Synthesis and memory characteristics of novel soluble polyimides based on asymmetrical diamines containing carbazole

Article abstract of DOI:10.1016/j.polymer.2016.03.067

Two novel asymmetrical carbazole-based diamines 9-(2-(1,1′-binaphthyl-4-yl)benzyl)-9H-carbazole-3,6-diamine (BNBCD) and 9-((4′-(9H-carbazol-9-yl)biphenyl-2-yl)methyl-9H-carbazole-3,6-diamine (CBMCD) were synthesized. A series of novel soluble aromatic polyimides were prepared from these diamines by poly-condensation with Pyromelitic dianhydride (PMDA) and 2,2′,3,3′-biphenyl tetracarboxylic dianhydride (BPDA) via a two-step procedure. The resulting polymers were fully characterized, they exhibited excellent organosolubility and high thermal stability with the temperature of 5% weight loss under nitrogen atmosphere over 400°C. Resistive switching devices with the configuration of Al/polymer/ITO were constructed from these polyimides by using conventional solution coating process. The memory devices based on PI-a, PI-b and PI-c exhibited a flash type memory capability, whereas the PI-d presented a write once read many times (WORM) memory capability.

Full text of DOI:10.1016/j.polymer.2016.03.067

Polymer 91 (2016) 118e127
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and memory characteristics of novel soluble polyimides
based on asymmetrical diamines containing carbazole
*
Jing Zhao, Ling Peng, Ya-Liang Zhu, Yang-Jun Song, Li-Jun Wang, Ying-Zhong Shen
Applied Chemistry Department, School of Material Science & Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing, 210016, PR China
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 21 November 2015
Received in revised form
Two novel asymmetrical carbazole-based diamines 9-(2-(1,1 -binaphthyl-4-yl)benzyl)-9H-carbazole-3,6-
0
diamine
(BNBCD)
and
9-((4 -(9H-carbazol-9-yl)biphenyl-2-yl)methyl-9H-carbazole-3,6-diamine
CBMCD) were synthesized. A series of novel soluble aromatic polyimides were prepared from these
diamines by poly-condensation with Pyromelitic dianhydride (PMDA) and 2,2 ,3,3 -biphenyl tetra-
carboxylic dianhydride (BPDA) via a two-step procedure. The resulting polymers were fully character-
ized, they exhibited excellent organosolubility and high thermal stability with the temperature of 5%
(
1
March 2016
0
0
Accepted 18 March 2016
Available online 23 March 2016
ꢀ
weight loss under nitrogen atmosphere over 400 C. Resistive switching devices with the configuration
Keywords:
of Al/polymer/ITO were constructed from these polyimides by using conventional solution coating
process. The memory devices based on PI-a, PI-b and PI-c exhibited a flash type memory capability,
whereas the PI-d presented a write once read many times (WORM) memory capability.
Asymmetrical diamines
Soluble polyimide
Memory materials
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
[15e17]. Polymer-based memory devices were reported to exhibit
many fascinating characteristics, including write-once read-many-
times (WORM) [18,19], flash (rewritable) [20e22], static random
access memory [23,24], and dynamic random access memory
(DRAM) [25,26] effects.
Recently, electrically bistable resistive switching organic mo-
lecular and polymeric materials for the application in memory
devices have been attracting tremendous attention [1e4]. There are
many charming qualities for polymeric materials, such as high
flexibility and processability, feasible manipulation of structures at
the molecular scale during synthesis, etc. Furthermore, polymeric
materials can be processed into three-dimensional (3D) structures
composing of layer-by-layer arrangements at low costs, which
makes fabrication of high-density memory devices economically
possible [5,6]. Therefore, a great deal of efforts have been devoted
to developing new polymer switching materials that may be uti-
lized for the sake of volatile or nonvolatile memory devices
designing. For memory devices based on inorganic materials, data
is stored and retrieved depending on the amount of input charge.
However, the storing of data in organic-type memory devices takes
a new form that is related to the electrical bistability under an
external electric field. A number of organic materials have been
investigated for memory applications such as conjugated molecules
and polymers [7e12], dopants [13,14], and complex systems
Among all of the studied polymer systems, electron donor/
acceptor polymers, especially functional polyimides obtained
additional investigation due to their excellent features, such as,
high thermal stability, good processability, superior mechanical
properties, as well as the electrical switching behavior [27e30].
Electron-donor/acceptor-containing polyimides exhibit bistable
resistive switching properties which stems from field-conduced CT
processes. Carbazole as a photoconducting compound has been the
subject of many studies. In addition the inherent electrodonating
nature of the nitrogen atom in the conjugated aromatic rings makes
it possible to form chargeetransfer complexes [31]. Carbazole and
its many derivatives have been covalently incorporated into poly-
meric systems as it can be functionalized [32]. As a result, carbazole
may exist in main-chain [33] as building blocks or in side-chains as
pendent groups [34]. Due to the existing carbazole conjugated
system, there is high probability that carbazole-containing polymer
will possess electro- and photochemical properties as well as high
thermal stability. Thus, the difunctionalizing of carbazole com-
pounds is of practical importance, such as the application in a high-
performance polyimide system.
*
Corresponding author.
E-mail addresses: zhaojing1076@126.com (J. Zhao), yz_shen@nuaa.edu.cn
When in most of organic solvents, the majority of rod-like
(
Y.-Z. Shen).
http://dx.doi.org/10.1016/j.polymer.2016.03.067
032-3861/© 2016 Elsevier Ltd. All rights reserved.
