Two quinoxaline derivatives designed from isomer chemistry for nonvolatile ternary memory device applications

Article abstract of DOI:10.1016/j.dyepig.2015.06.017

In this paper, two quinoxaline-based small organic molecules, which are conjugated and mutual isomers in vertical and horizontal directions, were successfully synthesized. By exploring isomers with identical functional groups, the impact from changing in

Full text of DOI:10.1016/j.dyepig.2015.06.017

Dyes and Pigments 122 (2015) 66e73
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Two quinoxaline derivatives designed from isomer chemistry for
nonvolatile ternary memory device applications
Erbo Shi, Hao Zhuang, Zhaojun Liu, Xuefeng Cheng, Haiyan Hu, Najun Li, Dongyun Chen,
Qingfeng Xu, Jinghui He, Hua Li**, Jianmei Lu*, Junwei Zheng
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow
University, Suzhou Industrial Park, 199 Ren'ai Road, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, two quinoxaline-based small organic molecules, which are conjugated and mutual isomers
in vertical and horizontal directions, were successfully synthesized. By exploring isomers with identical
functional groups, the impact from changing in molecular composition on the observed different per-
formance can be minimized. Their photoelectrical properties, thin-film morphology and the electrical
memory performance were comparatively investigated. The devices based on these two small molecules
both showed ternary WORM storage performance. Compared with vertical direction, horizontal mole-
cule had a lower threshold voltage and higher current ratio. The results of AFM and XRD showed that
horizontal molecule has better thin-film morphology and layered accumulation, which are conducive to
the migration of carriers. We hope this study can provide a new reference to the future elaborate design
and synthesis of novel small-molecule electrical memory materials.
Received 2 February 2015
Received in revised form
5 June 2015
Accepted 10 June 2015
Available online 22 June 2015
Keywords:
Quinoxaline
Memory device
Ternary
© 2015 Elsevier Ltd. All rights reserved.
Write once read many times
Horizontal conjugation
Cyano group
1. Introduction
isomers, the influence on device performance caused by the change
of molecular composition can be reduced to the lowest extent.
Organic and polymeric semiconducting materials are widely
studied for organic electronics due to their advantages [1e12], such
as low cost, adjustable structure, and excellent flexibility. Recently,
electrical memory devices based on organic molecules have been
reported and demonstrated promising storage properties [13e32].
Later, the study of new ultra-high density storage materials then
emerged as a possible solution to the challenge of vast information
produced every day. To obtain superior materials and devices,
many aspects had been studied such as adjusting the length of
conjugated system [24] and alkyl chain [33,34], tuning the molec-
ular planarity [35], optimizing the molecular or electrode deposi-
tion rate [36]. However, in most of these studies, the composition of
the molecules (functional groups or main framework) had been
changed, which would plainly cause differences in the performance
of the molecules. To circumvent this, isomers with same functional
groups could be employed in such study. By comparing these
In recent years, compared with benzothiadiazole and benzo-
triazole, the quinoxaline unit might be an ideal electron-accepting
building block because of its intermediate electron-withdrawing
ability. Quinoxaline-based conjugated small organic molecules
and polymers have been widely used in organic photovoltaic cells
(OPVs) [37e42] and dye-sensitized solar cells (DSSCs) [43e49]. As
demonstrated, some groups incorporate a quinoxaline unit as the
auxiliary acceptor to construct some novel molecules, which ex-
hibits two merits: (1) efficiently optimizing the molecular HOMO-
eLUMO energy band gap when combined with various donor
moieties; (2) conveniently tailoring the molecular structure due to
its multiple reaction sites. Recently, there are a few groups having
reported on two novel quinoxaline-based molecules with both
vertical and horizontal directions in OPVs [50] and DSSCs [43]. To
the best of our knowledge, organic memory devices based on
quinoxaline molecules have not been reported so far. Herein, we
designed and synthesized two quinoxaline-based isomers in ver-
tical (QU-1) and horizontal (QU-2) directions. Both of the ITO/small
organic molecule/Al sandwich devices showed ternary WORM
storage performance. In comparison, QU-2 demonstrated better
electrical performance with lower switching threshold voltage and
* Corresponding author. Tel.: þ86 512 65880368; fax: þ86 512 65880367.
** Corresponding author. Tel.: þ86 512 65880368; fax: þ86 512 65880367.
E-mail address: lujm@suda.edu.cn (J. Lu).
http://dx.doi.org/10.1016/j.dyepig.2015.06.017
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
67
higher ON/OFF current ratio. Atomic force microscopy (AFM) and X-
ray diffraction (XRD) results indicated that QU-2 has a smoother
film and a good layer-by-layer accumulation in the film, which
makes the migration of carriers more convenient. Therefore, QU-2
shows the better performance. We expect that this study of isomers
with different configurations will provide new insight for the
design of future high-performance data storage materials.
d
(ppm): 7.91 (s, 2H), 7.49 (d, J ¼ 8.5 Hz, 4H), 7.38 (d, J ¼ 8.5 Hz, 4H),
2.52 (s, 6H). HRMS-ESI (m/z): [M þ H]þ calcd for C22H17Br2N2:
466.9758; found: 466.9776.
