Effects of Pyrazine Derivatives and Substituted Positions on the Photoelectric Properties and Electromemory Performance of D–A–D Series Compounds

materials

Article Effects of Pyrazine Derivatives and Substituted Positions on the Photoelectric Properties and Electromemory Performance of D–A–D Series Compounds

Xuejing Song 1,2, Lingqian Kong 3, Hongmei Du 2, Xiangyu Li 2, Hanlin Feng 2, Jinsheng Zhao 2,* and Yu Xie 4,*

1 College of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan 523100, China; [email protected] 2 Department of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China; [email protected] (H.D.); [email protected] (X.L.); [email protected] (H.F.) 3 Dongchang College, Liaocheng University, Liaocheng 252059, China; [email protected] 4 Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China * Correspondence: [email protected] (J.Z.); [email protected] (Y.X.)

Received: 23 September 2018; Accepted: 17 October 2018; Published: 22 October 2018

Abstract: Pyrazine derivatives quinoxaline and pyridopyrazine were selected as the acceptors, and benzocarbazole was used as the donor to synthesize four different D–A–D compounds. The results showed that 2,3-bis(decyloxy)pyridine[3,4-b]pyrazine (DPP) exhibited stronger electron-withdrawing ability than that of 2,3-bis(decyloxy)quinoxaline (DPx), because DPP possesses one more nitrogen (N) atom, resulting in a red-shift of the intramolecular charge transfer (ICT) absorption bands and fluorescent emission spectra for compounds with DPP as the acceptor compared with those that use DPx as the acceptor. The band-gap energy (Eg) of the four D–A–D compounds were 2.82 eV, 2.70 eV, 2.48 eV, and 2.62 eV, respectively, for BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP. The solvatochromic effect was insignificant when the four compounds were in the ground state, which became significant in an excited state. With increasing solvent polarity, a 30–43 nm red shift was observed in the emissive spectra of the compounds. The thermal decomposition temperatures of the four compounds between 436 and 453 ◦C had very high thermal stability. Resistor-type memory devices based on BPC-2DPx and BPC-2DPP were fabricated in a simple sandwich configuration, Al/BPC-2DPx/ITO or Al/BPC-2DPP/ITO. The two devices showed a binary non-volatile flash memory, with lower threshold voltages and better repeatability.

Keywords: pyrazine; quinoxaline; benzocarbazole; donor-acceptor-donor; photoelectric properties; electromemory

1. Introduction Organic optoelectronic materials are mainly divided into two major categories according to the size of their molecules: small organic molecule materials and polymer materials. Small organic molecule materials have received considerable attention because of their advantages in determined molecular mass, easy puriﬁcation, high ﬂuorescent quantum yield, high electroluminescence, and low driving voltage [1–4]. Researchers have developed a wide variety of organic optoelectronic materials whose performance can be adjusted through molecular design [5–14]. For example, their solubility can be adjusted by changing the length of the alkyl substituents [4–6], and their

Materials 2018, 11, 2063; doi:10.3390/ma11102063 www.mdpi.com/journal/materials Materials 2018, 11, 2063 2 of 20 absorption spectrum, carrier mobility, and forbidden band width can be adjusted by changing the type of the substituent [7–10] and the link mode [11–14]. The combination of an electron-rich unit, i.e., an electron donor (D), with electron-donating ability, and an electron-deﬁcient unit, i.e., an electron acceptor (A), with electron-withdrawing ability to construct a D–A–D structure is referred as the D–A strategy [11–17]. In recent years, the construction of new optoelectronic materials using the D–A method by selecting different donors and acceptors has become an important research topic. The combination of different donors and acceptors resulted in a large number of molecules with different structures, which expands the range of spectral absorption to satisfy the needs of different applications. Intramolecular charge transfer (ICT) occurs in a D–A molecule, and it affects the molecule’s highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and band gap, thereby affecting various optoelectronic properties [11–19]. Combining D or A units with similar structures, changing the heteroatoms, or changing the substituents can affect the properties of D–A-type materials [7–10,20,21]. Commonly used donor units are benzothiophene [21], fluorene [22], carbazole [23], triphenylamine, and cyclopentadithiophene [24], and so on. Commonly used acceptor units are mainly heterocyclic rings containing O, N, or S, such as pyrrole [25], pyridine [26], benzopyrazine [9–13], thienopyrazine [27], pyridopyrazine [28], benzothiadiazole [29], benzotriazole [30], and phenanthroline [31] et al. Pyrazine is a diazine with two N atoms symmetrically positioned at the one and four positions. Pyrazine has a higher electron affinity than pyridine, which contains only one N atom. Thus, it is more suitable as an acceptor [25]. In addition, pyrazine is very easy to modify, thereby regulating its electron-withdrawing ability. For example, different substituents, such as alkyl chains, benzene rings [25], and thiophene rings [10,25], can be introduced at the two and three positions of pyrazine, and the pyrazine ring can be fused with an aromatic ring to obtain quinoxaline or pyridopyrazine [7–13,32,33]. Pyridine, quinoxaline, and pyridopyrazine contain one N atom, two N atoms, and three N atoms, respectively, and their electron-withdrawing ability gradually increase in this order (e.g., reduction potential versus Hg pool: −0.86 V (pyridopyrazine) > −1.10 V (quinoxaline) > −2.20 V (pyridine)) [33]. Researchers [22,25,32] have compared the optical and electrochemical properties of D–A photopolymers using pyridopyrazine, pyridine, and quinoxaline as the acceptors. The results showed that the ultraviolet-visible (UV-vis) ICT absorption band of the polymers using pyridopyrazine as the acceptor was shifted 30 nm in the longer wavelength direction compared with the polymer using pyridine or quinoxaline as the acceptor. The reason is that pyridopyrazine has a stronger electron-withdrawing ability than pyridine or quinoxaline. Gratzel et al. [7] constructed a variety of D–A structural compounds with pyrazine derivatives as the acceptor and compared the effect of electron-withdrawing ability of the acceptors, the substituents of acceptors, and the link mode between the donor and the acceptors on the optoelectronic properties of the obtained material. Park et al. [14] designed and synthesized two organic sensitizers with horizontally and vertically conjugated D–A–D structures using quinoxaline as the acceptor and triphenylamine as the donor. Dye-sensitized solar cells with high optoelectronic conversion efﬁciency (3.3% and 5.56% for horizontally and vertically conjugated structures, respectively) were obtained. Kai Pei et al. [8] studied D–A–π–A compounds with quinoxaline as the auxiliary acceptor, and indoline and triphenylamine as donors. The introduction of the auxiliary acceptor quinoxaline optimizes the energy level of the compound, expands the absorption range, promotes the distribution of electrons on the donor, and improves the photostability of the compound. Xuefeng Lu et al. [13] synthesized a series of D–A–D type compounds using pyrazine fused with different numbers of aromatic or thiophene rings as the acceptors, and using triphenylamine as the donor to achieve a controlled charge transfer. In the design and synthesis of D–A-type optoelectronic materials, the effects of the donor and acceptor types, link sites, and substituents on the performance of the obtained optoelectronic materials have been intensively studied. However, the effect of the substitution position of the acceptor on the donor on the performance of photoelectric active compounds has rarely been reported. There are no reports of a D–A–D-type optoelectronic compounds with benzocarbazole as the donor and quinoxaline Materials 2018, 11, x FOR PEER REVIEW 3 of 20 Materials 2018, 11, 2063 3 of 20 D–A–D-type structure were designed and synthesized using benzocarbazole as the donor and quinoxalineor pyridopyrazine or pyridopyrazine as the acceptor. as the In acceptor this study, (see four Figure compounds 1): 4,7-bis(9-phenyl-9 with the D–A–D-typeH-carbazol-2-yl)- structure were 2,3-bis(decyloxy)quinoxalinedesigned and synthesized using(BPC-2DPx, benzocarbazole the acceptor as the quinoxaline donor and quinoxaline substituted or at pyridopyrazine the 2-position as of the donorthe benzocarbazole); acceptor (see Figure 4.7-bis(9-phenyl-91): 4,7-bis(9-phenyl-9H-carbazol-3-yl)-2,3-bis(decyloxy)quinoxalineH-carbazol-2-yl)- 2,3-bis(decyloxy)quinoxaline (BPC-2DPx, (BPC-3DPx, the acceptorthe acceptor quinoxaline quinoxaline substituted substituted at at the the 3-positi 2-positionon of the donor benzocarbazole);benzocarbazole); 4.7-bis(9-phenyl- 4,7-bis(9-phenyl- 9H-carbazol-2-yl)-2,3-bis(decyloxy)pyridine9H-carbazol-3-yl)-2,3-bis(decyloxy)quinoxaline (BPC-3DPx,[3,4-b]pyrazine the acceptor (BPC-2DPP, quinoxaline substitutedthe atacceptor the pyridopyrazine3-position of substituted the donor benzocarbazole); at the 2-position 4,7-bis(9-phenyl-9 of the donor benzocarbazole);H-carbazol-2-yl)-2,3-bis(decyloxy)pyridine and 4,7-bis(9- phenyl-9[3,4-bH]pyrazine-carbazol-3-yl)-2,3-bis(decyloxy)pyridine (BPC-2DPP, the acceptor pyridopyrazine [3,4-b]pyrazine substituted (BPC-3DPP, at the 2-position the ofacceptor the H b pyridopyrazinedonor benzocarbazole); substituted and at the 4,7-bis(9-phenyl-9 2-position of the-carbazol-3-yl)-2,3-bis(decyloxy)pyridine donor benzocarbazole). The relationship [3,4- ]pyrazine between the (BPC-3DPP, the acceptor pyridopyrazine substituted at the 2-position of the donor benzocarbazole). structure and properties of the four D–A–D compounds were studied by density functional theory The relationship between the structure and properties of the four D–A–D compounds were studied by (DFT), UV-vis spectroscopy, fluorescent spectroscopy, and cyclic voltammetry. The effects of the density functional theory (DFT), UV-vis spectroscopy, fluorescent spectroscopy, and cyclic voltammetry. slightThe differences effects of in the the slight structures differences of the in theaccept structuresors and of the the link acceptors sites between and the linkthe sitesacceptor between and the donorthe on acceptor the optoelectronic and the donor properties on the optoelectronic of the resultant properties compounds of the resultantwere compared, compounds as were the compounds’compared, structure–property as were the compounds’ relationships. structure–property In addition, relationships. BPC-2DPx and In addition, BPC-2DPP BPC-2DPx were chosen and as electromemoryBPC-2DPP werematerials, chosen asfabricating electromemory sandwich materials, electromemory fabricating sandwich devices electromemory (Al/BPC-2DPx/ITO devices or Al/BPC-2DPP/ITO);(Al/BPC-2DPx/ITO their or Al/BPC-2DPP/ITO); morphology, memory their morphology, performance, memory and performance, electromemory and electromemory stability were investigated.stability were investigated.

