Self-assembly based on heterotriangulene derivatives: From nanowires to microrods

Article abstract of DOI:10.1039/b9nj00572b

Compound SA-6, SA-8 and SA-10 (totally called SAs) were synthesized by palladium-catalyzed Sonogashira cross-coupling reactions of core derivative 3 with three equiv. of 1-alkyloxy-4-ethynylbenzene 2 in good yields. After the dichloromethane solutions of SAs were mixed with methanol, two-level self-assemblies from nanowires to microrods based on SAs were obtained and characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The driving force of the two-level self-assembly is attributed to the strong π-π stacking ability of heterotriangulene cores and the hydrophobic interactions of alkyl chains with solvent molecules. These two processes may well be used for other molecules to fabricate new nanomaterials in the field of photoelectric devices. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b9nj00572b

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www.rsc.org/njc | New Journal of Chemistry
Self-assembly based on heterotriangulene derivatives: from nanowires
to microrods
Xiangjian Wan,a Huaqiang Zhang,b Yanqin Lib and Yongsheng Chen*a
Received (in Victoria, Australia) 16th October 2009, Accepted 15th January 2010
First published as an Advance Article on the web 10th February 2010
DOI: 10.1039/b9nj00572b
Compound SA-6, SA-8 and SA-10 (totally called SAs) were synthesized by palladium-
catalyzed Sonogashira cross-coupling reactions of core derivative 3 with three equiv. of
1-alkyloxy-4-ethynylbenzene 2 in good yields. After the dichloromethane solutions of SAs
were mixed with methanol, two-level self-assemblies from nanowires to microrods based on
SAs were obtained and characterized with scanning electron microscopy (SEM), transmission
electron microscopy (TEM) and X-ray diffraction (XRD). The driving force of the two-level
self-assembly is attributed to the strong p–p stacking ability of heterotriangulene cores and
the hydrophobic interactions of alkyl chains with solvent molecules. These two processes may
well be used for other molecules to fabricate new nanomaterials in the field of photoelectric
devices.
provided us with an excellent platform to investigate the
Introduction
self-assembly of 1D nanostructures. The main challenge of
In the past decades, one dimensional (1D) nanomaterials
including wires,1 tubes,2 rods,3 and belts4 have attracted
1D self assembly of molecules based on A lies in controlling
and optimizing the strong p–p interactions between the
aromatic cores in cooperation with the hydrophobic
extensive investigations, due to their unique properties and
applications in nanoscale devices. However, most researchers
interactions of the alkyl chains linked at the core with
have focused on inorganic compounds1–4 and polymers.5
solvent molecules.20 It is reported before that 1D assembly
Recently, organic nanomaterials based on small molecules
can simply be obtained by dispersing a concentrated solution
of a specific compound into a ‘‘poor’’ solvent.21 And when the
have attracted more and more attention, owing to their special
properties for use in electronic and optoelectronic nanodevices.6
molecules have long alkyl side chains, the aggregation often
Scientists have successfully constructed 1D organic nanostruc-
proceeds too fast for the molecular assembly to grow along
tures with the help of hydrogen-bonding,7–9 p–p stacking,10,11
one direction, thus leading to formation of chunky aggregates
electrostatic interactions,12,13 and other nonconvalent
rather than nanowires.22 In this regard, we report
interactions.14 Among them, self-assembly of p-conjugated
the development of a family of n-type semiconductors SA-6,
molecules is of extreme importance, and p–p stacking is
SA-8 and SA-10 (Chart 1) based on the core A at room
considered as one of the main driving forces for the formation
temperature and their different self-assembly behaviors with
of 1D assembly.15 This is particularly evident in the large
different alkyl side chains.
discotic molecules such as perylene tetracarboxylic diimide
(PTCDI) based derivatives,16,17 which form a unique class of
n-type semiconductor in comparison to the more common
p-type counterpart in organic semiconductors.16,17 As a part
of our efforts toward synthesizing new organic n-type materi-
als, we have identified heterotriangulene A (Chart 1) as a
promising candidate in our previous work.18 The three
carbonyl groups of A greatly reduce the electron density of
the aromatic core, a feature that has been shown to effectively
promote p–p interaction,19 which is important for the
conduction of electrons in n-type materials. The electron
deficient aromatic core of A and its C3 symmetry also
a Key Laboratory for Functional Polymer Materials and Center for
Nanoscale Science and Technology, Institute of Polymer Chemistry,
College of Chemistry, Nankai University, 300071, Tianjin, China.