2.2.2. 40,400-(6,7-Dimethylquinoxaline-2,3-diyl)dibiphenyl-4-
carbonitrile (QU-1)
Compound 2 (234 mg, 0.5 mmol), 4-cyanophenylboronic acid
(176 mg, 1.2 mmol), potassium carbonate (414 mg, 3.0 mmol) and
tetrakis(triphenylphosphine)palladium (20 mg) were mixed with
8 mL 1,4-dioxane and 4 mL water. After degassing for 20 min with
N2, the mixture was heated under N2 at 90 ꢀC for 5 h. After cooling
to room temperature, the mixture was poured into water and
extracted with DCM. The organic layers were separated, dried over
Na2SO4 and filtered. After evaporation of solvent, the residues were
subjected to column chromatography using PE:DCM ¼ 1:2 to pro-
duce 369 mg (72%) of QU-1 as a white solid. 1H NMR (400 MHz,
2. Experimental section
2.1. Materials
4,5-Dimethyl-1,2-benzenediamine, 4,40-Dibromobenzil, 4-
Cyanophenylboronic acid, Phenylboronic acid, 4,7-Dibromo-2,1,3-
Benzothidiazole, 2,3-Butanedione, Bromine, Iodine, Sodium boro-
hydride, Cobalt chloride hexahydrate, Tetrakis(triphenyl
phosphine)palladium, Potassium carbonate, Acetic acid glacial
were purchased from commercial sources (Alfa Aesar, TCI, and
SigmaeAldrich). Other solvents were purchased from Sinopharm
Chemical Reagent Co., Ltd. All chemicals were used as received
without further purification.
CDCl3)
d
(ppm): 8.08 (s, 2H), 7.73e7.69 (m, 12H), 7.62 (d, J ¼ 8.1 Hz,
4H), 2.57 (s, 6H); 13C NMR (100 MHz, DMSO-d6)
d
(ppm): 151.7,
144.1, 141.6, 140.0, 139.8, 138.6, 133.4, 131.1, 128.0, 127.3, 119.3, 110.8,
20.5. HRMS-ESI (m/z): [M þ H]þ calcd for C36H25N4: 513.2079;
found: 513.2067. M. P. 286e288 ꢀC. Elemental analysis: calcd for
C36H24N4: %C, 84.35; %H, 4.72; %N, 10.93; found: %C, 84.26; %H,
4.83; %N, 10.71.
2.2. Preparation of QU-1 and QU-2
The compounds QU-1 and QU-2 were prepared as detailed in
2.2.3. 4,7-Diphenylbenzo[c][1,2,5]thiadiazole (4)
Scheme 1.
A mixture of 3 (588 mg, 2.0 mmol), phenylboronic acid (536 mg,
4.4 mmol), potassium carbonate (1.1 g, 8.0 mmol), tetrakis(-
triphenylphosphine)palladium (40 mg), 10 mL toluene and 4 mL
water was heated at 90 ꢀC for 5 h under nitrogen. After cooling to
room temperature, the mixture was poured into water and
extracted with DCM. After drying over Na2SO4, the remaining liquid
was concentrated and purified by recrystallization (ethanol) to
produce 500 mg (86%) of 4 as a yellow solid. 1H NMR (400 MHz,
2.2.1. 2,3-Bis(4-bromophenyl)-6,7-dimethylquinoxaline (2)
Compound 1 (136 mg, 1.0 mmol) and 4,40-dibromobenzil
(368 mg, 1.0 mmol) were dissolved in 5 mL toluene and 3 mL acetic
acid. The mixture was refluxed for 5 h. After cooling to room
temperature, the resultant mixture was poured into water and
extracted with dichloromethane (DCM). After drying over Na2SO4,
the remaining liquid was concentrated and purified by column
chromatography (Petroleum ether (PE):DCM ¼ 5:1) to produce
400 mg (85%) of 2 as a while solid. 1H NMR (400 MHz, CDCl3)
CDCl3)
d (ppm): 7.97e7.95 (m, 4H), 7.78 (s, 2H), 7.57e7.53 (m, 4H),
7.48e7.44 (m, 2H). HRMS-ESI (m/z): [M þ H]þ calcd for C18H13N2S:
289.0799; found: 289.0797.
Scheme 1. Synthetic routes and molecular structures.

68
E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
2.2.4. 4,7-Bis(4-bromophenyl)benzo[c][1,2,5]thiadiazole (5)
about 100 nm, as measured by SEM through a cross section of the
device (Fig. 1b). Finally, a layer of Al was thermally evaporated onto
the film surface at about 5 ꢂ 10ꢁ6 Torr through a shadow mask to
yield the top electrode. The thickness of aluminum (Al) layer was
about 80 nm (Fig. 1b), and an active device area of 0.0314 mm2 was
obtained. All electrical measurements of the device were charac-
terized under ambient conditions, using a HP 4145B semiconductor
parameter analyzer.
A solution of bromine (1.2 mL, 23.3 mmol) in chloroform
(1.5 mL) was added dropwise to a mixture of 4 (288 mg, 1.0 mmol),
iodine (15 mg, 0.06 mmol) and chloroform (4 mL). The resulting
mixture was stirred at room temperature for 12 h. After workup,
the solid was filtered off and washed sequentially with aqueous
NaHSO3, water and methanol to afford the crude product. Recrys-
tallization in toluene gave the title compound as yellow needles 5.