0

J. Zhao et al. / Polymer 91 (2016) 118e127
119
polyimides are poorly soluble due to their rigid backbones and
strong interaction among chains, which restrains the application in
polyimides-based memory devices. In this respect, efforts have
been put onto the synthesis of new diamine and dianhydride
monomers to improve the solubility and in turn the processability.
The incorporation of asymmetric structure into rigid polymer
backbones was turned out to be a big success to overcome such a
problem. The introduction of geometrically or molecularly asym-
metric dimaine components into the polyimide main chain on one
hand ends up with new polyimide with improved solubility and
processability, on the other hand it leads to other desirable
properties.
differential refractive index detector. Monodispersed polystyrene
was used for the molecular weight standard. Thermo Gravimetric
studies was carried out with a NETZSCH STA 449 C with a constant
heating rate of 10 C min under nitrogen (1.0 cm min ). UVevis
absorption spectra were obtained on a UV-2550 UVevis spectro-
photometer. Atomic force micrographs (AFM) with the device
surface of the polymer films were obtained a Nano Scope IIIa AFM
operated in the tapping mode under room temperature.
ꢀ
ꢂ1
3
ꢂ1
2.3. Monomer synthesis
2.3.1. Synthesis of 3,6-Dinitrocarbazole (1)
In this report, two new cabazole-containing aromatic asym-
metric diamine monomers, BNBCD and CBMCD, were synthesized
and characterized. They were designed as promising monomer for
polyimides, which were featured with high solubility and good
thermal properties at the same time. Meanwhile, four functional
aromatic, organo-soluble PIs bearing asymmetric cabazole and
binaphthyl moieties were also synthesized from the reactions of
these diamines with two kinds of commercial dianhydrides via a
conventional two-stage process. These PIs were characterized by
elemental analysis, FT-IR, UVevis absorption, fluorescence spectra
gel permeation chromatography (GPC), thermogravimetric analysis
3 2 2
Cu(NO ) $H O (5.23 g, 22 mmol) was added into a mixture of
acetic acid (10 mL) and acetic anhydride (25 mL) at room temper-
ature. The mixture was stirred for 10e20 min, and to this solution
was then added carbazole (3.00 g, 18 mmol) slowly in portions over
5 min [32]. Heat was generated during the addition (temperature
ꢀ
rose to around 100 C), and an additional 10 mL of acetic acid was
added. The mixture was stirred at this temperature for 30 min and
then poured into distilled water (200 mL). The precipitate was
collected by filtration, washed with water (100 mL ꢁ 3). The still
wet product was then dissolved into a potassium alcoholic aqueous
solution (KOH 25 g, water 250 mL, and ethanol 250 mL). The so-
lution turned red. After being stirred for 30 min, the solution was
filtrated and the filtrate was acidified with concentrated hydro-
chloric acid. The yellow precipitate was then collected by filtration,
(
(
TGA), intrinsic viscosity measurement and cycle voltammogram
CV). Electronic switching devices were fabricated by placing the
PIs-based solution-cast film between the indium-tin oxide (ITO)
electrode and the Al electrode. The devices exhibited nonvolatile
electrical switching behavior and displayed WORM and flash
memory characteristics. All of them maintained a long retention
time in ambient conditions. The electrical characteristics make
these PIs potential candidates as organic bistable memory
materials.
ꢀ
ꢀ
washed with water and dried at 100 C under vacuum to give
ꢂ1
yellow solid 3.94 g (85%). Mp: 242e243 C; FT-IR (KBr, cm ): 3091
(NeH stretching), 1508, 1328 (eNO asymmetric and symmetric
stretching), 860 (CeN); H NMR (400 MHz, DMSO-d 11.10 (s,1 H,
2
1
6
) d
NH), 9.34 (d, 2 H, J ¼ 2.3 Hz), 8.40 (dd, 2 H, J ¼ 2.3 Hz, J ¼ 8.9 Hz),
7.78 (d, 2 H, J ¼ 8.9 Hz).
2
. Experiment
2.3.2. Synthesis of 9-(2-bromobenzyl)-3,6-dinitro-9H-carbazole (2)
Anhydrous potassium carbonate (6.941 g, 50 mmol) and (1)
(2.573 g,10 mmol) which was dissolved in DMF (35 mL) were added
to a 100 mL three-necked flask, and the mixture was stirred for
20 min to fully dissolved. Then 1-bromo-2-bromomethylbenzene
2.1. Materials
0
Carbazole, 2-bromotoluene, 1,1 -binaphthyl-4-ylboronic acid, 4-
9H-carbazol-9-yl)phenylboronic acid (99%, Hebei Delongtai
Chemicals Co.Ltd., China) and Pyromelitic dianhydride (PMDA)
ꢀ
(
was added into the mixture at one time and heated at 60 C for
4 h. After cooled to room temperature, the solution was poured into
0
0
were used as received. (Beta-4)-platinum and 2,2 ,3,3 -biphenyl
tetracarboxylic dianhydride (BPDA) were synthesized according to
the literature [35,36]. 1-bromo-2-bromothyl-benzene was synthe-
sized following an already reported procedure [37]. DMF (Guoyao)
200 mL water and collected by filtration. The solid was thoroughly
ꢀ
ꢀ
washed with water, and then dried at 60 C under vacuum to afford
ꢂ1
yellow solid 3.84 g (90% yield). Mp: 252 C. FT-IR (KBr, cm ): 1632
1
2
(CeC), 1521, 1331 (eNO ), 1026 (CeBr). H NMR (400 MHz, DMSO-
was
refined
by
reduced
pressure
distillation.
