Synthesis and properties of high-performance functional polyimides containing rigid nonplanar conjugated tetraphenylethylene moieties

Article abstract of DOI:10.1002/pola.26498

A simple diamine (TetraPEDA) containing rigid nonplanar conjugated tetraphenylethylene (TetraPE) moieties was designed and synthesized through Wittig-Horner and Suzuki coupling reactions. Four kinds of high-performance functional polyimides (PI) were thus

Full text of DOI:10.1002/pola.26498

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Synthesis and Properties of High-Performance Functional Polyimides
Containing Rigid Nonplanar Conjugated Tetraphenylethylene Moieties
Yiwu Liu, Yi Zhang, Qi Lan, Zexin Qin, Siwei Liu, Cunyuan Zhao, Zhenguo Chi, Jiarui Xu
PCFM Lab and DSAPM Lab, Materials Science Institute, State Key Laboratory of Optoelectronic Materials and Technologies,
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
Correspondence to: Y. Zhang (E-mail: ceszy@mail.sysu.edu.cn) or J. R. Xu (E-mail: xjr@mail.sysu.edu.cn)
Received 25 September 2012; accepted 16 November 2012; published online 18 December 2012
DOI: 10.1002/pola.26498
ABSTRACT: A simple diamine (TetraPEDA) containing rigid non-
planar conjugated tetraphenylethylene (TetraPE) moieties was
designed and synthesized through Wittig–Horner and Suzuki
coupling reactions. Four kinds of high-performance functional
polyimides (PI) were thus prepared by the polymerization of Tet-
raPEDA and four dianhydrides, respectively. Because of the intro-
duction of the aromatic rigid nonplanar TetraPE structure, the PI
exhibited special fluorescent characteristics, as the maximum flu-
orescence emission of the four PI was observed at 425–505 nm
in NMP solution and at 470–491 nm in film state. Also these
organo-soluble PI showed outstanding properties, such as low
dielectric constant (even without fluorinated substituent), light
color, high glass transition temperatures (382–443 ꢀC) and
thermal stability in air (Td5% up to 565 ꢀC), and excellent mechani-
cal properties. The polymer memory devices with the configura-
tion of indium tin oxide/PI/aluminum (ITO/PI/Al) exhibited distinct
volatile memory characteristics of static random access memory,
with an ON/OFF current ratio of 1 ꢁ 104 to 1 ꢁ 105. These func-
tional PI showed attractive potential applications in the field of
high performance flexible polymer photoconducting devices or
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fluorescent polymer memory devices. 2012 Wiley Periodicals,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1302–1314
KEYWORDS: fluorescent; functional polyimide; functionalization
of polymers; polyimides; polymer memory devices; synthesis;
tetraphenylethylene
INTRODUCTION Polyimides (PI) are widely used in micro-
electronics and aerospace industries as materials for elec-
tronic packaging and electrical insulation, because of their
excellent thermal and thermo-oxidative stability, solvent and
chemical resistance, electrical, and mechanical properties.1–3
Functional PI and their applications are emerging research
areas in recent years. Their application in the field of flexible
electronics and displays,4 flexible and thin film photovoltaics
(PV),5,6 xerography,7,8 light-emitting diodes,9 and so forth
have been explored and attracted growing interest, especially
in the field of polymer memories devices.10 Kang,11,12
Hsiao,13 and Ree14–15 have developed series of functional PI
containing both electron-donor (D) and electron-acceptor (A)
in the same macromolecule, which showed typical resistive
switching behaviors and significant advantages over inor-
ganic silicon- and metal-oxide-based memory materials. Com-
pared to other polymer memory materials, functional PI ex-
hibit not only the advantages of rich structure, flexibility,
low-cost, and three-dimensional stacking capability, but also
excellent thermal stability, dimensional stability, and favor-
able physical and chemical properties. However, these appli-
cations are limited by the infusible and insoluble nature and
dark color (yellow to brown) of the PI, which mainly result
from the rigid backbones and strong interchain packing, as
well as the formation of charge transfer complex (CTC)
between the electron-donating diamine moiety and the elec-
tron-accepting dianhydride moiety in polyimide molecular
chain.16–17 In this regard, many structural modifications of
the polymer backbone have been utilized to modify the
properties of PI,18–20 mainly focusing on the organo-solubil-
ity,21 colorlessness, and high transparency.22 Considerable
efforts have been made by introducing flexible linkages, fluo-
rinated substituent, unsymmetrical units or bulky pendent
groups into the polymer backbones.23–28 Among these strat-
egies, introduction of fluorinated substituent such as hexa-
fluoroisopropylidene linkages, perfluoroalkyl groups, and
pendent trifluoromethyl groups has been considered as one
of the most promising methods. However, its application is
limited because fluorochemicals are too expensive to be
widely used. Strategies of introducing flexible linkages,
unsymmetrical units, or bulky pendent groups have shown
certain benefits to improve the solubility and color of the
polymers, but usually led to reduction of thermal stability
and mechanical properties of PI.
The tetraphenylethylene (TetraPE) motif has attracted con-
siderable attention by virtue of its rich electrochemical29,30
Additional Supporting Information may be found in the online version of this article.
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bistable conductance switching effects, applicable to data
storage, such as polymer memory devices.45 Finally, the rigid
nature of this hexaphenylethylene structure is expected to
maintain a relatively high glass transition temperature, excel-
lent thermal stability, and mechanical properties. Such kind
of design philosophy can also be extended to the design and
synthesis of other high performance functional polymers,
such as aromatic polyamides, polyesters, and so forth, to
obtain good solubility and processability while maintaining
their excellent overall properties.
