Improving of molecular planarity via tailoring alkyl chain within the molecules to enhance memory device performance

Article abstract of DOI:10.1016/j.dyepig.2014.05.003

Three conjugated small molecules with different planarity which were tailored by the alkyl chain (from methyl to cyclic alkyl chain) were designed and successfully synthesized. The memory devices based on each molecule exhibited similar write-once-read-many-times characteristics but different switch threshold voltages. Atomic force microscopy and X-ray diffraction results indicated that the film surface morphology became more planar and intermolecular packing more closer as improving the molecular planarity, which led to the lower switch threshold voltages and the higher ON/OFF current ratios. Mechanism analysis demonstrated that one charge trap arising from the benzoquinoline group in the molecular backbone was injected by charge carriers as the external bias increased, resulting in OFF and ON distinct current states. This work not only demonstrated a new memory device with low-power consumption but also offered fundamental insight for the rational design of small molecules for organic memory devices.

Full text of DOI:10.1016/j.dyepig.2014.05.003

Dyes and Pigments 109 (2014) 155e162
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Improving of molecular planarity via tailoring alkyl chain within the
molecules to enhance memory device performance
,
Rongcheng Bo a, Hua Li a *, Hongzhang Liu a, Hao Zhuang a, Najun Li a, Qingfeng Xu a,
, b,
Jianmei Lu a *, Lihua Wang a
a 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
b State Key Laboratory of Treatments and Recycling for Organic Effluents by Adsorption in Petroleum and Chemical Industry, 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:
Three conjugated small molecules with different planarity which were tailored by the alkyl chain (from
methyl to cyclic alkyl chain) were designed and successfully synthesized. The memory devices based on
each molecule exhibited similar write-once-read-many-times characteristics but different switch
threshold voltages. Atomic force microscopy and X-ray diffraction results indicated that the film surface
morphology became more planar and intermolecular packing more closer as improving the molecular
planarity, which led to the lower switch threshold voltages and the higher ON/OFF current ratios.
Mechanism analysis demonstrated that one charge trap arising from the benzoquinoline group in the
molecular backbone was injected by charge carriers as the external bias increased, resulting in OFF and
ON distinct current states. This work not only demonstrated a new memory device with low-power
consumption but also offered fundamental insight for the rational design of small molecules for
organic memory devices.
Received 17 March 2014
Received in revised form
30 April 2014
Accepted 2 May 2014
Available online 21 May 2014
Keywords:
Memory device
Write-once-read-many-times
Planarity
Spacial distortion
Charge trap
© 2014 Elsevier Ltd. All rights reserved.
Benzoquinoline
1. Introduction
versatility in molecular design. However, until now most of the
researchers have only focused on the synthesis of new materials
With the rapid development of information technology, large
masses of data must be stored efficiently and safely, so it is very
important to develop excellent memory materials to live up to the
demand for data storage nowadays [1e3]. Electrical memory de-
vices based on organic memory materials have attracted increasing
interest for their significant advantages, such as good scalability
[4e8], the ease of materials processing, low-cost, simplicity in de-
vice structure and the potential for super-high data storage density
and studying their memory effects, design concept for organic non-
volatile memories is still a topic of active debate. For example,
clarifying the relationship between the molecular structure and
device properties is very important for promoting memory device
performance in the future. In our previous work, the influence of
the molecular length [11], the conjugated length [11], the alkyl-
chain length [40], the number and electron-withdrawing strength
of acceptor moieties [12] have been studied. Among those reports,
many researches only focused on the straight or branched alkyl
chain groups, the influence of cyclic alkyl groups and their spacial
distortion on film morphology, molecular packing and device per-
formance have been relatively ignored.
