An 18-Porphyrin Nanoring at the Size Limit for Global Aromaticity

What is the size limit for global aromaticity? How large can a macrocyclic π-system be and still exhibit an aromatic ring current around its circumference? We address this question by investigating a π-conjugated butadiyne-linked 18-porphyrin nanoring (diameter 8 nm). This nanoring was synthesized by two different strategies: classical template-directed synthesis, using a radial template with 18 pyridyl binding sites, and Vernier templating, using a small hexapyridyl template. Both strategies are effective when the porphyrins have octyloxy side chains, but classical templating is more effective than Vernier templating when the porphyrins have bulky trihexylsilyl substituents. The size and shape of the nanoring were confirmed by scanning tunneling microscopy. ¹⁹F NMR oxidation titrations on the nanoring bound to a fluorinated template revealed shoulder signals indicating weak global aromatic and antiaromatic ring currents in the 10+ and 12+ oxidation states, which have Hückel circuits of 242 and 240 π-electrons, respectively. These shoulder signals do not appear in control experiments with a split-ring complex consisting of the linear porphyrin 18-mer bound to the same fluorinated template, confirming that they arise from global ring currents. Nucleus-independent chemical shift calculations using the BLYP35 functional qualitatively predicted that the 18 porphyrin nanoring would exhibit global aromaticity in the 10+ and 14+ oxidation states and antiaromaticity in the 12+ and 16+ oxidation states, in keeping with the experimental results from NMR spectroscopy and with the Hückel rule. The experimental results show that the ring currents in this 18-porphyrin ring are weaker than those in the homologous 12-porphyrin ring by at least a factor of 2.