N,N-
d
6
,
d
, ppm): 9.61 (d, J ¼ 2.2 Hz, 2 H, ArH), 8.44 (d, J ¼ 2.3 Hz, 1 H,
Dimethylacetamide (DMAc, Guoyao) were distilled under reduced
pressure over KOH. All the other solvents and reagents were
analytical-grade and were used without further purification.
ArH), 8.42 (d, J ¼ 2.3 Hz, 1 H, ArH), 7.83 (s, 1 H, ArH), 7.81 (s, 1 H,
ArH), 7.76 (d, J ¼ 7.9 Hz, 1 H, ArH), 7.25 (m, 1 H, ArH), 7.17 (m, 1 H,
1
3
ArH), 6.41 (d, J ¼ 7.6 Hz, 1 H, ArH), 5.91 (s, 1 H, CH
2
). C NMR
(
101 MHz, DMSO-d
127.5, 123.4, 122.9, 122.4, 119.3, 111.6, 47.9. Anal. calcd. For
12BrN : C, 53.54; H, 2.84; N, 9.86. Found: C, 54.32; H, 2.58; N,
9.63.
6
, d, ppm): 145.3, 142.2, 135.1, 133.6, 130.2, 128.8,
2.2. Measurements
C
19
H
3 4
O
The NMR spectra were recorded on a Bruker Advance 400
1
spectrometer at resonant frequencies of 400 MHz for H and
1
01 MHz for 13C nuclei using DMSO-d
6
or CDCl
3
as the solvent.
2.3.3. Synthesis of 9-(2-bromobenzyl)-9H-carbazole-3,6-diamine
(3)
A three-necked flask was charged with compound (2) (4.26 g,
10 mmol) and 100 mL of acetic acid. The solution was stirred under
a nitrogen atmosphere until (2) dissolved completely, and then tin
powder (7.13 g, 60 mmol), 22 mL of concentrated hydrogen chloride
Fourier transform infrared (FTIR) spectra of intermediates and
monomers were obtained from KBr pellets with a Nicolet NEXUS-
470 FTIR spectrometer. Elemental analysis was performed on a
Perkine-Elmer 240C elemental analyzer. Melting points were
observed in sealed capillaries with uncorrected. The weight average
molecular weight (M
ꢀ
w
) and number average molecular weight (M
n
)
were added. The reaction mixture was stirred at 100 C under a
were determined by gel permeation chromatography (GPC) on a
Water GPC system equipped with four Waters Ultrastyragel col-
nitrogen atmosphere for 30 h. After cooling, the reaction mixture
was poured into sodium hydroxide solution and adjusted PH to
alkaline. The precipitate was collected, washed several times with
water and dried at 60 C under vacuum. After dried, the product
was extracted by tetrahydrofuran and then removed the solvent.
5
4
umns (300 ꢁ 7.5 mm, guarded and packed with 1 ꢁ 10 , 1 ꢁ 10 ,
3
ꢀ
1
1
ꢁ 10 , and 500 Å gels) in series. Tetrahydrofuran (THF,
ꢂ1
mL min ) was used as the eluent and was monitored by a

120
J. Zhao et al. / Polymer 91 (2016) 118e127
The residue was purified by recrystallization with ethanol and
water (Vethanol: Vwater ¼ 9: 1) to afford gray solid 2.67 g (73%).
mixture of equimolar acetic anhydride and pyridine was added to
the poly(amic acid) solution with stirring at room temperature for
2 h, and it was then heated to 100 C for 5 h. After the reaction was
ꢀ
ꢂ1
ꢀ
Mp: 246 C. FT-IR (KBr, cm ): 3286 and 3196 (eNH
2
asymmetric
1
and symmetric stretching), 1328 (CeN), 1026 (CeBr). H NMR
400 MHz, DMSO-d
, ppm): 7.67 (d, J ¼ 8.9 Hz, 1 H, ArH), 7.16 (m,
H, ArH), 7.14 (d, J ¼ 2.0 Hz, 2 H, ArH), 7.09 (m,1 H, ArH), 7.05 (s,1 H,
ArH), 7.02 (s, 1 H, ArH), 6.69 (d, J ¼ 2.1 Hz, 1 H, ArH), 6.67 (d,
J ¼ 2.1 Hz, 1 H, ArH), 6.25 (d, J ¼ 7.6 Hz, 1 H, ArH), 5.41 (s, 2 H, CH ),
, ppm): 141.6,
37.5, 134.3, 133.1, 129.5, 128.3, 123.2, 122.1, 115.4, 109.7, 104.6, 46.5.
completed, the mixture was cooled to room temperature. The
viscous solution of polymer was poured into methanol and
collected by filtration. The resulting polyimide was thoroughly
(
6
, d
1
ꢀ
washed with methanol and water, and then dried at 60 C under
2
vacuum for 12 h.
