Rigid dendritic donor-acceptor ensembles: Control over energy and electron transduction

Article abstract of DOI:10.1021/ja012694x

Several generations of phenylenevinylene dendrons, covalently attached to a C₆₀ core, have been developed as synthetic model systems with hierarchical, fine-tuned architectures. End-capping of these dendritic spacers with dibutylaniline or dode

Full text of DOI:10.1021/ja012694x

Published on Web 08/16/2002
Rigid Dendritic Donor-Acceptor Ensembles: Control over
Energy and Electron Transduction
Dirk M. Guldi,*,† Angela Swartz,† Chuping Luo,† Rafael Go´mez,‡
Jose´ L. Segura,*,‡ and Nazario Mart´ın*,‡
Contribution from the Radiation Laboratory, UniVersity of Notre Dame, Indiana 46556, and
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad Complutense,
E-28040 Madrid, Spain
Received December 12, 2001. Revised Manuscript Received June 26, 2002
Abstract: Several generations of phenylenevinylene dendrons, covalently attached to a C60 core, have
been developed as synthetic model systems with hierarchical, fine-tuned architectures. End-capping of
these dendritic spacers with dibutylaniline or dodecyloxynaphthalene, as antennas/electron donors, yielded
new donor-bridge-acceptor ensembles in which one, two, or four donors are allocated at the peripheral
positions of the well-defined dendrons, while the electron accepting fullerene is placed at the focal point of
the dendron. On the basis of our cyclic voltammetry experiments, which disclose a single anodic oxidation
and several cathodic reduction processes, we rule out significant, long-range couplings between the fullerene
core and the end-standing donors in their ground-state configuration. Photophysical investigations, on the
other hand, show that upon photoexcitation an efficient and rapid transfer of singlet excited-state energy
(6 × 1010 to 2.5 × 1012 s-1) controls the reactivity of the initially excited antenna portion. Spectroscopic
and kinetic evidence suggests that yet a second contribution, that is, an intramolecular electron-transfer,
exists, affording C60•- -dendron•+ with quantum yields (Φ) as high as 0.76 and lifetimes (τ) that are on the
order of hundreds of nanoseconds (220-725 ns). Variation of the energy gap modulates the interplay of
these two pathways (i.e., competition or sequence between energy and electron transfer).
Introduction
One of the most common problems in artificial model systems
is the lack of efficient solar light capture, because of insufficient
chromophore activity. To overcome this intrinsic limitation,
integration of several chromophores in “series” or “parallel”
holds particular promise for increased absorptive cross sections
and, thus, an efficient use of the solar spectrum. The strategy
of choice implies well-defined core-shell ensembles, in which
assemblies of chromophores are integrated in a “parallel”
configuration. This “parallel” design provides an enormous
advantage: The topography, that is, a central core surrounded
by a shell of chromophores, facilitates the mutual electronic
coupling with each individual constituent.
The intervening spacer is essential to guarantee the control
over the separation, electronic coupling, and composition in
donor-acceptor assemblies.4,5 Importantly, dendritic structures,
bearing an electro- and/or photoactive interior core, a closed-
packed shell, and a highly functional surface, furnish key criteria,
such as structural rigidity and long-range conjugation. Some of
their most fascinating features reach out to light-harvesting
arrays and electron-transfer relays of different complexities (i.e.,
enhancing the molecular function with increasing generation).
Cascades of light-driven energy and electron-transfer pro-
cesses, along well-defined energy and redox gradients, bear key
character in biology, chemistry, and materials science.1 Inspired
by the complex function of photosynthesis, that is, light harvest
by the antenna ensemble, transduction of excitation energy to
the photosynthetic reaction center, and charge-separation there-
after, numerous artificial model systems have been devised.2
Understanding the parameters that dictate, for instance, the
mutual interplay between efficient energy migration and uni-
directional electron transfer is a long-standing objective. A
crucial contention in this debate is to determine the factors that
activate these fundamentally different processes in a synchro-
nous (i.e., competition) or, alternatively, in an antichronous
manner (i.e., sequence). In the event of a sequential scenario,
optimizing each of the associated relay steps emerges as a
multifaceted and complex task. In principle, an optimal syner-
gism necessitates combining an appropriate energy gap with
an adequate spacing and orientation to contrive a good-
performing solar energy conversion system.3
* To whom correspondence should be addressed. E-mail: guldi.1@nd.edu
(D.M.G.); nazmar@quim.ucm.es (N.M.).
(3) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
† University of Notre Dame.
(4) (a) Fox, M. A.; Chanon, M. Photoinduced Electron Transfer; Elsevier:
Amsterdam, 1988. (b) Balzani, V. Electron Transfer in Chemistry Vol. I-V;
Wiley-VCH: Weinheim, Germany, 2001. (c) Carter, F. L. Molecular
Electronic DeVices; Marcel Dekker: New York, 1987.
‡ Universidad Complutense.
(1) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chichester, U.K., 1991; pp 161-196, 355-394. (b) Turro, N.
J. Modern Molecular Photochemistry; University Science Books: Mill
Valley, CA, 1991; pp 321-361. (c) Speiser, S. Chem. ReV. 1996, 96, 1953.
(2) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40 and
references therein
(5) (a) Stowell, M. H. B.; McPhillips, T. M.; Rees, D. C.; Soltis, S. M.; Abresch,
E.; Feher, G. Science 1997, 276, 812. (b) McDermott, G.; Prince, S. M.;
Freer, A. A.; Hawerthornthwaite-Lawlwss, A. M.; Paiz, M. Z.; Cogdell,
R. J.; Isaacs, N. W. Nature 1995, 374, 517.
9
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J. AM. CHEM. SOC. 2002, 124, 10875-10886
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A R T I C L E S
Guldi et al.
Figure 1. First (1a, 1b) and second (2a, 2b) generations of new C60-dendron dyads.
The features of the dendron skeleton can be fine-tuned at a
molecular level not only within a simple two-component system
(i.e., one donor-one acceptor) but, most significantly, within
multicomponent ensembles (i.e., several donors-one acceptor),
which ultimately can lead to high solar energy conversion
efficiencies.6
Previous work has focused mainly on encapsulating flexible
dendron units.7 Their overall flexibility precludes, however,
meaningful conclusions on effects that may relate to distance
and coupling between the donor and acceptor couples. Consid-
ering this deficiency, we drew our attention to the unique
features of stiff dendritic macromolecules.8 Their convergent
constitution facilitates the construction of well-defined energy
gradients via a site-specific functionalization. In turn, energy
might be funneled from, for example, an array of light-collecting
chromophores at the periphery toward a single energy/electron
“sink” in the core.
π-sphere, offers unique opportunities for stabilizing charged
entities. Most importantly, the small reorganization energy,
found for C60 in electron-transfer processes, accelerates the initial
charge-separation and shifts the strongly exothermic charge
recombination deep into the “inverted-region” decelerating its
rate. In fact, efficient relays of electron-transfer reactions, for
example, in fullerene-based dyad, triad, and tetrad ensembles,
has been accomplished along well-designed and fine-tuned redox
gradients leading to lifetimes as long as 0.38 s.9
In this paper, we describe several new D-B-A (donor-
bridge-acceptor) systems in which one (19), two (1a), or four
(2a) dibutylaniline or dodecyloxynaphthalene (1b, 2b) electron
donors are located at the peripheral positions of well-defined
phenylenevinylene-based dendrons and a fullerene acceptor at
the focal point of the dendrimer (Figure 1). Most importantly,
pursuing this strategy helps to alter the absorption cross section
of the antenna portion, that is, the dibutylaniline or dodecyloxy-
naphthalene periphery. We will demonstrate that, in these model
systems, modulation of the energy gap and the relative energetic
positioning of the states involved leads alternatively to a
sequence or a competition in energy- and electron-transfer
reactions. As far as the different generations of dendrons are
concerned, their rigid constitution creates diverse donor-
acceptor separation ranging from 8.56 Å (17) to 19.1 Å (2b).
