Hydrogen Bonding Interfaces in Fullerene?TTF Ensembles

Article abstract of DOI:10.1021/ja036358n

Novel thermodynamically stable supramolecular donor-acceptor dyads have been synthesized. In particular, we assembled successfully C₆₀, as an electron acceptor, with the strong electron donor TTF through a complementary guanidinium-carboxylate ion pair. Two strong and well-oriented hydrogen bonds, in combination with ionic interactions, ensure the formation of stable donor-acceptor dyads. The molecular architecture has been fine-tuned by using chemical spacers of different lengths (i.e., phenyl versus biphenyl) and functional groups (i.e., ester versus amide), thus providing meaningful incentives to differentiate between through-bond and through-space electron-transfer scenarios. In electrochemical studies, both the donor and acceptor character of the TTF and C₆₀ units, respectively, have been clearly identified. Steady-state and time-resolved emission studies, however, show a solvent-dependent fluorescence quenching in C₆₀?TTF dyads as well as the formation of the C_60.-?TTF^.+ radical ion pairs, for which we determined lifetimes that are in the range of hundred of nanoseconds to microseconds. The complex network that connects C ₆₀ with TTF in the dyads and the flexible nature of the spacer result in through-space electron-transfer processes. This first example of electron transfer in C₆₀-based dyads, connected by strong hydrogen bonds, demonstrates that this approach can add outstanding benefits to the construction of artificial photosynthetic systems that bear a closer resemblance to the natural one.

Full text of DOI:10.1021/ja036358n

Published on Web 11/15/2003
Hydrogen Bonding Interfaces in Fullerene•TTF Ensembles
Margarita Segura,† Luis Sa´nchez,‡ Javier de Mendoza,*,† Nazario Mart´ın,*,‡ and
Dirk M. Guldi*,§
Contribution from the Departamento de Qu´ımica Orga´nica, UniVersidad Auto´noma de Madrid,
Cantoblanco, E-28049, Madrid, Spain, Departamento de Qu´ımica Orga´nica I, Facultad de
Qu´ımica, UniVersidad Complutense, E-28040 Madrid, Spain, and Radiation Laboratory,
UniVersity of Notre Dame, Notre Dame, Indiana 46556
Received May 27, 2003; E-mail: javier.demendoza@uam.es; nazmar@quim.ucm.es; guldi.1@nd.edu
Abstract: Novel thermodynamically stable supramolecular donor-acceptor dyads have been synthesized.
In particular, we assembled successfully C60, as an electron acceptor, with the strong electron donor TTF
through a complementary guanidinium-carboxylate ion pair. Two strong and well-oriented hydrogen bonds,
in combination with ionic interactions, ensure the formation of stable donor-acceptor dyads. The molecular
architecture has been fine-tuned by using chemical spacers of different lengths (i.e., phenyl versus biphenyl)
and functional groups (i.e., ester versus amide), thus providing meaningful incentives to differentiate between
through-bond and through-space electron-transfer scenarios. In electrochemical studies, both the donor
and acceptor character of the TTF and C60 units, respectively, have been clearly identified. Steady-state
and time-resolved emission studies, however, show a solvent-dependent fluorescence quenching in C60•TTF
dyads as well as the formation of the C60•-•TTF•+ radical ion pairs, for which we determined lifetimes that
are in the range of hundred of nanoseconds to microseconds. The complex network that connects C60 with
TTF in the dyads and the flexible nature of the spacer result in through-space electron-transfer processes.
This first example of electron transfer in C60-based dyads, connected by strong hydrogen bonds,
demonstrates that this approach can add outstanding benefits to the construction of artificial photosynthetic
systems that bear a closer resemblance to the natural one.
Introduction
reversible, and in contrast to truly covalent bonds, their binding
energies are highly dependent on the chemical environment and
Photoinduced energy and electron-transfer processes bear
great significance in nature since they govern photosynthesis
in plants and bacteria.1 In the context of engineering extended
1-D, 2-D, or 3-D model systems, meaningful incentives are lent
from bacterial photosynthetic reaction centers. In these natural
analogues, the required well-defined architectures are often
achieved, with high directionality and selectivity, by multipoint
interactions between the individual building blocks, including
hydrogen bond motifs.2
temperature.
