Synthesis and memory characteristics of highly organo-soluble polyimides bearing a noncoplanar twisted biphenyl unit containing aromatic side-chain groups

Article abstract of DOI:10.1039/c0jm02547j

Two novel 3′,4′,5′-trifluorobiphenyl-based aromatic polyimide monomers, 2,2′-bis[4′-(3′′,4′′, 5′′-trifluorophenyl)phenyl]-4,4′-biphenyl diamine (BTFBPD) and 2,2′-bis[4′-(3′′,4′′,5′′- trifluorophenyl)phenyl]-4,4′,5,5′-biphenyltetracarboxylic dianhydride (B

Full text of DOI:10.1039/c0jm02547j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and memory characteristics of highly organo-soluble polyimides
bearing a noncoplanar twisted biphenyl unit containing aromatic side-chain
groups†
a
a
b
b
ab
b
Yueqin Li,ab Huihua Xu, Xian Tao, Kejia Qian, Shuang Fu, Yingzhong Shen
and Shijin Ding
*
*
Received 5th August 2010, Accepted 25th October 2010
DOI: 10.1039/c0jm02547j
Two novel 30,40,50-trifluorobiphenyl-based aromatic polyimide monomers, 2,20-bis[40-(300,400,500-
trifluorophenyl)phenyl]-4,40-biphenyl diamine (BTFBPD) and 2,20-bis[40-(300,400,500-
trifluorophenyl)phenyl]-4,40,5,50-biphenyltetracarboxylic dianhydride (BTFBPDA) were synthesized.
Two fluorinated polyimides (PIs), PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) were
prepared via a two-step procedure using BTFBPD reacting with 2,20-diphenyl-4,40,5,50-
biphenyltetracarboxylic dianhydride (DPBPDA) and BTFBPDA, respectively. BTFBPD was
characterized with single crystal X-ray diffraction analysis and the geometric parameters showed the
noncoplanar twisted character. PIs exhibited highly organo-solubility and thermal stability. The PI
films sandwiched between an indium-tin oxide(ITO) bottom electrode and Al top electrode exhibited
two accessible conductivity states and can be switched from the low-conductivity to the high-
conductivity. The memory devices with the configuration of Al/PI(BTFBPD-DPBPDA)/ITO exhibited
a flash type memory capability, whereas the Al/PI(BTFBPD-BTFBPDA)/ITO presented a write once
read many times (WORM) memory capability. The fabricated devices showed low turn-on threshold
voltages of ꢀ1.3 V (PI(BTFBPD-DPBPDA)) and ꢀ1.7 V (PI(BTFBPD-BTFBPDA)) and both ON/
OFF current ratios on the order of 103 to 104.
dynamic random access memory (DRAM), write-once-read-
many (WORM) and flash memory, etc. The mechanisms re-
sponsible for electrical conduction, switching and nonvolatile
effects were summarized as carrier trapping-detrapping, electric-
field induced charge transfer (CT), filamentary metal, confor-
mational behaviors and phase changes, and redox effects.22
Among the entire studied polymer systems, electron donor/
acceptor polymers, especially functional polyimides have been
widely investigated owning to their excellent thermal stability,
good processability, and superior mechanical properties besides
the electrical switching behavior.23–29 Polyimides containing
electron donor and electron acceptor groups exhibit bistable
resistive switching arising from field-conduced CT processes,
wherein imide groups acted as the electron acceptor. The reported
polyimides were poly(3,30-bis(diphenylcarbamyloxy)-4,40-biphenyl-
enehexafluoroisopropylidenediphthalimide) (6F-HAB-DPC PI),23
poly(4,40-aminotriphenylenehexafluoroisopropylidenediphthalimide)
(6F-TPA PI),13 poly(3,30-bis(N-ethylenyloxycarbazole)-4,40-
biphenylenehexafluoroisopropylidenediphthalimide) (6F-HAB-
CBZ PI),19 poly(3,30-di(4-(diphenylamino)benzylidenylimino-
ethoxy)-4,40-biphenylenehexafluoroisopropylidenediphthalimide)
(6F-HAB-TPAIE PI),24 poly(N-(N0,N0-diphenyl-N0-1,4-phenyl)-
N,N-4,40-diphenylenehexafluoroisopropylidenediphthalimide)
(6F-2TPA PI),25 poly(4,40-amino(4-hydroxyphenyl)-diphenyl-
enehexafluoroisopropylidenediphthalimide) (6F-HTPA PI),26
poly(2,7-bis(phenylenesulfanyl)thianthrenehexafluoroisopropy-
lidenediphthalimide) (APTT-6FDA PI) and poly(4,40-thiobis-
(p-phenylenesulfanyl)hexafluoroisopropylidenediphthalimide)
(3SDA-6FDA PI),27 poly(4-amino-40-(p-aminophenoxy)triphenyl-
aminehexafluoroisopropylidenediphthalimide)triphenylamine-
Introduction
Electrically switching organic molecules and polymeric materials
have attracted more and more attention as alternative or
supplemental materials to traditional inorganic materials in the
application of resistive random access memory (RRAM) in
recent years.1–4 Polymeric materials not only exhibit low-cost
potential, simplicity in structure, good scalability, and potential
for high density data storage in 3D arrays but also their prop-
erties can be easily tailored through chemical synthesis.5 There-
fore, there is currently much research into the development of
new polymer switching materials with properties and process-
ability that meet the requirements of nonvolatile or volatile
memory devices, including conjugated polymers,6–12 electron
donor/acceptor polymers,13,14 polymer nanocomposites embedded
metal nanoparticle and fullerene,1–3,15 and polymers with specific
pendant chromophores.16–20 Rather than encoding ‘‘0’’ and ‘‘1’’
as the amount of charge stored in a cell in silicon devices, poly-
mer memory stores data, for instance, based on the high and
low-conductivity response to an applied voltage.21 Different
kinds of memory devices have been also explored, such as
aApplied Chemistry Department, School of Material Science
Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing,
210016, P. R. China. E-mail: yz_shen@nuaa.edu.cn; Fax: +86-25-
52112626; Tel: +86-25-52112906-84765
bState Key Laboratory of ASIC and System, School of Microelectronics,
Fudan University, Shanghai, 200433, P. R. China. E-mail: sjding@fudan.
edu.cn; Fax: +86-21-55664845; Tel: +86-21-55664845
&
† Electronic supplementary information (ESI) available: FT-IR,
1H-NMR, 13C-NMR spectra of BDBPDA and BTFBPDA. CCDC
reference numbers 715156, 784374. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0jm02547j
hexafluoroisopropylidenediphthalimide)
(APT-TPA
PI),28
1810 | J. Mater. Chem., 2011, 21, 1810–1821
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poly(2,5-bis(p-aminophenoxyphenyl)-1,3,4-oxadiazolehexafluoro-
isopropylidenediphthalimide) (P(BPPO)-PI).29 The triphenyla-
mino (TPA) moieties as electron donor have been proposed to
enhance the electron donating and charge transport ability with
phthalimide moieties (electron acceptor). The 6F-TPA PI con-
taining TPA moieties has been reported to exhibit volatile memory
(i.e., dynamic random access memory (DRAM)) behavior with
bipolar ON- and OFF-switching characteristics.13 In contrast, the
TPA-PPF PI exhibits WORM behavior in negative voltage
sweeps, even though the polymer contains the same TPA moieties.
