Four-state conical intersections: The nonradiative deactivation funnel connected to O-O homolysis in benzene endoperoxide

► Influence of the electronic structure of endoperoxides in the UV absorption spectrum and photochemical paths leading to rearrangement products and singlet oxygen. ► O-O photolysis deactivation pathway in benzene endoperoxide. ► Characterization of a four state conical intersection. ► Obtention of the 9th dimensional branching space of the 4th order intersection point.