Chemistry Reference
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Figure 6.14 (a) Molecular structure of the [EuLaEu(L22)
3
]
9þ
helicate; (b) Eu(
5
D
0
7
F
0
)
excitation spectra at 15 K.
Redrawn with permission from [10]. Copyright 2004 WILEY-VCH
Verlag GmbH & Co. KGaA, Weinheim.
conclusion that scrambling among the sites occurs and that the populations are 0.74 : 0.26
(La:Eu) for the central sites and about 0.11 : 0.89 for the end sites. In solution, NMR spec-
troscopy points to the presence of three main species, EuLaEu (48%), EuEuEu (23%) and
LaLaEu (19%), which explains Eu occupancy of the central site and La occupancy of
about 10% in each of the terminal sites in the crystals.
To get more insight into this problem, the Eu(
5
D
0
!
7
F
0
) transition has been analysed
in detail for both the homometallic EuEuEu and the heterometallic EuLaEu helicates
(Figure 6.14). Gaussian decomposition allowed the authors to determine the relative emis-
sion efficiency r of the two sites in EuEuEu (Equation 6.13):
I
c
I
c
þ
r
c
r
c
þ
r
c
r
t
¼
I
t
¼
0
:
27
¼
therefore
0
:
74
ð
6
:
13
Þ
2
r
t
0.094. Taking the La population equal to
10% in each terminal site and setting a as being equal to the population of EuEuEu, one
gets Equation 6.14:
For the heterometallic helicate,
I
c
/(
I
c
þ
I
t
)
¼
I
c
I
c
þ
0
:
8
ar
c
I
t
¼
ð
6
:
14
Þ
0
:
8
ar
c
þ
1
:
8
r
t
which solves for a¼
0.26 is calculated for the Eu
occupancy of the central site corresponding to [La
0.94
Eu
2.06
(L22)
3
]
9þ
, a formula compati-
ble with elemental analysis and X-ray diffraction data. This means that crystals are a mix-
ture of 54% EuLaEu, 20% EuEuEu and 26% LaLaEu, matching very well the solution
speciation [10].
The bis(benzimidazole) framework can be expanded to feature four tridentate coordi-
nation sites as in L23 (Scheme 6.5). The ligand self-assembles in acetonitrile with
0.32. From this, a population of 0.8a¼
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