Chemical linkage functions of poly(ether imide)s on the resistive switching memory effects

Article abstract of DOI:10.1016/j.orgel.2014.10.050

A series of aromatic poly(ether imide)s, AZTA-PEIs containing triphenylamine and 1,2,4-triazole moieties are prepared and characterized. All the polymers with inherent viscosity from 0.58 to 1.1 dL/g show glass transition temperatures in the range of 250-278 °C. Resistive switching memory devices are constructed based on the processable poly(ether imide) (AZTA-PEIa). The device can be switched from the initial OFF state to the ON state under either positive or negative electrical sweep at about ±3.2 V. The ON state is nonvolatile and can maintain the high conducting state even turning off the electrical power and applying a reverse bias. The device fulfills the requirements of a write-once read-many times memory (WORM) with a high ON/OFF current ratio up to 10⁵ and a long retention time in both ON and OFF states. The bistable switching effects of the polymer result from the conformation-coupled charge transfer from electron donors (triazole-substituted triphenylamine moieties) to electron acceptors (phthalimide moieties). By comparing with the memory behaviors of analogue polymers, the functions of ether and imide in the chemical polymer structure on the memory behaviors are discussed.

Full text of DOI:10.1016/j.orgel.2014.10.050

Organic Electronics 16 (2015) 148–163
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Chemical linkage functions of poly(ether imide)s
on the resistive switching memory effects
Kun-Li Wang a, , I-Hao Shih a, Sheng-Tung Huang a, Hsin-Luen Tsai b
⇑
a Department of Chemical Engineering & Biotechnology, National Taipei University of Technology, 1, Sec 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan
b Department of Electronic Engineering, Kao Yuan University, 1821 Jhongshan Rd., Lujhu Dist, Kaohsiung 82151, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of aromatic poly(ether imide)s, AZTA-PEIs containing triphenylamine and 1,2,
4-triazole moieties are prepared and characterized. All the polymers with inherent viscos-
ity from 0.58 to 1.1 dL/g show glass transition temperatures in the range of 250–278 °C.
Resistive switching memory devices are constructed based on the processable poly(ether
imide) (AZTA-PEIa). The device can be switched from the initial OFF state to the ON state
under either positive or negative electrical sweep at about 3.2 V. The ON state is nonvol-
atile and can maintain the high conducting state even turning off the electrical power and
applying a reverse bias. The device fulfills the requirements of a write-once read-many
times memory (WORM) with a high ON/OFF current ratio up to 105 and a long retention
time in both ON and OFF states. The bistable switching effects of the polymer result from
the conformation-coupled charge transfer from electron donors (triazole-substituted
triphenylamine moieties) to electron acceptors (phthalimide moieties). By comparing with
the memory behaviors of analogue polymers, the functions of ether and imide in the
chemical polymer structure on the memory behaviors are discussed.
Received 19 September 2014
Received in revised form 25 October 2014
Accepted 30 October 2014
Available online 12 November 2014
Keywords:
Polymer memory
Nonvolatile
Data storage
Structure–property relationships
Resistive switching
Poly(ether imide)s
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
cessing, has allowed the simulation of various electronic
memory effects and devices [7], including dynamic ran-
Recently, resistive random access memory (RRAM) has
attracted great attention due to its potential for the
replacement of memory in next-generation memory appli-
cations [1,2]. Polymer electronic memories are potentially
an alternative or a supplementary technology to the
conventional memory technology facing challenges in
miniaturizing from micro- to nano-scale [1,2]. Polymer
memories can be processed at low cost and with their
use the multi-stack layer structures required for highly
dense memory devices can easily fabricated [3–6]. The
approach of properties-tuning by combined molecular
design and engineering, and materials synthesis and pro-
dom access memory (DRAM), [8–11] flash (rewritable)
memory, [12–18] write-one read-many-times (WORM)
memory [19–23] and static random access memory
(SRAM) [24–28]. With an understanding of the current
state of memory technology and the basic concepts of elec-
tronic memories, the historical development of polymer
electronic memories can be classified into three categories
by drawing the mechanistic analog between the polymer
memory element and one of the three primary circuit
elements, viz., capacitor, transistor and resistors.
Data storage in resistor-type memories is based on elec-
trical bistability (ON- and OFF-states) arising from changes
in intrinsic properties, such as charge transfer interaction,
phase/conformation changes and changes in redox states,
of the electro-active polymers in response to an applied
electric field. A number of polymer materials, including
conjugated polymers [29–31], polyimides [5,8,24,32–35],
⇑
Corresponding author. Tel.: +886 2 27712171x2558; fax: +886 2
27317117.
E-mail addresses: klwang@ntut.edu.tw, kunliwang@ntu.edu.tw
(K.-L. Wang).
http://dx.doi.org/10.1016/j.orgel.2014.10.050
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
149
poly(aryl ether)s [12], and others [4,36,37] have been
10 °C minꢀ1 and under a nitrogen flow rate of 20 cm3 minꢀ1
.
investigated for memory applications. Different kinds of
memory devices have been discovered and the driving
force behind these memory devices basically depends on
many kinds of mechanisms such as ‘‘trapping–detrapping’’,
