Electrosynthesis and Electrochromism of a New Crosslinked Polydithienylpyrrole with Diphenylpyrenylamine Subunits

polymers

Article Electrosynthesis and Electrochromism of a New Crosslinked Polydithienylpyrrole with Diphenylpyrenylamine Subunits

Yu-Ruei Kung 1,* , Sin-Yun Cao 2 and Sheng-Huei Hsiao 2,*

1 Department of Chemical Engineering and Biotechnology, Tatung University, Taipei 10452, Taiwan 2 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] * Correspondence: [email protected] (Y.-R.K.); [email protected] (S.-H.H.)

Received: 30 October 2020; Accepted: 18 November 2020; Published: 24 November 2020

Abstract: A new electroactive monomer with two 2,5-di(2-thienyl)pyrrole (SNS) units and one diphenylpyrenylamine (DPPA) subunit, namely N,N-bis(4-(2,5-di(2-thienyl)-1H-pyrrol-1-yl)-phenyl)- 1-aminopyrene (DPPA-2SNS), was synthesized from 1,4-di-(2-thienyl)butane-1,4-dione with N,N-di(4-aminophenyl)-1- aminopyrene through the Paal–Knorr condensation reaction. Visible and near-infrared (NIR) electrochromic polymer films could be facilely generated on the ITO-glass surface by the electrochemical polymerization of DPPA-2SNS in an electrolyte solution. The electrosynthesized polymer films exhibit multi-staged redox processes and multi-colored anodic electrochromic behavior. A multi-colored electrochromism, with yellowish orange, greyish blue, and purplish black colors, was observed in the polymer film by applying a positive potential. The polymer films exhibit reasonable coloration efficiency, fast response time, and good cycling stability, especially when switched between neutral and the first oxidation states. For comparison, N-(1-pyrenyl)-2,5-di(2-thienyl)pyrrole (Py-SNS) was also prepared and characterized with electrochemical and electro-optical properties.

Keywords: conjugated polymers; electrochromic polymers; 2,5-Di (2-thienyl)-1H-pyrrole; diphenylpyrenylamine

1. Introduction Electrochromism (EC) refers to a reversible change in the optical absorption or color of electroactive species as a result of electrochemical oxidation or reduction induced by a simple electric potential [1]. Over the past two decades, EC materials have received great interest and have been extensively investigated for many potential applications [2–7], such as smart windows, displays, automatically dimming mirrors, e-paper, self-tunable eyewear, adaptive camouflage, and energy storage devices. There are many types of EC materials that have been developed, including inorganic metal oxides, organic small molecules, organic π-conjugated polymers, and organic–inorganic coordination polymers [8]. In general, organic π-conjugated polymers demonstrate a number of advantages over inorganic and organic small molecule materials [9–12]. They possess great potential for the fabrication of flexible devices, diverse and multicolored electrochromisms, facile molecular design, and high coloration efficiency. Triphenylamine (TPA) and its derivatives are the important building blocks for the preparation of electroactive molecules because of their excellent electron donating nature, the easy oxidizability of their nitrogen center, and their ability to transport charge carriers with high stability [13–15]. Many triarylamine derivatives and polymers have been developed for potential optoelectronic

Polymers 2020, 12, 2777; doi:10.3390/polym12122777 www.mdpi.com/journal/polymers Polymers 2020, 12, 2777 2 of 18 applications such as organic light-emitting devices (OLEDs), organic field-effect transistors (OFETs), and solar cells [16–18]. Triarylamine-containing condensation polymers such as aromatic polyamides and polyimides have been developed as a type of promising high-performance functional material owing to several useful properties such as excellent thermal stability, high electrochemical stability, good electrochromic performance, and the formation of stable amorphous films for application in thin-film devices [19,20]. The electronic and photophysical properties of pyrene are very attractive for the design of new organic light-emitting and semiconductor materials. The long fluorescence lifetime, large Stokes shift, and the tunable intensity of the excimer make the pyrene unit a useful form of fluorophore labeling in metal ion or nucleic acid probes [21,22]. The derivatives, polymers, starbursts, and dendrimers of pyrene have been extensively investigated for opto-electronic applications such as OLEDs, because of their emissive properties combined with high charge carrier mobility [23]. Owing to the attractive properties associated with the pyrene and triarylamine moieties, we have developed many diphenylpyrenylamine (DPPA)-based polymers with dual fluorescent and electrochromic functions [24–26]. Thiophene, pyrrole, carbazole, viologen, triphenylamine and others are commonly used as building blocks in order to modify the electrochromic properties of the π-conjugated polymers [27–32]. Since 1986, poly(2,5-dithienylpyrrole)s (PSNS) have attracted particular interest, which could be easily synthesized via chemical and electrochemical polymerization of 2,5-dithienylpyrrole (SNS) unit [33–35]. In the past three decades, an exciting class of conducting polymer, PSNS derivatives, have been much investigated for their electrochromic properties [36–42]. Because of their thiophene–pyrrole–thiophene terarylenic structures, all the monomers and their corresponding polymers exhibited low oxidation potential. In general, the PSNS homopolymers displayed a yellow to blue coloration with a short switching time. In addition, the electrochemical and electrochromic properties can be easily adjusted by attaching a second electroactive moiety such as triarylamine on the side chain [43–45]. In this work, a new electropolymerizable monomer containing a diphenylpyrenylamine (DPPA) core and two SNS units, coded as DPPA-2SNS, was prepared from N,N-di(4-aminophenyl)-1-aminopyrene with two equivalent amounts of 1,4-di(2-thienyl)butane-1,4-dione via the Paal–Knorr pyrrole synthesis. The electrochemical and optical properties of this SNS monomer were investigated. Their corresponding PSNS films were prepared by electropolymerization, and their electrochemical, electrochromic, and fluorescent properties were investigated and compared to those from N-(1-pyrenyl)-2,5-di(2-thienyl)pyrrole (Py-SNS), which has been reported in the literature [46].

2. Experimental Section

2.1. Materials The synthetic details and characterization data of the starting materials including 1-aminopyrene, N,N-di(4-aminophenyl)-1-aminopyrene (DPPA-2NH2) have been reported previously [24]. The synthetic details and analysis of 1,4-di(2-thienyl)butane-1,4-dione (DTBDO), N-(1-pyrenyl)-2,5-di(2-thienyl)pyrrole (Py-SNS), N,N-bis(4-(2,5-di(2-thienyl)-1H-pyrrol-1-yl)phenyl)-1-aminopyrene (DPPA-2SNS) were summarized in the Supplementary Materials.

2.2. Electrochemical Polymerization Electrochemical polymerization was performed with a CH Instruments 750A electrochemical analyzer. The PSNS were prepared by repetitive CV cycling of the SNS monomers in acetonitrile (MeCN) or dichloromethane (CH2Cl2) solutions containing 0.1 M Bu4NClO4 at a scan rate of 50 mV/s in a suitable potential range for ten cycles. The polymer was deposited onto the surface of the working electrode (ITO/glass substrate, polymer ﬁlms area about 0.8 2.0 cm2), and the ﬁlm was rinsed with × plenty of acetone and dichloromethane for the removal of the un-reacted monomer, inorganic salts and other organic impurities formed during the process. Polymers 2020, 12, 2777 3 of 18

2.3. Measurements and Methods Infrared (IR) spectra were recorded on a Horiba FT-720 FT-IR spectrometer. 1H NMR spectra were measured on a Bruker Avance III HD-600 MHz NMR spectrometer with tetramethylsilane (TMS) as an internal standard. Ultraviolet-visible (UV-Vis) spectra of the synthesized compounds and polymer films were recorded on an Agilent 8454 UV-visible spectrometer. Electrochemistry was performed with a CHI 750A electrochemical analyzer (Austin, TX, USA). Voltammograms are presented with the positive potential pointing to the left and with increasing anodic currents pointing downwards. Cyclic voltammetry (CV) was conducted with the use of a three-electrode cell in which ITO (polymer films area about 0.8 2.0 cm2) was used as a working electrode. Ferrocene was used as × an external reference for calibration (+0.48 V vs. Ag/AgCl). Spectroelectrochemistry analyses were carried out with an electrolytic cell, which was composed of a 1 cm cuvette, ITO as a working electrode, a platinum wire as an auxiliary electrode, and an Ag/AgCl reference electrode. Absorption spectra in the spectroelectrochemical experiments were measured with an Agilent 8454 UV-Visible diode array spectrophotometer (Santa Clara, CA, USA). Color coordinates of the electrochromic films were measured on an Admesy Brontes colorimeter. Photoluminescence (PL) spectra were measured with a Horiba FluoroMax-4 Spectrofluorometer (Kyoto, Japan). Fluorescent quantum yields (ΦPL) of the samples in different solvents were calculated by comparing emission with that of a standard solution of 9,10-diphenylanthracene in cyclohexane (ΦPL = 90%) at room temperature.

2.4. Fabrication of Electrochromic Devices Electrochromic polymer ﬁlms were electrodeposited on the ITO-coated glass substrate by the electropolymerization method described above. A gel electrolyte based on PMMA (Mw: 120,000) and LiClO4 was plasticized with propylene carbonate (PC) to form a highly transparent and conductive gel. PMMA (1 g) was dissolved in dry MeCN (4 mL), and LiClO4 (0.1 g) was added to the polymer solution as a supporting electrolyte. Then, propylene carbonate (1.5 g) was added as a plasticizer. The mixture was then gently stirred until gelation. The gel electrolyte was spread on the polymer-coated side of the electrode, and the electrodes were sandwiched.