Yield: 350 mg (77%). 1H NMR (400 MHz, CDCl3)
d (ppm): 7.86 (d,
J ¼ 8.5 Hz, 4H), 7.77 (s, 2H), 7.68 (d, J ¼ 8.5 Hz, 4H). HRMS-EI (m/z):
2.4. Measurements
[M þ H]þ calcd for C18H11Br2N2S: 444.9010; found: 444.9016.
The 1H NMR (400 MHz) data were recorded on Varian 400 M
spectrometers using CDCl3 or DMSO-d6 as solvent and TMS as in-
ternal standard at room temperature. High resolution mass spectra
(HRMS) were performed on a Micromass GCT-TOF mass spec-
trometer with an ESI source. UVevis absorption spectra were car-
ried out with a PerkinElmer Lambda-17 spectrophotometer at room
temperature. Cyclic voltammetry (CV) measurements were carried
out using a platinumecarbon working electrode, an Ag/AgCl
reference electrode, and a counter electrode (Pt wire) at room
temperature on a Corr-Test CS Electrochemical Workstation, a 0.1 M
solution of tetrabutylammonium hexafluorophosphate in anhy-
drous dichloromethane was used. Atomic force microscopy (AFM)
measurements were performed using a MFP-3DTM (Digital In-
struments/Asylum Research) AFM instrument. Scanning electron
microscopy (SEM) images were taken on a Hitachi S-4700 scanning
electron microscope. X-ray diffraction (XRD) patterns were taken
on an X'Pert-Pro MPD X-ray diffractometer. The devices were
characterized under ambient conditions, using a HewlettePackard
4145B semiconductor parameter analyzer with a HewlettePackard
8110A pulse generator.
2.2.5. 5,8-Bis(4-bromophenyl)-2,3-dimethylquinoxaline (7)
Compound 5 (335 mg, 0.75 mmol), NaBH4 (2.81 mg, 7.5 mmol),
CoCl2$6H2O (3.6 mg, 0.015 mmol), 150 mL EtOH and 40 mL THF
were charged into a 500 mL flask. The mixture was refluxed for 3 h,
cooled to room temperature and filtered. The solvent was evapo-
rated, water was added and the reaction mixture was extracted
three times with Et2O. The combined organic layer was dried over
Na2SO4. Evaporation of the solvent gave 6 as a pale white solid. A
mixture of
6 (210 mg, 0.5 mmol), 2,3-butanedione (55 mL,
0.55 mmol), 5 mL toluene and 3 mL acetic acid was refluxed for 5 h.
After cooling to room temperature, the resultant mixture was
poured into water and extracted with DCM. After drying over
Na2SO4, the remaining liquid was concentrated and purified by
column chromatography (PE:DCM ¼ 1:1) to produce 180 mg (81%)
of 7 as a while solid. 1H NMR (400 MHz, CDCl3)
d (ppm): 7.72 (s, 2H),
7.63 (s, 8H), 2.68 (s, 6H).
2.2.6. 40,400-(2,3-Dimethylquinoxaline-5,8-diyl)dibiphenyl-4-
carbonitrile (QU-2)
Compound QU-2 was synthesized similarly as described for the
synthesis of QU-1, yield 79% (121 mg). 1H NMR (400 MHz, CDCl3)
3. Results and discussion
d
(ppm): 7.92 (d, J ¼ 8.2 Hz, 4H), 7.85 (s, 2H), 7.81e7.74 (m, 12H),
2.73 (s, 6H); 13C NMR (376 MHz, CF3COOD)
d
(ppm): 149.5, 141.3,
3.1. Optical properties and electrochemical properties
136.3, 132.4, 130.0, 129.9, 129.6, 128.4, 126.3, 123.5, 123.2, 104.9,
24.8. HRMS-ESI (m/z): [M þ H]þ calcd for C36H25N4: 513.2079;
found: 513.2067. M. P. 290e292 ꢀC. Elemental analysis: calcd for
Fig. 2 shows the UVevis absorption spectra of QU-1 and QU-2 in
dilute anhydrous tetrahydrofuran solution and in the film state.
QU-1 shows an absorption around 295 nm, which represents a
C
36H24N4: %C, 84.35; %H, 4.72; %N, 10.93; found: %C, 84.20; %H,
4.91; %N, 10.81.
pep* transition, while the absorption around 360 nm corresponds
to an intramolecular charge transfer (ICT) between the benzene
ring and the acceptor. Similarly, QU-2 has a peak around 295 nm as
well; but in contrast, QU-2 reveals a broader absorption band due
to efficient formation of an ICT state in the long-wave direction. The
absorption spectra of the vacuum-deposited thin films on ITO
substrates are also presented in Fig. 2. Compared with those ab-
sorption bands in the solution, both absorption peaks exhibited a
bathochromic shift (5 nm for QU-1) in the film state, demonstrating
that closer and more ordered accumulation was formed [51], which
would facilitate smooth carrier migration between neighboring
2.3. Fabrication of memory device
The memory devices that consisted of the QU-1 or QU-2 films
sandwiched between an ITO bottom electrode and an Al electrode
were fabricated (Fig. 1a). The indium tin oxide (ITO) glass substrate
was precleaned by ultrasonic bath with deionized water, acetone
and ethanol for 20 min, respectively. Then QU-1 and QU-2 were
deposited onto the ITO bottom electrode under a pressure of
10ꢁ6 Torr. The thickness of the small organic molecule layer was
Fig. 1. (a) Schematic diagram of the indiumetin oxide/organic film/aluminum device, (b) SEM image of the cross section of a storage cell.