On the other hand, the 6F-2TPA PI, which contains connected
two TPA moieties, reveals unipolar WORM characteristic as well
as DRAM behavior with polarity, depending on the film thick-
ness.25 The mono- or dual-mediated phenoxy linkages between the
TPA and phthalimide moieties, represented as (D-L-A)n
(PI(AAPT-6FDA)) or (L-D-L-A)n (PI(APT-6FDA)), was found
to significantly affect the volatile memory behaviors. PI(AAPT-
6FDA) exhibited DRAM performance, while PI(APT-6FDA)
showed the static random access memory (SRAM) behavior.28
The role of TPA moiety in the electrically bistable characteristics
of these polymers remain questionable and these results collec-
tively suggest that the electrical memory behavior of a polyimide is
sensitively dependent upon the chemical natures of the constituent
parts in the polymer. The development of highly stable memory
devices based on dimensionally and thermally stable polymers
remains in the exploration stage. Therefore, the relationship
between the polymer structure and memory characteristics
requires further exploration.
trifluorobiphenyl and phenyl moieties as pendants are expected
to be the electron donors and phthalimide moieties act as the
electron acceptors. The thermal, optical, and electrochemical
properties of PI(BTFBPD-DPBPDA) and PI(BTFBPD-
BTFBPDA) were also investigated. These PIs exhibit good solu-
bility in common organic solvents so they can be easily fabricated
by means of conventional solution spin-coating and subsequent
drying. The memory behavior was estimated by a simple sand-
wich device configuration consisting of polymer films as active
layer between ITO ground electrode and Al top electrode.
PI(BTFBPD-DPBPDA) exhibited a flash type memory behavior,
while PI(BTFBPD-BTFBPDA) presented a WORM memory
behavior. Both of them maintained a long retention time in
ambient conditions. The electrical characteristics suggested that
PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) were
expected candidates for organic bistable memory materials.
Experimental
Materials
Phenylboronic acid and 30,40,50-trifluorobiphenyl-4-ylboronic
acid (99%, Hebei Delongtai Chemicals Co., Ltd.) and 3,30,4,40-
biphenyltetracarboxylic dianhydride (BPDA, 99%, Changzhou
Wujin Linchuan Chemical Co., Ltd.) were used as received. 2,20-
Dibromobiphenyl-4,40-diamine was synthesized on the basis of
ref. 35. 4,40-Bis(N-methylphthalimide) was prepared according
to the literature.30 All the other reagents and solvents were
analytical-grade and were used without further purification.
Among various polyimide-based memory devices developed,
the 4,40-hexafluoroisopropylidenediphathalic anhydride (6FDA)
moiety played an important role in enhancing the electron
acceptor ability of imide group,5,13 and it was widely used as the
anhydride monomer to synthesize functional polyimides in
bistable memory devices. However, 2,20-position aryl-substituted
tetracarboxylic dianhydride and diamine as polyimide mono-
mers have not been explored for such applications. In general,
introduction of bulky substituent into 2,20-position of biphenyl
can produce high levels of ring-torsion and induce non-coplanar
structure. On one hand, the non-planar twisted biphenyl moieties
would push the neighboring chains apart to disrupt the crystal
packing and enhance solubility and sustain superior thermal
stability and mechanical property.30–33 On the other hand, the
non-planar configuration of organic data storage materials
would suppress intramolecular coupling for the energy barrier
induced by the torsion angle.34 Meanwhile, the intermolecular
charge transfer would be promoted and charge recombination
becomes difficult. Moreover, the noncoplanar configuration will
contribute to a large current ratio of ON/OFF because it per-
turbs conjugation in the molecular backbone and results in
a comparatively low conductance state (OFF).34 In this study, we
report the synthesis and fabrication of novel programmable
‘‘write–read–erase’’ memory devices based on nano-scale thin
films of soluble and thermally stable polyimides, such as
PI(BTFBPD-DPBPDA) derived from 2,20-bis[40-(300,400,500-tri-
fluorophenyl)phenyl]-4,40-biphenyl diamine (BTFBPD) reacting
with 2,20-diphenyl-4,40,5,50-biphenyltetracarboxylic dianhydride
(DPBPDA) and PI(BTFBPD-BTFBPDA) from BTFBPD and
2,20-bis[40-(300,400,500-trifluorophenyl)phenyl]-4,40,5,50-biphenyl
tetracarboxylic dianhydride (BTFBPDA) (Fig. 1). The 30,40,50-
Measurements
The FT-IR measurements (KBr pellets) were recorded in the
range of 400–4000 cmꢀ1 on the instrument of Thermo Nicolet
Nexus 670 infrared spectrometer. NMR spectra were recorded
on a Bruker Advance 300 spectrometer at resonant frequencies of
300 MHz for H and 75 MHz for 13C nuclei using CDCl3 and
1
DMSO-d6 as solvent and tetramethylsilane as the reference.
Weight average molecular weight (Mw) and number average
molecular weight (Mn) were determined by gel permeation
chromatography (GPC) on a Water GPC system equipped with
Fig. 1 Molecular structure of PIs and schematic diagram of the memory
devices consisting of a thin film of PIs (about 50 nm) sandwiched between
an ITO bottom electrode and an Al top electrode.
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2
3
four Water Ultrastyragel columns (300 ꢁ 7.5 mm, guarded and
Hz, JC–F ¼ 9.4 Hz, JC–F ¼ 4.1 Hz), 147.4, 142.3, 140.0, 138.1
(dt, 1JC–F ¼ 231.0 Hz, 2JC–F ¼ 15.7 Hz), 136.4, 133.9, 132.4, 129.3,
127.6, 125.6, 115.0, 113.5, 110.8 (dd, 2JC–F ¼ 15.0 Hz, 3JC–F ¼ 5.6
Hz). Anal. calcd. for C36H22F6N2: C, 72.48; H, 3.72; N, 4.70;
Found: C, 72.29; H, 3.40; N, 4.68.
5
4
3
ꢀ
packed with 1 ꢁ 10 , 1 ꢁ 10 , 1 ꢁ 10 , and 500 A gels) in series.
Tetrahydrofuran (THF, 1 mL minꢀ1) was used as the eluent. The
efluents were monitored by a UV detector (JMST Systems, VUV-
24, USA) at the wavelength of 254 nm. Monodispersed poly-
styrene was used as the molecular weight standard. Elemental
analysis was performed on a Perkin-Elmer 240 C elemental
analyzer. Thermo Gravimetric Analysis (TGA) and Differential
Scanning Calorimetric (DSC) studies were carried out with
4,40-Bis(2,20-diiodo-N-methylphthalimide) (1). 4,40-Bis(N-
methylphthalimide) (51.00 g, 0.16 mol) was added to concen-
trated H2SO4 (98 wt%, 700 mL) in a three-necked round-bottom
flask under vigorously stirring. When the solid dissolved
completely, KIO3 (137.00 g, 0.64 mol) and I2 (81.20 g, 0.32 mol)
were added to the mixture, with vigorous stirring for 24 h.
After the reaction finished the reaction mixture was poured
into ice-water containing the previously dissolved Na2SO3
(fume hood). Filtration and the collected precipitate was fil-
trated and washed thoroughly by water then purified by
recrystallization from ethanol to give a white product (73.8 g,
80%). FT-IR: nmax/cmꢀ1 1775 and 1712(C]O), 1378 (C–N),
1008 (C–I), 741 (imide ring deformation). 1H NMR (300 MHz,
dH/ppm in DMSO-d6): d 8.46 (s, 1 H), 7.71 (s, 1 H), 3.04 (s, 3 H,
CH3). 13C NMR (75 MHz, dH/ppm in DMSO-d6): d 167.3,
166.3, 132.9, 132.6, 131.8, 123.5, 107.0, 24.0. Anal. calcd. for
C18H10I2N2O4: C, 37.79; H, 1.76; N, 4.90; Found: C, 37.72; H,
1.66; N, 4.93.
