{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"779860a1f4c470324cd1cc66fa4918ffa93a05fb441101733069ed276fa9afde","created":"2026-10-03T07:17:58Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"ebefe0d6e849144647b8f8302ba0d27ff2c402ed1e7bd3f850265013f7e43f6a","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"chem.excited-states.inverted-singlet-triplet","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"In almost all molecules the lowest triplet excited state lies below the lowest singlet, a pattern usually attributed to Hund's rule. A few nitrogen-rich molecules built on heptazine (a flat ring system of carbon and nitrogen) reverse this order, which could make organic light-emitting diodes more efficient.","posed_since":"","precise":"For heptazine and a series of substituted derivatives, determine the adiabatic gap $\\Delta_{\\mathrm{ST}}=E(\\mathrm{S1})-E(\\mathrm{T1})$ to 0.05 eV, its sign across the series, and which electronic-structure ingredient (double excitations, spin polarization, dynamic correlation) is necessary to get the sign right. An answer is near-exact gaps for the series validated against measured delayed fluorescence and reverse intersystem crossing rates, with a design rule for negative $\\Delta_{\\mathrm{ST}}$.","problem_ref":null,"references":"","settled_by":"High-order coupled-cluster or selected configuration interaction gaps on the full series compared with spectroscopically measured gaps.","status_note":"In 2022 Aizawa et al. (Nature, https://doi.org/10.1038/s41586-022-05132-y) reported delayed fluorescence from a molecule with an inverted gap; quantitative prediction across derivatives remains method-dependent.","title":"Size and sign of inverted singlet-triplet gaps in heptazine 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