Page 209 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

194 Principles and Methods

Fullerene triplet quenching:Type II photosensitization. After excitation of C 60
to the triplet state via ISC (Eq. 83), the corresponding lifetime of the
triplet state is microseconds in absence of quenching by oxygen [61, 77,
81]. However, the lifetime also depends on phosphorescence (Eq. 91),
internal conversion (Eq. 92), self-quenching (Eq. 97), and triplet-triplet
annihilation (Eq. 98). Therefore, the triplet lifetime depends on concen-
. By measuring triplet lifetime at various concentrations,
tration of C 60
the effect of these alternative quenching mechanisms (Eqs. 97 and 98)
can be eliminated from lifetime calculations, allowing for an estimation
of the intrinsic triplet lifetime around 133 µs in nonpolar solvent [82].
More generally, the triplet lifetime tends to be around 40 µs in most
studies involving nonpolar solvents because a single concentration is
used to measure the triplet lifetime. These triplet lifetimes are excep-
tional, but along with type II photosensitization rates lifetimes change
dramatically when C 60 is suspended in the aqueous environment.
Encapsulating agents such as -cyclodextrin ( -CD), Triton-X, and
poly(vinylpyrrolidine) (PVP) increase the lifetime of the triplet state
(Eq. 93) up to 130 µs regardless of concentration [68 70, 83]. This is
likely due to the encapsulating agent’s ability to reduce contact between
fullerenes making self-quenching (Eq. 97) and triplet-triplet annihila-
tion (Eq. 98) less frequent (i.e., -CD encapsulation reduced triplet-
triplet annihilation by four times as compared with free C 60 in toluene
[69]). Photosensitization rates benefit from the increased lifetime of the
triplet state, but the same encapsulation effects reduce the rate of type II
singlet oxygen formation (Eq. 94). In the case of -CD, the triplet quench-
ing by oxygen was determined to be half that of free C 60 in toluene after
correction for oxygen diffusion rates [69]. In addition, illuminated
PVP/C 60 was monitored for the characteristic singlet oxygen emissions
band at 1270 nm. However, the IR emission was not observed [68] and
thus, it was concluded that type II sensitization (Eq. 94) was not occur-
ring or that it was taking place at a very low rate.
A more specific example, “mechanically entrapped C 60 ,” recently has
been developed for C 60 suspension [84]. The carbon cage is entrapped in
an all-silica zeolite Y supercage (Figure 5.26).
, the triplet lifetime (Eq. 93) is extended into the
In the absence of O 2
order of minutes. Presumably, this is due to the complete lack of quenching
mechanism activity and indicates the C molecules are more than likely
60
entrapped individually and not as clusters. Otherwise, triplet-triplet
annihilation (Eq. 98) and self-quenching (Eq. 97) would lower the triplet
lifetime significantly. Despite this, in the presence of oxygen, the type II
(Eq. 94) mechanism occurs effectively but at a slight rate decrease from
the diffusion-controlled quenching of free C .
60
Increasing the number of addends on the C cage decreases the quan-
60
tum yield of the triplet state (Eq. 85) and, as a result, the quantum yield