Balancing binding strength and charge transfer lifetime in supramolecular associates of fullerenes

Article abstract of DOI:10.1039/c1cc11693b

A macrocyclic exTTF host for fullerenes offers control over the electronic coupling between an electron donor and an acceptor, and stabilizes the charge separated state lifetimes into the range of 500 ps.

Full text of DOI:10.1039/c1cc11693b

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COMMUNICATION
Balancing binding strength and charge transfer lifetime in supramolecular
associates of fullereneswzy
a
b
c
bc
Bruno Grimm, Helena Isla, Emilio M. Perez, Nazario Martın* and Dirk M. Guldi*a
´
´
Received 24th March 2011, Accepted 11th May 2011
DOI: 10.1039/c1cc11693b
A macrocyclic exTTF host for fullerenes offers control over the
electronic coupling between an electron donor and an acceptor,
and stabilizes the charge separated state lifetimes into the range
of 500 ps.
provides the necessary means for preorganization and serves as a
source of positive van der Waals interactions. To this end, among
the macrocyclic hosts we have investigated,3 1 reveals by far the
best performance in terms of hosting C60.3a
We have previously demonstrated that upon light irradiation
of our tweezers-like host–guest systems, fully charge-separated
Investigation on the association of fullerenes by synthetic hosts
started immediately after the former became available in gram
quantities.1 Among the various fullerene hosts described to date,
macrocyclic structures have proven particularly effective in terms
of both affinity and selectivity.2 Following this strategy, some of
us have recently reported macrocyclic bis-exTTF hosts (Fig. 1)
capable of associating C60 and/or C70 with affinities as high as
micromolar.3 These macrocyclic hosts have been designed as an
evolution of our exTTF-tweezers.4 Importantly, the alkyl linker
ꢁ
+
ꢀ
C60 /exTTFꢀ
states are formed. These charge-separated
states are short-lived, with lifetimes in the range of around 10
picoseconds.4a Such photoinduced electron transfer processes
within exTTF/fullerene supramolecular species are poised for
numerous applications, especially in the context of constructing
more efficient optoelectronic devices5 through self-assembly.6
Here, we wish to report on the detailed investigations regarding
the electron transfer chemistry in 1ꢂC60 and 1ꢂC70.
Initially, we have employed absorption assays, in which
approximately 2.0 ꢃ 10ꢁ5 M of 1 was titrated against variable
concentrations of C60 or C70 (i.e. from 0 to 5.0 ꢃ 10ꢁ5 M). As a
general feature, facilely formed charge transfer complexes—1ꢂ
C
60 or 1ꢂC70—were seen to develop in a variety of solvents (i.e.
toluene, chlorobenzene, and benzonitrile, Fig. S1–S3, ESIy).
For C60 in benzonitrile, the growth of a new broad transition
at 475 nm goes hand in hand with the decrease of the exTTF
absorption at 430 nm. Concomitantly, a low energy isosbestic
point evolves around 460 nm. Relative to benzonitrile, hypso-
chromic shifts were noted in chlorobenzene and in toluene,
where the maxima for the charge-transfer band are discernible
at 470 and at 465 nm, respectively. The fact that the corre-
sponding maxima tend to blue-shift with decreasing solvent
polarity leads us to postulate a redistribution of charge density
in the ground state from the electron donating exTTF to the
electron accepting C60 or C70, to afford the corresponding
C60dꢁ/1d+ or C70dꢁ/1d+. Throughout the solvent change the
absorption maximum of exTTF is, however, not affected. For
C70, the maxima for the charge-transfer band were at 467 nm
in chlorobenzene and 473 nm in benzonitrile. In the next step,
we subjected the absorption changes at 470 nm during the
titration experiments to non-linear fitting functions assuming
formation of the 1 : 1 complex.3 Ka values of 4.2 ꢃ 104, 9.7 ꢃ
105, and 1.1 ꢃ 106 Mꢁ1 were calculated for the formation of 1ꢂ
C60 in toluene, chlorobenzene, and benzonitrile, respectively
(see ESIy). The corresponding 1ꢂC70 values were 5.6 ꢃ 105 Mꢁ1
in chlorobenzene and 5.8 ꢃ 105 Mꢁ1 in benzonitrile. These
values are in good agreement with those obtained by global
multivariable analysis.3
Fig. 1 Structures of exTTF-tweezers and exTTF-macrocycle 1.
a Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), Friedrich-Alexander-Universitat
¨
Erlangen-Nurnberg, Egerlandstraße 3, 91058 Erlangen, Germany.