EXPERIMENTAL
Materials
Diphenylmethane, n-butyllithium, 4-amino-phenylboronic
acid, tetrakis(triphenylphosphine) palladium, 4,40-dibromo-
benzophenone, and Aliquat 336 (tricaprylyl-methylammo-
nium chloride) were purchased from Alfa-Aesar company
and used as received. 3,30,4,40-biphenyltetra-carboxylic acid
dianhydride (BPDA), 3,30,4,40-benzophenone-tetracarboxylic
dianhydride (BTDA), 4,40-(hexafluoroiso-propylidene) diph-
thalic anhydride (6-FDA), and pyromellitic dianhydride
(PMDA) were purchased from National Pharmaceutical
Group Chemical Reagent Co., and were heated at 140 ꢀC
under vacuum for 12 h prior to use. Analytical grade di-
methyl formamide (DMF) was purified by distillation under
an inert nitrogen atmosphere. Tetrahydrofuran (THF) was
distilled from sodium/benzophenone. All other solvents and
reagents as analytical grade were purchased from Guangzhou
Dongzheng Company and used without further purification.
FIGURE 1 3D structure of TetraPEDA and its polyimide.
and excited state properties31,32 as well as its extensive
usage as an electron transfer catalyst in a variety of polymer-
ization,33 aggregation enhanced emission.34 In TetraPE, an
iconic aggregation-induced emission (AIE) chromophore, fast
rotation of the phenyl rings and partial twisting of the C¼¼C
bond quench its fluorescence in dilute solutions.35–37 In Tet-
raPE aggregates, short intermolecular interactions obstruct
the rotation of the phenyl groups and permit deactivation by
fluorescence. The unique luminescence behavior of TetraPE
and other rotors has been harnessed for the development of
biological sensors,38 solid-state lighting materials,39,40 and
luminescent polymers.35–37,41,42 Derivatives of TetraPE have
been incorporated into organic polymers and oligomer with
the resulting materials displaying fluorescence profiles that
can be perturbed according to the overall density of TetraPE
units within the polymer/oligomer and the environment sur-
rounding the bulk material.43,44
Instrumentation
All nuclear magnetic resonance spectra (NMR) were
recorded at room temperature using a Varian Unity Inova
1
500 NB spectrometer, operating at 499.774 MHz for H with
indirect detection probe and at 125.682 MHz for 13C with
broadband probe. Samples were prepared as solution of 5–
15 mg of each compound in 0.7 mL of deuterated dimethyl
sulfoxide (DMSO), using tetramethylsilane (TMS) as the in-
ternal reference. Inverse gated decoupling was employed for
quantitative carbon nuclei intensities at pulse angle of 248,
acquisition time of 0.45 s, and relaxation delay of 2.9 s for
about 5000 scans. Two-dimensional spectra were obtained
by using standard Varian software and the parameters used
were as follows: COSY spectra were collected with four tran-
sients for each of 256 increments and the spectral windows
were 4096.5 Hz in both dimensions. Mass spectra were
measured using a Thermo MAT95XP-HRMS spectrometer.
Elemental analysis was carried out on a CHNS Elemental An-
alyzer. Infrared spectra were recorded by a BRUKER TENSOR
27 Fourier-transform infrared (FTIR) spectrometer. Gel per-
meation chromatography (GPC) was carried out on a Waters
717 plus autosampler and Waters 1515 Isocratic HPLC
Pump. Two Waters columns, connected in series, were used
with DMF as the eluent and calibrated with polystyrene
standards having narrow polydispersity. UV–vis (UV) absorp-
tion spectra were recorded on a Hitachi UV–vis spectropho-
tometer (U-3900). Fluorescence spectra were measured
using a Shimadzu RF-5301PC spectrometer with a slit width
In this work, a novel diamine (TetraPEDA) containing Tet-
raPE moiety was designed and synthesized through Wittig–
Horner and Suzuki coupling reactions. The 3D chemical
structure of TetraPEDA and its polyimide are shown in
Figure 1. The TetraPEDA has a special twisted steric struc-
ture (as shown in the red circle), where the four benzene
rings are nonplanar, and the direct coupling of the six ben-
zene rings through the ethylene group builds the large con-
jugated structure. When such kind of structure is introduced
into the polyimide main chain, the nonplanar steric structure
minimizes the strong interaction and tight stacking of the
molecular chains. As a result, the increase in free volume of
the polymers favors solubility improvements in organic sol-
vents as well as lowering of the dielectric constant. In addi-
tion, the large nonplanar conjugated structure may evoke
special photoelectric properties, affording
a convenient
method to build novel functional PI. With TetraPEDA moi-
eties as electron donors (D) and the dianhydride moieties as
electron acceptors (A), the polyimide D–A molecules may
build electronic transition between the ground and excited
states and could be manipulated by the induced charge
transfer (CT) from donor (D) to acceptor (A) under applied
electric fields. For this reason, the PI are expected to exhibit
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of 5 nm for PI and 3 nm for monomer respectively, for emis-
sion on 365 nm. Cyclic voltammetry (CV) measurements
were carried out on a Shanghai Chenhua electrochemical
workstation (CHI660C) in a three-electrode cell with a Pt
disk working electrode, an Ag/AgCl reference electrode, and
a glassy carbon counter electrode. All CV measurements
were performed under an inert argon atmosphere with sup-
porting electrolyte of 0.1 M tetrabutyl-ammonium perchlo-
rate (n-Bu4NClO4) in N-methyl-2-pyrrolidone (NMP) at a
scan rate of 100 mV/s using ferrocene as standard. The
highest occupied molecular orbital (HOMO) energy levels
were obtained using the onset oxidation potentials from the
CV curves. The lowest unoccupied molecular orbital
(LUMO)/HOMO energy gaps (DEg) for the compounds were
estimated from the onset absorption wavelengths of the UV
absorption spectra. Thermogravimetric analyses (TGA) were
performed with a TA thermal analyzer (Q50) under N2/air
(flowing rate of 40 mL/min) at a heating rate of 20 ꢀC/min
from 30 to 1000 ꢀC. Differential scanning calorimetry (DSC)
curves were obtained with a NETZSCH thermal analyzꢀer
mixture was extracted with dichloromethane. The organic
layer was evaporated after drying with anhydrous sodium
sulfate and the resulting crude product was dissolved in tol-
uene (100 mL). Then p-toluenesulfonic acid (2.00 g, 11.6
mmol) was added, and the mixture was refluxed overnight
at 110 ꢀC under argon and cooled to room temperature.