E-mail: yschen99@nankai.edu.cn; Fax: +86 22 23499992;
Tel: +86 22 23500693
b Lanzhou Petrochemical Research Center, PetroChina,
Lanzhou 730060, China
Chart 1 Molecular structures of A and SAs.
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Results and discussion
Synthesis
Compound SAs were synthesized by palladium-catalyzed
Sonogashira cross-coupling reaction23 of core derivative 318
with three equiv. of 1-alkyloxy-4-ethynylbenzene 224 in good
yields (Scheme 1). As expected, SAs were readily soluble in
common organic solvents such as toluene, CH2Cl2, and THF.
Thermal analysis
Thermal stabilities of SAs were investigated with thermogravi-
metric analysis (TGA), as shown in Fig. 1. The onset decom-
position temperatures of the molecules decrease from SA-6 to
SA-10, indicating that the longer alkyl side chains lead to the
weaker intermolecular interactions from SA-6 to SA-10 in the
solid state. Even this, the onset decomposition temperatures of
the molecules are all around 400 1C in nitrogen, illustrating
that they are quite thermally stable and can be used for various
high-temperature processes in the construction of electronic
devices.
Fig. 1 TGA plots of SAs with a heating rate of 10 1C minÀ1 under N2
atmosphere.
the molecules in solid state, we investigated the absorption
behaviors of SAs in films. The film absorption results from
SA-6 to SA-10 are nearly the same, so herein, we take SA-6 as
an example to compare with its absorptions in solution. As
shown in Fig. 3, the spectrum of SA-6 exhibits a bathochromic-
shift in the solid state compared with that in solution, i.e.,
from wavelength of 458 nm in solution shifted to 472 nm in
film. Since the probability of assembling for aggregates of SA-
6 in the solid matrix increases, one may perceive that the
bathochromic-shift phenomenon is coming from the inter-
molecular p–p interactions of SA-6 in solid state. The batho-
chromic shift of UV absorption from solution to film also
suggests that the aggregation style of SA-6 in solid state is not
exact p–p stacking.27
The fluorescence spectra of SAs at the same concentration,
show structured emission bands around 382 nm and broad
featureless emission bands at 500 nm, when all excited at
348 nm (Fig. 4), which is in accordance with the result in
literature.19 The emission bands around 382 nm, showing
normal Stoke shift of 34 nm and the same emission intensity,
can be assigned to the locally excited state emissions of SAs.28
But it is very interesting to find that the band intensities at
500 nm decrease with the increasing length of the alkyl chains.
The different extent of p–p stacking for SAs was also
evidenced by the measurement of Differential scanning calori-
metry (DSC), as shown in Fig. 2. The DSC patterns of SAs
show only one phase transition on heating at 187, 180, 166 1C
from SA-6 to SA-10, respectively, which corresponds to their
melt points. The dramatically lower melting point from SA-6
to SA-10 indicates the weakened molecular stacking, which is
caused by the longer alky side chains from SA-6 to SA-10.
Optical properties
The absorption spectra from SA-6 to SA-10 (Fig. 3) in
dichloromethane show intensive transitions in the UV region,
with strong absorption bands in the range of 230–310 nm.
These bands are assigned to the p–p* transitions of subunits 2.
The absorption spectra of SAs show a maximum at 348 nm
with a well-resolved vibronic structure, which can be assigned
to the transition of core part according to the reported
literature.25 Apart from the characteristic absorptions of the
A core and the subunits 2, the solution spectra of these three
compounds also show a new broad band at longer wavelength
around 458 nm, which depends on their effective conjugation
lengths.26 To gain further insight into the interactions between
Scheme 1 The synthesis of SAs.
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Fig. 3 The absorption spectra of SAs (1.0 Â 10À6 M) in dichloro-
methane and SA-6 in thin film.
Fig. 4 Uncorrected emission spectra of SAs (lex = 348 nm) in
dichloromethane (5.0 Â 10À7 M).
Fig. 2 Differential scanning calorimetry (DSC) traces of SAs with a
heating/cooling rate of 5 1C minÀ1. The data shown are from the
second heating-cooling cycle.
