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Triggering WORM/SRAM Memory Conversion in a Porphyrinated Polyimide via Zn Complexation as the Internal Electrode Qudrat Ullah Khan, Nanfang Jia, Guofeng Tian, Shengli Qi, and Dezhen Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01732 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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The Journal of Physical Chemistry

Triggering WORM/SRAM Memory Conversion in a Porphyrinated Polyimide via Zn Complexation as the Internal Electrode Qudrat Ullah Khan, Nanfang Jia, Guofeng Tian, Shengli Qi*, and Dezhen Wu. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China; Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, Jiangsu, China. *E-mail: [email protected]. Phone: +86-10-6442-4654.

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The Journal of Physical Chemistry 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

Abstract We design two novel solution processable polyimides (PIs), NH-Por-6FDA and Zn-Por6FDA, with 5,15-bis(4,-aminophenyl)-10,20-diphenylporphyrin (trans-DATPP) (electron donor) and 4,4’-(hexafluoroisoprpoylidine)diphthalic anhydride (6FDA) (electron acceptor) as the building blocks for polymer memory applications. The chemical structures of the two polymers are mostly identical with the only difference lying in the zinc ion (Zn2+) insertion into the porphyrin core in the Zn-Por-6FDA. Electrical characterization indicates that the NH-Por-6FDA possesses bi-directional nonvolatile write-once read many times (WORM) memory behavior, while the Zn-Por-6FDA shows vastly different volatile static random access memory (SRAM) behavior. Both polymer memory devices show high ON/OFF current ratio up to 106 and exhibit excellent long term operation stability in 108 read cycles and retention time of 4000 s with no current degradation. The charge transfer (CT) and function of the donor/acceptor moiety in the polymers related with the electrical switching effect are elucidated on the basis of optical, electrochemical measurement and quantum simulation results. The inserted zinc ion in the porphyrin is suggested to form an internal electrode and act as a bridge during the electronic transition process, which facilitates both the CT and back CT, consequently triggering the WORM/SRAM conversion upon Zn complexation. The results observed here indicate the significance of metal-complexation on the memory effects, and will attract the attention of the researchers to use noble transition metals for the suitable expecting memory devices.


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1. Introduction Polymeric memories are promising alternative to conventional inorganic silicon-based electronics for the next-generation information storage, since they provide the advantages of easy processing, flexibility, light weight, low cost, good scalability and 3D-stacking capability, which enables the fabrication of flexible devices with the characteristics of ultrafast write/read speed, mass data storage capacity, low power consumption, long-term stability, superior shockresistance and durability.1-4 Entirely different from the traditional silicon memory cell, polymeric memory stores information on the basis of electrical bistability, i.e., the high (ON) and low (OFF) current response of the device to an external applied voltage.5-7 To achieve electrical bistability, aromatic polyimides (PIs) containing electron donor (D) and acceptor (A) within a single macromolecular chain have nowadays been the major topic of interest among researchers.1,

8-10

In addition to the intrinsic merits such as high-temperature

stability, structure diversity, and chemical resistance,11 these D-A PIs could readily form conjugated structures for charge transfer (CT), which then contribute to electronic transitions between the ground and excited states through induced CT complex, resulting in desirable memory effect.8, 12-13 The memory type is determined by the stability of the formed CT complex, which could be tuned by altering the electron pull-push effect between D and A.14 To this end, various electron-donating species with different strength, including triphenylamine9, carbazole19-21, ferrocene22-23, oxadiazole12,

16

, pyrene24-25 and anthracene13,

26

15-18

,

, have been

utilized and polymerized into the PI chain as electron donor. The synthesized PIs witness the achievement of memory behaviors from the volatile dynamic random access memory (DRAM) and static random access memory (SRAM) to the nonvolatile flash and write once read many times memory (WORM) upon structural variation, revealing the significance of the electron-



donating moieties.

