In comparison with the widely recognized π aromaticity, σ aromaticity is a less developed concept in chemistry, especially for unsaturated systems. Moreover, most studies on σ aromaticity have been mainly limited to the ground state of saturated systems; unsaturated species with σ aromaticity in the excited state have never been reported. Here we demonstrate that a recently synthesized tetraatomic boron species is σ aromatic not only in the lowest singlet state but also in the lowest triplet state according to density functional theory (DFT) calculations, supported by numerous aromaticity indices, such as nucleus-independent chemical shift (NICS), anisotropy of the induced current density (ACID), and electron density of delocalized bonds (EDDB). In addition, several isomers are predicted to possess a triplet ground-state character. Our findings not only highlight the importance of developing the concept of σ aromaticity in unsaturated systems, which are traditionally reserved as the domain of π aromaticity, but also extend the concept of adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) into the σ system.

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00181