Unlocking Inverted Singlet-Triplet Gap in Alternant Hydrocarbons with Heteroatoms

Abstract

Fifth-generation organic light-emitting diodes exhibit delayed fluorescence enabled by exothermic reverse intersystem crossing due to a negative singlet–triplet gap, where the first excited singlet lies below the triplet. This phenomenon, termed delayed fluorescence from inverted singlet and triplet states (DFIST), has been experimentally confirmed only in two triangular molecules with a 12-annulene periphery and a central nitrogen atom. Here, we report a high-throughput virtual screening of 30,797 BN-substituted polycyclic aromatic hydrocarbons (BNPAH) derived from 77 parent scaffolds with 2–6 rings. Using a multi-level workflow that combines structural stability criteria with accurate excited-state calculations, we identify the top 46 DFIST-BNPAH candidates with singlet-triplet gaps less than -0.015 eV. Notably, this set includes BN-helicenes, where inversion arises from through-space charge-transfer states. Our findings reveal new design motifs for DFIST that extend beyond known frameworks, thereby expanding the chemical space for next-generation emitters based on heteroatom-embedded aromatic systems.