New cholesteryl-containing bent core liquid crystals

Article abstract of DOI:10.2298/JSC120810114H

The paper presents the synthesis and mesomorphic behavior of two new series of bent core liquid crystalline compounds based on a 1,3-dihydroxybezene core and containing a cholesteryl 6-oxyhexanoate wing. The two series were obtained by esterification of t

Full text of DOI:10.2298/JSC120810114H

J. Serb. Chem. Soc. 78 (5) 669–680 (2013)
UDC 547.922+547.53:532.783:547.565.2
JSCS–4447
Original scientific paper
New cholesteryl-containing bent core liquid crystals
COSMIN-CONSTANTIN HUZUM, IRINA CARLESCU, GABRIELA LISA
and DAN SCUTARU*
Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical
University of Iasi, 71 D. Mangeron St., 700050 – Iasi, Romania
(
Received 10 August, revised 3 September 2012)
Abstract: The paper presents the synthesis and mesomorphic behavior of two
new series of bent core liquid crystalline compounds based on a 1,3-dihyd-
roxybezene core and containing a cholesteryl 6-oxyhexanoate wing. The two
series were obtained by esterification of the cholesteryl 6-(3-hydroxyphe-
noxy)hexanoate core with some 4-{[4-(n-alkyloxy)phenyl]azo}benzoic acids
(
n-alkyl = n-hexyl – n-dodecyl) or 4-{[4-(n-alkyloxy)benzoyl]oxy}benzoic
acids (n-alkyl = n-hexyl – n-decyl). The esterification reactions were performed
via the corresponding acyl chlorides or with the dicyclohexylcarbodiimide/4-
-
(N,N-dimethylamino)pyridine (DCC/DMAP) system. All the synthesized
compounds evidenced enantiotropic liquid crystalline properties with smectic
type textures when investigated by differential scanning calorimetry and pola-
rized optical microscopy. Isotropization and isotropic to liquid crystal tran-
sitions occurred at relatively low temperatures (between 89 and 146 °C). The
compounds containing the azo-aromatic linking group presented the largest
range of stability of the mesophases (between 42 and 87 °C). All the inves-
tigated compounds were thermally stable in the range of the existence of meso-
phases.
Keywords: liquid crystals; banana shaped; cholesterol; resorcinol.
INTRODUCTION
Liquid crystals have been intensively investigated for the last six decades,
resulting in the discovery of many applications in modern technologies. Choles-
teric liquid crystals (CLCs) are compounds with liquid crystalline properties that
contain at least one chiral carbon atom in their molecule. They are so-called
1
because such properties were first observed in cholesteric esters.
Due to their unique optical properties, such as selective reflection of circular
polarized light, high optical rotatory power and circular dichroism, CLCs have
2
,3
attracted the attention of chemists and physicists.
*
Corresponding author. E-mail: dscutaru@ch.tuiasi.ro
doi: 10.2298/JSC120810114H
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HUZUM et al.
Liquid crystalline compounds containing two identical or non-identical
mesogens united through a flexible spacer of varying length and parity are among
4
the large variety of non-conventional liquid crystals reported to date. Since the
5
discovery of cholesteryl benzoate, the first liquid crystal, more than 3300 mono-
mers, oligomers and polymers with cholesterol have been synthesized and cha-
6
racterized.
Chiral dimesogenic compounds consisting of two different mesogenic units
with at least one containing a cholesteryl unit interlinked through a spacer are a
relatively new class of liquid crystalline compounds that still require more
research in order to establish the relationship between their structure and their
7
liquid crystalline properties. Unsymmetric dimesogenic compounds containing a
cholesteryl esteric moiety and azobenzene mesogenic groups may be suitable for
8
,9
use in liquid crystalline displays and optical–electric functional devices. Mole-
cules containing azo groups show reversible trans/cis isomerization upon UV–
1
0,11
–
Vis irradiation.
Tamaoki and Mallia reported the synthesis of dimesogenic
compounds containing the cholesteryl-azobenzene moiety linked by a flexible
alkyl chain of 6 to 14 carbon atoms in length, the majority of these compounds
1
2
exhibited only the cholesteric mesophase.