The yield of PI-a was 89%. The number average molecular weight
n w
(M ) and weight average molecular weight (M ) values tested by
13
4
.67 (s, 4 H, NH
2
). C NMR (101 MHz, DMSO-d
6
,
d
1
Anal. calcd. For C19
H
16BrN
3
: C, 62.31; H, 4.40; N, 11.47. Found: C,
GPC were 16,800 and 25,200, respectively, with the dispersity
ꢂ1
6
2.96; H, 3.98; N, 11.23.
(DPI ¼ M
w
/M
n
) at 1.5. FTIR: (KBr, cm ) 1771 and 1720 (C]O
asymmetric and symmetric stretching), 1492, 1378 (CeN). Anal.
calcd. for (C51 : C, 81.47; H, 4.42; N, 5.59; Found: C, 81.54;
H 4.33; N, 5.61.
The yield of PI-b was 86%. The number average molecular
weight (M ) and weight average molecular weight (M ) values
0
2.3.4. Synthesis of 9-(2-(1,1 -binaphthyl-4-yl)benzyl)-9H-
33 3 4 n
H N O )
carbazole-3,6-diamine (BNBCD)
To a 100 mL three-necked round-bottomed flask, a mixture of 9-
(
1
2-bromobenzyl)-9H-carbazole-3,6-diamine (3) (0.8 g, 2.19 mmol),
,1 -binaphthyl-4-ylboronic acid (0.977 g, 3.28 mmol), Pd(PPh
n
w
0
3
)
4
tested by GPC were 17,400 and 28,300, respectively, with the dis-
ꢂ1
(
0.051 g, 0.044 mmol), Na
2
CO
3
(0.93 g, 8.76 mmol), toluene (22 mL)
persity (DPI ¼ M
O), 1593, 1441 (CeN). Anal. calcd. for (C57
4.50; N, 5.08; Found: C, 82.78; H, 4.39; N, 5.10.
The yield of PI-c was 92%. The number average molecular weight
) and weight average molecular weight (M ) values tested by
w
/M
n
) at 1.62. FTIR: (KBr, cm ) 1771 and 1720 (C]
ꢀ
and water (11 mL) was added and heated at 100 C under a nitrogen
atmosphere for 6 h. When the reaction was completed, the reaction
mixture was cooled to room temperature and filtered. The crude
product was achieved by extraction with toluene. After remove
solvent, the crude product was recrystallized from ethanol and
37 3 4 n
H N O ) : C, 82.69; H,
(M
n
w
GPC were 22,900 and 33,900, respectively, with the dispersity
ꢀ
ꢂ1
ꢂ1
water to afford 0.86 g (71%). Mp: 229 C. FT-IR (KBr, cm ): 3305,
(DPI ¼ M
1486, 1371 (CeN). Anal. calcd. for (C49
N, 7.56; Found: C, 79.12; H, 4.39; N, 7.84.
The yield of PI-d was 90%. The number average molecular
weight (M ) and weight average molecular weight (M ) values
w
n
/M ) at 1.48. FTIR: (KBr, cm ) 1771 and 1714 (C]O),
1
3
207 (eNH
2
), 1324 (CeN). H NMR (400 MHz, DMSO-d
6
,
d, ppm):
32 4 4 n
H N O ) : C, 79.44; H, 4.35;
8
.26 (s, 1 H, ArH), 8.00 (m, 3 H, ArH), 7.94 (d, J ¼ 8.3 Hz, 1 H, ArH),
7
.88 (d, J ¼ 8.4 Hz, 3 H, ArH), 7.74 (d, J ¼ 8.2 Hz, 2 H, ArH), 7.62 (d,
J ¼ 8.1 Hz, 2 H, ArH), 7.53 (m, 2 H), 7.24 (d, J ¼ 8.6 Hz, 2 H, ArH), 7.21
n
w
(
d, J ¼ 8.1 Hz, 2 H, ArH), 7.13 (s, 2 H, ArH), 6.73 (d, J ¼ 8.5 Hz, 2 H,
tested by GPC were 32,600 and 41,700, respectively, with the dis-
13
ꢂ1
ArH), 5.46 (s, 2 H, CH
2
), 4.67 (s, 4 H, NH
2
). C NMR (101 MHz,
persity (DPI ¼ M
w
/M
n
) at 1.28. FTIR: (KBr, cm ) 1777 and 1720 (C]
DMSO-d
32.7, 131.6, 127.9, 127.6, 126.9, 126.8, 126.7, 126.5, 126.0, 125.8,
25.5, 124.5, 124.3, 122.1, 114.2, 108.9, 103.4, 44.7. Anal. calcd. For
6
,
d, ppm): 140.3, 138.3, 138.1, 137.6, 137.5, 136.2, 133.4,
O), 1492, 1365 (CeN). Anal. calcd. for (C55
H
36
N
4
O
4
n
) : C, 80.87; H,
1
1
4.44; N, 6.86; Found: C, 80.91; H, 4.38; N, 6.88.
C
H
39 29
N
3
: C, 86.80, H, 5.42, N, 7.79. Found: C, 86.82, H, 5.38, N, 7.80.
