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Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device Yang Li, Zilong Wang, Cheng Zhang, Peiyang Gu, Wangqiao Chen, Hua Li, Jian-Mei Lu, and Qichun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05178 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018

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ACS Applied Materials & Interfaces

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device ⊥

⊥

Yang Li,†,‡, Zilong Wang,‡, Cheng Zhang,† Peiyang Gu,‡ Wangqiao Chen,‡ Hua Li,*,† Jianmei Lu,*,† and Qichun Zhang*,‡ †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou 215123, P. R. China ‡

School of Materials Science and Engineering, Nanyang Technological University, Singapore

639798, Singapore KEYWORDS: N-heteroacenes, conjugation, nonvolatile memory, donor−acceptor systems, multilevel resistive switching

Abstract: A novel thiadiazoloquinoxaline (TQ)-based donor−acceptor (D−A) type Nheteroacene (Py-1-TQ) has been demonstrated for promising application in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show FLASH-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile WORM-type ternary memory effects with high ON2/ON1/OFF current ratios (105.8/103.4/1), attributing to the

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different electron-withdrawing abilities between pyrazine unit and TQ species that can induce stepwise D−A charge transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage (UHDDS) devices through the rational D−A tuning.

1. INTRODUCTION The rapid growth of data information1 greatly demands a novel archival data-storage technology with lower-power consumption, faster speed, and higher density for future computing systems.2-7 Recently, memory switching behaviors of organic materials have attracted considerable attention because of their high flexibility, 3D stacking capability, and low cost.8-21 Among all organicbased memory materials, donor−acceptor (D−A) systems have emerged as an ideal platform for developing novel organic resistive memory devices due to their tunable structures, unique optoelectronic properties, and the possibility to induce multilevel memory behaviors through stepwise D−A charge transfer processes.22-29 Such multilevel memory performance holds great potential to realize ultrahigh 3n or larger data-storage density,22,24,28 which could satisfy the key requirements of future information-communication technologies (ICTs).6,30 In order to achieve high performance D−A systems, the rational selection of appropriate donor groups and acceptor species is critical but still challenging.31-34 Especially for the acceptors, their different electron-deficient abilities usually play an important role in molecular electronic properties.34-39 Recently, thiadiazoloquinoxaline (TQ)-based acceptor has emerged as a promising candidate in designing high performance D−A molecular systems.40-47 Compared with its counterpart of benzothiadiazole (BT) acceptor, TQ possesses a larger planarity and can be


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functionalized with various aryl or alkyl groups to enhance the solubility, thus improving the ability of molecular self-assembly.40,45 Due to their charming advantages, the significant progress in design, synthesis, and characterization of TQ-containing D−A materials have been witnessed in the past few years.40-47 Moreover, among these materials, TQ-based large N-heteroacenes have attracted particular attention due to their effective aromatic planarity and unique electrondeficient nature for realizing highly-efficient D−A ambipolar or n-type semiconductors.48-52 Currently, N-heteroacenes have become an important class of molecules that possess unusual properties compared with their pristine hydrocarbon structures.53-56 The properties of Nheteroacenes usually change a lot with different conjugation lengths as well as the number/valence/position of N atoms,53,56 which provides many possibilities of achieving exceptional optical and electronic performances. Therefore, N-heteroacenes have aroused great interests due to their fascinating optoelectronic properties and promising applications in organic electronics.53-56 To date, the TQ-based N-heteroacenes have been demonstrated to display promising applications in organic light-emitting diodes (OLEDs),49 organic field-effect transistors (OFETs),51,52 and organic solar cells (OSCs)48,50. However, their applications in resistive memory devices remain rarely explored. In this work, we report a novel TQ-based D−A N-heteroacene (Scheme 1, Py-1-TQ), which contains decyloxyl groups as electron donor, and two different types of acceptors (namely as pyrazine and TQ). We believe that this compound could exhibit the following merits: (1) the donor and acceptors are sequentially-annulated in one row, which are supposed to induce stepwise D−A charge-transfer processes that might realize multilevel memory switching behaviors;20,25 (2) the utility of cross-conjugated pyrene as a core backbone could enhance the stability of whole molecule both in the ground and excited states;56 (3) TQ acceptor can be


