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.