In recent years, materials containing the quinoline group with a
larger conjugated plane have been widely used in organic field
effect transistors (OFET) or organic light-emitting diodes (OLED)
[41e44]. In this paper, three conjugated small molecules con-
taining quinoline groups with different planarity were designed
and successfully synthesized. The device exhibited lower switch
threshold voltages (Vths) and higher ON/OFF current ratio with the
alkyl chain changed from cyclic alkyl chain to methyl. The AFM
and XRD results testified that the improved planarity of the
[9e14]. A wide variety of materials, such as
p conjugated polymers
[15e22], pendent polymers [23e32], dopant [33] and complex
systems [34,35] have been reported and exhibited good memory
performances. Among these organic memory materials, organic
small molecules were widely studied due to their well confined
structures which are important for molecular design and calcula-
tion [36e39], good stimuli-responsive, easy purification and
* Corresponding authors. 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. Tel.: þ86 512 65880368; fax: þ86 512 65880367.
E-mail addresses: jianmeilu_group@hotmail.com, lujm@suda.edu.cn (J. Lu).
http://dx.doi.org/10.1016/j.dyepig.2014.05.003
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

156
R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
molecule would achieve more planar film morphology and closer
intermolecular distance in the film. The improved film
morphology would reduce the charge injection barrier between
the film and the electrode, furthermore, the closer intermolecular
packing distance would facilitate the charge transport through the
neighboring molecules. We hope this work can offer a funda-
mental insight for the rational design of small molecules for
OMDs.
2.2. Synthesis of compounds
The synthetic routes are shown in Scheme 1.
Compounds 1 and 2 were synthesized according to the methods
described in literature [45,46].
4-Bromo-N,N-diphenylaniline (1). Yield (5.16 g) 80%, white solid.
1H NMR (400 MHz, CDCl3)
d 7.32e7.29 (m, 6H), 7.08e7.01 (m, 6H),
6.96e6.93 (m, 2H).
N,N-Diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ani-
line (2). Yield (3.44 g) 60%, white solid. 1H NMR (400 MHz, DMSO-
2. Experimental
d6)
d
7.55 (d, J ¼ 8.5 Hz, 2H), 7.36e7.32 (m, 4H), 7.11 (t, J ¼ 7.4 Hz,
2H), 7.05 (d, J ¼ 7.6 Hz, 4H), 6.90 (d, J ¼ 8.5 Hz, 2H), 1.28 (s, 12H).
Compounds 3e5 were synthesized according to the methods
described in literature [47,48].
2.1. Materials
Naphthalen-2-amine (98%, special caution: this compound is a
potent carcinogen and should be done enough protective measures,
such as wearing antigas mask and gloves and handling this material
in the hood, avoiding any skin contact), 4-bromobenzaldehyde
(98%), cyclooctanone (97%), cyclohexanone (99%), acetone (AR),
triphenylamine (97%), N-bromosuccinimide (98%), iodine (98%),
bis(pinacolato)diboron (98%) and other solvents were all purchased
from Energy Chemical China.
3-(4-Bromophenyl)-1-methylbenzo[f]quinoline (3). Yield (1.00 g)
29%, white solid. 1H NMR (400 MHz, DMSO-d6)
d
8.90 (d, J ¼ 8.2 Hz,
1H), 8.30 (d, J ¼ 8.2 Hz, 2H), 8.19 (s, 1H), 8.11 (t, J ¼ 8.3 Hz, 2H), 7.97
(d, J ¼ 8.9 Hz, 1H), 7.82e7.67 (m, 4H), 3.19 (s, 3H).
5-(4-Bromophenyl)-1,2,3,4-tetrahydrobenzo[a]phenanthridine
(4). Yield (1.50 g) 21%, white powder. 1H NMR (400 MHz, DMSO-d6)
d
8.83 (d, J ¼ 7.2 Hz, 1H), 8.13e8.06 (m, 1H), 8.03 (d, J ¼ 8.9 Hz, 1H),
Scheme 1. Synthesis scheme and molecular structures.