8, 10, 27

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However, due to the limited variety of D species, the design of

electroactive PIs with wanted memory characteristic is severely restricted. As a result, great efforts were made on decorating the typical D species via chemical modification, with the purpose of realizing memory alteration through tiny structure variation. The method utilized for D modification includes structural isomerization13, 27-28, combining different D species together as a new entity16,

29-30

, and introducing flexible linkages

31-33

or substitutions9,

34-35

. Some

successes have been achieved via this strategy. Whereas, chemical decorations of the D species are jobs of great difficulty and don’t always work, consequently resulting in limited regulation of the memory behaviors through the laborious structural tailoring. Thus, seeking novel D species and developing electroactive D-A PIs that provide favorable charge transport and controlled memory features remain a priority in this field of research. Porphyrin, as a typical organic semiconductor and photo-responsive unit, plays a vital role in the field of electronics and photonics.36-38 The porphyrin ring bears the ability to coordinate with over 56 different metals and therefore could be customized to realize diversified desirable functions by varying the central metal species.39-42 Keeping in mind of the highly π-π conjugated structure, the unique electron-rich ring and the excellent molar extinction coefficient, porphyrin and its derivatives have nowadays been widely explored for organic-field-effect transistors43, organic photovoltaic cell44-45, organic light emitting diodes46, and sensors47-48. And, a recent pioneering work has demonstrated the possibility of using porphyrin unit as the electrondonating species to realize polymer memory.49 However, literature survey indicates that works on this point were rare and further explorations are necessary to develop diversified porphyrinated PIs with controlled memory effects and elucidate the relationship between the polymer structure and device characteristics.


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In the present study, two novel porphyrin-containing polyimides abbreviated as NH-Por6FDA and Zn-Por-6FDA were designed and synthesized for memory applications. Figure 1 illustrates

the

chemical

structures

of

the

two

porphyrinated

PIs.

The

4,4’-

(hexafluoroisoprpoylidine)diphthalic anhydride (6FDA) was selected to work as the electronaccepting unit, and the 5,15-bis(4,-aminophenyl)-10,20-diphenylporphyrin (trans-DATPP) / 5,15-bis(4,-aminophenyl)-10,20-diphenyl-Zn-porphyrin (Zn-trans-DATPP) were designed to function as the electron-donating units. The structures of NH-Por-6FDA and Zn-Por-6FDA are basically identical with the only difference lying in the insertion of a Zn ion in the porphyrin ring in the Zn-Por-6FDA PI. The Zn ion was chosen to insert into the porphyrin ring since we believe that metal complexation would probably enhance the electron delocalization of the porphyrin ring. Moreover, it is supposed that the inserted metal ion might provide a channel and function as a bridge for electronic transition, which possibly lead to varied electrical performance and distinctive memory characteristics. Experimental results demonstrate that the insertion of Zn ion has triggered the electrical memory conversion of the porphyrinated PI from the nonvolatile WORM to the volatile SRAM, suggesting the significant role of the metal ion. Optical, electrochemical measurement and molecular simulation were carried out to clarify the electronic transition process and the memory mechanism. This work might pave the way for researchers to replace or insert the porphyrin core with a suitable metal atom for desirable application in various memory devices. 2. Experimental 2.1 Materials. The trans-DATPP was synthesized in our lab by nitration of the tetraphenylporphyrin (TPP) followed by reduction with hydrazine monohydrate (N2H4·H2O) on Pd/C catalyst. The Zn-trans5 ACS Paragon Plus Environment


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DATPP was subsequently obtained by dropwise addition of zinc acetate / methanol solution to the chloroform solution of trans-DATPP. Details of the synthesis and characterization are described

in

the

supporting

information

(SI)

(see

Figure

S1-S2).

