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Figure 6.2 Simplified scheme demonstrating sensitization of Ln III luminescence by a ligand.
Key: A¼absorption, E¼emission, G¼ground state, Ex¼excited state, D¼donor state; other
parameters are defined in the text.
extracted from absorption spectra (Equation 6.2) or, in the case of Eu III , from emission
spectra (Equation 6.3).
k obs ¼ t obs
k rad
Q L Ln ¼
ð
6
:
1
Þ
t rad
ð
cn 2
2
1
t rad ¼
8
p
n
ð
2 J
þ
1
Þ
2303
e ðnÞ
d
n
ð
6
:
2
Þ
2 J 0 þ
N A ð
1
Þ
1
t rad ¼
I tot
I MD
n 3
A MD;0
ð
6
:
3
Þ
with A MD,0 being equal to 14.65 s 1 ;( I tot / I MD ) is the total integrated emission from
the Eu( 5 D 0 ) level to the 7 F J manifold ( J
¼
0-6) divided by the integrated intensity of the
5 D 0 !
7 F 1 transition.
6.2 Homometallic Lanthanoid Helicates
Tridentate coordinating units of the C 2 -symmetrical ligands described in Scheme 6.2 are
connected by saturated methylene or cyclohexane units which are poor electronic relays. It
is therefore anticipated that the two ends of the helical edifices [Ln 2 (L) 3 ] n þ ( n
0or6)
will behave independently and that trends in their photophysical properties will parallel
those found in mononuclear complexes in which the metal ion has a similar environment
[38]. Ligands sketched in Schemes 6.1 and 6.2 have been tailored to self-assemble with
Ln III ions either in aprotic (L3, L4) or protic solvents leading to nine-coordinate chemical
environments with idealized trigonal symmetry. Single-crystal X-ray structure determina-
tions of dinuclear helicates with ligands L3 [6], L4 [39] and H 2 L5 [40] confirm coordina-
tion geometries derived from tricapped trigonal prismatic polyhedra. Furthermore, NMR
spectra and solution structure determination based on lanthanoid-induced shifts and relaxa-
tion times [41] fully agree with this picture [40,42]. The only exception is ligand H 2 L16
¼
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