Laminated nanotapes fabricated from conformation specific self-assembly of N-annulated perylene derivatives

Article abstract of DOI:10.1039/c3cc45011b

Laminated nanotapes were fabricated via conformation specific self-assembly of two N-annulated perylene derivatives (NPDs). The assemblies in solvated states were characterized using cryogenic transmission electron microscopy (cryo-TEM), and their nanostr

Full text of DOI:10.1039/c3cc45011b

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Laminated nanotapes fabricated from conformation
specific self-assembly of N-annulated perylene
derivatives†
Cite this: DOI: 10.1039/c3cc45011b
Received 4th July 2013,
Accepted 8th August 2013
Kai Sun, Yan Li* and Wenxin Fu*
DOI: 10.1039/c3cc45011b
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Laminated nanotapes were fabricated via conformation specific Moreover, we found that a slight variation of the molecular
self-assembly of two N-annulated perylene derivatives (NPDs). The conformation can switch the orientation of building molecules
assemblies in solvated states were characterized using cryogenic within nanotapes. Note that the cryo-TEM technique allowed
transmission electron microscopy (cryo-TEM), and their nanostruc- direct imaging of solvated supramolecular assemblies and
tures were modulated by the synergistic interactions of p–p stacking avoided artifacts to a great extent.
and hydrogen bonds.
Scheme 1 shows the chemical structures and preferred
conformation of two NPDs, which were synthesized according to
Highly ordered one dimensional supramolecular structures the procedure given in the literature11 (see Scheme S1 in the ESI†).
such as nanowires and nanotapes formed from organic semi- The only difference between 1 and 2 was that 2 had fused perylene
conductor molecules have attracted considerable research rings to give a completely planar conjugated molecule, while the
interests due to their potential in optoelectronic devices.1 perylene rings of 1 had a twisted angle to lower their free energy.
Constructing well-defined nanostructures from functionalized Such rotation will give rise to rotational entropy for 1,12 which
graphene was believed to be a strategy with great potential.2 thus had much better solubility in common organic solvents
Various nanostructures such as nanowires,3 nanobelts,4 nano- than 2.13 Both 1 and 2 can be easily dispersed in polar solvents,
sheets,5 and nanotubes6 have been constructed from self- such as THF, DMF, etc., but are insoluble in nonpolar solvents, such
assembly of aromatic organic molecules.7 Researchers could as hexane and toluene. Solution self-assemblies of 1 and 2 were
make full use of multiple intermolecular interactions, such as induced via direct dissolution of solid compounds into THF.
p–p stacking, hydrogen bonds, hydrophilic–hydrophobic inter- Cryo-TEM measurements revealed that both 1 and 2 formed
actions, van der Waals force, etc., to tune and optimize the nanotapes with width between 50 and 200 nm and length
assembled nanostructures.8 In addition, the substructure of ranging from 100 to 1000 nm (Fig. 1). More cryo-TEM images
building blocks and assembly conditions (solvent polarity and of the assemblies are shown in Fig. S1 (see ESI†).
concentration) were demonstrated to have profound effects on
the resultant morphology as well.9
Then, we investigated the effects of concentration on self-
assembly behaviors using UV/Vis absorption (Fig. 2) and fluores-
Recently, we reported a new efficient synthetic method to cence spectra (see Fig. S3 in the ESI†). From the absorption
prepare solvent processable bis-N-annulated quaterrylenes spectra, the main absorption peaks of both 1 and 2 did not move
(BNQs) and tri-N-annulated hexarylenes (TNHs) from easily with the increase of concentration, indicating insignificant change
available N-annulated perylene derivatives. BNQs and TNHs of their own coupling excitons,14 which were further confirmed by
exhibited strong fluorescence, large dipole moments and liquid
crystalline properties.10 Here, we report the formation of lami-
nated nanotapes from two NPDs, from which sub-millimeter
long one-dimensional structures were obtained under optimized
conditions. The nanostructures of assemblies can be modulated
by synergistic interactions of p–p stacking and hydrogen bonds.
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China. E-mail: fuwenxin@iccas.ac.cn,
yanli@iccas.ac.cn
† Electronic supplementary information (ESI) available: Details of synthesis,
Scheme 1 Chemical structures of 1 and 2 (Top). Optimized conformation of the
two perylene rings by Gaussian 03 for 1 (a) and 2 (b), respectively.
additional SEM images, cryo-TEM images, XRD patterns and SAED images. See
DOI: 10.1039/c3cc45011b
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Fig. 1 Cryo-TEM images of assemblies formed from 1 (a, b) and 2 (c) in 1 mM
THF and the corresponding spacing profile (d) of white rectangle in (c). The white
arrows in (b) and (c) represent the direction of hydrogen bonds between NPD
monomers.
Scheme 2 Schematic illustration of the self-assembly mechanism for 1 with the
