Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character

The origin of the diradical character dependence of the second hyperpolarizability (γ) of neutral singlet diradical systems is clarified based on the perturbation formula of γ using the simplest diradical molecular model with different diradical characters, i.e., H₂ under bond dissociation. The enhancement of γ in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, respectively, with the increase in diradical character. This feature confirms that open-shell singlet conjugated molecules with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose γ values can be controlled by the diradical character in addition to the conjugation length.