Assembly of dual π-conjugated units toward enhanced birefringence in a 2D organic sulfonate crystal

Birefringent crystals, which enable precise control and detection of light polarization, are indispensable in advanced laser technologies. However, achieving a birefringence exceeding 0.3 remains a significant challenge. Herein, we report a dual π -conjugated strategy to construct a novel two-dimensional (2D) purely organic sulfonate crystal, (m-AP)(3-pySO 3 ) (denoted as 3PYH; m-AP = m -aminophenol, 3-pySO 3 = 3-pyridinesulfonate), synthesized via a facile solvent evaporation method. 3PYH features a hydrogen-bonded 2D layered architecture assembled from π -conjugated [m-AP] + cations and [3-pySO 3 ] − anions. This crystal exhibits a short ultraviolet (UV) cutoff edge at 271 nm with a wide bandgap of 4.14 eV. Notably, it demonstrates a large visible-region birefringence of 0.36, which is significantly higher than those reported for most 2D sulfates and their derivatives. Combined structural analysis and first-principles calculations reveal that the exceptional birefringence stems from the synergistic enhancement of polarizability anisotropy by dual π -conjugated units, coupled with hydrogen-bond-mediated ordered orientation of aromatic rings, which induces pronounced electron clouds density anisotropy along different directions. This work identifies 3PYH as a promising UV birefringent material and validates the efficacy of the dual π -conjugated strategy for designing sulfonate crystals with strong optical anisotropy.