ORIGINAL PAPER

Multichromic, ferrocene clicked poly(2,5-dithienylpyrrole)s

Zeynep Bicil & Pinar Camurlu & Baris Yucel & Burcu Becer

Received: 18 March 2013 /Accepted: 17 July 2013 /Published online: 18 August 2013# Springer Science+Business Media Dordrecht 2013

Abstract Herein, we report the synthesis of a novel, clickfunctionalized 2,5-dithienylpyrrole (SNS-Fc) for the firsttime. Ferrocene was employed as an anchored unit becauseof its well-known physical and redox properties. SNS-Fc waselectrochemically polymerized and electrochromic propertiesof the resulting polymer were investigated in detail. In addi-tion, electrochemical copolymerization of SNS-Fc in the pres-ence of 3,4-ethylenedioxythiophene was performed. Theresulting copolymer displayed multichromic behavior rangingbetween orange, yellow, green and blue in different redoxstates. Cyclic voltammetry studies of both polymers revealedtwo reversible waves related to the polymer backbone and FeII/III redox couples, while showing the typical characteristic ofsurface-confined structures. Our studies have shown that theredox behavior, band gap and color of poly(2,5-dithienylpyrrole) derivatives are highly dependent on an-chored unit (ferrocene) and copolymerization.

Keywords Click chemistry . Dithienylpyrrole .

Electrochemical polymerization . Ferrocene . Optoelectronicproperties

Introduction

Over the last decade numerous studies have been devoted toconducting polymers since they have been successfullyemployed in the fields of electrochromic displays [1], solar cells[2], biosensors [3]. Among these polymers, poly(2,5-

dithienylpyrrole) derivatives (PSNS) have received considerableattention because of their low oxidation potentials and promisingelectrochromic properties [4–11]. In particular, copolymerizationof some 2,5-dithienylpyrrole (SNS) derivatives with 3,4-ethylenedioxythiophene (EDOT) led to an impressive set ofelectrochromic polymers having superior properties such as en-hanced optic contrast, switching time, multichromism and effec-tive band gap control [12, 13]. On the other hand, anothervaluable approach to control the electrochemical, optical andelectrochromic properties of conducting polymers is modifica-tion of polymer backbone by functional side groups [14, 15].Until now all PSNS derivatives were achieved via electrochem-ical or chemical polymerization of SNS monomers prepared byPaal-Knorr reaction generally in low yields. Therefore, structur-ally enrichment of classical SNS based polymers dependsmainlyon success in monomer synthesis which seriously limits theintroduction of different functional groups. In this manner, weenvisaged that the Click chemistry could be the most reliable andpractical strategy for elaboration of variously functionalized SNSderivatives [16, 17]. Click chemistry has widespread applicationsincluding preparative organic synthesis, modification of naturalproducts, polymer and dendrimer design [18–20]. However,employment of click reaction in the field of conducting polymersis rather new and limited mainly with preparation of PEDOTderivatives [21–26]. According to our best knowledge, clickreaction has never been utilized for functionalization of SNSmonomers or polymers (PSNS) by post-polymerization process.

Keeping these in mind, in this study we aimed to investigatethe optoelectronic properties of a PSNS having an active groupwhich was anchored to its structure through ‘Click Chemistry’.What we are looking for is to provide an effective approach forthe synthesis of SNS monomers that could be applied duringanchoring of various groups in future. In this pioneer study wepresented the synthesis of ferrocene functionalized 2,5-dithienylpyrrole monomer (SNS-Fc) via click chemistry(Scheme 1) and its electrochemical polymerization to achievea novel modified PSNS. Ferrocene was chosen as a functionalunit since conducting polymers having ferrocene functionalityexhibit valuable properties such as low redox potential and highelectron donor ability [26, 27]. We also presented a novel

Electronic supplementary material The online version of this article(doi:10.1007/s10965-013-0228-z) contains supplementary material,which is available to authorized users.

Z. Bicil : P. Camurlu (*) :B. BecerDepartment of Chemistry, Akdeniz University, 07058 Antalya,Turkeye-mail: pinarcamurlu@gmail.com

B. YucelDepartment of Chemistry, Istanbul Technical University,34469 Istanbul, Turkey

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multicolored electrochromic copolymer produced by electro-chemical copolymerization of SNS-Fc in the presence ofEDOT. Optoelectronic properties of both polymers were de-scribed via cyclic voltammetry, spectroelectrochemistry,switching studies and colorimetry measurements.

