Luminescence chemosensors, biological probes, and imaging reagents

In the last decade, there has been fast-growing interest in the development of photofunctional transition metal complexes for various biological applications due to their rich photophysical and photochemical properties. In this Chapter, we summarize the recent exploitation of luminescent complexes of transition metal centers having a d 6 , d 8 , and d 10 electronic configuration in living systems, with a focus on their use as chemosensors, biological probes, and imaging reagents. The structure–property relationships of transition metal complexes are described to shed light on how the lipophilicity, formal charge, molecular size, chirality, and counterion of the complexes affect their cellular uptake and localization properties. Methods to manipulate the cellular uptake and biocompatibility of the complexes are also discussed. The design strategies of transition metal complexes as chemosensors for analytes including ions (e.g., metal cations and inorganic anions), small molecules (e.g., amino acids and biothiols), reactive species (e.g., reactive oxygen, nitrogen, carbonyl, and sulfur species), nucleic acids, and enzymes are reviewed. Additionally, the development of transition metal complexes as biological probes for visualizing specific biomolecules and monitoring the intracellular microenvironment (e.g., oxygen level, pH, polarity, viscosity, and temperature) are summarized. Furthermore, the strategic design of transition metal complexes as imaging reagents of specific cellular structures are described.