Photoinduced electron transfer between C70 fullerene and 3,3',5,5'-tetramethylbenzidine studied by electron paramagnetic resonance.

Article abstract of DOI:10.1016/S1386-1425(00)00317-6

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3',5,5'-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation [C70-TMB] in benzonitrile. Upon selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gparallel - gperpendicular = 0.00165.

Full text of DOI:10.1016/S1386-1425(00)00317-6

Spectrochimica Acta Part A 56 (2000) 2729–2739
www.elsevier.nl/locate/saa
Photoinduced electron transfer between C70 fullerene and
3,3%,5,5%-tetramethylbenzidine studied by electron
paramagnetic resonance
Vlasta Brezova´ *, Dana Dvoranova´, Peter Rapta, Andrej Stasˇko
Department of Physical Chemistry, Faculty of Chemical Technology, Slo6ak Technical Uni6ersity, Radlinske´ho 9,
SK-812 37 Bratisla6a, Slo6ak Republic
Received 29 February 2000; accepted 23 May 2000
Abstract
Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron
transfer between 3,3%,5,5%-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in
UV-vis spectra pointed at ground state charge transfer complex formation {C70–TMB} in benzonitrile. Upon
selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR
singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ
reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen
1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR
spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gꢀ−gÞ=0.00165. © 2000
Elsevier Science B.V. All rights reserved.
Keywords: Anion radicals; C70 fullerene; Electron paramagnetic resonance; Photoinduced electron transfer
[17,19,22] and laser flash techniques [1,2,4,10–14].
In our previous EPR and Near-IR (NIR) investi-
1. Introduction
gations the formation of C60 anion radicals was
Numerous investigations of the unique photo-
observed [24–26], induced either by electron
chemical and photophysical properties of fullere-
transfer from photoactivated TiO2 particles [23],
nes have been carried out in recent years [1–14].
or by quenching the photoexcited triplet state of
The electron transfer to fullerene C60 was inten-
fullerene C60 and its derivatives. On the other
sively studied by electron paramagnetic resonance
hand, there is only limited information on the
(EPR) [15–21], UV-vis-Near-IR spectroscopy
generation of anion radicals of C70 and their
characterization [27–34].
Fullerene C70, similarly to C60, occurs in solu-
tions in a number of oxidation states including
* Corresponding author. Tel.: +421-7-52926032; fax: +
421-7-52493198.
E-mail address: brezova@cvtstu.cvt.stuba.sk (V. Brezova´).
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00317-6

