Mechanism Insights into Second-Order Nonlinear Optical Responses of Anionic Metal Clusters

We present the first-principle calculations on the electronic excitations and second-order properties in solution phase of two typical inorganic trinuclear anionic clusters, [MoCu₂S₄(SPh)₂]²⁻ and [Mo₂CuS₄]¹⁻(edt)₂(PPh₃) (edt=1,2-ethanedithiolato) in the framework of density functional theory (DFT). The computed excitation energies are in good agreement with the outcome of the measurements. The predicted values of the molecular quadratic hyperpolarizabilities are of the comparable order of those of the typical organometallic chromophores. We demonstrate the significant contributions to the second-order responses from the charge transfers between the metal centers (MMCT) which are ascribed to the direct metal–metal bonding interactions in these two charged clusters. This meaningful ligand-independent mechanism for the second-order response largely relates to metal–metal bonding strength, and the understanding will benefit to the future design of the new-generation molecular based nonlinear optical materials and optoelectronic devices by means of the conscious tuning of metal–metal interactions and metal-core structures of inorganic polynuclear clusters.