An Optically Anisotropic Crystal with Large Birefringence Arising from Cooperative π Orbitals.

Birefringent materials are highly demanded for high‐performance polarized optics. As compared with artificial anisotropic metamaterials, anisotropic crystals have advantages of low optical losses and easy processing, but their birefringence is still limited. Herein, based on first‐principles studies, we identified a new type of functional anion units, (HxC6N9)(3−x)− (x=0, 1, 2), and then successfully synthesized a new anisotropic crystal, namely, CsH2C6N9⋅H2O (I), whose crystal structure consists of (H2C6N9)− anions. Remarkably, I is ultraviolet transparent and exhibits very large birefringence of about 0.55@550 nm, which is much larger than those of commercial birefringent crystals. These results make I a candidate for highly efficient manipulation of optics and light in optical modulation devices. Theoretical calculations reveal that large birefringence mainly arises from the cooperative π orbitals in (H2C6N9)− anions. This work provides a new insight on the underlying structure–property relationships of anisotropic crystals. [ABSTRACT FROM AUTHOR]