Jiachen Lu, Chong-An Chen, Wei Zeng, Kang Min Ok
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 [C5H5NS] group as an effective birefringence-active group (BAG) via four mercaptopyridine (MP)-based anisotropic crystals: (4-MP)2HgCl2 (I), (4-MP)HgCl2 (II), (2-MP)HgCl2 (III), and [(2-MP)HgCl]2·HgCl2·2Cl (IV). A steric-guided coordination engineering strategy enables systematic study of structural evolution through stoichiometric modulation of n2-/4-C5H5NS:nHg [2:1 (I) → 2:2 (II) → 2:2 (III) → 2:3 (IV)], including a coordination transformation from Hg-S2 (double S atoms) to Hg-S1 (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 [Δn = 0.234 → 0.588 at 546 nm (I → IV)]. Crystal IV exhibits competitive birefringence (Δn = 0.588 @546 nm) among organic-inorganic hybrid metal halides (OIHMHs) crystals containing π-conjugated ring systems with comparable bandgaps. This work establishes [C5H5NS] as an effective BAG and demonstrates a viable strategy for tailoring optical anisotropy in OIHMHs crystals.