Dual-Emissive Cyclometalated Iridium(III) Polypyridine Complexes as Ratiometric Biological Probes and Organelle-Selective Bioimaging Reagents

In this Article,
we present a series of cyclometalated iridium­(III) polypyridine complexes
of the formula [Ir­(N^C)2(N^N)]­(PF6) that showed
dual emission under ambient conditions. The structures of the cyclometalating
and diimine ligands were changed systematically to investigate the
effects of the substituents on the dual-emission properties of the
complexes. On the basis of the photophysical data, the high-energy
(HE) and low-energy (LE) emission features of the complexes were assigned
to triplet intraligand (3IL) and triplet charge-transfer
(3CT) excited states, respectively. Time-dependent density
functional theory (TD-DFT) calculations supported these assignments
and indicated that the dual emission resulted from the interruption
of the communication between the higher-lying 3IL and the
lower-lying 3CT states by a triplet amine-to-ligand charge-transfer
(3NLCT) state. Also, the avidin-binding properties of the
biotin complexes were studied by emission titrations, and the results
showed that the dual-emissive complexes can be utilized as ratiometric
probes for avidin. Additionally, all the complexes exhibited efficient
cellular uptake by live HeLa cells. The MTT and Annexin V assays confirmed
that no cell death and early apoptosis occurred during the cell imaging
experiments. Interestingly, laser-scanning confocal microscopy revealed
that the complexes were selectively localized on the cell membrane,
mitochondria, or both, depending on the nature of the substituents
of the ligands. The results of this work will contribute to the future
development of dual-emissive transition metal complexes as ratiometric
probes and organelle-selective bioimaging reagents.