Liquid crystalline properties of copper(II) complexes derived from azo-containing salicylaldimine ligands

Article abstract of DOI:10.1016/j.poly.2005.12.016

The one-step metal promoted reaction between 5-((4-alkoxyphenyl)azo) salicylaldehyde (alkoxy = decyloxy, dodecyloxy, tetradecyloxy) and ethanolamine in the presence of Cu(CH₃COO) · 4H₂O yields new salicylaldimine based copper(II) bis(chelates) as a result of the [1 + 1] Schiff base condensation. The liquid crystalline character of these complexes was examined using differential scanning calorimetry (DSC) and a polarizing microscope equipped with a heating and cooling stage. All copper complexes form the enantiotropic Smectic C mesophase.

Full text of DOI:10.1016/j.poly.2005.12.016

Polyhedron 25 (2006) 1915–1920
www.elsevier.com/locate/poly
Liquid crystalline properties of copper(II) complexes derived from
azo-containing salicylaldimine ligands
a,
a
a
b
Zolfaghar Rezvani *, Baharak Divband , Amir Reza Abbasi , Kamellia Nejati
a
Department of Chemistry, Faculty of Science, Azarbaijan University of Tarbiat Moallem, Tabriz, Iran
Department of Chemistry, Payam Noor University-Tabriz Center, Emamieh, Hakim Nezami Street, Tabriz, Iran
b
Received 11 November 2005; accepted 6 December 2005
Available online 17 February 2006
Abstract
The one-step metal promoted reaction between 5-((4-alkoxyphenyl)azo) salicylaldehyde (alkoxy = decyloxy, dodecyloxy, tetradecyl-
oxy) and ethanolamine in the presence of Cu(CH3COO) Æ 4H2O yields new salicylaldimine based copper(II) bis(chelates) as a result of the
[1 + 1] Schiff base condensation. The liquid crystalline character of these complexes was examined using differential scanning calorimetry
(DSC) and a polarizing microscope equipped with a heating and cooling stage. All copper complexes form the enantiotropic Smectic C
mesophase.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Schiff bases; Salicylaldimine; Azo; Metallomesogen; Smectic C; Copper complexes
1. Introduction
Cu(II) bis(chelates) based on azo-linked bidentate salicyl-
aldimine [14–16] and tetradentate azo-linked N,N0 salicylide-
neiminato Ni(II), Cu(II) and VO(IV) complexes [17]. In this
work we report the synthesis and liquid crystalline properties
ofnew bis[5-((4-nalkoxyphenyl)azo)-N-ethanol salicylaldim-
inato]copper(II) complex homologues (see Scheme 1).
In recent years metallomesogens have seen systematic
research and development [1–5]. Considerable amounts of
metallomesogens are based on transition metal coordination
compounds with salicylaldimine-based ligands [6–8]. More-
over much attention has been paid to photonic materials
based on light-induced phase transitions because these types
of materials are very useful in new information processing
technology. Azo-containing polymeric liquid crystal systems
and metallomesogens containing uncomplexed azo moieties
are suitable for this research field because these materials are
able to change molecular shape via reversible cis–trans isom-
erization upon photoirradiation. These compounds have
potential applications in optical switching, high-density
optical data storage and optical computing [9,10].
2. Experimental
2.1. Reagents
All reagents and solvents were used as supplied by
Merck chemical company and were used without further
purification. 4-Alkoxynitrobenzene homologues were
obtained by reaction between 4-nitrophenol with 1-
bromooctan, 1-bromododecane or 1-bromotetradecane in
DMF as the solvent and K2CO3 as the base by refluxing
for 3 h [18] and then the crude 4-alkoxynitrobenze homo-
logues were purified by recrystallization from ethanol.
4-Alkoxyaniline homologues were prepared by reducing
the corresponding 4-alkoxynitrobenzene homologue as
described in the literature [19].
Recently, some metallomesogens containing isomerizable
azo moieties have been reported [11–13]. We also recently
reported the synthesis and liquid crystalline character of
*
Corresponding author. Tel./fax: +98 412 4524991.
