The present invention provides a luminogenic, in particular a phosphorogenic transition metal-based pyridyl complex containing a nitrone moiety, which nitrone moiety acts as a bioorthogonal functional group and an emission quencher, and can undergo cycloaddition reaction with a complementary bioorthogonal functional group coupled to a substrate. The transition metal is can be selected from iridium or ruthenium. Also disclosed is a method for preparing the transition metal-based pyridyl complex and a pharmaceutical composition comprising it. Still further provided is a method for bioorthogonal labeling of a biomolecule, a method for staining of a cell structure, a method for in vivo imaging of an organism, and a kit for in vivo imaging of an organism. The luminogenic properties and high reactivity of the complexes are highly advantageous for bioorthogonal labeling and imaging of biomolecules in their native biological environments at much lower costs than those of the existing commercial products.