Cationic‐Dual‐Modulated Linear and V‐Shaped Birefringent‐Active Unit Generates Remarkable Birefringence

Abstract Birefringent materials, characterized by their ability to split light into two orthogonal polarization states, have become indispensable components in various optical technological applications. The inherently constrained birefringence (Δ n ≈ 0.15−0.28) of commercially available crystals poses the critical challenge that fundamentally limits their technological evolution and practical deployment in advanced photonic systems. A dual‐modulated approach using cationic groups to direct linear/V‐shaped anionic groups' alignment for optimal birefringence is innovatively introduced, establishing the theoretical strategy for targeted design of birefringent materials. Through strategic construction of hydrogen‐bonding networks between different birefringent‐active units (BAUs), three novel birefringent crystals are assembled, denoted as (C 4 H 4.5 N 3 OI)(ICl 2 ) 0.5 ( 1 ), (C 4 H 5 N 3 OI)I 2 Cl 3 ·2H 2 O ( 2 ), and (C 4 H 5 N 3 OI)ICl 2 ·H 2 O ( 3 ), by integrating planar π‐conjugated group [protonated 5‐Iodocytosine cation (C 4 H 5 N 3 OI) + ], linear building block (ICl 2 − ) and V‐shaped motif (I 2 Cl 3 − ). This molecular engineering approach yield exceptional birefringence, with compounds 1 − 3 exhibiting the value of 0.675, 0.712, and 0.746@546 nm, respectively, which surpasses almost most reported hybrid crystals and outperforms all commercial inorganic birefringent materials. Theoretical calculations demonstrate that the exceptional birefringence originates from the ordered alignment of functional units within the crystals' framework.