Synthesis and mesomorphic behavior of two new homologous series containing azobenzene and 1,3,4-oxadiazole units

Article abstract of DOI:10.1080/15421406.2013.840944

Two new nonsymmetrical mesogenic homologous series of terminal substituent ether (series [V_n]) and carboxy (series [V_In]) incorporating azobenzene and 1,3,4-oxadiazole group were synthesized. Both series have been All compounds thus isolated were purified and characterized by elemental analysis, Fourier Transform Infrared Spectroscopy, 1H NMR, along with thermal analysis and texture observation using Differential Scanning Calorimetry (DSC) and Polarizing Optical Microscopy (POM), respectively. All compounds of the first series exhibited liquid crystalline properties. The homologues [V₁]-[V₃] display a nematic mesophase, the compounds [V₄]-[V₇] exhibit a dimorphism behavior, nematic (N) and smectic A (SmA) mesophases, the compounds [V₈] and [VI₉] display enantiotropic smectic A and C (SmA and SmC) phases and the last homologue [V₁₂] exhibits only a smectic C mesophase. All compounds of the second series [V_In] also show mesomorphic behavior, the compounds [VI₁]-[VI₃] and [VI₅] display the nematic mesophase, the homologue [VI₇] exhibits dimorphism behavior, nematic, and smectic A (SmA). The mesomorphic behavior has been analyzed in terms of structural property relationships.

Full text of DOI:10.1080/15421406.2013.840944

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Synthesis and Mesomorphic Behavior of
Two New Homologous Series Containing
Azobenzene and 1,3,4-Oxadiazole Units
Nisreen H. Karama, Amir T. Attoa & Ammar H. Al-dujailia
a Department of Chemistry, College of Education of Pure Science,
Ibn Al-Haytham, University of Baghdad, Baghdad, Iraq
Published online: 15 Dec 2014.
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To cite this article: Nisreen H. Karam, Amir T. Atto & Ammar H. Al-dujaili (2014) Synthesis and
Mesomorphic Behavior of Two New Homologous Series Containing Azobenzene and 1,3,4-Oxadiazole
Units, Molecular Crystals and Liquid Crystals, 605:1, 1-11, DOI: 10.1080/15421406.2013.840944
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Mol. Cryst. Liq. Cryst., Vol. 605: pp. 1–11, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.840944
Synthesis and Mesomorphic Behavior of Two New
Homologous Series Containing Azobenzene
and 1,3,4-Oxadiazole Units
NISREEN H. KARAM, AMIR T. ATTO,
AND AMMAR H. AL-DUJAILI∗
Department of Chemistry, College of Education of Pure Science,
Ibn Al-Haytham, University of Baghdad, Baghdad, Iraq
Two new nonsymmetrical mesogenic homologous series of terminal substituent ether
(series [Vn]) and carboxy (series [VIn]) incorporating azobenzene and 1,3,4-oxadiazole
group were synthesized. Both series have been All compounds thus isolated were puri-
fied and characterized by elemental analysis, Fourier Transform Infrared Spectroscopy,
1H NMR, along with thermal analysis and texture observation using Differential Scan-
ning Calorimetry (DSC) and Polarizing Optical Microscopy (POM), respectively. All
compounds of the first series exhibited liquid crystalline properties. The homologues
[V1]-[V3] display a nematic mesophase, the compounds [V4]-[V7] exhibit a dimorphism
behavior, nematic (N) and smectic A (SmA) mesophases, the compounds [V8] and [VI9]
display enantiotropic smectic A and C (SmA and SmC) phases and the last homologue
[V12] exhibits only a smectic C mesophase. All compounds of the second series [VIn]
also show mesomorphic behavior, the compounds [VI1]-[VI3] and [VI5] display the
nematic mesophase, the homologue [VI7] exhibits dimorphism behavior, nematic, and
smectic A (SmA). The mesomorphic behavior has been analyzed in terms of structural
property relationships.
Keywords Azobenzene; liquid crystals; nonsymmetrical-1,3,4-oxadiazole
Introduction
Many series of liquid crystalline compounds containing heterocyclic units have been syn-
thesized due to their potentially wide range of applications, such as in the optical, electrical,
and biological medical fields [1–4]. Interest in these compounds arises because the inclu-
sion of heteroatom can cause large changes in the kind of mesophase present or in the
physical properties of the phases.
Many series of liquid crystalline compounds containing heterocyclic groups have been
synthesized due to their potentially wide range of applications, such as in the optical, elec-
trical, and biological medical fields [5–7]. Usually five member heterocycles are involved
and they form part of the core in rod-shaped, bent-shaped, or disc-shaped molecules.
∗Address correspondence to Ammar H. Al-Dujaili, Department of Chemistry, College
of Education for Pure Science, Ibn Al-Haytham, University of Baghdad, Baghdad, Iraq; E-mail:
ammar.h.s.@ihcoedu.uobaghdad.edu.iq; ah.aldujaili1946@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
1

