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-H3L) birefringent crystal, which consists of pi-conjugated benzene rings and tetrahedral phosphonate groups interconnected via multiple H-bonding interactions. Interestingly, the birefringence is as large as triangle 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 pi-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. An anisotropic hydrogen-bonded organic framework birefringent crystal is prepared by means of hydrogen-bonding self-assembly strategy. It is found that hydrogen bonds can regulate the birefringence by altering dihedral angles between pi-conjugated organic groups and the crystal plane. image