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functionalized with tri-isopropylsilyl groups, thus remarkably improving the molecular solubility; and (4) the incorporation of alkyl chains could promote the self-assembly of ordered nanostructures, leading to high charge mobility.57,58 Meanwhile, we also synthesized two counterparts (Py-0-TQ and Py-2-TQ, Scheme 1), which only contain one acceptor (either pyrazine or TQ). As expected, the memory devices based on these three molecules show apparently different resistive memory behaviors, which demonstrate the potential of TQ-based N-heteroacenes as tunable ultrahigh-density data-storage (UHDDS) candidates. 2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of As-Prepared Molecules. Scheme 1 shows the synthetic routes to Py-0-TQ, Py-1-TQ and Py-2-TQ. Compounds 5, 4 and 2 were synthesized according to the previously-reported methods.27,51,59 The intermediate diketone (3) was harvested in 58% yield through the condensation reaction between diamine 5 and an excess amount of tetraketone 4. Py-0-TQ was obtained as yellow solid in 60% yield by reacting 4 with 2, while Py-1-TQ was harvested as dark-red solid in 80% yield via reacting 3 with 2, and Py-2-TQ was acquired as dark-red solid in 75% yield by reacting 5 with 4. Note that the synthesis of Py-2-TQ has been reported by Mateo-Alonso et al. during the preparation of this work.49 These three target products were confirmed by 1H and

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C NMR (Figure S1-S8), MALDI-TOF-MS (Figure

S9-S11), FT-IR (Figure S12-S14) and elemental analysis. It is noteworthy that these compounds are quite stable in the air condition. In addition, all materials show satisfactory thermal stability, with an onset decomposition temperature over 200 °C (considering 5% weight-loss, Figure S15). 2.2. Photophysical and Electronic Properties. The optical properties of Py-0-TQ, Py-1-TQ and Py-2-TQ were firstly studied. Figure 1 shows the UV-vis spectra of all three compounds in


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CH2Cl2 solution. All compounds exhibit two sets of absorption bands: one discernible set in the UV region between 240 and 375 nm, and the other in the visible region. Especially for the visible absorption part, Py-0-TQ merely exhibits two well-resolved narrow peaks at 411 and 435 nm with the onset absorption (λonset) at 445 nm. However, distinguished from Py-0-TQ, the lowenergy absorption bands of both Py-1-TQ and Py-2-TQ display a remarkable bathochromic shift from 440 to 600 nm, which could attribute to the specific electronic nature of TQ incorporation.39,52 Besides, the absorption edges of Py-1-TQ and Py-2-TQ are very close to each other (626nm for Py-1-TQ and 630 nm for Py-2-TQ). Thus, the optical band gaps (Egopt) of Py0-TQ, Py-1-TQ and Py-2-TQ are estimated to be 2.79, 1.98 and 1.97 eV (Egopt = 1240/λonset). To examine the molecular energy levels of all three materials, cyclic voltammetry (CV) was conducted to inspect the electrochemical properties of Py-0-TQ, Py-1-TQ and Py-2-TQ. As shown in Figure S16, all materials can display reduction waves, where Py-0-TQ exhibits one irreversible reduction peak, while Py-1-TQ exhibits one more quasi-reversible reduction peak, and Py-2-TQ exhibits two more quasi-reversible reduction peaks. These results are in good agreement with the redox behavior of TQ unit in some reported TQ-based molecules.49,51 Meanwhile, using the onset reduction potential, the LUMO energies of Py-0-TQ, Py-1-TQ and Py-2-TQ are estimated to be –3.57, –4.03, and –4.08 eV, respectively, according to the equation ELUMO = −(4.80 − EFc + Eredonset) eV. The HOMO energy levels of Py-0-TQ, Py-1-TQ and Py-2TQ are thus calculated to be –6.36, –6.01, and –6.05 eV, using the equation EHOMO = ELUMO – Egopt. The HOMO/LUMO (–6.05/–4.08 eV) energy levels of Py-2-TQ are in consistent with the values (–6.03/–4.07 eV) reported by Mateo-Alonso group. Notably, compared with Py-0-TQ, the LUMO energies of Py-1-TQ and Py-2-TQ are lower than –4.0 eV. Since TQ is a strong electron-withdrawing unit,41,42 the introduction of TQ species into the linearly-annulated skeleton