4,4’-

(hexafluoroisoprpoylidine)diphthalic anhydride (6FDA) was purchased from Sigma-Aldrich and purified by vacuum sublimation. Isoquinoline and m-cresol were purchased from J&K Scientific. The m-cresol was distilled over zinc powder before use. 2.2 Synthesis of the porphyrinated polyimides via one-step strategy. The two porphyrinated polyimides, NH-Por-6FDA and Zn-Por-6FDA, were synthesized via the typical one-step imidization process, as illustrated in Figure 1. Taking NH-Por-6FDA as an example, in a three-necked round bottom flask degassed with dry N2 for 15min, equimolar amount of trans-DATPP and 6FDA were mixed and vigorously stirred in m-cresol followed by the addition of isoquinoline. After reacting at 175 oC for 24 hrs, the resulting PI solution was allowed to cool to room temperature and then poured into 400 ml methanol under constant stirring, precipitating the polymer in fiber form. The synthesized PI was then washed thoroughly with hot methanol and collected by filtration. The two porphyrinated polyimides were then employed as the active layer and processed into sandwich memory devices with the configuration of ITO | porphyrinated PI | Al (as shown in Figure 1) for further test. Device fabrication and measurements are described in detail in SI.


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Figure 1. Synthetic procedure for the NH-Por-6FDA PI and Zn-Por-6FDA PI, and the schematic diagram of the sandwich memory device 3. Results and Discussion 3.1 Polyimide Synthesis and Characterization The formation of polyimide was confirmed by FTIR spectroscopy, and the results are shown in Figure S3. The NH-Por-6FDA PI shows the characteristic IR absorption peaks (Figure S3(a)) at 3300 (N-H stretching) and 965 (N-H bending), 1778 (asymmetric stretching of imide C=O), 1722 (symmetric stretching of imide C=O), 1370 (imide C-N stretching), and 725 cm-1 (imide ring deformation). The Zn-Por-6FDA PI exhibits similar IR absorption (Figure S3b) except for the absence of the N-H vibrations at 3300 and 965 cm-1 due to Zn complexation. Chemical structures of the porphyrinated PIs are further confirmed by 1H-NMR, the results of which are shown in Figure S4 and Figure S5, respectively. NH-Por-6FDA PI (see Figure S4). 1

H-NMR (d6-DMSO, δ, ppm,): 8.94 (d, 8H), 8.89 (d, 8H,), 8.41 (d, 8H), 8.24 (s, 2H), 8.23 (d,

8H), 8.16 (d, 2H), 8.05 (d, 2H,), 7.90 (d, 8H), 7.76 (m, 12H), –2.75 (s, 2H, NH). Zn-Por-6FDA PI (see Figure S5). 1H-NMR (d6-DMSO, δ, ppm): 8.854-8.833 (d, 8H, pyrrole), 8.381 (d, 2H), 8.202 (m, 6H), 8.129 (d, 2H), 7.990 (d, 2H), 7.922 (d, 4H), 7.822 (m, 6H). Signals of the internal 7 ACS Paragon Plus Environment


free N-H protons of the porphyrin ring disappear in 1H-NMR spectrum of the Zn-Por-6FDA PI because of Zn chelation. Figure 2 shows the TGA and DSC analysis for the two porphyrinated polyimides. The thermal decomposition temperatures (Td, 90wt% residual) of the NH-Por-6FDA and Zn-Por6FDA in air were determined to be 383 and 392 oC, respectively. It is obvious that the thermal stability of the porphyrinated polyimide was enhanced after Zn complexation. Besides, DSC analysis also shows that the glass transition temperature (Tg) of the Zn-Por-6FDA polyimide was promoted to 285 oC, which is 11 oC higher as compared to the NH-Por-6FDA polyimide (274 o

C). The better thermal properties of the Zn-Por-6FDA polyimide than those of the NH-Por-

6FDA are suggested to be attributed to the better coplanar structure and the enhanced molecular (chain) rigidity after Zn complexation, as supported by the molecular simulation results shown later (in Figure 8 and Figure 9). The higher Td and Tg of the two porphyrinated PIs confirm adequate thermal stability for the electronic memory device applications.

