Luminescent platinum(II) complexes with self-assembly and anti-cancer properties: hydrogel, pH dependent emission color and sustained-release properties under physiological conditions

Supramolecular interactions are of paramount importance in biology and chemistry, and can be used to develop new vehicles for drug delivery. Recently, there is a surge of interest on self-assembled functional supramolecular structures driven by intermolecular metal-metal interactions in cellular conditions. Herein we report a series of luminescent Pt(II) complexes [Pt(C<^>N<^>N-pyr)(C equivalent to NR)](+) [HC<^>N<^>N-pyr = 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine)] containing pincer type ligands having pyrazole moieties. These Pt(II) complexes exert potent cytotoxicity to a panel of cancer cell lines including primary bladder cancer cells and display strong phosphorescence that is highly sensitive to the local environment. The self-assembly of these complexes is significantly affected by pH of the solution medium. Based on TEM, SEM, ESI-MS, absorption and emission spectroscopy, and fluorescence microscopy together with cell based assays, [Pt(C<^>N<^>Npyr)(C equivalent to NR)](+) complexes were observed to self-assemble into orange phosphorescent polymeric aggregates driven by intermolecular Pt(II)-Pt(II) and ligand-ligand interactions in a low-pH physiological medium. Importantly, the intracellular assembly and dis-assembly of [Pt(C<^>N<^>N-pyr)(C equivalent to NR)](+) are accompanied by change of emission color from orange to green. These [Pt(C<^>N<^>N-pyr)(C equivalent to NR)](+) complexes accumulated in the lysosomes of cancer cells, increased the lysosomal membrane permeability and induced cell death. One of these platinum(II) complexes formed hydrogels which displayed pH-responsive and sustained release properties, leading to low-pH-stimulated and time-dependent cytotoxicity towards cancer cells. These hydrogels can function as vehicles to deliver anti-cancer agent cargo, such as the bioactive natural products studied in this work.