[Ag(NH3)2]2SO4: A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials

• Published in: Angewandte Chemie International Edition Vol. 60; no. 39; pp. 21216 - 21220
• Main Authors: Yang, Yi‐Chang, Liu, Xin, Lu, Jing, Wu, Li‐Ming, Chen, Ling
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 20.09.2021
• Edition: International ed. in English
• Subjects: Ammonia; Anisotropy; Birefringence; cationic moiety coordination; Cations; Coordination; Crystal optics; Crystal structure; Design; Design modifications; Dipoles; Electromagnetic radiation; Group theory; Incident light; noncentrosymmetric; nonlinear optical materials; Nonlinear optics; Optical materials; Optical properties; Optics; Phase matching; Potassium phosphate; Potassium phosphates; second harmonic generation
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202107780

Over recent decades, guided by the anionic group theory of nonlinear optical (NLO) materials, rational design strategies have been primarily focused on anionic moieties; consequently, structural modification and design of cationic moieties have long been neglected. Herein, we report a strategy for the coordination of cationic moieties that substantially enhances the optical properties of NLO materials. For an example with well‐known crystal structure, [Ag(NH3)2]2SO4, we demonstrate that the coordination of the Ag+ cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P4‾ 21c structure as a positive uniaxial crystal. The bending of the [Ag(NH3)2]+ cationic moiety parallel to the z‐axis generates an anisotropic arrangement of the dipoles, i.e., a dipole of 0.12 D along the z‐direction, yet zero dipole in the xy‐plane, which interacts anisotropically with the incident light oscillating electromagnetic wave, leading to optical anisotropy with a large birefringence. The incident beam of 589.3 nm normal to the (110) crystal plane measures Δnobv.=0.08, and [Ag(NH3)2]2SO4 also exhibits a phase‐matching NLO response 1.4 times that of KH2PO4 (KDP) (obv. 1.4×KDP @1064 nm; cal. d36=1.50 pm V−1). A new strategy for the coordination of cationic moieties offers a new possibility for the rational design and property modification of functional ionic materials, particularly NLO materials. As demonstrated by [Ag(NH3)2]2SO4 and Ag2SO4, the exceptional enhancements of the linear birefringence and nonlinear SHG properties are attributed to the unique [Ag(NH3)2]+ cation moiety.