Chemistry Reference
In-Depth Information
ISC
S
1
T
1
Absorption
h
ν
h
ν
'
Phosphorescence
S
0
h
ν
h
ν
'
N
N
N
N
M
(n+1)+
M
n+
FigUre 12.2
The electronic origin of luminescence from
d
6
polypyridyls.
easily reduced. Thus low oxidation state metal complexes (Ir
III
, Re
I
, Ru
II
) combined with highly conjugated ligands that can
easily accept an electron are good combinations for this application. A degree of tuning of the excitation and emission wave-
lengths is possible by structural changes to the ligand involved in accepting the electrons, or other ligands, because these change
the energy gaps involved. This can be used in sensing of intracellular species, but a detailed discussion of these photophysical
phenomena (which are in fact much more complex than this simplistic model suggests) is beyond the scope of this chapter [3].
12.2.2
iridium and rhodium Cyclometallates as imaging Agents
Complexes of the general formula [Ir(C^N)
2
(N^N)], where C^N represents a cyclometallating ligand such as 2-phenyl pyr-
idine, and N^N represents a chelating diimine such as 2,2′-bipyridine, have been the most widely applied iridium imaging
agents, although the neutral [Ir(C^N)
3
] analogues have been studied too. There have been many studies of the luminescence
of a large range of examples of these species including examples that show responses to analytes and thus represent sensors
[6]. These complexes are attractive for cell imaging applications due to their visible excitation and emission wavelengths,
which are highly tunable. In some cases these complexes show extremely long (ms) lifetimes arising from ligand-centred
triplet (
3
LC) emissive states. The bis-cyclometallated complexes [Ir(C^N)
2
(N^N)] are monocationic, which is likely to assist
in their cell uptake due to the membrane potential-driven preferential uptake of cations.
12.2.2.1 Synthesis
Typically, iridium imaging agents are prepared by the reaction of the cyclometallated dimer
[(C^N)
2
IrCl]
2
with the chelating diimine ligand, either under forcing thermal conditions or in the presence of an activating
agent such as a silver salt (Scheme 12.1).
12.2.2.2 Development of Iridium Imaging Agents
While the useful photophysical properties of iridium cyclometallates
have been known for many years, it was only in 2008 that the first report of the application of such species in cell imaging
appeared. Li designed a series of cyclometallated iridium species with fluorous substituents (to increase lipophilicity,
Figure 12.3) and showed that they were taken up by cells and that the Ir-based luminescence was retained
in vivo
, allowing
imaging by fluorescence microscopy [7].
There has now been a large range of Ir
III
complexes synthesised and applied to cell imaging, particularly by the Lo and Li
groups. A range of biologically relevant substituents have been attached to these complexes (e.g., biotin [8], indoles [9],
Figure 12.4), and extensive studies of their uptake and localisation in a range of mammalian cells have been carried out. In
general, iridium cyclometallates show reasonable-to-good cell uptake, which is attributed to their lipophilicity and (usually)
cationic nature. Predominantly, these complexes show accumulation in lipophilic structures of the cytoplasm, which again
is attributed to the extended hydrocarbon skeleton intrinsic to the cyclometallated structures. This predominance of cyto-
plasmic localisation means that the design of targeted iridium cyclometallated probes is a challenging area, although there
are some examples of specific localisation in organelles. For instance, complex
5
(Figure 12.5) has been shown by co-
localisation techniques to localise in the nucleoli (although there is also cytoplasmic staining) [10].
