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

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‐H 3 L) 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.