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Quadrupolar (A-#-D-#-A) Tetra-aryl 1,4-Dihydropyrrolo[3,2-b]pyrroles as Single Molecular Resistive Memory Devices: Substituent Triggered Amphoteric Redox Performance and Electrical Bistability Ram Kumar Canjeevaram Balasubramanyam, Rajnish Kumar, Samuel J Ippolito, Suresh K. Bhargava, Selvakannan R. Periasamy, Ramanuj Narayan, and Pratyay Basak J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 9, 2016

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The Journal of Physical Chemistry

Quadrupolar (A-π-D-π-A) Tetra-aryl 1,4Dihydropyrrolo[3,2-b]pyrroles as Single Molecular Resistive Memory Devices: Substituent Triggered Amphoteric Redox Performance and Electrical Bistability† Ram Kumar C. B.,1,4,5Rajnish Kumar,1,3Samuel J. Ippolito,4,5,6Suresh K. Bhargava,5,6 Selvakannan R. Periasamy,5,6 Ramanuj Narayan*,1,3, and Pratyay Basak*,2,3

1

Polymers and Functional Materials Division; RMIT-IICT Joint Research Centre, 2 Nanomaterials Laboratory, Inorganic and Physical Chemistry Division 3 Academy of Scientific & Innovative Research (AcSIR) † CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad-500 007, INDIA. Ph(O): 91-40-27193225 E-mail: [email protected]; [email protected] and 4

School of Electrical and Computer Engineering; 5 School of Applied Sciences; 6 Centre for Advanced Materials and Industrial Chemistry; Royal Melbourne Institute of Technology (RMIT), 124 La Trobe St, Melbourne VIC 3000, Australia.

*To whom all correspondences should be addressed

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KEYWORDS fused pyrroles, quadrupolar, structural isomers, amphotericity, intra-molecular charge transfer, charge traps, organic resistive memory

ABSTRACT A series of quadrupolar (A-π-D-π-A) tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) derivatives synthesized are showcased as potential organic resistive memory (ORM) devices for the first time. The experimental observations coupled with density functional theory (DFT) calculations probes in detail the role of terminal substituent groups (p-NH2, p-Cl, p-CN, p-NO2, m-NO2) on the optical and electrical properties. Electrochemical studies reveal that the 3- and 4-dinitro derivatives form an unusual class of tetra-aryl DHPPs that exhibit intrinsic amphoteric redox behavior contrary to the literature reports. The bipolar nature within a single molecule was harnessed to design operational ORMs. Interestingly, the memory devices fabricated using the structural isomersexhibited dissimilar memory characteristics. While the p-NO2 derivative displays permanent Write Once Read Many times (WORM) memory, its meta-counterpart represents a behavior akin to rewriteable flash memory. The noticeably higher ON/OFF ratio (~104) for the p-NO2 derivatives could be ascribed to their matched redox energy levels with the work-function of active electrodes favoring better charge injection. Rational interpretation of these findings strongly suggests that the choice and strategic positioning of terminal substituents can significantly alter the nature of "charge traps" affecting the device outcome. These encouraging findings open up a relatively less chartered territory of air stable fused pyrrole systems that holds great promise for realizing next generation organic memory devices.

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Introduction Conjugated small molecules (CSM`s) displaying unique optical and electrical properties have kept the researchers fascinated over the last couple of decades.1 Their potential applications can be as diverse as organic photovoltaics (OPVs),2,3 organic light emitting diodes (OLEDs),4,5 organic field effect transistors (OFETs)6,7 to organic resistive memory (ORMs)8,9 devices. Recent advances in synthetic strategies, ease of purification, scalability have made organic materials cost competitive over conventional inorganic counterparts promising industrial viability.10 Nevertheless, minor concerns regarding device performance, stability and life-time remains the roadblock to commercialization. Fused hetero-aromatic structures with a donor-acceptor (D-A) arrangement, commonly known as push-pull systems are an interesting class of CSMsthat have garnered considerable attention in recent years.6 Judicious choice of donor (D) and acceptor (A) fragments i.e."pick and impart" molecular designs offers rich structural flexibility to tailor the molecular framework at will. Tweaking the architecture while exercising control at the molecular level with simple and smart synthetic strategies can effectively alter the intrinsic electronic character. Conventional p-/n-type materials can be tuned appropriately to exhibit bipolar and/or ambipolar behavior.11 Additionally, the fine planarity inherent in these systems imparts excellent structural ordering and crystallinity when processed as thin-films.12 These underlying critical parameters associated with the organic molecule under study often determines the electro-active performance and suitability of the engineered device. Over the years various scaffolds with push-pull arrangements, such as, linear (D-π-A), quadrupolar (D-π-A-D or A-π-D-π-A) and octupolar/tripodal ((D-π)3-A or (A-π)3-D) have been documented.13 The extent of intra-molecular charge transfer (ICT) facilitated by the delocalized π-electrons defines their unique opto-electronic properties and efficiency. Literature suggests that considerable attention in designing the push-pull systems have primarily focused on thiophene/oligothiophene scaffolds with modest attempts directed towards using furan as the core.14 However, the concerns regarding electron -2ACS Paragon Plus Environment


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mobility achievable in thiophenes and the chemical instability associated with furan derivatives have limited their scope.14,15 The first successful synthesis of 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) reported in 197216 provided an exciting option to develop fused pyrrole systems as a promising alternative to furans and thiophenes. The hypothesis that NH-π interaction ensures conjugation of electrons in either directions, thereby increasing the charge mobility is an interesting proposition to explore further.15,17 Even so, the prospect remained largely unexploited until 2013, owing to the lack of an efficient synthesis strategy. A simple one-pot-domino reaction to synthesize DHPP`s with relatively moderate yields developed by Gryko et al.18 opened up renewed possibilities. The scaffolds were projected as linear / non-linear optical materials19, as well as active materials in OFETs.18 Albeit, to the best of our knowledge, DHPP`s have not been investigated as prospective organic resistive memory (ORM) devices till date. Organic resistive memory devices based on CSMs utilize the electrical bistability of the active materials to store data as high (OFF) and Low (ON) resistance states.20 The device can hence be programmed as "0" and "1" to encode digital memory in a cell. Charge transfer (CT) complexes owing to inter- / intra- molecular electron transfer under an induced electrical bias is responsible for triggering the change in resistivity. Depending on the stability of these CT-complexes, absence/presence of trap states, their concentration and depth, device characteristics can vary from volatile Static/Dynamic Random access Memory (S-RAM, D-RAM) to non-volatile Write Once Read Many Times (WORM) and Rewritable flash memory devices.21 It is now understood that simple structural modifications, such as, altering π-spacers (linkers), enhanced co-planarity in the conjugated backbone, change in substituent groups (associated inductive effects and resonance) all critically affect the electro-activity of the molecules designed.22 In our present effort, we have synthesized a series of tetra-aryl DHPP derivatives and for the first time attempted to showcase their feasibility as potential ORM devices. The study investigates in detail the role of terminal substituent groups (p-NH2, p-Cl, p-CN, p-NO2, m-NO2), on the optical and electrical -3ACS Paragon Plus Environment