2.5. Device fabrication and characterization
0
2
9
.3.5. Synthesis of 9-((4 -(9H-carbazol-9-yl)biphenyl-2-yl)methyl-
H-carbazole-3,6-diamine (CBMCD)
The indium-tin oxide (ITO, around 200 nm in thickness) coated
onto glass substrate was used as a bottom electrode. Ahead of
coating of the polymer film, the glass surfaces were cleaned with
deionized water, isopropanol and acetone by ultrasonication for
30 min, respectively. Then a solution of polyimide (16 mg/mL)
filtered through polytetrafluoroethylene (PTFE) membrane syringe
filters with a pore size of 0.22 mm which was spin-coated onto the
ITO substrate at a revolving speed of 1800 rpm for 1 min, and re-
A solution of (3) (0.60 g, 1.64 mmol), 4-(9H-carbazol-9-yl)phe-
nylboronic acid (0.71 g, 2.46 mmol), Pd(PPh
(0.75 g, 6.56 mmol) in toluene (20 mL) and
water (10 mL) was heated 110 C under an nitrogen atmosphere for
3
)
4
(0.038 g,
0
.033 mmol), Na
2
CO
3
ꢀ
6
h. When the reaction was completed, the reaction mixture was
cooled and filtered. The filtrate was condensed and recrystallized
from ethanol and water (Vethanol: Vwater ¼ 9: 1). The solid was
washed twice with water and ethanol respectively. The gray solid
was collected and dried in vacuum at 50 C to give product 0.76 g
88%). Mp: 283 C. FT-IR (KBr, cm ): 3354, 3321 (eNH
ꢀ
sidual solvent was removed under a vacuum at 60 C for 24 h [38].
The thickness of polymer films was determined to be around
50 nm. Finally, the thickness of Al top electrodes about 110 nm was
formed by thermal evaporation onto the polymer surface through a
ꢀ
ꢀ
ꢂ1
(
(
2
), 1324
1
ꢂ6
CeN). H NMR (400 MHz, CDCl
3
,
d
, ppm): 8.17 (d, J ¼ 7.8 Hz, 2 H,
shadow mask at a pressure of about 10 Torr with a uniform
ArH), 7.65 (d, J ¼ 9.3 Hz, 4 H, ArH), 7.48 (d, J ¼ 10.0 Hz, 3 H, ArH),
.45e7.38 (m, 3 H, ArH), 7.33 (d, J ¼ 8.0 Hz, 4 H, ArH), 7.16 (d,
J ¼ 8.3 Hz, 1 H, ArH), 6.97 (s, 2 H, ArH), 6.80 (d, J ¼ 8.2 Hz, 3 H, ArH),
depositing rate of 1e2 Å/s. Every electrical measurements of the
devices were characterized under ambient conditions, without any
encapsulation, using an Agilent B1500A semiconductor analyzer.
During the electrical measurements, ITO was maintained as the
ground electrode (the cathode). During the voltage sweep, Al was
set as the top electrode (the anode).
7
13
5
.43 (s, 1 H, CH
ppm): 140.8, 140.1, 139.8, 138.7, 136.9, 135.9, 135.0, 130.5, 130.1,
28.2, 127.2, 126.9, 126.7, 126.0, 123.5, 123.3, 120.3, 120.1, 115.7,
09.8, 109.3, 106.3, 45.2. Anal. calcd. For C37 : C, 84.06; H, 5.34;
N, 10.60. Found: C, 84.09; H, 5.42; N, 10.35.
2 2 3
), 3.55 (s, 2 H, NH ). C NMR (101 MHz, CDCl , d,
1
1
28 4
H N
3. Results and discussion
2.4. Polymer synthesis
3.1. Monomer synthesis
All polymers were synthesized via a two-step procedure [38]. In
detail, taking the preparation of PI-a as an example, PMDA (0.61 g,
mmol) was added into a solution of BNBCD (1.11 g, 2 mmol) in
dehydrated DMAc under nitrogen. The mixture was stirred at room
temperature for 24 h to form the solution of poly(amic acid). A
Two novel asymmetric carbazole-based dimines were synthe-
sized by Suzuki coupling reaction with the high yields of 71%e73%
as shown in Scheme 1. The traditional method to synthesize the 9-
position aryl-substituted carbazole was substituted in the 9-
position firstly, then nitrated to obtain 3,6-dinitrocarbazole, the
2

J. Zhao et al. / Polymer 91 (2016) 118e127
121
Scheme 1. Synthesis of diamines.
yield of each step was low. Thus, we have found a new method to
synthesize 9-position aryl-substituted carbazole. Carbazole was
firstly nitrated to give 3,6-dinitrocarbazole and then substituted in
the N-position to afford 9-(2-bromobenzyl)-3,6-dinitro-9H-carba-
zole (2), the yield was perfect and post-processing was also simple.
Reduction of (2) to afford 9-(2-bromobenzyl)-9H-carbazole-3,6-
diamine (3). Finally, the diamines of BNBCD and CBMCD were
prepared from arylboronic acid and (3) by Suzuki coupling reaction.
In FT-IR spectra, the characteristic absorption peaks near 3306 and
ꢂ1
3
203 cm are attributed to the NeH asymmetric and symmetric
ꢂ1
stretching. The peaks near 1310 cm are assigned to the CeN
stretching. Besides, the peak positions in the H NMR and C NMR
spectra of the 9-[2 -(aryl)phenyl]-3,6-diaminocarbazoles are also
consistent with the proposed structure. All the H NMR and
1
13
0
1
13
C
NMR spectra with the assignment of the observed resonances The
spectroscopic data were in good agreement with the expected
structures.
Scheme 2. Synthesis of polymers.