The spherical shape of fullerenes, giving rise to a delocalized
π-electron system, renders these carbon allotropes ideal com-
ponents for the construction of efficient electron-transfer model
systems. The delocalization of charges, electrons or holes, within
the giant, spherical carbon framework (diameter > 7.5 Å),
together with the rigid, confined structure of the aromatic
(6) For reviews on light-harvesting and photoactive dendrimers see: (a)
Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701. (b) Venturi,
M.; Serroni, S.; Juris, A.; Campagna, S.; Balzani, V. Top. Curr. Chem.
1998, 197, 193. (c) Vogtle, F.; Gestermann, S.; Hesse, R.; Schwierz, H.;
Windisch, B. Prog. Polym. Sci. 2000, 25, 987.
(7) (a) Nierengarten, J.-F. Chem.sEur. J. 2000, 6, 3667. (b) Hirsch, A.;
Vostrowsky, O. Top. Curr. Chem. 2001, 217, 51.
(8) (a) Xu, Z.; Moore, J. S. Acta Polym. 1994, 45, 83. (b) Devadoss, C.;
Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. (c) Shortreed,
M. R.; Swallen, S. F.; Shi, Z.-Y.; Tan, W.; Xu, Z.; Devadoss, C.; Moore,
J. S.; Kopelman, R. J. Phys. Chem. B 1997, 101, 6318. (d) Swallen, S. F.;
Shi, Z.-Y.; Tan, W.; Xu, Z.; Moore, J. S.; Kopelman, R. J. Lumin. 1998,
76-77, 193. (e) Avent, A. G.; Birkett, P. R.; Paolucci, F.; Roffia, S.; Taylor,
R.; Wachter, N. K. J. Chem. Soc., Perkin Trans. 2 2000, 1409. (f) Segura,
J. L.; Go´mez, R.; Mart´ın, N.; Luo, C.; Swartz, A.; Guldi, D. M. Chem.
Commun. 2001, 707.
Results
Synthesis. Synthesis of Functionalized Phenylenevinylene-
Based Dendrons. The synthetic procedure for the preparation
of C60-dendron dyads is based on the 1,3-dipolar cycloaddition
reaction10 of C60 with aldehyde-functionalized dendrons. There-
fore, the target molecules for our synthetic workup are phen-
(9) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617.
(10) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519.
9
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Rigid Dendritic Donor−Acceptor Ensembles
A R T I C L E S
Scheme 1
ylenevinylene-based dendrons carrying aldehyde functionalities
at their focal point. Three different synthetic routes (Scheme 1)
were pursued to get to the first generation dendron 5a, carrying
the required aldehyde functionality.
Access to the second-generation dendron 14a, with a nitrile
functionality at the focal point, was made possible by the
Wittig-Horner reaction between bis(phosphonate) 11a and
aldehyde 5a. Subsequent treatment of 14a with a stoichiometric
amount of 1.0 M solution of DIBAL-H (diisobutylaluminum
hydride) in dichloromethane furnished 15a as a yellowish solid
in a 78% yield (Scheme 2).
The good solubility of these compounds guaranteed full
spectroscopic characterization of the new dendrons. The FTIR
spectra show the fingerprints of the functional groups present
at the focal point: A band around 2220 cm-1 corresponds in
10a and 14a to the conjugated nitrile group, while in 5a and
15a a band at lower frequencies (1690 cm-1) represents the
aldehyde functionality.
of the peripheral benzene rings are grouped into an AA′BB′
system with a coupling constant (J) of 8.8 Hz. On the other
hand, the olefinic protons give rise to two doublets with a
coupling constant of 16.4 Hz, which provides further proof for
their E configuration. Similar δ values and coupling constants
for the peripheral benzene rings and the olefinic protons were
noted in the higher generation dendrimers. In addition, a series
of new signals were assigned to the inner benzene rings. The
alkyl chains give rise to a triplet at 3.2 ppm and several
multiplets, corresponding to the methylene group directly
attached to the nitrogen atoms, and the remaining methylene
and methyl groups, respectively.
In the 13C NMR spectra, the signals associated with the
aromatic carbon atoms (i.e., carrying the dibutylamino groups)
appear around 147 ppm. The rest of the signals observed are in
agreement with the proposed structures which are also confirmed
by mass spectrometry (5a: M+ 564; 15a: M+ 1228) and
elemental analyses.
The 1H NMR spectra for the E,E isomer (5a) show the signal
corresponding to the aldehyde group (9.98 ppm) together with
two other signals as singlets corresponding to the central benzene
moiety (7.79 and 7.76 ppm). The remaining hydrogen atoms
Synthesis of C60-Dendron. Treating aldehydes (5a,b and
15a,b) with [60]fullerene and an excess of sarcosine in refluxing
toluene for 24 h gave C60-dendron dyads (1a,b and 2a,b) (see
Schemes 1 and 2). Because of the presence of the long alkyl
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A R T I C L E S
Guldi et al.
Scheme 2
chains, these dyads are quite soluble in common organic
solvents.
cycloaddition reactions between C60 and aldehydes 9, 12, and
formaldehyde, respectively.
1
In CDCl3 solutions, the H NMR spectra of 1a, 1b, 2a, and
Electronic Absorption Spectra. The absorption maxima in
dichloromethane solutions of the formyl-substituted dendrons
(5a,b and 15a,b) and the respective dyads (1a,b and 2a,b) are
given in Table 1 along with references 16-18. In general, the
values for the dibutylaniline derivatives are around 370-380
nm, whereas the analogues bearing the alkoxynaphthalene
moiety appear at around 336-340 nm. A red shift is observed
in the λmax values in dibutylaniline derivatives in comparison
with the parent distyrylbenzene (λmax ) 360 nm) because of
the presence of the dialkylamino substituents.
2b confirm that the stereochemistry of the vinylenic double
bonds remains unaffected during the cycloaddition reaction (Jtrans
∼ 16 Hz). The fulleropyrrolidine signals appear at around 5
ppm (CH) as a singlet and a set of two doublets (CH2) with
coupling constants around J ) 9 Hz, integrating for one
hydrogen. On the other hand, the 13C NMR spectra show the
signals originating from the rigid stilbenoid dendron as well as
those from the fullerene system. Specifically, in addition to the
characteristic fullerene sp2-region, another four signals, which
correspond to the sp3-carbon atoms of the fulleropyrrolidine ring,
were noted between 69 and 84 ppm, and those of the alkoxy
chains, between 70 and 15 ppm.
