Chemistry Reference
In-Depth Information
6.3.2 Eu
III
-to-Cr
III
Energy Transfer
When one metal ion imbedded in the helicate plays the role of a donor for sensitizing the
emission of a second accepting metal ion, the characteristic lifetimes of the excited state
is affected by the intermetallic communication process according to Equations (6.5)-(6.8)
giveninSection6.2.4.Thechromiumhelicate[EuCr
III
(L25)
3
]
6þ
displays both intense
Cr
2
E
4
A
2
Þ
5
D
0
!
7
F
j
Þ
5
D
0
Þ
lifetime is considera-
bly shorter than in [EuZn(L25)
3
]
5þ
, which is assigned to unidirectional Eu
ð
!
and Eu
ð
emission at 10 K. The Eu
ð
Cr transfer
along the
C
3
axis [90]. Determination of h
et
(CrEu) for two types of crystalline samples
(anhydrous and hydrated, see Table 6.13) provides similar efficiencies for this transfer, in
the range 65-78%. The efficiency for the anhydrous sample is slightly temperature-
dependent while that for the hydrated sample is independent of temperature. The energy
transfer yield is also insensitive to the nature of the sample, as shown by the same
calculation carried out for 10
4
M solutions in acetonitrile using either quantum yields
(see Table 6.12),
!
66%, or lifetimes (65%), a proof of the dipole-dipolar
mechanism operating in these fairly rigid triple-helical structures. The calculated
R
0
dis-
tance is
h
et
(CrEu)
¼
10.3 A
. In the case of Tb
III
, the transfer is quantitative, no
5
D
4
luminescence
being observed even upon direct Tb
III
excitation, because of the near resonance between
the Tb(
5
D
4
)andCr
electronic levels. It is also noteworthy that the Cr
III
lumines-
cence is heavily quenched at room temperature, as shown with the GdCr helicate for
which the Cr
4
T
2
Þ
ð
2
E
lifetime dramatically decreases between 10 and 295 K.
Furthermore, divergent Cr
ð
Þ
Cr intramolecular axial energy transfers could be
evidenced in [Cr
2
Ln(L29)
3
](CF
3
SO
3
)
9
, with temperature-independent global yields of
90% (Eu) and
Ln
!
>
99.9% (Tb) [11]. These two situations are summarized in Figure 6.15.
n
þ
Table 6.12 Quantum yields and lifetimes of [MLnL
3
]
(Ln¼Eu, Tb) determined upon ligand
excitation.
t
(
5
D
J
) (ms)
a
c
(M)
b
Q
Ln
(%)
c
L
Ln
M
Refs.
10
4
L24
Eu
Zn
2.30(5)
0.01
[8,61]
Tb
Zn
1.17(4)
n.a.
[8]
10
3
L25
Eu
Zn
2.56(10)
4.2
[61]
10
4
9.3
[61]
Tb
Zn
1.89(6)
n.a.
[61]
Cr
III
0.55(4)
d
10
4
Eu
3.2
[90]
10
3
L26
Eu
Zn
2.35(2)
8.2
[95]
10
4
HL27
Eu
Zn
2.99(9)
32
[94]
2.43(2)
d
10
4
e
15
[94]
10
3
L28
Eu
Zn
2.63(1)
7.4
[96]
Fe
II
10
3
Eu
0.28(1)
0.03
[96]
a
Solid state sample, at 10 or 13 K, ligand excitation.
b
In acetonitrile.
c
In acetonitrile at room temperature; recalculated by using the most recent values reported for the [Ln
(tpy)
3
]
3
þ
internal references, 32
1% (Eu) and 35
1% (Tb) in acetonitrile. Uncertainty:
10-15%.
d
For [EuCr(L25)
3
](CF
3
SO
3
)
6
4H
2
O; 0.75(1) ms for [EuCr(L25)
3
](CF
3
SO
3
)
6
4MeCN; 0.87(4) ms for a solution
10
4
M in acetonitrile.
e
In H
2
O.
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