Single and double reduction of C60 in 2:1 γ-cyclodextrin/[60]fullerene inclusion complexes by cyclodextrin radicals

Article abstract of DOI:10.1016/j.chemphys.2008.10.014

Spectroscopic and chemical properties of γ-CD^{{radical dot}} radicals, resulting from the abstraction by HO^{{radical dot}} radicals of hydrogen atoms, have been investigated using pulse radiolysis. The reactions of γ-CD^{{radical dot}}

Full text of DOI:10.1016/j.chemphys.2008.10.014

Chemical Physics 354 (2008) 174–179
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Single and double reduction of C in 2:1
c-cyclodextrin/[60]fullerene
6
0
inclusion complexes by cyclodextrin radicals
Annamaria Quaranta , Yongmin Zhang , Yali Wang , Ruth Edge c,d, Suppiah Navaratnam c,e
a,*
b,*
b
,
f
a,
*
Edward J. Land , René V. Bensasson
a
Laboratoire de Chimie et Biochimie des Substances Naturelles, MNHN, UMR5154 CNRS-MNHN/USM50502, 63 rue Buffon, 75005 Paris, France
Université Pierre and Marie Curie, Laboratoire de Chimie Organique, UMR 7611 CNRS, Tour 44/45, C. 181, 4 place Jussieu, 75005 Paris, France
Free Radical Research Facility, STFC Daresbury Laboratory, Warrington,WA4 4AD, UK, and North East Wales Institute, Wrexham, LL11 2AW, UK
Free Radical Research Facility, STFC Daresbury Laboratory, Warrington WA4 4AD, UK
BioScience Research Institute, PEEL Building, University of Salford, Salford M5 4WT, UK
Lennard-Jones Laboratories, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
b
c
d
e
f
a r t i c l e i n f o
a b s t r a c t
Å
Å
Article history:
Received 6 June 2008
Accepted 15 October 2008
Available online 22 October 2008
Spectroscopic and chemical properties of
c
-CD radicals, resulting from the abstraction by HO radicals of
Å
hydrogen atoms, have been investigated using pulse radiolysis. The reactions of
c
-CD radicals with C60 in
2
the
:1
c
c
-CD/C60 inclusion complexes have been studied in aqueous solutions. It has been demonstrated that
-CD radicals are reducing species producing C6 monoanion radicals, as well as doubly reduced C
well characterised by their absorption spectra in the near IR. The oxidation potential of
Å
Åꢀ
2ꢀ
,
0
60
Å
c
-CD radical is
-CD radicals
Keywords:
Å
estimated to be more negative than ꢀ390 mV vs. NHE. The kinetics of the C60 reduction by
c
2
:1 c-Cyclodextrin/[60]fullerene inclusion
have been determined and compared with kinetics by other reducing species including the solvated elec-
complexes
Pulse radiolysis
Oxidised c-cyclodextrin
Fullerene reduction
ꢀ
Åꢀ
tron (e ) and CO2 radicals. It was observed that the method of preparation of the 2:1
complexes modifies the C60 reduction mechanism.
c
-CD/C60 inclusion
aq
Ó 2008 Elsevier B.V. All rights reserved.
Reduction potentials
Åꢀ
C
C
9
6
2
0
ꢀ
6
0
,10-Anthraquinone-2-sulfonate ion
1
. Introduction
as solvent of C60. These authors displayed the transient species
resulting from reactions of these complexes with HO radicals, in
Å
Cyclodextrins (CD), water-soluble cyclic oligomers of glucose,
have a hydrophobic cavity able to form inclusion complexes with
hydrophobic molecules [1]. They are employed as drug carriers
the spectral range 200–500 nm. From the data obtained, they sug-
gested two possible reactions: (i) addition of HO to C60 double
Å
bonds, and (ii) abstraction of an H atom from
c
-CD, leading to
c
-
Å
[
2–6], studied in recognition processes [7,8] and for applications
CD radicals which subsequently adds to C60, acting as a radical
sponge. These authors also indicate that the radical product of this
addition has a kmax at 290 nm and that its formation kinetics
matches a C60 bleaching at 330–335 nm.
in nanodevice construction [9]. These CD are also an interesting
matrix for spectroscopic studies since they are transparent
throughout a part of the IR, and in the visible and near–UV spectral
region. The internal diameters of the largest CD, the
are constituted by 8 glucose units, allow the formation of 2:1
CD/[60]fullerene inclusion complexes ( -CD/C60) [10,11].
