Changing molecular conjugation with a phenazine acceptor for improvement of small molecule-based organic electronic memory performance

Article abstract of DOI:10.1039/c7ra11932a

Two conjugated small molecules with different molecular conjugation, 4′,4′′-(diazene-1,2-diyl)bis(2′,3′,5′,6′-tetrafluoro-N,N-diphenyl-[1,1′-biphenyl]-4-amine) (TPA-azo-TPA) and 4,4′-(perfluorophenazine-2,7-diyl)bis(N,N-diphenylaniline) (TPA-ph-TPA), in which the electron donor triphenylamine moiety is bridged using different electron-accepting azobenzene or phenazine blocks, were designed and synthesized. The TPA-ph-TPA molecule with a larger conjugation acceptor regularly formed a nanocrystalline film and the as-fabricated memory devices exhibited outstanding non-volatile write once read many (WORM) memory effects with an ON/OFF ratio ten times higher than that of TPA-azo-TPA. Using theoretical calculations, it was speculated that the memory performance is a result of an electric field induced charge transfer effect and the enhanced device performance of the acceptor molecular conjugation is because of the presence of a strong charge transfer effect. The experimental findings suggest that the strategy of molecular conjugation may promote the performance of small molecule-based organic electronic memory devices by an enhanced a strong charge transfer effect.

Full text of DOI:10.1039/c7ra11932a

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Changing molecular conjugation with a phenazine
acceptor for improvement of small molecule-based
organic electronic memory performance†
Cite this: RSC Adv., 2018, 8, 805
*
Quan Liu,
Caibin Zhao, Guanghui Tian and Hongguang Ge
Two conjugated small molecules with different molecular conjugation, 40,400-(diazene-1,2-diyl)bis(20,30,50,60-
tetrafluoro-N,N-diphenyl-[1,10-biphenyl]-4-amine) (TPA-azo-TPA) and 4,40-(perfluorophenazine-2,7-diyl)
bis(N,N-diphenylaniline) (TPA-ph-TPA), in which the electron donor triphenylamine moiety is bridged using
different electron-accepting azobenzene or phenazine blocks, were designed and synthesized. The TPA-ph-
TPA molecule with a larger conjugation acceptor regularly formed a nanocrystalline film and the as-
fabricated memory devices exhibited outstanding non-volatile write once read many (WORM) memory
effects with an ON/OFF ratio ten times higher than that of TPA-azo-TPA. Using theoretical calculations, it
was speculated that the memory performance is a result of an electric field induced charge transfer effect
and the enhanced device performance of the acceptor molecular conjugation is because of the presence of
a strong charge transfer effect. The experimental findings suggest that the strategy of molecular conjugation
may promote the performance of small molecule-based organic electronic memory devices by an
enhanced a strong charge transfer effect.
Received 30th October 2017
Accepted 13th December 2017
DOI: 10.1039/c7ra11932a
rsc.li/rsc-advances
pull electron groups was changed and this lead to the “charge
trapping” hypothesis. Subsequently, tuning the length of the
Introduction
exible alkyl side chain, molecular planarity, and conjugation
length, was studied precisely to adjust the memory perfor-
mances.15–17 Azobenzene (azo) because of its week electron
withdrawing ability was commonly used for the “charge trap-
ping” group in small molecule memory material.18–20 However,
the stability and conjugate planarity of azobenzene are poor,
which greatly limits their use because of their data storage
performance. Phenazine (ph) derivatives play an important role
in organic semiconductors for their wide range of photoelectric
properties and stability.21–23 Therefore, it is important to design
a new phenazine-based conjugate skeleton rather than one
based on azobenzene.
Traditional magnetic or optical material based data storage
systems were limited by their memory capacity and cell size,
which greatly restricted their use in applications for high
density data storage.1–3 Organic electronic memory (OEM) is
a promising alternative to achieve high-performance data
storage and has gained signicant scientic interest over the
past few years. Many materials can be used for OEM, including
polymeric materials,4,5 composite materials6–8 and small mole-
cules.9–11 Among them, the organic small molecules have evoked
considerable research interest because of their low cost, well-
dened molecular structure, easy purication, and optoelec-
trical property tunability.12
In this work, rst molecules linked with azobenzene (TPA-
azo-TPA) were synthesized, and then the azobenzene was
replaced with phenazine (TPA-ph-TPA), as shown in Scheme 1.
