Synthesis of [60]fullerene derivatives bearing five-membered heterocyclic wings and an investigation of their photophysical kinetic properties

Article abstract of DOI:10.1016/j.jphotochem.2010.10.007

The preparation, characterization and photophysical properties of six new stable [6,6]-closed fullerene cycloadducts bearing five-membered heterocycles are described. The modified [60]fullerenes are obtained by a simple and rapid synthesis via a Bingel-ty

Full text of DOI:10.1016/j.jphotochem.2010.10.007

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis of [60]fullerene derivatives bearing five-membered heterocyclic wings
and an investigation of their photophysical kinetic properties
Leandro J. Santosa, Ana S.P. Gonc¸ alvesb, Klaus Krambrockb, Maurício V.B. Pinheirob,
Marcos N. Eberlinc, Boniek G. Vazc, Rossimiriam P. de Freitasd,∗, Rosemeire B. Alvesd
a Universidade Federal de Vic¸ osa, Campus de Florestal, Rodovia LMG, 818-km 6, CEP: 35690-000 s/n, Florestal, MG, Brazil
b Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Avenida Antônio Carlos 6627, CEP: 31270-901, Belo Horizonte, MG, Brazil
c Laboratório ThoMSon de Espectrometria de Massas, Instituto de Química, Universidade Estadual de Campinas, Campus Zeferino Vaz Bloco A6-111, CEP:13083-970 CP 6154, Barão
Geraldo, Campinas, SP, Brazil
d Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Avenida Antônio Carlos 6627, CEP: 31270-901, Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation, characterization and photophysical properties of six new stable [6,6]-closed fullerene
cycloadducts bearing five-membered heterocycles are described. The modified [60]fullerenes are
obtained by a simple and rapid synthesis via a Bingel-type reaction with tetrazole and oxadiazole mal-
onate derivatives. The photophysical kinetics of these new fullerene derivatives in toluene solution under
ultraviolet illumination (375 nm, UVA) are studied by electron paramagnetic resonance and free-radical
spin-trapping using ␣-phenyl-N-tert-butyl nitrone as a spin-trap. The results are compared with pure
[60]fullerene and [6,6]-phenyl C61 butyric acid methyl ester (C60-PCBM). It is concluded that for all six
new compounds as well as pure [60]fullerene and PCBM both superoxide and singlet oxygen are pro-
duced in the first stages of UVA illumination following the type I and II mechanisms, respectively. In
all cases singlet oxygen is produced as the primary dominant species; however, the type I mechanism
always occurs in parallel with type II. In the end, the superoxide is self-dismuted into hydroxyl radicals,
Received 26 January 2010
Received in revised form
14 September 2010
Accepted 11 October 2010
Available online 15 October 2010
Keywords:
Fullerene C60 derivatives
Electron paramagnetic resonance
Reactive oxygen species
Photosensitizers
thus yielding PBN-OH• spin adducts (g = 2.007 and ahf 14N) = 1.54 mT). The kinetic reaction constants and
(
their efficiencies in the production of reactive oxygen species at 375 nm and per mW of absorbed power
are determined. The experimental results are consistent with an autocatalytic reaction model in which
the system evolutes under UVA illumination, with superoxide catalyzing the conversion of singlet oxygen
into more superoxide.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
todynamic therapy in the visible spectral range) or by covalently
modifying the fullerenes to obtain, for example, polyhydroxilated
Since the discovery of fullerenes in 1985 [1] and their sub-
sequent availability in macroscopic quantities [2], the chemical
and physical properties of functionalized fullerenes have attracted
considerable attention. Fullerene derivatives have many applica-
tions, such as in non-linear optical devices [3,4], photosensitizers
[5–8], photovoltaic cells [9], and biomedicine [10–13]. To design
and construct new molecules with novel and attractive properties
and applications, judicious structural modifications of [60]fullerene
are required [14,15]. These modifications can be achieved either
by non-covalent binding with other molecules (for example, C60
and C70 fullerenes with a derivatized Zn-phthalocyanine [16] for
applications in hybrid photovoltaics or as photosensitizers for pho-
fullerenes with water-solubility and the free-radical scavenging
properties required for applications in anti-oxidant drugs [17].
