Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF)

Article abstract of DOI:10.1021/jp9941458

1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF = tetrathiafulvalene) to C₆₀ have been used to synthesize a series of novel donor-bridge-acceptor dyads. In these dyads the pyrrolidine[3′,4′: 1,2][60]fullerene is covalently attached to the electron donor TTF either directly (5) or alternatively through one (2a) or two (7) vinyl groups. In the ground state, dyads 2a, 5, and 7 undergo four quasireversible one-electron reductions and two reversible oxidation steps. The former are associated with the reduction of the C₆₀ core, whereas the latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively. Semiempirical PM3 calculations reveal donor-acceptor distances of 4.8 A (5), 7.6 A (2a), and 10.5 A (7), and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an N-methylfulleropyrrolidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is substantially reduced. Furthermore, the fluorescence quantum yield correlates well with the solvent dielectric constant and also with the spatial separation of the donor and acceptor moieties in the dyads. These correlation suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate the (C₆₀^?-)-(TTF^?+) pair. Pico- and nanosecond-resolved transient spectroscopy further substantiate a rapid transformation of the initially formed singlet excited state into the charge-separated radical pair with intramolecular rates ranging between 1.17 × 10¹⁰ s^-1 and 1.47 × 10⁹ s^-1. In all cases, the product of back electron transfer is the triplet excited state, which is generated with markedly high quantum yields (0.61-0.97). The latter is, in addition to the rapid primary intramolecular electron transfer, subject to a slower, secondary intermolecular electron transfer with rate constants of 7 × 10⁸ M^-1s^-1 (5) in benzonitrile and 1.6 × 10⁹ M^- s^-1 (5) in toluene.

Full text of DOI:10.1021/jp9941458

4648
J. Phys. Chem. A 2000, 104, 4648-4657
Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine
Dyads Containing Tetrathiafulvalene (TTF)
Nazario Mart´ın,* Luis Sa´nchez, and Ma Angeles Herranz
Departamento de Qu´ımica Orga´nica I, Facultad de Qu´ımica, UniVersidad Complutense,
E-28040 Madrid, Spain
Dirk M. Guldi*
Radiation Laboratory, UniVersity of Notre Dame, South Bend, Indiana
ReceiVed: NoVember 22, 1999; In Final Form: February 22, 2000
1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF ) tetrathiafulvalene) to C60 have been used to
synthesize a series of novel donor-bridge-acceptor dyads. In these dyads the pyrrolidine[3′,4′:1,2][60]fullerene
is covalently attached to the electron donor TTF either directly (5) or alternatively through one (2a) or two
(7) vinyl groups. In the ground state, dyads 2a, 5, and 7 undergo four quasireversible one-electron reductions
and two reversible oxidation steps. The former are associated with the reduction of the C60 core, whereas the
latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively.
Semiempirical PM3 calculations reveal donor-acceptor distances of 4.8 Å (5), 7.6 Å (2a), and 10.5 Å (7),
and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an
N-methylfulleropyrrolidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is
substantially reduced. Furthermore, the fluorescence quantum yield correlates well with the solvent dielectric
constant and also with the spatial separation of the donor and acceptor moieties in the dyads. These correlation
suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate
the (C60•-)-(TTF•+) pair. Pico- and nanosecond-resolved transient spectroscopy further substantiate a rapid
transformation of the initially formed singlet excited state into the charge-separated radical pair with
intramolecular rates ranging between 1.17 × 1010 s-1 and 1.47 × 109 s-1. In all cases, the product of back
electron transfer is the triplet excited state, which is generated with markedly high quantum yields (0.61-
0.97). The latter is, in addition to the rapid primary intramolecular electron transfer, subject to a slower,
secondary intermolecular electron transfer with rate constants of 7 × 108 M-1s-1 (5) in benzonitrile and 1.6
× 109 M-1 s-1 (5) in toluene.
Introduction
pendant fullerene acts as an electron-acceptor component. By
following these synthetic approaches, different donor fragments
have been attached to C60 to form C60-based dyads and triads.11
The rational design of molecules capable of achieving
photoinduced charge separation with a high quantum efficiency
has been an important task in chemistry for the last years.1-5
The advent of fullerenes6,7 as promising electroactive chro-
mophores opened up a new approach to this goal. The redox
properties of the ground and excited states of [60]fullerene have
been used for the design of devices with complex functions,
such as artificial photosynthetic systems and photoactive dyads.
This work evoked a lively interest to study fullerenes with regard
to efficient charge separation.8 Furthermore, the combination
of a spherical symmetry, strong pyramidalization of the
individual carbon atoms,9 and the low reorganization energy of
the carbon network,10 renders [60]fullerenes unique candidates
for building blocks of more complex systems. As a consequence,
a wide variety of electron-donor units have been covalently
attached to the C60 framework in recent years.11
Among the various methodologies that enable the linkage of
a broad spectrum of donor moieties to C60, cycloaddition
reactions play an outstanding role.12 Specifically, Diels-Alder
and 1,3-dipolar cycloadditions are particularly successful.13
Alternatively, the Bingel’s reaction14 [e.g., addition of bis-
(ethoxycarbonyl)methylene groups] proved to be another good
concept for the design and preparation of dyads in which the
Tetrathiafulvalenes (TTFs) are remarkable with respect to
their involvement in redox reactions. In their one-electron
oxidized form (namely, the 1,3-dithiolium cation), they display
a (4n + 2) aromatic character, in contrast to the ground state.
In view of electron-transfer processes, this gain of aromaticity
is an important requisite, assisting in the stabilization of charge-
separated radical pairs. Additionally, TTF and its derivatives
undergo two one-electron oxidation steps at well-defined redox
potentials to form the corresponding radical cations and di-
cations. The latter have been successfully employed as starting
materials for the preparation of electrically conducting and
superconducting TTF salts and TTF charge-transfer complexes.15
The systematic functionalization of TTF is a very active
research field of interdisciplinary interest. For example, 616 was
prepared in a one-pot synthesis involving monolithio-TTF and
5-(N,N-diethylamino)-4-pentadienal17 by following Jutz’s pro-
cedure.18 The latter concept has been extended to other
organolithium systems by Friedly et al.19
We have recently reported the synthesis of TTF-containing
organofullerenes 1 and 2, via a 1,3-dipolar cycloaddition of
azomethine ylides to C60 starting from a formyl-TTF precursor.20
10.1021/jp9941458 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/21/2000

One-Electron Transfers in Fulleropyrrolidine Dyads
J. Phys. Chem. A, Vol. 104, No. 19, 2000 4649
CHART 1
efficiency of intramolecular electron-transfer processes.24 In this
paper we report on the synthesis, electrochemistry, and photo-
physical properties of novel C60-TTF dyads 5 and 7, in addition
to the previously synthesized organofullerene 2a.20 In these
compounds, the TTF donor unit is attached to the fulleropyr-
rolidine moiety through a single σ-bond (5) and one (2a) or
two (7) vinyl spacers. In this context, effects will be elucidated
that show the influence of both the distance between donor and
acceptor moieties and the solvent polarity on intramolecular
dynamics and the efficiency of charge separation.