E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
69
i
EHOMO ¼ ꢁ½EOXðonsetÞ þ 4:8 ꢁ EFerrocene
ELUMO¼ EHOMOþEg
As shown inTable 1, the energy barriers of hole injection from the
ITO bottom electrode (work function: ꢁ4.8 eV) to the HOMO level of
the active layer were determined to be 1.09 eV (QU-1) and 0.89 eV
(QU-2). While the energy barriers of electron injection from the Al
electrode (work function: ꢁ4.3 eV) to the LUMO level of active layer
were estimated to be 1.53 eV (QU-1) and 1.57 eV (QU-2). Thus, both
compounds exhibit the p-type semiconductor characteristics and
the conduction process was dominated by hole injection [56].
3.2. Morphology of the thin films
Fig. 2. UVevis absorption spectra of the two compounds in THF solution and solid thin
films on quartz substrates.
To investigate the surface morphology of the electroactive
layers, atomic force microscopy (AFM) was used. The tapping mode
AFM images are shown in Fig. 4, the images clearly show that both
molecules formed two-dimensional nanograins in the solid films
with uniform size. This type of morphology can also be observed on
other substrates with different polarities (e.g., glass and quartz),
indicating that the self-assembly of the molecules is driven by the
intermolecular force rather than the substrate and molecule in the
process of vacuum deposition. However, the surface roughness of
these two films, film density and uniformity are significantly
different. In comparison, QU-2 film is more neat with a smaller
surface roughness (QU-1: 2.144 nm; QU-2: 1.183 nm), which can
facilitate the charge injection between the electrodes and the
organic thin film, thereby contributing to lower Vth values. The
higher-quality film morphology of QU-2 was also owing to the
better conjugation of the substituent of 5,8-sites. Additionally, the
compact film can effectively prevent the formation of conductive
filaments due to aluminum infiltration [34].
The XRD patterns of the two molecular films using ITO glasses as
substrates are shown in Fig. 5. Obviously, there were two related
diffraction peaks at 6.3ꢀ and 12.7ꢀ (7.0 Å) for the QU-2 film. But only
two independent diffraction peaks were observed at 8.2ꢀ and 10.0ꢀ
in the QU-1 film, which corresponds to a d-spacing of 8.8 Å. These
results indicate that QU-2 has a good layer-by-layer accumulation
in the film [57,58], which is beneficial for charge transport between
neighboring layers. Clearly, as the positions of substituent groups
changed from 2,3-sites to 5,8-sites, the molecules in the thin film
became more tightly accumulation, which enhanced the charge
carrier transport in the film and reduced the Vth values.
molecules. But for QU-2, a significantly broader, further red-shifted
absorption was observed. Obviously, there was evident vibronic
shoulder at about 360 nm in the absorption spectra of film state,
possibly owing to its better planarity with the two substituent
groups grafted onto the two sides of quinoxaline, indicating more
effective
pep stacking between the conjugated backbones
[52e54]. However, a similar vibronic shoulder was not observed in
the absorption spectrum of QU-1, which may due to the twisting of
the two substituent groups grafted onto the 2, 3-position of qui-
noxaline or the difficulty in forming better conjugation in the thin
film. The optical band gap of QU-1 and QU-2 can be estimated from
the absorption edges, which give the results 3.12 eV and 2.96 eV,
respectively.
In order to study the molecular energy levels, the electro-
chemical properties of the organic molecules were determined by
cyclic voltammograms (CVs) (Fig. 3) using tetrabutylammonium
hexafluorophosphate in anhydrous dichloromethane with the
concentration of 0.1 M, and the data are presented in Table 1. The
onset oxidation potentials of the QU-1 and QU-2 are 1.49 eV and
1.29 eV vs Ag/AgCl, respectively. The HOMO and LUMO levels were
calculated by the following equation [55]. Here the standard Efer-
was measured to be 0.40 eV. The HOMO values were deter-
rocene
mined to be ꢁ5.89 eV and ꢁ5.69 eV for QU-1 and QU-2. Therefore,
the LUMO levels of both molecules were measured to be ꢁ2.77 eV
and ꢁ2.73 eV.
3.3. Currentevoltage (IeV) characteristics
Fig. 6 shows the typical currentevoltage (IeV) characteristics of
the ITO/small organic molecule/Al device with a simple sandwich
structure. Initially, the memory device was in a low-conductivity
state (OFF or “0” state), when a negative bias was applied from
0 to ꢁ4 V (with ITO as anode and Al as cathode), an abrupt current
increase from the low-conductivity state to an intermediate-
conductivity state (ON1 or “1” state) was observed at ꢁ1.93 V,
and then another abrupt current increase was observed at ꢁ3.03 V,
indicating
a transition from a low-conductivity state to an
intermediate-conductivity state and then to a high-conductivity
state (ON2 or “2” state) (sweep 1). This transition from the low-
conductivity state to intermediate-conductivity state and further
to high-conductivity state could be regarded as the “writing” pro-
cess for data storage. The memory device remained in high-
conductivity state after a subsequent scan from 0 to ꢁ4 V (sweep
2). For another storage cell, it could be switched on at a threshold
voltage around ꢁ1.93 V with the scan from 0 to ꢁ2.5 V (sweep 3);
Fig. 3. Cyclic voltammetry (CV) curves of the solutions in anhydrous dichloromethane
with 0.1 M n-Bu4NPF6 as the supporting electrolyte. The inset is the CV curve of the
ferrocene standard swept in the same conditions as that for the two compounds ((a)
for QU-1, (b) for QU-2). The scan rate was 100 mV sꢁ1. The concentration is 1 mg/mL.

