Coordination-Induced Amplification of Birefringence and Nonlinear Optical Response in a Chiral Metal-Organic Framework.

Crystalline materials exhibiting programmable optical anisotropy are of great interest for advanced photonic applications but remain challenging. In this study, we introduce a coordination-driven approach that significantly enhances optical anisotropy by transforming a loosely packed molecular crystal (crystal-1) into a chiral metal-organic framework (MOF) (crystal-2). This structural transformation leads to a more than 20-fold increase in birefringence (Δn = 0.226 at 546 nm) and activates a measurable second-harmonic generation (SHG) activity. First-principles calculations suggest that the observed optical enhancement is driven by three main factors: coordination-driven enhancement of asymmetry, alignment of the π-conjugated framework, and anisotropic electron distribution. These findings highlight the potential of coordination polymerization as a versatile approach for designing hybrid optical materials with tailored anisotropy, providing a foundation for the development of MOF-based photonic devices.