A rational molecular design of β-phase polydiarylfluorenes: Synthesis, morphology, and organic lasers

Article abstract of DOI:10.1021/ma402585n

Rational molecular design allows for manipulating the chain conformations of polymer semiconductors by cooperative arrangement of bulky groups with steric hindrance effect and supramolecular groups with noncovalent attractions. Herein, a model polyfluorene with β-phase, poly[4-(octyloxy)-9,9- diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF), has been synthesized successfully via key Baeyer-Villiger rearrangement reaction. Its thin film exhibited excellent spectral stability without green band emission after thermal annealing at 200 C under air and nitrogen ambients. The β-phases of PODPF in the concentrated toluene solution, organogels, and films have been characterized and confirmed by UV absorption and PL spectra as well as grazing-incidence X-ray scattering. The results suggest that the octyloxy substituents enable backbone planarization via van der Waals forces of the in-plane alkyl chains to overcome intrachain repulsion between fluorene monomers. Organic lasers using β-phase PODPF exhibit lower threshold than those of poly(9,9-dioctylfluorene), suggesting promising optical gain media. This observation suggested that supramolecular steric hindrance (SSH) is a promising molecular design of polymer semiconductors, and supramolecular steric polymers are one kind of model to get insight into the structure-function relationships for electrically pumped organic lasers in organic electronic and photonics.

Full text of DOI:10.1021/ma402585n

Article
pubs.acs.org/Macromolecules
A Rational Molecular Design of β‑Phase Polydiarylfluorenes:
Synthesis, Morphology, and Organic Lasers
Jin-Yi Lin,† Wen-Sai Zhu,† Feng Liu,‡ Ling-Hai Xie,*,†,‡ Long Zhang,† Ruidong Xia,*,† Gui-Chuan Xing,‡
and Wei Huang*,‡,†
†Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics & Information Displays
(KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing, P. R. China
‡Jiangsu-Singapore Joint Research Center for Organic/Bio- Electronics & Information Displays, Institute of Advanced Materials,
Nanjing-Tech. University, Nanjing, P. R. China
S
* Supporting Information
ABSTRACT: Rational molecular design allows for manipulat-
ing the chain conformations of polymer semiconductors by
cooperative arrangement of bulky groups with steric hindrance
effect and supramolecular groups with noncovalent attractions.
Herein, a model polyfluorene with β-phase, poly[4-(octyloxy)-
9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenyl-
fluoren-2,7-diyl] (PODPF), has been synthesized successfully
via key Baeyer−Villiger rearrangement reaction. Its thin film
exhibited excellent spectral stability without green band
emission after thermal annealing at 200 °C under air and
nitrogen ambients. The β-phases of PODPF in the
concentrated toluene solution, organogels, and films have
been characterized and confirmed by UV absorption and PL spectra as well as grazing-incidence X-ray scattering. The results
suggest that the octyloxy substituents enable backbone planarization via van der Waals forces of the in-plane alkyl chains to
overcome intrachain repulsion between fluorene monomers. Organic lasers using β-phase PODPF exhibit lower threshold than
those of poly(9,9-dioctylfluorene), suggesting promising optical gain media. This observation suggested that supramolecular
steric hindrance (SSH) is a promising molecular design of polymer semiconductors, and supramolecular steric polymers are one
kind of model to get insight into the structure−function relationships for electrically pumped organic lasers in organic electronic
and photonics.
