Theoretical study on the relationship between diradical character and second hyperpolarizabilities of four-membered-ring diradicals involving heavy main-group elements.

By using spin-unrestricted density functional theory methods, the relationship between the diradical character y and the second hyperpolarizability γ (the third-order nonlinear optical (NLO) properties at the molecular scale) for four-membered-ring diradical compounds, that is, cyclobutane-1,3-diyl, Niecke-type diradicals, and Bertrand-type diradicals, were investigated by focusing on the substitution effects of heavy main-group elements as well as of donor/acceptor groups on the y and γ values. It has been found that i) γ is enhanced in the intermediate y region for these four-membered-ring diradicals, ii) Niecke-type diradicals with intermediate y values, which are realized by tuning the combination of the main-group elements involved, exhibit larger γ values than Bertrand-type diradicals, and iii) the y value and thus γ value can be controlled by modifying the both-end donor/acceptor substituents attached to carbon atoms in Nicke-type C2 P2 diradicals. These results demonstrate that four-membered-ring diradicals involving heavy main-group elements exhibit high controllability of the y and γ, which indicates the potential applications of four-membered-ring diradicals as a building block of highly efficient open-shell NLO materials.