Noncovalent Assembly Engineering Achieves Excellent Birefringence in Crystals

Birefringent crystals can manipulate the phase and polarization of light, and are therefore widely used as key components in various optical devices. In this study, the hydrogen-bonding interactions between the [C₈H₆FN₂O]⁺ donor and the anionic acceptors, namely [SiF₆]^2- and Cl^-, were utilized to achieve a high degree of coplanarity during crystal packing. The as-grown (C₈H₆FN₂O)₂SiF₆ and (C₈H₆FN₂O)Cl crystals exhibit significant birefringence (Δn_exp = 0.32 and 0.51 at 546 nm), surpassing that of all commercial crystals. Structural analysis reveals that this remarkable birefringence arises from the optimal assembly of the [C₈H₆FN₂O]⁺ units under the synergistic effect of hydrogen bonds and π-π interactions. This discovery provides a clear structural paradigm for rational design and synthesis of crystals with strong optical anisotropy. Additionally, the discussion on the development of raw material screening methods for developing large birefringence materials is also in-depth. The coplanar assembly and rational screening of functional units offer a promising approach for the development of birefringent crystals, which is expected to advance the field of optics and related applications.