FigureFigure 1. Structure 1. Structure of of the the four four investigated investigated D–A–D-type D–A–D-type compounds. compounds. 2. Results and Discussion 2. Results and Discussion 2.1. Synthesis of the Four Fluorescent Compounds 2.1. Synthesis of the Four Fluorescent Compounds Studies have shown that the presence of large volumes of alkyl or alkoxy substituents onStudies the have acceptor shown is conducivethat the presence to improve of large the vo optoelectroniclumes of alkyl conversion or alkoxy efficiency substituents ofthe on the acceptormaterial is conducive [8]. Therefore, to improve the the acceptor optoelectronic DPx or conversion DPP selected efficiency in this of the study material contained [8]. Therefore, two the acceptordecyloxyphenyl DPx or DPP substituents, selected e.g., in this 2,3-bis study (4-decyloxybenzene)-5,8-dibromoquinoxaline contained two decyloxyphenyl substituents, (compound e.g., 2,3-bis(4-8) and 2,3-bis(4-decyloxybenzene)-5,8- dibromopyrido[3,4-b]pyrazine (compound 9). The synthesis route decyloxybenzene)-5,8-dibromoquinoxaline (compound 8) and 2,3-bis(4-decyloxybenzene)- is shown in Scheme1d,e. The synthesis of acceptor compound 8 is divided into two steps. The first step 5,8-dibromopyrido[3,4-b]pyrazine (compound 9). The synthesis route is shown in Scheme 1d,e. The was reduction of 4,7- dibromo-2,1,3-benzothiadiazole (compound 1) by sodium borohydride (NaBH4) synthesisto obtain of acceptor 3,6-dibromo-1,2-phenylenediamine compound 8 is divided (compound into two 2steps.). Secondly, The first compound step was2 reacted reduction with 4,4-of 4,7- dibromo-2,1,3-benzothiadiazoledichenyloxybenzene (compound (compound7) to obtain compound1) by sodium8 [34]. Theborohydride synthesis of (NaBH acceptor4) compoundto obtain9 3,6- dibromo-1,2-phenylenediaminewas also carried out in two steps. (compound The first step 2 was). theSecondly, bromination compound reaction of2 3,4-diaminopyridine reacted with 4,4- dichenyloxybenzene(compound 3) to obtain (compound 3,4-diamino-2,5-dibromopyridine 7) to obtain compound (compound 8 [34]. The4). synthesis In the second of step,acceptor compound compound4 9 wasreacted also carried with compound out in two7 to steps. obtain The compound first step9 [ was35]. the bromination reaction of 3,4-diaminopyridine (compound 3) to obtain 3,4-diamino-2,5-dibromopyridine (compound 4). In the second step, compound 4 reacted with compound 7 to obtain compound 9 [35].

Materials 20182018,, 1111,, 2063x FOR PEER REVIEW 44 of of 20 Materials 2018, 11, x FOR PEER REVIEW 4 of 20 Br Br Br N NH2 NH NH Br NaHB4 Br 2 2 S (a) Br2 Br (b) N NH2 N NH N NH EtOHNaHB4 NH 2 2 S 2 NH2 Br2 NH2 Br Br (a) N N Br (b) N EtOH NH 2 NH2 NH2 Br1 Br2 3 Br 4 1 2 3 4

H CO HO C H O 3 OCH3 OH 10 21 OC10H21 H CO CH3COOH HO C10H21Br C H O 3 OCH3 OH 10 21 OC(c)10H21 C H Br C C CH48%3COOH HBr C C K CO , (C10 H21) NBr,DMF C C O O O O 2 3 4 9 4 O O (c) C C 48% HBr C C K CO , (C H ) NBr,DMF C C O 5 O O 67O 2 3 4 9 4 O O 5 67

Br Br Acetic acid 2 Br NNBr + 7 Acetic acid Ethanol NN (d) 2 + 7 Ethanol (d) OC H C10H21O 8 10 21 OC H C10H21O 8 10 21 N Br Br N Acetic acid Br NNBr 4 + 7 (e) AceticEthanol acid NN 4 + 7 Ethanol (e)

C H O OC10H21 10 21 9 C H O OC10H21 10 21 9 Scheme 1. Synthesis routes of the acceptors quinoxaline and pyridopyrazine. Scheme 1.1. Synthesis routes ofof thethe acceptorsacceptors quinoxalinequinoxaline andand pyridopyrazine.pyridopyrazine. The four D–A–D compounds were all obtained through one-step Suzuki coupling (Scheme 2) [6,9,13],TheThe andfour four the D–A–D D–A–D yields compounds were compounds between were were62–70%. all obtained all The obtained throughcompounds through one-step BPC-2DPx one-stepSuzuki and coupling Suzuki BPC-3DPx (Scheme coupling were 2) (Schemeobtained[6,9,13], 2 and)[by6 , 9Suzukithe,13 ]yields, and coupling thewere yields reactionbetween were betweenusing62–70%. compound 62–70%.The compounds The8 with compounds 9-phenyl-9 BPC-2DPx BPC-2DPxH -carbazol-2-yl-boronicand BPC-3DPx and BPC-3DPx were wereacidobtained (2-BPC) obtained by Suzukior by 9-phenyl-9 Suzuki coupling couplingH-carbazol-3-yl-boronic reaction reaction using using compound compound acid (3-BPC) 8 8withwith (Scheme9-phenyl-9 9-phenyl-9 2).HH The-carbazol-2-yl-boronic-carbazol-2-yl-boronic compounds BPC- acid2DPPacid (2-BPC)(2-BPC) and BPC-3DPP or or 9-phenyl-9 9-phenyl-9 wereH -carbazol-3-yl-boronicHprepared-carbazol-3-yl-boronic in the same acidmanner acid (3-BPC) (3-BPC) by replacing (Scheme (Scheme2 compound). The 2). compoundsThe 8compounds with BPC-2DPPcompound BPC- and92DPP. BPC-3DPP and BPC-3DPP were prepared were prepared in the samein the manner same manner by replacing by replacing compound compound8 with compound8 with compound9. 9.

X N Br Br N X X N N Br NNBr + Pd(PPh3)4 N X X=CH, BPC-2DPx NN (HO) B Pd(PPh ) NN + 2 K2CO3,(C4H9)34NF,4 toluene =N, BPC-2DPP N X=CH, BPC-2DPx (HO) B NN OC H 2 K2CO3,(C4H9)4NF, toluene =N, BPC-2DPP C10H21O 10 21 X=CH, 8 OC H C H O OC10H21 C10H21O =N, 9 10 21 10 21 X=CH, 8 OC H =N, 9 C10H21O 10 21

X Br Br Pd(PPh3)4 N X N X Pd(PPh ) Br NNBr + N K2CO3,(C4H9)4NF,3 4 toluene N X N NN NN + N K2CO3,(C4H9)4NF, toluene (HO)2B NN X=CH, BPC-3DPx OC H =N, BPC-3DPP C10H21O 10 21 (HO)2B X=CH, BPC-3DPx OC H X=CH, 8 C10H21O 10 21 OC H =N, BPC-3DPP C10H21O =N, 9 10 21 OC H X=CH, 8 C10H21O 10 21 =N, 9 Scheme 2. Synthesis routes of the four compounds. Scheme 2. Synthesis routes of the four compounds.

Materials 2018, 11, 2063 5 of 20 Materials 2018, 11, x FOR PEER REVIEW 5 of 20

2.2. Optimization2.2. Optimization of Molecular of Molecular Energy Energy Levels Levels The Thedensity density functional functional theory theory (DFT) (DFT) was was used used to tooptimize optimize the the structure structure of of the the compounds. compounds. The molecularThe molecular structure structure after optimization after optimization and the and HOMOs the HOMOs and and LUMOs LUMOs are are shown shown in in Figure Figure2 2..

FigureFigure 2. The 2. The frontier frontier molecular molecular orbital of of 4,7-bis(9-phenyl-9 4,7-bis(9-phenyl-9H-carbazol-2-yl)-2,3-bis(decyloxy)quinoxalineH-carbazol-2-yl)-2,3-bis(decyloxy) quinoxaline(BPC-2DPx) (BPC-2DPx) (a), 4.7-bis(9-phenyl-9 (a), 4.7-bis(9-phenyl-9H-carbazol-3-yl)-2,3-bis(decyloxy)quinoxalineH-carbazol-3-yl)-2,3-bis(decyloxy)quinoxaline (BPC-3DPx) (b), 4,7-bis (BPC- 3DPx)(9-phenyl-9 (b), 4,7-bis(9-phenyl-9H-carbazol-2-yl)-2,3-bis(decyloxy)pyridine[3,4-H-carbazol-2-yl)-2,3-bis(decyloxy)pyridine[3,4-b]pyrazine (BPC-2DPP)b]pyrazine (c) and (BPC-2DPP) 4,7-bis(9- (c) phenyl-9H-carbazol-3-yl)-2,3-bis(decyloxy)pyridine[3,4-b]pyrazine (BPC-3DPP) (d). and 4,7-bis(9-phenyl-9H-carbazol-3-yl)-2,3-bis(decyloxy)pyridine[3,4-b]pyrazine (BPC-3DPP) (d). As shown in Figure2, the distributions of π electrons in the HOMO of the four compounds were mainlyAs shown distributed in Figure on the2, the benzocarbazole distributions donor of π electrons unit, while in the the excited HOMO electrons of the infour the compounds LUMO of the were mainlyfour distributed compounds on were the mainlybenzocarbazole distributed donor on the unit acceptor, while moieties, the excited including electrons pyridopyrazine in the LUMO and of the fourquinoxaline. compounds This were indicates mainly that distributed ICT occurred on from the theacceptor ground moieties, state to the including excited state pyridopyrazine [8–13], and the and quinoxaline.electrons wereThis transferredindicates that from ICT benzocarbazole occurred from to pyridopyrazinethe ground state or quinoxaline.to the excited The state alkoxy [8–13], side and the electronschain had nowere obvious transferred electron from supply benzocarbazole effect. DFT-based to calculations pyridopyrazine were performed or quinoxaline. using Gaussian The alkoxy side softwarechain had at the no level obvious of theory electron B3LYP/6-31 supply g+(d) effect. [13 ],DFT-based and the calculated calculations values were of the performed HOMO level, using Gaussian software at the level of theory B3LYP/6-31 g+(d) [13], and the calculated values of the HOMO level, LUMO level, and band-gap energy (Eg) are shown in Figure 2. The HOMO levels in BPC-2DPx and BPC-2DPP only slightly differed, as did the HOMO levels in BPC-3DPx and BPC- 3DPP. However, the LUMO levels of compounds with DPP as the acceptor were lower than those with DPx as the acceptor. This difference indicates that DPP had a stronger electron-withdrawing ability than DPx, which resulted in stronger ICTs, the red-shifted absorption spectra, and lower energy gaps.

