Giant Birefringence Enabled by the Highly Anisotropic Linear IX 2 − (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-4APICl2 (1, 4AP = 4-aminopyridine), HDMA2ICl2·Cl (2, DMA = dimethylamine), H-4APIBr2 (3), and HDMA2IBr2·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.