E-mail address: z_rezvani@yahoo.com (Z. Rezvani).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.12.016

1916
Z. Rezvani et al. / Polyhedron 25 (2006) 1915–1920
8
H
HOCH2CH2NH2 , Cu(CH3COO)2.H2O
EtOH, Reflux
O
3
7
6
N
OH
9
10
CnH2n+1CH2O
N
1
2
5
4
n= 9 ; 1a
n=11 ; 1b
n=13 ; 1c
H
CH2CH2OH
N
OCH2CnH2n+1
N
O
O
N
Cu
N
N
N
CnH2n+1CH2O
HOCH2CH2
H
n=9; 2a
n=11; 2b
n=13; 2c
Scheme 1. Synthetic route to the copper complexes.
2.2. Physical measurements
Calc. for C23H30N2O3: C, 71.16; H, 7.39; N, 7.90. Found:
C, 70.71; H, 7.12; N, 7.54%. H NMR (400 MHz, CDCl3)
1
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 Finne-
gan-mat GC–MS–DS spectrometer model 8430. Infrared
spectra were taken with a FT-IR Bruker, vector 22 spec-
trometer using KBr pellets in the 400–4000 cmꢀ1 range.
The DSC thermograms of the compounds were obtained
on a Mettler–Toledo DSC 822e module, which was
calibrated with indium metal (T = 156.6 0.3, DH =
28.45 0.6 J gꢀ1). Samples of 2–5 mg in the solid form were
placed in aluminum pans (40 ll) with a pierced lid, and
heated or cooled at a scan rate of 10 °C minꢀ1 under a nitro-
gen flow. TGA was carried out on a Mettler–Toledo TGA
851e at a heating rate of 10 °C minꢀ1 under a nitrogen
atmosphere. The optical observations were made with a
Zeiss polarizing microscope equipped with a Linkam
THMSG 600 heating and cooling stage and a Linkam
THMS 93 programmable temperature-controller. 1H
NMR spectra were obtained on a Bruker FT-NMR AC-
400 (400 MHz) spectrometer with deutrated chloroform as
the solvent. All chemical shifts are reported in d(ppm) rela-
tive to tetramethylsilane as an internal standard.
d 11.27 (s, H-8), 10.02 (s, H-9), 8.18 (d, J 3.2 Hz, H-3), 8.14
(dd, J 3.1, 8.4 Hz, H-2), 7.91 (dd, J 3.1, 8.2 Hz, H-4, H-7),
7.10 (d, J 8.7, H-1), 7.01 (dd, J 3.6, 8.3 Hz, H-5,H-6), 4.05
(t, J 7.1 Hz, H-10), 1.81–1.00 (15H, alkyl chain).
2.3.2. 5-(4-Dodecyloxyphenylazo) salicylaldehyde (1b)
Yellow, yield 80%, m.p. 123 °C. MS m/z (relative inten-
sity): 411.6 (M+1,15), 410.6 (M, 45), 242.5 (MꢀC12H25,
25), 121.6 (MꢀC12H25OC6H4N2, 100). Anal. Calc. for
C25H34N2O3: C, 73.15; H, 8.35; N, 6.83. Found: C, 72.73;
1
H, 8.14; N, 6.47%. H NMR (400 MHz, CDCl3) d 11.26
(s, H-9), 10.02 (s, H-8), 8.15 (d, J 2.8 Hz, H-3), 8.13 (dd,
J 2.9, 8.2 Hz, H-2), 7.89 (dd, J 3.0, 7.9 Hz, H-4, H-7),
7.11 (d, J 8.1 Hz, H-1), 7.01 (dd, J 3.2, 7.9 Hz, H-5, H-
6), 4.04 (t, J 6.7 Hz, H-10), 1.84–0.87 (23H, alkyl chain).
2.3.3. 5-(4-Tetradecyloxyphenylazo) salicylaldehyde (1c)
Yellow, yield 80%, m.p. 123 °C. MS m/z (relative inten-
sity): 411.6 (M+1,15), 410.6 (M, 45), 242.5 (MꢀC14H29,
25), 121.6 (MꢀC12H25OC6H4N2, 100). Anal. Calc. for
C27H38N2O3: C, 73.15; H, 8.35; N, 6.83. Found: C, 72.73;
1
H, 8.14; N, 6.47. H NMR (400 MHz, CDCl3) d 11.26 (s,
H-9), 10.02 (s, H-8), 8.15 (d, J 2.8 Hz, H-3), 8.13 (dd, J
2.9, 8.2 Hz, H-2), 7.89 (dd, J 3.0, 7.9 Hz, H-4, H-7), 7.11
(d, J 8.1 Hz, H-1), 7.01 (dd, J 3.2, 7.9 Hz, H-5, H-6),
4.04 (t, J 6.7 Hz, H-10), 1.84–0.87 (23H, alkyl chain).
