Synergistic Effect of π‑Conjugated [C(NH₂)₃] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C­(NH₂)₃Sb₂F₇ and C­(NH₂)₃SbF₄, were obtained by conjunction of [C­(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb–F bonds and H–F hydrogen bonds, the crystal structure of C­(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C­(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C­(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C­(NH₂)₃SbF₄ were attributed to the synergy between the planar [C­(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.