Zeolitic Open-Framework Borates with Noncentrosymmetric Structures and Nonlinear Optical Properties

Three zeolitic pentaborates (H2en)[B5O8(OH)] (1, en = ethylenediamine), (Hen)2[Al(B5O10)] (2), (H2en)[NH(CH3)2][Al(B5O10)] (3) were solvothermally obtained by employing ethanediamine molecules as structure-directing agents (SDAs). The nonmetal borate of 1 features a noncentrosymmetric two-dimensional layered structure. Through introducing Al atoms as the linkers and regulating the reaction conditions of 1, two three-dimensional zeolitic open-framework aluminoborates, noncentrosymmetric (NCS) of 2 and centrosymmetric (CS) of 3, were achieved. Both 2 and 3 exhibit [Al(B5O10)] n2 n- zeolitic frameworks based on the same fundamental building blocks (FBBs). However, the different arrangements between two FBBs result in 2 showing a NCS framework with dia topology, while 3 presents CS framework with cag topology. The flexibility of linkages modes of the FBBs as well as the unique structure-directing functions play crucial roles in the different formations. Powder second-harmonic generation (SHG) measurements revealed that acentric 1 and 2 possess nonlinear optical activity and 2 is type I phase-matchable with SHG responses of ∼1.0 time for KDP (KH2PO4). Infrared and UV-vis diffuse reflectance spectroscopy, along with electronic structure calculations, were also performed for the materials.