Most of the reported fullerene-donor dyads (e.g., ferrocene
and aniline) display a back electron-transfer process following
photoinduced electron transfer that leads to a direct regeneration
of the singlet ground state. This process can be rationalized in
terms of the oxidation potentials of the ferrocene and aniline
donor moieties, which consequently move the energies of the
charge-separated states [(C60•-)-(Fc•+) and (C60•-)-(aniline•+)]
well below that of the fullerene triplet state. It will be shown
that in TTF-containing dyads 2a, 5, and 7, the triplet excited
state is actually the result of back electron-transfer. The high
quantum yield of the triplet excited state is beneficial by enabling
a slower, secondary intermolecular electron transfer in addition
to the rapid primary intramolecular electron transfer.
Results and Discussion
Synthesis. Target molecules 5 and 7 were prepared via 1,3-
dipolar cycloaddition of azomethine ylides to C60.25 Accordingly,
the N-unsubstituted dyad 5 was obtained, in moderate yield
(30% yield, 48% yield based on reacted C60), through a reaction
of formyltetrathiafulvalene (4)26 with glycine and [60]fullerene.
Similarly, dyad 7 was synthesized (27% yield, 53% yield based
on reacted C60) by reacting the formyl-containing TTF derivative
6 with sarcosine (N-methylglycine) and [60]fullerene.
Formyl-containing TTFs are usually prepared by lithiation
of TTF and further reaction with the formylating reagent.
Preparation of formyl-TTF vinylogues requires a subsequent
Wittig reaction of formyl-TTF with phosphoranes. Recently,
we have developed a facile one-pot synthesis of formyl-TTF
vinylogues (such as 6) in satisfactory yields from monolithio-
TTF and commercially available, or easily synthesized, unsatu-
rated N,N-dimethylaminoaldehydes.16
In these compounds, the TTF unit is either directly linked to
the fullerene pyrrolidine ring21 (1a,b) or through an ethylene
spacer (2a,b) (Chart 1). In the solid state, 1a and 2a form charge-
transfer (CT) complexes with strong electron-acceptors, such
as 2,3,5,6-tetrafluor-7,7,8,8-tetracyano-p-quinodimethane (TC-
NQF4), which at a 1:1 donor:acceptor stoichiometry display
semiconducting behavior.20
The C60-TTF donor-bridge-acceptor dyads of type 3 have
been prepared by Diels-Alder cycloaddition of the respective
o-quinodimethane analogues of TTF and [60]fullerene.22 The
cationic and anionic species of compounds 3a-c give rise to
electron paramagnetic resonance (EPR) signals and 32S hyperfine
coupling constants that reveal spin density distributions located
on the TTF and [60]fullerene moieties, respectively.23 In
nanosecond-resolved flash photolysis, these systems undergo a
rapid quenching of the triplet excited states generating, in the
case of 3a and 3b, transient charge-separated open-shell species
with lifetimes typically around 75 µs.23 No information is given,
however, with respect to the potential participation of the
fullerene singlet excited state in intramolecular electron transfer
processes. The scope of our current study is to apply a time
resolution of a few tens of picoseconds, to monitor the entire
dynamic range and to optimize the charge separation via a
controlled separation of the donor-acceptor moieties.
The structures of 5 and 7 were characterized by ultraviolet-
visible (UV-vis), Fourier transform infrared (FTIR), proton and
carbon-13 nuclear magnetic resonance (1H and 13C NMR,
respectively) and fast-atom bombardment (FAB) spectroscopy.
1
Specifically, the H NMR spectrum of 5 reveals a pyrrolidine
proton doublet at δ 5.01 and δ 4.77 (J ) 10.2 Hz, geminal
hydrogens) followed by a singlet at 5.63 (CH), whereas 7 shows
a doublet at δ 4.85 and δ 4.12 (J ) 9.3 Hz) and a doublet at δ
4.38 (CH, J ) 8.7 Hz).
The 13C NMR spectra of dyads 5 and 7 show that although
the resonances observed were less than expected due to the
superposition of several signals, the number of resonances
reveals the lack of symmetry in these compounds. In addition
to the N-Me group of 7, which appears at 40.1 ppm, there are
signals for the sp3 carbons of the pyrrolidine ring and for the
6,6-junction of the C60 framework. The latter appear at 77.2,
72.7, 71.4, and 61.0 ppm (for 5) and 81.8, 77.2, 69.7, and 69.3
ppm (for 7), which is in good agreement with other N-methyl
fulleropyrrolidine derivatives.25
The positive liquid secondary ion mass spectra (LSIMS) in
3-nitrobenzylic alcohol (NBA) matrix showed the molecular ions
for 5 and 7 at m/z 965 and 1031, respectively.
The nature and length of the spacer connecting both donor
and acceptor moieties exert a profound impact on the rate and

4650 J. Phys. Chem. A, Vol. 104, No. 19, 2000
Mart´ın et al.
TABLE 1: Redox Properties of Fullerene-TTF Dyads 2a, 5
In conclusion, the CV measurements confirm that both the
donor and acceptor components preserve their individual
electroactive identity and that they lack any mutual electronic
interaction in the ground state. These findings are in good
agreement with the assumption derived from the electronic
ground-state spectra as well as from Raman studies on dyads
1a21 and 3a-d.22,23
and 7a
compound E1
E2
E1
E2
E3
E4
red
1/2ox
1/2ox
red
red
red
5
2a
7
TTF
C60
0.51
0.43
0.48
0.37
0.87
0.76
0.83
0.70
-0.65 -1.08 -1.65 -2.12
-0.67 -1.08 -1.58 -2.19
-0.66 -1.04 -1.65 -2.17
-0.60 -1.00 -1.52 -2.04
PM3-Optimized Structures. Previously, we reported the
PM3-optimized structure of dyad 1a together with the calculated
molecular orbital (MO) distribution.28 In dyad 1a, the highest
occupied molecular oribtal (HOMO) is localized on the TTF
end with essentially the same orbital composition as the parent
TTF. In contrast to pristine C60, the three degenerate MOs are
split into three different orbitals in dyad 1a. However, the
LUMO is still located on the fullerene framework and lacks
any noticeable contribution from the organic addend.28 It is
important to note that this molecular distribution remains
unchanged in dyads 2a, 5, and 7.
a All potentials in V versus SCE; Tol:MeCN (4:1) as solvent; scan
rate, 200 mV/s; 0.1 mol‚dm-3 Bu4N+ClO4 as supporting electrolite;
-
GCE as working electrode.
