Phosphorescent Iridium(III) Complexes for Bioimaging

Phosphorescent iridium(III) complexes have received increasing attention in bioimaging applications owing to their advantageous photophysical properties and efficient internalization into live cells. In this chapter, we summarize the recent design of bioimaging reagents based on phosphorescent iridium(III) complexes. The utilizations of cationic, neutral, and zwitterionic phosphorescent iridium(III) complexes in bioimaging applications have been described first. Complexes showing aggregation-induced phosphorescence have also been included considering the absence of the commonly observed aggregation-caused quenching. Then we discuss the functionalization of iridium(III) complexes with biological substrates and reactive groups, which allows non-covalent and covalent interaction, respectively, with intracellular biomolecules. As the photophysical properties of iridium(III) complexes are very sensitive toward their surrounding ligands and microenvironment, the use of these complexes as intracellular sensors for gas molecules, ions, and amino acids has been summarized. Additionally, the incorporation of iridium(III) complexes into dendrimer, polymer, and nanoparticle systems providing attractive functionalities has been discussed. Furthermore, various strategies, including the use of near-infrared-emitting and two-photon excitable complexes, upconversion nanoparticles, and lifetime-based microscopy techniques, to enhance signal-to-noise ratios in bioimaging have been discussed. At last, the design of reagents for multi-mode imaging techniques involving phosphorescence and magnetic resonance imaging has been described.