ꢂ
a NETZSCH STA 449 C with a constant heating rate of 10 C
minꢀ1 under argon flow rate of 1.0 cm3 minꢀ1. X-ray powder
diffraction patterns were obtained with a Bruker D8-Advance
model X-ray diffractionmeter with a Ni-filter and graphite
ꢀ
monochromator, and Cu-Ka1 radiation (l ¼ 1.54056 A, 40 kV,
50 mA, 4ꢂ/min). The single X-ray structure measurement was
performed on a Rigaku SCXmini CCD, detector equipped with
ꢀ
graphite monochromatic Mo-Ka radiation (l ¼ 0.71073 A) at
297 K.‡ UV-vis absorption spectra were obtained on a UV-2550
UV-vis spectrophotometer and fluorescence spectra were
measured with an Eclipse fluorescence spectrometer. Melting
points were measured by WBR-1B melting point digital appa-
ratus. Cyclic voltammogram (CV) was recorded on a CHI 660C
electrochemical work station using a glass carbon electrode as
the working electrode, Pt plate as the counter electrode, and an
Ag/AgCl electrode as the reference. A 0.1 M tetra-n-buty-
lammonium perchlorate (n-Bu4ClO4) in anhydrous acetonitrile
was employed as the supporting electrolyte.
2,20-Diiodobiphenyl-4,40,5,50-tetracarboxylic acid (2). 4,40-
Bis(2,20-diiodo-N-methylphthalimide) (46.38 g, 0.081 mol),
KOH (22.68 g, 0.405 mol) dissolved in water (250 mL) was
heated to reflux for 24 h, and then made acidic by the addition of
6.5 N HCl to pH of 1.0. The white precipitate was collected by
Monomer synthesis
2,20-Bis[40-(300,400,500-trifluorophenyl)phenyl]-4,40-biphenyl diamine
(BTFBPD). A solution of 2,20-dibromobiphenyl-4,40-diamine (3.42
g, 0.01 mol), 4-(30,40,50-trifluoro)biphenylboronic acid (5.54 g, 0.022
mol), Pd(PPh3)4 (0.23 g, 0.2 mmol), Na2CO3 (4.24 g, 0.04 mol), in
ethanol (40 mL) and water (20 mL) was heated at 80 ꢂC under an
nitrogen atmosphere for 4 h. When the reaction was completed, the
reaction mixture was cooled and filtered under nitrogen. The
filtrate was condensed and extracted twice with 1,2-dichloroethane
(2 ꢁ 20 mL). The combined organic layer was washed twice with
saturated NaHCO3 solution (40 mL) and water respectively, and
then dried over anhydrous MgSO4. The solvent was evaporated to
obtain brown powders. The filter cake and the powders were
combined and recrystallized from 1,2-dichlorethane to give a white
product (4.5 g, 75%), mp 246 ꢂC (dec.). FT-IR: nmax/cmꢀ1 3389 and
3433 (N–H),1363 (C–N), 1041 (C–F). 1H NMR (300 MHz, dH/ppm
in DMSO-d6): d 7.68 (t, 2 H, 3JF–H ¼ 7.8 Hz, 4JF–H ¼ 4.1 Hz), 7.45
(d, 2 H, J ¼ 7.95 Hz), 6.91 (d, 1 H, J ¼ 8.15 Hz), 6.72 (d, 2 H, J ¼
ꢂ
filtration and dried in vacuum at 100 C to give teracarboxylic
acid (47.00 g, 99%). FT-IR: nmax/cmꢀ1 3432 (O–H), 1691and 1293
(C]O), 1086 (C–I). 1H NMR (300 MHz, dH/ppm in DMSO-d6):
d 8.47 (s, 1 H), 7.71 (s, 1 H). 13C NMR (75 MHz, dH/ppm in
DMSO-d6): d 167.4, 166.8, 149.4, 140.6, 134.9, 133.5, 131.5,
102.7. Anal. calcd. for C16H8I2O8: C, 33.02; H, 1.39; Found: C,
33.00; H, 1.41.
Tetraethyl 2,20-diiodobiphenyl-4,40,5,50-tetracarboxylate (3).
2,20-Diiodobiphenyl-4,40,5,50-tetracarboxylic acid (47.00 g,
0.08 mol), ethanol (20 mL, 0.35 mol), concentrated sulfuric
acid (10 mL) and benzene (260 mL) were charged into a 500 mL
three-necked flask equipped with a magnetic stirrer, Dean–Stark
trap and a reflux condenser. The reaction mixture was heated to
80 ꢂC, and maintained at this temperature for 4 h. The water
formed in the reaction was removed by azeotropic benzene. Then
the mixture was cooled to room temperature and washed by
saturated NaHCO3, brine, and water, respectively. The organic
layer was separated and dried over anhydrous MgSO4 and
concentrated to give yellow residue; it was recrystallized frꢂom
ethanol to afford white crystals (40.0 g, 72%), mp 94.1–94.6 C.
FT-IR: nmax/cmꢀ1 1728 (C]O), 1284 and 1135 (C–O–C), 1079
(C–I). 1H NMR (300 MHz, dH/ppm in DMSO-d6): d 8.25 (s, 1 H),
7.51 (s, 1 H), 4.32–4.43 (m, 4 H, CH2), 1.33–1.41 (m, 6 H, CH3).
13C NMR (75 MHz, dH/ppm in DMSO-d6): d 166.3, 165.9, 149.9,
139.4, 133.7, 131.9, 129.9, 102.2, 62.3, 62.1, 14.2, 14.1. Anal.
calcd. for C24H24I2O8: C, 41.52; H, 3.48; Found: C, 41.48; H,
3.50.
7.95 Hz), 6.53 (d, 1 H, J ¼ 7.9 Hz), 6.37 (s, 1 H), 5.07 (br, 2 H). 13
C
NMR (75 MHz, dH/ppm in DMSO-d6): d 150.6 (ddd, 1JC–F ¼ 223.5
‡ Crystal structure analysis of BTFBPD. C36H22F6N2, M ¼ 596.56,
ꢀ
crystal system: monoclinic, space group P21/c, a ¼ 12.097(8) A, b ¼
ꢂ
ꢂ
ꢂ
ꢀ
ꢀ
13.934(9) A, c ¼ 16.889(10) A, a ¼ 90 , b ¼ 94.519(12) , g ¼ 90 , V ¼
3
ꢀ
2838(3) A , T ¼ 297(2) K, Z ¼ 4. R (reflections) ¼ 0.0695 (3679), wR2
(reflections)
¼
0.1791 (6471). CCDC reference numbers 715156,
784374. Crystal structure analysis of tetraethyl 2,20-bis[40-(300,400,500-
trifluorophenyl)phenyl]biphenyl-4,40,5,50-tetracarboxylate. C48H36F6O8,
ꢀ
ꢂ
crystal system: monoclinic, space group P21/c, a ¼ 17.257(4) A, b ¼
ꢂ
ꢂ
ꢀ
ꢀ
ꢀ
17.152(3) A, c ¼ 15.542(3) A, a ¼ 90 , b ¼ 116.36(3) , g ¼ 90 , V ¼
3
4122.0(18) A , T ¼ 297(2) K, Z ¼ 4. R (reflections) ¼ 0.0969 (4849),
wR2 (reflections) ¼ 0.2426 (7489).