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V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
+
−
−6
’
’
’
C70, C70 up to C70 [15,35–37]. NIR spec-
troscopy represents a powerful tool in the identifi-
Our study was motivated by the differing data
in the EPR spectra of C70 anion radicals found in
the literature. Therefore, we investigated the pho-
toinduced electron transfer between C70 and an
electron-donating substrate TMB, applying
monochromatic excitation (u=546 nm) of
fullerene molecules in solvent systems with a dif-
ferent polarity, and we also generated C70 anion
radicals in the cathodic reduction.
’
cation of individual C70 species, as each oxidation
state possesses characteristic absorption bands
[36,38]. The experimental absorption spectrum of
−
’
C70 with a maximum at 1368 nm was explained
by assuming that the addition of one electron to
the C70 skeleton caused a lowering of the symme-
try from D5h to C56, and consequently that C70
exists as a dipolar anion [38]. The enhanced
’
’
’
reactivity Of C70 as compared to C60 in the
photoinduced electron transfer from 1-benzyl-1,4-
dihydronicotineamide was similarly ascribed to
the higher local densities of unpaired electron and
negative charge in C7−0 due to the loss of symmetry
[39]. The photoinduced electron transfer between
tetrathiafulvalenes and C70 in polar and nonpolar
solvents was investigated by nanosecond laser
photolysis combined with transient absorption
spectroscopy in the visible and NIR regions [40].
The formation of C7−0 was monitored at 1380 nm
[40]. Similar techniques were applied also in the
photoinduced electron transfer between C60/C70
and zinc tetraphenylporphyrin [41], or 1,1%-bi-
naphthyl-2,2%-diamine [42]. Baumgarten et al. pre-
2. Experimental
2.1. Materials
Fullerene C70 (Gold Grade) was purchased
from Hoechst (Germany) and TMB from Aldrich
was applied. The organic solvents 1,2-
dichlorobenzene (DCB), benzonitrile (BN), aceto-
nitrile (ACN) of analytical grade and
tetrabutylammonium
((TBA)PF6) salt were obtained from Fluka.
hexafluorophoshate
2.2. Photochemical reduction
’
pared
different
C70
anions
in
toluenetetrahydrofuran solutions by contact with
The solutions with the desired concentrations of
C70 and TMB were prepared immediately prior to
irradiation, carefully purged by argon and trans-
ferred into a quartz flat cell optimized for the
Bruker standard rectangular cavity. The cell was
placed in the Bruker TM cavity and continuously
irradiated by monochromatic light with wave-
length of 546 nm, which was selected from the
radiation flux of a medium-pressure mercury lamp
(Applied Photophysics, UK) using a glass filter
(Schott Glaswerke, Germany).
a potassium mirror, and the EPR and vis-NIR
−
’
spectra were recorded [36]. Mono-anion C70 char-
acterized in EPR by g=2.0022 and a line width
of 0.07 mT was identified in NIR spectra by an
absorption maximum at 1370 nm. Paramagnetic
2−
70
C
was described in EPR by a singlet with
g=2.0026, a line width of 0.06 mT, and a NIR
absorption band at 1171 nm [36]. Friedrich et al.,
applying electrochemical in situ reduction of C70,
−
’
measured one narrow EPR line for C70 character-
ized by g=2.0019 and a line width of 0.019 mT
’
[43]. Reduction of C70 by interaction with 1-
2.3. Cyclic 6oltammetry
amino-3-propanol and 1,5-diaminopentane was
evidenced by EPR and vis-NIR spectroscopy
[44,45]. The monitored NIR-characteristic absorp-
Cyclic voltammograms were measured at 295 K
under argon in 0.05 mM C70 fullerene, 0.1 M
(TBA)PF6 in DCB or BN solutions using a PAR
270 (Princeton Applied Research) electrochemical
system with the standard three-electrode cell with
platinum working and counter electrodes and a
saturated calomel reference electrode (SCE) at
−
2−
’
tions for C70 and C70 are in good agreement
with data published in [36,38], while the EPR
−
’
spectrum of C70 measured at 333 K was here
described by two lines, with g-values of 2.0004
−
’
and 2.0001, and an EPR spectrum of C70 ob-
served at 12.5 K was simulated applying gꢀ=
2.0000 and gÞ=1.994 [44,45].
scan rate of 100 mV s−1
.

