Lighting the way to see inside the live cell with luminescent transition metal complexes

The application of luminescent metal complexes to cell imaging is a rapidly emerging area. This review discusses the background to the field, the advantages that such complexes may offer over conventional fluorescent imaging probes, and the desirable requirements for successful applications.We particularly focus on a variety of applications that are offered by square planar platinum(II) complexes, including their use in time-resolved emission imaging microscopy (TREM). This emerging technique exploits the long luminescence lifetimes of such metal complexes to considerably extend the scope of conventional fluorescence lifetime imaging microscopy (FLIM), which is normally carried out on fluorophores that emit on the nanosecond timescale. Selected recent examples of complexes with d6 metal ions, Ir(III), Ru(II) and Re(I), are also discussed. The multitude of applications that transition metal complexes can offer – and which are not necessarily accessible with organic labels – are outlined, and linked to photophysical properties as well as to the modes of interaction between the label and the imaged object.

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► Luminescent transition metal complexes have opened up new horizons in bioimaging.
► Key advantages are high emission yield, synthetic versatility and long lifetime.
► Pt, Ir, Ru, Re complexes provide high contrast imaging of subcellular structures.
► Expanding time-resolved imaging to timescales orders of magnitude longer than FLIM.