Benzenetricarboxamide-cored triphenylamine dendrimer: Nanoparticle film formation by an electrochemical method

Article abstract of DOI:10.1039/c3ra42469c

By dropwise addition of a solution of tricarboxamide-cored triphenylamine dendrimer G1 in THF into water (5 × 10^-6 M), nanoparticles with average diameters of 80 ± 20 nm were formed and collected by centrifugation. The particles show aggregatio

Full text of DOI:10.1039/c3ra42469c

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Benzenetricarboxamide-cored triphenylamine
dendrimer: nanoparticle film formation by an
electrochemical method†
Cite this: RSC Adv., 2013, 3, 22219
ab
*
Man-kit Leung,
You-Shiang Lin,a Chung-Chieh Lee,a Chih-Cheng Chang,a
Yu-Xun Wang,a Cheng-Po Kuo,a Nirupma Singh,a Kun-Rung Lin,a Chih-Wei Hu,c
Chen-Ya Tseng and Kuo-Chuan Ho
c
bc
*
By dropwise addition of a solution of tricarboxamide-cored triphenylamine dendrimer G1 in THF into water
(5 ꢀ 10ꢁ6 M), nanoparticles with average diameters of 80 ꢂ 20 nm were formed and collected by
centrifugation. The particles show aggregation induced emission and emit green light under
Received 18th May 2013
photoluminescence conditions. The particles can be fused together by applying
a concept of
Accepted 30th August 2013
electrochemical curing; the G1 particles are coupled through electrochemical oxidation to form a film.
This method provides a fast assembling process for constructing films in a few seconds. Fabrication of
electrochromic and fluorescence switching devices was demonstrated.
DOI: 10.1039/c3ra42469c
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by chemical methods or by physical methods. When organic
molecules are dispersed in highly polar solvents, in particular in
Introduction
The bottom-up approach of assembling molecules and nano-
particles upon a surface is of fundamental importance for
constructing novel materials or devices.1 Due to the recent
advances of synthetic methodologies, large varieties of organic
or polymeric nanoparticles can be synthesized and used as
building blocks for construction of three-dimensional struc-
tures.2 Therefore, developments of assembling techniques for
organizing organic molecules or nanoparticles are of primary
interests in modern sciences and technologies. Despite their
noncovalent and reversible nature, molecular self-assembly
techniques have proven to be useful tools for the construction
of nano-scale structures. Many examples are known nowadays,
which contain strong (self-) complementary and unidirectional
intermolecular interactions to facilitate the self-assembled
architectures. Monolayers and Langmuir–Blodgett lms are
typical examples of nano-structures that could be achieved by
self-assembly.
aqueous media, the organic molecules tend to aggregate due to
the advantages of hydrophobic interactions.3 In the solid state,
strong intermolecular electronic interactions such as dipolar
interactions, hydrogen bond interactions and aromatic p–p
interactions would also lead to molecular aggregation. The
growth of the aggregates sometimes leads to nano-scale parti-
cles. Currently, a major challenge is the implementation of
physical functions into nano-materials, such as luminescence
ability, switching and electronic or photovoltaic properties.
However, the aggregation of organic molecules strongly affects,
or sometimes even alters the molecular properties. For example,
aggregate formation usually leads to quenching of light emis-
sion with red-shiing of the emission spectrum, which has
been a big challenge for the development of light-emitting
devices,4 such as biological sensors or organic light emitting
diodes. Recently, Tang reported observations of aggregation-
induced emission (AIE) for siloles and diphenyldibenzo-
fulvenes, which brought in a new dimension of research for
organic materials.5 The AIE phenomenon poses a challenge to
the current understanding of the luminescence process in the
solid state.
