Chemistry Reference
In-Depth Information
Fig. 14 Molecular model of
exTTF-crown ether/fullerene
ammonium salt complex
for the first dyad the formation of a charge separated state after irradiation with a
long lifetime of ~2.02
s was observed, the lifetime of the radical pair in the case of
the zinc phthalocyanine dyad was only 3 ns [
131
,
132
]. This difference has been
ascribable to a pronounced coupling between the ZnPc and C
60
in 37, which is
reflected in a large binding constant of 1.7
ʼ
10
7
M
1
(vs a
K
a
of 5.1
10
4
M
1
found for 36).
The six-point Hamilton array has also been used to form highly stable dyads
when interacting with a cyanuric acid moiety, with association constants usually in
the range of 10
4
-10
5
M
1
for monotopic receptors [
133
] and even higher when a
ditopic Hamilton receptor is employed [
134
]. In recent work, Hirsch et al. have
employed this Hamilton receptor together with metal complexation to control the
step-by-step assembly of the different components in triad 38 [
135
]. In this triad the
perylenediimide (PDI) moiety acts as a light harvester unit and, after selective
photoexcitation, an energy transfer to the porphyrin unit takes place, which has
been corroborated by steady-state and time-resolved measurements. The energy
transfer is followed by an electron transfer event from the porphyrin to the axially
complexated fullerene moiety driving to the formation of a radical ion pair with a
lifetime of 3.8 ns. This lifetime is longer than that of the corresponding dyad from
the porphyrin and the fullerene.
Tetrathiafulvalene (TTF) and
-extended tetrathiafulvalene (exTTF) have also
been used as the electron donor counterpart in D-A nanohybrids with fullerenes in
supramolecular assemblies. In pseudorotaxane-type structures between exTTF and
C
60
, it has been found that the interaction between the donor moiety of exTTF and
the acceptor unit of fullerene is stronger when a flexible spacer allows the involve-
ment of the intramolecular interaction between the convex fullerene surface and
the concave face of exTTF (Fig.
14
)[
136
]. In this case, a binding constant of
1.58
ˀ
10
6
M
1
in chlorobenzene was observed, more than two orders of magni-
tude higher than the
K
a
values obtained for related systems in which only the crown
ether-ammonium salt motif can interact. For this dyad a remarkable anodic shift
of ~ 100 mV of the oxidation potential of exTTF reflects the strong donor-acceptor
interaction. Transient absorption spectroscopy showed the formation of a radical
ion pair with a lifetime of 9.3 ps.
