Naphthalene-Like Unit That Induces Strong Optical Anisotropy in C10H8NO2Br and C10H8NO2Br·H2O

Two metal-free birefringent crystals, C10H8BrNO2 and C10H8BrNO2·H2O, which contain a new birefringence-active [C10H8NO2]+ gene, were synthesized via a mild solution evaporation method. In their crystal structures, the π-conjugated naphthalene-like [C10H8NO2]+ 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 [C10H8NO2]+ 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.