It is interesting to note that substitution of the fullenere unit
by the formyl group in the dibutylaniline derivatives results in
a slight hypsochromic shift, and the same effect is observed in
going from the first to the second dendron generation in dyads
1a (λmax ) 377 nm) and 2a (λmax ) 369 nm). A similar effect
As suitable model systems for the photophysical studies, 16,
17, and 18 (Scheme 2) were synthesized by using similar
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Rigid Dendritic Donor−Acceptor Ensembles
A R T I C L E S
Table 1. Absorption Maximaa and Redox Potentialsb of Reference
Dendrons (5a, 5b, 15a, 15b) and of Dyads (1a, 1b, 2a, 2b, 16, 17,
18)
Table 2. Photophysical Characteristics of Reference Dendrons
(5a, 5b, 15a, 15b)
5b
15b
5a
15a
1
1
2
3
4
λmaxa [nm]
E ap c [V]
E cpd[V]
E cp [V]
E cp [V]
E cp [V]
λ max
401 nma
(3.09 eV)
0.8
408 nma
(3.03 eV)
0.47
468 nmb
(2.65 eV)
0.17
454 nmb
(2.72 eV)
0.31
5a
1a
15a
2a
5b
1b
15b
2b
16
382
377
373
369
338
340
336
338
312
308
322
+0.88
+0.70
+0.79
+0.72
+1.30
+1.27
+1.32
1.30
fluorescence
-0.67
-0.73
-0.64
-1.08
-1.07
-1.03
-1.67
-1.69
-1.62
-2.16
Φ
fluorescence
τ
1.76 nsc
600 nm
1.85 nse
1.99 nsc
1.95 nsc
2.05 nsc
fluorescence
λ max
680, 950 nm 650, 945 nm 645, 945 nm
singlet-singletd
τ
1.97 nse
1.90 nse
2.01 nse
680 nm
-0.64
-0.65
-0.64
-0.71
-0.60
-1.03
-1.07
-1.04
-1.14
-1.07
-1.62
-1.64
-1.65
-1.75
-1.64
singletd
1.08
17
18
λ max
380, 580 nm 390, 590 nm 620 nm
510 nm 540 nm
triplet-tripletd
λmax
1.32
C60
-1.93
480, 820 nm 480, >900 nm
π-radical cationf
a In dichloromethane solutions b In toluene-acetonitrile solvent mixture
(4:1 v/v) using Bu4NClO4 (0.3 mg L-1), SCE reference electrode, and glassy
a Excitation wavelength 340 nm. b Excitation wavelength 380 nm.
c Measured at the fluorescence maximum, see entry 1. d Excitation wave-
length 355 nm. e Measured at the singlet-singlet maximum, see entry 4.
f Determined by pulse radiolysis.
carbon as working electrode. Scan rate: 200 mV s-1
d cp: cathodic peak.
.
c ap: anodic peak.
was also observed for the respective alkoxynaphthalene deriva-
tives, although to a lesser extent (see Table 1).
calculations (vide infra), suggests that an intramolecular singlet-
singlet energy transfer occurs, further examination of these
fullerene-based dendrons was deemed necessary. This led us
to characterize the photophysics of reference dendrons (5a,b
and 15a,b) by transient absorption changes, recorded with
several time delays after a short picosecond (Figure 3a) and
long nanosecond (Figure 3b) laser pulse, and compare them to
those of the corresponding dyads (1a,b and 2a,b) (Figure 4).
We turn first to the question of the reference dendrons (5a,b
and 15a,b). At early times (i.e., 50-100 ps), characteristically
strong transitions, which are short-lived (τ ∼ 2 ns), were found
in the visible region (600-750 nm). Photoexcitation of, for
example, 15b (2.0 × 10-5 M) led to the instantaneous formation
of a broad ∼680 nm maximum, which is ascribed to the singlet
excited-state absorption feature. A representative spectrum,
monitored with a 50 ps time delay, is given in Figure 3a. At
longer times, the singlet excited state deactivates via a mono-
exponential rate law and a rate of 5.0 × 108 s-1 to produce the
long-lived triplet excited state (Figure 3a, 5000 ps time delay).12
Similar properties were concluded for 5a,b and 15a in toluene.
In nanosecond experiments, 15b gave rise to spectral features
shown in Figure 3b. The spectrum, representative of 5a,b and
15a, displays maxima between 580 and 680 nm. Analysis of
the transient absorption changes throughout the 550-950 nm
region reveals a reasonably good spectral correlation with what
is seen to develop over the course of the picosecond time regime.
The triplet absorbances of 5a,b and 15a generally decayed via
dose-independent first-order kinetics and resulted in a complete
restoration of the ground state.
Electrochemistry. The electrochemical features of dyads 1a,
1b, 2a, 2b, 16, and 17 were probed by cyclic voltammetry at
room temperature (see Supporting Information, Figure S1). A
toluene-acetonitrile solvent mixture (4:1 v/v) and tetra-n-
butylammonium perchlorate (0.3 mg/mL) as supporting elec-
trolyte were used in a conventional three-compartment cell,
equipped with glassy carbon, SCE, and platinum wire as
working electrode, reference electrode, and auxiliary electrode,
respectively. The redox potentials, measured at 200 mV s-1
,
are collected in Table 1 and compared with those of the formyl-
substituted stilbenoid dendrons (5a,b and 15a,b) and unsubsti-
tuted fulleropyrrolidine (18).
As a general feature, dyads (1a,b and 2a,b) give rise to three
quasireversible one-electron reduction waves that reflect the first
three one-electron reduction steps of the fullerene cores. These
reduction potential values are shifted to more negative values
relative to pristine C60. The underlying cathodic shift stems from
the saturation of a double bond of the fullerene core, which,
accordingly, raises the lowest unoccupied molecular orbital
(LUMO) energy of the resulting fullerene derivative.11
Photophysics. Toluene: Fluorescence Spectroscopy. In
toluene, the high-energetic fluorescence (∼3.0 eV) exhibited
by the reference dendrons (5a,b and 15a,b), with quantum yields
as high as 0.80, is in the corresponding dyads (1a,b and 2a,b)
reduced by up to 3 orders of magnitude (see Tables 2 and 3
and Figure 2a). Taking the fluorescence quantum yields and
lifetime of the references into account, we extrapolated the
lifetime and, accordingly, the rate of deactivation for the
photoexcited dendrimer structure. Details on this determination
are listed in the footnote to Table 3. For example, a lifetime of
0.4 ( 0.01 ps is estimated for 1a in toluene, which corresponds
to an ultrafast intramolecular deactivation rate of 2.5 × 1012
s-1. Similar values were determined for 1b, 2a and 2b and are
collected in Table 3.
dendron
9
hν8 1*(dendron) ISC8 3*(dendron)
(1)
The picture associated with the picosecond absorption
spectroscopy of dyads 1a,b and 2a,b is drastically different from
the conclusion of reference dendrons 5a,b and 15a,b. Despite
the unequivocal excitation of the dendrimer moieties (λex ) 355
nm), no spectral evidence for the dendrimer’s singlet-singlet
absorption was found at any delay time following the short 18
ps laser pulse. Instead, the spectral features are identical with
Toluene: Transient Absorption Spectroscopy. Because the
synopsis of the emission studies, along with the thermodynamic
(11) (a) Suzuki, T.; Maruyama, T.; Akasaba, T.; Ando, W.; Kobayashi, K.;
Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359. (b) Chlistouff, J.; Cliffel,
D.; Bard, A. J. In Handbook of Organic ConductiVe Molecules and
Polymers Vol. 1; Nalwa, N. S., Ed.; John Wiley & Sons: New York, 1997;
Chapter 7. (c) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31,
593.
(12) It is worthwhile to underline that these intersystem crossing (ISC) kinetics
reveal an almost exact resemblance with those of the fluorescence lifetime
(see Table 2).
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A R T I C L E S
Guldi et al.