2.3. Materials
All homologue materials were prepared similarly.
2.3.1. 5-(4-Decyloxyphenylazo) salicylaldehyde (1a)
This compound was prepared as described in the litera-
ture [20]. Yellow, yield 80%, m.p. 126 °C. MS m/z (relative
intensity): 355.4 (M+1,15), 354.3 (M, 40), 241.2
(MꢀC10H21, 25), 121.0 (MꢀC10H21OC6H4N2, 100). Anal.
2.3.4. Syntheses of the copper complexes
The copper complexes were prepared in similar manners
using a method described elsewhere [21]. Thus, an alcoholic
solution containing 10 mmol of ethanolamine was added to
a solution of 5-alkoxyphenylazo salicylaldehyde (10 mmol)

Z. Rezvani et al. / Polyhedron 25 (2006) 1915–1920
1917
in 50 ml of boiling absolute ethanol. Then 10 mmol sodium
acetate in 10 ml water was added to the above reaction sys-
tem. Finally 50 ml boiling ethanol solution containing
10 mmol Cu(CH3COO)2 Æ H2O was added. The reaction
mixture was kept boiling for 25 min. Crystals precipitated
from the solution and recrystallization was performed from
ethanol/chloroform (1:1 v/v) solution. The copper com-
plexes were obtained as brown micro crystals. The micro
crystals were filtered off, washed with absolute ethanol
and then recrystallized from ethanol–chloroform (1:3 v/v).
Complex 2a: Brown, yield 80%. Anal. Calc. for
C50H68N6O6Cu: C, 65.83; H, 7.46; N, 9.22. Found: C,
65.4; H, 7.2; N, 9.2%.
Complex 2b: Brown, yield 85%. Anal. Calc. for
C54H76N6O6Cu: C, 66.98; H, 7.86; N, 8.68. Found: C,
66.6; H, 7.4; N, 8.2%.
Complex 2c: Brown, yield 75%. Anal. Calc. for
C58H84N6O6Cu: C, 68.00; H, 8.21; N, 8.21. Found: C,
67.6; H, 7.9; N, 8.0%.
group was observed at 3190 cmꢀ1 because the intramolecu-
lar hydrogen bonding between OH and the formyl group in
compound 1 leads to a decrease of the stretching frequency
of OH, and the carbonyl group was observed at 1666 cmꢀ1
.
In the IR spectra of the copper complexes (2a–c), the phe-
nolic OH group disappeared and a free lateral OH group
was observed as a broad peak centered at 3320 cmꢀ1 [21].
In addition, the C@N group was observed at 1620 cmꢀ1
.
It is well recognized that the C@N stretching frequency
in bidentate salicylidenic schiff base ligands with N-alkyl
structures appears in the 1635–1640 cmꢀ1 region [14–
17,22], so the appearance of C@N at 1622 cmꢀ1 (at a lower
wavenumber than the free schiff base ligands) in the copper
complexes demonstrates that C@N has been formed by a
[1 + 1] condensation reaction and that it is coordinated
to the copper center. On the other hand, the lack of a phe-
nolic OH group in the copper complexes indicates that the
OH group has been deprotonated and is coordinated to the
metal ion as –Oꢀ. Reduction of the double bond character
of the C@N bond, which is caused by the coordination of
nitrogen into the metal center, is in agreement with results
obtained from other similar complexes, described previ-
ously [14–17,22]. Based on these observations and the ele-
mental analyses results we conclude that the Schiff-base
ligands are coordinated to metal atoms as bidentate
(N–O) ligands in a 2:1 ratio.
On the basis of the literature data for copper complexes
with a similar coordination environment, the OH group is
free of coordination [21] and is not bonded to the metal
center. This fact may be related to the strong John–Teller
effect in d9 Cu+2, which avoids the coordination of OH
to the metal center. Therefore, it is suggested that the
Cu(II) centres have square planar or nearly square planar
coordination according to the common stereochemistry
of this kind of compound [8,23].
