Heterometal-driven symmetry breaking in metal-oxo clusters for highly nonlinear optical application

Abstract

Engineering geometric asymmetry in molecular architectures offers a powerful strategy for boosting third-order nonlinear optical (NLO) responses, yet such approaches have not been reported for metal-oxo clusters. Here, we synthesized three iron-oxo clusters [Fe₇(μ₄-O)₄(FcDCA)₆] (Fe₇OC), [Fe₆Ca(μ₄-O)₄(FcDCA)₆] (Fe₆CaOC), and [Fe₆Na(μ₃-O)₄(FcDCA)₆] (Fe₆NaOC) (ferrocene dicarboxylic acid = H₂FcDCA) with spatially distorted cluster cores via a heterometal incorporation strategy. Single-crystal X-ray diffraction analysis reveals incorporation of heterometal Ca or Na ions induces severe tetrahedral-core distortion through bond elongation, leading to breaking spatial symmetry in the geometric structure of the cluster core. Spectroscopy and Z-scan measurements demonstrated that the symmetry breaking of these geometric structures in the Fe-oxo clusters results in a narrowed optical band gap and a substantially increased third-order nonlinear absorption coefficient (β). Theoretical calculations indicate that the introduction of heterometals lowered the band gap of Fe-oxo clusters, enhanced the nonlinear hyperpolarizability, and improved the third-order NLO performance. Furthermore, the metal-oxo cluster-based polymer liquid crystal device prepared by mixing Fe₆NaOC clusters with nematic liquid crystals not only has high transmittance and electro-optical switch characteristics, but also has stronger optical limiting performance. This work not only develops new high-performance NLO metal-oxo cluster materials with geometric symmetry engineering, but also establishes metal-oxo cluster-based electro-optical smart devices for nonlinear optical limiting applications.