70
E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
Table 1
Optical and electrochemical properties of the two synthesized molecules.
Molecule
lonset (nm)
Eg (eV)a
Eox (onset) (eV)b
HOMO (eV)c
LUMO (eV)d
FITOeEHOMO (eV)
ELUMOeFAl (eV)
QU-1
QU-2
398
419
3.12
2.96
1.49
1.29
ꢁ5.89
ꢁ5.69
ꢁ2.77
ꢁ2.73
1.09
0.89
1.53
1.57
a
The data were calculated by the following equation: band gap ¼ 1240/lonset of the two synthesized molecular films.
b
c
Vs Ag/AgCl in DCM.
Calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV).
Calculated from the HOMO energy level and optical band gap.
d
Fig. 4. Tapping-mode height and typical cross section profiles of AFM topographic images of vacuum-deposited thin films on ITO-substrates: QU-1 (a); QU-2 (b).
this storage cell remained in this intermediate-conductivity state
during a subsequent scan from 0 to ꢁ2.5 V (sweep 4). The storage
cell shows another switching threshold voltage at ꢁ3.06 V during
the scan from 0 to ꢁ4 V (sweep 4). Therefore, this QU-1-based
device exhibited typical non-volatile ternary WORM memory
characteristics.
based on the density functional theory (DFT) with the Gaussian 03
program was carried out. The results of DFT molecular simulation
demonstrate that the molecular conjugated skeleton shows
continuous positive electrostatic potentials (ESP, the red areas),
which provide the open channel for the smooth migration of the
charge carriers. However, the mobility of charge carriers might be
reduced by the “charge trap” (the negative ESP regions in blue)
caused by electron-withdrawing groups cyano and quinoxaline.
When under a low bias, charge carriers do not have enough energy
to travel through the electroactive layer. When the applied voltage
reached the first threshold voltage, the “shallow trap” arising from
the quinoxaline moiety would be filled up quickly, resulting in the
As analog, the device based on ITO/QU-2/Al shows similar ten-
dency for the IeV characteristics (Fig. 6d). Under an external elec-
tric field, there are two abrupt current increases at ꢁ1.64 V (the
switch-ON1 voltage) and ꢁ2.79 V (the switch-ON1 voltage) with
ON/OFF current ratios around 3 ꢂ 106:103:1. In comparison, QU-2
shows a lower switching threshold voltage and higher ON/OFF
current ratio. Then the stability of the memory effects was also
investigated by the retention time and read cycles. The retention
time of both intermediate-conductivity state and high-conductivity
state have no obvious degradation in current under a constant
voltage of ꢁ1 V for over 200 min. The results are shown in Fig. 6(b)
and (e). In addition, Fig. 6(c) and (f) shows the read cycles under a
continuous read pulses of ꢁ1 V for the different conductivity states
of the two devices, respectively. And the intermediate and high-
conductivity states were also stable during the up to one million
read cycles, which is reasonably acceptable for memory storage
applications.
3.4. Proposed data-storage mechanism
In order to further understand the switching behavior of the
memory devices based on the two small molecules, calculation
Fig. 5. X-ray diffraction spectra of QU-1 and QU-2 films deposited on ITO glass
substrates.

E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
71
Fig. 6. (a), (d) Currentevoltage (IeV) characteristics of compounds QU-1 and QU-2; (b), (e) stability of the device in three states under a voltage of ꢁ1 V; and (c), (f) effect of read
cycles on three states under a constant “read” voltage of ꢁ1 V.
transition from the OFF state to the ON1 state. The “deep trap”
arising from the cyano moiety would also be filled up with the
increasing of the voltage, leading to switching from the ON1 state to
the ON2 state. And the memory device could remain in the high
conductivity state even when the voltage was continually
increased. The HOMO, LUMO and LUMO þ 1 orbitals for QU-1 and
QU-2 are shown in Fig. 7. It is clearly shown that the HOMOs
distributed along the whole conjugated benzene ring; the LUMOs
were localized around the quinoxaline moiety, and the LUMO þ 1s
were localized on the cyano group, respectively. Finally, the
HOMO / LUMO transitions led to the separation of the charges.
Thus, the injected carriers were fixed by the intramolecular and/or
intermolecular charge transfer and the captive carriers cannot
easily be released, even though the reverse voltage was applied.
Therefore, the memory devices show the typical non-volatile
ternary WORM storage behavior.
state. And what's more, the non-planar spacial distortion of the two
phenyl group would decrease the overlap of frontier orbitals and
reduce the electron delocalization ability. As a consequence, The
QU-1-based device shows the higher Vth values. Inversely, the
incorporated two similar substituent group on 5,8-site can improve
the film morphology of QU-2 and be beneficial for the overlap of
relevant frontier orbitals and facilitate electron delocalization,
thereby leading to a lower Vth values. In addition, according to the
HOMO energy levels of two molecules listed in Table 1 and the
work function of bottom electrodes (ITO), the hole injection barrier
for QU-2 (0.89 eV) was lower than QU-1 (1.09 eV), which would
also contribute to the observed lower threshold voltage of QU-2-
based device. Consequently, the different arrangements of the
functional groups in different isomers have a great influence on
device performance, which paves a way to optimize the device
performance in the future.