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properties. The experimental findings coupled with density functional theory (DFT) calculations provided pertinent clues in understanding the structure-property relationship of these organic moieties. A pronounced bathochromic shift in the absorption spectra of the 4-Nitro derivative probably is a direct manifestation of their quinoidal character leading to relatively smaller energy gap. Interestingly, the 3and 4-nitro derivatives exhibit amphoteric redox (bipolar) behavior contrary to literature reports on intrinsic donor characteristics of DHPPs.17 As a logical culmination to this observation, efforts were made to demonstrate the possibility of utilizing these derivatives as single molecule memory elements. Encouragingly, both 3- and 4-Nitro derivatives displayed impressive non-volatile memory performance with reasonable endurance and retention time. Literature reveals that structural isomers have rarely been studied to identify the effects of substituent positions on the device behavior and performance.23,24 Additionally the position of terminal substituents has a significant impact which was evident from the contrasting behavior of the 3- and 4-nitro derivatives (DN3PP, DNPP). While DN3PP exhibited a rewriteable flash memory characteristic, the DNPP exhibited Write Once Read Many time’s (WORM) memory behavior. Notably, the performance of the test cells were fabricated without the use of automation supported in-line assembly or encapsulation – rather the devices were tested in air at room temperature. We strongly believe that the possibility to considerably improve the performance levels further remains open by employing such sophisticated state-of-the-art fabrication techniques in future devices.

Experimental Materials 4-nitro benzaldehyde, 3-nitro benzaldehyde, 4-cyano benzaldehyde, tin chloride dihydrate, and acetic acid were purchased from Finar Scientifics and were used without further purification. 4-chloro benzaldehyde and 2,3-butane dione were purchased from Sigma-Aldrich. 4-butyl aniline was purchased



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from Merck and distilled prior to use. All solvents were purchased from Finar Scientifics and are of analytical grade. General Synthetic Protocol for 1,4-dihydropyrrolo[3,2-b]pyrroles In 50 mL round-bottom flask equipped with a reflux condenser and magnetic stir bar, glacial acetic acid (15 mL) was added followed by the addition of arylamine (10 mmol), aldehyde (10 mmol) and TsOH (1 mmol). The mixture was heated at 90 °C with stirring for 30 min. Butane- 2,3-dione (5 mmol) was slowly added and the resulting mixture was stirred at 90 °C for 3.5 h. The reaction mixture was then cooled to room temperature. The precipitate of the obtained dye was filtered under vacuum and washed with cooled glacial acetic acid. Re-crystallization from ethyl acetate and vacuum drying yielded pure product. (1) 1,4-bis(4-butylphenyl)-2,5-bis(4-nitrophenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole Deep wine red crystals, MP: 307.3 oC. 1H NMR (500 MHz, CDCl3) δ 8.06 (d, J = 9.0 Hz, 4H), 7.32 (d, J = 9.0 Hz, 4H), 7.24 (d, J = 8.4 Hz, 4H), 7.18 (d, J = 8.4 Hz, 4H), 6.53 (s, 2H), 2.67 (t, 4H), 1.69 – 1.61 (m, J = 15.4, 7.6 Hz, 4H), 1.45 – 1.35 (m, J = 14.7, 7.4 Hz, 4H), 0.97 (t, J = 7.4 Hz, 6H). 13C NMR (75 MHz, CDCl3) δ 145.42, 141.82, 139.59, 136.75, 135.17, 134.06, 129.57, 127.61, 125.25, 123.70, 96.42, 77.45, 77.03, 76.61, 35.20, 33.50, 22.41, 13.99 HRMS: calculated for C38H36N4O4, M+: 613.28024. Found: 613.28093. Anal. calculated for C38H36N4O4; C 74.37, N 9.12, H 6.07, O 10.42; found C 74.59, N 9.16, H 6.38, O 9.87. (2) 1,4-bis(4-butylphenyl)-2,5-bis(3-nitrophenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole Orange needle like crystals, MP: 208.4-208.7 oC. 1H NMR (300 MHz, CDCl3) δ 8.13 (s, 2H), 7.98 (d, J = 8.1 Hz, 2H), 7.43 (d, J = 7.9 Hz, 2H), 7.33 (t, J = 7.9 Hz, 2H), 7.24 – 7.14 (m, 8H), 6.50 (s, 2H), 2.65 (t, 4H), 1.72 – 1.53 (m, J = 15.5, 7.6 Hz, 4H), 1.46 – 1.30 (m, 4H), 0.95 (t, J = 7.3 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 148.28, 141.62, 136.76, 135.10, 134.13, 133.47, 132.75, 129.54, 128.98, 125.29, 122.26, 120.65, 95.48, 77.31, 77.06, 76.80, 35.21, 33.51, 22.35, 14.01 HRMS: calculated for C38H36N4O4, M+: 613.28002.


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Found: 613.28093. Anal. calculated for C38H36N4O4; C 74.37, N 9.12, H 6.07, O 10.42; found C 74.70, N 9.06, H 6.38, O 9.86. (3) 4,4'-(1,4-bis(4-butylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole-2,5-diyl)dibenzonitrile Fluorescent yellowish-green needle like crystals, MP: 272.5 oC. 1H NMR (500 MHz, CDCl3) δ 7.47 (d, J = 8.5 Hz, 4H), 7.27 (d, J = 8.6 Hz, 4H), 7.22 (d, J = 8.3 Hz, 4H), 7.16 (d, J = 8.4 Hz, 4H), 6.47 (s, 2H), 2.66 (t, 4H), 1.68 – 1.60 (m, J = 15.3, 7.5 Hz, 4H), 1.44 – 1.34 (m, 4H), 0.96 (t, J = 7.4 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 141.55, 137.70, 136.86, 135.04, 133.41, 131.93, 129.45, 127.82, 125.17, 119.16, 108.99, 95.91, 77.27, 77.02, 76.76, 35.18, 33.49, 22.38, 13.98 HRMS: calculated for C40H36N4, M+: 573.30059. Found: 573.30127. Anal. Calculated for C40H36N4; C 83.73, N 9.77, H 6.50; found C 84.09, N 9.79, H 6.88. (4)

1,4-bis(4-butylphenyl)-2,5-bis(4-chlorophenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole

Off-white

crystals, MP: 261.3 oC. 1H NMR (500 MHz, CDCl3) δ 7.20 – 7.10 (m, J = 8.7 Hz, 16H), 6.36 (s, 2H), 2.63 (t, 4H), 1.68 – 1.58 (m, J = 15.3, 7.6 Hz, 4H), 1.44 – 1.33 (m, J = 14.7, 7.4 Hz, 4H), 0.95 (t, J = 7.4 Hz, 6H).

