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Alkylphenoxy perylene bisimide ligands and their Pt(II) squares
0.94) in
chloroform or dichloromethane, while the ferrocene-substituted
perylene bisimide ligands and the corresponding squares
53
possess high fluorescence quantum yields (
F
= 0.86
−
F
54
are poorly fluorescent (
< 0.002) in dichloromethane. The
low fluorescence quantum yields are presumably due to the
deactivation of the photoexcited singlet state of perylene bisimide
through reductive electron transfer from ferrocenes, which is
supported by the following facts: For these ferrocene-containing
perylene bisimides, the energy level of the charge-separated
state (perylene
F
F
•−
−
•
+
) is about 1 eV, as calculated from the cyclic
voltammetric reduction and oxidation potentials. Because the lowest
singlet-excited state of these perylene bisimides lies ca. 2.15 eV above
the ground state, the photoinduced intramolecular electron-transfer
reaction is exothermic by ca. 1.15 eV (110 kJ mol
Fc
−
1
) and, therefore,
thermodynamically feasible. However, the singlet-excited state of
ferrocene (2.46 eV) is above that of the perylene chromophore (2.15
eV), thus, quenching by the perylene
ferrocene energy transfer
would be forbidden owing to the endothermic effect by 0.31 eV (30
kJ mol
→
−
1
). Moreover, ferrocene units have also been incorporated on
the metal center for architecting supramolecules for electrochemical
studies.
reported
by Sun and Lees, is particularly an interesting assembly from an
electrochemical point of view because it incorporates four redox-
active terpyridyl metal complexes as bridging units in addition to
other four ferrocene units at corners [52]. A DMF solution showed
a reversible wave for the ruthenium center oxidation (Ru
The heterometallic (Pd/Ru) molecular square
55,
2+
3+
/Ru
,
+
E
.
However, no appreciable communication between the metal centers
was observed in this assembly. When the Pd corners were replaced
by Re(CO)
= 0.7 V) and an irreversible oxidation wave at 0.88 V for Fc/Fc
1/2
Br corners, the oxidation potential for ruthenium center
was anodically shifted owing to the electron-withdrawing nature
of the rhenium corner. This observation implies that the redox
potential of the bridging metal complex can be modulated by subtle
environmental variations and suggests that the square
3
55
could be a
potential redox-sensing material.
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