conformation,23 resulting in an enhanced effective conjugation
INTRODUCTION
■
length,12 close molecular chain packing, and larger Forster radius
̈
Thin-film morphology of π-conjugated polymers is of great
importance to govern the fundamental exciton and carrier
of excitation transfer. The β-phase of PDAFs were widely
observed in solutions,4,24,25 thin films,26−28 nanostructures
behaviors, device performance, and stability in printed plastic
(nanoparticles29 and nanowires30,31), and gels.16,32−35 Such β-
electronics.1−3 However, it is thorny task to control over their
phase morphology was also achieved in PDAFs by variations of
morphologies owing to the conformation diversity, complex
external conditions, such as film postprocessing,28 and adding a
interchain interactions, and polymorphism phenomena.1,4,5
small amount of high-boiling-point 1,8-diiodooctane (DIO),8
Polymorphic behaviors are strongly dependent on film
nonvolatile polyphenyl ether,36 or liposome phospholipid
fabrication procedures such as film-processing methods, ink
bilayers into precursory solution.37 However, PDAFs have the
formulations, substrate types, and rheological kinetics.6−9 As one
inherent drawback with poor environmental stability, in which
typical example, poly(dialkylfluorenes) (PDAFs) exhibit various
morphologies, including amorphous phase,10,11 β-phase,12 liquid
the generation of ketone defects not only deteriorate the color
purification but also decrease the lifetime of devices.1 Therefore,
crystalline,13 semicrystalline α- and α′-phases,14,15 and network-
ing gel states.16 Among them, β-phase showed a series of the
it is significant to explore the novel β-phase polyfluorenes via
appealing features such as improved charge-carrier mobility,17
molecular design.
Up to date, many efforts have been made to investigate the
high current efficiency (CE), and excellent spectral stability in
polymer light-emitting diodes (PLEDs),9,18 efficient amplified
effects of molecular structure, such as molecular weight, alkyl
spontaneous emission (ASE), and low threshold lasers.19−21 In
substituent, and copolymerization, on the formation of β-phase
addition, β-phase morphology provides an opportunity to create
patterned thin films.19,22
Received: December 17, 2013
Revised: January 12, 2014
Published: January 30, 2014
PDAFs’ β-phase, especially poly(9,9-dioctylfluorene) (PFO),
stemmed from an intriguing “planar zigzag” (21 helix) chain
© 2014 American Chemical Society
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Scheme 1. (a) Schematic Two-Side Molecular Design Model of PODPF, (b) Fluorescent Photograph of Diluted (left) and β (right)
Phase Contained PODPF Solution, and (c) Synthetic Routes of PODPF
in polyfluorenes.4,17,38−48 It has been confirmed that the
prerequisite to achieve β-phase of PDAFs is the van der Waals
forces of side chains stronger enough to overcome the steric
repulsion and polarize polymer backbone.23 Furthermore, an
optimal chain length of PDAFs for the formation of β-phase is
regarded as an octyl side chain in PFO which is easier to form the
interdigitation packing than any other length and substituted
style.4 To date, most of examples show that the low-content β-
phase of PDAFs only keep after the introduction of chiral
groups,46 cross-linking networks,42 and surfactant zwitterion
groups.47 The delicate balances are easily destroyed by the
introduction of molecular segments. As a result, morphology-
directed molecular designs face severe challenge to achieve β-
phase homopolyfluorenes beyond PDAFs. In our previous
works, the attractive π−π stacking interactions at the cruciform-
shaped spirofluorenes have been proved to modulate the
luminescence and manipulate molecular assembly effec-
tively.49−52 Supramolecular steric hindrance (SSH)49,50 would
become one molecular design principle to control over molecular
conformation and thin film morphology in conjugated polymers.