Finally, the mixture was concentrated and the crude product
was purified by silica gel column chromatography using n-
hexane as eluent to yield a white powder (9.17 g, 86%). 1H
NMR (500 MHz, CDCl3, d): 6.84–6.88(d, 4H), 6.98–7.01(m,
4H), 7.10–7.13(m, 6H), 7.20–7.23 (d, 2H); 13C NMR (125
MHz, CDCl3, d): 120.69, 126.86, 127.89, 130.97, 131.10,
132.89, 138.35, 142.15, 142.26, 142.99; IR (KBr): m ¼ 698
(CABr stretching), 1495 (CAC stretching), 1097–702 cmꢂ1
(ArAH stretching); HRMS (ESI, m/z): [M þ H]þ calcd for
C26H18Br2: 490.23; found, 490. Anal. calcd for C26H18Br2: C
63.7, H 3.7, N 0.000; found: C 63.35, H 3.735, N 0.000.
Synthesis of (40,400-(2,2-Diphen-ylethene-1,1-diyl)dibi-
phenyl-4-amine) (TetraPEDA)
TetraPEDBr (7.3535 g, 15 mmol)) and 4-aminophenylboronic
acid (5.202 g, 30 mmol) were dissolved in THF (100 mL),
and then 2 M aqueous K2CO3 aqueous solution (45 mL) and
10 drops of Aliquat 336 were added. The mixture was
stirred for 45 min under argon at room temperature. Then
tetrakis(triphenyl-phosphine) palladium (Pd(Pph3)4) catalyst
(catalytic amount) was added in and the reaction mixture
was stirred at 75 ꢀC for 24 h. After being cooled to room
temperature, the product was concentrated and purified by
silica gel column chromatography using dichloromethane/
n-hexane (vol/vol ¼ 2/1). Yield of the product was about
85%. 1H NMR (500 MHz, DMSO-d6, d): 5.18 (s, 4H), 6.58–
6.63(d, 4H), 6.96–6.99(d, 4H), 7.00–7.03(d, 4H), 7.01–
7.11(m, 2H), 7.11–7.15(t, 4H), 7.29–7.34 (m, 8H); 13C NMR
(125 MHz, DMSO-d6, d): 114.12, 124.27, 126.31, 126.53,
126.79, 127.74, 130.63, 131.17, 138.39, 139.74, 140.15,
140.50, 143.52, 148.26; IR (KBr): m ¼ 1495 (CAC stretching),
1097–702 (ArAH stretching), 3037 (CAH stretching), 1285
(CAN stretching), 1617 (d NAH), 3368 cmꢂ1 (NAH stretch-
ing); HRMS (ESI, m/z): [M þ H]þ calcd for C38H30N2: 514.66;
found: 514. Anal. calcd for C38H30N2: C 88.68, H 5.876, N
5.44; found: C 88.15, H 5.962, N 5.508.
ꢀ
(DSC 204) at a heating rate of 25 C/min from 30 to 450 C
under flowing nitrogen. The glass transition temperatures
(Tg values) were taken from the second heating trace after
rapid cooling from 450 ꢀC at a cooling rate of 40 ꢀC/min
and read at the midpoints of the transitions in the heat
capacity. The dynamic mechanical (DMA) spectra of the sam-
ples were obtained by using NETZSCH DMA 242. The speci-
mens were analyzed in tensile mode at a constant frequency
of 1 Hz, static force of 0.1 N, dynamic force of 0.11 N, and a
temperature range from 30 to 480 ꢀC at a heating rate of 5
ꢀC/min. Tensile test was performed on samples cut from 35
to 40 mm thick sheet and tested using U-CAN UT-2060 (Tai-
wan, China) instrument according to ISO standard 37-2005.
The specimen size was 6 ꢁ 100 mm2 and Jaw separation
was 50 mm. Jaw speed was first set at 2 mm/min, and then
changed to 20 mm/min when elongation reached 1 mm. The
dielectric constant was measured at frequencies between
100 Hz and 1 MHz at 20 ꢀC and 50% relative humidity,
using Solartron SI 1260 Impedance/Gain Phase Analyzer to-
gether with two custom-built copper electrodes (10.2 ꢁ 10.2
mm2). The amplitude of the a.c. electric field was about 1 V/
cm. The samples were thin films in size of 12 ꢁ 12 mm2.
Silver paste was coated onto both side surfaces of the PI
film to ensure excellent contact between the electrodes and
PI film.
Synthesis of Polyimides
Synthesis of polyimide TetraPEDA-6FDA (PI-6FDA) was used
as an example to illustrate the general synthetic route. To a
solution of TetraPEDA (1.0293 g, 2 mmol) in 11.8 mL of puri-
fied DMF in a 50-mL flask, 6-FDA (0.8885 g, 2 mmol) was
added in, achieving a solid content of approximately 15 wt
%. The mixture was stirred at room temperature under ar-
gon for about 6 h to form a viscous poly(amic acid) (PAA) so-
lution. The weight average molecular weight (Mw) and the
polydispersity index (PDI) of the resultant PAA (PAA-6FDA)
estimated from GPC were 3.29 ꢁ 105 and 1.58, respectively,
measured in DMF at a concentration of 4 mg/mL at 50 ꢀC.