and ‘‘charge transfer’’, and ‘‘conformational change’’ [1,2].
Kang and co-workers reported a flash memory effect of
PVK modified with covalently bonded C60 [36] and devel-
oped PVK derivatives [37] with flexible spacers between
the carbazole pendant groups and polymer backbone, for
the conformation-induced WORM and volatile memory
device applications, respectively. The results suggested
that the flexible spacers play a key role in the observed
memory properties of PVK derivatives. Wang et al reported
that the polyimide (AZTA-PIa) [20] and poly(arylene ether)
(AZTA-PEa) [12] containing triazole structures in the main
chain and triphenylamine groups in the side chain as
shown below exhibited, respectively, a write-once-and-
read-many-times (WORM) switching behaviors and a flash
memory switching behaviors. It will be interesting
and important for polymer memory materials to realize
poly(ether imide)s (AZTA-PEIa) containing triazole struc-
tures in the main chain and triphenylamine groups in the
side chain with both imide and ether structures. Functional
groups are important to chemical reactions and shows dif-
ferent physical and chemical properties. However, to the
best of our knowledge, there is no literatures discussing
the memory behaviors of polymers with similar polymer
backbone but with different functional linkages.
In this work, a series of functional poly(ether imide)s,
AZTA-PEIs, containing both triphenylamine electron donor
and 1,2,4-triazole acceptor groups and both ether and
imide function structures were synthesized and character-
ized as well as the switching effects being discussed. The
electronic memory behaviors of organo-soluble poly(ether
imide), AZTA-PEIa, containing both phthalimide and ether
functional groups were compared with polyimide (AZTA-
PIa) containing imide structure, and poly(arylene ether)
(AZTA-PEa) containing ether structure (as shown in
Scheme 1). The previously reported polymer (OXTA-PIa)
[19] showing the similar switching behavior will also
compared. The imide and ether functional groups in the
poly(ether imide)s playing different roles on the memory
behaviors will be discussed.
Weight-average (Mw) and number-average (Mn) molecular
weights were determined by gel permeation chromatogra-
phy (GPC) on a Water GPC system equipped with four
Waters Ultrastyragel columns (300 ꢁ 7.5 mm, guarded
and packed with 105, 104, 103 and 500 Å gels in series).
N,N-dimethylformamide (DMF, 1 mL minꢀ1) was used as
the eluent which was monitored with a UV detector (JMST
Systems, VUV-24, USA) at 254 nm. Polystyrene was used as
the standard. UV–visible absorption and fluorescence
spectra were measured on a Shimadzu UV-NIR 1601 spec-
trophotometer and Shimadzu RF 5301PC fluorescence
spectrophotometer, respectively. Cyclic voltammetry (CV)
was performed on a CHI model 619A Electrochemistry
Workstation with ITO as the working electrode and a
platinum wire as the auxiliary electrode at a scan rate of
50 mV sꢀ1 against a Ag/Ag+ reference electrode in a 0.1 M
acetonitrile (CH3CN) solution of tetrabutylammo-nium
perchlorate (TBAP). The thickness of the polymer film cast
on the indium–tin oxide (ITO) coated glass substrate was
determined from the edge profile of the film, using the tap-
ping mode, on a Veeco multimode atomic force microscope
equipped with a Nanosensors PPP-NCHR silicon tip.
2.2. Materials for synthesis
Diphenylamine, 4-aminophenol, 4-fluorobenzoyl chlo-
ride, 4-fluoronitrobenzene, acetic anhydride, and pyridine
were purchased from Acros Chemical Co. and were used
as received. Phosphorus pentachloride, hydrazine monohy-
drate, and 10% palladium on activated carbon (Pd/C) were
purchased from Riedel-de Haën Chemical Co., Alfa Aesar
Chemical Co., and Merck Chemical Co., respectively, and
were used as received. TBAP was obtained from Acros
and recrystallized twice from ethyl acetate and then dried
in vacuum before use. The aromatic tetracarboxylic dian-
hydrides, 4,40-hexafluoroisopropylidene- diphthalic anhy-
dride (6FDA, 7a; from CHRISKEV), 4,40-sulfonyldiphthalic
anhydride (SDPA, 7b; from TCI), 4,40-oxydiphthalic anhy-
dride (ODPA, 7c; from TCI), 3,30,4,40-benzophenone tetra-
carboxylic dianhydride (BTDA, 7d; from CHRISKEV),
pyromellitic dianhydride (PMDA, 7e; from CHRISKEV),
3,30,4,40-biphenyltetra- carboxylic dianhydride (BPDA, 7f;
from TCI) and were sublimated before use. N-methyl-
2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc),
dimethyl sulfoxide (DMSO), and toluene were purchased
from Tedia Chemical Co. Tetrahydrofuran (THF) and DMF
were purchased from Echo Chemical Co. 1,3-Dimethyl-
3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) was pur-
chased from Aldrich Chemical Co.
2. Experimental section
2.1. Instrumentation
FT-IR spectra of the synthesized monomers and poly-
mers were recorded on a Perkin–Elmer GX FTIR spectro-
photometer. NMR spectra were measured on a Bruker
DRX-500 NMR spectrometer. Elemental analyses were
carried on a Perkin–Elmer 2400 elemental analyzer. The
inherent viscosities of polyimides were measured with an
Ubbelohde viscometer. Thermogravimetric analysis (TGA)
was conducted on a Perkin–Elmer Pyris 6 TGA thermogravi-
metric analyzer at a heating rate of 10 °C minꢀ1 and under a
nitrogen or air flow rate of 20 cm3 minꢀ1. Differential
scanning calorimetric (DSC) analysis was performed on a
Perkin–Elmer Pyris DSC 6 calorimeter at a heating rate of
2.3. Synthesis of the monomers and polymers
The synthesis routes for the monomers and polymers
are illustrated in Schemes 2 and 3, respectively. The
intermediate compounds 1–6 were prepared according to
previously reported literature [12].
2.3.1. Synthesis of the diamine (AZTAE)
A two-necked 50-mL glass reactor with Dean-Stark
trap was charged with 4-aminophenol (1.02 g, 9 mmol),