Table 3. Photophysical Characteristics of Dyads (1a, 1b, 2a, 2b, 16, 17)
17
1b
2b
16
1a
2a
Dendron
Φ
1.8 × 10-4 a
3.2 × 10-3 a
2.7 × 10-4 b
2.5 × 10-3 b
fluorescence
(toluene)
Φ
fluorescence
(THF)
1.6 × 10-4 a
0.4 ( 0.01 ps
3.0 × 10-3 a
13 ( 0.1 ps
1.9 × 10-4 b
3 ( 0.03 ps
2.2 × 10-3 b
16 ( 0.1 ps
τ
fluorescence
(toluene)c
Fullerene
Φ
6.0 × 10-4 a
5.6 × 10-4 a
4.3 × 10-4 a
3.4 × 10-4 a
1.42 ns
5.4 × 10-4 a
5.9 × 10-4 b
0.2 × 10-4 b
0.15 × 10-4 b
1.38 ns
1.6 × 10-4 b
0.2 × 10-4 b
0.15 × 10-4 b
3.9 × 10-4 b
0.6 × 10-4 b
0.5 × 10-4 b
1.48 ns
fluorescence
(toluene)
Φ
4.0 × 10-4 a
3.0 × 10-4 a
1.51 ns
fluorescence
(THF)
Φ
fluorescence
(benzonitrile)
τ
1.5 ns
fluorescence
(toluene)
τ
1.44 ns
1.39 ns
1.40 ns
ndd
ndd
ndd
fluorescence
(THF)
a Excitation wavelength 340 nm. b Excitation wavelength 380 nm. c Calculated values according to k ) [Φ(ref) - Φ(dyad)]/[τ(ref)Φ(dyad)]. d Not detectable;
lifetimes shorter than the time resolution of our apparatus (i.e., ∼ 0.1 ns).
the texture of the fullerene singlet excited state. Specifically, a
broad transition centering around 880 nm is a clear attribute of
the fullerene singlet-singlet absorption, as illustrated in Figure
4a.13 As can be seen by reference to Figure 4b, which displays
time profiles taken in toluene and benzonitrile, the fullerene
singlet excited state is formed instantaneously and in a single
step. The rise time of the 880 nm absorption, as a sensitive
marker for the actual energy-transfer event, is masked by the
instrument response time and, therefore, prevents an accurate
kinetic analysis of the intramolecular reaction, but it allows us
Third, and most importantly, in the dibutylaniline-based
systems (1a, 2a), the quantum yields of the fullerene fluores-
cence were as low as 0.15 × 10-4 (1a in benzonitrile).
THF and Benzonitrile: Transient Absorption Spectros-
copy. Upon 355 nm illumination (18 ps) of 1a,b and 2a,b in
THF and benzonitrile, the typical fullerene singlet-singlet
absorptions were seen, similar to the trend realized in toluene
(vide supra, compare to Figure 4).16
Interestingly, for 1b and 2b, no obvious impact on the singlet
lifetime was noted: The monoexponential rates (∼6 × 108 s-1),
as summarized in Table 4, are virtually identical, although the
fullerene fluorescence quantum yields drop to 50%.17 For
example, in Figure 4b, the time absorption profiles at 880 nm
are compared for dyad 2b in oxygen-free toluene and benzo-
nitrile. At the conclusion of the picosecond time scale, the only
recognizable product is the fullerene triplet excited state, judged
by the 700 nm triplet-triplet absorption, with a triplet lifetime
of ∼20 µs.
to estimate a lower limit, >5 × 1010 s-1 14
.
In principle, this
estimate corroborates the ultrafast deactivation, as we extra-
polated from the fluorescence experiments (vide supra). On the
longer time scale, the fate of the fullerene singlet excited state
is identical to that found for fulleropyrrolidine 18: An ISC
(intersystem crossing) (∼ 5 × 108 s-1), driven by a strong spin-
orbit coupling, governs the efficient transformation of the singlet
into the triplet excited state with λmax at 700 nm.
THF and Benzonitrile: Fluorescence Spectroscopy. When
more polar solvents, such as THF and benzonitrile, were used,
the steady-state fluorescence experiments (Table 3) gave rise
to several interesting trends.
First, the high-energetic dendrimer emission decreases in all
systems (1a,b and 2a,b) by about 10% in comparison to the
experiments conducted in nonpolar toluene.
Second, in the dodecyloxynaphthalene-based systems (1b,
2b), the fullerene fluorescence drops in benzonitrile by 45%
(3.0 × 10-4) relative to that in toluene (5.4 × 10-4). This
observation is consistent with the assumption that a competing
electron transfer may be activated (vide infra).15
The fluorescence experiments led us to postulate that in the
dibutylaniline-based systems 1a and 2a the lifetimes of the
fullerene singlet excited states are strongly affected by the
electron donor. In fact, transient absorption measurements
disclosed that the slow intersystem crossing dynamics (7.4 ×
108 s-1) do not govern the lifetime of the fullerene singlet
excited state but that a rapidly occurring decay of the singlet-
singlet transition does. Figure 6 illustrates the decay at the 880
nm maximum of 2a in toluene and THF.
The transient absorption changes, recorded at the conclusion
of this fast deactivation, are not superimposable with those
(15) Earlier, we noticed a similar trend in fullerene-oligomer ensembles, see
ref 35c.
(13) The slightly lower fluorescence quantum yield can be correlated with the
better electron-donor ability of dibutylaniline dendrons (5a, 15a) relative
to the dibutylaniline moiety in 16.
(14) Similar rise and decay kinetics were found upon analyzing at alternative
wavelengths in the near-infrared region.
(16) We like to emphasize that again the transduction of singlet excited-state
energy (eq 1) precedes the fullerene singlet excited-state information. This
process is covered by our instrumental time response of 18 ps. See also
previous discussion on toluene.
(17) Similarly, the fullerene fluorescence lifetimes are identical.
9
10880 J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002

Rigid Dendritic Donor−Acceptor Ensembles
A R T I C L E S
Figure 3. (a) Picosecond transient absorption spectrum (vis-NIR part)
recorded -50 ps, 50 ps (- - -), and 5000 ps (s) upon flash photolysis of
reference 15b (5.0 × 10-5 M) at 355 nm in deoxygenated toluene, indicating
the dendron singlet-singlet (λmax ) 680, 950 nm) and triplet-triplet features
(λmax ) 590 nm), respectively. (b) Nanosecond transient absorption spectrum
(vis-NIR part) recorded 50 ns upon flash photolysis of reference 15b (5.0
× 10-5 M) at 355 nm in deoxygenated toluene, indicating the dendron
triplet-triplet features (λmax ) 390, 590 nm).
Figure 2. (a) Emission spectra of dyad 2b and reference 15b in toluene
with matching absorption at the 340 nm excitation wavelength (i.e., OD340
nm ) 0.2) (emission spectrum of dyad 2b is amplified by a factor of 145)
and (b) excitation spectra (normalized to exhibit matching signals around
350 nm) of dyad 2b in toluene and reference 15b in toluene, monitoring
the fullerene emission at 715 nm and dendron emission at 410 nm,
respectively.
iodine. Trialdehyde (3) was obtained following a two-step
reaction sequence: Reduction of the commercially available
methyl 1,3,5-benzenetricarboxylate to the corresponding tri-
(hydroxymethyl) derivative and oxidation with pyridinium
chlorochromate.19 On the other hand, synthesis of 4-(N,N-
dibutylamino)benzyltriphenylphosphonium iodide (4) was com-
pleted via treatment of N,N-dibutylaniline with triphenylphos-
phine, potassium iodide, and formaline.20
recorded for the fullerene model, that is, the fullerene triplet
excited state (λmax ∼ 380 and 700 nm).18 Instead, the newly
formed species shows a sharp peak around 1000 nm, the
characteristic fingerprint of the one-electron reduced fullerene
π-radical anion. As far as the dibutylaniline moieties are
concerned, transient maxima in the visible around 820 nm
corroborate the oxidation of the donor and complete the
characterization of the charge-separated C60•- -dendron•+ radi-
cal pairs. Using the comparative method and the fullerene triplet
as a standard, quantum yields of the charge separation as high
as 0.76 were derived in dichloromethane. It is interesting to
note that in 1a and 2a C60•- -dendron•+ is the only product
formed: no evidence for a long-lived excited state was seen at
any time delay.