¨
E-mail: dirk.guldi@chemie.uni-erlangen.de
b
´
´
´
Departamento Quımica Organica, Facultad de Ciencias Quımicas,
Universidad Complutense de Madrid, 28040 Madrid, Spain.
E-mail: nazmar@quim.ucm.es; Fax: +34-91-394-4103
c IMDEA Nanociencia, Facultad de Ciencias,
´
Avda. Fco. Tomas y Valiente, 7, Ciudad Universitaria de
Cantoblanco, 28049 Madrid, Spain
w This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue.
z Dedicated to Professor Jochen Mattay, on the occasion of his
65th birthday.
y Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc11693b
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7449–7451 7449

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conclusion of the 387 nm laser pulse. Spectral characteristics
of this very short-lived excited state (1.2 ps) are transient
maxima around 465, 605, and 990 nm as well as transient
bleaching at o450 nm. The short lifetime is rationalized by the
presence of the sulfur atoms, with a strong second-order
vibronic spin–orbit coupling.8
The differential absorption changes, taken right after the
excitation of C60/C70 at 387 nm, show the instantaneous trans-
formation of high lying singlet excited states into the lowest
vibrational state of the associated singlet excited states. In
particular, marked transitions develop in the near-infrared region
with maxima at 960 nm for C60 and 860/1140 nm for C70. These
singlet excited states with energies of about 1.80 eV deactivate
slowly via intersystem crossing to the energetically lower lying
triplet excited state. The intersystem crossing lifetimes were
determined from multi-wavelength analyses as 1.6 ns. The newly
appearing band at 750 nm reflects the diagnostic signature of the
triplet excited state of C60, for which a lifetime of 45 ms has been
determined. For C70, the corresponding triplet excited state
spectrum is dominated by a maximum at 950 nm.9
Fig. 2 Emission spectra upon 450 nm excitation of a solution of 1 (1.56 ꢃ
10ꢁ5 M) in benzonitrile with variable concentrations of C60 (0; 3.36 ꢃ
10ꢁ7 M; 6.67 ꢃ 10ꢁ7 M; 9.96 ꢃ 10ꢁ7 M; 1.48 ꢃ 10ꢁ6 M; 1.96 ꢃ 10ꢁ6 M;
2.59 ꢃ 10ꢁ6 M; 3.22 ꢃ 10ꢁ6 M; 4.13 ꢃ 10ꢁ6 M; 5.28 ꢃ 10ꢁ6 M; 6.67 ꢃ
10ꢁ6 M; 8.28 ꢃ 10ꢁ6 M; 1.00 ꢃ 10ꢁ5 M; 1.2 ꢃ 10ꢁ5 M; 1.39 ꢃ 10ꢁ5 M
and 1.60 ꢃ 10ꢁ5 M). Inset: plot of the normalized intensity I/I0 at 462 nm
versus concentration of C60 used to determine the association constant.
Fig. 2, in which a complementary fluorescence assay is
displayed, documents that even in the excited state mutual
interactions prevail between the electron donating exTTF and
the electron accepting C60/C70. As in the absorption assays, a new
feature appears around 538 and 541 nm in the presence of
variable C60 or C70 concentrations at the expense of the exTTF
centered excited state features of 1, which were seen to diminish
around 470 nm. Again, we postulate that a charge transfer is
responsible for the newly developing features. Notable is the
mirror image between the charge transfer absorption and the
charge transfer emission. For 1ꢂC60 and 1ꢂC70, the C60dꢁ/1d+ and
C70dꢁ/1d+ features are discernible in non-polar as well as polar
solvents with emission quantum yields of about 10–3. A sub-
stantial red shift of 30 nm accompanies the solvent change, which
infers a significant solvent stabilization of the charge transfer
state. For 1ꢂC70 the most prominent C70d–/1d+ features are
maxima at 515 nm in chlorobenzene, and 541 nm in benzonitrile.