Both molecules have an electron-donating triphenylamine
moiety, which is bridged by blocks with different conjugation
abilities. The inuences of molecular conjugation changed
the lm morphology, and the memory device performance
were precisely and systematically investigated. The results
showed that the sandwich memory device with the larger
conjugation molecule (TPA-ph-TPA) as the active layer
exhibited better binary electrical conductance switching and
nonvolatile memory effects with long-term thermal stability
than the smaller conjugation molecule (TPA-azo-TPA)
(Scheme 2).
Recently, numerous high-performance organic semi-
conductors and various outstanding memory behaviors have
been successfully achieved using innovative molecular designs.
Yam et al. designed a series of subphthalocyanin-based and
boron(III) diketonate-based molecules with tunable memory
device performance.13,14 In previous work, the strength of push–
Shaanxi Province Key Laboratory of Catalytic Foundation and Applications, School of
Chemical and Environmental Science, Shaanxi University of Technology, Hanzhong,
723001, China. E-mail: liuq@snut.edu.cn; Tel: +86 916 2641660
† Electronic supplementary information (ESI) available: MS and NMR spectra of
compounds are listed in Fig. S2–S6. The I–V characteristics of the memory
devices using LiF as block layer are shown in Fig. S7. The long-term stability of
the memory devices under ambient conditions are shown in Fig. S8. The
memory behavior of the exible memory devices under different bending
conditions are shown in Fig. S9. See DOI: 10.1039/c7ra11932a
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chromatography/mass (GC/MS) spectrometer (Waters) with an
electrospray ionization source. The lm thickness was deter-
mined using a Form Talysurf Intra Proler (Taylor Hobson).
Ultraviolet-visible (UV-vis) absorption spectra were measured at
room temperature using a UV-3600 spectrophotometer (Shi-
madzu). Atomic force microscopy (AFM) measurements were
performed using
a MFP-3DTM AFM instrument (Digital
Instruments/Asylum Research). Thermogravimetric analysis
(TGA) was conducted using a Dynamic Rate method wiꢁth1 a TGA
ꢀ
2950 (TA Instruments) at a heating rate of 10 C min under
a nitrogen (N2) ow rate of 50 mL minꢁ1
.
Scheme 1 Molecular structures of the TPA-azo-TPA and TPA-ph-
TPA.
Fabrication of memory devices
The indium tin oxide (ITO) substrates were pre-cleaned
sequentially with ethanol, acetone and isopropanol in an
ultrasonic bath for 20 min. The benzene solution of TPA-ph-TPA
and TPA-azo-TPA was ltered through poly(tetrauoroethylene)
membrane micro-lters with a pore size of 0.32 mm. Then,
0.1 mL of solution was spin coated onto the substrates at
a spinning speed of 500 rpm for 10 s and then 2000 rpm for 30 s,
followed by vacuum drying at 80 ꢀC for 8 h. The thickness of the
lms was about 90 nm as measured using a model M2000 DI
spectroscopic ellipsometer (J. A. Woollam, USA). To construct
the aluminum (Al)/TPA-ph-TPA and TPA-azo-TPA/ITO struc-
tures, Al top electrodes were thermally evaporated onto the lm
surface under 2 ꢂ 10ꢁ6 Torr through a shadow mask with
a thickness of about 60 nm and an area of 0.20 mm2.
Fabrication of exible memory devices: using 30 ꢂ 30 mm
sized poly(ethylene terephthalate) (PET) as the exible
substrate, Al bottom electrodes were thermally evaporated onto
the PET substrate under 2 ꢂ 10ꢁ6 Torr through a shadow mask
with a thickness of about 100 nm and an area of 1 ꢂ 30 mm.
Then, an 80 nm thickness organic layer was vacuum deposited
onto the bottom Al electrodes as measured using the spectro-
scopic ellipsometer. The Al top electrodes were thermally
evaporated onto the organic layer through a shadow mask with
a thickness of about 100 nm and an area of 1 ꢂ 30 mm.
Scheme 2 Synthesis routes of TPA-azo-TPA and TPA-ph-TPA.
