Chemistry Reference
In-Depth Information
FIGURE 6.16
Transmission versus incident fluence at 532 nmof a sol-gel sample containing
compound
34
.
31
[51,53]. For the largest compounds, the resulting samples only contain well-
dispersed fullerodendrimer molecules. Measurements on the resulting doped samples
have revealed efficient optical limiting properties [51]. The transmission as a function
of the fluence of the laser pulses remains nearly constant for fluences lower than 5mJ/
cm
2
(Figure 6.16). When the intensity increases above this threshold, the effect of
induced absorption appears, and the transmission diminishes rapidly, thus showing
the potential of these materials for optical limiting applications.
C
60
is also an attractive functional core for the preparation of light harvesting
dendrimers [54-60]. Effectively, its first singlet and triplet excited states are relatively
low in energy and photo-induced energy transfer events have been evidenced in some
fullerene-based dyads [61]. In particular, photophysical investigations of some
fulleropyrrolidine derivatives substituted with OPVmoieties revealed a very efficient
singlet-singlet OPV
-
34
!
C
60
photo-induced energy transfer [54,62]. Based on this
observation, dendrimers
with a fullerene core and peripheral OPV subunits
(Figure 6.17) have been prepared [63].
The photophysical properties of fullerodendrimers
39
-
41
have been first inves-
tigated in CH
2
Cl
2
solutions. Upon excitation at the OPV band maximum, dramatic
quenching of OPV fluorescence is observed for all fullerodendrimers. At 394 nm
(corresponding to OPV band maxima) the molar absorptivities (
39
-
41
e
) of these full-
erodendrimers are 134,800M
1
cm
1
, 255,100M
1
cm
1
for
39
for
40
, and
730,400M
1
cm
1
for
of the ubiquitous N-methyl-fulleropyrrolidine
at 394 nm is only 7600 a remarkable light harvesting capability of the peripheral units
relative to the central core is evidenced along the series. UV-vis-NIR luminescence
and transient absorption spectroscopy have been used to elucidate in more details the
photo-induced processes in fullerodendrimers
41
. Since the
e
as a function of the dendritic
generation and of the solvent polarity (toluene, CH
2
Cl
2
, benzonitrile), taking into
account that the free energy change for electron transfer is the same along the series
due to invariability of the donor-acceptor couple. In any solvents, all of the full-
erodendrimers exhibit ultrafast OPV
39
-
41
!
C
60
singlet energy transfer (k
EnT
ca.
