Chemistry Reference
In-Depth Information
TABLE 6.2 Lifetime of the First Triplet Excited State of 31-38 in Air Equilibrated
Solutions Determined by Transient Absorption at Room Temperature
Compound
t
(ns) in PhMe
t
(ns) in CH
2
Cl
2
t
(ns) in CH
3
CN
%
a
31
279
598
32
304
643
330
33
318
732
412
34
374
827
605
35
288
611
314
36
317
742
380
37
448
873
581
38
877
1103
1068
a
Not soluble in this solvent.
with the solvent and from quenchers such as molecular oxygen. It must be however
emphasized that the triplet lifetimes of
in the three solvents are rather different
from each other, likely reflecting specific solvent-fullerene interactions that affect
excited state deactivation rates. This suggests that, albeit a dendritic effect is
evidenced, even the largest wedge is not able to provide a complete shielding of
the central fulleropyrrolidine core in
34
. The latter hypothesis was confirmed by
computational studies. The calculated structure of
34
reveals that the dendritic shell is
unable to completely cover the fullerene core. In contrast, the triplet lifetimes of
38
34
[52] in the three solvents lead toward a similar value suggesting that the fullerene
core is in a similar environment whatever the nature of the solvent is. In other words,
the C
60
unit is, to a large extent, not surrounded by solvent molecules but substantially
buried in the middle of the dendritic structure that is capable of creating a specific site-
isolated microenvironment around the fullerene moiety. The latter hypothesis is quite
reasonable based on the calculated structure of
38
showing that the dendritic branches
are able to fully cover the central fullerene core. The dendritic effect evidenced for
31
38
was found to be useful to optimize the optical limiting properties characteristic
of fullerene derivatives. Effectively, the intensity dependant absorption of fullerenes
originates from larger absorption cross sections of excited states compared to that of
the ground state [53], therefore the increased triplet lifetime observed for the largest
fullerodendrimers may allow for an effective limitation on a longer time scale. For
practical applications, the use of solid devices is largely preferred to solutions and
inclusion of fullerene derivatives in sol-gel glasses has shown interesting perspec-
tives [53]. However, faster de-excitation dynamics and reduced triplet yields are
typically observed for fullerene-doped sol-gel glasses when compared to solu-
tions [53]. The latter observations are mainly explained by two factors: (i) pertur-
bation of the molecular energy levels due to the interactions with the sol-gel matrix
and (ii) interactions between neighboring fullerene spheres due to aggregation [53].
Therefore, the encapsulation of the C
60
core evidenced by the photophysical
studies for both series of fullerodendrimers might also be useful to prevent such
undesirable effects. The incorporation of fullerodendrimers
-
in sol-gel glasses
has been easily achieved by soaking mesoporous silica glasses with a solution of
31
-
34