Experimental
Materials
4-Bromo-2,3,5,6-tetrauoroaniline, hypochlorous acid tert-butyl
ester (t-BuOH), [4-(diphenylamino)phenyl]boronic acid, palla-
dium diacetate, sodium iodide (NaI), potassium carbonate,
[1,10-bis(diphenylphosphino)ferrocene]dichloropalladium(II) and
potassium acetate were purchased from commercial sources
(TCI, Alfa Aesar, and Sigma-Aldrich). All solvents were
purchased from Sinopharm Chemical Reagent Co., Ltd. All
chemicals were used as received without further purication.
Synthetic procedures
Synthesis
of
1,2-bis(4-bromo-2,3,5,6-tetrauorophenyl)
was prepared according to
diazene (1). Compound
1
a previous method found in the literature.24 To a mixture of 4-
bromo-2,3,5,6-tetrauoroaniline (0.5 mmol) and NaI (1.0 mmol,
150.0 mg) in t-BuOH (3 mL), was added tert-butyl hypochlorite
(1.0 mmol, 108.6 mg) under an N2 atmosphere at room
temperature for 3 h. The mixture was stirred for the indicated
time and quenched with aqueous sodium thiosulfate (1.0 M,
10 mL), and the solution was extracted with dichloromethane
Characterization
All electrical measurements of the devices were performed (DCM (CH2Cl2); 20 mL ꢂ 3). The combined organic extracts
under ambient conditions without any encapsulation using an were dried over sodium sulfate and concentrated under vacuum
HP 4145B semiconductor parameter analyzer (Hewlett Packard) to give the crude product. Purication using ash column
equipped with an HP 8110A pulse pattern generator (Hewlett chromatography on silica gel [eluent: CH2Cl2 in petroleum
Packard). Nuclear magnetic resonance (NMR) spectra were ether (PE)] gave the product. 19F-NMR [282 MHz, deuterated
recorded using an Inova 400 MHz Fourier transform (FT)-NMR dimethylsulfoxide (DMSO-d6)] d (ppm): ꢁ131.23 (d, J ¼ 16.0 Hz,
spectrometer (Varian). High-resolution mass spectra were 2F), ꢁ133.64 (d, J ¼ 13.3 Hz, 2F), ꢁ147.70 (d, J ¼ 16.0 Hz, 2F),
acquired using a Micromass GCT-time-of-ight (TOF) gas ꢁ148.61 (d, J ¼ 13.3 Hz, 2F); MS: calc'd for C12Br2F8N2 [M + H]+
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483.9370, found 483.8380. Anal. found for C12Br2F8N2: C, 29.98;
Br, 32.84; N, 5.75.
Synthesis of 2,7-dibromo-1,3,4,6,8,9-hexauorophenazine
(2). The synthesis method of 2 is similar to the method of
synthesis for 1.
19F-NMR (282 MHz, DMSO-d6) d (ppm): ꢁ118.49 (d, J ¼
17.2 Hz, 2F), ꢁ125.15 (d, J ¼ 18.3 Hz, 2F), ꢁ151.82–151.95 (m,
2F). MS: calc'd for C12Br2F6N2 [M + H]+ 443.8332, found
443.8040. Anal. found for C12Br2F6N2: C, 32.32; Br, 35.84; N,
6.27.
Fig. 1 (a) TGA curves of TPA-azo-TPA, and TPA-ph-TPA at a heating
rate of 10 ꢀC minꢁ1 under a N2 atmosphere; (b) UV-vis absorption
spectrum of TPA-azo-TPA, and TPA-ph-TPA in DCM solution.
General procedure for the synthesis of TPA-azo-TPA and
TPA-ph-TPA. A solution of 1 or 2 (0.409 g, 1 mmol), 4-(diphe-
nylamino)phenylboronic acid (0.723 g, 2.5 mmol) and tetra-
kis(triphenylphosphine)palladium(0) (0.062 g, 0.05 mmol)
dissolved in toluene (15 mL) was stirred in a two-necked ask
under a N2 atmosphere for 30 min. To the reaction mixture,
potassium carbonate (0.28 g, 2 mmol) in distilled water (15 mL)
was added dropwise over a period of 20 min. The resulting
Photophysical and electrochemical properties
The TPA-ph-TPA and TPA-azo-TPA have good solubility and so
DCM was chosen as the solvent and a spin coating method was
used to fabricate the lm. Fig. 1b shows the optical absorption
spectra of TPA-ph-TPA and TPA-azo-TPA nano-lms on quartz
substrates. The strong absorption bands at approximately 400–
500 nm can be attributed to the n–p* transition (charge trans-
fer) of the azobenzene.28 Compared with that of TPA-azo-TPA,
the onset optical absorbance of TPA-ph-TPA exhibits a signi-
cant red-shi for 34 nm, which corresponds to the narrower
energy band gap of the phenazine structure. The optical band
gaps of the TPA-ph-TPA and TPA-azo-TPA molecules, estimated
from the absorption edges of the lms, were 2.03 eV and
2.15 eV, respectively. Therefore, increased conjugation with the
phenothiazine structure gives rise to a ground state charge
transfer complex in TPA-ph-TPA. This also suggests that the
TPA-ph-TPA lm forms an ordered stacking of the p-conjuga-
tion system, favouring an effective carrier migration.
