Page 208 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Reactive Oxygen Species Generation on Nanoparticulate Material 193

Figure 5.25 Increasing the amount
of addends enlarges the gap
between the HOMO and LUMO.

The cage structure can be altered by various chemical reactions that
perturb the extended -bonding network and subsequently raise the
energy of the LUMO electrons due to loss of conjugation [76] (Figure 5.25).
This increases the energy required to excite electrons across the band gap
and into their excited states; thus requiring higher energy light (Eq. 80)
and contributing to a reduction in quantum yield (Eq. 85) [73].
The photophysical properties of C , a higher order fullerene cage, are
70
influenced by its structure, which may have an oblong shape in order
to maintain the cagelike carbon structure. The singlet oxygen quantum
3
yield (Eq. 102) (a measure of the lower limit of C quantum yield [Eq. 85])
70
was found to be around 0.81, indicating light conversion was not as effi-
cient in this molecule [67]. This decrease is partially attributed to deac-
1
tivation of C 70 by alternative pathways such as internal conversion
(Eq. 82) that do not produce the triplet state. Thus, it appears that
either increasing the size of the fullerene cage or decreasing fullerene
symmetry––or both––may lead to a decrease in quantum yield (Eq. 85).

Intersystem crossing: Fullerene triplet-state formation. The characteristic
10
rates of ISC (k ) for C and C had been determined to be 3.0
10 s 1
70
isc
60
9 1
and 8.7
10 s , respectively [77]. Decay of the singlet-excited C is pre-
60
dominantly ISC to the triplet state (Eq. 83) [61, 67]. This phenomenon can
1
3
be explained in terms of small energy splitting between C and C , low
60
60
fluorescence (Eq. 81), and large spin-orbital interaction. The large diam-
eter and spherical nature of C promote these properties by lowering elec-
60
tron repulsion and the extended -bonding network, respectively. In C ,
70
the extended -bonding network seems to be perturbed enough to promote
internal conversion (Eq. 82) rather than ISC (Eq. 83); this results in
3
reduced C quantum yields (Eq. 85). Addends reduce the ISC rate (Eq. 83)
60
in the same way by reducing the amount of bonds on the surface of the
C cage. Since the singlet state cannot be as easily relaxed, its lifetime
60
is noticeably longer but still on the order of nanoseconds [78 80].