Reactions of photoexcited C60 with weak organic acids. Novel method for the derivatization of fullerenes

Article abstract of DOI:10.1016/S0009-2614(00)00049-X

Pyrex-filtered irradiation of C₆₀ solutions in liquid diphenylmethane in the absence of oxygen is shown to result in Ph₂CH radicals, which further react with the electron-deficient C₆₀ to give Ph₂CH-C₆₀H_n (n=1, 3, 5). The fullerene ground state to ³C₆₀ transition is suggested to be accompanied by the increase of the fullerene solvation affecting thus the acidity of methylene hydrogens in Ph₂CH₂ as well as the feasibility of their dissociation. Also, photolysis of C₆₀ solutions in weakly acidic phenylacetylene is shown to lead to the addition of a varying number of phenylethinyl groups to the fullerene core.

Full text of DOI:10.1016/S0009-2614(00)00049-X

17 March 2000
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Chemical Physics Letters 319 2000 309–317
www.elsevier.nlrlocatercplett
Reactions of photoexcited C60 with weak organic acids. Novel
method for the derivatization of fullerenes
b
c
d
c
M. Fedurco a,), K. Klostermann , U. Kirbach , D. Scheller , L. Dunsch
a
UniÕersity of GeneÕa, Department of Chemistry, 30 quai Ernest-Ansermet, CH-1211 GeneÕa, Switzerland
b
Technische UniÕersitat Dresden, Institut fur Analytische Chemie, D-01069 Dresden, Germany
IFW Dresden, Institut fur Festkorperforschung, Helmholtzstrasse 20, D-01171 Dresden, Germany
Technische UniÕersitat Dresden, Institut fur Organische und Farbenchemie, Mommsenstraße 13, D-01069 Dresden, Germany
¨
¨
c
¨
¨
d
¨
¨
Received 26 November 1999; in final form 11 January 2000
Abstract
Pyrex-filtered irradiation of C60 solutions in liquid diphenylmethane in the absence of oxygen is shown to result in
Ph2CH Ø radicals, which further react with the electron-deficient C60 to give Ph2CH–C60Hn ns1, 3, 5 . The fullerene
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.
3
ground state to C60 transition is suggested to be accompanied by the increase of the fullerene solvation affecting thus the
acidity of methylene hydrogens in Ph2CH2 as well as the feasibility of their dissociation. Also, photolysis of C60 solutions
in weakly acidic phenylacetylene is shown to lead to the addition of a varying number of phenylethinyl groups to the
fullerene core. q 2000 Elsevier Science B.V. All rights reserved.
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.
1. Introduction
ganic molecule target molecule as well as from the
initiator of the radical reactions. A similar problem
was encountered during the phenylation of C60 in the
Feasibility of multiple radical additions to
fullerene in the presence of free radical species has
been demonstrated in several studies in liquids 1–4
as well as in the gas phase 5 . A characteristic
feature of such reactions is the addition of an unusu-
ally large number of functional groups to the fullerene
core. For example, either 15 benzyl groups, or at
least 34 methyl groups, could be covalently attached
to fullerene during irradiation of di-tert-butylpero-
xide in toluene, depending on the reaction tempera-
Ž
.
presence of a Grignard reagent i.e., PhMgBr and
methyl iodide 7,8 . The latter usually leads to ring-
opening reactions and gives four different fullerene
isomers diphenyl fulleroids and methanofullerenes
9,10 . The fact that diphenylmethyl radicals can
indeed be generated from DPM and photoexcited BP
molecules comes from a recent study reported by
Saitow et al. 11 . These authors observed exciton-
assisted hydrogen abstraction from diphenylmethane
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.
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ture 6 . However, such reactions often lead to the
addition of radicals formed from the irradiated or-
doped in benzophenone crystal however, no such
.
studies have yet been reported in solution .
Diphenylmethyl radicals formed upon UV irradiation
of the crystal were stable in the solid state for more
than two weeks. Surprisingly, we find that the pres-
ence of benzophenone in liquid diphenylmethane is
)
Corresponding author. Fax: q41-022-702-6029; e-mail:
milan.fedurco@chiam.unige.ch
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 00 00049-X

