Ordering, flexibility and frustration in arrays of porphyrin nanorings

The two-dimensional packing of cyclic porphyrin polymers, ‘nanorings’, on a graphite surface shows a rich and complex behaviour due to their internal symmetry and flexibility. Depending on the number of porphyrin groups within the nanoring we observe either a highly ordered hexagonal phase or frustrated packing driven by directional interactions which for some arrangements is combined with the internal deformation of the cyclic polymer. We study nanorings, c-PN, with N = 10 or 12 linked porphyrins which are deposited from solution onto a graphite surface and investigated using scanning probe microscopy. For c-P12 we observe a close-packed hexagonal array in which the porphyrins on neighbouring rings are aligned, but frustration occurs in arrays of c-P10 since close packing and co-alignment of neighbouring groups cannot be simultaneously realised. These observations are in excellent agreement with the computational predictions of the local ordering, stabilisation effects due to deformation, and the intermolecular interactions in the studied aggregates.