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.