Fullerenes exhibit remarkable physicochemical features,
which render them promising building blocks for the integration
into functional molecular assemblies and supramolecular ar-
rays.5,6 Among several outstanding properties, the spherical
shape of fullerenes, giving rise to a delocalized π-electron
system, offers unique opportunities for stabilizing charged
entities. As a consequence, small reorganization energies are
found for C60 in electron-transfer processes, which accelerate
the initial charge separation, while the strongly exothermic
charge recombination is decelerated, due to being shifted deeply
into the “inverted region”. Despite the current interest to
incorporate fullerene components into supramolecular systems,7
only a few examples have been reported so far, in which
hydrogen bonding motifs dictate the structural arrangement8 or
the transduction of excited-state energy between donors and
C60.9
Owing to the versatility that biomimetic methodologies
provide as meaningful organization principles to regulate size,
shape, and function down to the nanometer scale, they are
widely spread in use for the preparation of simple artificial
photosynthetic systems3 or as models for electron-transfer
theory.4 A central benefit of biomimetic motifs is that they are
† Universidad Auto´noma de Madrid.
‡ Universidad Complutense.
§ Radiation Laboratory, University of Notre Dame.
(1) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (b) Moser, C. C.;
Keske, J. M.; Warncke, K.; Farid, R. S.; Dutton, P. L. Nature 1992, 355,
796-802. (c) Regan, J. J.; Onuchic, J. N. In The Reaction Centers of
Photosynthetic Bacteria; Michel-Beyerle, M. E., Ed.; Springer: Berlin,
1996.
In the context of preparing new photovoltaic materials, the
integration of C60 and tetrathiafulvalene (TTF)10 or C60 and
exTTF (exTTF, π-conjugated p-quinonoid-TTF) into synthetic
reaction center models (i.e., covalently linked C60-TTF and C60-
(2) (a) Lehn, J.-M. Supramolecular Chemistry; VCH: Wienheim, 1995. (b)
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9
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J. AM. CHEM. SOC. 2003, 125, 15093-15100
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A R T I C L E S
Segura et al.
exTTF)11 emerged as a particularly promising approach.12,13
Typically, photoexcitation of these C60-TTF and C60-exTTF
systems leads to highly energetic and long-lived charge separ-
ated states. The charge-separation lifetimes vary from a few
nanoseconds to hundreds of microseconds and can be controlled
at will: Important variables in control over the lifetimes are
solvent environment, donor-acceptor separation, electronic
coupling element, and so forth. Since a significant fraction of
the photon energy (up to 80%) is converted and stored in the
form of charge-separated states (∼1.1-1.5 eV), it permits the
possibility to minimize the loss of excited-state energy and,
thereby, to improve the conversion efficiency of solar energy
into electrical and chemical energy.
In the current work, we wish to report on the synthesis of
several new C60•TTF ensembles, in which a photoexcited-state
acceptor (C60) and an electroactive donor (TTF) are held together
through hydrogen bonding interactions that are based on the
complementary guanidinium and carboxylate motifs.14 To add
structural diversity to the C60•TTF ensembles, two different
functionalities, namely, ester or amide groups, have been
employed in connecting C60 and TTF to the guanidinium and
carboxylate moieties in several protocols. As a result, we isolated
a series of topologically different H-bonding dyads whose
physicochemical properties have been examined by cyclic
voltammetry, fluorescence spectroscopy, and transient absorp-
tion spectroscopy. From the spectroscopic characterization, we
conclude the formation of charge-separated radical pairs,
C60•-•TTF•+, that exhibit lifetimes in the range of hundreds of
nanoseconds, evolving from photoinduced electron-transfer
processes that occur, in large, via through-space interactions.
Results and Discussion
In the present study, we probed the assembly of the strong
electron acceptor C60 with the strong electron donor TTF,
through a guanidinium-carboxylate ion pair involving two strong
and well-oriented donor-donor-acceptor-acceptor (DD-AA)
hydrogen bonds in combination with an electrostatic interaction,
toward thermodynamically stable supramolecular donor-ac-
ceptor dyads. An important aspect of our work is the fine-tuning
of the molecular architecture, which was accomplished by using
spacers of different lengths and of different hydrogen bond
orientations (donors, 1a,b and 4; acceptors, 2a,b, 3a,b, 5, and
6; see Chart 1).
A closer inspection of the guanidinium/donor or acceptor
connection reveals some structural flexibility, and different
structural conformers are assumed to be present in the resulting
ensembles (Chart 2).
Charge-transfer interactions, as they may well prevail between
the donor (TTF) and acceptor (C60), impose, on the other hand,
a certain degree of control over orientation and structure. To
constitute a useful way for elucidating such structural conforma-
tions, we pursued the use of oppositely oriented hydrogen bond
motifs (see for illustration dyads 1a•5 and 2a•4) which should
result in different electron-transfer responses.
Synthesis. The synthesis of guanidinium-TTF derivatives
1a,b is summarized in Scheme 1. Both compounds were
prepared in moderate (49%) and good yields (72%), respectively,
from acyl chloride 1015 via coupling with the appropriate
guanidinium salt [alcohol 7(PF6-) or amine 9].16 The use of
reverse-phase silica gel chromatography for the purification of
ester 1a(PF6-) is central, since it prevented hydrolysis of this
otherwise relatively unstable derivative.
