Regulating the packing of π-conjugated ligands toward exceptional birefringence in magnesium phosphite hybrids

Developing phosphite-based birefringent materials with simultaneously large birefringence and wide optical transparency remains a significant challenge. Inspired by the large intrinsic polarizability anisotropy of planar π-conjugated ligands, we introduced 1,10-phenanthroline (phen) into the magnesium phosphite system to explore the influence of ligand packing on birefringence. Colorless crystals of Mg(phen)(HPO₃) (1) and Mg(phen)(H₂PO₃)₂·H₂O (2) were successfully obtained under different solvothermal conditions. Although both compounds crystallize in the same monoclinic space group C2/c and consist of one-dimensional magnesium phosphite chains, they exhibit markedly different packing arrangements of the phen ligands. In compound 1, adjacent phen ligands are nearly perpendicular with a dihedral angle of approximately 90°, resulting in a birefringence of 0.36 at 546 nm. In contrast, compound 2 features a nearly parallel packing of the phen ligands and exhibits a remarkably large birefringence of 0.55 at 546 nm, placing it among the most birefringent phosphite-based materials reported to date. Theoretical calculations reveal that the birefringence is dominated by the π-conjugated phen ligands, while the phosphite groups make a much smaller contribution. These findings demonstrate that regulating the packing mode of π-conjugated ligands is an effective approach to enhancing birefringence in metal phosphite hybrids.