Chemistry Reference
In-Depth Information
Scheme 6.3
Unsymmetrical ligands for heteronuclear 4f-4f helicates.
respectively). This is due to the very low quantum yield of Eu III in a N 9 coordination site
(see above). The D E (A 2 -E) splitting is comparable to the value found for LaEu while D E
(E-E) is larger and comparable to the splitting found for the helicates with symmetrical
ligands (Table 6.5). The observed lifetimes for these two helicates are the same,
2.20
0.05ms (EuEu) with a small temperature dependence,
again pointing to the observed emission spectra arising mainly from the Eu(N 6 O 3 ) sites.
0.01 (LaEu) and 2.26
6.2.4 Energy Transfer between Lanthanoid Ions
Bimetallic 4f-4f 0 helicates are well suited for studying the energy transfer between two
different lanthanoid ions. Since the intermetallic distance is in the order of 90 pm, the
transfer is likely to be governed by the dipole-dipole (Forster) mechanism. In the absence
of a transfer, both ions are excited by a transfer from ligand donor states and emit light
with their characteristic lifetime. When one ion transfers energy onto the acceptor, its life-
time is shortened, while the lifetime of the acceptor remains the same, provided that the
rate constant of the transfer process is larger than the rate constant of the deactivation of
the acceptor (Figure 6.6). Within this hypothesis, the following simplified equations hold
to estimate the efficiency of transfer between the donor ion D and the acceptor ionA
(Equations (6.5,6.6)):
1
1
t
k obs
k 0 ¼
k et ¼
obs
ð
6
:
5
Þ
D
0
t
k 0
D
obs
t
t
1
h et ¼
1
k obs ¼
1
0 ¼
ð
6
:
6
Þ
6
1þð
R DA =
R 0 Þ
0 are the lifetimes of the donor in the presence and in the absence
of the acceptor, respectively, R DA is the distance between the donor and the acceptor and
R 0 is the critical distance for 50% transfer, which depends on: (i) an orientation factor k
having an isotropic limit of 2/3, (ii) the intrinsic quantum yield Q D of the donor in absence
of the acceptor, which is sometimes estimated as being equal to
obs and
in which
t
t
D
D
,
(iii) the refractive index n of the medium and (iv) the overlap integral J ov between
t
0 ð
H 2 O
Þ=t
0 ð
D 2 O
Þ
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