1812 | J. Mater. Chem., 2011, 21, 1810–1821
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Tetraethyl 2,20-bis[40-(300,400,500-trifluorophenyl)phenyl]bipheny-
l-4,40,5,50-tetracarboxylate (4). A solution of tetraethyl 2,20-
diiodobiphenyl-4,40,5,50-tetracarboxylate (6.94 g, 0.01 mol),
30,40,50-trifluorobiphenyl-4-ylboronic acid (5.54 g, 0.022 mol),
Pd(PPh3)4 (0.23 g, 0.2 mmol) and Na2CO3 (4.24 g, 0.04 mol) in
toluene (40 mL) and water (20 mL) was heated to 110 ꢂC in
an atmosphere of nitrogen for 4 h. When the reaction was
completed, the reaction mixture was cooled and filtered. The
organic layer was washed with portions of saturated NaHCO3
solution, brine and water, respectively, and then dried over
anhydrous MgSO4. The solvent was evaporated; the brown
residue was recrystallized from ethanol to give a white product
(6.2 g, 73%), mp 199–200 ꢂC. FT-IR: nmax/cmꢀ1 1724 (C]O),
1.25–1.32 (m, 6 H, CH3). 13C NMR (75 MHz, dH/ppm in DMSO-
d6): d 166.5, 166.2, 143.3, 140.3, 137.9, 132.0, 131.9, 130.3, 129.9,
128.6, 128.0, 127.4, 61.5, 13.9, 13.8. Anal. calcd. for C36H34O8: C,
72.71; H, 5.76; Found: C, 72.64; H, 5.80. 2,20-Diphenyl-4,40,5,50-
biphenylꢂtetracarboxylic dianhydride was of yield 86% and mp
1
271–272 C. FT-IR, H NMR and 13C NMR data were seen in
the supporting information (see Fig. S1†) and in line with that
reported in the literature.25
Synthesis of PI(BTFBPD-DPBPDA). 2,20-Diphenyl-4,40,5,50-
biphenyltetracarboxylic dianhydride (0.4464 g, 1.00 mmol) was
added into a solution of BTFBPD (0.5965 g, 1.00 mmol) in
dehydrated DMAc (solid content 5 wt%) under nitrogen. The
reaction mixture was stirred at ambient temperature for 24 h
under a nitrogen atmosphere to form poly(amic acid) (PAA)
solution. A mixture of equimolar of acetic anhydride and pyri-
dine was added to the poly(amic acid) solution with stirring at
ambꢂient temperature for 1 h, and then the mixture was heated to
100 C and maintained for 3 h. After being cooled, the viscous
polymer solution was poured into methanol. The precipitate was
filtered, washed thoroughly with methanol and water, and dried
at 100 ꢂC under vacuum for 24 h. Yield: 93%. The number
average molecular weight (Mn) and weight average molecular
weight (Mw) values tested by GPC were 24880 and 39820,
respectively, with the polydispersity index (PDI ¼ Mw/Mn) at
1.6. FT-IR: nmax/cmꢀ1 1778 and 1724 (C]O), 1364 (C–N), 1041
(C–F). Anal. calcd. for (C64H32F6N2O4)n Calcd.: C, 76.32; H,
3.21; N, 2.78; Found: C, 76.63; H, 3.57; N, 2.59.
1
1294 and 1137 (C–O–C), 1043 (C–F). H NMR (300 MHz, dH/
ppm in DMSO-d6): d 7.93 (s, 1 H), 7.52 (s, 1 H), 7.11–7.18 (m, 4
H), 6.60 (d, 2 H, J ¼ 8.3 Hz), 4.35–4.48 (m, 4 H, CH2), 1.35–1.57
(m, 6 H, CH3). 13C NMR (75 MHz, dH/ppm in DMSO-d6):
d 166.8, 166.7, 151.0 (ddd, 1JC–F ¼ 245.1 Hz, 2JC–F ¼ 9.9 Hz, 3JC–
1
2
¼ 4.0 Hz), 142.8, 140.7, 138.7 (dt, JC–F ¼ 239.7 Hz, JC–F
¼
F
16.2 Hz), 138.4, 136.1, 132.4, 132.3, 129.6, 126.7, 125.6, 111.6
2
3
(dd, JC–F ¼ 15.7 Hz, JC–F ¼ 5.2 Hz), 62.0, 14.3, 14.2. Anal.
calcd. for C48H36F6O8: C, 67.45; H, 4.25; Found: C, 67.43;
H, 4.53.
2,20-Bis[40-(300,400,500-trifluorophenyl)phenyl]-4,40,5,50-biphenyl-
tetracarboxylic dianhydride (BTFBPDA) (5). KOH (1.96 g, 5 mmol)
was added to a solution of tetraethyl 2,20-bis[40-(300,400,500-trifluoro-
phenyl)phenyl]biphenyl-4,40,5,50- tetracarboxylate (6.00 g, 7 mmol)
dissolved in ethanol (100 mL) in a 250 mL of round flask. The
reaction mixture was heated to reflux for 24 h. The white
precipitate formed was collected by filtration, dissolved in water
and acidified by diluted HCl to a pH of 2.0. The light yellow
precipitate was collected by filtration and dried under vacuum
at 100 ꢂC for 24 h. The above obtained solid was charged in
a 100 mL three-necked flask, and a mixture of 20 mL acetic
anhydride and 20 mL acetic acid was added. The reaction
mixture was gradually heated to 135 ꢂC and maintained at this
temperature for 4 h. The yellow precipitate formed was filtered
and washed thoroughly by toluene. Dried under vacuum at
110 ꢂC for 24 h and obtained yellow powder (4.70 g, 95%). FT-
IR: nmax/cmꢀ1 1845 and 1782 (C]O), 1241 and 1111 (C–O–C),
1041 (C–F). 1H NMR (300 MHz, dH/ppm in DMSO-d6): d 8.43
(s, 1 H), 7.89 (s, 1 H), 7.69 (q, 2 H, 3JF–H ¼ 9.4 Hz, 4JF–H ¼ 6.7
Hz), 7.46 (d, 2 H, J ¼ 8.4 Hz), 6.63 (d, 2 H, J ¼ 8.4 Hz). 13C
NMR (75 MHz, dH/ppm in DMSO-d6): d 162.7, 162.7, 150.6
(ddd, 1JC–F ¼ 245.0 Hz, 2JC–F ¼ 10.3 Hz, 3JC–F ¼ 4.4 Hz), 147.3,
Synthesis of PI(BTFBPD-BTFBPDA). Using BTFBPDA as
an anhydride, PI(BTFBPD-BTFBPDA) was synthesized by
a procedure similar to that of PI(BTFBPD-DPBPDA). The yield
of PI(BTFBPD-BTFBPDA) was of 90%. Mn, Mw and PDI
values of PI(BTFBPD-BTFBPDA) were 21660, 30330 and 1.4,
respectively. FT-IR: nmax/cmꢀ1 1779 and 1726 (C]O), 1365
(C–N), 1044 (C–F). Anal. calcd. for (C76H34F12N2O4)n Calcd.:
C, 72.02; H, 2.71; N, 2.21; Found: C, 71.79; H, 2.97; N, 2.44.
