Chemistry Reference
In-Depth Information
Figure 6.20 Top trace (black): luminescence spectrum of Eu
2
(L33S)
3
]
6þ
. Bottom traces (col-
oured): CPL spectra of Eu
2
(L33R)
3
]
6þ
and Eu
2
(L33S)
3
]
6þ
in MeOH.
Reproduced with permis-
sion from [117]. Copyright 2009 American Chemical Society.
synthesis requires the availability of facial noncovalent tripodal receptors
P-
or
M-
[M
L
3
]
n
þ
with M being an inert transition metal ion to avoid scrambling in solution. First,
a racemic LnM
II
helicate is self-assembled, followed by oxidation into
rac
-LnM
III
. After
removal of the Ln
III
ion with edta, the racemic mixture is loaded onto an ion exchange
column and eluted with Na
2
Sb
2
[(
)-C
4
O
4
H
2
]
2
to yield the facial tripodal receptor
fac
-
[M
III
L
3
]
3þ
with the desired chirality. The latter is finally reacted with Ln
III
. Within the
range of investigated nd-4f compounds, both Co
III
and Cr
III
qualify, but the difficult oxi-
dation of the former leaves traces of Co
II
in solution which catalyses the
fac
-[Co(
L36
)
3
]
3þ
?
þ
mer
-[Co(
L36
)
3
]
3þ
isomerization, so that Cr
III
is preferred. Both
P
,
P
- and
M
,
M
-[EuCr
(L25)
3
]
6þ
isomers have been isolated in high purity and their CD and CPL spectra are
displayed in Figure 6.21. The two CD spectra are perfect mirrors of each other and the
resulting asymmetry factor at 340 nm amounts to 74M
1
cm
1
. When it comes to lumi-
nescence, both Cr
4
A
2
Þ
2
E
5
D
0
!
7
F
J
Þ
1-4, transitions display asymme-
try. The luminescence dissymmetry factors
g
lum
are very small for Cr
III
emission,
ð
!
and Eu
ð
,
J
¼
þ
0.01
0.01 (
P
,
P
), as well as for the Eu
III
(
M
,
M
) and
transitions to
J
¼
2(
0.07), 3 (
0.0009)
0.033), while they are sizeable for the magnetic dipole transition to
7
F
1
,
and 4 (
0.154
(
M
,
M
)and
0.163 (
P
,
P
). Correlation between molecular structure and CPL spectra is
presently still not well understood; for instance, in the present case, the signs and magni-
tudes of the dissymmetry factors do not correlate well with helicity, possibly because the
edifice deviates too much from
D
3
symmetry [120].
Bipyridine ligands derivatized with a chiral pinene unit are strong inductors of chirality in
transition metal complexes. Ligands ()-HL35 (Scheme 6.8) self-assemble with Ln
III
þ
ions
L35)
6
(m-OH)]
2þ
complexes with a propeller-like
arrangement of the ligands around the trinuclear metal core inducing helical chirality
[121,122]. These trinuclear edifices slowly convert into tetranuclear assemblies with the for-
mula [Ln
4
(
to quantitatively yield trinuclear [Ln
3
(
L35)
9
(m-OH)]
2þ
and both tri- and tetranuclear complexes display metal-centred
luminescence and chirality [123]. Quantum yields for [Ln
3
(
L35)
6
(m-OH)]
2þ
in methylene
chloride amount to 0.1, 13, 15 and 0.01% for Ln
¼
Nd (
isomer), Eu(
þ
), Tb(
þ
)andEr(
).
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