Previously, Priyadarsini et al. [12] performed pulse radiolysis
studies of -CD/C60 inclusion complexes prepared using methanol
c
-CD which
Our investigation has a triple aim: (i) extension of the 200–
500 nm spectral region, studied by Priyadarsini et al. [12], to
the IR, a spectral region where absorption changes are a probe
c-
c
Åꢀ
2ꢀ
for identifying the presence of
C60 anions,
C
and C
,
(ii)
6
0
60
Å
c
demonstration that
c
-CD radicals can be reducing species,
and discussion of the observed reactions using the analysis
of Swallow [13] and Adams et al. [14] on products of glucose
radiolysis in aqueous solutions, (iii) discussion of the reduction
mechanism of C60 in inclusion complexes for two different
preparation methods, using either methanol or toluene to
*
Corresponding authors. Present address: iBiTec-S, CEA Saclay, Bât. 532, 91191
Gif-sur-Yvette Cedex, France. Tel.: +33 1 69 08 26 33; fax: +33 1 69 08 87 17 (A.
Quaranta).
E-mail addresses: annamaria.quaranta@cea.fr (A. Quaranta), yongmin.zhang@
upmc.fr (Y. Zhang), rvb@mnhn.fr (R.V. Bensasson).
solubilise C60
.
0
301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2008.10.014

A. Quaranta et al. / Chemical Physics 354 (2008) 174–179
175
2
. Experimental
monochromator (bandwidths 10–40 nm) were observed as a func-
tion of time before and after the radiation pulse using photoelectric
detection. The output of the photomultiplier (EMI 9558Q) was dis-
played on a Tektronix TDS 380 digitizing oscilloscope. Data pro-
cessing was performed on a Dan 486DX33 PC (UK) fitted with
Pentium type processors, using software developed in house.
Absorbed doses, between 2 and 21 Gy, were determined from
2.1. Materials
Chromatographically pure fullerene, C60, was purchased from
Syncom (Groningen, Holland), the 9,10-anthraquinone-2-sulfonate
AQS) from Lancaster Research Chemicals (UK), and the -cyclodex-
trin ( -CD), as hydrate C48 O, from Acros (99% purity,
(
c
Åꢀ
ꢀ2
c
80
H O
40 ꢁ xH
2
the transient ðSCNÞ
2
formation in air-saturated 10 M KSCN as
value of
Geel, Belgium). Ultrapure water from a Pur1te Neptune Analytical
Process Flow model was used to make all aqueous solutions.
described by Adams et al. [14], but using the updated G
e
ꢀ
4
2 ꢀ1
2.59 ꢃ 10 m J obtained by Buxton and Stuart [22], G being
Åꢀ
the radiation chemical yield of ðSCNÞ and
e its molar absorption
2
2.2. Preparation of the
c
-CD/C60 inclusion complexes
-CD/C60 inclusion complexes
coefficient at 475 nm. Saturation of such solutions with N
2
O results
Åꢀ
in a doubling of the ðSCNÞ2 yield.
The method we used to make
c
ꢀ
Å
Å
was derived from the protocol of Yoshida et al. [15] and employs
2.5. Generation of the primary radicals (eaq, H and HO ) by pulse
toluene to solubilise C60. A mixture of two solutions consisting of
radiolysis
3
C
60 (20 mg, 27.8
l
mol) in toluene (10 cm ) and
c
-CD (67 mg,
3
ꢀ9
5
2
l
mol) in water (10 cm ) was refluxed at 118 °C for 30 h under
Within 10 s of high energy radiation deposition, the radiolysis
products of water present at neutral pH are the reducing radicals:
vigorous mechanical stirring. After cooling to room temperature,
the aqueous layer containing precipitated -CD/C60 complex was
centrifuged. The precipitate was then dissolved in cold H
ꢀ
Å
c
hydrated electron (eaq) and hydrogen atom H , the oxidising radical
Å
O
HO , and the molecular products: hydrogen peroxide and molecular
2
3
(
10 cm ) and centrifuged at 10 °C (for 30 min at 4000g), in order
hydrogen:
to remove as much free
cipitate was dried under vacuum for 15 h. The final
c
-CD as possible. After filtration, the pre-
Å
Å
ꢀ
H
2
O , H þ HO þ e þ H
2
þ H
2
O
2
ð2Þ
aq
c
-CD/C60 com-
plexes were obtained as purple crystals. The quality of the
complexes obtained was evaluated by the UV–visible spectra in
water, which displayed the characteristic absorption bands of C60
in organic solvents. It has to be noted that it is difficult to avoid
The radiolysis yields (G) expressed in radical or molecules
formed per 100 eV are [23]
ꢀ
Å
Å
e ðG ¼ 2:65Þ þ H ðG ¼ 0:60Þ þ HO ðG ¼ 2:8Þ þ H
2
ðG ¼ 0:45Þ
aq
þ H
2
O
2
ðG ¼ 0:68Þ
self-association of
c-CD in aqueous solutions [3].
