Chemistry Reference
In-Depth Information
lanthanoid triflates to give triple-stranded tetranuclear helicates [Ln
4
(L23)
3
](CF
3
SO
3
)
12
.
Similarly to the trinuclear helicate described above, microscopic stability constants have
been determined for Eu
III
, leading to a statistical analysis of the stepwise formation of the
helical structure and to the determination of the affinity of the sites for the metal ions. The
overall structure can be looked at as a pseudo-
D
3
edifice, an hypothesis which is substan-
tiated by the Eu(
5
D
0
) emission spectrum: from the fine structure of the
5
D
0
!
7
F
1
transi-
tion, one infers that the
7
F
1
level splits into two main components, labelled A
2
and E in
D
3
symmetry, the latter further displaying two subcomponents. At 10 K, D
E
(A
2
E)
¼
75
42 cm
1
for the N
6
O
3
terminal sites; these data are identical
to the ones for the trinuclear helicate at 10 K. High-resolution excitation spectra of the
5
D
0
cm
1
and D
E
(E
E)
¼
7
F
0
transition reveal a main component at 17 226 cm
1
(N
6
O
3
) at room tempera-
ture, with an energy virtually equal to the one of the corresponding site in the trinuclear
compound [Eu
3
(L22)
3
]
9þ
(17 225 cm
1
). The component from the N
9
central sites is
more difficult to detect at room temperature; it occurs at 17 235 cm
1
at 10 K, which com-
pares well with 17 238 cm
1
for [Eu
3
(L22)
3
]
9þ
at this temperature. Estimation of this
energy at room temperature and solving equations for the two sites leads to the conclusion
that the nephelauxetic effect generated by the pyridine groups is much larger than the one
stemming from the benzimidazole moieties [87].
6.3 Heterometallic d-f Helicates
The work performed on helicates containing f- and d-transition metal ions has been initi-
ated for several reasons: (i) facilitating the self-assembly process by prearranging the lig-
and with the help of the sterically-demanding d-transition metal ion (Cr
II
,Fe
II
,Co
II
,Zn
II
,
Ru
II
,Os
II
;Cr
III
) so that the
HHH
species is privileged, (ii) isolating enantiomerically pure
helicates with Cr
III
, (iii) controlling the Fe
II
spin-crossover parameters and (iv) tuning the
photophysical properties of the Ln
III
ions by the d-transition metal partner. To these ends,
pentadentate ditopic ligands were prepared (Scheme 6.6). The self-assembly of the
HHH
-
[MLnL
3
] helicates proceeds smoothly and with a very high thermodynamic control.
Structural aspects in solid state (X-ray crystallography) and solution (LIS NMR), as well
as paramagnetic properties have been thoroughly investigated [38]. Here, we focus on
photophysical properties only.
6.3.1 Basic Photophysical Properties
The ligand-centred photophysical properties of the helicates with a spectroscopically
silent nd-transition partner are comparable to those of the previously discussed homobi-
metallic helicates. When a nd-transition ion amenable to form MLCT states with the lig-
and strands is introduced into the helical edifices, specific absorption and, possibly,
emission bands from these states are also observed, as well as d-d transitions. Selected
data are collected in Table 6.10. Lanthanoid complexation has small influence on the lig-
and levels when M
Zn but a more pronounced one for the other transition metal ions.
In order to confirm the structural data obtained in solid state and/or solution, high-reso-
lution excitation spectra of the Eu(
5
D
0
¼
7
F
0
) transition and emission spectra of the Eu
(
5
D
0
!
7
F
1
) transition have been recorded. Relevant data are collected in Table 6.11. The
energies of the
5
D
0
7
F
0
transition calculated with Equation 6.4 and the parameters
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