Discovery of crystals functionalized by optically active units is of great significance to laser science and industry since these species have the potential for frequency conversion and light modulation. This spurred researchers on to find new optical crystals with such functionality, especially in new unexplored systems. Nowadays, boron-based compounds with multiple anionic units have been found and reported in other systems, like borosilicates, borophosphates, borosulfate and borogermanates, etc., in which short-wavelength UV NLO and birefringent crystals have been screened out as future optical materials.

　　Bis(oxalate)borates and difluoro(oxalato)borates represented by LiB(C₂O₄) and LiBF₂C₂O₄ applied as electrolytes in advanced lithium-ion batteries are important part of boron-based compounds. In the above two systems, the preliminary goal of introducing additional π-conjugated units in the B？O anionic skeleton is naturally achieved. From a structural perspective, two [C₂O₄] units that belong to [B(C₂O₄)₂] multiple anions are arranged vertically to each other, which is unfriendly to both birefringence and NLO coefficient for bis(oxalate)borates. Instead, this situation can be well solved in the difluoro(oxalato)borate, and thereby, they focused on this system to screen and explore short-wavelength UV optical crystals in this work.

　　Based on this, a research group led by Prof. Shilie Pan at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, synthesized three difluoro(oxalato)borates, namely LiBF₂C₂O₄, NaBF₂C₂O₄, and Sr(BF₂C₂O₄)(BF₄). Among them, the first two cases are known compounds that were extensively studied in lithium- and sodium-ion battery applications but their linear (like birefringence) and nonlinear optical properties are still blank. However, for Sr(BF₂C₂O₄)(BF₄), the first alkaline earth metal difluoro(oxalato)borate is discovered by us for the first time. Performance characterization combined with experimental and theoretical approaches indicates that all three difluoro(oxalato)borates are birefringent crystals with large optical anisotropy, whereas NaBF₂C₂O₄ is characterized as a short-wavelength UV NLO crystal. The priority of the difluoro(oxalato)borate system as a new source of optical crystals reflected by the microscopic properties of the [BF₂C₂O₄] chromophore is discussed in this study. More importantly, the [BF₂C₂O₄] multiple anion was proven to be a bifunctional unit for causing a considerable nonlinear optical effect and birefringence simultaneously, which also confirmed the feasibility of exploring novel optical candidates in difluoro-(oxalato)borate systems.

　　The paper was published on Chem. Mater. 2022, 34, 7516–7525 with the title of “Difluoro(oxalato)borates as Short-Wavelength Optical Crystals with Bifunctional [BF₂C₂O₄] Units”. This work was financially supported by National Key R&D Program of China, Young Elite Scientist Sponsorship Program by CAST, National Natural Science Foundation of China, and the West Light Foundation of CAS.

　　Article: https://pubs.acs.org/doi/10.1021/acs.chemmater.2c01831

Figure. The diagram of bifunctional [BF₂C₂O₄] unit