R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
157
7.83 (d, J ¼ 8.9 Hz, 1H), 7.71 (dd, J ¼ 8.5, 2.6 Hz, 4H), 7.53 (d,
J ¼ 8.4 Hz, 2H), 3.59 (s, 2H), 2.81 (d, J ¼ 6.2 Hz, 2H), 1.90e1.73 (m,
4H).
122.9, 31.1, 30.9, 30.7, 28.0, 27.2, 25.8. HRMS (ESI, m/z) calcd. for
C
43H37N2 (M þ Hþ) 581.2957, found 581.3107.
8-(4-Bromophenyl)-9,10,11,12,13,14-hexahydrobenzo[f]cycloocta
2.3. Fabrication of the memory device
[c]quinoline (5). Yeild (0.50 g) 20%, white powder. 1H NMR
(400 MHz, CDCl3)
d
8.82 (d, J ¼ 8.9 Hz, 1H), 8.10e7.84 (m, 3H),
The ITO glass substrates were precleaned in ultrasonic bath for
30 min each in deionized water, acetone, and ethanol. The elec-
troactive organic layer was vacuum-deposited under a pressure of
around 1.0 ꢁ 10ꢂ6 Torr and the thickness was 68 nm. The aluminum
electrodes (0.0314 mm2) were vacuum-deposited at 5.0 ꢁ 10ꢂ6 Torr
through a shadow mask to form top electrodes and the thickness
was 55 nm. Both of the thickness was obtained by the scanning
electronic microscopic (SEM). The schematic diagram of the device
and the SEM image of one storage cell were shown in Fig. 1.
7.71e7.64 (m, 2H), 7.62 (d, J ¼ 8.3 Hz, 2H), 7.39 (d, J ¼ 8.3 Hz, 2H),
3.62 (s, 2H), 2.98e2.93 (m, 2H), 2.31e2.22 (m, 2H), 1.73e1.64 (m,
4H), 1.55e1.52 (m, 2H).
40-(1-Methylbenzo[f]quinolin-3-yl)-N,N-diphenyl-[1,10-biphenyl]-
4-amine (6). A mixture of compound 2 (0.40g, 1.15 mmol), com-
pound 3 (0.43g, 1.15 mmol), Pd(PPh3)4 (0.04 g, 0.03 mmol), K2CO3
(0.47 g, 3.46 mmol), H2O (5 mL) and dioxane (30 mL) was stirred for
6 h under nitrogen at 80 ꢀC. After completion of the reaction, the
mixture was diluted with H2O and extracted with EtOAc
(3 ꢁ 50 mL). The combined organic layer was washed with brine
and dried over anhydrous sodium sulfate and then the organic
solvents were completely removed by rotary evaporation. The
residue was purified by column chromatography using petroleum
ether/ethylacetate (5/1; v/v) to give compound 6 (0.32 g, 50%) as
white powder. mp: 200e201 ꢀC. IR (in KBr), cmꢂ1: 3031, 1932, 1582,
1486, 1451, 1329, 1277, 1208, 1175, 821, 719, 694, 633. 1H NMR
2.4. Measurement
NMR spectra were obtained on Inova FT-NMR spectrometers. IR
spectra were obtained on Nicolet 6700 spectrometer. HRMS (ESI)
was determined by using micro TOF-Q II HRMS/MS instrument
(Bruker). UVeVis absorption spectra were measured at room
temperature by using a Shimadzu UV-3600 spectrophotometer.
SEM images were taken on a Hitachi S-4700 scanning electron
microscope. Cyclic voltammetry was performed at room tempera-
ture using an ITO working electrode, a reference electrode Ag/AgCl,
and a counter electrode (Pt wire) at a sweep rate of 100 mV sꢂ1 on a
CorrTest CS Electrochemical Workstation analyzer. A 0.1 M solution
of tetrabutylammonium perchlorate (TBAP) in CH3CN was used.
