Luminescent EuIII and TbIII bimetallic complexes of N,N′-heterocyclic bases and tolfenamic acid: structures, photophysical aspects and biological activity

Abstract

Four new homodinuclear isostructural luminescent Eu^III and Tb^III complexes containing N,N-heterocyclic bases (DPQ/DPPZ) and tolfenamic acid (TFA) presented as [Ln₂(DPQ/DPPZ)₂(TFA)₆], where Ln^III = Eu^III [DPQ (1), and DPPZ (2)] and Tb^III [DPQ (3) and DPPZ (4)], were studied in this work. The planar DPQ/DPPZ ligands used are well-known intercalators and photosensitizers. Tolfenamic acid (TFA) used as an anthranilic acid derivative-based NSAID, provides the hard carboxylate-O-donor for hard Ln^III Lewis acids, and can form a bridge between two Ln^III ions. The SC-XRD studies on the dinuclear complexes revealed their isostructural nature in the solid state. This is due to the similarity of both Ln^III ions, exhibiting identical ligand disposition in 3D and electronic interactions with the ligands. All the complexes possess C_i symmetry, making the coordination environment around each Ln^III identical. The two Ln^III ions in the dimeric structures are bridged by –COO⁻ groups of the four TFA molecules in a μ-η¹:η¹ coordination mode. Additionally, one TFA is coordinated to each Ln^III in an η²-chelating mode together with one bidentate N,N-donor DPQ/DPPZ ligand. Therefore, each Eu^III/Tb^III ion ultimately possesses an eight-coordinated {Ln₂N₂O₆} square antiprismatic geometry. The luminescent complexes showed the characteristic (⁵D_J → ⁷F_J) f–f transitions in solution with a quantum yield in the range of 8–20%, and thus were utilized for cellular fluorescence imaging studies. The affinity of the complexes with biological targets such as DNA and serum albumin was studied. The cellular uptake and bioimaging potential of the luminescent compounds in MCF-7 and HeLa cancer cells were probed using confocal fluorescence imaging studies. The sufficient internalization of the Ln^III-probes in the cells observed from the intrinsic luminescence from Eu^III/Tb^III demonstrated their non-selective cytosolic and nuclear localization. The complexes were photo-cytotoxic at 365 nm mediated by the ROS generated from the photo-excitation of the DPQ/DPPZ sensitizers.