150
K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
WORM
flash
N
N
N
N
O
O
CF3
CF3
CF3
CF3
N
O
N
O
O
N
N
N
N
O
n
n
AZTA-PEa
AZTA-PIa
N
O
WORM
WORM? flash?
N
N
N
N
O
O
N
O
O
CF3
CF3
CF3
CF3
N
O
O
O
N
O
N
N
N
O
O
n
AZTA-PEIa
n
OXTA-PIa
Scheme 1. Chemical structures of the analogue polymers and their memory types.
difluoro compound 6 (2.04 g, 4 mmol), a solvent mixture of
DMPU (16 mL) and trace toluene, and an excess of K2CO3
(4.07 g). The reaction mixture was then heated to 130 °C
and reacted for 12 h, and then heated to 170 °C for 48 h.
After the reaction was completely promoted, the reaction
mixture was poured into distilled water after cooling to
room temperature. The mixture was extracted with chloro-
form, and the extract was washed with distilled water,
dried with anhydrous magnesium sulfate, and then con-
centrated under reduced pressure. The crude products
were purified by column chromatography with an ethyl
acetate as an eluent. Evaporation of the eluent produced
a light yellow solid of diamine monomer, AZTAE (48%);
m.p. 265 °C (by DSC). FT-IR (KBr, cmꢀ1): 1199 (AOA);
3359, 3454 (ANH2). 1H NMR (500 MHz, [d6]DMSO, 25 °C,
TMS): d 5.03 (s, 4H, NH2), d 6.61–6.62 (t, 4H, ArH,
J = 8.7 Hz), d 6.74–6.76 (d, 4H, ArH, J = 8.7 Hz), d 6.81–
6.83 (d, 4H, ArH, J = 8.8 Hz), d 6.88–6.90 (d, 2H, ArH,
J = 8.8 Hz), d 6.98–7.00 (d, 4H, ArH, J = 7.8 Hz), d7.04–7.07
(t, 4H, ArH, J = 7.4 Hz), d 7.14–7.15 (d, 2H, ArH, J = 8.8 Hz),
d 7.24–7.27 (t, 4H, ArH, J = 7.8 Hz), d 7.35–7.37 (d, 4H,
ArH, J = 8.8 Hz). 13C NMR (DMSO, ppm): d 115.0, 115.9,
120.4, 121.4, 122.2, 124.1, 124.8, 128.0, 129.3, 130.0,
130.2, 144.6, 146.0, 146.4, 148.3, 153.9, 160.2. Anal Calac
for C44H34N6O2: C 77.86%, N 12.38%, H 5.05%. Found: C
77.62%, N 12.23%, H 5.08%.
(1 mmol) with stirring. The mixture was stirred at ambient
temperature for 4 h to form viscous poly(amic acid). Chem-
ical imidization was carried out by addition of 1.75 mL of
acetic anhydride and 0.85 mL of pyridine into the above
poly(amic acid) solution, followed by heating the mixture
at 120 °C for another 6 h. The polymer solution was poured
slowly into 300 mL of methanol with stirring. The precipi-
tate was filtered, washed with hot methanol, and dried at
100 °C under reduced pressure. The structure of the poly
(ether imide) (AZTA-PEIa) was identified by FT-IR and
NMR spectroscopes.
FT-IR (KBr, cmꢀ1) 1373 (CAN), 1723, 1782 (C@O); 1H
NMR (500 MHz, [d6]DMSO, 25 °C, TMS): 6.93–6.95 (d, 2H,
ArH, J = 7.3 Hz), 7.03–7.07 (m, 10H, ArH), 7.19–7.28 (m,
10H, ArH), 7.49 (m, 8H, ArH), 7.75 (s, 2H, ArH), 7.96–7.97
(d, 2H, ArH, J = 6.7 Hz), 8.16–8.18 (d, 2H, ArH, J = 7.4 Hz).
The other polyimides were prepared by similar proce-
dures using the corresponding dianhydride (7b–7e)
instead of 4,40-hexafluoroisopropylidene- diphthalic dian-
hydride (7a).
2.4. Device fabrication and characterization
The indium–tin oxide (ITO) coated glass substrate was
pre-cleaned sequentially with deionized water, acetone
and isopropanol in an ultrasonic bath for 15 min. A
100 l )
L DMAc solution of the polyimide (10 mg mLꢀ1
2.3.2. Synthesis of the poly(ether imide)s (AZTA-PEI)s
The poly(ether imide)s were all prepared in similar pro-
cedures. Typical procedures for AZTA-PEIa preparation is
shown as follow:
The dianhydride 6FDA (7a; 1 mmol) was added slowly
to a NMP solution (2 ml) of the novel diamine AZTAE
was spin-coated onto the ITO substrate at a spinning rate
of 1200 rpm, followed by solvent removal in a vacuum
oven at 10ꢀ5 Torr and 60 °C for 10 h. The thicknesses of
all the polyimide films were about 50 nm, as determined
by the AFM edge profiling. Finally, an Al top electrode of
about 400 nm in thickness was thermally deposited onto

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
151
N
N
N
H
+
SnCl2
NaH
DMF
ethanol
F
O2N
NH2
2
NO2
1
O H H O
NH2NH2H
2O
PCl5
F
COCl
F
F
C
N N C
NMP
Toluene
4
3
N
N
HO
NH2
Cl
C
Cl
C
2
F
F
N N
K2
CO
3, DMPU
xylene
5
F
F
N
N
6; AZTA
N
N
O
NH2
H2N
O
N
N
AZTAE
Scheme 2. Synthesis of the novel diamine AZTAE.
the polymer surface through a shadow mask at a pressure
of about 10ꢀ7 Torr. Electrical property measurements were
carried out on devices of 0.4 ꢁ 0.4 mm2, 0.2 ꢁ 0.2 mm2,
and 0.15 ꢁ 0.15 mm2 in size, under ambient conditions,
using an Agilent 4155C semiconductor parameter analyzer
equipped with an Agilent 41501B pulse generator. The cur-
rent density–voltage (J–V) data reported were based on
device units of 0.4 ꢁ 0.4 mm2 in size, unless stated other-
wise. ITO was maintained as the ground electrode during
the electrical measurements.
from the density function theory (DFT), using the Becke’s
three-parameter functional with the Lee, Yang and Parr
correlation functional method (B3LYP) and the basis set
6-31G(d) (in short, DFT B3LYP/6-31G(d)). Based on the
ground state optimized geometry, the optimized geometry,
dipole moment and ESP surface of the first singlet excited
state were calculated with configuration interaction with
single excitations (CIS) method [38]. For both molecular
simulations, vibrational frequencies were calculated ana-
lytically to ensure that the optimized geometries really
correspond to the total energy minima.
2.5. Molecular simulation
3. Results and discussion
Molecular simulations of the polyimide molecules were
carried out with the Gaussian 09 (Revision C. 01) program
package on an Intel Xeon EM64T workstation. The calcula-
tions for the ground and excited states were carried out
with 1 CPU and 2 GByte memory, and 4 CPUs and 8 GByte
memory, respectively. Electronic properties of the polyi-
mides at the ground state, including molecular orbitals
and electrostatic potential (ESP) surface, were calculated
3.1. Characterization of diamine monomer and polymers
The novel diamine (AZTAE), containing electron-rich tri-
phenylamine and electron-deficient triazole moieties and
flexible ether groups was prepared according to the syn-
thetic route shown in Scheme 2. The intermediates 1–6
were prepared according to the reported literature [12].

