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

Abstract

Birefringent crystals, owing to their capability to modulate the polarization state of light, have been widely utilized as polarizing components and optical devices in various key technological fields. In this work, three borate-squarate hybrid crystals, Rb₂(C₄O₄)[B(OH)₃]₂·2H₂O (I), Cs₂(C₄O₄)[B(OH)₃]₂·2H₂O (II) and Rb(C₄O₄)_0.5·B(OH)₃ (III), were synthesized by hydrogen-bonded synergistic assembly of [B(OH)₃] and [C₄O₄]²⁻. 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.