Synthesis and liquid-crystalline properties of N,N′-di-(5-(4- alkoxyphenyl)azo)-salicylidene-1,2-phenylene diiminato nickel(II) complexes

Article abstract of DOI:10.1080/15421406.2010.526554

The synthesis, characterization, and liquid-crystal properties of two new tetradentate N,N′-di-(5-(4-alkoxyphenyl)azo)-salicylidene-1,2 phenylene diimine ligands (alkoxy=octloxy, decyloxy) and their nickel(II) complexes are reported. The ligands were prep

Full text of DOI:10.1080/15421406.2010.526554

Mol. Cryst. Liq. Cryst., Vol. 533: pp. 16–26, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.526554
Synthesis and Liquid-Crystalline Properties of
N,N0-di-(5-(4-Alkoxyphenyl)azo)-Salicylidene-
1,2-Phenylene Diiminato Nickel(II) Complexes
ZOLFAGHAR REZVANI,1 KAMELLIA NEJATI,2
ESLAM ALIZADEH,1 AND RAZIE SAMMIMI1
1Faculty of Science, Department of Chemistry, Azarbaijan University of
Tarbiat Moallem, Tabriz, Iran
2Department of Chemistry, Payam Noor University–Tabriz Center,
Emamieh, Tabriz, Iran
The synthesis, characterization, and liquid-crystal properties of two new tetradentate
N,N0-di-(5-(4-alkoxyphenyl)azo)-salicylidene-1,2 phenylene diimine ligands (alkoxy ¼
octloxy, decyloxy) and their nickel(II) complexes are reported. The ligands were pre-
pared by the condensation of the 5-((4-alkoxyphenyl)azo)salicylaldehydes homologous
with 1,2 phenylene diamine. The ligands and their nickel complexes have been character-
ized by infrared (IR), 1H nuclear magnetic resonance (NMR), mass spectroscopy, and
elemental analyses. The liquid-crystalline properties of the ligands and the related nickel
complexes were studied by differential scanning calorimetry (DSC) and by using a
polarizing microscope equipped with a heating and cooling stage. None of the free
ligands exhibited liquid-crystalline behavior but the copper complexes demonstrated
an enantiotropic smectic A mesophase.
Keywords Liquid crystal; nickel complexes; salicylaldimine; Schiff base; smectic
A; tetradentate
Introduction
Metallomesogens occupy a specific place among a large number of presently known
liquid-crystalline substances [1–3]. These liquid crystals are coordination compounds
of metal cations with various organic chelating ligands. Their specific physical
properties give rise to a variety of applications in modern electronics. The chemistry
and physics of metallomesogens have recently been described in several review
articles and books [4–7].
In recent times, azo-containing liquid crystals have been extensively studied
for their various electro-optical characteristics, such as photochromism and photo-
isomerization [8–10]. On the other hand, metallomesogens containing uncomplexed
azo moieties, and therefore keeping their potential for photoisomerization, have
Address correspondence to Zolfaghar Rezvani, Faculty of Science, Department of
Chemistry, Azarbaijan University of Tarbiat Moallem, Km 35 Tabriz-Marageh Road, Tabriz,
Iran. E-mail: z_rezvani@yahoo.com
16

Synthesis and Liquid-Crystalline Properties of Nickel(II) Complex
17
Scheme 1. Synthetic route to the nickel complexes.
been attracting much attention recently due to their possible applications in the area
of photon-mode high-density data storage and photo-switching devices [11–15].
As a part of our systematic investigation [16–21] on structural and thermal
studies of Cuþ2, Niþ2, and VOþ2 metal complexes with azo-linked salicylaldimine
Schiff-base derivatives, we report here the results of our investigation on the
synthesis and liquid-crystalline character of tetradentate Schiff base N,N0-di-(5-(4-
alkoxyphenyl)azo)-salicylidene-1,2 phenylene diimine and the corresponding
nickel(II) complexes (see Scheme 1).
Experimental
Reagents
All reagents and solvents used were supplied by Merck Chemical Company
(Germany) and were used without further purification. 4-Alkoxynitrobenzene
homologues were obtained by reaction between 4-nitrophenol with 1-bromoalkane
in dimethyl formamide (DMF) as solvent and K2CO3 as base by refluxing for