Nevertheless, the covalent modification of [60]fullerene with
heterocyclic “wings” containing two or more nitrogen atoms has
only recently been explored. For example, the hexakis adduct 1
(Fig. 1) [18] was prepared under copper-mediated Huisgen 1,3-
dipolar cyclo-addition, and it displayed interesting electrochemical
properties. In addition, the fullerene adduct 2, bearing a triazole
group, was recently synthesized by Chen et al. [19]; it exhibited
electron transport properties that may be useful in the construction
of organo-electronic devices.
Recently, we have synthesized the first fullerene derivative
linked with a tetrazole unit 3 (Fig. 1) and determined that it exhibits
photophysical properties suitable for the production of reactive
oxygen species (ROS) by ultraviolet A (UVA) photosensitization
[20]. This derivative may have potential use as a photosensitizer
in photodynamic therapy (PDT). In continuation of that work, the
∗
Corresponding author. Tel.: +55 3134095721; fax: +55 3134095700.
E-mail addresses: rossipdf@yahoo.com.br (R.P. de Freitas),
brondi@netuno.lcc.ufmg.br (R.B. Alves).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.10.007

L.J. Santos et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
185
N
N
N
O
O
6
R
R
N
O
R
O
O
O
O
R
R
2
R
O
O
R
O
R
O
R
R
O
O
O
R
R
N
N
N
O
O
N
N
N
N
N
N
N
N
S
O
O
S
10
10
O
OMe
1
R =
3
Fig. 1. [60]Fullerene derivatives containing heterocycles.
synthesis, characterization and photophysical properties of six new
[60]fullerene derivatives containing either tetrazole or oxadiazole
units are reported here.
calibration was done with a 2,2-diphenyl-1-picrylhydrazyl (DPPH)
standard with g = 2.0037. The EPR measurements were performed
as a function of illumination time, and with a microwave power of
approximately 2 mW.
2. Experimental
2.2. EPR spin-trapping methodology
2.1. Materials and experimental setup
The generation of ROS assisted by C60 and its derivatives under
UVA illumination was studied by EPR using the spin-trapping tech-
nique. Each sample consisted of two compounds in an air-saturated
toluene solution: the spin trap PBN (␣-phenyl-N-tert-butyl nitrone)
and one of the synthesized fullerene derivatives with concentra-
tions of 300 mM and 1 mM, respectively. Two reference solutions,
containing either pure C60 or C60-PCBM, were prepared with the
same concentrations to compare their efficiency in ROS produc-
tion. The samples were prepared under ambient laboratory light.
A fixed volume of 100 ␮L of the above solutions was added to a
borosilicate EPR tube with a 3 mm inner diameter (Wilmad). An
UVA solid state laser (Power Technology Inc.), with a wavelength
of 375 nm and power of 16 mW, was used for illumination through
the open top of the sample tube placed inside the EPR cavity. The
spot had a diameter of 1.74 0.15 mm, and the illuminated vol-
ume of the sample was 33.3 2.9 ␮L. All samples were measured
under the same illumination conditions, with the exception of the
spin quantification calibration measurements that are described in
supplementary data.
Reagents and solvents were purchased as reagent grade and
used without further purification. Tetrahydrofuran (THF) was dis-
tilled over sodium benzophenone. Dry acetone was prepared after
agitation with potassium carbonate for 24 h at room temperature
and then distilled. The azoles were obtained from Sigma–Aldrich.
The C60 (99.5%) and C60-PCBM ([6,6]-phenyl C61 butyric acid
methyl ester) were obtained from M.E.R. Corporation and from SES
RESEARCH, respectively.
The melting points were determined on a Mettler FP80HT
apparatus and are reported uncorrected. The nuclear magnetic
resonance (NMR) spectra were recorded on Bruker Avance DRX-
200 or DRX-400 spectrometers. Chemical shifts are reported
in ı units downfield from tetra-methylsilane, and the coupling
constant (J) values are given in Hz. All of the compounds inves-
tigated in this work were characterized with optical absorption
measured with an HP/Agilent 8453 UV–vis spectrophotometer.
Fourier-transform mass spectroscopy (FT-MS) was performed with
a LTQ FT ULTRA (ThemoScientific-7T-Germany) instrument with
the TriVersa NanoMate system (Advion, USA) operating in the
chip-based infusion mode and in the positive ion mode using
a silicon-based integrated nanoelectrospray microchip. Column
chromatography was performed with silica gel 60, 70–230 mesh
(Merck). A commercial microwave oven (Panasonic Junior Smart
NNS53BH) was specially modified [21] for organic synthesis and
used to obtain the 12-bromododecane-1-ol.