3. Results and Discussion

3.1. Synthesis and Structural Characterization Due to the highly fluorescent property of pyrene moiety and redox-activity of triarylamine unit, incorporation of diphenylpyrenylamine (DPPA) subunit into the SNS monomer may lead to new compounds and PSNS with dual fluorescent and electrochromic functions. According to Scheme1, 1-aminopyrene (Py-NH 2) was synthesized starting from the nitration of pyrene, followed by Pd/C-catalyzed hydrazine reduction of the intermediate 1-nitropyrene (Py-NO2). N,N-Di(4-aminophenyl)-1-aminopyrene (DPPA-2NH2) was prepared by CsF-assisted N,N-diarylation reaction of 1-aminopyrene with p-fluoronitrobenzene, followed by Pd/C-catalyzed reduction of the intermediate dinitro compound N,N-di(4-nitrophenyl)-1-aminopyrene (DPPA-2NO2). The synthetic details and characterization data of the synthesized compounds were reported in our previous 1 publication [24]. The IR and H-NMR spectra of DPPA-2NH2 are demonstrated in the Supplementary Materials. The SNS-based monomers Py-SNS and DPPA-2SNS were prepared by the Paal–Knorr pyrrole synthesis from Py-NH2 and DPPA-2NH2, respectively, with 1,4-di(2-thienyl)butane-1,4-dione, using p-toluenesulfonic acid (PTSA) as an acid catalyst. The synthetic route is shown in Scheme2, and all the target compounds were characterized by FTIR and 1H NMR spectroscopy. Polymers 2020, 12, x FOR PEER REVIEW 4 of 19

Polymers 2020, 12, 2777 4 of 18 Polymers 2020, 12, x FOR PEER REVIEW 4 of 19

Scheme 1. Synthetic routes to 1-aminopyrene (Py-NH2) and N,N-di(4-aminophenyl)-1-aminopyrene

(DPPA-2NH2).

The SNS-based monomers Py-SNS and DPPA-2SNS were prepared by the Paal–Knorr pyrrole synthesis from Py-NH2 and DPPA-2NH2, respectively, with 1,4-di(2-thienyl)butane-1,4-dione, using p-toluenesulfonic acid (PTSA) as an acid catalyst. The synthetic route is shown in Scheme 2, and all 1 the targetSchemeScheme compounds 1. 1.Synthetic Synthetic were routesroutes characterized to to 1-aminopyrene 1-aminopyrene by FTIR (Py-NH (Py-NH and2)2) and andH NMRN N,N,N-di(4-aminophenyl)-1-aminopyrene-di(4-aminophenyl)-1-aminopyrene spectroscopy.

(DPPA-2NH(DPPA-2NH2).2).

SchemeScheme 2.2. SynthesisSynthesis ofof 1,4-di(thiophene-2-yl)butane-1,4-dione1,4-di(thiophene-2-yl)butane-1,4-dione (DTBDO),(DTBDO), Py-SNS,Py-SNS, andand DPPA-2SNS.DPPA-2SNS.

Figure S1 (Supplementary Materials) illustrates the FT-IR spectra of Py-NH2, DPPA-2NH2 and Figure S1 (Supplementary Materials) illustrates the FT-IR spectra of Py-NH2, DPPA-2NH2 and their nitro precursors. 1-Nitropyrene and N,N-di(4-nitrophenyl)-1-aminopyrene show the characteristic their nitro precursors. 1-Nitropyrene and N,N-di(4-nitrophenyl)-1-aminopyrene show the nitro absorption pair at 1586/1331 and 1577/1304 cm 1 ( NO asymmetric and symmetric stretching), − 2 −1 characteristic nitro absorption pair at 1586/1331 and 1577/1304− cm (−NO2 asymmetric and symmetric respectively. After reduction, the characteristic absorptions of the nitro group disappear and the stretching), respectively. After reduction, the characteristic absorptions of the nitro group disappear 1 amino compounds show the typical NH2 stretching absorptions in the region from 3200 to 3430 cm− . and the amino compounds show the− typical −NH2 stretching absorptions in the region from 3200 to IR spectraScheme of 1,4-di(2-thienyl)butane-1,4-dione 2. Synthesis of 1,4-di(thiophene-2-yl)butane-1,4-dione (DTBDO), Py-SNS, (DTB andDO), DPPA-2SNS Py-SNS, and are DPPA-2SNS summarized. in 3430 cm−1. IR spectra of 1,4-di(2-thienyl)butane-1,4-dione (DTBDO), Py-SNS, and DPPA-2SNS are Figure S2. Characteristic absorptions agree well with the desired molecular structures. The complete summarizedFigure S1 in (Supplementary Figure S2. Characteristic Materials) illustrates absorptions the FT-IRagree spectra well withof Py-NH the 2desired, DPPA-2NH molecular2 and conversion to the N-substituted pyrrole unit could be conﬁrmed by the disappearance of carbonyl structures.their nitro The precursors. complete1 conversion1-Nitropyrene to the and N-subs N,Ntituted-di(4-nitrophenyl)-1-a pyrrole unit couldminopyrene 1be confirmed show by the absorption at 1565 cm− and aliphatic C–H stretching absorption at 2922 cm− of DTBDO and the −1 −1 disappearancecharacteristic nitro of carbonyl absorption absorption pair at 1586/1331 at 1565 cm and 1577/1304and aliphatic1 cm C–H1 (−NO stretching2 asymmetric absorption and symmetric at 2922 primary amine absorption in the range of 3200 3430 cm− . The H NMR spectra of DPPA-2NH2, −1 − −1 1 DTBDO,cmstretching), of DTBDO and respectively. the and SNS the monomers Afterprimary reduction, Py-SNS amine absorptionthe and char DPPA-2SNSacteristic in the absorptions arerange illustrated of 3200 of − inthe3430 Figures nitro cm group S3–S7.. The disappear H All NMR the resonance and the amino peaks compounds can be well show assigned the typical to the − molecularNH2 stretching structures absorptions of the in synthesized the region compounds.from 3200 to −1 Although3430 cm the. IR resonance spectra of signals 1,4-di(2-thienyl)butane-1,4-dione of the pyrenyl protons are somewhat (DTBDO), complicated, Py-SNS, and full DPPA-2SNS assignments are of allsummarized peaks can bein done Figure with S2. the Characteristic aid of two-dimensional absorptions H-H agree COSY well NMR with spectra. the Thus,desired the molecular results of allstructures. the spectroscopic The complete analysis conversion suggest the to successfulthe N-subs preparationtituted pyrrole of the un targetedit could compounds.be confirmed by the disappearance of carbonyl absorption at 1565 cm−1 and aliphatic C–H stretching absorption at 2922 cm−1 of DTBDO and the primary amine absorption in the range of 3200−3430 cm−1. The 1H NMR

Polymers 2020, 12, x FOR PEER REVIEW 5 of 19

spectra of DPPA-2NH2, DTBDO, and the SNS monomers Py-SNS and DPPA-2SNS are illustrated in Figures S3–S7. All the resonance peaks can be well assigned to the molecular structures of the synthesized compounds. Although the resonance signals of the pyrenyl protons are somewhat complicated, full assignments of all peaks can be done with the aid of two-dimensional H-H COSY NMR spectra. Thus, the results of all the spectroscopic analysis suggest the successful preparation of Polymersthe targeted2020, 12 compounds., 2777 5 of 18

3.2. UV-Vis Absorption and Photoluminescence of DPPA-2SNS Monomer 3.2. UV-Vis Absorption and Photoluminescence of DPPA-2SNS Monomer UV-Vis absorption and photoluminescence (PL) spectra of DPPA-2SNS in dilute solution (~1 × 10−5 M)UV-Vis and the absorption PL imageand of the photoluminescence solution under 365 (PL)nm light spectra are shown of DPPA-2SNS in Figure 1. in Their dilute absorption solution 5 (~1and PL10 −dataM) andare thesummarized PL image ofin the Table solution S1. The under solutions 365 nm lightof DPPA-2SNS are shown in in Figure different1. Their solvents absorption show × andabsorption PL data maxima are summarized in the range in Tableof 326~328 S1. The nm. solutions The absorption of DPPA-2SNS maximum in dishowsfferent little solvents shift in show the absorptiontested solvents, maxima indicating in the rangethat the of solvent 326~328 polarity nm. The exerts absorption little effect maximum on its showsground-state little shift electronic in the testedtransition. solvents, DPPA-2SNS indicating is fluorescent that the solvent and exhibits polarity a exertsPL maximum little effect wavelength on its ground-state of 439 nm, electronic resulting transition.in blue emission DPPA-2SNS in toluene is fluorescent solution. and The exhibits PL em aission PL maximum of DPPA-2SNS wavelength displays of 439 a nm, slight resulting solvent- in bluepolarity emission dependence, in toluene revealing solution. a The dominant PL emission broa ofd DPPA-2SNSemission band displays that aundergoes slight solvent-polarity remarkable dependence,bathochromic revealing shifts with a dominant an increase broad in emission the solvent band thatpolarity. undergoes The remarkableemission wavelength bathochromic slightly shifts withincreased an increase with increasing in the solvent solvent polarity. polarity, The changing emission from wavelength PL λmax slightlyof 439 nm increased in toluene with to increasing453 nm in solventDMSO. polarity,The quantum changing yield fromof DPPA-2SNS PL λmax of diminished 439 nm in toluenesignificantly to 453 from nm 35% in DMSO. in toluene The to quantum 0.2% in yieldDMSO. of DPPA-2SNSThe fluorescence diminished solvatochromism significantly fromcan be 35% attributed in toluene to to the 0.2% fast in DMSO.intramolecular The fluorescence charge- solvatochromismtransfer process, resulting can be attributed in a large tochange the fast in intramoleculardipole moment charge-transferin the excited state. process, resulting in a large change in dipole moment in the excited state.

