Chemistry Reference
In-Depth Information
1
IL
30
k
ic
25
20
1
MLCT
k
isc
15
k
et
3
MLCT
k
p
3
MMCT
hν
k
nr
10
k
rxn
k
nr
'
5
1
GS
0
Figure 8.16
Jablonski-type diagram of [{(bpy)
2
Ru(dpp)}
2
RhCl
2
]
5+
. bpy = 2,2
-bipyridine,
dpp = 2,3-bis(2-pyridyl)pyrazine,
1
GS = singlet electronic ground state,
1
MLCT = singlet metal
to ligand charge transfer,
3
MLCT = triplet MLCT,
1
IL = singlet internal ligand excited state.
Relative energies adapted from Molnar, Jensen, Vogler, Jones, Laverman, Bridgewater, Richter
and Brewer.
41
′
tophysical properties. Metals bridged by 2,3-bis(2-pyridyl)pyrazine (dpp) electroni-
cally couple through the aromatic pyrazine ring.
40
Mononuclear Ru(II) complexes
with dpp have a strong, visible MLCT transition and a LUMO localized on the
pyrazine ring of the dpp ligand (e.g. [(bpy)
2
Ru(dpp)]
2+
,
λ
max
abs
480
in water).
Bridging the complex to another metal centre stabilizes the dpp(p * ) LUMO, shifting
the MLCT transition to lower energy (e.g. [(bpy)
2
Ru(dpp)Ru(bpy)
2
]
4+
,
=
nm
abs
526
).
Trimetallic complexes with Ru
II
(dpp) subunits bridged to a
cis
- Rh
III
Cl
2
centre such
as [{(bpy)
2
Ru(dpp)}
2
RhCl
2
]
5+
have interesting properties (Figure 8.16).
λ
max
=
nm
41
The HOMO
of these trimetallic complexes is localized on the Ru(dp) orbitals like other Ru(II)
polyazine complexes, but the LUMO is Rh(ds*) in nature. Direct Ru(dp )
Rh(d s * )
(metal-to-metal) charge transfer (MMCT) excitation is forbidden by symmetry and
the relatively weak HOMO-LUMO electronic coupling. Irradiation with visible
light, however, populates the
1
MLCT state. Intersystem crossing generates the
3
MLCT state, which is higher in energy than the
3
MMCT state.
42
Coupling of the
dpp(p * ) and Rh(d s*) acceptor orbitals facilitates intramolecular electron transfer
to give the
3
MMCT state. These trimetallic complexes have been shown to photo-
cleave DNA.
43,44
Substitution of the bridging ligand dpp for molecular bridges with
lower energy acceptor orbitals decreases the photochemical reactivity, implicating
the
3
MMCT state.
44
→
8.2.4 Bimolecular Excited State Interactions
Ground state light absorbers (LA) and their electronic excited states (*LA) are
interesting for study, but it is the interaction of *LA with substrate molecules that
interests PDT researchers. Bimolecular interactions resulting in quenching of *LA
following excitation (Equation 8.3) and competing with unimolecular radiative
decay (Equation 8.4) and nonradiative decay (Equation 8.5) can be classifi ed into
basic groups: bimolecular deactivation,
k
nr
(Equation 8.6); excited state electron
transfer,
k
et
, to (Equation 8.7) or from (Equation 8.8) *LA; excited state energy
′
