Designing Hybrid Sb-Based Nonlinear Optical Crystals with Large Birefringence by Combining the Lone Pair and π-Conjugated Genes

Abstract

The combination of two types of NLO-active units into one compound is beneficial to produce outstanding second harmonic generation (SHG) crystals. Herein, by incorporation of Sb³⁺ with SCALP electrons into planar π-conjugated 2-(aminomethyl)pyridine, two new noncentrosymmetric hybrid antimony (Sb³⁺)-based halides with zero-dimension (0D) structure, (C₆H₁₀N₂)₂SbX₆·X·H₂O (X = Cl and Br), have been synthesized. As expected, both compounds exhibit obviously SHG responses of 0.56 and 0.83 × KDP. Moreover, they also show clearly ultraviolet (UV) transparency with the ultraviolet cutoff edges at 360 and 393 nm, indicating their potential application as SHG crystals. Structural and theoretical analysis imply that their prominently optical properties are largely ascribed to the collaborative contribution of organic π-conjugated cations and the distorted Sb-halogenated polyhedrons. Additionally, a large birefringence of 0.156 and 0.147@1064 nm, which can be ascribed to the strong aeolotropic conjugated π electron distribution of their organic groups, were determined for (C₆H₁₀N₂)₂SbCl₆·Cl·H₂O and (C₆H₁₀N₂)₂SbBr₆·Br·H₂O, respectively. Intriguingly, (C₆H₁₀N₂)₂SbCl₆·Cl·H₂O also presents obvious photoluminescence, which mainly derives from the radiation recombination of triplet self-trapped excitons. This work proves that the combination of SCALP elements with organic π-conjugated cations is an effective strategy to explore novel NLO materials with high optical performances.