5 Figure 1. PL images, UV-vis absorption, and PL proﬁles of the dilute solutions (ca. 1 10 −5 M) of Figure 1. PL images, UV-vis absorption, and PL profiles of the dilute solutions (ca. 1× × 10− M) of DPPA-2SNS inin variousvarious solvents.solvents. PhotographsPhotographs werewere takentaken underunder illuminationillumination ofof 365365 nmnm UVUV light.light. 3.3. Electrochemical Activity and Polymerization of SNS Monomers

The electrochemical properties of Py-SNS and DPPA-2SNS2 were probed by cyclic voltammetry in 0.1 M Bu4NClO4 solution of acetonitrile (MeCN) or dichloromethane (CH2Cl2). Figure2 presents the cyclic voltammetry (CV) diagrams of 1 mM Py-SNS in 0.1 M Bu4NClO4/MeCN at a scan rate of 50 mV/s between 0.00 and 1.00 V. As shown in the first CV scan, the SNS unit started oxidation at about 0.7 V, followed by an oxidation peak at 0.94 V. In the second scan, a new oxidation peak appeared at around 0.64 V and intensified after each cycle, which indicated the occurrence of the coupling reactions between terminal thiophene cation radicals forming the bis- or oligo-SNS moiety. After 10 cycles of CV scan, a perceived yellowish orange polymer film was deposited on the ITO-glass (as shown in the inset Polymers 2020, 12, x FOR PEER REVIEW 6 of 19

3.3. Electrochemical Activity and Polymerization of SNS Monomers The electrochemical properties of Py-SNS and DPPA-2SNS2 were probed by cyclic voltammetry

in 0.1 M Bu4NClO4 solution of acetonitrile (MeCN) or dichloromethane (CH2Cl2). Figure 2 presents

the cyclic voltammetry (CV) diagrams of 1 mM Py-SNS in 0.1 M Bu4NClO4/MeCN at a scan rate of 50 mV/s between 0.00 and 1.00 V. As shown in the first CV scan, the SNS unit started oxidation at about 0.7 V, followed by an oxidation peak at 0.94 V. In the second scan, a new oxidation peak appeared at around 0.64 V and intensified after each cycle, which indicated the occurrence of the coupling reactions between terminal thiophene cation radicals forming the bis- or oligo-SNS moiety. After 10 Polymers 2020, 12, 2777 6 of 18 cycles of CV scan, a perceived yellowish orange polymer film was deposited on the ITO-glass (as shown in the inset of Figure 2c). The polymer film adhered firmly on the electrode surface and could ofnot Figure be removed2c). The from polymer the filmITO-glass adhered substrate, firmly onev theen after electrode being surface immersed and couldin water not for be removeda month; fromhowever, the ITO-glass the film could substrate, completely even after dissolve being in immersed NMP and in concentrated water for a month; sulfuric however, acid under the filmultrasonic could completelyoscillation in dissolve two hours. in NMP and concentrated sulfuric acid under ultrasonic oscillation in two hours.

Figure 2. (a) The ﬁrst CV scan, (b) the ﬁrst two CV scans, and (c) ten repetitive CV scans of 1 mM of Figure 2. (a) The first CV scan, (b) the first two CV scans, and (c) ten repetitive CV scans of 1 mM of Py-SNS in 0.1 M TBAP/MeCN in the potential range of 0.00–1.00 V at a scan rate of 50 mV/s. (d): DPV Py-SNS in 0.1 M TBAP/MeCN in the potential range of 0.00–1.00 V at a scan rate of 50 mV/s. (d): DPV scans at scan rate of 5 mV/s. scans at scan rate of 5 mV/s.

Figure S8 shows the CV diagrams of 1 mM Py-SNS in 0.1 M Bu4NClO4/CH2Cl2 at a scan rate of 50Figure mV/s betweenS8 shows 0.00 the andCV diagrams 1.10 V. Similar of 1 mM to thatPy-SNS observed in 0.1 inM acetonitrile,Bu4NClO4/CH a new2Cl2 at oxidation a scan rate peak of at50 0.66mV/s V between was observed 0.00 and in the 1.10 second V. Similar CV scan; to that however, observed the in current acetonitrile, density a ofnew the oxidation redox waves peak did at not0.66 increase V was observed with successive in the second scans, CV and scan; the monomer however, didthe notcurrent give density the corresponding of the redox polymer waves did on not the electrodeincrease with surface successive in dichloromethane. scans, and the The monomer result is similar did not to give the finding the corresponding reported in the polymer literature on [46the]. electrode surface in dichloromethane. The result is similar to the finding reported in the literature Thus, the electropolymerization of monomer Py-SNS was carried out in 0.1 M Bu4NClO4/MeCN. [46]. DPPA-2SNSThus, the electropolymerization is hardly soluble in acetonitrile, of monomer but Py-SNS it is readily was carried soluble out in dichloromethane. in 0.1 M Bu4NClO Therefore,4/MeCN. the CVDPPA-2SNS experiments is ofhardly this SNS soluble monomer in acetonitrile, were performed but it in is dichloromethane. readily soluble Figurein dichloromethane.3 illustrates the CVTherefore, diagrams the of CV DPPA-2SNS experiments (0.2 of mM) this inSNS a 0.1 monome M Bu4NClOr were4 /performedCH2Cl2 solution in dichloromethane. over a range of Figure applied 3 potentialsillustrates ofthe 0.00–1.70 CV diagrams V. There of DPPA-2SNS are two oxidation (0.2 mM) waves in ata 0.1 around M Bu 0.874NClO and4/CH 1.512Cl V2 observed solution onover the a firstrange CV of cycle, applied which potentials can be attributedof 0.00–1.70 to V. the There oxidation are two of the oxidation SNS moiety waves and at thearound combined 0.87 and oxidation 1.51 V reactionsobserved ofon the the amino first CV center cycle, of DPPAwhich andcan thebe attr pyreneibuted group, to the respectively. oxidation of Due the to SNS the inmoiety situ coupling and the reactioncombined of oxidation the SNS units reactions during of the the amino first CV center cycle, of a DPPA new redox and the couple pyrene appeared group, atrespectively. 0.66 and 0.49 Due V in the subsequent cycle. The oxidation peak at 1.07 V in the second CV cycle should be related to the oxidation of the DPPA amino center. In addition, the gradual broadening of the redox waves could be clearly seen in successive scans, indicating the gradual deposition of electroactive species on the working electrode. After ten CV scans, a visible dark blue polymer film formed on the ITO electrode. The electrodeposited film was insoluble in NMP and concentrated sulfuric acid because of the crosslinking structure of the polymer matrix. The polymer film can be stripped off from the ITO-glass after being immersed in water. Polymers 2020, 12, x FOR PEER REVIEW 7 of 19 to the in situ coupling reaction of the SNS units during the first CV cycle, a new redox couple appeared at 0.66 and 0.49 V in the subsequent cycle. The oxidation peak at 1.07 V in the second CV cycle should be related to the oxidation of the DPPA amino center. In addition, the gradual broadening of the redox waves could be clearly seen in successive scans, indicating the gradual deposition of electroactive species on the working electrode. After ten CV scans, a visible dark blue polymer film formed on the ITO electrode. The electrodeposited film was insoluble in NMP and concentrated sulfuric acid because of the crosslinking structure of the polymer matrix. The polymer Polymers 2020, 12, 2777 7 of 18 film can be stripped off from the ITO-glass after being immersed in water.

Figure 3. (a) The ﬁrst CV scan, (b) the ﬁrst two second CV scan, and (c) ten repetitive CV scans of Figure 3. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 0.2 0.2 mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.70 V at a scan rate mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.70 V at a scan rate of of 50 mV/s. 50 mV/s. However, the applied voltage of 1.7 V may cause an over-oxidation of the deposited polymer. As shownHowever, in Figures the applied4 and5, voltage monomer of DPPA-2SNS1.7 V may cause could an undergo over-oxidation electrochemical of the deposited polymerization polymer. by Asrepeated shownPolymers CV in 2020 scanningFigures, 12, x FOR 4 inPEERand a rangeREVIEW5, monomer of applied DPPA-2SN potentialsS could of 0.00–1.10 undergo or electroc 0.00–1.30hemical V. polymerization8 of 19 by repeated CV scanning in a range of applied potentials of 0.00–1.10 or 0.00–1.30 V.

Figure 4. (a) The ﬁrst CV scan, (b) the ﬁrst two second CV scan, and (c) ten repetitive CV scans of Figure 4. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 0.2 0.2 mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.10 V at a scan rate mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.10 V at a scan rate of of 5050 mV mV/s./s.

Figure 5. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 0.2

mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.30 V at a scan rate of 50 mV/s.

Polymers 2020, 12, x FOR PEER REVIEW 8 of 19

Figure 4. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 0.2

mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.10 V at a scan rate of Polymers 2020, 12, 2777 8 of 18 50 mV/s.

Figure 5. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of Figure 5. (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 0.2 0.2 mM of DPPA-2SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.30 V at a scan rate PolymersmM 2020 of ,DPPA-2SNS 12, x FOR PEER in REVIEW0.1 M Bu NClO /CH Cl in the potential range of 0.00–1.30 V at a scan rate of9 of 19 of 50 mV/s. 4 4 2 2 50 mV/s. 3.4. UV-vis Absorption of the PSNS Films 3.4. UV-vis Absorption of the PSNS Films The UV-vis absorption spectra of the solutions of monomers Py-SNS (in MeCN) and DPPA- The UV-vis absorption spectra of the solutions of monomers Py-SNS (in MeCN) and DPPA-2SNS 2SNS (in CH2Cl2) and their corresponding polymer films of P1 and P2 on an ITO-glass substrate are (in CH2Cl2) and their corresponding polymer films of P1 and P2 on an ITO-glass substrate are depicted depicted in Figure 6. The spectra of the monomers show absorption bands with maximum peaks at in Figure6. The spectra of the monomers show absorption bands with maximum peaks at 342 and 342 and 326 nm and absorption onsets at 390 and 428 nm, respectively. The polymer films of P1 and 326 nm and absorption onsets at 390 and 428 nm, respectively. The polymer films of P1 and P2 P2 show absorption maxima at 350 and 329 nm and absorption onsets at 569 and 547 nm, respectively. show absorption maxima at 350 and 329 nm and absorption onsets at 569 and 547 nm, respectively. The red-shift in absorption maximum and onset of the polymer films compared to the monomers The red-shift in absorption maximum and onset of the polymer films compared to the monomers imply imply an extended π-conjugation length. As mentioned earlier, the dilute solutions of DPPA-2SNS an extended π-conjugation length. As mentioned earlier, the dilute solutions of DPPA-2SNS in less polar in less polar solvents such as toluene showed a moderate fluorescence intensity. However, the solvents such as toluene showed a moderate fluorescence intensity. However, the electrodeposited electrodeposited film of P2 revealed a very weak fluorescence emission, possibly due to a high degree film of P2 revealed a very weak fluorescence emission, possibly due to a high degree of π-π stacking of of π-π stacking of the aromatic units in the solid state. the aromatic units in the solid state.

