Materials Sciences and Applications, 2011, 2, 555-563
doi:10.4236/msa.2011.26074 Published Online June 2011 (
Copyright © 2011 SciRes. MSA
Polymer-Controlled Synthesis of
1-(2-Pyridylazo)-2-naphthol Hierarchical
Jie Leng1, Qiulin Liao1, Yong Gao1, Huaming Li1,2
1College of Chemistry, Xiangtan University, Xiangtan, China; 2Key Laboratory of Polymeric Materials & Application Technology of
Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Envi-
ronment-friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, China.
Received March 27th, 2011; revised April 12th, 2011; accepted April 30th, 2011.
The self-assembly of organic 1-(2-pyridylazo)-2-naphthol (PAN) into hierarchical architectures, such as microfibers, mi-
crorods, and sheaflike structures, in solution was successfully achieved by reprecipitation method with the assistance of
thermoresponsive diblock copolymer poly(N,N-dimethylacrylamide)-b-poly(N-isopropylacrylamide) (PDMA-b-PNIPAM).
It was found that the morphology modification can be readily controlled by varying the polymer concentrations. The
optical absorption and fluorescence emission properties of the as-prepared PAN architectures were investigated.
Time-dependent spectra of the precipitating solution for sheaflike structures formation were measured to monitor the
self-assembly process of PAN molecules. The results showed that the PAN microstructures exhibited intense fluores-
cence emission, indicating an unusual aggregation-induced emission enhancement (AIEE) phenomenon for PAN, which
has great potential for future use in optoelectronic microdevices.
Keywords: Organic Microstructures, Block Copolymer, Self-Assembly, Reprecipitation
1. Introduction
Nano/microstructures based on small organic molecules
have attracted considerable attention during the past few
years due to the potential applications in diverse fields
such as color-tunable display, field-effect transistors,
chemical sensors, and optical waveguides [1-7]. There-
fore, the fabrication of organic nano/microstructures with
controlled sizes and shapes have been an intense and
rapidly developing field of research since the properties
are intimately related to its morphology [8-18]. Recently,
extensive efforts have been devoted to the development
of synthetic strategies for preparing organic nano/micro-
structures such as reprecipitation [19-21], solvent evapo-
ration [22], physical vapor deposition (PVD) [23], and
template-directed method [24,25]. For example, tris(8-
hydroxyquinoline)aluminum nanowires were success-
fully synthesized by adsorbent-assisted PVD [23]. Addi-
tionally, 1,4-bis(2-(5-phenyloxazolyl))benzene nanowires
were also prepared using anodic aluminum oxide (AAO)
membrane as a template [25]. However, it is still a big
challenge to develop a simple and easy-tuned route for
the fabrication of organic hierarchical architectures be-
cause of the uncontrollability of intermolecular interac-
tions and complexities of self-assembly process [18,26].
In recent articles dealing with the preparation of or-
ganic nano/microstructures, the reprecipitation method
has become one of the most popular methods because of
its easy and versatile operation [27,28]. This facile
method is based on solvent displacement by pouring mi-
croamounts of organic compound solution into ma-
croamounts of poor solvent. The sudden changes of en-
vironment then induce the self-assembly of organic
molecules. It has been reported that surfactant [24,29]
and polymer micelles [19,30] can be employed as a addi-
tive or template for the preparation of organic nano/mi-
crostructures. For instance, Qi [19] and co-worker pre-
pared uniform dye Sudan microrods with the assistance
of Pluronic F127.
In our previous paper [31], we have reported the syn-
thesis of diblock copolymer PDMA-b-PNIPAM via re-
versible addition fragmentation chain transfer (RAFT)
polymerization. The expected transition from molecu-
larly dissolved unimers at low temperature to aggregated
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures
micelles above its critical micelle temperature was ob-
served upon heating the copolymer aqueous solution.
From the viewpoint of applications, it would be greatly
beneficial if the micelles self-assembled in aqueous me-
dia may respond to the external changes. This copolymer
is well suited to act as a template for the fabrication of
organic architectures. The present work is designed to
fabricate 1-(2-pyridylazo)-2-naphthol (PAN) hierarchical
architectures by reprecipitation method. Although PAN
for analytical purposes has been investigated and widely
applied in liquid-liquid extraction separation [32-33] and
spectrometric determination of metal ions [34-36], there
is little work on the synthesis of morphology-controlled
PAN superstructures. In this work, hierarchical architec-
tures of organic PAN were prepared through a tem-
plate-assisted reprecipitation method. By tuning the tem-
plate concentrations in the reaction system, the mor-
phologies of PAN structure can be manipulated from
fibers to rods, and then to sheaflike structures. The opti-
cal properties of the prepared products were investigated
by absorption and fluorescence spectroscopy.
