Crystal clear: unveiling giant birefringence in organic-inorganic cocrystals.

Coplanar groups with large anisotropic polarizability are suitable as birefringence-active groups for investigating compounds with significant birefringence. In this study, the organic coplanar raw reagent, -CHNO (4HP), was selected as an individual complement. Utilizing the cocrystal engineering strategy, we successfully designed two cocrystals: [LiNO·HO·4HP]·4HP (Li-4HP2) and [Mg(NO)·6HO]·(4HP) (Mg-4HP), and one by-product: LiNO·HO·4HP (Li-4HP), which were grown using a mild aqua-solution method. The synergy of the coplanar groups of NO and 4HP in the structures resulted in unexpectedly large birefringence values of 0.376-0.522@546 nm. Furthermore, the compounds exhibit large bandgaps (4.08-4.51 eV), short UV cutoff edges (275-278 nm), and favorable growth habits, suggesting their potential as short-wave UV birefringent materials.