In this work, we explored the conformational stabilization of β-
chains in polyfluorenes via the noncovalent forces at steric
hindrance. A molecular building block was designed where bulky
groups with the feature of steric repulsions and alkyl groups with
the van der Waals attractive forces are arranged into two side of
fluorene monomer, respectively (Scheme 1a). In consideration
of bulky groups with the merits of high thermal and chemical
stability, poly(diarylfluorenes) have the obvious advantages with
the more environmental stability against aggregation and
oxidation over PFO. One model polyfluorene, poly[4-
(octyloxy)-9,9- diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-
diphenylfluoren-2,7-diyl] (PODPF), has been synthesized in
which diphenyl moieties lied at the 9-positon of fluorene and the
alkyl side chains substituted at 4-position of fluorene. The β-
phase of PODPF has been obtained in solutions (Scheme 1b),
organogels, and films. It has been characterized by UV−vis
electronic absorption, photoluminescence (PL) spectra, and
grazing incidence X-ray diffraction (GIXD). Lasers from β-phase
PODPF exhibited lower threshold with respect to PFO-based
laser.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of PODPF. As a model
polymer, PODPF was prepared via Yamamoto-type polymer-
ization of key monomer that was synthesized by the Baeyer−
Villiger rearrangement reaction (Scheme 1c). Concretely,
dibromofluorenone was transformed into 3,8-dibromobenzo-
chromen-6-one (1) after the Baeyer−Villiger rearrangement
reaction at the yield of 75%. The Grignard reaction of 1 gave the
tertiary alcohol 2,4′-dibromo-2′-(hydroxydiphenylmethyl)-
biphenyl-2-ol, followed by nucleophilic substitution of 1-
bromooctane to obtain (4,4′-dibromo-2′-(octyloxy)biphenyl-2-
yl)diphenylmethanol (3). The monomers 4 were prepared by
BF3·Et2O-catalysted Friedel−Crafts reaction according to our
previous work.53 The chemical structure was fully characterized
and confirmed by nuclear magnetic resonance (NMR) and gel
permeation chromatography (GPC) (Figures S1 and S2). GPC
measurement revealed the molecular weight (Mn) of PODPF to
be 5.60 × 104 (PDI = 1.33). PODPF exhibited good solubility in
common organic solvents, such as chloroform (CHCl3) and
tetrahydrofuran (THF). The decomposition temperature (Td) is
up to 405 °C, and glassy transition temperatures (Tg) is 108 °C
(Figures S3 and S4). Cyclic voltammetry (CV) study shows the
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of
PODPF are −2.38 and −5.55 eV, respectively (Figure S5).
Optical Properties of PODPF. We investigated the optical
properties of PODPF solution and films and uncovered the
effects of film thermal postprocessing on the formation of
PODPF β-phase to examine the capability of conformational
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Figure 1. Optical properties of PODPF in various states. UV−vis (a) and PL spectra (b) of PODPF in toluene solution (0.01 mg/mL), organogels, and
films (films A and B were spin-coated from toluene and CHCl3, 10 mg/mL). UV−vis and PL spectra of PODPF film spin-coated from CHCl3 solution
after thermal annealing at 200 °C in N2 (c) and in air (d) for 0, 6, and 12 h. PL and transient decay spectra of PODPF film drop-coated from CHCl3 (e)
and toluene (f) solution (3 mg/mL).
planarization. PODPF adapted β-phase structures in solvents
such as toluene (>0.01 mg/mL), rather than in dichloromethane,
CHCl3, and THF (Figure S6), and PODPF exhibited β-phase
conformation in films spin-coated from toluene or mixed
solvents of CHCl3 and methylcyclohexane (1:2) but not in the
film spin-coated from CHCl3. Meanwhile, β-phase can also be
observed in PODPF organogels in toluene or toluene/
methylcyclohexane, which were formed using the heating−
cooling method at the concentration of 15 mg/mL at −20 °C in
toluene or 10 mg/mL in toluene/methylcyclohexane (1:1) at
room temperature (Figure S7). In addition, PODPF films adapt
the β-chain conformation and phase through thermal annealing,
as shown in Figure 1c,d. Furthermore, similar to PFO, PODPF
would self-assemble into nanowires or nanoribbons by drop-
coating from dilute toluene solution (0.01 mg/mL) (Figure S8).