2
N. H. Karam et al.
Among the heterocyclic mesogenes, much attention has paid to 2,5-disubstuted 1,3,4-
oxadiazoles owing to their rich mesophases and good thermal stability. In addition, this
type of liquid crystal emits strong fluorescent light with high quantum yield and many
potentially be used as functional materials [8–10].
Cionaca et al. [11] have described the liquid-crystalline properties of some 2,5-
asymmetric disubstituted 1,3,4-oxadiazole derivatives containing an azo and an ester link-
age obtained through esterification of 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-1,3,4-
oxadiazole with a series of 4-(4-alkoxyphenyl)-benzoic acids containing 6–10 and 18
aliphatic carbon atoms. All reported compounds present liquid crystalline properties, with
nematic and smectic C type structures, with very large range of stability of mesophases.
Bai et. al. [12] have reported the synthesis and mesomorphic behavior of a series of
nonsymmetric liquid crystal dimmers with long terminal chains containing 1,3,4-oxadiazole
group and azobenzene group as the mesogenic units. These nonsymmetric liquid crystal
dimers are evidenced to display monolayer smectic A phase. Microphase segregation and
specific intermolecular interactions between the two different mesogenic units are the major
driving forces for the formation of the monolayer smectic A structure.
Continuing our work on the LC possibilities of oxadiazole and thiadiazole systems
[13,14], we describe in this paper the synthesis and liquid crystalline properties of two new
homologous series containing azobenzene and 1,3,4-oxadiazole group.
Experimental
Materials
All chemicals and solvents were of reagent grade (Aldrich Chemicals Co.) and used without
further purification.
Instrumentation
Elemental analyses (CHN) were carried out using a Perkin-Elmer model 2400 instrument.
Infrared spectra were recorded with a Shimadzu 8000 FT-IR spectrophotometer in the
wave number range 4000–400 cm−1 with samples embedded in KBr disc. 1H NMR spectra
were obtained with Bruker spectrometer model ultra shield at 300 MHz. The compounds
were dissolved in DMSO-d6 solution with the TMS as internal standard. Mass spectra
were recorded on IEOL JMS-7 high resolution instrument. Transition temperatures and
enthalpies were scanned in TA instruments Q1000 DSC, differential scanning calorimeter
with a heating rate of 10.0◦C/min in air and it was calibrated with indium (156.6◦C, 28.45
J/g). The temperatures were read as the maximum of the endothermic peaks. The textures
of the mesophases were studied with Olympus BX40 microscope equipped with a Leitz
Laborlux 12 Pols hot stage and PR600 controller. The samples were investigated using cross
polarizers at room temperature with a magnification of 2.5× and a magnification 10× or
20× was at elevated temperature. The samples were mounted on quartz slides of 26 26 mm
with 1 mm thickness. Procedure: The substance to be examined is heated up to the isotropic
liquid point on a microscopic slide and a cover slip is pushed into the melt in such a way
that thin layer is formed between the slide and cover slip. The slide was cooled thoroughly
and placed on the heating stage of the polarizing microscope. The temperature initially
was raised rapidly (5◦C/min) and the slide was then recooled. Afterward, the heating was
then continued at a rate of 1◦C/min and cooling too was carried out at the same rate. The
mesomorphic transition temperatures and the textures under the microscope were recorded
for the compounds.

