Chemistry Reference
In-Depth Information
Table 6.6 Parameters for the calculation of
R
DA
in [Ln
2
(L12a,b)
3
] helicates by Eu
III
to Nd
III
energy transfer in aqueous solutions [51].
Parameter
a
(L12a)
2
(L12b)
2
Parameter
a
(L12a)
2
(L12b)
2
Eu
0
(ms)
t
1.48
1.56
Q
Eu
0.40
0.80
Eu
obs
(ms)
10
17
J
(cm
6
mol
1
)
t
1.26
0.25
1.22
1.22
R
0
(A
)
h
et
0.15
0.843
9.90
9.85
r
DA
(A
)
k
et
(s
1
)
118
3359
13.2
7.4
a
Estimated uncertainties:
5-10% on decay rates,
12% on
Q
Eu
and
J
,
0.8A
on
r
DA
. Note: a change in
R
0
by a factor of 2 leads to
r
DA
estimate changing by only 12%.
the homometallic helicate, a biexponential decay was found for Tb
III
, corresponding to a
long lifetime identical to the one of the homometallic complex and to a shorter one evi-
dencing Tb
III
to Eu
III
energy transfer. In view of the peculiar properties of this system,
with
E
0-0
(T
) close to Tb(
5
D
4
), measurements have been performed on frozen acetonitrile
solutions at 77 K using both direct f-f excitation and excitation in the ligand bands (data
within parentheses). Tb
III
lifetimes are 0.66 and 0.16 ms (0.71 and 0.20 ms), which trans-
lates into h
et
¼
76% (72%) and given the Tb-Tb distance determined by X-ray crystallog-
raphy, 8.876 A
,
R
0
is calculated as being 10.7 A
(10.4A
). The latter distance is slightly
larger compared with observed range of values for Tb
III
-Eu
III
pairs (6-9 A
)[30].This
stems from the special arrangement of the Ln
III
ions on the symmetry axis of the mole-
cule, which results in little interference from ligands along the energy-transfer path.
Another study involved Eu
III
to Nd
III
energy transfer to determine intermetallic dis-
tances in [EuNd(L12a,b)
3
] in aqueous solution [51]. The data obtained (Table 6.6) are in
reasonable agreement with model calculations based on molecular mechanics assuming
ionic bonding between the metal ion and the ligands: 9.9 and 8.0 A
for helicates with
(L12a)
2
and (L12b)
2
, respectively.
More energy transfer phenomena will be discussed in Sections 6.3.2 and 6.3.3.
6.2.5 Lanthanoid Luminescent Bioprobes
In addition to displaying sharp, therefore easily recognizable, emission lines, analytical
probes containing trivalent lanthanoid ions have the advantage of suffering little or no
photobleaching and presenting long lifetimes of the metal-centred luminescence, in the
milli- to microsecond range. This decisive advantage translates into the possibility of
using cheap equipment for time-resolved detection of the luminescence. Both bioanalyses
such as immunoassays [56,69-71] and bioimaging [56,71-75] benefit from this feature.
Since dinuclear helicates contain two luminescent ions and since they are easy to
assemble in aqueous solutions under physiological conditions, it was logical to test if
they would be good bioprobes. To this end, their solubility has been improved by the
addition of short polyoxyethylene fragments on the benzimidazole moiety (H
2
L6; refer to
Scheme 6.2) or a pyridine group (H
2
L
i
,
i
7-11). Several properties of the resulting heli-
cates have to be checked before using them as bioprobes. The first is their thermodynamic
stability, which has been assessed by the determination of conditional stability constants
at pH 7.4 in 0.1M Tris-HCl. In most cases, the triple-stranded helicate is the main species,
with a speciation larger than 92%, except for [Eu
2
(L6)
3
] and [Eu
2
(L9)
3
]. In view of the
¼
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