The surface morphology of films was measured on a MFD-3D-SA
Asylum Research's atomic force microscope (AFM). X-ray diffrac-
tion (XRD) patterns were taken on an X'Pert-Pro MPD X-ray
diffractometer. All electrical measurements of the devices were
characterized under ambient conditions without any encapsulation
using a HP-4145B Semiconductor device Analyzer.
(400 MHz, CDCl3)
d
8.85 (d, J ¼ 8.6 Hz 1H), 8.30 (d, J ¼ 8.0 Hz 2H),
8.21e8.11 (m, 1H), 8.02e7.96 (m, 2H), 7.89 (s, 1H), 7.76 (d, J ¼ 8.1 Hz
2H), 7.73e7.66 (m, 2H), 7.58 (d, J ¼ 8.5 Hz 2H), 7.29 (t, J ¼ 7.8 Hz,
3H), 7.18e7.15 (m, 7H), 7.05 (t, J ¼ 7.2 Hz, 2H), 3.24 (s, 3H). 13C NMR
(300 MHz, CDCl3)
d 154.9, 149.6, 147.5, 147.4, 145.5, 141.3, 137.3,
134.3, 132.8, 131.1, 130.8, 129.5, 129.3, 129.0, 127.7, 127.4, 126.9,
126.4, 126.3, 124.6, 124.5, 123.7, 123.0, 122.5, 27.0. HRMS (ESI, m/z)
calcd. for C38H29N2 (M þ Hþ) 513.2331, found 513.2405.
Compounds 7 and 8 were synthesized using a similar procedure
for compound 6.
N,N-Diphenyl-40-(1,2,3,4-tetrahydrobenzo[a]phenanthridin-5-yl)-
[1,10-biphenyl]-4-amine (7). Yeild (0.25 g) 50%, white powder. mp:
243e244 ꢀC. IR (in KBr), cmꢂ1: 3031, 2937, 2865, 1589, 1500, 1433,
1316, 1278, 1119, 1003, 827, 715, 696. 1H NMR (400 MHz, CDCl3)
3. Results and discussion
d
8.81 (s, 1H), 7.98 (s, 2H), 7.72e7.61 (m, 6H), 7.55 (d, J ¼ 8.6 Hz, 2H),
7.47 (s, 1H), 7.31e7.27 (m, 5H), 7.17 (s, 3H), 7.16 (s, 2H), 7.05 (t,
J ¼ 7.3 Hz, 2H), 3.67 (s, 2H), 2.96 (s, 2H), 1.91(s, 4H). 13C NMR
3.1. Photophysical and electrochemical properties
(300 MHz, CDCl3)
d 158.7, 147.6, 147.2, 140.3, 134.7, 133.3, 132.1,
Fig. 2(aec) shows the UVevis spectra of compounds 6, 7 and 8 in
DCM solution and in the film state, respectively. The maximum
absorption peaks of the three compounds in DCM are at 363 nm,
340 nm and 337 nm, respectively, which could be assigned to the
132.0, 130.1, 129.2, 129.0, 128.9, 128.5, 128.4, 127.8, 126.6, 126.2,
125.6, 124.8, 124.4, 123.8, 122.9, 33.3, 28.4, 23.1, 22.2. HRMS (ESI, m/
z) calcd. for C41H33N2 (M þ Hþ) 553.2644, found 553.2765.
40-(9,10,11,12,13,14-Hexahydrobenzo[f]cycloocta[c]quinolin-8-yl)-
N,N-diphenyl-[1,10-biphenyl]-4-amine (8). Yeild (0.20 g) 56%, white
powder. mp: 245e246 ꢀC. IR (in KBr), cmꢂ1: 3031, 2919, 2850, 1590,
1498, 1435, 1327, 1279, 1005, 828, 750, 695. 1H NMR (400 MHz,
pep* transition of the benzoquinolines group within the molecular
backbone. The maximum absorption peaks of the three compounds
in the film state are at 373 nm, 347 nm and 340 nm, respectively.