5 Figure 6. UV-vis absorption spectra of Py-SNS and DPPA-2SNS in THF (ca. 1 10 −5 M) and their Figure 6. UV-vis absorption spectra of Py-SNS and DPPA-2SNS in THF (ca. 1× × 10− M) and their electrodeposited ﬁlmsfilms onon ITO-glass.ITO-glass.

3.5. Redox Response of Polymers The electrochemical properties of the polymer films P1 and P2 electrodeposited on ITO glass were investigated by CV in a 0.1 M Bu4NClO4/MeCN monomer-free solution. As illustrated in Figure 7a, the P1 film electropolymerized from Py-SNS exhibited one reversible redox couple (E1/2 = 0.68 V) with Epa = 0.81 V and Epc = 0.54 V when the applied potential was scanned between 0 and 0.9 V. The reversible oxidation process should be associated with the oxidation of the SNS units in the polymer chain. When the potential was scanned to 2.0 V, two oxidation waves were observed at 0.77 and 1.87 V. However, the oxidation processes were irreversible due to over-oxidation of the polymer. Similarly, the CV diagram of the P2 film from DPPA-2SNS showed one reversible oxidation process (E1/2 = 0.61 V, Epa = 0.77 V and Epc = 0.44 V) in the potential range of 0.0–1.0 V, attributed to the oxidation reaction of the SNS units (see Figure 7b). Two semi-reversible oxidation processes with oxidation peaks at 0.70 and 1.44 V were observed in the CV diagram with the scanning range of 0.0–1.7 V. As the potential was scanned up to 2.5 V, P2 showed two or three oxidation waves. The first oxidation process is reversible; however, the latter oxidation processes are irreversible, probably due to over- oxidation of the polymer.

Polymers 2020, 12, 2777 9 of 18

3.5. Redox Response of Polymers The electrochemical properties of the polymer films P1 and P2 electrodeposited on ITO glass were investigated by CV in a 0.1 M Bu4NClO4/MeCN monomer-free solution. As illustrated in Figure7a, the P1 film electropolymerized from Py-SNS exhibited one reversible redox couple (E1/2 = 0.68 V) with Epa = 0.81 V and Epc = 0.54 V when the applied potential was scanned between 0 and 0.9 V. The reversible oxidation process should be associated with the oxidation of the SNS units in the polymer chain. When the potential was scanned to 2.0 V, two oxidation waves were observed at 0.77 and 1.87 V. However, the oxidation processes were irreversible due to over-oxidation of the polymer. Similarly, the CV diagram of the P2 film from DPPA-2SNS showed one reversible oxidation process (E1/2 = 0.61 V, Epa = 0.77 V and Epc = 0.44 V) in the potential range of 0.0–1.0 V, attributed to the oxidation reaction of the SNS units (see Figure7b). Two semi-reversible oxidation processes with oxidation peaks at 0.70 and 1.44 V were observed in the CV diagram with the scanning range of 0.0–1.7 V. As the potential was scanned up to 2.5 V, P2 showed two or three oxidation waves. The first oxidation process is reversible; however, the latter oxidation processes are irreversible, probably due to over-oxidation of the polymer. Figure S9 compares the CV behaviors of two polymer films of P2 prepared by the different potential scanning ranges of 0.0–1.1 V (film A) and 0.0–1.3 V (film B) for ten repetitive CV scans. These two films show similar CV behaviors, as the voltage was applied between 0 and 1.2 or 1.3 V. Two redox pairs were observed in their CV diagrams, which could be ascribed to the oxidation reactions of the SNS moiety and the DPPA amino center, respectively. As can be seen from the CV diagrams cycled in the 0.0–1.7 V range, the polymer film A seems to reveal a higher electrochemical activity of the DPPA core (Epa at around 0.95 V) than polymer film B. Thus, the polymer P2 from DPPA-2SNS prepared by repeated CV scanning in the applied potential range of 0.0–1.1 V was expected to possess better electrochromic performance. Redox waves at lower potentials of these two polymers are apparently attributable to reversible electrochemical oxidation processes of the SNS units, and the second redox wave at around 0.94 V in the CV diagram of P2 is ascribed to the oxidation of amino center in the DPPA core. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the corresponding polymers were estimated from the oxidation Eonset values (Table1). Assuming that the HOMO energy level for the ferrocene /ferrocenium (Fc/Fc+) standard is 4.80 eV with respect to the zero vacuum level, the HOMO levels for P1 and P2 were calculated to be 4.84 and 4.83 eV (relative to the vacuum energy level), respectively. Their LUMO energy levels were estimated to be 2.66 and 2.56 eV, respectively, deduced from the bandgap calculated from the optical − − absorption edge. Figure S10 displays the CV curves of the polymer films at different scanning rates between 30 and 300 mV/s in 0.1 M Bu4NClO4/MeCN, and the redox current density presents a linear growth with the increasing scan rate. This indicates that the polymer films were firmly coated on the ITO electrode and they showed a non-diffusional redox behavior. All the polymer films still remained a very high redox-activity after 100 repetitive CV sans in the potential ranges of 0.0–0.9 V or 0.0–1.0 V (see Figure8), especially with electron-rich triarylamine unit, indicating high electrochemical stability in the first oxidized state. However, the second oxidized state of polymer P2 is less stable as seen from the decrease of redox current density upon long-term CV cycling. For commercial electrochromic application, long-term cycling stability is a key issue for materials and their derived devices. Figure9 shows that the P2 film has a reasonable stability, since it retains 68% of its electroactivity after 1000 repetitive switches between neutral state and first oxidized states. Polymers 2020, 12, 2777 10 of 18 Polymers 2020, 12, x FOR PEER REVIEW 10 of 19

FigureFigure 7. 7. CyclicCyclic voltammorgrams voltammorgrams of of the the electrodeposited electrodeposited films ﬁlms of of ( (aa)) P1P1 fromfrom Py-SNS Py-SNS and and ( (bb)) P2P2 fromfrom

DPPA-2SNSDPPA-2SNS on on the the ITO-coated ITO-coated glass glass slide slide in in 0.1 M Bu 44NClONClO44/MeCN/MeCN at at a a scan scan rate rate of of 50 50 mV/s. mV/s.

Figure S9 compares the CV behaviors of two polymer films of P2 prepared by the different potential scanning ranges of 0.0–1.1 V (film A) and 0.0–1.3 V (film B) for ten repetitive CV scans. These two films show similar CV behaviors, as the voltage was applied between 0 and 1.2 or 1.3 V. Two redox pairs were observed in their CV diagrams, which could be ascribed to the oxidation reactions of the SNS moiety and the DPPA amino center, respectively. As can be seen from the CV diagrams cycled in the 0.0–1.7 V range, the polymer film A seems to reveal a higher electrochemical activity of the DPPA core (Epa at around 0.95 V) than polymer film B. Thus, the polymer P2 from DPPA-2SNS prepared by repeated CV scanning in the applied potential range of 0.0–1.1 V was expected to possess

Polymers 2020, 12, x FOR PEER REVIEW 11 of 19 Polymers 2020, 12, x FOR PEER REVIEW 11 of 19 better electrochromic performance. Redox waves at lower potentials of these two polymers are betterapparently electrochromic attributable performance. to reversible Redox electrochemical waves at oxidationlower potentials processes of ofthese the SNStwo units,polymers and arethe apparentlysecond redox attributable wave at around to reversible 0.94 V electrochemicalin the CV diagram oxidation of P2 isprocesses ascribed ofto thethe SNSoxidation units, of and amino the secondcenter inredox the DPPA wave core.at around 0.94 V in the CV diagram of P2 is ascribed to the oxidation of amino centerThe in theenergy DPPA levels core. of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecularThe energy orbital levels (LUMO) of the of highestthe corresponding occupied mole polymerscular orbital were estimated (HOMO) fromand lowestthe oxidation unoccupied Eonset molecularvalues (Table orbital 1). (LUMO)Assuming of thatthe correspondingthe HOMO en polymersergy level were for estimatedthe ferrocene/ferrocenium from the oxidation (Fc/Fc Eonset+) valuesstandard (Table is 4.80 1). eVAssuming with respect that tothe the HOMO zero vacuum energy level,level forthe theHOMO ferrocene/ferrocenium levels for P1 and P2(Fc/Fc were+) standardcalculated is to 4.80 be eV4.84 with and respect4.83 eV to(relative the zero to vacuumthe vacuum level, energy the HOMO level), levelsrespectively. for P1 andTheir P2 LUMO were calculatedenergy levels to be were 4.84 estimated and 4.83 eVto (relativebe −2.66 toand th e− 2.56vacuum eV, respectively,energy level), deduced respectively. from Theirthe bandgap LUMO energycalculated levels from were the opticalestimated absorption to be − 2.66edge. and Figures −2.56 S10 eV, displays respectively, the CV deduced curves of from the polymerthe bandgap films calculatedat different from scanning the optical rates between absorption 30 andedge. 300 Figures mV/s inS10 0.1 displays M Bu4NClO the CV4/MeCN, curves and of the the polymer redox current films atdensity different presents scanning a linear rates growth between with 30 and the 300increasing mV/s in scan 0.1 M rate. Bu 4ThisNClO indicates4/MeCN, that and thethe polymerredox current films densitywere firmly presents coated a linear on the growth ITO electrode with the and increasing they showed scan rate. a non-diffusional This indicates redox that the behavior. polymer films werePolymers firmly2020 coated, 12, 2777 on the ITO electrode and they showed a non-diffusional redox behavior. 11 of 18 Table 1. Optical and electrochemical properties of the electrosynthesized polymers. Table 1. Optical and electrochemical properties of the electrosynthesized polymers. UV-Vis Table 1. Optical and electrochemicalOxidation properties Potential of the electrosynthesizedOptical polymers. AbsorptionUV-Vis HOMO LUMO Polymers Structures Oxidation(V) Potential b BandgapOptical Absorptiona d d (nm)UV-Vis b HOMO(eV) LUMO(eV) Polymers Structures (V) Bandgapg b c a a OxidationOx1 PotentialOx2 ( (V)E ) (eV) Opticald d Absorptionλmax(nm) λ onset (nm) Eonset E1/2 E1/2 (eV) HOMO(eV) LUMO Polymers Structures g c Bandgap d d Ox1 Ox2 (E ) (eV) (eV) (eV) λmax λonset Eonset E1/2 E1/2Ox1 Ox2 c λmax λonset Eonset E1/2 E1/2 (Eg) (eV) P1 350 569 0.48 0.68 – 2.18 −4.84 −2.66 P1P1 350 569 569 0.48 0.480.68 0.68 – 2.18– 2.18−4.84 −2.664.84 2.66 − −