ꢀ
solution was reuxed overnight at 80 C. The reaction mixture
was extracted with dichloromethane (DCM) and the organic
layer was separated. The organic layer was evaporated with
a rotary evaporator, and then the resulting powdery product was
puried using column chromatography with DCM : PE (1 : 5) as
the eluent and a crystalline solid was obtained.
Spectral data
40,4000-(Diazene-1,2-diyl)bis(20,30,50,60-tetrauoro-N,N-diphenyl-
[1,10-biphenyl]-4-amine) (TPA-azo-TPA). Yield 74%, 1H-NMR [400
MHz, deuterated chloroform (CDCl3)] d (ppm): 7.41–7.31 (m,
12H), 7.20–7.05 (m, 16H); 13C-NMR (100 MHz, CDCl3) d (ppm):
147.3, 146.9, 132.0, 131.0, 129.5, 129.4, 129.3, 125.5, 125.4,
124.9, 124.1, 123.4, 122.1, 121.3; 19F-NMR (376 MHz, CDCl3)
d (ppm): 111.6, 127.1, 144.2, 149.6; TOF-MS: calc'd for
Fig. 2 shows the cyclic voltammetry (CV) measurements of TPA-
azo-TPA and TPA-ph-TPA on an ITO glass substrate in
C
C
48H28F8N4 [M + H]+ 812.2186, found 812.2000; anal. found for
48H28F8N4: C, 70.65; H, 3.49; N, 6.92.
4,40-(peruorophenazine-2,7-diyl)bis(N,N-diphenylaniline) (TPA-
a
0.1 mol Lꢁ1 solution of tetrabutylammonium hexa-
uorophosphate (TBAPF6) in anhydrous acetonitrile (CH3CN)
solution and measurements were taken with a scan rate of
100 mV sꢁ1. The onset oxidation (Eoonx set) of TPA-azo-TPA is
approximately 0.85 V versus silver/silver chloride (Ag/AgCl). The
onset oxidations (Eooxnset) of TPA-ph-TPA were 0.35 V and 0.83 V
versus Ag/AgCl, respectively. The oxidation potential onset of
ferrocene EoFneset was 0.44 V versus Ag/AgCl in acetonitrile with bare
ph-TPA). Yield: 82%, 1H-NMR (400 MHz, CDCl3) d (ppm): 7.70 (s,
2H), 7.54–7.52 (m, 4H), 7.34–7.33 (m, 6H), 7.24–7.18 (m, 12H),
7.15–7.13 (m, 4H); 13C-NMR (100 MHz, CDCl3) d (ppm): 146.4,
144.4, 131.0, 129.1, 128.9, 125.2, 123.7, 120.8, 104.5; 19F-NMR
(376 MHz, CDCl3) d (ppm): 129.2, 130.9, 153.1; TOF-MS: calc'd
for C48H28F6N4 [M + H]+ 774.2218, found 774.2210; anal. found for
C48H28F6N4: C, 74.70; H, 3.66; N, 7.26.
ITO glass. The estimated highest occupied molecular orbital
onset
(HOMO) levels can be calculated from the E
using the
ox
onset
following formula: HOMO ¼ ꢁ[E
+ 4.8 ꢁ EFc] eV. The lowest
ox
unoccupied molecular orbital (LUMO) levels are not detectable
using CV and so the values of the LUMO levels were estimated
using the following formula: LUMO ¼ [HOMO + Eg] eV. The
HOMO levels of TPA-azo-TPA and TPA-ph-TPA were ꢁ5.21 eV and
ꢁ4.72 eV, respectively. The determined LUMO levels of TPA-
azo-TPA and TPA-ph-TPA were ꢁ3.06 eV and ꢁ2.69 eV, respec-
tively. TPA-ph-TPA has lower LUMO levels than TPA-azo-TPA,
because of the charge transfer process, and this result was in
agreement with the UV spectroscopic data.
