Excitation Energies and Optical Properties of Open-Shell Singlet Diradicaloids

Abstract

This chapter provides theoretical basis for the electronic states and optical properties of singlet diradicaloids using the valence configuration interaction model and high-precision quantum chemical calculations. The physical quantities are described by “diradical character”, which is a quantum chemically well-defined index for “bond weakness” in chemical sense or “electron correlation” in physical sense. The concept of a singlet diradicaloid was proposed nearly 50 years ago and is characterized by “two weakly interacting electrons with similar energy”. As mentioned in previous sections, the diradical character has a chemical concept of “instability of chemical bonds” proposed by Yamaguchi, and it is easily estimated by an approximate spin-projected UHF method. The present indices are also related to the effective neutral and ionic chemical bond orders, and they describe the degree of electron correlation for arbitrary electronic states, including ground and excited states, and thus for any superposition of electronic states.