18
Z. Rezvani et al.
3 h [22] and then crude 4-alkoxy nitro benzene homologues were purified by recrys-
tallization from ethanol. 4-Alkoxy aniline homologues were prepared by reducing of
the corresponding 4-alkoxynitrobenzene as described in the literature [23].
Physical Measurements
Elemental (C, H, and N) analyses were carried out on a Perkin-Elmer automatic
equipment model 240B. Electron impact (70 eV) mass spectra were recorded on a
Finnegan-mat GC-MS-DS spectrometer model 8430. Infrared spectra were taken
with a Bruker, vector 22 fourier transform infrared (FTIR) spectrometer using
KBr pellets in the 400–4,000 cmꢀ1 range. The differential scanning calorimetry
(DSC) thermograms of the compounds were obtained on a Mettler-Toledo
DSC 822e module, which was calibrated with indium metal (T ¼ 156.6 ꢁ 0.3,
DH ¼ 28.45 ꢁ 0.6 J gꢀ1). Samples of 2–5 mg in solid form were placed in aluminuꢀm1
pans (40 mL) with a pierced lid and heated or cooled at a scan rate of 10ꢂC min
under a nitrogen flow. Thermogravimetric analysis (TGA) was carried out on a
Mettler-Toledo TGA 851e at a heating rate of 10ꢂC minꢀ1 under a nitrogen atmos-
phere. The optical observations were made with a Zeiss polarizing microscope
equipped with a Linkam THMSG 600 heating and cooling stage and Linkam THMS
1
93 programmable temperature controller. H nuclear magnetic resonance (NMR)
spectra were obtained in deutrated chloroform as solvent on a Bruker FT-NMR
AC-400 (400 MHz) spectrometer. All chemical shifts are reported in d(ppm) relative
to the tetramethylsilane as internal standard. X-ray powder diffraction patterns were
˚
recorded on a Bruker D8 powder diffractometer (Cu Ka : 1.541 A).
Materials
All homologue materials were prepared as described in the literature [24].
5-(4-Decyloxyphenylazo)salicylaldehyde (1A). Yellow, yield 80%, mp 124ꢂC. MS
m=z (relative intensity): 383.4 (M þ 1, 14), 382.3 (M, 38), 121 (M-C10H21OC6H4N2,
100). Anal. Calc. for C23H30N2O3: C, 72.19; H, 7.84; N, 7.32; Found: C, 71.8; H,
1
7.5; N, 7.0. H NMR (400 MHz, CDCl3) d 11.27 (s, H-8), 10.02 (s, H-9), 8.18 (d,
J ¼ 3.2 Hz, H-3), 8.14 (dd, J ¼ 3.1, 8.4 Hz, H-2), 7.91 (dd, J ¼ 3.1, 8.2 Hz, H-4,
H-7), 7.10 (d, J ¼ 8.7, H-1), 7.01 (dd, J ¼ 3.6, 8.3 Hz, H-5, H-6), 4.05 (t, J ¼ 7.1 Hz,
H-10), 1.81–1.00 (19H, alkyl chain). IR (KBr): n ¼ 3,186 (O-H), 3,065 (C-H,
=
=
aromatic), 2,923, 2,957 (C-H, aliphatic), 1,661 (C O), 1,5ꢀ919 (C C, aromatic),
=
1,501 (N N), 1,245 (C-O, ethereal), 1,149 (C-O, phenolic) cm
.
5-(4-Dodecyloxyphenylazo)salicylaldehyde (1B). Yellow, yield 80%, mp 123ꢂC. MS
m=z (relative intensity): 411.6 (M þ 1,13), 410.6 (M, 42), 121 (M-C12H25OC6H4N2,
100). Anal. Calc. for C25H34N2O3: C, 73.06; H, 8.28; N, 6.82; Found: C, 72.8; H,
1
8.2; N, 6.5. H NMR (400 MHz, CDCl3) d 11.26 (s, H-9), 10.02 (s, H-8), 8.15 (d,
J ¼ 2.8 Hz, H-3), 8.13 (dd, J ¼ 2.9, 8.2 Hz, H-2), 7.89 (dd, J ¼ 3.0, 7.9 Hz, H-4,
H-7), 7.11 (d, J ¼ 8.1 Hz, H-1), 7.01 (dd, J ¼ 3.2, 7.9 Hz, H-5, H-6), 4.04 (t,
J ¼ 6.7 Hz, H-10), 1.84–0.87 (23H, alkyl chain). IR (KBr): n ¼ 3,190 (O-H), 3,065
=
=
(C-H, aromatic), 2,921, 2,851 (C-H, aliphatic), 1,661 (C O), 1,600 (C C,
ꢀ1
=
aromatic), 1,500 (N N), 1,245 (C-O, ethereal), 1,148 (C-O, phenolic) cm
.

