Conjugated donor-acceptor-acceptor (D-A-A) molecule for organic nonvolatile resistor memory

Article abstract of DOI:10.1002/asia.201402981

A new donor-acceptor-acceptor (D-A-A) type of conjugated molecule, N-(4-(N′,N′-diphenyl)phenylamine)-4-(4′-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic monoimide (TPA-NI-DCN), consisting of triphenylamine (TPA) donors and naphthalimide (NI)/dicyanovinylene (DCN) acceptors was synthesized and characterized. In conjunction with previously reported D-A based materials, the additional DCN moiety attached as end group in the D-A-A configuration can result in a stable charge transfer (CT) and charge-separated state to maintain the ON state current. The vacuum-deposited TPA-NI-DCN device fabricated as an active memory layer was demonstrated to exhibit writeonce- read-many (WORM) switching characteristics of organic nonvolatile memory due to the strong polarity of the TPA-NI-DCN moiety.

Full text of DOI:10.1002/asia.201402981

COMMUNICATION
DOI: 10.1002/asia.201402981
Conjugated Donor-Acceptor-Acceptor (D-A-A) Molecule for Organic
Nonvolatile Resistor Memory
[
a]
[b]
[a]
[b]
[c]
Lei Dong, Guangwu Li, An-Dih Yu, Zhishan Bo,* Cheng-Liang Liu, and
[
a]
Wen-Chang Chen*
Abstract: A new donor-acceptor-acceptor (D-A-A) type of
conjugated molecule, N-(4-(N’,N’-diphenyl)phenylamine)-4-
producibility when compared to the polymeric counter-
[2–4]
parts.
However, a rational molecular design for establish-
(
4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic
ing structure-memory property relationships and modulat-
ing/optimizing functions has not been fully explored yet.
Organic materials with a donor-acceptor (D-A) structure
were developed to be switched between at least two differ-
monoimide (TPA-NI-DCN), consisting of triphenylamine
(
(
TPA) donors and naphthalimide (NI)/dicyanovinylene
DCN) acceptors was synthesized and characterized. In con-
[1e,2–4]
junction with previously reported D-A based materials, the
additional DCN moiety attached as end group in the D-A-A
configuration can result in a stable charge transfer (CT) and
charge-separated state to maintain the ON state current.
The vacuum-deposited TPA-NI-DCN device fabricated as
an active memory layer was demonstrated to exhibit write-
once-read-many (WORM) switching characteristics of or-
ganic nonvolatile memory due to the strong polarity of the
TPA-NI-DCN moiety.
ent resistance states.
Tailoring of molecular planari-
[2a–c]
[2d]
[2e–g]
ty,
conjugated backbone length, alkyl chain length,
[2h]
spacer linkage,
and number/strength of donors and ac-
[2i–m,3,4]
ceptors
were explored to manipulate the memory per-
formance. In particular, the different arrangement regarding
to structural design on D-A small molecules used in
memory devices, such as the asymmetric polar push-pull
system of D-A-A type molecule, is relatively rare reported.
By following these design strategies, herein we report a N-
(
4-(N’,N’-diphenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)-
phenyl)naphthalene-1,8-dicarboxylic monoimide (see
Recently, organic-based resistor memory for resistance
switching effect has emerged as one of promising candidates
as next-generation data storage devices mainly because of
Figure 1) with a specific D-A-A configuration to construct
nonvolatile memory coded as TPA-NI-DCN, in which an
electron-donating triphenylamine (TPA) moiety is connect-
ed to an electron-withdrawing dicyanovinylene (DCN)
through another electron-withdrawing naphthalimide (NI)
block. The additional aromatic phenyl ring inserted between
DCN and NI units can increase the length of p-conjugated
[1]
the structural flexibility and electrical property tunability.