Synthesis and Mesomorphic Behavior of Two New Homologous Series
3
O
C
OC2H5
H2N
NH2NH2.H2O
O
NH2
NH
C
[I]
H2N
CS2/KOH
N
N
SH
H2N
O
[II]
C4H9Br/KOH
N
N
H2N
SC4H9
O
[III]
+
/NaNO2
H
Phenol
N
N
N
N
HO
SC4H9
O
[IV]
K2CO3/alkyl halide
Pyridine/acid chloride
O
N
N
N
N
N
N
SC4H9
CnH
CnH2n+1
O
N
2n+1 C
O
N
SC4H9
O
O
[Vn]
[VIn]
n =
1-9 or12
n =
1,2,3,5 or 7
Scheme 1.
Synthesis
The reaction sequences leading to the formation of compounds of the two series 2-
butylthio-5-[4-(4’-alkoxyphenyl) azophenyl]-1,3,4-oxadiazol [Vn] and 2-butylthio-5-[4-
(4’-carboxyalkyphenyl) azophenyl]-1,3,4-oxadiazol [VIn] are outlined in Scheme 1, where
n designate the number of carbon atom in terminal alkoxy or carboxyalkyl group
substituent.
Synthesis of 4-aminophenyl hydrazide [I]. This compound was synthesized by the method
described in references [15,16].

4
N. H. Karam et al.
Synthesis of 2-thio-5-[4-aminophenyl]-1,3,4-oxadiazole [II]. This compound was synthe-
sized by the method described in references [17,18].
Synthesis of 2-butylthio-5-(4-aminophenyl)-1,3,4-oxadiazole [III]. Potassium hydroxide
(0.67 g, 12 mmol) dissolved in minimum amount of water, was added dropwise to a
stirred solution of (II) (1.93 g, 10 mmol) in 10 mL of dioxane at 5◦C. After heating the
mixture for 15 min and cooling, n-butyl bromide (1.37 g, 10 mmol) was added dropwise.
The solution was refluxed for 2 hr; afterward the solvent was evaporated on a rotatory
evaporator. Ice water (100 mL) was added; the resulting precipitate was collected, and
recrystallized from ethanol, yield (78%). M.P. 115◦C. Calc. for C12H15ON3S;, C, 57.83; H,
6.02; N, 16.87; Found: C, 57.03; H, 5.94; N, 16.76. FT-IR (KBr, cm−1): 3350–3440 (νNH
,
2
amine); 2990–2950 (νC-H, aliphatic); 1620 (νC=N, oxadiazole ring). 1H NMR (DMSO):
δ = 6.5–7.8 ppm (d-d, C6H4); 6.0–6.5 (b, NH2); 3.1–3.5 (t, S-CH2); 1.3–1.9 (m, CH2CH2);
0.9–1.2 (t, CH3). MS: m/z+1 = 250.
Synthesis of 2-butylthio 5-[4-(4’-hydroxyphenyl) azophenyl]-1,3,4-oxadiazole [IV]. A
mixture of (6.23 g, 25 mmol) of [III] and 15 mL of concentrated H3PO4 was gently
heated and stirred until all was dissolved; it was then cooled to 0◦C and 5 mL of con-
centrated HNO3 was added. Sodium nitrite (1.76 g, 25 mmol) dissolved in a very small
amount of water was added portion-wise. During this addition the temperature was kept
between –5◦C and 0◦C. After vigorously stirring for 10 min, (2.54 g, 27 mmol) of phenol
were added quickly; stirring and the low temperature were maintained for 3 hr. The reac-
tion mixture was then poured over ice, giving the crude product which was then filtered
and recrystallized from ethanol, yield (90%). M.P. 170–173◦C. Calc. for C18H18O2N4S;, C,
61.01; H, 5.08; N, 15.82; Found: C, 60.66; H, 5.94; N,15.25. FT-IR (KBr, cm−1): 3180 (νOH
,
hydroxy); 3010–2990 (νC-H, aliphatic); 1660 (νC=N, oxadiazole ring). 1H NMR (DMSO):
δ = 6.5–7.8 ppm (d-d, C6H4); 6.0–6.5 (b, NH2); 3.1–3.5 (t, S-CH2); 1.3–1.9 (m, CH2CH2);
0.9–1.2 (t, CH3). MS: m/z+1 = 393. MS: m/z+1 = 355.
Synthesis of 2-butylthio-5-[4-(4’-alkoxyphenyl) azophenyl]-1,3,4-oxadiazol [Vn]. A mix-
ture of [IV] (3.54 g, 10 mmol), anhydrous potassium carbonate (5.53 g, 40 mmol), cyclo-
hexane 35 mL and n-alkyl bromide or alkyl iodide (18 mmol) were refluxed and stirred
vigorously. Alkylation was completed after 3 hr. The solution filtered from potassium car-
bonate, the solvent was distilled off, and the product was extracted by adding 25 mL of
10% aqueous KOH and 50 mL of ethyl acetate. The organic layer was dried over anhydrous
MgSO4; ethyl acetate was evaporated to give the compounds [Vn].
n
1
85
2
92
3
98
4
96
5
94
6
96
7
95
8
92
9
90
12
80
Yields (%)
Data Compound [V1]
Yield (85%). M.P. 112oC. Calc. for C19H20O2N4S;, C, 61.96; H, 5.43; N, 15.22; Found:
C, 62.12; H, 4.64; N, 15.80. FT-IR (KBr, cm−1): 2995–2850 (νC-H, aliphatic); 1620 (νC=N
,
oxadiazole ring); 1070 (νC−O−C, OCH3). 1HNMR (DMSO): δ = 7.1–8.1 ppm (d-d, C6H4);
3.83 (s, OCH3); 3.39 (t, S-CH2); 1.2–1.9 (m, CH2CH2); 0.95 (t, CH3). MS: m/z+1 = 369.