3. Experimental results and discussion
3.1. Synthesis and structural characterization of fullerene
derivatives
The synthesis of key malonate derivatives containing a five-
membered heterocyclic ring for the preparation of the new
fullerene derivatives is achieved by two methods. The first method
involves a [2+3] cycloaddition followed by reaction with commer-
cial malonyl chloride, which produces the new tetrazole malonates.
The second and simpler method is the alkylation of commer-
cial tetrazoles and oxadiazole, which is followed by reaction with
malonyl chloride to produce unknown azole malonates. Fig. 2 sum-
marizes the synthesis of tetrazole malonates 10 and 11 and their
use in the Bingel reaction to obtain fullerene derivatives. Initially,
commercially available diol 4 is converted into its bromo deriva-
All electron paramagnetic resonance (EPR) measurements were
done at room temperature with a custom-built X-band spectrome-
ter (9.38 GHz) using a commercial cylindrical cavity (Bruker) and a
klystron (Varian). The magnetic field was produced by an electro-
magnet (Varian) and an automated current source (Heizinger). For
detection, standard lock-in techniques (EG&G) and 100 kHz field
modulation were employed (0.1 mT). Each spectrum had a scan
time of 150 s. The microwave frequency was stabilized by an auto-
matic frequency control up to five digits and the frequency was
measured with a digital frequency meter (PTS). The magnetic field

186
L.J. Santos et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
i
v
iii
N
N
N
HO
R
HO
HO
OH
HO
v
HO
+
N
N
N
10
10
H
10
10
10
N
N
N
N
N
N
5R = Br
7
4
8
9
ii
Ph
v
Ph
6R = CN
O
O
O
O
O
N
N
N
N
O
O
O
O
O
O
O
O
N
N
10
10
10
N
N
N
10
10
10
10
N
N
N
N
N
N
N
N
10
11
N
N
Ph
Ph
Ph
Ph
Ph
vi
vi
O
O
O
N
N
N
N
N
N
N
10
N
N
N
N
N
N
Ph
Ph
Ph
13
12
Fig. 2. Preparation of compounds 12 and 13. Reagents and conditions: (i) HBr (48%), n-Bu4NBr, MW, 4 min (51%); (ii) KCN, 18-crown-6 ether, CH3CN, 80 ◦C, 48 h (76%); (iii)
NaN3, NH4Cl, DMF, 140 ◦C, 20 h (100%); (iv) BnBr, K2CO3, acetone, 60 ◦C, 24 h, 8 (46%) and 9 (42%); (v) malonyl dichloride, pyridine, THF, room temperature, 5 h, 10 (50%) and
11 (51%) and (vi) C60, I2, DBU, toluene, room temperature, 5 h, 12 (31%, 65% based on recovered C60) and 13 (40%, 70% based on recovered C60).
tive 5 with a 51% yield [22]. Treatment of 5 with potassium cyanide
and 18-crown-6 ether in CH3CN [23] gives a 76% yield of nitrile 6.
The [2+3] cycloaddition of sodium azide and nitrile 6 with ammo-
nium chloride in DMF [24] provides the 5-substituted tetrazole
7 in a quantitative yield. The structure of 7 is confirmed by 13C
NMR, and the typical chemical shifts of C-tetrazolic are observed
at 157.90 ppm [25]. Alkylation of 7 with benzyl bromide in dry
acetone and anhydrous K2CO3 produces mixtures of N(1)- and N(2)-
alkylation isomers [26]. After silica chromatography purification,
two isomers, 8 and 9, are isolated in 46% and 42% yield, respec-
tively. These isomers are distinguished by the 13C chemical shifts
of the N-alkyl group and verified with heteronuclear multiple bond
correlation (HMBC) experiments. The HMBC contour plot shows the
cross correlation between the tetrazole carbon and the CH2 group
linked to the vicinal nitrogen, due only to the 3J coupling constant
for 8. Isomers 8 and 9 are esterified separately with commercial
malonyl dichloride in pyridine and THF at room temperature, pro-
viding 10 and 11 in yields of 50% and 51%, respectively. Malonates
10 and 11 are subsequently transformed into their [60]fullerene
mono-adducts 12 and 13, respectively, via a Bingel reaction [27],
following the conditions described by Nierengarten et al. [28].