P2P2 329329 547 547 0.47 0.470.59 0.591.15 1.15 2.27 2.27−4.83 −4.832.56 2.56 P2 329 547 0.47 0.59 1.15 2.27 −4.83 −2.56 − Polymers 2020, 12, x FOR PEER REVIEW 12 of 19 Notes:Notes: a aUV-visUV-vis absorption absorption maximum maximum and and onse onsett wavelengths wavelengths for for the the polymer polymer films; films; b Calculatedb Calculated from from first CV c Notes:firstscans, CV a versus UV-vis scans, Ag versus absorption/AgCl Ag/AgCl in MeCN maximum in at MeCN a scan and at rateonse a scan oft wavelengths 50 rate mV /ofs; 50Optical mV/s; for the bandc Opticalpolymer gap band calculated films; gap b Calculated calculated from absorption fromfrom edge of the d polymer film: Eg = 1240/λonset; The HOMO and LUMOd energy levelsc were calculated from Eonset values of CV firstabsorption CV scans, edge versus of the Ag/AgCl polymer in film:MeCN Eg at= a1240/ scanλ rateonset; of The50 mV/s; HOMO Optical and LUMO band gap energy calculated levels fromwere diagrams and were referenced to ferrocene (4.8 eV relative to vacuum energy level; Eonset = 0.44 V; E1/2 = 0.52 V in g d absorption edge ofonset the polymer film: E = 1240/λonset; The HOMO and LUMO energy levels were calculatedCH Cl ). fromE E = Evaluesonset + 4.8of CV0.44 diagrams (eV); Eand were= –E referencedEg .to ferrocene (4.8 eV relative to Polymers2 2 2020− HOMO, 12, x FOR PEER REVIEW− − LUMO HOMO − 12 of 19 calculatedvacuum energy from level;Eonset valuesEonset = 0.44of CV V; diagramsE1/2 = 0.52 Van ind wereCH2Cl referenced2). −EHOMO =to E onsetferroce + 4.8ne − (4.80.44 eV(eV); relative −ELUMO to = vacuum energy level; E = 0.44 V; E = 0.52 V in CH Cl ). −E = E + 4.8 − 0.44 (eV); −E = –EHOMO − Eg. onset 1/2 2 2 HOMO onset LUMO –EHOMO − Eg. All the polymer films still remained a very high redox-activity after 100 repetitive CV sans in the potentialAll the ranges polymer of 0.0–0.9 films still V orremained 0.0–1.0 aV very (see highFigure redox-activity 8), especially after with 100 electron-rich repetitive CV triarylamine sans in the potentialunit, indicating ranges ofhigh 0.0–0.9 electrochemical V or 0.0–1.0 stabilityV (see Figure in the 8), first especially oxidized with state. electron-rich However, triarylamine the second unit,oxidized indicating state of highpolymer electrochemical P2 is less stable stability as seen in fromthe firstthe decreaseoxidized of state. redox However, current density the second upon oxidizedlong-term state CV cycling.of polymer For P2commercial is less stable electrochromic as seen from application, the decrease long-term of redox cycling current stability density is upona key long-termissue for materialsCV cycling. and For their commercial derived electrochromic devices. Figure application, 9 shows thatlong-term the P2 cycling film has stability a reasonable is a key issuestability, for sincematerials it retains and 68%their of derived its electroactivity devices. Figure after 1000 9 shows repetitive that switchesthe P2 film between has a neutral reasonable state stability,and first sinceoxidized it retains states. 68% of its electroactivity after 1000 repetitive switches between neutral state and first oxidized states.

Figure 8. CV diagrams of (a) P1 and (b) P2 on the ITO-coated glass slide in 0.1 M Bu NClO /MeCN at 4 4 Figure 8. CV diagrams of (a) P1 and (b) P2 on the ITO-coated glass slide in 0.1 M Bu4NClO4/MeCN at a scanFigure rate of 8. 50CV mV/s. diagrams of (a) P1 and (b) P2 on the ITO-coated glass slide in 0.1 M Bu NClO /MeCN at a scan rate of 50 mV/s. 4 4 a scan rate of 50 mV/s.

Figure 9. Electrochemical stability of P2 on the ITO-coated glass slide in 0.1 M Bu4NClO4/MeCN FigureFigure 9. Electrochemical 9. Electrochemical stability stability of of P2 P2 on on the the ITO-coated ITO-coated glass glass slide slide in in 0.1 0.1 M MBu 4BuNClO4NClO4/MeCN4/MeCN after after after 1000 repetitive switching by a CV method with scan rate at 50 mV/s. Ia: anodic peak current, 1000 1000repetitive repetitive switching switching by bya CVa CV meth methodod withwith scan raterate atat 50 50 mV/s. mV/s. Ia : Ianodica: anodic peak peak current, current, Ic: Ic: Ic: cathodic peak current. cathodiccathodic peak peak current. current.

3.6. Spectroelectrochemical Properties of Polymers 3.6. Spectroelectrochemical Properties of Polymers The spectroelectrochemical properties of the electrodeposited polymer films were examined for Thevarious spectroelectrochemical redox states by using theproperties changes inof thethe el electronicectrodeposited absorption polymer spectra under films variouswere examined applied for variouspotentials. redox states In the neutralby using form, the the changes P1 film in exhibited the electronic strong absorption absorption in thespec UVtra region under at variouswavelength applied potentials.352 nm In and the a neutral medium form, absorption the P1 in film the visibleexhibited region strong at 460 absorption nm; thus, the in film the UVappeared region as at yellowish wavelength 352 nmorange and ina medium color (Figure absorption 10). When in the the voltage visible was region gradually at 460 raised nm; thus, to 0.90 the V, film the absorption appeared intensityas yellowish orangeat in460 color nm dropped(Figure 10).obviously When andthe voltagea new broad was grbandadually appeared raised in tothe 0.90 near-infrared V, the absorption (NIR) region intensity at 460centered nm dropped at 1000 obviouslynm, corresponding and a new to the broad formatio bandn of appeared the charge in carriers. the near-infrared During oxidation, (NIR) the region centeredpolymer at 1000 film nm,induced corresponding a color change to from the yellowishformation orange of the ( Lcharge * = 46, acarriers. * = 11, b *During = 26) to oxidation,blue (L * = the 39, a * = 0, b * = −1). polymer film induced a color change from yellowish orange (L * = 46, a * = 11, b * = 26) to blue (L * = 39, a * = 0, b * = −1).

Polymers 2020, 12, 2777 12 of 18

3.6. Spectroelectrochemical Properties of Polymers The spectroelectrochemical properties of the electrodeposited polymer films were examined for various redox states by using the changes in the electronic absorption spectra under various applied potentials. In the neutral form, the P1 film exhibited strong absorption in the UV region at wavelength 352 nm and a medium absorption in the visible region at 460 nm; thus, the film appeared as yellowish orange in color (Figure 10). When the voltage was gradually raised to 0.90 V, the absorption intensity at 460 nm dropped obviously and a new broad band appeared in the near-infrared (NIR) region centered at 1000 nm, corresponding to the formation of the charge carriers. During oxidation, the polymer film induced a color change from yellowish orange (L * = 46, a * = 11, b * = 26) to blue (L * = 39, a * = 0, Polymers 2020, 12, x FOR PEER REVIEW 13 of 19 b * = 1). −

Figure 10. Spectroelectrograms, color changes and transmittance changes of P1 thin films on ITO-coated Figure 10. Spectroelectrograms, color changes and transmittance changes of P1 thin films on ITO- glass in 0.1 M Bu4NClO4/MeCN at various applied voltages. coated glass in 0.1 M Bu4NClO4/MeCN at various applied voltages. The spectral changes in the P2 film electro-deposited from DPPA-2SNS correlated with applied The spectral changes in the P2 film electro-deposited from DPPA-2SNS correlated with applied potentials are presented in Figure 11. P2 revealed a strong UV absorption at 329 nm and a medium potentials are presented in Figure 11. P2 revealed a strong UV absorption at 329 nm and a medium absorption in the visible region of 400–500 nm in the neutral state and appeared as yellowish orange absorption in the visible region of 400–500 nm in the neutral state and appeared as yellowish orange (L * = 54, a * = 2, b * = 40). As the voltage was slowly increased from 0.00 to 0.90 V, the absorbance (L * = 54, a * = 2, b * = 40). As the voltage was slowly increased from 0.00 to 0.90 V, the absorbance at at 329 and 450 nm diminished, and a broad absorption in the near infrared (NIR) region emerged at 329 and 450 nm diminished, and a broad absorption in the near infrared (NIR) region emerged at around 900 nm, accompanied by a color change of the film to greyish blue (L * = 52, a * = 2, b * = 7). around 900 nm, accompanied by a color change of the film to greyish blue (L * = 52, a * = 2, b * = −7). The spectra and color change of the P2 film can be explained by the formation of polaron charge carriers The spectra and color change of the P2 film can be explained by the formation of polaron charge caused by oxidation of the SNS units. As the applied voltage was raised to 1.60 V, the NIR absorbance carriers caused by oxidation of the SNS units. As the applied voltage was raised to 1.60 V, the NIR slightly dropped, and the growth of new broad band in the visible region of 400–800 nm was observed, absorbance slightly dropped, and the growth of new broad band in the visible region of 400–800 nm which indicated the formation of bipolaron charge carriers caused by further oxidations of the amino was observed, which indicated the formation of bipolaron charge carriers caused by further center of DPPA group and the SNS moiety; meanwhile, the P2 film changed color from greyish blue to oxidations of the amino center of DPPA group and the SNS moiety; meanwhile, the P2 film changed purplish black (L * = 14, a * = 3, b * = 5). color from greyish blue to purplish black− (L * = 14, a * = 3, b * = −5).