Synthesis and Mesomorphic Behavior of Two New Homologous Series
5
Synthesis of 2-butylthio-5-[4-(4’-carboxyalkyphenyl) azophenyl]-1,3,4-oxadiazol [VIn].
To a stirred solution of compound [IV] (3.54 g, 10 mmol), triethylamine (2.02 g, 20 mmol)
in dried mixture of (5 mL DMF + 10 mL THF), was added dropwise carboxylic acid
chloride (10 mmol) at 0–5◦C. After addition has been completed the resulting suspension
was stirred at the same temperature for 3 hr. The triethylamine hydrochloride salt was
precipitated. It was filtered and filtrate was poured with stirring into 100 mL ice water,
then the mixture was extracted by adding 50 mL of diethyl ether. The ether solution was
evaporated to give a residue which was recrystallized from ethanol/water, yield 72–88%.
n
1
2
3
5
7
Yields (%)
83
72
86
88
80
Data Compound [VI1]
Yield (83%). M.P. 119◦C. Calc. for C20H20O3N4S;, C, 60.61; H, 5.05; N, 14.14; Found:
C, 61.51; H, 5.00; N, 13.77. FT-IR (KBr, cm−1): 2950–2850 (νC-H, aliphatic); 1757 (νC=O
,
carboxy); 1070 (νC−O−C, OCH3). 1HNMR (DMSO): δ = 7.1–8.1 ppm (d-d, C6H4); 3.53 (s,
C(O)CH3); 3.39 (t, S-CH2); 1.88 (m, CH2CH2); 0.93 (t, CH3). MS: m/z+1 = 397.
Result and Discussion
Characterization of Compounds
The synthetic routes used for the preparation of the 15 compounds of the two series [Vn] and
[VIn] are outlined in Scheme 1. The elemental analysis of series [Vn] and [VIn] compounds
synthesized above is listed in Table 1. The observed values are in well agreement with theo-
retical values indicating structure of respective compounds. The structures of final products
were confirmed by 1HNMR and FT-IR spectroscopy. In 1HNMR spectra, the synthesized
[Vn] compounds showed peaks of the terminal oxy ether spacer (-OCH2(CH2)nCH3) at
≈4.37–3.59 ppm and for the [VIn] compounds the peaks of the terminal thio ether spacer
(-S(CH2)nCH3) at ≈3.38–1.11 ppm. The IR spectral frequencies of the synthesized [Vn]
and [VIn] compounds were obtained using KBr pellets. For [Vn] compounds the absorption
peaks of C-O-C and C = N were observed at 1070 and 1620 cm−1, respectively, along with
aliphatic C-H stretching frequencies at 2995–2850 cm−1. For [VIn] compounds The absorp-
tion peak of carboxy group was observed at 1757 cm−1 and C-O-C stretching frequencies
occurred at 1070 cm−1 along with aliphatic C-H stretching frequencies at 2950–2850 cm−1
.
Liquid-Crystalline Properties
The mesomorphic properties for these two ([Vn] and [VIn]) series were determined by
means of DSC and POM. DSC is a valuable aid by which phase transition temperature
can be conveniently measured. This techniques offers a direct and complimentary (to
microscopy) evaluation of thermal behavior.
The thermal and thermodynamic data for the compounds of [Vn] and [VIn] series
are gathered in the formalTables 2 and 3, respectively. A plot of transition temperature
against the number (n) of carbon atoms in the lateral alkoxy chain is shown in Figs. 1
and 2. The effects of the terminal chain length on the transition temperatures and phases
behavior observed in this series are in accordance with those observed for conventional