The phenyl-C60 derivatives (Scheme 2) were obtained by
PyBOP (benzotriazole-1-yl-oxytripyrrolidinophosphonium hexaflu-
orophosphate) activation of carboxylic acid 12, which was
synthesized from p-formylbenzoic acid (11), C60, and N-
octylglycine in 52% yield.17 In a succeeding reaction with the
chiral guanidinium salt [7(Cl-) or 9], 2a,b were isolated as
hexafluorophosphate salts with good yields.
(8) (a) Diederich, F.; Echegoyen, L.; Go´mez-Lo´pez, M.; Kessinger, R.; Stoddart,
J. F. J. Chem. Soc., Perkin Trans. 2 1999, 1577-1586. (b) Rispens, M. T.;
Sa´nchez, L.; Knol, J.; Hummelen, J. C. Chem. Commun. 2001, 161-162.
(c) Gonza´lez, J. J.; Gonza´lez, S.; Priego, E. M.; Luo, C. P.; Guldi, D. M.;
de Mendoza, J.; Mart´ın, N. Chem. Commun. 2001, 163-164. (d) Sa´nchez,
L.; Rispens, M. T.; Hummelen, J. C. Angew. Chem., Int. Ed. 2002, 41,
838-840. (e) Beckers, E. H. A.; Schenning, A. P. H. J.; van Hal, P. A.;
El-ghayoury, A.; Sa´nchez, L.; Hummelen, J. C.; Meijer, E. W.; Janssen,
R. A. J. Chem. Commun. 2002, 2888-2889.
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A.; Peeters, E.; Rispens, M. T.; Hummelen, J. C.; Meijer, E. W.; Janssen,
R. A. J. J. Mater. Chem. 2002, 12, 2054-2060. (b) Guldi, D. M.; Ramey,
J.; Mart´ınez-D´ıaz, M. V.; de la Escosura, A.; Torres, T.; Da Ros, T.; Prato,
M. Chem. Commun. 2002, 2774-2775. (c) Guldi, D. M.; Gouloumis, A.;
Va´zquez, P.; Torres, T. Chem. Commun. 2002, 2056-2057.
(10) For a review on recent applications of TTF, see: Segura, J. L.; Mart´ın, N.
Angew. Chem., Int. Ed. 2001, 40, 1372-1409.
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Materials and DeVices; Nalwa, H. S., Ed.; Academic Press: New York,
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J. J. Org. Chem. 2000, 65, 1978-1983. (b) Mart´ın, N.; Sa´nchez, L.;
Herranz, M. A.; Guldi, D. M. J. Phys. Chem. A 2000, 104, 4648-4657.
(c) Mart´ın, N.; Sa´nchez, L.; Guldi, D. M. Chem. Commun. 2000, 113-
114. (d) Segura, J. L.; Priego, E. M.; Mart´ın, N.; Luo, C. P.; Guldi, D. M.
Org. Lett. 2000, 2, 4021-4024. (e) Herranz, M. A.; Mart´ın, N.; Ramey,
J.; Guldi, D. M. Chem. Commun. 2002, 2968-2969. (f) Gonza´lez, S.;
Mart´ın, N.; Guldi, D. M. J. Org. Chem. 2003, 68, 779-791. (g) Gonza´lez,
S.; Mart´ın, N.; Swartz, A.; Guldi, D. M. Org. Lett. 2003, 5, 557-560. (h)
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2457-2468.
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Moore, T. A.; Gust, D. HelV. Chim. Acta 2001, 84, 2765-2783. (b) Kreher,
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Albert, J. S.; Goodman, M. S.; Hamilton, A. D. J. Am. Chem. Soc. 1995,
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In a similar way, biphenyl-C60 compounds 3a,b were prepared
from acid 15. The latter was synthesized in racemic form by
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Hydrogen Bonding Interfaces in Fullerene•TTF Ensembles
A R T I C L E S
Chart 1. Structures of Guanidinium Compounds 1-3a,b and Carboxylates 4-6
with C60 and N-octylglycine and subsequently deprotected in
an acidic medium (Scheme 3).
All new compounds were fully characterized by a variety of
disclose evidence for the pyrrolidine protons, that is, a doublet
at ca. 5.1 and 4.1 ppm (CH2, J ≈ 9.4 Hz; geminal hydrogens)
and a singlet at ca. 5.1 ppm (CH). Similar to 1a and 1b, the
link between the fullerene and guanidinium fragments was
established by the presence of the corresponding signals (δ )
65.5 ppm for both esters 2a and 3a; δ ) 43.5 ppm for both
amides 2b and 3b) in the 13C NMR spectra.