Polymers 2020, 12, 2777 13 of 18 Polymers 2020, 12, x FOR PEER REVIEW 14 of 19

Figure 11. Spectroelectrograms and color changes of P2 thin films on ITO-coated glass in 0.1 M Figure 11. Spectroelectrograms and color changes of P2 thin films on ITO-coated glass in 0.1 M Bu4NClO4/MeCN at various applied voltages. Bu4NClO4/MeCN at various applied voltages. 3.7. Electrochromic Switching 3.7. Electrochromic Switching Electrochromic switching studies were carried out in order to elucidate the response times and opticalElectrochromic contrast of the switching polymer films. studies The were absorbance carried out of the in polymerorder to elucidate films at an the absorption response maximum times and wasoptical monitored contrast as of a the function polymer of timefilms. when The absorban the film wasce of switched the polymer between films neutral at an absorption and oxidized maximum states. was Figuremonitored S11 depictsas a function the optical of time transmittance when the film of P1wasfilm switched at 460 andbetween 1000 nmneutral as a and function oxidized of time states. by applyingFigure square-wave S11 depicts potential the optical steps transmittance between 0 andof P1 0.90 film V at for 460 a pulseand 1000 width nm of as 10 a s.function The response of time timeby applying was calculated square-wave as the potential time required steps between 90% of full0 and switches 0.90 V for between a pulse their width colored of 10 s. and The bleached response stagestime was because calculated after thisas the point time the required naked eyes 90% could of full not switches sense the between changes their in thecolored color. and The bleached optical stages because after this point the naked eyes could not sense the changes in the color. The optical contrast (measured as Δ%T) of P1 between neutral orange and oxidized blue states was found to be

Polymers 2020, 12, x FOR PEER REVIEW 15 of 19 Polymers 2020, 12, 2777 14 of 18

Polymers 2020, 12, x FOR PEER16% REVIEWat 460 nm and 32% at 1000 nm. The response time for the coloring and15 bleaching of 19 process is about contrast (measured as3 and∆%T) 1 s, of respectively.P1 between neutral orange and oxidized blue states was found to be 16% at 460 nm and 32% atWhen 1000 nm. the The TheP2 film response response was switched ti timeme for the between coloring 0.00 and and bleaching 0.90 V with process a pulse is about width of 20 s, the optical 3 and 1 s, respectively.transmi ttance at 450 and 900 nm was examined. As shown in Figure 12, the optical contrast of P2 When the P2 filmfilmbetween was switched neutral betweenyellow and 0.00 oxidized and 0.90 greyish V with blue a pulse states width was offound 20 s, to the be optical 28% at 450 nm and 58% at transmittance at 450900 and nm. 900 The nmnm coloring waswas examined.examined. response As As time shown shown was in in measured Figure Figure 12,12 as, the 3.7 optical s at 450 contrast nm and of 4.7 P2 s at 900 nm, and the between neutral yellowbleaching and oxidized response greyish time was blue recorded statesstates was as found5.6 s at to 450 be nm 28% and at 450 4.6 nms at and900 nm. 58% As at the applied voltage 900 nm. The The coloring coloringwas response response stepped time time from was was 0.00 measured measured to 1.60 V, as asthe 3.7 3.7 film s s at exhibited 450 nm and optical 4.7 contrasts at 900 of nm, 29% and at the650 nm for the oxidized bleaching response timepurplish was recordedblack state as and 5.6 re s atquired 450 nm 8.1 andands for 4.64.6the ss coloring atat 900900 nm.nm. step As and the 15.3 applied s for the voltage bleaching step, as shown was stepped from 0.00in to(Figure 1.60 V, S12). thethe However, filmfilm exhibitedexhibited the P2 opticaloptical film revealed contrastcontrast ofa substantial 29% at 650 loss nm forof optical the oxidized contrast after 10 switching purplish black state andcycles required between 8.18.1 the ss for neutral the coloring and the step second and 15.3oxidized s for thestates bleaching (from 29% step, to as 1 3%).shown in (Figure S12). However, However, the P2 filmfilm revealed a substantial loss of optical contrast after 10 switching cycles between the neutral and the second oxidizedoxidized statesstates (from(from 29%29% toto 13%).13%).

Figure 12. Potential step absorptiometry of the P2 film (from DPPA-2SNS) on the ITO-glass slide (in

Figure 12. Potential stepMeCN absorptiometry with 0.1 M Bu of4NClO the P24 asﬁlm a supporting (from DPPA-2SNS) electrolyte) on by the applying ITO-glass a slidepotential step: (a) 0.00 V ↹

(inFigure MeCN 12. withPotential 0.1 M step Bu0.904 absorptiometryNClO V (204 as cycles) a supporting withof the a pulseP2 electrolyte) film width (from byof DPPA-2SNS) 20 applying s at λmax a potential= on900 the nm ITO-glass step:and ( (ba)) 0.00 0.00slide V (in↹ 0.90 V (20 cycles) with 0.90MeCN V (20with cycles) 0.1 M with Bu4 aNClOa pulsepulse4 widthaswidth a supporting of of 20 20 s s at atλ λelectrolyte)maxmax = 450900 nm. nm by andThe applying (opticalb) 0.00 a contrast Vpotential0.90 and V step: (20 responsecycles) (a) 0.00 withtimes V ↹ a were calculated for the pulse0.90 V width (20 cycles) of 20 with s at λ firstamax pulse =switching450 width nm. of Thecycle. 20 opticals at λmax contrast = 900 nm and and response (b) 0.00 times V ↹ were0.90 V calculated (20 cycles) for with the ﬁrsta pulse switching width of cycle. 20 s at λmax = 450 nm. The optical contrast and response times were calculated for the first switching cycle. The electrochromic coloration efficiency (CE) can be calculated by the equation: CE = ΔOD/Qd, where ΔOD is the optical absorbance change, and Qd is the inject/ejected charge during redox step. The electrochromic coloration efficiency (CE) can be calculated by the equation: CE = ΔOD/Qd, where ΔOD is the optical absorbance change, and Qd is the inject/ejected charge during redox step.

Polymers 2020, 12, 2777 15 of 18

The electrochromic coloration eﬃciency (CE) can be calculated by the equation: CE = ∆OD/Qd, where ∆OD is the optical absorbance change, and Qd is the inject/ejected charge during redox step. As can be seen from Table2, the CE values of P1 were measured as 108 cm2/C at 1000 nm and 62 cm2/C at 460 nm. Those of P2 were calculated as 224 cm2/C at 900 nm and 117 cm2/C at 450 nm.

Table 2. Electrochromic properties of the polymer ﬁlms.

Polymers 2020, λ12, x FORa PEER REVIEW Response Time b Q d 16 of CE19 e max ∆OD c d Polymers (nm) ∆%T (mC/cm2) (cm2/C) As can be seen from Table 2, the CE valuestc (s) of P1 were tb (s) measured as 108 cm2/C at 1000 nm and 62 2 2 2 cm /C at 460 1000nm. Those of P2 32 were calculated 3.3 as 224 cm 1.0/C at 900 nm 0.30 and 117 cm /C 2.76 at 450 nm. 108 P1 460 16 1.3 2.9 0.17 2.76 62 Table 2. Electrochromic properties of the polymer films.

900 58 4.7 4.6b 0.46 2.05 224 Response Time d d e a c Q CE P2 Polymers450 λmax 28(nm) Δ%T 5.6 3.7ΔOD 0.24 2 2.052 117 tc (s) tb (s) (mC/cm ) (cm /C) 650 45 8.1 15.3 0.59 9.09 65 1000 32 3.3 1.0 0.30 2.76 108 P1 Notes: a Wavelength of absorption460 maximum; 16 b Time1.3 for 90% 2.9 of the full-transmittance0.17 2.76 change; 62 c Optical density change (∆OD) = log[Tbleached/Tcolored900 ], where 58 Tcolored4.7and Tbleached 4.6 are the0.46 maximum 2.05 transmittance 224 in the oxidized and neutral states, respectively; d Q is ejected charge, determined from the in situ experiments; e Coloration P2 450 d 28 5.6 3.7 0.24 2.05 117 eﬃciency (CE) = ∆OD/Q . d 650 45 8.1 15.3 0.59 9.09 65 Notes: a Wavelength of absorption maximum; b Time for 90% of the full-transmittance change; c As mentioned earlier, polymer P2 seemed to exhibit a longer response time than polymer P1. Optical density change (ΔOD) = log[Tbleached/Tcolored], where Tcolored and Tbleached are the maximum

