Controlling aromaticity across electronic states is crucial for designing novel species. While aromaticity typically could be achieved in either the lowest singlet state (S0) or the lowest triplet state (T₁), dual-state aromaticity or antiaromaticity remains less developed. Herein, we demonstrate that NO-substitution uniquely induces antiaromaticity in both S0 and T1 states of 1,2-BN-doped cyclobutadiene (1,2-BN-CBD), initially nonaromatic in S0 and weakly aromatic in T1.

Cyclobutadiene (CBD) displays aromaticity in the lowest-lying triplet excited state (T1) according to Baird's 4n electron rule. Hence, antiaromatic CBD in the T1 state has never been reported so far. Here we demonstrate via density functional theory (DFT) calculations that the CBD ring could possess dual antiaromaticity in the lowest singlet state (S0) and T1 states (termed as adaptive antiaromaticity), which is supported by various aromaticity indices including NICS, ACID, ΔBL, ELF and ISE.