To determine the distance between the TTF donor and the
C60 acceptor framework, we calculated the structures of 2a, 5,
and 7, in a similar fashion, by semiempirical PM3 geometry
minimizations with Hyperchem 5.1.31 The optimized geometries
of dyads 2a, 5, and 7 are shown in Figure 2. The bond numbers
between the C60 surface and the closest sulfur atoms are 3, 5,
and 7, which leads to through-bond distances of 4.8 Å (5), 7.6
Å (2a), and 10.5 Å (7). In addition, the interchromophore
distances from the closest sulfur atom of the TTF moiety to the
C60 surface amount to 3.8, 5.2, and 7.2 Å in dyads 5, 2a, and
7, respectively.
Figure 1. Cyclic voltammetry of fulleropyrrolidine 5 at 200 mV/s in
-
toluene:MeCN (4:1, v/v) at room temperature using Bu4N+ClO4 as
supporting electrolyte.
Finally, a deviation from planarity of ∼30° is noted between
the TTF unit and the vinylogous spacers leading to a slightly
distorted configuration that has no significant impact on the
calculated distances between the two centers. The energy of
the latter is lowered by 0.1-0.2 kcal/mol relative to a planar
configuration.
Photophysical Measurements. Steady-State Emission Stud-
ies: Intramolecular Electron-Transfer EVent. The combination
of an electron acceptor that exhibits a low reorganization energy
and an electron donor that gains aromaticity on oxidation is
appealing in light of photoactive materials that are designed
for rapid intramolecular electron-transfer processes. A conve-
nient means to probe such electron-transfer events, involving,
for example, the fullerene singlet excited state in dyads 2a, 5
and 7 and a covalently attached electron donor (TTF), is steady-
state fluorescence at ambient temperature or in frozen matrixes.
The UV-vis spectra of 5 and 7 display an absorption band
around 430 nm, which is a characteristic signature of [6,6]-
closed fullerene derivatives. Accordingly, this signature unmis-
takably confirms the linkage of the organic TTF-addends at the
6,6-ring junction of the C60 framework.27 In room-temperature
solutions, the singlet ground-state spectra of dyads 1 and 2
provide no evidence for a charge-transfer interaction between
the two electroactive moieties. In contrast to the condensed
phase, solid-state EPR spectra of 1a (both at room temperature
and 5.2 K) give rise to weak signals that were assigned to solid-
state interaction.28 Furthermore, these observations resemble
those noticed in EPR studies carried out with dyads 3a-c.23
Electrochemistry. The electrochemical properties of dyads
2a, 5, and 7 have been studied by cyclic voltammetry at room
temperature. The corresponding data are collected in Table 1
along with the redox potentials of TTF and the C60 reference.
Dyads 2a, 5, and 7 give rise to four quasireversible one-
electron reduction waves that reflect the individual reduction
steps of their fullerene cores (Figure 1). In general, these
reduction potential values are shifted to more negative values,
relative to [60]fullerene. This shift stems from the saturation
of a double bond in the C60 core that raises the lowest
unoccupied molecular orbital (LUMO) energy of the resulting
organofullerene accordingly.29 It should be noted that the first
reduction potential values for these dyads (5, 2a, and 7) are all
quite similar. The noted resemblance indicates that the presence
of the vinylogous spacers between the donor and acceptor
moieties has no measurable effect on the electron-accepting
properties of the C60 moiety.
Considering its simplicity, an N-methylfulleropyrrolidine was
chosen as an internal reference. It is well known that the
nitrogen-free lone pair, located at the pyrrolidine ring of this
derivative, does not engage in any electron-donating events with
the fullerene core. Thus, the noticeably higher fluorescence
quantum yield of this fullerene reference (Φ ) 6.0 × 10-4
related to the partially broken symmetry of the former.32
)
relative to pristine [60]fullerene (Φ ) 1.5 × 10-4) has been
The room-temperature fluorescence (i.e., emission from the
singlet excited state) of dyads 2a, 5, and 7 (9 × 10-6 M) in
toluene solutions, together with the aforementioned fullerene
reference, are summarized in Figure 3. In general, the emission
spectra of dyads 2a, 5, and 7 exhibit structural patterns
superimposable on the fullerene reference. Specifically, a strong
On the anodic side, two reversible oxidation waves are
observed at positive potentials, corresponding to the radical
cation and dication species of the TTF fragment, respectively.
Compared with TTF, the oxidation potentials are, however,
anodically shifted by ∼100 mV. This shift can be rationalized
in terms of the substitution effects on the TTF unit.30
*
*0f0 transition at 710 nm is followed by a series of 0f1,
*0f2, etc. emissions. These emissions reflect the texture of the
(C2v′) symmetry of the fullerene core and, more importantly,
are virtual mirror images of the singlet ground state absorption
bands at 622, 634, 653, 666, 677, 687, and 699 nm. Hereby,

One-Electron Transfers in Fulleropyrrolidine Dyads
J. Phys. Chem. A, Vol. 104, No. 19, 2000 4651
Figure 3. Fluorescence spectra (τexc ) 398 nm) of N-methylfullero-
pyrrolidine (b) and dyads 7 (O), 2a (×), and 5 (+) in toluene solutions
(9 × 10-6 M) at room temperature.
(1.76 eV), which rules out an alternative singlet energy transfer
mechanism from the singlet excited fullerene state to the TTF
moiety.
It is interesting to note that the fluorescence intensities of
dyads 2a, 5, and 7 reveal a good correlation with their spacer
length; that is, the spatial separations between the electron-
acceptor (fullerene) and electron-donor moiety (TTF) of 4.8 Å
(2a), 7.6 Å (5), and 10.5 Å (7).
The dielectric continuum model helps to quantify intramo-
lecular electron-transfer processes and in essence predicts a
strong impact of the solvent polarity on the redox potential of
the donor-acceptor couple and thus on the free energy change
of the reaction (-∆G°).33 In particular, the exothermicity is
expected to increase with the solvent dielectric constant. Thus,
in the current context, the fluorescence quantum yields of dyads
2a, 5, and 7 were compared in methylcyclohexane, toluene,
dichloromethane, and benzonitrile solutions with dielectric
constants of 2.02, 2.38, 9.81, and 25.9, respectively. Corre-
sponding experiments in these solvents engenders the expected
trend; namely, the stepwise increase of the quenching efficiency
on progressing solvent polarity (Table 2). This result clearly
supports an intramolecular electron-transfer mechanism (eq 1).
Table 2 lists the calculated (-∆G°) values for the electron-
transfer evolving from the fullerene singlet excited state. In
general, a good correlation between the (-∆G°) values and the
quantum yields is noted. From the quantum yields (Φ) of the
fluorescence and the lifetime (τ) of the fullerene reference (N-
methylfulleropyrrolidine; see Table 2), we are able to extract
the singlet lifetimes (τSINGLET) and the rate constants (kET) of
the corresponding electron-transfer reaction of dyads 2a, 5, and
7 according to the following expression (see Table 2):4
Figure 2. PM3 calculated geometries for dyads 5, 2a, and 7, showing
the spatial distances between the TTF and C60 chromophores.
the energy of the spectroscopically important *S1fS0 transition,
which depicts the singlet excited-state energy, is a fundamental
parameter for quantifying the envisaged electron-transfer proc-
esses (vide infra).
