Supramolecular compounds structural variations

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

Association of molecules changes their As as has been proven for metal complexes, charge transfer complexes, Lewis acids, etc. In any case, complex formation can be followed by the variation of the CD. Molecular association is the first step to an aggregation and, furthermore, is the presupposition for supramolecular structures. Compounds with inter-molecular exciton interaction, e.g., carotenoids or cyanine dyes, have been successfully analyzed. The spontaneous association to chiral associates from achiral compounds, e.g., cyanines, have been proven and analyzed with the help of ECD spectroscopy. In this context, the successful CD analyses with Langmuir-Blodgett films as well as the analyses of membranes have to be mentioned where suprastruc-tural chirality can also emerge from achiral monomers. 

The remarkable structural diversity of supramolecular aggregates of organozinc alkoxides and related compounds is possible because of the variations in the coordination numbers and geometry of the metal it is favored by the relatively strong donor-acceptor bonds formed between oxygen and zinc. Most of the supramolecular aggregates are preserved in hydrocarbon solution (e.g. benzene), when there is no competition from the solvent for the vacant coordination sites of the metal. 

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