V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
2731
generation of a sufficiently high concentration of
the radical species of interest, the sample tempera-
ture was lowered to 110 K using a Bruker variable
temperature control unit ER 4111 VT. The EPR
spectra were recorded without further irradiation
raising the temperature to the desired values in
the temperature range from 110 to 210 K.
2.4. Photochemical in situ EPR
A Bruker 200D EPR spectrometer, assembled
with an Aspect 2000 computer was used to mea-
sure EPR spectra. The solutions of fullerene C70
and TMB were continuously irradiated (u=546
nm) in the cavity of the spectrometer and the
EPR spectra were monitored in situ. The standard
EPR experiments were performed at 290 K. Typi-
cal settings for the series of EPR experiments were
as follows: gains 104–105; time constant 50 or 100
ms; sweep time 20 or 50 s; modulation amplitude
0.005–0.05 mT; microwave power 10 mW. The
g-value was determined with an uncertainty of
90.0001 using a marker containing 1,1-diphenyl-
2-picrylhydrazyl (DPPH) built into the spectrome-
ter. EPR spectra were simulated using Bruker
WIN-EPR SimFonia.
2.5. Electrochemical in situ EPR
Amperostatic cathodic reduction was carried
out directly in the cavity of the EPR spectrometer
with solutions as specified in the cyclic voltamme-
try section, using a Varian electrolytic cell with
platinum net working, gold counter, and Ag/AgCl
reference electrodes. This galvanostatic procedure
allows a better potential control at the entire
platinum grid than by standard potentiostatic
techniques.
The measurements of EPR spectra at low tem-
peratures were carried out in 3 mm o.d. quartz
EPR tubes. In situ irradiation of fullerene solu-
tion was performed at 200 K. Immediately after
2.6. UV-6isible spectroscopy
UV-vis spectra of C70 and TMB solutions were
recorded under argon using a diode array UV-vis
spectrometer PC 1000 (Ocean Optics).
3. Results and discussion
Fig. 1 shows the individual UV-vis spectra of
C70 and TMB in BN, along with the spectra of the
solutions measured under argon at a constant C70
concentration and at the changed TMB concen-
trations. The formation of charge-transfer com-
plexes between the ground state in C70 and also in
C60 fullerenes and amines was previously demon-
strated by UV-vis spectroscopy, as the absorption
spectra of fullerenes undergo characteristic
changes upon the addition of amines to the solu-
tion [46–50]. Spectra presented in Fig. 1 indicate
absorption spectra of C70 alteration similar to
those observed previously in the presence of dif-
ferent electron donating substrates [47–50]. The
detected spectral changes caused by the presence
of TMB pointed at ground state charge transfer
complex formation between C70 and TMB in BN
solvent. As TMB has only negligible absorption in
the range 500–600 nm (Fig. 1), in the steady-state
Fig. 1. UV-vis spectra of individual compounds: [TMB]=1.4
mM (1); [C70]=0.34 mM (2) measured under argon in BN
solutions, along with spectra observed at constant [C70]=0.34
mM and variable TMB concentrations: 0.05 (3); 0.12 (4); 0.24
(5) and 0.48 mM (6). The inset shows the dependence of
absorbance at u=470 nm evaluated from spectra 2–6 on
TMB concentration.

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V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
Fig. 2. EPR spectra obtained at 290 K during continuous irradiation (u=546 nm) of 0.48 mM TMB in BN solutions containing
various concentrations of C70: (a) 0 mM; (b) 0.23 mM; (c) 0.34 mM.
increased upon progressing irradiation (Fig. 2b,c).
We assigned this signal to C70 mono-anion, taking
into account the fact that the UV-vis experiments
provided evidence of ground state charge transfer
complex {C/70(SUB)-TMB} formation in BN (Fig.
1). This solvent is characterized by a relative
photochemical experiments using monochromatic
light of 546 nm, only C70 molecules were selectively
excited.
Fig. 2 shows EPR spectra measured before and
during a continuous irradiation (u=546 nm) of
TMB solutions in BN containing various concen-
trations of C70. The TMB molecule is not excited
by the applied radiation in fullerene-free solutions,
and no EPR signals are evident (Fig. 2a). Adding
C70 to such donor solutions, a sharp EPR singlet
M, characterized by g-value, gM=2.0004 and
peak-to-peak line-width, ppM=0.013 mT, can be
observed already before starting irradiation, and it
permittivity of 25.7, and allows the ground state
−
’
separation and stabilization of C70 according to
Eq. (1) [14]:
’
’
C70+TMBl{C70−TMB}lC7−0 +TMB+
(1)
Upon starting the radiation, the intensity of the
EPR signal M increased, and formation of a new

