Achieving record birefringence in metal-free semiorganic sulfate crystals via tetrahedral anionic group substitution

Birefringent crystals with the ability to precisely modulate polarized light are indispensable optical devices in modern optics and optoelectronics. However, achieving high birefringence in such crystals remains a challenge. Integration of the π-conjugated 2-amino-5-methylpyridine (AMPy), which has large polarizability anisotropy, with non-π-conjugated tetrahedral groups produces two birefringent crystals, AMPy·PO₄ and AMPy·SO₄. AMPy·PO₄ has a three-dimensional hydrogen-bonding structure, whereas AMPy·SO₄ exhibits a two-dimensional hydrogen-bonded layer. Remarkably, upon substituting phosphate with sulfate, the experimental birefringence increases 3.8-fold, yielding a giant birefringence of 0.506 at 550 nm coupled with broad optical transparency and establishing AMPy·SO₄ as a new benchmark for metal-free sulfate crystals. Structural and computational investigations reveal that the outstanding birefringence of AMPy·SO₄ mainly originates from two key factors: the formation of a layered structure where tetrahedral sulfate anions modulate the arrangement of optically anisotropic AMPy cations to achieve nearly parallel alignment via hydrogen bonding, and the dense stacking of planar π-conjugated AMPy moieties. This work not only offers a universal strategy for designing semiorganic birefringent crystals via synergistically combining π-conjugated groups and non-π-conjugated rigid tetrahedral moieties but also pushes the birefringence limit of metal-free sulfate crystals to a new level.