Unearthing Hydrogen Bonded Metal‐Free Crystal with Remarkable Optical Anisotropy

Abstract Birefringent crystals are crucial in modulating the polarization state of light within contemporary technological applications. Organic molecules with π‐conjugated structure are known to exhibit significant birefringence effect owing to their unique electronic properties. This study demonstrates that 4, 5‐dichocyanimidazole can be used as a new crystal building block for birefringent crystals. Crystals of C 5 N 4 H 2 ( DCI ) and C 5 N 4 H 2 ·H 2 O ( DCI‐w ) are obtained by evaporation at room temperature. It is worth noting that DCI and DCI‐w exhibit significant birefringence. At 546 nm, the experimental birefringence of DCI is 0.417 and the calculated birefringence of DCI‐w reaches 0.618. The birefringence of DCI‐w is significantly larger than those of all commercial birefringent crystals and exceeds most metal‐free birefringent crystals. First‐principles calculations and structural analysis show that this remarkable birefringence is mainly due to the strong anisotropy of the planar π‐conjugated C 5 N 4 H 2 molecules and the well‐organized arrangement of the groups. This discovery opens up new possibilities for the development of high‐performance birefringent optical materials.