Chemistry Reference
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the pyridine in L25 (Scheme 6.6), the resulting [LnFe(L25)
3
]
5þ
helicate displays
spin-crossover behaviour. In acetonitrile at room temperature, the high-spin fraction
of [EuFe(L25)
3
]
5þ
amounts to 15% and the transition temperature is estimated to be
359K by extrapolation of data recorded in the range 243-333K. The LMCT of the
pure low-spin complex [EuFe(L25)
3
]
5þ
, accounting for its violet colour, extends
from 430 to 630 nm, with a molar absorption coefficient reaching 5800M
1
cm
1
at
530nm.Asaresult,the
5
D
0
luminescence in [EuFe(L25)
3
]
5þ
is totally quenched as
a consequence of the overlap between the Eu
III
emission spectrum and the broad
LMCT absorption band [89]. A similar quenching also happens for [EuFe(L24)
3
]
5þ
[88]. In contrast, the steric constraint linked to the introduction of the methyl group
in the 6-position of the pyridine in L28 leads to the exclusive formation of the high-
spin [EuFe(L28)
3
]
5þ
helicate, both in solution and in the solid state. This is con-
firmedbytheweakLMCTabsorptionband(
500M
1
cm
1
) around 450 nm,
responsible for the yellow colour of the helicate and the faint d-d transitions at 910
(14M
1
cm
1
) and 1130 nm (11M
1
cm
1
). Consequently, [EuFe(L28)
3
]
5þ
is lumi-
nescent, even at room temperature, yet less than the corresponding Zn
II
complex, as
indicatedbytheEu
e
5
D
0
Þ
lifetime which drops from 2.63ms (ligand excitation, solid
state, 13 K) in the zinc helicate to 0.28ms in the iron complex due to partial direc-
tional Eu
III
ð
Fe
II
HS
energy transfer, the latter ion acting as a semi-transparent part-
ner. The situation is summarized in Figure 6.16.
!
6.3.3.2 Tuning the Lifetime of NIR-Emitting Ln
III
Ions
The
2
E
4
A
2
transitions of Cr
III
around 700 nm are of great importance
in technology and are used in ruby lasers and to sensitize the luminescence of Nd
III
in
yttrium aluminium garnet (YAG) lasers since the corresponding emission bands overlap
considerably with the absorption spectrum of neodymium. The Cr
4
A
2
and
4
T
2
!
!
2
E
level can be tuned
by ligand-field effects and routinely activates the luminescence of NIR-emitting ions
(Nd
III
,Er
III
,Yb
III
) [36]. Depending on the energy of the 3d levels, Cr
III
can also sensitize
the luminescence of visible-emitting ions such as Eu
III
and Tb
III
, an example being heter-
onuclear complexes CrLn
2
with a 2-methyl-8-hydroxyquinoline bridge featuring Cr-Eu
distances of 3.3-3.4 A
[98].
Because of its transparency to biological tissue, NIR light is appreciated in bioprobes
and many Ln
III
ions seem to be ideally suited for this purpose. There are however two
problems with these probes [36]: (i) due to the small energy gap, the excited states are
readily de-activated by all kind of vibrational oscillators, even by those residing in the
outer coordination sphere, and (ii) the excited state lifetime is usually short, limiting the
ease of application of time-resolved detection. The latter inconvenience may be remedied
by populating the excited state of the NIR-emitting Ln
III
ions by a slow emitting donor.
Transition metal ions such as Cr
III
meet the necessary criteria and the triple helical recep-
tors dealt with in this chapter are adequate hosts for inducing such an energy transfer. The
[LnCr
III
(L25)
3
]
6þ
and [LnRu
II
(L25)
3
]
5þ
complexes (Ln
ð
Þ
Nd, Gd, Er, Yb), have therefore
been proposed for this purpose [14,92]. Relevant data are reported in Table 6.14.
The situation can be mathematically described as follows. In the absence of energy trans-
fer, the excited states of the metal ions have deactivation rates
k
Ln
and
k
M
which are the
sum of the radiative and nonradiative rate constants. When the energy transfer rate
¼
M;Ln
et
h
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