Linear and hyperbranched electron-acceptor supramolecular oligomers

Article abstract of DOI:10.1002/asia.201000832

The nanometric organization of p-type and n-type materials is expected to facilitate charge mobility in OSCs, thereby preventing recombination and enhancing efficiency. Herein, the synthesis and self-assembly of monomers comprising one or two units of a T

Full text of DOI:10.1002/asia.201000832

FULL PAPERS
DOI: 10.1002/asia.201000832
Linear and Hyperbranched Electron-Acceptor Supramolecular Oligomers
Josꢀ Santos,[a] Emilio M. Pꢀrez,[a, b] Beatriz M. Illescas,[a] and Nazario Martꢁn*[a, b]
On the occasion of the 150th anniversary of the Department of Chemistry, The University of Tokyo
Abstract: The nanometric organization
of p-type and n-type materials is ex-
pected to facilitate charge mobility in
OSCs, thereby preventing recombina-
tion and enhancing efficiency. Herein,
the synthesis and self-assembly of mon-
omers comprising one or two units of a
TCAQ tweezer covalently connected
to a fullerene derivative is reported.
Such molecules self-assemble in solu-
tion (NMR) and on surfaces (AFM) to
form linear and hyperbranched supra-
molecular oligomers, respectively. The
self-assembly process was also found to
have very little influence on the elec-
tronic properties of the electroactive
units of the monomers (CV and UV/
Vis), as was expected from their match-
ing character. These polymers consti-
tute two of the first examples of n-type
supramolecular polymers reported to
date.
Keywords: fullerenes · host–guest
systems
assembly
mers
·
pi interactions
supramolecular poly-
· self-
·
Introduction
Within self-assembled architectures, supramolecular poly-
mers are receiving a great deal of attention as a new type of
material.[3] Most examples of supramolecular polymers re-
ported to date rely on hydrogen bonding,[3e,4,5] metal coordi-
nation (including metal–organic frameworks (MOFs))[6–8]
solvophobic interactions,[3b,9] or a combination of these[3c] as
cohesive forces between the molecular monomers. Despite
One of the factors that limit the efficiency of organic solar
cells (OSCs)[1] is charge recombination, a process in which
the negatively and positively charged species recombine in
the active layer before the charges proceed to the electro-
des. The nanometric organization of p-type and n-type mate-
rials is expected to facilitate charge mobility in OSCs, thus
preventing recombination and enhancing efficiency. The
thermodynamically controlled self-assembly of judiciously
designed molecular fragments presents a number of advan-
tages for achieving this aim, including the possibility of
processing the active components from solution and of fine-
tuning the geometry and/or size of the self-assembled struc-
tures.[2]
À
the very prominent role that p p interactions play in other
areas of noncovalent chemistry, the number of supramolec-
À
ular polymers in which p p interactions are the dominating
force for association are much more scarce.[10] We recently
described the self-assembly of linear[11] and hyper-
branched[12] donor–acceptor supramolecular polymers based
À
on p p and van der Waals interactions. These polymers
made use of the attractive noncovalent interactions estab-
lished between the concave aromatic surface of the electron
rich 2-[9-(1,3-dithiol-2-ylidene)anthracen-10ACTHNUTRGNE(NUG 9H)-ylidene]-
1,3-dithiole (exTTF) and the convex exterior of C60.[13] Our
initial receptors featured a simple tweezers-like design and
showed binding constants of log Ka =3.5 toward C60 in PhCl
at room temperature.[13i] More recently, we have been able
to reach binding constants towards C60 of up to log Ka =6.5
in PhCl at room temperature by making use of macrocyclic
structures.[13e,f] We have also shown that 11,11,12,12-tetracya-
no-9,10-anthraquinodimethane (TCAQ), which is electron
poor in character but conserves the concave geometry of
exTTF, can also be utilized in the construction of tweezers-
like receptors, although the binding constant is approximate-
ly halved in comparison to the exTTF counterpart.[14]
[a] Dr. J. Santos, Dr. E. M. Pꢀrez, Dr. B. M. Illescas, Prof. Dr. N. Martꢁn
Departamento de Quꢁmica Orgꢂnica
Facultad de C.C. Quꢁmicas
Universidad Complutense de Madrid
Av. Complutense s/n, 28040, Madrid (Spain)
Fax : (+34)913944103
E-mail: nazmar@quim.ucm.es
[b] Dr. E. M. Pꢀrez, Prof. Dr. N. Martꢁn
IMDEA Nanoscience
Av. Francisco Tomꢂs y Valiente 7
Ciudad Universitaria de Canto Blanco, 28049, Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000832.