Materials 2018, 11, 2063 6 of 20

LUMO level, and band-gap energy (Eg) are shown in Figure2. The HOMO levels in BPC-2DPx and BPC-2DPP only slightly differed, as did the HOMO levels in BPC-3DPx and BPC-3DPP. However, the LUMO levels of compounds with DPP as the acceptor were lower than those with DPx as the Materialsacceptor. 2018 This, 11, x difference FOR PEER REVIEW indicates that DPP had a stronger electron-withdrawing ability than6 DPx, of 20 which resulted in stronger ICTs, the red-shifted absorption spectra, and lower energy gaps. FigureFigure 22 alsoalso shows shows the the dihedral dihedral angle angle data data between between the acceptor the acceptor and the and adjacent the adjacent donor in donorthe four compounds,in the four compounds,enabling comparisons enabling of comparisons their coplanarity. of their The coplanarity. dihedral angles The dihedral of the two-substituted angles of the compoundstwo-substituted were compounds greater than were those greater of thanthe three- thosesubstituted of the three-substituted isomers, indi isomers,cating indicatingthat the three- that substitutedthe three-substituted compounds compounds exhibited be exhibitedtter coplanarity better coplanarity than the two-subs than thetituted two-substituted compounds. compounds. In the case ofIn DPx the caseas the of acceptor, DPx as the there acceptor, was no there significant was no diffe signiﬁcantrence in difference the dihedral in the angles dihedral between angles the between acceptor andthe acceptorthe two donors, and the indicating two donors, that indicating both BPC-2DPx that both and BPC-2DPx BPC-3DPx and BPC-3DPxhave good havesymmetry. good symmetry. However, inHowever, the case inofthe DPP case as ofthe DPP acceptor, as the acceptor,the dihedral the dihedralangles between angles between the receptor the receptor and the and two the donors two substantiallydonors substantially differed differedfrom the from right the to the right left, to which the left, may which be related may be to related the asymmetry to the asymmetry of the pyridine of the unitspyridine in the units DPP in molecular the DPP molecular structure. structure.

2.3.2.3. UV-vis UV-vis Absorption Absorption Spectra Spectra

TheThe UV-vis UV-vis absorption spectraspectra ofof thethe fourfour compounds compounds at at a concentrationa concentration of of 1 ×1 10× −105 −mol/L5 mol/L solutionsolution of of n-hexane, n-hexane, dichloromethane, dichloromethane, chloroform chloroform,, toluene, toluene, tetrahydrofuran, tetrahydrofuran, acetonitrile, acetonitrile, N, N- dimethylformamide,N, N-dimethylformamide, and dimethyl and dimethyl sulfoxide sulfoxide are areshown shown in inFigure Figure 3,3, andand thethe corresponding corresponding absorptionabsorption positions positions are are shown in Table1 1..

FigureFigure 3. 3. Ultraviolet-visibleUltraviolet-visible (UV-vis) (UV-vis) absorption spectra ofof BPC-2DPxBPC-2DPx ((aa),), BPC-3DPx BPC-3DPx ( b(b),), BPC-2DPP BPC-2DPP ((cc),), and and BPC-3DPP BPC-3DPP ( (d)) in different solvents.solvents.

TheThe UV-vis UV-vis absorption absorption spectra of the fourfour compoundscompounds werewere similarsimilar becausebecause ofof theirtheir similar similar structures.structures. In In Figure Figure 3,3 ,all all four four compounds compounds had had two two significant signiﬁcant characteristic characteristic absorption absorption peaks peaks in differentin different solvents. solvents. The The absorption absorption peak peak be betweentween 300–340 300–340 nm nm is is attributed attributed to to the ππ––ππ** electronelectron transition of the conjugated skeleton, and the other peak (between 350–500 nm) is attributed to the ICT (π*–π*) between the electron donor BPC and the electron acceptor DPx or DPP. It can be seen that the absorption peak is not affected by solvent polarity (solvents are sorted according to their polarity [36] in ascending order). This insignificant solvatochromic effect is typical for the ground state of D–A compounds [12,37,38]. In the ground state, D–A-type compounds have lower polarity, and the effect of the solvent polarity on the electrons is insignificant [12,37,38], which results in a weak solvatochromic effect.

Materials 2018, 11, 2063 7 of 20 transition of the conjugated skeleton, and the other peak (between 350–500 nm) is attributed to the ICT (π*–π*) between the electron donor BPC and the electron acceptor DPx or DPP. It can be seen that the absorption peak is not affected by solvent polarity (solvents are sorted according to their polarity [36] in ascending order). This insigniﬁcant solvatochromic effect is typical for the ground state of D–A compounds [12,37,38]. In the ground state, D–A-type compounds have lower polarity, and the effect of the solvent polarity on the electrons is insigniﬁcant [12,37,38], which results in a weak solvatochromic effect.

Table 1. The max absorption wavelengths (λmax, nm) of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP in different solvents.

Solvent BPC-2DPx BPC-3DPx BPC-2DPP BPC-3DPP Hex 311, 379 297, 388 296, 403 319, 396 PhH 314, 383 299, 390 298, 408 321, 399 THF 313, 383 298, 390 297, 408 319, 399 DCM 314, 383 298, 390 297, 408 320, 400 TCM 315, 384 299, 391 299, 410 320, 403 DMF 314, 383 297, 389 297, 407 319, 401 DMSO 316, 385 298, 391 298, 410 322, 403 ACN 312, 378 296, 386 295, 402 316, 395

The substitution position strongly influences the locations of the compounds’ absorption peaks, as eVident from the results that are summarized in Table1. Comparing the absorption peaks of BPC-2DPx and BPC-3DPx, the two absorption bands shifted in opposite directions. The lower wavelength absorption band blue-shifted, and the higher wavelength absorption band red-shifted. Compared with the acceptor substituted at the two-position of the donor, its isomer, which the acceptor substituted at the two-position of the donor, processed better coplanarity between the donor and acceptor. Therefore, in the case of the three-position substitution, stronger interaction occurred between the donor and acceptor, which is more conducive to the occurrence of ICT and resulted in a red-shift of the longer wavelength absorption peak associated with π*–π* electron transfer. In addition, the decrease of electron density on the donor DPx ring caused by ICT resulted in a blue shift of the lower wavelength absorption peak. When DPP was used as the acceptor, the change in the absorption band shift was also associated with the substitution position, but was opposite to that observed for DPx. Compared to DPx, the structure of a DPP is to replace a benzene ring on DPx with a pyridine ring. There is a sp2 hybridization orbital on the N atom of the pyridine ring, which is not involved in the bond forming, and is occupied by a pair of lone electrons, making the pyridine possess some basicity. So, the electronegativity of the N atom in the pyridine ring has a great influence on the density distribution of the electron cloud on the ring. The π electron cloud shifts to the N atom in the pyridine ring on the DPP unit; as a result, the density of the electron cloud around the N atom is higher than the carbon atoms in the pyridine ring, especially the carbon atoms on the ortho-position and para-position of the nitrogen atom. So, the pyridine molecule has less aromaticity than that of benzene. The above difference between DPx and DPP may have a different effect on the adjacent linked groups, and then explained the differences in the UV-vis spectra of DPP or DPx as the acceptor. The effect of the acceptor on the absorption band of the compounds can be eValuated by comparing the absorption bands of BPC-2DPx and BPC-2DPP, as well as those of BPC-3DPx and BPC-3DPP in Table1. For the BPC2-based compounds, when DPP was used as the acceptor instead of DPx, the resultant BPC-2DPP experienced a blue-shift for its high energy absorption and a red-shift for its low energy absorption, respectively. Compared with DPx, DPP had one more electron-deficient N atom, and thus had stronger electron-withdrawing ability, which is beneficial for ICT. Hence, the longer-wavelength absorption bands of the compounds with DPP as the acceptor exhibited red-shifts. In the case of the BPC3 as the donor unit, both absorption peaks of BPC-3DPP Materials 2018, 11, 2063 8 of 20 showed obvious red-shifts compared with those of BPC-3DPx. These shifts were due to the stronger electron-withdrawing ability of DPP. The Eg values of the four compounds were calculated from their onset wavelength of the absorption (Eg = 1241/eonset [13]), and the data are 2.82 eV, 2.70 eV, 2.48 eV, and 2.62 eV, respectively, for BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP, which are dissolved in chloroform. The Eg values of the four compounds were much lower than the Eg of carbazole (3.2 eV) [39]. In the case of the compounds with DPP as the acceptor, which exhibited stronger electron-withdrawing ability than the acceptor of DPx, the Eg values of the compounds were smaller. Therefore, the introduction of a strong Materials 2018, 11, x FOR PEER REVIEW 8 of 20 electron-withdrawing unit between two BPC units as the acceptor led to a lower band gap compared comparedwith carbazole with derivatives. carbazole derivatives. The substitution The ofsubstituti an acceptoron of at an the acceptor three-position at the ofthree-position the donor is helpfulof the donorto improve is helpful the coplanarity to improve of the the coplanarity formed molecule, of the formed which is molecule, theoretically which beneficial is theoretically to reduce beneficial the Eg of the molecule. to reduce the Eg of the molecule. 2.4. Fluorescent Emission Spectra 2.4. Fluorescent Emission Spectra BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP were excited at the maximum absorption peak BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP were excited at the maximum absorption to obtain the fluorescent emission spectra in eight different common solutions, as shown in Figure4. peak to obtain the fluorescent emission spectra in eight different common solutions, as shown in The positions of the fluorescent emission peaks are shown in Table2 (solvents are sorted according to Figure 4. The positions of the fluorescent emission peaks are shown in Table 2 (solvents are sorted their polarity [34] in ascending order). according to their polarity [34] in ascending order).

