Giant Birefringence and Solar‐Blind Ultraviolet Transmission by Coordination‐Induced Assembly of π‐Conjugated Pyridine–Carboxylate

Abstract Giant birefringent crystal with large bandgap is of paramount importance for modern optical devices but simultaneous achievement of giant birefringence and large bandgap is a great challenge stemming from their inherent contradictory relationship according to Kramers‒Kronig equation. Herein, an organic–inorganic hybrid uniformly‐stacked 2D crystal [Be 2 (μ‐OH) 2 (C 7 H 3 NO 4 )(H 2 O)]·H 2 O ( I ) was successfully synthesized by a molecular crystal engineering involving hydrogen bond synthons. Layered I features new [Be 2 O 2 N(μ‐OH) 2 (H 2 O)] ∞ helical chain composed of Be II tetrahedral coordination, and the strong covalent Be II cations coordinate with the O and N atoms on birefringence‐active π‐conjugated (C 7 H 3 NO 4 ) 2− organic groups with optimal spatial arrangement. As a result, I demonstrates the largest birefringence of 0.6 in the solar‐blind UV spectral region, with large bandgap of 4.75 eV and short UV absorption edge of 261 nm. First‐principles calculations and structural analyses uncover that the superior optical properties mainly originate from collective effects of highly‐active (C 7 H 3 NO 4 ) 2− groups, distorted Be‐tetrahedral units, and their uniform stacking. This work not only provides a potential solar‐blind UV birefringent crystal for rapidly increasing number of optical applications but also a new chance for the rational design of wide UV giant birefringent crystals.