Very characteristic for liquid crystals with strong chiral pro-mesogenic units
as in the case of cholesterol with 8 chiral centers) are the blue phases (BP I, BP
(
II, BP III). They appear between the isotropic and chiral nematic or smectic tex-
1
3,14
tures, within a narrow temperature range
(
and the twist grain boundary phase

TGBA, TGBC, TGBC ), observed at the phase transition from isotropic liquid
or chiral nematic to the smectic A or chiral smectic C phases.
1
3
TGB and BP phases are examples of frustrated mesophases and derive from
an antagonistic situation when the molecules are trying to form a helical structure
with the chiral nematic helix axis perpendicular on the long axes of the molecules
simultaneously with their tendency to form a lamellar structure.15 The discovery
of these types of new mesophases raises up the interest for future practical ap-
1
6
plications of liquid crystals containing chiral groups in their molecules.
The new field of bent core liquid crystals has also tried to exploit the pro-
perties that may be induced by the presence of a cholesteric moiety. Generally,
the synthesized structures are quite complex, two typical rigid mesogenic groups
being attached to the core molecule while the cholesteryl part acts more as a ter-
6
minal chain.
In order to develop the class of cholesterol-containing bent core liquid crys-
tals, this paper presents the synthesis, structural characterization and mesomor-
phic properties of two new series of chiral liquid crystals based on a resorcinol
core. In order to obtain as simple as possible bent core liquid crystalline com-
pounds containing cholesteric units, the synthesized compounds possess only one
typical mesogenic wing, formed by two aromatic rings, connected via ester or
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CHOLESTERYL BENT CORE LIQUID CRYSTALS
671
azo linking groups, and containing alkyloxy terminal flexible chains. The other
arm is formed by a cholesteryl unit linked by a pentamethylen flexible spacer to
the core molecule.
EXPERIMENTAL
Materials, instruments and methods
All reagents and solvents were purchased from Aldrich or Merck and were used without
further purification unless otherwise noted. Cholesteryl 6-bromohexanoate was synthesized
1
7
18
accordingly to a literature procedure. The 4-{[4-(alkyloxy)phenyl]azo}benzoic acids and
1
9
the 4-[4-(alkyloxy)benzoyloxy]benzoic acids were obtained by adapting literature data. All
reactions involving dicyclohexylcarbodiimide (DCC) and 4-(N,N-dimethylamino)pyridine
(
DMAP) were performed in anhydrous dichloromethane, under a dry nitrogen atmosphere.
Silica gel 60 (Merck) was used for column chromatography.
®
Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance DRX
4
00 MHz spectrometer. Chemical shifts were reported in ppm relative to tetramethylsilane
®
(
TMS) as an internal standard. The infrared (IR) spectra were recorded using a Nicolet
Magna 550 FT-IR spectrometer (NaCl crystal window). The mass spectra were recorded on a
quadrupole–time of flight mass spectrometer equipped with an electrospray ion source (Agi-
®
lent 6520 Accurate Mass Q-TOF LC/MS). The transition temperatures were determined
using a Linkam heating stage and Linksys 32 temperature control unit in conjunction with an
Axioscop 40 Zeiss polarizing optical microscope and Qimaging/Retiga-1000R camera for
image capture. The transitions were confirmed by DSC analysis (Mettler Toledo DSC1).
-
1
Heating and cooling cycles were run at rates of 10 °C min under a nitrogen atmosphere. The
samples were measured in closed lid aluminum pans. Mesophase type was assigned by visual
2
0
comparison (under the microscope) with known phase standards.
All the thermal gravimetric analyses were performed on 2.5–4.5 mg samples on a
®
e
Mettler-Toledo TGA SDTA851 instrument under a dynamic N atmosphere, flow rate of 20
2
-
1
-1
ml min , at a heating rate of 10 K min from 25 to 900 °C. In order to obtain comparable
data, constant operational parameters were employed for all samples.
The melting points were recorded using a melting point meter Krüss Optotronic KSPI – N
and are uncorrected.