Fig. 3 depicts the alkylation of the commercial tetrazoles 14, 22,
23 and oxadiazole 15 with 12-bromododecane-1-ol or commer-
cial 6-chloroexan-1-ol. These reactions produce the alcohols 16, 17,
24(N-2), and 25(N-2) in high yields, which are esterified with com-
mercial malonyl dichloride to form the malonates 18, 19, 26(N-2),
and 27(N-2), respectively. These malonates are subsequently trans-
formed into their respective mono-adduct fullerene derivatives 20,
21, 28, and 29 by the Bingel-type reaction [28], respectively, with
yields between 36% and 42%. The structure and purity of all of
the mono-adducts are confirmed by 1H and 13C NMR spectroscopy
(these results are listed in the supporting information).
O
O
N
N
O
O
N
N
N
or
i
iii
N
iv
N
N
HO
R =
R
N
R
O
O
R
R
N
Ph
Ph
4
O
O
R
HS
HS
O
4
4
N
4
4
20R =
N
Ph
S
N
14
15
N
N
N
16
17
N
Ph
S
18 R =
19 R =
N
Ph
S
N N
R =
21
Ph
S
O
N N
O
N N
O
R =
S
Ph
+
S
Ph
iii
O
O
O
O
O
N
N
N
Y
N
N
ii
N
N
iv
N
N
5
X
R
R
2
O
O
R
O
R
5
X
10
10
N
10
5
10
1
Y
1
N
N
X
Y
N
N
N
24(N-1) X = Ph, Y =
26(N-2)
27(N-2)
24(N-2) X = Ph, Y =
25(N-2) X = H, Y =
R =
OH
OH
OH
OH
N
22 X = Ph,Y = H
23 X = H, Y = H
10
10
10
10
2
N
Ph
25(N-1) X = H, Y =
N
N
N
N
R =
N
N
N
28R =
Ph
2
N
N
N
29
R =
N
Fig. 3. Preparation of compounds 20, 21, 28 and 29. Reagents and conditions: (i) 6-chloroexan-1-ol, K2CO3, acetone, 60 ◦C, 48 h, 16 (90%), 17 (60%); (ii) 12-bromododecane-
1-ol, K2CO3, acetone, 60 ◦C, 24 h, 24(N-1) (10%), 24(N-2) (90%), 25(N-1) (38%), 25(N-2) (57%); (iii) malonyl dichloride, pyridine, THF, room temperature, 5 h 18 (48%), 19 (60%),
26(N-2) (55%), 27(N-2) (35%) and (iv) C60, I2, DBU, toluene, room temperature, 5 h, 20 (36%, 77% based on recovered C60), 21 (42%, 52% based on recovered C60), 28 (39%, 43%
based on recovered C60), and 29 (38%, 50% based on recovered C60).

L.J. Santos et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
187
25
20
15
10
5
150 min
30 min
dark
325
330
335
0
Magnetic Field (mT)
0
2
4
6
8
10
Illumination Time (x103 s)
Fig. 4. Time evolution of the absolute PBN spin adduct concentration in an illumi-
nated (UV laser: 375 nm, 16 mW, room temperature) toluene solution containing
1 mM C60 (ꢀ) or C60-PCBM (᭹), and 300 mM of the spin trap PBN. The insert shows
the EPR spectra for the C60 solution before illumination (dark) and after 30 and
150 min of illumination.
The 13C NMR spectra of the mono-adducts show the char-
acteristic signal [29] of the sp3-hybridized fullerene C atoms in
the cyclopropane ring between 70 and 85 ppm, in addition to
the signals between 136 and 150 ppm corresponding to the sp2-
hybridized fullerene sphere. The methano bridge signal is observed
in the region from 52 to 53 ppm.
The molecular compositions are also confirmed by ultra-high-
resolution Fourier transform mass spectrometry with electrospray
ionization. Molecular masses of all isotopologue ions within the
[M+H]+ clusters agree exceptionally well with calculated values
and are within less than 1 ppm of those values. The MS/MS anal-
ysis of the protonated molecules via dissociations induced either
by collisions or photons also shows structurally diagnostic dissoci-
ation patterns involving the heterocyclic wings, corroborating the
assigned structures of the new fullerene derivatives.