head-to-head and tail-to-tail models. In order to compare to 2 (in Scheme S2,
ESI†), we simplify the twisted perylene rings of 1 as a plane.
nanotapes illustrated in Scheme 2 and Scheme S2 (see ESI†). From
the molecular structure, the width of the perylene ring is about
0.5 nm, so two head-to-head packing perylenes will give a width
of B1 nm as the J-type aggregation.15 In addition, the length of
fully extended C-10 alkyl side chains is around 1.2 nm provided
that they adopt the zig-zag conformation. Partially interdigita-
tion of two alkyl chains can result in a dimension of 1.5 nm,
which had great consistency with the cryo-TEM results (Fig. 1).
Based on the above information, we proposed that the perylene
rings adopted a face-up orientation as illustrated in Scheme 2.
Also note that the two NPDs have different orientations
within flat nanotapes, namely along and perpendicular to the
Fig. 2 UV/Vis absorption spectra of
1 (a) and 2 (b) in THF at different
concentrations.
cryo-TEM measurements at low concentrations (see Fig. S2 in long axis of perylene rings for 1 and 2, respectively. We believed
the ESI†). It was also interesting to emphasize that both 1 and 2 that it was induced by different growth kinetics. Firstly, both 1
had large absorption coefficients and formed aggregates even and 2 preferred stacking perpendicular to the flat tape via p–p
at 10ꢀ6 M THF, indicating that they had strong intermolecular interactions, which were supported by XRD characterizations
interactions (p–p stacking, hydrogen bonds and dipole–dipole (see Fig. S4 in ESI†). Secondly, sample 2 with flat conformation
interactions), which agreed well with the previous results.10 In might need less energy to arrange a head-to-head packing in
contrast, the fluorescence spectra of 2 displayed a slightly batho- polar solvent medium. In contrast, sample 1 will have extra
chromic shift with the increase of concentration from 1.25 to entropy penalty arising from the rotational freedom between
100 mM, probably due to the inner filter effects (reabsorption). two perylene rings that prefer having twisted arrangements. As
Interestingly, careful examination using high resolution a result, growing along carboxylic acid will be preferred for 1. In
transmission electron microscopy (HRTEM) revealed that both contrast, the nanotape’s growth direction was perpendicular to
1 and 2 actually formed lamellar nanostructures (Fig. 1b and c). the hydrogen bonds for 2.
As the samples were still in solvated states, we attributed the
More importantly, the hydrogen bonds were also critical for
dark region to the planar aromatic ring domains and the grey forming laminated nanotapes. We prepared samples 10 and 20,
region to alkyl side chain domains considering their electron which had similar structures to 1 and 2 except that 10 and 20
density differences. TEM and FFT transfer showed that the dark contained two ester bonds instead of carboxylic acid. Surpris-
region was about 1.2 nm wide and the grey region was about ingly, both 10 and 20 only formed amorphous structures regard-
1.5 nm wide. It is well-known that the distance of p–p stacking less of self-assembly conditions (see Fig. S5 in ESI†).
is about 0.35 nm, we thus assumed that laminated structures
had to adopt a distinct packing motif.
Considering that long range ordering is desirable for nano-
devices, we attempted to employ a selective solvent to promote
Considering the molecular dimensions of 1 and 2, we proposed the growth of preformed nanotapes. Toluene (a poor solvent for
a head-to-head and tail-to-tail packing model for their resulting the two NPDs) was added to sample THF solutions to induce
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through two stages, from disassembly to amorphous aggregates.
In contrast, for twisted perylene rings 1, the p–p interaction
between toluene and 1 was not strong enough to break that
among molecules. Hence, as a poor solvent, toluene just induced
the preformed nanotapes to further reassemble larger ones.
In summary, we have demonstrated that two N-annulated
perylene derivatives with different conformations have different
self-assembly behaviors. Molecular self-assembly revealed that
they have different abilities to form large nanotapes with variable
sizes under different conditions, which provides an insight into
control and optimization of molecular parameters and solvent
effects to tune resulting morphologies.
This work was supported by National Natural Science Foun-
dation of China (51225306, 91027043 and 21204091). We thank
Prof. Zhibo Li and Prof. Zhaohui Wang for their advice on
synthesis and characterization.
Notes and references
Fig. 3 SEM (a, b) and cryo-TEM (c) images of supramolecular assemblies formed
from the two NPDs: (a, c) 1 in THF–toluene (1/19, v/v) mixed solvents (C = 0.05 mM);
(b) 2 in THF–toluene (1/9, v/v) mixed solvents (C = 0.01 mM); (d) the corresponding
spacing profile of a white rectangle in (c). The white arrow in (c) represents the
direction of hydrogen bonds between NPD monomers. The SEM sample was
prepared by drop-casting the assembly suspension on a silicon wafer.
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