Nature of the “sudden polarization” effect and its role in photochemistry

The “sudden polarization” effect is described within the framework of the four-state model of asymmetrical diradicals. Its applicability for photochemical predictions is illustrated by the results obtained from the ab initio minimal-basis-set calculations followed by a very limited CI. The analysis of the reasons for the large charge separation in the low-lying singlet excited states and their particular polarities has been performed with the PPP full CI approach. The connection between an alternant-symmetry braking within the π-electron model and the origin of the “sudden polarization” effect for the hydrocarbon intermediates is demonstrated. A possible extension of similar phenomena to the excited states of molecular system with pronounced polar character is discussed.