The simple device cell configuration is built in a form of
metal-insulator-metal (MIM) where the insulator acts as the
active memory material. To date, studies regarding the for-
mation of electrical properties of nonvolatile/volatile
memory using organic small molecules have been carried
out based on their attractive advantages of well-defined mo-
lecular structures, easy purification, and batch-to-batch re-
+
+
[
a] L. Dong, A.-D. Yu, Prof. W.-C. Chen
Department of Chemical Engineering
National Taiwan University
Taipei 10617 (Taiwan)
E-mail: chenwc@ntu.edu.tw
+
[
b] G. Li, Prof. Z. Bo
Beijing Key Laboratory of Energy Conversion and Storage Materials
College of Chemistry
Beijing Normal University
Beijing 100875 (China)
E-mail: zsbo@bun.edu.cn
[
c] Prof. C.-L. Liu
Department of Chemical and Materials Engineering
National Central University
Taoyuan 32001 (Taiwan)
+
[
] These authors contributed equally to this work.
Figure 1. Scheme of cross-point memory device and optimized geometry,
molecular orbitals and ESP surface of D-A-A type TPA-NI-DCN, and
structure of D-A type TPA-NI as comparison.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402981.
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Wen-Chang Chen, Zhishan Bo et al.
spacers. Consequently, the new D-A-A configuration of
TPA-NI-DCN for specifically enhancing the device storage
lifespan exhibits a write-once-read-many (WORM) memory
characteristic through the physical origin of resistance
switching. Furthermore, it would be of interest to compare
the electrical characteristics of TPA-NI-DCN with that of
[2a]
TPA-NI reported in the literature to clarify the role of an
additional acceptor of DCN on the memory characteristics.
The molecular structure of targeted D-A-A type conjugat-
ed TPA-NI-DCN materials is outlined in Figure 1. The de-
tailed synthesis procedures are described in Scheme 1. Start-
Figure 2. (a) UV/Vis absorption spectra and (b) cyclic voltammogram of
TPA-NI-DCN.
long-wavelength region due to the CT from D to A. Besides,
the absorption peak maximum on the evaporated thin film
(
on quartz substrate) of TPA-NI-DCN in the visible region
features a bathochromic shift of about 10 nm and broaden-
ing as compared to the peaks in solution (in diluted chloro-
form), which might be related to the formation of molecular
interaction and increased polarizability in the solid state. As
a result, the optical band gap estimated from the absorption
onset is 2.88 eV. The electrochemical properties of TPA-NI-
DCN were investigated by cyclic voltammetry (Figure 2(b)).
TPA-NI-DCN exhibits a quasi-reversible oxidation wave,
which implies that the HOMO level of TPA-NI-DCN can
be estimated to À5.27 eV based on the onset potential of
the forward oxidation peak. As expected, the two strong
electron-withdrawing cyano arms attached to the backbone
significantly lower the LUMO energy level of À2.39 eV,
which was determined from the difference between the opti-
cal band gap and the HOMO level.
The standard two-terminal sandwich configuration of sili-
con wafer/Al/TPA-NI-DCN/Al cross-point resistor-type
memories, where all the active and electrode layers were
thermally evaporated, was fabricated to characterize the
electrical switching behavior, as shown in Figure 1. Our
device is integrated by the logically organized cells on the
square with a bit line (as lower electrode), active memory
layer, and some word lines (as the upper electrode), thus ef-
fectively avoiding the sneak current to the unselected cell.
Each memory cell can store one binary bit discussed as fol-
lows. During the electrical characterization, a two-terminal
scheme was used and the bottom electrode was grounded
while quasi-DC sweeps with steps of 0.05 V were applied to
the top electrode. All the electrical measurements were per-
Scheme 1. Synthesis of TPA-NI-DCN. Reagents and conditions: i) N
O, Pd-C, EtOH, 808C; ii) 4-bromo-1,8-naphthalic anhydride, EtOH,
08C; iii) [Pd (dba) ], P(o-tol) , 4-formylphenylboronic acid, NaHCO
THF/H O, reflux, 908C; iv) malononitrile, NEt , CHCl , rt.
