Crystal clear: unveiling giant birefringence in organic–inorganic cocrystals

Abstract

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, o-C₅H₅NO (4HP), was selected as an individual complement. Utilizing the cocrystal engineering strategy, we successfully designed two cocrystals: [LiNO₃·H₂O·4HP]·4HP (Li-4HP2) and [Mg(NO₃)₂·6H₂O]·(4HP)₂ (Mg-4HP), and one by-product: LiNO₃·H₂O·4HP (Li-4HP), which were grown using a mild aqua-solution method. The synergy of the coplanar groups of NO₃⁻ 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.