152
K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
N
AZTAE
+
O
O
O
O
N
O
O
N
H
N
H
Ar
Ar
O
O
NMP, r.t
N
N
COOH
HOOC
O
O
AZTAE-PAA
n
7
N
O
O
N
Acetic anhydride
Pyridine, 120oC
N
Ar
N
O
O
N N
O
O
n
AZTA-PEI
7
Ar:
CF3
C
CF3
O
S
O
O
a
6FDA
SDPA
PMDA
b
ODPA
c
O
C
d
BTDA
e
BPDA
f
Scheme 3. Synthesis of polyimides (AZTA-PEI)s by two-step polymerization.
The synthesis of the diamine was obtained from difluoro
compound (6) and 4-aminophenol, which is based on the
nucleophilic displacement of the active fluoro substituents
by potassium phenoxide, in polar aprotic solvents such as
DMPU. The 1,2,4-triazole moiety can accept a negative
charge and lower the activation energy for the displacement
of the p-substituted fluoro groups through a Meisenheimer
complex. The prepared intermediates and desired diamine
were identified by elemental analysis, FT-IR, 1H NMR and
13C NMR spectroscopes, as well as two-dimensional NMR
techniques. The characteristic absorption at 1079 cmꢀ1 for
the fluoro group of compound 6 disappeared, while a new
absorption at 1199 cmꢀ1 for ether group and two
absorption bands at 3359 and 3454 cmꢀ1 for amino
group appeared in the FT-IR spectra. The 1H, 13C and
two-dimensional HMQC NMR spectra of the novel AZTAE
diamine with peak assignments are shown in Fig. 1. A new
chemical shift at 5.03 ppm, characteristic peak of the amino
groups, appeared in the 1H NMR spectrum (Fig. 1(a)). The
assignments and integrals of the peaks in NMR spectra are
consistent with the proposed structure of AZTAE diamine.
There are 17 peaks in the 13C NMR spectrum (Fig. 1(b)) for
the proposed AZTAE diamine. The peak at 153.9 ppm for
the carbon in the triazole ring (C9 in the 13C NMR spectrum)
supports the successful cyclocondensation of the triazole
ring. The peak assignments and atom connectivity were
confirmed again by the HMQC spectrum (Fig. 1(c)). The
FT-IR, NMR spectroscopies and elemental analysis results
clearly confirm that the AZTAE diamine synthesized herein
is consistent with the proposed structure.

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
153
Fig. 1. (a) 1H (b) 13C and (c) two dimensional HMQC NMR spectra of AZTAE.

154
K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
The synthesized ether diamine was used in the prepara-
presence of the flexible phenoxyl group [42]. On the other
hand, in comparison to the previously reported poly(aryl
ether)s (AZTA-PEs; 187–234 °C) [12], AZTA-PEI polymers
exhibit higher glass transition temperatures due to
their rigid imide structure in the main chains. That is, for
the glass transition temperatures, AZTA-PIs > AZTA-PEIs >
AZTA-PEs due to their rigid imide and flexible ether main
chain linkages. The thermal stability of the polyimides
was evaluated under a nitrogen atmosphere at a heating
rate of 10 °C minꢀ1. The temperatures for 10% weight loss
of the poly(ether imide)s in nitrogen atmosphere ranged
from 450 to 526 °C. The char yields of the polymers at
800 °C in nitrogen ranged from 41 to 55 wt%.
tion of poly(ether imide)s by a conventional two-step poly-
condensation with various commercially available aromatic
dianhydrides as shown in Scheme 3. The polycondensation
was carried out at ambient temperature to form the corre-
sponding poly(amic acid)s, and followed by chemical cyc-
lodehydration. The polymer structures were confirmed by
FT-IR and NMR spectra. All the polyimides showed the char-
acteristic absorption bands of the imide ring at around
1780 cmꢀ1 (asymmetric C@O stretch) and 1725 cmꢀ1 (sym-
metric C@O stretch), while the characteristic absorptions at
3300–3400 cmꢀ1 for the amino groups of the diamine
monomer (AZTAE) disappeared. The 1H and COSY NMR
spectra of the soluble AZTA-PEIa are shown in Fig. 2. The
disappearance of amino peak for the AZTAE diamine at
5.03 ppm in 1H NMR spectrum suggests that polymeriza-
tion has been carried out efficiently. Assignments of each
proton are also presented in Fig. 2(a), and the spectrum
agrees well with the proposed polymer structure. The two
dimensional COSY spectrum in Fig. 2(b) confirms again
the assignments and purposed polymer structure. The
inherent viscosity and molecular weights of the prepared
poly(ether imide)s are summarized in Table 1. The poly(-
ether imide)s obtained by chemical imidization had inher-
ent viscosities of 0.58–1.1 dL gꢀ1 in NMP. The number
average molecular weights (Mn) and weight average molec-
ular weights (Mw) were 3.6–8.0 ꢁ 104 and 7.1–15.8 ꢁ 104,
respectively, and the polydispersity index (PDI = Mw/Mn)
was from 1.95 to 1.98, as measured by GPC and referenced
to polystyrene standards.
The solubility of the prepared polyimides is summarized
in Table 2. The solubility of polyimides depends on their
chain packing abilities and intermolecular interactions,
which in turn are influenced by the rigidity, symmetry,
and regularity of the polymer backbone [39]. These pre-
pared poly(ether imide)s showed better solubility than
those analogous polyimides (AZTA-PIs) [19] containing
1,2,4-triazole structures in the main chain and triphenyl-
amine groups in the side chain due to the flexible ether moi-
eties in the main chain. The polymer AZTA-PEIa exhibited
excellent solubility in a variety of solvents, such as NMP,
DMAc, DMF, DMSO, chloroform, and 1,4-dioxane at room
temperature. The excellent solubility of AZTA-PEIa results
from not only the flexible ether group but also from the
introduction of 4,40-hexafluoroisopropylidenediphthalic
dianhydride (6FDA), which prohibits a close packing of the
chains, and increases the affinity of the chains to the polar
solvents [39–42].
The optical properties of poly(ether imide)s were inves-
tigated by UV–visible absorption and photoluminescence
(PL) spectroscopy in chloroform. The optical absorption
and photoluminescence of the polymers are almost the
same due to having the same chromophore, triphenyl-
amine-substituted triazole, in the polymer structures.
Fig. 3 shows the UV–visible absorption and PL spectra of
AZTA-PEa, AZTA-PIa and AZTA-PEIa in chloroform with
that of triphenylamine-substituted triazole difluoro com-
pound (6; AZTA) in chloroform as the reference. As shown
in Fig. 3(a), the AZTA difluro compound in chloroform
exhibits a maximum absorption peak at 300 nm, which is
mainly attributed to the
p ? p elec-
p⁄ transition of the
tronic system delocalized along the triphenylamine-substi-
tuted triazole moieties. The absorption of AZTA-PEa and
AZTA-PIa in chloroform shows a maximum absorption
respectively, at 291 and 290 nm, which is blue shifted by
about 10 nm from that of AZTA after polycondensation.
The absorption maximum in the film states are very close
to that in solutions. The absorption edge of AZTA-PIa film
extends to about 352 nm, from which the band gap energy
(Eg) of AZTA-PIa is determined to be about 3.52 eV. In
Fig. 3(b), the PL spectra of AZTA-PEa, AZTA-PIa and
AZTA-PEIa in chloroform are compared to that of the AZTA
monomer. All the emission spectra were obtained with an
excitation wavelength of 280 nm. AZTA and AZTA-PEa
exhibit emission peaks at about 418 nm, which is red-
shifted by 64 nm in comparison to the emission (354 nm)
of 1,2,4-triazole gro>up [31]. This observation suggests
that the emission of AZTA and AZTA-PEa originates from
the intramolecular charge transfer (CT) between the tri-
phenylamine and triazole group [32]. The fluorescence of
AZTA-PIa and AZTA-PEIa, however, are significantly
quenched, indicating a considerable decay of the excited
singlet state of the AZTA moieties, probably by charge or
energy transfer between AZTA and the phthalimide moie-
ties. The poor match between the emission spectrum of
AZTA and the absorption spectrum of 6FDA [24] rules out
the possibility of energy transfer [43]. CT between AZTA
and the phthalimide moieties is thus accountable for the
fluorescence quenching in AZTA-PIa and AZTA-PEIa. This
donor–acceptor interaction process will be studied further
by molecular simulation (see below).
The thermal properties of the poly(ether imide)s were
evaluated by DSC and TGA. The results are summarized in
Table 3. The glass transition temperatures (Tg) of the
poly(ether imide)s were found to range from 250 to
278 °C. The polymer, AZTA-PEIc, obtained from ODPA (7c)
showed the lowest Tg (250 °C) because of the presence of
a flexible ether linkage between the phthalimide units.
The polymer, AZTA-PEIe, obtained from PMDA (7e) showed
the highest Tg (278 °C) due to its rigid backbone. In compar-
ison to the previously reported polyimides (AZTA-PIs; 262–
314 °C) [19], AZTA-PEI polymers with similar molecular
structures exhibit lower glass transition temperatures,
arising from its more flexible molecular chain due to the
3.2. Electrochemical properties
The electrochemical property of the soluble polyimide,
AZTA-PEIa, was investigated by cyclic voltammetry