Figure 4. FluorescentFluorescent emission emission spectra spectra of BPC-2DPx BPC-2DPx ( aa),), BPC-3DPx BPC-3DPx ( (bb),), BPC-2DPP BPC-2DPP ( (cc)) and and BPC-3DPP BPC-3DPP (d) in different solutions.

Unlike the UV-vis absorption spectra, the fluorescentfluorescent emission spectra of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP in each solution showed on onlyly one one well-defined well-defined separate separate peak. peak. In In addition, addition, theirtheir fluorescent fluorescent emission emission peaks peaks exhibited exhibited a a very very la largerge Stokes Stokes shift shift (ranging (ranging from from 86 86 nm nm to to 153 153 nm) nm) compared with with the UV-vis absorption peaks. The The re resultssults in in Figure Figure 44 showshow thatthat thethe positionspositions ofof thethe emission peaks of the four D–A–DD–A–D compoundscompounds werewere substantiallysubstantially red-shiftedred-shifted withwith thethe increasingincreasing polarity of of the the solvent, solvent, except except for anhydrous for anhydrous magnesium magnesium sulfate (ACN), sulfate but (ACN), the trend but remains the trend unchanged. remains The fluorescent emission peaks of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP in DMSO were red- shifted 35 nm (from 465 nm to 500 nm), 42 nm (from 487 nm to 529 nm), 43 nm (from 520 nm to 563 nm), and 30 nm (from 503 nm to 533 nm), respectively, compared with those of the same compounds in n- hexane. These results demonstrate a strong solvatochromic effect. This phenomenon is easily explained. For a compound with a D–A structure, electrons in the excited state of the compound transfer from the HOMO to the LUMO, which increases the polarity of the excited state (S1). A more polar solvent is beneficial to the electron delocalization [12,37,38,40]. The excited state in the higher- polarity solvent can easily relax to a lower-energy state, which results in a red-shift of the fluorescent emission peak. The difference in the solvatochromic phenomena between the ground state and the excited states arises from the greater polarity of the excited states [12,37,38,40].

Materials 2018, 11, 2063 9 of 20 unchanged. The fluorescent emission peaks of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP in DMSO were red-shifted 35 nm (from 465 nm to 500 nm), 42 nm (from 487 nm to 529 nm), 43 nm (from 520 nm to 563 nm), and 30 nm (from 503 nm to 533 nm), respectively, compared with those of the same compounds in n-hexane. These results demonstrate a strong solvatochromic effect. This phenomenon is easily explained. For a compound with a D–A structure, electrons in the excited state of the compound transfer from the HOMO to the LUMO, which increases the polarity of the excited state (S1). A more polar solvent is beneficial to the electron delocalization [12,37,38,40]. The excited state in the higher-polarity solvent can easily relax to a lower-energy state, which results in a red-shift of the fluorescent emission peak. The difference in the solvatochromic phenomena between the ground state and the excited states arises from the greater polarity of the excited states [12,37,38,40].

Table 2. The maximum ﬂuorescent emission wavelengths (λmax, nm) of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP in different solvents.

Solvent BPC-2DPx BPC-3DPx BPC-2DPP BPC-3DPP Hex 465 487 520 503 PhH 473 500 530 511 THF 478 510 536 517 DCM 485 518 549 520 TCM 486 518 542 519 DMF 492 522 551 522 DMSO 500 529 563 533 ACN 490 525 553 524

The fluorescent emission spectra in dichloromethane were used as an example. A comparison of the fluorescent emission spectra of BPC-2DPP and BPC-2DPx, which have different acceptors substituted at the same position on the donor, revealed that the fluorescent emission bands of the two compounds were 549 nm and 485 nm, respectively, and that the former was red-shifted by 64 nm compared with the latter. Similarly, a red-shift was also observed between the compounds of BPC-3DPP and BPC-3DPx. The red-shift trend of the fluorescent emission spectrum was the same as that observed in the UV-vis absorption spectra, and is attributed to the stronger electron-withdrawing effect of DPP than that of DPx, which strengthens the D–A interaction and facilitates the intramolecular charge transfer. The π*–π* electron transfer of the corresponding D–A–D structure compound was enhanced, resulting in a red-shift in the fluorescent emission spectrum. The substitution position also affects the fluorescent emission spectrum of a compound. Upon comparing BPC-3DPx and BPC-2DPx, the fluorescent emission peaks were located at 518 nm and 485 nm (in dichloroform solution), respectively, where the former was red-shifted by 33 nm compared with the latter. When DPP was used as the acceptor, the peak in the fluorescent emission spectrum of BPC-3DPP was blue-shifted by 29 nm compared with that of BPC-2DPP, which is opposite to the substituent effect observed when DPx was used as the acceptor. The fluorescent emission spectra of these two groups of compounds were shifted in different directions, which is consistent with the variation rule of UV-vis spectra, and the reason is the same.

2.5. Cyclic Voltammetry The electrochemical characteristics of four D–A–D compounds were studied by cyclic voltammetry (CV) using a 0.2 mol/L ACN solution of tetrabutylammonium hexaﬂuoride as the electrolyte [13], and the HOMO and LUMO levels were estimated from the results. The cyclic voltammograms (Figure5) of the four compounds were similar, with two pairs of irreversible redox peaks. These peaks corresponded to the losing their electrons to form stable radical cations of the two electron donors (two benzocarbazoles) on each compound. The redox peaks of the compound BPC-2DPx were located at 1.59/1.33 V and 1.36/1.12 V, the redox peaks of BPC-3DPx were located at 1.52/1.32 V and 1.23/1.11 V, the redox peaks of BPC-2DPP were located at 1.62/1.40 V and Materials 2018, 11, 2063 10 of 20

1.28/1.07 V, and the redox peaks of BPC-3DPP were located at 1.65/1.41 V and 1.39/1.11 V. The onset of the oxidation potentials (Eonset) of BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP were 1.13 V, Materials1.07 2018 V,, 1.12 11, x V, FOR and PEER 1.26 REVIEW V, respectively. 10 of 20

FigureFigure 5.5.The The cyclic cyclic voltammetry voltammetry (CV) (CV) curves curves of BPC-2DPx of BPC-2DPx (a), BPC-3DPx (a), BPC-3DPx (b), BPC-2DPP (b), (BPC-2DPPc), and BPC-3DPP (c), and (d). BPC-3DPP (d). The HOMO levels (−5.55 eV, 5.49 eV, −5.54 eV, and −5.68 eV, respectively) of BPC-2DPx, TheBPC-3DPx, HOMO BPC-2DPP, levels (− and5.55 BPC-3DPPeV, 5.49 eV, were −5.54 calculated eV, and on the−5.68 basis eV, of respectively) the formula HOMO of BPC-2DPx, = −e (Eonset BPC- + 0.02 + 4.4) [13], where Eonset is the onset oxidation potential of the compounds. According to the 3DPx, BPC-2DPP, and BPC-3DPP were calculated on the basis of the formula HOMO = −e (Eonset + equation LUMO = HOMO + Eg, the LUMO energy levels of the compounds were calculated to be 0.02 + 4.4) [13], where Eonset is the onset oxidation potential of the compounds. According to the −2.73 eV, −2.79 eV, −3.06 eV, and −3.06 eV, respectively. Thus, the Eonset of the four compounds equation LUMO = HOMO + Eg, the LUMO energy levels of the compounds were calculated to be were similar, and they also had similar HOMO values. Compared with the LUMO levels of the two −2.73 compoundseV, −2.79 eV, with −3.06 DPx eV, as the and acceptor, −3.06 eV, those respectively. of BPC-2DPP Thus, and BPC-3DPP the Eonset of were the lower, four whichcompounds resulted were similar,in aand higher they ICT, also a red-shift had similar of the ICTHOMO absorption values band,. Compared and a lower with energy the LUMO gap. The levels energy of bands, the two compoundsLUMO/HOMO with DPx values, as the and acceptor, other relevant those of data BP forC-2DPP the four and compounds BPC-3DPP are were summarized lower, which in Table resulted3. in a higher ICT, a red-shift of the ICT absorption band, and a lower energy gap. The energy bands, LUMO/HOMOTable 3. values,The electrochemical and other andrelevant optical data property for datathe offour compounds, compounds include are dihedral summarized angles, Eonset in Table, 3. λmax, λonset, Eg, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO). Table 3. The electrochemical and optical property data of compounds, include dihedral angles, Eonset, b c d e e e λmax, λonsetDihedral, Eg, highest Angles (deg) occupiedλmax (nm)/ molecularλonset (nm) Eorbitalg (eV) (HOMO),HOMO (eV) andLUMO lowest(eV) unoccupiedEg (eV) HOMO molecular(eV) LUMO orbital(eV) BPC-2DPx 41.77, 40.92 314, 383/440 2.82 −5.55 −2.73 3.32 −5.37 −2.05 (LUMO).BPC-3DPx 37.28, 36.86 298, 390/459 2.70 −5.49 −2.79 3.09 −5.09 −2.00 BPC-2DPP 39.42, 19.42 297, 408/501 2.48 −5.54 −3.06 2.94 −5.35 −2.41 BPC-3DPP 36.21,Dihedral 15.23 λmax 320, (nm) 400/473/ Eg b 2.62 HOMO−5.68 c LUMO−3.06 d 2.80Eg e HOMO−5.13 e −LUMO2.33 e a b DihedralAngles angle (deg) between theλonset central (nm) acceptor (eV) and the adjacent(eV) benzene(eV) ring. Band(eV) gap calculated(eV) from Eonset(eV), c d Eg = 1241/Eonset. Calculated from Eonset of the compounds, HOMO = −e(Eonset + 0.02 + 4.4). Estimated using BPC-2DPx 41.77, 40.92 314, 383/440 e 2.82 −5.55 −2.73 3.32 −5.37 −2.05 empirical equations LUMO = HOMO + Eg. Eg, HOMO and LUMO were calculated based on the density functional BPC-3DPxtheory (DFT). 37.28, 36.86 298, 390/459 2.70 −5.49 −2.79 3.09 −5.09 −2.00 BPC-2DPP 39.42, 19.42 297, 408/501 2.48 −5.54 −3.06 2.94 −5.35 −2.41 BPC-3DPP 36.21, 15.23 320, 400/473 2.62 −5.68 −3.06 2.80 −5.13 −2.33 The HOMO levels and the LUMO levels from the quantum calculations, as well as the E values a Dihedral angle between the central acceptor and the adjacent benzene ring. b Band gap calculatedg deviated from the experimental values, are all presented in Table3. The theoretical calculation values from Eonset, Eg = 1241/Eonset. c Calculated from Eonset of the compounds, HOMO = −e(Eonset + 0.02 + 4.4). d

Estimated using empirical equations LUMO = HOMO + Eg. e Eg, HOMO and LUMO were calculated based on the density functional theory (DFT).

