18.3, 22.5, 27.6, 31.1,
31.5, 35.6, 89.6, 89.9, 91.1, 93.6, 121.6, 121.9, 122.3,
123.6, 126.1, 126.8, 128.9, 130.0, 130.2, 131.1, 132.0,
132.1, 133.5, 135.2, 136.3, 137.5, 141.1, 142.7, 146.2. IR
(KBr) nmax (cm21): 2964, 2926, 2196, 2070, 1659, 1463,
1408, 1261, 1094, 833. MS m/z 523 (M+), 508. Elemental
analysis: calc. for C37H33NS, C 84.85, H 6.35, N 2.67;
found, C 84.69, H 6.42, N 2.72%.
2.2.9. 2-Bromo-6-ethoxynaphthalene, 51. 6-Bromo-2naphthol (10.0 g, 44.82 mmol), bromoethane (6.35 g,
58.79 mmol),
potassium
carbonate
(12.39 g,
89.64 mmol), potassium iodide (1.48 g, 8.91 mmol) and
acetonitrile (80 ml) were mixed and heated under reflux
with constant stirring for more than 16 h. The solvent
was removed under vacuum and the reaction mixture
poured into water. The product was extracted with
ether (3640 ml) and treated with dilute hydrochloric
acid, washed with water and dried. The crude product
was purified by column chromatography using hexane
as eluent to give a white solid 51; yield 11.2 g (99.5%).
1
H NMR (d, CDCl3): 1.20–1.70 (t, 3H), 3.80–4.30 (t,
2H), 7.05–7.88 (m, 6H). IR (KBr) nmax (cm21): 2963,
2938, 1265, 1064, 852.
2.2.10. 4-[2-(6-Ethoxy-2-naphthyl)-1-ethynyl]-2-ethylaniline,
55. By following the procedure for compound 9 and
using 51 and 40 as starting materials, compound 55 was
obtained as a brown solid; yield: 6.46 g (51.4%). 1H
NMR d51.30–1.50 (t, 3H), 2.45–2.52 (q, 2H), 3.78 (s,
2H), 4.0–4.20 (q, 2H), 7.06–7.91 (m, 9H). IR (KBr) nmax
(cm21): 3359, 3059, 2980, 2934, 2210, 1622, 1599, 1471,
1385, 1256, 1041, 858.
2.2.11. 2-[2-(3-Ethyl-4-iodophenyl)-1-ethynyl]-6-ethoxynaphthalene, 59. By following the procedure for
compound 13 and using 55 as starting material,
compound 59 was obtained as a yellow solid; yield
5.31 g (65.5%). 1H NMR d51.13–1.26 (t, 3H), 1.30–1.50
(t, 3H), 2.45–2.52(q, 2H), 4.00–4.20 (q, 2H), 7.07–7.91
(m, 9H). IR (KBr) nmax (cm21): 2920, 2197, 1626, 1480,
1390, 1258, 1017, 856.
2.2.12. 2-Ethyl-{4-[2-(6-ethoxy-2-naphthyl)-1-ethynyl]}49-amino-39-fluorobistolane, 67. By following the
procedure for compound 9 and using 59 and 15 as
starting materials, compound 67 was obtained as a
brown solid; yield 3.04 g (59.8%). 1H NMR d51.23–1.32
(t, 3H), 1.44–1.50 (t, 3H), 2.81–2.89 (q, 2H), 3.89 (s, 2H),
4.10–4.17 (q, 2H), 7.09–7.95 (m, 12H). IR (KBr) nmax
(cm21): 3337, 2962, 2205, 1630, 1471, 1384, 1254, 1041,
856.
2.2.13. 2-Ethyl-{4-[2-(6-ethoxy-2-naphthyl)-1-ethynyl]}39-fluoro-49-isothiocyanatobistolane, 75. By following the
procedure for compound 27 and using 67 as starting
material, compound 75 was obtained as a white solid;
yield 2.51 g (76.3%). 1H NMR d51.23–1.32 (t, 3H),
1.44–1.50 (t, 3H), 2.81–2.89 (q, 2H), 4.10–4.17 (q, 2H),
7.05–7.95 (m, 12H). 13C NMR (d, CDCl3): 14.5, 14.8,
18.3, 27.6, 31.1, 63.5, 88.9, 90.7, 92.0, 92.3, 106.6, 118.9,
119.8, 120.5, 123.6, 124.2, 126.8, 128.0, 128.4, 128.8,
128.9, 129.3, 131.1, 131.4, 132.2, 134.3, 142.0, 146.4,
156.4, 158.0, 159.8. IR (KBr) nmax (cm21): 2923, 2195,
2093, 1622, 1475, 1390, 1256, 1040, 834. Elemental
analysis: calc. for C37H33NS, C 78.29, H 4.84, N 2.95;
found, C 78.27, H 4.66, N 2.95%.
