Chemistry Reference
In-Depth Information
decrease of the reorganization energy upon enlargement of the molecular architec-
ture. Structural factors are also at the origin of an unprecedentedOPV
!
C
60
electron
transfer observed for
40
and
41
in apolar toluene, whereas in benzonitrile electron
transfer occurs in all cases.
6.3.2 Fullerene-Rich Dendrimers
Whereas the main part of the fullerene-containing dendrimers reported so far have
been prepared with a C
60
core, dendritic structures with fullerene units at their surface
or with C
60
spheres in the dendritic branches have been much scarcely considered.
This is mainly associated with the difficulties related to the synthesis of fullerene-rich
molecules [7]. Indeed the two major problems for the preparation of such dendrimers
are the low solubility of C
60
and its chemical reactivity limiting the range of reactions
that can be used for the synthesis of branched structures bearing multiple C
60
units.
Over the past years, several examples of fullerene-rich dendrimers have been
reported. These results will be summarized in the following sections.
6.3.2.1 Divergent Synthesis
The divergent preparation of fullerodendrimers by
grafting C
60
units on the peripheral reactive groups of commercially available
dendrimers is the easiest way to produce large dendritic structures. This synthetic
approach was applied to prepare compounds
42
44
-
(Figure 6.18) from fullerene
derivative
and polypropyleneimine (PPI) dendrimers [64].
The choice of the appropriate activating group for the carboxylic acid function of
the C
60
building block
13
was the key to this synthesis. Effectively, the reaction
conditions for the activation stepmay not be strongly acid or basic to preserve the ester
functions. Furthermore, the grafting onto the dendritic polyamines requires an
extremely efficient reaction to obtain the corresponding functionalized derivatives
with good yields and to avoid the formation of defected dendrimers. The preparation
of the pentafluorothiophenolester derived from compound
13
and pentafluorothio-
phenol under N,N
0
-dicyclohexylcarbodiimide (DCC)-mediated esterification condi-
tions appeared as a good choice. Indeed, the reaction conditions for the preparation of
this activated acid are mild and the efficient grafting of pentafluorothiophenolesters
onto PPI dendrimers has already been reported [65]. The activated acid was obtained
in nearly quantitative yields by reaction of carboxylic acid
13
with pentathiofluor-
othiophenol in the presence of DCC and a catalytic amount of 4-dimethylaminopyr-
idine (DMAP). Subsequent reaction of the resulting pentafluorothiophenolester with
the PPI dendrimers of first-, second- and third generation in the presence of
triethylamine provided the corresponding dendritic derivatives
13
in good yields.
Owing to the presence of four pendant alkyl chains per fullerene moiety, compounds
42
42
-
44
are all well soluble in common organic solvents such as CH
2
Cl
2
, CHCl
3
, THF,
or toluene, and spectroscopic characterization was easily achieved. The
1
H-NMR
spectra of dendrimers
-
44
show the typical pattern of the fullerene cis-2 bis-adduct
with the expected additional signals arising from the PPI skeleton. The integration
ratio are also consistent with the proposed molecular structures. The structure of
42
-
44
42
and
43
was further confirmed by MALDI-TOF mass spectrometry. The expected
