Different conjugated system Cd(II)/Hg(II) Schiff base complexes: syntheses, supramolecular metal−organic frameworks, luminescent properties and DFT study

A series of different conjugated systems of 2D/3D supramolecular metal-organic frameworks (SMOFs) are constructed by C/O−H⋯Cl hydrogen bonds and π⋯π interactions. These complexes, [HgL1Cl2] (1), [HgL2Cl2] (2), [HgL3Cl2] (3), [CdL4Cl4]2 (4), and [CdL5Cl2(CH3OH)] (5), have been synthesized and characterized by single-crystal X–ray diffraction, 1H NMR, FT–IR, and EA. The X-ray diffraction analyses reveal that 1 features a 3D supramolecular framework with {44·66} topology structure, while 2, 3, and 5 exhibit 3D 6-connected {412·63} topology structures. Complex 4 shows a two-dimensional layer with 44 topology structure. Based on these varied structures caused by different conjugated system, the emission maximum wavelengths of 1–5 can be tuned in a large range of 492–587 nm. Both electron-donating ability and the conjugated system in general can support λem shift to red direction. In order to have better understanding of electronic transitions of the complexes, a time-dependent DFT study has been performed. The enhancement of the fluorescence intensities for the complexes compared to the ligands indicates potential to serve as photoactive materials.