Synthesis, crystal structures, optoelectronic properties and resistive memory application of π-conjugated heteroaromatic molecules

Article abstract of DOI:10.1016/j.tet.2020.131471

Three π-conjugated small molecules DPBT, DEPBT and DEPBPy, based on bithiazole or bipyridine were designed and synthesized. In DEPBT and DEPBPy, the central core and terminal phenyl groups are connected by C[dbnd]C double bonds, whereas in DPBT, they are combined through C–C single bonds. The effects of the molecular structural changes on their crystal structures, optoelectronic properties and thin film morphologies were comparatively studied. In addition, their resistive memory performance was primarily investigated for the first time. Both DEPBT- and DEPBPy-based devices exhibit typical binary memory behavior, while DPBT-based device presents no memory property. The experimental results confirm that the introduction of C[dbnd]C bridged bonds into molecules could enhance the molecular interaction, which plays an important role in the molecular arrangement on the thin film, and has a significant contribution to charge transport in memory devices. This work demonstrates that aryl vinyl terminated bithiazoles and bipyridines both are good promising candidates for organic memory storage.

Full text of DOI:10.1016/j.tet.2020.131471

Tetrahedron 76 (2020) 131471
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis, crystal structures, optoelectronic properties and resistive
memory application of
p-conjugated heteroaromatic molecules
,
, c, ***
Ming Jia a c, Yabo Li b, Mengmeng Huang b, Jung Keun Kim b, Yingliang Liu a
,
Yangjie Wu b **, Shaokui Cao a
,
, c, *
a School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450001, PR China
b College of Chemistry, Institute of Green Catalysis, Zhengzhou University, Zhengzhou, 450052, PR China
c Henan Key Laboratory of Advanced Nylon Materials and Application, Zhengzhou University, Zhengzhou, 450001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three
p-conjugated small molecules DPBT, DEPBT and DEPBPy, based on bithiazole or bipyridine were
Received 20 May 2020
Received in revised form
10 July 2020
Accepted 2 August 2020
Available online 15 August 2020
designed and synthesized. In DEPBT and DEPBPy, the central core and terminal phenyl groups are
connected by C]C double bonds, whereas in DPBT, they are combined through CeC single bonds. The
effects of the molecular structural changes on their crystal structures, optoelectronic properties and thin
film morphologies were comparatively studied. In addition, their resistive memory performance was
primarily investigated for the first time. Both DEPBT- and DEPBPy-based devices exhibit typical binary
memory behavior, while DPBT-based device presents no memory property. The experimental results
confirm that the introduction of C]C bridged bonds into molecules could enhance the molecular
interaction, which plays an important role in the molecular arrangement on the thin film, and has a
significant contribution to charge transport in memory devices. This work demonstrates that aryl vinyl
terminated bithiazoles and bipyridines both are good promising candidates for organic memory storage.
© 2020 Elsevier Ltd. All rights reserved.
Keywords:
Bithiazole
Bipyridine
C¼C double bond
Organic memory device
Film-forming ability
1. Introduction
property [3c,4]. Thus, massive efforts have been paid to enhance
intermolecular interaction of organic small molecules, such as
Organic memory devices (OMDs) have attracted particular
attention due to their potential advantages of flexibility, low
fabrication cost and ultrahigh-density capacity [1]. Among the
potential materials for OMDs, organic small molecules with good
memory property have earned significant progress in the past few
decades [2]. Organic small molecules with well-defined structures
are coalesced via intermolecular force, such as van der Waals force,
introducing heteroatoms (N, Si, O or S, etc.), C]C double bonds or
C^C triple bonds into the conjugated backbone, and great success
has been achieved in the promotion of OMDs performance [5].