2 4
H
H
8
2
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
A
H
U
G
R
N
U
G
3
3
,
2
3
3
ing from diphenylamine, reaction for small molecules pro-
ceeded smoothly with a high yield. The newly synthesized
1
TPA-NI-DCN was characterized and verified by H NMR
13
and C NMR spectroscopies as well as elemental analysis
see Figures S1 and S2 in the Supporting Information). The
(
thermal properties were also investigated by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(
DSC). The thermogram of TPA-NI-DCN shows about 5%
weight loss at a temperature as high as 4128C (Figure S3(a)
in the Supporting Information), which can probably be at-
tributed to the rigid rings. No obvious transition peak was
detected in the DSC curve at the temperature range of À40
to 3408C, indicating that the as-prepared TPA-NI-DCN
solid is in the amorphous state due to its twisted geometry
(
Figure S3(b) in the Supporting Information). The good
thermal stability of TPA-NI-DCN suggests it as a potential
candidate for achieving thermally durable organic electron-
ics devices. The absorption and charge transfer (CT) charac-
teristics were studied by UV/Vis absorption spectroscopy
formed at room temperature under a N -filled atmosphere
2
using a Keithley 4200 semiconductor analyzer. Figure 3(a)
shows the current–voltage (I-V) characteristics of a vertical
resistor memory device for an optimized TPA-NI-DCN
thickness of 40 nm and a cell active joint area of 0.6ꢁ
(
Figure 2(a)). TPA-NI-DCN exhibits a two-band absorp-
2
tion, with one band at the short-wavelength region assigned
to aromatic TPA and NI moieties and the other band at the
0.6 mm . At the beginning, the device is in its high-resistance
state (HRS; OFF state) and the current increases slowly
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Wen-Chang Chen, Zhishan Bo et al.
and NI groups. Therefore, the switching mechanism of TPA-
NI-DCN can be explained by the electric field-induced CT
effect between the donors and acceptors. Under excitations
with sufficient energy, electrons are possibly transferred
from the electron-donating TPA to the electron-accepting
DCN/NI moieties. Therefore, the CT interaction gives rise
to the charge separation state that creates the effective
charge transport channel and reduces the resistance
throughout the TPA-NI-DCN layer. Charge hopping over
the thermal barrier between the neighboring local sites is
a principal charge conduction mechanism in thin films of or-
ganic conjugated materials employed in devices. Color-
coded computer graphics stimulation shows a representation
of the ESP surface of TPA-NI-DCN. Note that ESP illus-
trates the charge distribution of molecules three dimension-
ally and enables to visualize variably negatively and posi-
tively charged regions. Accordingly, a channel flow inside
the film for migration of charge carriers between the bottom
and top electrodes is formed in the continuous positive ESP
through the conjugated backbone, leading to a resistance
switching from HRS to LRS. The negative ESP suggests
that this electrophilic region acts as a trap site to localize
the charge carrier that possibly induces the related memory
[5]
effects and charge retention.
However, the WORM
Figure 3. (a) I–V characteristics and (b) retention time test of the TPA-
NI-DCN memory device.
memory behavior of TPA-NI-DCN is supported by the fol-
lowing factors. First, the large calculated dipole moment
(
4.48 Debye) of the molecule, contributed by the strong
with the applied voltage. However, the current jumps
electron-withdrawing DCN groups, suggests a high polarity
that implies a strong intermolecular interaction in the D-A-
A architecture. Second, the transferred electrons can be
more significantly delocalized in the DCN moiety with
a strong electron affinity that provides a deep trapping bar-
rier and stable CT state. Third, the introduction of aromatic
TPA donors linked with acceptors suppresses strongly the
intramolecular coupling induced by the twisted conforma-
tion after turning off the power. Consequently, all these fea-
tures lead to a more stable/promoted CT state, and an ap-
propriate charge delocalization in the separated moieties
leads to the storage of the former transferred charge. It also
prevents the segregated cationic and anionic charges from
recombination even after power-off or the application of
a reverse polarity bias. As a result, the quasi-permanent
conductance is observed in the LRS of the fabricated non-
volatile device.
À8
À5
abruptly from 5ꢁ10 A to 1ꢁ10 A at a threshold voltage
of about À3.5 V, indicating that the device is switched from
the HRS to the low-resistance state (LRS; ON state). The
operation for the electrical transition is defined as the “writ-
3
ing” process, with an ON/OFF (HRS to LRS) ratio of ~10 .
