Luminescent Iridium(III) and Rhenium(I) Complexes as Biomolecular Probes and Imaging Reagents

There is an emerging interest in the applications of luminescent transition metal complexes as biological probes and imaging reagents due to their interesting photophysical properties, which include high photostability, long emission lifetimes, large Stokes’ shifts, and highly environment-sensitive emission. The involvement of heavy metal implies that the quantitation of cellular uptake can be conveniently performed by inductively coupled plasma mass spectrometry. For more than a decade, we have developed a number of luminescent transition metal complexes that carry a reactive functional group for bioconjugation. Additionally, we have incorporated different biological substrates within transition metal complexes that show highly environment-sensitive emission and exploited them as noncovalent probes for biomolecules such as nucleic acids and proteins. In the past several years, we have also developed an interest in the cellular studies of related complexes. Different functional moieties have been appended to luminescent transition metal complexes with a variety of spacer-arms to afford new cellular reagents. The functional moieties include biotin, indole, estradiol, alkyl and fluorous chains, intercalating units, dendritic skeleton, reactive functional groups such as isothiocyanate and aldehyde, ion and molecule sensing units, polymeric chains such as poly(ethylene glycol) and poly(ethyleneimine), carbohydrates, and chemical probes for bioorthogonal labeling. In this chapter, we have summarized the molecular design and photophysical properties of a selection of these metal complexes developed in our laboratory. The cellular uptake, cytotoxicity, intracellular trafficking and localization, and the potential biological applications (such as sensing, bioimaging, and photoinduced cytotoxicity) of these complexes have been described.