Chemistry Reference
In-Depth Information
It is worth pointing out here that redox cycling to produce topological reorganization
processes in appropriate scaffoldings is a concept, which has led to the discovery of
molecular hysteresis [47,48], molecular memory [49], molecular machines [50], and the
precursors of molecular electromechanical motors [51]. In particular, redox-induced
movement occurring in multifunctional systems incorporating multistrand arms with dis-
tinct (soft or hard donors) coordination sites has been found by changing the oxidation
state of iron cation [52,53]. As well, electrochemically induced molecular and ring-glid-
ing motions in pseudo-rotaxane and catenate derivatives have been observed in copper(I)
complexes containing different interlocking rings [54].
One clear advantage of these imino-based systems is that very stable complexes can be
formed and that the terminal imino group can be readily functionalized without disruption
to the helicate structure. This provides the key element for inclusion of the local molecu-
lar architecture into an organized macroscopic ensemble, such as calamitic mesophases.
7.2.1 Liquid Crystals from Imino-Polypyridine Based Helicates
Metallomesogens (liquid crystals containing metal ions) have become subject to increas-
ing interest, since the introduction of transition metal centers in particular into liquid-
crystalline material which may results in the significant modification of physical propert-
ies such as color, conduction, magnetism, or redox behavior [55,56]. The ability of transi-
tion metal ions to adopt different coordination geometries and to organize elemental
synthons around a central core in principle permits the preparation of a wide variety of
novel metallomesogens. To date, many different metallic centers and coordination geom-
etries have been incorporated into metallomesogens [57].
Chemical information, as expressed through molecular recognition, provides a means
to direct the spontaneous formation of supramolecular species from complementary com-
ponents. Although control of the supramolecular structure is still a demanding challenge,
careful choice of the molecular building blocks makes it possible to predict the nature of
theemergentassembly.Oneofthemajorchallenges remaining in the area of supra-
molecular chemistry concerns the identification of viable applications (macroscopic func-
tion), other than by analytical chemistry, via the integration of individual supramolecular
species into an organized network that can be addressed macroscopically [58]. The chal-
lenge here is to obtain more functional systems by selective coordination processes
around metallic cores, in order to build sophisticated molecular scaffolds (such as helices,
grids, ladders, cyclic helicates, numerous polyhedral species) and to provide access to
even more sophisticated systems mimicking basic biological functions. Additionally, the
conjunction of molecular processes (e.g., metal-induced organization of ligands around
metallic cations to produce selectively targeted assemblies) and self-assembly of these
units at the macroscopic level to form liquid-crystalline materials offers many interesting
features, where unexpected properties could emerge.
Indeed, major goals for the ever-developing field of supramolecular chemistry are:
(i) to relate local molecular architecture to macroscopic ordering of the system, (ii) to
identify useful applications other than analytical chemistry, and (iii) to integrate individ-
ual supramolecular species into organized networks. Such large-scale organization is
probably essential for the construction of practical devices from intricate molecular units
[59]. Despite this realization little genuine progress has been made with regard to the
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