Two-dimensional self-organization of rectangular OPE amphiphiles into microcrystalline lamellae

Article abstract of DOI:10.1039/b916418a

Nanographite-like lamellae are obtained by the self-assembly of rectangular OPE amphiphiles peripherally decorated with polar and paraffinic chains. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b916418a

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Two-dimensional self-organization of rectangular OPE amphiphiles
into microcrystalline lamellaew
a
a
a
b
´ ´ ´
´
Gustavo Fernandez, Fatima Garcıa, Fatima Aparicio, Emilio Matesanz and
a
´
Luis Sanchez*
Received (in Cambridge, UK) 10th August 2009, Accepted 28th September 2009
First published as an Advance Article on the web 14th October 2009
DOI: 10.1039/b916418a
Nanographite-like lamellae are obtained by the self-assembly of
rectangular OPE amphiphiles peripherally decorated with polar
and paraffinic chains.
that appear at 2920 (nanti) and 2852 (nsym) cmÀ1 (Fig. S1w) are
diagnostic of interdigitation of the alkyl chains and can be
considered a first indication of the formation of organized
structures from amphiphiles 1 and 2.1b,4b
The aggregation features of compound 1 in solution have
been studied by concentration-dependent UV-Vis experiments
in polar 2-propanol (2-PrOH) and apolar methylcyclohexane
(MCH) (Fig. S2w). For polar 2-PrOH, the application of
the indefinite or isodesmic model to the molar extinction
The self-assembly of biomolecules, bio-inspired compounds or
synthetic building blocks to form complex supramolecular
structures is being utilized for the fabrication of optoelectronic
devices or for biomedical purposes.1,2 Monodisperse amphiphilic
p-systems self-organize into a variety of structures due to their
highly reproducible and dynamic self-assembly.3 The final
morphology and the function exerted by the aggregated
species are conditioned by the hydrophilic/hydrophobic ratio
and also by subtle changes in external factors like concentration
and/or polarity of the solvent.3 In this regard, most of
these objects self-assemble into one-4,5 or three-dimensional
architectures.6,7 Nonetheless, fewer examples have been
reported on the anisotropic aggregation of amphiphiles to
form two-dimensional (2D) structures despite their potential
applicability to develop nanocargos for drug delivery,2 or
highly organized active layers in organic electronics.8 Many
of the reported 2D networks have been prepared from peptidic
building blocks,9 whilst those obtained from synthetic starting
materials have been studied to a lesser extent.10
coefficient13 gave a binding constant (Ka) value of 3 Â 105 MÀ1
.
p–p stacking aromatic interactions together with a remarkable
solvophobic contribution from the apolar aromatic moiety
and the decyl chains account for this high Ka value.13b
The more favourable solvent–solute interactions and,
consequently, reduced solvophobic contribution results in a
smaller Ka value of 8.7 Â 103 MÀ1 when MCH is utilized. The
small size of the ethyleneoxide chains in 2 compared to 1
reduces the Ka value in polar 2-PrOH (Ka = 1.5 Â 105 MÀ1
)
and increases it in apolar MCH (Ka = 5.7 Â 105
M
À1),
the trend being similar to that observed for amphiphile 1
(Fig. S2w).
Dynamic light scattering measurements complement the
study of the self-assembly of 1 in water solution (B10À5 M).
The CONTIN analysis of the correlation functions shows
three different contributions centered at hydrodynamic radii
(RH) of B300 nm, B890 nm and at values higher than B1 mm
(Fig. S3aw), in good agreement with referable systems.10a The
diffusion coefficient value has been accurately calculated
as 0.276 Â 10À8 cm2 sÀ1 for the contribution centered
at B890 nm, which represents B85% of the population, by
Herein, we report on the spontaneous assembly of
rectangular amphiphilic oligo(phenylene ethynylenes) (OPEs)
into nanographite-like microcrystalline 2D networks guided
by a delicate synergy of p–p stacking, solvophobic and van der
Waals interactions. Amphiphiles 1 and 2 are based in the
1,2,4,5-tetrakis(2-phenylethynyl)benzene aromatic backbone
and are peripherally substituted with two aliphatic decyloxy
and two polar oligoether chains of different length (Fig. 1).
