Chemistry Reference
In-Depth Information
display triplet-state phosphorescence at room temperature, a reason why 77K data are
used. The dependence of
E
0-0
(T
) upon temperature is not precisely known (typically
1000-3000 cm
1
between 77 and 295 K) but one may hypothesize that in the case of
[Eu
2
(L5)
3
] it shifts the triplet state to an energy more favourable for energy transfer.
Another explanation could be found in the fact that the overall quantum yield depends on
two main factors: the energy transfer efficiency from the ligand and the ability of the
edifice to prevent nonradiative deactivation. The latter is not taken into consideration in
the correlation. However, looking into the lifetime data one sees that they are very similar
(2.2-2.5ms) for all helicates with the exception of [Eu
2
(L9)
3
], meaning that radiationless
processes are very comparable so that this factor does not seem to play a role for
[Eu
2
(L5)
3
] having a large quantum yield. But the explanation for the short lifetime of
[Eu
2
(L9)
3
] lies in the resonance between the triplet state energy and the Eu
5
D
1
Þ
ð
level,
leading to energy back transfer, short lifetime and low quantum yield.
That the simple relationship between
E
0-0
(T
) and
Q
Eu
cannot rationalize all quantum
yield values is illustrated by data for the other helicates reported in Table 6.1 which would
not fit well into the monotonous correlation. Solvent effects have also to be taken into
consideration. Finally, deviations from the monotonous correlation are found within the
phosphonate helicate series. For instance, [Eu
2
(L14a)
3
] is an exception similar to
[Eu
2
(L5)
3
], while the quantum yield of the other chelates is very low with respect to their
triplet state energy.
Among the cationic helicates with neutral ligands L3-L4 measured in acetonitrile,
[Eu
2
(L4-NCS)
3
]
6þ
has much larger quantum yield than predicted by the correlation,
while [Eu
2
(L4)
3
]
6þ
and [Eu
2
(L4-Cl)
3
]
6þ
have slightly smaller quantum yields. The case
of [Eu
2
(L3)
3
]
6þ
is similar to [Eu
2
(L9)
3
], with a triplet state energy close to the energy of
the Eu
5
D
1
Þ
level. In addition, it has been shown for a mononuclear analogue that the
EuN
9
coordination favours a low-lying LMCT state [53] which strongly deactivates
the ligand singlet state [54].
A more detailed way of looking into the data reported in Table 6.1 is to evaluate the
radiative lifetimes and intrinsic quantum yields, and, subsequently, the sensitization effi-
ciencies. These parameters are reported in Table 6.2 for helicates with carboxylate and
phosphonate ligands. Interestingly, the radiative lifetimes for the seven solutions of dicar-
boxylate helicates lie in a narrow 6.2-6.9ms range, with an average of 6.6ms. This is by
no means unexpected since they all feature the same N
6
O
3
chemical environment with
very similar Eu-ligand distances. In turn, the intrinsic quantum yields
Q
E
Eu
are very simi-
lar, 35-40% (Figure 6.3, see triangles), except for [Eu
2
(L9)
3
]forwhich
Q
E
Eu
is much
smaller; this is due to the short observed lifetime discussed above. For the other dicarbox-
ylate helicates, differences in overall quantum yields have therefore to be traced back to
differences in energy transfer efficiencies (h
sens
; Figure 6.3, circles) which vary depend-
ing on subtle electronic differences in the ligands. In effect, h
sens
and
Q
Eu
value depen-
dences are perfectly parallel and the large quantum efficiency displayed by [Eu
2
(L5)
3
]
with respect to
E
0-0
(T
) can be entirely ascribed to a more efficient energy transfer. How-
ever, the very poor quantum yield of [Eu
2
(L9)
3
] results both from an inefficient energy
transfer and from large nonradiative deactivation rate constants, as reflected by the very
small value of
Q
E
Eu
.
Although radiative lifetimes for helicates with phosphonate ligands are only avail-
able for two chelates, an interesting observation can be made: they are much shorter
ð
Search WWH ::
Custom Search