Atom-Response-Theory-Guided Design of Chiral Niobium Halides with both a Large Nonlinear Coefficient and Strong Chiroptical Nonlinearity

Chiral hybrid organic-inorganic metal halides (HOIMHs) with noncentrosymmetric structure have attracted significant attention for nonlinear optics (NLO). Herein, guided by atom response theory (ART), we screened representative lead-free metal halide polyhedra and found that [NbCl₆]^- exhibits the highest linear polarizability. Therefore, we constructed the first chiral niobium-based HOIMHs, (<i>R</i>/<i>S</i>-3BrMBA)NbCl₆ (<i><b>R</b></i><b>/</b><i><b>S</b></i><b>-Nb</b>), and investigated their NLO properties. As expected, <i><b>S</b></i><b>-Nb</b> exhibits efficient second harmonic generation (SHG) under 770-1000 nm excitation. Most importantly, the effective second-order NLO coefficient (<i>d</i>_eff) of <i><b>S</b></i><b>-Nb</b> reaches 6.77 pm/V, representing the highest value among the reported zero-dimensional chiral HOIMHs. Moreover, <i><b>S</b></i><b>-Nb</b> demonstrates strong SHG circular dichroism (SHG-CD) under circularly polarized excitation, with an anisotropy factor (<i>g</i>_SHG-CD) of 1.37, which is one of the highest values among chiral HOIMHs. Our results highlight the potential of chiral niobium halides toward nonlinear chiroptical applications and provide an effective ART-guided strategy for designing high-performance NLO material through polarizability and band-gap engineering.