Electron transfer from C76 (C(2v)) and C78 (D2) to radical cations of various arenes: Evidence for the Marcus inverted region

Full text of DOI:10.1021/ja970397f

5744
J. Am. Chem. Soc. 1997, 119, 5744-5745
Electron Transfer from C76 (C2W′) and C78 (D ) to
Radical Cations of Various Arenes: Evidence for
the Marcus Inverted Region
2
6πηri) into the Smoluchowski equation which yields {kdiff )
2
3
/3RT/η10 [(r1 + r2)/r1 + (r1 + r2)/r2]}. The rate constant
assumes its lowest value for equally sized reaction partners (r1
r2) but increases with increasing difference between r1 and
)
r2. In combination with solvents of relatively low viscosity, a
significantly large enough difference between kdiff and kact at
higher -∆G is ensured. Another possibility is to look for
systems with a small solvent reorganization energy, λs, because
this facilitates reaching the maximum of the exothermic electron
transfer process at lower -∆G (and thus reaching the inverted
region at lower energy). It appears that fullerenes are not only
suited to improve on kdiff but also on λs. Concerning λs the
pronounced possibility for charge and energy delocalization
within the fullerene moiety may be expected to exert an effect
in the desired direction as it minimizes vibrational differences
between the reaction partners in the ground and transition state.
Fullerenes are also excellent candidates for the purpose of
this investigation since they readily undergo redox reactions
yielding easily detectable radical anions and cations. For the
envisaged electron transfer reactions, we have chosen C76 (C2V′)
Dirk M. Guldi* and Klaus-Dieter Asmus
Radiation Laboratory and Department of
Chemistry and Biochemistry
UniVersity of Notre Dame, Notre Dame, Indiana 46556
ReceiVed February 6, 1997
One of the most intriguing aspects of electron transfer theories
is the characteristic parabolic dependence of the corresponding
1
rate constants on the free energy changes. The range in which
the rate constants actually decrease with increasing the free
energy toward the highly exothermic region is generally referred
to as the Marcus inverted region. Experimental proof of it has
been provided, for example, by some elegant pulse radiolysis
studies by Miller et al. using donor-acceptor systems with rigid
frameworks or matrices.2
-11
Employing a constant distance
and C78 (D2), which exhibit particularly low ionization potentials
between the two components eliminates diffusion processes and,
consequently, converts the electron transfer to a truly unimo-
lecular reaction controlled only by the activation energy of the
electron transfer. Rate constants for bimolecular reactions, on
the other hand, consist not only of an activation component (kact)
but also include a diffusion related term (kdiff), with the latter
defining the upper limit of the observable rate. The bimolecular
rate constant thus first increases with increasing driving force
19,20
of 7.10 and 7.05 eV, respectively,
quantities.
are available in sufficient
The reaction under investigation here is the electron transfer
from the ground state of these C76 and C78 fullerenes to radical
cations of various arenes, i.e.,
•+
•+
•+
C /C + (arene) f (C ) /(C ) + arene
(1)
7
6
78
76
78
(
i.e., when kdiff > kact) before reaching the diffusional limit (at
This reaction yields the radical cation of the fullerene and
can conveniently be monitored through either the formation of
the latter or the decay of the arene radical cations, since both
species exhibit distinct optical absorptions. The technique
chosen for experimental verification was pulse radiolysis, known
to be one of the most powerful tools to investigate reactive
intermediates.
kdiff ≈ kact). However, in most cases, it does not decrease again
at higher -∆G but stays at this level. Reports on an inverted
region for true bimolecular electron transfer have been scarce
and practically limited to back electron transfer reactions within
2
1
12-18
the charge-separated radical pair.
The difficulty in deter-
mining the inverted region may have various reasons such as a
low kdiff obscuring the decline of the overall rate and/or a high
reorganization energy shifting the maximum of the Marcus
relationship to higher -∆G.
One promising approach to encounter the difficulties is to
elevate the diffusion-controlled limit by selecting a system of a
large-sized electron donor and small-sized electron acceptor
couple. This becomes evident by introducing the Stokes-
Einstein relationship for the diffusion coefficient (Di ) kbT/
The electron transfer studies were carried out in deoxygenated
CH2Cl2 which, upon radiolysis, leads to the generation of the
•
+
solvent radical cation ([CH2Cl2] ). This species is a strong
oxidant capable of oxidizing many organic compounds, includ-
2
2-24
ing arenes and fullerenes.
(
Direct oxidation of C76 and C78
-
5
•+
4.0 × 10 M) by [CH2Cl2] , conducted in deoxygenated
solutions via the sequence of reactions shown in eqs 2 and 3,
has been carried out to characterize the radical cations of the
21,25-27
(
1) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-89.
fullerenes.
Differential absorption spectra recorded upon
(
2) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am.
