Phosphorescent iridium(III) complexes as lifetime-based biological sensors for photoluminescence lifetime imaging microscopy

Luminescent iridium(III) complexes exhibit high phosphorescence quantum yields and photostability, good cell membrane permeability, rich and sensitive photophysical properties, and thus have been extensively studied as intracellular sensors for photoluminescence microscopy imaging. The microenvironmental parameters and interactions of the complexes with cellular biological species can be readily reflected by changes in luminescence properties. In many studies, such changes are limited to responses in the phosphorescence intensity and wavelength; the phosphorescence lifetime is often ignored because it is invisible and difficult to analysis in cellular conditions. In recent decades, photoluminescence lifetime imaging microscopy (PLIM) has been emerging and fast developed, which allows visualizing and mapping the decays of luminescent dyes in the time domain during cellular imaging. In this review article, we focus on the design and applications of lifetime-responsive phosphorescent iridium(III) complexes as imaging reagents for mapping environmental parameters and sensing analytes of interest in intracellular conditions. These parameters and analytes either interact with the probes in the excited state to affect their decay rates or directly react with the probes yielding new products. The utilization of the phosphorescent complexes to visualize intracellular labeling of biomolecules via PLIM are also discussed.