Donor-acceptor dyads 1a•5, 1b•5, 1a•6, and 1b•6 (Chart
2), in which the electron donor (TTF) is directly attached to
the guanidinium scaffold and the electron acceptor (phenyl-C60
or biphenyl-C60) is connected to the carboxylate, were prepared
by single liquid-liquid extraction. In particular, an aqueous
phase of the sodium carboxylates 5(Na+) or 6(Na+) was brought
in contact with a dichloromethane solution of the guanidinium
hexafluorophosphate salts 1a(PF6-) and 1b(PF6-) (see Experi-
mental Section in the Supporting Information for details).
spectroscopic and spectrometric techniques. In particular, the
1H NMR spectra of ester 1a(PF6-) and amide 1b(PF6
)
-
derivatives, recorded in [D6]acetone, show the presence of the
guanidinium NH protons at 7.30/7.14 ppm for the ester and 7.19/
7.13 ppm for the amide analogue, chemical shifts that are typical
for guanidinium hexafluorophosphate salts.16 In addition, signals,
which correspond to the TTF hydrogens (δ ) 7.76/6.69 ppm
for 1a and δ ) 7.36/6.66 ppm for 1b), are also discernible.
The ester or amide junctions were mainly confirmed by the
existence of two discrete methylene signals at 67.6 and 46.0
ppm, respectively, in the 13C NMR spectra.
With respect to phenyl-C60 (2a,b) and biphenyl-C60 deriva-
tives (3a,b), inspection of the 1H NMR spectra (see Figure 1a)
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A R T I C L E S
Segura et al.
Chart 2. Structures of Hydrogen Bonded C60•TTF Dyads
Scheme 1. Synthesis of TTF Derivatives 1a,b(PF6-)a
Scheme 2. Synthesis of Phenyl-C60 Derivatives 2a,b(PF6-)a
a (a) CH2Cl2/2 M aq KOH (×2); then 0.1 M aq NH4PF6 (×2), 100%.
(b) 10, N,N-diisopropylethylamine (DIPEA), CH2Cl2, room temperature,
12 h; then CH2Cl2/0.1 M aq NH4PF6 (×2), 49%. (c) N-Methylmorpholine
(NMM), Ms2O, THF, room temperature, 1 h; then CH2Cl2/0.1 M aq NH4PF6
(×2), 96%. (d) 30% NH3, MeOH, room temperature, 30 min, 100%. (e)
10, DIPEA, CH2Cl2, room temperature, 12 h; then CH2Cl2/2 M aq KOH
(×2) and 0.1 M aq NH4PF6 (×2), 72%.
a (a) C60, N-octylglycine, o-dichlorobenzene (ODCB), reflux, 16 h, 52%.
(b) PyBOP, DIPEA, 7(Cl-), CH2Cl2, 80 °C, 3 days; then CH2Cl2/2 M aq
KOH (×1) and 0.1 M aq NH4PF6 (×2), 58%. (c) PyBOP, DIPEA, 9,
CH2Cl2, room temperature, 12 h; then CH2Cl2/2 M aq KOH (×2) and 0.1
M aq NH4PF6 (×2), 73%.
Analogously, acceptor-donor dyads 2a•4, 2b•4, 3a•4, and 3b•4,
with the donor subunit attached to the carboxylate, were
prepared by single liquid-liquid extraction of an aqueous
solution of lithium TTF-carboxylate 4(Li+)19 with dichlo-
romethane solutions of the guanidinium salts [2a(PF6-), 2b(PF6-),
3a(PF6-), and 3b(PF6-)].
It is worth mentioning that the formation of compounds 2a,b
and 3a,b, as well as their corresponding supramolecular dyads,
should lead to the expected diastereomeric mixture provided
that a new stereogenic center is generated in the pyrrolidine
1
ring. However, even after registering the H NMR spectra of
these compounds at high resolution (500 MHz), we were not
able to distinguish these diastereomers. The large distance
between the pyrrolidine ring and the guanidinium group could
account for this experimental finding.
In all the dyads, significant downfield shifts (ca. 4 ppm in
CDCl3) were observed for the guanidinium NH protons (see,
for example, Figure 1b), which is consistent with the existence
of a strong DD-AA hydrogen bonding arrangement in the new
guanidinium-carboxylate ion pair formed.14
(19) Gar´ın, J.; Orduna, J.; Uriel, S.; Moore, A. J.; Bryce, M. R.; Wegener, S.;
Yufit, D. S.; Howard, J. A. K. Synthesis 1994, 489-493.
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Hydrogen Bonding Interfaces in Fullerene•TTF Ensembles
A R T I C L E S
Figure 1. 1H NMR spectra (500 MHz, CDCl3) of compound 3a(PF6-) (a) and dyad 3a•4 (b) at room temperature: (b) guanidinium NH’s; (9) CH TTF;
(2) CH pyrrolidine; (*) CH2 pyrrolidine; ([) CH2O.
