Cyclometalated Platinum(II) Complexes Bearing Bidentate O,O′‑Di(alkyl)dithiophosphate Ligands: Photoluminescence and Cytotoxic Properties

Mononuclear complexes [Pt­(ppy)­(S^∧S)] (1a, S^∧S = O,O′-di­(cyclohexyl)­dithiophos­phate (ctp); 2a, S^∧S = O,O′-di­(butyl)­dithiophos­phate (btp)) and [Pt­(bzq)­(S^∧S)] (1b, S^∧S = ctp; 2b, S^∧S = btp) have been prepared by the reaction of precursor complexes [Pt­(C^∧N)­Cl­(dmso)], C^∧N = deprotonated form of 2-phenylpyrdine (ppy) and 7,8-benzoquinoline (bzq), and potassium salt of S^∧S ligands. All complexes were characterized by NMR spectroscopy, and the structure of 2b was further identified by single crystal X-ray determination. Although the complexes are not luminescent in solution at ambient temperature, they become strong emissive materials (bright green) in solid state (at room temperature) with high quantum yields and long lifetimes in the microsecond domain. In frozen glass state or at low temperature (solid state), these complexes become better emissive in relation to room temperature. UV–vis spectra, supported by TD-DFT calculations, indicate that ¹ILCT (intraligand charge transfer) predominates over the other transitions (L = C^∧N cyclometalated ligand). Accordingly, 1 and 2 exhibit structured emission bands which display a large involvement of ³LCCT (ligand-centered charge transfer) with lower contribution of ³MLCT (metal to ligand charge transfer) transition in the excited states. Also, biological activities of 1 and 2 were evaluated against three human cancer cell lines including A549 (human lung cancer), SKOV3 (human ovarian cancer), and MCF-7 (human breast cancer). 2a presented an effective potent cytotoxic activity regarding to the cell lines. The cellular localization of 1a and 2a in MCF-7 human cells was investigated by fluorescence microscopy.