Planar inorganic five-membered heterocycles with σ + π dual aromaticity in both S₀ and T₁ states

Cyclic species being aromatic in both the lowest singlet and triplet electronic states (so-called adaptive aromaticity) are scarce. To date, the reported systems have been mostly organometallic heterocycles with the aromaticities in the two states having the same origin of either σ- or π-electron delocalization (i.e., adaptive σ or π aromaticity). Herein, an exhaustive density functional theory study was conducted for 90 planar inorganic five-membered heterocycles in the forms of XY₄ and XY₂Z₂ (X = O or S; Y and Z = N, P, As, Sb or Bi). They all contain 6π electrons and thus should be aromatic and antiaromatic in the lowest singlet and triplet states, respectively, according to classical Hückel's 4n + 2 and Baird's 4n π-electron rules. To our surprise, however, several of them (e.g., ON₂As₂, ON₂Bi₂ and SAs₂Sb₂) exhibit considerable aromatic characters in both S₀ and T₁ states, as confirmed by multiple aromaticity indices. More interestingly, further analyses reveal that their aromaticities in the two states may unprecedentedly stem from both σ- and π-electron delocalization. Thus, they likely bear unusual adaptive σ + π dual aromaticity. By finding adaptive aromatics in rather simple inorganic unsaturated systems, our work extends this emerging aromaticity concept to the big inorganic world.