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

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 ]2 SO4 , we demonstrate that the coordination of the Ag+ cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P 4 ‾ 21 c 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 ]2 SO4 also exhibits a phase-matching NLO response 1.4 times that of KH2 PO4 (KDP) (obv. 1.4×KDP @1064 nm; cal. d36 =1.50 pm V-1 ).