2. Experimental Section
2.1. Materials
PAN was recrystallized twice from EtOH. Block co-
polymer PDMA-b-PNIPAM and homo-PDMA were
synthesized by RAFT polymerization as described else-
where [31,37]. Copolymer with a molecular weight of
54,000 g/mol (denoted as DMA268-NIPAM243) was used
in this study.
2.2. Characterization
Scanning electron microscope (SEM) images were re-
corded using a field emission scanning electron micro-
scope (JEOL, JSM-6360LV), and the samples were
loaded on the mica surface, previously sputter-coated
with a homogeneous gold layer for charge dissipation
during the SEM imaging. The powder X-ray diffraction
(XRD) patterns of the samples were measured on a Japan
Rigaku D/max-2500 diffractometer with CuKa radiation
(λ = 1.5418 Å). FTIR spectra in KBr pellets were re-
corded on a PE Spectrum One FTIR spectrophotometer.
UV-Vis spectra were measured using PE Lamada 25
spectrometer. Fluorescence spectra were recorded on a
PE LS-55 luminescence spectrometer. Dynamic light
scattering (DLS) was performed on a Brookhaven In-
struments BI-200 SM equipped with a BI-APD correlator
and a crystalaser GCL-100-L operating at 532 nm with a
fixed scattering angle of 90˚, and the temperature was
controlled by a PolyScience 9102 digital temperature
controller. To ensure that DLS measurements were not
affected by dust, all solution samples were filtered
through 200 nm membrane filters.
2.3. Synthesis of PAN Hierarchical Architectures
The organic hierarchical architectures of PAN were pre-
pared through reprecipitation method. In a typical prepa-
ration, a solution of PAN in ethanol (2 mM, 400 mL) was
injected into 5 mL of aqueous DMA268-NIPAM243 solu-
tion (10 mg/mL) with vigorous stirring at 50˚C. After
stirring for 5 min, the sample was left undisturbed for
about 8 days. The resultant precipitate was centrifuged,
washed thoroughly with deionized water, and dried under
vacuum at room temperature for characterization. Repre-
cipitation was also carried out at varied DMA268-NI-
PAM243 concentrations and temperatures. For compari-
son purposes, copolymer DMA268-NIPAM243 was re-
placed by homo-PDMA for the reprecipitation of PAN.
3. Results and Discussion
3.1. Preparation of Polymeric Micelles
Polymer micelles were employed to assist the fabrication
of PAN hierarchical architectures in this study. To obtain
PDMA-b-PNIPAM micelles, aqueous solution of the
diblock copolymer DMA268-NIPAM243 with different
concentrations was raised from room temperature to their
micellization temperature. The expected transition from
molecularly dissolved unimers at low temperature to ag-
gregated micelles above their critical micelle temperature
(CMT) was observed by DLS measurements. Above the
CMT, the NIPAM segment became dehydrated due to an
entropy gain resulting from the release of water mole-
cules upon association of the isopropyl groups [38]. The
diameter of micelles was also determined by DLS. It is
found that the CMT and the micelle diameter are sensi-
tive to the concentration of the diblock copolymer. For
example, the CMTs of the DMA268-NIPAM243 aqueous
solution at concentrations of 0.5, 2.0, 5.0 and 10.0
mg/mL are approximately 37˚C, 36˚C, 34˚C and 33˚C,
respectively, while the hydrodynamic diameters of the
formed micelles are approximately 63, 58, 54 and 48 nm,
respectively (Figure 1). It is important to note that the
temperature induced association/dissociation process was
reversible over numerous heating and cooling cycles, and
micelle sizes remained approximately constant. These
observations are consistent with a recent report by
McCormick and coworkers [37]. In the following study,
the DMA268-NIPAM243 micelles were used to assist the
reprecipitation of PAN hierarchical architectures.
