Chemistry Reference
In-Depth Information
Table 6.3 Photophysical parameters at 295 K (unless otherwise stated) for triple-stranded
[Ln
2
(L)
3
] (Ln6¼Eu) dinuclear helicates upon ligand excitation.
a
E
0-0
(
T
)
(cm
1
at 77 K)
Q
Ln
(%)
Ligand
Ln
Solvent
t
obs
(
m
s)
Ref.
L4 Tb MeCN 20 930 61(3) 9.8
b
[39]
(L5)
2
Sm H
2
O, pH 7.5 20 660 42.0(4) 0.14 [55]
Tb H
2
O 20 660 50 1.2
b
[40]
Yb D
2
O, pD7.5 20 660 40(2) 1.8 [55]
(L6)
2
Tb H
2
O, pH 7.4 22 100 390(40) 0.34(4) [45]
(L7)
2
Nd H
2
O, pH 7.4 22 050
c
0.21(2) 0.031(6) [46]
Sm H
2
O, pH 7.4 22 050
c
30.4(4) 0.38(6) [46]
Tb H
2
O, pH 7.4 22 050 650(20) 11(2) [46]
Yb H
2
O, pH 7.4 22 050
c
4.40(7) 0.15(3) [46]
(L8)
2
Tb H
2
O, pH 7.4 21 900 660(20) 10(1) [49]
(L9)
2
Tb H
2
O, pH 7.4 19 550 10.5(8)
d
[50]
Yb H
2
O, pH 7.4 19 550 4.28(2) 0.15(2) [50]
(L10)
2
Tb H
2
O, pH 7.4 21 150 120(10) 2.5(3) [50]
(L11)
2
Tb H
2
O, pH 7.4 20 800 40(3) 0.31(5) [50]
Yb H
2
O, pH 7.4 20 800 4.33(3) 0.16(2) [50]
(H
2
L13a)
2
Tb H
2
O, pH 7.4 22 570
e
220(50) 2.7(4) [52]
(H
2
L13b)
2
Tb H
2
O, pH 7.4 22 285 n.a. 0.20(7) [52]
(L14a)
2
Tb H
2
O, pH 7.4 20 400
e
130(10) 2.5(4) [52]
(L14b)
2
Tb H
2
O, pH 7.4 21 050 460(10) 1.6(6) [52]
a
Whenever available, standard deviations (2
s
) are given within parentheses for lifetime data while uncer-
tainties on quantum yields are in the range
10-15%.
b
Some originally published quantum yield data were in error because the value used for the reference
[Tb(tpy)
3
]
3
þ
was largely underestimated due to an instrumental problem; the correct value is 35(1)%.
Consequently, quantum yield data have been recalculated.
c
Estimated from Tb value.
d
Too small to be measured.
e
Estimated from Lu value.
rationalize as far as absolute values of
Q
Tb
are concerned, but from lifetime dependence,
it is also clear that back transfer is operating.
The three quantum yields available for Yb
III
helicates in water are the same
(0.15-0.16%), consistent with the large energy difference between the Yb
2
F
5=2
5=2
Þ
level and
E
0-0
(T
), 10-12 000 cm
1
, so that small variations in
E
0-0
(T
) have little influ-
ence. Quantum yield values are essentially determined by nonradiative deactivations
which are quite efficient in view of the small energy gap between Yb
ð
2
F
5=2
5=2
Þ
ð
and
2
F
7=2
7=2
Þ
, approximately 10 000 cm
1
. Similar considerations apply to Sm
III
Yb
ð
(gap
7500 cm
1
) and Nd
III
(5400 cm
1
).
6.2.2 Radiative Lifetime and Nephelauxetic Effect
It is clear from Equation 6.1 that, if one wishes to increase the intrinsic quantum yield,
one has to maximize
t
rad
.
The first action is rather well understood and mastered, at least for visible-emitting ions,
that is for those ions which have a large energy gap between the emitting level and the
highest sublevel of the ground (or receiving) state. High energy vibrations are relatively
t
obs
by minimizing nonradiative deactivation and/or to decrease
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