The switching voltage of the TPA-NI-DCN device can be
reduced if the work function of the deposited electrode is
close to the HOMO level of the TPA-NI-DCN. In the LRS,
the current then increases smoothly with the applied volt-
age. The device remains in its LRS once the applied voltage
is turned off and then subsequently reapplied to the device
nd
rd
either at negative (2 sweep) or positive voltage (3 sweep).
This device cannot return to the HRS even after switching
th
th
off the power for 1 day in both polarities (4 or 5 sweep).
This indicates that the TPA-NI-DCN device belongs to the
nonvolatile WORM memory. To investigate the stability of
the small molecule-based device, the retention test was con-
ducted with a reading bias of À1 V, as shown in Figure 3(b).
It is seen that the device exhibits good retention abilities:
both the ON and OFF currents do not degrade and thus the
ON/OFF ratio does not show any serious deterioration. The
Moreover, the current at the LRS increases as the elec-
trode size increases in a nearly linear dependence, which
can exclude the switching phenomena from random forma-
[
2b]
tion and rupture of conducting metal filaments.
Besides,
the obtained vapor phase-deposited TPA-NI-DCN thin film
exhibits an extremely smooth surface, with a root-mean-
square (RMS) roughness of about 0.38 nm as obtained from
the AFM image (Figure S4 in the Supporting Information).
This also suggests that the uniform and crack-free surface
minimizes the possibility of filamentary resistive switching
since no effective pathway can be situated for filaments em-
4
test can last for a period of over 10 s and it also proves the
nonvolatile nature and non-destructive reading.
The optimized molecular geometries and electronic prop-
erties such as HOMO and LUMO energy levels and electro-
static potentials (ESP), calculated by density functional
theory, are shown in Figure 1. It is seen that HOMO orbitals
are localized at the TPA electron donors while LUMO orbi-
tals are mainly localized at the electron acceptors, the DCN
[
3]
bedded in the TPA-NI-DCN film. To elucidate the con-
duction mechanism of resistive switching, the plots of the I-
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Wen-Chang Chen, Zhishan Bo et al.
TPA-NI-DCN device can be proven to exhibit a WORM-
type switching behavior due to the attached terminal strong
electron-withdrawing strength of DCN in terms of D-A-A
configuration and formation of a stable CT complex.
Here, the role of the D-A-A motif was designed for the
following characteristics: (1) Relative to the single D-A
system for memory switching, the formation of stable CT
and charge-separated state in a D-A-A framework are
adopted and can favor the nonvolatile WORM memory
characteristics accordingly. In previous studies, numerous
single D-A molecules exhibited weak retention ability of
volatile characteristics or poor stability of the charge-sepa-
[2a,c,i,l]
rated state of erasable flash characteristics.
(2) Both D-
A-A and A-D-A molecules with the same D/A ratio of 1/2
have been used as active materials and exhibit WORM be-
havior. However, the elaborate combination of two connect-
ed acceptors in D-A-A perhaps enables the overlapping of
trapping potential caused by the neighboring spatial loca-
[
2m]
[2h]
tion
and possess binary memory states as compared to
[
2m]
multinary states of the A-D-A counterpart.
(3) Modifica-
tion in D and A structures play an important role in the
data storage properties (volatile vs. nonvolatile), especially
in the D/A strength and relative ratio of D/A moieties in
the molecular backbone. (4) D-A-A small molecules where
the electron donor DCN and electron acceptor TPA are
linked by NI units have advantages of reproducible memory
switching over the phase-separated blends system.
Figure 4. Experimental and fitted electrical characteristics of the TPA-
NI-DCN memory device in the (a) HRS and (b) LRS.
V curves for both LRS and HRS in negative voltage regions
were theoretically fitted (Figure 4). Evidently, the I–V char-
acteristic of the LRS shows a linear plot for log (I/V) versus
In conclusion, our Al/TPA-NI-DCN/Al device shows a bi-
stable WORM memory characteristic. The theoretical quan-
tum calculation and physical models can well explain resist-
ance switching in terms of the electric induced CT and trap-
ping. The additional acceptors of central NI and terminal
DCN units adjacent to the TPA donor in a p-conjugation
can strengthen the stability in the LRS. Specifically, the D-
A-A configuration is effectual for molecular engineering in
the design of nonvolatile memory materials.
