Synthesis, characterization and photoluminescent properties of Zn-based mono- and hetero-MOFs containing the R-isophthalate (R = methyl or tert-butyl) ligands

Abstract

Four novel Zn-based homo- and hetero-metal organic frameworks (MOFs), [Zn₂(H₂O)(mip)₂]_n (Zn-mip), [ZnMg(H₂O)₂(mip)₂·3(H₂O)]_n (ZnMg-mip), [Zn₃(OH)₂(tbip)₂]_n (Zn-tbip) and [Zn₂Ba(O)(H₂O)₂(tbip)_2.5·(H₂O)]_n (ZnBa-tbip), have been hydrothermally synthesized by employing the single ligand 5-methyl isophthalic acid (H₂mip) or 5-tert-butyl isophthalic acid (H₂tbip). The structures of all the MOFs are determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, inductively coupled plasma (ICP) analysis, FT-IR spectroscopy, DTA/TG analyses and X-ray powder diffraction (XRPD) techniques. The results indicate that all the homo- and hetero-MOFs exhibit the 3D open frameworks with the unique Zn-based cluster second building units (SBUs). The influence of the coordination geometries of the hetero-metal centers on the structures of Zn-based mono-MOFs is realized through replacing the five (or four)-coordinated Zn atoms with the alkaline-earth metal ions. Moreover, compared with the free ligands, the photoluminescent properties of the Zn-mip, ZnMg-mip, Zn-tbip and ZnBa-tbip are also studied in the solid state at room temperature. The different emission bands of the homo- and hetero-MOFs demonstrate that the substituent effect of alkaline-earth metal ions plays a key role in directing the related photoluminescent properties of the Zn-based MOFs.