Inverting singlet–triplet gaps by design

Inverted singlet–triplet (INVEST) emitters offer a promising path toward efficient, metal-free organic light-emitting diodes (OLEDs) by enabling exothermic reverse intersystem crossing. However, rational design of such materials remains elusive due to the complex interplay of structural and electronic factors. Here, we present a robust computational framework that screens 212 derivatives of phenalene, uthrene, and zethrene cores using a multi-tiered quantum chemical workflow (TD-DFT, SA-CASSCF, and SC-NEVPT2). We identify 15 novel INVEST molecules with negative singlet–triplet energy gaps and reveal generalisable design principles involving compact π-conjugation, heteroatom doping, and strategic fluorination. Crucially, solvent modeling confirms that INVEST behavior is retained across diverse environments, highlighting solution-processable potential. These findings advance the molecular design of metal-free triplet harvesters and offer a predictive toolkit for next-generation OLED materials.