A High‐Performance Nonlinear Optical Crystal with a Building Block Containing Expanded π‐Delocalization

Common nonlinear optical (NLO) crystals consist of traditional functional building blocks with inherent optical limitation. Herein, inspired by traditional (B3O6)3− inorganic building block, we theoretically identified a new type of organic functional building blocks and then successfully synthesized the first cyamelurate NLO crystal, Ba(H2C6N7O3)2 ⋅ 8 H2O. To our surprise, the constituent (H2C6N7O3)− building block is not in structurally optimal arrangement, but Ba(H2C6N7O3)2 ⋅ 8 H2O exhibits excellent optical properties including wide band gap of 4.10 eV, very large birefringence of 0.24@550 nm, and exceptionally strong second‐harmonic generation (SHG) response of about 12×KH2PO4. Both the SHG response and birefringence are much larger than those of commercial NLO crystal β‐BaB2O4 with optimally aligned (B3O6)3− building block. Theoretical calculations suggest that the expanded π‐conjugation delocalization within (H2C6N7O3)− vs (B3O6)3− should be responsible to the enhanced performance. This work implies that there is still much room to develop new NLO crystals with excellent functional building blocks that may be longly neglected. Expanded π‐conjugation delocalization from traditional functional building block (B3O6)3− to the long overlooked (HxC6N7O3)(3−x)− (x=0, 1, 2) induces significantly enhanced second‐order nonlinear optical response of Ba(H2C6N7O3)2 ⋅ 8 H2O.