was added to the organic extracts and the solution cooled to
Results and Discussion
−
20 °C for ca. 2–3 h. The resulting precipitate was collected,
Bu-azo-5-Pyr–4-octyloxybenzoic acid mixtures
washed with light petroleum and dried under vacuum. The
product was recrystallised from ethanol. Yield: 3.93 g, 55%. d
Bu-azo-5-Pyr and 4-octyloxybenzoic acid were miscible over
the complete composition range and this was presumably
attributable to the formation of a hydrogen bond between the
unlike components in the mixture. This view was supported
by IR spectroscopy; specifically, the spectra of the complexes
contain bands centred at ca. 2490 and 1900 cm−1, indicative
of strong hydrogen bonding.10–13 In addition, the carbonyl
band has a shoulder at ca. 1690 cm−1, a characteristic value
for free carbonyl groups. Representative IR spectra of the
individual components and the equimolar complex are shown
as Fig. 1. Unfortunately, the complexity of the spectrum of the
complex is such that it prevents even a semi-quantitative
assessment of the equilibrium constant for the formation of
the complex.
H
(
CDCl ; J values in Hz throughout) 7.9, 7.8, 7.3, 7.0 (m,
3
aromatic, 8H), 4.1 (t, ArOCH , 2H, J 6.3), 3.4 (t, CH Br, 2H,
2
2
J 6.7), 2.7 (t, ArCH , 2H, J 7.8), 1.3–2.0 [m, CH CH CH ,
2
3
2
2
OCH CH ) CH Br, 10H], 1.0 (t, CH , 3H, J 7.3).
2
2 3
2
3
1
-Bromo-6-[4-(4-butylphenylazo)phenoxy]hexane, 6
Compound 6 was prepared using the procedure described for
5
. Yield: 5.54 g, 71%. d (CDCl ) 7.9, 7.8, 7.3, 7.0 (m, aromatic,
H
3
8
H), 4.0 (t, ArOCH , 2H, J 6.4), 3.4 (m, CH Br, 2H), 2.7 (t,
2
2
ArCH , 2H,
J
7.6),
1.3–2.0 [m, CH CH CH ,
2
3
2
2
OCH (CH ) CH Br, 12H], 0.9 (t, CH , 3H, J 7.3).
2
2 4
2
3
Bu-azo-5-Pyr, 1
The dependence of the transition temperatures determined
using diÂerential scanning calorimetry on the mole fraction of
Bu-azo-5-Pyr in the mixture is shown in Fig. 2. Bu-azo-5-Pyr
melts directly into the isotropic phase at 89 °C and can be
supercooled to ca. 70°C without the observation of liquid
crystallinity. Similarly, the mixtures containing greater than
0.5 mol fraction of Bu-azo-5-Pyr do not exhibit liquid crystal-
line behaviour. In comparison, the remaining mixtures are
liquid crystalline; specifically nematic, smectic C and smectic A
phases are observed. These phases were identified on the basis
of the observation of clear characteristic optical textures when
viewed through the polarised light microscope.14,15 Specifically,
A mixture containing 5 (3.8 g, 9.5 mmol), 4-hydroxypyridine
(
0.92 g, 9.7 mmol) and caesium carbonate (15.6 g, 47.7 mmol)
in N,N-dimethylformamide (DMF) (50 ml) was refluxed with
stirring overnight. The reaction mixture was allowed to cool,
poured into ice cold water (1 l) and stirred for ca. 30 min. The
mixture was extracted with chloroform; the organic layer was
washed with water, dried and the chloroform removed using
a rotary evaporator. The crude product was passed through
silica gel using acetone as the eluent. Yield: 0.6 g, 14.9%. Mp
8
5–86 °C. d (CDCl ) 8.4, 7.9, 7.3, 6.9 (m, aromatic, 12H), 4.0
H
3
(
m, 2 ArOCH , 4H), 2.7 (t, ArCH , 2H, J 7.6), 1.2–2.0 [m,
2
2
3
(
CH ) , CH CH , 10H], 1.0 (t, CH , 3H, J 7.3).
2
3
2
2
Bu-azo-6-Pyr, 2
Bu-azo-6-Pyr was prepared using the procedure described for
Bu-azo-5-Pyr. Yield 0.53 g, 9.7%. Mp 118–120°C. d (CDCl )
H
3
8
2
0
.4, 7.8, 7.3, 7.0, 6.8 (m, aromatic, 12H), 4.0 (m, ArOCH , 4H),
2
.7 (t, ArCH , 2H, J 7.6), 1.3–1.9 [t, (CH ) , CH CH , 12H],
2
2 4
2
2
.9 (t, CH , 3H, J 7.3).
3
4
-Octyloxybenzoic acid, 3
4
-Octyloxybenzoic acid (Aldrich) was recrystallised from etha-
nol prior to use. The transition temperatures were in good
agreement with those reported elsewhere:9 crystal–smectic C,
1
01 °C, smectic C–nematic, 108 °C, and nematic–isotropic,
Fig. 1 IR spectra of (a) Bu-azo-5-Pyr, (b) 4-octyloxybenzoic acid and
1
47 °C.
(c) the equimolar complex
Preparation of complexes
The mixtures were prepared by codissolving the components
in pyridine and the solvent allowed to evaporate slowly. The
complexes were dried under vacuum for at least 24 h prior to
characterisation.
Characterisation
The proposed structures of all the compounds were verified
using 1H NMR and IR spectroscopy. 1H NMR spectra were
measured in CDCl on a Bruker AC-F 250 MHz NMR
3
spectrometer. IR spectra were recorded using a Nicolet 205
FTIR spectrometer.
Thermal characterisation
The thermal behaviour of the materials was characterised by
diÂerential scanning calorimetry using a Mettler-Toledo DSC
8
20 system equipped with an intracooler accessory and cali-
Fig. 2 Dependence of the transition temperatures on the mole fraction
of Bu-azo-5-Pyr 1 for mixtures of Bu-azo-5-Pyr and 4-octyloxybenzoic
acid. The broken line represents the melting point; (# )
smectic A–isotropic transition; (1 ) smectic C–nematic transition, (% )
nematic–isotropic transition; (( ) smectic C–smectic A transition. C=
crystal; N=nematic; S =smectic C; S =smectic A; I=isotropic.
brated using an indium standard. The heating and cooling
rates in all cases were 10 °C min−1. Phase identification was
performed by polarised light microscopy using an Olympus
BH-2 optical microscope equipped with a Linkam THMS 600
heating stage and TMS 91 control unit.
C
A
Crystal–crystal transitions have been omitted for the sake of clarity.
1
164
J. Mater. Chem., 1997, 7(7), 1163–1167