Achieving Ultra‐High Optical Anisotropy via Structural Regulation and Coplanar Engineering of Extended π‐Conjugated [H _2–x C ₃ N ₃ S ₃ ] (x = 1, 2) Groups

Abstract Optical anisotropy, arising from the spatially asymmetric interaction between light and matter, manifests as birefringence and dichroism and plays a pivotal role in controlling light polarization in advanced optical technologies. Realizing large birefringence remains challenging and relies on designing birefringence‐active groups (BAGs) with strong anisotropy and optimized spatial arrangements. Herein, the thio substitution strategy is adopted to maximize optical polarization through the realization of a perfectly coplanar configuration, resulting in the successful construction of six birefringent materials (I−VI) via a facile aqueous solution method. These materials exhibit large birefringence Δ n exp 0.604 for I , and Δ n calc 0.627, 0.283, 0.692, 0.267, and 0.558 @546 nm in II−VI . This enhanced birefringence is attributed to the large polarization anisotropy of thiocyanuric acid [TCA] BAGs, facilitated by their coplanar alignment and dense packing, which is governed by synergistic covalent and weak hydrogen‐bonding interactions. First‐principles calculations and structural analyses uncover that the superior optical properties mainly originate from [TCA] in I−VI , while [C(NH 2 ) 3 ] + , [CN 4 H 7 ] +, and [TCA] BAGs collaborate to govern optical properties. By enabling structural regulation to boost optical anisotropy, this finding offers new opportunities for advancing high‐performance birefringent materials in the UV region.