Fabrication and electrical characterization of memory devices.
The indium-tin oxide (ITO, 200 nm in thickness) coated onto
glass substrate was used as a bottom electrode. Prior to coating
of the polymer film, the ITO surface was cleaned sequentially
with deionized water, acetone and isopropanol by sonication for
15 min. Subsequently, a DMAc solution of PI (9 mg mLꢀ1
)
filtered through polytetrafluoroethylene (PTFE) membrane
microfilters with a pore size of 0.22 mm was spin-coated onto the
ITO substrate at a rotation rate of 2000 rpm for 60 s, followed
by solvent removal in a vacuum at 60 ꢂC for 16 h. The thickness
of the polymer layer was about 50 nm, as determined by the
Microfigure Measuring Instrument (Surforder ET-3000).
Finally, an Al top electrode with an area of 0.0314 mm2 and
a thickness of about 110 nm was formed by thermal evaporation
onto the polymer surface through a shadow mask at a pressure of
about 10ꢀ6 Torr. All electrical measurements of the devices were
characterized under ambient conditions, without any encapsu-
lation, using a HP 4145B semiconductor analyzer. ITO was
maintained as the ground electrode during the electrical
measurements and Al was set as the cathode during the voltage
sweep.
1
2
145.1, 137.1, 136.8 (dt, JC–F ¼ 141.1 Hz, JC–F ¼ 10.1 Hz),
136.4, 131.6, 130.3, 129.8, 129.4, 128.9, 126.5, 126.3, 111.4 (dd,
2JC–F ¼ 14.8 Hz, 3JC–F ¼ 6.5 Hz). Anal. calcd. for C40H16F6O6:
C, 68.00; H, 2.28; Found: C, 68.27; H, 2.16.
2,20-Diphenyl-4,40, 5,50-biphenyl tetracarboxylic dianhydride
(DPBPDA). 2,20-Diphenyl-4,40,5,50-biphenyl tetracarboxylic
dianhydride was synthesizd by the same method. Tetraethyl 2,20-
diphenyl bipꢂhenyl-4,40,5,50-tetracarboxylate was of 82% yield,
mp 160–161 C. FT-IR: nmax/cmꢀ1 1724 (C]O), 1242 and 1136
1
(C–O–C). H NMR (300 MHz, dH/ppm in DMSO-d6): d 7.84
(s, 1 H), 7.46 (s, 1 H), 7.22 (t, 1 H, J ¼ 14.7 Hz), 7.11 (t, 2 H, J ¼
14.9 Hz), 6.56 (d, 2 H, J ¼ 8.4 Hz), 4.25–4.34 (m, 4 H, CH2),
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Results and discussion
Monomer synthesis
In this study, a new functional compound, BTFBPD was
successfully synthesized by Suzuki reaction with a high yield of
75% (Scheme 1), which contains 30,40,50-trifluorobiphenyl moie-
ties. The FT-IR spectrum (Fig. 2) of the obtained compound
presents the characteristic absorption of N–H asymmetric
stretching and symmetric stretching located at 3443 and 3356 cmꢀ1
and the characteristic absorption due to the C–F at 1240 cmꢀ1. 1H
and 13C NMR spectra of BTFBPD are shown in Fig. 3 and Fig. 4,
respectively, with the assignment of the observed resonance. For
BTFBPD, the proton peaks of aromatic ring appeared at 6.37–
7.68 ppm, the coupling constants of 3JF–H and 4JF–H were 7.8 and
4.1 Hz, respectively; the carbon peaks of aromatic ring appeared
1
2
Fig. 3 1H NMR spectrum of BTFBPD.
at 110.8–150.6 ppm, the coupling constants of JC–F, JC–F and
3JC–F were 223.5, 9.4 and 4.1 Hz, respectively. All the spectro-
scopic data were in good agreement with the expected structure.
The molecular structure of the diamine was also confirmed by
single crystal X-ray diffraction. Single crystal of BTFBPD could
be grown by evaporation of saturated 1,2-dichloroethane solu-
tion. The molecular structure of BTFBPD was demonstrated in
Fig. 5. The dihedral angle of the two central nitrogen-connected
phenyl rings (plane 1: C13–C18 and plane 2: C19–C20) was 54.4ꢂ.
And the dihedral angle between 1,4-disubstituted phenyl ring
(plane 3: C7–C12) and the central nitrogen-connected phenyl
ring (plane 1) was 50.8ꢂ, while a delicate difference dihedral angle
Fig. 4 13C NMR spectrum of BTFBPD.
between plane 1 and plane 5 (C25–C30) was 48.8ꢂ on the other
side. Meanwhile, the dihedral angle between plane 3 and plane 4
(C1–C6) was 25.6ꢂ, which is smaller than that of plane 5 and
plane 6 (C31–C36) 34.6ꢂ. Most of all, the dihedral angle between
plane 3 and plane 5 was 15.5ꢂ, which are close to parallel. As
shown in Fig. 5, compound BTFBPD displayed a rod-like
Scheme 1 The synthetic route of BTFBPD.
Fig.
5 ORTEP drawing of BTFBPD, showing 30% probability
Fig. 2 FT-IR spectrum of BTFBPD.
displacement ellipsoids and the hydrogen atoms are omitted for clarity.
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1
twisted geometry configuration. This three-dimensional structure
together with the bulky pendant groups in the tetracarboxylic
dianhydride will hinder the close packing of polymer chain and
enhance the solubility of formed polyimides.
products in each step were confirmed by FT-IR, H NMR, 13C
NMR and elemental analysis. Detailed characterization of
DPBPDA and BTFBPDA were presented in Fig. S1† and
Fig. S2†, respectively. For BTFBPDA, the proton peaks of
aromatic ring appeared at 6.63–8.43 ppm, the coupling constants
of 3JF–H and 4JF–H were 9.4 and 6.7 Hz, respectively; the carbon
peaks of carbonyl group appeared at 162.7 ppm and those of
aromatic ring appeared at 110.8–150.6 ppm, the coupling
constants of 1JC–F, 2JC–F and 3JC–F were 245.0, 10.3 and 4.4 Hz,
respectively. In the FT-IR spectrum, the characteristic peak of
a carbonyl group of tetracarboxylic dianhydrides appeared at
1787 and 1845 cmꢀ1. The molecular structure of tetraethyl 2,20-
bis[40-(300,400,500-trifluorophenyl)phenyl]biphenyl-4,40,5,50-tetra-
carboxylate was also characterized by single-crystal X-ray
diffraction. Single crystal of this compound could be grown by
evaporation of saturated 1,2-dichloroethane and ethyl acetate
solution. The molecular structure was demonstrated in Fig. 6.
The dihedral angle of the two central phenyl rings (plane 1: C4–
C12 and plane 2: C28–C36) was 54.58ꢂ. And the dihedral angle
between 1,4-disubstituted phenyl ring (plane 3: C13–C18) and
the central phenyl ring (plane 1) was 53.41ꢂ, while a obvious
difference dihedral angle between plane 2 and plane 5 (C38–C42)
was 42.13ꢂ on the other side. Meanwhile, the dihedral angle
between plane 3 and plane 4 (C19–C24) was 49.67ꢂ, which is
The synthetic route for BTFBPDA tetracarboxylic dianhy-
dride is shown in Scheme 2. The aromatic side-substituted tet-
racarboxylic dianhydride were synthesized via a very important
intermediate
tetraethyl-2,20-diiodobiphenyl-4,40,5,50-tetra-
carboxylate, which was synthesized from BPDA through four
steps. Firstly, BPDA was converted into N-methyl imide, and
then iodinated by I2/KIO3 in concentrated H2SO4 under vigor-
ously stirring at room temperature to give 4,40-bis(2,20-diiodo-N-
methylphthalimide). This iodinated imide was ring-opened by
KOH and then acidified to form 2,20-diiodobiphenyl-4,40,5,50-
tetracarboxylic acid, and followed by esterification with ethanol
to give tetraethyl-2,20-diiodobiphenyl-4,40,5,50-tetracarboxylate.