These results suggested that synergic induction occur that are
favorable for β-chain conformations via the two-side balanced
introduction of bulky groups with steric repulsion and alkyl
chains with the van der Waals forces.54
the 436 nm peak at the absorption spectra of polyfluorenes) in
the absorption spectra in Figure 1a,c confirms the formation of
PODPF β-phase in concentrated toluene solutions (>0.01 mg/
mL), organogels, and films. It is easy to find that the content of β-
phase in the drop-coated film is higher than those in spin-coated
films and organogels. However, this shoulder peak was absent in
the absorption spectra of pristine films spin-coated (Figure 1a,
film B, and Figure 1c,d) or drop-coated (not shown) from CHCl3
solution, suggesting no β-phase formed in film spin- and drop-
coated from CHCl3 solution. PL spectra of PODPF in diluted
toluene solution (<0.01 mg/mL) exhibited three well-resolved
emission bands at 432, 456, and 483 nm that are assigned to the
0−0, 0−1, and 0−2 interaction singlet transition in an isotropic
phase, according to the literature (Figure S6).16 For the
formation of β-phase in organogel, there are four emission
bands at 432, 458, 486, and 515 nm. However, the peak at 432
nm was slightly quenched than those of PODPF diluted solution
(Figure 1b and Figure S7), implying energy transfer from
isotropic phase to β-phase, similar to PFO/toluene gels.16,33
Figure 1b shows that this energy transfer was significantly
stronger in the PODPF film spin-coated from toluene solution,
of which the PL spectra consisted of three peaks at 456, 486, and
518 nm similar to emission of β-phase PFO according to the
reports by Khan et al.,56 corresponding to 0−0, 0−1, and 0−2
vibration transition of β-phase. For PODPF drop-coated films
from toluene solution, there are three featured emission peaks at
Optical analyses were employed here to probe the formation
of β-phase in various states (Figure 1). The maximum absorption
peak of PODPF in various states located at 390 nm is attributed
to main-chain backbones of polyfluorenes.55 Previous study
suggested that the small peak at ∼436 nm in absorption spectra
indicated the formation of polyfluorenes β-chain phase.12 The
appearance of the shoulder peaks at 444 nm (similar position to
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486, 515, and 555 nm. With regard to low-content β-phased
films, the 0−0 vibration transition at ∼456 nm of β-phase
becomes very weak. A peak profile at 486 nm is similar to the
emission spectra of PFO/DCE gel according to our previous
report.16 The excitation spectra of PODPF β-phase films spin- or
drop-coated from toluene presented an excitation peak at 446
nm below the π−π* edge, further confirming the classical well-
resolved sharp feature of β-phase polyfluorenes (Figure S9).28
From Figure 1c,d, there was no shoulder peak at 444 nm in
absorbance spectra of the pristine PODPF film spin-coated from
CHCl3 suggesting not β-phase in the film. But the films exhibited
the featuring shoulder peak at 444 nm in the absorption spectra
and emission peaks at 458, 483, and 515 nm after thermal
annealing (either in N2 or in air), suggesting the formation of β-
phase. Moreover, the low-energy green bands at the center of
530−550 nm have no obvious changes in PL profile of PODPF
films after samples annealed at 200 °C for 12 h in air or in
nitrogen. These results suggest that PODPF have advantages
over PFO in terms of high spectral stability and antioxidation
(Figure S10). It has been observed that the β-phase
conformation gives rise to orders of magnitude enhancement
in photophysical stability in terms of increased lifetime.57 Time-
resolved PL (Figure 1e,f and Table 1) showed that the
very broad, with a slight enhancement of intensity in the out-of-
plane direction, indicating that the π−π stacking is weakly face-
on orientated. A weak hump at 0.85 A−1 (corresponding to 7.4 Å)
in the in-plane line cut profile. This most probably comes from
the alkyl chain spacing along the polymer backbone as indicated
from Figure 3g or simply the spacing of main chain repeat unit.
By using an all trans-conformation, the alkyl chain spacing was
calculated to be about 8.0 Å, the same as the length of fluorene
along the backbone,12 corresponding with GIXD results. For the