The exponential quenching of the steady-state fluorescence of 1
was further used to quantify the association between 1 and C60 or
1 and C70.7 In particular, we have plotted the emission intensity
gathered at 462 nm versus the concentration of C60/C70 (Fig. 2,
inset). A nonlinear curve fitting allowed the association constants
for the formation of 1ꢂC60 to be estimated as 5.1 ꢃ 104 (toluene)/
1.04 ꢃ 106 Mꢁ1 (benzonitrile) and 3.4 ꢃ 105 (chlorobenzene) and
3.8 ꢃ 106 Mꢁ1 for 1ꢂC70 (benzonitrile).
The differential absorption changes obtained upon photo
exciting 1ꢂC60 in benzonitrile into the charge transfer features at
480 nm are gathered in Fig. 3. The spectral features clearly reveal
the instantaneous (i.e. o1 ps) formation of photoexcited
C60d–/1d+ with features that include maxima at 540, 655, 950,
and 1080 nm. The instantaneous appearance of the C60d–/1d+
In the next step, with the help of femtosecond laser pulses at
387 nm the fate of localized exTTF and C60/C70 singlet
excited states was probed. In addition, 480 nm photoexcitation
experiments were conducted to direct the excitation into the
charge transfer state. Please note in this context the lack of ground
state absorption of exTTF and C60 at the 480 nm excitation
wavelength. Following the time evolution has emerged in our
earlier work as a convenient approach to identify spectral features
of the initial excited state and the resulting charge transfer
products, and to determine absolute rate constants for their
formation as well as their decay.4a
Fig. 3 Upper part: differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (480 nm) of 1ꢂ
C60 in Ar-saturated benzonitrile with time delays between 0.1 and
6000 ps at room temperature. Lower part: time-absorption profile at
655 nm, reflecting the transformation of C60dꢁ/exTTFd+ into the fully
In transient absorption measurements with 1 only one
transient intermediate evolved. This excited state intermediate
is centered on exTTF and appears simultaneously with the
ꢁ
+
ꢀ
C60 /1ꢀ charge separated state and charge recombination kinetics.
c
7450 Chem. Commun., 2011, 47, 7449–7451
This journal is The Royal Society of Chemistry 2011

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excited state further confirms the intimate interactions between
60 and exTTF in 1ꢂC60. The C60d–/1d+ excited state is, however,
chain in 1 is key: it provides an alternative source for positive
non-covalent interactions and assures that C60 or C70 remains
encapsulated by the macrocycle despite weaker electronic coupling
with the exTTF units.
C
ꢁ
ꢀ
short-lived and transforms within 6.7 ꢄ 0.5 ps into a fully C60
/
1ꢀ+ charge separated state. In this regard, the transient centred at
around 665 nm is assigned to the one-electron oxidized radical
cation of exTTF. This assignment is in accordance with previous
studies conducted with photolytic and radiolytic techniques.10
The one-electron reduced radical anion of C60, on the other
hand, shows up in the near-infrared (i.e. 1080 nm).11 The charge
separated state lifetimes, as determined from a multiwavelength
analysis (i.e. decay at 665, 950, and 1080 nm), were 157 ꢄ 31 ps.
Products of the charge recombination are the energetically lower
lying C60 singlet excited state (1.8 eV) and C60 triplet excited state
(1.55 eV) as identified by maxima at 960 and 750 nm, respec-
tively. In complementary experiments, chlorobenzene was
probed as solvent. Essentially, the same spectral features of the
C60d–/1d+ excited state were seen as it transforms into the fully
Financial support by the MICINN of Spain (CTQ2008-00795/
BQU, PIB2010JP-00196, and CSD2007-00010), FUNMOLS
(FP7-212942-1), and the CAM (MADRISOLAR-2 S2009/
PPQ-1533) is acknowledged. We also thank the Deutsche
Forschungsgemeinschaft (SFB583) and the Office of Basic
Energy Sciences of the US. HI and EMP thank the MICINN
for a FPU studentship and a Ramon y Cajal fellowship
´
cofinanced by the European Social Fund, respectively. B. G.
acknowledges generous support from the FCI and the GSMS.