Compounds 1 and 2 were readily obtained starting from
previously reported 1,2-dibromo-4,5-diiodobenzene.11 The
different chemoselectivity of bromine and iodine atoms under
Sonogashira–Hagihara conditions has allowed the modulated
peripheral substitution with aliphatic and polar chains (see
Scheme S1w).12 Compounds 1 and 2 have been fully characterized
by a number of spectroscopic techniques (see ESIw). The two
sharp IR bands corresponding to CH2 stretching vibrations
a Departamento de Quı´mica Orga´nica, Facultad de Ciencias Quı´micas,
Ciudad Universitaria s/n, 28040 Madrid, Spain.
E-mail: lusamar@quim.ucm.es; Fax: +34 913944103
b
´
C.A.I. Difraccion de Rayos X, Facultad de Ciencias Quımicas,
Ciudad Universitaria s/n, 28040 Madrid, Spain
Fig. 1 (a) Chemical structure and molecular dimensions of 1 and 2.
(b) Illustration of the anisotropic 2D self-assembly of 1 and 2 to form
microcrystalline lamellae.
w Electronic supplementary information (ESI) available: Fig. S1–S12,
Table S1 and S2, and experimental section. See DOI: 10.1039/b916418a
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a linear fit to a plot of the inverse of time versus the square of
the scattering vector (Fig. S3bw).
Unlike our previously reported rectangular OPE decorated
with four peripheral TEG chains, which form vesicles or toroids
from polar solutions,6b the anisotropic self-organization of 1
onto surfaces forms microcrystalline lamellar superstructures
that can be visualized by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) techniques. A
number of thick stratified sheets, distributed on large areas of
the surface, have been observed when a B10À5 M solution of 1
in water is utilized (Fig. S4a and S5w). The amphiphilicity of 1
also allows the formation of overlapped polyhedral lamellae,
thinner than those observed for water, by using a solution of 1
in apolar MCH at B10À4 M (Fig. S4b and S6w).
The different thicknesses of the 2D sheets observed in both
water and MCH could be justified by considering the higher
fraction of n-mer aggregates (an)6b,13 (Fig. S4cw) and average
amphiphile number per stack (N)6b,13 (inset in Fig. S4cw)
calculated for polar 2-PrOH in comparison with MCH. The
lower an and N values determined for 1 in MCH could also
be related to the appearance of the sheets at the edge of the
deposited droplet onto the surface. In these regions, the
dewetting process that increases the local concentration in
these areas results in an enhanced self-assembly process of
compound 1.14 The dewetting process has been confirmed
by confocal fluorescence microscopy (Fig. S7aw). Light
irradiation of a droplet of a B10À4 M solution of 1 in MCH
deposited onto a glass plate generates rapid evaporation of the
solvent that gives rise to concentric circular regions with a high
population of fluorescent objects. An expanded view of these
objects demonstrates the formation of overlapping sheets
(Fig. S7bw).
Fig. 2 TEM images showing the 2D sheets formed from 1. (a) Sheets
deposited from B10À5 M aqueous solution onto carbon-coated
copper grids. (b) HR-TEM images showing several overlapping
lamellae. (c) HR-TEM of the expanded region of image (b). The inset
in (c) depicts the density profile of the lattice highlighted with the
black line.
solutions requires the use of a THF–H2O (1 : 1) mixture as
solvent to obtain TEM images. Again, these TEM images
reveal the presence of stratified sheets (Fig. S11w).