case of PODPF films spin-coated from toluene (Figure 2c,d),
there are a broad peak at Q = 1.46 A−1 (d = 4.30 Å) in out-of-
plane profiles, a peak at Q = 1.36 A−1 (d = 4.62 Å), and a peak at Q
= 0.85 A−1 (d = 7.39 Å) in in-plane profiles. The peak at 1.36 A−1
comes from the amorphous average chain spacing of the polymer
resulting from bulky diphenyl groups and distributes randomly
and overlaps with the π−π peak. For the PODPF films drop-
coated from toluene (Figure 2e,f), the structure of the material
becomes even more complicated. In the out-of-plane direction,
there is a small 0.48 A−1 lamellae spacing peak. In the high angle
region, there are peaks at 1.36 and 1.51 A−1. The 0.48 A−1
lamellae spacing enhances significantly and shows as a sharp peak
in the in-plane direction, suggesting the crystal form in face-on
orientation. The peak at Q = 0.48 A−1 (d = 13.09 Å) in the drop-
coated sample is very similar to the distinguished “β-phase”
reflection mode of PFO, showing a reduced lamellar spacing in
the crystal. The new peak of 1.51 A−1 (4.16 Å) with high intensity
in the in-plane direction is from a better stacked π−π plane. This
peak also arises from the β-phase as observed in PFO β-phase
system a distinctive 1.50 Å−1 is shown. The angular distribution
of this peak enhances in mainly in-plane direction with also a
trace of enhancement in out-of-plane direction, as shown in the
color coding in the diffratogram. These results indicate this
ordered structure takes place in both face-on and edge-on
orientation. Compared to the drop-coated sample, spin-coated
film exhibits less order of the PODPF chain. This is due to the
formation of the film being too fast in a spin-coated film, which
does not give the polymer chains enough time to dewell and
order, thus reducing the crystalline structure. The rich scattering
peaks and humps in both in-plane and out-of-plane direction
indicate there are more than one ordered structure in the toluene
drop-casted sample. It contains a weak aggregated structure
similar to β-phase. On the basis of these structural analyses, we
proposed a schematic model of PODPF with a face-on
arrangement of polyfluorene backbones in β-domain (Figure
2g,h). It should be noted that PODPF spin-coated films have β-
phase, as observed by optical characterizations, but no featuring
peak at Q = 0.48 A−1, indicating a ribbon type of assemblies made
up of enhanced π−π stacking similar to β-phase. The alky side
chains are frustrated under the fast drying process and cannot
form a good order.
Table 1. Photophysical Data of PODPF in Solution and Thin
Films
d
λabs
(nm)
T
(ps)
ΦPLQY
polymer
PODPF
solvent
toluene
states
λPL (nm)
(%)
a
solution
390
432, 456,
483
412 81
b
film
395,
444
438, 456,
482, 518
500 39
(30 )
396 80
c
a
CHCl3
solution
390
430, 452,
481
b
film
390
445, 465,
502
300 46
(33 )
c
a
b
c
Concentration of 0.01 mg/mL. Spin-coated thin films. Drop-coated
d
thin films. PFO amorphous film with ΦPLQY of about 43% at the same
conditions.
fluorescent longevity is longer for β-phased film (500 ps) than
that of the pristine film (300 ps). The PL quantum yields
(ΦPLQY) of PODPF are ∼80% in solution, ∼46% in amorphous
films, ∼39% in β-phase films, and 30% in drop-coated film, which
are equivalent to those of PFO at the same states.
Molecular Arrangements of PODPF Films. Grazing
incidence wide-angle X-ray diffraction (GIXD) was carried out
to characterize the molecular ordering and orientation of
PODPF film on Si substrates. Figures 2a,c,e display 2D GIXD
patterns of the PODPF films spin-coated from CHCl3 solution,
and spin-coated and drop-coated from toluene solution,
respectively, while Figures 2b,d,f show their corresponding out-
of-plane (qz) and in-plane (qxy) profiles. The diffusive ring with a
broad peak at Q = 1.45 A−1 (d = 4.33 Å, according to d = 2π/q) in
Figure 2a is ascribed to the π−π stacking distances from adjacent
polymer chains. This value is much larger than the prototype
P3HT layer stacking distance.58 In fact, the value is close to the
highly branching side chain substituted poly[N-9″-heptadecanyl-
2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadia-
zole)] (PCDTBT) (1.42 A−1).59 The larger π−π distance can be
explained by the steric hindrance of 9,9-diphenyl moieties at the
9-position of fluorene, which block the interchain interaction.