We have observed that organic solvents present in 2:1
c
-CD/C60
Å
For generating almost exclusively HO radicals (ꢂ92%), aqueous
inclusion complexes cannot be easily eliminated. For this reason,
ꢀ
solutions were saturated with N
2
O which converts e into hydro-
aq
we have preferred the Yoshida to Priyadarsini preparation because
xyl radicals by dissociative electron attachment
Å
residual methanol competing with
c
-CD in the reaction with HO
N
2
O þ eꢀaq þ H
2
O ! N
2
þ HO þ HO
Å
ꢀ
ð3Þ
Å
radicals, might give CH
2
OH radicals, reducing agents (E (CH
2
O,
+
Å
2
H / CH OH) = ꢀ920 mV [16]) able to reduce C60 via
In such conditions the remaining radicals (ꢂ8%) are reducing
Å
OH ! C Å6 ꢀ
CD þ HCOH þ Hþ
hydrogen radicals [24].
C
60
=
c
CD þ CH
2
=c
ð1Þ
0
For generating almost exclusively hydrated electrons, e , HOÅ
ꢀ
aq
Å
Å
radicals are removed by tert-butanol to form CH (CH ) COH radi-
In the Yoshida preparation, the HO radical may react with
2
3 2
cals which are normally very unreactive.
residual toluene mainly by addition to a double bond to form
Å
OH CH
6
CH
3
[17,18] which is not a reducing radical in contrast to
Å
Åꢀ
Å
2.6. Generation of
c
-CD and CO2 radicals
2
CH OH radicals generated in the case of the Priyadarsini
preparation.
Å
2.6.1. Generation of
c
-CD radicals
In irradiated water saturated with N
2
O, primary hydrogen (HÅ)
2
.3. Spectrophotometry
Å
and hydroxyl (HO ) radicals may react with
c-CD to produce c-
Å
CD radicals. This reaction will be discussed in detail in Section 3.2.1.
Ground state absorption spectra were recorded on a Varian Cary
spectrophotometer (bandwidth: 2 nm) using 10 mm quartz cuv-
ette. The concentration of -CD/C60 inclusion complexes in water
was estimated by assuming for C60
being similar to C60 in a non-polar environment like cyclohexane,
Åꢀ
2
.6.2. Generation of CO2 radicals
c
ꢀ1
ꢀ1
In irradiated water saturated with N O and in presence of a high
concentration (0.1 M) of sodium formate, the HO radicals produce
CO radicals, which are reducing species
,
e
G
(330) ꢂ 50,000 M cm
,
2
Å
Åꢀ
ꢀ
1
ꢀ1
e
c
G
(330) = 49,500 M cm [19]. Yoshida et al. [15] obtained for
2
ꢀ1
ꢀ1
-CD/C60
e
G
(330) = 42,700 M cm
.
O þ eꢀ þ H
Å
ꢀ
N
2
2
O ! N
2
þ HO þ HO
ð4Þ
ð5Þ
aq
2.4. Pulse radiolysis set up
Å
ꢀ
O þ CO Å2 ꢀ
HO þ HCO ! H
2
2
Å
Åꢀ
The pulse radiolysis experiments were carried out with a
2 MeV Radiation Dynamics Ltd. (UK) electron linear accelerator.
The concentration of HO and CO radicals was estimated to be
2
1
between 0.8 and 5
. Results and discussion
.1. Ground state absorption spectra of
The ground state absorption spectra of the
lM.