It was worth noting that the Vth values of the device based on
QU-2 were lower than the QU-1-based device, possibly due to the
distortion of the two phenyl groups grafted onto the 2,3-sites of
quinoxaline. As a result, the spacial distortion led to poor film
morphology and hindered charge carriers migration in the thin film
4. Conclusions
In summary, two quinoxaline-based mutual isomers of small
organic molecules (QU-1 and QU-2) were synthesized, and their

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E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
Fig. 7. LUMOs þ 1, LUMOs and HOMOs of (a) QU-1 and (b) QU-2 in their optimized ground-state structures.
[7] Lin YW, Lin CJ, Chou YH, Liu CL, Chan HC, Chen WC. Nonvolatile organic field
effect transistor memory devices using one-dimensional aligned electrospun
nanofiber channels of semiconducting polymers. J Mater Chem C 2013;1:
5336e43.
[8] Chen Y, Wan X, Long G. High performance photovoltaic applications using
solution-processed small molecules. Acc Chem Res 2013;46:2645e55.
[9] Lin WP, Liu SJ, Gong T, Zhao Q, Huang W. Polymer-based resistive memory
materials and devices. Adv Mater 2014;26:570e606.
[10] Zhang W, Liu Y, Yu G. Heteroatom substituted organic/polymeric semi-
conductors and their applications in field-effect transistors. Adv Mater
2014;26:6898e904.
[11] Coughlin JE, Henson ZB, Welch GC, Bazan GC. Design and synthesis of mo-
lecular donors for solution-processed high-efficiency organic solar cells. Acc
Chem Res 2014;47:257e70.
photoelectrical properties, thin-film morphology and the electric
memory performance were comparatively investigated. The
assembled device exhibited ternary WORM memory performance
based on sandwich structure of ITO/small organic molecule/Al.
Compared with QU-1, QU-2 had a lower threshold voltage and
higher current ratio. In addition, the film based on the linear QU-2
packed closer, and had smaller surface roughness and better layer
accumulation. All of these are conducive to the migration of car-
riers. We hope this study can provide a beneficial reference to the
future elaborate design and synthesis of novel small-molecule
electrical storage materials.
[12] Roncali J, Leriche P, Blanchard P. Molecular materials for organic photovol-
taics: small is beautiful. Adv Mater 2014;26:3821e38.
[13] Ling QD, Chang FC, Song Y, Chan DSH, Kang ET, Neoh KG, et al. Synthesis and
dynamic random access memory behavior of a functional polyimide. J Am
Chem Soc 2006;128:8732e3.
[14] Lim SL, Ling Q, Teo EYH, Chan DSH, Kang ET, Neoh KG, et al. Conformation-
induced electrical bistability in non-conjugated polymers with pendant
carbazole moieties. Chem Mater 2007;19:5148e57.
[15] Kronemeijer AJ, Akkerman HB, Kudernac T, Feringa BL, Blom PWM, Boer B,
et al. Reversible conductance switching in molecular devices. Adv Mater
2008;20:1467e73.
[16] Liu G, Ling QD, Teo EY, Chan DS, Kang ET, Neoh KG, et al. Electrical conduc-
tance tuning and bistable switching in poly(n-vinylcarbazole)ecarbon nano-
tube composite films. ACS Nano 2009;3:1929e37.
Acknowledgments
This work was financially supported by the Chinese Natural
Science Foundation (21176164, 21206102 and 21336005), the NSF of
Jiangsu Province (BE2013052), a project of the Department of Ed-
ucation of Jiangsu Province (12KJB430011), Suzhou Nanoproject
(ZXG2012023) and Project supported by the Specialized Research
Fund for the Doctoral Program of Higher Education of China (Grant
no. 20113201130003 and 20123201120005).
[17] Kim TW, Choi H, Wang G, Kim DY, Hwang H, Lee T, et al. One transistoreone
resistor devices for polymer non-volatile memory applications. Adv Mater
2009;21:2497e500.
Appendix A. Supplementary data
[18] You NH, Chueh CC, Liu CL, Ueda M, Chen WC. Synthesis and memory device
characteristics of new sulfur donor containing polyimides. Macromolecules
2009;42:4456e63.
[19] Lee J, Chang H, Kim S, Bang GS, Lee H. Molecular monolayer
nonvolatile memory with tunable molecules. Angew Chem Int Ed 2009;48:
8501e4.
[20] Zhuang XD, Chen Y, Liu G, Kang ET, Neoh KG, Zhang B, et al. Conjugated-
polymer-functionalized graphene oxide: synthesis and nonvolatile rewritable
memory effect. Adv Mater 2010;22:1731e5.
[21] Son DI, Kim TW, Lee DU, Lee JM, Park WI, Choi WK, et al. Flexible organic
bistable devices based on graphene embedded in an insulating poly(methyl
methacrylate) polymer layer. Nano Lett 2010;10:2441e7.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.06.017.
References
[1] Yang Y, Ouyang J, Ma L, Tseng RJH, Chu CW. Electrical switching and bistability
in organic/polymeric thin films and memory devices. Adv Funct Mater
2006;16:1001e14.
