Chemistry Reference
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70
60
50
40
30
5
7
8
20
10
6
11
10
9
0
19.5
20.0
20.5
21.0
21.5
22.0
E
0-0
(T*) / 10
3
cm
-1
Figure 6.3 Quantum yields for aqueous solutions of the dicarboxylate [Eu
2
L
3
] helicates ver-
sus E
0-0
(T
) at 77 K. Ligand numbering according to Table 6.1. Black squares: overall quantum
yields. Red triangles: intrinsic quantum yields. Blue circles, sensitization efficiencies.
the
5
D
0
excited state,
7
F
0
transition which reflects the nephelauxetic effect created by the metal-ion surroundings.
Referring to Figure 6.2, the main donor state in the case of Eu
III
and Tb
III
is often a
ligand triplet state so that attempts have been made to relate the energy of this state with
the value of the overall quantum yield. To put it simple, if there is no other energy migra-
tion paths operating,
E
0-0
(T
) should lie at least 1500 cm
1
above the emitting level to
avoid too much back transfer and the ideal energy gap between donor and receiving
levels lies between 2000 and 4000 cm
1
. This however has to be taken as a guideline
only as illustrated by the graph in Figure 6.3 in which quantum yields are reported
versus triplet state energies. The correlation is good for the helicates with the dicarbox-
ylate ligands and generally speaking, the quantum yield increases monotonically up to
a triplet state energy of 22 000 cm
1
, which is about the energy of the Eu
t
obs
, the overall quantum yield
Q
Eu
and the energy of the
5
D
0
!
5
D
2
Þ
level. If
the main energy transfer mechanism is a double electron exchange (Dexter mecha-
nism), which is usually the case in coordination compounds, the corresponding selec-
tion rule on
J
quantum number is D
J
ð
J
0
¼
0 excluded. This means that
energy will be mainly funnelled onto the
5
D
1
level located at 19 000 cm
1
, either from
7
F
0
or from
7
F
1
(which has about 30% population at 295 K). Effectively,
Q
Eu
values
start to be sizeable when
E
0-0
(T
)
¼
0,
1 with
J
¼
20 800 cm
1
, corresponding to an energy difference
of about 1800 cm
1
with respect to the
5
D
0
level. The quantum yield, however, contin-
ues to increase with increasing energy gaps larger than 1800 cm
1
, which means that
transfer onto higher excited state(s) also occurs, involving for instance
5
D
2
.
There is, however, a serious exception for [Eu
2
(L5)
3
], the reason for which is difficult
to explain. The triplet state energy for [Eu
2
(L5)
3
] has been determined at 10 K while the
data for the other helicates are at 77 K, but it is doubtful that there is a very large energy
shift between these two temperatures. Further, one may note that, since quantum yields
are determined at room temperature, triplet state energies should also be measured at this
temperature, which is often not possible since emission spectra of the helicates rarely
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