Scheme 3. Synthesis of Biphenyl-C60 Derivatives 3a,b(PF6-)a
Table 1. Redox Potentials of New TTF and C60 Derivatives
1
2
3
4
1/2
1/2
compda
E red
E red
E red
E red
E
E
2,ox
1,ox
1a
1b
0.55
0.51
0.99
0.94
2a
2b
3a
3b
C60
TTF
-0.58
-0.60
-0.59
-0.60
-0.54
-0.97
-0.98
-0.99
-0.99
-0.94
-1.52
-1.52
-1.52
-1.52
-1.43
-1.95
-1.98
-1.94
-1.97
-1.91
0.44
0.81
a Experimental conditions: V vs Ag/Ag+; ODCB/MeCN (4:1) as solvent;
GCE as working electrode; Bu4N+ClO4- (0.1 M) as supporting electrolyte;
scan rate, 100 mV/s.
a (a) C60, N-octylglycine, ODCB, reflux, 16 h, 51%. (b) HCl, AcOH,
ODCB, reflux, 72 h, 65%. (c) PyBOP, DIPEA, 7(Cl-), CH2Cl2, 80 °C, 3
days; then CH2Cl2/2 M aq KOH (×1) and 0.1 M aq NH4PF6 (×2), 67%.
(d) PyBOP, DIPEA, 9, CH2Cl2, room temperature, 12 h; then CH2Cl2/2 M
aq KOH (×2) and 0.1 M aq NH4PF6 (×2), 72%.
noted for the amide group, thus slightly decreasing the electron
donor ability of the TTF unit.
[60]Fullerene derivatives, on the other hand, which are linked
to the guanidinium cations through either an ester (2a, 3a) or
amide group (2b, 3b) show four quasireversible reduction waves.
These reduction waves are cathodically shifted in comparison
with the parent C60 molecule, measured under the same
experimental conditions. Earlier, this electrochemical shift,
which is characteristic for most 1,2-dihydrofullerenes, has been
rationalized by the saturation of a C60 double bond and
consequently raising the LUMO energy of the C60 derivative.20
Electrochemistry. The redox properties of 1a,b, 2a,b, and
3a,b were measured at room temperature by cyclic voltammetry
experiments. We selected a 4:1 o-dichlorobenzene/acetonitrile
solvent mixture to ensure solubilization of all the fullerene
derivatives. The data are collected in Table 1 along with those
of the parent TTF and C60 moieties, which serve as references.
TTF derivatives 1a,b, that is, those bearing guanidinium
cations, show the presence of two reversible oxidation waves,
corresponding to the formation of the radical cation and dication
species. These waves are anodically shifted in comparison with
the parent TTF, which can be accounted for by the electronic
effect of the electron-withdrawing ester and amide carbonyl
groups attached to the 1,3-dithiole ring of the TTF moiety. The
values determined for the first oxidation potential in 1a,b reveal
the stronger deactivation of the ester group, relative to what is
Interestingly, the fullerene centered reduction potential peaks
appear at nearly identical values. This observation prompts to
a lack of electronic communication between the guanidinium,
ester or amide group, with C60 in the ground state. A similar
trend evolves when comparing the phenyl- and biphenyl-based
ensembles, which exhibit practically the same reduction potential
values (Table 1).
(20) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593-601.
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A R T I C L E S
Segura et al.
Table 2. Photophysical Features of Hydrogen Bonded C60•TTF Dyads
toluene
chloroform
dichloromethane
fluorescence fluorescence
fluorescence
quantum yield
fluorescence
lifetime (ns)
fluorescence
quantum yield
fluorescence
lifetime (ns)
quantum yield
lifetime (ns)
1a•5
1b•5
1a•6
1b•6
2a•4
2b•4
3a•4
3b•4
3.6 × 10-4
3.4 × 10-4
4.2 × 10-4
4.2 × 10-4
4.2 × 10-4
3.9 × 10-4
4.2 × 10-4
4.0 × 10-4
1.05
1.05
1.25
1.26
1.19
1.13
1.22
1.24
2.8 × 10-4
2.8 × 10-4
3.3 × 10-4
3.2 × 10-4
0.86
0.83
1.06
1.08
2.7 × 10-4
2.7 × 10-4
3.3 × 10-4
3.2 × 10-4
3.6 × 10-4
3.4 × 10-4
3.0 × 10-4
3.1 × 10-4
0.82
0.80
1.09
1.10
1.02
0.97
1.01
1.06
Accordingly, the more polar solvents amplify the fullerene
fluorescence quenching (from 3.4 × 10-4 in toluene to 2.7 ×
10-4 in dichloromethane for dyad 1b•5, Table 2).
