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.