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).8H(2)O. To our surprise, the constituent (H2C6N7O3)(-) building block is not in structurally optimal arrangement, but Ba(H2C6N7O3)(2).8H(2)O 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 12xKH(2)PO(4). Both the SHG response and birefringence are much larger than those of commercial NLO crystal beta-BaB2O4 with optimally aligned (B3O6)(3-) building block. Theoretical calculations suggest that the expanded pi-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.

Automated summary

In this study, a new type of organic functional building block was identified and synthesized to create the first cyamelurate nonlinear optical crystal. The crystal exhibited excellent optical properties, including a wide band gap, large birefringence, and strong second-harmonic generation response. This work highlights the potential for developing new nonlinear optical crystals with superior functional building blocks.