The PAA was subsequently coated uniformly on a clean and
dry glass plate with a controlled film thickness, and then
thermally imidized in a vaꢀcuum oven wꢀith temperature pro-
Synthesis of 1-Bromo-4-(1-(4-bromophenyl)-2,2-
diphenylvinyl) benzene (TetraPEDBr)
Diphenylmethane (6.18 mL, 36.8 mmol) and anhydrous tet-
rahydrofuran (100 mL) were mixed in a 500-mL flask and
ꢀ
continuously stirred for 40 min under argon at 0 C. A 2.2 M
solution of n-butyllithium in hexane (21.7 mL, 47.74 mmol)
was added into the flask. After stirring for 40 min at that
temperature, 4-(4-bromophenyl) benzophenone (10 g, 21.7
mmol) was added and the reaction mixture was stirred for
another 10 h allowing the temperature to rise gradually to
room temperature. Then the reaction was quenched with an
aqueous solution of ammonium chloride (100 mL) and the
ꢀ
ꢀ
gram of 100 C (1 h)/200 C (1 h)/300 C (1 h)/350–400 C
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Fabrication and Characterization of the Memory Devices
The memory devices were fabricated on an indium-tin oxide
(ITO)-coated glass, with the configuration of ITO/PI/Al.
Before the fabrication, the ITO glass was precleaned by ultra-
sonication with water, acetone, and isopropanol each for 20
min. A PAA solution in DMF (10 mg/mL) was spin-coated
onto the clean ITO glass at a spin speed of 2000 rpm for 45
s and then thermal imidization was performed as mentioned
above. The thickness of the thin film was determined to be
around 80–110 nm. Finally, an 80-nm-thick Al top electrode
was thermally evaporated through the shadow mask
(recorded device units of 0.2 ꢁ 0.2 mm2 in size) at a pres-
sure of 10ꢂ7 Torr with a uniform depositing rate of 3–5 Å/s.
The electrical characterization of the memory device was
performed by a Keithley 4200-SCS semiconductor parameter
analyzer equipped with a Keithely 4205-PG2 arbitrary wave-
form pulse generator. ITO was used as the cathode, and Al
was set as the anode during the voltage sweep. The probe
tip was a tungsten wire (10 lm in diameter) attached to a
tinned copper shaft with a point radius < 0.1 lm (GGB
Industries, Inc.).
SCHEME 1 Synthesis routes of TetraPEDA.
(1 h) to produce PI (TetraPEDA-6FDA). The polyimide film
was removed from the glass substrate after the oven cooled
1
to room temperature. H NMR (500 MHz, DMSO-d6, d): 7.05–
7.09(d, 4H), 7.10–7.19(m, 10H), 7.47–7.57(m, 8H), 7.70–
7.77(d, 4H), 7.77–7.81(m, 2H), 7.92–7.97(d, 2H), 8.11–8.18
(d, 2H); IR (KBr): m ¼ 1776 and 1718 (CO stretching), 1495
(CC stretching), 1097–702 cmꢂ1 (ArAH stretching); Anal.
calcd for C57H32F6N2O4: C 73.58, H 4.013, N 3.205; found: C
74.18, H 3.495, N 3.035. All of the other PI (PI-BTDA, PI-
BPDA, and PI-PMDA) were synthesized by using a procedure
similar to that described above.
Molecular Simulation
Molecular simulations of the PI were carried out with the
Gaussian 03 (revision E.03) program package. Equilibrium
ground state geometry and electronic properties of basic
unit of the PI were optimized by means of the density func-
tional theory (DFT) method at the B3LYP level of theory
FIGURE 2 (a) 1H NMR, (b) 13C NMR, (c) H-H COSY, (d) C-H QC, and (e) C-H BC spectra of TetraPEDA in DMSO-d6.
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TABLE 1 Thermal and Photophysical Properties of TetraPEDA
Td (ꢀC)b
k
(nm)
em
max
Compd
Tg (ꢀC)a 5% 10%
k
(nm)c THFd Powder
abs
max
TetraPEDA 144
368 389
393
485
463
a
Measured by DSC at a heating rate of 25 ꢀC/min.
Measured at a heating rate of 20 ꢀC/min under N2.
TriPEDA concentration of 1 ꢁ 10ꢂ5 mol/L in THF.
b
c
d
Measured in THF solution at a concentration of 1 ꢁ 10ꢂ5 mol/L.
(Beckes-style three-parameter DFT using the Lee-Yang-Parr
correlation functional) with the 6-31 G(d) basic set.
RESULTS AND DISCUSSION
Synthesis and Characterization of TetraPEDBr and
TetraPEDA
The synthesis of TetraPEDA is illustrated in Scheme 1, where
dibromide (TetraPEDBr) is prepared under Wittig–Horner
reaction conditions from the reaction of diphenylmethane
with 4,4-dibromobenzo-phenone, followed by the Suzuki
reaction with 4-aminophenyl-boronic acid to yield the
desired diamine TetraPEDA. 1H NMR and 13C NMR, mass
spectra, FTIR, and elemental analysis were employed to iden-
tify the structures of the synthesized dibromide intermediate
SCHEME 2 Synthesis of polyimides.
resonance signals at around 5.2 ppm corresponding to the
amino protons.
The thermal and photophysical properties of the synthesized
compound TetraPEDA (Table 1) analyzed by TGA and DSC
revealed that TetraPEDA had good thermal stability. The 5%
and 10% weight-loss temperatures of TetraPEDA in nitrogen
were 368 and 389 ꢀC, respectivꢀely, and its glass transition
temperature (Tg) was about 144 C.