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
155
Fig. 2. (a) 1H and (b) COSY NMR spectra of AZTA-PEIa.
conducted in 0.1 M dry acetonitrile solution of TBAP under
a nitrogen atmosphere for the cast film on an ITO-coated
glass substrate. The polymer exhibited a stable reversible
redox couple at E1/2 = 0.65 V. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecu-
lar orbital (LUMO) energy levels can be calculated from the
redox couple and the band gap of the copolymer based on
the reference energy level of ferrocene (4.8 eV below the

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K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
Table 1
the low-conductivity (OFF) state. The current density
remained low during the 1st negative sweep (with Al as
the cathode and ITO as the anode) from 0 to ꢀ4 V until
the switching threshold voltage of ꢀ3.2 V was reached. At
this voltage, the current density was observed to increase
abruptly from 10ꢀ7 A cmꢀ2 to 10ꢀ2 A cmꢀ2, indicating the
device transition from the initial OFF state to the high-con-
ductivity (ON) state. This electrical transition is equivalent
to the ‘‘writing’’ process in a digital memory cell. The device
exhibited good stability in the ON state during the subse-
quent negative (the 2nd sweep) and positive sweeps (the
3rd sweep), and it did not return to the OFF state even after
turning off the power or upon applying a reverse sweep.
The non-volatile and non-rewritable natures of the ON
state demonstrate that the AZTA-PEIa-based device exhib-
its write-once read-many-times (WORM) type memory
effects. The ON/OFF current ratio is in the order of 105 at
ꢀ1 V. The devices with different active areas of
0.4 ꢁ 0.4 mm2, 0.2 ꢁ 0.2 mm2, and 0.15 ꢁ 0.15 mm2 show
almost the same J–V characteristics, indicating that the
current density is independent on the device areas. This
area-independent current density, good reproducibility
and stability of the memory effects, together with the
molecular structure-dependent electrical switching behav-
iors, rule out the possibility of filament conduction [44] or
polymer degradation effects in the present devices.
In addition to the memory effect, other parameters, such
as stability, ON/OFF current ratio and read cycles at room
temperature, are equally important to the performance of
a memory device. Fig. 4(b) and (c) shows, respectively,
the effect of operation time and read pulses on the memory
devices at room temperature. Under a constant stress of
ꢀ1 V, or read pulses of ꢀ1 V, no obvious degradation in cur-
rent density was observed for both the ON and OFF states of
the present device as shown in Fig. 4(b). The ON and OFF
states are also stable up to 1 ꢁ 108 read pulses of ꢀ1 V, as
shown in Fig. 4(c). The results reveal the stability of the
device under ambient conditions. The switching effect of
the device was also tested with an initial positive electrical
sweep, as shown in Fig. 4(d). During the 1st positive sweep
from 0 to 4 V, the current density remained low, until the
switching threshold voltage of 3.2 V was reached, where
an abrupt increase in the current density was observed.
The results indicate that the AZTA-PEIa device can be writ-
ten bi-directionally, with comparable positive and negative
switching threshold voltages; while the ON state cannot be
turn back to OFF state even applying a reverse bias. The sta-
bility of the AZTA-PEIa device under elevated temperature
was also tested. Both the ON and OFF states were stable at
100 °C under ambient conditions over a 6-h period, as
shown in Fig. 5(e), although the current curve at ON and
OFF states vary a little bit zigzag as comparing to that at
room temperature (Fig. 4(b)). This result indicates that
the present memory device possesses good stability and
performance at 100 °C. However, when the device in the
ON state was heated at 150 °C for 10 min and then cooled
to room temperature, the device converted to OFF state as
shown in the fourth sweep of Fig. 4(f). The device in the ini-
tial OFF state was switched to ON state at ꢀ3.2 V and
retained in the ON state as described above (first to third
sweeps). Interestingly, after being heated at 150 °C for
Inherent viscosity and molecular weights of prepared polyimides.
Polymer
ginh (dL/g)a
Mn (ꢁ104)b
Mw (ꢁ104)b
15.8
7.1
PDI
AZTA-PEIa
AZTA-PEIb
AZTA-PEIc
AZTA-PEId
AZTA-PEIe
AZTA-PEIf
0.88
0.58
0.86
8.0
3.6
7.8
1.98
1.97
1.95
15.2
c
d
d
d
–
–
–
–
–
–
–
–
–
–
–
c
d
d
d
d
d
d
1.1
a
Inherent viscosity of the poly(ether imide) precursor measured at a
concentration of 0.5 g/dL in NMP at 30 °C.
b