Experimental

General

All chemicals were purchased from Aldrich, Merck Chemical asanalytical grade. LiClO4 was electroanalytical grade and thio-phene, succinyl chloride, ethanolamine, p-toluene sulfonic acid(PTSA), triethylamine (TEA), dimethylaminopyridine(DMAP), p -toluene sulfonyl chloride (TsCl), sodium azide,copper(II)sulfate pentahydrate, sodium ascorbate, 3,4-ethylenedioxythiophene (EDOT) were used as received. Ace-tonitrile (ACN) was distilled over calcium hydride and kept on4 Å molecular sieves. 1,4-di(2-thienyl)-1,4-butanedione [28],ethynylferrocene [29] and 1-(2-azido-ethyl)-2,5-dithiophen-2-yl-1H-pyrrole (SNS-N3) [30] were synthesized according to liter-ature. Electrochromic measurements; spectroelectrochemistry,and switching studies of the polymer films deposited on ITOwere performed in a UV-cuvette with three-electrodes placed inthe sample compartment of a spectrophotometer.

Equipments

NMR spectra were recorded both on Bruker Avance Spectrom-eter at 300 MHz for 1H NMR and 75 MHz for 13C NMR andBruker Spectrospin Avance DPX-400 spectrometer at 400 MHzfor 1H NMR and 100 MHz for 13C NMR and Varian Inova 500spectrometer at 500 MHz for 1H NMR and 125 MHz for 13CNMR. Chemical shifts (δ) were given relative totetramethylsilane (TMS) as the internal standard. High resolutionmass spectra (HRMS) were obtained on a Waters Synapt Q-TOF-MS spectrometer. FTIR spectra were recorded on aBruckerTensor 27 spectrometer. Ivium stat potentiostat/galvanostat wasused to supply a constant potential during electrochemical syn-thesis and cyclic voltammetry. Spectroelectrochemical and kinet-ic studies of the polymers were performed on Thermo EvolutionArray UV–Vis spectrophotometer. Colorimetry measurementswere recorded on a Minolta CS-100A Chroma Meter with a 0/0 (normal/normal) viewing geometry as recommended by Com-mission Internationale de l’Eclairage (CIE). Elemental analysesof the polymers were performed on a LECO, CHNS-932.

Synthesis of 4-ferrocenyl-1-[2-(2,5-di-thiophene-2-yl-pyrrole-1-yl)-ethyl]-1H-[1,2,3]triazole (SNS-Fc)

SNS-N3 (0.5 mmol) and ethynylferrocene (0.5 mmol) weredissolved in H2O/THF (1:1, 10 mL), aqueous CuSO4. 5H2O

(0.42 mL, 1.0 M) and sodium ascorbate (0.86 mL, 1.0 M) wereadded and the reaction mixture was stirred at room temperaturefor 3 h. The resulting mixture was diluted with DCM (100 mL)and the organic phase was separated. The aqueous phase wasextracted with DCM (2×50 mL). The combined organic phaseswere washedwith brine and dried overMgSO4 and filtered. Afterremoval of the solvent under reduced pressure, the residue wassubjected to purification by chromatography on silica gel using1:3 ethyl acetate/hexane as eluent to yield SNS-Fc (0.243 g, 95%,orange solid). 1H-NMR (500 MHZ, CDCl3): δ 7.34 (dd, J=0.9,5.0 Hz, 2 H), 7.09 (dd, J=3.5, 5.0 Hz, 2 H), 6.90 (dd, J=0.9,3.5 Hz, 2 H), 6.61 (s, 1 H), 6.41 (s, 2 H), 4.68 (t, J=6.0 Hz, 2 H),4.60 (t, J=2.1 Hz, 2 H), 4.38 (t, J=6.0 Hz, 2 H), 4.26 (t, J=2.1 Hz, 2 H), 4.01 (s, 5 H). 13C-NMR (100 MHZ, CDCl3): δ147.03, 133.61, 128.65, 127.81, 126.63, 125.19, 119.52, 111.93,75.39, 69.52, 68.56, 66.72, 49.13, 44.56; FTIR (KBr, cm−1):3127, 3106, 3066, 2958, 1594, 1451, 1431, 1408, 1344, 1360,1308, 1260, 1221, 1203, 1189, 1106, 159, 1045, 1026, 878, 843,812, 771, 689. HRMS [TOF MS ES+]: m/z [M+H]+ calcd. forC26H23FeN4S2 511.0714, found 511.0690 (−4.7 ppm).