1
product and diamine monomer. The H and 13C NMR spectra
of TetraPEDBr are shown in Supporting Information Figure
S1, whereas panels (a) and (b) in Figure 2 illustrate the 1H
and 13C NMR spectra of TetraPEDA. Assignments of each car-
bon and proton were assisted by the two-dimensional (2D)
COSY, CAH QC, and CAH BC NMR spectra shown in Figure
2(c–e). The spectral results were in agreement with the pro-
posed molecular structure of TetraPEDA, and the 1H NMR
spectra confirmed that the diamine was successfully synthe-
sized by the mentioned three-step reactions by the
Figure 3 shows the photoluminescence (PL) spectra of Tetra-
PEDA (10ꢂ5 M) in THF solution with varying amounts of
water (vol %). Because of the rigid aromatic non-coplanar
3D structure and large conjugated properties, the novel dia-
mine displayed the expected photophysical properties and
remarkable aggregation-induced emission enhancement
(AIEE) properties.46,47 The maximum UV absorption wave-
length of TetraPEDA was 393 nm in THF, which can be
attributed to the p–p* transition of the p electronic system
delocalized along the hexaphenylethylene conjugated moi-
eties. When TetraPEDA was excited at the maximum absorp-
tion wavelength, its maximum PL emission in THF solution
and powder state appeared at wavelength of 485 and at 463
nm, respectively, indicating a significant blue-shift in the
maximum emission for powder TetraPEDA compared to Tet-
raPEDA in solution. The PL intensity of the TetraPEDA
increased with the addition of water (poor solvent for Tetra-
PEDA). It reached the maximum when water content was
90%. However, the overall PL intensities of TetraPEDA (1 ꢁ
10ꢂ5 M) in THF with addition of water were much higher
than that without the addition of water, suggesting the AIEE
properties of TetraPEDA.
FIGURE 3 PL spectra of TetraPEDA (1 ꢁ 10ꢂ5 M) in THF with
varying amounts of water (vol %). (The inset depicts the
changes of PL peak intensity with water fraction, and the pic-
tures are UV irradiation excited at 365 nm.)
Synthesis and Characterization of the Polyimides
Four kinds of PI were prepared in a conventional two-step
procedure by the reaction of equal molar amounts of
diamine TetraPEDA with commercially available aromatic
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FIGURE 4 (a) Chemical structure of PI-6FDA, (b) 1H NMR, and (c) H-H COSY spectra of PI-6FDA in DMSO-d6.
tetracarboxylic dianhydrides (PMDA, BPDA, BTDA, and
6FDA) in DMF to form precursor PAAs, followed by thermal
cyclodehydration, as shown in Scheme 2. The chemical struc-
tures of the PI derived from TetraPEDA were confirmed by
FTIR spectra in Supporting Information Figure S2. Compared
to the spectra of TetraPEDA, the characteristic absorption at
3368 and 1617 cmꢂ1 (NAH stretching) disappeared in the
spectra of PI, meanwhile the characteristic absorption peaks
of the imide group at 1776 cmꢂ1 (asymmetrical stretching of
carbonyl) and 1718 cmꢂ1(symmetrical stretching of
TABLE 2 Molecular Weight of the PAAs and the Solubility Behaviors of the Corresponding Polyimides
GPCa
Solventb
DMF
Polymer
Mw ꢁ 105
PDI
NMP
DMAc
DMSO
m-Cresol
PI-6FDA
PI-BPDA
PI-BTDA
PI-PMDA
Kapton
a
3.29
2.68
3.46
2.31
1.58
1.87
1.60
1.93
þþ
þþ
þ
þþ
ꢂ
þþ
þ
þþ
þꢂ
þ
þþ
þþ
þ
þꢂ
ꢂ
þ
þꢂ
þꢂ
þꢂ
ꢂ
þ
þ
ꢂ
þ
ꢂ
With respect to polystyrene standards, DMF as the eluent at flow rate
of 1 mL/min and test temperature of 50 ꢀC.
heating (90 ꢀC); ꢂ, insoluble even on heating. The solubility was deter-
mined by adding 10 mg of PI in 1 mL of solvent.
b
Solubility: þþ, Soluble at room temperature; þ, partially soluble at
room temperature, soluble on heating(90 ꢀC); þꢂ, partially soluble on
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TABLE 3 Thermal Properties of the PIs
Td5% (ꢀC)
Td10% (ꢀC)
N2 Air
Tg
Tg
Char Yield
(wt %)c
Polymers (ꢀC)a (ꢀC)b N2
Air
PI-6FDA
PI-BPDA
PI-BTDA
PI-PMDA
a
371
408
377
434
382
422
390
443
540 534 559 554 69
567 565 584 588 69
546 542 567 571 67
546 516 571 555 64
Measured by DSC at a heating rate of 25 ꢀC/min.
Measured by DMA at 1 Hz and at a rate of 4 ꢀC/min.
Residual weight percentage at 800 ꢀC in nitrogen.
b
c
carbonyl), and the CAN bond at around 1373 cmꢂ1 were
clearly shown. This indicated the successful reaction
between TetraPEDA and the four dianhydrides, and the com-
plete imidization reaction of the PAA. Moreover, the desired
formation of all polymers was confirmed by 1H NMR, and
the spectra of PI-6FDA as an example are shown in Figure 4.
FIGURE 5 DMA curves of the PI films.
