Mesogenic binuclear oxamide derivatives with discotic and calamitic properties

Article abstract of DOI:10.1039/b712174a

Series of binuclear copper and nickel complexes having two tetradental enaminoketone centres have been synthesized and examined. The rectangular shape of the mesogenic core allows the formation of lamellar (SmA) or columnar (Col_h, Col_r

Full text of DOI:10.1039/b712174a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Mesogenic binuclear oxamide derivatives with discotic and calamitic
properties†
a
a
b
a
a
*
´
ꢀ
Jadwiga Szyd1owska, Adam Krowczynski, Renata Bilewicz, Damian Pociecha and qukasz G1az
Received 8th August 2007, Accepted 21st January 2008
First published as an Advance Article on the web 4th February 2008
DOI: 10.1039/b712174a
Series of binuclear copper and nickel complexes having two tetradental enaminoketone centres have
been synthesized and examined. The rectangular shape of the mesogenic core allows the formation of
lamellar (SmA) or columnar (Colh, Colr) phases depending on the number of attached aliphatic chains.
In some complexes the columnar phases have molecules tilted in columns. An unexpected phase
sequence Colr upper–Colh–Colr lower, upon lowering temperature, was detected, which is a type of
re-entrancy. The crystallographic unit cells of the rectangular columnar phases are not the same.
Intramolecular interaction between metallic centres was found in ESR studies and confirmed by cyclic
voltammetry.
metallomesogens the mesogenic cores was a triangle constructed
from enaminoketone and phenyl rings and having inner
hydrogen bonds.15–16 The effective shape of the molecules was
adjusted by connection of aliphatic chains. Different numbers
and positions of substituted chains result in either rod-like or
disc-like molecules so the synthesised compounds can give
smectic A as well as hexagonal columnar phases. However,
most synthesised complexes with this particular mesogenic core
and having intermediate shape do not form any LC phases,
they melt directly to the isotropic phase.
1. Introduction
The structure of
a liquid crystalline phase—lamellar or
columnar—depends on the effective molecular shape, rod-like
or disc-like. However, there are also substances that exhibit
both these phases.1–4 The largest group of these bimesomorphic
compounds consists of those which can form columnar or
lamellar aggregates due to specific intermolecular interac-
tions.5–6 Usually these compounds belong to the homologous
series in which the length of one of the substituted alkyl chains
has a significant effect on molecular aggregation.6 In particular,
calamitic bolamphiphilic compounds with various lateral chains6
or elongated tri- and tetra-catenars,7–9 or di-alkoxystilbazoles10
show calamitic or disk-like properties within one homologous
series. Among the compounds which can arrange into supramo-
lecular assemblies are dendrimers. They can create smectic
phases but when the amount of peripheral chains increases the
columnar phase is favoured.11
In this contribution we present liquid crystalline complexes
having a novel mesogenic core of intermediate shape, which is
more or less rectangular. The applied core seems to promote
columnar as well as lamellar liquid crystalline phases. The mate-
rials form both types of phases. A more elongated rhombic
mesogenic core was also tested. However, only columnar phases
were detected, in some cases a re-entrant sequence of columnar
rectangular–hexagonal–rectangular phases was observed. The
complexes that are expected to have calamitic shape exhibit
melting points above their decomposition temperatures.
Other groups of bimesomorphic liquid crystalline substances
contain in the molecules two types of mesogenic cores—discotic
and calamitic.12–14 As an example triphenylene derivatives have
been reported. To the disc-shaped triphenylene centre six calam-
itic azobenzene or biphenyl units are connected through flexible
chains. The stable molecular conformation can be either
elongated or disc-like, so nematic (N)12 or discotic nematic
(ND)13 phases appear. Moreover, one of the homologues exhibits
both smectic and columnar (Colh) phases.14
The described mesogenic core contains two metallic centres
separated only by four molecular bonds. Such a close distance
allows the metal centres to interact, resulting in an exchange
coupled system. This interaction was proved by ESR spectros-
copy17 and confirmed by cyclic voltammetry.18–22
It is also possible to obtain mesogens showing this exceptional
feature of bimesomorphism when the molecular core has non-
decisive rod-like or disc-like shape. In the previously synthesised
2. Experimental
2.1 Measurements
The mesophase identification was based on microscopic exami-
nation of the liquid crystalline texture. A Zeiss Jenapol-U polar-
izing microscope equipped with a Mettler FP82 HT hot stage
was use. When the results of the microscopic observation were
doubtful the X-ray studies gave the final phase identification.
