Heavy main-group elements in supramolecular chemistry and nonlinear optical materials

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How far can we push chemical self-assembly? This is one of the 25 current "Big Questions" highlighted by the journal Science to mark its 125th anniversary (vol. 309, No. 5731, 1 July 2005, p. 95). During the last 30 years scientists have made astonishing progress in using the interactions between molecules to construct supramolecular structures with increasing complexity. This approach is often inspired by nature: hydrogen bonds (present in liquid water and DNA) and the coordination of metal ions (as observed in the active site of haemoglobin) have been the most-commonly used forces to drive the spontaneous organisation of small building blocks into large, complex, and sometimes functional structures. In a vertical direction, the goal is to build bigger and hierarchical structures. In the horizontal, the intention is to expand the self-assembly paradigm to take advantage of other interactions. The use of "pi-stacking" of rings (the "glue" that agglomerates carbon nanotubes) and "van der Waals forces" (the reason wax is solid under ambient conditions) is now well documented. Other intermolecular forces have not been used mostly because they have been poorly understood until now, as is the case for the inter-atomic contacts made by the heavy p-block elements (those on the lower right hand side of the periodic table). The succession of names given to this phenomenon parallels the evolution of our understanding: the initial "non-bonded contact", became "weakly bonding interaction" and now the term "secondary bonding interaction" (abbreviated SBI) is gaining increasing acceptance. This application seeks funding for a research program that pursues the use of SBIs of heavy p-block elements in supramolecular chemistry and molecular self-assembly. SBIs have distinctive features that can be exploited to control the properties of self-assembled materials. For example, while hydrogen bonds are barriers to electron mobility, SBIs enhance conductivity. In addition, SBIs can stabilise compounds with unusual compositions. A related line of research is concerned with advanced optical properties of compounds of heavy main-group elements that will permit transmission and processing of large amounts of information as light pulses.