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Chem. Asian J. 2011, 6, 1848 – 1853

Herein, we present the synthesis and self-assembly of
purely electron accepting C60-TCAQ hybrids as monomers
for the construction of linear and hyperbranched supra-
molecular oligomers in solution and polymers in the solid
state.
either one or two units of the tweezer-like TCAQ receptor
through ester linkages. Based upon our previous find-
ings,[11,12] we expected 1 to self-assemble to form linear and/
or cyclic oligomeric structures, and 2 to form dendrimer-like
hyperbranched associates. The short linker between the self-
recognizing units guarantees that the intramolecular self-as-
sociation is negligible compared to intermolecular processes.
The syntheses of 1 and 2 were carried out following stan-
dard protection/deprotection and esterification procedures.
The key intermediates were 2-hydroxymethyl-TCAQ,
TBDMS-protected 5-hydroxy isophthalic acid and PC61BA
which were all obtained following the previously reported
procedures.[15] Both of the final compounds and the synthet-
ic intermediates showed satisfactory spectroscopic and ana-
lytical data. For full experimental details, see the Supporting
Information.
Results and Discussion
The design of monomers 1 and 2 mirrors that of the donor-
acceptor monomers based on exTTF reported previous-
ly,[11,12] but the electron-donating units have been substituted
with the electron-acceptor TCAQ. The structure of the mon-
omers and their schematic representation are shown in
Scheme 1. They consist of one unit of PC61BA connected to
The first indication of the ability of 1 to form high molec-
ular weight aggregates in solution was its relatively broad
1H NMR spectrum (18.4 mm, CDCl3, 300 MHz, 298 K). On
dilution, all NMR signals sharpen up, and are slightly de-
shielded (Figure 1). We have previously shown this behavior
Figure 1. Partial 1H NMR (CDCl3, 300 MHz, 298 K) of monomer 1 at
several concentrations, showing the aromatic region.
Scheme 1. Chemical structure and cartoon representation of monomers 1
and 2. Key hydrogen atoms (see main text) are marked as empty (1) or
filled (2) squares, stars, and circles.
to be a reliable sign of head-to-tail self-association.[11] To un-
ambiguously discard aggregation through non-directional
fullerene–fullerene interactions and/or signal broadening
caused by changes in viscosity, we studied structurally relat-
ed molecules that lacked the tweezers-like recognition
motif, such as PC61BM (the methyl ester of PC61BA) which
showed perfectly defined spectra at concentrations as high
as 100 mm.[11]
Abstract in Spanish: La organizaciꢄn de los materiales de
tipo p y n a escala nanomꢀtrica facilita la movilidad de
carga en las cꢀlulas solares orgꢂnicas, impidiendo la recom-
binaciꢄn y aumentando asꢁ su eficiencia. En este trabajo se
describe la sꢁntesis y el auto-ensamblaje de monꢄmeros con
una o dos pinzas de TCAQ conectadas al fullereno. El estu-
dio de las interacciones supramoleculares en disoluciꢄn
(RMN) y sobre superficie (AFM) demuestra que estas mo-
lꢀculas se autoensamblan formando oligꢄmeros lineales y ra-
mificados, respectivamente. Este proceso de auto-ensambla-
je tiene poca influencia en las propiedades electrꢄnicas de
las unidades electroactivas de los monꢄmeros (CV y UV/
Vis), como cabꢁa esperar dado su carꢂcter electroaceptor.
Los ejemplos aquꢁ descritos constituyen dos de los primeros
polꢁmeros supramoleculares de tipo n descritos hasta la
fecha.
