Structural polymorphism and luminescence properties of zinc(ii) and cobalt(ii) MOFs with rigid and flexible ligands†

Abstract

Here, we present a series of Zn(II) and Co(II) coordination polymers containing two types of ligands: sterically rigid terephthalate derivatives (bdc-NO₂²⁻ and bdc-Br²⁻) and flexible bis(2-methylimidazolyl)propane (bmip). The combination of two types of ligands allowed us to obtain and characterize both single and mixed-metal compounds by single crystal and powder X-ray diffraction, FT-IR, and elemental analysis. These 4-fold interpenetrated frameworks have a diamond topology and do not reveal any structural transformations despite the presence of a flexible ligand. Luminescence spectra for all the Zn-containing compounds were recorded. All the compounds exhibit a strong dependence of emission maxima on excitation wavelength, which is characteristic of the bmip ligand and is enhanced in coordination polymers. Compound [Zn(bdc-Br)(bmip)] demonstrates bright luminescence with a high quantum yield of 39%. Mixed-metal Zn(II)/Co(II) compounds have redshifted luminescence in comparison to [Zn(bdc-NO₂)(bmip)] and compound [Zn_0.6Co_0.4(bdc-NO₂)(bmip)] is characterized by near white emission under 460 nm excitation.