3.2. Reprecipitation of PAN from Aqueous
Copolymer Solution
Due to the insolubility of PAN in water, we turn to ther-
moresponsive amphiphilic PDMA-b-PNIPAM polymer
Copyright © 2011 SciRes. MSA
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures
Copyright © 2011 SciRes. MSA
Figure 4 depicts the FTIR spectrum of the as-prepared
PAN microstructures (Figure 3(b)), which was almost
identical to that of the PAN raw powders (Figure 3(a)),
suggesting that the polymer can be completely removed
by washing with water.
surfactants as a more facile means for maximizing the
solubility of PAN particles in water during assembly.
Because these aggregates are grown from particles in
micelle, the surfactants may favorably prolong their solu-
tion lifetime, and sustain growth to larger objects while
maintaining the narrow distribution. Upon injecting a 2
mM ethanolic solution of PAN into a hot solution of
diblock copolymer DMA268-NIPAM243 (10 mg/mL), and
allowing the resulting mixture to precipitate at 50˚C,
narrowly dispersed sheaf-like architectures are obtained
(Figure 2(a)). The product looks like ribbon-sheaf with
two fantails consisting of a bundle of outspread ribbons,
which are closely bonded to each other in the middle, so
we call it a “ribbon-sheaf structure”. Careful observa-
tions (Figures 2(b)-(d)) reveal that the individual rib-
bon-sheaf has a length in the range of 5 - 15 μm and a
middle diameter in the range of 0.5 - 1 μm. Interestingly,
the architectures are in fact built from ribbons with
widths of 200 - 300 nm and lengths of 5 - 15 µm, respec-
3.3. Polymeric Micelle-Templated Synthesis of
Sheaflike Architectures
As mentioned previously, PDMA-b-PNIPAM can form
The crystalline phase of the as-prepared PAN archi-
tectures was identified by powder X-ray diffraction. The
XRD patterns in Figure 3 clearly show that the PAN
product with a sheaflike structure have an identical crys-
talline phase similar to that of PAN raw powder, al-
though the precipitated products are not well crystallized.
Figure 1. Hydrodynamic diameter (Dh) versus temperature
for DMA268-NIPAM243 aqueous solution for the concentra-
tion of 0.5, 2.0, 5.0 and 10.0 mg/mL, respectively.
Figure 2. SEM images at different magnifications (a)-(d) of the PAN sheaflike architectures precipitated from 10 mg/mL of
MA268-NIPAM243 aqueous solution at 50˚C. D
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures
Figure 3. XRD patterns of (a) PAN raw powders, and (b)
PAN sheaflike structures precipitated from 10 mg/mL of
DMA268-NIPAM243 aqueous solution.
Figure 4. FTIR spectra of (a) PAN raw powders, and (b)
sheaflike architectures precipitated from 10 mg/mL of
DMA268-NIPAM243 aqueous solution. The inset shows the
chemical structure of PAN.
thermally responsive micelles. The structures of copoly-
mer micelles have been well described by the core-shell
model, in which a spherical core composed of PNIPAM
is surrounded by a shell composed of Gaussian chains of
strongly hydrated PDMA [37,39]. In this work, DMA268-
NIPAM243 spherical micelles with a diameter approxi-
mately 50 nm were used to control the reprecipitation
process of PAN molecules. The presence of DMA268-
NIPAM243 micelles was deemed to favor the formation of
PAN sheaflike architectures. This argument was demon-
strated by a series of controlled experiments. Figure 5
shows the SEM images of PAN raw powders and prod-
ucts prepared from pure water or aqueous polymer solu-
tions by reprecipitation method. It is clearly seen that the
raw powders consist of irregular plates ranging from 0.5
to 2 μm in width, and 1 - 4 μm in length (Figure 5(a)).
Upon reprecipitation from pure water at 50˚C, the pow-
ders turn to become long fibers with diameters of 150 -
220 nm and typical lengths ranging from 3 to 10 μm (Fig-
ure 5(b)). Figure 5(c) shows the sample prepared from
10.0 mg/mL of DMA268-NIPAM243 solution at 25˚C, from
which fiber-like architectures were observed. It were fur-
ther verified by another controlled experiment in which
PDMA homopolymer with equal molecular weight
(DMA535, Mn = 53000 g/mol) was employed instead of
DMA268-NIPAM243 copolymer. Not surprisingly, SEM
image shows that the morphology of the sample prepared
from aqueous DMA535 solution at 50˚C is almost identical
to that prepared from DMA268-NIPAM243 aqueous solution
at 25˚C (Figure 5(d)). These results indicated that the
PAN molecules have the tendency to self-assembly in wa-
ter, but it is difficult to prepare uniform architectures by
simple reprecipitation, therefore, it is more important to
find suitable ways for the facile fabrication of novel and
morphology-controlled organic architectures.
