Modulating helicity through amphiphilicity - Tuning supramolecular interactions for the controlled assembly of perylenes

Article abstract of DOI:10.1039/c1cc10220f

Here we show that it is possible to modulate the supramolecular assembly of designed H-bonding amphiphilic perylene-based materials through simple solvent interactions. These modulated supramolecular interactions have been translated to and observed in macroscopic properties, and provide new pathways to the preparation of switchable interfaces based on designed supramolecular interactions.

Full text of DOI:10.1039/c1cc10220f

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COMMUNICATION
Modulating helicity through amphiphilicity—tuning supramolecular
interactions for the controlled assembly of perylenesw
Yongwei Huang,ab Jianchen Hu,a Wenfeng Kuang,a Zhixiang Wei*a and Charl F. J. Faulc
Received 12th January 2011, Accepted 13th March 2011
DOI: 10.1039/c1cc10220f
Here we show that it is possible to modulate the supramolecular
assembly of designed H-bonding amphiphilic perylene-based
materials through simple solvent interactions. These modulated
supramolecular interactions have been translated to and
observed in macroscopic properties, and provide new pathways
Scheme
1 Chemical structure of the PTCDI–HAG amphiphile
to the preparation of switchable interfaces based on designed
supramolecular interactions.
molecule.
with slight changes leading to different aggregate morphologies.
Understanding and tuning of these factors should therefore
lead to pathways for the controlled production of chiral
superstructures.
Helical self-assembly by noncovalent interactions is a widely
observed feature of natural biomacromolecules that directs the
formation of highly ordered structures, e.g., the spontaneous
self-assembly of DNA into a double helix and assembly of
proteins and polysaccharides into a-helices. Inspired by the
unique features of fascinating biological superstructures,
chemists have been able to design a variety of aesthetically
appealing helical supramolecular assemblies by elegantly utilizing
cooperative noncovalent and covalent forces, such as hydrogen
bonding, p–p stacking, van der Waals and chirality.1 In this
context, the control of the supramolecular organization of
p-conjugated systems into helices of nanoscopic dimensions is
of fundamental importance,2–4 as the resulting structures
could find application in the emerging area of (supramolecular)
electronics and photonics because of their unique electronic
and optical properties.
In the present study, a new sugar-based amphiphilic
perylene diimide derivative N-(1-hexylheptyl)-N0-((4-amino-
phenyl)-a-D-glucopyranoside)–perylene-3,4,9,10-tetracarboxyl-
bisimide (PTCDI–HAG, Scheme 1) was synthesized and its
aggregate morphologies in different solvents studied (see ESIw
for the synthesis and characterization of PTCDI–HAG).
PTCDI was selected as the central building block for self-
assembly on the basis of geometry (strong, well-studied p–p
interactions),4 function (optoelectronic activity),8 and the
possibility to produce asymmetric molecular structures.8,9
The combination here with carbohydrates provides, at the
same time, a source of chirality and biocompatibility for
functional supramolecular materials.10 Although the self-
assembly of perylene diimide amphiphilic molecules into
nanofibers in solution has been extensively studied,11 limited
investigations have been performed on the control of helical
morphologies.12 Therefore, it was expected that the aggregate
morphologies of the amphiphilic perylene moieties could be
tuned by solvents with different polarities.
Chiral amphiphiles provide a rich library of chiral building
blocks, which make them attractive candidates for application
in the design of functional self-assembled materials, even more
so if they exhibit biocompatibility.5 Chiral amphiphilic molecules
often assemble in solution to form aggregates with high aspect
ratios such as rods, tapes, or tubes, suggesting that chirality is
intimately associated with growth and stability of self-
assembled fibers of small organic molecules.6 However, the
aggregate morphologies of the amphiphile molecules are often
influenced by environmental parameters such as the nature of
the solvent, the ratio of the good/poor solvent, or temperature,7
To trace the effect of the solvents on the aggregation
behavior and elucidate the self-assembly mechanism of the
helical nanowires, CD, UV-Vis and fluorescence measure-
ments were conducted to monitor the self-assembly process.
Binary solutions with a final PTCDI–HAG concentration of
0.1 mgꢀmLꢁ1 were prepared by injecting the selected poor
solvents (n-octane or water) into good solvent (chloroform or
THF) solutions of PTCDI–HAG, respectively. After vigorous
stirring for 10 minutes, the samples were left to stand overnight
at room temperature.
a National Center for Nanoscience and Technology, Beijing 100090,
China. E-mail: weizx@nanoctr.cn; Fax: +86 10 62656765;
Tel: +86 10 82545565
b Medical College, Henan University, Kaifeng 475004, China.
E-mail: hywei79@126.com
c School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
E-mail: charl.faul@bristol.ac.uk
Fig. 1a and b show the CD and UV-Vis spectra of the
PTCDI–HAG aggregates at various chloroform/n-octane
volume ratios, respectively. When n-octane was gradually
added into the chloroform solution, a well-resolved bisignate
w Electronic supplementary information (ESI) available: Experiments
for the synthesis and characterization of PTCDI–HAG and additional
XRD, TEM and PL data. See DOI: 10.1039/c1cc10220f
c
5554 Chem. Commun., 2011, 47, 5554–5556
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Fig. 1 (a, c) CD and (b, d) UV-Vis spectra of the PTCDI–HAG
molecules in different volume ratios of CHCl3/n-C8H18 or THF/H2O.