The preparation of 3,5-bis(triphenylphosphoniummethyl)-
benzonitrile (8) and the related bis(phosphonate) 11a emerged
as a valuable alternative. In particular, radical bromination of
3,5-dimethylbenzonitrile led to the corresponding 3,5-di(bro-
momethyl)benzonitrile (7a),21 and, upon treatment with tri-
phenylphosphine in dimethylformamide, to derivative 8. Alter-
natively, treating 7a with an excess of trimethyl phosphite
afforded the corresponding bis(phosphonate) 11a (84% yield)
as a stable white solid. Upon Wittig reaction between 8 and
4-(N,N-dibutylamino)benzaldehyde (9) in dry ethanol, using
lithium ethoxide as a base, 10a was obtained as a mixture of
configurational isomers. Iodine-catalyzed thermal isomerization
of the mixture helped to convert 10a into the pure all-trans
isomer. This compound could also be directly separated as the
Discussion
Synthesis. The first approach involves Wittig reaction
between 1,3,5-triformylbenzene (3) and p-(N,N-dibutylamino)-
benzylphosphonium derivative 4 under stoichiometric control.
The disadvantage of this approach is that compound 5a was
isolated as a stereoisomeric mixture. Transforming the stereoi-
somers into the pure E,E isomer necessitated refluxing the
material in p-xylene in the presence of catalytic amounts of
(19) Formigue´, M.; Johannsen, I.; Boubekeur, K.; Nelson, C.; Batail, P. J. Am.
Chem. Soc. 1993, 115, 3752.
(20) Bredereck, H.; Simchen, G.; Gribenow, W. Chem. Ber. 1973, 106, 3732.
(21) Bodwell, J. G.; Bridsom, J. N.; Houghton, T. J.; Yarlagadda, B. Tetrahedron
Lett. 1997, 38, 7475.
(18) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
9
J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002 10881

A R T I C L E S
Guldi et al.
Table 4. Photophysical Characteristics, Related to the Fullerene
Features, of Dyads (1a, 1b, 2a, 2b, 16)
1b
2b
16
1a
2a
τ
1.59 ns 1.49 ns 1.55 ns 0.73 ns
1.40 ns
singlet
(toluene)
τ
singlet
(THF)
τ
singlet
(benzonitrile)
Φ
triplet
(toluene)
Φ
triplet
(THF)
Φ
triplet
(DCM)a
Φ
triplet
(benzonitrile)
τ
1.50 ns 1.51 ns 0.04 ns 0.05 ns
0.16 ns
0.09 ns
0.29
1.47 ns 1.42 ns
0.03 ns
0.15
0.79
0.69
0.6
0.43
radical pair radical pair
Φ
0.47
Φ
0.41
radical pair radical pair
Φ
0.74
Φ
0.76
0.59
0.3
radical pair radical pair
Φ
Φ
0.46
350 ns
0.53
220 ns
triplet
triplet
radical pair
(THF)
τ
radical pair
(DCM)a
τ
250 ns
360 ns
580 ns
725 ns
triplet
triplet
radical pair
(benzonitrile)
Figure 4. (a) Picosecond transient absorption spectrum (vis-NIR part)
recorded -50 ps, 50 ps (- - -), and 5000 ps (s) upon flash photolysis of
dyad 2b (5.0 × 10-5 M) at 355 nm in deoxygenated toluene, indicating the
fullerene singlet-singlet (λmax ) 880 nm) and triplet-triplet features (λmax
) 700 nm), respectively. (b) Time-absorption profiles at 880 nm of dyad
2b in toluene (1.49 ns) and benzonitrile (1.42 ns) monitoring the fullerene
singlet-singlet decay (ø2-values for a monoexponential fit were 0.98 and
0.95 in toluene and benzonitrile, respectively).
a DCM ) dichloromethane.
N,N-dimethylformamide in the presence of a catalytic amount
of sodium iodide, was then performed en route to the cyano
analogue (10b). By following a similar strategy as that previ-
ously described for the dibutylaniline-containing analogues,
aldehyde-functionalized first- and second-generation dendrons
were obtained. Subsequent reaction of 5b with bis(phosphonate)
11a yielded the cyano-substituted second-generation dendron
14b, which upon further reduction with DIBAL-H leads to the
second-generation dendron 15b (Scheme 2).
Electronic Spectra and Electrochemistry. The ground-state
absorption spectra of all dyads are a superposition of the spectra
of the individual chromophores. However, the λmax values in
the dyads are hypsochromically shifted relative to those of the
respective formyl-substituted dendrons, which could be ac-
counted for by the loss of conjugation in going from the carbonyl
group to the fullerene unit. More fundamental is the observation
that the higher generations exhibit enhanced absorption cross
sections. For example, extinction coefficients of 3680 M-1 cm-1
(5a) and 7800 M-1 cm-1 (15a) of the visible transition (i.e.,
∼380 nm) clearly parallel the increase of light-absorbing units,
thus giving rise to a useful antenna effect.
A closer inspection of the electrochemical data reveals that
the first reduction potential values of these dyads are all very
similar (see Table 1). Furthermore, they compare quite well with
that of the fulleropyrrolidine reference (18) in which the same
double bond of the fullerene core is saturated.
The redox behavior of the stilbenoid dendrons shows an
increase of the donor strength when going from the dodecyl-
oxynaphthalene-based (5b, 15b: Eox ∼ 1.30 V vs SCE) to the
dibutylaniline-based systems (5a, 15a: Eox ∼ 0.7 V vs SCE).
This fact is in agreement with the better donor ability of the
pure all-trans isomer, in 67% yield, by Wittig-Horner reaction
between bis(phosphonate) 11a and aldehyde 9 in dry THF at 0
°C, using potassium tert-butoxide as a base. Final treatment of
a dichloromethane solution of 10a with a stoichiometric amount
of 1.0 M solution of DIBAL-H in dichloromethane then afforded
5a in a 73% yield as the pure all-trans isomer.
Because of the enhanced fluorescent properties exhibited by
naphthalenevinylene derivatives,22 we have also carried out the
synthesis of first- and second-generation dendrons bearing
dodecyloxynaphthalene moieties on the peripheral positions as
depicted in Schemes 1 and 2.
Radical bromination of 3,5-dimethylbromobenzene (6b)
yielded 3,5-di(bromomethyl)bromobenzene (7b)23 and, upon
heating with trimethyl phosphite, bis(phosphonate) 11b. Sub-
sequent Wittig-Horner reaction between 11b and 2-dodecyloxy-
6-formylnaphthalene (12)24 allows us to isolate the first-
generation dendron, endowed with a bromine atom at the focal
point (13). The Rosenmund von Braun reaction25 of bromo
derivative 13, namely, the treatment with copper cyanide in dry
(22) (a) Segura, J. L.; Mart´ın, N.; Hanack, M. Eur. J. Org. Chem. 1999, 643.
(b) Do¨ttinger, S. E.; Hohloch, M.; Segura, J. L.; Steinhuber, E.; Hanack,
M.; Tompert, A.; Oelkrug, D. AdV. Mater. 1997, 9, 233. (c) Hohloch, M.;
Segura, J. L.; Do¨ttinger, S. E.; Hohnholz, D.; Steinhuber, E.; Spreitzer,
H.; Hanack, M. Synth. Met. 1997, 84, 319.
(23) (a) Steenwinkle, P.; James, S. L.; Grove, D. M.; Veldman, N.; Speck, A.
L.; van Koten, G. Chem.sEur. J. 1996, 2, 1440. (b) Paugan, M. F.; Bien,
J. T.; Smith, B. D.; Christoffels, L. A. J.; Dejong, F.; Reinhoudt, D. N. J.
Am. Chem. Soc. 1996, 118, 9820.
(24) Go´mez, R.; Segura, J. L.; Mart´ın, N. J. Org. Chem. 2000, 65, 7501.
(25) Ellis, G. P.; Romney-Alexander, T. M. Chem. ReV. 1987, 87, 779.
