A new exTTF-crown ether platform to associate fullerenes: Cooperative n-π and π-π Effects

Article abstract of DOI:10.1021/ja108744a

A new and readily available exTTF-bis(crown ether), 1, efficiently recognizes C₆₀ as well as C₇₀ by means of cooperative π-π and n-π interactions. The geometrical (concave-convex) and electronic (donor-acceptor) complementarity accounts on one hand for remarkable binding strengths, with association constants reaching 10⁷ M ^-1 in benzonitrile, and on the other hand for lifetimes of the photogenerated radical ion pair state on the order of 45 ps.

Full text of DOI:10.1021/ja108744a

Published on Web 11/22/2010
A New exTTF-Crown Ether Platform To Associate Fullerenes: Cooperative
n-π and π-π Effects
Bruno Grimm,† Jose´ Santos,‡ Beatriz M. Illescas,‡ Antonio Mun˜oz,‡ Dirk M. Guldi,*,† and
Nazario Mart´ın*,‡,§
Institute of Physical and Theoretical Chemistry and Interdisciplinary Center for Molecular Materials, UniVersity of
Erlangen, 91058 Erlangen, Germany, Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad
Complutense, 28040 Madrid, Spain, and Instituto Madrilen˜o de Estudios AVanzados en Nanociencia
(IMDEA-Nanociencia), Campus UAM, Cantoblanco, E-28049 Madrid, Spain
Received September 28, 2010; E-mail: dirk.guldi@chemie.uni-erlangen.de; nazmar@quim.ucm.es
Scheme 1. exTTF-bis(crown ether) 1 and Its Complexes with C60
and C70
Abstract: A new and readily available exTTF-bis(crown ether),
1, efficiently recognizes C60 as well as C70 by means of coopera-
tive π-π and n-π interactions. The geometrical (concave-convex)
and electronic (donor-acceptor) complementarity accounts on
one hand for remarkable binding strengths, with association
constants reaching 107 M-1 in benzonitrile, and on the other hand
for lifetimes of the photogenerated radical ion pair state on the
order of 45 ps.
Energy- and electron-transfer processes in supramolecular elec-
tron donor-acceptor nanohybrids offer important incentives for
designing efficient electron-transfer mimics of the photosynthetic
apparatus and for constructing artificial devices, namely, photo-
voltaic cells.1 To this end, nanohybrids of fullerenes (as electron
acceptors) with porphyrins,2 tetrathiafulvalenes (TTFs),3 phthalo-
cyanines,4 and ferrocenes or oligomers5 (as electron donors) have
been widely studied. On the other hand, we have focused on
exploring 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene
(exTTF) as a template to realize a variety of supramolecular
nanohybrids with C60 and/or C70. For example, we have exploited
both hydrogen bonding in preorganized systems with ammonium
crown ether recognition motifs6 and π-π interactions making
use of the match between the concave aromatic surface of exTTF
and the convex surface of fullerenes.7 Whereas systems that are
based purely on π-π interactions require at least two exTTF
groups to form stable complexes with fullerenes, 1:1 complexes
(exTTF · C60) could be obtained by introducing single ammonium
crown ethers. Recently, we reported the formation of room-
temperature-stable supramolecular complexes with a 1:1 stoi-
chiometry, exTTF•C60 (association constant Ka ≈ 1.6 × 106 in
chlorobenzene), employing the cooperativity between hydrogen
bonding and π-π interactions.6c
terization of 1 was performed using standard spectroscopic and
analytical techniques (see the Supporting Information).
We started with absorption assays in which 1.4 × 10-5
M
solutions of 1 were titrated against variable concentrations of C60
(i.e., 0 to 2.2 × 10-5 M), giving rise to a rapidly formed charge-
transfer complex. An illustration thereof is shown in Figure 1.
During the titration, the evolution of a new broad transition at 480
The first-ever reported molecular receptor for fullerenes consisted
of aza-crown ethers carrying suitable alkyl chains on the nitrogens.8
Ample evidence suggests the involvement of electron-transfer
processes with either C60 or C70.9
In the current work, we designed exTTF-bis(crown ether) 1, a
new exTTF platform for binding C60 and C70, by means of
introducing two crown ethers as recognition motifs to optimize the
complex stability (Scheme 1). 1 was synthesized by esterification
of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene-2,6-diol10
with the carboxylic acid of benzo-18-crown-6. Complete charac-
Figure 1. Absorption spectra of a dilute benzonitrile solution of 1 (1.4 ×
10-5 M) with variable concentrations of C60 (0, 5.3 × 10-7, 1.0 × 10-6
1.55 × 10-6, 2.0 × 10-6, 3.0 × 10-6, 4.0 × 10-6, 5.7 × 10-6, 6.9 × 10-6
8.5 × 10-6, 9.9 × 10-6, 1.2 × 10-5, 1.3 × 10-5, 1.5 × 10-5, 1.6 × 10-5
,
,
,
† University of Erlangen.
1.8 × 10-5, 1.9 × 10-5, 2.1 × 10-5, and 2.3 × 10-5 M). Arrows indicate
‡ Universidad Complutense.
§ Instituto Madrilen˜o de Estudios Avanzados en Nanociencia.
the progression of the titration. The inset shows the binding isotherm.
9
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J. AM. CHEM. SOC. 2010, 132, 17387–17389 17387