Compared with the absorption spectra in solution, the maximum
absorption peaks of the three compounds in film states show a red-
shift of 10 nm, 7 nm and 3 nm, respectively. These changes indicate
DMSO-d6)
d
8.83 (d, J ¼ 7.5 Hz 1H), 8.09 (d, J ¼ 9.4 Hz 1H), 8.05 (d,
J ¼ 9.3 Hz 1H), 7.85 (t, J ¼ 7.4 Hz 1H), 7.80e7.67 (m, 6H), 7.55 (d,
J ¼ 8.1 Hz 2H), 7.35 (t, J ¼ 7.8 Hz 4H), 7.09 (d, J ¼ 8.1 Hz 8H), 3.69 (s,
2H), 3.00 (s, 2H), 2.19 (s, 2H), 1.64 (s, 4H), 1.49 (s, 2H). 13C NMR
that the three molecules happened regular arrangement or
pep
stacking in the film, which promise a smooth charge carrier
transport between neighboring molecules [49]. The absorption
spectrum of compound 6 has the largest red-shift than that of
(300 MHz, CDCl3)
d 147.6, 147.2, 140.1, 134.7, 133.4, 133.2, 130.6,
130.0, 129.2, 129.0, 127.7, 127.6, 126.4, 126.2, 124.6, 124.4, 123.9,
Fig. 1. (a) Schematic diagram of the indium-tin oxide/organic film/aluminum device, (b) SEM image of the cross section of a storage cell.

158
R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
Fig. 2. (aec) UVevisible absorption spectra of the three molecules in CH2Cl2 solution and on films; (def) cyclic voltammograms of the films deposited on ITO glass in CH3CN.
compounds 7 and 8 may due to its best planarity among the three
molecules. In addition, according to the UVevis spectrum the op-
tical bandgaps (Eg) of compounds 6, 7, 8 can be estimated to be
about 2.89 eV, 3.08 eV and 3.17 eV, respectively. The electro-
chemical properties of the three compounds were measured by
cyclic voltammetry (CV). As shown in Fig. 2(def), the first oxidation
peak of compounds 6, 7 and 8 is 0.52 eV, 0.54 eV, 0.56 eV,
respectively. The HOMO and LUMO energy levels can be calculated
from the onset oxidation potential with reference to ferrocene
(4.8 eV) by the following equation [50]:
respectively. We have known ITO and Al has a work function
of ꢂ4.8 eV and ꢂ4.3 eV, thus the energy barriers of hole injection
from the ITO electrode to HOMO levels (Ф1) of compounds 6, 7 and
8 were estimated to be 0.02 eV, 0.04 eV and 0.06 eV, respectively,
while the energy barrier of electron injection from the Al electrode
to the LUMO levels (Ф2) of compounds 6, 7 and 8 were estimated to
be 2.37 eV, 2.54 eV and 2.61 eV, respectively (Al as cathode and ITO
as anode). It indicates that hole injection is easier than electron
Table 1
Optical and electrochemical properties of the three synthesized molecular films.
EHOMO ¼ ꢂ½EoxðonsetÞ þ 4:8 ꢂ EFerroceneꢃ
ELUMO ¼ EHOMO þ Eg
Compounds lonset/nm Ega/eV Eox(onset)/eV HOMO/eV LUMO/eV
Ф1/eV Ф2/eV
6
7
8
428
402
391
2.89 0.52
3.08 0.54
3.17 0.56
ꢂ4.82
ꢂ4.84
ꢂ4.86
ꢂ1.93
ꢂ1.76
ꢂ1.69
0.02 2.37
0.04 2.54
0.06 2.61
As shown in the Table 1, the HOMO and LUMO values of com-
pound 6 are ꢂ4.82 eV and ꢂ1.93 eV, compound 7 are ꢂ4.84 eV
a
The data were calculated using the following equation: bandgap ¼ 1240/lonset of
and ꢂ1.76 eV, compound
8
are ꢂ4.86 eV and ꢂ1.69 eV,
the three synthesized molecular films.

