Four Benzoguanamine-Based Birefringent Crystals with Excellent Optical Anisotropy

Large optical anisotropy stands as the paramount characteristic of birefringent crystals, and the introduction of planar π-conjugated groups is a widely recognized approach for synthesizing crystals with excellent optical anisotropy. However, the pivotal challenge lies in controlling the orientation and spatial arrangement of the birefringent active groups. Through the introduction of a novel phenyl-melamine [C₉H₁₀N₅]⁺ unit, four birefringent crystals─namely C₉H₁₁N₅(NO₃)₂, (C₉H₁₀N₅)Br, (C₉H₁₀N₅)Cl·H₂O, and (C₉H₁₀N₅)BF₄·H₂O─have been successfully synthesized. These crystals exhibit notable birefringence values of 0.27, 0.49, 0.42, and 0.57 at 550 nm, respectively. Notably, the birefringence values of the latter three compounds surpass those of commercially available birefringent materials, as well as the majority of melamine-based materials. In addition, an examination of the crystal structures revealed that varying the anionic components can alter the orientation of the birefringent active groups, thereby influencing the optical anisotropy of the crystals. In this study, additional properties, such as thermal stability and UV–vis-NIR diffuse reflectance spectra, were also investigated. These results verified that adopting the phenyl substitution strategy to modify the existing birefringent active groups can effectively optimize the polarization anisotropy of the units, thereby constructing crystal materials with excellent performance.