Organic nanoparticles are another kind of interesting
structure that have attracted a lot of attention during the last
decades. Organic nanoparticles can usually be prepared either
Carefully reviewing the features of nano-materials reveals
aDepartment of Chemistry, National Taiwan University, 1, Roosevelt Road Section 4,
Taipei, Taiwan 106, Republic of China. E-mail: mkleung@ntu.edu.tw
bInstitute of Polymer Science and Engineering, National Taiwan University, 1,
˚
˚
¨
that their sizes are quite different, ranging from Angstroms (A)
to hundreds of nanometers (nm). For example, self-assembled
monolayers usually have a lm thickness on the nm scale.6
Layer by layer approaches could provide thin lms with thick-
nesses on the 10 nm scale.7 Electropolymerization could give
rise to relatively thick lms, provided that the morphologies
and homogeneity of the lms could be well controlled.8 Nano-
particles, on the other hand, are relatively large species, with
Roosevelt Road Section 4, Taipei, Taiwan 106, Republic of China
cDepartment of Chemical Engineering, National Taiwan University, 1, Roosevelt Road
Section 4, Taipei, Taiwan 106, Republic of China
† Electronic supplementary information (ESI) available: Experimental procedures
including synthesis, photophysical measurements, AIE, electrochemical studies,
and the electrochromic cell; NMR spectra, UV-vis spectra and CV. See DOI:
10.1039/c3ra42469c
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diameters up to the hundred nm scale. These particles can be including hydrogen bonding, p–p stacking, and solvophobic
used as building blocks to construct three-dimensional arrays.9 interactions, are formed.
Our recent interest is to develop techniques that could help
to integrate self-assembled mono-layers, layer-by-layer electro- C3-symmetrical
By combining all the features we have mentioned before, the
dendritic 1,3,5-benzenetricarboxylamides
chemical deposited lms, and nanoparticles to form optoelec- (BTCAs) G0 and G1, as shown in Scheme 1 and 2, were designed
tronic devices. To achieve this purpose, we rst have to select because they could show a combination of hydrogen bond
appropriate organic molecules that could help to nish the interactions as well as aromatic p–p interactions, with an
assembling.
integration of photoluminescence, electroluminescence, and
Dendritic molecules have attracted much research interest electrochemical activities. First, the central core contains a
for a variety of high-tech applications such as dye-sensitized C3-symmetrical tricarboxamide unit, which provides the
solar cells,10 organic memory devices,11 light harvesting mate- opportunity for aggregate formation through hydrogen bond
rials,12 and as components in drug or gene delivery systems.13 interactions. The presence of the electron-withdrawing carbox-
Among the family of dendritic molecules, dendrimers have well yamide groups on the benzene ring makes the central domain
dened and unique macromolecular structures. Their highly electron-decient. Therefore, when the electron-donating tri-
branched and globular structures lead to a number of impor- arylamine dendrons are introduced as the outer sphere, an
tant physical and chemical properties that contrast to the linear electronic-gradient would be established. In addition, the
polymers of analogous molecular weight.14,15 For example, highly aromatic exterior would provide strong aromatic p–p
dendrimers demonstrate signicantly increased solubility that interactions, which will be benecial for the intermolecular
can be readily tuned by derivatizing the periphery and they also aggregation. The electron-rich triarylamino dendrons are elec-
exhibit very low intrinsic viscosities when compared with their trochemically active and could be electrochemically polymer-
linear analogues.
ized under appropriate conditions.
Dendrimers containing triphenylamine (TPA) moieties are
Our team is interested in electrochemical techniques for thin
well reported in the literature.16–18 It is known that TPA-cored lm deposition. Herein we demonstrate the use of electro-
organic compounds are excellent hole-transport materials for chemical curing of nanoparticles of G1 as a fast assembling
optoelectronic applications. This family of dendrimers technique for optoelectronic device fabrication. The fabrication
intrigues us because they have high branching numbers and are strategies involve self-assembly layer formation, layer-by-layer
supposed to be more sensitive to electro-polymerization than electrochemical deposition, and electrochemical cross-linking
the linear family, from a statistical point of view.