Bithiazole (BT) and bipyridine (BPy) both are important aro-
matic heterocycles, which have many advantages including excel-
lent chemical stability, energy level tunability and easily-modified
structure [6]. Organic semiconductor materials based on bithiazole
or bipyridine moieties have been widely applied in organic field-
effect transistors (OFETs) [7], organic solar cells (OSCs) [8] and
organic light-emitting diodes (OLEDs) [9]. Nevertheless, the use of
them in OMDs has still been very limited. To the best of our
knowledge, only a few ruthenium complexes with bipyridine as
ligands have been used in OMDs [10]. In addition, the resistive
memory application of bithiazole-based small molecules has not
been reported. Therefore, it is a significant work to develop diver-
sified bithiazole- or bipyridine-based organic memory materials
and study the relationship between molecular structure and device
performance.
p-p stacking or hydrogen bonding interactions, instead of strong
covalent bonds in inorganic semiconductors [3]. As we all known
that enhancing intermolecular interaction could increase the
ordering on a molecular scale and lead to distinct solid-state or-
ganization, which often results in excellent charge-transport
* Corresponding author. School of Materials Science and Engineering, Zhengzhou
University, Zhengzhou, 450001, PR China.
** Corresponding author.
*** Corresponding author. School of Materials Science and Engineering, Zhengz-
hou University, Zhengzhou, 450001, PR China.
E-mail addresses: liuylxn@zzu.edu.cn (Y. Liu), wyj@zzu.edu.cn (Y. Wu),
caoshaokui@zzu.edu.cn (S. Cao).
Herein, we synthesized two phenyl vinyl-bridged 4,40-bithia-
zole- or bipyridine-based
p-conjugated small molecules, namely,
https://doi.org/10.1016/j.tet.2020.131471
0040-4020/© 2020 Elsevier Ltd. All rights reserved.

2
M. Jia et al. / Tetrahedron 76 (2020) 131471
2,20-di((E)-styryl)-4,40-bithiazole (DEPBT) and 4,40-di((E)-styryl)-
2,20-bipyridine (DEPBPy). In contrast, the CeC single bond com-
pound 2,20-diphenyl-4,40-bithiazole (DPBT) was also prepared, as
shown in Scheme 1. The effects of the molecular structural changes
on their crystal structures, optoelectronic properties and film-
forming abilities were comparatively studied. In addition, their
resistive memory performance was primarily investigated. It is
proved that the C]C bridged bond extends molecular conjugated
backbone and enhances intermolecular interaction, which is
beneficial to improve the film quality as well as the electrical
memory performance. Both DEPBT- and DEPBPy-based devices
exhibit typical binary memory behavior, while DPBT-based device
presents no memory property. This work demonstrates that aryl
vinyl terminated bithiazoles and bipyridines both are good prom-
ising candidates for organic memory storage.
2. Results and discussion
2.1. Synthetic procedures
Fig. 1. Single crystal structures: (a) The front and side view of DEPBT; (b) Intermo-
lecular interaction of DEPBT; (c) The front and side view of DEPBPy; (d) Intermolecular
interaction of DEPBPy.
The phenyl terminated compound DPBT and the intermediate 3
were prepared through the Hantzsch reaction according to the
literatures [11]. The styrene-terminated target molecules DEPBT
and DEPBPy were prepared through the aldol-type condensation
reaction according to the published procedure [12]. The detailed
synthesis routes are depicted in Scheme 1. The molecular structures
of DEPBT (CCDC 1507339) and DEPBPy (CCDC 1580398) have been
successfully verified by X-ray single crystal diffraction.
backbone, thereby extending the p-conjugated area and enhancing
close packing between the adjacent parallel molecules [4a,4c].
Meanwhile, the neighboring molecular columns of DEPBT are
linked by multiple CeH/N and CeH/S hydrogen bonds.
