Fundamental of Diradical-Character-Based Molecular Design for Singlet Fission

The fundamental of diradical-character-based molecular design for singlet fission is clarified through the correlation between the diradical character, the first singlet (S-1) and triplet (T-1) excitation energies, the frontier orbital energy gap, and the energy level matching condition (2E(T-1) - E(S-1) approximate to 0 or < 0) for singlet fission by using the analytical solution of the electronic structure for a model system with two electrons in two orbitals. Moreover, the S-1-T-1 gap is found to be a key factor for governing the amplitude of E(T-1) for 2E(T-1) - E(S-1) approximate to 0. These findings are indeed justified by the spin-flip time-dependent density functional theory calculations for a series of typical alternant/nonalternant hydrocarbons, that is, phenacenes, acenes, and isobenzofulvene. The present results demonstrate that a weak diradical character is the underlying concept for efficient singlet fission molecules.