Hydrogen-Bond-Directed Assembly of Polyanionic Cluster [Mo ₂ O ₅ (IO ₃ ) ₄ ] ²– for Nonlinear Optical Crystal Design

Iodates are promising candidates for second-order nonlinear optical (NLO) materials due to the stereochemically active lone pair of I 5+ in their functional units [(IO 3 ) − /(IO 4 ) 3– ], which confers large second-harmonic generation (SHG) coefficients and high birefringence. However, their natural tendency to crystallize in centrosymmetric structures, driven by electrostatic stabilization, severely restricts the design and development of high-performance nonlinear optical crystals. Although incorporating d 0 transition metals or other NLO-active units has been studied, the rational design of hybrid iodates through polyanionic cluster modulation with organic cations remains underexplored. Electrostatic potential analysis revealed the [Mo 2 O 5 (IO 3 ) 4 ] 2– cluster as an optimal building block with multiple hydrogen-bonding sites. By employing distinct planar π-conjugated cations to modulate the polyanionic building blocks, two organic–inorganic hybrid molybdenyl iodates were successfully synthesized: centrosymmetric (C 5 H 6 NO) 2 Mo 2 O 5 (IO 3 ) 4 ·2H 2 O ( 1 ) and noncentrosymmetric (C 2 H 4 N 3 ) 2 Mo 2 O 5 (IO 3 ) 4 ·4H 2 O ( 2 ). 2 exhibits a strong SHG response of 4.2 times that of KH 2 PO 4 (KDP) and high birefringence of 0.298 at 546 nm, highlighting its potential for UV linear and nonlinear optical applications.