Results and discussion
Thermal stability
The molecules TPA-ph-TPA and TPA-azo-TPA can be obtained in
moderate yields using a multistep Suzuki cross-coupling reac-
tion. The thermal decomposition temperature of the TPA-azo-
TPA is 261 ꢀC, however, TPA-ph-TPA exhibits good thermal
stability with a decomposition temperature above 300 ꢀC
(Fig. 1a). Compared with the currently reported azo-based small
molecules,25–27 the stability of TPA-ph-TPA was good. This sug-
gested that increasing the degree of conjugation of the molec-
Morphology of the thin lm
ular framework leads to the further improvement of the To investigate the surface morphology and lm microstructure,
molecular heat resistance.
AFM measurements were made on the TPA-azo-TPA and TPA-
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Fig. 4 I–V characteristics of the small molecule-based memory
devices: (a) Al/TPA-azo-TPA/ITO and (b) Al/TPA-ph-TPA/ITO; the
effect of retention time of (c) the Al/TPA-azo-TPA/ITO and (d) the Al/
TPA-ph-TPA/ITO memory devices under a constant stress of ꢁ1.0 V.
Fig. 2 Cyclic voltammograms of TPA-azo-TPA and TPA-ph-TPA film
were measured in 0.1 mmol Lꢁ1 TBAPF6/CH3CN solution with Ag/AgCl
as reference electrode and platinum wire as the counter electrode. A
scan rate of 100 mV sꢁ1 was used.
The ON state was maintained in the nal sweep from 0 to ꢁ4 V
(sweep 3). Therefore, the TPA-azo-TPA-based device exhibited
a typical nonvolatile write once read many (WORM) memory
behavior. Compared with the Al/TPA-azo-TPA/ITO devices, the
Al/TPA-ph-TPA/ITO devices exhibited much smoother I–V
characteristics during the switching process. As shown in
Fig. 4b, the Al/TPA-ph-TPA/ITO memory cell was in the OFF
state. When a negatively biased potential sweep from 0 to ꢁ4 V
was applied (sweep 1 in Fig. 4b), the current increased consec-
utively from 10ꢁ6 A to 10ꢁ2 A, indicating that the device had
been set to the ON state by the negative forward sweep. The
device remained in the ON state during the subsequent voltage
sweep (sweep 2). In the following positive sweep from 0 to 4 V
(sweep 3), the device remained in the ON state. The value of the
ON/OFF ratio for TPA-ph-TPA-based devices was 104. The TPA-
azo-TPA-based device also showed a typical nonvolatile WORM
memory performance. Both of these two devices have potential
applications for non-volatile data storage. Compared with the
Al/TPA-azo-TPA/ITO device, the TPA-ph-TPA-based device has
a higher ON/OFF ratio, which is helpful for avoiding false
programming and error readout problems.
In order to exclude the possibility of the inuence of metal
laments in the memory behaviours, sandwich structured
memory devices: Al/LiF/small molecules/ITO were fabricated
using the same conditions. The LiF layer (5 nm) acted as
a buffer layer to prevent the Al nanoparticles penetrating into
the active lms. The I–V results indicated the WORM memory
behaviours of TPA-azo-TPA and TPA-ph-TPA based memory
devices, as shown in Fig. S1 and S2 (ESI†), which demonstrated
that the WORM memory behaviours were not dependent on the
metal lament.
ph-TPA lm. As seen from the AFM height images (Fig. 3), both
of the lms showed a relatively smooth surface without obvious
defects. The TPA-azo-TPA lm clearly showed two-dimensional
grains in its solid state at room temperature with uniform
size and the surface root-mean-square (RMS) roughness was
1.8 nm. The TPA-ph-TPA lm showed a denser needle
morphology, and had a RMS roughness of 0.9 nm, which was
smaller than that of TPA-azo-TPA. Therefore, the lm quality
was improved signicantly.