The most probabletransmittance reason in mightthe oxidized be thatand neutral the cross-linked states, respectively; structure d Qd is ejected of P2 charge,is not determined conducive from to countering + e ion (TBA ) penetration.the in situ experiments; Therefore, Coloration the electrolyte efficiency (CE) was = ΔOD/Q changedd. to lithium perchlorate (LiClO4) with a smaller counter ion Li+. As shown in Figure S13 and Table S2, a faster response time was observed As mentioned earlier, polymer P2 seemed to exhibit a longer response time than polymer P1. when the switchingThe most probable test of thereasonP2 mightfilm wasbe that carried the cross-linked out in a LiClO structure4/MeCN of P2 solution.is not conducive to countering ion (TBA+) penetration. Therefore, the electrolyte was changed to lithium perchlorate 3.8. Electrochromic(LiClO4) with Devices a smaller counter ion Li+. As shown in Figure S13 and Table S2, a faster response time was observed when the switching test of the P2 film was carried out in a LiClO4/MeCN solution. Based on the foregoing results, it can be concluded that the electrochemically generated polymer films can be3.8. used Electrochromic in the construction Devices of electrochromic devices and the optical display. Therefore, we fabricated single-layerBased on the electrochromic foregoing results, it cells can be as concluded preliminary that the investigations. electrochemically generated The polymer polymer films were electrodepositedfilms can onto be used ITO-coated in the constructi glass,on thoroughlyof electrochromic rinsed, devices and and then the optical dried. display. Afterward, Therefore, the we transparent and conductivefabricated gel single-layer electrolyte electrochromic was spread oncells the as polymer-coatedpreliminary investigations. side of theThe electrodepolymer films and were the electrodes electrodeposited onto ITO-coated glass, thoroughly rinsed, and then dried. Afterward, the were sandwiched.transparent To and prevent conductive leakage, gel electrolyte an epoxy was resin spre wasad on applied the polymer-coated to seal the side device. of the The electrode electrochromic devices usingand theP1 andelectrodesP2 films were assandwiched. active layers To prevent were fabricatedleakage, an epoxy (Figure resin 13 ).was The appliedP1-based to seal devicethe is pale yellowishdevice. orange The in electrochromic the neutral devices form. using When P1 and the P2 applied films as active voltage layers was were increased fabricated (Figure to 1.94 13). V, the color changed toThe greenish P1-based blue.device Inis pale the yellowish other case, orange the inP2 the-based neutral device form. When is yellowish the applied orange voltage inwas the neutral increased to 1.94 V, the color changed to greenish blue. In the other case, the P2-based device is form. Whenyellowish the applied orange in voltage the neutral was form. increased When the to applied 1.83 andvoltage 2.38 was V, increased the color to 1.83 changed and 2.38 to V, greenish the blue and purplishcolor black, changed respectively. to greenish blue and purplish black, respectively.

Figure 13. Photos of sandwich-type ITO-coated glass electrochromic devices using (a) P1 and (b) P2 ﬁlms as active layers (active area ca. 20 mm 20 mm). × Polymers 2020, 12, x FOR PEER REVIEW 15 of 19

16% at 460 nm and 32% at 1000 nm. The response time for the coloring and bleaching process is about 3 and 1 s, respectively. When the P2 film was switched between 0.00 and 0.90 V with a pulse width of 20 s, the optical transmittance at 450 and 900 nm was examined. As shown in Figure 12, the optical contrast of P2 between neutral yellow and oxidized greyish blue states was found to be 28% at 450 nm and 58% at 900 nm. The coloring response time was measured as 3.7 s at 450 nm and 4.7 s at 900 nm, and the bleaching response time was recorded as 5.6 s at 450 nm and 4.6 s at 900 nm. As the applied voltage was stepped from 0.00 to 1.60 V, the film exhibited optical contrast of 29% at 650 nm for the oxidized purplish black state and required 8.1 s for the coloring step and 15.3 s for the bleaching step, as shown in (Figure S12). However, the P2 film revealed a substantial loss of optical contrast after 10 switching cycles between the neutral and the second oxidized states (from 29% to 13%).

Polymers 2020, 12, 2777 16 of 18

4. Conclusions In this work, a new SNS-based electroactive molecule DPPA-2SNS, bearing triarylamine subunits, was synthesized and then directly deposited onto the ITO-glass substrate as a robust polymeric film by repetitive CV scanning. The CV diagrams of the electrochemically generated polymers revealed multiple oxidation processes, and the first two oxidation reactions were reversible. A multi-colored electrochromism with yellowish orange, greyish blue, and purplish black colors was observed in the spectroelectrochemical measurement of the electrodeposited thin films by applying a positive potential. These polymers also showed strong near-infrared absorption upon oxidation. The polymer films generally exhibited reasonable coloration efficiency and optical contrast, together with high switching stability, especially at the first oxidized state. Thus, the prepared polymers can be promising candidates for electrochromic applications.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/12/ 2777/s1, Figure S1: IR spectra of the 1-nitropyrene, 1-aminopyrene, N,N-di(4-nitrophenyl)-1-aminopyrene, and N,N-di(4-aminophenyl)-1-aminopyrene. Figure S2: IR spectra of the 1,4-di(thiophen-2-yl)butane-1,4-dione, 1 Py-SNS, and DPPA-2SNS. Figure S3: H NMR spectrum of N,N-di(4-aminophenyl)-1-aminopyrene in DMSO-d6. 1 1 Figure S4: H NMR spectrum of 1,4-di(thiophen-2-yl)butane-1,4-dione in CDCl3 (* solvent peak). Figure S5: H 1 NMR and H-H COSY spectra of Py-SNS in DMSO-d6. Figure S6: H NMR spectrum of DPPA-2SNS in CDCl3 (* solvent peak). Figure S7: H-H COSY spectra of DPPA-2SNS in CDCl3 (* solvent peak). Figure S8: (a) The first CV scan, (b) the first two second CV scan, and (c) ten repetitive CV scans of 1 mM of Py-SNS in 0.1 M Bu4NClO4/CH2Cl2 in the potential range of 0.00–1.10 V at a scan rate of 50 mV/s. Figure S9: Cyclic voltammorgrams of the electrodeposited films of DPPA-2SNS on the ITO-coated glass slide in 0.1 M Bu4NClO4/MeCN at a scan rate of 50 mV/s. (a) Film A prepared by repeated CV scanning between 0 and 1.1 V and (b) film B prepared by repeated CV scanningFigure between12. Potential 0 and step 1.3 absorptiometry V for ten cycles. of Figure the P2 S10: film Scan (from rate DPPA dependence-2SNS) on of the polymer ITO-glass films slide (a) (in P1 and (b) P2 on the ITO-glass substrate in MeCN containing 0.1 M Bu4NClO4 at different scan rates from 30 to 300 mV/s. FigureMeCN S11: Potential with 0.1 stepM Bu absorptiometry4NClO4 as a supporting of the P1 film electrolyte) (from Py-SNS) by applying on the ITO-glassa potential slide step: (in (a MeCN) 0.00 V ↹ with 0.1 M Bu4NClO0.90 V4 (20as acycles) supporting with a electrolyte) pulse width by of applying 20 s at λmax a potential = 900 nm step: and ( (a)b) 0.00 V ↹ 0.90 V (20 cycles) cycles) with with a pulse widtha pulse of 10 width s at λofmax 20 =s at1000 λmax nm = 450 and nm. (b) The optical optical switching contrast at and potential response 0.00 times V were0.90 Vcalculated (20 cycles) for the with a pulse width of 10 s at λ = 460 nm. The optical contrast and response times were calculated for the first first switchingmax cycle. switching cycle. Figure S12: Potential step absorptiometry of the P2 film (from DPPA-2SNS) on the ITO-glass slide (in MeCN with 0.1 M Bu4NClO4 as a supporting electrolyte) by applying a potential step 0.00 V 1.60 V (10 cycles) withThe a pulse electrochromic width of 30 s atcolorationλmax = 650 efficiency nm. The optical (CE) can contrast be calculated and response by timesthe equa weretion: calculated CE = ΔOD/Qd, for the firstwhere switching ΔOD cycle. is the Figure optical S13: absorbance Potential step change, absorptiometry and Qd is of the the P2inject/ejected film (from DPPA-2SNS) charge during on the redox step. ITO-glass slide (in MeCN with 0.1 M LiClO4 as a supporting electrolyte) by applying a potential step: (a) 0.00 V 0.90 V (20 cycles) with a pulse width of 15 s at λmax = 900 nm and (b) 0.00 V 0.90 V (20 cycles) with a pulse width of 15 s at λmax = 450 nm. The optical contrast and response times were calculated for the first switching cycle. Table S1: Optical properties of DPPA-2SNS in different solvents. Table S2: Electrochromic properties of the polymer films of P2 with different electrolyte. Author Contributions: Investigation, S.-Y.C. and Y.-R.K.; supervision, S.-H.H.; writing—original draft preparation, S.-H.H.; writing—review and editing, Y.-R.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science and Technology, Taiwan, Republic of China, grant number MOST 107-2218-E-036-003-MY2 and MOST 108-2221-E-027-023-MY3. Acknowledgments: The authors are grateful for the financial support from the Ministry of Science and Technology, Taiwan, Republic of China. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Rosseinsky, D.R.; Mortimer, R.J. Electrochromic systems and the prospects for devices. Adv. Mater. 2001, 13, 783–793. [CrossRef] 2. Granqvist, C.G. Electrochromics for smart windows: Oxide-based thin ﬁlms and devices. Thin Solid Films 2014, 564, 1–38. [CrossRef] 3. Beaujuge, P.M.; Ellinger, S.; Reynolds, J.R. The donor-acceptor approach allows a black-to-transmissive switching polymeric electrochrome. Nat. Mater. 2008, 7, 795–799. [CrossRef][PubMed] 4. Malti, A.; Brooke, R.; Liu, X.; Zhao, D.; Ersman, P.A.; Fahlman, M.; Jonsson, M.P.; Berggren, M.; Crispin, X. Freestanding electrochromic paper. J. Mater. Chem. C 2016, 4, 9680–9686. [CrossRef] Polymers 2020, 12, 2777 17 of 18

