Naphthalene-Like Unit That Induces Strong Optical Anisotropy in C 10 H 8 NO 2 Br and C 10 H 8 NO 2 Br·H 2 O.

Two metal-free birefringent crystals, C 10 H 8 BrNO 2 and C 10 H 8 BrNO 2 ·H 2 O, which contain a new birefringence-active [C 10 H 8 NO 2 ] + gene, were synthesized via a mild solution evaporation method. In their crystal structures, the π-conjugated naphthalene-like [C 10 H 8 NO 2 ] + groups are basically aligned, which induces high optical anisotropy; i.e., the title compounds exhibit large birefringences of 0.36 and 0.41 at 550 nm according to first-principles calculations. Moreover, the UV-vis-near-IR diffuse-reflectance spectra suggest that they have similar optical band gaps. Structural analysis and theoretical calculations show that the [C 10 H 8 NO 2 ] + unit is responsible for the good optical anisotropy. These results make the naphthalene-like motif a good structural gene to search for new birefringent crystals.