In addition, the similarity of the emission spectra confirms
the selective excitation of the fullerene core in dyads 2a, 5,
and 7. Under identical scanning conditions, all three dyads give
rise to substantially lower emission yields relative to the
reference. Considering the moderate oxidation potential of the
TTF moiety, which is comparable to electron donors such as
ferrocene and aniline,32 we tentatively ascribe the observed
emission quenching to the following photoinduced electron-
transfer (ET) event:
kET ) [Φ(ref) - Φ(x)]/[τ(ref) Φ(x)] + kISC
(2)
A closer look reveals that the calculated thermodynamic
driving force (-∆G°) and the measured singlet lifetimes of the
two vinyl-linked dyads 2a and 7 do not follow exactly the trend
noticed for the directly linked dyad 5. In this context, and for
the sake of simplicity, it is important to note that the calculations
just presented have been based on the spatial distance between
the two redox active moieties (therefore, any difference in
electronic coupling is neglected). Accordingly, a possible
rationale for the difference in the deactivation of the singlet
excited states of 2a and 7 stems from the deviation from
planarity between the TTF unit and the vinylogous spacers of
C60-TTF
9
hν8 (1*C60)-TTF
9
ET8 (C60•-)-(TTF•+) (1)
On the other hand, the lowest singlet excited state of TTF
(2.8 eV) is higher than the lowest excited state of the fullerene

4652 J. Phys. Chem. A, Vol. 104, No. 19, 2000
Mart´ın et al.
TABLE 2: Photophysical Parameters of Fullerene Reference and Fullerene-TTF Dyads 2a, 5 and 7
solvent
toluene
parameter
(∆G°)ET Intraa
-0.512 eV
0.36 × 10-4
9.1 × 109
129 ps
-0.179 eV
1.6 × 10-4
1.6 × 109
395 ps
-0.008 eV
ΦFLUORESCENCE
6.0 × 10-4
2.58 x 10-4
7.7 × 108
680 ps
b
kET [s-1
]
τSINGLET
c
k
ET [s-1
]
3.45 × 108f
7.8 × 109
not resolved
0.96
2.5 × 109
not resolved
0.97
1.5 × 109
not resolved
0.96
-0.27 eV
2.5 × 109
1.8 × 109
-0.714 eV
1.92 × 10-4
1.2 × 109
450 ps
kBET [s-1
]
c
c
ΦTRIPLET
0.96
CH2Cl2
(∆G°)ET Interd
-0.25 eV
1.6 × 109
2.0 × 109
-0.880 eV
0.24 × 10-4
1.4 × 1010
112 ps
-0.31 eV
e
k
ET [M-1 s-1
]
kOXYGEN [M-1 s-1
(∆G°)ET Intraa
]
2.2 × 109
-0.833 eV
1.32 × 104
2.5 × 109
289 ps
ΦFLUORESCENCE
kET [s-1
]
b
τsinglet
c
k
ET [s-1
]
8.9 × 109
6.9 × 108
0.88
-0.669 eV
3.9 × 109
-0.990 eV
0.12 × 10-4
2.8 × 1010
85 ps
3.5 × 109
5.4 × 108
0.96
2.2 × 109
5.1 × 108
0.97
-0.689 eV
3.6 × 109
-0.937 eV
0.98 × 10-4
2.9 × 109
259 ps
kBET [s-1
]
c
ΦTRIPLET
(∆G°)ET Interd
-0.729 eV
3.2 × 109
-0.999 eV
0.74 × 10-4
4.1 × 109
184 ps
k
OXYGEN [M-1 s-1
]
benzonitrile
(∆G°)ET Intraa
ΦFLUORESCENCE
b
kET [s-1
]
τSINGLET
c
k
ET [s-1
]
1.2 × 1010
4.6 × 108
0.61
-0.768 eV
7.0 × 108
3.1 × 109
5.4 × 109
4.2 × 108
0.67
3.9 × 109
3.7 × 108
0.66
-0.788 eV
9.1 × 108
3.0 × 109
c
kBET [s-1
]
ΦTRIPLET
(∆G°)ET Interd
-0.827 eV
e
k
ET [M-1 s-1
]
kOXYGEN [M-1 s-1
]
3.0 × 109
a This determination was performed following the Continuum model: e.g., ∆GET ) ∆E0-0 - EOX + ERED - ∆GS; ∆GS ) e2/(4∆ꢀ0) [(1/2R+
+
1/2R- - 1/RD-A) 1/ꢀS - (1/2R+ + 1/2R-) 1/ꢀR ]; EOX ) E1/2 (TTF/TTF•+); ERED ) E1/2 (C60/C60•-); R+ ) radius TTF (7.6 Å); R- ) radius C60 (4.8
Å); ꢀS ) dielectric constant of solvent used for photophysical studies; ꢀR ) dielectric constant of solvent used for measuring the redox potential,
namely, toluene:acetonitrile, 4:1 (ꢀR ) 9.24); ∆E0-0 ) excited-state energy of C60 (1.762 eV). b Intramolecular electron transfer calculated from
1
emission studies (eq 2) and assuming that the natural rate constants are the same as that of the reference compound. c Intramolecular electron and
back electron transfer measured from picosecond experiments. d Determined as described in footnote a, but assuming 5 Å separation (RD-A) and
∆E0-0 ) excited-state energy of C60 (1.50 eV). e Intermolecular electron transfer measured from nanosecond experiments. τISC measured from
3
f
picosecond experiments.
∼30° in dyads 2a and 7. This distortion certainly impacts the
coupling in these conjugated systems (2a and 7) relative to 5
and, in turn, may slow the electron transfer.
Transient Spectroscopic Techniques: Intramolecular Electron-
Transfer EVent. The lower fluorescence quantum yields (i.e.,
the rapid deactivation of the photoexcited fullerene moieties)
in dyads 2a, 5, and 7 relate to their effective engagement with
the TTF centers. Pico- and nanosecond-resolved photolysis
spectroscopic techniques are powerful means to characterize
dynamic processes that are associated with the generation and
fate of photoexcited states and, thus, complement the emission
studies. With the scope to shed further light on the deactivation
of the fullerene singlet excited-state, we probed these C60-TTF
dyads in transient flash photolysis following 18 ps 355 nm and
8 ns 337 nm laser pulses.
Picosecond photolysis of N-methyl fulleropyrrolidine (2.0 ×
10-5 M) in oxygen-free toluene solutions leads to transient
Figure 4. Time-resolved difference absorption spectra of excited singlet
absorption changes revealing a maximum around 895 nm
states and excited triplet states of N-methylfulleropyrrolidine (2.0 ×
10-5 M) recorded 0, 50, 100, 500, 1000, 2000, 3000, and 4000 ps after
(Figure 4). Typically, this formation process is completed ∼50
excitation (355 nm) in oxygen-free toluene solution.
ps after the 355 nm laser excitation. These changes resemble
the general observation for pristine C60 in toluene and are,
accordingly, attributed to the singlet excited-state absorption (*S1
f *Sn). Its decay follows clean first-order kinetics and results
in the formation of the (T1 f Tn) absorption centered around
710 nm. It should be noted that for this fullerene reference, the
associated singlet-triplet conversion (ISC) occurs with a rate
(kISC) of 3.45 108 s-1
.

