Unlocking Inverted Singlet-Triplet Gap in Alternant Hydrocarbons with Heteroatoms

Abstract

Fifth-generation organic light-emitting diodes exhibit delayed fluorescence even at low temperatures, enabled by exothermic reverse intersystem crossing from a negative singlet-triplet gap (STG), where the first excited singlet lies anomalously 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 derived from 77 parent scaffolds (2--6 rings). Using a multi-level workflow combining structural stability criteria with accurate L-CC2 excited-state calculations, we identify 72 heteroaromatic candidates with STGs$<0$. Notably, this includes BN-helicenes, where inversion arises from through-space charge-transfer states. Several systems exhibit non-zero oscillator strengths, supporting their potential as fluorescent emitters. Our findings reveal new design motifs for DFIST beyond known frameworks, expanding the chemical space for next-generation emitters based on heteroatom-embedded aromatic systems.