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M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
not really necessary, since C60 itself assists photode-
composition of the latter molecule.
While electric field desorption of Ph2CH–C60–H
from carbon whiskers occurs at ;6 keV and does
not require methane gas, CI MS has the advantage of
higher sensitivity, but the sample has to be heated to
Ž
2. Experimental
at least 5008C temperatures up to 15008C were used
.
in this study in order to get fullerene derivatives
0.5 mM solution of C60 in pure diphenylmethane
was placed in a quartz cuvette containing a 5 cm
long tube open on one end. The end of the tube was
tightly closed with numerous layers of Parafilm
into the gas phase. The addition of alkyl groups from
methane plasma to Ph2CH–C60–H as well as to C60
could not be avoided in this case.
Cyclic voltammetry was carried out with a PAR
273 potentiostat interfaced to a microcomputer. The
working electrode was mechanically polished Pt wire
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.
American Can . Two syringes were introduced into
the cuvette in order to allow inlet and outlet of
argon. Longer of the syringes was drawn directly
into the solution. Before the photolysis, fullerene
solution was purged intensively with the inert gas for
;5 min. Intense purging of the solution was also
maintained during the irradiation of the solution in
order to allow solution mixing and to remove traces
of oxygen. The absence of oxygen in the system was
checked using UV–visible and NIR spectroscopy in
the following way. Triethylamine was added to the
fullerene solution in diphenylmethane and this mix-
ture was irradiated for several minutes using a
medium-pressure mercury lamp. Absorbance of C60
monoanion at 1075 nm formed upon such photolysis
did not decrease more than 10% within 1 h after
turning the mercury lamp off, indicating the absence
of oxygen or impurities in the solvent which would
react further with fullerene anions.
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.
1 mm in diameter melted in glass. The counter
2
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.
electrode Pt sheet ;4 cm was separated from the
cathodic compartment using the glass tube with a
ceramic frit. All the electrochemical experiments
were done at room temperature in DPM containing
20% vrv acetonitrile and 0.1 M tetrabutylammo-
q
y
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.
nium hexafluorophosphate TBA PF6 . All the
electrode potentials are reported against the AgrAgq
Ž
reference electrode in acetonitrile containing 0.01 M
AgClO4 and 0.09 M TBAqPF6y
.
.
1H NMR studies were done for HPLC purified
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.
DPM–C60 photolysis of C60 in DPM for 20 min
recorded in CS2 and containing deuterated acetone
Ž
using 200 and 90 MHz NMR instruments Bruker,
.
Germany .
Samples containing products of fullerene photoly-
3. Results and discussion
Ž
sis were purified by means of analytical HPLC Cos-
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.
mosil column, toluene:acetonitrile 9:1 was used as
Fig. 1 shows that C60 dissolved in pure diphenyl-
methane gives rather weak, orbitally forbidden sin-
glet–singlet transitions in the visible range of the
electronic spectrum, as observed for C60 dissolved in
the mobile phase; flow rate: 0.8 mlrmin; detection
.
wavelength: 340 nm . Diphenylmethane was re-
moved from the sample by distillation under reduced
pressure at 1008C and the reaction products were
dissolved in toluene prior to HPLC separation. Effi-
ciency of fullerene to Ph2CH–C60–H conversion,
upon 15 min of photolysis, was ;60%. Each frac-
tion obtained during the photolysis was analysed by
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other aromatic solvents 12,13 . However, Pyrex-
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filtered irradiation medium-pressure Hg lamp, times
.
shorter than 15 min of 0.5 mM C60 solution in
DPM was accompanied by pronounced changes in
the visible range of the spectrum with the appearance
of new bands at 705, 480 and 435 nm, respectively
Ž
.
MS techniques MAT 95 Finnigan . An exact mea-
surement of molecular mass was done using positive
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.
Fig. 1a . Isobestic points at 620.3 and 580.6 nm
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.
mode field desorption mass spectrometry FD MS
suggest the presence of two absorbing species in
solution. The typical chestnut brown colour of the
photolysis product and 435 and 705 nm bands indi-
cate that the monoadduct H–C60 –R was formed
on carbon whiskers using fluorinated alkanes as an
internal standard. Computer simulation based on the
carbon isotope pattern supported the compound com-
position. Negative mode chemical ionisation mass
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upon photolysis of C60 in DPM 14 . Apparently,
photoexcitation of C60 with energies exceeding 1.56
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.
spectrometry CI MS was done for comparison.