One-Electron Transfers in Fulleropyrrolidine Dyads
J. Phys. Chem. A, Vol. 104, No. 19, 2000 4653
Figure 6. Transient absorption spectrum (UV-vis part) recorded 50
ns (b) and 20 µs (O) after flash photolysis of 2.0 × 10-5 M 5 at 337
nm in deoxygenated benzonitrile.
lifetime of the radical pair can be rationalized, at least in part,
by the small spacer separating the electron-acceptor and electron-
donor moieties in dyad 5. In contrast, the larger spacers in dyads
2a and 7 enhance the lifetime of (C60•-)-(TTF•+), with rates
(kBET) ranging between 5.35 × 108 and 3.73 × 108 s-1. The
strong coupling induced by the π-conjugation in these rigidly
spaced dyads (2a and 7) undoubtedly mediates the fast back
electron transfer (BET) in the latter two.
In all cases, the product of BET is the fullerene triplet excited-
state characterized, as already mentioned, by a (T1 f Tn)
absorption around 710 nm. The spectral assignment is further
substantiated by the concurrent decay of the (C60•-)-(TTF•+)
radical pair absorption between 800 and 900 nm and the growing
in of the (3*C60)-TTF excited-state absorption at 710 nm
(Figure 5b).
Figure 5. Time-resolved difference absorption spectra of excited singlet
states and excited triplet states of dyad 5 (2.0 × 10-5 M) recorded (a)
0, 50, 300, and 500 ps after excitation (355 nm) and (b) 0 and 3500 ps
after excitation in oxygen-free benzonitrile solution.
(C60•-)-(TTF•+) BET8 (3*C60)-TTF
(3)
To illustrate the generation of the triplet excited state, Figure
6 depicts the differential absorption changes recorded for 5 (2.0
× 10-5 M) in oxygen-free benzonitrile solutions immediately
(50 ns) after a 337 nm laser pulse. The set of two maxima at
380 and 710 nm is in close agreement with the triplet-triplet
characteristics of earlier published spectra,34 substantiating the
generation of the triplet excited state. Probing a similar solution
of the fullerene reference allowed the determination of the
quantum yield of the fullerene triplet in dyads 2a, 5, and 7 via
relative actinometry. Relative to the fullerene reference (Φ )
0.96), the photoexcited dyads gave rise to similar triplet quantum
yields in dichloromethane solutions (Φ ) 0.88-0.97). In
benzonitrile, however, the quantum yields were slightly lower
(Φ ) 0.61-0.87). The lower energy and longer lifetime of the
(C60•-)-(TTF•+) pair in benzonitrile solutions is a possible
rationale for this difference.
Intermolecular One-Electron Transfer. The triplet lifetimes
of the fullerene core in dyads 2a, 5, and 7 exhibit a surprising
dependence on the dyad concentration. In light of the oxidative
power of the TTF addends, this observation suggests that the
triplet excited state may be subject to an additional quenching
reaction in addition to any triplet-triplet annihilation processes,
namely, an intermolecular process. To support this hypothesis,
we analyzed the differential absorption changes <20 µs after
the laser pulse. Indeed, the 710 nm absorption band is absent
in the visible region (Figure 6, 20 µs), but a new maximum
around 440 nm and a set of weaker absorption bands at 510,
600, and 640 nm develop. The 440 nm peak, whose growth
A similar formation of the fullerene singlet excited state (λmax
) 890 nm) is found in complementary picosecond experiments
with dyads 2a, 5, and 7 in all investigated solvents (i.e., toluene,
dichloromethane and benzonitrile; Figure 5a). The singlet
lifetimes of the photoexcited fullerene core are significantly
affected by the TTF moiety and also by the relative distance
between the two redox-active sites. Specifically, in nonpolar
toluene solutions, singlet lifetimes of 129, 395, and 680 ps were
measured for dyads 2a, 5, and 7, respectively. Furthermore,
increasing the solvent polarity leads to a further shortening of
the singlet lifetime for dyad 7, for example. In conclusion, the
distance and solvent dependence of the observed intramolecular
rates resemble the results of the emission studies (see Table 2).
The transient absorption changes, typically recorded after the
completion of the intramolecular transfer process, display no
resemblance to the triplet excited state of N-methylfullero-
pyrrolidine; that is, the (T1 f Tn) absorption at 710 nm is clearly
absent (see Figure 5; 5 in benzonitrile). Instead, the spectrum
is characterized by a broad absorption (800-900 nm) that is
similar to the one observed following intermolecular electron
transfer (vide infra; Figure 7). Thus we tentatively ascribe this
transient species to a charge-separated radical pair, namely,
(C60•-)-(TTF•+).
In general, the lifetime of the (C60•-)-(TTF•+) pair in dyads
2a, 5, and 7 is very short. In dyad 5, for example, back electron
transfer rates (kBET) of 6.89 × 108 and 4.59 × 108 s-1 were
measured in CH2Cl2 and benzonitrile, respectively. The short

4654 J. Phys. Chem. A, Vol. 104, No. 19, 2000
Mart´ın et al.
matches the decay of the triplet absorption (710 nm), is in good
agreement with the radiolytically generated TTF π-radical
cation, namely, TTF•+ (see Pulse Radiolysis). More importantly,
the spectral region between 750 and 1100 nm shows the time-
resolved growth of a characteristic near-IR absorption band
(Figure 7), whose formation match the accelerated decay of the
710 nm absorption. The identity of this band with the diagnostic
peak of various fullerene π-radical anions35 unquestionably
confirms the formation of a charge-separated radical pair [e.g.,
(C60•-)-(TTF)/(C60)-(TTF•+)], evolving from a photoinduced
electron transfer from the triplet and subsequent diffusional
dissociation of the contact pair. (The absorption band around
810 nm helps to characterize the charge-separated radical pair
on the picosecond time scale.)
(3*C60)-TTF + C60-TTF
9
ET8
Figure 7. Transient absorption spectrum (near-IR part) recorded 20
µs (b) after flash photolysis of 2.0 × 10-5 M 5 at 337 nm in
deoxygenated benzonitrile.
