Giant Birefringence and Solar-Blind Ultraviolet Transmission by Coordination-Induced Assembly of π-Conjugated...

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 [Be₂(μ-OH)₂(C₇H₃NO₄)(H₂O)]·H₂O (I) was successfully synthesized by a molecular crystal engineering involving hydrogen bond synthons. Layered I features new [Be₂O₂N(μ-OH)₂(H₂O)]_∞ 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 (C₇H₃NO₄)^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 (C₇H₃NO₄)^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.