To deeply explore the origin of the band at 500 nm, we also
measured the fluorescence of SA-6 (take for example in Fig. 5)
in dichloromethane with different concentrations. It was found
that the emission intensity ratio for band around 382 nm
versus band at 500 nm showed apparent concentration depen-
dence. With the increase of concentration of SA-6, the band
intensities at 500 nm increase while those around 382 nm show
common features of concentration quenching. These results
also illustrated that the band at 500 nm may originate from the
intermolecular aggregation and can be assigned to the excimer
emissions.
Fig. 5 Uncorrected emission spectra of SA-6 (lex = 348 nm) in
dichloromethane with different concentrations.
Electronchemical properties
occupied and lowest unoccupied molecular orbital (HOMO
and LUMO) parameters, which indicate the facility to inject
In view of the potential application of such molecules in
optoelectronics, it is important to consider the relative highest
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holes and electrons respectively in the material. Cyclic voltam-
metry (CV) was carried out to obtain the electronic levels of
SAs (Fig. 6). All these compounds have two irreversible
reduction peaks. So we take SA-6 for example to calculate
its HOMO and LUMO. A LUMO level of À3.44 eV for SA-6
was calculated from the onset value of the reduction peak
which enables effective molecular aggregation into well-
organized nanowires and microrods, as evidenced by the
optical spectral measurements below. In SEM images
(Fig. 7), self-assemblies of SAs as many microrods, with
average diameters of 10 mm and lengths of several millimetres
were observed. TEM imaging gave a deeper insight about the
structure of the microrods. From the TEM images (Fig. 8), we
could find that the microrods are made of straight nanowires
packed side by side. The typical width of these nanowires is ca.
3 nm. Thus, a two-level self-assembly process may exist in the
formation of the microrods based on SAs. The first level is due
to the strong intermolecular p–p interactions between the large
A cores, where the molecules assemble along the axis direction
of the nanowires. Then, these nanowires assemble in the
orthogonal direction of the axis to form the microrods. The
driving force for the second level assembly is believed to come
from the hydrophobic interactions of alkyl chains with solvent
molecules.30
(Eonset = À1.36 V versus Fc/Fc+). With the optical band-
red
gap of 2.5 eV (from onset wavelength of the UV-vis spectrum
of SA-6 in solid state), the HOMO level of À5.94 eV for SA-6
has been calculated.29
Self assembly properties
The self-assemblies of SAs were performed using a diffusion
process in a binary solvent of dichloromethane–methanol,
Identification and unequivocal assignment of the self-
assembly of SAs were finally achieved by X-ray diffraction
(XRD) and small angle X-ray scattering (SAXS) (Fig. 9).
In the wide-angle region, features have been observed
corresponding to various short-range interactions. First, at
d = 3.4 A, a relatively sharp scattering was observed,
corresponding to the p stacking distance of the large flat A
cores along the 1D assembly columnar axis (schematic illus-
tration shown in Fig. 10). This is also supported by the single
crystal X-ray data of the parent compound heterotriangulene
A, where the molecules pack into a 1D column with a
stacking distance of 3.5 A.19,31 The broadening peak at
3.4 A suggests that either the stacking is less regular or that
the A cores are slightly tilted with respect to the columnar
axis.32 Then, a large broad and diffuse scattering centered
3.7–4.3 A was visible, which reflects the short-range correla-
tions of the alkyl chains from SA-6 to SA-10. In the slightly
lower angle region, another weak diffuse scattering peak
corresponding to a distance of ca. 9.0 A was attributed to
some correlations of alkyl chains between neighboring
molecules. In the small-angle region, a sharp reflection peak
at 2y = 3.411 was detected (take SA-6 for example), from
which a spacing of a = 25.9 A was estimated. This value
corresponds to the spacing between the lateral 2D arrays.
The sharpness and intensity of the small-angle reflection at
2y = 3.411 indicate clearly the long-range second level
assembly between the 1D assembly columns, which is formed
by the first-level assembly of SA-6 due to the intermolecular
p–p interactions of A cores. Overall, the two-level self-
assembly process can be illustrated in Fig. 10.