C O M M U N I C A T I O N S
nm in benzonitrile at the expense of the 438 nm absorption
maximum of exTTF was observed, along with a low-energy
isosbestic point around 460 nm. The latter revealed a considerable
hypsochromic shift in going from polar benzonitrile to less polar
chlorobenzene, where the isosbestic point was found to be at 453
nm. Hypsochromic shifts were also noted for the maximum, which
was found to be at 488 nm in benzonitrile, while in chlorobenzene
it appeared at 480 nm. It is notable, however, that the absorption
maximum of exTTF was not affected by the solvent change. The
fact that the corresponding maxima tend to be blue-shifted with
decreasing solvent polarity leads us to postulate a redistribution of
charge density from the electron-donating exTTF moiety to the
solvents with emission quantum yields of ∼10-3 and emission
lifetimes shorter than our time resolution of 100 ps. Interestingly,
a fairly large red shift of ∼35 nm (515 nm in chlorobenzene to
550 nm in benzonitrile) reflects a significant change in solvent
stabilization of the charge-transfer state.
electron-accepting C60, affording C60δ-/exTTFδ+
.
Treating the absorption changes at 450 nm for 1·C60 in the
concentration range from 0 to 2.3 × 10-5 M with an exponential
fitting function allowed the association constant, Ka, to be calculated.
Good fits to the data were observed when the formation of a 1:1
complex was assumed. In fact, the Job’s plot confirmed the 1:1
stoichiometry. With this assumption, a Ka value of 5.00 × 106 M-1
was calculated for the formation of 1·C60 in benzonitrile. In
chlorobenzene, where the charge-transfer band evolves at 478 nm,
the Ka value was determined to be 5.01 × 106 M-1 (see the
Supporting Information).
Notably, these Ka values are among the highest reported for the
complexation of C60.7,11,12 It is important to note that 1 features
only a single exTTF, in sharp contrast to recent work on molecular
tweezers with two or more exTTFs. In addition, 106 M-1 is
comparable to values realized with highly preorganized exTTF
architectures.7f In fact, performing analogous absorption assays with
C70 instead of C60 (Figure S1 in the Supporting Information)
afforded a Ka value of 2.57 × 107 M-1 for 1·C70 in benzonitrile.
Figure 3. (top) Differential absorption spectra (visible and near-IR) obtained
upon femtosecond flash photolysis (480 nm) of 1·C60 in benzonitrile with
time delays between 0.1 and 1.4 ps at room temperature. (middle)
Differential absorption spectra (visible and near-IR) obtained upon femto-
second flash photolysis (480 nm) of 1·C60 in benzonitrile with time delays
between 1.6 and 92 ps at room temperature. (bottom) Time-absorption
profile at 684 nm, reflecting the transformation of C60δ-/exTTFδ+ into the
fully charge-separated C60•-/exTTF•+ state and the charge-recombination
dynamics.
Figure 2. Emission spectra upon 450 nm excitation of a dilute benzonitrile
solution of 1 (1.4 × 10-5 M) with variable concentrations of C60 (0, 5.3 ×
10-7, 1.0 × 10-6, 1.55 × 10-6, 2.0 × 10-6, 3.0 × 10-6, 4.0 × 10-6, 5.7 ×
10-6, 6.9 × 10-6, 8.5 × 10-6, 9.9 × 10-6, 1.2 × 10-5, 1.3 × 10-5, 1.5 ×
10-5, 1.6 × 10-5, 1.8 × 10-5, 1.9 × 10-5, 2.1 × 10-5, and 2.3 × 10-5 M).
Arrows indicate the progression of the titration.
The gradual quenching of the steady-state fluorescence intensity
of 1 was used to quantify the association between 1 and C60 or
C70. Emission intensity data collected at 680 nm were plotted versus
the concentration of C60 or C70, and nonlinear curve fitting allowed
the association constant for the formation of 1·C60 in benzonitrile
to be estimated as 2.2 × 105 M-1. The corresponding Ka value for
Figure 2 and Figure S4 document that fluorescence assays, as a
complement to the above absorption assays, confirmed mutual
interactions between the electron-donating exTTF and the electron-
accepting C60 and C70 in 1·C60 and 1·C70, respectively. As in the
absorption assays, the exTTF-centered excited-state features of 1
at 470 nm decreased in the presence of increasing C60 concentrations
concomitantly with the rise of a new feature centered around 550
nm. Again we conclude that charge transfer is responsible for the
emissive features. Notable is the mirror image between charge-
