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Accepted · first revision · record

Title: Size and sign of inverted singlet-triplet gaps in heptazine molecules
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.
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 $\Delta_{\mathrm{ST}}$.
Settled by: High-order coupled-cluster or selected configuration interaction gaps on the full series compared with spectroscopically measured gaps.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: partially-resolved
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.
Posed since: (empty)