Synthesis and Liquid-Crystalline Properties of Nickel(II) Complex
Syntheses of the Ligands
19
All homologue materials were prepared in a similar manner. Thus, 0.02 mol of the
1,2-phenylene diamine and 0.04 mol of related 5-(4-alkoxyphenylazo)salicylaldehyde
were dissolved in 100 mL absolute ethanol with a few drops of glacial acetic acid as a
catalyst. The solution was refluxed for 1 h and then left at room temperature. After
cooling, the ligands were obtained as yellow microcrystals. The microcrystals were
filtered off, washed with 15 mL of cold absolute ethanol, and then recrystallized
several times in ethanol-chloroform (1:2, v=v).
N,N0-Di-(5-(4-decyloxyphenyl)azo)-salicylidene-1,2 phenylene diimine (2A). Red,
yield 80%, mp 164.5ꢂC. MS m=z (relative intensity): 857.7 (M þ 1, 13), 856.7 (M,
42), 332.4 (M-2 ꢃ C10H21OC6H5N2, 100). Anal. Calc. for C52H64N6O4: C, 72.84; H,
1
7.47; N, 9.81; Found: C, 72.4; H, 7.0; N, 9.5. H NMR (400 MHz, CDCl3) d 13.85
(s, H-9), 8.81 (s, H-8), 8.03–8.05 (H-2, H-3), 7.90–7.92 (dd, J ¼ 7.8, 2.4 Hz, H-4,
H-7), 7.42–7.45 (H-11), 7.33–7.35 (H-12), 7.20 (d, J ¼ 8.2 Hz, H-1), 7.03 (d,
J ¼ 8.1 Hz, H-5, H-6), 4.07 (t, J ¼ 6.5 Hz, H-10), 0.90–1.88 (19H, alkyl chain). IR
(KBr): n ¼ 3,416 (O-H), 3017 (C-H, aromatic), 2,850–2,920 (C-H, aliphatic), 1,617
=
=
=
(C N), 1,600 (C C, aromatic), 1,502 (N N), 1,252 (C-O, ethereal), 1,149 (C-O,
phenolic) cmꢀ1
.
N,N0-Di-(5-(4-dodecyloxyphenyl)azo)-salicylidene-1,2 phenylene diimine(2B). Red,
yield 77%, mp 154.5ꢂC. MS m=z (relative intensity): 913.7 (M þ 1, 14), 912.7 (M,
40), 332.4 (M-2 ꢃ C12H25OC6H5N2, 100). Anal. Calc. for C56H72N6O4: C, 73.63;
1
H, 7.89; N, 9.20; Found: C, 73.2; H, 7.5; N, 8.9. H NMR (400 MHz, CDCl3) d
13.83 (s, H-9), 8.80 (s, H-8), 8.03–8.05 (H-2, H-3), 7.90–7.92 (dd, J ¼ 7.7, 2.4 Hz,
H-4, H-7), 7.42–7.45 (H-11), 7.33–7.35 (H-12), 7.19 (d, J ¼ 8.1 Hz, H-1), 7.03 (d,
J ¼ 8.1 Hz, H-5, H-6), 4.07 (t, J ¼ 6.5 Hz, H-10), 0.89–1.88 (19H, alkyl chain). IR
(KBr): n ¼ 3,418 (O-H), 3,016 (C-H, aromatic), 2,852–2,922 (C-H, aliphatic), 1,619
=
=
=
(C N), 1,600 (C C, aromatic), 1,502 (N N), 1,251 (C-O, ethereal), 1,148 (C-O,
phenolic) cmꢀ1
.
Syntheses of the Nickel Complexes
Nickel complexes were prepared in a similar manner using the method described
elsewhere [17]. Thus, a solution of 0.004 mol of Ni(OAc)2 ꢄ 4H2O in 10 mL of ethanol
was added to an ethanol-chloroform (1:1 v=v) solution containing 0.004 mol of
ligand and refluxed for 2 h. The obtained solution was left at room temperature.
Nickel complexes were obtained as red microcrystals. The microcrystals were filtered
off, washed with absolute ethanol, and then recrystallized from ethanol-chloroform
(1:3 v=v).
3A. Red, yield 77%. Anal. Calc. for C52H62N6O4Ni.1.5C2H6O (981.7): C, 67.23; H,
7.23; N, 8.56; Found: C, 66.8; H, 6.8; N, 8.2. IR (KBr): n ¼ 3,417 (O-H, alcoholic),
3,223 (OH, phenolic), 3,069 (C-H, aromatic), 2,955, 2,918, 2,849 (C-H, aliphatic),
=
=
=
1,638 (C N), 1,604 (C C, aromatic), 1,503 (N N), 1,251 (C-O, ethereal), 1,152
(C-O, phenolic) cmꢀ1
.
3B. Brown, yield 80%. Anal. Calc. for C56H70N6O6Ni.1.5C2H6O: C, 68.23; H, 7.61;
N, 8.09; Found: C, 67.8; H, 7.2; N, 7.7. IR (KBr): n ¼ 3,417 (O-H, alcoholic), 3,223
(OH, phenolic), 3,069 (C-H, aromatic), 2,955, 2,918, 2,849 (C-H, aliphatic), 1,638

