Unraveling the flexible aromaticity of CH: a 2D superatomic-molecule theory.

Phenalenyl (CH) is the smallest triangular unit of a graphene nanosheet, and has been experimentally verified to be stable in radical (CH˙), cationic (CH), and anionic (CH) states. All these three species feature high symmetry and stability as well as delocalized π electrons, a visible sign of aromaticity, but their aromatic origin remains a challenge. This work reports new chemical insights into the π electrons of CH and deciphers their aromaticity using a recently emerged two-dimensional (2D) superatomic-molecule theory. 12π-CH, 13π-CH˙, and 14π-CH are seen as triangular 2D superatomic molecules O, O, and O, respectively, where O denotes a 2D benzenoid superatom bearing 4 π electrons. Visualized superatomic Lewis structures show that each O can dynamically adjust its π electrons to satisfy the superatomic sextet rule of benzene superatomic lone pairs and covalent bonds. CH are representatives of adaptive aromaticity in the 2D superatomic-molecule system, exhibiting flexible π electronic structures to achieve shell-closure. Moreover, we specially adopt a progressive methodology to study the evolution of 2D periodic materials, by applying this theory to the similar family of CHN, CHN and graphitic carbon nitride (g-CN) crystals, and meanwhile accounting for the special stability of g-CN. This work enriches 2D superatomic bonding chemistry and provides a useful strategy to design new 2D functional nanostructured materials.