Chemistry Reference
In-Depth Information
8
bpy(
π→π
*)
N
7
N
N
N
Ru
II
6
N
5
N
4
3
2
Ru(dπ)→bpy(π*) CT
1
0
200
300
400
500
600
Wavelength (nm)
Figure 8.9
Electronic absorption spectrum of tris(2,2
′
-bipyridine)ruthenium(II), [Ru(bpy)
3
]
2+
,
in CH
3
CN at RT
1
IL
30
k
ic
25
1
MLCT
k
isc
3
LF
20
3
MLCT
15
k
nr
h
ν
'
k
p
h
ν
10
k
nr
'
5
1
GS
0
Figure 8.10
Jablonski-type diagram of [Ru(bpy)
3
]
2+
. bpy = 2,2
-bipyridine,
1
GS = singlet elec-
tronic ground state,
1
MLCT = singlet metal to ligand charge transfer excited state,
3
MLCT =
triplet MLCT excited state,
1
IL = singlet internal ligand excited state,
3
LF = triplet ligand fi eld
excited state. Relative energies are adapted from Juris, Balzani, Barigletti, Campagna and
Belser.
19
′
3
MLCT state and lower-lying
3
LF state (Figure 8.11). Excitation directly into higher
LF states has been linked to photoaquation of complexes with the
cis
- Ru
II
(TL)
2
moiety, where complexes such as [(bpy)
2
RuCl
2
] photochemically generate
[(bpy)
2
RuCl(OH
2
)]
+
and [(bpy)
2
Ru(OH
2
)
2
]
2+
.
23
Tris(chelate) polyazine complexes of Os(II) generally absorb and emit visible
light, but at longer wavelengths and with decreased emission effi ciency than the
Ru(II) - centred analogs.
24,25
A shorter
3
MLCT state lifetime (and smaller F
p
) is a
result of several contributing factors. The energy gap law states that as the difference
in energy of two electronic states decreases, the vibronic coupling of the states
increases, enhancing
k
nr
and leading to more effi cient ES deactivation.
24
Another
factor to consider is the large spin-orbit coupling of Os(II) versus Ru(II). A larger
spin-orbit coupling constant relaxes the spin selection rule, increasing the intensity
of spin forbidden transitions. This gives rise to a low energy 'tail' of the visible region