The molecule weight of the poly(ether imide)s was measured by GPC
in DMF.
c
The polymer was insoluble in NMP.
The polymer was insoluble in DMF.
d
Table 2
Solubilitya of prepared poly(ether imide)s.
Polymer
Solvents
DMAc THF CHCl3 DMF NMP DMSO DOX
AZTA-PEIa ++
AZTA-PEIb ++
AZTA-PEIc ++
AZTA-PEId
AZTA-PEIe
AZTA-PEIf
++
ꢀ
++
ꢀ
++
++
++
++
++
++
++
++
ꢀ
++
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
++
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
++
ꢀ
Abbreviations: NMP: N-methyl-2-pyrrolidinone; DMAc: N,N-dimethyl-
acetamide; DMF: N,N-dimethylformamide; DMSO: dimethylsulfoxide;
THF: Tetrahydrofuran; DOX: 1,4-Dioxane.
a
Solubility: measure in the concentration of 1 mg/1 mL; ++, soluble at
room temperature; +, soluble on heating; , partially soluble on heating;
ꢀ, insoluble on heating.
Table 3
Thermal properties of prepared poly(ether imide)s.
a
b
Polymer
Tg (°C)
Td5
Td10
Char yield (%)c
AZTA-PEIa
AZTA-PEIb
AZTA-PEIc
AZTA-PEId
AZTA-PEIe
AZTA-PEIf
273
277
250
262
278
266
501
418
458
497
471
497
526
450
481
526
507
518
55
43
41
46
53
50
a
Glass transition temperature (Tg) measured by DTG at a heating rate
of 10 °C/min.
b
Decomposition temperature recorded on DTG at a heating rate of
10 °C/min under nitrogen flow.
c
Char yield (%) at 800 °C under nitrogen flow.
vacuum level, which is defined as zero). The HOMO energy
level of AZTA-PEIa was evaluated to be ꢀ5.08 eV. The
LUMO level of the conjugated polymers was calculated
according to the equation: LUMO = HOMO + Eg. From the
optical bandgap of 3.52 eV, the LUMO level for the polymer
was calculated to be ꢀ1.56 eV.
3.3. Electrical switching effects
Electrical switching behavior and memory effects of
AZTA-PEIa were demonstrated by current density–voltage
(J–V) characteristics of an ITO/AZTA-PEIa/Al sandwich. As
shown in Fig. 4(a) the as-fabricated device was initially in

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
157
Fig. 3. (a) UV–visible absorption spectra of AZTA-PEa, AZTA-PIa and AZTA-PEIa in chloroform, with that of AZTA as the reference. The concentration of
AZTA was about 5 ꢁ 10ꢀ7 mol Lꢀ1. The concentrations of the rest three polyimides were adjusted in order for the amount of their repeat units to be
comparable to that of AZTA. All the absorption spectra are normalized to the maximum absorption peak of AZTA for ease of comparison. (b)
Photoluminescence spectra of AZTA-PEa, AZTA-PIa and AZTA-PEIa in chloroform, as well as that of AZTA compound as reference. All the emission spectra
were obtained with the excitation wavelength of 280 nm.
10 min, the device in the ON state was found to return to its
OFF state (the fourth sweep). The device could be switched
again to the ON state at the threshold voltage of ꢀ3.3 V and
remained in the ON state. It could not be returned to the
OFF state even after turning off the power (the fifth sweep)
or upon applying a reverse sweep (the sixth sweep). The
results indicate that the turn-on phenomenon do not orig-
inate from chemical reactions. The reasons are discussed in
the switching mechanism section.
In previously reported memory devices made from aro-
matic polyimide (AZTA-PIa) and poly(arylene ether)
(AZTA-PEa), AZTA-PIa exhibited WORM memory behav-
iors [19] while AZTA-PEa showed flash memory behaviors
[12]. The electrical switching behaviors and memory
effects of the poly(ether imide), AZTA-PEIa, are very simi-
lar with AZTA-PIa, but very different from AZTA-PEa. This
suggests that the phthalimide group plays an important
role on the memory effects. The phenomena is also consis-
tent with the photoluminescent behavior (Fig. 3(b)).
3.4. Switching mechanism
The electrical switching behavior and memory effects of
AZTA-PEIa demonstrated by current density–voltage (J–V)
characteristics of an ITO/AZTA-PEIa/Al sandwich device
exhibits similar WORM type memory effects as the devices
fabricated from AZTA-PIa [20] and OXTA-PIa [19] with
comparable ON and OFF current densities. The only differ-
ence consists in their switching threshold voltages, with
1.8 V for OXTA-PIa, 2.5 V for AZTA-PIa and 3.2 V for
AZTA-PEIa. Therefore, the switching mechanism is same
with the AZTA-PIa, which is based on a conformation-cou-
pled charge transfer process induced by the applied elec-
tric field [20]. The different switching threshold voltages
associated with the difference in their molecular structures
will be elucidated with molecular simulation.
To obtain a better understand of the switching behavior
of the polymer-based memory device, the electronic prop-
erties of the polyimide were studied by density functional