PI-a
3.2. Polyimides synthesis
The functional polyimides PI-a to PI-d in Scheme 2 were pre-
pared from two new diamines (BNBCD, CBMCD) with the respec-
tive aromatic dianhydrides (PMDA, BPDA) via a two-step poly-
condensation method as shown in Scheme 2. In this procedure, the
first step was to form the poly(amic-acid)s at room temperature
PI-b
PI-c
(
r.t.). Then, a dehydrating agent, such as a mixture of acetic anhy-
dride and pyridine was added into the system, formed effective
chemical imidization to obtain the polyimides. The FTIR spectra of
polymers confirmed effective formation of polyimides as shown in
PI-d
ꢂ1
Fig.1. The characteristic bands at 1788 and 1721 cm are attributed
to the asymmetric and symmetric stretching of imide carbonyl
4000
3500
3000
2500
2000
1500
1000
500
ꢂ1
groups, respectively, and a band at 1388 cm
due to CeN
-
1)
Wavenumbers cm
(
stretching which confirmed the formation of imide structure. The
elemental analysis results of C, H, and N content are in a fair
agreement with the theoretical content. The FT-IR and elemental
analysis suggest the successful preparation of the target
polyimides.
Fig. 1. FT-IR spectra of PIs.
methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and THF
at room temperature or upon heating at 80 C. The PIs demon-
strated moderate to high inherent viscosities of 0.63e0.71 dL g in
ꢀ
ꢂ1
3.3. Organo-solubility and thermal properties of polymers
DMAc, as shown in Table 1. The good solubility was attributed to the
structural modification through the incorporation of the bulky and
non-coplanar units in the side chain [39], which renders the
polymer non-coplanar and also reduces inter-chain interactions.
The solubility and inherent viscosities of the synthesized PIs in
common solvents are summarized in Table 1. These polymers
exhibited good solubility in various polar solvents such as DMAc, N-

122
J. Zhao et al. / Polymer 91 (2016) 118e127
ꢀ
Table 1
800 C. Ascribed the high anaerobic char yields of these PIs to their
The inherent viscosity and solubility of PIs.
high aromatic content. The introduction of non-coplanar bulky
aromatic side group at the 9-position on the carbazole structure
aggrandize the size and sharp anisotropy of the diamine with 9-
substituted aryl group and impact arrangement of molecules,
which could improve the processing characteristics of these rela-
tively intractable polymers. These results show that the new pol-
yimides are thermally stable and dimensionally polymers, which
satisfy the requirement of heat resistance in memory devices.
Sample
h
inh(dL/g)
DMAc
DMF
NMP
THF
Acetone
Toluene
PI-a
PI-b
PI-c
PI-d
0.65
0.71
0.68
0.63
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
þþ
þþ
þþ
þþ
þþ
þ
þ
e
þ
þ
þ
þ
þ
þþ
þþþ
þþþ
ꢂ1
h
inh, intrinsic viscosity measured at a concentration of 0.5 g$dL
C; þþþ, soluble at room temperature; þþ, soluble on heating; þ, slightly sol-
in DMAc at
ꢀ
30
uble on heating; e, insoluble even on heating.
3.4. Atomic Force Microscope (AFM) morphology of the polymer
films
PI-a
PI-b
PI-c
PI-d
AFM was used to characterize the surface morphologies and
roughness of polymer films. The morphology of the polymer films
plays an important role in determining the device performance. As
shown in Fig. 4, the three-dimensional images of different poly-
imides. The thicknesses of polymer films are around 20 nm eval-
uated by AFM, the RMS (root-mean-square) roughness ranges from
1.542 to 2.27 nm and the Z-ranges from 2.12 to 11.24 nm, demon-
strating the excellent ability of film-forming for these polyimides.
As the optimized geometry of the copolymers suggested, the two-
dimensional structure combined with twisted conformation of the
side groups would create large free volume and disturb the regu-
larity of polymer molecule, thus prevent the polymer from severe
aggregation [38].
0
10
20
30
40
50
60
70
80
90
3.5. Optical and electrochemical properties
2
(degree)
Fig. 5a shows the UVevis absorption spectra and photo-
luminescence (PL) of the PIs in THF solution. The concentrations of
Fig. 2. Diffraction patterns of PIs.
the polyimides for the UVevis absorption test were about
ꢂ7
ꢂ1
8
ꢁ 10 mol L . PI-a and PI-c show the absorption peak maxima
max) at 226 and 223 nm, which is ascribed the * transition of
the -electronic system delocalized along the asymmetry twisted
These PIs were investigated by X-ray powder diffraction. From all
these diffraction patterns (Fig. 2), it was observed that all the PIs
displayed a typical amorphous pattern and not able to show any
crystallinity. Obviously, the amorphous nature of the asymmetry
aromatic-substituted carbazole-based polyimides was also re-
flected in their good dissolvability [19]. The thermal properties of
the polyimides were evaluated by TGA in the atmosphere of
nitrogen (Fig. 3). The 5% weight loss temperatures (T5%) of polymers
(l
p/p
p
structure between side chain binaphthyl and the main chain phenyl
aromatic ring. In comparison to PI-a and PI-c, two additional groups
of phenyl and carbazole were incorporated into PI-b and PI-d,
resulting in the lmax at 239 and 236 nm. When the polyimides
were excited at maximum absorption wavelength, the emission
peaks were observed at 383, 364,382, and 365 nm, respectively. The
higher fluorescence intensity of PI-a and PI-b compared to PI-c and
PI-d may be attributed to the deference of anhydrides. Moreover,
the optical band gap (Eopt) of the PIs from the UVevis absorption
edges are estimated to be 4.44, 4.29, 4.47 and 4.38 eV, respectively.
