CdF(C6H4NO2)(H2O): a UV nonlinear optical material with unprecedented SHG and birefringence via π-conjugated rings and a unique “Warren truss structure”

Abstract

We report the design and synthesis of a novel ultraviolet (UV) nonlinear optical (NLO) material, CdF(C₆H₄NO₂)(H₂O), featuring a unique “Warren truss structure”. This material exhibits a two-dimensional (2D) layered architecture structure composed of highly polarized [CdNO₂F₃] octahedra and π-conjugated organic rings (C₆H₄NO₂)⁻. Notably, CdF(C₆H₄NO₂)(H₂O) demonstrates exceptional second-harmonic generation (SHG) response, with an intensity 3.2 times that of KH₂PO₄ (KDP), and a large birefringence of 0.26@546 nm, which is highly unusual for UV fluorides with a bandgap of >4.2 eV. Theoretical calculations and structural analysis reveal that the introduction of (C₆H₄NO₂)⁻ into CdF₂ induces significant structural distortion and polarization, leading to the formation of a non-centrosymmetric “Warren truss structure”. This structure aligns [CdNO₂F₃] octahedra and organic rings in a highly ordered manner, which is crucial for the enhanced SHG and large birefringence. Our findings provide a new strategy for designing high-performance UV NLO materials by leveraging organic–inorganic hybrid structures.