20
Z. Rezvani et al.
=
=
(C N), 1,604 (C C, aromatic), 1,503 (N N), 1,251 (C-O, ethereal), 1,152 (C-O,
=
phenolic) cmꢀ1
.
Results and Discussion
Synthesis
The Schiff-base ligands were synthesized in a four-step process, in which the
hydroxyl group in 4-nitrophenol is first replaced by an alkoxy chain followed by
reduction of the nitro group to an amine. In the third step, salicylaldehyde is coupled
with the diazonium chloride obtained from the 4-alkoxyaniline. Subsequent reaction
of the thus-formed 5-(4-alkoxyphenylazo) salicylaldehyde with 1,2-phenylene
diamine (Scheme 1) by refluxing in absolute ethanol using a few drops of acetic acid
as catalyst afforded the Schiff-base ligands, which were purified by repeated crystal-
lization from an ethanol=chloroform mixture. 5-(4-decyloxyphenylazo) salicylalde-
hyde (1A), 5-(4-Dodecyloxyphenylazo)salicylaldehyde (1B), and the Schiff-base
1
ligands (2A and 2B) were characterized by IR and H NMR spectroscopy, mass
1
spectrometry, and elemental analyses. The H NMR proton numbering scheme of
1A, 1B, 2A, and 2B is represented in Scheme 1. The nickel complexes were character-
ized by C, H, N elemental analysis and IR spectroscopy. Some physical and charac-
terization data for the ligands and complexes are given in the Experimental section.
The elemental analyses and spectroscopic data for the new compounds are in good
agreement with the proposed formula.
In the compounds 1A and 1B, the hydroxyl group was observed at 3,186–90 cmꢀ1
because the intramolecular hydrogen bonding between OH and the formyl group in
compound 1 leads to decreasing the stretching frequency of OH, and the carbonyl
group was observed in the region 1,660–1,666 cmꢀ1 [21]. The total absence of
=
n(C O) absorption in the IR spectra of 2A and 2B together with the presence of
ꢀ1
=
new n(C N) absorption at 1,617–19 cm clearly indicated that a new Schiff-base
ligand had formed in each case. Comparison of the IR spectra of the metal complexes
=
with those of the free ligands shows that the stretching frequency of the C N bond is
shifted to lower wavenumbers (11–12 cmꢀ1) after coordination. This shift is due to
=
reduction of the double-bond character of the C N bond as a result of coordination
of the nitrogen to the metal center and is in agreement with the results obtained for
other similar complexes described previously [25–27]. The broad band of medium
intensity in the 3,404–3,450 cmꢀ1 region is due to the O-H stretching vibration of
the lattice ethanol. Based on these observations and the results of elemental analyses,
we conclude that the Schiff-base ligands are coordinated to metal atoms as tetraden-
tate (ONNO) ligands in a 1:1 ratio.
Mesomorphism
The thermal stability of these materials was checked by TGA. All other thermal char-
acterizations were then carried out below the decomposition temperature. Figures 1a
and 1b show the TGA and DSC thermograms of compound 2A at a heating rate of
10ꢂC=min. Compound 2B has similar DSC and TGA thermograms. For Schiff-base
ligands (2A, 2B) the decomposition process starts at 220ꢂC as confirmed by TGA and
DSC thermograms. The mesomorphic properties of 5-(4-alkoxyphenylazo)
salicylaldehyde homologues (1A, 1B), the Schiff ligands (2A, 2B), and related Ni(II)