Fig. 6 shows the cyclic voltammograms (CVs) of the PIs on platinum
electrode in 0.1 M acetonitrile solution of tetrabutylammonium
hexauorophosphate. The oxidation halfwaves potentials (E1/2) for
PIs were determined to be 1.76 V, 1.96 V, 1.79 V, and 1.90 V vs. Hg/
ꢀ
were investigated by TGA in the range of 388e447 C. The anaer-
obic char yield of these polymers in nitrogen was around 60% at
1
1
10
00
PI-a
PI-b
PI-c
PI-d
9
8
7
6
5
4
0
0
0
0
0
0
Hg
redox standard potential, E1/2, was measured to be 0.49 V vs. Hg/
Hg Cl in acetonitrile. Assuming that the HOMO level for the Fc/
2
Cl
2
, respectively. The external ferrocene ferrocenium (Fc/Fcþ)
2
2
Fc þ standard is ꢂ4.8 eV with respect to the zero vacuum level, the
HOMO levels for the polyimides a-d are determined to
be ꢂ6.07 eV, ꢂ6.27 eV, ꢂ6.10 eV, and ꢂ6.21 eV, respectively.
Therefore, the LUMO levels of the polyimides a-d are estimated to
be ꢂ1.63 eV, ꢂ1.98 eV, ꢂ1.63 eV, and ꢂ1.83 eV, respectively. This
result indicates that PI-b could provide more stable and lower
HOMO energy level. The adscititious twisted side aromatic groups
in PI-a and PI-c enhanced the LUMO energy level slightly, probably
because of the non-planar or twisted conformation of the polymer.
In the Al/PI-a/ITO device, the energy barrier to hole injection from
electrode to the active layer was estimated to be 0.87 eV from the
0
200
400
600
800
o
Temperature
(
C)
work function (
F) of ITO (ꢂ4.80 eV) and the HOMO of the active PI
ꢀ
ꢂ1
.
Fig. 3. TGA curves of PIs under nitrogen atmosphere at a heating rate of 10.0 C min
layer, and the energy barrier to electron injection from the

J. Zhao et al. / Polymer 91 (2016) 118e127
123
2
Fig. 4. Atomic Force Microscope three-dimensional images (surface area of 0.5 ꢁ 0.5
mm ) of PI-a, PI-b, PI-c and PI-d.
Fig. 5. UVevis absorption spectra and fluorescence spectra of PIs in THF solution.
ꢂ7
ꢂ3
electrode to the active layer was estimated to be 2.57 eV from the
F
about ꢂ4.7 V from 10 to 10 , indicating the device is switched
from a low conductivity state (OFF state) to a high conductivity
state (ON state), which equivalent to the “writing” process for the
memory device. After the transition, the device remained in the
ON-state even after bias interruption. Further, during the voltage is
swept from 0 to þ5.0 V, the device is still in the ON-state. However,
as the applied voltage is swept from þ5.0 V to 0, an sharp current
decrease was detected at a voltage of about þ1.9 V. The transition
suggests that the device is converted back into an original OFF-
state, which equivalent to the “erasing” process for the memory
device. Moreover, this type of OFF-state can be maintained during
the sweep 4 and after the power is turned off. These results indicate
that the devices are nonvolatile memory and possess the ability of
writing, reading, and erasing cycles to meet the requirement of a
flash memory device. Further, the retention characteristics of the
ON/OFF states for Al/PI-b/ITO memory cell are shown in Fig. 7d. It is
of Al (ꢂ4.20 eV) and the LUMO of the active layer. This shows that
hole injection from ITO into the HOMO of PI-a is easier than elec-
tron injection from Al into the LUMO of PI-a. As a result, PI-a is a p-
type material, and the other polymers also display the similar p-
type material properties.
3.6. Electrical characteristics of the PI-based memory devices
The memory behaviors and electrical switching of the PIs was
tested by the currentevoltage (IeV) measurements of an Al/poly-
mer/ITO sandwich device [40e43]. The memory effects for PI-a, PI-
b and PI-c are similar, and the typical IeV characteristics of the
memory devices in semilog scale of the devices are shown in Fig. 7.
Taking the PI-b (Fig. 7b) as an example, during voltage sweeping
from 0 to ꢂ5.0 V, the current exhibits a abrupt increase at

124
J. Zhao et al. / Polymer 91 (2016) 118e127
4 4
Fig. 6. CV responses of (a) PI-a, (b) PI-b, (c) PI-c, and (d) PI-d films coated ITO electrodes in 0.1 M n-Bu NClO /acetonitrile solution with a platinum plate counter electrode and an
Ag/AgCl (3.8 M KCl) reference electrode. A scan rate of 50 mV/s was used.
Fig. 7. (a) IeV characteristics of Al/PI-a/ITO device with an electrode area of 0.0314 mm2 (b) IeV characteristics of Al/PI-b/ITO device with an electrode area of 0.0314 mm2 (c) IeV
2
characteristics of Al/PI-c/ITO device with an electrode area of 0.0314 mm . (d) Effect of operation time on the ON and OFF states of the Al/PI-b/ITO device under a constant stress
of ꢂ0.5 V at room temperature.
found that the memory cell exhibits two stable states of ON/OFF
within 104 s, and the resulting ON/OFF ratio is as large as 10
under ꢂ0.5 V. No degradation was observed during the test.
