Tuning Birefringence in Supramolecular Mg-PDA Frameworks: Modulating Bonding and Layer Connectivity

Herein, we prepared two novel hydrogen-bonded supramolecular frameworks within the Mg-pyridine dicarboxylic acid (Mg-PDA) system: [Mg(H₂O)₆][(C₇H₄NO₄)·H₂O]₂ (1) and Mg(C₇H₃NO₄)(H₂O)₂ (2). In compound 1, the 2D [(C₇H₄NO₄)·H₂O]_∞^- anionic layer is formed by O/N-H···O hydrogen bonding connecting (C₇H₄NO₄)^- anions with free water molecules. In contrast, the 2D [Mg(C₇H₃NO₄)(H₂O)₂]_∞ neutral layer in compound 2 is connected by Mg-O/N coordination bonds, which promote the coplanar arrangement of π-conjugated six-membered pyridine rings. This structural arrangement enhances the disparity between in-plane and out-of-plane bonding interactions in the 2D [Mg(C₇H₃NO₄)(H₂O)₂] layer. Consequently, both experimental data and theoretical calculations indicate that compound 2 shows strong optical anisotropy, with giant birefringence (Δn = 0.44 @ 546 nm) surpassing that of compound 1 (Δn = 0.34 @ 546 nm) and exceeding commercially available α-BaB₂O₄ (Δn = 0.122 @ 546 nm). More importantly, in combination with their short-wave UV cutoff edges (279 nm for 1 and 273 nm for 2), compound 2 features the most promising birefringent properties among all UV Mg-containing crystals.