NO-modulated triplet ground state and two-state antiaromaticity in BN-doped cyclobutadienes: a combined DFT and machine learning study

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. Unlike attachment to nitrogen or carbon, NO bonding to boron (2) induces two-state antiaromaticity, as confirmed by Nucleus-Independent Chemical Shift (NICS), Electron Localization Function (ELF_π), NICS-grid, and Isomerization Stabilization Energy (ISE) analyses. Furthermore, compounds with NO at boron (2 and 10) exhibit triplet ground states. Spin density mainly localizes on NO, driving antiaromaticity in T₁. Principal Interaction Orbital (PIO) and Principal Interacting Spin Orbital (PISO) analyses reveal that exocyclic BN double bond formation enforces planarization and enables localization in the S₀ and T₁ states, leading to two-state antiaromaticity in 2. K-means clustering combined with principal component analysis (one of the most commonly used unsupervised machine learning algorithms) classified BN-doped CBDs based on their electronic and structural properties, uniquely isolating 2 due to its distinct substituent positions and aromaticity behaviors. These findings highlight an important role of the substituent position in tuning electronic and aromatic properties.