5. Osterholm, A.M.; Shen, D.E.; Kerszulis, J.A.; Bulloch, R.H.; Kuepfert, M.; Dyer, A.L.; Reynolds, J.R. Four shades of brown: Tuning of electrochromic polymer blends toward high-contrast eyewear. ACS Appl. Mater. Interfaces 2015, 7, 1413–1421. [CrossRef][PubMed] 6. Koyuncu, S.; Koyuncu, F.B. A new ITO-compatible side chain-functionalized multielectrochromic polymer for use in adaptive camouflage-like electrochromic devices. React. Funct. Polym. 2018, 131, 174–180. [CrossRef] 7. Yang, P.; Sun, P.; Mai, W. Electrochromic energy storage devices. Mater. Today 2016, 19, 394–402. [CrossRef] 8. Mortimer, R.J. Electrochromic materials. Annu. Rev. Mater. Res. 2011, 41, 241–268. [CrossRef] 9. Amb, C.M.; Dyer, A.L.; Reynolds, J.R. Navigating the color palette of solution-processable electrochromic polymers. Chem. Mater. 2011, 23, 397–415. [CrossRef] 10. Gunbas, G.; Toppare, L. Electrochromic conjugated polyheterocycles and derivatives—highlights from the last decade towards realization of long lived aspirations. Chem. Commun. 2012, 48, 1083–1101. [CrossRef] 11. Neo, W.; Ye, Q.; Chua, S.-J.; Xu, J. Conjugated polymer-based electrochromics: Materials, device fabrication and application prospects. J. Mater. Chem. C 2016, 4, 7364–7376. [CrossRef] 12. Lv, X.; Li, W.; Ouyang, M.; Zhang, Y.; Wright, D.S.; Zhang, C. Polymeric electrochromic materials with donor-acceptor structures. J. Mater. Chem. C 2017, 5, 12–28. [CrossRef] 13. Thelakkat, M. Star-shaped, dendrimeric and polymeric triarylamines as photoconductors and hole transport materials for electro-optical applications. Macromol. Mater. Eng. 2002, 287, 442–461. [CrossRef] 14. Shirota, Y. Photo- and electroactive amorphous molecular materials—molecular design, syntheses, reactions, properties, and applications. J. Mater. Chem. 2005, 15, 75–93. [CrossRef] 15. Shirota, Y.; Kageyama, H. Charge carrier transporting molecular materials and their applications in devices. Chem. Rev. 2007, 107, 953–1010. [CrossRef] 16. Ning, Z.; Tian, H. Triarylamine: A promising core unit for efficient photovoltaic materials. Chem. Commun. 2009, 5483–5495. [CrossRef] 17. Iwan, A.; Sek, D. Polymers with triphenylamine units: Photonic and electroactive materials. Prog. Polym. Sci. 2011, 36, 1277–1325. [CrossRef] 18. Liang, M.; Chen, J. Arylamine organic dyes for dye-sensitized solar cells. Chem. Soc. Rev. 2013, 42, 3453–3488. [CrossRef] 19. Yen, H.-J.; Liou, G.-S. Recent advances in triphenylamine-based electrochromic derivatives and polymers. Polym. Chem. 2018, 9, 3001–3018. [CrossRef] 20. Yen, H.-J.; Liou, G.-S. Design and preparation of triphenylamine-based polymeric materials towards emergent optoelectronic applications. Prog. Polym. Sci. 2019, 89, 250–287. [CrossRef] 21. Park, S.Y.; Yoon, J.H.; Hong, C.S.; Souane, R.; Kim, J.S.; Mattews, S.E.; Vicens, J. A pyrenyl-appended triazole-based calix[4]arene as a fluorescent sensor for Cd2+ and Zn2+. J. Org. Chem. 2008, 73, 8212–8218. [CrossRef][PubMed] 22. Conlon, P.; Yang, C.J.; Wu, Y.; Chan, Y.; Martinez, K.; Kim, Y.; Stevens, N.; Marti, A.A.; Jockusch, S.; Turro, N.J.; et al. Pyrene excimer signaling molecular beacons for probing nucleic acids. J. Am. Chem. Soc. 2008, 130, 336–342. [CrossRef][PubMed] 23. Figueira-Duarte, T.M.; Mullen, K. Pyrene-based materials for organic electronics. Chem. Rev. 2011, 111, 7260–7314. [CrossRef][PubMed] 24. Kung, Y.-C.; Hsiao, S.-H. Fluorescent and electrochromic polyamides with pyrenylamine chromophore. J. Mater. Chem. 2010, 20, 5481–5492. [CrossRef] 25. Kung, Y.-C.; Hsiao, S.-H. Solution-processable, high-Tg, ambipolar polyimide electrochromics bearing pyrenylamine units. J. Mater. Chem. 2011, 21, 1746–1754. [CrossRef] 26. Hsiao, S.-H.; Huang, Y.-P. Redox-active and fluorescent pyrene-based triarylamine dyes and their derived electrochromic polymers. Dye. Pigment. 2018, 158, 368–381. [CrossRef] 27. Kumar, A.; Welsh, D.M.; Morvant, M.C.; Piroux, F.; Abboud, K.A.; Reynolds, J.R. Conducting poly(3,4-alkylenedioxythiophene) derivatives as fast electrochromics with high-contrast ratios. Chem. Mater. 1998, 10, 896–902. [CrossRef] 28. Walczak, R.M.; Reynolds, J.R. Poly(3,4-alkylenedioxypyrrole)s: The PXDOPs as versatile yet underutilized electroactive and conducting polymers. Adv. Mater. 2006, 18, 1121–1131. [CrossRef] 29. Mert, O.; Demir, A.S.; Cihaner, A. Pyrrole coupling chemistry: Investigation of electroanalytic, spectroscopic and thermal properties of N-substituted poly(bis-pyrrole) films. RSC Adv. 2013, 3, 2035–2042. [CrossRef] Polymers 2020, 12, 2777 18 of 18

30. Witker, D.; Reynolds, J.R. Soluble variable color carbazole-containing electrochromic polymers. Macromolecules 2005, 38, 7636–7644. [CrossRef] 31. Shah, K.W.; Wang, S.-X.; Soo, X.Y.; Xu, J. Viologen-based electrochromic materials: From small molecules, polymers and composites to their applications. Polymers 2019, 11, 1839. [CrossRef][PubMed] 32. Beaupre, S.; Dumas, J.; Leclerc, M. Toward the development of new textile/plastic electrochromic cells using triphenylamine-based copolymers. Chem. Mater. 2006, 18, 4011–4018. [CrossRef] 33. Mcleod, G.G.; Mahboubian-Jones, M.G.B.; Pethrick, R.A.; Watson, S.D.; Truong, N.D.; Galin, J.C.; Francois, J. Synthesis, electrochemical polymerization and properties of poly(2,5-di(2-thienyl)pyrrole). Polymer 1986, 27, 455–458. [CrossRef] 34. Ferraris, J.P.; Skiles, G.D. ‘Substitutional alloys’ of organic conductors. Polymer 1987, 28, 179–182. [CrossRef] 35. Ferraris, J.P.; Hanlon, T.R. Optical, electrical and electrochemical properties of heteroaromatic copolymers. Polymer 1989, 30, 1319–1327. [CrossRef] 36. Cihaner, A.; Algi, F. Processable electrochromic and fluorescent polymers based on N-substituted thienylpyrrole. Electrochim. Acta 2008, 54, 665–670. [CrossRef] 37. Camurlu, P.; Karagoren, N. Clickable, versatile poly(2,5-dithienylpyrrole) derivatives. React. Funct. Polym. 2013, 73, 847–853. [CrossRef] 38. Camurlu, P. Polypyrrole derivatives for electrochromic applications. RSC Adv. 2014, 4, 55832–55845. [CrossRef] 39. Pandule, S.; Oprea, A.; Barsan, N.; Weimar, U.; Persaud, K. Syntheses of poly(2,5-di(thiophen-2-yl)-1H-pyrrole) derivatives and the effects of the substituents on their properties. Synth. Met. 2014, 196, 158–165. [CrossRef] 40. Sefer, E.; Bilgili, H.; Gultekin, B.; Tonga, M.; Koyuncu, S. A narrow range multielectrochromism from 2,5-di-(2-thienyl)-1H-pyrrole polymer bearing pendent perylenediimide moiety. Dye. Pigment. 2015, 113, 121–128. [CrossRef] 41. Soganci, T.; Soyleyici, H.C.; Giziroglu, E.; Ak, M. Processable amide substituted 2,5-bis(2-thienyl)pyrrole based conducting polymer and its fluorescent and electrochemical properties. J. Electrochem. Soc. 2016, 163, H1096–H1103. [CrossRef] 42. Su, Y.-S.; Chang, J.-C.; Wu, T.-Y. Applications of three dithienylpyrroles-based electrochromic polymers in high-contrast electrochromic devices. Polymers 2017, 9, 114. [CrossRef][PubMed] 43. Koyuncu, S.; Zafer, C.; Sefer, E.; Koyuncu, F.B.; Demic, S.; Kaya, I.; Ozdemir, E.; Icli, S. A new conduction polymer of 2,5-bis(2-thienyl)-1H-(pyrrole) (SNS) containing carbazole subunit: Electrochemical, optical and electrochromic properties. Synth. Met. 2009, 159, 2012–2021. 44. Lai, J.-C.; Lu, X.-R.; Qu, B.-T.; Liu, F.; Li, C.-H.; You, X.-Z. A new multicolored and near-infrared electrochromic material based on triphenylamine-containing poly(3,4-dithienylpyrrole). Org. Electron. 2014, 15, 3735–3745. [CrossRef] 45. Cai, S.; Wen, H.; Wang, S.; Niu, H.; Wang, C.; Jiang, X.; Bai, X.; Wang, W. Electrochromic polymers electrochemically polymerized from 2,5-dithienylpyrrole (DTP) with different triarylamine units: Synthesis, characterization and optoelectrochemical properties. Electrochim. Acta 2017, 228, 332–342. [CrossRef] 46. Tirke¸s,S.; Mersini, J.; Özta¸s,Z.; Algi, M.P.; Algi, F.; Cihaner, A. A new processable and fluorescent polydithienylpyrrole electrochrome with pyrene appendages. Electrochim. Acta 2013, 90, 295–301.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).