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leads to lower LUMO of the conjugated molecules. It has been reported that the low-lying LUMO levels (< –4.0 eV) play a significant role in guaranteeing air-stable electron transport,33,60 which suggests the potential of Py-1-TQ and Py-2-TQ as efficient ambipolar or n-type candidates. The deep LUMO levels could make the organic materials invulnerable to the ambient trapping species such as O2 and H2O, which is beneficial for enhancing the stability of their corresponding devices in air operation. 2.3. Thin Film Self-Assembled Morphology and Nanostructure. AFM (Atomic force microscopy) analysis was carried out to examine the film-forming behaviors of all three molecules. As indicated in Figure 2a-c, the AFM images of Py-0-TQ, Py-1-TQ and Py-2-TQ display quite different self-assembled morphologies. Py-0-TQ molecules tend to form orientation-selective nanobelts via spin-coating and the average width of the nanobelts is more than 100 nm (Figure 2a). The RRMS (root-mean-square roughness) of this thin film is 7.13 nm. Yet, for the film formed by Py-1-TQ, the molecules self-assemble into a rice-grain-like nanostructure (Figure 2b). These “rice-grains” are uniform with an average length of 350 nm and the width of 150 nm, as well as high crystallinity (RRMS = 9.72 nm). For the film formed by Py2-TQ, the AFM image indicates many granular grains with partial crystalline regions (Figure 2c). The size of these granular domains is ~ 50–60 nm in range with roughness of 3.24 nm. More interestingly, the 3D-AFM images of Py-0-TQ, Py-1-TQ and Py-2-TQ suggest that the molecular self-assembly orientation significantly changes from in-plane (parallel to the substrate) to out-of-plane (perpendicular to the substrate) orientation along with the TQ incorporations (Figure 2d-f). Such different self-assembly behaviors of Py-0-TQ, Py-1-TQ and Py-2-TQ are believed to actuate distinct electronic properties.


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To further study the internal molecular packing of all three molecules in films, power XRD tests were conducted (Figure S17). The film of Py-0-TQ shows a primary diffraction peak at 2θ = 7.24°, corresponding to a d-spacing of 12.20 Å, which might originate from the nanobelts.24,61 One additional diffraction peak with a low intensity is also observed at 2θ = 14.5°, which corresponds to the second order Bragg reflection (d = 6.10 Å), indicating a high degree of crystallinity.62 For Py-1-TQ, the XRD pattern exhibits two sets of multiple Bragg reflections. The first set of diffraction peaks locate at 2θ = 4.43°, 8.85° and 13.4°, which correspond to the first, second and third order Bragg reflections of 19.92 Å, 9.98 Å and 6.60 Å, respectively. The other set locates at 2θ = 5.14° and 16.3°, which could correspond to the first and third order Bragg reflections (d = 17.17 Å and 5.43 Å). Both two sets of diffraction peaks suggest that Py-1TQ molecules form an ordered arrangement and layer-by-layer packing in film that is favorable for highly-efficient charge transportation.37,63 For Py-2-TQ, the powder XRD spectrum displays a main diffraction peak at 2θ = 4.46°, corresponding to a d-spacing of 18.58 Å. The intensity of such diffraction is lower than those of Py-0-TQ and Py-1-TQ, which implies lower degree of molecular crystallinity that is consistent with the result observed by AFM. These distinct morphology and nanostructures of Py-0-TQ, Py-1-TQ and Py-2-TQ strongly prompt us to investigate their resistive memory device performances. 2.4. Resistive Memory Device Performance. To examine the electrical switching properties of these molecules, organic memory devices were fabricated and measured by I–V (current– voltage) characteristics. During the measurement, the scan step of the I–V characteristics was 0.01 V. Figure S18 illustrates the memory device architecture, using ITO as a bottom electrode, thermally-deposited aluminum as a top electrode, and thin films of Py-0-TQ, Py-1-TQ and Py2-TQ as the active layers, respectively. As illustrated in Figure 3a, the Py-0-TQ based device


