Steric‐Guided Coordination Engineering Enables Significantly Optimized Optical Anisotropy via Sulfur‐Driven [C ₅ H ₅ NS] Birefringence‐Active Groups

ABSTRACT Birefringent crystals are essential electro‐optical materials for manipulating polarized light, yet achieving programmable optical anisotropy through rational design remains challenging. Herein, we identify the [C 5 H 5 NS] group as an effective birefringence‐active group (BAG) via four mercaptopyridine (MP)‐based anisotropic crystals: (4‐MP) 2 HgCl 2 ( I ), (4‐MP)HgCl 2 ( II ), (2‐MP)HgCl 2 ( III ), and [(2‐MP)HgCl] 2 ·HgCl 2 ·2Cl ( IV ). A steric‐guided coordination engineering strategy enables systematic study of structural evolution through stoichiometric modulation of n 2‐/4−C5H5NS : n Hg [2:1 ( I ) → 2:2 ( II ) → 2:2 ( III ) → 2:3 ( IV )], including a coordination transformation from Hg‐S 2 (double S atoms) to Hg‐S 1 (single S atom) environments. Interestingly, this process progressively relieves steric hindrance and drives dihedral angle ( θ ) tuning from 78.26° to 0°, leading to a significantly enhanced birefringence [Δ n = 0.234 → 0.588 at 546 nm ( I → IV )]. Crystal IV exhibits competitive birefringence (Δ n = 0.588 @546 nm) among organic–inorganic hybrid metal halides (OIHMHs) crystals containing π ‐conjugated ring systems with comparable bandgaps. This work establishes [C 5 H 5 NS] as an effective BAG and demonstrates a viable strategy for tailoring optical anisotropy in OIHMHs crystals.