Figure 2. (a) TGA thermograms and (b) DSC curves of the porphyrinated polyimides measured at a heating rate of 10 oC min-1. (TGA analysis was performed in air, and DSC curves were recorded under N2.) 8 ACS Paragon Plus Environment

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3.2 Memory Effects and Device Performance The electrical memory behavior of the two porphyrinated polyimides were characterized by the current-voltage (I-V) curves of the ITO | porphyrinated PI | Al sandwich devices. Figure 3 displays the I-V results of the NH-Por-6FDA based memory device, which manifests a typical nonvolatile WORM behavior. As observed in Figure 3a, the device is initially at the low conductivity state (the OFF state). During the 1st positive voltage sweep, a sharp increase in current is observed at a switching voltage of about 2.6 V, indicating the transition of the device from the low conductivity state to the high conductivity state (ON state) with an ON/OFF current ratio up to 106. After this transition, the device maintains the ON state during the followed positive scan (2nd, 4th sweep) and negative scan (3rd sweep), and could not be retrieved to its initial OFF state even after turning off the power (5th, 6th sweep), revealing its irreversible nonvolatile feature and WORM memory characteristic. To evaluate the stability of the memory effect, Figure 3b and Figure 3c display the effect of operation time and stimulus read pulse on the ON and OFF states of the memory device. Both ON and OFF states are conducted under constant voltage stress and continuous pulse cycles. No degradation in current is observed, and both the ON and OFF current response are quite stable in up to 108 continuous read pulses, demonstrating excellent stability of the memory device. WORM memory behavior was also observed when the device was operated with an initial negative voltage sweep, as depicted in Figure 3d, indicating the bi-directional feature of the NH-Por-6FDA based memory device.



Figure 3. (a) Current-voltage (I-V) characteristics of the ITO | NH-Por-6FDA | Al sandwich device with an initial positive voltage sweep. (b) Retention time of the ON and OFF states of the ITO | NH-Por-6FDA | Al sandwich device tested at 1 V under ambient condition. (c) Stimulus effect of 1 V read pulse on the ON and OFF states of the memory device. The inset shows the pulse shapes used in the measurement. (d) I-V curves of the ITO | NH-Por-6FDA | Al sandwich device with an initial negative voltage sweep.


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Figure 4. (a) I-V characteristics of the ITO | Zn-Por-6FDA | Al sandwich device. (b) The effect of the operation time on the ON and OFF states of the ITO | Zn-Por-6FDA | Al sandwich device tested at a constant stress of 1 V under ambient conditions. Figure 4 shows the I-V curves of the Zn-Por-6FDA based sandwich device. Totally different from the NH-Por-6FDA polyimide, the Zn-ion-inserted polyimide manifests a reversible volatile SRAM memory behavior, implying the significant effect of Zn in the porphyrinated polyimide. As indicated in Figure 4a, during the 1st positive scan from 0 to 5 V, the device keeps its OFF state until a threshold voltage of 3.2 V is reached. The current then abruptly increases from the initial 10-10 A to 10-4 A, turning the device to the ON state. The device steadily remains at the ON state during the continued positive and negative sweep (2nd, 3rd sweep). However, the subsequent tests show that the ON state could only be retained temporarily, and will relax to the initial OFF state voluntarily once turning off the power for over 5 min, reflecting its actually “volatile” nature. Then in the 4th sweep, the device could be switched on again, indicating its SRAM memory characteristics. Further tests show that the SRAM memory device is bidirectionally operable with comparable ON/OFF ratio and threshold voltage. A voltage scan in negative direction (7th sweep) also switches the device to the ON state at about -3.3 V. Similarly,



the ON state could only be maintained in 5 min but could be retrieved by applying a further scan (10th sweep). The I-V characteristics in Figure 4 demonstrate that the Zn-por-6FDA polyimide possesses volatile but reprogrammable and bi-directionally accessible electrical bistability, which can be used in SRAM memory device in digital information technology. 3.3 Optical Properties.