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M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
311
.
no oxygen present . However, the strong interactions
of photoexcited fullerene with aromatic rings of
weakly acidic diphenylmethane molecules affect
acidity of methylene group hydrogens and, therefore,
makes the C–H bond scission more facile. The
resulting Ph2CH Ø and H Ø radicals then readily add
to fullerene yielding Ph2CH–C60–H derivative. The
latter molecule gave characteristic bands at 435 and
705 nm in the UV–visible spectrum, which can also
be seen from its quantitative HPLC separation from
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.
C60 Fig. 1b .
Irradiation of C60 solution in DPM for longer
times than 15 min resulted in the disappearance of
isobestic points at 620 and 580 nm and caused a
Ž
continuous decrease in the absorbance at 705 nm as
.
shown in the inset of Fig. 1a . On the other hand, the
intensity of bands at 435 and 480 nm grew continu-
ously with the photolysis time. This might indicate
that further additions of radicals to C60 have oc-
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.
curred see below .
Positive mode field desorption mass spectrometric
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.
measurements Fig. 2a confirmed the presence of
the C60 derivative having a molecular mass of
888.154, suggesting the compound composition
C73 H12 , corresponding to the formula Ph2CH–C60–
H. Irradiation of C60 in DPM for longer times than
15 min resulted in a more extensive hydrogenation
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.
of the fullerene derivative Fig. 2b , and MS mea-
surements have also confirmed the presence of
Ph2CH–C60 H3 in the photolysed solution. This di-
rectly confirms that reactive reaction intermediates
are indeed radical species. Apparently, such addi-
tional hydrogenation of the C60 monoadduct is re-
sponsible for the disappearance of isobestic points in
the visible range of the spectrum at longer irradiation
times than 15 min. Note that following 1 h photoly-
Ž .
Fig. 1. a UV–Vis spectra recorded during the irradiation of C60
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.
in diphenylmethane DPM for 0, 2, 4, 6, 8, 10, 13 and 15 min,
respectively. Inset shows absorbance changes measured at 705 nm
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plotted against time during the photolysis of C60 in DPM Pyrex
. Ž .
filter . b UV–Vis spectra for C60 and Ph2CH–C60 –H derivative
recorded upon their HPLC separation.
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.
eV 36 kcalrmol , as used in the present study,
3
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results in formation of the long-lived triplet C60
sis, the 886 mass unit signal weaker in intensity
.
state which is a powerful electron acceptor, with a
than that for Ph2CH–C60–H appears for the first
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reduction potential near 1.14 V vs. SCE 15,16 . It
has been demonstrated in several studies that C60
time in the FD MS spectra. The latter signal is likely
3
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to belong to diphenyl fulleroid Ph2C61 cyclopropane
.
can be quenched by electron donors such as amines,
bridge-containing fullerene , a structure previously
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described by Suzuki et al. 9 . Interestingly, even on
prolonged photolysis i.e., exceeding 1 h , exclu-
sively a single biphenyl addition occurred to the
fullerene core. As one would expect, diphenylme-
thine adduct appeared to be very unstable under
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x
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pyrene, pyridine 17 or dimethylsulphoxide 18 ,
resulting in the corresponding cation and C60 radical
anion. Since diphenylmethane does not contain a
lone pair of electrons, electron transfer from DPM to
3C60 is not likely to occur. Indeed, ESR experiments
did not show any sign of DPM photodecomposition
Ž
.
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.
chemical ionization CI MS conditions. Since the
886 mass unit signal did not occur in FD MS spectra
Ž
in the absence of fullerene Pyrex-filtered Hg lamp,

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M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
Ž .
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.
Fig. 2. a Field desorption mass spectra positive mode for the sample of Ph2CH–C60–H obtained after 15 min of C60 irradiation in DPM,
Ž .
b and that for derivatives formed upon 1 h of Pyrex-filtered irradiation of C60 in DPM.
during the short photolysis, it is concluded that
hydrogen removal from the methine bridge in
Ph2CH–C60–H followed by the cyclopropanation
step occurred only in the methane plasma under CI
conditions.
in diphenylmethane showed no sign of chemical
reactivity between the fullerene anions and diphenyl-
Ž
methane in the range of electrode potentials from 0
to y2.0 V vs. AgrAgq electrode . However, irradi-
.
ation of the electrochemical cell containing 0.38 mM
Ž
.
Ž .
Cyclic voltammograms Fig. 3A, curve a recorded
during the C60 reduction to mono-, di- and trianion
C60 in DPM Hg lamp, argon purged solutions for
Ž
20 min resulted in fullerene derivatization Fig. 3A,

