Coordination Geometry-Directed Optical Anisotropy and Phase-Matched Nonlinear Optics in Chiral Metal–Organic Frameworks

Optical anisotropy and phase-matching capability are critical requirements for high-performance nonlinear optical (NLO) crystals, yet they are inherently difficult to achieve simultaneously. Chiral metal-organic frameworks (CMOFs), with tunable coordination environments and crystal symmetry, provide a promising platform for addressing this challenge. Herein, a coordination geometry-directed strategy is proposed to regulate optical anisotropy and phase-matched nonlinear optical behavior in CMOFs. By employing the same chiral ligand while varying the metal centers (Zn²⁺ versus Cd²⁺), two CMOFs featuring tetrahedral and octahedral coordination geometries were constructed, leading to distinct crystal symmetries and lattice anisotropies. Structural analysis reveals that the octahedrally coordinated Cd-based framework exhibits symmetry lowering and pronounced unit-cell anisotropy, resulting in a markedly enhanced birefringence (Δn = 0.113 experimentally and 0.198 theoretically at 546 nm), nearly three times that of its Zn analogue. As a consequence, effective phase-matchable second-harmonic generation is achieved, with an SHG efficiency comparable to that of KDP. Density functional theory calculations further demonstrate that the distorted octahedral coordination geometry and coordination-enhanced charge redistribution give rise to strong electronic anisotropy, polarizability anisotropy, and hyperpolarizability. This work establishes coordination geometry as a decisive structural parameter for directing optical anisotropy and nonlinear optical performance in CMOFs.