[2] Lee J, Lee E, Shultz DA, Schmidt RD, Forbes MDE, Lee H, et al. Nitronyl nitroxide
radicals as organic memory elements with both n- and p-type properties.
Angew Chem Int Ed 2011;50:4414e8.
[22] Li H, Xu QF, Li NJ, Sun R, Ge JF, Lu JM, et al. A small-molecule-based ternary
data-storage device. J Am Chem Soc 2010;132:5542e3.
[3] Chou YH, Lee WY, Chen WC. Self-assembled nanowires of organic n-type
semiconductor for nonvolatile transistor memory devices. Adv Funct Mater
2012;22:4352e9.
[23] Lee JS, Kim YM, Sim JS, Shin H, Sohn BH, Jia Q, et al. Multilevel data storage
memory devices based on the controlled capacitive coupling of trapped
electrons. Adv Mater 2011;23:2064e8.
[4] Liu SJ, Wang P, Zhao Q, Dong XC, Lin WP, Huang W, et al. Single polymer-
based ternary electronic memory material and device. Adv Mater 2012;24:
2901e5.
[24] Bo RC, Li H, Xu QF, Li NJ, Zhuang H, Lu JM, et al. Improving of molecular
planarity via tailoring alkyl chain within the molecules to enhance memory
device performance. Dyes Pigments 2014;109:155e62.
[5] Tee BCK, Wang C, Allen R, Bao Z. An electrically and mechanically self-healing
composite with pressure- and flexion-sensitive properties for electronic skin
applications. Nat Nanotechnol 2012;7:825e32.
[25] Liu SJ, Wang P, Sun HB, Dong XC, Lin WP, Huang W, et al. Single polymer-
based ternary electronic memory material and device. Adv Mater 2012;24:
2901e5.
[6] Liu J, Yin Z, Cao X, Wang L, Huang W, Zhang H, et al. Fabrication of flexible, all-
reduced graphene oxide non-volatile memory devices. Adv Mater 2013;25:
233e8.
[26] Ye C, Peng Q, Li M, Luo J, Tang Z, Song Y, et al. Multilevel conductance
switching of memory device through photoelectric effect. J Am Chem Soc
2012;134:20053e9.

E. Shi et al. / Dyes and Pigments 122 (2015) 66e73
73
[27] Ko YG, Kwon W, Hahm SG, Lee TJ, Liou GS, Ree M, et al. Various digital
memory behaviors of functional aromatic polyimides based on electron donor
and acceptor substituted triphenylamines. Macromolecules 2012;45:
3749e58.
[44] Pei K, Wu YZ, Zhang Q, Han LY, Tian H, Zhu WH, et al. Constructing organic
DeAe -A-featured sensitizers with a quinoxaline unit for high-efficiency
p
solar cells: the effect of an auxiliary acceptor on the absorption and the en-
ergy level alignment. Chem Eur J 2012;18:8190e200.
[28] Ahn B, Kim DM, Hsu JC, Kim J, Chen WC, Ree M, et al. Tunable film mor-
phologies of brush-linear diblock copolymer bearing difluorene moieties yield
a variety of digital memory properties. ACS Macro Lett 2013;2:555e60.
[29] Li G, Zheng K, Leck S, Hu F, Sun XW, Zhang Q, et al. Synthesis and nonvolatile
memory behaviors of dioxatetraazapentacene derivatives. ACS Appl Mater
Interfaces 2013;5:6458e62.
[45] Shi J, Chen JN, Chai ZF, Peng T, Li QQ, Li ZJ, et al. High performance organic
sensitizers based on 11,12-bis(hexyloxy) dibenzo[a,c]phenazine for dye-
sensitized solar cells. J Mater Chem 2012;22:18830e8.
[46] Lu XF, Feng QY, Lan T, Zhou G, Wang ZS. Acid degradable cross-linked micelles
for the delivery of cisplatin: a comparison with nondegradable cross-linker.
Chem Mater 2012;24:3179e211.
[30] Gu PY, Zhou F, Gao J, Li G, Wang C, Lu JM, et al. Synthesis, characterization, and
nonvolatile ternary memory behavior of a larger heteroacene with nine lin-
early fused rings and two different heteroatoms. J Am Chem Soc 2013;135:
14086e9.
[47] Li SR, Lee CP, Kuo HT, Ho KC, Sun SS. High-performance dipolar organic dyes
with an electron-deficient diphenylquinoxaline moiety in the
p-conjugation
framework for dye-sensitized solar cells. Chem Eur J 2012;18:12085e95.
[48] Pei K, Wu YZ, Zhang Q, Han LY, Tian H, Zhu WH, et al. Constructing high-
[31] Wang G, Miao SF, Zhang QJ, Li H, Li NJ, Lu JM, et al. Effect of a
p-spacer be-
efficiency DeAepeA-featured solar cell sensitizers: a promising building
tween a donor and an acceptor on small molecule-based data-storage device
performance. Chem Commun 2013;28:9470e2.
[32] Liu HZ, Bo RC, Liu HF, Li H, Li NJ, Lu JM, et al. Study of the influences of mo-
lecular planarity and aluminum evaporation rate on the performances of
electrical memory devices. J Mater Chem 2014;2:5709e16.
block of 2,3-diphenylquinoxaline for antiaggregation and photostability. ACS
Appl Mater Interfaces 2013;5:4986e95.