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M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
313
Fig. 3. A a Cyclic voltammograms CVs for the reduction of C60 on Pt electrode in DPM containing 0.1 M TBAqPF6y and 20% vrv
Ž
. Ž .
Ž
.
Ž .
acetonitrile at a scan rate of 100 mVrs. b CV recorded after the irradiation of 0.38 mM C60 in DPM for 20 min in the absence of oxygen.
Ž .
B
Ž .
CVs recorded in a course of irradiation of C60 in DPM Pyrex filter : 0, 5, 10, 15 and 20 min of irradiation. Scan rate: 100 mVrs.
.
curve b . The newly formed reaction products of
such photoelectrolysis were found to be reducible at
;100 mV more negative electrode potential than
C60 molecules in DPM. Redox reactions of new
formed derivative, namely, Ph2CH–C60–H, were
found to be electrochemically reversible at scan rates
not exceeding 1 Vrs. Progressive decrease in the
cathodic currents during the C60 reduction to mono-,
di- and trianions in DPM, under the concomitant
irradiation of the solution for a period of 20 min, is
shown in Fig. 3B. Under such conditions, ;60% of
fullerene converted into Ph2CH–C60–H, as con-
firmed by HPLC. The formal redox potential for the
Ph2CH–C60–H then appears to be quite close to that
Ž
for the diphenyl fulleroid, Ph2C61 note: formal re-
0ry
dox potential for C
couple in DPMracetonitrile
60

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314
M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
Ž
.
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x
w x
9:1 mixture is close to that in toluene:acetonitrile
ppm 19 , C60 H4, ds5.06–5.50 ppm 25 , or ds
q
y
Ž
.
Ž
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x
9:1 mixture containing 0.1 M TBA PF6 being
4.6–5.4 ppm 26 , and C60 H18, ds3.42–5.43 ppm
.
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x.
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.
only 16 mV more negative for the latter system .
Infrared spectra for biphenyls linked to the
fullerene core via cyclopropane, and via a methine
bridge are expected to differ from each other because
of differing symmetries of both molecules and due to
27 . The signals at 4.18 dd, Jf6 Hz, Jf2 Hz
and 4.23 ppm t, Jf7 Hz cannot be assigned to
protons directly connected to the C60 . The coupling
usually measured in hydrogenated C60 is )9 Hz in
all cases reported in the literature 19,25–27 . The
transition of protons on the methylene group in
diphenylmethane solvent is usually observed at ;
4.0 ppm. If one of the three signals in the range from
4.1 to 4.4 ppm be due to an additional hydrogen on
the C60 , it is only possible if it sits symmetrically
with respect to other addends the only way to
appear as a singlet . Interesting in this respect is that
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.
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x
Ž .
the presence of hydrogen s on the fullerene core in
the former case but not in the latter. As expected, the
Ž
.
Ž
.
FTIR spectrum not shown of samples of C60 in
DPM irradiated for 15 min showed high-intensity
stretching modes around 2920 cmy1, typical for
hydrogenated fullerene and a weak C–H stretching
mode for the methine bridge. Bands between 1300
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.
and 1500 cmy1 deformations of aromatic C–H
Ž
hydrogen trotting on the fullerene core has been
detected in the case of the morpholine adduct of C60
28 .
Our preliminary experiments on the irradiation of
groups and strong signals in the 700–750 cmy1
.
Ž
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x
region out-of-plane vibrations of aromatic hydro-
.
gens were also detected. Bands at 2850 and 2825
cmy1, typical for C60 H2 19 or 2912, 2847 and
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x
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.
C60 irradiation time was 1 h in phenylacetylene
2827 cmy1 characteristic of C60 H36 20 were absent
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x
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.
PA , followed by CI MS analysis of reaction prod-
ucts, revealed that up to 11 phenylethinyl radicals
add to C60 under similar conditions as utilized in the
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in the IR spectrum of diphenyl fulleroid 9 .
We have attempted to separate Ph2CH–C60–H
Ž
.
from the solvent following the photolysis of C60 in
case of fullerene photolysis in DPM Fig. 5 . We
assume, that in each case, the acidic C–H bond in
PA gets labilized due to a strong interaction of
photoexcited fullerene with the delocalized system of
p-electron over the phenyl ring. Here, two possibili-
ties can be considered. First, C–H bond scission, as
discussed above, would lead to a direct C[C bond
introduction onto the C60 core and one hydrogen
atom on an adjacent carbon on the fullerene. A
second alternative would rely on cycloaddition of PA
to C60 . The corresponding derivative formed under
such conditions would have a similar molecular mass
as in the previous case but the structure would
contain a double bond connected to fullerene via two
1
Ž
.
diphenylmethane ;20 min for the H NMR exper-
iment. However, in order to distil the solvent out
Ž
.
b.p.s2648C , solutions had to be heated to rela-
Ž
tively high temperatures ;1008C under the reduced
pressure . Under such conditions, some decomposi-
.
tion of fullerene derivative andror their further
chemical reactions could have occurred in a process.
1
Fig. 4 shows H NMR spectra for such a mixture of
reaction products recorded in the region from 4.0 to
8.5 ppm. In the region of aromatic protons, only the
signal at ;8.18 ppm is compatible with the chemi-
cal shift of ortho protons of phenyl rings that are
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bent toward the surface of the C60 9,21–23 . The
aromatic signals occurring in the 6.85–7.75 ppm
region are difficult to assign without additional 13C
NMR experiments. They might be, at least in part,
due to the rotation of phenyl rings in Ph2CH–C60–H,
and also due to aromatic proton signals of more than
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.
single bonds with no hydrogen directly on the
fullerene core . FTIR experiments performed on
products formed upon fullerene photolysis in pheny-
lacetylene and deuterated phenylacetylene suggest
the presence of H or D atoms on the fullerene core
and, therefore, the first possibility seems to be more
plausible. The identification of asymmetric and sym-
metric stretches typical for the C[C bond around
1600 cmy1 is not straightforward in this case since
these bands are often quite weak.
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one kind of phenyl ring 24 . The signals at 6.03,
6.17 and 4.92 ppm should correspond to protons that
are directly bound to the C60 moiety. Note that all
the signals are singlets. They may be due to a proton
near the C60 –methine bond, or due to pairs of nearby
protons that are chemically equivalent since no cou-
As manifested in the present work, photolysis of
fullerene solutions in DPM allows one to attach
Ž
pling structures are observable: C60 H2 ds5.94

