Chemistry Reference
In-Depth Information
a
1g
,t
1u
t
1u
σ*
M
σ
np
a
1g
e
g
ns
*
M
t
1g
,t
2g
,
t
1u
,t
2u
π
*
L
t
1u
,t
2u
,t
1g
,t
2g
π*
LF
MLCT
t
2g
,e
g
t
2g
(n-1)d
π
M
IL
t
1u
,t
2u
,t
1g
,t
2g
t
1u
,t
2u
,t
1g
,t
2g
π
π
L
a
1g
,t
1u
,e
g
a
1g
,t
1u
,e
g
σ
σ
L
Metal
Orbitals
Ligand
Orbitals
Molecular
Orbitals
Figure 8.7
Molecular orbital diagram of an octahedral metal complex, depicting common
electronic transitions associated with transition metal (M) complexes with polyazine ligands
(L) (IL = internal ligand, MLCT = metal to ligand charge transfer, LF = ligand fi eld)
employed. Combining chromophores into mixed-metal supramolecules gives com-
plexes with photophysical properties that are unique compared to the mononuclear
analogues.
20
Tuning the ES properties of the transition metal polyazine complexes
allows for tuning of their photochemical properties. Table 8.1 summarizes the pho-
tophysical properties of transition metal polyazine complexes used in the study of
photochemical degradation of DNA or light activated cell studies.
Ruthenium( II ) and Osmium( II ) Polyazine Complexes
Ru(II) and Os(II) complexes are interesting as photosensitizers as they possess
reactive ES that are easily tuned by modifi cation of their ligand coordination
sphere.
19
Mononuclear Ru(II) polyazine complexes typically have an absorption
band in the 400-500 nm region due to the Ru(d
TL( p * ) charge transfer transi-
tion (MLCT), where TL = terminal polyazine ligand (Figure 8.7). The electronic
absorption spectrum of [Ru(bpy)
3
]
2+
, bpy = 2,2
p )
→
-bipyridine, is presented in Figure
8.9. The initially populated
1
MLCT state undergoes intersystem crossing with unit
effi ciency (F
isc
′
21
For tris(chelate) polyazine
complexes of Ru(II), the
3
MLCT state is long-lived at room temperature (t
0
= 10 -
1000 ns) and displays strong phosphorescence (F
p
≈
1) to give the
3
MLCT state (Figure 8.10).
≈
0.10).
19
The ES populated fol-
lowing metal - centred d
d excitation is the ligand fi eld state (LF). Though higher
in energy, the nonemissive
3
LF state is thermally accessible at room temperature
from the
3
MLCT state. Thermal population of the
3
LF state competes with phos-
phorescence or nonradiative decay pathways of the
3
MLCT state as a decay
pathway.
Modifi cation of the coordination environment of Ru(II) polyazine systems
modulates the electronic properties of the chromophore. Bis(chelate) polyazine
complexes of Ru(II) with a
cis
- Ru(TL)
2
moiety tend to have absorption spectral
→
