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 yi [defined by the occupation numbers of the LUNO (= Lowest Unoccupied Natural Orbital) + i (i = 0, 1, ...), where 0 ≤ yi ≤ 1 and yi ≥ yj (i > j)] using the linear H4 full configuration interaction model. The diradical characters suited for singlet fission are found to lie in the region with y0 > 0.10 except for y0 ∼ y1, though its energy efficiency is better in case of smaller y0, 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.