As depicted in Fig. 1c, the difference between the molecular
structures of DEPBT and DEPBPy is the central core. DEPBPy
crystallizes in the triclinic space group P-1, with unit cell parame-
ters a ¼ 9.4384(8) Å, b ¼ 9.4791(7) Å, c ¼ 12.8095(10) Å,
2.2. X-ray crystallogtaphy
To study the molecular structure-packing mode correlations, the
single crystals of DEPBT and DEPBPy were successfully obtained in
the process of vacuum sublimation, which could be compared to
that of known compound DPBT [13]. The single crystal structures of
DEPBT and DEPBPy are shown in Fig. 1aec. Similar with DPBT,
DEPBT crystallizes in the monoclinic space group P21/c, with unit
cell parameters a ¼ 15.3811(6) Å, b ¼ 5.38288(18) Å, c ¼ 11.1802(5)
a
¼ 105.343(7)ꢀ,
b
¼ 95.778(7)ꢀ,
g
¼ 117.504(8)ꢀ (Fig. S10 and
Table S2 in SI). Surprisingly, there are two different twisted con-
formations in crystal structures of DEPBPy. For both conforma-
tional molecules, the bipyridine cores are also in a trans coplanar
configuration, and the dihedral angles between the bipyridine
plane and phenyl rings increase to 25.13ꢀ and 24.73ꢀ, respectively.
In the crystal packing (Fig. 1d), there are also multiple CeH$$$p and
CeH/N interactions that exist in neighboring molecules. Different
with DEPBT, the adjacent parallel molecules of DEPBPy only
Å,
a
¼ 90ꢀ,
b
¼ 100.416(4)ꢀ,
g
¼ 90ꢀ (Fig. S9 and Table S1 in Sup-
porting Information). As displayed in Fig.1a, the bithiazole core is in
a trans coplanar configuration, and the lateral phenyl rings twist
from the bithiazole plane in the same direction with small dihedral
angle of ~16.22ꢀ. In the crystal packing (Fig. 1b), DPBT and DEPBT
molecules both adopt a stacking mode of J-type aggregation along b
slightly overlaps, resulting in a weak
p-p interaction. Different
packing modes of the three compounds may lead to the differences
in their film-forming and intermolecular charge transfer abilities
[4a,5c,14].
axis through intermolecular
p-p interactions. For DEPBT, the
introduction of C]C double bonds extends the conjugated
2.3. Thermal property
The thermal properties of the three synthesized compounds
were evaluated by thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC) measurements, as shown in
Scheme 1. Synthesis routes and molecular structures. Reagents and conditions: (i) Br2,
CHCl3, reflux, 3 h; (ii) MeOH, reflux, 3 h; (iii) Acetic anhydride, reflux, Ar, 24 h.
Fig. 2. (a) TGA graphs of DPBT, DEPBT and DEPBPy; (b) Normalized UVeVis absorp-
tion spectra of DPBT, DEPBT and DEPBPy in DMF.

M. Jia et al. / Tetrahedron 76 (2020) 131471
3
Fig. 2a and Fig. S11. For DPBT, DEPBT and DEPBPy, the decompo-
sition temperatures (Td, 5% weight loss temperatures) are 276.9,
328.6 and 343.4 ꢀC, the melting temperatures (Tm) are observed at
189.8, 242.8 and 265.0 ꢀC, respectively (Table 1). It shows that all
the three compounds exhibit good thermal stability for device
applications. Td and Tm of DEPBT and DEPBPy are obvious higher
than that of DPBT, which is probably attributed to the stability of
C]C double bonds [4c,15].
2.4. Optical and electrochemical properties
Fig. 3. (a) DPV curves of DPBT, DEPBT and DEPBPy in DMF; (b) DPV curve of ferrocene
in DMF.
The UVeVis absorption spectra of the three compounds in DMF
solution (10ꢁ5 M) and thin film are exhibited in Fig. 2b and Fig. S12.
In DMF solution, DPBT and DEPBT both present two absorption
bands. The first strong absorption band appears at 270 nm for DPBT
and 286 nm for DEPBT, owing to the existing phenyl substituents.
