Quantifying the electronic dimension of global aromaticity in macrocycles

Characteristic magnetic shielding patterns are commonly associated with ring currents, and therefore aromaticity, in large molecular structures. However, the relationship between ring currents and aromaticity is controversial, and this problem is not helped by the fuzzy definition of aromaticity. Cycloparaphenylene and porphyrin nanorings exhibit macrocyclic ring currents and/or aromatic stabilization energy in some oxidation states. Here, we extend the analysis of the aromaticity in these molecules to include the electronic delocalization criterion. Using the recently developed inter-fragment delocalization index, we show the macrocycles which have been previously described as aromatic based on NMR evidence also show dramatically enhanced electronic delocalization compared to non-aromatic references. By changing the DFT functional, we demonstrate that the magnitude of the macrocyclic ring current is correlated with the amount of macrocyclic electron delocalization. Our results strengthen past assignments of (anti)aromaticity in cycloparaphenylenes and porphyrin nanorings by evaluating their cyclic electronic delocalization, which is the hallmark of (anti)aromaticity.