13

C NMR (126 MHz, CDCl3) δ 140.75, 137.32, 134.76, 132.18, 131.86, 129.19, 129.15,

128.32, 125.08, 94.62, 77.29, 77.03, 76.78, 35.19, 33.53, 22.42, 14.01 HRMS: calculated for C38H36Cl2N2 M+: 591.23088 Found: 591.23283. Anal. calculated for C38H36Cl2N2; C 77.02, N 4.73, H 6.29, Cl 11.96; found C 81.82, N 9.90, H 7.80, Cl 11.60. (5) 4,4'-(1,4-bis(4-butylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole-2,5-diyl)dianiline In a 100 mL, 2necked round bottom flask equipped with a reflux condenser and magnetic stir bar tin (II) chloride dihydrate (5.5g, 24.4 mmol) was added in THF (50 mL). The mixture was stirred and heated to reflux to obtain a clear solution. Compound 5 (1.5 g, 2.44 mmol, 3 portions) was added slowly over 30 min to the refluxing solution. After the addition, the mixture was allowed to reflux for 3 hours. Then, 5 mL conc. HCl was added and the resulting solution was continued to reflux for 30 min and cooled to room temperature. The hydrochloride salt precipitated as an off-white solid during cooling. The mixture was



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further cooled to 15 °C and the precipitate was collected by vacuum filtration, washed with water (2 x 25 mL), and suction dried thoroughly. The hydrochloride salt was transferred to a 100 mL 2-necked round bottom flask equipped with magnetic stir bar and to which 50 mL of NaOH solution (2N) was added and stirred for 2 hours at room temperature. The compound was extracted with ethyl acetate (2 x 50 mL) washed with brine and the combined layers of organic phase was dried over MgSO4. The solvent was removed under reduced pressure to afford Compound 1 as a pure pale brown solid (1.05 g, 78%). MP: 246.4-247.6 oC. 1H NMR (500 MHz, CDCl3) δ 7.18 (d, J = 8.4 Hz, 4H), 7.13 (d, J = 8.4 Hz, 4H), 7.01 (d, J = 8.5 Hz, 4H), 6.54 (d, J = 8.5 Hz, 4H), 6.26 (s, 2H), 3.62 (s, 4H), 2.61 (t, 4H), 1.67 – 1.55 (m, J = 15.4, 7.6 Hz, 4H), 1.47 – 1.30 (m, J = 14.6, 7.3 Hz, 4H), 0.94 (t, J = 7.4 Hz, 6H).13C NMR (126 MHz, CDCl3) δ 144.59, 139.88, 137.99, 135.50, 130.64, 129.45, 128.81, 124.98, 124.73, 114.90, 93.37, 77.32, 77.06, 76.81, 35.21, 33.58, 22.44, 14.05 HRMS: calculated for compound C38H40N4, M+: 553.32938. Found: 553.33257. Anal. calculated for C38H40N4; C 82.42, N 10.11, H 7.46; found C 81.82, N 9.90, H 7.80.

Characterization 1

H NMR was recorded on Bruker Avance (300MHz), and Varian (500 MHz) NMR spectrometer. 13C-

NMR was proton decoupled and recorded on a 125 MHz Bruker NMR spectrometer. Both were measured in CDCl3 using TMS as an internal standard. Chemical shifts are reported in parts per million (ppm). The following abbreviations were used to describe the peak multiplicities. S: singlet, d: doublet, t: triplet, m: multiplet. High-resolution mass spectra (HRMS) were recorded on ESI-QTOF mass spectrometry. FTIR spectra were obtained using Perkin Elmer FT-IR 400 spectrometer. UV-Vis and Fluorescence spectra were recorded using Shimadzu UV-1800 spectrophotometer and Cary eclipse fluorescence spectrophotometer. Thermal analysis was carried using TAQ50 TGA analyser. Melting points were calculated using Contech programmable melting point apparatus.


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Cyclic voltammetry measurements were recorded on Zahner Zennium electrochemical workstation using a three electrode system: glassy carbon as working electrode, platinum wire as counter electrode and Ag/AgCl as reference electrode in dry DCM (dichloromethane) with 0.1M tetrabutyl ammonium hexaflurophosphate (TBAPF6) as supporting electrolyte. The solvents used were freshly dried and a dry nitrogen purge for approximately 30 minutes was carried prior to each experiment. The HOMO and LUMO levels were calculated using Ferrocene/Ferrocenium redox couple as an external standard using the formula EHOMO= - 4.4- EOx Onset ELUMO= - 4.4-Ered Onset Device Fabrication and Performance Evaluation The flexible ITO coated PET substrates used for making memory devices were masked and patterned into two ~ 0.8 cm wide lines. Later, the patterned substrates were ultra-sonicated in deionized water for 15min followed by 15min each in acetone, ethanol and isopropanol. 6 mg/mL THF solution of DHPP derivatives were prepared and 150µL of the solution was spin-coated onto the patterned ITO coated PET substrate at 1500 rpm for 40 s and later vacuum dried at 70 oC overnight. Finally, a 500-nm-thick Al top electrode was thermally evaporated through the shadow mask at a pressure of 10-6 torr with a depositing rate of 3-5 Å.s-1 using Vacuum box coater BC 300. The active area of device determined through shadow mask was found to be ca. 0.64 cm2. Additional devices with different electrode area (0.20 cm2 and 0.08 cm2) and thickness of the organic layer (~ 120nm, 300nm, 450nm and 600nm) was also fabricated (see SI, Figure-S16 and Figure-S17). Post optimization, a thickness, t = 300 nm and an area, A = 0.64 cm2 was maintained through-out the study unless specified otherwise. The electrochemical characterization (I-V, cycling and impedance) of the two terminal memory devices was performed using Zahner Zennium electrochemical workstation. During all measurements, ITO was used as bottom electrode and Al was the top electrode for applying voltage. All electrochemical measurements were done in ambient conditions with a voltage scan rate of 100 mV.s-1 on at least three devices (N = 3) were -8ACS Paragon Plus Environment


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fabricated under similar conditions for each study. The J-V characteristics of ITO/DNPP/Al and ITO/DN3PP/Al were evaluated for N = 21 and 24 devices, respectively to ascertain reproducibility and current distribution. The current distribution plots were obtained by reading the observed current at two fixed voltages i.e. at OFF and ON stages respectively. For DNPP, a voltage value of -0.8V represents the OFF state and -1.0V the ON state for all the devices. In the case of DN3PP, the OFF state is considered at -1.5V while -2.0V was opted as ON state. The histograms represent the current density and % frequency at these voltage values. The respective current density responses were then concatenated in the form of a matrix to generate the respective 3D-contour plots that gives a virtual representation of the spatial current distribution for OFF and ON states. The cycling performance of ITO/DN3PP/Al were carried out for N = 9 devices to ascertain stability and reliability of ON/OFF ratio.