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K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
Fig. 4. Memory effects of a 0.4 ꢁ 0.4 mm2 ITO/AZTA-PEIa/Al device. The sequence and direction of each sweep are indicated by the respective number and
arrow. (a) Current density–voltage (J–V) characteristics with an initial negative electrical sweep. (b) Effect of operation time on the ON and OFF states of the
memory devices under a constant stress of ꢀ1 V at room temperature. (c) Effect of read pulse of ꢀ1 V on the ON and OFF states of the memory devices. (d) J–
V characteristics with an initial positive electrical sweep. (e) The stability of ON and OFF states of the memory devices under a constant stress of ꢀ1 V at
100 °C. (f) J–V characteristics with an initial negative electrical sweep. The 4th–6th sweeps were conducted sequentially after heating the device at 150 °C
for 10 min under vacuum.
theory (DFT). Calculations of the molecular orbitals of the
basic units (BU), taking all of the component functional
moieties into account, i.e., the triphenylamine, phthalimide
and triazole moieties, were carried out at the B3LYP/
6-31G(d) level with the Gaussian 09 program package.
The highest occupied molecular orbital (HOMO) and low-
est unoccupied molecular orbitals (LUMOs) of the BU are
presented in Fig. 5, along with the plausible electronic
transitions under excitation. The HOMO of the polyimide
is located mainly on the triphenylamine moieties, while

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
159
Fig. 5. Calculated molecular orbitals of the basic unit (BU) and the plausible electronic transitions of AZTA-PEIa under the excitation of an electric field.
the two lower LUMOs (LUMO1 and LUMO2) is located on
the phthalimide moieties, indicating that the triphenyl-
amine moieties act as the main electron donors and the
phthalimide moieties act as the electron acceptors in the
polymer. In addition to LUMO1 and LUMO2, the higher
LUMO3 is located on the triazole moieties, suggesting that
the triazole moieties in AZTA-PEIa can serve as mediators
to facilitate the intramolecular CT. Upon excitation, elec-
trons in the HOMO can transit locally to LUMO4 on the tri-
phenylamine moieties, forming a locally excited (LE) state.
AZTA-PIa [20], the twisted molecular chain in the confor-
mation-coupled charge transfer state, which is referred to
as the twisted intramolecular charge transfer (TICT), can
give rise to a large potential energy barrier for the dissoci-
ation of CT complexes. As a result, the conductive CT state
is stabilized and the high-conductivity (ON) states is
retained. Once a high-conductivity state or path is formed,
carriers can move in both directions. Thus, a reverse bias of
the same magnitude will not return the device to the low-
conductivity state. Such non-volatile and non-rewritable
WORM type memory effects are also observed in the previ-
ous polyimide (OXTA-PIa) and the present poly(ether
imide) (AZTA-PEIa) memory devices. The memory behav-
iors are compared further by molecular simulation.
Molecular simulation of the excited state of the polymer
was carried out to further elucidate the conformation-cou-
pled charge transfer process. The optimized geometry and
electrostatic potential (ESP) surface of the BU in the excited
state were studied at the CIS/6-31G(d) level with the
Gaussian 09 program package. Fig. 6 summarizes the
molecular simulation results of both the ground and
excited states of the BU. The molecular simulation of
previously reported polyimides (OXTA-PIa and AZTA-PIa)
with similar WORM behaviors was also shown in Fig. 6
Excitation of the triphenylamine moieties leads to
a
decrease in ionization potential and consequently pro-
motes intramolecular CT. As a result, the LE state is easily
transformed to the CT state through the relaxation of elec-
trons from LUMO4 to LUMO1 via LUMO2 and LUMO3. The
CT state can also form through the direct transition of elec-
trons from HOMO to LUMO1. Under an electric field, the
generated holes can delocalize among the triphenylamine
donor moieties, generating an open channel in the HOMO
for the charge carriers (holes) to migrate through. Thus,
the current density increases rapidly to switch the device
to the high-conductivity (ON) state.
According to the previously reported ‘‘electric field
induced conformation-coupled charge transfer’’ for the

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K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
for comparison. The ESP surfaces show that the polyimide
consists mainly positive ESP region in both states (red
color), with some minor negative ESP regions (blue color)
arising from the sp2 hybridized O atoms in the phthalimide
group and sp2 hybridized N atoms in the oxadiazole and
triazole group. These negative ESP regions suggest concen-
tration local electron densities near the sp2 hybridized
atoms that possess a certain electron-withdrawing ability.
In comparison to the ground state, the BU in the excited
state exhibits larger negative ESP regions around the O
atoms in the phthalimide groups and smaller negative
regions around the N atoms in the oxadiazole and triazole
groups, indicating that under excitation, electrons will
transfer from the oxadiazole or triazole moieties to the
phthalimide moieties. This electronic process is consistent
with the electric field induced electron transfer from the
triazole-substituted triphenylamine donor moieties to the
phthalimide acceptor moieties, as mentioned above.
Following the above CT process, the polymer molecules
also undergo chain twisting under the applied electric
field, as indicated by their different conformations at the
ground and excited states. Due to the steric effect of the
carbonyl groups in the phthalimide moieties, the three
polymers all exhibit pre-twisted conformations, with the
dihedral angles between the phthalimide plane and the
adjacent benzene ring (h1) being 42.2°, 42.9°, and 41.1°
for OXTA-PIa, AZTA-PIa, and AZTA-PEIa, respectively.
Upon excitation, the dihedral angles exhibit increase to
58.8°, 60.3°, and 54.9°, respectively. Moreover, an addi-
tional twisting between the phenoxyl plane and the tria-
zole plane is observed in AZTA-PEIa, with the dihedral
angle (h2) increasing from 66.7° in the ground state to
72.8° in the excited state. The additional twisting between
the phenoxyl plane and the triazole plane in AZTA-PEIa
may increase the barrier to form electric field induced con-
formation-coupled charge transfer and then increase the
switching voltage, although the dihedral angle between
the phthalimide plane and the adjacent benzene ring
(h1 = 41.1) is smaller than that of AZTA-PIa (h1 = 42.9).
The highly twisted conformation in the excited state
decouples the donor and acceptor moieties, and generates
an energy barrier which prevents the back transfer of
charges. As a result, the conductive CT state (ON state) is
sustained, and cannot be dissociated even by a reverse
bias. However, the polymer has higher specific volume
and higher kinetic energy at higher temperature to over-
come the energy barrier. Therefore, at higher temperature
(eg. 150 °C) the energy can overcome the energy barrier,
therefore the back transfer of charges occurred and turn
back to the initial state (OFF state) even though the tem-
perature is lower than its glass transition temperature.
The device based on AZTA-PEIa exhibits similar non-
volatile memory effects with those previously reported
polyimides, AZTA-PIa and OXTA-PIa, except for the values
of their switching threshold voltages, which increase in a
sequence of OXTA-PIa (ꢀ1.8 V), AZTA-PIa (ꢀ2.5 V), and
Fig. 6. Electrostatic potential (ESP) surfaces, optimized geometries, dihedral angles (h1) between the phthalimide plane and the adjacent benzene ring, and
dihedral angles (h2) between the phenoxyl plane and the triazole plane (for AZTA-PEIa only) at the ground and excited states. For the ESP surfaces, the
positive ESP regions are in red, while the negative ESP regions are in blue. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)

