In this study we have shown that substitution of chloride ligand for imidazole (Im) ring in cyclometalated platinum complex, Pt(phpy)(PPh3)Cl (1; phpy = 2-phenylpyridine; PPh3 = triphenylphosphine), which is non-emissive in solution, switches on phosphorescence of the resulting compound. Crystallographic and NMR spectroscopic studies of the substitution product showed that the luminescence ignition is a result of Im coordination to give the [Pt(phpy)(Im)(PPh3)]Cl complex. The other imidazole-containing biomolecules such as histidine and histidine-containing peptides and proteins also trigger luminescence of the substitution products. The complex 1 proved to be highly selective towards the imidazole ring coordination that allows site-specific labelling of peptides and proteins with 1 using the route, which is orthogonal to the common bioconjugation schemes via lysine, aspartic/glutamic acids or cysteine and does not require any preliminary modification of a biomolecule. The utility of this approach was demonst