Chem. Soc. 1989, 111, 3751-3.
radiolysis
•
+
-
(
3) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J. Am. Chem. Soc. 1983,
05, 670-1.
4) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984,
06, 3047-9.
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CH Cl
8 [CH Cl ] + e
(2)
(3)
2
2
2
2
1
1
5
(
•
+
•+
•+
[
CH Cl ] + C /C f CH Cl + (C ) /(C )
2 2 76 78 2 2 76 78
(
353-4.
(
6) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-1.
7) Miller, J. R.; Beitz, J. V.; Huddleston, R. H. J. Am. Chem. Soc. 1984,
pulse radiolysis for the C76-containing solutions exhibited the
(
1
06, 5057-68.
(
(19) Yang, Y.; Arias, F.; Echegoyen, L.; Chibante, L. P. F.; Flanagan,
S.; Robertson, A.; Wilson, L. J. J. Am. Chem. Soc. 1995, 117, 7801-4.
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Chem. Phys. Lett. 1995, 234, 455-9.
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Herman, M. S. J. Am. Chem. Soc. 1989, 111, 1917-9.
(
9) Gould, I. R.; Farid, S. J. Am. Chem. Soc. 1988, 110, 7883-5.
(
10) Ohno, T.; Yoshimura, A.; Shiyoama, H.; Mataga, N. J. Phys. Chem.
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Chem. 1993, 97, 11258-64.
1
987, 91, 4365-70.
(
11) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S. J. Am. Chem. Soc.
987, 109, 3794-6.
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Phys. 1988, 127, 249-61.
13) Mataga, N.; Kanda, Y.; Asahi, T.; Miyasaka, H.; Okada, T.; Kakitani,
(22) Shank, N. E.; Dorfman, L. M. J. Chem. Phys. 1970, 52, 4441-7.
(23) Arai, S.; Ueda, H.; Firestone, R. F.; Dorfman, L. M. J. Chem. Phys.
1969, 50, 1072-7.
1
(
(24) Guldi, D. M.; Neta, P.; Asmus, K.-D. J. Phys. Chem. 1994, 98,
4617-21.
(
T. Chem. Phys. 1988, 127, 239-48.
(25) Kato, T.; Kodama, T.; Oyama, M.; Shida, T.; Nakagawa, T.; Matsui,
Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett.
1991, 180, 446-50.
(
14) Asahi, T.; Mataga, N. J. Phys. Chem. 1989, 93, 6575-8.
(
15) Gould, I. R.; Moody, R.; Farid, S. J. Am. Chem. Soc. 1988, 110,
7
242-4.
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4169-70.
(
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17) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B.
(
(27) The electron, after solvation, will react with the solvent to yield
Cl- and CH2Cl with the latter possibly adding to the fullerenes. Spectro-
scopically, these processes are clearly distinguishable from the radical cation
formation.
•
J. Am. Chem. Soc. 1985, 107, 1080-2.
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S0002-7863(97)00397-1 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 24, 1997 5745
Table 1. Rate Constant for Electron Transfer from C76 (C2V′) and
•
+
C78 (D
2
) to Various (Arene) in Dichloromethane at Room
Temperature
monitored
wavelength
[nm]b
1
k
obs [M- s-1
]
IP
[eV]
a
compound
C
76
C
78
9
9
durene
8.2
6.6 × 10 6.9 × 10
330
400
410
400
340
400
380
400
480
400
460
430
440
400
460
9
9
naphthalene
m-terphenyl
biphenyl
hexamethylbenzene
triphenylene
phenanthrene
fluorene
8.15 8.9 × 10 9.3 × 10
1
0
10
10
10
10
10
10
10
10
10
10
10
8.01 1.1 × 10 1.5 × 10
1
0
7.95 2.0 × 10 2.5 × 10
Figure 1. Plot of ln kobs for electron transfer from (a) C76 (C2V′) and
1
0
•+
7.9
2.1 × 10 2.0 × 10
(b) C78 (D
2
) to (arene) in dichloromethane at room temperature as a
1
0
7.86 2.9 × 10 2.3 × 10
function of the free energy changes for the reactions.
1
0
7.85 3.0 × 10 2.5 × 10
1
0
7.78 3.2 × 10 2.9 × 10
1
0
excellent agreement with those calculated from the classical
9
-anthraldehyde
7.69 2.9 × 10 3.0 × 10
10
chrysene
9
anthracene
pyrene
coronene
7.59 4.5 × 10 4.2 × 10
dielectric continuum model (eq 5) for the different arenes (λs
1
0
0
-1
,10-dibromoanthracene 7.58 2.8 × 10 2.7 × 10
) 11.14-16.33 kcal mol ) with the radii of the reactant (r1
1
7.45 1.5 × 10 1.7 × 10
and r2), refraction index (n ) 1.424) of the solvent, and dielectric
constant (ꢀ ) 9.08) of the solvent. Furthermore, it is noted
that the total reorganization energy is relatively small. A similar
conclusion has recently been forwarded by Imahori et al. for
1
0
7.41 1.0 × 10 1.1 × 10
9
7.29 3.6 × 10
c
8
9
9
-anthracenemethanol
7.21 7.8 × 10 2.0 × 10
a
Taken from ref 29. b Arene radical cation absorption. Not mea-
c
30
C60-related electron transfer reactions.
sured.