The second absorption band at 295 nm for DPBT and 335 nm for
DEPBT and DEPBPy films are better. This result is in accordance
with the SEM measurements in Fig. S14. Subsequently, XRD ex-
periments were carried out to measure the crystalline ordering
within the films. According to the XRD patterns (Fig. 4d), only
DEPBT film shows obvious diffraction peaks, no XRD peaks are
observed from DPBT and DEPBPy films. These results suggest that
DEPBT films form a more ordered crystalline arrangement [19].
DEPBT is corresponding to a
backbone [7c,16]. DEPBPy has one characteristic absorption at
317 nm due to a * transition, with a slight blue-shift compared
p-p* transition within the conjugated
p-p
There are diffraction peaks at 2
q
¼ 5.85ꢀ, 17.53ꢀ and 23.48ꢀ for
with DEPBT. This result may be due to the different electron donor-
acceptor effect of the two compounds [10b]. Compared with DPBT,
the absorption peaks of DEPBT and DEPBPy are significantly shift to
lower energy band, which can be attributed to the extending con-
jugated backbone by C]C bridged bonds [4]. For all the three
compounds (Fig. S12), the thin films show obvious broaden and
red-shift peaks compared to that peaks in solution, indicating that a
tight packing or molecular aggregation occurred in the solid state
DEPBT, corresponding to a d-spacing of 15.08 Å and its multiple
diffractions (5.05 Å and 3.78 Å), respectively. The obtained d-
spacing of DEPBT is consistent with the long axis (a axis, ~15.38 Å)
of unit cell obtained from single crystal X-ray diffraction, indicating
DEPBT has a close p-p stacking in the condensed aggregate [20]. It
is noted that the different morphology of the films could lead
different charge transfer ability, which gives a different device
performance [4,5b,19].
[17]. The absorption peaks (l) and the absorption onsets (lonset) for
estimating the optical band gaps (Eogpt) are summarized in Table 1.
Due to bithiazole derivatives do not present good electro-
chemical activity in the cyclic voltammetry (CV) [16,17b], the
different pulse voltammetry (DPV) was used to examine the elec-
trochemical property for the synthesized compounds, as displayed
in Fig. 3aeb. All potentials of the three compounds were calibrated
with the ferrcocene/ferrocenium couple (Fcþ/Fc) as internal stan-
dard in the DMF solution of 10ꢁ3 M with 0.1 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) as a supporting electrolyte.
In contrast to DPBT (Eox ¼ 0.98 eV), the other two compounds
DEPBT (Eox ¼ 1.30 eV) and DEPBPy (Eox ¼ 1.47 eV) exhibit an
obvious positive shift in the first oxidation potential. This phe-
nomenon indicates that C]C double bonds increase the molecular
electrochemical stability [4c,18]. Oxidation onset potentials (Eoonxset),
the highest occupied molecular orbitals (HOMOs) and the lowest
unoccupied molecular orbitals (LUMOs) are summered in Table 1.
2.6. Resistive memory evaluation
The memory properties of the studied compounds were pri-
marily determined by investigating the current-voltage (IeV)
characteristics of “Al/Organic/ITO” sandwich devices, as shown in
Fig. 5aed and Fig. S15. Considering a thicker film can be avoided a
metal-filament effect [21], all devices with an active thickness of
about 120 nm was directly selected without further optimization
(Fig. 5d). As shown in Fig. 5aec, the IeV characteristics of “Al/
DEPBT/ITO” and “Al/DEPBPy/ITO” devices both exhibited flash-
type binary memory behavior, while the DPBT-based device pre-
sents no memory characteristic. The resistive memory properties of
the three compounds are in correspondence with their film-
forming abilities, which shows that the introduction of C]C dou-
ble bonds will affect the morphology of molecular films as well as
the electrical memory performance. In addition, the reproducibility
of the memory devices was examined on 50 independent cells of
each compound. The proportions of DEPBT- and DEPBPy-based
devices with bistable memory behavior are about 60% and 66%,
respectively. The retention performance of “Al/Organic/ITO” de-
vices were evaluated by a reading voltage of þ0.1 V in the “ON” and
“OFF” states under ambient conditions as depicted in Fig. S16. The
current ON/OFF ratios (ION/IOFF) are about 104 for both devices of
2.5. Morphology of the thin films
To investigate the surface morphology and film microstructure,
atomic force microscopy (AFM), scanning electron microscope
(SEM) and X-ray diffraction (XRD) measurements were performed
on the films of DPBT, DEPBT and DEPBPy. As shown in Fig. 4aed
and Fig. S13, the AFM image of DPBT forms long strips and deposits
loosely, leading to a non-continuous film, while the quality of
Table 1
A summary of thermal, optical and electrochemical data of DPBT, DEPBT and DEPBPy.
