Decoding Aromaticity in the [24]paracyclophanetetraene Anions

Paracyclophanetetraene (PCT) anions are known for their extraordinary stability that determines cycling performance under fast-charge/fast-discharge conditions, making PCT a suitable material for energy storage in batteries. In previous studies, it was suggested that the unique energy-harnessing capabilities of PCT are associated with the redox-based aromaticity switching from local 24𝜋-aromaticity (PCT) through global 26𝜋-aromaticity (PCT2-) to 28𝜋-antiaromaticity (PCT4-), in accordance with the famous 𝜋-electron bookkeeping rules by Hückel ("4n+2" / "4n"). In this work, however, we call into question this interpretation by showing that the ability of PCT to accumulate up to four electrons is associated with the superposition of the ionic forms that represent the migration of multiple 𝜋-sextets in accordance with Clar's rule. Our findings challenge existing paradigms and offer insight into the resonance-electronic structure of the conjugated macrocycles to better understand the origins of their exceptional electron-transport capabilities and thermodynamic stability.