1/2
V , which suggests that Poole–Frenkel (PF) emission is
obeyed. On the other hand, the I–V characteristic of the
HRS is more complicated and can be divided into two re-
gions. At the low-voltage region of HRS (<1 V), the linear
proportion for current to voltage corresponds to an Ohmic
conduction behavior. After that it gradually changes to the
3
.4
Childꢂs law region, with a linear relation between I and V
in the higher voltage region of HRS. This behavior is quali-
tatively interpreted to match the trap-associated space-
charge-limited-conduction (SCLC) theory with a simple
Experimental Section
n [6]
equation of I(V)=aV+bV . The trap-controlled SCLC de-
Chemicals and Instrumentation
scribes the charge conduction via charge trapping/detrap-
ping where the negative ESP from the NI and DCN strong
acceptors can serve as trap sites to result in the HRS fol-
lowed by driving the resistive switching through the PF
emission. The occurrence of PF emission is probably attrib-
uted to charge transport of organic materials filled with
All the reagents were used as received from the commercial suppliers
without further purification. Dehydrated tetrahydrofuran (THF) and di-
ethyl ether (Et
indicator under nitrogen atmosphere. Chloroform (CHCl
2
O) were distilled from sodium with benzophenone as an
) was distilled
3
before use. The final products were confirmed by using NMR spectrosco-
py (1D NMR spectra were recorded on a Bruker AV 400 or DM 300
1
spectrometer at the resonant frequencies of 400 MHz for
H and
[
2b,2h]
13
charge traps.
The enhanced molecular polarity through
100 MHz for C nuclei, whereas 2D NMR spectra including HSQC and
NOESY were recorded on a Bruker AVIII 500 MHz spectrometer at the
resonant frequencies of 500 MHz for H and 125 MHz for C nuclei) and
elemental analysis (Flash EA1112 analyzer). Thermal properties were es-
timated from a Seiko TG/DTA 6300 thermogravimetric analysis system
(TGA) and a TA Instruments DSC-Q100 differential scanning calorime-
the strong donor–acceptor interaction in D-A-A structure
arrangement can greatly stabilize the charge-separated state.
As a result, the irreversible WORM-type memory behavior
is observed in our newly synthesized TPA-NI-DCN. Accord-
ing to the previously reported TPA-NI D-A type molecule
1
13
ter (DSC) under a nitrogen atmosphere at a heating rate of 10 and
À1
5
8Cmin , respectively. Optical absorption spectra were obtained on
(
without the DCN end group) by Lu and co-workers (the
a Shimadzu UV-1601 PC UV/Visible spectrometer. The electrochemical
behavior of TPA-NI-DCN thin film was investigated by using CV (CHI
[2a]
structure is shown in Figure 1), this device exhibits a non-
volatile flash memory that indicates CT complex decomposi-
tion after applying an opposite voltage. Once again our
6
30A electrochemical analyzer) with a standard three-electrode electro-
chemical cell in a 0.1m tetrabutylammonium tetrafluoroborate solution in
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Wen-Chang Chen, Zhishan Bo et al.