1
1
1
1
1
2
formation of a transient with a distinct maximum at 960 nm,
accompanied by additional absorptions at 550 and 770 and a
shoulder at 1050 nm. In analogy to those of C60 and C70, these
absorption bands are attributed to the π-radical cations of the
λs )
+
+
-
(∆ze)
(5)
{
r12} {n
2
ꢀ}
2r1 2r2
1
0
fullerenes, formed with a rate constant k3 (C76) > 2.0 × 10
Since the total reorganization energy consists of two major
contributions, namely, the solvent-related term (λs) and a
parameter (λν) reflecting the changes in internal vibrational
modes it appears that the latter are of lesser importance in our
systems. Minor vibrational differences in the oxidized state of
the fullerene relative to its ground state can be rationalized in
terms of the large degree of electron and energy delocalization
within the resonance structure of the π-system of the fullerene.
This is also manifested in small Raman and Stokes shifts, as
verified, for example, upon reduction and photoexcitation of
-
1
-1
M
s . The spectral assignment is further substantiated by
similar absorption features recently reported for a synthetic
•
+
-
28
[
(C76) ][CB11H6Br6 ] salt (hexabromocarborane).
Corre-
•+
sponding near IR features for C78 were much weaker showing,
however, also a distinct band around 980 nm. The assignment
of the infrared bands to the radical cations of the fullerenes is
further corroborated by the decay kinetics of the latter.
In analogy to eq 3, arene radical cations were generated via
eq 4, e.g., with deoxygenated CH2Cl2 solutions containing 2 ×
-
2
1
0
M of the respective arenes. The formation and lifetime
31,32
fullerenes.
The relatively low reorganization energy is clearly beneficial
for the possibility of establishing a Marcus-inverted region.
Equally favorable is undoubtedly the selection of relatively
large-sized donor molecules (C76 lattice constant of 15.3 ( 0.1
Å), small-sized acceptor molecules, and a solvent of relatively
low viscosity (η ) 0.414 at 25 °C) which elevate the diffusion-
controlled barrier and separate it sufficiently from the actual
electron transfer rates. For C76 and naphthalene, for example,
•
+
•+
[
CH Cl ] + arene f CH Cl + (arene)
(4)
2
2
2
2
of the various (arene)•+ radical cations were monitored via their
respective absorptions in the UV-vis region (see Table 1).
Addition of variable amounts of C76 and C78 in the 10
-
5
M
concentration range resulted in an accelerated decay of the
respective radical cation absorption of the arenes with rates
linearly depending on the fullerene concentration. At the same
time, formation of the fullerene radical cations was observed
in the near IR, suggesting the underlying process to be an
intermolecular electron transfer according to eq 1. The bimo-
lecular rate constants k1 obtained for a large number of arenes
with ionization potentials ranging from 7.21 eV (9-anthracen-
emethanol) to 8.2 eV (durene) are listed in Table 1.
As a consequence of the irreversible character of most of the
oxidation processes of the arenes in electrochemical experiments,
it is probably not feasible to correlate any measured half-wave
potentials with -∆G. The driving forces of eq 1 were,
therefore, related to the difference in the respective ionization
potentials of the arenes and the fullerenes (-∆G ) ∆IP ) IParene
1
0
-1 -1
a value of kdiff ) 3.0 × 10
M
s
can be computed from
Smoluchowski’s theory of diffusion-controlled reactions. This
is clearly higher than the observed rate constants identifying
the latter as a true measure of kact. The decline in rate constants
at higher -∆G is thus considered to reflect a true Marcus-
inverted behavior. The ratio between the maximum rate and
the rate at the high exothermic end (durene) in our set of
experiments is 7. This is only somewhat smaller than the factor
of 30 reported for intramolecular electron transfer reactions in
rigid spaced donor-bridge-acceptor dyads.
2
9
29
Acknowledgment. This work was supported by the Office of Basic
Energy Sciences of the U.S. Department of Energy. This is contribution
no. NDRL-3985 from the Notre Dame Radiation Laboratory.
-
IPfullerene). The corresponding relationship of the measured
rate constants for the electron transfer reactions, k1, versus these
∆G values are shown in Figure 1a,b for C76 and C78,
respectively. In both cases, a pronounced parabolic dependence
is observed as it is expected for a Marcus-type behavior. Both
maxima appear around 0.6 eV, corresponding to a total
-
JA970397F
(
30) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545-
-
1
reorganization energy of 13.8 kcal mol . This value is in
50.
(
31) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc.
(
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(