Synthesis of cholesteryl 6-(3-hydroxyphenoxy)hexanoate (1)
A mixture of resorcinol (0.91 g, 8.25 mmol) and K CO (4.55 g, 33 mmol) in 2-butanone
2
3
(
(
40 mL) was stirred at reflux temperature for 30 min and then cholesteryl 6-bromohexanoate
3.1 g, 5.5 mmol) dissolved in 2-butanone (10 mL) was added dropwise with a syringe. The
reaction mixture was kept over night at reflux temperature, cooled down to room temperature,
filtrated and concentrated under vacuum. Purification by column chromatography on silica gel
using a mixture of hexane : ethyl acetate 3:1 as eluent afforded 1 as a pure substance. The
physical and spectral data for 1 are given in the Supplementary material to this paper.
General method for the synthesis of compounds 4a–f
Acyl chlorides 2a–f were prepared from the corresponding acids by reaction with thionyl
chloride and were used immediately for the synthesis of 4a–f. A mixture of one equivalent of
1, 1.08 equivalents of 4-{[4-(alkyloxy)phenyl]azo}benzoyl chloride 2 and tetrabutylammo-
nium hydrogensulfate (TBAHS) in dichloromethane (40 mL) and 1.28 equivalents of potas-
sium carbonate in water (10 mL) were vigorously stirred for 24 h at room temperature. The
organic layer was separated, washed several times with distilled water, dried over anhydrous
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HUZUM et al.
magnesium sulfate and concentrated on a rotary evaporator. Compounds 4a–f were purified
by column chromatography on silica gel using a mixture of dichloromethane : ethyl acetate
2
0:1 as eluent.
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(hexyloxy)phenyl]azo}benzoate (4a)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4-{[4-(hexyloxy)phenyl]azo}benzoyl chlo-
ride (0.130 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40 mL),
and K CO (0.48 g, 0.45 mmol) in water (10 mL).
2
3
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(heptyloxy)phenyl]azo}benzoate (4b)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4-{[4-(heptyloxy)phenyl]azo}benzoyl
chloride (0.135 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40
mL), and K CO (0.48 g, 0.45 mmol) in water (10 mL),
2
3
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(octyloxy)phenyl]azo}benzoate (4c)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4{[4-(octyloxy)phenyl]azo}benzoyl chlo-
ride (0.141 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40 mL),
and K CO (0.48 g, 0.45 mmol) in water (10 mL).
2
3
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(nonyloxy)phenyl]azo}benzoate (4d)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4-{[4-(nonyloxy)phenyl]azo}benzoyl chlo-
ride (0.146 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40 mL),
and K CO (0.48 g, 0.45 mmol) in water (10 mL).
2
3
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(decyloxy)phenyl]azo}benzoate (4e)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4-{[4-(decyloxy)phenyl]azo}benzoyl chlo-
ride (0.151 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40 mL),
and K CO (0.48 g, 0.45 mmol) in water (10 mL).
2
3
3-{[6-(cholesteryloxy)-6-oxohexyl]oxy}phenyl 4-{[4-(dodecyloxy)phenyl]azo}benzoate (4f)
Quantities: compound 1 (0.21 g, 0.35 mmol), 4-{[4-(dodecyloxy)phenyl]azo}benzoyl
chloride (0.162 g, 0.38 mmol) and TBAHS (5.7 mg, 0.017 mmol) in dichloromethane (40 mL)
and K CO (0.48 g, 0.45 mmol) in water (10 mL). The physical and spectral data for 4a–f are
2
3
given in the Supplementary material to this paper.
General method for the synthesis of compounds 5a–e
A mixture of 1 equivalent of 1, 1.1 equivalents of 4-{[4-(alkyloxy)benzoyl]oxy}benzoic
acid and 0.2 equivalents of DMAP dissolved in dry dichloromethane was stirred for a 15–20
min at room temperature, cooled to 0 °C on an ice bath and then 1.2 equivalents of DCC
dissolved in dry dichloromethane were added dropwise. After 30 min, the ice bath was
removed and the reaction mixture was stirred for 48 h at room temperature after which the
precipitated N,Nʹ-dicyclohexylurea (DCU) was filtered off. The solvent was evaporated in
vacuum and the solid residue was chromatographed on silica gel using a 3:1 mixture of hex-
ane:ethyl acetate as eluent. White products were obtained.
4-{[3-((6-(cholesteryloxy)-6-oxohexyl)oxy)phenoxy]carbonyl}phenyl 4-(hexyloxy)benzoate (5a)
Quantities: compound 1 (0.20 g, 0.35 mmol), 4-{[4-(hexyloxy)benzoyl]oxy}benzoic acid
(
0.134 g, 0.39 mmol), DCC (0.087 g, 0.42 mmol), DMAP (0.0085 g, 0.07 mmol), dry di-
chloromethane (40 mL).
