Effect of Multiple Hydrogen Bonding on the π-Conjugated Groups and Optical Properties in Molecular Crystals

Birefringent crystals are widely used in modern optical devices for their ability to modulate the polarization state of light. Exploring birefringent crystals with large birefringence (Δn > 0.3) and short ultraviolet (UV) cutoff edge is still a challenge. Protonated melamine groups are candidates for designing birefringent crystals with excellent performance due to its high microscopic polarizability anisotropy. However, its potential for constructing large birefringent crystals has not been fully exploited due to limitations in the density and arrangement of its lattice structure. This study employed a strategy of multiple hydrogen bond-induced molecular ordered assembly to successfully synthesize three excellent birefringent crystals with supramolecular framework: (C₃N₆H₇)BF₄ (MelBF), (C₃N₆H₇)H₂PO₄ (MelPO), and (C₃N₆H₇)₂PO₃F·4H₂O (MelPOF). It is worth noting that new compound MelBF exhibits a large birefringence of 0.44 at 546 nm and a short UV cutoff edge of 228 nm. Here, tetrahedral anions containing highly electronegative O/F atoms synergistically regulate and optimize the dihedral angle and density of the protonated melamine planar ring groups through multiple hydrogen bonding interactions to synthesize crystals with both high birefringence and short UV cutoff edges. This study confirms the potential of multiple hydrogen bonds-driven molecular assembly for designing advanced birefringent crystals with excellent optical properties.