A highly birefringent metal-free crystal assembled by cooperative non-covalent interactions

Birefringent crystals can manipulate the phase and polarization of light, so they are widely used as essential components in various optical devices. Common strategies to construct birefringent crystals are introducing metal cations that are either able to realize favorable coordination with functional anionic units or are susceptible to polarizability anisotropy. Herein, we report a metal-free crystal, NH₄(H₂C₆N₇O₃)·2H₂O, synthesized using the facile solution method. In the crystal structure of NH₄(H₂C₆N₇O₃)·2H₂O, (H₂C₆N₇O₃)^- functional units are assembled in an optimal manner by cooperative non-covalent interactions, i.e., hydrogen bonding and π-π interactions. As a result, this metal-free crystal possesses exceptional birefringence up to 0.54@550 nm, which is larger than those of most metal-containing birefringent crystals. In addition, the interference color of this crystal does not change obviously from 243 K to 313 K, indicating that the birefringence is robust at different temperatures. This work will inspire useful insights into the role of non-covalent interactions in designing outstanding birefringent crystals for efficient polarized optical devices.