The thermal properties of the PIs were investigated by TGA,
DSC, and DMA, and the results of TGA analyses (Table 3 and
Supporting Information Fig. S3) revealed that all the poly-
mers exhibited good thermal stability with insignificant
weight loss up to 500 ꢀC in nitrogen or air. The 5% and
10% weight-loss temperatures (Td5% and Td10%) of the poly-
mers in nitrogen were in the range of 571–592 and 594–610
ꢀC, respectively. The amount of carbonized residue (char
yield) of these polymers in nitrogen atmosphere was more
than 69% at 800 ꢀC, and the high char yields can be
ascribed to the high aromatic content of the polymer. The Tg
values of the polymers (Table 3) measured from DSC ther-
mograms were in the range of 371–434 ꢀC, or measured
The molecular weight of the PAAs was determined by GPC
and the qualitative solubility behavior of the corresponding
PI is summarized in Table 2. The Mw of the PAAs ranged
from 2.31 ꢁ 105 to 3.46 ꢁ 105, with PDIs ranging from 1.58
to 1.93, indicating the synthesized PI had a very high molec-
ular weight and a narrow molecular weight distribution. It is
well-known that aromatic PIs generally show poor solubility
in organic solvents, especially for those derived from rigid
monomers, such as PMDA, BTDA, BPDA, PPD, and ODA. As
shown in Table 2, commercial aromatic polyimide (Kapton
film) was insoluble in all the test solvents even on heating,
while the four synthesized PI were soluble in polar solvents
such as N-methylpyrrolidone (NMP), dimethylacetamide
(DMAc), DMSO, DMF, and m-cresol, even without fluorinated
substituent. The enhanced solubility can be attributed to the
introduction of the twisted nonplanar conjugated TetraPE
group into the main chain of polyimide macromolecule. Such
a structure may increase the steric hindrance moieties, and
thus reduce the interactions between the molecular chains.
The excellent solubility makes these polymers as potential
candidates for practical application in microelectronics by
spin- or dip-coating processes.
ꢀ
from DMA curves (Fig. 5) were ranged 382–443 C. The dif-
ferent values of Tg measured by DSC and DMA may be attrib-
uted to the different heating stories and the distinct natures
of these two testing methods. The thermal stability and Tg of
the synthesized PI are much higher than those of the
reported Al-PIs (Td5%: 425–457 ꢀC, Tg: 220–322 ꢀC),48
because of the aromatic nature of TetraPEDA. Moreover, all
the polymers showed no clear melting endothermic peaks
up to the decomposition temperatures on their DSC thermo-
grams (not shown here), and this is in agreement with
the wide-angle X-ray diffraction (WAXD) results (see Sup-
porting Information Fig. S4), demonstrating the amorphous
TABLE 4 Optical Properties of the Polyimides
In solutions (nm)a
Films (nm)
b
b
abs
max
abs
onset
PL
max
c
abs
max
abs
onset
PL
max
d
Polyimide
k
k
k
k0
k
k
k
Eg
PI-6FDA
PI-BPDA
PI-BTDA
PI-PMDA
a
321
309
342
362
383
391
393
403
425
426
505
494
418
427
432
430
412
424
425
424
456
472
504
520
471, 483, 493
470, 483, 493
473, 484, 494
471, 483, 493
3.24
3.17
3.15
3.08
Polymer concentration is 1 ꢁ 10ꢂ2 mg/mL in NMP.
Calculated from polymer solution (1 ꢁ 10ꢂ2 mg/mL in NMP) using the
d
b
c
Excited at absmax in both solid and solution states.
equation: Eg ¼ 1,240/konset
.
The cutoff wavelengths (k0) from the transmission UV–vis absorption
spectra of polymer films (thickness about 10 lm).
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FIGURE 7 Absorptivity of the PIs in NMP solution (1 ꢁ 10ꢂ2
FIGURE 6 UV–vis transmission spectra of the PI films in thick-
ness of ꢃ10 lm.
mg/mL).
nature of these PI based on the rigid nonplanar conjugated
TetraPE moieties.
effect and thus effectively suppress the formation of intermo-
lecular CTC of polymer chains.
Mechanical properties of functional PI are rare reported in
the literatures, maybe due to the brittle characteristic. How-
ever, the PI in this work could afford flexible and tough films
via solution casting and these films were subjected to tensile
testing, yielding the tensile strength and tensile modulus in
the range of 85.6–108.5 MPa and 1.9–2.9 GPa, respectively
(Supporting Information Table S1). The excellent mechanical
properties are of vital importance to the practical application
of these functional PI. Additionally, in comparison with the
commercial Kapton films (dielectric constant of 3.49), the PI
showed low electrical properties with the dielectric constant
in the range of 2.58–3.11 (Supporting Information Fig. S5).
The high Tgs, excellent thermal stability, favorable mechanical
properties, and electrical properties of the soluble PIs are
probably due to the aromatic rigid nonplanar TetraPE struc-
ture in the PI main chains, and are expected to meet the
requirements of heat resistance, high processing, and appli-
cation temperatures in the microelectronics industry.
The absorptivity spectra of PI measured in NMP solution (1
ꢁ 10ꢂ2 mg/mL) exhibited the maximum absorption wave-
length (kambasx) at around 309–362 nm (Fig. 7), which can be
attributed to the p–p* transition of the p electronic system
delocalized along the hexaphenylethylene conjugated moi-
eties. The absorption edge of the PI extended to about 383–
403 nm, from which the band gaps (Eg, the difference
between HOMO and LUMO) of the four PI were estimated at
3.24, 3.17, 3.15, and 3.08 eV, respectively (Table 4). Com-
abs
pared to the diamine TetraPEDA (k
393 nm), the four PI
max
abs
showed k
with a blue-shift of 72 nm, 84 nm, 51 nm, and
max
31 nm for PI-6FDA, PI-BPDA, PI-BTDA, and PI-PMDA, respec-
tively. This observation indicated that the chemical structure
of the dianhydride affected the electron-withdrawing
(acceptor) property of the imide group in the TetraPEDA-
based PIs. On the other side, the maximum absorption peaks
of PI-6FDA, PI-BPDA, PI-BTDA, and PI-PMDA films were 412,
424, 425, and 424 nm, respectively, with the absorption
Optical Properties of the Polyimides
The optical and electrochemical properties of the PI (Table
4) were investigated by UV–vis and PL spectroscopy, in both
the solution and solid state (film). The transmission UV–vis
spectra of the polyimide films observed in the visible region
showed the cut off wavelengths (k0) were in the range of
418–432 nm (Fig. 6). These polyimide films were in light-
color with good transparency (more than 85% transmittance
in the visible region), compared with conventional PI such as
Kapton, which is deep brown (Supporting Information Fig.