On the other hand, in order to elucidate the effect of
copolymer concentrations on the growth of PAN archi-
tectures, the preparations were also carried out at differ-
ent copolymer concentrations under otherwise identical
conditions. As shown in Figure 6(a), microfibers of
PAN with lengths of 20 - 50 µm were obtained at a low
polymer concentration (0.5 mg/mL), which is somewhat
similar to the sample precipitated from pure water. Inter-
estingly, one-dimensional double stranded helix archi-
tectures were also observed. Upon increasing the
DMA268-NIPAM243 concentrations to the range of 2.0 -
5.0 mg/mL, microrods with diameters about 320 - 420 nm
became predominant products, which have an average
length less than 20 µm (Figures 6(b) and (c)). However,
when the polymer concentration was increased to 8.0
mg/mL, the corresponding bundle-like architectures of
PAN can be observed from Figure 6(d). PAN sheaflike
architectures were obtained when the polymer concentra-
tion was further increased to 10.0 and 15.0 mg/mL (Fig-
ures 6(e) and (f)). These results indicate that the co-
polymer concentration is crucial for the formation of
PAN architectures. With the increase of polymer concen-
tration, the morphology of PAN architectures can be ma-
nipulated from fibers to rods, and finally to sheaflike
On the basis of the above results, it can be concluded
that the DMA268-NIPAM243 micelles play a key role in
the preparation of PAN sheaflike architectures. However,
it remains unclear how the block copolymers micelles
induce the formation of PAN fiber-like branches and the
final sheaflike architectures. In fact, similar nanoparti-
cle-based hierarchical superstructures of organic crystals
Copyright © 2011 SciRes. MSA
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures559
Figure 5. SEM images of (a) PAN raw powders, and the PAN products precipitated from (b) pure water at 50˚C, (c) 10
mg/mL of DMA268-NIPAM243 aqueous solution at 25˚C, and (d) 10 mg/mL of DMA535 aqueous solution at 50˚C.
under the direction of block copolymers have been pre-
viously observed [30,40-42]. It can also be recalled that,
a two-stage growth mechanism for the polymer-directed
synthesis of penniform BaWO4 nanostructures have been
proposed by Shi et al. [43], implying that polymer mi-
celles played a very important role in the formation of
hierarchical architectures. For example, Nguyen and
co-workers had demonstrated the growth of narrowly
dispersed porphyrin nanowires and their hierarchical
assembly into macroscopic columns with the assistance
of an amphiphilic Pluronic F127 [30,41]. In our present
study, sheaflike architectures of PAN were obtained
when the copolymer concentration was above 8.0 mg/mL.
The presence of copolymer micelles provided suitable
sites for the nucleation and growth of PAN aggregates,
inducing the formation of sheaflike PAN architectures.
Nevertheless, more detailed researches are required to
examine the growth process for the formation of PAN
hierarchical architectures under the direction of PDMA-
3.4. Time-Dependent Absorption Spectra of PAN
Products Self-Assembled from Copolymer
To better understand the growth process of PAN hierar-
chical architectures, the optical properties of both the
PAN molecules dissolved in ethanol and the as-prepared
PAN architectures dispersed in water were characterized.
Figure 7 displayed the UV-Vis absorption spectra of raw
PAN ethanol solution and the PAN sheaflike architec-
tures dispersed in water. The spectrum of PAN ethanol
solution (20 mM, curve a) exhibits a peak at 460 nm and
two shoulders at 550 nm and 398 nm, which might be
attributed to the π-π* and n-π* transitions (Figure 7(a)).
The absorption spectrum of PAN sheaflike structures
precipitated from 10 mg/mL of DMA268-NIPAM243
aqueous solution and redispersed in water is character-
ized by two peaks at 393 nm and 468 nm (Figure 7(b)).
