Achieving large birefringence in oxalate crystals by a hydrogen bond-directed strategy

Rational design of birefringent crystals with high birefringence remains a critical challenge. Herein we present two oxalate crystals of (C6N2H11)(HC2O4) (1) and (C4N2H4)(H2C2O4) (2) (H2C2O4 = oxalic acid, C6N2H11 = 2-ethyl-4-methylimidazolium cation and C4N2H4 = pyrazine). Remarkably, crystal 2 exhibits an unprecedentedly large birefringence of 0.422 at 550 nm, which surpasses all commercial birefringent crystals. The formation of the directional hydrogen bonds between oxalate and planar pyrazine constituents facilitates the adoption of a planar configuration by oxalic units possessing large polarizability anisotropy (Δα = 21.72). Whereas the distorted configuration of oxalate groups with a relatively small Δα of 13.95 induced by the non-coplanar arrangement of the imidazole planes of C6N2H11 leads to moderate birefringence (0.144@550 nm) for 1. Computational analyses reveal that the birefringent superiority of 2 originates from the synergistic effect of the π-conjugated oxalate and pyrazine units in a parallel arrangement directed by hydrogen bonds. This work breaks the record of birefringence in oxalates. It also develops a powerful hydrogen bond-directed strategy to modulate the configuration of oxalate groups, enabling its use as a tunable anisotropic structural unit for constructing birefringent crystals.