3.
3.1.
Results and discussion
Thermal transitions and mesomorphic properties
The chemical structure, phase transition temperatures,
associated enthalpies, and optical anisotropy values for
the reported novel compounds and some known
materials for comparison are shown in tables 1–5.
Table 2 summarizes the phase transitions of compounds
24–28. In all these bistolanes, a methyl lateral substituent was introduced at the cental phenyl ring to
obtain lower melting point LCs. All these compounds
exhibit an enantiotropic nematic phase. Compounds
24–27 contain the same terminal isothiocyanato group
on the right-hand side and different alkyl chain length
on the left. Their phase transitions are plotted against
513
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Super high birefringence LCs
Table 1.
Purity and elemental analysis data for some representative bistolanes.
Elemental analysis, found (calc.)/%
Compound
24
25
26
27
28
36
37
38
47
48
49
50
71
72
73
74
75
76
77
78
Purity/%
C
H
N
99.0%
99.0%
98.6%
99.1%
98.7%
98.4%
98.8%
98.8%
99.0%
99.0%
98.8%
99.1%
99.0%
99.3%
99.3%
99.0%
99.4%
98.6%
99.0%
98.6%
78.96(79.17)
79.19(78.95)
79.40(79.77)
79.60(79.72)
83.02(82.71)
76.51(76.08)
73.50(73.37)
73.50(73.15)
84.83(85.04)
81.94(82.17)
84.87(84.47)
84.85(84.69)
81.37(80.93)
81.50(81.22)
81.62(81.90)
81.73(81.26)
78.29(78.27)
78.50(77.94)
78.70(78.29)
78.89(78.53)
4.59(4.40)
4.92(5.06)
5.24(4.96)
5.53(5.46)
6.01(6.22)
5.04(5.39)
4.63(5.02)
4.63(4.74)
6.13(6.01)
5.73(5.74)
6.56(6.50)
6.35(6.42)
5.07(5.06)
5.34(5.36)
5.60(5.43)
5.85(6.04)
4.84(4.66)
4.94(5.22)
5.20(5.07)
5.45(5.61)
3.54(3.52)
3.42(3.48)
3.31(3.18)
3.20(3.23)
3.34(3.25)
3.19(2.91)
3.06(2.96)
3.06(3.15)
2.75(2.81)
2.65(2.45)
2.60(2.76)
2.67(2.72)
3.06(2.99)
2.97(2.92)
2.88(2.81)
2.80(2.62)
2.95(2.95)
2.86(2.79)
2.78(2.77)
2.71(2.72)
the carbon number of the left-hand alkyl chain in
figure 1. Both melting and clearing temperatures
decrease gradually with increasing carbon number.
Compound 28 contains similar structures to those of
compound 27, without an additional lateral fluoro
group at the C-3 position of the right-hand phenyl ring.
For comparison purposes, we synthesized compound 27
which contains a lateral fluoro group at the C-3 position
of the phenyl ring. Its clearing point is lower than those
of compound 28. It seems that the lateral group can
hinder molecular packing and thus decrease transition
temperatures.
Table 2. Phase transition temperature (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 24–28. Cr5crystal, N5nematic, I5isotropic.
Compound
n
X
Y
24
2
H
F
25
3
H
F
26
4
H
F
27
5
H
F
28
5
H
H
Table 3 lists the phase transition temperatures of
compounds 36–38. Their alkyl side chains were changed
to the equivalent alkoxy groups. In all these bistolanes a
fluoro lateral substituent was introduced at the central
phenyl ring, and they all contain the same terminal
iscthiocyanato group. Compound 36 exhibits enantiotropic nematic and smectic phases. The introduction of
a lateral fluoro group at the C-3 position of the phenyl
ring in compound 37 did not suppress the smectic phase
by destroying the symmetry, in fact it enhanced the
melting point to 150uC. The position of the fluorine
atom was changed from C-3 to C-2 to provide a pure
nematic phase in compound 38; as expected, a wide
nematic range from 127 to 259uC was obtained.
