Role of the Radical\nCharacter in Singlet Fission:\nAn Ab Initio and Quantum Chemical Topology Analysis

The radical character of molecules exhibiting singlet\nfission is\nrelated to the energy level matching relationships that facilitate\nthis process. Using a linear H<sub>4</sub> model molecule, we employ\nquantum chemical topology descriptors based on full configuration\ninteraction calculations to rationalize singlet fission. In this context,\nthe influence of the closed-shell to diradical and diradical to tetraradical\ncharacter on the singlet fission energy matching conditions is analyzed.\nWe find that in the diradical limit the singlet fission efficiency\ncan be manipulated considering the active molecule coupled to an excited\ndiradical, while in the diradical to tetraradical limit, the efficiency\nis dependent only on the gap between the lowest-lying excited singlet\nand triplet state. Furthermore, our results reveal possible design\nstrategies for molecules with radical character exhibiting efficient\nsinglet fission.