Theoretical study on the diradical characters and third-order nonlinear optical properties of transition-metal heterodinuclear systems

We investigate the correlation between the diradical characters and the second hyperpolarizabilities (γ) of several transition-metal heterodinuclear systems using the spin-unrestricted coupled-cluster method. Due to asymmetry, the longitudinal γ values are enhanced by the primary contribution of the dσ electrons with intermediate diradical character. In particular, large enhancements of γ amplitudes and/or change of signs are found for the systems composed of different group metal atoms due to the large asymmetric charge distribution originating from the difference in their ionization potentials. This result indicates a possibility of a novel class of efficient nonlinear optical materials based on transition-metal heterodinuclear complexes.