Table 3. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 36–38. Cr5crystal, Sm5smectic, N5nematic,
I5isotropic.
heating scan
cooling scan
Cr 162:8ð3:04Þ N 216:8ð0:03Þ I
I 193:8ð0:02Þ N 103:8 ð2:26Þ Cr
Cr 172:2ð3:67Þ N 217:9ð0:01Þ I
I 196:2ð0:01Þ N 130:4 ð3:05Þ Cr
Cr 130:1ð2:79Þ N 202:8ð0:05Þ I
I 202:2ð0:04Þ N 71:1 ð2:05Þ Cr
Cr 103:5ð6:30Þ N 188:3ð0:16Þ I
I 186:7ð0:14Þ N 60:8 ð6:50Þ Cr
Cr 103:1ð5:81Þ N 225:0ð0:22Þ I
I 223:5ð0:24Þ N 79:7 ð4:59Þ Cr
Compound X Y
36
H H
37
H F
38
F H
heating scan
cooling scan
Cr1 96:0ð0:98Þ Cr2 126:2ð1:18Þ SmA 160:8ð0:26Þ N 231:6ð0:11Þ I
I 229:2ð0:02Þ N 157:6ð0:14Þ SmA 123:1ð0:82Þ Cr2 60:8ð0:57Þ Cr1
Cr 150:9ð9:14Þ SmA 174:9ð0:20Þ N 231:6ð0:29Þ I
I 245:9ð0:13Þ N 160:8ð0:18Þ SmA 117:5ð8:76Þ Cr
Cr1 98:0ð2:05Þ Cr2 126:6ð2:01Þ N 259:2ð0:01Þ I
I 252:8ð0:01Þ N 70:4ð1:24Þ Cr
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Y.-M. Liao et al.
Table 4. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol 21) in parentheses, for
Compounds 47–50. Cr5crystal, N5nematic, I5isotropic.
heating scan
cooling scan
Compound X
Y
47
H
H
48
H
F
49
H
C2H5
50
CH3
H
Cr 115:9ð10:08ÞN 241:3ð0:38Þ I
I 234:8ð0:21Þ N 89:7ð15:44Þ Cr
Cr 107:3ð10:82ÞN 241:4ð0:22Þ I
I 229:5ð0:18Þ N 60:8ð9:01Þ Cr
Cr1 44:0ð3:80ÞCr2 80:3ð8:90Þ N 225:6ð2:09Þ I
I 224:2ð1:85Þ N 37:9ð4:57Þ Cr
Cr 114:7ð42:90ÞN 240:9ð0:29Þ I
I 229:4ð0:17Þ N 62:1ð35:03Þ Cr
Table 4 lists the phase transition temperatures of
isothiocyanato biphenyl-bistolanes 47–50. In this series
a pentyl group was placed on the biphenyl ring, and
different lateral groups were introduced on the righthand side containing an isothiocyanato phenyl ring. All
the listed compounds exhibit a pure nematic phase
within the whole mesomprphic region. Moreover, the
smectic phases which normally appear in NCS-based
biphenyl tolanes or tolanes are completely suppressed.
Such an unusual behaviour is believed to be due to the
laterally substituted ethyl chain(s) and/or the single
fluorine atom. The lateral substitutions increase molecular separation and break the molecular symmetry so
that formation of the smectic phase is inhibited.
Table 5. Phase transition temperatures (uC) and corresponding enthalpy changes (kcal mol21) in parentheses, for compounds 71–78. Cr5crystal, N5nematic, I5isotropic.
Compound
n
X
Y
71
2
H
H
72
3
H
H
73
4
H
H
74
5
H
H
75
2
H
F
76
3
H
F
77
4
H
F
78
5
H
F
a
Overlapped transition.
Figure 1. Dependence of transition temperatures on the
number of alkyl chain carbon atoms for compounds 24–27.
Compound 47 shows the highest transition temperature
because of its right-hand side phenyl ring without lateral
substitutents. In compound 48 a fluoro lateral substituent was introduced at the C-3 position of the righthand side phenyl ring, reducing the melting point from
115.9 to 108uC. In compound 49 an ethyl group was
introduced at the C-3 position of the right-hand side
phenyl ring, and both melting and clearing points were
reduced. In compound 50 a methyl lateral substituent
was introduced at the C-2 position of the right-hand
side phenyl ring; the resulting transition temperature
was similar to that of compound 47.