The X-ray measurements were taken from powdered samples
with Bruker diffractometers: a wide angle D8 DISCOVER
equipped with a cryostream temperature attachment and small
angle NANOSTAR which was used in gathering spectra at
aLaboratory of Dielectrics and Magnetics, Chemical Department Warsaw
University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland. E-mail:
jadszyd@chem.uw.edu.pl; Tel: +048228220211
bLaboratory of Theory and Applications of Electrodes, Department of
Inorganic and Analytical Chemistry, Chemistry Department, Warsaw
University, Pasteura, 1, Warsaw, Poland
† Electronic supplementary information (ESI) available: Synthesis
procedures, NMR spectra and elemental analysis results of the studied
compounds. See DOI: 10.1039/b712174a
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continuously changing temperature. The detected diffraction
signals were indexed in the centred rectangular lattice for Colr
as well as for Colh phases. In Colh phase the known relation
a ¼ bO3 was fulfilled. Phase transition temperatures and
enthalpy changes were determined by calorimetric measurements
performed with a DSC-7 Perkin-Elmer setup. When the phase
transition temperatures were close to the sample decomposition
temperature, heat effects were not detected and the temperatures
were taken from microscopic observation. ESR investigations
were performed using an X-band Bruker spectrometer
ELEXYS-500 equipped with a nitrogen flow temperature
controller. ESR spectra were taken for samples which were
powdered or dissolved in xylene. Cyclic voltammetry experi-
ments were done using a CHI potentiostat (Model CHI, version
5.19) in three electrode arrangement with a silver/silver chloride
(Ag/AgCl) reference electrode, a platinum wire counter electrode
and a glassy carbon (GCE Detector, 2 mm diameter) working
electrode. The reference electrode was separated from the
working environment by an electrolytic bridge filled with 1 M
tetrabutylammonium tetrafluoroborate (TBATFB)–methylene
chloride solution. The methylene chloride solvent containing 1
M TBATFB was used as the supporting electrolyte solution.
1H and 13C NMR spectra were recorded using a Varian UNITY
plus operating at 500 MHz. The compounds were dissolved at
room temperature in CDCl3 and for poorly soluble substances
ꢀ
toluene-d8 at 90 C was used as solvent. Tetramethylsilane was
applied as internal standard. The molecular dimensions were
estimated by using the modelling HyperChem MM+ program.
2.2 Structure of the complex
The rectangular mesogenic core of the synthesised compounds is
constructed from two tetradentate chelating centres (Scheme 1).
Each of the centres consists of enaminoketone hexagonal and
pentagonal rings. Two pentagonal rings are formed from one
shared oxamide moiety. The two nickel(II) or copper(II) ions
introduced in the chelating centres in the tetragonal surrounding
make the whole structure planar. Substitution of different
numbers of chains having equal length modifies the molecular
shape. It results in rod-like or disc-like molecules (compounds
1–4; Table 1). Joining at the enaminoketone ring (at the carbonyl
moiety) the phenyl ring, which is trioctyloxy substituted, makes
the mesogenic core more extended and gives molecules with disc-
like shape (compounds 5, 6).
The synthetic procedure of the studied compounds and their
NMR spectra as well as the results of elemental analyses are
presented as ESI.†
3. Results and discussion
The mesomorphic properties of the studied compounds 1–6, the
phase sequence, phase transition temperatures and enthalpy
changes are gathered in Table 1.
The molecules 1 have four aliphatic chains attached to the
mesogenic core: an octyloxy chain at each of the diaminophenyl
rings at the 4-position and a nonyl chain at each of the carbonyl
groups of the enaminoketone rings. This substitution results in
molecules which have an elongated shape and can form
a lamellar phase. The length-to-width ratio for the conformation
shown in Fig. 1 is 2.6. The di-nickel complex 1-Ni exhibits a high
temperature smectic A phase. Although its temperature range is
rather narrow, the phase is enantiotropic. Very high melting and
clearing temperatures probably result from the condensed hexag-
onal and pentagonal ring system and strong intralayer molecular
interactions. The di-copper complex 1-Cu does not create liquid
Scheme 1 General formula of the di-metallic complexes.