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shows a FLASH-type binary memory behavior. During the first sweep from 0 to −5.0 V, an abrupt increase in current occurs at ~ −1.85 V, which implies that the device switches from a high-resistance (OFF) state to a low-resistance (ON) state. This OFF-to-ON state transition serves as a “writing” process within a digital memory cell. The device retains its ON state during the next negative voltage scan (sweep 2) and even after the removal of the electric field, manifesting the NVM (nonvolatile memory) nature. In addition, a positive voltage bias can program the ON state into the original OFF state (sweep 3), which could be regarded as an “erasing” process in the data-storage device. The recovered OFF state can be read during the next positive scan (sweep 4) and re-programmed to the ON state (sweep 5) in the following reverse pulse. This “write−read−erase−read−rewrite” cycle demonstrates the binary FLASH memory behavior17,63 of Py-0-TQ based devices with the ON/OFF current ratio of ~ 103. Figure 3b describes the I−V characteristics of the memory device employing Py-1-TQ as the active layer. Notably, Py-1-TQ-based devices exhibit an exceptionally different resistive memory behavior. When applying a negative voltage sweep, two distinct electrical transitions in current appear at –2.62 and –4.11 V. Both transitions indicate that the device switches from OFF state to an intermediate-resistance (ON1) state, and further to a low-resistance (ON2) state, which corresponds to a ternary memory behavior.22,28 Subsequent negative and reverse voltage scans (sweep 2 and 3) reveal that the device can maintain its ON2 state but could not be erased, suggesting a typical WORM (i.e. write-once-read-many-times) memory character. The OFF, ON1 and ON2 currents are well-separated by over two orders of magnitude with the current ratios of about 105.8/103.4/1, which ensures the high-resolution and low-error device operation. Such ternary memory performance holds great promise to realize ultrahigh density data-storage application.12,13


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Figure 3c shows the electrical resistive switching property of the Py-2-TQ-based devices, which also exhibit FLASH-type binary memory performance relative to that of Py-0-TQ. The I−V curve displays similar “write−read−erase−read−rewrite” cycle with the ON/OFF current ratio of about 103. Moreover, the stabilities of Py-0-TQ, Py-1-TQ and Py-2-TQ based memory devices have been studied through retention time test under a constant stress of –1.0 V. No significant degradation of these devices in ON and OFF states is detected for longer than 2×103 s (Figure 3d-f), which shows that these devices are endurable under constant voltage stress and read pulses. Furthermore, fifty independent units of Py-0-TQ, Py-1-TQ and Py-2-TQ based memory devices were measured to investigate their cell-to-cell reproducibility. It is found that for Py-0TQ, about 76% device cells show FLASH-type binary memory behaviors, while for Py-1-TQ, about 70% device cells exhibit WORM-type ternary memory effects, and for Py-2-TQ, about 72% device cells show FLASH-type binary memory behaviors, which indicates the satisfactory reproducibility. Besides, we illustrated the statistic SET voltage (VSET) distributions of these devices by an error bar. As shown in Figure S19, the VSET of Py-0-TQ-based memory device distributes between –1.8 V and –3.0 V, while the VSET of Py-2-TQ-based memory device distributes between –3.2 V and –4.2 V. Especially for Py-1-TQ-based memory device, the first SET voltage (VSET1) and second SET voltage (VSET2) are well-separated. VSET1 locates between –1.4 V and –2.7 V whilst VSET2 distributes between –3.7 V and –4.8 V. The narrow distributions of SET voltages also demonstrate the high reproducibility of Py-0-TQ, Py-1-TQ and Py-2-TQ based memory devices. 2.5. Proposed Resistive Switching Mechanism. To understand the different resistive switching behaviors of Py-0-TQ, Py-1-TQ and Py-2-TQ, DFT (density functional theory)