onset
b
Compound
T
d (ꢀC)
Tm (ꢀC)
lonset (nm)
Eogpta (eV)
E
(eV)
EHOMO (eV)
ELUMOc (eV)
ox
DPBT
DEPBT
DEPBPy
276.9
328.6
343.4
189.8
242.8
265.0
346
385
346
3.58
3.22
3.58
0.83
1.20
1.39
ꢁ5.29
ꢁ5.64
ꢁ5.83
ꢁ1.71
ꢁ2.42
ꢁ2.25
a
Eg estimated from the onset absorption (Eg ¼ 1240/lonset eV).
b
c
onset
EHOMO ¼ - (E
e EþFc/Fc þ 4.8) eV.
ox
ELUMO ¼ EHOMO þ Egopt
.

4
M. Jia et al. / Tetrahedron 76 (2020) 131471
Fig. 4. (a) AFM image (20
patterns for DPBT-, DEPBT- and DEPBPy-based films on the ITO substrate.
m
m2 ꢂ 20
m
m2) of DPBT-based film, (b) AFM image (4
m
m2 ꢂ 4
m
m2) of DEPBT-based film, (c) AFM image (4
m
m2 ꢂ 4
m
m2) of DEPBPy-based film, (d) XRD
Fig. 5. IeV characteristic of memory devices: (a) DPBT, (b) DEPBT and (c) DEPBPy. And (d) Cross-section SEM image of the device.
DEPBT and DEPBPy. The statistic distribution of VSET were provided
mechanism of the aryl vinyl terminated bithiazoles- or bipyridines-
in Fig. S17. These results denote the reliable memory performance
of the devices, which implies that the synthesized compounds
could function as a potential storage material [2b,2c,19a]. Based on
the experimental results and the literatures [15b,22], the electrical
switching behaviors of the synthesized compounds may be illus-
trated by a possible “traps” mechanism. The detailed memory
based OMDs is under way in our laboratory.
3. Conclusions
Three bithiazole- or bipyridine-based
p-conjugated small mol-
ecules, DPBT, DEPBT and DEPBPy, were synthesized and

M. Jia et al. / Tetrahedron 76 (2020) 131471
5
characterized. Their single crystal structures, photophysical and
electrochemical properties have been systematically investigated.
In addition, their resistive memory characteristics were primarily
studied for the first time. Both DEPBT- and DEPBPy-based devices
exhibit typical binary memory behavior, while DPBT-based device
presents no memory property. The experimental results confirm
that the introduction of C]C bridged bonds into molecules could
enhance the molecular interaction, which plays an important role
in the molecular arrangement on the thin film, and has a significant
contribution to charge transport in memory devices. This work
demonstrates that aryl vinyl terminated bithiazoles and bipyridines
both are good promising candidates for organic resistive storage,
and could be considered as effective structural units for polymeric
materials. The synthesis of a series of new aryl vinyl terminated
bithiazoles and their application in organic resistive memory are
under way in our laboratory.
solvent was removed under vacuum, and the residue was washed
with an excess amount of CHCl3. The crude product was further
purified by crystallized from CHCl3 and dried to offer 2, as a light-
yellow crystalline solid (7.90 g, 56%). M.p.: 117e119 ꢀC. 1H NMR
(400 MHz, CDCl3)
d 4.35 (s, 4H).
