Steric‐Guided Coordination Engineering Enables Significantly Optimized Optical Anisotropy via Sulfur‐Driven [C 5 H 5 NS] Birefringence‐Active Groups
Jiachen Lu, Chong‐An Chen, Wei Zeng, Kang Min OkABSTRACT
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.