Figure 5. (a) UV-vis absorption spectra for the synthesized NH-Por-6FDA and Zn-Por-6FDA polyimides measured in DMAc solution (10 µM); (b) Fluorescence emission spectra for the NHPor-6FDA and Zn-Por-6FDA polyimides in DMAc solution (10 µM) excited at 440 nm. Figure 5a shows the UV-vis absorption spectra of the two porphyrinated polyimides. The NH-por-6FDA and Zn-Por-6FDA PIs exhibit a prominent absorption peak at 420 and 428 nm, respectively, both of which are corresponding to the π-π* transition of porphyrin electrons. It is clear that the π-π* absorption of the Zn porphyrinated PI is red shifted, indicating that the insertion of Zn ion in the porphyrin core has facilitated the π-π* electron transitions of the porphyrin, for which one reason is suggested to be due to the improved molecular co-planarity of the porphyrin ring after Zn complexation (shown later in Figure 8 and 9). However, Figure 5a shows that the onset absorption wavelength (λedge) of the Zn-Por-6FDA PI is blue shifted to 12 ACS Paragon Plus Environment

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607nm from 656nm of the NH-Por-6FDA PI, based on which the optical energy band gap (Eg) were estimated to be 2.034 and 1.890 eV, respectively, manifesting an increased energy band gap (Eg) after Zn complexation. The blue shift of the λedge of the Zn-Por-6FDA is suggested to be due to the significant Zn d π to porphyrin π* orbital interaction (metal to ligand π-back bonding).50 Similar red shift of the maximum absorption and blue shift of λedge were also observed on the UV-vis spectra of the porphyrinated PIs in solid thin film state, as shown in Figure S6. Whereas, relative to that in solution state, the absorption transitions in solid state are wholly red-shifted to higher wavelength region, implying the possible formation of polymer chains aggregation, which will facilitate charge transfer (CT) in the PI bulk when the external voltage is applied. Figure 5b shows the fluorescence (FL) spectra of the NH-Por-6FDA and Zn-Por-6FDA polyimides. It is clear that the FL emission spectra of Zn-Por-6FDA is blue shifted and exhibits shorter emission wavelength than that of the NH-Por-6FDA polyimide, confirming the enlarged Eg (i.e., S1-S0) after Zn complexation, consistent with the UV results. It is suggested that the inserted Zn ion plays a key role on this point and has significantly altered the electronic transition process. The shorter FL emission wavelength and the larger Eg of Zn-Por-6FDA indicate its higher energy barrier for charge transfer (CT) and its weaker tendency to form strong CT complexes as compared to the Zn-free NH-Por-6FDA polyimide, providing hints for its volatile memory characteristics observed in Figure 4, as will be discussed later.



3.4 Electrochemical Behaviors.

Figure 6. Cyclic voltammograms (CV) of the NH-Por-6FDA and Zn-Por-6FDA polyimides coated on an ITO electrode measured in 0.1 M n-Bu4BF4/acetonitrile solution with Ag/AgCl as reference electrode and Pt wire as counter electrode. (Scan rate: 100 mV s-1.) The electrochemical behaviors of the NH-Por-6FDA and Zn-Por-6FDA polyimides were investigated by cyclic voltammetry (CV) measurement, the results of which are shown in Figure 6 and summarized in Table 1. As observed, the NH-Por-6FDA polyimide exhibits an onset ionization potential (Eox(onset)) of 0.97 V, while the Zn-Por-6FDA polyimide exhibits an Eox(onset) of merely 0.70 V. It is clear that the onset oxidation potential of the porphyrinated polyimide is significantly reduced (~ 0.27 V degraded) after Zn complexation, indicating that the synthesized polyimide has become much easier to donate electrons after Zn chelation. This seems unreasonable since it is contradictory to the previous UV and FL results, where Zn complexation has resulted in an enlarged Eg. To this point, it could be confirmed that it must be the inserted Zn ion that plays a key role and has significantly altered the electronic transition process. The function of the Zn ion is considered to be that it acts as a bridge or an internal electrode, which provides more convenient path for charge transition and consequently facilitates