Results and Discussion Assessing Optical and Electrochemical Properties A series of tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrroles with varying terminal substituent groups (pNH2, p-Cl, p-CN, p-NO2, m-NO2) were synthesized following reported protocols18 (see SI). The optical absorption and emission spectra of these DHPP derivatives (compounds 1-5, Figure 1) were carried out to evaluate their photo-physical properties. UV-Vis absorption spectra were recorded in both solution and solid state to understand molecular transitions (π-π*) and change in inter-molecular interactions when deposited as films. The absorption spectra in THF solution (10µM) and thin films spin coated on quartz substrates are depicted in Figure 2(a) and 2(b). Corresponding fluorescence emission spectra collected in solution are provided in the supporting information. As apparent from both absorption and emission spectra, with increase in electron-withdrawing ability of the terminal substituent and extended conjugation, a progressive bathochromic shift is observed. In particular, the 4-nitro phenyl derivative (DNPP) exhibited a pronounced red shift having broad absorption at λmax ca. 478nm. The observation is similar to earlier reports on dinitro derivatives of oligothiophenes,25 wherein the quinoidal contribution -9ACS Paragon Plus Environment

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is ascribed to play a key role. In the present study, a contrary observation for 3-nitro phenyl derivative (DN3PP) with a λmax at ~362nm, lends added credence to this argument. The significant blue shift in absorption for DN3PP is a direct consequence of constrained conjugation. It is rationalized that quinoidal forms lead to a more polarized excited state that is supported by theoretical studies as well. Further, when compared to other analogues the relatively broad absorption band observed for the psubstituted dinitro derivative (DNPP) suggests a strong intra-molecular charge transfer (ICT) within the system.13 Similar trend is retained in the solid state studies, with an additional shoulder appearing at the higher wavelengths of DNPP and DCNPP indicating the effects of molecular aggregations.26 A significant red shift in the onset of the absorption band edge owing to these intermolecular interactions is evidenced. The absorption onset from the solid-state spectra was considered for estimating the optical

band gaps (E ) of the DHPP derivatives are summarized in Table 1. The electrochemical window and redox behavior of these materials were assessed using detailed cyclic voltammetry (CV) studies. Typically, CV measurements were carried out in anhydrous dichloromethane (DCM) containing 0.1 M tetra butyl ammonium hexaflurophosphate (TBAPF6) as supporting electrolyte with glassy carbon as working electrode (WE), platinum wire as counter electrode (CE) and Ag/AgCl as reference electrode (RE). Representative voltammetric scans within the electrochemical potential window (±1.5 V) obtained at 50 mV.s-1 sweep for the derivatives are presented as stack plots in Figure 2(c). As observed, molecules DAPP, DClPP and DCNPP exhibit only one reversible redox wave in the positive potential window characteristic of electron rich fused pyrroles (DHPP donor). The p- and m-nitro-substituted derivatives, DNPP and DN3PP displays reversible redox behavior in both positive and negative potential windows as expected of nitro derivatives. The current density observed in CV plots also indicate that compounds DNPP and DN3PP are potentially better acceptors, which correlates well with observations made by Gudieka et al.27 To the best of our knowledge, such an observation is hitherto undocumented for 1,4-dihydropyrrolo[3,2-b]pyrrole or its -10ACS Paragon Plus Environment


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derivatives. This attribute is typical of amphoteric redox (bipolar) molecules which possess dual donor and/or acceptor capabilities within its architecture.28,29 The observation is of particular significance and opens up prospects of designing single molecular devices. Ionization Potentials (HOMO), and Electron Affinity(LUMO) versus vacuum were estimated from the onset potentials of oxidation/reduction with reference to energy level of Ferrocene/Ferrocenium redox couple as an external standard.30 The onset potential of oxidation in the +ve window (anodic sweep) and onset of reduction peaks in the -ve potential window (cathodic sweep) were used where ever possible. Since only DNPP and DN3PP shows dual characteristics as both donor and acceptor, the HOMO and LUMO could be estimated directly from the voltage sweeps. For molecules DAPP, DClPP and DCNPP, the estimation of LUMO levels was aided by optical measurements in conjunction with CV and is summarized in Table 1. Interestingly, the onset of oxidation potential is observed to be more +ve with increase in electron-withdrawing ability of the terminal substituent that effectively lowers the HOMO levels. Concurrently, an apparent suppression of the redox peak in the reverse sweep is also quite evident. This is possibly owing to a destabilizing effect of the terminal acceptor groups and their interaction with the π-conjugated backbone.28 Molecular Simulations based Theoretical Studies To gain a better perspective into the electronic structures of the synthesized tetra-aryl 1,4dihydropyrrolo[3,2-b]pyrrole derivatives, density functional theory (DFT) and time-dependent DFT calculations were carried out using B3LYP/6-31G (d, p) basis set for geometry optimizations, single point energy calculations and charge transfer transitions. Gaussian 03 ab initio software package31 was used for all theoretical calculations and the predicted transitions in the absorption spectra (300-500nm) were studied using GaussSum 3.0 software. To reduce the computational complexity, the butyl chains have been substituted by smaller methyl groups. Post energy minimization, the optimized geometries obtained was utilized to calculate frontier molecular orbitals (FMO). The molecular orbital iso-surfaces, -11ACS Paragon Plus Environment

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ground-state dipole moments and electro-static potentials (ESP) for the optimized geometries of the synthesized tetra-aryl DHPP derivatives are summarized in Table 2. Additional data pertaining to the major contributions of the respective molecular orbitals, oscillator strengths, density of states, probable absorption wavelengths corresponding to the allowed transitions, excited state dipole moments, assessed for the synthesized derivatives are all provided in Table S1 and Table S2 (SI). As evident from the FMO isosurfaces (Table 2), the HOMO is distributed primarily along the πdelocalized chromophore with a significantly larger contribution from the fused pyrrole rings (donor). The LUMO on the other hand is apparently seen to differ considerably with the change in the terminal substituents (p-NH2, p-Cl, p-CN, p-NO2, m-NO2). For DAPP and DClPP, the LUMO is observed to be delocalized throughout the molecule, implying that they have a weak intra-molecular charge transfer.21 It was also observed that the increasing electron withdrawing ability for the terminal substituents in DCNPP, DN3PPand DNPP, the LUMO inclines to populate the acceptor groups. These findings suggest that a relatively enhanced intra-molecular charge separation is more probable for these three 1,4dihydropyrrolo[3,2-b]pyrrole derivatives. It can be rationalized that depending on the extent of ICT, the HOMO to LUMO excitation would readily result in the shift of electron density from the fused pyrroles to the acceptor moieties leading to a more polarized excited state.32 This implies a substantial lowering of the energy gap between the HOMO and LUMO as reflected in the electronic absorption spectra. Further, the optimized geometries were used to estimate the most favorable transitions in the 300-500 nm range using time-dependent density functional theory (TD-DFT) studies. For DAPP, DClPP, DCNPP and DNPP, the most intense absorptions (λmax) should occur at ca. 363, 362, 406 and 481 nm, respectively, corresponding to the HOMO–LUMO excitation with highest oscillator strengths (see Table S1, SI). However, for DN3PP the most intense absorption band predicted at 361 nm results from a favored HOMO to LUMO+2 transition. As discussed in the previous section, the progressive