Synthesis of P(SNS-Fc)

P(SNS-Fc) films were prepared potentiodynamically on ITOelectrodes with a thickness around 150 nm, using 0.01 Mmonomer (SNS-Fc) in acetonitrile (ACN) solution containingLiClO4. A platinum wire and an Ag/Ag+ electrode were usedas the counter and reference electrodes respectively.

Synthesis of the copolymer (P(SNS-Fc-co-EDOT))

SNS-Fc (50 mg) was dissolved in 5 mL of ACN and 5 μL ofEDOT was introduced into the electrolysis cell containingLiClO4. The film was prepared dynamically scanning thepotential between 0.0 V and 1.0 V on ITO glass electrodeswith a thickness around 200 nm.

Results and discussion

Synthesis of SNS-Fc

Ferrocene functionalized SNS monomer (SNS-Fc) was synthe-sized by five-step synthetic route, as shown in Scheme 1.Initially, 1,4-di(2-thienly)-1,4-butanedione was synthesized bythe reaction of thiophene and succinyl chloride in the presenceof AlCl3 in dry DCM [28]. The second step involves a Paal–Knorr reaction of 1,4-di(2-thienly)-1,4-butanedione with etha-nolamine in the presence of catalytical amount of PTSA. Treat-ment of the resulting alcohol (SNS-ethanol) with an equimolaramount of TsCl produced tosylated SNS derivative (SNS-OTs)in 90% yield and then it was converted to SNS-N3 [30] with anexcess amount of NaN3 by heating at 50 °C for 6 h. Finally,

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SNS-N3 was clicked with ethynylferrocene to furnish the de-sired monomer (SNS-Fc) as an orange solid in 95 % yield. Thestructure of the monomer (SNS-Fc) was rigorously proved by1H, 13C-NMR, FTIR spectra and HR-MS analysis.

Redox properties of SNS-Fc

During the first anodic scan of SNS-Fc a reversible oxidation at0.32 V and an irreversible oxidation peak at 0.78 V wasobserved (Fig. 1a). In accordance with the literature the irre-versible oxidation peak was attributed to formation of theradical cation of SNS unit [6] and the reversible redox couplewas associated with ferrocene unit [12]. Starting from thesecond scan, two reversible redox couples at Ep½=0.28 V,Ep½=0.45 V were observed and the current intensity of thesereversible peaks increased with successive scanning along with

increase in thickness of the polymer film. In order to assesredox processes of P(SNS-Fc), cyclic voltammogram ofP(SNS-Fc) and P(SNS-N3) (a polymer having analogous poly-mer backbone, except the ferrocene and triazole units) werealso recorded under same conditions and CVof both P(SNS-Fc)and P(SNS-N3) are given in Fig. 1b at a scan rate of 100 mV/s.As seen, P(SNS-N3) displayed a single quasi-reversible redoxprocess at Ep½=0.51 V, indicating its classical p-type dopingnature. The difference in redox behavior of these two polymersat 100 mV/s evidently comprehended that the reversible redoxwave observed at Ep½=0.31 V, Ep½=0.55 Vare due to ferro-cene units and p-doping of P(SNS-Fc), respectively. More-over, redox behavior of P(SNS-Fc) was evaluated as a func-tion of scan rate (Fig. 1c). Existence of a linear dependencebetween peak current intensity and scan rate during dopingprocess brought convincing evidence that P(SNS-Fc) is an

Scheme 1 The synthetic route of SNS-Fc

Fig. 1 Cyclic voltammogram ofa) SNS-Fc, b) P(SNS-Fc) andP(SNS-N3) at a scan rate of100 mV/s, c) P(SNS-Fc) atvarious scan rates in LiClO4/ACN

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electrode supported, electroactive film having non diffusion-limited the p-doping process.

