Global Aromaticity in Neutral Porphyrin Nanobelts

The ability of a ring-shaped molecule to sustain a global ring current, when placed in a magnetic field, indicates that its electronic wavefunction is coherently delocalized around its whole circumference. Large molecules that display global ring currents are attractive components for molecular electronic devices because it should be possible to control charge transport through these molecules using quantum interference. Here, we use theoretical methods to investigate how the global ring currents in molecular nanobelts built from edge-fused porphyrins evolve with increasing ring size. Our predictions were validated by using coupled clusters to construct a density functional approximation (denoted OX-B3LYP) that is specifically tailored to accurately describe these nanobelts. The results indicate that a global ring current persists in neutral belts consisting of up to about 22 porphyrin units, with Hückel circuits of 220 electrons (circumference 18.6 nm), which is surprising because global ring currents have not previously been reported in neutral macrocycles of this size.