(C 3 N 6 H 7 ) 2 SbF 5 ·H 2 O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C 3 N 6 H 7 ) + Groups.

An antimony fluoride melamine birefringent crystal, (C 3 N 6 H 7 ) 2 SbF 5 ·H 2 O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C 3 N 6 H 7 ] ∞ chains further connected by hydrogen bonds originated from [SbF 5 ] 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 (C 3 N 6 H 7 ) 2 SbF 5 ·H 2 O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO 3 crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.