Precise Singlet–Triplet Gap of Heptazine (7AP)

Determine the precise value of the singlet–triplet energy gap (S1–T1) of heptazine (7AP) to establish the exact magnitude and sign of the inversion in its lowest excited states.

Background

Heptazine (7AP) is among azaphenalenes reported to exhibit an inverted singlet–triplet gap, with theoretical best estimates (TBEs) predicting negative gaps and experimental observations on derivatives suggesting inversion. However, a precise, definitive value for the unsubstituted 7AP remains undetermined.

In this work, the authors note that while TBEs corrected for 0–0 excitation suggest a value around −0.083 eV, this differs from fluorescence measurements performed on a substituted 7AP (~−0.011 eV). The discrepancy underscores the need for an accurate determination of the STG for the parent 7AP molecule.

References

As the precise value of the STG of 7AP is unknown, the TBE values corrected for 0-0 excitation amounts to $-0.083$ eV, which is lower than $-$0.011 eV from the fluorescence measurements of a substituted 7AP.

— Influence of Pseudo-Jahn-Teller Activity on the Singlet-Triplet Gap of Azaphenalenes  (2407.04552 - Majumdar et al., 2024) in Section 3.2 (Geometries and Singlet-Triplet gaps)