Diradical Character View of Singlet Fission

The feasibility conditions of singlet fission on the excitation energy differences are revealed as functions of the multiple diradical characters y i [defined by the occupation numbers of the LUNO (= Lowest Unoccupied Natural Orbital) + i ( i = 0, 1, ...), where 0 ≤ y i ≤ 1 and y i ≥ y j ( i > j )] using the linear H 4 full configuration interaction model. The diradical characters suited for singlet fission are found to lie in the region with y 0 > 0.10 except for y 0 ∼ y 1, though its energy efficiency is better in case of smaller y 0, to which diradical and multiradical compounds with low/intermediate diradical characters such as open-shell singlet polycyclic aromatic hydrocarbons belong. These findings indicate that the multiple diradical character is an effective indicator for exploring molecular systems for efficient singlet fission.