For optical characterization, the fullerene solutions (0.02 mM
in toluene) are placed in a standard quartz cuvette (10-mm thick-
ness), and their spectra are recorded at room temperature. A pure
air-saturated toluene sample is used for the baseline. All of the
spectra are very similar. In the visible range (wavelength above
400 nm), the absorption is weak, but it increases drastically at
shorter wavelengths (in the UVA and UVB ranges). The molar
extinction coefficient (ε) is calculated for all samples at 375 nm,
the wavelength of the laser diode used in the spin-trapping exper-
iments. With the extinction coefficients ε, the amount of the laser
power absorbed in the illuminated fraction of the sample vol-
ume of the EPR borosilicate sample tube (0.33 mL) is calculated
by the Beer–Lambert law, taking into account the laser spot diam-
eter (1.74 0.15 mm) and the height of the solution in the tube
(14.0 0.5 mm) (these results are listed in supporting information).
Fig. 5. Time evolution of the absolute PBN spin adduct concentration in the illumi-
nated toluene solutions containing fullerene derivatives (at 1 mM) and the spin trap
PBN (300 mM): (a) compounds 20 (᭹), 21 (ꢁ) and 28 (ꢀ); (b) compounds 12 (ꢀ), 13
(ꢀ) and 29 (ꢂ). The insert shows the EPR spectra for the compounds 21 (a) and 13
(b) solutions before illumination (dark) and after 150 min of illumination.
the same EPR hyperfine triplet due to the nitrone group in the PBN
molecule (14N, I = 1 and ∼100% natural abundance) with a hyper-
fine splitting aN of 1.54 0.01 mT and a g-factor of 2.007 0.001.
This hyperfine splitting is characteristic of final PBN-OH• spin
adducts, which are ex•p−ected after self-dismutase of superoxide
[30]. Both OH• and O2
PBN-adducts have been unambiguously
identified by their superhyperfine splitting in 17O-enriched sys-
•−
tems, i.e., aN = 1.48 mT for PBN-O2
and aN = 1.53–1.55 mT for
PBN-OH• [31].
The insert in Fig. 4 shows three typical EPR spectra, one in the
dark before illumination and two others after 30 and 150 min of
UVA laser illumination (375 nm and 16 mW). The error bars in the
absolute spin adduct concentration are derived from the uncertain-
ties in the fit parameters of the TEMPOL calibration curve.
Fig. 5(a) and (b) shows the time evolution curves for the con-
centration of the photogenerated PBN spin adducts for all of
the fullerene derivatives synthesized in this work. The same EPR
parameters (microwave frequency and power) and concentrations
are used for all of the fullerene derivative solutions. While Fig. 5(a)
shows the three compounds that have lower efficiency in generat-
ing free radicals and thus PBN spin adducts, Fig. 5(b) shows those
with higher efficiencies.
3.2. EPR spin-trapping studies
Fig. 4 shows the absolute concentration of PBN spin adducts
generated in a toluene solution of C60 or commercial C60-PCBM
as a function of illumination time. The EPR spectra are measured at
room temperature. The C60 and C60-PCBM concentrations are1 mM,
whereas the PBN concentration is 300 mM. The curves are calcu-
lated from a series of EPR spectra after different illumination times
using the TEMPOL calibration data. The EPR spectra from C60 and
all of the other investigated fullerene derivatives are dominated by

188
L.J. Santos et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
According to the well-known photo-oxidation type II mecha-
amount of superoxide produced at the same time (but to a lesser
degree by the type I mechanism) catalyzes the reduction of the
singlet oxygen into superoxide; and (iii) the formation of the PBN-
OH• spin adducts after the superoxide self-dismutase into OH•. For
higher concentrations of superoxide, the reduction runs faster. In
the end, all of the singlet oxygen is converted into superoxide, and
all of the dismuted superoxide generates PBN-OH• spin adducts.
