Theoretical Design of Open-Shell Singlet Molecular Systems for Nonlinear Optics

Design principles toward open-shell singlet molecular systems exhibiting remarkable nonlinear optical (NLO) responses are derived based on a two-site diradical model within the valence configuration interaction scheme. In this scheme, the concept of diradical character (y), a chemical index for bond weakness in the ground state, enables to classify singlet molecular systems into three categories: (i) closed-shell (y = 0), (ii) intermediate diradical (0 < y < 1), and (iii) pure open-shell (y = 1) systems. It is found that the second hyperpolarizabilities (and also the first hyperpolarizabilities for asymmetric systems) are enhanced in the intermediate diradical character region with respect to the other ones. This leads to a new class of open-shell singlet NLO systems, which are expected to outstrip traditional closed-shell NLO systems. On the basis of this principle, practical molecular design guidelines for tuning the diradical characters and, therefore, for achieving large NLO properties are discussed o...