This compound was respectively reacted with phenylboronic
acid and 30,40,50-trifluorobiphenyl-4-ylboronic acid to generate
aromatic side-substituted biphenyl tetraethyl tetracarboxylates,
and then hydrolyzed by alkali to remove the tetraethyl ester
groups and acidified to generate relative biphenyl tetracarboxylic
acids. The tetracarboxylic dianhydrides were obtained by dehy-
dration of biphenyl tetracarboxylic acids with acetic anhydride/
acetic acid. The yields in each step were high, and the obtained
Scheme 2 The synthetic route of BTFBPDA.
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diamine (BTFBPD) with tetracarboxylic dianhydrides (DPBPDA
and BTFBPDA) in two steps, as shown in Scheme 3. First,
diamine and tetracarboxylic dianhydride was dissolved in
N,N-dimethylacetamide (DMAc) (solid content 5 wt%) under
nitrogen to yield a viscous PAA solution. Then, chemical imid-
ization was carried out by adding equimolar of acetic anhydride
and pyridine into the PAA solution to produce the polyimide.
The two PIs were characterized by the FT-IR spectra (Fig. 7),
which shown the typical absorption peaks of the imide moiety at
about 1780 cmꢀ1 (asymmetrical stretching of C]O), 1720 cmꢀ1
(symmetrical stretching of C]O), the C–N bond at about
1360 cmꢀ1 and the imide ring deformation at about 740 cmꢀ1. In
addition, the characteristic absorption of the C–F group located
at about 1040 cmꢀ1 can also be observed. In the case of polymers
with amide linkages in the main chain, the FT-IR spectra are not
sufficient to confirm the complete imidization. Full imidization in
poly(amide imides) was confirmed by elemental analysis.
Polymer properties
Organo-solubility and thermal properties of polymers. The
solubility properties of the polyimides in several common
organic solvents at 10% (w/v) are also presented in Table 1. These
polymers exhibited good solubility in a variety of polar solvent
such as DMAc, N,N-dimethylformamide (DMF), N-methyl-
pyrrolidone (NMP), and THF at room temperature or upon
heating at 80 ꢂC; however, they were slightly soluble or insoluble
in less polar solvent like CHCl3, acetone and toluene. Further-
more, the intrinsic viscosities of PI(BTFBPD-DPBPDA) and
PI(BTFBPD-BTFBPDA) were of 0.78 and 0.64 dL gꢀ1, respec-
tively. The high solubility may be attributed to the introduction
of bulky side-chain substituted groups in the polymer structure.
Fig. 6 ORTEP drawing of tetraethyl 2,20-bis[40-(300,400,500-trifluoro-
phenyl)phenyl]biphenyl-4,40,5,50-tetracarboxylate, showing 30% proba-
bility displacement ellipsoids and the hydrogen atoms are omitted for
clarity.
larger than that of plane 5 and plane 6 (C43–C48) 34.6ꢂ. Most of
all, the dihedral angle between plane 3 and plane 5 was 27.26ꢂ. As
shown in Fig. 6, this compound also displayed a rod-like twisted
geometry configuration.
The polymers, PI(BTFBPD-DPBPDA) and PI(BTFBPD)-
BTFBPDA) were prepared by the polycondensation of the
Scheme 3 Synthesis of PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA).
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Fig. 7 FT-IR spectra of PI(BTFBPD-DPBPDA) and PI(BTFBPD-
BTFBPDA).
Fig.
8
TGA and DSC curves of PI(BTFBPD-DPBPDA) and
PI(BTFBPD-BTFBPDA) under argon atmosphere at a heating rate of 10
ꢂC minꢀ1
.
The PIs were characterized with X-ray powder diffraction
studies. From the diffraction pattern (Fig. S3†), it was observed
that PIs displayed a nearly completely amorphous pattern and
failed to show any crystallinity. Apparently the amorphous
nature of these aromatic-substituted biphenyl-based polyimides
was reflected also in their good solubility. This property provides
good quality thin film by means of a conventional solution
spin-casting process. The thermal properties of the PIs were
investigated by TGA (Fig. 8) and DSC (Fig. S4†). The 5%
weight-lost temperature (Td 5%) of PI(BTFBPD-DPBPDA) and
PI(BTFBPD-BTFBPDA) were 556 ꢂC and 548 ꢂC, respectively.
These polymers afforded high anaerobic char yield of around
70% at 800 ꢂC in a argon atmosphere. From the DSC curves, no
Tg signals of the polymers were detected. These results confirm
that the new PIs are dimensionally and thermally stable poly-
mers, which meet the requirement of heat resistance in the elec-
tronics industry.
and 426 nm, respectively. The higher fluorescence intensity of
PI(BTFBPD-BTFBPDA) compared to PI(BTFBPD-DPBPDA)
could be attributed to the incorporation of 30,40,50-tri-
fluorobiphenyl chromophore. In addition, from Fig. 9, the
optical band gap (i.e., the difference between the highest occu-
pied molecular orbital (HOMO) level and the lowest unoccupied
molecular orbital (LUMO) level) of PI(BTFBPD-DPBPDA) and
PI(BTFBPD-BTFBPDA) from the absoption edges of polymer
films are estimated to be 3.66 eV and 3.40 eV, respectively.
Therefore, the smaller optical band gap of PI(BTFBPD-
BTFBPDA) than that of PI(BTFBPD-DPBPDA) might result
from the more twisted configuration by introducing one more
3,4,5-trifluorophenyl moiety. The cyclic voltammetry curves of
PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) are
shown in Fig. 9, with the PI films coated on a glass carbon
electrode in anhydrous acetonitrile containing 0.1 M tetrabuty-
lammonium perchlorate as supporting electrolyte. The oxidation
Optical and electrochemical properties. The spectra and fluo-
rescence of the studied polymers in THF solution and thin film
state were shown in Fig. 9. The concentrations of the polymers
halfwave potential (E1/2
) for PI(BTFBPD-DPBPDA) and
PI(BTFBPD-BTFBPDA) were determined to be 1.67 V and
1.50 V vs. Ag/AgCl, respectively. The external ferrocene/ferro-
cenium (Fc/Fc+) redox standard potential, E1/2, was measured to
be 0.53 V vs. Ag/AgCl in acetonitrile. Assuming that the HOMO
level for the Fc/Fc+ standard is ꢀ4.80 eV with respect to the zero
vacuum level, the HOMO levels for the PI(BTFBPD-DPBPDA)
and PI(BTFBPD-BTFBPDA) are determined to be ꢀ5.94 eV
and ꢀ5.77 eV, respectively. Therefore, the LUMO levels of the
PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) are esti-
mated to be ꢀ2.28 eV and ꢀ2.37 eV, respectively. The results
suggest that PI(BTFBPD-DPBPDA) could provide a lower and
for the UV-vis absorption test were about 10ꢀ6 mol Lꢀ1
.
