Unraveling the flexible aromaticity of C13H9+/0/-: a 2D superatomic-molecule theory

Phenalenyl (C13H9) is the smallest triangular unit of a graphene nanosheet, and has been experimentally verified to be stable in radical (C13H9), cationic (C13H9+), and anionic (C13H9-) states. All these three species feature high symmetry and stability as well as delocalized pi electrons, a visible sign of aromaticity, but their aromatic origin remains a challenge. This work reports new chemical insights into the pi electrons of C13H9+/0/- and deciphers their aromaticity using a recently emerged two-dimensional (2D) superatomic-molecule theory. 12 pi-C13H9+, 13 pi-C13H9, and 14 pi-C13H9- are seen as triangular 2D superatomic molecules O-LOZENGE(3), O-LOZENGE(3)-, and O-LOZENGE(3)2-, respectively, where O-LOZENGE denotes a 2D benzenoid superatom bearing 4 pi electrons. Visualized superatomic Lewis structures show that each O-LOZENGE can dynamically adjust its pi electrons to satisfy the superatomic sextet rule of benzene via superatomic lone pairs and covalent bonds. C13H9+/0/- are representatives of adaptive aromaticity in the 2D superatomic-molecule system, exhibiting flexible pi 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 C6H3N7, C18H6N22 and graphitic carbon nitride (g-C3N4) crystals, and meanwhile accounting for the special stability of g-C3N4. This work enriches 2D superatomic bonding chemistry and provides a useful strategy to design new 2D functional nanostructured materials.

Automated summary

This study reveals the aromaticity of C13H9+/0/- using a two-dimensional superatomic-molecule theory, and shows that they can adjust their pi electron structures to achieve shell-closure through superatomic lone pairs and covalent bonds. C13H9+/0/- are representatives of adaptive aromaticity in the 2D superatomic-molecule system. The research also investigates the evolution of 2D periodic materials by studying the similar family of C6H3N7, C18H6N22 and graphitic carbon nitride (g-C3N4) crystals, enriching the field of 2D superatomic bonding chemistry and providing a useful strategy for designing new 2D functional nanostructured materials.