In addition, the flexible nature of the spacer between TTF
and C60 allows a high degree of conformational freedom. Thus,
two contrasting scenarios, that is, a through-bond or a more
plausible through-space supported electron transfer, might be
accountable for the photoreactivity of the C60•TTF dyads. To
shed light onto the mechanism in question and differentiate
between them, we probed several spacers, since their chemical
nature and composition help to modulate the electronic coupling.
In the case of simple ester and amide linkages, both spacers
place donor and acceptor in approximately the same spatial
separation. However, we found no significant differences
between esters and amides, 4.2 × 10-4 for both 1a•6 and 1b•6,
and the fullerene deactivation remains unaffected regardless of
the solvent. We conclude, from this trend, that a through-space
mechanism might be operative. This scenario appears credible
in light of a simple molecular modeling, which suggests that
donor and acceptor are placed in close proximity to each other.22
In the second set of dyads, we compared the impact of
biphenyl and phenyl spacers on the photoreactivity of the
C60•TTF dyads: Using the rigid biphenyl separation instead of
phenyl leads to a small but appreciable impact on the fullerene
singlet excited-state deactivation. More precisely, a 20%
reactivation of the fullerene fluorescence from 3.4 × 10-4 to
4.2 × 10-4 (1b•5 vs 1b•6, Table 2 and Figure 2) is seen. This
again speaks against a through-bond electron transfer but should
be considered in support of a through-space electron-transfer
scenario. In fact, in case of a through-bond electron transfer,
implementation of four additional bonds into the hydrocarbon
bridge should have affected the electron-transfer rate by 2 orders
of magnitude (i.e., decrease in rate of nearly 1 order of
magnitude per introduction of two bonds into the hydrocarbon
bridge).23 The observed rates and yields, however, are in contrast
to the increment of bonds in the spacer. Although the fluores-
cence patterns of the different C60•TTF dyads are nearly
Figure 2. Room-temperature fluorescence spectra of a N-methylfullero-
pyrrolidine reference, dyad 1b•6, and dyad 1b•5 in toluene solutions.
Excitation wavelength is 360 nm.
Steady-State and Time-Resolved Fluorescence Studies. We
started the steady-state fluorescence experiments with dyads
(1a•5, 1b•5, 1a•6, 1b•6, 2a•4, 2b•4, 3a•4, and 3b•4, Chart 2)
by probing solvents which support the well-directed hydrogen
bond motif, such as toluene, chloroform, and dichloromethane.
Benefits of this solvent variation are the control over the free
energy gap for photoinduced charge-transfer processes: In the
more polar solvents, a better solvation of charged entities, such
as fullerene radical anions and the TTF radical cations, prevails
and, subsequently, lowers the energy of the radical ion pairs
relative to that of the singlet ground state.
Notably, the TTF electron donor features much weaker
absorptions in the visible region than the fullerene core which
leads, upon photoexcitation in the 300-400 nm range, to the
exclusive fullerene singlet excited-state population (i.e., <95%).
All details about the photophysical parameters are summarized
in Table 2. The most important observation is that the room-
temperature fullerene fluorescence, with its characteristic maxi-
mum at 715 nm in toluene, is in C60•TTF dyads (for example,
3.4 × 10-4 for 1b•5) quenched relative to that of N-methyl-
fulleropyrrolidine (6.0 × 10-4), which was used as a reference.
This trend can also be seen in Figure 2. For a given solution,
the fluorescence intensity is known to be proportional to the
intensity of the exciting light and the molar extinction coef-
ficient. Since these parameters were kept constant, it is conceiv-
able to attribute the decrease in fluorescence intensities to
reductive quenching of the fullerene excited singlet state by the
TTF moiety via electron transfer.
(21) For example, see: Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J.
Am. Chem. Soc. 1997, 119, 974-980.
(22) Due to the high flexibility of each component, an accurate estimation of
the minimum distance between TTF and C60 in each dyad is difficult. The
following minimal distances between the centers of the fullerene and the
outer TTF ring in ester or amide derivatives were evaluated from the in
vacuo optimized structures: 6.55 Å for dyads 1a•5 and 2a•4; 7.84 Å for
dyad 3a•4; and 9.30 Å for dyad 1a•6.
(23) (a) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.;
Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. Soc. 1987, 109,
3258-3269. (b) Oevering, H.; Verhoeven, J. W.; Paddon-Row, M. N.;
Warman, J. M. Tetrahedron 1989, 45, 4751-4766. (c) Kroon, J.; Verho-
even, J. W.; Paddon-Row, M. N.; Oliver, A. M. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1358-1361. (d) Paddon-Row, M. N. Acc. Chem. Res. 1994,
27, 18-25. (e) Shephard, M. J.; Paddon-Row, M. N. Aust. J. Chem. 1996,
49, 395-403.