4.3.2. 2,20-diphenyl-4,40-bithiazole (DPBT) [23]
1,4-Dibromobutane-2,3-dione (2, 1.22 g, 5 mmol) was added in
one portion to a solution of thiobenzamide (1.37 g, 10 mmol) in
MeOH (30 mL), and the mixture was refluxed 3 h. The precipitate
was filtered, washed with MeOH and dried to offer the product, as
an off-white crystalline solid (1.31 g, 82%). The product was suffi-
ciently pure to proceed to the next step. 1H NMR (400 MHz, CDCl3)
d
8.07e8.02 (m, 4H), 7.91 (s, 2H), 7.50e7.42 (m, 6H). 13C NMR
(100 MHz, CDCl3)
HRMS (ESI): calcd. for
321.0518.
d 168.3, 151.5, 133.5, 130.1, 128.9, 126.6, 115.3.
C
18H13N2S2 [MþH]þ 321.0515; found
4. Experimental section
4.3.3. 2,20-dimethyl-4,40-bithiazole (3) [24]
4.1. Materials
Compound 3 was obtained in a similar way to DPBT, as an off-
white solid in 67% yield and used without further purification.
Biacetyl (98%) was purchased from Alfa-Aesar (USA). Ethane-
thioamide (98%) and benzothioamide (98%) were purchased from
Sinopharm Chemical Reagent Co., Ltd. (China). 4,40-Dimethyl-2,20-
bipyridine was purchased from J&K (China). Benzaldehyde (99%)
were purchased from Beijing Innochem Science & Technology Co.,
Ltd. (China). All the solvents were purchased from Tianjinzhiyuan
Chemical Reagent Co., Ltd. (China). All the reagents and solvents
were of analytical grade without any purification.
M.p.: 156e158 ꢀC. 1H NMR (400 MHz, CDCl3)
d
7.56 (S, 2H), 2.73(S,
6H). 13C NMR (100 MHz, CDCl3)
d 166.2, 150.2, 114.3, 19.2.
4.3.4. 2,20-di((E)-styryl)-4,40-bithiazole (DEPBT)
Compound 3 (98.1 g, 0.5 mmol) and benzaldehyde (127
mL,
1.25 mmol) were dissolved in 200 mL of acetic anhydride under
argon atmosphere. The reaction mixture was heated to reflux for
24 h. After cooling to ambient temperature, the off-white product
was isolated by centrifugation and washed with alcohol. The crude
product was sublimated twice to give bright white crystal
4.2. Characterization and instruments
(119.2 mg, 64%). 1H NMR (400 MHz, DMSO-d6)
d 7.99 (s, 2H), 7.75 (d,
1H NMR and 13C NMR spectra were recorded on 400 and
100 MHz NMR instruments using CDCl3 and DMSO-d6 as the sol-
vent and TMS as the internal standard. High resolution mass
spectra (HRMS) was obtained on an Agilent LC-MSD-Trap-XCT
spectrometer with micromass MS software using electrospray
ionisation (ESI). Single crystal X-ray diffraction was conducted on a
Bruker SMART CCD diffractor, using graphite monochromated Mo K
J ¼ 7.2 Hz, 4H), 7.58 (s, 4H), 7.40 (dt, J ¼ 24.9, 7.0 Hz, 6H). 13C NMR
(100 MHz, DMSO-d6)
d 166.7, 150.5, 135.4, 134.5, 129.1, 128.9, 127.4,
121.3, 116.0. HRMS (ESI): calcd. for C22H17N2S2 [MþH]þ 373.0828;
found 373.0831.
