Luminescent Cyclometallated Pt(II) Complexes for Cellular Imaging

The photoluminescence properties of cyclometallated platinum(II) complexes make them potential candidates for use in a range of applications such as triplet harvesting agents in OLEDS, as photosensitizers, and in solar cells. Also, the strong luminescence of these complexes gives them much potential for use in chemosensing applications and as bioimaging agents. This final topic is an area of increasing study as a result of the diverse and tuneable excited states of such molecules, and their advantages over conventional organic emitters such as reduced photobleaching and long luminescence lifetimes allowing time-resolved imaging to be performed. This research project aimed to develop luminescent polyamine appended Pt(II) complexes to target and image synaptic vesicles. Polyamines are frequently found in high concentrations in cancer cells which have an up-regulated polyamine transport system (PTS). The exploration of PTS as a vector to selectively target live cells to understand the cellular uptake is an important aspect of research for this project. Intensely luminescent cyclometallated platinum(II) complexes have been prepared containing N^C^N-coordinating ligands, based on 1,3-di(2-pyridyl)benzene, 1,3-di(1,2,3-triazole)benzene and 2-(3-(1,2,3-triazol)phenyl)pyridine derivatives. Tridentate ligand functionalisation has been employed to obtain complexes that features either one of or a combination of hydrophilic and lipophilic groups. Substitution of the chloride ancillary ligand for pyridine or alkyne derivatives has been achieved. A variety of synthetic strategies were employed in the synthesis of the proligands, for which the main strategies involve either one of or a combination of metal catalysed cross-coupling and click reactions. A series of monodentate ligands with an alkyne functionality were synthesised via different methods such as amine or PEG alkylation, amide coupling and reductive amination of a Schiff base. All the ligands and Pt(II) complexes were characterised by 1H and 13C NMR spectroscopy, mass spectrometry and X-ray analysis.1,3- di(2-pyridyl)benzene based Pt(II) complexes are intensely luminescent, in their own right, with significantly high quantum yields and lifetimes (Φ = 0.039-0.13), (τ = 0.50-0.85 μs). Whereas, 1,3-di(1,2,3-triazole)benzene -type cyclometalated Pt(II) complex, the high energy of the π* orbitals of the 1,2,3-triazole units resulted in the absence of detectable emission; this was attributed to an inadequate energy separation between the emissive state and the d−d states. However, when formally replacing only one pyridine ring in Pt(II) complexes of 1,3-di(2- pyridyl)benzene ligands by a 1,2,3-triazole, an intense green emission could be achieved (Φ = 0.09, τ = 0.85 μs) in aerated DCM. Finally, Substitution of the chloride ancillary ligand for pyridine led to further blue-shifts, whereas alkyne substitution led to red-shifts.