Chemistry Reference
In-Depth Information
yields are largely modulated by the alkaline ion in KLn
2
chelates, RbYb
2
having the larg-
est value, almost twice as large as KYb
2
.
6.4 Chiral Helicates
In view of their importance in functional biomolecules, chiral structures have always fas-
cinated chemists and biologists. Many chiral polynuclear helical systems and coordina-
tion polymers (see next section) have been proposed, particularly with the aim of gaining
control on helix inversion [110] but, apart from CD spectra, few quantitative photophysi-
cal data are at hand. For instance, trinuclear Zn
2
Ln [111] and tetranuclear Zn
3
La metal-
lohelicenes [110,112] have been designed for this purpose, but they are not helicates since
the metal ions are not located on an internuclear axis. Generally speaking, enantiomeri-
cally pure lanthanoid complexes are not easy to isolate in view of the large lability of
these trivalent cations. Chiral isomers are characterized by circular dichroism (CD) and
circularly polarized luminescence (CPL). CD probes the ground state chirality through
measurement of either the coordinated chromophore absorption or the intra-configura-
tional f-f transitions. The extent of the effect is characterized by the absorption dis-
symmetry factor defined from the difference in molar absorption coefficient D
e
between
left (
L
) and right (
R
) circularly polarized light [113] (Equation 6.23):
2
D
e
e
¼
2
ð
e
L
e
R
Þ
e
L
þ
e
R
g
abs
¼
ð
6
:
23
Þ
CPL is the emissive counterpart of CD and therefore probes the excited state chirality;
it also reflects the molecular motions taking place between absorption and emission. In
this case, the parameter of interest is the luminescence dissymmetry factor defined as a
function of the emission intensities (Equation 6.24):
2
D
I
I
¼
2
ð
I
L
I
R
Þ
g
lum
¼
ð
6
:
24
Þ
I
L
þ
I
R
Theoretically,
g
lum
can be related to the electric and magnetic dipole transition
moments m
ge
and
m
ge
(
g
denotes the ground state and
e
the excited state; Equation 6.25):
4
f
CPL
ðlÞ
f
TL
ðlÞ
m
ge
m
ge
g
lum
ðlÞ¼
ð
6
:
25
Þ
2
ðm
ge
Þ
where
f
CPL
(l)and
f
TL
(l) are the line shapes for CPL and total luminescence (TL)
signals. Since
ge
is much larger than
m
ge
, CPL usually focuses on Laporte's
allowed magnetic dipole transitions. For lanthanoids,
m
the best suited transitions
4
G
5=2
5=2
!
6
H
J
;
5
D
0
!
7
F
1
Þ
5
D
4
!
7
F
J
;
are Sm
ð
J
¼
7
=
2
;
5
=
2
Þ
,Eu
ð
,Tb
ð
J
¼
3
5
Þ
,
8
S
7=2
7=2
!
6
P
7=2
7=2
Þ
4
F
9=2
9=2
!
6
H
11=2
11=2
Þ
2
F
5=2
5=2
!
2
F
7=2
7=2
Þ
Gd
.One
advantage of lanthanoid complexes over organic chiral probes is their often large lumines-
cence dissymmetry factors, which can easily reach
ð
,Dy
ð
and Yb
ð
0.5. A common standard for
CPL, tris(3-trifluoroacetyl-
d
-camphorato)europium has
g
lum
¼
0.78 [114], whereas
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