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

Abstract

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 adaptive antiaromaticity, as confirmed by Nucleus-Independent Chemical Shift (NICS), Electron Localization Function (ELFπ), 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 adaptive antiaromaticity in 2. K-means clustering algorithm 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.