Noncentrosymmetric (C 3 H 7 N 6 ) 6 (H 2 PO 4 ) 4 (HPO 4 )·4H 2 O and Centrosymmetric (C 3 H 7 N 6 ) 2 SO 4 ·2H 2 O: Exploration of Acentric Structure by Combining Planar and Tetrahedral Motifs via Hydrogen Bonds.

The combination of organic and inorganic nonlinear active units to obtain organic-inorganic hybrid materials has been proved to be a very effective method to obtain nonlinear optical (NLO) materials with excellent properties. Herein we reported two hybrid melamine-based compounds, namely, acentric (C 3 H 7 N 6 ) 6 (H 2 PO 4 ) 4 (HPO 4 )·4H 2 O ( 1 ) and centrosymmetric (C 3 H 7 N 6 ) 2 SO 4 ·2H 2 O ( 2 ), which were synthesized via facile solvent evaporation method. Compound 1 features a three-dimensional (3D) network structure composed of ∞ [(H 2 PO 4 ) 4 (HPO 4 )(H 2 O) 4 ] 6- layers which are further linked with ∞ [(C 3 H 7 N 6 ) 6 ] 6+ layers via hydrogen bonds. Compound 2 displays a 3D structure composed of [(C 3 H 7 N 6 ) 2 (SO 4 )(H 2 O) 2 ] ∞ layers further linked with each other by hydrogen bonds. Compound 1 presents a second harmonic generation signal of about 0.1 × KDP. Furthermore, UV-vis and infrared spectra, thermal analyses, and theoretical calculation were also adopted to evaluate its NLO performance. The theoretical calculations showed that the SHG response and large birefringence of 1 were primarily caused by the (C 3 H 7 N 6 ) + , (H 2 PO 4 ) - , and (HPO 4 ) 2- groups.