Fig 8a exhibits the typical IeV characteristics of the memory
devices fabricated with the PI-d film. Initially, the as-fabricated
device was in the low-conductivity (OFF) state. During the first
negative sweep from 0 to ꢂ5.0 V, the current remained low until
the switching threshold voltage of ꢂ4.1 V was reached. At the
4

J. Zhao et al. / Polymer 91 (2016) 118e127
125
2
Fig. 8. (a) IeV characteristics of Al/PI-d/ITO device with an electrode area of 0.0314 mm . (b) Effect of operation time on the ON and OFF states of the Al/PI-d/ITO device under a
constant stress of ꢂ0.5 V at room temperature.
ꢂ6
ꢂ3
voltage, the current sharp increased from 10 to 10 , indicating
the device transition from initial OFF state to the high-conductivity
(
Electric density contours and the corresponding energy levels are
presented in Fig. 9, respectively. The calculated energy levels
(HOMO, LUMO) of PI-a, PI-b and PI-c were of (ꢂ5.98, ꢂ2.13),
(ꢂ5.87, ꢂ2.31) and (ꢂ5.82, ꢂ2.11) eV, respectively, which are
consistent with those determined from the CV measurement
(HOMO, LUMO) energy levels of (ꢂ6.07, ꢂ1.63), (ꢂ6.27, ꢂ1.98) and
(ꢂ6.10, ꢂ2.63) eV, respectively. In addition, the HOMO represents
electron density localization for an electron donor in the ground
state, while LUMO stands for the electron density localization in the
excited state. The LUMO energy level of the donor must have a
proper match with that of the acceptor to offer enough driving
force for charge separation [29]. The HOMOs of the PIs are mainly
on the carbazole part, the carbazole and aryl pendant moieties are
the main electron donors and phthalimide moieties are the elec-
tron acceptors. For the BU of PI-a, the HOMO is located on carbazole
of the amine parts. The LUMO is distributed on anhydride unit. The
electronic transition corresponds to promotion of an electron at the
excited state. The energy barrier for hole injection from work
function of ITO to the HOMO of the active layer is lower than the
energy barriers between the work function of Al and HOMO.
Therefore, hole injection is a favored process under a negative bias.
It is noteworthy that the threshold voltage may be related to HOMO
energy level [45], but this not the deciding factor, Because some
studies suggested that driving voltage of the device associated with
the structure of dianhydride monomer [46]. In the PI-a and PI-c,
ON) state, which serves as the “writing” process for the memory
device. After the transition, the device remained in the ON-state
even after bias interruption. Interestingly, it is shows that the de-
vice could not convert back into the OFF-state during the subse-
quent negative and positive sweeps, and it also did not return to the
low conductivity state even turning off the power or upon the using
a reverse sweep. The inerasable and nonvolatile nature of the ON
state indicates that the Al/PI-d/ITO device exhibits WORM type
memory. Further, it is found that an ON/OFF current ratio as high as
0 is achieve under a constant stress of ꢂ0.5 V, and also exhibits
3
1
superior stability, as shown in Fig. 8b.
3
.7. Electrical switching mechanism of the PI-based memory
devices
The switching mechanism of the PIs is supported by the field
induced CT effect from the theoretical calculations as described
below. Molecular simulations, on the basic unit (BU) of PIs were
carried out by density functional theory (DFT) theoretical calcula-
tions. Calculations of the molecular orbitals and optimized geom-
etry of the BU were carried out at the WB97XD/6-31G(D, P) level
with the Gaussian 09 program package [44], and then the molec-
ular orbital of HOMOs and LUMOs were predicted at the same level.
Fig. 9. LUMO and HOMO energy levels for the three molecules, calculation based on DFT.

126
J. Zhao et al. / Polymer 91 (2016) 118e127
Fig. 10. LUMO and HOMO energy level for PI-d, calculation based on DFT.
they have the same diamine monomer in the polymer structure,
but they drive threshold voltages are ꢂ2.0 V and ꢂ2.3 V respec-
tively. So we consider that the structure of dianhydride monomer
has a great influence on the driving voltage. As can be seen from
HOMO energy level of PI-b and PI-d, the HOMOs of the PIs are
mainly on the carbazole part of side chain not the main chain. The
higher threshold voltages of PI-b and PI-d compared to PI-a and PI-c
may be attribute to the incorporation of carbazole in the polymer
side chains.
Acknowledgments
A Project Funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2016.03.067.
However, on the basic unit (BU) of PI-d was carried out by
density functional theory (DFT) theoretical calculation is definitely
different from others as shown in Fig. 10. The HOMO of the PI-d are
mainly on the carbazole part of polymer side chain and the LUMO is
distributed on the carbazole part as well. This could explain the
charge transfer effect is not the main reason of conduction mech-
anism. Some polymers containing carbazole groups in the side
chain, can exhibit electrically induced conformational changes
between a regiorandom and a regioregular structure, resulting in
the variation in conductivity state [47,48]. The introduction of aryl
pendants, such as 9-p-tolyl-9H-carbazole and non-coplanar dia-
nhydride, which may introduce more torsional conformation in the
polymer backbone. This torsional conformation at the excited state
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