Strategically designed anisotropic organic–inorganic metal halides (OIMHs) with superior balance between bandgap and birefringence†

Herein, we report two novel organic–inorganic metal halides (OIMHs) with outstanding ultraviolet birefringences, namely Cd(C₆H₄NO₂)I·H₂O (1) and Cd(C₆H₅NO₂)₂I₂ (2). Both feature a 1D neutral chain, in which the 1D [Cd(C₆H₄NO₂)I·H₂O]_∞ is built from Cd₂I₂N₂O₆ dimers and (C₆H₄NO₂)⁻ anions via the connection of Cd–N bonds, whereas each pair of antiparallel (C₆H₅NO₂) groups hang at 1D [CdI₂]_∞, forming a 1D [Cd(C₆H₅NO₂)₂I₂]_∞ chain. Importantly, birefringence measurements indicate that 1 and 2 feature exceptional birefringent properties with values of 0.243@550 nm and 0.388@550 nm, respectively, which are not only greater than those of commercial materials (α-BaB₂O₄, 0.122@546 nm), but also 8.1 and 12.8 times that of inorganic CdI₂. Additionally, the UV-Vis spectra show that 1 and 2 have short adsorption edges of 280 nm and 284 nm, relating to wide bandgaps of 4.23 eV and 3.94 eV, respectively. Therefore, they, especially 2, are strong candidates for potential ultraviolet birefringent materials. We innovatively propose a design strategy for the NA-Cd–I system, providing new insights into the exploration of novel high-performance birefringent crystals.