Synthesis and Liquid-Crystalline Properties of Nickel(II) Complex
21
Figure 1. (a) TG curve of 2A and (b) DSC curve of 2A.
complexes (3A, 3B) have been studied by polarizing optical microscopy observations
using a heating–cooling stage and the phase transition temperatures and enthalpies
were obtained by DSC. Compounds 1A, 1B, 2A, and 2B were not mesomorphic.
These compounds clearly melted and transformed into isotropic liquids and no meso-
phase was observed in the heating or cooling cycle on optical observations.
The thermal gravimetry (TG) and differential thermal gravimetry (DTG) curves
for the nickel complexes (3A and 3B) are shown in Figs. 2a and 2b. The thermal beha-
vior of two nickel complexes is similar. Two major mass loss steps are observed for
each complex. The first mass loss stage has a decomposition temperature range of
90–130ꢂC, with a mass loss of 7.68% for 3A and 6.72% for 3B, which corresponds
to the loss of 1.5 molecules of lattice ethanol (calcd. 7.03% for 3A and 6.65%
for 3B). The percentages of calculated mass losses are in a good agreement with
experimental values. The second mass loss step within the temperature range of
335ꢂC–440ꢂC is attributed to decomposition of the complex.
The nickel complexes (3A, 3B) display liquid-crystalline character as revealed by
polarizing optical microscopy and DSC thermograms. Phase transition temperatures

22
Z. Rezvani et al.
Figure 2. TG and DTG thermogram of (a) 3A and (b) 3B.
along with the corresponding enthalpy values for the Schiff-base ligands (2A, 2B)
and the nickel complexes (3A, 3B) are summarized in Table 1. Figure 3 shows a
complete set of DSC curves for the nickel complexes.
The thermal behaviors of the complexes are similar as is apparent from the DSC
heating scan curves. Here we describe the behavior of compound 3A as a represen-
tative. Compound 3A showed four endothermal peaks at 110.4ꢂC, 182.8ꢂC, 315.9ꢂC,
and 325.9ꢂC (Fig. 3a) in the first heating scan.
The first peak at 110.4ꢂC associated with the TG mass loss within the tempera-
ture range 90ꢂC–130ꢂC is attributed to the removal of lattice ethanol molecule. The
peak at 182.8ꢂC with DH values of 25.4 kJ molꢀ1 is related to solid–solid transition.
The two endothermal peaks at 315.9ꢂC and 325.9ꢂC are attributed to the crystal-to-
mesophase and mesophase-to-isotropic liquid transitions, respectively. The texture
of the mesophase was investigated by polarizing optical microscopy (POM). For