Table 1 Phase transition temperatures and thermal effects for compounds 1–6
Series
R1
R2
R3
M
Phase sequence, T/ꢀC (thermal effect/J gꢁ1
)
1-Ni
1-Cu
2-Ni
2-Cu
3-Ni
4-Ni
4-Cu
5-Ni
5-Cu
6-Ni
6-Cu
C9H19
C9H19
C9H19
C9H19
C9H19
C9H19
C9H19
4-H17C8Ph
4-H17C8Ph
2,3,4-(H17C8O)3Ph
2,3,4-(H17C8O)3Ph
H
H
H
H
H
H
C8H17
C8H17
C8H17
C7H15
C7H15
H
Ni
Cu
Ni
Cu
Ni
Ni
Cu
Ni
Cu
Ni
Cu
Cr–283.1 (31.5)–SmA–286.6 (13.3)–Iso
Cr–280.7 (45.5)–Iso
Cr–170.2 (49.3)–Iso
Cr–177.6 (48.3)–Iso
Cr–176.6 (34.2)–Iso
OC8H17
OC8H17
H
OC8H17
OC8H17
OC8H17
OC8H17
OC8H17
OC8H17
Cr–139.1 (16.0)–Colr–208a–Colhb–220.2 (8.2)–Iso
Cr–138.9 (9.7)–Colr–215.0 (3.1)–Iso
Cr–225.9 (32.7)–Colr–234.6 (5.87)–Iso
Cr–245 (31.7)–Iso
Colr–70.3 (3.06)–Colh–86.7 (0.076c)–Colr–197.4 (5.0)–Iso
Colr–80.7 (2.3)–Colh–192.2 (4.8)–Iso
H
From X-ray studies. Co-existence of both Colh and Colr0 phases in the temperature range 208–214 ꢀC. Jump of specific heat (J gꢁ1 Kꢁ1).
a
b
c
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5-position of the diaminophenyl rings (2) or octyl chains on
the enaminoketone rings at olefinic carbon atoms (3). Such mole-
cules cannot form any conformation giving decisive disc-like or
rod-like molecular shape and neither lamellar nor columnar
phases appear although the melting temperatures are substan-
tially lower than those for complexes 1.
In compounds 4 two chains were incorporated at each of the
core rings: two octyloxy chains at the diaminophenyl rings and
two alkyl chains (octyl and nonyl) at the enaminoketone rings.
Applying eight chains gives the disc-like molecular shape and
results in the formation of columnar phases. The synthesized
copper complex 4-Cu exhibits an enantiotropic rectangular
columnar phase (Colr) whereas the nickel complex forms both
Colr and Colh phases. In microscopic observation of the latter
complex, at a temperature just below the clearing point, a black
homeotropic texture or spherulitic domains with extinction
brushes parallel to the direction of polarizers were found
(Colh). At a lower temperature, characteristic textures of both
columnar phases appear all together, suggesting the co-existence
of the two phases Colh and Colr0. In calorimetric measurements
there are no enthalpy changes at the Colh–Colr0 phase transition.
At the transition point to the low temperature Colr phase no
thermal effects are observed, either. In the X-ray intensity
pattern there is a 4 K temperature range where signals coming
from Colh and Colr0 appear together and this behaviour is
repeatable in a cycle of cooling and heating the sample (Fig. 2
and Table 2). Probably the co-existing Colr0 and Colh phases
form a non-equilibrium system due to the slow kinetics of phase
transition, in which the values of the thermodynamic potential of
both phases are very similar. A similar effect of phase coexistence
can be observed for synclinic and anticlinic subphases of the B2
phase in banana compounds.23,24 On heating, the Colr0 phase
disappears and only the Colh phase remains. Below the two
phase region, the X-ray signals of the low temperature Colr
phase differ from those of the Colr0 phase. In the upper Colr0
phase the (200) reflection comes from less distant crystallo-
graphic planes than the (110) signal, which gives a planar crystal-
lographic cell with smaller a parameter than for the hexagonal
structure (a < ahex). In the lower Colr phase the weaker signal
(200) relates to a longer dimension and points to elongation of
the crystallographic a parameter (a < ahex). The intensity ratios
of the (110) and (200) signals in the Colr0 and Colr phases are
similar. At 206 ꢀC (Colr0) it is I(110)/I(200) ¼ 1.3 : 1 and at 156
ꢀC (Colr) I(110)/I(200) ¼ 1.56 : 1. These values are slightly smaller
than expected for an ideal rectangular lattice made of points, for
which I(110)/I(200) ¼ 2 : 1. For a real molecular arrangement the
intensity ratio is strongly influenced by the electron distribution,
Fig. 1 Proposed molecular conformation of the 1-Ni complex in the
SmA phase.
crystalline phases although its melting point is not higher than
the melting temperature of the di-nickel complex (1-Ni).
