Exploring non-covalent interactions for creating complex molecular architectures and assemblies

S. Ramakrishnan, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore

Molecules are generally constructed by linking the constituent atoms through covalent bonds that provide only a limited flexibility to the atomic subunits in terms of their relative positions. Covalent bonds being relatively strong, the limited conformational freedom that molecules do enjoy arises mainly from the facile rotation around certain types of covalent bonds, namely σ-bonds. The presence of other types of bonds, such as π-bonds, lead to significantly restricted bond rotation and consequently the shape and size of the molecules become increasingly rigid and fixed. Non-covalent bonds, on the other hand, permit a much greater degree of flexibility as they are relatively weak and can often be broken and re-formed at room temperature. This reversibility provides for dynamic control and modulation of the structure and shape of molecular entities, which can serve as an “error-correction” mechanism for the final optimization of a specific function. The importance of such weak reversible non-covalent interactions has been long recognized as being central to biological structure and function.

More than a decade ago, Lehn ushered in a new era that brought to focus the range of possibilities that exist for the design and construction of “Supramolecular” systems, which utilized several types of weak non-covalent inter-molecular interactions for the control of size, shape and properties of molecules and molecular assemblies. Since then the field has witnessed rapid progress with chemists exploring this realm for a variety of applications, such as - (a) building macromolecules wherein non-covalent interactions are used to assemble the long polymer chains (supramolecular polymers), (b) the control of solution conformation and properties of macromolecules (Foldamers), (c) assembly of functional molecular entities (d) the generation of “molecular imprints” in polymer matrices, etc. A key feature in many of the above approaches is the utilization of multiple weak interactions that collectively aid in the structuring/assembly process. In my introductory presentation, I shall briefly review the evolution of this field using some illustrative examples and shall conclude by discussing possible scenarios for future development.

References:
1. Zemb, Thomas and Blume, Alfred., Self-assembly: weak and specific intermolecular interactions at work. Current Opinion in Colloid and Interface Science 8 (2003) 1–42.

2. Lehn, Jean Marie, Toward complex matter: Supramolecular chemistry and self-organization, PNAAS, April 16, 2002, vol. 99, no. 8, 4763–4768.

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