Synthesis and Liquid-Crystalline Properties of Nickel(II) Complex
23
Table 1. Thermal transitions and thꢀer1modynamic data of ligands and Ni complexes,
heating and cooling rate: 10ꢂC min
Phase transitions (ꢂC) and corresponding
enthalpies (kJ molꢀ1, in parentheses)a
First
heating scan
First
cooling scan
Second
heating scan
Compound
2A
2B
3A
Cr1 112(3.2) Cr2 164.5(51)I
Cr1 109(3.1) Cr2 154.5(47)I
Cr1 100.2, 110.4b (54.8) Cr2
182.8(25.4) Cr3 315.9(20.3)
SmA 325.9(5.2)I
I, 323.9(4.6) SmA
304.2(4.7) Cr3
Cr3 314.9(17.6)
SmA 323.2(3)I
3B
Cr1 118.3(60) Cr2 154.2(17.8)
Cr3176.6(2.6) Cr4 304.9(19.5)
SmA 315.2(4.85)I
I, 314.3(5) Cr6 106.4(1.98)
SmA 304.2(20) Cr6 303.4(19.33)
Cr5 63.5(4.7) Cr6 SmA 312.2(4.5)I
aCr ¼ crystal; I ¼ isotropic.
bOverlapped with a previous transition.
two nickel complexes, a viscous mesophase appears as a focal conic texture after
melting process or cooling from the isotropic liquid, which indicates the formation
of an enantiotropic smectic A(SmA) mesophase. Continued heating of complexes
3A or 3B causes a considerable amount of decomposition at around 335ꢂC, which
was detected by exothermic peaks in the DSC trace, TGA thermograms, and optical
observations. In the first cooling scan, Compound 3A showed two exothermal peaks
at 323.9ꢂC and 304.2ꢂC (Fig. 3a), corresponding to the isotropic liquid to the smectic
A mesophase and the smectic A mesophase to the crystalline phase, respectively. In
the second heating scan, two endothermal peaks at 314.9ꢂC and 323.2ꢂC with
enthalpy values of 17.6 and 3 kJ molꢀ1 are ascribed to the crystal-to-mesophase
and mesophase-to-isotropic liquid transition. The structural changes during
desolvation and solid–solid transitions have been confirmed by using powder
X-ray diffraction. A microscopic picture from the SmA mesophase for complex 3A
is illustrated in Fig. 4 as a representative example.
These complexes represent a further example of a mesomorphic complex derived
from a nonmesogenic ligand. Based on our knowledge, nickel complexes based on
tetradentate salicylidenic ligands containing an uncomplexed azo moiety with a
1,2-phenylene diamine bridge have not been reported as liquid crystalline materials
up to the present. However, the nickel complexes reported in this work are the first
example of metallomesogens based on an azo-linked salicylidenic Schiff-base ligand
system with a 1,2-phenylene diamine bridge. This complex represents a further
example of a mesomorphic complex derived from a nonmesogenic ligand.
Recently, some mesogenic nickel complexes of a similar structure to the title com-
plexes of this work have been reported. We have recently reported the investigation of
liquid-crystalline properties of bis(alkoxyphenylazo)-substituted N,N0-salicylidene dii-
minato Ni(II), Cu(II), and VO(IV) complexes based on 1,2-diaminoethane and 1,3-
diaminopropane bridges [17]. These complexes exhibitd smectic A liquid-crystalline
character and undergo a considerable amount of decomposition near the transition
to the isotropic liquid. Similar mesomorphic properties were previously reported by

24
Z. Rezvani et al.
Figure 3. DSC thermograms of (a) 3A and (b) 3B.
Aiello et al. [28] for nickel complexes with a similar coordination environment based
on tetradentate salicylaldimine Schiff-base ligands but with a 4-alkyphenylazo linkage.
In the case of the nickel complexes reported in Aiello et al. [28], the complexes undergo
a considerable amount of decomposition near the transition to the isotropic.
In addition, Singh et al. [29] have reported the nematic liquid-crystalline properties
of nickel complex with a N,N0-di-4-(40-entyloxybenzoate)salicylidene diaminoethane
Schiff-base ligand. This complex exhibits low transition temperatures in comparison
with the title complexes of this work and undergo decomposition near the transition
to the isotropic liquid. Finally, Aiello et al. [30] reported a new type of zinc(II)
complexes derived from tetradentate salen-like Schiff-base ligands containing

Synthesis and Liquid-Crystalline Properties of Nickel(II) Complex
25
Figure 4. Optical texture observed for the copper complex 3a through crossed polarizers at
320ꢂC.
uncomplexed alkylphenylazo moiety with 1,3-diaminopropane and 1,3-diamino-2,2-
dimethylpropane bridges. These complexes show nematic and smectic C(SmC) meso-
phases [30]. However, the complexes described in this work are more thermally stable
than those based on ethylene and trimethylene bridges. This fact may be related to the
presence of the phenylene bridge in the structure of complexes.
Conclusion
In this work we have prepared two nickel complex homologues derived from tetra-
dentate N,N0-di-(5-(4-alkoxyphenyl)azo)-salicylidene-1,2 phenylene diamine ligands.
Though the Schiff-base ligands do not show any liquid-crystalline character, the
nickel complexes have mesomorphic properties and form a smectic A mesophase
as confirmed by the microscopic textures. For two nickel complexes decomposition
occured several degrees after the clearing point. The mesophases have a narrow tem-
perature range. The nickel complexes undergo a solid–solid transition before melting
to the liquid-crystalline phase as confirmed by powder X-ray diffraction patterns.
Acknowledgment
The authors sincerely appreciate the financial support from Azarbaijan University of
Tarbiat Moallem, Research Vice Chancellor, and Faculty of Basic Science.
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