Earlier reports on intramolecular electron transfer in donor-
bridge-acceptor dyads demonstrated the dependence of the rate
of electron transfer upon the solvent dielectric constant.21
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Hydrogen Bonding Interfaces in Fullerene•TTF Ensembles
A R T I C L E S
identical, it should be noted that, parallel to the fluorescence
reactivation, the associated maxima shows a bathochromic shift
from 715 to 718 nm, as compared to that determined for the
N-methylfulleropyrrolidine reference. This suggests alteration
of the through-space communication and confirms the creation
of through-space electronic interactions between the two chro-
mophores in the dyads.
An independent test on the validity of the fluorescence
quantum yields involved fluorescence lifetime measurements,
by recording the kinetic time profiles of the 715 or 718 nm
fluorescence maxima upon 337 nm fullerene excitation. For the
N-methylfulleropyrrolidine reference, we determined a lifetime
of 1.5 ns in toluene and dichloromethane, which is notably
longer than the corresponding values in all of the C60•TTF
dyads. As can be seen from a closer inspection of Table 2, the
values determined for the different dyads and/or in the different
solvents agree quantitatively and qualitatively with the quantum
yields.
Figure 3. Differential absorption changes recorded 50 ns after a 337 nm
laser excitation of N-methylfulleropyrrolidine (dashed spectrum) and dyad
1b•5 (solid spectrum) in deoxygenated dichloromethane.
When comparing the fluorescence data for the dyads in which
donors and acceptors are placed in a converse fashion, for
example, for shorter contact distances, compare 1a•5 with 2a•4
(or 1b•5 with 2b•4), we note that in all solvents employed the
interactions were stronger in the 1 series (TTF at the guani-
dinium) than in the 2 series (C60 at the guanidinium), whereas,
in the biphenyl series, in which the spacer keeps both compo-
nents wide apart, for example, 1a•6 versus 3a•4, no significant
differences were observed.
Therefore, hydrogen bonds emerge as an effective means to
hold the electron donor (TTF) and the electron acceptor (C60)
in spatially predetermined positions. To manipulate the orga-
nization principle, a protic solvent, such as hexafluoro-2-
propanol (HFIP), which interferes with the existing -NH‚‚O-
C- network, was combined with dichloromethane. The addition
of HFIP resulted in a quantitative recovery of the fullerene
fluorescence; that is, the quantum yields (6.0 × 10-4) are now
comparable to that of the N-methylfulleropyrrolidine reference.
This result confirms the complete interruption of the integrative
hydrogen bond network and the C60‚TTF complex dissociation
into both independent fragments (i.e., C60 and TTF). Similarly,
the fluorescence lifetime of 1.4 ns in the HFIP-dichloromethane
mixtures becomes almost identical with that seen for the
N-methylfulleropyrrolidine reference under identical experi-
mental conditions (Figure S2).
Figure 4. Time-absorption profile of the C60 radical anion absorption decay
(i.e., at 1000 nm) as recorded upon a 337 nm laser excitation of dyad 1b•5
in deoxygenated dichloromethane.
nm is discernible, which resembles the characteristic fullerene
radical anion marker. In addition, we found the 450 nm transition
of the TTF radical cation. This spectroscopic evidence confirms
the metastable electron transfer, evolving from the initially
excited fullerene core.
Fitting the decay dynamics of C60 and TTF•+ to a first-
•-
order rate law, we determined lifetimes of 115 ns (dyad 1a•5,
not shown) and 124 ns (dyad 1b•5, Figure 4) for the radical
ion pairs C60•-•TTF•+ in dichloromethane, which corresponds
to charge-recombination rates of (8.4 ( 0.3) × 106 s-1. More
drastic is the impact of the distance dependence, that is, phenyl
versus biphenyl spacer. In the latter systems, the charge-
recombination process is considerably decelerated. For example,
a rate of 1.0 × 106 s-1, measured for the 1a•6 dyad, results
from a smaller electronic coupling between donor and accep-
tor.24
Addition of the hydrogen bond breaking solvent HFIP
changes the fullerene reactivity. Most importantly, right after
the laser pulse, no evidence for the C60•-•TTF•+ radical pair
formation was seen. Instead, the sole species formed is the
fullerene triplet in 95% efficiency, similar to what is shown for
the N-methylfulleropyrrolidine reference in Figure 3. Due to
the presence of the non-hydrogen bonded electron donor, the
fullerene triplet excited state is, however, subject to an
Transient Absorption Spectroscopy. To confirm the fluo-
rescence behavior we employed transient absorption spectro-
scopic technique with the N-methylfulleropyrrolidine reference
and the different C60•TTF dyads (1a•5, 1b•5, 1a•6, 1b•6, 2a•4,
2b•4, 3a•4, and 3b•4). This allows, besides establishing intra-
and intermolecular dynamics, such as charge-recombination
rates, identifying the product evolving from the initial fullerene
deactivation, suggested by the fluorescence experiments.