(C60•-)-TTF + C60-(TTF•+) (4)
Enlarging the dyad (2a, 5, and 7) concentration and, therefore,
quencher concentration, resulted in an increasingly faster
deactivation of the triplet excited state. The applied dyad
concentrations [(1.0-0.05) × 10-4 M] gave rise to a linear
dependence of the decay rate that, in turn, further supports a
reductive quenching mechanism involving (3*C60)-TTF. In
benzonitrile, for example, an intermolecular quenching rate of
7 × 108 M-1 s-1 was determined for 5. A similar deactivation
of the triplet excited state was noted in corresponding experi-
ments in toluene and dichloromethane solutions. Stabilization
of the radical pair is, however, largely hampered by the
insufficient solvent polarity of these solvents. Accordingly,
kinetic measurements in the concentration window (1.0-0.05)
× 10-4 M led to an accelerated decay of the triplet absorption,
yielding the singlet ground state with a rate constant of 1.6 ×
109 M-1 s-1 in toluene. The excited-state behavior of dyads 2a
and 7 also follows this trend.
Figure 8. Transient absorption spectrum (UV-vis region) obtained
after pulse radiolysis of TTF (6.0 × 10-5 M) in oxygenated dichloro-
methane solutions.
In benzonitrile solutions, the charge-separated radical pair is
relatively stable and decays slowly over a few hundred
microseconds, quantitatively yielding the singlet ground state.
The lifetime of the (C60•-)-TTF depends mainly on the dyad
concentration [(2.0-0.5) × 10-5 M]. Specifically at high dyad
concentrations that lead to a higher yield of charge-separated
radicals, the lifetime is reduced. In fact, the decay of (C60•-)-
TTF (5) was found to be a second-order reaction, according to
particular, pulse radiolysis of 5.0 × 10-5 M TTF in dichloro-
methane leads to differential absorption changes depicted in
Figure 8. They disclose a pronounced maximum at 430 nm,
similar to those seen in the nanosecond photolytic experiments
(see Figure 6). Applying TTF at variable concentrations (e.g.,
ranging between 5.0 × 10-5 and 2.0 × 10-3 M) resulted in an
accelerated formation of the corresponding (TTF•+) absorption
at 430 nm. The observed rate (kobs ) ln 2/τ1/2) is linearly
dependent on the TTF concentration, indicating that the
underlying process is due to the following oxidation reaction
(with a rate constant of 4.2 × 108 M-1 s-1):
eq 5, with k2 ) 3.9 × 109 M-1 s-1
.
(C60•-)-TTF + C60-(TTF•+) BET8
C60-TTF + C60-TTF (5)
In the presence of molecular oxygen, the electron-transfer
route from the triplet state competes with an intermolecular
energy transfer to generate singlet oxygen (1O2) (eq 6). At dyad
concentrations of 3.0 × 10-5 M (5), the fullerene triplet reacts
with oxygen with rate constants of 2.0 × 109, 3.9 × 109, and
3.1 × 109 M-1 s-1 in toluene, dichloromethane, and benzonitrile,
respectively. It should be noted that similar rate constants were
also derived for the fullerene reference and dyads 2a and 7.
TTF + •O2CHCl2/•O2CH2Cl f (TTF•+)
(7)
It should be noted that the initially formed solvent radical
cation, [CH2Cl2]•+, despite being a powerful oxidant, does not
engage with the TTF moiety on the monitored time-scale (up
to 200 µs) because of the too short lifetime of [CH2Cl2]•+.
Summary and Conclusions
(3*C60)-TTF + O2 f C60-TTF + 1O2
(6)
Novel C60-based dyads, in which the C60 core is covalently
linked to the electron donor tetrathiafulvalene (TTF), have been
synthesized by 1,3-dipolar cycloadditions of in situ-generated
TTF-containing azomethine ylides to C60. Both donor and
acceptor moieties are connected through a pyrrolidine ring (5)
and one (2a) or two (7) additional vinyl groups. The electro-
chemical study of these dyads reveals, regardless of the spacer,
Finally, in an attempt to confirm the absorption of the singly
oxidized TTF moiety, pulse radiolytic oxidation experiments
were carried out in oxygenated dichloromethane solutions.
Radiolysis of this medium is known to provide the means for
a selective one-electron oxidation process (see eq 7).36 In

One-Electron Transfers in Fulleropyrrolidine Dyads
J. Phys. Chem. A, Vol. 104, No. 19, 2000 4655
SCHEME 1
SCHEME 2
a very similar behavior. In general, all dyads show four reduction
waves, corresponding to the first four reduction steps of the
C60 core, and two oxidation waves at positive values, due to
the formation of the radical cation and dication species of the
TTF unit. Semiempirical calculations at the PM3 level show
the spatial interchromophore distances and reveal a configuration
in which the TTF fragment and the vinylogous spacers deviate
from planarity but exhibit an energy similar to that calculated
for the planar configuration.
Steady-state and time-resolved photolysis reveal that the
fullerene singlet excited states in dyads 2a, 5, and 7 are subject
to rapid intramolecular electron-transfer events yielding a short-
lived charge-separated radical pair, namely, (C60•-)-(TTF•+).
This result can be rationalized by the close distance of the
donor-acceptor couple, for example in dyad 5, or the strong
coupling, induced by the vinylogous spacers in dyads 2a and
7. In contrast to fullerene-ferrocene and fullerene-aniline
dyads, back electron transfer in 2a, 5, and 7 proceeds mainly
via formation of the fullerene triplet excited state. The energy
level of the latter (∼1.50 eV) is still sufficient to activate a
second, intermolecular electron transfer, as noticed earlier in
bimolecular quenching between the triplet excited state of
pristine C60 and TTF. Similarly, it is safe to assume that an
intermolecular triplet quenching of the N-methyl fulleropyrro-
lidine (i.e., the fullerene core in dyads 2a, 5, and 7) takes place;
Scheme 2 illustrates this for dyad 2a in toluene solutions.
In benzonitrile solutions, the energy level of the intramo-
lecular radical pair in dyads 2a, 5, and 7 (∼0.8 eV) falls,
nevertheless, below that of the fullerene triplet excited state (1.50
eV). Despite the energetic argument, the nanosecond measure-
ments provide unambiguous evidence for the fullerene triplet
excited state, with lower quantum yields relative to that in
toluene solutions.
radical pair. We are currently intensifying our work to calculate
the solvation energies of the (C60•-)-(TTF•+) and (C60•-)/
(TTF•+) pairs more precisely. A more likely reason for this
surprising observation may arise from an electron-transfer
process in the singlet excited state that proceeds with an
efficiency less than unity.
Experimental Section
1
General Details. The H NMR and 13C NMR spectra were
obtained with a Varian VXR 300. Mass spectra were recorded
with a VC Autospec EBE spectrometer operating at 30 kV, using
a bombardment of Cs+ ions and 2-nitrophenyloctyl ether (2-
NPOE) or 3-NBA as matrix. The FTIR spectra were recorded
with a Nicolet Magna-IR spectrometer 5550. Cyclic voltam-
metry measurements were performed on an EG&G PAR
Versastat potentiostat using 250 Electrochemical Analysis
software. A Metrohm 6.0804.C10 glassy carbon electrode was
used as indicator electrode in voltammetric studies 1 × 10-5
M solutions of the compound in dichloromethane, 0.1 M Bu4-
NClO4 as the supporting electrolyte; platinum working and
One argument that could be forwarded is the energy of the
intermolecular radical pair. In the context of stabilizing an
intermolecular radical pair in reference to an intramolecular pair,
the solvation energy of both species should be considered. In
general, the free ions are better solvated than the intramolecular

4656 J. Phys. Chem. A, Vol. 104, No. 19, 2000
Mart´ın et al.
counter electrode; saturated calomel electrode (SCE) as reference
electrode at 20 °C. All chromatography was performed with
Merck silica gel (70-230 mesh). All reagents were used as
purchased unless otherwise stated. All solvents were dried
according to standard procedures. All reactions were carried out
under an atmosphere of dry argon.