V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
2733
radical D (gD=2.0007; ppD=0.011 mT) is evi-
dent, which is attributed to C70 di-anion (Fig.
2b,c). The formation of C70 di-anion under the
of TMB in our experiments resulted in the forma-
tion of C70 anion radicals M and D (Fig. 3b,c).
Despite higher concentrations of electron donat-
ing TMB (ꢀ10−3 M) used in the DCB solvent,
the relative intensities of EPR signals M and
especially D are low (Fig. 3d). This low efficiency
of electron transfer is presumably caused predom-
inantly by the low stability of C70 anion radicals
in DCB.
Because the solvent polarity and relative per-
mittivity determine the stability of ion radicals
[39–42,53], we attempted to improve the electron
transfer efficiency by the addition of ACN or salt
with a large cation as (TBA)PF6 to DCB. The
relative permittivity of binary solvents DCB/ACN
was calculated by algebraic averaging based on
the mole fraction, as recommended by Sun et al.
[54].
The time dependence of mono-anion M forma-
tion during irradiation (u=546 nm) and after
stopping irradiation in DCB and DCB/ACN (6:1;
v/v) solutions of C70/TMB monitored by EPR is
shown Fig. 4. The relative EPR intensity of signal
M reached a stationary value after 1 min in DCB
solvent, and its fast decay after stopping irradia-
tion can be observed, characterized by a half-time
of 5.3 s. The presence of ACN in DCB signifi-
cantly enhanced the relative mono-anion M inten-
sity during irradiation (Fig. 4). The decay profiles
of signal M in both solvent systems were analyzed
by the non-linear minimization procedure of least-
squares using the program SCIENTIST (Micro-
Math) in order to determine decay kinetics (inset
in Fig. 4). In DCB solutions the best results were
obtained by the application of first-order kinetics
(fit correlation=0.986), but in binary solvent
DCB/ACN (6:1; v/v) a better fit was calculated by
the combination of first-order and second-order
kinetics (fit correlation=0.992).
given experimental conditions can be explained
−
’
either by C70 excitation and subsequent electron
−
’
transfer from TMB (Eq. (2)) or by C70 dispropor-
tionation [16,20] (Eq. (3)):
TMB, hv
“
’
2−
70
C7−0
C
(2)
(3)
2C−70lC70+C
2−
70
The intensities of EPR signals M and D in BN
sensitively reflected the solution composition (C70
and TMB initial concentrations), as well as the
irradiation period. It should be noted here that
radicals M and D photogenerated in BN solutions
were relatively stable after the cessation of radia-
tion (half-times ~M=12 min and ~D=5 min, re-
spectively). The high stability in polar BN solvent
’
supported our assignment of EPR signals to C70
anion radicals, because the EPR signals of photo-
generated radical adducts on C70 are transient,
and decayed immediately after stopping illumina-
tion [51]. Additionally, when we exposed the irra-
diated C70/TMB solutions to oxygen, both EPR
signals disappeared, due to the well-known elec-
tron transfer from fullerene anions to molecular
oxygen, forming super-oxide anion radical
[20,40,52] (Eq. (4)):
’
’
C7−0 +O2“C70+O2−
(4)
Replacing BN by less polar DCB (mr=9.95)
significantly lowered the intensity of both C70
anion radicals, as well as their stability. Fig. 3
illustrates a set of EPR spectra recorded upon
continuous irradiation (u=546 nm) of fullerene
C70 solutions in DCB with various concentrations
of TMB. The excitation of TMB-free solutions of
C70 at 290 K by monochromatic light caused the
excitation of C70 to the paramagnetic triplet state
[3,5]. However, in our cw-EPR experiments at 290
K the triplet state cannot be observed (Fig. 3a),
because of its low intensity and large line width
[5]. The analogous situation was described in a
study of the photochemical reactions of fullerenes
with anthracenes [51]. The detection of the C70
triplet state requires application of pulsed EPR
techniques [3,5]. Photoexcitation in the presence
Sasaki et al. investigated photoinduced electron
transfer between C60 and TMB in different sol-
vents by nanosecond laser photolysis [53]. In non-
polar and less polar solvents they confirmed that
−
+
’
’
C60 and TMB decline with first-order kinetics,
indicating that the back electron transfer reaction
−
+
’
’
)
ion
take place within the ion pair (C60 , TMB

