Ultrawide Bandgap and Record-High Birefringence in a Covalent Beryllium-Organic Framework

The rational design of advanced optical crystals is fundamentally constrained by the intrinsic trade-off between a wide bandgap and large birefringence: structural motifs that provide the requisite electronic anisotropy usually lead to low-energy electronic transitions and a narrow energy gap. Overcoming the trade-off demands decoupling the anisotropic polarizability from band-edge states. To address this, we developed a quantitative screening strategy for functional groups in short-wave ultraviolet (UV)-transparent crystals (λ < 300 nm). The validated 1,3,5-benzenetricarboxylate (C₉H₃O₆)^3- octupolar motifs and tetrahedral Be²⁺ cations were integrated to produce uniformly directionally stacked covalent beryllium-organic framework [Be₃(μ-OH)_4.5(C₉H₃O₆)_0.5]·0.5H₂O (I), in which all carboxylate oxygen sites are coordinated by highly UV-transparent Be²⁺ ions to form a novel fully coupled [Be₆(μ-OH)₁₅(C₉H₃O₆)]^6- octupolar supermotif. Strong Be-O covalent interactions result in two critical electronic optimizations: elimination of nonbonding electrons and enhancement of π-conjugation delocalization, which significantly improves both the energy gap and polarization anisotropy. Accordingly, crystal I shows an ultrawide bandgap of 5.00 eV, a short-wave UV cutoff at 248 nm, and a record-high birefringence of 0.60 in the short-wave UV region. Moreover, crystal I exhibits outstanding overall optical performance, including an exceptional quality factor Q of 3.00─the highest value reported among all short-wave UV materials. This work breaks the intrinsic performance bottleneck for birefringent crystals and establishes a viable ultrawide bandgap birefringent platform for next-generation photonic applications.