Yin–Yang Complementarity Strategy Achieving Giant Optical Anisotropy in a Metal-free Birefringent Material C(NH2)3(HC4O4)

The Yin–Yang complementarity strategy was first proposed to successfully develop a superior metal-free birefringent material C(NH2)3(HC4O4), which was fabricated by two kinds of planar π-conjugated units containing triangle [C(NH2)3]+ cations and square [HC4O4]− anions. C(NH2)3(HC4O4) features a giant birefringence (0.313@546 nm), which is significantly enhanced compared to the commercially available birefringent materials.