PI(BTFBPD-DPBPDA) shows the absorption peak maxima
(lmax) at 273 and 269 nm in solution and thin film state, while
those of PI(BTFBPD-BTFBPDA) were at 276 and 278 nm,
respectively. These were assignable to the p–p* transition, which
was attributed to the twisted structure between side chain phenyl
or biphenyl and the main chain biphenyl aromatic rings. When
these PIs of about 10ꢀ7 mol Lꢀ1 were excited at the maximum
absorption wavelength, the emission peaks of PI(BTFBPD-
DPBPDA) and PI(BTFBPD-BTFBPDA) were observed at 416
Table 1 Inherent viscosity and volubility behavior of polymers
a
Polymer
hinh (dL gꢀ1
)
DMAc
DMSO
DMF
NMP
THF
CHCl3
Acetone
Toluene
b
b
c
c
c
c
c
e
e
e
e
e
PI(BTFBPD-DPBPDA)
PI(BTFBPD-BTFBPDA)
0.78
0.64
b
b
b
d
a
inherent viscosity measured at a concentration of 0.5 g dLꢀ1 in DMAc at 30 ꢂC. b : soluble at room temperature. c : soluble on heating 80 ꢂC. d : slightly
soluble on heating 80 ꢂC. e : insoluble even on heating.
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more stable HOMO energy level. The additional twisted side-
groups in PI(BTFBPD-BTFBPDA) increased the LUMO energy
level slightly, probably due to the nonlinear or twisted confor-
mation of the polymer. In the Al/PI(BTFBPD-DPBPDA)/ITO
device, the energy barrier to hole injection from the electrode to
the active layer was estimated to be 0.74 eV from the work
function (F) of ITO (ꢀ5.20 eV) and the HOMO of the active PI
layer, and the energy barrier to electron injection from the elec-
trode to the active layer was estimated to be 1.92 eV from the F
of Al (ꢀ4.20 eV) and the LUMO of the active layer. This indi-
cates that hole injection from ITO into the HOMO of
PI(BTFBPD-DPBPDA) is easier than electron injection from
Al into the LUMO of PI(BTFBPD-DPBPDA). Therefore,
the PI(BTFBPD-DPBPDA) is a p-type material and holes
predominate the conduction process. In the Al/PI(BTFBPD-
BTFBPDA)/ITO device, the energy barrier to hole injection from
the electrode to the active layer was estimated to be 0.57 eV and
the energy barrier to electron injection from the electrode
to the active layer was estimated to be 1.83 eV, indicating that
the PI(BTFBPD-BTFBPDA) also a p-type material and the
conduction process in the device is dominated by hole injection
as well.
Moreover, this type of low conductivity state can be maintained
during the subsequent positive sweep (see sweep 4) and after
the power is turned off. Further, the retention characteristics of
the ON/OFF states for the Al/PI(BTFBPD-DPBPDA)/ITO
memory cell are shown in Fig. 10b. It is found that the memory
cell exhibits two stable states of ON and OFF within 200 min,
and the resulting ON/OFF ratio is as large as ꢃ103 under ꢀ1.0 V.
No degradation was observed during the test.
The electrical behavior of the Al/PI(BTFBPD-BTFBPDA)/
ITO device was also investigated as shown in Fig. 10c. During
voltage sweeping from 0 to ꢀ4.0 V, the current exhibits a sharp
increase at around ꢀ1.7 V, i.e., from 10ꢀ6 A to 10ꢀ2 A. This
indicates that the device is switched from a low conductivity state
(OFF state) to a high conductivity state (ON state), which serves
as the ‘‘writing’’ process for the memory device. After the tran-
sition, the device is still in the ON-state even after bias inter-
ruption (i.e., 1 min), as revealed by sweep 2. Interestingly, it is
observed that the device could not be switched to the low-
conductivity state during voltage sweeping from 0 to +6 V, as
indicated by sweep 3. The storage cells in device show high
repeatable I–V characteristics (sweep 4, sweep 5 and sweep 6).
The behavior of this memory device which could not be returned
to the OFF state by turning off the power and application of
a reverse bias exhibits that it is characteristic of a WORM
memory device. Further, it is found that an ON/OFF current
ratio as high as ꢃ104 is achieved under a constant stress of ꢀ1.0
V, and also exhibits superior stability, as displayed in Fig. 10d.
Further information about the charge transport mechanism
can be obtained from the I–V curves in OFF and ON states
according to various conduction models reported in the litera-
ture.36–39 As shown in Fig. 11, the OFF states for the Al/
PI(BTFBPD-DPBPDA)/ITO device exhibits space charge
limited current (SCLC, voltage from 0 to ꢀ1.3 V), which can be
modeled by the equation as follow:40 I ¼ 9A330mV2/8d3, where 330
is the absolute permittivity of PI(BTFBPD-DPBPDA), d is the
thickness of the polymer film, and m is the carriers mobility.
However, that for the Al/PI(BTFBPD-BTFBPDA)/ITO in the
OFF state can be elucidated as Poole–Frenkel (PF)41 emission in
terms of the plots of log(I/V) vs. V1/2 found to be linear in the
voltage rang of ꢀ0.25 to ꢀ1.6 V. On the other hand, the currents
in the ON state of both devices are almost linear in dependence
on applied voltage just as in metallic conduction (Fig. 11b,d); this
indicates that in the ON state the charge transport is dominated
Electrical characteristics of the PI-based memory devices. The
memory effect of PI(BTFBPD-DPBPDA) and PI(BTFBPD-
BTFBPDA) was revealed by the current–voltage (I–V)
measurements of an Al/polymer/ITO structure. Fig. 10a presents
the typical I–V curves of the memory devices fabricated with
PI(BTFBPD-DPBPDA). During voltage sweeping from 0 to
ꢀ4.0 V, the resulting current exhibits a sharp increase by two
orders of magnitude at ꢀ1.3 V. This indicates that the device is
switched from a low conductivity state (OFF state or ‘‘0’’ signal
in data storage) to a high conductivity state (ON-state, ‘‘1’’ signal
in data storage), which is equivalent to a ‘‘writing’’ process in
a digital memory cell. When the voltage is swept backward from
ꢀ4.0 V to 0 V, the device still remains in a high conductivity state.
Further, as the applied voltage is swept from zero to a positive
bias, the device is still in a high conductivity state in the initial
stage (i.e., corresponding to a larger current). However, the
resultant current decreases abruptly by two orders of magnitude
when the voltage is increased up to +4.0 V. This indicates that the
device is restored to an original low conductivity state, which
corresponds to the ‘‘erasing’’ process for the memory device.
Fig. 9 (a) UV-vis and photoluminescence spectra of PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) in THF solution and thin film state. (b)
Cyclic voltammograms of PI(BTFBPD-DPBPDA). (c) Cyclic voltammograms of PI(BTFBPD-BTFBPDA). PI films were coated on a glass carbon
electrode in acetonitrile containing 0.1 M tetrabutylammonium perchlorate with a Ag/AgCl (3.8 M KCl) reference electrode and a platinum plate
counter electrode. A scan rate of 50 mV sꢀ1 was used.