The HOMO levels and the LUMO levels from the quantum calculations, as well as the Eg values deviated from the experimental values, are all presented in Table 3. The theoretical calculation values of Eg of the D–A–D compounds with DPP as the acceptor were smaller than those with DPx as the acceptor, which is consistent with the experimental values.

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of Eg of the D–A–D compounds with DPP as the acceptor were smaller than those with DPx as the Materialsacceptor, 2018, which11, x FOR is consistentPEER REVIEW with the experimental values. 11 of 20 Materials 2018, 11, x FOR PEER REVIEW 11 of 20 2.6.2.6. Thermostability Thermostability Analysis Analysis 2.6. ThermostabilityThe thermal Analysis stability of BPC-2DPx was measured by thermogravimetric analysis (TGA) in The thermal stability of BPC-2DPx was measured by thermogravimetric analysis (TGA) in an N2 an N atmosphere, and its TGA curves are shown in Figure6. The TGA curves of BPC-3DPx, atmosphere,The2 thermal and its stability TGA curvesof BPC-2DPx are shown was measured in Figure by 6. thermogravimetricThe TGA curves of analysis BPC-3DPx, (TGA) BPC-2DPP, in an N2 BPC-2DPP, and BPC-3DPP are shown in Figure S5a–c. These four compounds showed only a one-stage andatmosphere, BPC-3DPP and are its shownTGA curves in Figure are shown S5a–c. in FigureThese 6.four The compoundsTGA curves ofshowed BPC-3DPx, only BPC-2DPP, a one-stage anddecomposition BPC-3DPP pattern.are shown They in all Figure had a highS5a–c. thermal These decompositionfour compounds temperature showed (Tonlyd, deﬁned a one-stage as the decomposition pattern. They all had a high thermal decomposition temperature (Td, defined as the temperature of the weightlessness was 5%), which were 445 ◦C, 448 ◦C, 453 ◦C, and 453 ◦C, respectively. decomposition pattern. They all had a high thermal decomposition temperature (Td, defined as the temperatureTheir maximum of the decomposition weightlessness rates was were 5%), 476 ◦whichC, 477 ◦wereC, 512 445◦C, and°C, 510448◦ C,°C, respectively, 453 °C, and with 453 very °C, respectively.temperature Theirof the maximum weightlessness decomposition was 5%), whichrates were 445476 °C,°C, 448477 °C,°C, 453 512 °C, °C, and and 453 510 °C, °C, respectively.high thermal Their stability. maximum decomposition rates were 476 °C, 477 °C, 512 °C, and 510 °C, respectively, with very high thermal stability. respectively, with very high thermal stability.

FigureFigureFigure 6. 6. TGATGATGA curvecurve curve of of BPC-2DPx BPC-2DPx.BPC-2DPx. .

2.7.2.7.2.7. Performance Performance Performance of of ofthe the the Memory Memory Memory DevicesDevices Devices Figure7 is the scanning electronic microscopic (SEM) image of the cross-sectional view of a FigureFigure 7 7is is the the scanning scanning electronicelectronic microscopic (SEM)(SEM) image image of of the the cross-sectional cross-sectional view view of ofa a memory device with the aim to determine the thickness of the organic layer. From bottom to top, memorymemory device device with with the the aim aim toto determinedetermine the thicknethicknessss of of the the organic organic layer. layer. From From bottom bottom to top,to top, the the the layers are the ITO (indium-tin oxide) glass, BPC-2DPx, or BPC-2DPP film, and the aluminum layerslayers are are the the ITO ITO (indium-tin(indium-tin oxide)oxide) glass, BPC-2DPx,BPC-2DPx, oror BPC-2DPPBPC-2DPP film, film, and and the the aluminum aluminum electrode,electrode, respectively. respectively. As As can can be be seen seen from from Figu Figurere 77,, the thickness of the BPC-2DPx and BPC-2DPP electrode, respectively. As can be seen from Figure 7, the thickness of the BPC-2DPx and BPC-2DPP filmsfilms were were 119.9 119.9 nm nm and and 143.6 143.6 nm, nm, respectively. respectively. The The th thinin film film is is beneficial beneficial for for the the injection injection of of holes holes films were 119.9 nm and 143.6 nm, respectively. The thin film is beneficial for the injection of holes andand electrons electrons during during electrical electrical st storageorage [41,42]. [41,42]. The The flatness flatness of of the the to topp electrode electrode also also influences influences the the and electrons during electrical storage [41,42]. The flatness of the top electrode also influences the performanceperformance of of the the device. device. The The flatness flatness of of the the top top electrode electrode can can be be controlled controlled by by the the deposition deposition rate rate performance of the device. The flatness of the top electrode can be controlled by the deposition rate andand the the time. time. The The thickness thickness can can be becontrolled controlled by th bye thelevel level of the of vacuum the vacuum and the and deposition the deposition rate. The rate. andhigherThe the higher time. the vacuum, The the vacuum,thickness the more the can uniform more be controlled uniform the depositio theby th depositionne rate.level The of rate.thealuminum vacuum The aluminum top and electrode the deposition top was electrode deposited rate. was The −4 higherunderdeposited the the vacuum, pressure under the theof 5.0 pressuremore × 10 uniform−4 Pa; of 5.0in this the× 10 case, depositio Pa;the indepositionn this rate. case, The rate thealuminum was deposition maintained top rateelectrode at was about maintained was 5 nm/min. deposited at −4 underAfterabout the 50 5 min,pressure nm/min. the thickness of After 5.0 × 50 10 min,of thePa; the Alin thicknessthistop electrodecase, ofthe the depositionwas Al about top electrode 230 rate nm. was was maintained about 230 nm.at about 5 nm/min. After 50 min, the thickness of the Al top electrode was about 230 nm.

FigureFigure 7. SEM images of of a a cross-sectional cross-sectional vi viewew of of devices devices based based on on BPC-2DPx BPC-2DPx (a ()a and) and BPC-2DPP BPC-2DPP (b ().b ). Figure 7. SEM images of a cross-sectional view of devices based on BPC-2DPx (a) and BPC-2DPP (b). TheThe device device performance performance is is highly highly correlated correlated with with the the surface surface morphology morphology of of the the organic organic semiconductingsemiconductingThe device performanceactive active layer layer [42,43 [42is ,43highly]. ].To To investigate investigatecorrelated the thewith surface surface the root surface root mean mean squaremorphology square roughness roughness of the(RMS) (RMS)organic of semiconductingthe films, atomic active force layermicroscopy [42,43 ].(AFM) To investigate measurem theents surface were carried root mean out on square the BPC-2DPx roughness and (RMS) BPC- of 2DPP films. The 3D-AFM topography images are shown in Figure 8. The scanning range was 4.0 µm the films, atomic force microscopy (AFM) measurements were carried out on the BPC-2DPx and BPC- × 4.0 µm, and the distribution of the compound was relatively uniform. The RMS of BPC-2DPx film 2DPP films. The 3D-AFM topography images are shown in Figure 8. The scanning range was 4.0 µm was 25.42 nm, while the RMS of BPC-2DPP film was 27.65 nm. The two films had high smoothness, × 4.0 µm, and the distribution of the compound was relatively uniform. The RMS of BPC-2DPx film which is favorable for the devices to exhibit good memory performance. was 25.42 nm, while the RMS of BPC-2DPP film was 27.65 nm. The two films had high smoothness, which is favorable for the devices to exhibit good memory performance.

Materials 2018, 11, 2063 12 of 20

of the films, atomic force microscopy (AFM) measurements were carried out on the BPC-2DPx and BPC-2DPP films. The 3D-AFM topography images are shown in Figure8. The scanning range was 4.0 µm × 4.0 µm, and the distribution of the compound was relatively uniform. The RMS of BPC-2DPx film was 25.42 nm, while the RMS of BPC-2DPP film was 27.65 nm. The two films had high smoothness, Materials 2018, 11, x FOR PEER REVIEW 12 of 20 which is favorable for the devices to exhibit good memory performance.

Figure 8. Three-dimensional (3D) atomic force microscopy (AFM) topography of BPC-2DPx (a) and Figure 8. Three-dimensional (3D) atomic force microscopy (AFM) topography of BPC-2DPx (a) and BPC-2DPx (b) films. BPC-2DPx (b) films. The memory behavior of the BPC-2DPx and BPC-2DPP films was demonstrated by the Thecurrent memory−voltage behavior (I−V) characteristics of the BPC-2DPx of ITO/BPC-2DPx/Al and BPC-2DPP or ITO/BPC-2DPP/Al films was demonstrated sandwich devices. by the current−Figurevoltage9 shows (I−V) the characteristics I–V electrical switching of ITO/BPC-2DPx/Al of the memory devices; or ITO/BPC-2DPP/Al the two devices present sandwich obvious devices. bistability resistance characteristics. In the case of the device based on BPC-2DPx (Figure9a), Figure 9 shows the I–V electrical switching of the memory devices; the two devices present obvious the voltage swept from −1 V to 1 V. In the negative scan from 0 V to −1 V, the device remained bistabilityin theresistance “OFF” or characteristics. “0” state up to − 0.38In the V at case first, atofwhich the device the device based was on in a BPC-2DPx low-conductivity (Figure state. 9a), the voltage Asswept the scan from continued −1 V to to 1 the V. negative In the voltages,negative the scan current from increased 0 V to abruptly, −1 V, andthe thedevice device remained switched in the “OFF” orto “0” the “ON”state orup “1” to state,−0.38 withV at the first, property at which of high the conductivity. device was Inin the a low-conductivity above process, the abruptstate. As the scan continuedchange in to the the current negative density voltages, represents the the writingcurrent process increased of the abruptly, device, and and the ON/OFF the device switching switched to ratio in current was about 103. Then, during the continuous bias sweep, the device remained in the the “ON” or “1” state, with the property of high conductivity. In the above process, the abrupt change high conductivity state until the voltage increased to 0.54 V, at which point the current suddenly in the currentsharply density decreased, represents and the device the returnedwriting to process the low-conductivity of the device, state, and which the can ON/OFF be regarded switching as the ratio in currenterasure was processabout for103 data. Then, storage. during Similarly, the continuous the scan voltage bias range sweep, of the the device device based remained on BPC-2DPP in the high conductivitywas − state2 V–2 until V, and the the correspondingvoltage increased I−V curve to 0. is54 shown V, at inwhich Figure point9b. At the the beginning,current suddenly the device sharply decreased,was and in the the OFF device state. When returned the bias to increased the low-co to aboutnductivity 1.4 V, that is,state, the thresholdwhich can voltage, be theregarded current as the 2 erasure processsuddenly for sharply data increasedstorage. andSimilarly, changed the to thescan ON voltage state, andrange the of switching the device ratio based was about on BPC-2DPP 10 . The process of data writing was carried out. In the subsequent bias sweep, the device remained in the was −2 V–2 V, and the corresponding I−V curve is shown in Figure 9b. At the beginning, the device ON state. When the bias reached −1.4 V, the current decreased to the OFF state, and the switching was in theratio OFF was state. about When 103. the bias increased to about 1.4 V, that is, the threshold voltage, the current suddenly sharply increased and changed to the ON state, and the switching ratio was about 102. The process of data writing was carried out. In the subsequent bias sweep, the device remained in the ON state. When the bias reached −1.4 V, the current decreased to the OFF state, and the switching ratio was about 103.