4-{[3-((6-(cholesteryloxy)-6-oxohexyl)oxy)phenoxy]carbonyl}phenyl 4-(heptyloxy)benzoate (5b)
Quantities: compound 1 (0.20 g, 0.35 mmol), 4-{[4-(heptyloxy)benzoyl]oxy}benzoic
acid (0.140 g, 0.39 mmol), DCC (0.085 g; 0.41 mmol), DMAP (0.0085 g, 0.07 mmol), dry di-
chloromethane (40 mL).
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CHOLESTERYL BENT CORE LIQUID CRYSTALS
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4-{[3-((6-(cholesteryloxy)-6-oxohexyl)oxy)phenoxy]carbonyl}phenyl 4-(octyloxy)benzoate (5c)
Quantities: compound 1 (0.20 g, 0.35 mmol), 4-{[4-(octyloxy)benzoyl]oxy}benzoic acid
(
0.145 g, 0.39 mmol), DCC (0.085 g, 0.41 mmol), DMAP (0.0085 g, 0.07 mmol), dry dichlo-
romethane (40 mL).
4-{[3-((6-(cholesteryloxy)-6-oxohexyl)oxy)phenoxy]carbonyl}phenyl 4-(nonyloxy)benzoate (5d)
Quantities: compound 1 (0.20 g, 0.35 mmol), 4-{[4-(nonyloxy)benzoyl]oxy}benzoic acid
(
0.150 g, 0.39 mmol), DCC (0.085 g, 0.41 mmol), DMAP (0.0085 g, 0.07 mmol), dry dichlo-
romethane (40 mL).
4-{[3-((6-(cholesteryloxy)-6-oxohexyl)oxy)phenoxy]carbonyl}phenyl 4-(decyloxy)benzoate (5e)
Quantities: compound 1 (0.20 g, 0.35 mmol), 4-{[4-(decyloxy)benzoyl]oxy}benzoic acid
(
0.155 g, 0.39 mmol), DCC (0.085 g, 0.41 mmol), DMAP (0.0085 g, 0.07 mmol), dry dichlo-
romethane (40 mL).
RESULTS AND DISCUSSION
A two-step reaction was necessary to obtain the desired compounds (Scheme
1
). In the first step, cholesteryl 6-(3-hydroxyphenoxy)hexanoate (1) was obtained
by refluxing cholesteryl 6-bromohexanoate and resorcinol (in a 1:1.5 molar ratio)
1
7
in 2-butanone, in the presence of potassium carbonate. In the second step, the
free phenolic group of 1 was esterified with two series of mesogenic acids con-
taining two aromatic rings, connected via an azo or an ester linking group and
containing alkyloxy ending chains. For the esterification reactions, two methods
were used. In the case of the 4a–f series, poor results were obtained when the
more convenient DCC/DMAP system was used. In this case, the esterification
reactions were realized with the corresponding acid chlorides in aqueous K CO /
2
3
/
dichloromethane at room temperature for 24 h, using tetrabutylammonium hyd-
2
1
rogen sulfate (TBAHS) as a phase transfer catalyst. In the case of 5a–e series,
the esterification was performed with DCC and DMAP in dry dichloromethane at
1
9
room temperature for 48 h. All the obtained compounds were purified by co-
lumn chromatography using dichloromethane:ethyl acetate (20:1) or hexane:ethyl
acetate (3:1) as eluents. The yields were similar for both series (around 50 %).
The structure and purity of the obtained final compounds were checked and con-
1
13
firmed by H-NMR, C-NMR, FT-IR and mass spectrometry (data given in the
Supplementary material to this paper).
Taking into consideration the importance of thermal stability for the whole
interval of the existence of the mesophase, thermogravimetric studies were per-
formed for both the 4a–f and 5a–e series. Thermogravimetric data evidenced
very good thermal stability for all the compounds, the Tonset values (temperatures
at which the degradation processes begin) being more than 150 °C higher than
the isotropization values (Tonset values in Tables I and II). These values are
comparable with those other classes of bent core compounds containing azo or
1
8,22
ester linking groups.
