(C3N6H7)2SbF5·H2O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C3N6H7)+ Groups

An antimony fluoride melamine birefringent crystal, (C3N6H7)2SbF5·H2O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C3N6H7]∞ chains further connected by hydrogen bonds originated from [SbF5]2- 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 (C3N6H7)2SbF5·H2O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO3 crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.