We used single pulses of duration 0.22–2 ls and with a peak cur-
rent of about 30 mA. The accelerator is normally operated at 10
pulses per second but the single pulse mode is achieved by modi-
fying the pulses to the electron gun [20]. The detection system con-
sisted of a Xe arc lamp and pulsing unit, high radiance Kratos
monochromator and quartz optics. The sample cell, constructed
by Spectrosil quartz, had an optical path length of 25 mm [21].
Optical transmissions at various wavelengths selected with the
3
3
c-CD/C60 inclusion complexes
c
-CD/C60 inclusion
complexes prepared by the Yoshida and Priyadarsini methods are
reported in Fig. 1a and b.

1
76
A. Quaranta et al. / Chemical Physics 354 (2008) 174–179
Å
Å
These spectra exhibit three intense peaks at 213, 260, 332, a
weaker peak around 408 nm and a large band between 410 and
00 nm, which are spectral features related to (i) the allowed sin-
glet–singlet transitions of the C60 molecule observed in the 210-,
60-, 330- and 404–408 nm regions in n-hehane, cyclohexane, 1-
In irradiated water saturated with N
erated by reactions (2) and (3), abstract H atoms which are
hydroxyl group in -CD, as in the case of simple alcohols [31]. This
abstraction gives three types of
low only one glucose monomer out of the 8 units constituting
2
O, H and HO radicals, gen-
a- to a
7
c
Å
c
-CD radicals by reaction (6). Be-
2
octanol solutions, both in energy and transition strength [25,26]
and (ii) a set of weak forbidden transitions observed in the 410–
the
c-CD molecules is represented
7
00 nm observed in n-hexane, toluene and micellar solutions
25,26].
The
days at room temperature, and for more than three months at
°C, as shown by their unmodified ground state absorption spec-
tra. Several systems might coexist depending on the relative con-
centration of cyclodextrin and C60, the type of preparation, the
time and temperature of storage, and the presence or exclusion
of air [10,27–29]. In agreement with previous studies, we con-
firmed that mainly two forms of complexes resulted between C60
[
ð6Þ
c
-CD/C60 inclusion complexes remained stable for several
Å
4
These three
c
-CD radicals are those suggested by Swallow [13] for
Å
the reaction of HO with glucose. The rate constants of reaction of
Å
Å
7
ꢀ1 ꢀ1
glucose with H and HO are, respectively, k = 6.1ꢃ10 M
s
9
ꢀ1 ꢀ1
[32], and k = 1.5 ꢃ 10 M
s
[32]. Thus, the reaction of
c
-CD with
Å
HO radicals is preponderant not only because of the faster rate con-
stant but also because HO radicals are ꢂ10 times more concen-
Å
Å
and
two
c
c
-CD: (i) one featuring the C60 included in the cavities of
-CD, the solution having a magenta colour, typical of
trated than H .
Å
Å
ꢀ5
Reaction of HO with
c
-CD (3 ꢃ 10 M) gives the transient
non-aggregated C60, with the ground state absorption spectrum
exhibiting a small, sharp and characteristic peak at 408 nm and
spectrum shown in Fig. 2, and occurs with a rate constant
9
ꢀ1 ꢀ1
k = 0.9ꢃ10 M
s , which is lower than that reported in the liter-
9
ꢀ1 ꢀ1
(
[
ii) a second form, partially consisting of non-inclusion complexes
30], where the C60 molecules are associated in clusters surrounded
ature (k = 5ꢃ10 M
s
) [33]. The average molar absorption coef-
Å
Å
ficient
e
k
(c
-CD) of the three
c
-CD radicals at wavelength k was
by several
c-CD [10,29], the solution being yellowish-brown, a typ-
calculated by the Beer–Lambert expression below:
ical colour of C60 aggregates without the 408 nm peak in the
absorption ground state spectrum. In all our investigations, we
exclusively used the first type of complex.
DA
k
HO ꢄ‘
Å
e
k
ð
c
-CDÞ ¼
þ
e
k
ðc
-CDÞ
ð7Þ
Å
½
where
molecule,
e
k
(
c
D
-CD) is the molar absorption coefficient of the parent
Å
Å
3
C
.2. Spectroscopic and chemical properties of
c
-CD radicals in
c
-CD/
Ak is the maximum transient absorbance of the
c
-CD
Å
60 inclusion complexes prepared via the Yoshida protocol
radical at wavelength k, l is the optical path in cm, and [HO ] is
the molar concentration of the hydroxyl radical.