6
N. H. Karam et al.
Table 1. Elemental analysis data for the series [Vn] and [VIn]
Analysis% found (calcd.)
Molecular
formula
Molecular
weight (g/mol)
Compound
C
H
N
[V1]
[V2]
[V3]
[V4]
[V5]
[V6]
[V7]
[V8]
[V9]
[V12]
[VI1]
[VI2]
[VI3]
[VI5]
[VI7]
C19H20O2N4S
C20H22O2N4S
C21H24O2N4S
C22H26O2N4S
C23H28O2N4S
C24H30O2N4S
C25H32O2N4S
C26H34O2N4S
C27H36O2N4S
C30H42O2N4S
C20H20O3N4S
C21H22O3N4S
C22H24O3N4S
C24H28O3N4S
C26H32O3N4S
368
382
396
410
424
438
452
466
480
522
396
410
424
452
480
62.12 (61.95) 4.64 (5.43) 15.80 (15.21)
63.20 (62.82) 5.89 (5.75) 14.51 (14.65)
64.23 (63.63 5.98 (6.06) 14.64 (14.14)
65.17 (64.39) 5.72 (6.34) 14.61 (13.65)
64.42 (65.09) 6.09 (6.60) 12.18 (13.09)
65.20 (65.75) 7.20 (6.84) 12.23 (12.78)
66.67 (66.37) 7.08 (7.07) 12.66 (12.38)
66.88 (66.95) 7.36 (7.29) 12.04 (12.01)
67.99 (67.50) 7.64 (7.50) 11.38 (11.66)
69.34 (68.96) 8.34 (8.04) 11.41 (10.82)
61.51 (60.60 5.00 (5.05) 13.77 (14.14)
60.65 (61.46) 5.71 (5.36) 14.02 (13.65)
62.26 (62.36) 6.39 (5.66) 13.98 (13.20)
62.95 (63.51) 6.88 (6.19) 13.15 (12.28)
64.15 (65.000 6.68 (6.66) 12.20 (11.66)
low molar mass mesogens. A graphical representation of the enthalpy changes for series
[Vn] compounds associated with Cr→N, N→I, SmA→N, Cr→SmA, and Cr→SmC phase
transitions as a function of the number (n) of carbon atoms in the lateral alkoxy chain is
given in Fig. 3. The odd-even effect for N→I and SmA→N transitions was observed.
The enthalpy changes associated with crystallization decreased from 28.29 to 17.01 kJ/mol
indicating effect of chain length on the crystallization behavior. Even though enthalpy
Figure 1. Dependence of transition temperatures on the increasing number of carbon atoms (n) in
the terminal alkoxy chains for the [Vn] series compounds.

Synthesis and Mesomorphic Behavior of Two New Homologous Series
7
Table 2. Phase transition temperatures and enthalpies data of series [Vn] compounds
Compound
Transition
Temperature ◦C
ꢀH kJ mol−1
ꢀS J K−1 mol−1
[V1]
Cr→Cr
Cr→N
80
−
−
112
155
108
160
111
152
108
123
156
100
128
150
104
136
154
109
138
151
96
28.95
0.59
27.49
0.65
26.14
0.97
19.37
4.36
0.79
17.01
3.20
1.19
18.24
3.28
1.39
21.73
2.49
0.99
18.70
2.07
1.27
17.03
4.26
−
75.24
1.40
N→I
[V2]
[V3]
[V4]
Cr→N
72.14
1.54
68.19
2.30
77.16
43.54
1.86
45.65
2.98
2.81
48.46
5.67
3.25
56.95
3.63
N→I
Cr→N
N→I
Cr→SmA
SmA→N
N→I
[V5]
[V6]
[V7]
[V8]
[V9]
[V12]
Cr→SmA
SmA→N
N→I
Cr→SmA
SmA→N
N→I
Cr→SmA
SmA→N
N→-I
2.35
50.66
4.97
Cr→SmC
SmC→SmA
SmA→I
Cr→-SmC
SmC→SmA
SmA→I
Cr→SmC
SmC→I
144
152
90
144
148
74
2.98
46.92
10.23
−
20.10
7.74
57.96
18.45
146
Table 3. Phase transition temperatures and enthalpies data of series [VIn] compounds
Compound
Transition
Temperature ◦C
ꢀH kJ mol−1
ꢀS J K−1 mol−1
[VI1]
Cr→N
N→I
119
158
116
168
112
130
99
121
103
123
112
24.60
4.59
19.25
0.29
17.41
3.06
18.07
0.59
62.77
29.10
49.50
0.66
45.22
23.55
48.55
1.50
[VI2]
[VI3]
[VI5]
[VI7]
Cr→N
N→I
Cr→N
N→I
Cr→N
N→I
Cr→SmA
SmA→N
N→I
16.48
0.58
0.78
43.84
1.47
1.84

