Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of À 0.23 vs. Fc/ Fc + , and are partially oxidized in ambient conditions. Addi-tional oxidation to the dication results in aromaticity switching to a global 30πe À aromatic state, as indicated by the formation of a strong diatropic current observed in the 1 H NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.

Global aromaticity is commonly observed in charged macrocycles, but rarely in their neutral state. Therefore, the significance of global effects in neutral macrocycles, as well as their size dependency, remains an open question. Here we present a series of furan-acetylene macrocycles, ranging from pentamer to octamer, which display alternating contributions of the global aromatic and antiaromatic ring currents at their neutral state. Calculations and X-ray structures reveal relatively planar geometries regardless of the macrocycle size. We find that the odd-membered macrocycles display global aromatic characteristics, while the even-membered macrocycles display contributions from globally antiaromatic ring current. These factors are expressed electronically (oxidation potentials), optically (emission spectra), and magnetically (chemical shifts). DFT calculations support the experimental findings, showing HOMO and LUMO degeneracy for the odd-membered macrocycles and HOMO-1 and LUMO+1 degeneracy for antiaromatic macrocycles. The global characteristics are more pronounced for small macrocycles and are attenuated for larger members, establishes furan-acetylene macrocycles as shape-persistent models for the study of global aromaticity.

Aromaticity can be assigned by Hückel’s rule, which predicts that planar rings with delocalized (4n + 2) π-electrons are aromatic, whereas those with 4n π-electrons are antiaromatic. However, for neutral rings, the maximal value of “n” to which Hückel’s rule applies remains unknown. Large macrocycles exhibiting global ring current can serve as models for addressing this question, but the global ring current are often overshadowed in these molecules by the local ring current of the constituent units. Here, we present a series of furan-acetylene macrocycles, ranging from the pentamer to octamer, whose neutral states display alternating contributions from global aromatic and antiaromatic ring currents. We find that the odd-membered macrocycles display global aromatic characteristics, whereas the even-membered macrocycles display contributions from globally antiaromatic ring current. These factors are expressed electronically (oxidation potentials), optically (emission spectra), and magnetically (chemical shifts), and DFT calculations predict global ring current alternations up to 54 π-electrons.