Cyclometallated platinum and iridium complexes with thiolate ligands, and novel bioimaging agents inspired by dynamic disulfide interchange reactions

Abstract

The photoluminescence properties of cyclometallated second and third row transition metal 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.

The synthesis of cyclometallated complexes of the type [Pt(N^C^N)Cl], [Ir(N^C)2(L)Cl], and [Ir(N^C^N)(N^X)Cl]n+ (where N^C^N and N^C are terdentate and bidentate cyclometallating aromatic ligands; L = monodentate pyridine ligand; X = heterocyclic N or cyclometallated aryl C; n = 0,1) is described and their reactions with thiolates are explored with the aim of producing new classes of thiol sensors. Thus, novel monometallic complexes of the type [Pt(N^C^N)(thiolate)] and [Ir(N^C^N)(N^C)(thiolate)] are reported as well as a novel series of dimers with the general structure [Ir(N^C)2(μ- thiolate)]2 and one example of a [Ir(N^C^N)(μ-thiolate)(thiolate)]2 dimer. The effect of the introduction of a thiolate ligand on the excited states of the Pt(N^C^N) complexes is also described.

In addition, a series of luminescent compounds based on the dansyl group or iridium complexes are reported for use as bioimaging agents and their utility in cellular assays has been investigated. These probes incorporate molecular
structures inspired by the ability of small molecules to mimic the activity of certain redox active proteins and hence have the potential to engage in disulfide exchange processes in vivo. These compounds have been shown to
display acceptable uptake into cells without excessive cytotoxicity and have been used to obtain fluorescence microscopy images of live CHO cells.