Electronic structure of the para-benzoquinone radical anion revisited

Abstract

Photoinduced dynamics of the para-benzoquinone anion features a subtle interplay between autodetachment and non-adiabatic transitions involving a dense manifold of resonances. We report the results of a multistate multireference perturbation theory study of the electronic structure of the para-benzoquinone anion in the ground, several low-lying excited electronic states, and in the lowest electron-detached state (the ground state of the neutral molecule). The electronic structure calculations revealed non-planar equilibrium geometry of the ²A_u excited state of the anion, but the effects of non-planarity on the shape of the absorption spectrum are found to be minor. Despite the large differences in the vertical excitation energies for the two lowest bright excited states, ²A_u (2.55 eV) and ²B_3u (2.93 eV), the simulated absorption spectra significantly overlap for the photon energies below 2.7 eV. Relevant minimum energy crossing points have been located using the CASSCF method. Excited-state deactivation channels are discussed in the context of accurate energetics and recent spectroscopic studies of the para-benzoquinone anion.