Size-Dependency of Hückel’s Rule in Neutral Molecules: Alternating Behavior in Global Ring Current of Furan-Acetylene Macrocycles

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.