Chemistry Reference
In-Depth Information
n
þ
Table 6.2 Derived photophysical parameters for triple-stranded [Eu
2
(L)
3
]
dinuclear
helicates in H
2
O at pH 7.0-7.4 and 295 K unless otherwise stated.
t
rad
(ms)
a
Q
E
Eu
(%)
a
h
sens
(%)
a
Ligand
Ref.
(L5)
2
6.8
37
67
[40]
(L6)
2
6.2
36
30
[45]
(L7)
2
6.9
36
58
[46]
(L7)
2
b
4.9
48
50
[47]
(L7)
2
c
3.9
d
60
38
[48]
(L7)
2
e
3.6
d
63
44
[48]
(L8)
2
6.6
37
52
[49]
(L9)
2
6.8
8
4
[50]
(L10)
2
6.4
40
38
[50]
(L11)
2
6.7
35
26
[50]
(H
2
L13b)
2
4.2
45
13
[52]
(L14a)
2
4.8
67
37
[52]
a
Estimated errors:
t
rad
12%, Q
E
Eu
12%,
h
sens
16%.
b
Solid state sample.
c
Monodisperse [Eu
2
(L7)
3
]@SiO
2
nanoparticles (55
5 nm).
d
Recalculated, see Section 6.2.2.
e
Monodisperse [Eu
2
(L7)
3
]@SiO
2
-NH
2
nanoparticles (90
10 nm).
than for the dicarboxylate complexes (about
30%) despite a seemingly similar
coordination environment (N
3
O
6
). But phosphonate groups are more strongly coordi-
nating than carboxylates, which explains this shortening. In addition, the observed
lifetimes are somewhat longer, probably due to the larger hydrophobicity of phos-
phonate versus carboxylate anions, resulting in substantially larger
Q
E
Eu
values [52].
Further, the sensitization efficiency has a tendency to be rather low, so that overall
quantum yields are smaller than those of the carboxylate helicates or at most compa-
rable, in the case of [Eu
2
(L14a)
3
].
Photophysical parameters for the [Ln
2
(L)
3
]
n
þ
edifices with lanthanoid ions other than
Eu
III
are listed in Table 6.3. The more abundant data are found for Tb
III
and the
Q
Ln
versus
E
0-0
(T
) correlation is depicted in Figure 6.4 for helicates encompassing this ion and dis-
solved in water. The first excited level of Tb
III
,
5
D
4
, is located at an energy close to
20 500 cm
1
, therefore helicates with ligands having
E
0-0
(T
)
21 500 cm
1
are likely to
be poorly luminescent, which is indeed the case for ligands H
2
L
i
with
i
<
5, 9, 10, 11, 14a
and 14b. Energy back-transfer occurs, as demonstrated by the Tb(
5
D
4
) lifetime, which is
short at room temperature (10-120 ms for the dicarboxylate helicates) but considerably
longer at 77 K with values between 1.9 and 2.6 ms, in line with a temperature-dependent
nonradiative deactivation process. When the energy gap is around 1500 cm
1
,for
[Tb
2
(L7)
3
] and [Tb
2
(L8)
3
], the edifices are consequently more luminescent, although life-
times are also quite temperature-dependent, but their room temperature values are around
650-660 ms. Moreover [Tb
2
(L
i
)
3
](
i
¼
6, 13a, 13b) do not fit at all into the correlation. For
[Tb
2
(L6)
3
], lifetime dependence is a bit comparable to those with ligands H
2
L
i
(
i
¼
7, 8),
t
obs
increasing from 0.39ms at room temperature to1.85ms at 77K, so that onemay
additionally invoke a less good energy transfer to explain the very low quantum yield
value of only 0.34%. The cases of the helicates with H
4
L13a,b are equally difficult to
¼
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