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.