CH
3
CN at room temperature at atmosphere with a scanning rate of
À1
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The details of synthetic routes for 4-nitrophenyldiphenylamine (1), 4-di-
phenylaminophenylamine (2), N-(4-(diphenylamino)phenyl)-4-bromo-
naphthalene-1,8-dicarboxylic imide (3), and N-(4-(diphenylamino)phen-
yl)-4-(4-formylphenyl)naphthalene-1,8-dicarboxylic imide (4) are de-
scribed in the Supporting Information. The final product, N-(4-(N’,N’-di-
phenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-di-
carboxylic monoimide (TPA-NI-DCN), was synthesized as described
below. A solution of compound 4 (0.50 g, 0.9 mmol) in dry chloroform
[
2] a) W. Ren, H. Zhuang, Q. Bao, S. Miao, H. Li, J. Lu, L. Wang, Dyes
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(
50 mL), a few drops of Et
3
N, and malononitrile (0.067 g, 1.0 mmol) were
stirred under N at room temperature for 30 min. The solvent was re-
moved under reduced pressure and the residue was purified by column
chromatography on silica gel to afford TPA-NI-DCN as a yellow solid
Yield: 0.47 g (86%); H NMR (400 MHz, CDCl
2
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3
): d=8.63–8.69 (m, 2H),
8
.16 (d, J=8.4 Hz, 1H), 8.05 (d, J=8.2 Hz, 2H), 7.84 (s, 1H), 7.64–7.76
(
m, 4H), 7.18–7.28 (m, 4H), 7.07–7.17 (m, 8H), 7.00 ppm (t J=7.3 Hz,
1
3
2
1
1
1
3
H); C NMR (100 MHz, CDCl ): d=164.34, 164.13, 158.80, 148.07,
47.44, 145.03, 144.63, 131.94, 131.88, 131.11, 131.02, 130.95, 129.70,
29.36, 129.14, 129.03, 128.55, 128.00, 127.60, 125.01, 123.41, 123.37,
24.14, 123.07, 113.48, 112.46, 83.91 ppm; elemental anal. calcd (%) for
40 24 4 2
C H N O : C 81.07, H 4.08, N 9.45; found: C 81.19, H 4.16, N 9.38.
Device Fabrication and Measurements
The memory device was fabricated in a cross-point sandwiched architec-
ture with the configuration of Al/TPA-NI-DCN/Al. Before deposition of
the Al bottom electrode, the silicon wafer was pre-cleaned by an ultra-
sonic cleaning process with toluene, acetone, and isopropanol, each for
1
7125–17132; m) Z. Su, H. Zhuang, H. Liu, H. Li, Q. Xu, J. Lu, L.
1
5 min. Al bottom electrode lines with a thickness of 30 nm were deposit-
Wang, J. Mater. Chem. C 2014, 2, 5673–5680; n) H. Li, Z. Jin, N. Li,
Q. Xu, H. Gu, J. Lu, X. Xia, L. Wang, J. Mater. Chem. 2011, 21,
À1
ed via thermal evaporation at a depositing rate of 1 ꢃs under a pressure
of around 1ꢁ10 torr. An active layer of the investigated organic small
À6
5
860–5862; o) H. Li, Q. Xu, N. Li, R. Sun, J. Ge, J. Lu, H. Gu, F.
molecule was successively deposited on top of the bottom electrode by
thermal evaporation with a targeted thickness of 40 nm, as determined in
situ by a calibrated quartz crystal microbalance (QCM). Top electrode
lines were thermally deposited through the shadow mask at the same
condition, aligned perpendicular to the bottom lines, leading to cross-
point arrays of memory cells. All the electrical characteristics of the fab-
ricated flexible memory devices were measured by using a Keithley 4200-
SCS semiconductor parameter analyzer. All electrical measurements
Yan, J. Am. Chem. Soc. 2010, 132, 5542–5543.
[
[
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were performed in a N -filled glove box.
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Acknowledgements
6
W.-C.C. thanks the financial support from Ministry of Science and Tech-
nology (MOST) of Taiwan. Z.B. acknowledges the funding assistant from
National Natural Science Foundation of China (No. 21161160443).
Received: August 22, 2014
Revised: September 3, 2014
Published online: && &&, 0000
Keywords: charge transfer
· donor-acceptor systems ·
imides · memory · resistance switching · triphenylamine
Chem. Asian J. 2014, 00, 0 – 0
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

COMMUNICATION
Donor-Acceptor Systems
Lei Dong, Guangwu Li, An-Dih Yu,
Zhishan Bo,* Cheng-Liang Liu,
Wen-Chang Chen*
&&&& — &&&&
Conjugated Donor-Acceptor-Acceptor
D-A-A) Molecule for Organic Nonvo-
latile Resistor Memory
The early bird catches the WORM: A
donor-acceptor-donor (D-A-A) type
conjugated molecule based on linear
p-bridging triphenylamine (TPA)
donors and naphthalimide (NI)/dicya-
novinylene (DCN) acceptors was pre-
pared and shown to possess nonvola-
tile WORM (write-once-read-many)
memory behavior.
(
&
&
Chem. Asian J. 2014, 00, 0 – 0
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!