Ttlie:Designing promising ultraviolet (UV) birefringent crystals with different hydrogen-bonded phosphate frameworks
Abstract: In this study, we computationally identified using first-principles calculations two pyridine derivatives with high polarizability anisotropy: (C6H7N2O)+ (protonated nicotinamide, abbreviated as 3AP+) and (C6H6NO2)+ (protonated nicotinic acid, abbreviated as 3CP+). Subsequently, we synthesized two novel semiorganic crystals: (3AP) (H2PO4) (I) and (3CP) (H2PO4) (II). Both crystals incorporate hydrogen-bonded phosphate frameworks (HPFs), which are constituted by H2PO4− anions linked through O–H⋯O hydrogen bonds. Compound I (Fdd2) features one-dimensional (1D) hydrogen-bonded phosphate frameworks (HPFs) that interconnect with 2D [(3AP)(H2PO4)] layers, forming a complex 3D network. In contrast, compound II (Pbca) possesses 2D HPFs that are integrated with 3CP+ cations to form a 2D layered network. Compound I exhibits a moderate second-harmonic generation (SHG) effect (1 × KH2PO4), and a significant birefringence (Δncal.: 0.191@1064 nm). Furthermore, compound II exhibits both a broad bandgap (4.24 eV) but also an exceptional birefringence (Δnexp.: 0.284@546 nm), which is the highest value reported among all semiorganic phosphates. This suggests that II could be a promising candidate for use as an outstanding ultraviolet (UV) birefringent material.
