Enhanced inverted singlet-triplet gaps in azaphenalenes and non-alternant hydrocarbons

Inverted singlet-triplet gaps may lead to novel molecular emitters if a rational design approach can be achieved. We uncover a substituent strategy that enables tuning of the gap and succeed in inducing inversion in near-gapless molecules such as azupyrene. Based on known inverted-gap emitters, such as heptazine, we design substituted analogs with even more negative singlet-triplet gaps than in the parent systems. The inversion is lost if the reverse substituent-strategy is used. We thus demonstrate a definite set of conceptual design rules for inverted gap molecules.