3. Results and discussion
3.1. Synthesis
5-(4-Alkoxyphenylazo) salicylaldehyde homologues
(1a–c; see Scheme 1) were synthesized in a three-step pro-
cess, in which the hydroxy 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
was coupled with the diazonium chloride obtained from
the 4-alkoxyaniline. 1a–c were purified by repeated crystal-
lization from ethanol to give a single spot on thin layer
chromatography, and characterized by IR, 1H NMR, mass
spectroscopy and elemental analyses. The copper com-
plexes were obtained in good yield and purity, and were
characterized by C, H, N elemental analyses and IR spec-
troscopy. These copper compounds are all paramagnetic
3.2. Mesomorphism
1
and their H NMR spectra display only broad alkoxy sig-
nals. All other proton signals close to the paramagnetic
copper centers are unobserved. The elemental analyses
and spectroscopic data for the new compounds gave satis-
factory results. Some physical and characterization data
for 5-(4-alkoxyphenylazo) salicylaldehyde homologues
and the copper complexes are given in the experimental
section and selected IR data are tabulated in Table 1.
The IR spectral frequencies of the synthesized compounds
were obtained using KBr pellets as described in the exper-
imental section. In the compounds 1a–c, the hydroxyl
The mesomorphic properties of 5-(4-nalkoxyphenylazo)
salicylaldehyde homologues (1a–c) and the Cu(II) com-
plexes have been studied by polarizing optical microscopy
observations using a heating–cooling stage and the phase
transition temperature and enthalpy were obtained by dif-
ferential scanning calorimetry (DSC). Phase transition tem-
peratures along with the corresponding enthalpy values for
the copper complexes are summarized in Table 2. The alde-
hyde homologues (1a–c) did not show any liquid crystalline
character. These compounds clearly melted and transformed
Table 1
Selected IR data for the Schiff base ligands and metal complexes
Compound
m (cmꢀ1
)
O–H
C–H (aromatic)
C–H (aliphatic)
C@N
C–O (etheric)
C@O
1(a–c)
2(a–c)
3185–90(br, m)
3220–40(br, m)
3069–71(m)
3050–60(m)
2950–2850(s)
2955,2860(s)
1241
1255–86(s)
1665–6(s)
1621–3(s)
s, strong; m, medium; br, broad.

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Z. Rezvani et al. / Polyhedron 25 (2006) 1915–1920
Table 2
texture [24] was observed for all copper complexes under
POM. The fourth peak, having a relatively small DH value,
corresponds to the transition of the mesophase to an iso-
tropic liquid. This process can also be observed optically.
A relatively larger clearing enthalpy for the mesophase-
to-isotropic transition was observed at higher temperatures
(203–240 °C) indicating that the mesophases were highly
ordered. Results from DSC and POM therefore suggest
the presence of a SmC mesophase. A microscopic picture
from the SmC mesophase for complex 2a is illustrated in
Fig. 2.
Continued heating after the clearing point causes a con-
siderable amount of decomposition immediately after isot-
ropization, which has been detected by exothermic peaks in
the DSC trace and optical observations. The subsequent
cooling and heating processes do not give reproducible
Transition temperatures and enthalpy changes of the copper(II) complexes
Compound
Transitiona
Tb (°C)
DHb (kJ molꢀ1
)
2a
Cr1–Cr2
Cr2–Cr3
Cr3–SmC
SmC–I(dec)c
56.1
167.0
204.3
240.3
1.61
1.59
53.43
5.27
2b
2c
Cr1–Cr2
Cr2–SmC
SmC–I(dec)
173.6
193.8
235.1
9.58
51.10
3.54
Cr1–Cr2
Cr2–Cr3
Cr3–SmC
SmC–I(dec)
107.4
171.3
191.4
230.9
1.67
17.06
49.08
13.41
a
Cr, crystal; SmC, smectic C; I, isotropic liquid.
b
Data obtained from first DSC cycle. Temperature data are at peak
onset.
c
dec: decomposition occurred immediately at clearing point.
into isotropic liquids and no mesophase was observed on
the heating or cooling cycle from optical observations.
The melting points of the ligands decrease with increasing
n value (alkyl chain length, see Scheme 1).