(1H, d, J ) 15.3), 6.28 (3H, m), 4.85 (1H, d, J ) 9.3), 4.38
(1H, d, J ) 8.7), 4.12 (1H, d, J ) 9.3), 2.87 (3H, s); 13C NMR
(75 MHz, CDCl3/CS2 1/1) δ 147.2, 146.7, 146.3, 146.2, 146.0,
145.2, 145.1, 145.0, 144.6, 144.3, 144.1, 143.1, 142.6, 142.5,
142.1, 142.0, 140.2, 136.3, 135.5, 135.1, 133.5, 132.7, 130.6,
128.9, 128.1, 125.4, 125.2, 125.0, 123.9, 120.8, 119.5, 119.0,
118.9, 118.6, 111.2, 110.6, 81.8, 77.2, 69.7, 69.3, 40.1; FTIR
(KBr) 1634, 1462, 1330, 1230, 180, 1027, 976, 795, 767, 729,
639, 598, 574, 562, 553, 526 cm-1; UV-vis (CH2Cl2) λmax (log
ꢀ): 232 (3.97), 258 (5.20), 318 (1.83), 432 (0.97) nm; MS
(FAB+) (m/z): 1031 (M+, 19%), 720 (98%).
Semiempirical calculations were carried out at the PM3/RHF
level with the Hyperchem 5.1 program package.31
Picosecond laser flash photolysis experiments were carried
out with 355-nm laser pulses from a mode-locked, Q-switched
Quantel YG-501 DP ND:YAG laser system (pulse width ∼18
ps, 2-3 mJ/pulse). The white continuum picosecond probe pulse
was generated by passing the fundamental output through a D2O/
H2O solution. Nanosecond laser flash photolysis experiments
were performed with laser pulses from a Molectron UV-400
nitrogen laser system (337.1 nm, 8 ns pulse width, 1 mJ/pulse)
in a front-face excitation geometry. The photomultiplier output
was digitized with a Tektronix 7912 AD programmable digi-
tizer.38 Pulse radiolysis experiments were accomplished with
50 ns pulses of 8 MeV electrons from a model TB-8/16-1S
electron linear accelerator. Dosimetry was based on the oxidation
of SCN- to (SCN)2•-, which, in N2O-saturated aqueous solu-
tions, takes place with G ∼ 6 (G denotes the number of species
per 100 eV, or the approximate micromolar concentration per
10 J of absorbed energy). The radical concentration generated
per pulse amounts to (1-3) × 10-6 M for all systems
investigated in this study.39 Absorption spectra were recorded
with a Milton Roy Spectronic 3000 Array spectrophotometer.
Emission spectra were recorded on a SLM 8100 Spectrofluo-
rimeter.
Acknowledgment. We are indebted to DGICYT (Project
PB95-0428-CO2) for financial support. M.A.H. acknowledges
M.E.C. of Spain for a research fellowship. Some of this work
was supported by the Office of Basic Energy Sciences of the
U.S. Department of Energy (contribution no. NDRL-4107) from
the Notre Dame Radiation Laboratory). We thank Dr. G. Hug
for many helpful discussions and Dr. I. Carmichael for the PM3
calculations of the TTF HOMO and LUMO.
References and Notes
(1) Wasielewski, M. R. in Photoinduced Electron Transfer, Fox, M.
A.; Chanon, M., Eds., Elsevier: Amsterdam, 1988.
(2) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
(3) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26,
198.
(4) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18.
(5) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34,
849.
(6) Kroto, H. W.; Heath, J. R.; O′Brien, S. C.; Curl, R. F.; Smalley,
R. E. Nature 1985¸ 318, 162.
(7) Kra¨tsmer, W.; Lamb, L. D.; Fostiropoulus, K.; Huffman, D. R.
Nature 1990, 347, 354.
(8) Sun, Y.-P. In Molecular and Supramolecular Photochemistry;
Ramamurthy, V., Schanze, K., Eds.; Marcel Dekker: New York, 1997;
Vol. 1. Prato, M. J. Mater. Chem. 1997, 7, 1097. Imahori, H.; Sakata, Y.
AdV. Mater. 1997, 9, 537.
(9) Williams, R. M.; Koeberg, M.; Lawson, J. M.; An, Y.-Z.; Rubin,
Y.; Paddon-Row, M. N.; Verhoeven, J. W. J. Org. Chem. 1996, 61, 5055.
(10) Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1997, 119, 5744.
Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.; Okada,
T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545.
(11) For a very recent review on C60-based electroactive organo-
fullerenes, see: Mart´ın, N.; Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. ReV.
1998, 98, 2527.
N-Methyl-2′-[2-(tetrathiafulvalenyl)ethenyl]pyrrolidino-
[3′,4′:1,2][60]fullerene. (2a). This compound was synthesized
by following the previously reported procedure.20,28
2′-(Tetrathiafulvalenyl)pyrrolidino[3′,4′:1,2][60]fuller-
ene (5). An ODCB solution of C60 (100 mg, 0.138 mmol),
formyl-TTF (4) (64.8 mg, 0.277 mmol), and glycine (20.85 mg,
6.277 mmol) was heated to reflux under argon atmosphere for
2 h. The solvent was removed under reduced pressure, and the
crude material was carefully chromatographed on a silica gel
column using cyclohexane and a cyclohexane:toluene (1:1)
mixture as eluent. Further purification was accomplished by
repetitive precipitation and centrifugation using methanol as
(12) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519. Diederich,
F.; Thilgen, C. Science 1996, 271, 317. Taylor, R.; Walton, D. R. M. Nature
1993, 363, 685.
1
solvent. 30% yield (43% based on recovered C60); H NMR
(300 MHz, CDCl3/CS2 1/1) δ: 6.73 (1H, s), 6.31 (2H, s), 5.63
(1H, s), 5.01 (1H, d, J ) 10.2), 4.77 (1H, d, J ) 10.2); 13C
NMR (75 MHz, CDCl3/CS2 1/1) δ: 155.0, 153.5, 152.4, 151.5,
148.0, 147.1, 146.8, 146.1, 146.0, 145.9, 145.8, 145.5, 145.4,
145.3, 145.1, 144.5, 142.5, 142.1, 141.8, 141.5, 134.7, 119.1,
118.9, 117.5, 117.4, 77.2, 72.7, 71.4, 61.0; FTIR (KBr): 2850,
(13) For a recent review on addition reactions of Buckminsterfullerene-
C60, see: Hirsch, A. Synthesis 1995, 895. Diels-Alder reactions were
systematically introduced into fullerene chemistry by Rubin and Mu¨llen;
see: Rubin, Y.; Khan, S.; Freedberg, D. I.; Yeretzian, C. J. Am. Chem.