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the electron-donating process, which will be further discussed later. In addition, as displayed in Figure 6, the NH-Por-6FDA polyimide exhibits an irreversible p-doping CV behavior during the anodic scan from 0 to +2.0 V. Two distinct oxidation peaks with comparable intensity were observed at 1.375 V (peak 1) and 1.635 V (peak 2), and there is no reduction peak appearing when scanning backward from +2 to 0 V. For the Zn-Por-6FDA polyimide, during the anodic scan from 0 to +2 V, two oxidation peaks were also observed but at relatively low potentials of 0.975 V (peak 3) and 1.385 V (peak 4), respectively. And, it is noted that the oxidation peak at 1.385 V (peak 4) is significantly suppressed after Zn complexation. More importantly, two clear but moderate reduction peaks were present at 1.10 V and 0.82 V when scanning backward from +2 to 0 V, demonstrating a quasi-reversible p-doping feature of the Zn-Por-6FDA polyimide. The results shown here indicate that both the trans-DATPP and the Zn-trans-DATPP units (see Figure 1) in the two porphyrinated polyimides possess a strong tendency to donate electrons and readily function as the hole-transporting sites in the synthesized materials as anticipated. Roughly speaking, the two couples of ionization peaks observed on the CV spectra of NH-Por6FDA (peak 1 and 2) and Zn-Por-6FDA (peak 3 and 4) correspond to withdrawing electrons from HOMO and HOMO-1, respectively, as illustrated in the inset scheme in Figure 6. For the NH-Por-6FDA without Zn, ionization peak 1 and peak 2 could be principally ascribed to the electron excitation of HOMO → vacuum level and HOMO-1 → vacuum level. Due to the small deviation of energy barriers between HOMO → vacuum level and HOMO-1 → vacuum level, ionization peak 1 and peak 2 possess basically equal probability and therefore exhibit comparable intensity. Whereas, when Zn ion was encapsulated in the porphyrin cavity, it will function as an internal bridge for electron transition. Accordingly, the corresponding oxidation peak 3 and peak 4 are assignable to electron excitation of HOMO → Zn → vacuum level and



HOMO-1 → Zn → vacuum level, respectively. Thus, ionization of Zn-Por-6FDA becomes easier and occurs at relatively lower potential. Besides, since the energy barrier of HOMO → Zn is lower than that of HOMO-1 → Zn and the difference becomes not negligible in such situation, it is clear that electron transition of HOMO → Zn will have higher probability. Therefore, it is observed in Figure 6 that the ionization peak 3 becomes predominant and peak 4 is significantly suppressed. As a summary, Zn complexation has facilitated electron donating of the porphyrinated polyimide, and endows the Zn-Por-6FDA with a quasi-reversible p-doping electrochemical feature. Here, the irreversible and quasi-reversible electrochemical features of the NH-Por-6FDA and Zn-Por-6FDA polyimide are supposed to be partially responsible for the irreversible nonvolatile WORM (see Figure 3) and reversible volatile SRAM (see Figure 4) memory behaviors of the two porphyrinated polyimides.


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Table 1. Optical and electrochemical properties of the porphyrinated polyimides UV-vis (nm) Polyimides

Eox(onset) (eV) (from CV)

Eg (eV)

HOMO (eV)

HOMO-1

LUMO (eV)

Exp.a Calc.d

Exp.b Calc.d

Calcd

Exp.c Calc.d

λmax

λedge

NH-por-6FDA

420

656

0.97

1.89

2.10

-5.38

-4.73

-5.11

-3.49

-2.26

Zn-Por-6FDA

428

607

0.70

2.04

2.17

-5.11

-4.74

-5.06

-3.07

-2.57

a

Estimated from the UV-Vis absorption edge wavelength (λedge) by using the Planck equation Eg = 1240/λedge.

b

The HOMO energy levels were calculated from CV onset oxidation potential (Eox(onset)) and were referenced to ferrocene (4.8 eV

below the vacuum level). HOMO = -[(Eox(onset) - Eferrocene) + 4.8] (eV). Eferrocene is determined to be 0.38 V vs. Ag/AgCl. c

Determined from the equation: LUMO = Eg + HOMO.

d

Obtained by molecular simulation at DFT/B3LYP/6-31G(d) theory level, as displayed in Figure 7.