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Fig. 10 (a) Current–voltage (I–V) characteristics of a Al/PI(BTFBPD-DPBPDA)/ITO device with an electrode area of 0.0314 mm2.(b) Retention
characteristics of both ON and OFF states for the Al/PI(BTFBPD-DPBPDA)/ITO device under a constant stress (ꢀ1.0 V) at room temperature. (c)
Current–voltage (I–V) characteristics of a Al/PI(BTFBPD-BTFBPDA)/ITO device with a electrode area of 0.0314 mm2. (d) Retention characteristics of
both ON and OFF states for the Al/PI(BTFBPD-BTFBPDA)/ITO device under a constant stress (ꢀ1.0 V) at room temperature.
by the ohmic model. Moreover, the current level of our devices in
the ON state was found not to be dependent on the device cell
size (data not shown), which is indicative of heterogeneously
local filament formation.
barrier for hole injection is lower than that for electron injection,
so the conduction process in devices are dominated by hole
injection. Thus the pendant units might act as hole-trapping sites,
which would explain the observed memory behavior of the PIs-
based devices. The pendant units in the PI films are enriched with
holes when a bias is applied. At the same time, the imide rings in
the PI films are enriched with electrons. When the applied bias
reaches the threshold voltage, the trapped charges are able to
move through the trapped sites by means of a hopping process
(i.e. through filament formation), which results in current
flow between the bottom and top electrodes. However, switch-
ing behaviour of PI(BTFBPD-DPBPDA) and PI(BTFBPD-
BTFBPDA) are different. The reason why the device with
PI(BTFBPD-DPBPDA) film exhibits excellent flash memory
features, whereas that with PI(BTFBPD-BTFBPDA) film
exhibits very stable WORM memory behaviors, may be attrib-
uted to the difference in the fluorine atom population per unit
volume and the different tensional angles between the phenyl
rings in the tetracarboxylic dianhydride moiety. Firstly, consid-
ering the chemical structures, the population of fluorine atom,
which can be able to withdraw electrons and at the same time be
holder for holes, is quite different in the polymers. The highly
elelctronegative nature of the fluorine substitutent draws electron
density from the aromatic p-system, redering it electron-deficient
The above results suggest that the unipolar and bipolar ON
and OFF switching behaviors of PI flims can be fitted pretty
well according to proper theoretical model. Surprisingly, the
programmable, digital unipolar and bipolar bistable memory
behaviors of PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA)
in our study are quite different from the DRAM and WORM
characteristics of PIs which contain the trifluoromethyl groups
and triphenylamino moieties, such as 6F-TPA PI,13 6F-HAB-
TPAIE PI,24 and 6F-2TPA PI.25 These interesting bistable
memory behaviors may be attributed to differences between the
chemical environments of the hole and electron trapping sites in
the polymers. The trapping sites might arise due to the chemical
composition of the PIs chain, which have different type aryl
pendants and two imide rings per unit of the backbone. The aryl
pendants are electron donors and thus probably act as nucleo-
philic sites, whereas imide rings are electron acceptors and
probably act as electrophilic sites, so that all repeat units have
electric polarization characteristics. Thus, all these groups are
likely to act as charge-trapping sites, depending on their associ-
ations. As discussed above, for the PIs in this study the energy
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Fig. 11 Experimental and fitted data of I–V curves for the memory devices in the OFF and ON states. Panels (a) and (b) are the OFF state with the
SCLC model and the ON state with the ohmic current model for the Al/PI(BTFBPD-DPBPDA)/ITO device. Panels (c) and (d) are the OFF state with
the Poole–Frenkel model (ꢀ0.25 to ꢀ1.6 V) and the ON state with the ohmic current model for the Al/PI(BTFBPD-BTFBPDA)/ITO device.
and more suitable for electron accepting and transporting
applcations.42–45 In particular, the introduction of electron-
withdrawing sustituents such as fluorine atoms is expected to
lower the LUMO level in polyconjugated systems facilitating the
electron injection.46 So, the difference in the fluorine atom pop-
ulation per unit volume may be strongly related to the observed
memory behaviors. Secondly, PI(BTFBPD-BTFBPDA) have
two more 3,4,5-trifluorophenyl moieties in the repeat units than
PI(BTFBPD-DPBPDA). From the single X-ray analysis of tet-
going back to the initial state even after the application of reverse
bias.34,47 As a result, the PI(BTFBPD-DPBPDA) could relax to
the initial conformation after the removal of the applied electric
field for a short period of time. The dihedral angles between the
ring moieties reduce to the original state, and also, the potential
barriers of back CT disappear. Hence, the back CT occurs from
the phthalimide to the pendants (reset to the OFF state) and
confirms the volatile switching behavior. Contrarily, the
conductive CT state (ON state) of PI(BTFBPD-DPBPDA) is
sustained, and cannot be dissociated by a reverse bias without the
accompanied conformation relaxation and thus exhibits the
WORM switching behavior. Moreover, the more torsional
conformation contributes to a comparatively low conductance
state (OFF) and results in a larger current ratio of ON/OFF.
raethyl
2,20-bis[40-(300,400,500-trifluorophenyl)phenyl]biphenyl-
4,40,5,50-tetracarboxylate, the dihedral angles between the
30,40,50-trifluorophenyl ring and the 1,4-substituted phenyl ring
were up to 49.67ꢂ, which may cause more torsional conformation
in the PI(BTFBPD-BTFBPDA) backbone. The torsional
conformation at the excited state reduces the effective conjuga-
tion of the p electrons between neighboring aromatic moieties,
and generate an intramolecular potential barrier which prevents
the back transfer of charges. In a ring-containing organic
molecule, the torsional angle between two aromatic moieties will
increase under a static electric field, resulting in an enhanced
potential energy barrier for the intramolecular CT and possibly
prevent the recombination of segregated charges.47 This electrical
switching under the applied negative and positive threshold
voltage is suggested to form a stable environment temporarily
and store the transferred charges. It can prevent the charge from
Conclusions
In summary, we have synthesized and characterized a novel 2,20-
position aryl substituted diamine (BTFBPD) and anhydrides
(DPBPDA and BTFBPDA). Furthermore, these monomers were
used for the first time to synthesize two new polyimides,
PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA), which
not only exhibit good organosolubility, high thermal and
dimensional stability, but also present electric bistable memory
characteristics when using the polyimides as an active layer in
1820 | J. Mater. Chem., 2011, 21, 1810–1821
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memory devices. The fabricated Al/PI(BTFBPD-DPBPDA)/
ITO devices have demonstrated a flash type memory behavior
with the ‘‘write’’ and ‘‘erase’’ voltages of about ꢀ1.3 V and 4.0 V,
respectively. Whereas the Al/PI(BTFBPD-BTFBPDA)/ITO
devices exhibited a WORM type memory behavior with the
‘‘write’’ voltage of around ꢀ1.7 V. In addition, the ON/OFF
current ratios of 103–104 are observed in both memory devices.
The I–V curves in the OFF and ON state were fitted according to
various theoretical models, and it was found that the charge
transport in the OFF state was governed by the SCLC model
when the active layer was PI(BTFBPD-DPBPDA) film and the
PF model when the active layer was PI(BTFBPD-BTFBPDA)
film, while those in the ON state were both governed by ohmic
model. We envision these thermally, dimensionally stable
PI(BTFBPD-DPBPDA) and PI(BTFBPD-BTFBPDA) poly-
mers are promising materials for mass production at low cost for
high-performance, programmable, nonvolatile memory devices
that can be operated with low power consumption in unipolar
and bipolar switching modes.
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Acknowledgements
We are grateful to the financial support from State Key Labo-
ratory of ASIC & System, Fudan University (09KF001).
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