Chemistry Reference
In-Depth Information
association constants have been reported for supramolecular systems resulting from
the axial coordination of a bis-Zn(II)-porphyrinic receptor to substrates bearing two
pyridine subunits [23].
Supramolecular assembly [(
)
12
] combining C
60
units and porphyrin moi-
eties [21] is also a photochemical molecular device. Indeed, the photophysical
properties of this system have been studied in detail and an almost quantitative
intramolecular photo-induced electron transfer from the photoexcited porphyrins to
the C
60
units evidenced by means of steady-state emission spectroscopy and
nanosecond flash photolysis measurements. Excited-state dynamic studies have been
carried out to investigate both charge-separation and charge-recombination events in
[(
17
)
(
18
)
12
]. The charge-separation rate constants (k
CS
) and the charge-recombina-
tion rate constants (k
CR
) have been thus deduced. Importantly, the k
CS
/k
CR
ratio for
[(
17
)
(
18
)
12
] is more than an order of magnitude greater than those reported for
precedent porphyrin-fullerene supramolecular dyads and triads [24]. It is obvious that
a larger number of the fullerene units in [(
17
)
(
18
)
12
] can enhance the probability of
the electron transfer from the zinc porphyrin units. However, in addition to this, one
can also presume that an efficient energy migration along the densely packed Zn(II)
porphyrin array [21] may enhance the opportunity for this electron transfer.
17
)
(
18
6.3 COVALENT FULLERENE-CONTAINING DENDRIMERS
6.3.1 Dendrimers with a Fullerene Core
C
60
itself is a convenient core for dendrimer chemistry [6] and the functionalization of
C
60
with a controlled number of dendrons dramatically improves the solubility of the
fullerenes [25-31]. Furthermore, variable degrees of addition within the fullerene
core are possible and its almost spherical shape leads to globular systems even with
low-generation dendrons [32]. In this respect, fullerene hexakis-adducts with a T
h
-
symmetrical octahedral addition pattern are of particular interest [33]. Whereas such
compounds can be prepared in good yields from relatively simple malonates [33],
structurally more complicated systems are generally obtained in rather low yields. In
order to overcome this problem, Nierengarten and coworkers have recently developed
a simple C
60
hexakis-adduct derivative bearing 12 alkyne subunits and shown that the
copper-mediated Huisgen 1,3-dipolar cycloaddition of azides and alkynes resulting in
1,2,3-triazoles is an ideal tool to efficiently produce dendritic hexa-substituted
fullerenes
(Figure 6.9) [34,35].
The 1,2,3-triazole-linked fullerodendrimers
19
and
20
were thus obtained in
good yields. It is worth noting that a first-generation dendritic C
60
hexa-adduct
similar to
19
and
20
was already reported by Hirsch and coworkers [32]. The latter was
obtained in a low yield (5%) from C
60
and the corresponding dendritic malonate,
furthermore preparative HPLC was required for its purification. In contrast, com-
pound
19
20
was prepared in a good yield (84%) and its purification was easily achieved
on a silica-gel column. Furthermore, the second-generation derivative (
20
) was
also obtained in a good yield (67%) under the copper-mediated Huisgen reaction