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V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
−
’
separation. This is illustrated in Fig. 5 where the
EPR-relative intensities of mono-anion M are
higher in the mixed DCB/ACN solvents, and they
pair. On the other hand, in the polar media C60
and TMB+ the decay kinetics changed to second-
order, indicating that the back electron transfer
reaction occurred on separate solvated anions
’
increased
upon
progressing
irradiation.
C60− and TMB+ [53]. Our results here using
Analogously, the additional saturation of DCB
’
’
−
’
’
C70 agree well with these suggestions; conse-
solvent with (TBA)PF6 salt stabilized C70 and
quently we proposed that in less polar DCB sol-
further improved the yield of mono-anion forma-
tion (Fig. 5).
Mono-anion radical M was also generated at
low temperature by electron transfer between C70
−
’
vent photogenerated ions exist as ion pairs (C70
,
TMB+
larity boosted the C70 and TMB solvation and
)
ion pair. However, an increased solvent po-
’
+
’
’
Fig. 3. EPR spectra measured at 290 K during continuous irradiation (u=546 nm) of 0.4 mM C70 in DCB solutions containing
various concentrations of TMB: (a) 0 mM; (b) 2 mM; (c) 3 mM; (d) 4 mM.

V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
2735
pendicular peak-to-peak line width obtained from
simulations of line M is plotted in Fig. 7b. With
increased temperature the line M became nar-
rower, and a fit of the temperature dependence of
the peak-to-peak width by least-squares analysis
resulted in an activation energy of 11 meV.
The low-temperature EPR spectrum of C70
−
’
measured at 120 K in frozen CH2Cl2 glass by
Dubois et al. was quite anisotropic, and was
interpreted using three g-values at g=2.007,
2.003 and ꢀ2.000 [15]. The greater anisotropy
for C−70 comparing with C60 was explained in view
of the lowered symmetry of C70 molecule [15].
−
’
Lobach et al. investigated the EPR spectra of C70
in a large temperature range from 12 to 350 K. At
the lowest temperature they observed an axial
anisotropy of g-factor; however above 70 K the
anisotropy was averaged out, presumably by
Jahn–Teller distorsions [44,45].
In order to prove that in the photochemical in
situ EPR experiments C70 anion radicals were
generated, we completed these investigations with
a cathodic reduction.
Fig. 4. The time dependence of the EPR-relative intensity of
line M observed during irradiation ( , u=546 nm) and after
stopping irradiation (¡) in 0.8 mM C70 and 1 mM TMB
solutions in different solvent systems: DCB (1); DCB/ACN
(6:1; v/v; mr=16.31) (2). The inset shows the experimental (ꢀ,
ꢁ) and calculated (—) dependence of radical M intensity in
different solvent systems after stopping irradiation: DCB (ꢀ)
and DCB/ACN (6:1; v/v; mr= 16.31) (ꢁ).
and TMB irradiating (u=546 nm) frozen DCB
solution at 200 K (Fig. 6a). The quartz tube
containing the sample was irradiated for 6 min,
and at that moment the sample temperature was
immediately lowered to 110 K. Then, the solid
state EPR spectra were recorded without further
irradiation raising the temperature to the desired
value in the temperature range from 110 to 210 K
(over 210 K the EPR line M expired without
irradiation in DCB solvent). As can be seen from
Fig. 6b, upon lowering the temperature, the EPR
line M became broader and asymmetric. Conse-
quently, the corresponding simulations of low-
temperature EPR spectra were performed
assuming g-value and line-width anisotropy (Fig.
6b). The data extracted from simulations are pre-
sented in Fig. 7. The lower temperatures enhanced
the gꢀ−gÞ difference (Fig. 7a), hence at 110 K
the experimental EPR spectrum was simulated by
an axially symmetric g-matrix with gꢀ−gÞ=
0.00165, whereas at 210 K this difference is only
0.00025. The temperature dependence of the per-
Fig. 5. The time dependence of the EPR-relative intensity of
line M observed during irradiation (u=546 nm) in 0.4 mM
C70 and 0.74 mM TMB solutions in different solvent systems:
DCB (ꢂ); DCB/ACN (5:2; v/v; mr=21.36) ( ); DCB/ACN
(4:3; v/v; mr=25.17) (ꢃ); DCB saturated with by (TBA)PF6
(ꢁ).

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V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
Fig. 7. The temperature dependence of gꢀ and gÞ-values (a) and perpendicular peak-to-peak line width (b) extracted from the
simulations of the anisotropic EPR line M measured in the frozen DCB solutions within the temperature range from 110 to 210 K.

