Chemistry Reference
In-Depth Information
Figure 6.6 Ln
III
to Ln
III
energy transfer in an heterobimetallic helicate
(the crystal structure is
drawn after data reported for [LaTb(L17)
3
]
6
þ
in Ref. [64]).
the emission spectrum
E
ðnÞ
of the donor and the absorption spectrum
e
ðnÞ
of the acceptor
(Equations (6.7,6.8)):
R
0
¼
10
25
2
n
4
8
:
75
ðk
Q
D
J
ov
Þ
ð
6
:
7
Þ
Ð
e
ðnÞ
ðnÞðnÞ
4
d
E
n
Ð
E
J
ov
¼
ð
6
:
8
Þ
ðnÞ
d
n
D
obs
t
can be substituted with the intensity ratio
I
obs
I
0
In Equation 6.6, the ratio
t
. An esti-
0
mate of
R
0
is therefore accessible from the experimental optical and structural properties
of the system. If a crystal structure is at hand, the problem simplifies in that
R
DA
is known
and, if the lifetimes of Equation 6.6 can be measured, then calculation of
R
0
is straightfor-
ward. These equations can also be used when the donor and/or the acceptor are other
entities, such as organic chromophores. The methodology allows one to determine dis-
tances between chromophores and metal ion sites in biological molecules [67], particu-
larly in metalloproteins, for instance when Ca
II
or Zn
II
are replaced with Ln
III
ions [68].
Tb
III
with an emissive level
5
D
4
located at 20 500 cm
1
is well suited for transferring
energy on a number of other lanthanoid ions, in particular Eu
III
, the energy difference
with Eu
and Eu(
5
D
0
) being 1500 and 3250 cm
1
, respectively, that is within an opti-
mal range to avoid back energy transfer. An early study involved helicates with ligand L3
(Scheme 6.1) which was reacted with equimolar amounts of Eu
III
and Tb
III
in acetonitrile,
yielding solutions with a mixture of [Eu
2
(L3)
3
]
6þ
, [Tb
2
(L3)
3
]
6þ
and [EuTb(L3)
3
]
6þ
spe-
cies in a 1/3, 1/3, 1/3 proportion, as determined by ES-MS [6]. While the luminescence
decay for Eu(
5
D
0
) is monoexponential with an associated lifetime matching the one of
5
D
1
Þ
ð
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