Luminescent cyclometalated iridium (III) polypyridine complexes appended with a sydnone moiety as novel bioorthogonal probes

Abstract

Two novel luminescent cyclometalated iridium(III) polypyridine complexes appended with a sydnone moiety, [Ir(N^C)2(bpy-Ph-sydnone)](PF6) (bpy-Ph-sydnone = 4-(-5-oxo-1,2,3-oxadiazol-3-yl)phenylcarboxylaminomethyl)-4’-methyl-2,2’-bipyridine; HN^C = 2-(2,4-difluorophenyl)pyridine (Hdfppy) (1a) and methyl 2-phenyl-4-quinolinecarboxylate (Hpqe) (2a)) and their sydnone-free counterparts, [Ir(N^C)2(bpy-Ph)](PF6) (bpy-Ph = 4-phenylcarbonylaminomethyl-4’-methyl-2,2’-bipyridine; HN^C = Hdfppy (1b) and Hpqe (2b)) were synthesized and characterized. Their electronic absorption and photophysical properties were also studied. Upon photoexcitation, these complexes displayed intense and long-lived green to red luminescence in fluid solutions under ambient conditions. The emission was ascribed to a triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Ir)  π*(N^N and N^C)) excited state. Interestingly, the sydnone complexes 1a and 2a displayed lower emission quantum yields and shorter emission lifetimes than their sydnone-free counterparts 1b and 2b. This phenomenon was tentatively ascribed to the quenching property of the appended sydnone moiety. Additionally, the reactivity of the sydnone complexes toward (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN) was examined. The second-order rate constants (k2) of complexes 1a and 2a with BCN were determined to be 3.17  102 and 2.83  102 M−1 s1, respectively, which is similar to that of phenyl sydnone with BCN (5.4  1022 M1 s1). The bioorthogonal labeling properties of the complexes were also investigated. SDS-PAGE analyses revealed that the sydnone complexes selectively labeled BCN-modified proteins. All these results demonstrated the potential applications of the sydnone complexes as luminescent bioorthogonal probes for cyclooctyne-modified biomolecules.