Third-Order Nonlinear Optical Properties of Open-Shell Systems: Diradical Character and Spin State Dependences

The third-order nonlinear optical (NLO) properties of open-shell singlet molecular systems are investigated using model and real molecular systems by the ab initio molecular orbital (MO) and density functional theory (DFT) approaches. The diradical character dependence of the second hyperpolarizability (γ)—the microscopic origin of the third-order NLO properties—is elucidated as well as its spin state dependence: γ values for the singlet systems with intermediate diradical character are remarkably enhanced as compared to those of closed-shell and pure diradical systems, and the γ values for intermediate diradical systems are significantly reduced by changing from singlet to triplet state. Such features are exemplified by two-site diradical model systems, diphenalenyl diradicaloids, and nanographenes. The γ values of multi-radical systems including one-dimensional diphenalenyl diradical clusters are also examined, while the intermolecular interaction effects and the size dependence of γ per unit are discussed toward realizing a new class of open-shell NLO materials.