Materials 2018, 11, 2063 13 of 20

Materials 2018, 11, x FOR PEER REVIEW 13 of 20

FigureFigure 9. 9. Current–voltageCurrent–voltage (I-V) characteristics of (a) ITO (Indium-Tin Oxide)Oxide) glass/BPC-2DPx/Alglass/BPC-2DPx/Al and and ((bb)) ITO/BPC-2DPP/Al ITO/BPC-2DPP/Al memory memory devices. devices.

TheThe energy energy level di differencesfferences at the interface of the bottombottom ITO electrodeelectrode ((−−4.84.8 eV) eV) and and the the depositeddeposited films films of of the the compounds compounds were were calculated, calculated, and the data data were were 0.75 0.75 eV eV and and 0.74 0.74 eV eV respectively, respectively, forfor ITO/BPC-2DPx ITO/BPC-2DPx and and ITO/BPC-2DPP. ITO/BPC-2DPP. At At the the same same time, time, the the energy energy level level differences differences (energy barrier) atat the the interface of the LUMO level of the compounds and the top AlAl electrodeelectrode ((−−4.284.28 eV) eV) were were also also calculated;calculated; thesethese were were 1.55 1.55 eV eV (Al/BPC-2DPx) (Al/BPC-2DPx and) and 1.22 eV1.22 (Al/BPC-2DPP). eV (Al/BPC-2DPP). The energy The barrierenergy betweenbarrier betweenthe interface the interface of the ITO of electrodethe ITO electrode and the organicand the layerorganic indicated layer indicated the hole injectionthe hole injection barrier between barrier betweenthe ITO electrodethe ITO electrode and these and two these layers. two The layers. electron The injectionelectron injection barrier between barrier between the interface the interface of the Al ofelectrode the Al electrode and the organicand the thinorganic films thin could films also could be obtained also be obtained from the from energy the level energy difference level difference between betweenthe Al electrode the Al electrode and these and two these layers. two layers. In this In case, this thecase, hole the injectionhole injection barriers barriers for both for both of the of twothe twodevices devices are muchare much lower lower than than those those of the of electron the elec injectiontron injection barriers. barriers. Therefore, Therefore, bothorganic both organic layers layershavethe have p-type the p-type property, property, and the and hole the injection hole injection process process dominates dominates the writing the writing and erasing and erasing process processof the devices of the devices [41–44]. [41–44]. In D–A–D In D–A–D compounds, compounds, the electron the electron acceptor acceptor cloud cloud functions functions as carrier as carrier traps. traps.When When bias is bias applied is applied between between the electrodes, the electrodes, the carriers the firstcarriers fill the first trap fill sites; the trap after sites; the traps after were the traps filled, werean abrupt filled, increase an abrupt in currentincrease occurs, in current corresponding occurs, corresponding to the OFF to to ON the state OFF transition. to ON state The transition. existence Theof carrier existence traps of resultscarrier intraps the results dipole formation,in the dipole and formation, an internal and electric an internal field electric is created, field maintaining is created, maintainingthe high conductivity the high conductivity state. Under state. a reversed Under a bias, reversed the charge bias, the traps charge loses traps the loses charge, the thecharge, internal the internalelectric fieldelectric immediately field immediately disappears, disappears, and the device and the returns device to returns its initial to lowits initial conductivity low conductivity state (OFF statestate), (OFF exhibiting state), exhibiting binary non-volatile binary non-volatile FLASH memory FLASH memory behavior behavior [45]. From [45] the. From view the of view the energy of the energybarriers barriers discussed discussed just as above, just as the above, ITO/BPC-2DPx/Al the ITO/BPC-2DPx/Al device should device have should a slightly have a higher slightly switching higher switchingvoltage (the voltage voltage (the from voltage “ON” from to “OFF” “ON” or to the “OFF” opposite or the change). opposite However, change). the However, actual phenomenon the actual phenomenonis the opposite, is i.e.,the opposite, the ITO/BPC-2DPx/Al i.e., the ITO/BPC-2DPx/Al device had adevice much had lower a much switching lower voltage switching than voltage that of thanthe ITO/BPC-2DPP/Al, that of the ITO/BPC-2DPP/Al, which suggested which suggested that the switching that the switching voltages arevoltages also determinedare also determined by other byfactors other besides factors thebesides molecular the molecular structure structure of the organic of the organic layer. Compared layer. Compared with the with other the non-volatile other non- volatilememory memory device reporteddevice reported previously, previously, these two these devices two devices still belong still tobelong the low-voltage to the low-voltage driven devices,driven devices,and have and certain have applicationcertain application prospects. prospects. The BPC3-based The BPC3-based two other two compounds other compounds are also are used also as used the asorganic the organic layer to layer construct to construct the corresponding the correspondi devices,ng devices, which are which designated are designated to ITO/BPC-3DPx/Al to ITO/BPC- 3DPx/Aland ITO/BPC-3DPP/Al, and ITO/BPC-3DPP/Al, respectively. respectively. Their Their performance performance were were also also examined examined (Figure (Figure S6 inS6the in thesupplementary supplementary information), information), and and the the data data showed showed that that the the difference difference on substitutionon substitution positions positions did didnot obviouslynot obviously affect affect the performancethe performance of the of devices, the devi whichces, which was different was different with its with effect its oneffect the UV-vison the UV-visabsorption absorption and fluorescence and fluorescence emission emission spectra. spectr In viewa. In of view this consideration,of this consideration, the in-depth the in-depth analysis analysiswas not was undertaken not undertaken for getting for thegetting relationship the relation betweenship between the performances the performances of the four of the devices four devices and the andmolecular the molecular structures structures of the four of the compounds. four compounds. The stability of data storage is an important index for evaluating the performance of memory devices. As shown in Figure 10, when a continuous read voltage of 0.5 V or 1 V was applied on the device of ITO/BPC-2DPx/Al or ITO/BPC-2DPP/Al, respectively, the current density of the device in ON and OFF states does not change significantly during the test time of 1300 s, revealing that the two devices possess good stability of data storage under ambient air conditions [41,43].

Materials 2018, 11, 2063 14 of 20

The stability of data storage is an important index for eValuating the performance of memory devices. As shown in Figure 10, when a continuous read voltage of 0.5 V or 1 V was applied on the device of ITO/BPC-2DPx/Al or ITO/BPC-2DPP/Al, respectively, the current density of the device in ON and OFF states does not change signiﬁcantly during the test time of 1300 s, revealing that the two Materialsdevices 2018, 11 possess, x FOR goodPEERstability REVIEW of data storage under ambient air conditions [41,43]. 14 of 20

FigureFigure 10. Retention 10. Retention time time measurement measurement for thethe ON ON and and OFF OFF states states of the of ( athe) ITO/BPC-2DPx/Al (a) ITO/BPC-2DPx/Al and and (b) ITO/BPC-2DPP/Al devices under a under a continuous voltage of 0.5 V and 1 V, respectively. (b) ITO/BPC-2DPP/Al devices under a under a continuous voltage of 0.5 V and 1 V, respectively. 3. Experimental 3. Experimental 3.1. Materials

3.1. Materials4,4-dimethoxy benzil (98%), liquid bromine (99.0%), sodium borohydride (NaBH4, 98%), sodium carbonate (Na2CO3, 99.8%), 1-bromodecane (98%), anhydrous potassium carbonate (K2CO3, 99.8%), 4,4-dimethoxyanhydrous magnesium benzil sulfate,(98%), acetonitrile liquid bromine (ACN, AR),(99.0%), dichloromethane sodium borohydride (DCM, AR), chloroform(NaBH4, 98%), (TCM, sodium carbonateAR), terahydrofuran(Na2CO3, 99.8%), (THF, 1-bromodecane AR), dimethyl formamide (98%), anhydrous (DFM, AR), potassium dimethyl sulfoxide carbonate (DMSO, (K2CO AR),3, 99.8%), anhydroustetra-n-butyl magnesium bromide sulfate, (99%), glacial acetonitrile acetic acid (ACN, (AR), anhydrousAR), dichloromethane ethanol (EtOH, 99.9%), (DCM, toluene AR), (PhH,chloroform (TCM,AR), AR), n-hexane terahydrofuran (AR), tetra (THF, (triphenylphosphine) AR), dimethyl palladium formamide (Pd(PPh (DFM,3)4), AR), and tetra-n-butylammonium dimethyl sulfoxide (DMSO, AR), tetra-n-butylﬂuoride (TBAF, bromide AR) were (99%), purchased glacial fromacetic Aladdin acid (AR), Industrial anhydrous Corporation ethanol (Shanghai, (EtOH, 99.9%), China). toluene 3,4-diaminopyridine, 4,7-dibromo-2,1,3-benzothiadiazole (98%), 9-phenyl-9H-carbazol-2-yl-boronic (PhH, AR), n-hexane (AR), tetra (triphenylphosphine) palladium (Pd(PPh3)4), and tetra-n- acid (98%, BPC2), or 9-phenyl-9H-carbazol-3-yl-boronic acid (98%, BPC3) were purchased from Aldrich butylammonium fluoride (TBAF, AR) were purchased from Aladdin Industrial Corporation Chemical Company and were used without further treatment. The thin layer chromatography silica (Shanghai,gel (GF254) China). and 3,4-diaminop the column chromatographyyridine, 4,7-dibromo-2,1,3-benzothiadiazole silica gel (200-300 mesh) were provided (98%), by Qingdao 9-phenyl-9H- carbazol-2-yl-boronicOcean Chemical Plant. acid (98%, BPC2), or 9-phenyl-9H-carbazol-3-yl-boronic acid (98%, BPC3) were purchased from Aldrich Chemical Company and were used without further treatment. The thin layer chromatography silica gel (GF254) and the column chromatography silica gel (200-300 mesh) were provided by Qingdao Ocean Chemical Plant.