of nanoparticles of G1 on an indium tin oxide (ITO) surface. The
During the past decades, we have put effort into under- performances of novel electrouorescence devices (EFDs),30
standing how to control the electrochemical behaviors of TPA with uorescence-switching and electrochromic properties,
derivatives. The TPA core exhibits excellent coplanarity of the have also been studied.
central nitrogen and the three coordinating carbons. The
nitrogen lone-pair electrons can maintain uninterrupted
conjugation with the aromatic arms. Therefore, the TPA core
Results and discussion
Synthesis of G0 and G1
usually functions as a strong electron donor to conjugated
systems.19,20 In addition, TPA derivatives possess high charge-
While 1 was known in literature,31 2 was prepared through a
three-step synthetic sequence (Scheme 1). Goldberg condensa-
tion32 was adopted to facilitate the mono C–N coupling of 3 and
4, using CuI (30 mol%)–N,N0-dimethyl-1,2-diamine (DMDA,
60 mol%)–K3PO4 (1.5 equiv.) as catalyst at 140 ꢃC for 24 h to give
5. A Suzuki coupling reaction would then proceed smoothly to
give 6, followed by deprotection of the phthalimide with
NH2NH2 to give 2. Dendrimers G0 and G1 were prepared from
8 and the corresponding arylamines 1 and 2 in the presence
of DMAP–Et3N. Compound 9 was prepared from 7 and 8 as
reference for comparison.
carrier mobility and low ionization potential; their hole trans-
porting rates are about 1 ꢀ 10ꢁ3 to 10ꢁ4 cm V sꢁ1
.
These
21
properties make TPA derivatives very promising hole-transport
materials in organic light emitting device applications. Rey-
nolds reported the use of TPA containing hyperbranched
conjugated polyelectrolyte bilayers for solar cell applications.22
TPA-cored dendrimers afford enhanced two-photon absorption
performance with locked molecule planarity.23–25 TPA-cored
nonconjugated polymers demonstrate a new approach for
electropolymerization that provides good hole injection and
transport performance in PLED, including low turn-on voltage
and high brightness.6d
The study of the hydrogen bond interactions
Besides the electrochemical properties, our target
compound has to demonstrate self-assembly capability. Among 1,3,5-Tricarboxylamide derivatives favor the formation of
the systems we searched, hydrogen bonded C3-symmetrical hydrogen bond networks through intermolecular hydrogen
1
molecules that associate into supramolecular stacks through bond interactions. In the NMR study, extremely broadened H
hydrogen bonds and p–p interactions have caught our atten- NMR signals for G0 and G1 were observed in CDCl3, indicating
tion.26–29 Many interesting properties such as ber and organic that the free rotation of the amido side-chains is restricted by
gel formation from discotic tricarboxamides and trisureas have intermolecular hydrogen bond interactions. This is probably
been recently investigated. In the organogels, three-dimensional due to the self-assembly of G0 and G1 into columnar
entangled networks of bers, held by noncovalent forces stacks at NMR concentration. However, in a mixed solvent of
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Scheme 1 Synthesis of G0, G1, and 9.
CDCl3–DMSO-d6 (0.5 ml : 0.2 ml), the hydrogen bonding network substituted for methyl substituents, 9 could only act as the
was broken-up. Dissolution of the stacks and dissociation of the hydrogen bond acceptor. On addition of 9 to the NMR sample of
hydrogen bond aggregates led to well-resolved 1H NMR spectra G1 (1 ꢀ 10ꢁ3 M), downeld shis of the amido protons were
1
with sharpened signals: a broad singlet at d 10.19 (3H, –NH–), a clearly observed. As shown in Fig. 1, the trends of the H NMR
multiplet at 8.62–8.64 (3Hc of the central benzene ring), a multi- shiing pattern t well with the equation33
plet at 7.69–7.71 (6Ha aromatic), a multiplet at 7.35–7.41 (22H
aromatic), a multipletat7.06–7.18 (24H aromatic)anda multiplet
at 6.90–7.04 (68H aromatic). The observation of the downeld
D
¼ ðꢁKSL ꢀ DÞ þ ðKSL ꢀ DSLÞ
½Lꢄ
shi of the amido protons above 9 ppm indicates that the amido where D ¼ d ꢁ d0 and DSL ¼ dSL ꢁ d0. The formation constant
protons are hydrogen bonded to DMSO-d6.
