Self-organization of electroactive materials: A head-to-tail donor-acceptor supramolecular polymer

Article abstract of DOI:10.1002/anie.200703049

(Figure Presented) Fullerenes do the conga! Synthesis and self-recognition of a monomer featuring a specially designed receptor for [60]fullerene are described. The monomer self-assembles in a head-to-tail fashion through π-π interactions, thus forming a linear oligomeric species in solution (see image). The formation of this redox-amphoteric supramolecular polymer represents a new approach to the controlled organization of electroactive materials.

Full text of DOI:10.1002/anie.200703049

Communications
DOI: 10.1002/anie.200703049
Supramolecular Polymers
Self-Organization of Electroactive Materials: A Head-to-Tail Donor–
Acceptor Supramolecular Polymer**
Gustavo Fernµndez, Emilio M. PØrez, Luis Sµnchez, and Nazario Martín*
Supramolecular polymers[1] consist of repeated units of
molecular monomers designed to self-assemble in solution
or in the bulk. One of the main approaches to build this kind
of architectures is based on the introduction of self-recogniz-
ing hydrogen-bonding motifs.[1b] The pioneering work led by
Lehn,in which three hydrogen-bonding groups are used [2] and
which was later expanded to four hydrogen bonds by Meijer
and co-workers[3],has proven to be particularly fruitful.
Another common strategy employed to construct supra-
molecular polymers makes use of metal coordination.[4]
Finally,a variety of supramolecular polymers based on
solvophobic interactions—primarily containing cyclodextrins
as hosts[1c,5]—has also been reported. To date,little effort has
been devoted to the study of supramolecular polymers based
on p–p aromatic interactions. Recently,Fukazawa and co-
workers described the realization of supramolecular poly-
meric nanonetworks based on this kind of noncovalent
interactions.[6]
recognizing units are complementary both in a supramolec-
ular and an electronic sense.
We designed monomer 6 to self-assemble in a head-to-tail
fashion by covalently connecting a C60 derivative with our
pincerlike fullerene receptor. To increase the solubility,we
chose PCBA (compound 5 in Scheme 1) as the fullerene-
containing fragment. The synthesis of monomer 6 is depicted
in Scheme 1.
Figure 1 shows the 1H NMR spectra of monomer 6
(CDCl3,300 MHz,298 K) at concentrations between 0.6 and
30mm. As the concentration increases,a slight shielding of
most of the resonances can be observed. This phenomenon is
accompanied by a broadening of all signals,which indicates
We recently described the first receptor for [60]fullerene,
which is based on a p-extended analogue of tetrathiafulva-
lene (TTF),2-[9-(13, -dithiol-2-ylidene)anthracen-10(9 H)-yli-
dene]-1,3-dithiole (exTTF).[7] Our receptor is composed of
two units of exTTF connected through an isophthalic diester
spacer. The large and concave aromatic surface of the exTTF
units serves as a recognizing motif for the convex surface of
C60,and binding constants of greater than 10 À3 mÀ1 were
measured. This finding prompted us to exploit this host–guest
system for building novel organized donor–acceptor arrays.
Several covalent[8a] and supramolecular[6,8] polymers con-
taining C60 have been described to date. However,to the best
of our knowledge,none of them are redox-amphoteric.
Herein,we report a fullerene–exTTF supramolecular poly-
mer,based on p–p aromatic interactions,in which the
[*] G. Fernµndez, Dr. E. M. PØrez, Dr. L. Sµnchez, Prof. Dr. N. Martín
Departamento de Química Orgµnica
Facultad de Ciencias Químicas
Ciudad Universitaria s/n. 28040 Madrid (Spain)
Fax: (+34)913-944-103
E-mail: nazmar@quim.ucm.es
Homepage: http://www.ucm.es/info/fullerene
[**] We are very thankful to Dr. F. Ortega (UCM) for the DLS
experiments, Dr. A. SoubriØ (UCM) for AFM imaging, and Dr. M.
Alonso (UAM) for MALDI-TOF spectra. This work has been
supported by the MEC of Spain and the Comunidad de Madrid
(CTQ2005-02609/BTQ and P-PPQ-000225-0505). E.M.P. and G.F.
are indebted to MEC for a Juan de la Cierva post-doctoral contract
and an FPU studentship, respectively.
Scheme 1. Synthesis of monomer 6. TBDMS=tert-butyldimethylsilyl,
EDC=N-ethyl-N’-(3-dimethyldiaminopropyl)carbodiimide hydrochlo-
ride, DMAP=4-dimethylaminopyridin.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. Partial 1H NMR spectra of 6 at different concentrations
(CDCl3, 300 MHz, 298 K).
the formation of high-molecular-weight aggregates at high
monomer concentrations.[9]
The competition between linear chain extension and
oligomeric cyclization[10] of monomer 6 was studied by means
1
of variable-temperature H NMR spectroscopy experiments
(CDCl3,300 MHz) performed at a concentration of 6m m. At
1
room temperature and above,the H NMR spectra show well-
defined sharp signals (see Figure 2),which indicates a low
concentration of high-molecular-weight species. As the tem-
perature decreases,most of the aromatic signals are shifted
upfield (Dd ꢀ À0.05 ppm) whereas the dithiole resonances
are shifted downfield (Dd ꢀ + 0.05 ppm). This pattern mirrors
that previously reported for a similar family of TTF-derived
fullerene receptors upon complexation[11] and is indicative of
a preferential association of fullerene on the aromatic side of
exTTF. These shifts are accompanied by a progressive
broadening of the resonances,although to a lesser extent
than in the concentration experiment. As expected,the
concentration of aggregates increases upon lowering the
temperature. However,the most remarkable feature is the
significant shielding of the alpha-methylene group with
respect to the PCBA ester unit (Dd ꢀ À0.13 ppm),a charac-