4.3.5. 4,40-di((E)-styryl)-2,20-bipyridine (DEPBPy) [25]
Compound 4 (92.1 g, 0.5 mmol) and benzaldehyde (127
mL,
radiation (l) 0.071073 nm. Melting points were measured using a
1.25 mmol) were dissolved in 200 L of acetic anhydride under
m
WC-1 microscopic apparatus and were uncorrected. Thermogravi-
metric analysis (TGA) and differential scanning calorimetry (DSꢁC1)
were taken on a STA449F5 analyzer at a heating rate of 10 ꢀC$min
and under a N2 flow rate of 60 mL minꢁ1. UVeVis spectra was
recorded in N,N-dimethylformamide solution of 10ꢁ5 M with Carry
5000 UVeViseNIR spectrometer. Different pulse voltammetry
(DPV) was carried out by a CHI66E0 electrochemical analyzer with
a three-electrode cell at a scan rate of 50 mV sꢁ1. A Pt wire was used
as the counter electrode and an Ag/AgCl electrode was used as the
reference electrode. Atomic force microscopy (AFM) images were
recorded by a Bruker Multimode 8 microscope in tapping mode
under atmospheric environment. Scanning electron microscope
(SEM) images were performed on FEI Quanta 250 FEG scanning
electron microscope. X-ray diffraction (XRD) patterns were
argon atmosphere. The reaction mixture was heated to reflux for
24 h. After cooling to ambient temperature, the off-white product
was isolated by centrifugation and washed with alcohol. The crude
product was sublimated twice to give bright white crystal
(138.6 mg, 77%). 1H NMR (400 MHz, CDCl3):
d
8.68 (d, J ¼ 5.1 Hz,
2H), 8.57e8.56 (m, 2H), 7.58 (d, J ¼ 7.2 Hz, 4H), 7.47 (d, J ¼ 16.4 Hz,
2H), 7.44e7.37 (m, 6H), 7.36e7.30 (m, 2H), 7.15 (d, J ¼ 16.3 Hz, 2H).
13C NMR (100 MHz, CDCl3):
d 156.6, 149.6, 145.8, 136.3, 133.4, 128.9,
128.7, 127.1, 126.2, 121.1, 118.3. HRMS (ESI): calcd. for C26H21N2
[MþH]þ 361.1699; found 361.1705.
4.4. Memory device fabrication
Indium-tin-oxide (ITO) glass was pre-cleaned in turn with
distilled water, acetone and alcohol, in an ultrasonic bath for
20 min. By physical vacuum vapor deposition method, the organic
molecules that had been sublimated were heated in a quartz cru-
cible, and the vacuum of the deposition system was about
7.0 ꢂ 10ꢁ4 Pa. The beginning deposition rate of DPBT, DEPBT and
DEPBPy were about 0.3e0.7 Å$sꢁ1, and then the deposition rate
were controlled by current properly with the film thickness
increased. The deposited film thickness were finally about 120 nm.
The Al electrode about 90 nm in thickness was thermally deposited
onto organic surface at about 7.0 ꢂ 10ꢁ4 Pa through a shadow mask.
The active device area of ~0.0314 mm2 was obtained.
measured on a Bruker D8-A X-ray diffractometer. The 2q angle was
scanned from 5ꢀ to 30ꢀ. All electrical measurements of the devices
were characterized under ambient conditions with an Agilent
B1500A semiconductor parametric analyzer.
4.3. Synthetic procedures
4.3.1. 1,4-dibromobutane-2,3-dione (2) [11a]
Biacetyl (5.0 g, 58.1 mmol) was dissolved in CHCl3 (20 mL), Br2
(6.0 mL, 116.2 mmol) was added dropwise while maintaining the
reaction mixture was refluxed. The mixture was stirred for 3 h. The

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The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the financial support from the
National Natural Science Foundation of China (NSFC, Nos.
U1304212, 21274133 and 21374106).
Appendix A. Supplementary data
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