R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
159
and compact shape, which would reduce the charge barrier be-
tween the film and the electrode. This may be as a result of the
spacial distortion of the cyclic alkyl chain to reduce the film-
forming property.
3.3. Electrical switching effects and memory performance
The electrical switching and memory effects of the three mol-
ecules were illustrated by IeV characteristics. As shown in Fig. 5a,
when a voltage sweep was applied from 0 to ꢂ4 V (with Al as
cathode and ITO as anode), the device was initially in the low
conductivity (“0” signal or OFF) state, the current of the Al/com-
pound 6/ITO device increased slowly with increasing the applied
voltage, until the threshold voltage of ꢂ1.53 V was reached, the
current increased abruptly frome10ꢂ5 toe10ꢂ2 A, indicating device
transition from the low conductivity state to the high conductivity
state(“1” signal or ON) state, with an ON/OFF current ratio of
7.6 ꢁ 103. The ON state retained its high conductivity state in the
subsequent forward and reverse biased sweeps (0 to 4 V), or even
the power supply was turned off, suggesting the device based on
compound 6 is typical non-volatile WORM electronic memory
characteristics. The Al/compound 7/ITO and Al/compound 8/ITO
devices exhibited a similar electrical conductivity switching and
associated WORM memory performance, the former with a higher
switching threshold voltage of ꢂ1.87 V and lower ON/OFF current
ratio of 3.5 ꢁ 101, the latter with the highest switching threshold
voltage of ꢂ2.01 V and the lowest ON/OFF current ratio of 2.3 ꢁ 101.
The electrical transition from the OFF state to the ON state served as
the "writing" process for the memory devices. The stability of the
memory effects were also evaluated, Fig. 5def show the retention
time under a constant stress of ꢂ1.50 V for both the ON and OFF
states of the three devices, respectively. The ON/OFF current ratios
could be maintained and no obvious degradation in current was
observed for both the ON and OFF states during periods of at least
105 s.
Fig. 3. XRD patterns of the three thin films.
injection in the ITO/compound 6 or 7 or 8/Al devices and the hole
injection dominates the whole charge transfer process.
3.2. Morphology of films and molecular packing
The XRD patterns of the three films using ITO glasses as sub-
strates are shown in Fig. 3. Compound 6 thin film exhibited a
diffraction peak at 2
q
¼ 22.75ꢀ (3.91 Å) while the compound 7 is at
2
q
¼ 20.84ꢀ (4.26 Å), which illustrates the compound 6 has a closer
intermolecular stacking distance [51,52]. However, the thin film
containing compound 8 has no obvious diffraction peaks. These
results indicate that the intermolecular
pep stacking become
stronger as the alkyl chain changed from cyclic alkyl to methyl
which would facilitate charge transport through the neighboring
molecules.
Atomic force microscopy (AFM) was used to characterize the
surface morphology of the electroactive layers. AFM height images
show that the compound 6 (Fig. 4a and d) film, the compound 7
(Fig. 4b and e) film and the compound 8 (Fig. 4c and f) film have the
RMS (roughness morphology square) of about 2 nm, 5 nm,
20 nm, respectively, as the alkyl chain changed from cyclic to
methyl chain, the surface morphology becomes more continuous
3.4. Proposed storage mechanism
In order to understand the electrical switching behavior of the
small-molecule-based memory device, theoretical calculations
Fig. 4. (aec) Tapping-mode AFM height images of the three thin films vacuum-deposited onto ITO substrate; (def) typical cross section profile of AFM topographic images. Scale
bars are equal to 1 m.
m

160
R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
Fig. 5. (aec) Currentevoltage (IeV) characteristics of compounds 6, 7 and 8; (def) effect of the operation time on the device current in the OFF and ON states of the three devices
under a constant stress of ꢂ1.50 V.
