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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.