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bathochromic shifts for these molecules noticed in our experimental observations (UV-Vis studies) are in good agreement with the theoretical approximations.13,25,26 Device Fabrication and Performance Evaluations The preliminary optical and electrochemical assessments on the synthesized tetra-aryl 1,4dihydropyrrolo[3,2-b]pyrrole derivatives coupled with the molecular simulation studies encouraged us to explore device feasibility. Particularly, the bipolar nature of DNPP and DN3PP indicated their potential to function as a single molecular device. In this attempt, efforts were directed towards fabricating organic resistive memory devices (ORMs) using DNPP, DN3PP and DCNPP as the active layers. For devices to operate under ambient conditions, i.e. in presence of air/O2/moisture, HOMO levels of the active molecules lower than -5.2 eV is a desirable pre-requisite.33 Encouragingly, the estimated HOMO levels of the synthesized molecules DNPP, DN3PP and DCNPP are significantly lower than the air oxidation threshold (~ -5.2 eV) justifying the excellent air stability observed for these compounds. Thermogravimetric studies (see SI) on these molecules also revealed their remarkable thermal stability (To > 280 oC) that offers several options in processing. Test-cell devices were fabricated by sandwiching the active organic layer/s between indium tin oxide (ITO) and Aluminum (Al) as the two active electrodes.34 Flexible ITO coated PET substrates (ITO-poly(ethylene terephthalate)) were spin-coated with the active layer from dilute THF solutions (6 mg/mL) followed by overnight annealing at 70 oC (see SI). These substrates suitably masked, were coated with aluminum by thermal evaporation. The ITO/Active layer/Al device configuration utilized is schematically illustrated in Figure 3(b). The average thickness of the organic layer was ~ 300 nm as seen from the cross-sectional FESEM micrograph (depicted as Inset, Figure 3(b) and Figure S15). All the measurements were carried out using Al as the top electrode and ITO as bottom for applying voltage in either direction under ambient conditions.


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The representative I-V characteristic of a typical ITO/DNPP/Al device so fabricated is illustrated in Figure 3(a). During the first sweep from 0 to -4.0 V (sweep 1), the device initially remained in the low conducting state (OFF) exhibiting current around 10-9 A. When the external bias reached a threshold voltage (Vth) of ~ -1.0 V (see SI, Figure-S18), an abrupt increase in current (10-5 A i.e. 4-fold jump) marks a switched ON state. This transition serves as data writing process, which can be encoded as “0” and “1” in a digital memory.35 Subsequent sweeps, Sweep 2 (immediately done afterwards) and Sweep 3 (carried out after the power was turned off for 25 mins) suggests the non-volatile nature of this transition.32 Thereafter, positive potential scans (Sweep 4, Sweep 5) was performed to check whether the device could be switched back to the OFF state (data erase process).26 Interestingly, the ITO/DNPP/Al device showed no change-over to the high resistivity state even with a scan potential of +10 V (see SI, Figure-S19). Successive scans to validate retention and stability demonstrates that the device retains information for 106 s without any significant degradation in the ON state (Figure 3(c)). Encouragingly, the high ON/OFF ratio of 104 avoids misreading, thus enabling the device to maintain two distinct states and this was validated for consistency over 21 independent devices analyzed (see SI, Figure-S18). The 3D-contour plots in the OFF and ON states depicting the current distribution of all the devices are presented in Figure 3(d)-(e). The non-erasable nature of this device conforms to a non-volatile WriteOnce-Read-Many-times (WORM) class and hence can be exploited in applications such as low cost electronic tags possessing permanent memory.36 When DN3PP was used as the active layer, the I-V characteristics of ITO/DN3PP/Al device showed a remarkable change (Figure 4(a)). As evidenced, the device transitioned to an ON state when the external bias reaches a threshold voltage (Vth) of ~ -1.9 V (see SI, Figure-S20), which is almost 0.9 V higher than that observed for DNPP. However, the ON/OFF ratio was considerably low compared to the ITO/DNPP/Al device. The switched ON state was retained in the immediate reverse scan (from -3.0 V to 0 V). Subsequently, the ON state can be switched back to the OFF state during a positive sweep at ~2.0 V, and can again be switched back ON in the next cycle, a characteristic behavior of non-volatile -14ACS Paragon Plus Environment


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flash memory.26 The validation was carried out on 24 independent devices fabricated under similar set of conditions (see SI, Figure-S20). The device was found to preserve the stored information for > 104s under retention tests performed under ambient condition in open air, while the endurance performance (reliability) of 500 cycles observed with ON/OFF ratio maintained was quite appreciable given the lack of device encapsulation (Figure 4(b)-(e)). The current distribution for all the devices in the OFF and ON states are presented in Figure 4(f)-(g). These results hint a clear possibility of a much improved device ON/OFF ratio, retention and endurance performance if state-of-the-art fabrication and encapsulation techniques were used to produce the device. As a case study, when cyano-substituted derivative lacking obvious bipolarity was studied under similar device configuration, i.e., ITO/DCNPP/Al only a gradual and non-linear change in the current was observed. However, as illustrated in Figure-S21, a hysteresis is apparent in the reverse sweep. Although the device showed traits similar to flash memory, the switch-on/switch-off voltages could not be ascertained in the sweeps. Even after a rigorous test of 50 read/write cycles the device retained its characteristics. For the ITO/DClPP/Al and ITO/DAPP/Al devices no indication of switching was observed in the experimental window (see supporting information, Figure-S22). Additionally, to exclude any possible contribution of metallic ions from the metal filament (Al) used, we sandwiched the active layers between two ITO electrodes to form a symmetrical cell i.e., ITO/Organic layer/ITO. Since, ITO is considered to be a relatively inert electrode the origin of molecular switching characteristics can be validated. Symmetrical device configurations yielded similar results, albeit at higher operating threshold voltages with relative low On/OFF ratio compared to that of ITO/Organic layer/Al (Figure-S23). This can be ascribed to the different work function of the ITO electrodes resulting in a significant increase of the charge injection barriers. Encouragingly, the nonvolatile memory traits of DNPP and DN3PP are observed to be retained, confirming the resistive switching nature an inherent property of the organic material.


