Nannulene

Nannulene, or [n]annulene, refers to a family of monocyclic, fully conjugated hydrocarbons composed of n carbon atoms arranged in a ring, with n pi electrons delocalized around the loop. The general formula is often cited as CnHn, reflecting one hydrogen on each ring carbon and a continuous pathway of p orbitals for electron delocalization. The chemistry of nannulenes is closely tied to aromaticity and antiaromaticity as described by Hückel’s rule.

According to Hückel theory, planar cyclic systems are aromatic if they contain 4m+2 pi electrons. For nannulenes, this means rings with n = 6, 10, 14, 18, and so on, would be candidates for aromatic stabilization if they could adopt planarity. In practice, many nannulenes with 4m pi electrons (such as [8]annulene) are destabilized by antiaromatic character and tend to adopt nonplanar, puckered conformations to relieve electronic strain. Consequently, the existence and structure of real, isolable nannulenes depend on a balance between aromatic stabilization and the strain associated with ring size and geometry.

Synthesis and characterization of nannulenes become progressively challenging with increasing ring size due to strain, reactivity, and conformational flexibility. Nevertheless, researchers study these systems to probe the limits of aromaticity, to understand how planarity and topology influence electronic structure, and to explore potential applications in materials science and molecular engineering.