Elevating Nonlinear Optical Response Through D-Electron Modulation in Metal-Organic Frameworks

Electronic structure and excited state behavior is of pronounced influence on regulation of nonlinear optical (NLO) response. Herein, a serials of transition metal ions bearing different d-electron numbers were in situ coordinated within porphyrinic metal-organic frameworks (MOFs), creating NLO-responsive M-metal (metal=Fe, Co, Ni, Cu, and Zn) frameworks. It demonstrated that the NLO properties can be optimized with the increased occupancy of the d-shell, which enhances the degree of delocalization. Specifically, the full-filled (d¹⁰) electron configuration of Zn²⁺ stabilizes the electronic structure, combination with π-π* local excitation character of M-Zn, promoting charge transfer process and resulting in outstanding NLO properties. Moreover, parameters related to the nonlinear process (β, n₂, Imχ⁽³⁾, Reχ⁽³⁾ and χ⁽³⁾) of M-Zn are calculated to be higher than those of other materials, consistent with theoretical calculations. This work paves the way for NLO modulation based on electronic analysis and provides a promising approach for constructing high-performance NLO materials.