K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
161
Fig. 7. Molecular orbitals and potential energy levels of the OXZ-TPA, TAZ-TPA, and TAZ-TPA-R modal materials. The dihedral angle between the
triphenylamine and oxadiazole/triazole moieties in OXZ-TPA and TAZ-TPA are set the same as those of the corresponding segments in OXTA-PI and AZTA-PI,
respectively, while the dihedral angle of TAZ-TPA-R is set the same as that of OXZ-TPA.
AZTA-PEIa (ꢀ3.2 V). As the oxadiazole and triazole groups
possess comparable ionization potential (IP) and electron
affinity (EA) [40,41], their effects on the intramolecular
CT are similar, ruling out their possibility to affect the
switching voltage thresholds. As indicated by the
optimized geometries of OXTA-PIa and AZTA-PIa, the
main difference consists in the torsional angles between
the triphenylamine moiety and the oxadiazole or triazole
moiety. The larger torsional angle (85.7°) in AZTA-PIa in
comparison to that (1.7°) in OXTA-PIa can place a higher
energy barrier for the intramolecular CT, accounting prob-
ably for the larger switching threshold voltage of the
AZTA-PIa device. To figure out the above assumption,
molecular orbitals and potential energies were calculated
for the oxadiazole-triphenylamine (OXZ-TPA) and tria-
zole-triphenylamine (TAZ-TPA) modal materials (molecu-
lar structures shown in Fig. 7), of which the torsional
angles are set the same as those of the corresponding
segments in OXTA-PIa (1.7°) and AZTA-PIa (85.7°),
respectively. Another triazole-triphenylamine modal
material (TAZ-TPA-R) with the same torsional angle (1.7°)
as that of OXZ-TPA was studied as the reference. As shown
in Fig. 7, OXZ-TPA and TAZ-TPA-R with similar planar con-
formation are found to possess comparable HOMO and
LUMO energy levels and thus the HOMO–LUMO energy
gaps (3.66 eV and 3.67 eV, respectively), further suggesting
similarity in the electronic properties of the oxadiazole and
triazole groups. The HOMO–LUMO energy gap of TAZ-TPA
(4.42 eV), however, is much larger, due to its nearly
orthogonal conformation. As the HOMOs of the modal
materials are mainly located at the triphenylamine moiety,
and the LUMOs are located at the oxadiazole or triazole
moiety, the HOMO–LUMO energy gap corresponds to the
energy barrier for the intramolecular CT between the tri-
phenylamine donor and the oxadiazole or triazole acceptor
moieties. Accordingly, the larger HOMO–LUMO energy gap
of TAZ-TPA indicates that a larger energy is required to
trigger the corresponding intramolecular CT, accounting

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K.-L. Wang et al. / Organic Electronics 16 (2015) 148–163
for the larger switching threshold voltage of the AZTA-PIa
device (ꢀ2.5 V) than that of OXTA-PIa device (ꢀ1.8 V). In
addition, the HOMO–LUMO energy gap of TAZ-TPA is
0.76 eV larger than that of OXZ-TPA, which is consistent
with the 0.7 V larger switching threshold voltages for the
AZTA-PIa device in comparison to that of the OXTA-PIa
device. It might be a coincidence, but it might be also a
possible reason for the difference of the switching thresh-
old voltage between AZTA-PEIa and OXTA-PIa.
induced by the applied electric field. The WORM properties
based on conformation-coupled CT process are related
with the chemical structures. This study gives some hints
about structure design on the control of WORM effects
for polymer memory materials.
Acknowledgements
The authors would like to thank the National Science
Council (NSC) of Taiwan for financial support (NSC100-
2221-E-027-024-MY3) and Taipei Medical University for
NMR technical support.
For AZTA-PIa and AZTA-PEIa, following CT from HOMO
of the triphenylamine moiety to LUMO of the triazole moi-
ety, the electrons will further transfer to LUMO of the
phthalimide moiety, giving rise to the final CT state. In
comparison to AZTA-PIa, the AZTA-PEIa molecule has an
additional phenoxyl group between the triazole and
phthalimide moieties, which has a higher LUMO energy
level (ꢀ0.03 eV) and can place an additional energy barrier
for the above intramolecular electron transfer. Thus, a lar-
ger applied electric field is required to overcome the extra
energy barriers, resulting in the larger switching threshold
voltage for the AZTA-PEIa device. That is, the switching
voltage for the ON state is determined by the ease of CT
processes. The ease of the CT processes is determined by
the barrier energy, which come from the energy gap and
the structure planarity (torsional angle) of CT process route
in this case. The strong electron-withdrawing phthalimide
group plays a role to keep the charge with the a conforma-
tion change to make the charge separation state stable
(WORM type). The additional phenoxyl group between
the triazole and phthalimide moieties in AZTA-PEIa
increase the barrier energy and make the switching voltage
higher than AZTA-PIa, even though the AZTA-PIa has
higher energy gap.
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