Fig. 4. 1H NMR spectra obtained for the Ph2CH–C60 –H derivative dissolved in CS2.

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M. Fedurco et al.rChemical Physics Letters 319 2000 309–317
Fig. 5. Negative mode chemical ionization MS of products formed during the photolysis of 0.5 mM C60 in phenylacetylene in quartz upon
continuous illumination of the solution using a mercury lamp for 1 h.
biphenyl group to C60 via the methine bridge. It is
interesting, in this respect, that our efforts to intro-
duce the Ph2CH– group during the electrochemical
reduction of Ph2CH–Cl on a mercury cathode, as
well as during the irradiation of such solutions in the
presence of C60 , have failed. In fact, benzylated
fullerene in the former case and diphenyl fulleroid in
phenylacetylene reported here or phenyl radicals re-
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x
ported previously 30 . The fact that photolysis of
C60 in diphenylmethane solutions leads to the addi-
tion of a single Ph2CH– group to C60 is rather
surprising, and might simply be due to the more
effective competition of hydrogen radicals than
Ph2CH Ø radicals for the fullerene.
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x
the latter case were synthesized 29 . Reactions of
photoexcited C60 molecules with weak aromatic acids
open up new attractive routes to fullerene derivatiza-
tion, and might be applicable to other organic acids
as well. This method especially is valuable since it
does not require any promoters of radical reactions
which are often involved in further chemical side
reactions with fullerene. Furthermore, reactive radi-
cals to be added to fullerene are generated from the
solvent itself so that purification of reaction products
might in some cases be simplified. In general, such
reactions follow pseudo-first-order kinetics since sol-
vent will always be in a large excess in respect to
Acknowledgements
We thank Dr. D. Jeannerat for helpful discussions
1
concerning H NMR measurements. This work was
supported by the Deutsche Forschungsgemeinschaft,
the Sachsisches Staatsministerium fur Wissenschaft
¨
¨
und Kunst.
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