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calculations were conducted using a GGA (generalized gradient approximation) method with DMol3 mode at BLYP/DNP set. DFT-calculated molecular electrostatic potential (ESP) isosurfaces (see Figure 4, top) reveal that an open channel is generated throughout the linearlyannulated molecular surface with continuous positive ESP (in yellow), through which the charge carriers can transport fluently. Yet, the negative ESP regions arose from the electron-deficient acceptors (pyrazine (in blue A) unit and TQ (in blue B) specie), could act as “charge traps” to hinder the charge carrier transportation.17,22,25 For Py-0-TQ and Py-2-TQ, the “traps” are caused by pyrazine unit and TQ group, respectively. When the voltage bias is low, charge carriers can hardly acquire enough energy to overcome the injection barrier between the donor and the acceptor. Thus, the memory device stays at the OFF (high-resistance) state. As the voltage increases, the charge carriers steadily collect adequate energy and inject from the donor to the acceptor to fill the trap, resulting in the device switching from the OFF state to the ON state. The calculated HOMO and LUMO orbitals of Py-0-TQ and Py-2-TQ are shown in Figure 4a and 4c. Upon the HOMO to LUMO transition, the electrons can move from the donor side to the acceptor part and become more localized. The trapped charge carriers could be stabilized by an intra-molecular charge transfer (ICT) process, which forms a charge-separated state.24,27 Besides, both Py-0-TQ and Py-2-TQ only possess one type of electron-withdrawing group in their linearly-annulated molecular backbone and DFT calculations also present that there is almost no twist configuration among the skeletons of these molecules. Therefore, the ICT process might not be strong enough to persist the charge-separated state.17,63 When applying a reverse voltage pulse, the trapped charge carriers could be de-trapped to the original state and FLASH-type binary switching behaviors are observed.


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For Py-1-TQ, the molecular isosurface displays two kinds of negative ESP regions (blue A and B, Figure 4b) aroused from pyrazine unit and TQ group. These negative areas might function as “trap” sites to stepwise impede the charge transport. When the voltage bias increases, the traps originated from two different acceptors might not be occupied simultaneously: the trap from pyrazine unit is filled while the trap from TQ group remains unfilled, corresponding to the OFFto-ON1 transition. This process can imply that the complete filling of TQ group requires more injection energy than pyrazine unit.12,22,28 As the bias goes forward, the charge carriers might ultimately gather sufficient energy to reach the trap of TQ, and hence bring out an ON2 state with “trap-free” environment. Moreover, because of the syngeneic effect of pyrazine unit and TQ group, the as-formed charge-separated state could be stabilized by the stronger ICT process of Py-1-TQ.24,37,64 The trapped charge carriers might not be easily de-trapped after withdrawing the power supply or under opposite voltage pulse. Thus, the Py-1-TQ based device exhibits WORM-type ternary memory characteristics. 3. CONCLUSIONS We have successfully prepared a TQ-based D−A N-heteroacene Py-1-TQ and its two counterparts of Py-0-TQ and Py-2-TQ. Compared with Py-0-TQ and Py-2-TQ, Py-1-TQ contains one type of donor (decyloxyl) and two different types of acceptors (pyrazine and TQ) sequentially-fused in one row, which can trigger stepwise charge transfer processes. All three molecules exhibit distinct self-assembly behaviors and optoelectronic properties. Although Py-0TQ and Py-2-TQ based memory devices show FLASH-type binary memory behaviors, Py-1TQ displays excellent WORM-type ternary memory effects, which hold a great potential to realize the UHDDS application. Our results illustrate the potential of TQ-based N-heteroacenes


