Cost-Effective Digital Discovery of Aza-Hydrocarbons via Isomerization for Inverted Singlet-Triplet Emitters

Hund's rule, derived from direct exchange (K), dictates that triplet (T₁) lies below singlet (S₁), leading to the 25% limit for electrofluorescence. Kinetic exchange (-t²/U) from charge-transfer perturbation can stabilize S₁ over T₁, enabling inverted singlet-triplet (IST) gaps and offering a promising route to triplet harvesting. Such materials are exceedingly rare, and systematic design remains elusive. Here, we employ newly developed target-state optimized density functional theory (TSO-DFT) for cost-effective excited-state calculations, yielding reliable IST signatures with the B3LYP functional. Subsequently, we apply a high-throughput virtual screening (HTVS) to explore nitrogen-doped hydrocarbons derived from C₁₀H₈, C₁₄H₁₀, C₁₆H₁₀, C₁₈H₁₂, and C₂₀H₁₂ molecular cores via isomerization. This workflow screens 118 081 viable structures down to 34 potential candidates exhibiting negative S₁-T₁ gaps, an appreciable oscillator strength of ≥0.01, and fast RISC rates (∼10⁸ s^-1). This study provides a discovery pipeline for novel IST emitters and a curated library of visible-light candidates for next-generation optoelectronics.