Compounds 2 and 3 have the molecules broadened by adding
two side chains at the rectangular core: octyloxy chains at the
Fig. 2 Intensity map of the low angle X-ray diffraction in changing
temperature for the 4-Ni complex.
˚
Table 2 X-Ray diffraction signals for the 4-Ni complex and corresponding crystallographic distances (in A) obtained from diffractograms and (in
)
parentheses) calculated from assumed rectangular crystallographic cells with dimensions a and b; dc ¼ distance between chains; r ¼ density (in g cmꢁ3
4-Ni
(200) (110) (310)
(400)
(020)
(220)
(510)
(001) diffused dc diffused
a
b
r
a
Colh 216 ꢀC 20.82 20.82
41.64 24.04
41.68 24.06
39.0 25.7
a
Colh 206 ꢀC 20.84 20.84
a
Colr0 206 ꢀC 19.50 21.46
Colrb 170 ꢀC 23.16 19.62 12.59 (12.57) 11.56 (11.58) 10.85 (10.83) 9.82 (9.81) 8.54 (8.52) 3.8
4.5
44.6 21.7 1.34
a
b
Data taken from small angle NANOSTAR. Wide angle D8 DISCOVER.
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Fig. 3 Structure of the low temperature Colr phases of the 4-Ni
complex.
which depends on the peculiar molecular shape and their mutual
orientations (structure factor). In the wide angle X-ray diffracto-
gram for the Colr phase there are only reflections for which the
h + k ¼ 2n relation is fulfilled (Table 2), this points to c2mm
symmetry of the phase.25 This symmetry results from identical
molecular orientation in crystallographic centers and usually is
not observed in Colr phases. The more usualt Colr phases have
the molecules organized in a herringbone arrangement and are
described by the p2gg symmetry symbol. Moreover, the appear-
ance of the (020) signal (Table 2) suggests a relatively dense
distribution of X-ray scattering centres along the a direction
although a > ahex. This may be due to the peculiar molecular
alignment with the molecular core parallel to the a direction.
The latter is in agreement with c2mm symmetry and suggests
the crystallographic cell structure shown in Fig. 3. It is worth
noticing that the obtained density in Colr phases is unexpectedly
high, higher than usual in enaminoketone liquid crystalline
phases. This may result from densely packed metallic ions in the
centre of the molecule.
Fig. 4 Spherulitic domains in microscopic texture of Colr phase of the
5-Ni complex.
along the polarizer directions (Fig. 5a), whereas for tilted mole-
cules the brushes are inclined from the polarizers (Fig. 5b).
Joining two octyloxy chains at each of the diaminophenyl rings
and three octyloxy chains at each of the aroyl rings gives
complexes 6. Attachment of ten aliphatic chains at the molecule
strongly decreases the melting points which are below room
temperatureforbothnickelandcoppercomplexes. Simultaneously
the isotropisation temperatures are high so the temperature range
of LC phases is almost 200 degrees. The synthesised substances
have discotic molecules and they form both types of columnar
phases: rectangular (Colr) and hexagonal (Colh) (Table 1).
The di-nickel complex 6-Ni exhibits an unexpected phase
sequence: Colr upper–Colh–Colr lower phases appear on
decreasing temperature. In all three columnar phases, micro-
scopic textures of a thin (ꢂ5 mm) one surface free sample show
some fragments of bright, contrasting spherulitic domains
In a further modification the molecular core was extended by
phenyl rings substituted at the carbonyl groups of the enamino-
ketone rings. This results in a core with a more elongated
rhombic shape. Our attempts to use this broadened core to
obtain calamitic mesogenic compounds were not successful.
Repeating the molecular structure similar to that of group 1
with four aliphatic chains gives nickel and copper complexes
having melting points higher than the decomposition tempera-
tures. The clearing point becomes lower when the number of
chains substituted increases.
In compounds 5 eight aliphatic chains were applied: two octy-
loxy chains at each of the diaminophenyl rings, one heptyl chain
at each of the enaminoketone rings and one para-octyl chain at
each of the aroyl rings. It gives an effective discotic molecular
shape and the synthesized di-nickel complex 5-Ni exhibits a rect-
angular columnar phase (Table 1). The phase is enantiotropic
although the clearing and melting temperatures are very high.
