Luminescent EuIII and TbIII Bimetallic Complexes of N,N’-Heterocyclic bases and Tolfenamic acid: Structures, Photophysical Aspects and Biological Activity

Four new homodinuclear isostructural luminescent EuIII and TbIII-complexes containing tolfenamic acid (TFA) and N,N-heterocyclic bases (DPQ/DPPZ) presented as [Ln2(DPQ/DPPZ)2(TFA)6] where, LnIII = EuIII [DPQ (1), and DPPZ (2)] and TbIII [DPQ (3) and DPPZ (4)] were studied in this work. The planar DPQ/DPPZ ligands used are well-known intercalator cum photosensitizers, while tolfenamic acid (TFA), a NSAID with a carboxylate group, provides hard O-donor for hard Lewis acidic LnIII ions and can form a bridge between the two LnIII ions. The dinuclear complexes were structurally characterized using X-ray crystallography showing their isostructural behavior. This is due to the resemblance of the LnIII experiencing identical ligand disposition in 3D and electronic interactions from ligands and LnIII. All complexes possess a Ci symmetry element, therefore, making the coordination environment around each LnIII remain identical. The two LnIII ions in the dimeric structure are bridged by -COO- groups of the four TFA molecules in a μ-η1:η1 coordination mode. Additionally, one TFA is coordinated to each LnIII in η2-chelating mode along with one bidentate N,N-donor DPQ/DPPZ ligand. Therefore, each EuIII/TbIII ion ultimately possesses eight-coordinated {Ln2N2O6} square antiprismatic geometry. The luminescent complexes showing characteristic (5DJ→7FJ) f-f transitions in solution with quantum yield ranging from 8-20% were utilized for cellular imaging studies. The affinity of the complexes with biological targets like DNA and serum albumin protein was studied. The cellular uptake and bio-imaging potential of the luminescent complexes in MCF-7 and HeLa cancer cells have been probed using confocal fluorescence imaging studies. The sufficient internalization of the LnIII-probes into the cells observed from intrinsic luminescence originating from EuIII/TbIII demonstrates the cytosolic and nuclear localization. The complexes exhibit photo-induced cytotoxicity upon exposure at 365 nm UV-A light presumably because of the ROS generation from photoexcitation of the DPQ/DPPZ sensitizers.