S6) and shows very low transparency in the visible region
with k0 of 513 nm.49 Other reported functional PI containing
triphenylamine (TPA) moieties also appeared as dark red
brown to deep brown, with k0 higher than 500 nm.50,51 The
improvement in color and transparency of the TetraPEDA-
based PI may be due to the large aromatic rigid nonplanar
conjugated structure directly connected with the imide unit,
which may induce a high steric hindrance and low inductive
FIGURE 8 Absorptivity of the polyimide films.
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respect to the zero vacuum level, the HOMO energy levels
for the four PI were determined to be ꢂ4.98, ꢂ5.04, ꢂ4.92,
and ꢂ5.00 eV, respectively. Furthermore, the LUMO energy
levels of the PI were estimated to be ꢂ1.86, ꢂ2.03, ꢂ2.04,
and ꢂ2.24 eV (Supporting Information Table S2). The HOMO
and LUMO energy levels of the synthesized PI were compa-
rable to those of reported functional PI,10,52,54 so the synthe-
sized PI can be expected to exhibit the similar electrical
switching behavior and to be used in polymer memory
devices.
Electrical Switching Effects and Memory Performances of
the ITO/Polyimide/Al Device
The electrical switching behavior of the PI can be demon-
strated by the current–voltage (I–V) characteristics of ITO/
PI/Al sandwich device, as shown in Figure 10. Figure 11
exhibits the typical I–V curves of the electrical switching and
memory devices fabricated with PI-6FDA, PI-BPDA, PI-BTDA,
and PI-PMDA. During the first positive sweep from 0 to 6–10
V (with Al as the anode and ITO as the cathode), the four
devices were initially in the low-conductivity (OFF) state and
then switched to the high-conductivity (ON) state at 2.7, 4.2,
4.7, and 6.5 V with an ON/OFF current ratio on the order of
1 ꢁ 104 to 1 ꢁ 105. The polymeric memory devices can
store data based on the high-conductivity (ON) and low-con-
ductivity (OFF) responses to the external applied voltages.
And the driving voltages of the memory devices are related
with the chemical structure of the dianhydride monomers.
FIGURE 9 PL of the polyimide films in thickness of ꢃ10 lm.
edges extending to 456–520 nm (Fig. 8). The different
absorption behaviors of the PI in solid states versus in solu-
tion indicated that the chemical and steric structure of the
dianhydrides and the aggregation structure of the polymers
may affect the optical properties of the PI.
Aromatic PI with fluorescent characteristic are rarely
reported in the literature. It has been reported that the PI
containing fluorescent chromophore, such as oxadiazole52
and TPA53 derivatives, usually show quenching of PL, which
is due to the re-absorption by the CT complexing between
the donor structure and the acceptor imide units. In this
work, as aforementioned, the hexaphenylethylene moiety
was an efficient fluorescence chromophore, and the synthe-
sized TetraPEDA exhibited an intense emission peak at 485
nm in THF and 463 nm in solid state. After introducing of
TetraPEDA into the polyimide main chain, the obtained aro-
matic PI also showed obvious fluorescent characteristics,
exhibiting the maximum fluorescence emission (kPmLax) at
around 425–505 nm in NMP solution and around 470–494
nm in film state (Fig. 9 and Table 4), respectively. The retain-
ing of fluorescent characteristic in the PI system can be
mainly attributed to the rigid nonplanar and large conju-
gated structure in the main chain, which can lower the inter-
molecular CT effect between the electron-donating diamine
moiety and the electron-accepting dianhydride moiety and
consequently, can effectively avoid the quenching of PL
caused by reabsorption through the CTC. These high-
performance fluorescent aromatic PI may have great poten-
tial application in the field of optics and microelectronics.
For the case of PI-BPDA [Fig. 11(b)], the device was initially
in the OFF state (‘‘0’’ signal in data storage) with a current
range of 1 ꢁ 10ꢂ10 to 1 ꢁ 10ꢂ12 A as the voltage swept
from 0 to 4.2 V (first sweep). When the voltage increased
further, an abrupt increase in the current was induced at a
threshold voltage of about 4.2 V, being indicated by a transi-
tion from the OFF state to ON state (‘‘1’’ signal in data stor-
age). This electronic transition from OFF state to ON state in
the first sweep can serve as the ‘‘writing’’ process, with the
ON/OFF current ratio of the studied memory device as high
as 1 ꢁ 105, suggesting a low misreading rate for memory
applications. In addition, the device could be kept in the ON
Electrochemical Properties of the Polyimides
The CV of the synthesized PI in NMP was measured at a
scan rate of 100 mV/s using tetrabutylammonium perchlo-
rate (n-Bu4NClO4, 0.1 M) as the supporting electrolyte and
ferrocene as standard (Supporting Information Fig. S7). The
onset oxidation (EoOnXset) of the four PI exhibited at 0.75, 0.81,
0.69, and 0.77 eV, respectively. The external ferrocene and
ferrocenium (Foc/Focþ) redox standard E1/2 was measured
to be 0.56 V vs. Ag/AgCl in NMP. By assuming that the
HOMO level for the Foc/Focþ standard was ꢂ4.8 eV with
FIGURE 10 Schematic diagram of the memory device.
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FIGURE 11 Current density–voltage (I–V) characteristics of ITO/PI/Al devices.
state during the subsequent second sweep from 7 to 0 V,
and the third sweep (from 0 to ꢂ7 V) was conducted after
turning off the power for about 2 min. Moreover, the device
could be reprogrammed, starting from the OFF state, to the
ON state again with an accurate threshold voltage of ꢂ4.0 V
and being kept in the ON state (the third and fourth
sweeps). It suggests that the ON state could be retained for
a short period of time after the removal of power and would
relax to the original OFF state eventually. And the positive
switching phenomena in the negative sweep were also
observed.
turned off. However, the memory is volatile and the data will
eventually disappear if the memory remains in the power-off
state.