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Proposed Mechanism The observed device behavior for the synthesized dinitro (p-, m-) and dicyano (p-) substituted tetraaryl 1,4-dihydropyrrolo[3,2-b]pyrrole derivatives can be rationalized on the following grounds. The HOMO and the LUMO estimated from the electrochemical and optical studies summarized in Table 1, can be represented with energy level diagram with the work functions (ϕ) of ITO and Al in device model (Figure 5(a)). The molecular simulation studies in conjunction with appropriate theoretical models, UVVisible studies and electrochemical impedance for the device OFF and ON states provided important leads in understanding the plausible charge transport mechanism involved. The device response under externally applied bias could be predicted by considering the energy barriers, injection of electrons/holes, trap states involved, dipole moments, role of space-charge polarizations that significantly contributes at various stages of the sweep.26,35 As seen from the DFT studies, all the molecules studied possess continuous electrostatic potential (ESP), which are colored in blue, show an open channel for charge migration along the pi-delocalized system (A-π-D-π-A). However, the presence of terminal acceptor groups does create negative ESP regions in each of the molecules, which are colored in red. These regions can potentially impede the motion of charge carriers and serves as “traps” that localize the charge thereby triggering a memory effect.37,38 The charge transport mechanism for molecules DNPP and DN3PP are shown as a graphic illustration in Figure 5(b). For ITO/DNPP/Al, comparable charge injection barriers at the ITO (ϕ)→HOMO (∆~ 0.69 eV) and the Al (ϕ)→LUMO (∆~ 0.67 eV) implies equal possibility of both hole and electron injections. At potentials < -0.5 V, Schottky barriers exist39 and the device remains in the OFF state (high resistivity at the active electrode-organic layer interface). Experimental data in this region of the J-V curve fits well with Ohmic conduction model (I α V). As the external bias exceeds -0.5 V, the applied electric field overcomes the Schottky barriers initiating injection of holes into the HOMO and electrons into the LUMO (initiating formation of molecular dipoles). The slow accretion of charges -16ACS Paragon Plus Environment


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leads to a growing space-charge polarization within the device. The Ohmic current at this juncture changes to a Space-Charge-Limited-Current (SCLC) current37 (I α V2) which is noticeable in this region of the J-V curve (Figure 5(c)). Close to the turn-on voltage (ca. -1.0 V) the double injection of charges from ITO and Al fills the “traps”, thereby creating a trap free environment.37,40 Analogous to reaching an end-point, a sudden opening up (enhanced carrier mobility) of the channel coupled with already high carrier concentration present,39 leads to a sharp shoot-up in the observed current (inter-molecular charge transfer process). This transition to the high conductivity state is characterized asthe device switch-ON state.With further increase in bias (> -1.0V),the current yet again follows the Ohmic conduction model. A non-volatile permanent memory behavior observed doesn’t allow the device to revert back to the OFF state even with an extreme positive sweep up to +10V (see SI, Figure-S19) supports a permanent WORM nature for this device. That the relatively strong electron withdrawing ability coupled with the inductive effect of the p-NO2 group act as a deep trap sites is very apparent from the ESP and the FMO isocontours of DNPP. UV-Visible studies in the OFF and ON states lends further support to the fieldinduced charge transfer interactions (Figure-6)41 along with additional clues obtained from the impedance studies (see SI, Figure-S24). We believe that the charge separated state once formed in the solid-state is stabilized by resonance (p-NO2) in addition to good intra-/inter- molecular interactions and probably aided by highly ordered arrangements. The solid-state UV-Vis and FESEM micrographs (Figure-S25) do strongly hint this possibility in the thin films, which helps in retaining the trapped charges thus exhibiting appreciable non-volatile memory. Devices based on m-NO2 terminally substituted derivative, i.e. ITO/DN3PP/Al, a comparably higher charge injection barrier (∆~ 0.8 eV) exists between the LUMO-DN3PP and Al(ϕ). Hence, at initiation of the negative sweep, injection of holes at the ITO electrode (ITO (ϕ)→HOMO; ~ 0.6 eV) is more favorable than the electrons pumped in from Al electrode. Similar to the ITO/DNPP/Al device, an Ohmic behavior is observed at potentials < -0.6V due to the presence of Schottky barriers and the device -17ACS Paragon Plus Environment

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remains in the high resistivity state. At voltages > -0.6 V, the barrier is overcome and a slow accumulation of space charges leads to a Space-Charge-Limited-Current (SCLC) current (I α V2). When a trap free environment is achieved close to V ~ 1.9 V, the device quickly switches-ON to a high conductivity state. The similarity of ITO/DN3PP/Al device to the ITO/DNPP/Al system however ends here. When ITO/DN3PP/Al is subjected to a positive potential sweep (~ 2.0 V), the device reverts back to the original OFF state. The change of substituent position to meta- weakens the extent of conjugation in these quadrupolar A-π-D-π-A molecules exerting constrains on the electron withdrawing ability and inductive effects of NO2 groups. Consequentially, the restricted inter-/intra- molecular interactions allows relatively shallower trap sites at the terminal ends as supported by the UV-Visible studies in the OFF and ON states (Figure-6(b)). This is also quite evident from the DFT studies (ESP and FMO isocontours, Table 2). Nevertheless, unlike the ITO/DNPP/Al device which behaved as permanent nonvolatile memory due to deep traps, the DN3PP device was observed to display a metastable non-volatile memory possibly because of the inherently higher dipoles in these molecules.42 The shallower traps permits the device to display the prospects of Write, Read, Erase and Read (WRER) cycles which is typical of FLASH memory devices.