Soc. 1993, 115, 344. Belik, P.; Gu¨gel, A.; Spickermann, J.; Mu¨llen, K.
Angew. Chem., Int. Ed. Engl. 1993, 32, 78. Gu¨gel, A.; Kraus, A.;
Spickermann, J.; Belik, P.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1994,
33, 559. For recent examples of 1,3-dipolar cycloadditions to C60 see:
Tetrahedron 1996, 52, 4925-5262 (special issue on “Fullerene Chemistry”,
Smith, A. B., III, Ed.).
1736, 1494, 1462, 1261, 1081, 966, 795, 753, 640, 526 cm-1
;
UV-vis (CH2Cl2) λmax (log ꢀ): 232 (4.86), 258 (5.01), 316
(4.63), 430 (3.72) nm; MS (FAB+) (m/z): 965 (M+, 23%), 720
(58%).
(14) Bingel, C. Chem. Ber. 1993, 126, 1957.
(15) Bryce, M. R. J. Mater. Chem 1995, 5, 1481. See also: Handbook
of Organic ConductiVe Molecules and Polymers; Nalwa, N. S., Ed.; John
Wiley & Sons: New York, 1997; Vol. 1.
(16) Gonza´lez, M.; Mart´ın, N.; Segura, J. L.; Gar´ın, J.; Orduna, J.
Tetrahedron Lett. 1998, 39, 3269.
(17) Becher, J. Synthesis 1980, 589.
(18) Jutz, C. Chem. Ber. 1958, 91, 1867.
(19) Friedly, A. C.; Yang, E.; Marder, S. R. Tetrahedron 1997, 53, 2717.
(20) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Andreu, R.; Gar´ın, J.; Orduna,
J. Tetrahedron Lett. 1996, 37, 5979.
(21) Prato, M.; Maggini, M.; Giacometti, C.; Scorrano, G.; Sandona´,
G.; Farnia, G. Tetrahedron 1996, 52, 5221.
(22) Hudhomme, P.; Boulle´, C.; Rabreau, J. M.; Cariou, M.; Jubault,
M.; Gorgues, A. Synth. Met. 1998, 94, 73. Boulle´, C.; Rabreau, J. M.;
Hudhomme, P.; Cariou, M.; Jubault, M.; Gorgues, A.; Orduna, J.; Gar´ın, J.
N-Methyl-2′-[4-(tetrathiafulvalenyl)butadienyl]pyrroli-
dino[3′,4′:1,2][60]fullerene (7). A toluene solution of C60 (200
mg, 0.33 mmol), TTF derivative (6) (80 mg, 0.28 mmol), and
sarcosine (20.85 mg, 6.277 mmol) was heated to reflux for 24
h. The solvent was removed under reduced pressure, and the
crude material was carefully chromatographed on a silica gel
column with a cyclohexane:toluene (1:1) mixture as eluent.
Further purification was accomplished by repetitive precipitation
and centrifugation: 27% yield (53% based on recovered C60);
1H NMR (300 MHz, CDCl3/CS2 1/1) δ: 7.21 (1H, d, J ) 6.9),
7.13 (1H, d, J ) 6.9), 6.72 (1H, dd, J1 ) 15.3, J2 ) 10.2), 6.43

One-Electron Transfers in Fulleropyrrolidine Dyads
J. Phys. Chem. A, Vol. 104, No. 19, 2000 4657
Tetrahedron Lett. 1997, 38, 3909. Llacay, J.; Mas, M.; Molins, E.; Veciana,
J.; Powell, D.; Rovira, C. Chem. Commun. 1997, 659.
(30) Khodorkovsky, V.; Becker, J. I. In Organic Conductors: Funda-
mentals and Applications; Farges, J.-P., Ed.: Marcel Dekker: New York,
1994; Chapter 3.
(23) Llacay, J.; Veciana, J.; Vidal-Gancedo, J.; Bourdelande, J. L.;
Gonza´lez-Moreno, R.; Rovira, C. J. Org. Chem. 1998, 63, 5201.
(24) Balzani, V. In Supramolecular Photochemistry; D. Reidel: Dor-
drecht, The Netherlands, 1987. Gust, D.; Moore, T. A. Photoinduced
Electron Transfer; Mattay, J., Ed.; Spinger: Berlin, 1991; Vol. III. Carter,
F. L. Molecular Electronic DeVices; Marcel Dekker: New York, 1987.
(25) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(26) Green, D. C. J. Org. Chem 1979, 44, 1476. Gar´ın, J.; Orduna, J.;
Uriel, S.; Moore, A. J.; Bryce, M. R.; Wegener, S.; Yufit, D. S.; Howard,
J. A. K. Synthesis 1994, 489.
(27) Maggini, M.; Karlsson, A.; Scorrano, G.; Sandona´, G.; Farnia, G.;
Prato, M. J. Chem. Soc., Chem. Commun. 1994, 589.
(28) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Andreu, R.; Gar´ın, J.; Orduna,
J.; Ort´ı, E.; Viruela, P. M.; Viruela, R. J. Phys. Chem. Solids 1997, 58,
1713.
(29) Suzuki, T.; Maruyama, Y.; Akasaba, T.; Ando, W.; Kobayashi, K.;
Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359. Chlistouff, J.; Cliffel, D.;
Bard, A. J. In Handbook of Organic ConductiVe Molecules and Polymer;
Nalwa, N. S., Ed.; John Wiley & Sons: New York; 1997, Vol. 1, Ch. 7.
Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593.
(31) HyperChem 5.1, Hypercube, Inc., Waterloo, Ontario N2L 2 × 2,
Canada, 1997.
(32) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem.
Soc. 1997, 119, 974. Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J.
Am. Chem. Soc. 1995, 117, 4093.
(33) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11 771.
(34) Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101, 1472.
(35) Kato, T.; Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki,
S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 180,
446. Guldi, D. M.; Hungerbu¨hler, H.; Janata, E.; Asmus, K.-D. J. Phys.
Chem. 1993, 97, 11 258.
(36) Guldi, D. M.; Neta, P.; Hambright, P. J. Chem. Soc., Faraday Trans.
1992, 88, 2013.
(37) Maciejewski, A.; Steer, R. P. Chem. ReV. 1993, 93, 67.
(38) Ebbesen, T. W. ReV. Sci. Instrum. 1988, 59, 1307. Nagarajan, V.;
Fessenden, R. W. J. Phys. Chem. 1985, 89, 2330.
(39) Asmus, K.-D. Methods Enzymol. 1984, 105, 167.