Current–voltage (I–V) characteristics of the memory devices
The memory effect of the TPA-azo-TPA devices was explored
rst, and the current–voltage (I–V) characteristics are shown in
Fig. 4a. Under a negatively biased voltage sweep, the current in
the fabricated Al/TPA-azo-TPA/ITO memory devices increased
abruptly from 10ꢁ5 A to 10ꢁ2 A at around 3.0 V, indicating that
the device has been switched from an OFF state to an ON state
(sweep 1 of Fig. 4a). This transition can be dened as the
“Write” process and an ON/OFF ratio of 103 can be obtained.
The device remained in the ON state, even aer the power was
turned off or during the subsequent voltage sweep (sweep 2).
The retention times of the ON state and the OFF state of the
Al/TPA-azo-TPA/ITO and Al/TPA-ph-TPA/ITO device is shown in
Fig. 4c and d. Under a constant stress of ꢁ1 V, TPA-ph-TPA-
based devices were observed to have more stability and no
signicant degradation in current for any of the states at least
Fig. 3 Tapping mode (5 mm ꢂ 5 mm) AFM phase of TPA-azo-TPA (a)
and TPA-ph-TPA (b) film on ITO substrates.
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106 s during the readout test. The long-term stability of two
molecular-based devices under air and humidity conditions was
measured, and the results of this are shown in Fig. S3 and S4
(ESI†). The test results show that the TPA-ph-TPA-based devices
have better long-term stability than the TPA-ph-TPA-based
devices. This may be because the phenazine group is more
stable than the azobenzene group in air and humidity
conditions.
Because of the rigid planarity of the glass substrates and the
brittleness of the ITO electrodes, which limits its application on
exible devices. Flexible substrates such as PET were used to
replace the ITO glasses and to fabricate exible memory
device.29–31 The specic fabricated method is described in the
Experimental section. The exible memory device can be bent
under different conditions and the degree of bending was
dened by the distance (D) between the ends of the arc. The top
and bottom Al electrodes of this exible device were aligned
perpendicularly to each other, and the voltage bias was applied
to the memory devices at the end of the top Al electrode
(cathode).
To conrm the feasibility of exible memory devices, the
sandwich structured exible memory device with a 10 (word
line) ꢂ 10 (bit line) crossbar array is shown in Fig. 5a. The
electric properties were measured under different bending
conditions. For the TPA-azo-TPA-based exible memory
device (Fig. S5; ESI†), the storage units of different regions
Fig. 5 (a) The model of the sandwich structure flexible memory device
consisting of an active layer between the top and bottom Al electrodes
(left), the calculation of the bending degree of the flexible memory
device (right); (b) the memory effects of the TPA-ph-TPA-based
flexible memory devices under different degrees of bending and the
memory device can maintain the memory effect when the distance
between the ends of the arc reaches 10 mm. (c) The TPA-ph-TPA-
based device can maintain memory performance within 500 bending
cycles in the maximum bending condition.
Memory mechanism
could show stable binary memory behavior even when the To better understand the electronic process occurring inside
distance between the ends of the arc bending was 20 mm. the thin lm, theoretical calculations were performed using
When D reached 10 mm, the storage units of the TPA-azo-TPA- density-functional program DMol3.32,33 The hybrid functional
based exible memory device became unstable. Additionally, B3LYP,34,35 together with a double numerical plus polarization
the I–V curves of the Al/TPA-ph-TPA/Al/PET structured (DNP) basis set, was used. Comparing the Egap of TPA-azo-TPA
memory device were also measured to test its exibility. As and TPA-ph-TPA, the TPA-ph-TPA has a lower Egap, which makes
shown in Fig. 5b, the device also maintained stable binary charge carrier migration easier. The molecular orbitals (HOMO,
memory characteristics when D was 10 mm. The electrical LUMO) and the electrostatic surface potential (ESP) of TPA-azo-
characteristics of each curve are summarized from results TPA and TPA-ph-TPA molecules were plotted, and are shown in
obtained from 20 different cells under each bending Fig. 6.
condition.