Notes and references
1 For reviews, see: (a) T. Kawase and H. Kurata, Chem. Rev., 2006,
106, 5250; (b) K. Tashiro and T. Aida, Chem. Soc. Rev., 2007, 36, 189;
ꢁ
+
ꢀ
C60 /1ꢀ charge separated state with kinetics of 9.0 ꢄ 0.5 ps
before charge recombination populates the C60 triplet excited
state with 137 ꢄ 11 ps. Longer lifetimes in benzonitrile compared
to chlorobenzene are in line with dynamics that are placed in the
normal region of the Marcus parabola. In other words, the large
reorganization energy that exTTF reveals in electron transfer
reactions is insufficiently compensated by the exceptionally small
reorganization energy of C60.10,11 In addition, the low driving
forces for the charge recombination (i.e. o1.0 eV) prevent the
processes to be beyond the thermodynamic maximum.12
(c) E. M. Pe
2 (a) A. R. Mulholland, C. P. Woodward and S. J. Langford, Chem.
Commun., 2011, 47, 1494; (b) G. Gil-Ramırez, S. D. Karlen,
rez and N. Martın, Chem. Soc. Rev., 2008, 37, 1512.
´ ´
´
A. Shundo, K. Porfyrakis, Y. Ito, G. A. D. Briggs, J. J.
L. Morton and H. L. Anderson, Org. Lett., 2010, 12, 3544;
(c) Y. Shoji, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2010,
132, 5928; (d) M. Yanagisawa, K. Tashiro, M. Yamasaki and
T. Aida, J. Am. Chem. Soc., 2007, 129, 11912.
3 (a) H. Isla, M. Gallego, E. M. Pe
N. Martın, J. Am. Chem. Soc., 2010, 132, 1772; (b) D. Canevet,
M. Gallego, H. Isla, A. de Juan, E. M. Perez and N. Martın, J. Am.
Chem. Soc., 2011, 133, 3184.
4 (a) S. S. Gayathri, M. Wielopolski, E. M. Pe
L. Sanchez, R. Viruela, E. Ortı, D. M. Guldi and N. Martı
Angew. Chem., Int. Ed., 2009, 48, 815; (b) E. M. Perez, L. Sanchez,
G. Fernandez and N. Martın, J. Am. Chem. Soc., 2006, 128, 7172.
5 For reviews, see: (a) M. R. Wasielewski, Chem. Rev., 1992, 92, 435;
(b) L. Sanchez, N. Martın and D. M. Guldi, Angew. Chem., Int.
rez, R. Viruela, E. Ortı and
´ ´
´
´
´
Turning to the photoexcitation of 1ꢂC70, the C70d–/1d+ features
were seen to evolve in chlorobenzene with maxima at 540, 665,
860, and 1140 nm. Following its instantaneous formation, trans-
´
rez, G. Ferna
´
ndez,
´
´
´
n,
´
´
´
ꢁ
formation to the corresponding C70 /1ꢀ+ charge-separated state
takes over with 5.1 ꢄ 1.5 ps. Evidence for the formation of the
latter is based on the signatures of the one-electron-reduced
radical anion of C70 at 880 and 1220 nm, in line with pulse
radiolytic investigations,13 and of one-electron oxidized radical
cation of exTTF at 665 nm. A multi-wavelength analysis of the
ꢀ
´
´
Ed., 2005, 44, 5374; (c) F. D’Souza and O. Ito, Coord. Chem. Rev.,
2005, 249, 1410; (d) M. R. Wasielewski, J. Org. Chem., 2006, 71,
5051; (e) N. Martın, L. Sanchez, M. A. Herranz, B. Illescas and
´ ´
D. M. Guldi, Acc. Chem. Res., 2007, 40, 1015.
6 For examples of self-assembled materials based on exTTF–C60 inter-
actions, see: (a) G. Ferna
Angew. Chem., Int. Ed., 2008, 47, 1094; (b) J. Santos, B. Grimm,
B. M. Illescas, D. M. Guldi and N. Martın, Chem. Commun., 2008,
ndez, E. M. Perez, L. Sanchez and N. Martın,
´ ´ ´ ´
ꢁ
+
ꢀ
C70 /1ꢀ features at 503, 665, 950, and 1200 nm helped to
´
establish a lifetime of 371 ꢄ 27 ps. In line with what has been
ꢁ
5993; (c) B. Grimm, J. Santos, B. M. Illescas, A. Munoz, D. M. Guldi
and N. Martin, J. Am. Chem. Soc., 2010, 132, 17387.