9
10882 J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002

Rigid Dendritic Donor−Acceptor Ensembles
A R T I C L E S
dibutylaniline group in comparison with that of dodecyloxy-
naphthalene.26
In summary, both electronic spectra and electrochemical data
reveal the absence of a significative interaction between the C60
and electron donor units in the ground state.
Photophysics: Toluene. Inspection of the low-energy region
of the emission spectra (see Figure 2a) helped to identify the
species evolving from the dendrimer’s singlet excited-state
quenching in 1a,b and 2a,b and, furthermore, to probe the
underlying mechanism. In particular, a fluorescence maximum
at 715 nm, resembling that of fulleropyrrolidine 18,18 was
produced, despite the near exclusive excitation of the dendron
moieties at 360 nm (.90%).27 The fact that we realize nearly
comparable quantum yields, relative to that of reference 18, lets
us conclude that the fullerene singlet excited state (1.76 eV) is
populated in quantitative yields. Evidence that this newly formed
state is produced from an energy-transfer reaction, rather than
evolved from a direct excitation, was lent from a set of excitation
spectra. The complementary excitation spectrum of the fullerene
emission in dyads 1a,b and 2a,b (Figure 2b) reflects nearly
exactly (i) the ground-state absorption of the dendron moieties
in the near-visible region and (ii) the excitation spectra of 5a,b
and 15a,b with maxima ∼ 350 nm. These analyses underscore
the hypothesis that the origin of excited state energy is
unequivocally that of the dendron systems.
Figure 5. Nanosecond transient absorption spectrum (vis-NIR part)
recorded 50 ns upon flash photolysis of dyad 2b (5.0 × 10-5 M) at 355 nm
in deoxygenated toluene, indicating the fullerene triplet-triplet features
(λmax ) 380, 700 nm).
C60-dendron 9
hν8 C60-1*(dendron) f 1*(C60)-dendron (2)
To correlate this interesting phenomenon, we examined
references 16 (dibutylaniline-based system) and 17 (dodecy-
loxynaphthalene-based system),28 under nearly identical condi-
tions. The high-energy regime of the naphthalene-based system
(17) is dominated by a significant quenching (∼97%) of the
dodecyloxynaphthalene emission, while in the low-energy region
the fullerene fluorescence appears. Because the dibutylaniline
group in 16 lacks the strong absorption seen in the dendron
analogues 5a, 15a, 1a, and 2a,29 only the fullerene fluorescence
was probed (Table 3). Although the fullerene fluorescence
quantum yields are in both systems (i.e., 16,17) comparable to
that of fulleropyrrolidine reference 18, different conclusions are
evoked. In the naphthalene-based system (17), it confirms a
quantitative transfer of singlet excited-state energy from the
dodecyloxynaphthalene unit to the fullerene, in sound agreement
with the corresponding dendrons (1b,2b). On the other hand,
in 16, it only infers that any energy transferred to the fullerene
remains there and an intramolecular electron evolving from the
fullerene singlet excited state is thermodynamically precluded.
This conclusion agrees well with the thermodynamic argument
suggesting an endothermic electron transfer in toluene, -∆G
< 0 (vide infra).13
Figure 6. Time-absorption profiles at 880 nm of dyad 2a in toluene (1.40
ns) and THF (0.16 ns) monitoring the fullerene singlet-singlet decay
(ø2-values for a monoexponential fit were 0.98 and 0.99 in toluene and
THF, respectively).
and quantum yield) were compared in nanosecond experiments,
as an example, following 8 ns pulses at 355 nm. First, the
triplet-triplet maxima (Figure 5) were in perfect accord with
the spectral features observed at the end of the picosecond time
scale (compare to Figure 4). Second, the triplet lifetimes (ca.
20 µs) match those seen for reference 18.
As far as the references 16 and 17 are concerned, pumping
light into the fullerene ground state in 16 leads to the population
of the singlet excited state. The lifetime of this intermediate
state is, however, not seemingly impacted by the presence of
the electron-donating dibutylaniline group. Instead, the lowest
vibrational state of the singlet excited state in 16 decays rapidly
and quantitatively to the energetically lower lying triplet seen
in reference 18. Furthermore, the differential absorption changes,
recorded immediately after an 8 ns pulse, showed the same
spectral features of the fullerene triplet excited state as observed
at the end of the picosecond experiments. A set of maxima
located at 380 and 700 nm was observed, which is in good
agreement with the features recorded for 18.30
As a final test in support of the energy-transfer hypothesis,
the triplet characteristics (i.e., triplet-triplet maximum, lifetime
(26) The presence of a formyl group on the stilbenoid moiety in 5a and 15a
has a stronger impact on the oxidation potentials than the nonconjugated
fullerene unit in 1a and 2a. Therefore, the anodic peaks appear in 5a and
15a anodically shifted relative to those of 1a and 2a. This electronic effect
is also observed in the naphthalene derivatives (5b, 15b) although to a
notably lesser extent.
Photophysics: THF and Benzonitrile. Again, 16 (i.e.,
dibutylaniline-based system) renders a particularly important
reference: Its reactivity assists in shedding light on the
mechanism, which might be applicable in deactivating the
photoexcited dendron moieties in 1a and 2a. In fact, reference
16 reveals a similar low fullerene fluorescence quantum yield,
(27) Reference dendrons (5a,b and 15a,b) did not show any emission in this
spectral region, see Figure 2a.
(28) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995,
117, 4093.
(29) This hampered the direct excitation of the donor, and only the fullerene
emission was subjected to this reference experiment.
(30) A similar reactivity was found for 17.
9
J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002 10883

A R T I C L E S
Guldi et al.
for example, in benzonitrile (0.15 × 10-4). Considering that
the singlet and triplet excited-state energies of aniline derivatives
are 3.7 ( 0.1 and 3.0 ( 0.1 eV,31 respectively, energy transfer
from the fullerene singlet excited state (1.76 eV) in 16 would
be highly endothermic and, therefore, improbable to take place.
Consequently, this leaves a very rapid intramolecular electron
transfer as the only likely pathway. In fact, in earlier work, it
was shown that 16 shows photoinduced electron transfer only
in polar media.28 From this comparison, we reach the tentative
conclusion that the fullerene singlet excited state, if produced
in the dibutylaniline-based systems (1a, 2a), would be subject
to an intramolecular electron transfer.
hν8 C60-1*(dendron) f
1*(C60)-dendron f C60•- -dendron•+ (3)
Figure 7. Transient absorption spectrum (vis-NIR part) recorded 50 ns
upon flash photolysis of dyad 2a (2.0 × 10-5 M) at 355 nm in deoxygenated
benzonitrile, indicating the charge-separated radical pair features (λmax at
820 and 1000 nm).
C60-dendron
9
By contrast, in dodecyloxynaphthalene-based systems 1b and
2b, this sequential pathway (i.e., transfer of singlet excited-
state energy followed by that of an electron, eq 3) plays no
major role. The only conceivable rationale for the moderate
fluorescence quenching of ∼45% might involve a competitive
scenario, that is, a direct electron transfer from the photoexcited
dendron.
C60-dendron 9
hν8 C60-1*(dendron) f C60•--dendron•+ (4)
Correlating the transient absorption experiments for 1b and
2b with the emission data, we gather that an intramolecular
electron transfer competes (i.e., eq 4) with the dominant singlet-
singlet energy transfer (i.e., eq 1). In the context of confirming
the electron-transfer contribution, the strong triplet-triplet
absorption, which largely dominates the pico- and nanosecond
spectra, necessitated its subtraction from, for example, the
nanosecond spectrum. Indeed, the corrected spectra (Figure 8)
revealed the following characteristics of the C60•- -dendron•+
radical pair: The fullerene π-radical anion displayed its typical
NIR-fingerprint absorption at 1000 nm plus a maximum around
400 nm,32 while the one-electron oxidized form of the den-
drimers was evidenced through its strong absorption in the
visible with maxima around 510 (1b) and 540 nm (2b).33 On
the basis of the relative yields of triplet versus radical pair, we
approximate the quantum yield of charge separation in ben-
zonitrile to be ∼10%.