We have measured at 320 nm the
for different -CD concentrations, considering that a sample of
CD hydrate contains 12.9% by weight of water molecules on average
e
k
(c-CD) in aqueous solutions
Å
3
.2.1. Generation and spectroscopic properties of
c
-CD radicals
c
c-
Å
The reaction of HO radicals with
c
-CD/C60 inclusion complexes
was investigated using complexes made via the Yoshida method.
ꢀ1
ꢀ1
[1]. The values obtained for e320
CD] = 10 M, 60 M cm for [c-CD] = 10 M, and 20 M cm
(
c
-CD) are: 360 M cm for [
c
-
ꢀ5
ꢀ1
ꢀ1
ꢀ4
ꢀ1
ꢀ1
ꢀ
3
for a concentration of [c-CD] = 10 M. The decrease of the
e
k
value
observed when
c
-CD concentration increases, called hypochro-
a
0
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
.00
mism, is observed in dimers and in chromophore aggregates for
stacked molecules of adenine, tyrosine, tryptophan, retinol, por-
phyrin and anthracene. Quantum mechanical treatment of hypo-
chromism theory has been described by Cantor and Schimmel
3
2
1
ꢀ
5
[34]. Using for
e
320
(c
-CD) the value found for [
c
-CD] = 10 M, the
Å
molar absorption coefficient of the (
c-CD) radical at 320 nm
4
00
500
600
700
800
0
0
0
0
0
.010
.008
.006
.004
.002
0
3
0
.4
.3
.2
.1
.0
b
0
0
0
0
2
1
400
500
600
700
800
0
2
00
300
400
500
600
700
800
0.000
3
20
340
360
380
400
420
440
wavelength / nm
wavelength / nm
Fig. 1. Absorption spectra of 2:1
prepared by (a) Yoshida method followed by one (gray) or two centrifugations
black), and (b) Priyadarsini method.
c-CD/C60 inclusion complexes in aqueous solution
Fig. 2. Differential spectrum obtained upon pulse radiolysis of a N
aqueous solution containing 30 M of -CD, 200 s after pulse.
2
O saturated
(
l
c
l

A. Quaranta et al. / Chemical Physics 354 (2008) 174–179
177
Å
Å
e
320
(
c
-CD)
was
estimated
to
be
approximately
tion of hexoses with HO , in the absence of O
glucosones.
2
, are mainly
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢅ760 M cm ± 150 M cm with the assumption that every
Å
HO hydroxyl radical reacts with one
c-CD molecule.
The observation of reaction (8) allows us to assess that the oxi-
Å
dation potential of the
c
-CD radical is more negative than
Å
Å
3
.2.2. Reaction of
In irradiated N
-CD/C60 inclusion complexes prepared by the Yoshida method,
s after
c
-CD radicals acting as reducing agents of C60
ꢀ250 mV because
c
-CD is capable of reducing the inclusion com-
-CD/C60 which has a reduction potential
O saturated aqueous solution containing 14
lM
plex 2:1
c
2
of
c
Åꢀ
E
7
ð2 : 1
c-CD=C60=2 : 1
c
-CD=C Þ ¼ ꢀ250 mV vs: NHE ½33ꢄ:
6
0
we observed the transient difference spectra, at 9 and 439
l
the electron pulse, shown in Fig. 3. These spectra exhibit (a) large
The rate constant of formation of the
C
60 anion is (i)
>1010
M
ꢀ1
s
ꢀ1
by solvated electrons in 2-propanol [38] and (ii)
absorptions at ꢂ350 nm, (b) broad low absorptions in the region
1
0
ꢀ1 ꢀ1
3
50–700 nm, (c) two infrared bands with kmax at 940 and
about 1.9 ꢃ 10
M
s
by hydrated electrons in the 2:1 c-CD/
1
080 nm (Fig. 3) and (d) kinetic differences at 940 and 1080 nm
C
c
60 inclusion complex [12]. For the C60 internal reduction by the
Å
suggesting the presence of two species, which will be discussed be-
low in the kinetic analysis section.