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Scheme 1. Synthesis of the cholesteric liquid crystals; 2a–f, X = –N=N–, Z = –Cl, n = 6–10,
2, aq. K CO , TBAHS, 24 h; 3a–e, X = –COO–, Z = – OH, n = 6–10, DCC, DMAP, CH Cl ,
1
2
3
2
2
4
8 h; 4a–f, X = –N=N–, n = 6–10, 12; 5a–e, X = –COO–, n = 6–10; Chol = cholesteryl.
-
1
TABLE I. Transition temperatures (°C), and associated transition enthalpies (J g ) for
compounds 4a–f; abbreviations: Cr, crystalline; LC, liquid crystal; I, isotropic; tonset: the
initial temperature at which the degradations processes begin
-
1
T / °C [ΔH / J g ]
Heating
LC/I
Cooling
LC/Cr
tonset
°C
Cmpd.
Cr/LC
59
Mesophase
I/LC
Mesophase
interval, °C
interval, °C
4a
4b
4c
4d
4e
4f
146
87
131
[1.57]
118
[3.21]
130
[5.28]
125
[4.02]
115
–
–
–
–
327
326
329
318
326
329
[
[
[
[
[
[
–24.94] [–2.45]
83 142
–25.66] [–1.89]
77 133
–22.66] [–6.28]
78 133
–35.58] [–3.66]
80 122
–36.05] [–2.55]
82 97
–39.45] [–1.12]
59
56
55
42
15
–
–
8
117
71
46
[1.26]
34
[19.38]
43
[2.37]
89
[0.55]
[15.08]
The phase transition temperatures and the associated enthalpies for 4a–f are
presented in Table I. During the DSC investigations, compounds 4a–f showed
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CHOLESTERYL BENT CORE LIQUID CRYSTALS
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HUZUM et al.
only two transitions on both the heating and cooling cycles. For the first three
compounds of this series, the crystallization temperature could not be determined
either by DSC or polarized optical microscopy (POM) observations because the
crystallization occurred very slowly. Generally, because of their high viscosity,
all these samples crystallized very slowly at much lower temperatures than the
Cr/Lc temperature transitions.
With the exception of compound 4f, the stability range of the mesophases,
on heating, is reasonably large in the case of the series of compounds 4a–f
(
between 42 and 87 °C).
The DSC curves of compound 4e (first heating and first cooling) are illus-
trated in Fig. 1, as examples. The POM investigations evidenced only the pre-
sence of smectic type mesophases. No typical cholesteric oily streaks textures
could be observed.
Fig. 1. DSC curves for compound 4e: 1 – first heating, 2 – first cooling.
In the case of compound 4e, polarized optical microscopy evidenced a smec-
ticX texture between 80–122 °C during the heating process, (Figs. 2a and 2d). On
cooling, smectic textures appeared again between 115–35 °C, (Figs. 2b and 2c).
The compounds of the 5a–e series, obtained by esterification of cholesteryl
6-(3-hydroxyphenoxy)hexanoate with 4-{[4-(alkyloxy)benzoyl]oxy}benzoic acids
presented enantiotropic behaviors with the existence of a stability range of meso-
phases of around 20 °C on heating (Table II). Compounds 5a–e presented a rich
polymorphism on heating with several crystalline/crystalline or liquid crystal/li-
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CHOLESTERYL BENT CORE LIQUID CRYSTALS
677
quid crystal transitions. On cooling, only two liquid crystal/liquid crystal transi-
tions could be seen for all compounds 5a–e (Table II). As was the case for com-
pounds 4a–f, no typical cholesteric textures could be observed.
(
a)
(b)
(c)
(d)
Fig. 2. Microphotographs of the mesophase textures observed for compound 4e: a) first
heating, 104 °C, b) first cooling, 111 °C, c) first cooling, 42 °C, d) second heating, 105 °C.
For the first three compounds of the series, no crystallization temperatures
could be observed on the DSC curves on cooling, probably for similar reasons as
was the case for compounds 4a–c.
The DSC thermogram of compound 5e, as a typical example for all the com-
pounds of series 5a–e, is presented in Fig. 3. On the second heating curve, the
peaks at 24 and 45 °C corresponded to crystalline/crystalline transitions, while
the peaks at 80, 82 and 100 °C corresponded to different smectic liquid crystal-
line transitions. On cooling, similar transitions at very similar temperatures could
be observed on the DSC curves.
