NaK(CO)(NO)X·6HO (X = NO, Cl, Br): Exploring High-Performance UV Birefringent Crystals Induced by Coplanar π-Conjugated CO and NO Groups.

Planar π-conjugated groups, like CO, NO, and BO triangles, are ideal functional units for designing birefringent materials due to their large optical anisotropy and wide band gap. The key point for designing birefringent crystals is to select appropriate functional building blocks (FBBs) and the proper arrangement mode. It is well known that the substitution strategy has proven to be a promising and accessible approach. In this work, alkali metals were chosen to regulate and control two different π-conjugated groups, CO and NO, to build new compounds with large birefringence. Subsequently, three new compounds, NaK(CO)(NO)X·6HO (X = NO, Cl, Br), were successfully synthesized using the hydrothermal method. The aliovalent substitution between the [NO] anionic group and halogen anions [Cl]/[Br] has been achieved in these compounds. NaK(CO)(NO)X·6HO feature the well-coplanar CO and NO groups in their crystal structure. This coplanar arrangement mode may effectively enhance the anisotropic polarizability of NaK(CO)(NO)X·6HO. And their experimental birefringence can reach 0.094-0.131 at 546 nm. Diffuse reflectance spectra demonstrate that these compounds exhibit short ultraviolet (UV) absorption edges of ∼235 nm. Meanwhile, NaK(CO)(NO)X·6HO also have an easily grown capacity under facile conditions. This work not only reports three new potential UV birefringent crystals but also provides a strategy to make the π-conjugated MO group coplanar.