-CD radical in the inclusion complex, we obtained a first order
rate constant of 1 ꢃ 10 s
4
ꢀ1
.
The infrared peaks at 940 and 1080 nm are characteristic of the
anion in benzonitrile [35], dichloromethane [36], meth-
Åꢀ
Å
C
3.2.3. Reaction of
c
-CD radicals acting as reducing agents of AQS
6
0
Å
yltetrahydrofuran [37], toluene [38,39], toluene/2-propanol/ace-
tone (8:1:1) [38], 2-propanol [40] and toluene/CH Cl mixtures
-CD/C60 inclusion com-
In order to confirm that
c-CD radicals are reducing species, we
replaced C60 with another electron acceptor, 9,10-anthraquinone-
2-sulfonate ion (AQS). We irradiated by pulse radiolysis an N O-
saturated AQS aqueous solution in the presence and in the absence
2
2
[
41]. They have also been observed for
c
2
ꢀ
plexes reduced by e [40]. The full width at half maximum
aq
(
FWHM) at 1080 nm in our experimental conditions (40 nm mono-
of
c
-CD, and observed the differential spectra shown in Fig. 4.
Å
chromator bandwidth in the IR) is ꢂ100 nm, with a ratio of the
The spectrum of the species produced by HO radicals in the
absorbance maxima,
D
A at 1080 nm/
D
A at 940 nm ꢂ 3. It is inter-
presence of
c
-CD (10 mM) and AQS (10
l
M) (Fig. 4a) is identical
Åꢀ
ꢀ
esting that the C6 absorption band profile at 1080 and 940 nm is
not significantly modified by the
pared to organic solvents. The
to that observed upon reaction of hydrated electron e with AQS
(Fig. 4b). This spectrum is that of AQS anion previously character-
ised [16,43]. In the absence of
with AQS gives a spectrum (Fig. 4c) different from that of AQS .
Therefore AQS reduction can be attributed to
possible to establish that the oxidation potential of the
0
eq
Åꢀ
c
-CD environment when com-
Å
Å
c
-CD radical is transformed, after
c
-CD, the reaction of HO radicals
Åꢀ
reduction of C60, into a non-radical species
c
-CDox via:
Å
c
-CD radicals. It is
Å
-CDox=C þ Hþ
Åꢀ
c
-CD =C60
!
c
ð8Þ
Å
6
0
c-CD rad-
where
c
-CDox is a non-radical species resulting from two consecu-
icals is more negative than the reduction potential of AQS, E
7
(AQS/
Åꢀ
tive oxidations of a glucose unit of
H atom by HO leading to the
c
-CD after (i) abstraction of an
AQS ) = ꢀ390 mV vs. NHE [44].
Å
Å
Å
c
-CD radical via reaction (6), and
Reduction of AQS by the radical
c
-CD occurs with a bimolecular
Å
8
ꢀ1 ꢀ1
(
ii) transfer of an H atom from the
c
-CD radical to C60 followed
rate constant of ꢂ6.2 ꢃ 10 M
s , lower than that by the hy-
Å
ꢀ
by a deprotonation of C60H (or electron transfer from
c
-CD radical
drated electron,
e
aq
,
for which the rate constant is
1
0
ꢀ1 ꢀ1
to C60 followed by deprotonation). Reaction (8) leads to the three
possible -CD oxidised products -CD (2-dehydro- -glucose, 3-
dehydro- -glucose and 6-dehydro- -glucose) shown below,
where only one doubly oxidised glucose unit is represented:
2.7 ꢃ 10
M
s . This is due to the difference between the one-
electron oxidation potentials of the two reducing species: the
CD radical and the solvated electron.
c
c
a-
D
c-
Å
a-
D
a-
D
Å
3
.2.4. Addition of
c
-CD radicals to C60
Å
It can be considered that
c-CD radicals may also add to C60 dou-
ð9Þ
ble bonds as demonstrated for alkyl [45,46], benzyl [45,47,48],
methyl [47,49], phenyl [50], peroxyl [51] radicals. Under our
experimental conditions, it is difficult to assess the fraction of
The formation of these doubly oxidised glucose units is suggested
by the observation of Phillips [42], that the final products of reac-
0
0
.025
.020
4
9
4
µs
39 µs
3
2
1
0
1
2
b
a
0.015
c
0
0
0
.010
.005
.000
.
a
b
AQS +γCD radical
-
aq
AQS + e
.