Fig. 3. DSC curves for compound 5e: 1 – first heating, 2 – first cooling.
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HUZUM et al.
Some microphotographs taken during the POM investigations for compound
5
e are presented in Fig. 4.
(
a)
(b)
(c)
(d)
Fig. 4. Microphotographs of the textures observed for compound 5e: a) first cooling, 60 °C,
b) first cooling, 27 °C, c) second heating, 84 °C, d) second heating, 98 °C.
On both the heating and cooling cycles, smectic type textures were evi-
denced (Fig. 4). On the first cooling, the smectic type textures appeared at around
00 °C and remained until crystallization (Figs. 4a and 4b) and on the second
1
heating again the same type of mesophases was seen (Figs. 4c and 4d).
CONCLUSIONS
Two new series of bent core liquid crystals based on a resorcinol core and
containing a cholesteryl moiety connected to the core via a pentamethylen
flexible spacer have been synthesized and characterized. The second arm consists
of two aromatic rings connected via azo or ester groups and containing an alkyl-
oxy terminal chain.
The liquid crystalline properties were investigated by polarized optical mic-
roscopy in association with differential scanning calorimetry. All the synthesized
compounds presented enantiotropic liquid crystalline properties, with smectic
textures. For all synthesized compounds, due to the presence of the cholesteric
moiety, both main liquid crystalline transitions (isotropization and isotropic to
liquid crystal) are relatively low (between 89 and 146 °C). Due to the presence of
the cholesteric moiety, some difficulties were encountered in evidencing the
crystallization temperature on cooling, in both the POM and DSC experiments.
The largest stability range of the mesophases was met in the case of the com-
pounds 4a–f (between 42 and 87 °C).
The tonset values obtained from thermogravimetric studies evidenced a very
good thermal stability for all compounds; the degradation processes beginning
more than 150 °C higher than the isotropization temperatures.
SUPPLEMENTARY MATERIAL
Physical and spectral data for the prepared compounds are available electronically from
http://www.shd.org.rs/JSCS/, or from the corresponding author on request.
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CHOLESTERYL BENT CORE LIQUID CRYSTALS
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Acknowledgements. This work was supported by CNCS –UEFISCDI (Romania), Project
No. PNII – IDEI 356/2008. This paper was realized with the support of EURODOC “Doctoral
Scholarships for research performance at European level” project financed by the European
Social Fond and the Romanian Government.
И З В О Д
НОВИ ТЕЧНИ КРИСТАЛИ КОЈИ САДРЖЕ ХОЛЕСТЕРИЛ-ГРУПУ
COSMIN-CONSTANTIN HUZUM, IRINA CARLESCU, GABRIELA LISA и DAN SCUTARU
Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical University
of Iasi,71 D. Mangeron St., 700050 – Iasi, Romania
У овом раду приказана је синтеза и мезоморфно понашање две нове серије једи-
њења течних кристала базираних на 1,3-дихидроксибензеновом језгру, која садрже холе-
стерил 6-оксихексаноатно крило. Две серије су добијене естерификацијом холестерил-
-6-(3-хидроксифенокси)хексаноатног језгра са 4-{[4-(n-алкилокси)фенил]азо}бензоевим
киселинама (n-алкил = n-хексил – n-додецил) или 4-{[4-(n-алкилокси)бензоил]окси}
бензоевим киселинама (n-алкил = n-хексил – n-децил). Реакције естерификације су
изведене преко одговарајућих ацил-хлорида или са DCC/DMAP системом. Све синтети-
сана једињења су показивала енантиотропне особине течних кристала, са текстурама
смектитног типа, приликом испитивања диференцијалном сканирајућом калориметри-
јом и поларизованом оптичком микроскопијом. Изотропизација и прелаз из изотропне
фазе ка фази течних кристала су биле и на релативно ниским температурама (између 89
и 146 °C). Једињења која садрже азо-ароматичну везујућу групу показују највећи опсег
стабилности мезофаза (између 42 и 87 °C). Сва испитивана једињења су термално ста-
билна у опсегу у којем постоје мезофазе.
(
Примљено 10. августа, ревидирано 3. септембра 2012)
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