Molecular quantum ring formed from a pi-conjugated macrocycle

The electronic structure of a molecular quantum ring (stacks of 40-unit cyclic porphyrin polymers) is characterised via scanning tunnelling microscopy and scanning tunnelling spectroscopy. Our measurements access the energetic and spatial distribution of the electronic states and, utilising a combination of density functional theory and tight binding calculations, we interpret the experimentally obtained electronic structure in terms of coherent quantum states confined around the circumference of the π-conjugated macrocycle. These findings demonstrate that large (53 nm circumference) cyclic porphyrin polymers have the potential to act as molecular quantum rings.