Conclusions In summary, we report for the first time the amphoteric redox (bipolar) behavior in fused pyrrole systems and demonstrate their possibility to function as single molecule organic resistive memory devices. A family of substituted tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole derivatives were synthesized and evaluated for their optical and electrochemical properties. The structure-property relationship assessed suggests a very strong dependence on the nature of the terminal groups. Increase in electron withdrawing ability alters the extent of intra-/inter- molecular charge transfer resulting in progressive red shift in the electronic absorption spectra. A particularly pronounced bathochromic shift for the pNO2 derivative emphasizes the role of quinoidal contribution. Electrochemical studies show a noticeable -18ACS Paragon Plus Environment


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shift in the onset of oxidation towards higher potentials with increased inductive strength and resonance in the terminal substituents. It is observed that the p- and m-dinitro derivatives display bipolarity within the same molecule. The results are appropriately supported with detailed theoretical simulations. This unusual observation of amphoteric redox characteristics in fused pyrrole systems was exploited to develop single molecule organic resistive memory devices. Flexible devices fabricated using ITO-PET and Aluminum as the active electrodes, i.e. ITO/DNPP/Al and ITO/DN3PP/Al validates the operational feasibility. As exemplified, the change in peripheral substituent position alters the device behavior significantly. Devices with DNPP as the active layer displayed permanent Write Once Read Many times (WORM) memory, while its structural isomer DN3PP exhibits rewriteable flash memory. It is realized that the redox energy levels suitably aligned with the work-function of active electrodes favors better charge injection thereby contributing to a higher ON/OFF ratio. This is quite evident in the J-V plots for the p-NO2 derivatives, which shows >104ordersof change in current density than its meta- counterpart. A plausible mechanism has been discussed in detail aided with experimental evidences, DFT calculations and appropriate physical models responsible for the observed memory effect. Rational interpretation of these findings strongly suggest that the choice, and strategic positioning of terminal substituents can significantly influence the nature of "charge traps" as well as inter-molecular interactions. To the best of our knowledge, structural isomers demonstrating such contrasting memory behavior is undocumented till date. We believe this will generate considerable interest in the scientific community on fused pyrrole systems to customize the molecular architecture and optimize the device performance via smart chemistry approach. Acknowledgements. The authors acknowledge XII-FYP project: M2D (CSC-0134), Council of Scientific and Industrial Research (CSIR, New Delhi, India)and CSIR-IICT, Hyderabad for the research grant and support. RK also acknowledges RMIT for the doctoral research fellowship. The authors sincerely appreciate the encouragement and considerable help received from Dr. S.V. Manorama, Dr. -19ACS Paragon Plus Environment

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K.V.S.N. Raju, and Dr. S.P. Singh during the course of this investigation. The authors declare no competing financial interest. Supporting Information. The experimental details on synthesis, structural characterization, device fabrication protocols, additional experimental data on spectroscopy, molecular simulation, thermogravimetry, cyclic voltammetry, comprehensive I-V studies as a function of thickness/area, statistical reproducibility, impedance analysis, and FESEM images are provided as supplementary information. This material is available free of charge via the Internet at http://pubs.acs.org.



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(39) Ling, Q. D.; Liaw, D. J.; Teo, E. Y. H.; Zhu, C.; Chan, D. S. H.; Kang, E. T.; Neoh, K. G. Polymer Memories: Bistable Electrical Switching and Device Performance. Polymer (Guildf). 2007, 48, 5182–5201. (40) Zhou, F.; He, J.-H.; Liu, Q.; Gu, P.-Y.; Li, H.; Xu, G.-Q.; Xu, Q.-F.; Lu, J.-M. Tuning Memory Performances from WORM to Flash or DRAM by Structural Tailoring with Different Donor Moieties. J. Mater. Chem. C 2014, 2, 7674-7680. (41) Cui, B.-B.; Mao, Z.; Chen, Y.; Zhong, Y.-W.; Yu, G.; Zhan, C.; Yao, J. Tuning of Resistive Memory Switching in Electropolymerized Metallopolymeric Films. Chem. Sci. 2015, 6, 1308–1315. (42) Chen, C.-J.; Yen, H.-J.; Chen, W.-C.; Liou, G.-S. Resistive Switching Non-Volatile and Volatile Memory Behavior of Aromatic Polyimides with Various Electron-Withdrawing Moieties. J. Mater. Chem. 2012, 22, 14085-14093.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 28 of 36

Table 1. Summary of the estimated optical and electrochemical properties for the synthesized tetra-aryl 1,4dihydropyrrolo[3,2-b]pyrrole derivatives assessed using UV-Vis spectroscopy and cyclic voltammetry.

Oxidation (V)a

Reduction (V)a

λmaxb

λemib

e

E

E d

HOMO (eV) c

LUMO

(eV)f

(eV)c

Solution

film

447

398

405

3.06

-

-4.88

1.82

-

360

428

400

411

3.01

-

-5.28

2.27

-

-1.05

361

423

405

420

2.95

1.98

-5.43

2.48

-3.49

-

-

407

467

445

458

2.71

-

-5.45

2.74

-

-0.79

-1.02

479

603

538

599

2.07

1.88

-5.49

3.42

-3.61

/

/

DAPP

0.48

0.54

-

-

359

DCLPP

0.88

0.98

-

-

DN3PP

1.03

1.22

-0.91

DCNPP

1.05

1.18

DNPP

1.09

1.21

Vs Ag/AgCl in anhydrous CHCl2 with 0.1M TBAP as supporting electrolyte at a scan rate of 50 mV.s-1 b Electronic absorption spectra recorded in dilute THF solutions; Wavelength in nm c HOMO and LUMO energy levels calculated from cyclic voltammetry using Ferrocene/Ferrocenium redox couple as an external standard d E = HOMOEC- LUMO e Calculated from the of thin films spun casted onto quartz substrates ( = 1240/ ) a

f

LUMO

LUMO calculated from the difference between HOMOEC and E -27-

ACS Paragon Plus Environment

Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2.Calculated Molecular orbitals, their isocontours and Electrostatic Potential (ESP) for the synthesized tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole derivatives at DFT-B3LYP/6-31G (d,p) level.

ESP

LUMO +2

LUMO +1

LUMO

HOMO

HOMO -1

Dipole moment

2.1137 D

0.0100 D

1.3294 D

5.9782 D

1.3929 D

DAPP

DCLPP

DCNPP

DN3PP

DNPP



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

Figure 1.Illustration of the terminal substituted tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole derivatives


Solution

1.2

DAPP DClPP DCNPP DN3PP DNPP

(c)

DNPP -2

0.8

Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current Density, J (mA.cm )

Page 31 of 36

(a)

0.4

0.0

300

1.2

400

500

600

700

DAPP DClPP DCNPP DN3PP DNPP

Thin-film

0.8

(b)

0.4

DN3PP

DCNPP DClPP DAPP -2

0.25 mA.cm

TBAPF6-DCM

0.0 300

400

500

600

700

2

1

Wavelength (nm)

0

-1

-2

V vs Ag/AgCl

(a) Figure 2. Electronic absorption spectra of compounds: (a) in THF solution (10µM); (b) as thin films spin coated on quartz substrates;(c) cyclic voltammetry of DHPP derivatives obtained in anhydrous DCM with 0.1 M TBAPF6 as supporting electrolyte at a scan rate of 50 mV.s-1.