-
-
c
AQS + HO radical
3
50
400
450
500
550
600
3
00 400 500 600 700 800 900 1000 1100
wavelength / nm
wavelength / nm
Fig. 4. Differential spectra obtained upon pulse radiolysis of 10
solutions (a) in presence of 10 mM -CD in a N
pulse: 400 s); (b) in N saturated solution with 1% tert-butanol (h, delay from
pulse: 50 s); (c) in N s).
O saturated solution (4, delay from pulse: 284
lM AQS aqueous
Fig. 3. Differential spectrum obtained upon pulse radiolysis of a N
aqueous solution containing 14 M of -CD/C60 inclusion complex prepared by
Yoshida’s method at a dose of 20 Gy.
2
O saturated
c
2
O saturated solution (d, delay from
l
c
l
2
l
2
l

1
78
A. Quaranta et al. / Chemical Physics 354 (2008) 174–179
Å
these
c
-CD radicals which would add to C60 and thus would not
3.4. Kinetic analysis of the reduction of C60 in
complexes by other species
c-CD inclusion
act as a reducing species.
ꢀ
Åꢀ
3
.2.5. Comparison with the pulse radiolysis results obtained by
Reactions of
c
-CD/C60 with reducing species e and CO2 were
aq
Åꢀ
Priyadarsini et al. and Dimitrijevi ´c and Kamat
also examined and confirm the formation of C 0, accompanied by
6
2ꢀ
As mentioned in Section 1, Priyadarsini et al. [12] have previ-
the doubly reduced C , at doses higher than 12 Gy.
60
Å
ously studied the reaction of HO radical with
complexes, prepared using methanol as solvent. These authors
investigated this system in the spectral region from 200 to
c
-CD/C60 inclusion
ꢀ
3.4.1. Reduction by the hydrated electron, e
aq
The transient absorption spectrum obtained after reaction of
ꢀ
5
00 nm and did not include spectral observations in the IR, which
the hydrated electron, e , with a solution of 26
l
M of
c
-CD/C60
aq
Å
here allowed us to discover that
species for C60 as well as for AQS molecules. In the region between
c
-CD radicals can act as reducing
complexes is similar to that shown in Fig. 3 for reduction by
CD radicals. Similarly, at radiation doses higher than 12 Gy be-
c-
Å
3
00 and 500 nm, the spectral features which we have observed for
tween 6 and 89 ls a decay of absorbance at 830 nm concomitant
with a growth at 1080 nm were observed, as well as an unchanged
Å
a C60 reduction by
c-CD are rather similar to those observed by
Dimitrijevi c´ and Kamat [33] for a C60 reduction by hydrated elec-
absorbance at 940 nm. These modifications are attributed once
Å
2ꢀ
Åꢀ
trons and (CH
3
)
2
COH radicals.
more to the transformation of C60 into C , via reaction (10). The
60
Åꢀ
molar absorption coefficient of the C6 at 1080 nm, calculated by
0
3
.3. Kinetic analysis of the reduction of C60 in
c
-CD inclusion
equation (11)
Å
complexes by c-CD radicals
DA
k
½eaqꢄ‘
Åꢀ
e
k
ðC Þ ¼
ð11Þ
6
0
ꢀ
Å
At radiation doses below 12 Gy, reaction of
embedded in
c
-CD with C60
ꢀ
1
ꢀ1
c-CD leads to the concomitant formation of the two
is e1080 P 11,000 M cm , consistent with the values found for
4
ꢀ1
Åꢀ
ꢀ1
ꢀ1
infrared absorption bands at 940 and 1080 nm (4 ꢃ 10 s ) due
C
2 2
in dichloromethane (12,000 M cm ) [36] and CH Cl /tolu-
6
0
Åꢀ
ꢀ
1
ꢀ1
exclusively to the monoanion C6
0
.
ene (12,000 M cm ) [41] solutions.