Under a microscope with crossed polarizers, the observed
textures of the Colr phase have spherulitic domains with extinc-
tion brushes inclined from the directions of the polarizers
(Fig. 4). It suggests that the molecular discs in the Colr phase
are tilted with respect to the columnar axis.23 Under crossed
polarizers dark areas are created in the regions where molecules
have their axes aligned parallel or perpendicular to the polarizing
directions. When the disc-like molecules are perpendicular to the
columns, in circular domains the extinction brushes draw lines
Fig. 5 Directions of extinction brushes (green lines) in spherulitic
domains of columnar phases;23 a ¼ molecules perpendicular to the
columnar axis, b ¼ molecules tilted; A, P ¼ directions of polarizers.
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Table 4 X-Ray diffraction signals for the 6-Cu complex and corre-
˚
sponding crystallographic distances (in A) obtained from diffractograms
and (in parentheses) calculated from assumed rectangular crystallo-
2
˚
graphic cells with dimensions a and b; s ¼ a ꢃ b (in A )
Cu
(200)
(110)
23.43
23.81
(210)
diffused
4.66
Colh (150 ꢀC)
23.43
—
a ¼ 46.86, b ¼ 27.05, s ¼ 1268
Colr (60 ꢀC)
26.57
a ¼ 53.14, b ¼ 26.63, s ¼ 1415
11.99 (11.90)
4.6
planar space group. Interesting behaviour of the columnar
phases studied was found in observation of the continuous
evolution of the (200) and (110) signal intensity with increasing
temperature (Fig. 7). At 40 ꢀC, for the low temperature Colr
phase the signals are almost equal, I200 : I110 ¼ 2.3 : 2.6, although
the multiplicity factor for (110) is two times higher. Because the
(110) peak is relatively weak, its second harmonic (220) is not
recorded (Table 3). The reflection (020), which is perpendicular to
Fig. 6 Microscopic bright spherulitic domains at the upper temperature
Colr phase for the 6-Ni complex.
(Fig. 6). The extinction brushes of these domains are inclined
from the polarizers more or less by the angle 45ꢀ. The inclination
angle does not vary with temperature. This indicates that in all
the phases the tilt of the molecular discs to the columnar axis
is about 45ꢀ. The bright domains are the sharpest in the lower
temperature Colr phase. At the phase transition from the lower
Colr to the Colh phase the extinction brushes of the domains
become less distinctive which suggests some molecular reorgan-
isation. In the calorimetric scan, at the transition point from
the Colh to the upper temperature Colr phase there is no enthalpy
change. Instead a jump of specific heat was detected. The final
phase identification performed in wide angle X-ray studies is
shown in Tables 3 and 4. Upon increasing the temperature the
obtained a dimension of the crystallographic cells decreases
whereas the b size increases. Taking into account the molecular
tilt in the columns (45ꢀ), the measured distance dd between discs
gives the c dimension along the columns, c ¼ ddO2. The esti-
mated density of the materials r is close to 1, much less than
that for the 4-Ni complex. Although the 2-D unit cell parameters
change monotonically with temperature, the smallest volume of
the crystallographic cells was found for the Colh phase, this is the
phase with the highest density. In both rectangular phases there
are no X-ray reflections with indexes h + k ¼ 2n + 1. This indi-
cates that in both Colr phases the herringbone order is absent.
Columns form the 2-D crystallographic lattice of the c2mm
Fig. 7 Intensity map of the low angle X-ray diffraction in changing
temperature for the 6-Ni complex.
˚
Table 3 X-Ray diffraction signals for the 6-Ni complex and corresponding crystallographic distances (in A) obtained from diffractograms and
(in parentheses) calculated from assumed rectangular crystallographic cell with dimensions a and b; dd and dc ¼ distances between discs and among
3
ꢁ3
˚
chains respectively; v ¼ volume of crystallographic cell (in A ); r ¼ density (in g cm
)
Ni
(200)
22.56
(110)
23.58
(310)
(020)
(220)
(400)
(510)
(130)
(620)
(530)
(600)
dd diff.
dc diff.