Proposed Switching Mechanism by the Theoretical
Analysis
To better understand the possibility of CT processes and the
switching behavior of the synthesized PI and their memory
devices, the electronic properties of the PIs at the ground
state were studied by DFT. The molecular orbital and dipole
moment of the basic repetitive units (BRU) of the PI were
calculated by DFT/B3LYP/6-31G(d) level with the Gaussian
03 program package. Figure 13 shows the charge density
isosurfaces of the basic unit of the PI with the most energeti-
cally favorable geometry. The HOMO isosurfaces were mainly
located on the diamine moiety, while the LUMO, LUMO1, and
LUMO2 were mainly located on the dianhydride moieties.
For the higher LUMO orbital, such as LUMO3, LUMO4, and
LUMO5, the four PI showed different distribution of the elec-
tron cloud at the molecular orbital, indicating that the chemi-
cal and steric structure of the four dianhydrides and their
electronic effects obviously affected the electronic transition
behavior of the polymers.
Figure 12 shows the retention times and stress tests of both
the ON and OFF states of the PIs. Initially, the memory devi-
ces started to turn ON or OFF to a high or low conductivity
state. Under a constant stress of ꢂ1 V, no obvious degrada-
tion in current was observed for both ON and OFF states for
at least 2 h during the test and the PI memory devices were
stable for at least 108 continuous read pulses of ꢂ1 V. The
‘‘remanent,’’ yet volatile nature of the ON state, as well as
the randomly accessible ON and OFF states in each ITO/PI/
Al device, are similar to the data remanence behavior of a
static random access memory (SRAM).52 In a SRAM, the
memory can be written into or read out in any order, regard-
less of the memory location last accessed, and the stored
data can be retained temporarily after the power has been
In this work, PI-BPDA was chosen as an example to illustrate
the mechanism of field-induced conductivity of the PI, as
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FIGURE 12 Retention time on the ON and OFF states of the ITO/PI/Al devices under a constant readout voltage (ꢂ1 V).
FIGURE 13 Electronic density contours of molecular orbital of the basic repetitive units of the four PI.
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moiety to the hexaphenylethylene moiety, and consequently
show SRAM characteristic.
CONCLUSIONS
In summary, a novel, simple but efficient diamine (Tetra-
PEDA) with TetraPE rigid nonplanar conjugated moiety was
successfully prepared via Witting–Honer and Suzuki reac-
tions. A series of PI produced by the polymerization of Tetra-
PEDA and four dianhydrides respectively exhibited special
fluorescent and resistive switching (ON-OFF) characteristic,
low dielectric constant, exceptional thermal stability, and
excellent mechanical properties, mainly due to their rigid
nonplanar conjugated structure. The PI also showed
enhanced solubility in common organic solvents, light color,
and high transparency. The memory devices with the config-
uration of ITO/PI/Al exhibited distinct volatile memory char-
acteristics of SRAM. The theoretical analysis showed that the
chemical and steric structure of the dianhydride may affect
the distribution of the electron cloud at the molecular
orbital, and the CT mechanism could be used to explain the
memory characteristics of the PI.
ACKNOWLEDGMENTS
The financial support by the National 973 Program of China
(2011CB606100), the National Natural Science Foundation of
China (51173214, 51233008, J1103305), Doctoral Fund of
Ministry of Education of China, the Fundamental Research
Funds for the Central Universities, and the State Key Laboratory
for Modification of Chemical Fibers and Polymer Materials of
Dong Hua University are gratefully acknowledged. Authors
thank Ningsheng Xu, Shaozhi Deng, and Juncong She from State
Key Laboratory of Optoelectronic Materials and Technologies
for their help in the measurement of the memory devices.
FIGURE 14 Molecular orbital (left) of the basic repetitive units
PI-BPDA and the transitions (right) from ground state to the CT
state induced by the electric field.
shown in Figure 14. The calculated energy levels are com-
parable to the HOMO (ꢂ5.04 eV) and LUMO2 (ꢂ2.03 eV)
energy levels of PI-BPDA, measured by CV (Supporting In-
formation Table S2). The HOMO locating on the hexaphenyl-
ethylene moiety may act as a donor (D), while the first
three LUMOs locating on the phthalimide moiety may act as
an acceptor (A). At the threshold voltage, electrons at the
HOMO with sufficient energy transit to LUMO4 within the
hexaphenylethylene moiety (Donor) to give rise to excited
state. Excitation of the donor could lead to a decrease in
ionization potential and consequently promote intra- or
intermolecular CT at the excited state. CT can occur indi-
rectly from the LUMO4 to the LUMO2 or LUMO1, then to
LUMO of the acceptor, or directly from the HOMO to the
LUMO2, LUMO1 and LUMO at the exited state, to form a
conductive complex. These proposed processes have been
supported by the electronic absorption spectrum of PI-
BPDA (Fig. 7 and Table 4). The maximum absorption and
absorption edge at about 307 nm (4.04 eV), 340 nm (3.65
eV), and 391 nm (3.17 eV) corresponded to the transitions
from HOMO to LUMO4 (3.98 eV, highest probability), HOMO
to LUMO2 (3.48 eV), and HOMO to LUMO1 (2.74 eV),
respectively. Once the conductive CT states are formed
under an applied electric field, they will facilitate electrical
conduction under both biases and the device cannot be
switched off by applying a reverse bias of the same magni-
tude. Nevertheless, the polymer will relax to its original
conformation sometime after removal of the applied electric
field. As the potential barrier disappears, the CT state will
dissociate via back electron transfer from the phthalimide
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