Chemistry Reference
In-Depth Information
media used to pre-treat the cells was increased from 5 to 120
M, the number of
viable cells following photolysis approached zero. The Os complex, [{(bpy)
2
Os(dpp)}
2
RhCl
2
]Cl
5
, exhibited decreased phototoxicity relative to the Ru complex, with the
ratio of cells present before and after photolysis decreasing toward unity as a func-
tion metal complex concentration. The difference in phototoxicity is quite surprising
given the similar DNA photoreactivity. This may be due to the lifetime of the reac-
tive state of each complex, with the Os systems providing shorter ES lifetimes.
The cells studies of these metal complexes show great promise. Neither study
of multimetallic complexes could comment on the mechanism of cell killing. Both
report the photosensitizers of interest to be effi cient DNA photocleavage
agents.
33,43,44,83
µ
8.6 Conclusions and Future Directions
Platinum group metal polyazine complexes have pronounced DNA photochemistry
and show unusual promise as photodynamic therapy (PDT) agents. Recent discov-
eries contribute to the understanding of the electronic excited state interactions of
these systems with DNA. Studies of photoactivated interactions of platinum group
complexes with biomolecules also provide insight into the basic photophysics of
these metal complexes. DNA can be a good probe of the excited state dynamics of
metal complexes.
Polyazine complexes of transition metal centres, especially ruthenium and osmium,
exhibit strong light absorbing properties in the UV and in the visible region, with
interesting photoreactivity. Excitation populates reactive electronic excited states with
properties very different than the ground state molecules. Ruthenium(II) polyazine
complexes are noted visible light absorbers with typically long - lived Ru(d
p * )
charge transfer (
3
MLCT) states known to photosensitize
1
O
2
. Osmium(II) polyazine
complexes have similar light absorbing properties to ruthenium(II), but with elec-
tronic transitions shifted to lower energy. The phosphorescence of
3
MLCT states
provides a convenient handle to reactivity of these
3
MLCT states.
Rhodium(III) polyazine complexes, though poor visible light absorbers, have
interesting reactive internal ligand (
3
IL) and ligand fi eld (
3
LF) electronic excited
states that photocleave and photobind DNA. The studies to date provide signifi cant
motivation for continued study of Rh(III) complexes. Polyazine complexes incor-
porating dirhodium(II) centres possess electronic excited states arising from the
delocalized metal- metal(d p*) highest occupied molecular orbital, giving them
potent photochemical reactivity. Polyazine bridged complexes of ruthenium(II) and
rhodium(III) are both visible light absorbers and possess the reactive Rh(III) centre.
Systems can be designed to have a lowest lying Ru(dp )
p )
→
dpp(
* ) charge transfer
(
3
MMCT) state that is reactive with, among other substrates, DNA. Structural modi-
fi cations of these assemblies provide a unique forum to modulate properties.
The forum of photomodifi cation of DNA by metal polyazine complexes has
been developing over the last three decades. Researchers in the fi eld have discov-
ered multiple modes of photochemical action, including metal complex mediated
→
Rh(d
σ
