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 on the basis of first-principles calculations performed on realistic open-shell singlet molecular systems.