Giant Birefringence Enabled by the Highly Anisotropic Linear IX₂ ^- (X = Cl, Br) Building Blocks

Birefringent crystals are essential for polarized optical devices, yet achieving large birefringence through rational design remains challenging. The key lies in constructing birefringence-active groups (BAGs) with giant polarization anisotropy and optimal spatial arrangements. Here, we report the successful construction of linear interhalogen BAGs, IX₂ ^- (X = Cl, Br), enabling giant polarization anisotropy. This was accomplished by simple halogenation of IO₃ ^- groups in an aqueous solution. Four novel birefringent crystals were synthesized: [H-4AP][ICl₂] (1, 4AP = 4-aminopyridine), [HDMA]₂[ICl₂]·Cl (2, DMA = dimethylamine), [H-4AP][IBr₂] (3), and [HDMA]₂[IBr₂]·Br (4). In all these compounds, the linear IX₂ ^- BAG adopts parallel arrangements, effectively maximizing synergistic polarization anisotropy. As a result, compounds 1-4 exhibit giant birefringence values in both the visible (0.647, 0.585, 0.836, and 0.782 at 546 nm) and near-infrared (NIR) regions (0.510, 0.356, 0.762, and 0.509 at 1064 nm), surpassing commercial birefringent crystals and many state-of-the-art materials. Furthermore, these compounds achieve an optimal balance between giant birefringence and moderate bandgap among linear BAG-based materials. Detailed theoretical calculations confirm that the IX₂ ^- BAGs play a dominant role in this exceptional birefringence. This study demonstrates the remarkable potential of linear interhalogen anions for developing high-performance birefringent crystals.