8
N. H. Karam et al.
Figure 2. Dependence of transition temperatures on the increasing number of carbon atoms (n) in
the terminal alkoxy chains for the [VIn] series compounds.
changes does not identify the mesophases, it can give an indication about existence different
type of mesophases and further enthalpy changes associated with the crystalline phase are
generally higher than that observed for mesophase as observed here.
All of the compounds in series [Vn] show mesomorphic properties. The first three
members of the series (n = 1–3) display an enantiotropic nematic (N) mesophase. The
member with longest alkoxy chain (n = 4–7) exhibit an enantiotropic dimorphism smectic
A (SmA) – N. For the members with n = 8 and 9 the nematic phase is not observed, those
Figure 3. Dependence of enthalpy changes (ꢀH) associated with transition temperatures on the
increasing number of carbon atoms (n) in the terminal alkoxy chain for the [Vn] series compounds.

Synthesis and Mesomorphic Behavior of Two New Homologous Series
9
Figure 4. Mesophase texture of the N phase obtained on cooling. A schlieren texture and the typical
nematic droplets (200× magnification) for the compound [V3] at 137◦C.
compounds show an anantiotropic dimorphism SmA and smectic C (SmC) mesophases,
the member with n = 12 display only a SmC mesophase.
All the compounds of series [VIn] also show mesomorphic properties. The compounds
with n = 1–3 and 5 display an enantiotropic nematic. The last member (n = 7) exhibits
exhibit an enantiotropic dimorphism nematic and SmA. These results illustrate the impor-
tance of the influence of terminal chain length on the occurrence of smectic order. It is
likely that the lateral interaction giving rise to a layered smectic order is more favored for
the compounds n ≥ 4 (series [Vn]) and compound n = 7 (series [VIn]), due to major volume
occupied by the longer flexible alkoxy chain.
Figure 5. Mesophase texture of the SmA phase obtained on cooling. A typical focal-conic fan texture
(200× magnification) for the compound [VI7] at 120◦C.

10
N. H. Karam et al.
Figure 6. Mesophase texture of the SmC phase obtained on cooling. A broken focal-conic (200×
magnification) for the compound [V12] at 135◦C.
Compounds of series [Vn] and [VIn] have the same central azo rigid core and the
same 2-butylthio-1,3,4-oxadiazole moiety. The difference between those two series is in
the lateral substituted chain. The results show that the alkoxy and carboxy alkyl chain has a
profound influence on the mesomorphic properties. While compounds of series [Vn] exhibit
smectic mesophase starting with compound n = 4, compounds of series [VIn] display a
smectic phase starting with compound n = 7. It seems likely that, in comparison, this is
due to the carbonyl group which is coplanar with the aromatic rigid core; these compounds
therefore have a major conjugated character and a major polarizability, making the lateral
interaction in alkoxy chain more than that with carboxy alky chain which favors mesophase
smectic phases formation [19].
The SmA, SmC, and N phases of the compounds of series [Vn] and [VIn] were de-
termined from textural observations by thermal microscopy under POM using heating and
cooling cycles. Phase transition temperatures observed by POM were found to be in reason-
able agreement with the corresponding DSC thermograms. The mesophases exhibited by
[Vn] and [VIn] compounds were identified from their optical textures, which were observed
by POM, using the classification systems reported by Sackmann and Demus and Richter
[20,21] and Gray and Goodby [22].
The nematic phase showed the typical schlieren texture with characteristic two- and
four-brush singularities and the typical nematic droplets (Fig. 4). The SmA phase was
characterized by the formation of a typical focal-conic fan texture (Fig. 5). Fig. 6 display
the SmC mesophase texture which was identified by the appearance of a broken focal-conic
texture.
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