were performed using density functional theory. The HOMO and
LUMO surfaces for compounds 6, 7 and 8 are shown in Fig. 6. The
HOMO is localized on the electron-donor side and the LUMO is on
the electron-acceptor side. As shown in Fig. 6, the molecular sur-
faces with the continuous electrostatic potentials (ESP) in the red
area. It indicates that charges can migrate through this open
channel. However, there are some negative ESP regions (blue area),
which are associated with the electron-acceptor groups. In the OFF
state these negative regions can serve as a “trap” to localize the
charge, reduce the mobility of the charge carriers, and the device
current was low, with the sweep voltage increasing, the trap are
filled completely, the device transforms the low conductivity state
into the high conductivity state, indicating the device switches
from OFF state to ON state. Compared to compounds 7 and 8,
compound 6 based memory device has a lower threshold voltage
and higher ON/OFF current ratio due to its improved planarity of
the molecules which will promote the film-forming property and
facilitate the carrier mobility of the films. As shown in Fig. 4, it is
obviously to see that the film surface morphology and the RMS of
compounds 7 and 8 are poorer than that of compound 6 due to the
Fig. 6. DFT molecular simulation results (B3LYP/6-311G (d) level) of compounds 6, 7 and 8: molecular optimized geometry, ESP, HOMO and LUMO molecular orbitals.

R. Bo et al. / Dyes and Pigments 109 (2014) 155e162
161
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spacial distortion of the cyclic alkyl chain of compounds 7 and 8
(Fig. 6, molecular optimized geometry). In addition, the 2 degree of
q
compound 7 film is smaller than compound 6 film and even dis-
appeared in compound 8 film with the alkyl chain changed from
methyl to cyclic alkyl chain, indicating that the arrangement of
molecules in the films become more and more loose or relative
disorderly. Thus, with the improving planarity of the three mole-
cules, the molecule generated a highly ordered arrangement and
facilitated the close packing between molecules and decreased the
hole-injection energy barrier, increased charge carrier transport
between neighboring molecules in the film, leading to a lower
threshold voltage and higher ON/OFF current ratio.
[15] Ling QD, Chang FC, Song Y, Zhu CX, Liaw DJ, Chan DSH, et al. Synthesis and
dynamic random access memory behavior of a functional polyimide. J Am
Chem Soc 2006;128:8732e3.
4. Conclusions
[16] Park S, Lee TJ, Kim DM, Kim JC, Kim K, Kwon W, et al. Electrical memory
characteristics of a nondoped
eties. J Phys Chem B 2010;114:10294e301.
p-conjugated polymer bearing carbazole moi-
In summary, three conjugated small molecules with different
planarity were designed and successfully synthesized. The
sandwich-structure memory devices based on the three molecules
were fabricated and their electrical characteristics were investi-
gated. All the devices exhibited WORM memory behavior, and the
ON/OFF current ratios can be maintained and no obvious degra-
dation in current was observed for both the ON and OFF states
during periods of at least 105 s under a constant read voltage stress
of ꢂ1.5 V. With the alkyl chain changed from cyclic alkyl chain to
methyl, the planarity of the molecules was obviously improved,
which generated a highly ordered molecule arrangement in the
film and decreased the hole injection energy barrier between the
film and the electrode, leading to the lower threshold voltages and
higher ON/OFF current ratios. The results were consistent with the
data revealed by cyclic voltammetry, UVevisible absorption spec-
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Acknowledgments
This work was financially supported by the NSF of China
(21176164, 21206102 and 21336005), the NSF of Jiangsu Province
(BE2013052), a Project of the Department of Education of Jiangsu
Province (12KJB430011), Suzhou Nano-project (ZXG2012023) and
Project supported by the Specialized Research Fund for the Doctoral
Program of Higher Education of China (Grant no. 20113201130003
and 20123201120005).
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