Colr (120 ꢀC)
13.20
(13.21)
13.80
(13.83)
11.81
(11.79)
11.26
(11.28)
8.56
(8.58)
3.74
4.65
a ¼ 45.12, b ¼ 27.66, c ¼ 5.29, v ¼ 6600, r ¼ 0.97
Colh (72 ꢀC)
23.03
23.03
13.31
(13.30)
13.31
(13.30)
11.53
(11.52)
11.53
(11.52)
8.70
(8.70)
8.70
(8.70)
3.75
3.76
4.55
4.46
a ¼ 46.06, b ¼ 26.59, c ¼ 5.30, v ¼ 5716, r ¼ 1.12
Colr (25 ꢀC)
24.74
21.00
13.49
(13.44)
—
—
12.39
(12.37)
9.18
(9.10)
7.6
(7.7)
6.77
(6.72)
6.19
(6.09)
8.25
(8.25)
a ¼ 49.48, b ¼ 23.19, c ¼ 5.32, v ¼ 6100, r ¼ 1.05
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Fig. 8 Low angle X-ray signals in the 6-Ni complex.
(200), is not detected either. This suggests that along the b direc-
tion electron density modulation is less pronounced than along
the a direction. This further implies that the rhombic molecular
cores, that are main scattering centres, are aligned with their long
axes along the b direction. Simultaneously, the position of the
(200) reflection gives an a cell dimension longer than it should
be for the Colh phase (a > ahex). After the sharp transition to
the Colh phase the signals (200) and (110) converge. At still
higher temperature the Colh phase smoothly transforms into
the upper temperature Colr phase in which the single peak splits
into two again. Now the (200) signal relates to the shorter
distance, so the a parameter becomes smaller than in the Colh
phase, a < ahex. About 14 K above the transition point the inten-
sity of the (200) peak is equal to or even a little higher than that
for the (110) signal (Fig. 8). On further heating the latter peak
ꢀ
increases and at 130 C the intensity ratio becomes I200 : I110
¼
1.5 : 3.4, which suggests that the density of X-ray scattering
centres may be slightly higher on the (110) than on the (200)
plane.
Possible crystallographic structures of the detected columnar
phases of the 6-Ni complex are proposed in Fig. 9. In all the
phases the molecular cores are synclinically tilted to the
columnar axis, however, the direction of the tilt is not clear.26–28
The c2mm symmetry of the 2-D crystallographic lattices allows
the molecules to tilt toward the a or b directions. In the complex
studied the molecular core is not circular (Fig. 9a) and a tilt
toward the a direction changes the columnar cross section less
than a tilt toward b, when compared to the non-tilted molecules.
In the drawn pictures the molecules have cores tilted toward the
a direction. This orientation fulfils the requirement of the dense
core packing along the b dimension where the longest size of the
core is aligned parallel to the b dimension. We suppose that in
the whole phase sequence the molecular cores are oriented in
this way.
Fig. 9 Proposed structure of the columnar phases of the 6-Ni complex;
a supposed molecular conformation in the lower Colr phase; b–d molec-
ular arrangements in subsequent phases.
centres (molecular cores) is more than two times larger for the
(200) than for the (110) plane. Upon increasing the temperature
to the Colh phase the aliphatic chains surrounding the core
become more uniformly distributed forming columns with
circular cross section (Fig. 9c). On further heating the aliphatic
chains become melted and wrap the core giving a molecular
shape that reflects the shape of the core (Fig. 9d). This results
in the flattened columns and the rectangular 2-D structure of
the upper Colr phase that have the squeezed a and elongated
b lattice parameters. Simultaneously, the longest size of the
molecular core remains oriented along the b direction and the
X-ray scattering centres seem to be still more densely packed
along the b axis. This means that the intensity of the (200) signal
As was found for the low temperature Colr phase, the a param-
eter of the 2-D crystallographic cell is relatively longer than in
the Colh phase. This structure may result from a peculiar molec-
ular conformation (Fig. 9a) having aliphatic chains stiff and
stretched along the a direction (Fig. 9b). For this extreme struc-
ture, the roughly estimated contribution of X-ray scattering
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can be comparable to or even larger than that of (110) (Fig. 8, 90
ꢀC). When the temperature increases the b parameter increases
and the a parameter decreases, and the (110) peak intensity
ꢀ
becomes stronger (Fig. 8, 130 C).
Comparing some properties of the columnar phases for both
nickel 6-Ni and 4-Ni complexes one can see that their 2-D crys-
tallographic lattices have c2mm symmetry and parameters a and
b change with temperature in a similar way. Irrespective of the
molecular core orientation in the 2-D lattice the a parameter is
the larger at low temperature and decreases with increasing
temperature whereas the b parameter increases. However, the
found densities of these two materials are not similar. For the
4-Ni complex the density of the Colr phase is rather high. It could
be estimated as lower if a tilt of the molecules was assumed.
