5. Diradical Character View of Singlet Fission

Singlet fission is one of the internal conversion process in which a singlet exciton splits into two triplet excitons having long lifetimes. This phenomenon is expected to be useful for significantly improving the photoelectric conversion efficiency in organic photovoltaic cells. In this chapter, we present diradical character based molecular design guidelines for efficient singlet fission molecules based on the energy level matching conditions between the lowest singlet and triplet excited states, which are found to be described by the multiple diradical characters. A simple model, i.e., tetraradical hydrogen cluster, is investigated in order to reveal the multiple diradical character dependences of relative excitation energies and to build a diradical character based design guideline. On the basis of this guideline, several candidate molecules are proposed.