Figure 8. Nanosecond transient absorption spectrum (vis-NIR part)
recorded 50 ns upon flash photolysis of dyad 2b (5.0 × 10-5 M) at 355 nm
in deoxygenated benzonitrile before (s) and after (- - -) subtraction of the
triplet component.
recombination dynamics. The dependence of charge-recombina-
tion rates versus free energy changes indicates stabilizing effects
for the charge-separated state in solvents of higher polarity in
both systems. These are clear attributes describing the normal
region of the classical Marcus parabola, in sound agreement
with the thermodynamic considerations (vide infra).
Thermodynamic Considerations. Upon photoexcitation, an
efficient and rapid energy transfer dominates the deactivation
of the initially excited dendron moieties in all dyads (i.e., 1a,b
and 2a,b). Spectroscopic and kinetic evidence (vide supra)
suggests that yet a second contribution, namely, an intramo-
lecular electron transfer, exists.
Photophysics: Charge Recombination. Both fingerprints
(i.e., ∼820 and 1000 nm) were employed as reliable probes, to
determine the lifetime of the associated C60•- -dendron•+ state
in 1a and 2a (see Table 4 and Figure 7). The decay curves were
well-fitted by a single-exponential decay component. In par-
ticular, lifetimes that are on the order of hundreds of nano-
seconds (220-725 ns) were derived from the decays of the
oxidized donor and reduced acceptor absorptions at 820 and
1000 nm, respectively.34 The larger donor-acceptor separation
in 2a (16.7 Å), relative to 1a (11.5 Å), decelerates the charge-
In dyads 1b and 2b, the thermodynamic driving forces (Table
5), associated with an intramolecular electron transfer between
the dendron singlet excited state and the fullerene electron
acceptor, match the reorganization energies quite well (-∆GET°
‡
∼ λ). This leads subsequently to very small ∆GCS values, on
the order of 0.004 eV and less. One expected consequence infers
that nearly ideal conditions exist for an electron-transfer event,
that is, no significant activation barrier. It is nonetheless
important to note that the energetic position of the charge-
separated state (∼1.66 eV) brings it close to that of the fullerene
singlet excited state (1.76 eV). Electron transfer evolving from
the fullerene singlet excited state in dyads 1b and 2b is
(31) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker, Inc.: New York, 1993.
(32) Guldi, D. M.; Hungerbu¨hler, V.; Janata, E.; Asmus, K.-D. J. Chem. Soc.,
Chem. Commun. 1993, 84.
(33) The spectral features of the one-electron oxidized products (see Table 2)
were charcterized by pulse radiolytic oxidation experiments. This technique
was chosen because it is known to be one of the most powerful tools to
investigate reactive intermediates, evolving from the selective one-electron
transfer, for example, to [CH2Cl2]•+ and •OOCH2Cl/•OOCHCl2 peroxyl
radicals in oxygenated dichloromethane solutions.
(34) Because of the presence of the bulky substituents, cis-trans isomerization,
prior to the rapid deactivation of the dendron singlet excited state (∼1 ps),
plays no major role.
9
10884 J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002

Rigid Dendritic Donor−Acceptor Ensembles
A R T I C L E S
) for Intramolecular
‡
a
Table 5. Driving Force Dependence (∆GS, -∆GCR°; -∆GCS°)a and Thermodynamic Parameters (λ; ∆GCS
Electron-Transfer Events and Energy Transfer in Fullerene-Based Dyads (1a, 1b, 2a, 2b, 16, 17)
∆G [eV]
S
−∆GCR° [eV]
−∆G °c [eV]
λ [eV]
∆G ‡ [eV]
∆GENERGY [eV]
CS
CS
distanceb [Å]
toluene
THF
bzcn
toluene
THF
bzcn
toluene
THF
bzcn
bzcn
bzcn
17
1b
2b
16
1a
2a
R-: 4.4
R+: 3.5
R+-: 8.56
R-: 4.4
R+: 3.5
R+-: 14.0
R-: 4.4
R+: 3.5
R+-: 19.1
R-: 4.4
R+: 3.7
R+-: 8.62
R-: 4.4
R+: 3.7
R+-: 11.5
R-: 4.4
R+: 3.5
0.48
-0.132
-0.328
2.43
1.82
1.91
1.94
1.23
1.29
1.35
1.62
1.01
(-0.67)
1.58
(-0.07)
1.82
(0.14)
1.16
1.39
1.49
1.09
1.30
1.42
0.09
(0.22)
1.68
0.78
0.91
0.46
0.64
0.82
-0.041
-0.015
-0.137
-0.076
-0.023
-0.301
-0.288
-0.32
2.73
2.86
1.83
2.01
2.19
1.65
1.66
1.05
1.06
1.08
0.33
(-0.97)
1.16
(-0.15)
1.41
(0.11)
0.0001
(0.29)
1.3
0.14
(-1.1)
1.06
(-0.18)
1.34
(0.1)
0.004
(0.32)
1.24
(-0.07)
(0.53)
(0.71)
0.033
-0.306
-0.288
0.88
(-0.25)
1.6
(0.47)
1.83
(0.7)
0.06
(0.07)
1.13
0.99
0.56
(-0.43)
1.4
(0.41)
1.67
(0.68)
0.01
(0.09)
R+-: 16.7
a See for details: Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118,
11771. b (R+) radius donor; (R-) radius acceptor; (R+-) donor-acceptor separation. c In parentheses, the values are given for intramolecular reactions
evolving from the fullerene singlet excited state.
endothermic in toluene and THF and only slightly exothermic
in polar benzonitrile. Thus, the thermodynamic conclusion
supports the spectroscopic finding: A competition between
intramolecular electron transfer and singlet-singlet energy
transfer.
alternative intramolecular electron transfer, from which an
energetic C60•- -dendron•+ radical pair evolves. Most impor-
tantly, variation of the energy gap modulates the character of
this electron-transfer reaction: Depending on the energetic
position of the C60•- -dendron•+ radical pair, either a competi-
tive (i.e., 1b, 2b) or a sequential pathway (i.e., 1a, 2a) is
activated.
In retrospect, our strategy to devise 1st and 2nd generations
of C60-(dendron) ensembles clearly demonstrates that signifi-
cantly higher absorption cross sections (2a,2b) and stabilization
of an efficiently formed C60•- -dendron•+ radical pair (2a) are
achieved. Considering the overall efficiency of 76% for (i)
funneling light from the antenna chromophores to the fullerene
core and (ii) charge separation evolving from the fullerene
singlet excited state, this model system reproduces natural
photosynthesis very well. Clearly, these performances open the
way for the preparation of integrated photosynthetic assemblies
and photovoltaic devices. Currently, we are pursuing the
incorporation of chromophores that absorb more strongly in the
visible region of the solar spectrum.
The picture is quite different for dyads 1a and 2a: Impor-
tantly, the charge-separated states are significantly lower in
energy (1.06-1.08 eV) than those of the fullerene singlet excited
states (1.76 eV). This renders electron transfer from both singlet
excited states (i.e., dendron and fullerene) energetically feasible.