teristic that is not observed in the concentration experiments
and points to the formation of more rigid,cyclic oligomers (a
type of association in which the methylene groups of PCBA
play a more prominent role than in the linear chain
extension).
Despite the high molecular weight of 6,we observed
aggregates up to the pentamer under MALDI-TOF mass-
spectrometry conditions. Experiments with 6 were carried out
using dithranol as matrix. Besides the molecular peak
corresponding to the monomer mass (m/z 1845),we observed
signals at m/z 3689 (dimer),5535 (trimer),7380 (tetramer),
and 9225 (pentamer; see the Supporting Information).
The self-diffusion coefficients of 6 were estimated by
pulse-field-gradient NMR (PFG-NMR) studies of two differ-
ent monomer solutions (1 and 15mm,CHCl 3,298 K). As
expected,the diffusion coefficients decreased with an
Figure 2. Partial VT-1H NMR spectra of 6 (CDCl3, 300 MHz, 6mm)
showing a) the aromatic region, the dithiol groups, and the exTTF
methylene groups, and b) the methylene signals of the PCBA unit.
increase in the concentration as a result of the larger size of
the oligomers of 6. Dynamic light-scattering measurements
(DLS) were carried out on solutions of 6 in chloroform
(0.5mm) to obtain an estimation of the molecular weight of
the polymers. A broad particle-size distribution was found—
ranging from 2 nm (for the monomer) to several micrometers
(greater than 400 mer),with a maximum at 378 nm (160 mer;
see the Supporting Information). Considering the association
constant between the parent receptor and [60]fullerene,[7] the
species found in the DLS experiment are somewhat larger
than expected,which might be attributed to the formation of
nonspecific oligomer aggregates.
The influence of the self-assembly process on the elec-
tronic properties of 6 was analyzed through cyclic voltamme-
try performed at different concentrations (in tetrahydrofuran,
THF,using Ag/AgNO as the reference electrode,glassy
3
carbon as the working electrode,and Bu 4NClO4 as the
supporting electrolyte; scan rate: 100 mVsÀ1; temperature:
298 K). Figure 3a shows the cyclic voltammograms (CVs) of 6
at 0.2,0.4,0.6,and 1.0m m. The CV of PCBM,that is,the
methyl ester of PCBA (measured at 1.0mm) is also shown as
reference. Upon concentration of 6,the reduction processes
become more energetic and irreversible. Analogously,the
exTTF oxidation wave is shifted anodically as concentration
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Figure 4. AFM images (tapping mode, air, 298 K) of a drop-cast of a
dichloromethane solution of 6 on mica. a) 277ꢀ277 nm;
b) 80ꢀ65 nm; c) profile of the structure shown in (b) (black line);
d) 3D image of (b).
of PCBM samples,obtained under experimentally equivalent
conditions,show randomly distributed and approximately
circular aggregates of variable height and diameter (less than
50 nm,see the Supporting Information).
In conclusion,we have described a head-to-tail linear
supramolecular polymer based on a new host–guest ful-
lerene–exTTF system. As demonstrated by 1H NMR and
PFG-NMR spectroscopy,MALDI-TOF mass spectrometry,
Figure 3. a) Cyclic voltammograms of PCBM (1mm) and 6 (0.2, 0.4,
0.6, and 1.0mm) in THF using Ag/AgNO3 as the reference electrode,
glassy carbon as the working electrode, and Bu4NClO4 as the support-
ing electrolyte (scan rate: 100 mVsÀ1; temperature: 298 K). b) Elec-
tronic absorption spectra (CHCl3, 298 K) of 4 (a), 6 (c), and
PCBM (g).
DLS,CV,UV/Vis spectroscopy,and AFM,monomer
6
readily self-assembles through noncovalent interactions to
form multimeric species both in solution and on a solid
surface. Besides being one of the first examples of supra-
molecular polymers in which p–p aromatic interactions are
the chief driving force for the self-association of the mono-
mer,[6] this system represents a fundamentally new approach
to the organization of electroactive donor–acceptor frag-
ments.[12] Its validity for the construction of more-efficient
optoelectronic devices will be investigated in the near future.
increases. The reduction of the positively charged species
generated upon oxidation of the exTTF units becomes highly
irreversible and occurs nearly at the same potential as the first
reduction process of the C60 cage (at about À0.8 V). All these
data indicate that electronic communication between the
electroactive units takes place in the ground state and is
enhanced by complexation,a fact that is also reflected in the
UV/Vis spectrum of 6 by the presence of a charge-transfer
band at lmax = 487 nm (see Figure 3b). Qualitatively identical
changes occur in the previously reported receptor upon
complexation with fullerene.[7]
Final evidence to support the formation of oligomeric
linear species was obtained by means of atomic force
microscopy (AFM). A solution of 6 (1.0mm) in dichloro-
methane was drop-casted onto mica. Tapping-mode AFM
images were then recorded under air at room temperature.
The images show long (15–300 nm),winding,necklacelike
fragments of 1.7–2.5 nm in height. These dimensions are in
accordance with those expected for associated fragments of 6
(see Figure 4 and cartoon in Scheme 1). An insight into a 77-
nm-long tubular formation (which corresponds to the 35 mer)
is shown in Figure 4b–d. In sharp contrast,the AFM images
Received: July 9,2007
Revised: September 5,2007
Published online: November 8,2007
Keywords: p interactions · fullerenes · host–guest systems ·
.
self-assembly · supramolecular polymers
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we carried out H NMR spectroscopy control experiments with
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