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as promising high-density data-storage candidates, and open up many new possibilities to prepare high-performance D−A N-heteroacene systems through rational molecular design. 4. EXPERIMENTAL SECTION 4.1. Materials. 4,7-Dibromobenzo[c][1,2,5]thiadiazole, 10% Pd/C, and hydrazine monohydrate were bought from Alfa Aesar company. Pyrene, ruthenium(III) chloride, trifluoroacetic acid, Niodosuccinimide, camphor sulfonic acid, acetonitrile, triisopropylsilylacetylene, palladium(II) bis(triphenylphosphine) dichloride, catechol, 1–bromodecane, and glacial acetic acid were purchased

from

Sigma-Aldrich

company.

Compounds

bis((triisopropylsilyl)ethynyl)benzo[c][1,2,5]thiadiazole-5,6-diamine bis((triisopropylsilyl)ethynyl)pyrene-4,5,9,10-tetraone

(4),

and

(5),

4,72,7-

1,2-bis(decyloxy)-4,5-

diaminobenzene (2) were synthesized in light of the reported procedures.27,51,59 All other solvents and reagents were used as received without further purification. 4.2. Synthesis. Synthesis of 2,7,10,14-tetrakis((triisopropylsilyl)ethynyl)phenanthro[4,5abc][1,2,5]thiadiazolo[3,4-i]phenazine-4,5-dione

(3).

Under

argon

protection,

4,7-

bis((triisopropylsilyl)ethynyl)benzo[c][1,2,5]thiadiazole-5,6-diamine (5, 263 mg, 0.5 mmol) and 2,7-bis((triisopropylsilyl)ethynyl)pyrene-4,5,9,10-tetraone (4, 311 mg, 0.5 mmol) were added into a mixed solvent containing CHCl3 (30 mL) and acetic acid (30 mL). The as-obtained mixture was degassed for 20 min and heated to reflux for 24 h. Then, the reaction mixture was poured into methanol, evaporated and the resulting residue was purified through silica gel column chromatography on using CH2Cl2/hexane as eluent to get dark-green compound 3 (320 mg, 0.29 mmol, yield 58%). 1H NMR (300 MHz, CDCl3) δ (ppm): 9.56 (d, J = 1.8 Hz, 2H), 8.56 (d, J = 1.8 Hz, 2H), 1.32 – 1.23 (m, 42H), 1.17 – 1.09 (m, 42H). 13C NMR (151 MHz, CDCl3) δ


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(ppm): 178.60, 155.27, 142.77, 141.42, 137.88, 135.48, 130.88, 130.29, 130.13, 125.88, 114.82, 111.84, 104.63, 101.37, 96.83, 18.99, 18.76, 11.49, 11.45. Elemental analysis (%) calcd for C66H88N4O2SSi4: C, 71.17; H, 7.96; N, 5.03; found: C, 70.54; H, 7.86; N, 4.58. Synthesis

of

6,7,15,16-tetrakis(decyloxy)-2,11-

bis((triisopropylsilyl)ethynyl)quinoxalino[2',3':9,10]phenanthro[4,5-abc]phenazine

(Py-0-

TQ). Compound 4 (311 mg, 0.5 mmol) and Compound 2 (420 mg, 1.0 mmol) were added into a mixed solvent containing CHCl3 (30 mL) and AcOH (30 mL) and refluxed under argon at 80 oC for 30 h. Then the solvent was evaporated through rotary evaporation, and the as-resulted mixture was purified through column chromatography on silica gel using CH2Cl2/hexane as eluent. The as-obtained product was dissolved in CHCl3, re-precipitated by methanol (50 mL), and washed with methanol two times to afford pure yellow compound Py-0-TQ (420 mg, 0.3 mmol, yield 60%). 1H NMR (300 MHz, CDCl3) δ (ppm): 9.70 (s, 4H), 7.62 (s, 4H), 4.32 (t, J = 6.3 Hz, 8H), 2.09 – 1.91 (m, 8H), 1.53 – 1.02 (m, 98H), 0.94 – 0.83 (m, 12H).

