Dibenzo-p-dioxin. Twisted and puckered excited state molecular geometries

[Display omitted] •Excited state molecular geometries calculated with a variety of theoretical models.•Predictions at variance with previous assumptions of planar geometries.•Twisted D2 symmetrical structure predicted for the S1 state of dibenzo-p-dioxin.•Puckered dienediyl-like structure predicted for the T1 state of dibenzo-p-dioxin. The title compound is generally acknowledged to have a planar D2h symmetrical molecular geometry in the ground state S0, and previous theoretical and experimental investigations seem to support the assumption of similar planar geometries in the excited singlet and triplet electronic states, S1 and T1. But a variety of theoretical models predict non-planar equilibrium geometries for these states: In the lowest excited singlet state (S1) a twisted, propeller-like geometry with D2 symmetry is predicted, while a strongly puckered, biradicaloid dienediyl-like structure is predicted for the triplet state (T1).