As far as the fulleropyrrolidines are concerned, they exhibit
on the nano-/microsecond time scale transient transitions
ascribable exclusively to the long-lived (i.e., 20 µs) triplet
excited state. These are sets of maxima at 360 and 700 nm; see
the Supporting Information. Figure 3 illustrates the 700-1100
nm region, recorded for a dichloromethane solution of N-
methylfulleropyrrolidine and 1b•5. For the dyad, the differential
absorption changes are fundamentally different from what is
seen for the fulleropyrrolidine. Clearly, a new feature at 1000
(24) In line with the moderate electron-transfer efficiency, also the product of
the fullerene intersystem crossing, the triplet excited state with its
characteristic 700 nm fingerprint is discernible. In chloroform or dichlo-
romethane, the triplet quantum yields are, however, on the order of 30%.
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A R T I C L E S
Segura et al.
groups (i.e., ester versus amide) and length spacer (i.e., phenyl
versus biphenyl), allowed the construction of topologically
different ensembles. This provided meaningful incentives to
differentiate between through-bond and through-space supported
electron-transfer interactions.
Steady-state and time-resolved emission studies show a clear
trend toward fluorescence quenching in the C60•TTF dyads
whose efficiency depends predominantly on the solvent polarity.
Electron transfer, as the cause of fluorescence quenching, was
confirmed independently by transient absorption spectroscopy.
In particular, the rapid formation of the characteristic fullerene
radical anion (1000 nm) and TTF radical cation (450 nm)
transitions clearly attests C60•-•TTF•+. Charge recombination
rates in the C60•TTF ensembles are typically around 106 s-1
depending on electronic coupling between donor and acceptor.
Owing to the complexity of the network that connects C60 with
TTF in our dyads and the flexible nature of the spacer, through-
space electron-transfer interactions are operative to create the
charge separated C60•-•TTF•+ state, although this photophysical
process is not very efficient. Addition of the hydrogen bonding-
breaking solvent HFIP, on the other hand, results in a change
of the photochemical reactivity, in which no evidence for
forming an intramolecular CS state is observed. Alternatively,
an intermolecular electron-transfer process takes place.
Figure 5. Plot of kobs versus C1b•5 at various concentrations in deoxygenated
HFIP-dichloromethane (1:1).
intermolecular electron-transfer quenching. From a concentration
dependence (i.e., (0.2-5.0) × 10-5 M for dyad 1b‚5), we
derived a rate constant of 4.5 × 109 M-1 s-1 in the HFIP-
dichloromethane mixture (Figure 5). This value is in good
agreement with intermolecular electron-transfer rates measured
for fullerene triplets with diverse electron donors, including TTF
itself.23
Conclusions
In retrospect, we demonstrated that hydrogen bonding motifs
confer outstanding benefits for constructing photo- and elec-
troactive donor-acceptor models as novel artificial photosyn-
thetic systems and possible photovoltaic applications.
Acknowledgment. We are indebted to MCyT of Spain
(Projects BQU-2002-00855 and PB98-0088) and the Office of
Basic Energy Sciences of the U.S. Department of Energy
(Contribution No. NDRL-4490 from the Notre Dame Radiation
Laboratory) for financial support. A grant from Comunidad
Auto´noma de Madrid (to M.S.) is also gratefully acknowledged.
The authors would like to acknowledge Dr. Pilar Prados and
Dr. Michiel Van Gool for helpful discussions and suggestions.
In the current work, we reported on the design, synthesis,
and physicochemical study of new supramolecular fullerene
architectures. These architectures are built on highly directional
and selective hydrogen bonding as a biomimetic organization
principle. We started with synthesizing a series of novel TTF
and C60 precursors that carry either a complementary guani-
dinium or carboxylate entity. A complementary array of
hydrogen bonds, donor-donor-acceptor-acceptor (DD-AA),
opens the way to incorporate both moieties into well-ordered
donor-acceptor arrays: 1a•5, 1b•5, 1a•6, 1b•6, 2a•4, 2b•4,
3a•4, and 3b•4. Varying the nature and composition of the
spacer connecting TTF and C60, by using different functional
Supporting Information Available: General experimental
procedures for the synthesis of guanidinium salts (1-3) and
C60-based carboxylic acids (12, 15) as well as supramolecular
dyads. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA036358N
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15100 J. AM. CHEM. SOC. VOL. 125, NO. 49, 2003