Considering the calculated free energy changes, -∆GET°, of
1.67 eV (2a) and 1.83 eV (1a) in, for example, benzonitrile, it
is clear that they are substantially larger than the reorganization
energies (λ ∼ 1.40 eV). Thus, the sequential route in 1a and 2a
emerges to avoid this strongly exothermic electron transfer,
which would be buried deep in the inverted region (-∆GET° )
λ). Instead, the electron transfer occurs from the fullerene singlet
(-∆GET° ∼ 0.7 eV), product of the initial transduction of
excited-state energy, and is consequently placed into the normal
region.
A final comment should address the comparison between
C60-dendron dyads and previously investigated C60-oligomer
systems.35-37 First, the meta-linkage of the stilbene units in the
dendron breaks the π-conjugation.38 This allows the control over
the effective conjugation length. Second, the rigid dendrons
function as efficient mediators for the electron coupling between
In summary, variation of the energy gap (see Table 5)
modulates the character of the electron-transfer reaction: De-
pending on the energetic position of the radical pair in reference
to the dendron and fullerene singlet excited states, either a
competitive or a sequential pathway is activated.
Conclusions
(35) (a) Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.; Krasniko,
V.; Hadziioannou, G. Chem. Commun. 1999, 617. (b) Armaroli, N.;
Barigelletti, F.; Ceroni, P.; Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.
Chem. Commun. 2000, 599. (c) Segura, J. L.; Gomez, R.; Martin, N.; Luo,
C.; Guldi, D. M. Chem. Commun. 2000, 701. (d) Eckert, J.-F.; Nicoud,
J.-F.; Nierengarten, J.-F.; Liu, S.-G.; Echegoyen, L.; Barigelletti, F.;
Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem.
Soc. 2000, 122, 7467.
(36) Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.;
Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174.
(37) Marcos Ramos, A.; Rispens, M. T.; van Duren, J. K. J.; Hummelen, J. C.;
Janssen, R. A. J. J. Am. Chem. Soc. 2001, 123, 6714.
(38) Additional proof for an intramolecular charge recombination stems from a
set of experiments, conducted with different laser power and different dyad
concentrations: Decreasing the radical-pair concentration in various
increments by up to 65% failed to lead to any notable changes in the charge-
recombination kinetics.
A new convergent route was developed to design a series of
C60-based dendrimers carrying dibutylaniline or dodecyloxy-
naphthalene donors. The dendrimers function as rigid molecular
scaffolds, locking the dibutylaniline/dodecyloxynaphthalene
electron donors and the fullerene acceptor in spatially well-
separated positions. In all dyads, an efficient and rapid energy
transfer, as confirmed in a series of steady-state and time-
resolved photolytic experiments, dominates the deactivation of
the initially excited dendron antennas, generating the fullerene
singlet excited state in nearly quantitative yields. A detailed
spectroscopic and kinetic analysis prompts, nonetheless, an
9
J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002 10885

A R T I C L E S
Guldi et al.
T-formal sample compartment equipped with a stroboscopic detector.
Details of the Laser Strobe systems are described on the manufacturer’s
web site, http://www.pti-nj.com.
electron donor and electron acceptor. As a consequence, highly
stabilized and spatially well-separated charge separated states
are generated, for example, in 1a and 2a, which give rise to a
profound distance behavior. Finally, energy transfer dominates
the deactivation of photoexcited C60-oligophenylenevinylene
dyads35 with the exception of trimeric/tetrameric R,ω-dimethyl-
2,5-bis(2-(S)-methylbutoxy)-1,4-phenylene species36 and poly-
mer with pendant methanofullerenes,37 for which, however, no
quantum yields of charge separation are reported.
Emission spectra were recorded with a SLM 8100 spectrofluorom-
eter. The experiments were performed at room temperature. When
measuring the fullerene emission in the 700 nm region, a 570 nm long-
pass filter in the emission path was used in order to eliminate the
interference from the solvent and stray light for recording the fullerene
fluorescence. Each spectrum was an average of at least 5 individual
scans, and the appropriate corrections were applied. The fluorescence
quantum yields were determined with a 9,10-diphenylanthracene
reference (Aldrich, 99+%) (Φ ) 1) and an average value of 3
fluorophore concentrations with ODs at the excitation wavelength
ranging from 0.1 to 0.5.
Experimental Section
Cyclic voltammograms were recorded on a potentiostat/galvanostat
equipped with software for electrochemical analysis by using a GCE
(glassy carbon electrode) as working electrode, SCE as reference
electrode, Bu4NClO4 as supporting electrolyte, toluene and acetonitrile
as solvents, and a scan rate of 200 mV/s.
All melting points were measured with a melting point apparatus
and are uncorrected. FTIR spectra were recorded as KBr pellets. 13C
1
1
and H NMR spectra were recorded with a 300 MHz (for H) and 75
MHz (for 13C) spectrometer. Chemical shifts are given as δ values
(internal standard: TMS).
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).
Nanosecond laser experiments were performed with laser pulses from
a Molectron UV-400 nitrogen laser system (337.1 nm, 8 ns pulse width,
1 mJ/pulse) or from a Qunta-Ray CDR Nd:YAG system (355 nm, 20
ns pulse width). The photomultiplier output was digitized with a
Tektronix 7912 AD programmable digitizer. The details of the
experimental set-up and its operation have been described elsewhere.39
The quantum yields of the triplet excited states (ΦTRIPLET) were
determined by the triplet-triplet energy-transfer method using â-car-
otene as an energy acceptor. The quantum yields of the charge
separation were measured using the comparative method. In particular,
the strong fullerene triplet-triplet absorption (ꢀ700nm ) 16 100 M-1
cm-1; ΦTRIPLET ) 0.98)40 served as a probe to obtain the quantum yields
for the charge-separated state, especially for the fullerene π-radical anion
(ꢀ1000nm ) 4700 M-1 cm-1).41 For all photophysical experiments, an
error of 10% must be considered.
3,5-Dimethylbromobenzene (6b) and N,N-dibutylaminobenzaldehyde
(9) are commercially available and were used without further purifica-
tion. 1,3,5-Triformylbenzene (3),11 N,N-dibutylamino-4-(triphenylphos-
phoniummethyl)benzene iodide (4),20 3,5-dimethylbenzonitrile (6a),42
3,5-di(bromomethyl)benzonitrile (7a),13 and 6-dodecyloxy-2-formyl-
naphthalene (12)23 were obtained by following previously described
synthetic procedures. Dichloromethane was distilled from CaCl2, and
ethanol was distilled from Mg and iodine.
Acknowledgment. This work has been supported by the
DGESIC of Spain (Project BQU2002-00855) and by the
European Comission (Contract JOR3CT980206) and the Office
of Basic Energy Sciences of the U.S. Department of Energy
(Contribution NDRL-4392 from the Notre Dame Radiation
Laboratory). We are also indebted to Centro de Espectroscop´ıa
de la UCM.
Fluorescence lifetimes were measured with a laser strope fluores-
cence lifetime spectrometer (Photon Technology International) with 337
nm laser pulses from a nitrogen laser fiber-coupled to a lens-based
Supporting Information Available: Synthetic section and
cyclic voltammetry (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(39) Thomas, M. D.; Hug, G. L Comput. Chem. 1998, 22, 491.
(40) Luo, C.; Fujitsuka, M.; Watanabe, A.; Ito, O.; Gan, L.; Huang, Y.; Huang,
C.-H. J. Chem. Soc., Faraday Trans. 1998, 94, 527.
(41) Luo, C.; Fujitsuka, M.; Huang, C.-H.; Ito, O. Phys. Chem. Chem. Phys.
1999, 1, 2923.
JA012694X
(42) Gryszkiewicz-Trochimowski, H. M. E.; Schmidt, W.; Gryszkiewicz-
Trochimowski, G. T. O. Bull. Chim. Fr. 1948, 593.
9
10886 J. AM. CHEM. SOC. VOL. 124, NO. 36, 2002