The kinetic equation related to the reduction is
nism [32], the first step for the production of singlet oxygen (1O2)
is photoexcitation of the photosensitizers, which are the fullerene
compounds in this study. The photoexcited fullerenes enter their
higher excited singlet states and are converted by intersystem-
crossing into long-lived triplet states. From the triplet state, the
available excess energy is transferred to oxygen molecules, thus
forming 1O2. The fullerenes decay to their singlet ground state,
where they remain unchanged after this cycle. Although it is well
established that the 1O2 is the main ROS produced by the photoexci-
tation of fullerenes [32], the formation of superoxide (O2•−) radicals
may occur because of two reasons: (i) photo-oxidation mechanism
type I, which is the photo-assisted transfer of an electron from the
fullerene to the oxygen molecule during photoexcitation (which
may also occur in parallel to the type II mechanism) or (ii) the singlet
oxygen reduction process, which is its conversion into superoxide
after accepting an electron from another electron donor (such as the
photosensitizer molecule). The reduction process of singlet oxygen
into superoxide has been previously observed in biological systems
[33]. The dismutase of superoxide into highly reactive OH• [34,35],
in the absence of anti-oxidants, is what causes escalating damage
in biomolecules, cells and tissues by OH•, fundamental for PDT.
In a recent paper [20], we have indirectly observed the first
stages of this process (conversion of 1O2 into O2•−) for a tetrazole-
fullerene derivative in air-saturated toluene solutions (compound
5 in [20]), which is similar to 3 of this study (differing only in the
•
−
•
•
•
∂[O2
]
−
= K[1O2][O2 −] = K[O2 ](N − K[1O2][O2 −]),
(2a)
(2b)
∂t
•
[O2 −] = [PBN − OH−],
where K is the reaction time constant and N is the total number of
produced ROS species that are conserved in a closed system:
•
•
−
−
0
N = [1O2] + [O2 ] = [1O2]0 + [O2
]
= [PBN − OH• ].
(3)
The time evolution of the superoxide concentration or the PBN spin
adducts is given by the solution of Eq. (2):
•
[O2 −] = [PBN − OH• ] =
,
(4)
N
1 + R0e−KNt
] is the ratio between the amount of sin-
0
where R0 = [1O2]0/[O2
•−
glet oxygen and superoxide produced at the beginning of the
reduction. Instead of a single exponential curve, as considered in
our previous work [20], the sigmoid functions characteristic of
autocatalytic reactions show a better fit for all of the time evolution
curves. These are totally consistent with the type I and II reaction
mechanisms for the production of superoxide and singlet oxygen,
respectively, and they also take into account the efficient reduction
of singlet oxygen into superoxide. The fit parameters for the illu-
mination curves for all of the investigated compounds are shown
in Table 1.
Notably, the total amounts of ROS produced for all com-
pounds under illumination at saturation, measured in terms of spin
adducts/mL, are on the same order, between 0.7 and 3.4 × 1016
(column N, Table 1). These results confirm that the exohedral func-
tionalizations of the fullerenes via cyclopropanation with different
organic moieties do not significantly change the photochemi-
cal and photophysical properties of the fullerene molecule. This
number, however, does not reflect the individual efficiencies in
the formation of ROS because we illuminated the solutions with
monochromatic light (375 nm) and their molar extinction coeffi-
cients differ slightly (Table 1 and Fig. 4). Therefore, the absorbed
energy is also different for each sample, and the total amount of spin
adducts for each compound must be normalized by the absorbed
laser power. If we divide the absolute amount of spin adducts gen-
erated at saturation by the absorbed power and the number of
fullerene molecules, a merit figure can be calculated that allows
for comparison of the overall photosensitizer efficiencies for ROS
production for each fullerene compound. We refer to this merit
figure as the absolute ROS photogeneration efficiency, which is the
absolute amount of ROS produced for each fullerene molecule per
length of the side-chains). From these experiments we concluded
•−
that the main formation of O2
occurred via the type II mecha-
nism followed by the reduction of singlet oxygen into superoxide.
The type I mechanism also occurred but to a lesser degree. For that
reason, and because PBN-1O2 paramagnetic adducts are not known
[36], we have performed PBN spin-trapping experiments under
UVA illumination in the presence of two distinct ROS inhibitors:
superoxide dismutase (for O2•−) and beta-carotene for (1O2).
