Giant Birefringence Induced by Intermolecular Aromatic Interactions in Hydrogen-Bonded Organic Frameworks

Birefringent crystals are crucial optical materials that play a pivotal role in modulating and detecting the polarization state of light. However, there is still a significant challenge in regulating the assembly of π-conjugated active units to construct optical crystals with giant birefringence. In this study, a hydrogen-bonding self-assembly strategy is proposed to construct an aromatic π-conjugated hydrogen-bonded organic framework birefringent crystal (HOFBC) which exhibits a remarkable birefringence value of 0.763@546 nm, surpassing all reported organic birefringent crystals and significantly exceeding that of any commercial birefringent crystals. Theoretical calculation and structural analyses indicate that this exceptional birefringence arises from high polarizability anisotropy of aromatic π-conjugated units and intermolecular aromatic interactions. Interestingly, when HOFBC-1 is heated to 250 °C and transforms into HOFBC-1-250, the crystal retains its crystalline structure and layer-by-layer assembly mode, achieving a giant birefringence value of 0.800@546 nm. This work presents a novel strategy for developing giant birefringent materials, opening up new opportunities for the application of HOF crystals in birefringence.