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

Abstract

The fundamental of diradical-character-based molecular design for singlet fission is clarified through the correlation between the diradical character, the first singlet (S1) and triplet (T1) excitation energies, the frontier orbital energy gap, and the energy level matching condition (2E(T1) – E(S1) ≈ 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 S1–T1 gap is found to be a key factor for governing the amplitude of E(T1) for 2E(T1) – E(S1) ≈ 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.