CO(NH2)2·NH4Cl: an ultraviolet birefringent material with conjugated CO groups†
Birefringent materials play a pivotal role in optical communications and the laser industry, and can modulate the polarization of light. However, only a few birefringent materials can be practically used in the ultraviolet (UV) region (λ < 400 nm) due to the strict requirements, such as the UV transparency range and ease of growth. As a novel functional unit, the CO group is proposed for designing birefringent materials, which is different from the traditional BO3, CO3 or VO4 groups, etc. Hence, we synthesized a birefringent crystal (CO(NH2)2·NH4Cl) with a size of up to 14 × 3 × 2 mm3 at room temperature by the slow evaporation method, which has excellent optical properties. It not only possesses a large birefringence and short UV cutoff edge, but can also be easily grown under facile conditions. Moreover, theoretical calculations were carried out to analyze the relationship between the electronic structure and optical properties. The results demonstrate that CO(NH2)2·NH4Cl is a prominent candidate for a UV birefringent material, and the introduction of conjugated CO groups is an effective synthesis strategy to accelerate the design and development of UV birefringent materials.