Design of borate-squarate hybrid crystals with large birefringence by synergistic assembly of planar π-conjugated groups

. They exhibit large refractive index differences of 0.281, 0.262, and 0.328 at 546 nm, respectively. Notably, III is obtained by dehydrating I at 100 °C in an air atmosphere. The coplanar alignment of π-conjugated groups in III boosts optical anisotropy efficiently. This study provides valuable guidance for the rational design and further exploration of short-wavelength birefringent materials.