(CNH)SbF·HO Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (CNH) Groups.

An antimony fluoride melamine birefringent crystal, (CNH)SbF·HO, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [CNH] chains further connected by hydrogen bonds originated from [SbF] groups with lone pairs. The experimental optical band gap (4.74 eV) allows it to be used in the ultraviolet region. The first-principles calculations suggest that (CNH)SbF·HO exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.