Tuning Birefringencein 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 (<b>1</b>) and Mg(C₇H₃NO₄)(H₂O)₂ (<b>2</b>). In compound <b>1</b>, 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 <b>2</b> 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 <b>2</b> shows strong optical anisotropy, with giant birefringence (Δ<i>n</i> = 0.44 @ 546 nm) surpassing that of compound <b>1</b> (Δ<i>n</i> = 0.34 @ 546 nm) and exceeding commercially available α-BaB₂O₄ (Δ<i>n</i> = 0.122 @ 546 nm). More importantly, in combination with their short-wave UV cutoff edges (279 nm for <b>1</b> and 273 nm for <b>2</b>), compound <b>2</b> features the most promising birefringent properties among all UV Mg-containing crystals.