3.5 Molecular Simulation and Electrical Switching Mechanism.

Figure 7. The calculated molecular orbitals (MOs) and corresponding energy levels of the (a) NH-Por-6FDA and (b) Zn-Por-6FDA polyimides. The arrows and illustrations in the inset show the charge transition (CT) process in the two porphyrinated polyimides. To get deep insight into the entirely different electrical characteristics of the NH-Por-6FDA PI and Zn-Por-6FDA PI, molecular simulation on the basic unit was carried out at DFT/B3LYP/6-31G(d) level. Figure 7 shows the charge density iso-surfaces and the energy 18 ACS Paragon Plus Environment

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levels of the model compounds. As indicated, for both PIs, the HOMO is mainly located on the porphyrin moieties, whereas the LUMO is located on the phthalimides unit, confirming the electron donor nature (D) of the porphyrin units and the electron acceptor nature (A) of the 6FDA unit. Besides, it is observed that, in both PIs, the LUMO and LUMO+1 are basically degenerate and act as equal sites for electron-withdrawing. The LUMO+2 and LUMO+3 are also degenerate and totally located on the porphyrin moieties. In addition, it should be noted that the HOMO-1 orbital overlaps with the HOMO, which is also fully confined on the porphyrin units, and more importantly, does not show significant energy deviation from the HOMO, implying its high capability to donate electrons during the electron excitation process. And, the calculated HOMO-1 energy levels (-5.13 eV, -5.05 eV) are comparable to the experimentally-determined HOMO levels (-5.38 eV, -5.11 eV) of NH-Por-6FDA and Zn-Por-6FDA, measured by UV and cyclic voltammetry (see Table 1). It is suggested that, when the external electric field reaches the threshold voltage, some of the electrons at the HOMO-1 and HOMO gain sufficient energy and transit to the LUMO+2 and LUMO+3 within D to form an excited state. According to FrankCodon theory, this transition is supposed to occur with the highest probability since HOMO-1, HOMO, LUMO+2, and LUMO+3 are all constrained within D and therefore possess the largest orbital overlapping. Excitation of D would decrease the oxidation potential and consequently lead to enhanced intra- or inter-molecular charge transfer (CT) at excited state. CT then occurs indirectly from LUMO+2 and LUMO+3 of D to LUMO+1 and LUMO of A, forming a conductive CT complex. Direct CT from the HOMO-1 and HOMO of D to the LUMO and LUMO+1 of A could also occur but at low probability. Electronic absorption spectra in Figure 2a of the porphyrinated PIs provide strong evidences for the above CT process. The predominant absorption maximum at ~420 nm (2.95 eV) and ~428 nm (2.90 eV) agree well with the HOMO-



