Enhancing Birefringence of Sulphates by Polarity Modification in Planar Cations

Sulphates are regarded as important optical materials owing to their broad transparency, yet their application is limited by weak birefringence from highly symmetric [SO₄] units. Herein, we enhance the birefringence in sulphates via cation polarity modification by introducing 1,1-dimethylguanidinium ([C₃N₃H₁₂]⁺, GC₂ ⁺), which shows twofold higher polarizability anisotropy than the parent planar guanidinium ([CN₃H₆]⁺, or G⁺). High-quality C₃N₃H₁₂HSO₄ crystals grown from a facile and safe aqueous route exhibit a large birefringence of 0.105 @ 546 nm, 73% higher than that of the guanidine-based [CN₃H₆]HSO₄. First-principles calculations confirm that GC₂ ⁺ not only boosts polarizability anisotropy but also promotes the oriented alignment of [SO₄] units via an extensive hydrogen-bonding network, leading to optimal structural ordering. This work highlights a synergistic strategy using polar cations and hydrogen-bond-directed assembly to design high-performance birefringent materials.