Multifunctional Cd‐based Hybrid with Large Birefringence Achieving Excellent Second Harmonic Generation and Reversible Temperature‐Activated Fluorescence for Anti‐counterfeiting

Abstract The integration of two kinds of NLO‐active genes, particularly organic π‐conjugated and inorganic polyhedral groups, is conducive to construct excellent nonlinear optical (NLO) materials. Based on this inspiration, an excellent NLO crystal, namely (4‐AMP)CdBr₄·H₂O (4‐AMP = 4‐(Aminomethyl)pyridine), is constructed via the combining new π‐conjugated gene (C₆H₁₀N₂)²⁺ and distorted tetrahedron (CdBr₄)²⁻. This compound with 0D structure that belongs to non‐centrosymmetric P2₁ space group is grown by solution evaporation method. Notably, the excellent second harmonic generation (SHG) response (≈1.5 × KDP), suitable large birefringence (≈0.158@546 nm) and short ultraviolet (UV) absorption cutoff edge (≈226 nm) of (4‐AMP)CdBr₄·H₂O, indicate its potentiality as a UV NLO and birefringent crystal. Theoretical calculations and structural analysis demonstrate convincingly that SHG behavior and optical anisotropy are mainly attributed to the interactive effect of π‐conjugated (C₆H₁₀N₂)²⁺ groups and distorted (CdBr₄)²⁻ tetrahedrons. Intriguingly, the compound presents a thermal‐activated reversibly PL switching, and further graphic patterns for anti‐counterfeiting are designed to achieve information encryption. This work not merely demonstrates a novel multi‐functional optoelectronic crystal, but provides new perspective in constructing multi‐functional metal halide materials.