1 → LUMO+2/3 transition of the NH-Por-6FDA PI and Zn-Por-6FDA PI (ca. 3.01 eV). Conduction path was then formed along the CT complexes via charge transfer interaction, consequently turning the device to a high-conductivity state (ON state). The results in Figure 7 show that the MOs and energy levels of the two porphyrinated PIs are basically identical and no distinct difference could be differentiated, indicating that the inserted Zn ion has limited influence on the electronic structure of the Zn-Por-6FDA PI. However, the CT process for the Zn-Por-6FDA PI could not be identical with that for the NHPor-6FDA PI, in terms of the vastly different I-V, UV, and CV features observed previously. Locating in the center of the electron-donating porphyrin ring, the encapsulated Zn ion (Zn2+) must play a significant role in the charge transition process. As in Figure 7b, it is noted that the inserted Zn2+ is within D and surrounded simultaneously by the HOMO-1, HOMO, LUMO+2 and LUMO+3. Thus, when the electrons are excited from HOMO-1 or HOMO within D, they will firstly transit to the Zn2+ due to its electron-deficient nature and the low energy barrier, forming a Zn bridge at -4.3 eV (i.e., work function of Zn), which subsequently works as an internal electrode and transports the trapped electron further from Zn to LUOMO+2/3 to form the excited state. Broadly speaking, formation of the internal Zn electrode facilitates the electronic transition and could be considered as promoting the HOMO and HOMO-1 orbitals to a higher level, which accounts for the significantly reduced ionization potential in CV and the red shift of the main absorption in UV. However, it should be reminded that the generated internal Zn electrode will also facilitate the back CT process, therefore leading to the dissociation of the CT complex and returning the device to the initial OFF state, interpreting the consequent volatile SRAM memory behavior of the Zn-Por-6FDA PI as compared to the nonvolatile WORM behavior of the NH-Por-6FDA PI.


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Figure 8. Optimized geometry and dihedral angles of the (a) NH-Por-6FDA and (b) Zn-Por6FDA polyimides

Figure 9. Electrostatic potential (ESP) maps of the (a) NH-Por-6FDA and (b) Zn-Por-6FDA polyimides Besides, Zn complexation also results in more co-planar molecular geometry and enhanced electron delocalization. As shown in Figure 8, the dihedral angle (θ) between porphyrin ring and 6FDA is reduced from 43o to 39o after Zn complexation, manifesting better co-planar structure of the Zn-Por-6FDA PI. Further, the ESP maps in Figure 9 indicate clearly that the inserted Zn ion has significantly enhanced the electron delocalization of the porphyrin structure, generating more uniform ESP distribution in the Zn-Por-6FDA PI. The better coplanar structure and the enhanced electron delocalization both favor the occurring of back CT12, volatile feature of the Zn-Por-6FDA memory.


31, 51

, further rationalizing the


4. Conclusions Two porphyrinated polyimides, NH-Por-6FDA and Zn-Por-6FDA, were successfully synthesized in this work for polymeric memory applications. The NH-Por-6FDA PI and Zn-Por6FDA PI have been demonstrated to exhibit irreversible nonvolatile WORM and reversible volatile SRAM memory behaviors, respectively. The electronic transition and switching mechanism were investigated by molecular simulation. The complexation of Zn ion in the porphyrin core has resulted in more co-planar molecular geometry and enhanced electron delocalization. More importantly, the inserted Zn forms an internal electrode and functions as a bridge during the electronic transition process, which facilitates both the forth and back CT, consequently triggering the WORM/SRAM memory conversion between the NH-Por-6FDA PI and Zn-Por-6FDA PI. Besides, Zn complexation has further enhanced the excellent thermal stability of the porphyrinated PI, which ensures adequate heat-resistance stability for future electronic device applications. This work demonstrates the availability of porphyrinated PIs in electronic memory and verifies the significance of metal complexation on the memory effects, which would probably ignite the interest of researchers to prepare desirable memories by using porphyrins chelated with different metal species.

Associated content Supporting information: Synthetic route of the monomer, 1H NMR spectra and FTIR spectra of the diamine and polyimides, instruments and measurements, fabrication of sandwich memory devices, GPC results, UV-vis spectra of NH-Por-6FDA and Zn-Por-6FDA film, SRAM properties of a batch of


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Zn-Por-6FDA based memory devices, experimental and fitted data of the OFF state of two devices, XRD patterns, and AFM results. Acknowledgement The authors sincerely appreciate the financial support from the National Key Basic Research Program of China (973 Program, 2014CB643604), National Natural Science Foundation of China (51673017), the Foundation Research Project of Jiangsu (Natural Science Foundation for Distinguished Young Scholars, BK20140006), Changzhou Sci & Tech Program (CZ20150001) and the support from CHEMCLOUDCOMPUTING@BUCT.



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