Many of the remarkable properties of molecular nanostructures are cooperative effects. A system is described as cooperative when it behaves differently from expectations based on the properties of its individual components. Multivalent cooperativity is crucial for biological molecular recognition, yet the factors determining the magnitude of this effect are poorly understood. Excitonic cooperativity is exploited in sensitive detectors for explosives, and is the basis of photosynthetic light harvesting. Electronic cooperativity is illustrated on the molecular scale by the phenomenon of aromaticity, and on a larger scale by metallic conductivity. Magnetic properties provide many examples of cooperativity. The magnitude of cooperative effects increases with the strength of coupling between the individual components, and with the number of coupled components. Cooperative systems exhibit sharp changes in behavior in response to small changes in conditions, such as transitions from free to bound, fluorescent to non-fluorescent, or conductive to insulating. The tendency towards an “all-or-nothing” response is often useful; in the limit of a very large ensemble, it leads to phase transitions. The CoSuN project will extend methodology developed in Oxford to create large monodisperse supramolecular nanostructures which are uniquely suited for exploring multivalent, excitonic and electronic cooperativity. The template-directed synthesis of these nanostructures is made possible by strong multivalent cooperativity, while the electronic coupling between the individual subunits results in other cooperative phenomena. This project will clarify understanding of cooperative molecular recognition. It will also help to solve some of the mysteries of photosynthesis and reveal the first molecular manifestations of coherent quantum mechanical phenomena, such as Aharonov-Bohm effects.
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