Electrochemical copolymerization

In order to provide another versatile, ferrocene containingpolymer and enhance the color pallet of PSNS derivatives,we attempted to incorporate EDOT within SNS-Fc unitsthrough electrochemical copolymerization [31, 32]. Copoly-merization of SNS-Fc was realized in the presence of EDOTin LiClO4/ACN system at a scan rate of 100 mV/s. When thecopolymer was anodically cycled in monomer-free electrolyte

solution (Fig. 2a), it exhibited two well-defined reversibleredox couples at Ep,1/2=0.38 V, 0.57 V. Among those theformer redox couple could be attributed to ferrocene moietiespresent within the copolymer structure. Besides the totalcharge difference during redox switching, the change in posi-tions and relative intensities of the peaks between P(SNS-Fc)and the copolymer (see Figs. 1c and 2a) could be related toformation of a true copolymer [33, 34]. Moreover, FTIRspectra of the copolymer confirmed existence of asymmetricand symmetric C–O–C stretching of etheric units at 1,205–1,110 cm−1, which is attributed to presence of EDOT units inits structure [35]. As seen in Fig. 2b both the anodic andcathodic current densities of the copolymer during p-dopingshow linear dependence with the scan rate. Such observationindicates that migration of the electroactive species is notdiffusion controlled, the electroactive polymer is well adheredand electrode supported. Thus, all these results imply forma-tion of a true, ferrocene containing, surface confined copoly-mer having different redox behavior from PEDOT andP(SNS-Fc). Furthermore, in order to provide more insightabout the composition of the copolymer we performed ele-mental analysis. Within the limits of experimental error ourstudies have shown that the resultant copolymer contained22.12, 3.84, 1.87 and 7.02 % of C,H,N and S, respectively.The homopolymer, on the other hand, revealed 32.34, 4.93,2.91, 5.24 % of C,H,N and S, respectively. When we considerthe S/C and N/C ratio of these two polymers we could infer therelative increase of the thiophene units compared to the pyr-role units upon copolymerization. Since thiophene/pyrroleratio is constant for the monomer we could deduce effectiveincorporation of EDOT units within the PSNS backbonethrough copolymerization.

Fig. 2 Cyclic voltammogram of the copolymer at various scan rates inLiClO4/ACN (inset: plot of anodic and cathodic peak current density vs.scan rate)

Fig. 3 Spectroelectrochemistry of P(SNS-Fc) film on an ITO coatedglass slide in monomer-free LiClO4/ACN electrolyte solution at appliedpotentials: (a) 0.6 V, (b) 0.7 V, (c) 0.8 V, (d) 0.9 V, (e) 1.0 V, (f) 1.1 V, (g)1.2 V, and (h) 1.3 V

Fig. 4 Spectroelectrochemistry of P(SNS-Fc-co-EDOT) film on an ITOcoated glass slide in monomer-free LiClO4/ACN electrolyte solution atapplied potentials: (a) −0.2 V, (b) 0.0 V, (c) 0.2 V, (d) 0.3 V, (e) 0.4 V, (f)0.5 V, (g) 0.6 V, (h) 0.7 V, (i) 0.8 V, (j) 0.9 V, (k) 1.0 V, (l) 1.1 Vand (m)1.2 V

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Electrochromic properties

The spectroelectrochemical experiments, which show changes inoptical properties of polymer upon applied potential, were carriedout in a monomer free electrolyte system between 0.6 V and1.3 V for P(SNS-Fc) (Fig. 3). At neutral state λmax of thehomopolymer, which corresponds to the π-π* transition, wasfound to be 345 nm and the optical band gap (Eg) was calculatedas 2.7 eV. Upon increase of the applied potential, the intensity ofπ-π* transition waned and a new peak aroused at about 600 nmdue to the evolution of charge carrier bands. However, it was notpossible to full oxidize P(SNS-Fc) through electrochemical pro-cess. Absence of isosbestic point during electrochemical oxida-tionmight indicate the occurrence of more than two spectroscop-ically distinct charge carriers or the dielectric effect in condensedphases leading to disappearance of the isosbestic point in two-component systems as in the case of other PSNS derivatives [36].The homopolymer appeared in yellow, green and blue at neutral,intermediate and oxidized states, respectively. Coloration kineticsof P(SNS-Fc) was investigated via alternation of the appliedpotential in square wave form between 0.0 and 0.9 V, whilesimultaneously monitoring the percent transmittance at 600 nm.Switching time (at 90% of ultimate contrast) and optical contrast

of P(SNS-Fc) were estimated as 0.95 s and 27.1 %, respectively.When the redox process for the film is repeated, the film switchesfairly reversibly between 0.0 and 0.9 Vup to 50 cycles. Howeverpotentials beyond 0.9 V results in gradual decline inelectroactivity of the polymer film starting from 5th cycle.

