Strongly Phosphorescent Transition-Metal Complexes with N-Heterocyclic Carbene Ligands as Cellular Probes

The development of functionalized luminescent transition-metal complexes that selectively target specific cellular structures (proteins, organelles, or nucleic acids) has been and continues to be an area of burgeoning interest in the design of luminescent probes and/or anticancer agents. The prevalence of deriving phosphorescent probes is owing to their long emission lifetimes (100 ns to ms), tunable emission wavelength, and fairly large Stokes shifts (hundreds of nm). However, metal complexes often suffer from the obstacles including low stability, inadequate water solubility, and low cellular uptake. The charge neutral and strong σ donor properties of N-heterocyclic carbene (NHC) ligands render NHCs as excellent ancillary ligands to form cationic metal complexes with high stability. More importantly, coordination of transition-metal ion with NHC ligand leads to the destabilization of the strongly antibonding metal (ndx2 -y2)-ligand orbital; consequently the non-radiative decay of the emissive excited state is strongly suppressed with concomitant enhancement of both the emission quantum yield and emission lifetime. In this review, we highlight recent works on luminescent transition-metal complexes with NHC ligands and their applications as cellular probes and bio-imaging reagents. In particular, examples of luminescent transition-metal complexes with planar d8 (e.g., Pt2+, Au3+), linear d10 (e.g., Au+), and octahedral d6 (e.g., Ir3+) metal centers are focused. The structural features of luminescent metal complexes in relation to their cellular uptake, localization properties, and designated biomolecular interactions for specific analytical and biological applications are discussed.