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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.
Minami et al. (Fri,) studied this question.