Electronic Absorption Spectra of Planar, Conjugated Biradicals in the Lowest Triplet State

Abstract

Abstract The semiempirical PPP‐SCF‐SCI method has been adapted to even‐electron, open‐shell π‐systems by performing the SCF calculation on the lowest triplet configuration and choosing a limited set of single electronic excitations from the latter for configuration interaction. The basic approximations and parameters of the standard PPP model were retained. The results were compared with experimental triplet‐triplet absorption spectra of aromatic hydrocarbons and with more elaborate calculations available for selected systems. The reliability of the model was found to be comparable to its well‐known performance in predicting the absorption spectra of closed‐shell π‐systems and of conjugated radicals. Triplet‐state absorption spectra for various planar, conjugated biradicals were calculated and, where possible, compared with experimental data.