ChemComm
Communication
Table 2 Mean emission intensities of HeLa cells incubated with blank medium
and the iridium(III) complexes (20 mM) at 37 1C and 4 1C for 2 hours as
determined using flow cytometry
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Mean emission intensity after incubation
Complex
At 37 1C
At 4 1C
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1
2
3
4
4352
4495
4267
5956
6491
60
61
60
60
60
56
55
5
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Blank medium
(the results are shown in Table 2) demonstrate that living HeLa
cells incubated with complexes 1–5 (20 mM) for 2 hours at 37 1C
displayed intense intracellular luminescence. However, almost
no intracellular luminescence was observed when the same
incubation was carried out at 4 1C, suggesting that an energy-
dependent process is the cellular uptake mechanism of these
five complexes with NHC ligands.
It is worth noting that while the cyclometalated iridium(III)
complexes are being extensively studied as imaging reagents,
the research studies have been predominantly directed towards the
cationic compounds with ancillary N4N bidentate ligands. The
electronically neutral iridium(III) complexes were rarely reported.40–42
The first one, bis(2-phenylbenzothiazole)(acetyl-acetonato)iridium(III),
was reported by Wu et al.40 Through the introduction of functional
groups, Steunenberg et al.41 have succeeded in increasing the cellular
uptake of the neutral tris(2-phenylpyridine)iridium(III) complex and
thus observed the luminescence of the amino acid-functionalized
homoleptical neutral complexes from living cells. Our results demon-
strated that without the need for ligand functionalization, the neutral
carbene complexes, 1–3, can penetrate the cell membranes and stain
the cytoplasm exclusively.
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In summary, a series of iridium(III) biscarbene complexes
with an ancillary ligand were synthesized and characterized
photophysically and electrochemically. Based on their emissive
and biological properties, we demonstrated the applicability of
these luminescent metal carbene complexes as multicolor
living cell imaging reagents. The low cytotoxicity and good
cell-permeability make biscarbene iridium(III) complexes,
cationic and neutral, promising for further development.
The financial support for this work from National Science
Foundation of China (NSFC) under Project No. 20902066,
20975074 and 21072218 is gratefully acknowledged. The authors
thank Anxin Wang and Rui Cao for flow cytometry studies.
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.