Redox‐Driven Strategy Enabling Leapfrog Enhancement of Birefringence in Layered Tin‐Based Optical Crystals

ABSTRACT Significantly improving birefringence performance remains a significant challenge in the field of optical crystals. Layered organic‐inorganic hybrid systems with significant structural anisotropy are candidates for constructing large birefringent crystals. This study employs an innovative redox‐driven strategy, successfully designed and synthesized two tin‐based compounds with distinct coordination geometries‐[(C 5 N 2 H 7 )SnCl 3 ] ( I ) and [(C 5 N 2 H 7 ) 2 SnCl 6 ] ( II ). Compound II exhibits a significant performance leap with a birefringence of 0.45, which is 118 times that of SnCl 4 . Redox‐driven strategy realizes the conformational transition from [SnCl 3 ] − claw triple‐coordinated configuration to [SnCl 6 ] 2− octahedral hexa‐coordination configuration by directionally modulating the valence state of the tin ions (Sn 2 + /Sn 4 + ), and produces the different space groups P 2 1 /c in I and P ī in II . Notably, the protonated 4‐aminopyridinium (C 5 N 2 H 7 ) + ring (4AP) and the [SnCl 6 ] 2− octahedra achieve an ideal parallel arrangement, forming a highly ordered layered supramolecular framework for compound II . In addition, millimeter‐scale single crystals of compound II were successfully grown (5.2 × 3.5 × 2.1 mm 3 ). This work not only establishes a structure‐property model of “coordination geometry—group modulation—optical properties”, but also provides a new design strategy for the development of high‐performance birefringent crystals.