In this work, we examine in detail the formation of singlet
oxygen and superoxide under UVA illumination of toluene solu-
tions containing the new fullerene derivatives and compare the
results to pure C60 and C60-PCBM solutions. All of the illumination
curves (Figs. 4 and 5) formed after the self-dismutase of superoxide
(and therefore the evolution of the amount of identified PBN-OH•
adducts), as a function of illumination time, are carefully fitted
to a sigmoid function, which is characteristic of irreversible auto-
catalytic reactions A + B → 2B [37]. In our case, the autocatalytic
reaction can be viewed as the reduction of the singlet oxygen into
superoxide in a closed system, with a further one-to-one conver-
sion of superoxide into OH•, which is captured by the PBN. The
autocatalytic reaction is given by
•
−
O12 + O•− + e− → 2O2
,
(1)
2
For this model, a number of assumptions are made: (i) that the type
II mechanism is dominant at the beginning of the illumination, pro-
ducing much more singlet oxygen than superoxide; (ii) the small
Table 1
Fit parameters for the illumination curves using an autocatalytic sigmoidal curve: the total number of ROS species produced N, the kinetic constant K, the ratio between the
amount of singlet oxygen and superoxide produced at the beginning of the reduction R0, the monochromatic efficiency and the coefficient of determination of the fit r2.
Compound
N (1016 spin
adducts/mL)
K (104 mL/mol s)
R0
Efficiency at 375 nm
r2
(10−6 mW−1
)
28
20
21
29
13
12
C60
C60-PCBM
1.31(2)
0.70(4)
2.22(7)
2.78(3)
2.38(9)
3.34(4)
1.84(4)
2.04(6)
2.7(2)
3.6(7)
1.5(1)
1.40(6)
1.4(2)
1.64(9)
2.1(1)
2.2(2)
10.8(7)
3.4(4)
16(2)
8.1(4)
14(2)
13(1)
9.2(6)
10(1)
0.58(2)
0.17(2)
1.13(6)
1.32(4)
1.82(9)
2.39(6)
0.92(4)
0.54(4)
0.995
0.989
0.996
0.950
0.992
0.998
0.987
0.996

L.J. Santos et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 184–190
189
1 mW of monochromatic illumination at 375 nm. Despite the fact
that this merit figure varied roughly within an order of magnitude,
as shown in Table 1, we can still draw some conclusions comparing
all of the investigated fullerene compounds: (i) pure fullerene is
not the most efficient photosensitizer among the molecules inves-
with the latter being dominant in the initial stage of illumination.
In the saturation stage, all ROS produced are converted into OH•
via the self-dismutase of superoxides. These results show that the
hydrophobic fullerene compounds are potential photosensitizers
for topical PDT.
tigated; (ii) the more efficient photosensitizers, compared to C60
,
are compounds 12 and 13. Before further discussing these conclu-
sions, we must also note that the more efficient photosensitizers
also have smaller kinetic reaction constants K, indicating that they
are faster in the production of ROS and therefore saturate more
rapidly. In addition, R0, which quantifies how much of the type II
mechanism occurs in comparison with type I, is also higher for 12
and 13.
Acknowledgements
The authors are grateful to the Brazilian research foundations
CAPES, CNPq, FAPEMIG, FAPESP and the Instituto de Nanotecnolo-
gia (MCT) for research grants and financial support.
Appendix A. Supplementary data
Pure fullerene, with its high efficiency in the production of sin-
glet oxygen, is not the most efficient photosensitizer among those
we have investigated. The fact that the [60]fullerene may aggregate
in solution diminishes the exposition of the molecules to the dis-
solved oxygen, thus reducing its efficiency in the formation of ROS.
Therefore, functionalization with long moieties likely hinders the
aggregation and enhances ROS production. Although compounds
12 and 13 have similar structures, both with a benzyl group bound
to the tetrazol unit by an exposed carbon, they exhibit higher ROS
photogeneration efficiencies. Apparently, the presence of a benzyl
group is important for increasing the production of ROS, which may
be due to a higher solubility of these compounds or to the special
reactivity of the benzyl group.
For all of the functionalized fullerene compounds, the dominant
photophysical process is a type II mechanism in non-polar solvents,
leading to the production of singlet oxygen, even though it was
recently shown that the type I mechanism is dominant in aqueous
solutions [38]. Despite this fact, superoxide radicals (and OH•) are
also final products even in organic solvents. Here it is important to
remember that fullerenes are good electron acceptors in dark con-
ditions; however, under UVA light, they may also donate electrons
created by photoionization. The high efficiency for ROS production
in these compounds makes them promising photosensitizers for
topic PDT and, in particular, for special pharmaceutical formula-
tions in sunscreens or other lotions that do not require high water
solubility [39–42].
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.10.007.
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