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C NMR (151

MHz, CDCl3) δ (ppm): 153.93, 140.47, 139.50, 130.23, 128.91, 125.53, 123.08, 107.39, 106.72, 92.29, 69.37, 31.96, 29.66, 29.62, 29.44, 29.39, 28.94, 26.10, 22.69, 18.89, 14.14, 11.53. Elemental analysis (%) calcd for C90H134N4O4Si2: C, 77.64; H, 9.70; N, 4.02; found: C, 77.10; H, 9.38; N, 4.05. Synthesis

of

6,7-bis(decyloxy)-2,11,14,18-

tetrakis((triisopropylsilyl)ethynyl)quinoxalino[2',3':9,10]phenanthro[4,5abc][1,2,5]thiadiazolo[3,4-i]phenazine (Py-1-TQ). Compound 3 (223 mg, 0.2 mmol) and Compound 2 (84 mg, 0.2 mmol) were added into a mixed solvent containing CHCl3 (30 mL) and AcOH (15 mL) and refluxed under argon at 80 oC for 30 h. Then, the solvent was evaporated through rotary evaporation, and the mixture was purified through column chromatography on


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silica gel using CH2Cl2/hexane as eluent. The as-obtained product was dissolved in CHCl3, reprecipitated by methanol (50 mL), and washed with methanol two times to afford pure compound Py-1-TQ (245 mg, 0.16 mmol, yield 80%) as dark-red solid. 1H NMR (300 MHz, CDCl3) δ (ppm): 9.76 (d, J = 1.7 Hz, 2H), 9.72 (d, J = 1.7 Hz, 2H), 7.63 (d, J = 5.6 Hz, 2H), 4.34 (t, J = 6.5 Hz, 4H), 2.07 – 1.98 (m, 4H), 1.44 – 1.19 (m, 112H), 0.92 – 0.86 (m, 6H). 13C NMR (151 MHz, CDCl3) δ (ppm): 154.09, 153.18, 143.80, 140.79, 139.64, 137.90, 131.50, 129.53, 128.90, 128.73, 125.86, 122.89, 113.36, 109.74, 105.70, 100.66, 92.41, 68.40, 30.92, 30.57, 28.68, 28.58, 28.36, 27.92, 25.10, 21.63, 18.03, 17.88, 13.09, 10.62, 10.54. Elemental analysis (%) calcd for C92H132N6O2SSi4: C, 73.74; H, 8.88; N, 5.61; found: C, 73.26; H, 9.24; N, 5.43. Synthesis

of

2,5,9,12,15,19-hexakis((triisopropylsilyl)ethynyl)-

[1,2,5]thiadiazolo[3'',4'':6',7']quinoxalino[2',3':9,10]phenanthro[4,5abc][1,2,5]thiadiazolo[3,4-i]phenazine (Py-2-TQ). Compound 5 (210 mg, 0.4 mmol) and Compound 4 (124 mg, 0.2 mmol) were added into a mixed solvent containing CHCl3 (30 mL) and AcOH (30 mL) and refluxed under argon at 80 oC for 24 h. Then the solvent was evaporated by rotary evaporation, and the mixture was purified through column chromatography on silica gel using CH2Cl2/hexane as eluent. The as-obtained product was dissolved in CHCl3, reprecipitated by methanol (50 mL), and washed with methanol two times to afford pure compound Py-2-TQ (240 mg, 0.15 mmol, yield 75%) as dark-red solid. 1H NMR (300 MHz, CDCl3) δ (ppm): 9.72 (s, 4H), 1.40 – 1.22 (m, 126H).

