Synthesis, structure, photophysical properties and evaluation of in vitro cytotoxic activity of homoleptic dipyrrin based palladium complexes

Abstract

The synthesis of bis(dipyrrinato)palladium(II) complexes (PD1-PD4) from functionalized dipyrromethanes was achieved via a simple and versatile route involving the oxidation of meso-substituted dipyrromethane followed by complexation with palladium acetate. All the complexes were characterized in detail using various spectroscopic techniques and their structures explicitly confirmed by X-ray crystallography. The crystal structures of the complexes PD1 and PD4 revealed an overall ‘zig-zag’ arrangement for the molecules due to the bent geometry of the meso-substituents with respect to the dipyrrin plane. The photophysical properties were meticulously studied; the complexes exhibit weak luminescence in the blue-green spectral region (λem = 500–575 nm) with lifetime of 2.5–3.8 ns. A short triplet state lifetime of 28.97–67.61 ns was observed for the complexes, as determined by nanosecond transient absorption spectroscopy. All the complexes were tested for their cytotoxicity through a trypan blue-based in vitro assay on DLA cancer cell lines and the results were compared with normal spleen cells. Interestingly, low IC50 values were obtained for PD3 against the cancerous cells, whereas the complexes are inactive towards normal cell lines. The antiproliferative activity revealed the potential of these complexes for further development as in vivo anticancer agents.