All the copper complexes reported in this work exhibit
liquid crystalline behavior. Fig. 1(2a–2c) shows the DSC
curves of the copper complexes. The thermal behaviors of
the complexes were similar as it is apparent from the
DSC curves; therefore we describe here the behavior of
compound 2a as a representative example. The first heating
cycle exhibits four endothermic transitions at 56.1, 167.0,
204.3 and 240.3 °C with DH values of 1.61, 1.59, 532.43
and 5.27 kJ molꢀ1, respectively. The first two transitions
correspond to the transition of a crystal phase to another
crystal phase and cannot be observed optically. The third
peak corresponds to the transition of a crystal phase to a
mesophase. The mesophase was examined by optical obser-
vation. A highly viscous mesophase with a typical Schlieren
Fig. 2. Optical texture observed for 2a through crossed polarizers at
210 °C.
Fig. 1. DSC thermograms of the copper complexes.

Z. Rezvani et al. / Polyhedron 25 (2006) 1915–1920
1919
Fig. 3. TGA curve of 2a.
results because thermal decomposition occurs during the
first heating cycle. All the other members of this series of
compounds also exhibit thermal decomposition behavior.
Therefore, in Table 1, phase transition temperatures of
the copper complexes are reported from the first heating
cycle of the DSC thermogram. In general, melting temper-
atures of these copper compounds decrease slightly by
increasing the alkyl chain length. The variation of clearing
temperatures with increasing chain length has similar
trends. In the first heating scan if the temperature increase
is within the clearing point, in the first cooling stage the
transition of isotropic liquid to mesophase is observed at
241 °C and the mesophase starts to crystallize at 224 °C,
as detected by DSC and confirmed by POM, but after
the clearing point the sample is decomposed and in the first
cooling cycle no transitions was observed.
[27] and Barbera [28] have reported the nematic liquid crys-
talline properties of bis[4-((4-alkoxy)benzoyloxy)-salicyl-
aldimine-N-alkyl]copper(II) complexes, but the complexes
with an OH lateral group reported in this work exhibit a
high order enantiotropic (thermodynamically stable) SmC
phase. The comparison of mesogenic behavior of N-alkyl
complexes with N–CH2CH2OH complexes demonstrates
the role of the OH group on the mesogenic character. We
can conclude that the presence of the OH group on the
ligands considerably improves the liquid crystalline behav-
ior from monotropic nematic to enantiotropic SmC phase
in this type of metallomesogenic systems. This fact may
be related to the attractive intermolecular interactions
between HOꢁ ꢁ ꢁCu and /or HOꢁ ꢁ ꢁHO.
4. Conclusion
To understand the thermal decomposition process, the
copper compounds were examined by thermogravimetric
analysis (TGA). The TGA thermogram of the copper com-
pounds at a heating rate of 10 °C/min shows that decom-
position begins at 260 °C, which is near the temperature
of clearing to the isotropic liquid. All the copper com-
pounds have similar TGA thermograms. Fig. 3 shows the
TGA thermogram of 2a as a representative example.
The copper complexes reported in this work are the first
example of metallomesogens based on two ring salicylide-
nic schiff base ligand system with an azo moiety and a polar
OH on the ligands (N–CH2CH2OH). Recently some meso-
genic copper complexes which have a similar structure to
that of the title complex of this work have been reported.
We have recently reported the investigation of liquid crys-
talline properties of bis[5-((4-nalkoxyphenyl)azo)-N-
(nalkyl)-salicylaldiminato]copper(II) complex homologues
[15]; metallomesogens based on two ring salicylidenic schiff
base ligand with an azo moiety and an alkyl group on the
ligands (N-alkyl). These complexes exhibited monotropic
nematic (thermodynamically unstable) liquid crystalline
character. Furthermore, Hoshino [25], Cruso [26], Marcos
In this work we have prepared copper complexes derived
from 5-((4-nalkoxyphenyl)azo)-N-(ethanol)-salicylaldimine
homologues as the first example of azo-containing metallo-
mesogens with a lateral OH group. In summary, the study
outlined above has shown that all complexes exhibit an
enantiotropic smectic C mesophase. Melting and clearing
temperatures of the complexes slightly decrease with
increasing alkoxy chain length. The mesomorphism can
be greatly improved by introducing the lateral OH group
close to the central core in this type of copper-containing
metallomesogenic system.
Acknowledgement
Financial support from Azarbaijan University of Tarb-
iat Moallem research council is greatly appreciated.