For the Colr phases of the 4-Ni and 6-Ni complexes, the similar
averaged distance between aliphatic chains, dc, may suggest
similar molecular tilts in the columns.27 However, we do not
have firm proof of the tilted structure for the 4-Ni complex.
The detected texture with spherulitic domains does not exclude
the molecular tilt in the columns. The spherulitic texture with
brushes parallel to the polarizers could be formed when mole-
cules are anticlinically tilted in neighbouring columns. Because
the molecules in columns are not ordered, anticlinic molecular
arrangement does not disturb the symmetry of the 2-D crystallo-
graphic lattice. In this case the electron distribution is not
influenced by the tilt direction and the c2mm symmetry remains
unchanged.
Fig. 10 ESR spectra of the 1-Cu complex.
3.1 ESR studies
coupling, taken from magnetic susceptibility measurements, is
negative and equal to J ¼ ꢁ581 cmꢁ1. This indicates that the
triplet state has higher energy than the singlet state. Similar
bleaching of the ESR signal at low temperature is well known
for another coupled copper–copper system, e.g. for di-copper
tetraacetate hydrate.30
Unusual ESR properties were found for the studied materials
having copper ions. The 1-Cu complex in xylene solution gives
one broad signal without hyperfine structure at room and higher
temperatures (Fig. 10a). The lack of hyperfine structure results
from an exchange narrowing effect. Under heating close to the
boiling point of the solvent the recorded signal becomes stronger.
For a powdered sample, when cooled to liquid nitrogen temper-
atures, the single broad feature in the ESR spectrum decreases
and at about 120 K only a tiny signal of the spectrum related
to the Cu(II) ions was detected (Fig. 10b). This spectrum prob-
ably comes from traces of an inorganic copper salt used for the
synthesis of the complex. In the investigated substance two
copper ions are very close and the distance between them is
only four molecular bonds. In such a short path electron
exchange coupling can be created between the two copper ions
which give electronic singlet and triplet states. The ESR active
state is the triplet state. However, the characteristic doubled
structure of ESR peaks for dissolved samples in the triplet state
is not detected. The recorded single broad signal results, as previ-
ously, from an exchange narrowing effect. The increasing inten-
sity of the recorded peak at higher temperature points to the
increasing population of the triplet state. Disappearance of the
ESR signal at lower temperature shows that the triplet state is
nearly empty and the singlet state becomes more populated.
Electron exchange coupling through an oxamide bridge has
been reported previously for a copper–copper system.29 However,
the previously described structure is a set up of a few molecules
chelating the copper ions and was studied in the crystalline phase
only. For that substance the J constant of the electron exchange
3.2 Cyclic voltammetry
In cyclic voltammetry studies compounds 6, nickel and copper
complexes, do not exhibit a single well-defined peak which is
characteristic for oxidation of the separated metallic ions in
the transition process from Ni(II) to Ni(III) or Cu(II) to Cu(III).
Instead, the first forward scan shows multiple poorly developed
signals in the range of positive potentials. The splitting of the
anodic peaks into two is better developed for the nickel than for
the copper complex. This type of voltammetric signal resembles
the signals obtained for multi-nuclear systems, and also points to
the presence of interactions between the metal centres inserted in
the mesogenic core.18–20 The appearance of two peaks instead of
one for two identical metallic centres suggests that oxidation of
one of them makes the process more difficult for the second
centre. This indicates that two nickel ions interact more effec-
tively than copper ions. This is probably because of the smaller
size of the coordination plane in this type of nickel complex.31
In the backward half-scan, the reduction peak is not well
developed for the Ni complex whereas the copper complex shows
one broad cathodic peak. For the nickel complex, the next
anodic peak appearing at ca. 1.5 V may be ascribed to the
1114 | J. Mater. Chem., 2008, 18, 1108–1115
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Cu–Cu structure with paramagnetic properties at high and
diamagnetic properties at low temperatures.
Acknowledgements
Financial support for this work was granted by project no.
COST/52/2006.
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The metallic centres contained in the bi-nuclear complexes
exhibit intramolecular interactions, which were detected in
ESR spectroscopy and confirmed in voltammetric studies. The
described di-copper oxamide complexes are a new example of
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