Global aromaticity in 2D macrocyclic polyradicaloids and 3D fully conjugated molecular cages

Aromaticity is a vital concept in organic chemistry to govern the electronic properties of π-conjugated molecules. Over a century, different types of aromaticity rules including Hückel, Möbius, and Baird aromaticity rules have been proposed and validated, depending on the number of delocalized π-electrons, topological structures, and spin states. On the other hand, π-conjugated systems with open-shell character have attracted increasing attentions recently, and many stable diradicaloids/polyradicaloids have been prepared. In particular, moderate antiferromagnetic coupling between the frontier π-electrons in a type of two-dimensional (2D) open-shell macrocycles would facilitate efficient π-electron delocalization and lead to unique global aromaticity. In this chapter, global aromaticity in 2D macrocyclic polyradicaloids is discussed, with emphasis on the fundamental relationships to molecular geometry, spin state, and radical character. Moreover, three-dimensional (3D) global aromaticity in fully conjugated molecular cages is also elaborated, and a perspective on possible spherical aromaticity in 3D π-conjugated system is presented.