Chemiluminescent transition metal complexes: Mechanisms and applications

Chemiluminescence is an enchanting phenomenon involving the emission of photons intrinsically excited by chemical reactions, which has been broadly applied for analytical and bioanalytical sensing as well as theranostic applications. Luminescent transition metal complexes are competitive alternatives to traditional organic fluorophores, as they inherently possess multiple modifiable variations (metal centers, coordination ligands, and metal-ligand interplays) which can be intricately tuned to afford a large number of fascinating combinations with diverse functions and distinct mechanisms of action, providing unprecedented opportunities for innovative theranostic design. Transition metal complexes, like Ru(II), Ir(III), Cr(III), etc., have been employed for constructing chemiluminescence reaction systems. Interestingly, in addition to the merits exhibited by pure organic chemiluminophores, organometallic chemiluminescence systems displayed exceptional advantages, such as distinctive redox reactions on metal centers, easy-to-modify, tunable emission colors/brightness, adjustable reaction kinetics, excellent thermal- and photo-stability, and traceable cellular uptake regardless of metabolisms. However, compared to the thriving studies on pure organic chemiluminophores, the research on chemiluminescent transition metal complexes is still in its early stages though having a long history. Therefore, in this review, we summarize the reported designing principles and unique mechanisms of action exerted by various types of chemiluminescent transition metal complexes, with a focus on their applications in bioimaging and biomedical realms. The remaining challenges and future perspectives for chemiluminescence studies are also discussed. We believe this review can serve as a theoretical guide and application reference for the molecular design of innovative chemiluminophores for a wide variety of bio-applications.