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Figure 9.5
Emission spectra of the supramolecular complexes
8-11
.
of this compound was gradually quenched when titrated with
Mn
2+
3+
2+
,
2+
ions (Fig. 9.6). However, it was observed
that metal ions with completely filled d orbitals do not contribute
to any variations in the fluorescence patterns of the rectangle
, Fe
, Ni
,
and Cu
11
.
2+
The fluorescence intensity of the Ni
bound to N
pocket of imine
4
nitrogens in rectangle
11
increased when titrated with stronger
¢
chelating 2,2
-bipyridine ligand (Fig. 9.7). All these observations
can be explained on the grounds of photoinduced electron-transfer
mechanism (PET) [29,30], which goes via excited state electron
transfer. The rectangle
can be considered as an ideal combination
of fluorophore-receptor-fluorophore, where the organometallic
“clip” units are the fluorophores and the imines N
11
pocket is the
receptor. Upon excitation, the incompletely filled d orbital of 3d
transition metal ions can drag the electrons from the excited state
and can go to lower oxidation state(s). However, in case of Zn
4
2+
ion
the nonavailability of the variable oxidation states as well as due to
having stable d
10
electronic configuration, it does not consume the
electron and hence PET does not occur. Hence, the fluorescence of
11
2+
¢
was not quenched in the presence of Zn
. Moreover, when 2,2
-
2+
bipyridine was added to the solution of [
11
+ Ni
], the fluorescence
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