The photophysics of Ir(III) cyclometalated complexes containing the 2-(2-pyridyl)benzimidazole ancillary ligand: Protonation effect and their potential as specific lysosome probes in cells

In this study, the role of the protonation of the coordinated 2-(2-pyridyl)benzimidazole (pbi) ligand on the photophysical properties of Ir(III) bis-heteroleptic complexes were investigated. Three conjugated acid-base pairs were synthesized and named as Ir1/Ir1-H+, Ir2/Ir2-H+, and Ir3/Ir3-H+ as a function of the cyclometalated (C<^>N) ligand employed: 2-phenylpyridine (ppy) for Ir1/Ir1-H+, 2-phenylquinoline (pq) for Ir2/Ir2-H+, and 1-phenylpyrazole (ppz) for Ir3/Ir3-H+. Temperature-dependent photophysical studies along with electrochemical measurements were explored to rationalize the structural dependence of the lower emitting state with further support of TD-DFT calculations. The parental Ir1/Ir1-H+ pair exhibits a large shift (75 nm) in the emission maxima as a function of the protonation, which is due to the change of the nature of the emitting state from intraligand (3IL) state in Ir1 to metal-to-ligand charge transfer (3MLCT) state in Ir1-H+. In the Ir2/Ir2-H+ pair, the extended conjugation of the 2-phenylquinoline ligand stabilizes the 3IL state and as a result, protonation does not lead to considerable changes in the emission maximum, although an extended absorption to the visible region could be observed. On the other hand, in the novel Ir3/Ir3-H+ pair, the 3MLCT state is the emitting state in both species with different emission lifetime (tau = 11 mu s and 5.25 mu s for Ir3 and ir3-H+, respectively) and quantum yield (phi em = 8 and 4 % for Ir3 and Ir3-H+, respectively). These complexes were explored as lysosomal markers. Notably, the Ir3/Ir3-H+ pair exhibits an incorporation rate above 40 % without causing damage to the HeLa cells. The Ir2/Ir2H+ pair have shown smaller incorporation rates but exhibits specific staining for lysosomes. This result along with the extended visible light absorption indicate that the Ir2/Ir2-H+ pair possesses the highest potential to be applied as lysosomal probe.