(KSL) for 1 : 1 complex formation (S + L ¼ SL) was therefore
To further explore the hydrogen bond character of the den- measured as 665 ꢂ 25 Mꢁ1. These observations are consistent
drimers, a proton-NMR titration was carried out using the tri- with the assumption of hydrogen bond network formation of
carboxamide 9 as the titrant (Fig. 1). With the amido protons tricarboxamide derivatives proposed in literature.
Scheme 2
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Fig. 1 Proton-NMR titration of G1 with 9.
The strong hydrogen bond interactions are also reected
in their high thermal stability. G0 and G1 form amorphous
organic glass in the solid state with very high glassy transition
temperatures of 153 ꢃC and 185 ꢃC, and decomposition
(5% weight loss) temperatures of 431 ꢃC and 510 ꢃC, respectively.
In addition, other hydrogen bond related self-assembly
properties of G0 and G1, including the physical sorption on the
ITO surface as well as nanoparticle formation will be discussed
in later sections.
The photophysical properties of G0 and G1
Pertinent spectral data for G0, G1, 9, 1, and 2 are summarized in
Table 1 and Fig. 2. G0 shows a characteristic absorption peaking
at 300 nm along with a broad-shoulder at 340 nm. Perhaps due
to the three-fold C3 concentric symmetry, solvatochromic effects
on the absorption are small.34 The major absorption of G0 at
300 nm overlaps with that of 1, indicating that this absorption
mainly arises from the dendritic side-arms. The extinction
coefficient (3) of G0 (5.8 ꢀ 104 Mꢁ1 cmꢁ1) is about 2.6 times the 3
value of 1 (2.2 ꢀ 104 Mꢁ1 cmꢁ1), suggesting that electronic
perturbations from the BTCA unit are limited. On the other
hand, the shoulder band is due to the charge transfer (CT)
absorption from the dendritic triarylamine (TAA) branches to
the BTCA core. G1 shows split absorption peaks at 300–400 nm,
which covers the spectral region of 2. G1 has the rst band at
310 nm, which is well overlayed with that of 2, and has an 3
value of 12.2 ꢀ 105 Mꢁ1 cmꢁ1, which is about 2.5 times larger
than that of 2 (3 ¼ 4.9 ꢀ 104 Mꢁ1 cmꢁ1) . On the other hand,
Fig. 2 Comparison of UV-vis spectra: (a) G0, 1, and 9 in CH2Cl2; (b) G1 and 2 in
CH2Cl2.
Fig. 3 Pictorial diagrams of the HOMO and LUMO of G1 calculated by PM5
due to the contributions from the CT band, G1 shows stronger methods.
Table 1 Photophysical data of G0 and G1
FL
Fluorescence l (nm) (QY)e
max
lambasxa (nm)
3b
Solutionc
Stokes shi
THF glass
Nanoparticled
G0
G1
300
310, 365
5.8
12.0, 19.9
513 (0.0011)
520 (0.0027)
215
155
495 (0.11)
500 (0.14)
515 (0.026)
520 (0.048)
a
b
c
d
e
Measured in CH2Cl2. 104 Mꢁ1 cmꢁ1
.
Measured in THF. Measured in THF/H2O ¼ 1 : 9. QY (quantum yield) is quantied in THF against
coumarin 1 (0.85) as standard.35
22222 | RSC Adv., 2013, 3, 22219–22228
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