Copper and zinc bis(thiosemicarbazonato) complexes with a fluorescent tag: Synthesis, radiolabelling with copper-64, cell uptake and fluorescence studies

The synthesis of new copper(II) bis(thiosemicarbazonato) complexes with an appended pyrene chromophore and their zinc(II) analogues is reported. The new proligands and their copper(II) and zinc(II) complexes were characterised by a combination of NMR, EPR, high performance liquid chromatography, mass spectrometry, electronic spectroscopy and electrochemical measurements. The new copper(II) complexes are fluorescent as a consequence of an appended pyrene substituent that is separated from the sulphur coordinating to the metal ion by five bonds. The emission from the pyrene substituent is concentration- and solvent-dependent with characteristic formation of excimer aggregates. A radioactive ⁶⁴Cu complex has been prepared. Cell permeability, intracellular distribution and importantly the ability to cross the nuclear membrane to target DNA were investigated using confocal fluorescence microscopy in a human cancer cell line under normal oxygen conditions and hypoxic conditions. In both cases, there was no evidence of uptake of the copper(II) bis(thiosemicarbazonato) complexes in the area of the cell nucleus.