At doses between 12 and 21 Gy, we observe between 9 and
39 s that the absorbance at 825 nm decreases, at 940 nm re-
mains almost constant and at 1080 nm grows (Fig. 3). The spec-
Åꢀ
4
l
3
.4.2. Reduction by CO2 radicals
The transient absorption spectrum obtained after reaction of
CO radicals with a solution of 60
shown in Fig. 6 is also similar to that observed for reduction by
-CD radicals (Fig. 3). At radiation doses lower than 12 Gy, growth
Åꢀ
trum observed at 9
l
s can be interpreted as due to the presence
l
M of
c
-CD/C60 complexes
2
Åꢀ
of C6 monoanion characterised by its 1075 nm band and of the
0
2
6
ꢀ
Å
doubly reduced C , characterised by its absorption bands at 950
0
c
4
ꢀ1
and 830 nm [52]. The spectrum observed at 439
l
s is that of the
at 950 and 1080 nm occurs with the rate constant ꢂ1.5 ꢃ 10 s
.
Åꢀ
C
radical monoanion.
Fig. 5 shows the absorption spectra of C60 and of C60 which
60
For dose of 2 Gy, a molar absorption coefficient is estimated to
Åꢀ
2ꢀ
ꢀ1
ꢀ1
ꢀ1
ꢀ1
be
ical of C60 monoanion absorption in organic solvents. At higher
radiation doses (12–21 Gy), for -CD/C60 concentration varying be-
tween 7.5 and 75 M, and for pH varying between 5 and 9, C60
reduction invariably occurs in two steps: following a fast formation
e1080 = 12,225 M cm and e980 = 3700 M cm , values typ-
2
6
ꢀ
superpose in the region 800–1050 nm. The doubly reduced C
0
present at 9
l
s disappears with time and generates monoanions
c
Åꢀ
C
via a reduction of neighbouring inclusion complexes:
60
l
2
6
ꢀ
Åꢀ
c-CDox=C
þ
c
-CD=C60 ! 2ð
c
-CD=C Þ
ð10Þ
0
60
4 ꢀ1
(
1.5 ꢃ 10 s ), a further slow absorbance growth is observed at
2
6
ꢀ
In Fig. 3, absorbance around 825 nm, mainly due to C , de-
1080 nm while the transient absorbance around 940 nm and
0
2
ꢀ
creases and is accompanied by an absorbance growth of the band
800 nm, due partially to C , decays as shown in Fig. 6. The exper-
60
imental conditions of this reduction by CO2 radicals shows more
clearly the 940 nm band decay than in the case of the reduction
Åꢀ
Åꢀ
peaking at 1080 nm, due to C60, while the absorbance around
9
40 nm remains almost constant. In this latter spectral region,
2ꢀ
Åꢀ
Å
ꢀ
the C6 absorbance decay is compensated by a C60 absorbance
by
c
-CD radicals or by the hydrated electron e
.
0
aq
growth.
2
2
1
1
5000
0000
5000
0000
.
-
C60
0
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
.00
2
-
C60
3
2
55µs
.2ms
5
000
0
7
00
800
900
1000
1100
1200
wavelength / nm
7
00
800
900
1000
1100
Fig. 5. Graphical representation of the molar absorption coefficients vs. wavelength
wavelength / nm
Åꢀ
2ꢀ
of C6 anions (h) and of C60 anion (4), adapted from spectra given in Ref. [52], with
0
ꢀ
1
ꢀ1
Åꢀ
0
molar absorption coefficients at their maxima being
e
1075 = 12,000 M cm for C6
Fig. 6. Differential spectra obtained upon pulse radiolysis of a N
2
O saturated cCD/
ꢀ
1
ꢀ1
2ꢀ
Åꢀ
and
e940 = 21,000 M cm for C60
.
C
60 water solution (60
l
M) in presence of 5
l
M of CO2
.

A. Quaranta et al. / Chemical Physics 354 (2008) 174–179
179
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[
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It can be emphasised that the
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Acknowledgments
1980. pp. 349–408.
The authors acknowledge for financial support the European
Commission (Contract ‘‘WONDERFUL”, HPRN-CT-2002-00177)
and the French Agence Nationale de la Recherche (No. ANR-05-
JCJC-0211-03). Pulse radiolysis was carried out at the Free Radical
Research Facility in the Synchrotron Radiation Department of the
STFC Daresbury Laboratory, Warrington, UK, with the support of
the European Commission through the ‘‘Improving Human Poten-
tial Transnational Access to major Research Infrastructures” (Con-
tract HPRI-CT-2002-00183). The authors thank Professor Yong
Chen of Nankai University (China) for fruitful discussions.
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