3.2. Characterization

1H and 13C NMR were measured by Varian AMX 400, where CDCl3 was used as the solvent and tetramethylsilane (TMS) was calibrated as the internal standard. MALDI-TOF MS was measured by Bruker Daltonics Flexanalysis. Elemental analysis was recorded by the Thermo Finnigan Flash EA 1112 analyzer. The thermal stability was measured by NETZSCH STA 499C TG-DSC in an N2 atmosphere, with a heating rate of 15 K/min. UV-vis data were collected by Varian Cary 5000. Fluorescent spectra were measured on an LS55 fluorescent photometer (Perkin Elmer). The electrochemical properties of the compounds were measured by workstation CHI 760C. A Canon Power Shot A3000 IS was used to take pictures of the four compounds and their solutions. SEM images were captured on a Hitachi S 4700 scanning electron microscope. Atomic force microscopy (AFM) measurements were performed

Materials 2018, 11, 2063 15 of 20

3.2. Characterization

1 13 H and C nmR were measured by Varian AMX 400, where CDCl3 was used as the solvent and tetramethylsilane (TMS) was calibrated as the internal standard. MALDI-TOF MS was measured by Bruker Daltonics Flexanalysis. Elemental analysis was recorded by the Thermo Finnigan Flash EA 1112 analyzer. The thermal stability was measured by NETZSCH STA 499C TG-DSC in an N2 atmosphere, with a heating rate of 15 K/min. UV-vis data were collected by Varian Cary 5000. Fluorescent spectra were measured on an LS55 ﬂuorescent photometer (Perkin Elmer). The electrochemical properties of the compounds were measured by workstation CHI 760C. A Canon Power Shot A3000 IS was used to take pictures of the four compounds and their solutions. SEM images were captured on a Hitachi S 4700 scanning electron microscope. Atomic force microscopy (AFM) measurements were performed using a MFP-3DTM instrument. Performance of the memory devices was tested with a Keithley 4200 instrument under ambient air conditions. Elemental analysis was performed on Thermo Finnigan Flash EA 1112 analyzer.

3.3. Synthesis

3.3.1. Synthesis of the Acceptor Unit The synthesis method of acceptor 2,3-bis(4-decyloxybenzene)-5,8-dibromoquinoxaline (compound 8) can be found in the literature [35]. First, 4,7-dibromo-2,1,3-benzothiadiazole was suspended in anhydrous ethanol, and sodium borohydride was used as the reducing agent to get 3,6-dibromo-1,2-phenylenediamine (compound 2) at a yield of 75% (Scheme1a). compound 7 was then prepared via the ether bond-breaking reaction in the presence of glacial acetic acid and 48% hydrogen bromide with 4,4-dimethoxybenzil (compound 5) as the raw material, with a yield of 66% (Scheme1c). Ultimately, compound 8 (yield 70%) was synthesized via a condensation reaction 1 between compounds 2 and 7 (Scheme1d). H nmR (CDCl3, 400 MHz, ppm): δH = 7.85 (s, 2H, ArH), 7.65 (dd, 4H), 6.88 (dd, 4H), 3.98 (t, 4H), 1.84–1.74 (m, 4H), 1.52–1.19 (m, 28H), 0.88 (t, 6H). 13C nmR (CDCl3, δ, ppm): δ = 160.39, 153.53, 138.94, 132.44, 131.63, 130.23, 123.36, 114.30, 68.04, 31.85, 29.53, 25.98, 22.65, 14.10. The preparation of acceptor 2,3-bis(4-decyloxybenzene)-5,8-dibromopyrido[3,4-b]pyrazine (compound 9) was described in the literature [39]. 3,4-diamino-2,5-dibromopyridine (compound 4, yield 52%) was ﬁrst synthesized by bromination of 3,4-diaminopyridine (Scheme1b). compound 9 (yield 63%) was then prepared via a condensation reaction between compounds 4 and 7 (Scheme1e). 1 H nmR (CDCl3, 400 MHz, ppm): δH = 8.68 (s, 1H, ArH), 7.66 (dd, 4H), 6.88 (dd, 4H), 3.99 (t, 4H), 13 1.86–1.73 (m, 4H), 1.48–1.20 (m, 28H), 0.88 (t, 6H). C nmR (CDCl3, δ, ppm): δ = 161.50, 156.77, 136.82, 132.14, 131.87, 129.80, 120.16, 114.75, 68.43, 32.11, 29.57, 26.24, 22.89, 14.32.

3.3.2. Synthesis of the Four D–A–D Compounds The four D–A–D compounds—BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP—were obtained by the Suzuki coupling reaction (Scheme2). Specifically, 3 g of BPC2, 3.93 g of compound 8, 40 mL of sodium carbonate solution (2 M), 60 mL of toluene, catalytic amount of Pd (PPh3)4 (5% of the molar mass of the reactants), and a little amount of the transfer agent TBAF were placed in a round-bottomed flask. The whole reaction system was heated at reflux under an argon atmosphere for 48 h. After completion of the reaction, the solvent was distilled under reduced pressure and washed twice with distilled water, and then extracted with chloroform. The organic phase was collected and dried over anhydrous magnesium sulfate to obtain the BPC-2DPx precursor. The precursor was separated and purified by silica-gel column chromatography using a mixed solution of n-hexane–dichloromethane (3:1 volume ratio) as the eluent to obtain a bright-yellow powdery solid 1 of BPC-2DPx. Yield: 64%. H nmR (CDCl3, 400 MHz, ppm): δ = 8.29 (d, 2H, ArH), 8.23 (d, 2H), 8.08 (s, 2H), 7.95 (s, 2H), 7.75 (d, 2H), 7.65 (d, 4H), 7.54–7.38 (m, 14H), 7.34 (dd, 2H), 6.77 (d, 4H), 13 3.95 (t, 4H), 1.78 (m, 4H), 1.49–1.24 (m, 28H), 0.91 (t, 6H) (see Figure S1a in ESI). C nmR (CDCl3, δ, Materials 2018, 11, 2063 16 of 20 ppm): δ = 159.59, 151.00, 141.31, 140.60, 139.41, 138.54, 137.68, 136.19, 131.63, 131.33, 129.81, 129.75, 127.30, 127.18, 125.86, 123.28, 122.78, 122.61, 120.42, 119.90, 119.80, 114.02, 113.00, 109.72, 67.96, 31.88, 29.56, 29.54, 29.38, 29.31, 29.19, 26.01, 22.67, 14.13 (see Figure S1b in ESI). MS (MALDI-TOF): m/z: calcd for 1076.60; found 1076.66. Elemental analysis calcd (%): C 84.72, H 7.11, N 5.20, O 2.97; found: C 84.63, H 7.14, N 5.23, O 3.00. The synthesis of BPC-3DPx was similar to that of BPC-2DPx. The precursor was separated and purified by silica-gel column chromatography using a mixed solution of n-hexane-dichloromethane 1 (2:1 volume ratio) as eluent to obtain a yellow, powdery solid. Yield: 62%. H nmR (CDCl3, 400 MHz, ppm): δ= 8.77 (d, 2H, ArH), 8.28 (d, 2H), 7.99 (m, 4H), 7.67 (dd, 12H), 7.58 (d, 2H), 7.53–7.44 (m, 6H), 7.37 (m, 2H), 6.80 (d, 4H), 3.93 (t, 4H), 1.75 (m, 4H), 1.47–1.24 (m, 28H), 0.91 (t, 6H) (see Figure S2a 13 in ESI). C nmR (CDCl3, δ, ppm): δ = 159.66, 150.42, 141.23, 140.42, 138.92, 138.42, 137.78, 131.65, 131.35, 130.38, 129.87, 129.43, 129.17, 127.39, 127.10, 125.81, 123.88, 123.28, 123.06, 120.24, 120.05, 114.12, 109.86, 109.17, 67.94, 31.88, 29.56, 29.54, 29.38, 29.31, 29.20, 26.00, 22.68, 14.13 (see Figure S2b in ESI). MS (MALDI-TOF): m/z: calcd for 1076.60; found 1076.63. Elemental analysis calcd (%): C 84.72, H 7.11, N 5.20, O 2.97; found: C 84.60, H 7.16, N 5.25, O 2.99. The precursor of BPC-2DPP was similarly obtained. The precursor was separated and purified by silica-gel column chromatography. The eluent was a mixed solution of n-hexane–dichloromethane (2:1 volume ratio). After purification, an orange powdery solid was obtained. Yield: 65%. 1H nmR 1 (CDCl3, 400 MHz, ppm): δ = 9.12 (s, 1H, ArH), 8.81 (d, H), 8.53 (dd, 1H), 8.31 (dd, 2H), 8.01 (dd, 1H), 7.74 (m, 4H), 7.69–7.45 (m, 17H), 7.39 (m, 2H), 6.86 (dd, 4H), 3.96 (q, 4H), 1.78 (m, 4H), 1.49–1.24 13 (m, 28H), 0.91 (m, 6H) (see Fig. S3a in ESI). C nmR (CDCl3,δ, ppm): δ = 160.41, 160.10, 157.79, 154.25, 151.75, 145.64, 141.52, 141.40, 141.28, 140.68, 137.65, 133.56, 131.57, 131.43, 131.33, 131.07, 130.78, 129.98, 129.89, 129.57, 128.87, 127.52, 127.50, 127.12, 126.00, 124.12, 123.72, 123.53, 123.29, 122.93, 120.42, 120.31, 114.33, 114.27, 109.94, 109.55, 109.22, 68.04, 31.88, 29.54, 29.37, 29.30, 29.19, 26.00, 22.67, 14.12 (see Figure S3b in ESI). MS (MALDI-TOF): m/z: calcd for 1077.59; found 1077.72. Elemental analysis calcd (%): C 83.53, H 7.01, N 6.49, O 2.97; found: C 83.40, H 7.07, N 6.55, O 2.98. The precursor of BPC-3DPP was obtained by the same method used to prepare the precursor of BPC-2DPP. The precursor was separated and purified by silica-gel column chromatography. The eluent was a mixed solution of n-hexane–dichloromethane (1.5:1 volume ratio). After purification, a yellow, 1 powdery solid was obtained. Yield: 70%. H nmR (CDCl3, 400 MHz, ppm): δ = 9.06 (s, 1H, ArH), 8.49 (s, 1H), 8.34 (t, 1H), 8.24 (m, 1H), 8.10 (d, 1H), 7.79 (t, 1H), 7.67 (t, 4H), 7.57–7.37 (m, 17H), 7.35 (dd, 2H), 6.79 (d, 4H), 3.97 (q, 4H), 1.80 (s, 4H), 1.50–1.24 (m, 28H), 0.88 (m, 6H) (see Figure S4a in 13 ESI). C nmR (CDCl3, δ, ppm): δ = 160.32, 159.97, 158.57, 154.88, 152.42, 145.88, 141.62, 141.51, 141.37, 140.77, 140.50, 137.69, 137.59, 132.81, 131.89, 131.52, 131.25, 130.93, 130.77, 129.78, 127.35, 127.24, 127.15, 126.10, 124.00, 123.16, 122.43, 120.70, 120.51, 120.13, 119.82, 114.23, 114.16, 113.63, 112.94, 109.78, 68.04, 31.87, 29.54, 29.37, 29.29, 29.17, 26.01, 22.66, 14.11 (see Figure S4b in ESI). MS (MALDI-TOF): m/z: calcd for 1077.59; found 1077.72. Elemental analysis calcd (%): C 83.53, H 7.01, N 6.49, O 2.97; found: C 83.43, H 7.05, N 6.56, O 2.96.