The angular dependence of this scattering peak (Figure 2b) is
Lasing Properties of PODPF β-Phase. It has been reported
that the β-phase of PFO exhibited efficient amplified
spontaneous emission (ASE) as a gain medium for polymer
laser. Therefore, β-phase contained PODPF film was studied to
explore its suitability as optical gain media. In order to investigate
the optical gain properties, slab waveguide samples were
fabricated and excited with the 355 nm beam from a Q-switched
Nd:YAG laser pumped optical parametric amplifier (Spectron
SL450), 10 Hz, 4.2 ns pulsed excitation. A rectangular spot of
dimensions was about 4 mm × 0.4 mm stripe for ASE
measurement. By increasing the excitation energy, we observed
ASE centered at 461 nm with a spectral half-width below 4.6 nm
as shown in Figure 3a, which is near the 0−0 vibronic transition
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Figure 2. Grazing-incidence-X-ray scattering (GIXD) images, X-ray profiles, and schematic models of PODPF films on the Si substrates deposited via
different method at room temperature: (a, b) spin-coated from CHCl3 solution; (c, d) spin-coated from toluene solution; (e, f) drop-coated from
toluene solution; (g) schematic model of in-plane chain arrangements of and (h) face-on orientation in the out-of-plane in the β-phase domain. The
regioregular structure is shown for the sake of drawing in (g) and (h), but the real structure is regiorandom.
Figure 3. (a) Amplified spontaneous emission (ASE, 461 nm) and laser spectra of a one-dimensional DFB laser comprising PODPF films. (b) Input−
output characteristics of the 1D DFB lasers at lasing wavelengths of 461 nm.
of the β-phase PODPF. In order to better quantify the ASE
behavior, we defined a threshold pulse energy (EASEth) at which
the fwhm of the emission spectrum drops to half its low
excitation pulse energy PL value. This energy is usually
accompanied by a kink in the input versus output intensity
graphs of the ASE. The ASE thresholds of PODPF for 461 nm is
251 nJ/pulse (15.69 μJ/cm2), lower than reported thresholds for
PFO film60 measured on the same experimental apparatus.
Distributed feedback (DFB) lasers were also fabricated by spin-
coating the polymer film on the grating-patterned fused silica
substrates. For a grating with period 290 nm, fill factor 25%, and
etch depth 90 nm, the lowest lasing threshold of 41.9 nJ/pulse
was achieved for a laser operating at 462 nm using a 160 nm
thickness PODPF film (Figure 3b). The results suggest that
PODPF is a promising optical gain media for using in solid state
amplifiers and lasers.
CONCLUSIONS
■
In conclusion, we demonstrated the rational molecular design of
supramolecular steric hindrance (SSH) to achieve β-phase
polyfluorene by means of the introduction of hydrophobic
interactions of the in-plane alkyl chains cross the steric
hindrance. PODPF as a model with the alkyloxy side chains at
the 4-position of fluorene was synthesized via key Baeyer−
Villiger rearrangement. Its β-phase morphology was achieved in
toluene solution, organogels, and films by spin- or drop-coating
from toluene solution as well as in amorphous thin films by the
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stability without green band emission under thermal annealing in
air and N2 at 200 °C. GIXD analysis suggested the featuring peak
at Q = 0.48 A−1 corresponds to the interdigitation distance of
13.09 Å, suggesting a possible chain arrangement of face-on
packing style relative to the silicon substrates. Organic lasers of
PODPF exhibited low threshold that will be potential optical
gain media toward electrical injection lasers. Supramolecular
steric hindrance will be a promising molecular design principle in
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monomer and PODPF, together with their NMR spectra,
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AUTHOR INFORMATION
Corresponding Authors
*E-mail iamlhxie@njupt.edu.cn (L.H.X.).
*E-mail iamrdxia@njupt.edu.cn (R.D.X.).
*E-mail iamwhuang@njupt.edu.cn (W.H.).
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(24) Knaapila, M.; Dias, F. B.; Garamus, V. M.; Almas
́
y, L.; Torkkeli,
The project was supported by the National Natural Science
Funds for Excellent Young Scholar (21322402), National
Natural Science Foundation of China (21274064, 21144004,
61376023, 61136003, 60876010, 61177029, 51173081), The
Program for New Century Excellent Talents in University
(NCET-11-0992), Doctoral Fund of Ministry of Education of
China (20133223110007), Excellent science and technology
innovation team of Jiangsu Higher Education Institutions
(2013), Natural Science Foundation of Jiangsu Province,
China (BK2011761, BK2008053, SJ209003, BM2012010),
Program for Postgraduates Research Innovation in University
of Jiangsu Province (CXLX11_0413, CXLX11_0423), and
Natural Science Foundation of Nanjing University of Posts and
Telecommunications (Grant No. NY2012013). Project was
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions, PAPD (YX03001).
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