CD signal (positive Cotton effect at 450 nm, negative at
503 nm and 543 nm) was observed for an 80/20 chloroform/
n-octane ratio, and became more pronounced until a 50/50
ratio was reached. The bisignate CD intensities changed
continuously until reaching a 50/50 ratio, with more n-octane
causing rapid precipitation. The bisignate positive/negative
Cotton effect with increasing wavelength indicates that the
PTCDI–HAG molecules adopt a left-handed or counter-
clockwise helical arrangement during aggregation.13 The
shape of the UV-Vis spectra in chloroform/n-octane also
change with an increase in the n-octane ratio. The isosbestic
point at 540 nm suggests that the monomeric amphiphile
molecules self-assemble into aggregates,12 which was further
supported by fluorescence spectra (Fig. S2 in ESIw). The
results of the CD and UV-Vis analysis show clearly that the
PTCDI–HAG molecules, while optically inactive in good
solvents, formed optically active left-handed helical aggregates
in a mixed chloroform/n-octane solvent system.
Fig. 2 TEM images of the PTCDI–HAG helical nanowires obtained
from (a) chloroform/n-octane (80/20, v/v), (b) THF/H2O (80/20, v/v),
(c) chloroform/n-octane (50/50, v/v) and (d) THF/H2O (50/50, v/v).
controlled process during solvent evaporation. The sample
obtained from the 50/50 chloroform/n-octane binary solution
consists of a high concentration of left-handed nanowires with
an average diameter of ca. 40 nm and several micrometres in
length. However, the nanostructures obtained from a 50/50
THF/H2O solution exhibited right-handed super-helical
morphologies with an average diameter of approximately
100 nm (Fig. 2c and d and enlarged images in Fig. S3 in
ESIw). Closer investigation of these larger fibers indicated they
are constructed of bundles of helical nanowires. The obvious
difference in handedness of the nanowires indicates that the
solvent plays a critical role in determining the packing
architecture of the amphiphile molecules. The handednesses
of the nanostructures are consistent with that in solution,
indicating they are formed by a thermodynamic process.
These remarkable differences can be explained by the
following arguments: it is well known that both possible
helical conformations (i.e. M for right handed and P for left
handed) necessarily have equal intermolecular interaction
energies in molecules in the absence of chiral units, therefore,
a racemic mixture of the helical assemblies can be expected. By
means of the chiral galactosyl moiety, the M/P chemical
equilibrium can be biased and the aggregate may adopt a
preferential screw sense helical conformation owing to side
chain interactions. However, solvent molecules often influence
P–M helical inversion, and solvent–solute interactions
profoundly affect the stability of conformationally flexible
molecules and their higher-order assemblies.14,15
However, when dissolved in
a tetrahydrofuran/water
(THF/H2O) solution, PTCDI–HAG aggregation exhibited exactly
the opposite behaviour in terms of chirality, and produced
mirror image Cotton effects, as shown in Fig. 1c and d. No
optical activity was observed in pure THF, with a CD signal
appearing with the addition of water as a poor solvent. When
a THF/H2O volume ratio of 40/60 was reached, well-resolved
bisignate CD signals were observed (negative Cotton effect at
470 nm, positive at 520 nm and 560 nm). This indicated that
the PTCDI–HAG molecules adopted a right-handed helical
arrangement.13 The CD spectra of the PTCDI–HAG molecules
in solution were also consistent with the TEM observations, as
will be discussed below.
Samples for TEM investigations were prepared by dropping
1 mL of the aged solution onto carbon-film-covered copper
grids, followed by evaporation of the solvent in air at room
temperature. Fig. 2a and b present the typical TEM images of
PTCDI–HAG obtained from the two binary solvent mixtures
at volume ratios of 80/20, respectively. Nanofibers were
formed in both mixtures, and at 80/20 THF/H2O a left-handed
helical morphology was observed. Since almost no aggregation
of PTCDI–HAG was observed by CD and UV-Vis in these
solutions, these nanofibers were formed by a dynamic
The amphiphilic molecules in our study are unique in that
their amphiphile structure consists of a hydrophobic perylene
diimide scaffold with a 1-hexylheptyl group and hydrophilic
galactosyl residue on either side. When dissolved in chloroform,
the galactosyl group, with addition of octane, will aggregate
due to its low solubility in the latter solvent. The chiral group
will induce the arrangement of the perylene diimide scaffold
with a left-handed screw sense. In contrast, the multiple O–HꢀꢀꢀO
hydrogen bonds between the OH groups from the galactosyl
c
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Chem. Commun., 2011, 47, 5554–5556 5555

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In conclusion, we have shown that it is possible to modulate
the supramolecular assembly of H-bonding amphiphilic
perylene-based materials through simple solvent interactions.
These modulated supramolecular interactions have been trans-
lated to and observed in macroscopic properties, and provide
new pathways to the preparation of switchable interfaces
based on designed supramolecular interactions.
We gratefully acknowledge the Royal Society (International
Joint Project), the National Natural Science Foundation of
China (Nos: 20974029, 91027031), the Ministry of Science and
Technology of China (Nos: 2009CB930400, 2010DFA64680,
2011CB932300) and Chinese Academy of Science for financial
support.
Notes and references
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Fig. 3 (a) Schematic representation of the self-assembly process of
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5556 Chem. Commun., 2011, 47, 5554–5556
This journal is The Royal Society of Chemistry 2011