Conclusion
Due to the p–p stacking interactions of A cores and the
hydrophobic interactions of alkyl chains with solvent mole-
cules, compound SAs could self-assemble into supramolecular
organizations from straight nanowires to well-organized
microrods through a two-level self-assembly process. This
process may well be used for other molecules to fabricate
new nanomaterials in the field of photoelectric devices.
Fig. 6 Cyclic voltammograms of SAs in 0.1 M Bu4NPF6–CH2Cl2,
with a glassy carbon electrode at a scan rate of 100 mV sÀ1. A
platinum wire and an SCE was used as the counter and reference
electrode, respectively.
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Fig. 7 SEM images of self-assemblies of SA-6 (a), SA-8 (b) and SA-10 (c).
Fig. 8 TEM images of self-assemblies of SA-6 (a), SA-8 (b) and SA-10 (c).
Fig. 10 Schematic illustration of possible molecular arrangements for
self-assembly of SA-6.
parts per million relative to the internal standard TMS. High
resolution mass spectra (HRMS) were recorded on a VG
ZAB-HS mass spectrometer. UV/Vis spectra were measured
on a JASCO V-570 spectrophotometer in a 1 cm width cell.
Fluorescence spectra were measured on a JOBIN YVON
HORIBA fluorescence spectrophotometer in a 1 cm width
cell. Cyclic voltammetry (CV) measurement was performed on
an LK98B II Microcomputer-based Electrochemical Analyzer
at room temperature with a three-electrode cell in a solution of
Bu4NPF6 (0.1 M) in dichloromethane at a scanning rate of
100 mV sÀ1. A platinum wire was used as a counter electrode,
and an SCE electrode was used as a reference electrode. After
measurement, the reference electrode was calibrated with
ferrocene (Fc). Thermogravimetric analysis (TGA) measure-
ment was performed on a TA instrument SDT-TG Q600
Fig. 9 XRD patterns of self-assemblies of SA-6 (a), SA-8 (b), SA-10
(c) and the SAXS pattern of self-assembly of SA-6 (d) at room
temperature.
Experimental
General information
Unless stated otherwise, all chemicals and reagents were
purchased reagent grade and used without further purification.
Solvents were purified by standard methods. All manipulations
were performed under a dry Argon atmosphere using standard
Schlenk techniques.
under an atmosphere of N2 at a heating rate of 10 1C minÀ1
.
Scanning electron microscopy (SEM) analysis was carried on a
Hitachi S-3500N scanning electron microscope at an acceler-
ating voltage of 20 kV. Transmission electron microscopy
(TEM) images were performed using a Tecnai 20 at an
1H NMR and 13C NMR spectra were recorded on a Bruker
AC-300 Spectrometer. Chemical shifts d, were reported in
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accelerating voltage of 200 kV. A 200 kV JEOL 2010F was
used for elemental mapping. X-ray diffraction (XRD) experi-
ment was performed on a Rigaku D/max-2500 X-ray powder
diffractometer with Cu-Ka radiation (l = 1.5406 A) at a
generator voltage of 40 kV and a current of 100 mA. SAXS
experiments were performed on a Bruker NanoStar SAXS
system (Cu-Ka radiation source at a voltage of 40 kV and a
current of 35 mA).
Technology (#2006CB932702) and Natural Science Founda-
tion of Tianjin City (#07JCYBJC03000, #08JCZDJC25300).
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graphy on silica (CH2Cl2/Hexane = 1 : 2) to obtain the pure
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(6.H, d, J = 8.7 Hz, ArH), 6.87 (6 H, d, J = 8.7 Hz, ArH),
3.98 (6 H, t, J = 6.5 Hz, OCH2), 1.86–1.77 (6 H, m, CH2),
1.50–1.34 (18 H, m, (CH2)3), 0.93 (9 H, t, J = 6.8 Hz, CH3).
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135.61, 133.39, 123.08, 122.16, 114.55, 113.91, 93.35, 85.29,
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1.86–1.76 (6 H, m, CH2), 1.49–1.25 (30 H, m, (CH2)5), 0.91
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173.89, 159.54, 135.85, 134.94, 133.24, 122.62, 121.82, 114.27,
113.802, 93.19, 85.21, 68.01, 31.87, 29.67, 29.48, 29.28, 26.05,
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We gratefully acknowledge the financial support from the
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