13

C NMR (151 MHz, CDCl3) δ (ppm):

154.29, 143.12, 140.59, 132.27, 129.05, 127.38, 123.74, 113.54, 110.31, 104.83, 100.68, 93.46, 18.00, 17.85, 11.02, 10.51. Elemental analysis (%) calcd for C94H130N8S2Si6: C, 70.36; H, 8.17; N, 6.98; found: C, 69.92; H, 7.93; N, 6.52.


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4.3. Film Deposition and Fabrication of the Memory Devices. The memory devices based on Py-0-TQ, Py-1-TQ and Py-2-TQ were fabricated using indium-tin-oxide (ITO)-coated glass as substrates. The ITO-coated substrates were pre-cleaned with water and ultrasonicated in deionized water, acetone, and ethanol for 20 min sequentially. Afterwards, the organic solutions of these small molecules in ortho-dichlorobenzene with a concentration of 10 mg/mL were spincoated onto ITO substrates via a speed of 300 RPM for 6 s, and then 1500 RPM for 30 s to obtain thin films, respectively. The films were subsequently delivered into an evaporation chamber and evacuated to a vacuum of ~ 10-6 torr. Eventually, an aluminum layer of about 100 nm in the thickness was thermally deposited onto the organic layer by a shadow mask of circle patterns. The active area of a single device cell was about 0.126 mm2. Under atmospheric condition, current–voltage tests of the non-packaged memory devices were performed using a Keilthley 4200-SCS semiconductor characteristic system. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1H and 13

C NMR spectra, MALDI-TOF-MS, FT-IR spectra, thermal properties, cyclic voltammogram

curves, X-ray diffraction patterns, schematic illustration of the memory device architecture, and optical and electrochemical properties (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.L.).


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*E-mail: [email protected] (J.L.). *E-mail: [email protected] (Q.Z.). Author Contributions ⊥

Y.L. and Z.W. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Q.Z. acknowledges financial support from AcRF Tier 1 (RG 111/17, RG 2/17, RG 8/16 and RG 114/16), Singapore. J.L. acknowledges financial support from the NSF of China (21206102 and 21336005). H.L. thanks the National Excellent Doctoral Dissertation funds (201455), China. Y.L. thanks the China Scholarship Council (201606920044) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYZZ16_0086). REFERENCES (1)

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Scheme 1. Synthetic route of as-prepared compounds Py-0-TQ, Py-1-TQ and Py-2-TQ. (i) acetic acid/chloroform, 58%; (ii) acetic acid/chloroform, 80%; (iii) acetic acid/chloroform, 75%; (iv) acetic acid/chloroform, 60%.


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Figure 1. Optical absorption spectra of Py-0-TQ, Py-1-TQ and Py-2-TQ in CH2Cl2 solution.

Figure 2. (a-c) AFM images of Py-0-TQ (a), Py-1-TQ (b) and Py-2-TQ (c) based thin films on ITO-coated substrates. (d-f) 3D-AFM images of Py-0-TQ (d), Py-1-TQ (e) and Py-2-TQ (f) based thin films on ITO substrates; the surfaces exhibit uniform in-plane to out-of-plane selfassembly orientations transition.


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Figure 3. (a-c) Current–voltage (I–V) characteristics of Py-0-TQ (a), Py-1-TQ (b) and Py-2-TQ (c) based memory devices; the numbers denote the sweeping sequences. (d-f) Retention stabilities of Py-0-TQ (d), Py-1-TQ (e) and Py-2-TQ (f) based memory devices at “ON” and “OFF” states under a constant “read” voltage of –1.0 V; the current states were well-separated with narrow fluctuations within one order of magnitude.


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Figure 4. DFT simulated molecular ESP and electron-density isosurfaces (HOMO and LUMO orbitals) of Py-0-TQ (a), Py-1-TQ (b) and Py-2-TQ (c) in their optimized geometries.


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