Unraveling the flexible aromaticity of C 13 H 9 +/0/- : a 2D superatomic-molecule theory.

Phenalenyl (C 13 H 9 ) is the smallest triangular unit of a graphene nanosheet, and has been experimentally verified to be stable in radical (C 13 H 9 ˙), cationic (C 13 H 9 + ), and anionic (C 13 H 9 - ) 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 C 13 H 9 +/0/- and deciphers their aromaticity using a recently emerged two-dimensional (2D) superatomic-molecule theory. 12π-C 13 H 9 + , 13π-C 13 H 9 ˙, and 14π-C 13 H 9 - are seen as triangular 2D superatomic molecules ◊ O 3 , ◊ O 3 - , and ◊ O 3 2- , 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 via superatomic lone pairs and covalent bonds. C 13 H 9 +/0/- 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 C 6 H 3 N 7 , C 18 H 6 N 22 and graphitic carbon nitride (g-C 3 N 4 ) crystals, and meanwhile accounting for the special stability of g-C 3 N 4 . This work enriches 2D superatomic bonding chemistry and provides a useful strategy to design new 2D functional nanostructured materials.