Cd7I12Q·As4Qn (Q = S, Se; n = 3, 4): In Situ Alteration of Functional Molecules in Polar Metal Inorganic Framework Tailoring High‐Performance Infrared Nonlinear Optical Crystals

Exploring new functional motifs is vital for the design of high‐performance infrared (IR) nonlinear optical (NLO) crystals. Asymmetric As4Qn (Q = S, Se; n = 3, 4) inorganic molecules are excellent IR NLO motifs, but the synthesis and design of related NLO crystals remain substantial challenges. Herein, an in situ functional molecule altering strategy is proposed to design As4Qn‐based IR NLO crystals with precisely tunable optical and NLO properties, and report the first synthesis of non‐centrosymmetric As4Qn‐based inorganic adducts: Cd7I12Q·As4Qn (Q = S, Se; n = 3, 4) harnessing soft solid‐state reactions. With the alteration of As4Qn functional molecules in identical Cd7I12Q polar metal inorganic framework, the NLO performance, birefringence and infrared transparency range of adducts can be tuned. Remarkably, Cd7I12Se·As4Se4 exhibits balanced comprehensive optical performances including a SHG effect of 1.3 × AgGaS2, a LDT of 11.2 × AgGaS2, an optical bandgap of 2.46 eV, birefringence of 0.045@2050nm/0.051@546nm and IR transparency range of 0.50–17.8 µm. The structure‐property relations analysis reveals that the macroscopic optical properties are dominated by the cavity‐filling molecular groups with varying polarizability and vibration frequency, which supports the tailoring effect of the functional molecular groups. This research expands IR NLO motifs and also establishes a protocol for construction of crystalline materials with tunable optical properties.