Table 5 summarizes the phase transition temperatures of isothiocyanato naphthyl-bistolanes 71–78. In
this series a pentyl group was placed on the biphenyl
ring, and different lateral groups were introduced on
the right-hand side containing an isothiocyanato
phenyl ring. Compounds 71–74 contain the same
terminal isothiocyanato group on the right-hand side
phenyl ring and alkoxy chains of varying length on the
naphthyl ring. Their phase transitions are plotted
against the carbon number of the alkyl chain in
heating scan
cooling scan
Cr 154:1ð2:85Þ N 278:1ð0:01Þ I
I 274:9ð{Þ N 114:5ð0:71Þ Cr
Cr 137:9ð4:25Þ N 279:5ð0:2Þ I
I 274:9ð{Þ N 137:1ð{Þ Cr
Cr 157:0ð1:96Þ N 266:1ð{Þ I
I 247:7ð{Þ N 104:0ð{Þ Cr
Cr 119:9ð2:0Þ N 187:2ð{Þ I
I 160:8ð{Þ N 60:5ð{Þ Cr
Cr 153:9ð8:66Þ N 283:6ð0:36Þ I
I 282:8ð{Þ N 144:6ð5:94Þ Cr
Cr 144:4ð2:78Þ N 277:7ð{Þ I
I 268:4ð{Þ N 125:8ð2:73Þ Cr
Cr 135:3ð2:32Þ N 259:0ð{Þ I
I 255:3ð{Þ N 78:9ð0:26Þ Cr
Cr 122:3ð3:32Þ N 237:3ð0:13Þ I
I 235:0ð{Þ N 72:8ð2:01Þ Cr
Figure 2. Dependence of transition temperatures on the
number of alkyl chain carbon atoms for compounds 71–74.
515
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Super high birefringence LCs
75–78 contain similar structures to those of compounds 71–74, with an additional lateral fluoro group
at the C-3 position of the right-hand side phenyl ring.
Both melting and clearing points decrease gradually
with increasing carbon number, see figure 3. To
reduce the transition temperatures further, a lateral
fluoro group was introduced at the C-3 position of the
right-hand side phenyl ring, but the transition
temperatures of most of the compounds were not
reduced as expected. Only the melting point of
compound 76 was decreased to that of compound
72, from 157.0 to 135.3uC.
Figure 3. Dependence of transition temperatures on the
number of alkyl chain carbon atoms for compounds 75–78.
figure 2. It can be seen that melting points show an
odd–even effect with increasing carbon number, while
clearing temperatures decrease gradually. Compounds
Table 6.
3.2.
Optical anisotropy
The Dn value, defined as the difference between the two
principal refractive indices of a uniaxial material, was
estimated by a guest–host method. The Dn value of a
guest–host system can be approximated from the
Dn value for single compound. Data calculated from guest-host systems.
Compound
Structure
Dn
28
0.47a
37
0.49a
38
0.49a
47
0.73b
48
0.67b
49
0.55b
50
0.62b
74
0.61b
a
Host ZLI 1132. bHost mixture E-63.
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516
Y.-M. Liao et al.
following equation:
ðDnÞgh ~xðDnÞg zð1{xÞðDnÞh
References
ð1Þ
where the subscripts g, h and gh denote guest, host, and
guest–host cells, respectively; x is the concentration (in
wt %) of the guest compound. By comparing the
measured results for the guest–host mixtures with those
of the host mixture, the Dn values of the guest
compounds can be extrapolated.
The Dn values of some synthesized compounds are
listed in table 6. Commercial LCs ZLI-1132 or E-63
was used as host mixture. It can be seen that the Dn
values of these compounds are in the range 0.5 to 0.8.
For those compounds with biphenyl and naphthyl
moieties, the Dn value is higher than that for the
corresponding phenyl moiety. This is because biphenyl
and naphthyl moieties have a more extended electron
conjugation than the phenyl moiety. The lateral
substitutions affect the Dn values significantly. The
Dn values of compounds 28, 37 and 38, were similar,
approaching to 0.5. Compound 49 has the lowest
birefringence among the compounds 47–50 due to its
two laterally substituted ethyl chains, which would
lead to a higher Dn value due to the molecular packing
density effect. The Dn values of compounds 48, 50 and
74 are 0.67, 0.62 and 0.61, respectively. Compound 47
has the highest Dn of 0.73 and is expected to be useful
for PDLC, cholesteric display, and laser beam steering
applications.
4.
Conclusions
Four series of novel super high Dn bistolane laterally
substituted liquid crystals with terminal isothiocyanato
groups were synthesized. Lateral alkyl or fluoro groups
were introduced to modify the LC properties. The
extrapolated Dn of some of these compounds is greater
than 0.7. Some of these compounds exhibit an odd–even
effect in their phase transition temperatures. A eutectic
mixture consisting of these compounds and some NCS
tolanes was developed. The Dn value was determined by
the guest–host method using a commercial LC as host
matrix. Compounds containing biphenyl and naphthyl
moieties have a high Dn value; compound 47 has the
highest Dn of 0.73, making it a good candidate for many
display applications.
Acknowledgement
The authors would like to thank the National Science
Council (NSC) of the Republic of China (NSC 94-2216E-009-001) for financial support of this research.
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