References
[1] A.M. Giroud, Coord. Chem. Rev. 180 (1998) 1485.
[2] R. Ziessel, Coord. Chem. Rev. 216 (2001) 195.

1920
Z. Rezvani et al. / Polyhedron 25 (2006) 1915–1920
[3] K. Binnemans, C. Go¨rller-Warland, Chem. Rev. 102 (2002) 2303.
[4] R.W. Date, E.F. Iglesias, K.E. Rowe, J.M. Elliott, D.W. Bruce, J.
Chem. Soc., Dalton Trans. (2003) 1914.
[5] R. Gimenez, D.P. Lydon, J.L. Serrano, Curr. Opin. Solid State
Mater. Sci. 6 (2002) 527.
[6] N. Hoshino, Coord. Chem. Rev. 174 (1998) 77.
[7] R. Paschke, D. Balkow, E. Sinn, Inorg. Chem. 41 (2002) 1949.
[8] A.B. Blake, J.R. Chipperfield, W. Hussain, R. Paschke, E. Sinn,
Inorg. Chem. 34 (1995) 1125.
[18] R.V. Deun, K. Binnemans, J. Alloys Comp. 303–304 (2000)
146.
[19] E.C. HorningOrganic Syntheses, collective vol. III, Wiley, New York,
1955, p. 130.
[20] A.A. Khandar, Z. Rezvani, K. Nejati, A.I. Yanovsky, J.M. Martinez,
Acta Chim. Slov. 49 (2002) 733.
[21] L. Chen, H. Xu, X. Yu, Y. Zhu, C. Yang, J. Polym. Sci. A 34 (1996)
721.
[22] M. Lee, Y.S. Yoo, M.G. Chol, Macromolecules 32 (1999)
2777.
´
[9] L. Laesker, T. Fischer, S.R. Marder, W.E. Torruellas, M. Blan-
chard-Desce, V. Ricci, G.I. Stegeman, S. Gilmour, J.L. Bredas, J. Li,
G.U. Bublitz, S.G. Boxer, Science 276 (1997) 1233.
[10] W.M. Gibson, P.J. Shannon, S.T. San, B.J. Swetlin, Nature 351
(1991) 49.
[23] R. Paschke, S. Liebsch, C. Tschierske, M.A. Oakley, E. Sinn, Inorg.
Chem. 42 (2003) 8230.
[24] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim,
2003.
[11] I. Aiello, M. Ghedini, M. La Deda, D. Pucci, O. Francescangeli,
Eur. J. Inorg. Chem. (1999) 1367.
[25] N. Hoshino, R. Hayakawa, T. Shiboya, Y. Matsunaga, Inorg. Chem.
29 (1990) 5129.
[12] I. Aiello, M. Ghedini, F. Neve, D. Pucci, Chem. Mater. 9 (1997) 2107.
[13] P.K. Sudhadevi Antharjanam, V. Ajay Mallia, S. Das, Chem. Mater.
14 (2002) 2687.
[14] A.A. Khandar, Z. Rezvani, Polyhedron 18 (1998) 129.
[15] Z. Rezvani, L. Rahimi Ahar, K. Nejati, S.M. Seyedahmadian, Acta
Chim. Slov. 51 (2004) 675.
[16] Z. Rezvani, A.R. Abbasi, K. Nejati, S.M. Seyedahmadian, Polyhe-
dron 24 (2005) 1467.
[17] K. Nejati, Z. Rezvani, New J. Chem. 27 (2003) 1665.
[26] (a) U. Caruso, A. Roviello, A. Sirigu, Liq. Cryst. 7 (1990) 421;
(b) U. Caruso, A. Roviello, A. Sirigu, Liq. Cryst. 7 (1990) 431;
(c) U. Caruso, A. Roviello, A. Sirigu, Liq. Cryst. 10 (1991) 85.
[27] (a) M. Maros, P. Romero, J.L. Serrano, J. Chem. Soc., Chem.
Commun. (1989) 1641;
(b) M. Maros, P. Romero, J.L. Serrano, J. Barbara, A.M. Levelut,
Liq. Cryst. 7 (1990) 421.
[28] J. Barbara, A.M. Levelut, M. Maros, P. Romero, J.L. Serrano, Liq.
Cryst. 10 (1991) 119.