[49] Yang J, Ganesan P, Teuscher J, Zakeeruddin SM, Tian H, Gratzel M, et al. In-
€
fluence of the donor size in DepeA organic dyes for dye-sensitized solar cells.
J Am Chem Soc 2014;136:5722e30.
[33] Ren WS, Zhu YX, Li H, Zheng JW, Li NJ, Lu JM, et al. Bistable memory devices
with lower threshold voltage by extending the molecular alkyl-chain length.
Phys Chem Chem Phys 2013;15:9212e8.
[34] Zhang YH, Zhuang H, Li H, Xu QF, Li NJ, Lu JM. Thermally stable ternary data-
storage device based on twisted anthraquinone molecular design. J Phys
Chem C 2012;116:22832e9.
[50] Hou L, Tan Z, Wang X, Zhou Y, Han M, Li Y. Novel two-dimensional
donoreacceptor conjugated polymers containing quinoxaline units: synthe-
sis, characterization, and photovoltaic properties. J Polym Sci Part A: Polym
Chem 2008;46:4038e9.
€
€
[51] Gsanger JH, Oh M, Hoen HW, Krause AM, Bao Z, Wurthner F, et al. A crystal-
engineered hydrogen-bonded octachloroperylene diimide with twisted
a
[35] Miao SF, Li H, Xu QF, Zheng JW, Li NJ, Lu JM, et al. Tailoring of molecular
planarity to reduce charge injection barrier for high-performance small-
molecule-based ternary memory device with low threshold voltage. Adv
Mater 2012;24:6210e5.
[36] Wei DC, Wu B, Guo YL, Yu G, Liu YQ. Controllable chemical vapor deposition
growth of few layer graphene for electronic devices. Acc Chem Res 2013;46:
106e15.
core: an n-channel organic semiconductor. Angew Chem Int Ed 2010;49:
740e3.
[52] Kim Y, Cook S, Tuladhar SM, Mcculloch I, Ha CS, Ree M, et al. A strong
regioregularity effect in self-organizing conjugated polymer films and high-
efficiency polythiophene:fullerene solar cells. Nat Mater 2006;5:197e203.
[53] Matsuo Y, Sato Y, Niinomi T, Soga I, Tanaka H, Nakamura E. Columnar struc-
ture in bulk heterojunction in solution-processable three-layered p-i-n
organic photovoltaic devices using tetrabenzoporphyrin precursor and silyl-
methyl[60]fullerene. J Am Chem Soc 2009;131:16048e50.
[54] Liu Y, Wan X, Zhou J, Long G, Tian J, Chen Y, et al. High-performance solar cells
using a solution-processed small molecule containing benzodithiophene unit.
Adv Mater 2011;23:5387e91.
[37] Kitamura C, Tanak S, Yamashita S. Design of narrow-band gap polymers.
Syntheses and properties of monomers and polymers containing aromatic-
donor and o-quinoid-acceptor units. Chem Mater 1996;8:570e8.
€
[38] Mastalerz M, Fischer V, Ma Q, Janssen RAJ, Bauerle P. Conjugated oligothienyl
dendrimers based on a pyrazino[2,3-g]quinoxaline core. Org Lett 2009;11:
4500e3.
[39] Wang E, Hou L, Wang S, Hellestram S, Zhang F, Inganas O, et al. An easily
synthesized blue polymer for high-performance polymer solar cells. Adv
Mater 2010;22:5240e4.
[55] Liu G, Zhang B, Chen Y, Zhu CX, Zeng L, Chan DS. Electrical conductivity
switching and memory effects in poly(N-vinylcarbazole) derivatives with
pendant azobenzene chromophores and terminal electron acceptor moieties.
J Mater Chem 2011;21:6027e33.
[40] Zhang J, Cai W, Wang M, Duan C, Yang T, Cao Y, et al. Synthesis of quinoxaline-
based donoreacceptor narrow-band-gap polymers and their cyclized de-
rivatives for bulk-heterojunction polymer solar cell applications. Macromol-
ecules 2011;44:894e901.
[56] Li H, Li NJ, Gu HW, Xu QF, Yan F, Lu JM, et al. Two different memory char-
acteristics controlled by the film thickness of polymethacrylate containing
pendant azobenzothiazole. J Phys Chem C 2010;114:6117e22.
[57] Loser S, Bruns C, Miyauchi H, Facchetti A, Stupp S, Marks T, et al.
[41] Perepichka IF, Perepichka DF, Meng H, Wudl F. Light-emitting polythiophenes.
Adv Mater 2005;17:2281e305.
A naphthodithiophene-diketopyrrolopyrrole donor molecule for efficient
solution-processed solar cells. J Am Chem Soc 2011;133:8142e5.
[58] Li P, Gao K, Schuermann S, KDe Cola, Müllen K, Chi L, et al. Controllable
growth and field-effect property of monolayer to multilayer microstripes of
an organic semiconductor. J Am Chem Soc 2010;132:8807e9.
[42] Perepichka II , Perepichka IF, Bryce MR, Palsson L. Dibenzothiophene-S,S-
dioxideefluorene co-oligomers. Stable, highly-efficient blue emitters with
improved electron affinity. Chem Commun 2005:3397e9.
[43] Chang DW, Lee HJ, Kim JH, Park SM, Dai LM, Baek JB, et al. Novel quinoxaline-
based organic sensitizers for dye-sensitized solar cells. Org Lett 2011;13:
3880e3.