Theoretical study of third-order nonlinear optical properties in square nanographenes with open-shell singlet ground states

Using spin-unrestricted density functional theory methods, the second hyperpolarizabilities, γ, of square nanographenes presenting different diradical characters are investigated from the viewpoint of open-shell singlet nonlinear optical systems. For nanographenes with intermediate diradical character, the γ component along the armchair edges is enhanced and amounts to about four times the γ component along the zigzag edges, whereas these γ components are of similar amplitudes when the system displays nearly pure diradical character. These observations are in conformity with the diradical character dependence of γ predicted in our previous studies.