Self-assembled Transition Metal Coordination Frameworks of ...

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harvesting systems, (d) redox active molecular squares for electrochemical sensing
and (e) molecular squares as catalysts. Due to the photo- and electrochemical
activities of incorporated transition metals and chromophoric ligands,
metallosupramolecular squares possess also considerable potential for applications in
molecular electronics [18]. Through metal-mediated self-assembly, it should be
possible to move from discrete molecular squares to more complex infinite 2D square
grids and networks, which might involve in the promising application as porous
functional materials also.
1.2.1. The method of self-assembly
The term ‘self-assembly’ is generally agreed to involve the spontaneous
assembly of molecules into stable, noncovalently joined aggregates displaying distinct
3-D order [11,19]. While coordinate bonds are highly directional and of greater
strength (bond energies ca. 10-30 kcal mol'l) than the weak interactions of biology
(bond energies ca. 0.6-7 kcal mol‘), they are nevertheless noncovalent in nature.
Indeed, they can be considered to have intermediate properties when compared to
covalent bonds (strong and kinetically inert) and the interactions of biology (weak and
kinetically labile) [ll]. Supramolecular chemistry can be defined as the chemistry
beyond the covalent bond or the chemistry of associates with a well-defined structure
[6,2O]. ln this regard, in supramolecular coordination chemistry, self-assembly is a
powerfiil approach which involves the encoding of coordination information into a
ligand, and then using a metal ion to interpret and use this information, according to
its own coordination preferences, in order to organize the growth of large polynuclear
metal ion arrays. Strategies to produce desired self-assembled coordination
frameworks of transition metal centers include design and synthesis of a
polyfunctional ligand and judicial utilization of its organizing ability to suitable metal
ions. Building grids or supramolecular architectures of nanoscopic dimensions from
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