Steric-Guided Coordination Engineering Enables Significantly Optimized Optical Anisotropy via Sulfur-Driven [C5H5NS] Birefringence-Active Groups

초록

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 & centerdot;HgCl2 & centerdot;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-S2 (double S atoms) to Hg-S1 (single S atom) environments. Interestingly, this process progressively relieves steric hindrance and drives dihedral angle (theta) tuning from 78.26 degrees to 0 degrees, leading to a significantly enhanced birefringence [Delta n = 0.234 -> 0.588 at 546 nm (I -> IV)]. Crystal IV exhibits competitive birefringence (Delta n = 0.588 @546 nm) among organic-inorganic hybrid metal halides (OIHMHs) crystals containing pi-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.

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