Nanoscale aromaticity and supramolecular electronic materials
ARO-MAT will target emergent cooperative electronic and magnetic phenomena in molecules with dimensions of 5–25 nm (i.e. as big as many proteins). The project will develop supramolecular architectures with large pi-systems and well-defined geometries, in which the frontier orbitals coherently delocalize charge over the whole nanostructure. Aromaticity is a key emergent phenomenon; it can be defined as the ability of a cyclic molecule to sustain a ring current when placed in a magnetic field. Until recently, it was thought that aromaticity is restricted to small molecules, with circuits of less than about 22 pi-electrons. Anderson has shown that circuits of more than 160 pi-electrons (circumference > 15 nm) can exhibit strong aromatic ring currents. Testing even larger rings will elucidate the link between aromaticity and the persistent currents found in non-molecular mesoscopic rings (diameter 50–500 nm). ARO-MAT will explore the effects of molecular size and topology on nanoscale aromaticity. Other emergent phenomena to be addressed include the formation of open-shell singlet polyradical ground states, magnetic bistability in systems with many paramagnetic metal centers, and the control of charge transport through single-molecule devices by quantum interference. This multidisciplinary project combines organic synthesis, supramolecular chemistry, theory, electronic structure calculations, NMR and EPR spectroscopy, magnetochemistry, molecular electronics and low-temperature charge transport experiments. The core objective is to create low band gap materials with unprecedented electronic and magnetic properties, and to understand the structure-property relationships governing the behavior of these new materials. Most of the target structures are based on metalloporphyrins because of their redox activity, stability, structural versatility, suitability for template-directed synthesis and ability to position multiple strongly coupled paramagnetic metal centers.