P₄S_n (n = 3, 5, 7): Large Birefringence Enhanced by 3D Aromaticity and Packing Effect Induced Directional π–π Noncovalent Interactions

Abstract Birefringent materials play a crucial role in advanced applications such as nonlinear optics, high‐density data storage, and display technologies. Three birefringent compounds, P₄S₃ (1), P₄S₅ (2), and P₄S₇ (3), are studied, all exhibiting broad transmission in the range of ≈0.38–14 µm, with remarkable birefringence values of 0.178, 0.173, and 0.194 @546 nm, respectively, surpassing that of commercial α‐BaB₂O₄ (0.124 @546 nm). The relationships between their topological atomic and bonding properties and birefringent performance are established by high‐resolution experimental charge density. The unusual phenomenon of considerable differences in single molecule polarizability anisotropy among 1–3, despite their similar crystal birefringence, can be attributed to 3D aromaticity and packing effects, which induce enhanced electron delocalization and directional noncovalent π–π interactions, unprecedently in the field of birefringent materials. This work provides valuable insights for the design of new birefringent materials by leveraging packing effects to induce favorable charge density characteristics.