2,3

-4

4,5

Curent Density, J (A.cm )

-5

ON "1"

Active layer ITO-PET

-6

10

Fabrication of Memory Devices

(a)

(b)

-7 -2

Curent Density, J (A.cm )

10

-8

10

1

-9

10

Sweep 1 Sweep 2 Sweep 3 (25min) Sweep 4 Sweep 5

OFF "0"

-10

-3

-2

-1 0 1 Voltage (V)

(d)

-3

-4

10

-5

10

2

40

-5

10

-7

10

-9

10

3

3

10

ON State @ -1 V

30

(e)

20 10 0

100.0µ 200.0µ 300.0µ

-6

10

-7

Frequency (%)

10

-8

10

-9

10

-4

-3

-2

-1

0

40

(f)

20 0

3.010E-04

-1

10

2.260E-04 -4

1.510E-04

10

7.600E-05

-7

10

1.000E-06

-10

(g) ON

10

0 1.0

5 1.2

0 1.5

5 1.7

0 2.0

8.040E-08 6.045E-08

-1

4.050E-08

10

2.055E-08

-4

10

6.000E-10 -7

10

(h) OFF

-10

10

0 1.0

0.0

30.0n

60.0n

90.0n

5 1.2

0 1.5

1.2

5 1.7

2.0

Current Density, J (A.cm-2)

1.6

Voltage (V)

OFF State@ -0.8V

6

10

4 2.

-5

60

5

1.8


4

10 10 Retention Time (s)

1 2.

0.0

ON OFF

(c)

1.2


10

Frequency (%)

10

-3

10

Current Density, J(A.cm-2)

-2

10

I-V

Al

10

1.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

Figure 3. (a) Typical Current-Voltage (J-V) curves of a ITO/DNPP/Al device exhibiting characteristic WORM behavior; (b)Cartoon representation of the device architecture with insets showing an image of the actual test cell fabricated along with a birds-eye view of the cross-section under FESEM; (c) Retention test at a read voltage of -2.0 V demonstrates appreciable stability under the electrical stress. A total of 21 individual devices were studied to confirm reproducibility: (d) Averaged J-V curves obtained for all the devices are plotted with standard deviation (±1σ) at each voltage; Histograms for current density at (e) ON and (f) OFF states; Spatial current distribution for (g) ON and (h) OFF states, respectively.


Page 33 of 36

Current (µA)

-4

Potential (V)

10

Erase -5

Read

2

2

40.0µ

0

0.0

-2

40.0µ 0.0

-40.0µ -40.0µ

(b)

-4

5

10

-80.0µ

15

3

10

-5

-6

10

-7

10

10

Retention @ -1.0V -8

10

(a)

-9

-9

10

-4

-2

0

2

4

200

300

400

500

(e)

1

10

Device 1 Device 5 Device 10 Device 12 Device 15

-1

10

(d)

10

100

Number of Cycles

Write

-8

0

Time (s)

2

J (A/cm ) 10

-7

10

(c)

-80.0µ

100ms

0

-6

10

Read

80.0µ

ON/OFF Ratio

Curent Density, J (A/cm )

10

80.0µ

100ms

4

Device 2 Device 7 Device 11 Device 14

-3

2

8x10

Voltage (V)

3

9.0x10

10

4

1.2x10

0

200 400 500 Cycle Number

Time (s)

7.380E-05

(g)

20 10 0

0.0

20.0µ

40.0µ

60.0µ

-7

Frequency (%)

10

-8

10

0

2

4

(h)

20 0

0.0

2.0µ 4.0µ 6.0µ 8.0µ 10.0µ

(i) ON

-10

10

-13

10.00 1

5 1.2

0 1.5

5 1.7

0 2.0

9.100E-06 6.850E-06 4.600E-06

-1

10

2.350E-06 -4

10

1.000E-07

-7

(j) OFF

10

-10

10

-13

10 0 1.0

5 1.2

-2

Current Density, J(A.cm )

0 1.5

5 1.7

0 1.5 5 1.2 0 1.0

Voltage (V)

3.800E-06 -7

10

50 2.

-2

OFF State @ -1.5V

40

10

5 2.2 0 2.0 5 1.7

-4

60

2.130E-05

-4

80.0µ -2

Current Density, J(A.cm )

-6

10

3.880E-05

10

Current Density, J(A.cm-2)

30

4 2. 2.1


-5

10

Frequency (%)

5.630E-05

(f)

-4

10

-1

ON State @ -2V

1.8 1.5 1.2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Representative J-V characteristics of ITO/DN3PP/Al device depicting the Write-ReadErase-Read (WRER) FLASH memory behavior. (b) Dual-axis plots showcasing the applied potential pulse (V) and the generated current response (µA) as a function of time (s) over a few WRER cycles. A dwell time of 100 ms was maintained between each -ve/+ve potential sweep. (c) A typical endurance test carried on an ITO/DN3PP/Al device followed over 500 WRER cycles. Similar studies were carried out on at least 7 devices taken in random. (d) Retention test performed at -1.0 V under ambient conditions. (e) The plot summarizes the ON/OFF ratio as a function cycle number on nine ITO/DN3PP/Al devices. A total of 24 individual devices were studied to confirm reproducibility: (f) Averaged J-V curves obtained for all the devices are plotted with standard deviation (±1σ) at each voltage; Histograms for current density at (g) ON and (h) OFF states; 3D-contour plots of current distribution for (i) ON and (j) OFF states, respectively. -32ACS Paragon Plus Environment


-9

6.0x10

-2

Current Density, J (A.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 34 of 36

-9

4.0x10

OFF State Experimental Data I α V, Ohmic Fit 2 I α V , SCLC Fit

-9

2.0x10

-5

3.0x10

-5

2.0x10

-0.4

-0.6

-0.8

ON State Experimental Data I α V, Ohmic Fit

-5

1.0x10

(c)

-0.4

-0.6

-0.8

Voltage (V)

Figure 5.(a)Representation of energy levels of the tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole derivatives along with the workfunction (Φ) of the active electrodes; (b) A graphic illustration of the plausible charge injection mechanism, carriers involved and status of the traps during device operation at various stages of the applied bias; (c) Potential region (zoomed in) depicting the experimental data fitted with theoretical models for OFF and ON states prior to and after switching.

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0.6

0.20 ON OFF

ON OFF

0.10 0.05

(a)

0.00

Absorbance

0.15

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

300

0.2

350

400

(b)

0.0 300

400

500

600

700

Wavelength (nm)

800

300

400

500

600

700

800

Wavelength (nm)

Figure 6. Representative UV-Visible absorbance plots of (a) an ITO/DNPP/ITO device and (b) an ITO/DN3PP/ITO device in the OFF and ON states respectively. The signature broadening and red-shift are clearly noticeable in the ON states signifying the formation of a field induced charge transfer interactions (at ca. 360-400 nm) and formation of an intra-molecular charge separated state (at ca. 600-700 nm).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical Abstract (TOC)


Page 36 of 36

Quadrupolar (A-Ï-D-Ï-A) - ACS Publications - American Chemical

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