Utilizing a series of fac-Re(CO)3 core based quinonoid containing complexes for photophysical and cell imaging studies

A series of N boolean AND O bis-chelating bridging quinone ligands (H2L1, H2L2 and H2L3) have been utilized to synthesize four dinuclear fac-Re(CO)(3) based neutral complexes (1-4) by their reaction with Re(CO)(5)(OTf), where the N-donors 4-dimethylaminopyridine (for 1), 3,5-dibromopyridine (for 2) and acetonitrile (for 3 and 4) act as ancillary ligands. The complexes were characterized by FTIR, H-1 NMR, ESI-MS and elemental analysis. The structures of complexes 1-3 were analysed by X-ray diffraction methods. Photophysical investigations revealed that all the complexes display strong visible-light absorption with absorption maxima at around 500 nm (epsilon = 40000-50000 M-1 cm(-1)), whereas in the solid state the complexes display high intensity broad bands in the entire UV-Vis-NIR region. Complexes 1-4 were found to be luminescent in fluid solutions at room temperature. Favorable photophysical properties allowed their utilization as luminescent probes for live-cell imaging using HeLa cells, where complexes 2 and 4 were found to display good cellular uptake with co-localization in lysosomal compartments, which was observed as yellow fluorescence. Complexes 2 and 4 displayed very low cytotoxicity, as suggested by LDH production experiments. The photophysical properties, uptake pathway, localization properties and cytotoxicity of the complexes are discussed in detail. (C) 2015 Elsevier Ltd. All rights reserved.