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integration of local order into large-scale multidimensional arrays. In marked contrast,
tremendous advances have been made with respect to the construction of exotic super-
molecules by stepwise accretion of simple building blocks.
While numerous examples of copper(I) helicates are known it has proved extremely
difficult to assemble them into organized assemblies. Special attention has to be given to
the number, length and nature of the lipid-like chains and it appears clear that there is a
high barrier to arrange self-organized structures into liquid-crystalline material. In this
respect, the onset of liquid-crystalline behavior can be seen as a fine balance between
order (helix) and chaos (alkyl chains). The net result of optimizing this balance is to pro-
duce low-temperature metallomesogens [26]. The initial step in the overall process
involves formation of a stable metallohelicate and is itself a challenging proposition in
view of the bulky alkyl chains that complicate the gathering of the ligands around the
cations. The helix provides essential rigidity that favors subsequent stacking of the aro-
matic cores.
A simple strategy is to assemble non-mesomorphic but lipid-like organic strands
(polycatenary imino-bipyridine ligands) around d-block transition metals to promote
formation of a liquid-crystalline state. Our discovery briefly described in the previ-
ous section (see above) that a stable copper(I) helicate containing bridging bis-
imino-bipyridine subunits is formed selectively and quantitatively by a cooperative
process prompted exploration of polycatenary substituted imino-bipyridine ligands as
building blocks. These non-discoidal units represent the key component for inclusion
of the local molecular architecture (double helix) into an organized macroscopic
ensemble. Many kinds of Schiff-base bipy ligands bearing an increasing number of
flexible chains and aromatic cycles have been prepared and studied (Figure 7.3). All
these new frameworks display well defined melting points (non-mesomorphic mate-
rial) and formed selectively deep-green dinuclear complexes with copper(I) precur-
sors. Even the presence of very bulky gallate-ether substituents does not significantly
perturb the helicoidal arrangement, which was deduced, by analogy with standard
ligands, from NMR data. A prototypical example is given in Figure 7.7.
In the case of complex
15
, formed from ligand
14
and copper(I) salts, a well defined
columnar mesophase was formed and observed at room temperature [26].
Cr
accounts for
crystal,
Col
h
for columnar phase with an hexagonal symmetry and
Iso
for isotropic melt.
D
H
is the enthalpy changes measured during the phase transition Cr 25
C(DH 132 kJ
mol
1
) Col
h
181
C(DH 3.1 kJ mol
1
) Iso.
This system, being the first liquid-crystalline metallohelicate, is a rare example of a
room temperature metallomesogen and illustrates the exceptional organizational ability
of copper(I) cations. It is, in fact, remarkable that this tiny cation is able to induce order
at both molecular and supramolecular levels considering the large volume and structural
disorder inherent to the non-disklike ligand. This point is well illustrated by comparing
molar volumes calculated for the ligand (
V
m
¼1356 cm
3
mol
1
) and the cation (
V
m
¼2.2
cm
3
mol
1
) [26]. The strength of the bonds between the copper(I) cation and the hetero-
cyclic amine certainly also contribute to the formation of the final edifice.
The key element of this approach lies with the helix providing rigidity and polarizabil-
ity to counterbalance the flexibility and non-polarizability of the paraffinic chains. It is
this subtle balance between organized and chaotic domains that controls the fate of the
mesomorphic material. The ability of imino-bipyridine ligands to form copper(I) helicates
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