A Robust Phosphonate‐Based Hydrogen‐Bonded Organic Framework Birefringent Crystal

Abstract Birefringent crystals as an important optical anisotropic material play a crucial role in modulating the polarization state of light. To date, most birefringent crystals are purely inorganic compounds with small birefringence. Developing large birefringent crystals remains a great challenge primarily because of the absence of tunable anisotropic structural units for inorganic materials. Herein, a hydrogen‐bonding self‐assembly strategy is reported to construct a robust hydrogen‐bonded organic framework (HOF, namely m‐H₃L) birefringent crystal, which consists of π‐conjugated benzene rings and tetrahedral phosphonate groups interconnected via multiple H‐bonding interactions. Interestingly, the birefringence is as large as ∆n = 0.17@550 nm and catches up with those of commercial birefringent crystals. A combination of theoretical calculation and single‐crystal structural analyses reveals that the H‐bonding interactions control the dihedral angle of π‐conjugated benzene rings and the smaller the dihedral angle, the greater the birefringence. This discovery opens the door for the tunable HOFs to be used as a promising new class of birefringence materials.