To better understand the charge carrier behavior of TPA-azo-
The I–V characteristics of exible memory device over TPA and TPA-ph-TPA based devices, the work functions (F) of Al
bending cycles of 500 times were also measured. The degree of top and ITO bottom electrodes were compared with molecular
bending was from the at state to the maximum degree of HOMO and LUMO orbital. The energy barrier between F of the
bending (D ¼ 10 mm) and as the number of bending cycles ITO (ꢁ4.8 eV) and the HOMO energy level was 0.35–0.41 eV,
increased, the TPA-azo-TPA-based exible memory device which is much lower than the energy barrier of 1.21–1.39 eV
cannot maintain stable binary memory behaviour, as shown in between the F of the Al (ꢁ4.3 eV) and the LUMO energy level.
Fig. S6 (ESI†). However, the TPA-ph-TPA-based exible This suggests that hole injection from ITO into the HOMO of
memory device showed stable binary memory behaviour up to TPA-azo-TPA or TPA-ph-TPA (corresponding to ITO as the
500 bending cycles of, as shown in Fig. 5c. In addition, the anode) is easier than electron injection from Al into the LUMO
threshold voltage is a little larger than the previous bending of TPA-azo-TPA or TPA-ph-TPA. Thus, TPA-azo-TPA and TPA-ph-
and therefore shows a little shiing aer several cycles of TPA are p-type materials and holes predominate the conduction
bending. This is probably because of an increase in the process (as shown in the inset of Fig. 6).
number of bending times, and the lm cannot withstand the
The ESP plots of TPA-azo-TPA and TPA-ph-TPA molecules
mechanical stretch and the ordered molecular stacking might both show an open channel along the molecular backbone with
be somehow damaged. Thus, the charges may have difficulty a continuous positive electrostatic potential, providing a path to
in moving through the lms and much energy is needed to allow charge carrier migration. For the TPA-azo-TPA molecule,
overcome the barrier. For bending cycles above 500, the TPA- electrons transit readily from the HOMO orbital to the LUMO
ph-TPA-based memory device showed no visible binary orbital under an external electric eld, forming the locally
memory behaviour.
excited state. As a result, a charge-transfer interaction can occur
in the TPA-azo-TPA molecule between the electron donor
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(TPA-ph-TPA) were synthesized. The photophysical, electro-
chemical properties and memory behaviors of these donor–p–
acceptor molecules were investigated and compared. This
comparative study of tuning the properties of the conjugated
D–A–D molecules using an aromatic acceptor may be an
alternative approach for the design and study of future high-
performance memory devices based on new materials.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors thank the Chinese Natural Science Foundation
(21503125); the Scientic Research Program of the Shaanxi
Provincial Education Department (2013JK0658 and No.
15JF013); the Brainstorm Project on Social Development by the
Department of Science and Technology of Shaanxi Province
(2015SF270) and the Doctoral Program of Shaanxi University of
Technology (SLGQD17-15).
Fig. 6 Simulated electrostatic potential (ESP) plots of TPA-azo-TPA (a)
and TPA-ph-TPA (b); HOMO, LUMO and energy level diagrams for the
ITO/TPA-azo-TPA and TPA-ph-TPA/Al devices (c).
moieties and the electron acceptor moieties. For the TPA-ph-
TPA molecule, a similar charge transfer process is also formed.
However, the phenazine conjugation structure enhances the
electron-withdrawing ability of the acceptor moiety, so the
HOMO and LUMO orbitals of TPA-ph-TPA molecules show more
obvious separation in the ground state, with HOMO mostly
localizing on the donor areas and LUMO mostly localizing on
the acceptor areas. It was speculated that the mechanism of the
memory performance of the TPA-azo-TPA and TPA-ph-TPA
devices could be the electric eld induced charge transfer effect.
Without an external electric eld, the electrons in the TPA-azo-
TPA and TPA-ph-TPA molecules were stable, and the as-
fabricated device was in the OFF state. When a negative bias
is applied, the charge transport pathways will form, and will
switch the Al/TPA-azo-TPA or TPA-ph-TPA/ITO device from the
OFF state to the ON state. However, the phenazine conjugation
structure can signicantly enhance the electron-withdrawing
ability of the molecule, resulting in a stronger intramolecular
charge transport in TPA-ph-TPA. It has been reported that
changing the charge transfer ability of the D–A molecules can
tune memory effects. So the better performance (larger ON/OFF
ratio and more stability with each electric conductive state) of
the TPA-ph-TPA-based device could be because of the stronger
intramolecular charge transfer effect. The nonvolatile nature of
the ON state is because of the intensive electron delocalization
in the acceptor moieties which stabilized the conductive charge
transfer state.
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