High resolution TEM (HR-TEM) images confirm the
presence of overlapping lamellae of microcrystalline material
(Fig. 2b and Fig. S12w). Many regions show layered columnar
structures characterized by periods of sublayers with a
common stacking direction and separated by B3.5
A
(Fig. 2c and Fig. S11w). This distance, typical of p–p inter-
actions, resembles that observed for nanographenes,16 which
hold great promise for the fabrication of nanoelectronic and
spintronic devices.17
To investigate the organization of amphiphiles 1 and 2
within the lamellae, we have studied the bulk-state structure
of the aggregates by X-ray diffraction experiments. Both 1 and
2 present a high number of reflections (Fig. 3 and Fig. S13 and
Table S1 and S2w), those observed at lower angles being
diagnostic of lamellar organization.
The strong tendency of compound 1 to form 2D supra-
molecular structures has been confirmed by TEM imaging
either from cryo-ultramicrotomed films or from B10À5
M
aqueous solution. TEM images of microtomed films of
untreated 1 embedded into an epoxy resin show dark fringes
of different thicknesses. Several of these fringe regions are
constituted of thin 2D lamellae of around 10 nm thickness
(Fig. S8w). Further evidence of the anisotropic 2D aggregation
of 1 into sheets has been extracted from TEM micrographs
obtained from B10À5 M aqueous solutions of 1. These images
show flat and stratified lamellae with a high degree of
crystallinity (Fig. 2 and Fig. S9w). In fact, the electron diffraction
pattern shows concentric and dotted rings at 2.1, 1.2, 1.0, 0.8,
0.7 and 0.6 nm (Fig. S10w). Unexpectedly, the TEM image in
A two-dimensional oblique network—with lattice para-
meters of a = 4.8 nm, b = 2.4 nm and g = 91.51, for 1,
and a = 3.8 nm, b = 2.2 nm and g = 92.31, for 2—can be
assigned upon indexing the observed reflections. These cell
Fig. 2a also presents areas identified as Moire fringes that
´
result from overlapping and rotated 2D crystalline layers
formed upon self-organization of 1 in aqueous solution
(Fig. 2a). Despite the fact that Moire patterns have been
´
observed for physisorbed atoms or organic molecules by
scanning tunneling microscopy,15a and for block copolymer
assemblies15b and single wall carbon nanotubes15c by TEM, to
the best of our knowledge, this is the first time that this
phenomenon has been observed for supramolecular structures
formed by aggregation of a discrete amphiphile and it is an
unambiguous indication of the extremely high organization
levels reached by these 2D sheets. Amphiphile 2 also forms
layered architectures, although its low solubility in aqueous
Fig. 3 X-Ray diffraction patterns (298 K) of 1 plotted against the
angle 2y. The inset shows the expanded region of large angles and a
schematic illustration of the p2 oblique unit cell.
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dimensions can accommodate two molecules per unit cell if
they have opposing orientations and if there is some inter-
digitation of the coincident lateral chains (insets in Fig. 3 and
Fig. S13w). This organization makes the proposed oblique cell
compatible with a p2 plane group, although the less symmetric
p1 plane group cannot be totally ruled out.
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For the case of amphiphile 1, the comparison between the
values for the diagonal extracted from the lattice parameters
of the p2 oblique unit cells (5.45 nm) and that measured from
CPK modeling (6.0 nm), suggests some interdigitation of the
peripheral chains. Furthermore, the value of 5.45 nm implies a
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In conclusion, the peripheral decoration of a rigid 1,2,4,5-
tetrakis(2-phenylethynyl)benzene moiety with polar TEG and
apolar decyl chains results in the anisotropic 2D assembly
of these OPE-based amphiphiles to form highly organized
microcrystalline lamellae. Unlike many other examples of
amphiphilic systems, in which the 2D sheets fold into 3D
micelles or vesicles, the tendency of the rod aromatic units and
the alkyl chains to crystallize frustrates this folding process.
Interestingly, the 2D lamellar organization is observed for
both polar and apolar solvents due to the presence of hydro-
philic and aliphatic chains. X-Ray diffraction analyses in the
bulk-state demonstrate that van der Waals interactions
between the peripheral chains give rise to oblique unit cells.
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aromatic interactions and form nanographitic structures that
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Chem. Commun., 2009, 7155–7157 | 7157