7 (a) B. Valeur, Molecular Fluorescence, Wiley-VCH, Weinheim, 2002;
(b) H. J. Schneider, in Frontiers in Supramolecular Organic Chemistry
seen for C60 /1ꢀ+, studies in benzonitrile revealed an even
longer lifetime of 482 ꢄ 36 ps (Fig. S5, ESIy). Notable is that
charge recombination in both solvents affords the C70 triplet
excited state.
ꢀ
and Photochemistry, ed. H. Durr, Wiley-VCH, Weinheim, 1991.
¨
8 (a) A. Maciejewski and R. P. Steer, Chem. Rev., 1993, 93, 67;
(b) H. Nishikawa, S. Kojima, T. Kodama, I. Ikemoto, S. Suzuki,
K. Kikuchi, M. Fujitsuka, H. Luo, Y. Araki and O. Ito, J. Phys.
Chem. A, 2004, 108, 1881.
9 (a) N. M. Dimitrijevic and P. V. Kamat, J. Phys. Chem., 1992, 96,
4811; (b) K. G. Thomas, V. Biju, D. M. Guldi, P. V. Kamat and
M. V. George, J. Phys. Chem. B, 1999, 103, 8864; (c) D. M. Guldi
and M. Prato, Acc. Chem. Res., 2000, 33, 695.
In a nutshell, a macrocyclic exTTF host efficiently incorporates
C60 or C70 with binding constants that range from 4.2 ꢃ 104 to
3.8 ꢃ 106 Mꢁ1. The binding is driven in large by charge transfer
interactions and, in turn, dominates the electronic ground state
with the unambiguous formation of C60d–/1d+ or C70d–/1d+
charge transfer states. These charge transfer states reveal distinct
absorption and emission featuresꢁand transform in the excited
10 (a) D. M. Guldi, L. Sanchez and N. Martın, J. Phys. Chem. B, 2001,
´ ´
105, 7139; (b) A. E. Jones, C. A. Christensen, D. F. Perepichka,
A. S. Batsanov, A. Beeby, P. J. Low, M. R. Bryce and A. W. Parker,
Chem.–Eur. J., 2001, 7, 973.
ꢁ
+
+
ꢀ
ꢀ
state into fully C60 /1ꢀ or C70 /1ꢀ charge separated states.
Remarkable are the charge separated state lifetimes reaching into
the 500 ps domain, which correlate with a weak electronic
coupling between an electron acceptor (i.e. C60/C70) and an
electron donor (i.e. exTTF). Support for this hypothesis comes
from extinction coefficients of C60dꢁ/1d+ in 1ꢂC60 relative to that
seen in exTTF bis-crown etherꢂC60 with values of 4000 and
5500 L molꢁ1 cmꢁ1, respectively (see Fig. S4, ESIy). In
comparison to exTTF bis-crown ether and exTTF-tweezers,4a,6c
which feature lifetimes on the 10 to 50 ps time scale, the alkyl
11 (a) T. Kato, T. Kodama, T. Shida, T. Nakagawa, Y. Matsui,
S. Suzuki, H. Shiromaru, K. Yamauchi and Y. Achiba, Chem. Phys.
Lett., 1991, 180, 446; (b) D. M. Guldi, H. Hungerbuhler, E. Janata
and K.-D. Asmus, J. Chem. Soc., Chem. Commun., 1993, 84.
12 Excitation, on the other hand, at 387 nm is the inception to
transform the locally excited exTTF excited state into the same
ꢁ
+
ꢀ
C60 /1ꢀ charge separated state seen in the aforementioned
480 nm excitation experiments.
13 D. M. Guldi, H. Hungerbuhler, M. Wilhelm and K.-D. Asmus,
¨
J. Chem. Soc., Faraday Trans., 1994, 90, 1391.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7449–7451 7451