Triple Stranded Porphyrin Nanobelts

Molecular nanobelts are fascinating analogues of carbon nanotubes. Their rigid geometries and strongly coupled π-electrons have the potential to generate a wavefunction resembling that of a quantum ring. Here we report the synthesis of triple-stranded nanobelts consisting of 8–12 edge-fused porphyrin units with diameters of 21–32 Å. These nanobelts were synthesized by nickel-mediated coupling of meso-bromoporphyrins, to form singly-linked nanorings, followed by oxidation with gold(III) chloride. Experimental 1H NMR spectra, supported by computational simulations, reveal that belts containing odd numbers of porphyrins, with circuits of 90 or 110 π-electrons, display global aromatic ring currents, whereas even-numbered belts, with 80, 100 or 120 π-electrons, are globally antiaromatic. These are the largest neutral molecules yet to have been found to sustain global ring currents.