At the neutral state P(SNS-Fc-co-EDOT) displayed a verybroad absorption at around 420 nm (Fig. 4). While incremen-tally increasing the applied potential, intensity of π-π* transi-tion waned and a new absorption peak developed at about750 nm due to the evolution of polaron bands. The opticalband gap of the copolymer, which was estimated to be around1.9 eV, is in midway between P(SNS-Fc) and PEDOT(1.6 eV) [37, 38]. The copolymer displayed distinctmultichromism, revealing dark orange, yellow, green, bluecolors in neutral, mid, oxidized and highly oxidized states,respectively. Due to subjective nature of color, objective,quantitative color analyses of both P(SNS-Fc) and P(SNS-Fc-co-EDOT) were carried out under CIE recommended con-ditions and the related data are tabulated in Table 1. Thepercent transmittance, applied potential profile and the currentdensity of P(SNS-Fc-co-EDOT) as a function of time aregiven Fig. 5a–c, respectively. The switching time of the co-polymer at 90 % of ultimate contrast was estimated as 2.24 s.

Fig. 5 a T %, b applied potential, c current density, during repetitive electrochromic switching of poly(SNS-Fc-co-EDOT), monitored at 468 nm inLiClO4/ACN

Table 1 Electrochromic properties of the polymers

Material λmaxa (nm) Eg

b (eV) Switching time (s)c t100, t95, t90 Optical contrast L, a, bd

PSNS-Fc 356 2.61 3.12, 1.33, 0.95 27.1 (n) 96, 5, 9

(i) 90,3,2

(o) 86, 6, −1

P(SNS-Fc-co-EDOT) 415 1.88 4.55, 2.75, 2.24 20.9 (n) 96, 11, 20

(i) 98, 4, 17

(i) 94, 2, 6

(o) 87, 5, −1

a For the neutral polymer filmsb Band gap, estimated from the optical absorption band edge of the filmsc The switching time of the copolymers at t100 : 100 %, t95 : 95 %, t90 : 90 % of ultimate contrastd Colorimetry study results at n:neutral, i: intermediate, o: oxidized states

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Compared with P(SNS-Fc) the copolymer displayed higherswitching time and lower optical contrast. However, P(SNS-Fc-co-EDOT) showed far more reversible and reproducibleswitching between its ultimate colored states, which wasnot only confirmed by the spectroscopic but also chro-noamperometry studies. Such superior attributes the copolymermay steam from the incorporation of EDOT units within thepolymer backbone, whose homopolymer (PEDOT) is knownfor its superior attributes such as the high stability in redoxswitching.

Conclusion

Until now almost all PSNS derivatives were achieved thoughelectrochemical or chemical polymerization of SNS monomerswhich were mainly synthesized through Paal-Knorr type reac-tion. Despite its effortless reaction conditions this reaction isknown to be significantly low in yield, which seriously limitsthe available structures, especially in case of bulky groups. In thisstudywe disclosed a robust approach for functionalization of 2,5-dithienylpyrroles via click reaction for the first time. For thispurpose a ferrocene functionalized, new SNS monomer (4-ferrocenyl-1-[2-(2,5-di-thiophene-2-yl-pyrrole-1-yl)-ethyl]-1H-[1,2,3]triazole, SNS-Fc) was synthesized. SNS-Fc was elec-trochemically polymerized to achieve a new conducting polymerP(SNS-Fc) exhibiting attractive electrochromic properties. Addi-tionally, a copolymer of SNS-Fc was obtained byelectropolymerization in the presence of EDOT. The resultingcopolymer displayed multichromism including four differentcolors (orange, yellow, green and blue) in different redox states.Our studies have shown that the redox behavior and band gap ofpoly(2,5-dithienylpyrrole) derivatives are highly dependent onanchored unit (ferrocene) and copolymerization. Copolymeriza-tion not only enhances the color pallet of the polymers but alsoimparts higher reversibility and stability to PSNS family. Ourresults imply that both polymers P(SNS-Fc) and P(SNS-Fc-co-EDOT) hold high potential for electrochromic devices. Owing tothese promising outcomes, currently we are preparing variouslyfunctionalized SNS monomers by ‘Click Chemistry’ and theircorresponding electroactive polymers. Further results will bereported in due course.

Acknowledgment We are grateful to TUBITAK (Project No:110T640) for the support of this study.

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