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V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
Fig. 6. (a) EPR spectra generated upon continuous monochromatic irradiation (u=546 nm) at 200 K in frozen DCB solutions
containing 0.4 mM C70 and 4 mM TMB. (b) Experimental (solid line) and simulated (dotted line) EPR spectra of photogenerated
radical M observed at different temperatures in frozen DCB solutions.
In Fig. 8 the EPR spectra measured by in situ
cathodic reduction of C70 in DCB solvent in the
region of the first reduction wave, were compared
with those measured by photoinduced electron
transfer from TMB. The inset in Fig. 8 represents
the corresponding cyclic voltammogram measured
under argon in 0.05 mM C70 DCB solution in the
presence of (TBA)PF6 supporting electrolyte at
295 K. Under the given experimental conditions,
in DCB, as well as in BN solvents, three reversible
redox waves were observed, characterized by the
half-wave potentials: E1/2 (C70 in DCB)= −0.57;
−0.97 and −1.39 V versus SCE; E1/2(C70 in
BN)= −0.47; −0.87 and −1.30 V vs. SCE. The
shift in half-wave potentials probably reflects dif-
ferent properties of the solvent used, as already
reported in the study of solvent, and supporting
electrolyte, effects on the electrochemical proper-

2738
V. Brezo6a´ et al. / Spectrochimica Acta Part A 56 (2000) 2729–2739
ties of C70 [37]. The simulations of spectra unam-
biguously confirmed that the EPR spectra gener-
ated in both (photochemical and electrochemical)
systems are identical and originate from C70 anion
radicals.
tramethyl-benzidine resulted in the charge transfer,
quenching the fullerene excited states and forming
the corresponding C70 anion radicals. The detected
changes in UV-vis spectra of C70 in the presence of
TMB pointed at ground state charge transfer
complex formation between C70 and TMB in BN
solvent.
4. Conclusions
Two EPR signals were observed upon in situ
irradiation of C70/TMB solutions in the cavity of
an EPR spectrometer. The EPR singlet M, charac-
terized by g-value, gM=2.0004 and peak-to-peak
line width, ppM=0.013 mT, was assigned to the
C70 monoanion. Signal D (gD=2.0007; ppD=
0.011 mT) was attributed to the C70 di-anion. The
stabilization of photogenerated anion radicals is
significantly dependent on solvent polarity. The
shortest lifetime of the radicals was found in DCB,
and it increased considerably upon the addition of
(TBA)PF6 salt or ACN to the C70/DCB solution,
or if DCB was replaced by BN. The concentration
of radicals M and D formed during irradiation
sensitively reflected the composition of the solution
irradiated (solvent, C70 and TMB concentrations)
and the irradiation period.
The photoexcitation of fullerene C70 by
monochromatic light (u=546 nm) in the presence
of electron donating substrate 3,3%,5,5%-te-
Mono-anion M was generated also in the solid
state, irradiating frozen C70 and TMB solutions in
DCB at a temperature of 200 K. Lowering the
temperature, the EPR line M became broader and
asymmetric. At 110 K the observed EPR spectrum
can be simulated by an axially symmetric g-matrix
with gꢀ−gÞ=0.00165.
The EPR spectra recorded by the in situ cathodic
reduction and also by the in situ photochemical
reduction of C70 in DCB are in both experimental
systems identical, and consequently, justify their
assignment to the anion radicals of C70.
Acknowledgements
Fig. 8. Experimental (solid line) and simulated (dotted line)
EPR spectra measured at 290 K by in situ photochemical
electron transfer in 0.4 mM C70 and 0.74 mM TMB in
DCB/ACN (4:3;v/v; mr=25.17), along with the EPR spectra
obtained by in situ cathodic reduction of 0.05 mM C70, 0.1 M
(TBA)PF6 in DCB in the region of the first reduction wave (*).
The inset represents the cyclic voltammogram obtained in 0.05
mM C70, 0.1 M (TBA)PF6 in DCB under argon at 295 K (scan
rate of 100 mV s−1).
We thank the Slovak Grant Agency (Project
VEGA 1/4206/97) for financial support.
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