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 (Δn_cal.: 0.191@1064 nm). Furthermore, compound II exhibits both a broad bandgap (4.24 eV) but also an exceptional birefringence (Δn_exp.: 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.