transfer absorption and charge-transfer emission. For 1 · C60, the
C60δ-/exTTFδ+ features were discernible in nonpolar as well as polar
1·C70 is 9.44 × 106 M-1
.
In transient absorption measurements with 1, only one transient
evolved. This excited state transient was centered on exTTF and
appeared simultaneously with the conclusion of the 387 nm laser
pulse. Spectral characteristics of this very short lived excited state
(1.2 ps) included transient maxima around 465, 605, and 990 nm
as well as transient bleaching at <450 nm. The short lifetime is
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rationalized by the presence of the sulfur atoms, which give rise to
strong second-order vibronic spin-orbit coupling.
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Figure 3 shows the differential absorption changes obtained upon
photoexcitation of 1·C60 in benzonitrile into the charge-transfer
bands at 480 nm. The spectral features clearly reveal the instan-
taneous (>1 ps) formation of photoexcited C60δ-/exTTFδ+, with
features that include a sharp minimum at 540 nm followed by
maxima at 648, 950, and 1120 nm. In this context, the lack of
ground state absorption of 1 at the 480 nm excitation wavelength
on one hand and the significant differences relative to those seen
upon excitation of C60 on the other hand should be noted. The
instantaneous appearance of the C60δ-/exTTFδ+ state further
confirms the intimate interactions in 1·C60. The latter is, however,
short-lived and transforms within 6.5 ps into the fully charge-
separated C60•-/exTTF•+ state. In particular, the transient centering
at ∼695 nm relates to the one-electron-oxidized radical cation of
the tweezer, in accordance with previous photolytic and radiolytic
studies.13 The one-electron-reduced radical anion of C60, on the
other hand, shows up in the near-IR (i.e., 1100 nm). The lifetime
of the charge-separated state, as determined from a multiwavelength
analysis (i.e., decay at 695, 950, and 1100 nm), was very short (45
ps). Products of the charge recombination are the energetically
lower-lying C60 singlet excited state and C60 triplet excited state,
as identified by maxima at 960 and 750 nm, respectively.14,15
In summary, exTTF-bis(crown ether) 1, a new exTTF platform
with a concave geometry, very efficiently recognizes C60 and C70.
The resulting association constants, which were determined both
by absorption and fluorescence titrations, are as high as 107 M-1
in benzonitrile and thus are among the highest ever reported for
fullerenes. These values are particularly remarkable in view of the
low degree of preorganization in 1 and the presence of a single
exTTF. We attribute the stability of 1·C60 and 1·C70 to cooperative
effects stemming from π-π and n-π interactions.16 The latter
should be considered as one of the most efficient means for binding
these carbon allotropes.
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Acknowledgment. Financial support by the MICINN of Spain
(CTQ2008-00795/BQU), the CAM (MADRISOLAR-2 S2009/PPQ-
1533), Consolider Ingenio 2010 (CSD 2006-0003), Fonds der
Chemischen Industrie (FCI), the Deutsche Forschungsgemeinschaft
(SFB 583: Redoxaktive Metallkomplexe - Reaktivita¨tssteuerung
durch molekulare Architekturen), and Exzellenzcluster EAM-
Engineering of Advanced Materials is acknowledged.
(14) Upon excitation at 387 nm, the locally exTTF excited state transforms into
the same C60•-/exTTF•+ charge-separated state seen in the aforementioned
480 nm excitation experiments.
(15) For 1 · C70, the corresponding C70•-/exTTF•+ charge-separated state [with a
signature of the one-electron-reduced radical anion of C70 at 1290 nm
(Figure S5)] decays with a lifetime of 77 ps in benzonitrile.
(16) In a control experiment carried out using the same exTTF without crown
ethers, no significant changes were found in the electronic spectra of pristine
exTTF (chlorobenzene, 298 K, 4.09 × 10-5 M) upon addition of C60
(chlorobenzene, 298 K, 4.05 × 10-3 M). See ref 7a.
Supporting Information Available: Experimental details and
Figures S1-S5. This material is available free of charge via the Internet
at http://pubs.acs.org.
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