Variable temperature (VT) 1H NMR spectroscopy of 1
(5.3 mm, CDCl3, 300 MHz), also provided valuable informa-
tion on the self-assembly process. It is known that on lower-
ing the temperature, more-ordered, entropically disfavored,
cyclic oligomeric structures become favored.[16] The results
of these experiments are shown in Figure 2. Besides the pro-
gressive broadening of all of the signals upon cooling, char-
acteristic of the formation of high-molecular-weight aggre-
gates, the most-prominent feature in the VT-NMR experi-
ments was the very significant shielding of the alpha-methyl-
ene with respect to the PCBA ester (indicated by a square),
which was not observed in the concentration experiments
and is consistent with the formation of more-rigid, cyclic
oligomers, a type of association which involves more-signifi-
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N. Martꢁn et al.
FULL PAPERS
1
Figure 2. Partial H NMR (5.3 mm, CDCl3, 300 MHz) of monomer 1 at several temperatures. Note that, for clarity, both the scale and intensity are differ-
ent in the aromatic (left) and alkyl (right) part of the spectra. Key signals are marked corresponding to the assignment in Scheme 1.
cant changes to the electronic environment of the methyl-
enes of the PCBA. Also worth mentioning is the coales-
cence (258 K) and splitting (from 248 K) of the signal corre-
sponding to the 2-isophthalic position (marked with a five-
point star). The ratio of the integrals of both signals was
almost exactly 1:1, which gives an indication of the bound/
unbound ratio of the monomers at this concentration, in
very good agreement with the binding constant determined.
With regards to the branched monomer 2, the changes in
its 1H NMR spectrum with increasing concentration were
less obvious (Figure 3). Although broadening did occur,
owing to solubility limits, we were unable to surpass concen-
trations of approximately 12 mm.
Figure 4. AFM topographic images of (tapping mode, air, 298 K) of a
drop-cast of a dichloromethane solution of 1 on mica. a) 1 mmꢅ1 mm;
b) height profile of the trace marked in (a); c) 250 nmꢅ250 nm.
Figure 4a, shows a typical micrograph (1 mmꢅ1 mm) of
Figure 3. 1H NMR (CDCl3, 300 MHz, 298 K) of monomer 1 at several
the drop-casted sample of 1. The majority of the sample has
concentrations, showing the aromatic region. Key signals are marked ac-
the appearance of a noodle soup, being composed of wind-
cording to the assignment in Scheme 1.
ing and relatively long linear associates of 1. The heights of
the associates was uniform (0.8–1.2 nm, Figure 4b) and in
Investigation under atomic force microscopy (AFM) pro-
vided definitive evidence of the formation of oligomers of
both 1 and 2 and their geometry. Studies were carried out in
tapping mode (air) on samples drop-cast from solutions of 1
and 2 in dichloromethane at concentration of circa 1ꢅ
10À3 mm on freshly exfoliated mica. Figure 4 and 5 show mi-
crographs of 1 and 2, respectively.
very good accordance with that expected for oligomers of 1.
Moreover, several individual linear oligomers (marked in
white circles) were also observed. Two of these individual
linear oligomers are shown in Figure 4c at higher magnifica-
tion. The oligomers extend for lengths up to 150-200 nm,
equivalent to a 50-75-mer. These results confirm that the de-
crease in association constant from the exTTF to the TCAQ
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Chem. Asian J. 2011, 6, 1848 – 1853

Linear and Hyperbranched Electron-Acceptor Supramolecular Oligomers
these, seemed to predominate (Figure 5a, white circles). A
higher magnification micrograph of one of these oligomers
of 2, reminiscent of a bunch of grapes, is shown in Figure 5c.
These results are nearly identical to those observed for the
case of the donor–acceptor supramolecular hyperbranched
polymer.[12]
The cyclic voltammograms (CV) of 1 and 2 (o-DCB/
CH3CN 4:1, a Bu4N·ClO4 0.1m support electrolyte, glassy
carbon working electrode, Ag/AgNO3 reference electrode,
Pt counter-electrode, scan speed 100 mVsÀ1, room tempera-
ture) are shown in Figure 6a. Both monomers show very
similar electrochemistry, thus confirming their electron-ac-
ceptor character. In particular, we observed up to five re-
Figure 5. AFM topographic images (tapping mode, air, 298 K) of a drop-
cast of a dichloromethane solution of 2 on mica. a) 1 mmꢅ1 mm; b) height
profile of the trace marked in (a); c) 250 nmꢅ250 nm.
