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not all the 12 nitrogens of the three cyclam cores are engaged in metal ion
coordination. However, upon metal coordination, the exciplex emission band
completely disappears, as is observed upon acid titration. Clearly, as is also
shown by NMR results, the presence of the 3+ ion is “felt” by all the nitrogens of
the three cyclam moieties, thereby raising the energy of the exciplex excited state
above that of the naphthyl-based one. For all the lanthanide complexes of 1 and 2,
no sensitized emission from the lanthanide ion was observed. Therefore, energy
transfer from either the
S
1
or
T
1
excited state of the naphthyl units of 1 and 2 to the
lanthanide ion is inefficient. By contrast, efficient energy transfer from naphtha-
lene-like chromophores to Eu
3+
has been reported in the case in which naphthalene
is linked through an amide or carboxylate bond to the lanthanide [
44
]. Apparently,
the nature of the coordination sphere plays an important role in energy transfer
efficiency.
To overcome this problem a supramolecular approach has been followed [
6
a]. It
was known that complexes of Ru
2+
containing 2,2
0
-bipyridine (bpy) and cyanide
ligands, i.e., [Ru(bpy)
2
(CN)
2
] and [Ru(bpy)(CN)
4
]
2
, are luminescent and can play
the role of ligands giving rise to supercomplexes [
45
,
46
]. Titration of an acetoni-
trile:dichloromethane 1:1 (v/v) solution of [Ru(bpy)
2
(CN)
2
] with Nd
3+
causes
changes in the absorption spectrum and quenching of the Ru
2+
complex emission,
accompanied by sensitized Nd
3+
emission, demonstrating the ability of [Ru
(bpy)
2
(CN)
2
] to complex the lanthanide metal ion. Titration of a 1:1 mixture of
dendrimer 2 and [Ru(bpy)
2
(CN)
2
] in acetonitrile:dichloromethane 1:1 (v/v) with
Nd(CF
3
SO
3
)
3
brings about changes in the absorption and emission spectra. The
lowest energy absorption band is blue-shifted, as observed for the titration of [Ru
(bpy)
2
(CN)
2
] in the absence of dendrimer. Upon excitation at 260 nm, where most
of the light is absorbed by dendrimer 2, the intensity of the naphthyl monomer
emission at 337 nm does not show a monotonous increase, as observed in the
absence of the [Ru(bpy)
2
(CN)
2
] complex, reaches a maximum at 0.5 equivalents,
and then decreases up to about 1.0 equivalent of Nd
3+
to rise again for higher metal
ion concentration. The emission intensity at 1.0 equivalent is lower than that
observed in the absence of [Ru(bpy)
2
(CN)
2
]. These results show that a three-
component system {2
l
Nd
3+
l
[Ru(bpy)
2
(CN)
2
]} (Fig.
9
) is formed in which the
dendrimer emission is quenched. The three-component system can be disassembled
by addition of an excess of each component or cyclam. The main photophysical
processes of the {2
l
Nd
3+
l
[Ru(bpy)
2
(CN)
2
]} adduct are summarized in Fig.
6
,
which shows the energy levels of the three components. In the two-component
dendrimer-Nd
3+
system, energy transfer from either the lowest singlet (
S
1
) or triplet
(
T
1
) excited state of the naphthyl units of the dendrimer to the lanthanide ion does
not occur. Sensitization of the Nd
3+
emission upon dendrimer excitation in the
three-component system is mediated by the [Ru(bpy)
2
(CN)
2
] component. Compar-
ison between the emission quantum yield of [Ru(bpy)
2
(CN)
2
] upon excitation at
260 nm (dendrimer absorption) and 450 nm ([Ru(bpy)
2
(CN)
2
] absorption) has
allowed to estimate that the energy transfer efficiency from the
S
1
excited state
of the naphthyl groups to the
1
MLCT excited state of [Ru(bpy)
2
(CN)
2
] is about
60% (Fig.
10
). The energy transfer efficiency from the
3
MLCT excited state of
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