F−/OH− terminal modification of [B3O6] clusters: Constructing deep-ultraviolet hydroxyfluorooxoborates with tunable optical performance

A systematic comparison of F − /OH − modified [B 3 O 6 ] 3- clusters shows that properties evolve with F − /OH − content. • By changing the type and arrangement of the fundamental building blocks, a breakthrough in dimensionality was achieved. • Tunable optical properties were achieved, namely, the birefringence across the series ranges from 0.013 to 0.112 at 546 nm. Borates stand out as a prominent class of compounds in structural chemistry, acclaimed for their extraordinary structural diversity stemming from the versatile B–O fundamental building blocks (FBBs), and terminal oxygen modification has emerged as a powerful and effective strategy for tailoring their structures and optimizing functional properties. Herein, we report the synthesis of seven novel hydroxyfluorooxoborates with different dimensional framework via F − /OH − terminal modification of the isolated [B 3 O 6 ] 3- cluster, aiming to explore structural diversity and structure-property relationships. These compounds are classified by their FBBs: Compounds I-IV (A 7 [B 3 O 3 F 4 OH] 2 (PF 6 ) 3 , A = Rb, K, Cs, NH 4 ) with [B 3 O 3 F 4 (OH)] 2- ; Compounds V-VI (Cs 1.2 (NH 4 ) 0.8 [B 3 O 3 F 2 (OH) 2 ] 2 and Rb 1.7 K 0.3 [B 3 O 3 F 2 (OH) 2 ] 2 ) with [B 3 O 3 F 2 (OH) 2 ] - ; and Compound VII (Rb 7 [B 3 O 3 F 2 (OH) 2 ] 2 [B 3 O 3 F 4 (OH)](NO 3 ) 2 BF 4 ) with coexisting [B 3 O 3 F 4 (OH)] 2- and [B 3 O 3 F 2 (OH) 2 ] - . The birefringence across the series ranges from small (∼0.013 - 0.024 at 546 nm for I-IV) and moderate (∼0.051 at 546 nm for VII) to large (∼0.063 - 0.112 at 546 nm for V and VI). Structural and properties analyses reveal that F − /OH − substitution eliminates B–O dangling bonds of [B 3 O 6 ] 3- , leading to varied [B 3 O 3 F 2x (OH) 3−x ] (x = 1, 2, 3) and [B 3 O 3 (OH) x ] (x = 3, 4, 5, 6), featuring a distinct structure-property relationship. In the [B 3 O 3 (OH) x ] series, both polarizability anisotropy and band gap decrease with rising hydroxyl content, while hyperpolarizability increases. In contrast, the mixed F − /OH − series exhibit the opposite trend. In conclusion, this work significantly provides a rational synthetic strategy for tailoring FBBs and constructs deep-ultraviolet hydroxyfluorooxoborates with tunable optical performance. Through F − /OH − terminal modification of [B 3 O 6 ] clusters, a structural breakthrough from 0D clusters to a 2D layer was achieved, along with tunable optical properties. Download: Download high-res image (251KB) Download: Download full-size image