3.4. Memory Device Fabrication A schematic sandwich structure comprised of a pair of electrodes and a layer of D–A–D compounds is shown in Figure 11. Firstly, the ITO glass was cleaned with water, and then sonicated in acetone and ethanol for 30 min, respectively. Then, the ﬁlms of BDP-DPx and BDP-DPP were vapor-deposited on ITO glass under a pressure of around 1.0 × 10−3 Pa and at a temperature of 185 ◦C. Finally, the Al top electrode was thermally eVaporated onto the ﬁlm surface through a shadow mask with a thickness around 100 nm and area of 0.20 mm2 at a pressure of 5.0 × 10−4 Pa with a uniform depositing rate of 4 nm/min. Materials 2018, 11, x FOR PEER REVIEW 16 of 20

130.38, 129.87, 129.43, 129.17, 127.39, 127.10, 125.81, 123.88, 123.28, 123.06, 120.24, 120.05, 114.12, 109.86, 109.17, 67.94, 31.88, 29.56, 29.54, 29.38, 29.31, 29.20, 26.00, 22.68, 14.13 (see Figure S2b in ESI). MS (MALDI-TOF): m/z: calcd for 1076.60; found 1076.63. Elemental analysis calcd (%): C 84.72, H 7.11, N 5.20, O 2.97; found: C 84.60, H 7.16, N 5.25, O 2.99. The precursor of BPC-2DPP was similarly obtained. The precursor was separated and purified by silica-gel column chromatography. The eluent was a mixed solution of n-hexane–dichloromethane (2:1 volume ratio). After purification, an orange powdery solid was obtained. Yield: 65%. 1H NMR (CDCl3, 400 MHz, ppm): δ = 9.12 (s, 1H, ArH), 8.81 (d, 1H), 8.53 (dd, 1H), 8.31 (dd, 2H), 8.01 (dd, 1H), 7.74 (m, 4H), 7.69–7.45 (m, 17H), 7.39 (m, 2H), 6.86 (dd, 4H), 3.96 (q, 4H), 1.78 (m, 4H), 1.49–1.24 (m, 28H), 0.91 (m, 6H) (see Fig. S3a in ESI). 13C NMR (CDCl3,δ, ppm): δ = 160.41, 160.10, 157.79, 154.25, 151.75, 145.64, 141.52, 141.40, 141.28, 140.68, 137.65, 133.56, 131.57, 131.43, 131.33, 131.07, 130.78, 129.98, 129.89, 129.57, 128.87, 127.52, 127.50, 127.12, 126.00, 124.12, 123.72, 123.53, 123.29, 122.93, 120.42, 120.31, 114.33, 114.27, 109.94, 109.55, 109.22, 68.04, 31.88, 29.54, 29.37, 29.30, 29.19, 26.00, 22.67, 14.12 (see Figure S3b in ESI). MS (MALDI-TOF): m/z: calcd for 1077.59; found 1077.72. Elemental analysis calcd (%): C 83.53, H 7.01, N 6.49, O 2.97; found: C 83.40, H 7.07, N 6.55, O 2.98. The precursor of BPC-3DPP was obtained by the same method used to prepare the precursor of BPC-2DPP. The precursor was separated and purified by silica-gel column chromatography. The eluent was a mixed solution of n-hexane–dichloromethane (1.5:1 volume ratio). After purification, a yellow, powdery solid was obtained. Yield: 70%. 1H NMR (CDCl3, 400 MHz, ppm): δ = 9.06 (s, 1H, ArH), 8.49 (s, 1H), 8.34 (t, 1H), 8.24 (m, 1H), 8.10 (d, 1H), 7.79 (t, 1H), 7.67 (t, 4H), 7.57–7.37 (m, 17H), 7.35 (dd, 2H), 6.79 (d, 4H), 3.97 (q, 4H), 1.80 (s, 4H), 1.50–1.24 (m, 28H), 0.88 (m, 6H) (see Figure S4a in ESI). 13C NMR (CDCl3, δ, ppm): δ = 160.32, 159.97, 158.57, 154.88, 152.42, 145.88, 141.62, 141.51, 141.37, 140.77, 140.50, 137.69, 137.59, 132.81, 131.89, 131.52, 131.25, 130.93, 130.77, 129.78, 127.35, 127.24, 127.15, 126.10, 124.00, 123.16, 122.43, 120.70, 120.51, 120.13, 119.82, 114.23, 114.16, 113.63, 112.94, 109.78,68.04, 31.87, 29.54, 29.37, 29.29, 29.17, 26.01, 22.66, 14.11 (see Figure S4b in ESI). MS (MALDI- TOF): m/z: calcd for 1077.59; found 1077.72. Elemental analysis calcd (%): C 83.53, H 7.01, N 6.49, O 2.97; found: C 83.43, H 7.05, N 6.56, O 2.96.

3.4. Memory Device Fabrication A schematic sandwich structure comprised of a pair of electrodes and a layer of D–A–D compounds is shown in Figure 11. Firstly, the ITO glass was cleaned with water, and then sonicated in acetone and ethanol for 30 min, respectively. Then, the films of BDP-DPx and BDP-DPP were vapor-deposited on ITO glass under a pressure of around 1.0 × 10−3 Pa and at a temperature of 185 °C. Finally, the Al top electrode was thermally evaporated onto the film surface through a shadow maskMaterials with2018 a, 11thickness, 2063 around 100 nm and area of 0.20 mm2 at a pressure of 5.0 × 10−4 Pa17 with of 20 a uniform depositing rate of 4 nm/min.

FigureFigure 11. 11. SchemeScheme of the ITO/D-A-DITO/D-A-D compound/Alcompound/Al memorymemory device. device. 4. Conclusions Four D–A–D compounds were synthesized using a one-step Suzuki coupling reaction with DPx or DPP as the acceptor, and BPC2 or BPC3 as the donor. The acceptor DPP had one more electron-deficient N-atom than DPx, which resulted in a larger red-shift of both the ICT absorption band and the fluorescent emission spectrum of compounds with DPP as the acceptor compared with those of the compounds with DPx as the acceptor, indicating that the ICT effect between DPP and the donor is stronger than that between DPx and the donor. The molecules obtained with BPC3 as the donor exhibited greater coplanarity than those with BPC2 as the donor. When DPx was used as the acceptor, more significant red-shifts occurred for both the ICT absorption band and the fluorescent emission spectra of the BPC3-based compounds compared with those of the BPC2-based compounds. When DPP was used as the acceptor, a larger blue-shift occurred for both the ICT absorption band and the fluorescent emission spectra of the BPC3-based compounds compared with those of the BPC2-based compounds. The replacement of the benzene ring with the pyridine ring in the DPP unit alters the direction of the electron cloud density of the DPx unit, and then leads to the different substituent effect with the adjacent linked groups. The Eg values of the four D–A–D compounds—BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP—were 2.82 eV, 2.70 eV, 2.48 eV, and 2.62 eV, respectively. In summary, compared with DPx, DPP exhibits stronger electron-withdrawing ability. When used for constructing D–A–D compounds with the same donor, molecules with DPP as the electron acceptor have lower Eg values. Better coplanarity between the donor and acceptor is beneficial for improving the degree of conjugation, and in theory may reduce the Eg value. This discovery aids in adjusting the Eg of optoelectronic materials by enabling the selection of an appropriate donor and acceptor moiety during the construction of the D–A–D compound to meet different application requirements. The memory devices fabricated with BPC-2DPx and BPC-2DPP as the active material were found to exhibit a binary non-volatile flash memory nature. They also exhibited a high ON/OFF current ratio and respectable retention ability under ambient conditions.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/11/10/2063/ 1 13 s1, Figure S1. (a) H nmR spectrum of BPC-2DPx in CDCl3 solution (b) C nmR spectrum of BPC-2DPx in CDCl3 1 13 solution, Figure S2. (a) H nmR spectrum of BPC-3DPx in CDCl3 solution, (b) C nmR spectrum of BPC-3DPx 1 13 in CDCl3 solution, Figure S3. (a) H nmR spectrum of BPC-2DPP in CDCl3 solution, (b) C nmR spectrum of 1 13 BPC-2DPP in CDCl3 solution, Figure S4. (a) H nmR spectrum of BPC-3DPP in CDCl3 solution, (b) C nmR spectrum of BPC-3DPP in CDCl3 solution, Figure S5. (a) TGA curves of BPC-3DPx, (b) TGA curves of BPC-2DPP, (c) TGA curves of BPC-3DPP, Figure S6. Current-voltage (I-V) characteristics of (a) ITO/BPC-3DPx/Al and (b) ITO/BPC-3DPP/Al memory devices. Author Contributions: X.S. synthesized the compounds and drafted the manuscript; L.K. fabricated the memory device; H.D. conducted the spectrum analysis test; X.L. and H.F. conducted the test of the memory device; J.Z. provided the idea of the manuscript; Y.X. reviewed and edited the paper. Acknowledgments: The work was ﬁnancially supported by the National Natural Science Foundation of China (51473074, 21,601,079 and 21667019), and thanks to Edward C. Mignot, Shandong University, for linguistic advice. Materials 2018, 11, 2063 18 of 20

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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