tweezers is not sufficient to prevent self-association of the
monomers. It is important to note that, for solubility rea-
sons, the association constants for the parent receptors are
determined in either PhCl or mixtures of solvents containing
a significant amount of CS2, both of which are very good
solvents for C60 and its derivatives. In contrast, all the self-
association studies presented in this work are carried out in
pure chloroform or dichloromethane, only modest solvents
for the fullerenes and their derivatives. It is well-known that
this change in solvent should result in a significant increase
in the association constant,[13f] which provides an explana-
tion for the formation of oligomers of longer lengths than
might be initially expected. Likewise, the increase in con-
centration owing to the evaporation of the solvent of the
drop-casted solution before observation by AFM surely
plays a role in the formation of such long oligomers. In fact,
even longer oligomers were observed in the case of the
donor–acceptor monomer.[11]
On the other hand, the images of 2 are not so regular,
and show associates of several geometries (Figure 5a). The
heights of the aggregates were again uniform and in very
good accordance with that expected for oligomers of 2 (0.8–
1.0 nm). Although several shapes were observed, an approx-
imately triangular form in which two vertices are rounded
off, with a smaller angle of circa 50–608, or composites of
Figure 6. a) Cyclic voltammograms of
1 and 2 (o-DCB/CH3CN 4:1,
Bu4N·ClO4 0.1m support electrolyte, glassy carbon working electrode,
Ag/AgNO3 reference electrode, Pt counter-electrode, scan speed
100 mVsÀ1, room temperature). b) UV/Vis spectrum (CH2Cl2, room tem-
perature) of 1 (solid line) and its synthetic precursors (dashed lines).
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N. Martꢁn et al.
FULL PAPERS
duction processes for each of 1 and 2. The first of these ap-
pears as an intense and reversible reduction wave at
À0.581 V for 1 and À0.586 for 2, and corresponds to the
TCAQ units. The CV of pristine TCAQ showed a reversible
wave involving a two-electron reduction process to form the
dianion species.[18] Second, third, fourth, and fifth reduction
processes are quasi-reversible and were ascribed to the full-
erene cage. The second reduction potential appears at
À0.857 V for 1 and À0.874 V for 2.[11] These values indicate
that the self-assembly process have very little influence on
the electronic properties of the electroactive units, as could
be expected given their matching electronic character. This
result is in contrast with the analogues of 1 and 2 featuring
exTTF electron-donor units instead of the TCAQ electron
accepting moieties, where we observed that both the reduc-
tion and oxidation processes became more energetic and ir-
reversible upon self-assembly.[11,12]
Acknowledgements
Financial support by the MICINN of Spain (CTQ2008-00795/BQU,
CTQ2006-14987-C02-02/BQU, and CSD2007-00010), the CAM (MA-
DRISOLAR-2 S2009/PPQ-1533), and European FEDER funds is ac-
knowledged. J.S. thanks the MEC for a FPI studentship. E.M.P. thanks
the MICINN for a Ramꢄn y Cajal fellowship, co-financed by the Europe-
an Social Fund.
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Conclusions
In summary, we describe the synthesis and self-assembly of
two n-type supramolecular polymers. To achieve these, we
have connected covalently one (linear) or two (branched)
units of molecular tweezers based on the TCAQ electron-ac-
ceptor moiety, and a fullerene derivative (PC61BA). Despite
the reduced binding constant that the TCAQ tweezers show
towards [60]fullerene with respect to their electron-donor
counterparts, based on exTTF,[14] we have shown that the ex-
pected supramolecular association of the all-acceptor mono-
mers takes place in solution. AFM micrographs of the
supramolecular polymers demonstrate that each of the mon-
omers associates preferentially with the expected geometry:
linear in the case of 1 and hyperbranched in the case of 2.
We have also shown that the self-assembly process has very
little influence on the electronic properties of the electroac-
tive units of the monomers, as was expected from their
matching electronic character.
These polymers constitute two of the first examples of
purely electron-acceptor supramolecular polymers reported
to date, thereby increasing the toolbox of self-organized ma-
terials that might find use in the construction of more-effi-
cient OSCs.
Constructing the actual devices based on these materials,
as well as synthesizing second-generation supramolecular
polymers based on our more-efficient macrocyclic hosts[13e,f]
are our most-immediate goals.
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Received: November 23, 2010
Published online: March 4, 2011
Chem. Asian J. 2011, 6, 1848 – 1853
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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