The formation process of PAN sheaflike architectures
was studied by time-dependent absorption spectra as
shown in Figure 8. Immediately after the injection of
400 mL ethanol solution of PAN (2 mM) into the aque-
ous solution of copolymer DMA268-NIPAM243 (10
mg/mL), an absorption peak and two absorption shoul-
ders were observed at 468 nm, 562 nm and 398 nm, re-
spectively. However, with the increase of aging time,
these two absorption bands decreased gradually, accom-
panied by the final disappearance of the absorption
shoulder at 562 nm, indicating the existence of aggrega-
tion for PAN molecules, which may result from the
changed solvent polarizability and intermolecular inter-
actions between PAN molecules. Meanwhile, the base-
lines of spectra were gradually heightened with the ex-
tension of aging time, implying that the scattering from
Copyright © 2011 SciRes. MSA
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures
Figure 6. SEM images of PAN products precipitated from DMA268-NIPAM243 aqueous solutions with different polymer con-
centrations: (a) 0.5; (b) 2; (c) 5; (d) 8; (e) 10; and (f) 15 mg/mL.
larger particles could also be detected for longer aging
times. Upon further aging of the precipitating solution for
8 days, PAN sheaflike architectures were obtained (Fig-
ure 7(b)), which exhibit two absorption peaks at 393 nm
and 468 nm.
The fluorescence emission properties of both the PAN
ethanol solution and PAN sheaflike architectures redis-
persed in water were also characterized as shown in Fig-
ure 9. The PAN ethanol solution shows two emission
peaks around 330 and 350 nm when excited at 273 nm
(Figure 9(a)). Different from that of PAN ethanol solu-
tion, two main emission peaks at 332 and 423 nm are
observed in the spectrum of PAN sheaflike architectures
when excited at 273 nm (Figure 9(b)). It is important to
note that there is a strong red shift of emission peak from
350 to 423 nm.
To gain insight into the above mentioned phenomenon,
we investigated the time-dependent fluorescence emis-
sion spectra obtained after injecting 400 mL ethanol solu-
tion of PAN (2 mL) into an aqueous solution of
DMA268-NIPAM243 (10 mg/mL), which is shown in Fig-
ure 10. Immediately after the injection, three very weak
emission peaks at around 332, 423, and 484 nm were
observed. With increasing aging time, the emission in-
tensities of spectra increase gradually but the peak posi-
tions are immovable, suggesting an unusual aggregation
induced emission enhancement (AIEE) process, which is
consistent with the previous observation [44-46]. Both of
the decreasing rotations of rigid molecular planes and the
change of solvent polarity are considered to be beneficial
Copyright © 2011 SciRes. MSA
Polymer-Controlled Synthesis of 1-(2-Pyridylazo)-2-naphthol Hierarchical Architectures561
Figure 7. UV-Vis absorption spectra of (a) PAN ethanol
solution, and (b) PAN sheaflike architectures precipitated
from 10 mg/mL of DMA268-NIPAM243 aqueous solution and
redispersed in water.
Figure 8. Time-dependent absorption spectra of PAN prod-
ucts self-assembled from 10 mg/mL DMA268-NIPAM243
aqueous solution.
Figure 9. Fluorescence spectra of (a) PAN ethanol solution,
and (b) PAN sheaflike architectures precipitated from 10
mg/mL of DMA268-NIPAM243 aqueous solution and re-
dispersed in water. λex = 273 nm.
Figure 10. Time-dependent fluorescence spectra of PAN
products self-assembled from 10 mg/mL of DMA268-NI-
PAM243 aqueous solution. λex = 273 nm.
for the enhancement of fluorescence intensity in this
4. Conclusions
PAN hierarchical architectures were synthesized by
reprecipitation method with the assistance of thermore-
sponsive diblock copolymer PDMA-b-PNIPAM in this
study. By varying the copolymer concentrations, the
morphology of PAN can be readily changed from mi-
crofibers to microrods and sheaflike architectures. It is
revealed that the formation of copolymer micelles play a
important role in the growth of PAN sheaflike architec-
tures but the detailed formation mechanism should be
further investigated. The PAN sheaflike architectures
exhibit an unusual aggregation-induced emission en-
hancement (AIEE) phenomenon compared with the PAN
ethanol solution. Furthermore, the self-assembly process
for PAN molecules was investigated by the time-de-
pendent absorption spectra. The approach reported here
may provide an effective method for packing small or-
ganic molecules into desired microstructures, which may
find potential application in optoelectronic microdevices.
It is also worthwhile to explore the extension of the pre-
sent technique to the preparation of other organic hierar-
chical architectures.
5. Acknowledgements
Financial support from program for NCET (NCET-07-
0731), Key Project of Chinese Ministry of Education
(209086) and Scientific Research Fund of Hunan Provin-
cial Education Department (08B085) is greatly acknowl-
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