In situ Raman and UV–vis spectroscopic studies of polypyrrole and poly(pyrrole-2,6-dimethyl--cyclodextrin)

Polypyrrole (PPy) and poly(pyrrole-2,6-dimethyl--cyclodextrin) [P(Py--DMCD)] films prepared by potential cycling in aqueous acidic solutions on indium tin oxide (ITO)-coated glass and gold electrodes were studied by in situ UV–vis and Raman spectroscopy. Characteristic UV–vis and Raman bands were identified and their dependencies on the electrode potential have been discussed. Spectroelectrochemical results reveal differences both in the position of the spectral bands and their potential dependence for PPy and P(Py--DMCD) films indicating interactions between polymer chains and CDs during electropolymerization process. The films were also characterized by cyclic voltammetry and FT-IR spectroscopy.

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Spectrochimica Acta Part A 78 (2011) 1–6 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journa l homepage: www.e lsev ier .co Review article In situ R poly(py Jalal Arjo a Faculty of Che b Department o c National Cent d Faculty of Che e Technische Un a r t i c l Article history: Received 8 Jun Received in re Accepted 7 De Keywords: Spectroelectro -Dimethyl cy Polypyrrole Conducting po Electropolymerization Contents 1. Introd 2. Exper 2.1. 2.2. 3. Result Ackno Refer 1. Introdu Polypyrr materials o use energy trochemica spectroelec structural e intrinsically onance Ram to study ch [25–27]. SE ∗ Correspon E-mail add 1386-1425/$ – doi:10.1016/j.uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 imental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Preparation of (pyrrole-cyclodextrin) complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Polypyrrole and P(pyrrole-cyclodextrin) film electrosynthesis in aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 s and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 wledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 ction ole is one of the most extensively studied functional f the conducting polymers family due to its wide storage devices, microelectronic devices and elec- l/chemical sensors [1–5]. In situ optical and Raman trochemical methods have been very useful for the lucidation and study of optoelectronic properties of conducting polymers [6–24]. Surface Enhanced Res- an Spectroscopy (SERS) has been used extensively emically and electrochemically polymerized PPy films RS can be used to identify oxidation (doping) state and ding author. Tel.: +98 8118270820. ress: jalal.arjomandi@s2004.tu-chemnitz.de (J. Arjomandi). the doping level in a conducting polymer film. As an example SERS can selectively enhance the Raman bands associated with the oxi- dized species of polythiophene [28]. In recent years, oxidation reactions of host–guest compounds have attracted enormous research interest by several authors [29–31]. Cyclodextrins (CDs) are cyclic oligosaccharides composed of from 1,4-glucopyranose units that exhibit a torus-shaped struc- ture with a hydrophobic cavity and a hydrophilic exterior. They can form inclusion complexes only with guest molecules of the proper sizes [32]. This has led to considerable interest in using CDs in the synthesis of conducting polymers. It has been found that CDs bound to the electrode acted as molecular templates to restrict the growth sites of PPy within the -CDs cavities [33]. Several reports have focused on the synthesis and preparation of an inclusion complex of poly(aniline--cyclodextrin) [34], poly(bithiophene- hydroxypropyl--cyclodextrin) [35], poly(pyrrole-sulfonated - see front matter © 2009 Elsevier B.V. All rights reserved. saa.2009.12.026aman and UV–vis spectroscopic studies of polypyrrole and rrole-2,6-dimethyl--cyclodextrin) mandia,∗, Anwar-ul-Haq Ali Shahb, Salma Bilal c, Hung Van Hoangd, Rudolf Holzee mistry, Buali Sina University, Hamedan 6517838683, Iran f Chemistry, Abdul Wali Khan University Mardan, N.W.F.P., Pakistan er of Excellence in Physical Chemistry, University of Peshawar, N.W.F.P., Pakistan mistry, Hanoi National University of Education, 136 Xuan Thuy, Hanoi, Viet Nam iversität Chemnitz, Institut für Chemie, Straße der Nationen 62, D-09111 Chemnitz, Germany e i n f o e 2009 vised form 3 December 2009 cember 2009 chemistry clodextrin lymer a b s t r a c t Polypyrrole (PPy) and poly(pyrrole-2,6-dimethyl--cyclodextrin) [P(Py--DMCD)] films prepared by potential cycling in aqueous acidic solutions on indium tin oxide (ITO)-coated glass and gold electrodes were studied by in situ UV–vis and Raman spectroscopy. Characteristic UV–vis and Raman bands were identifiedand their dependencies on the electrodepotential havebeendiscussed. Spectroelectrochemical results reveal differences both in the position of the spectral bands and their potential dependence for PPy and P(Py--DMCD) films indicating interactions between polymer chains and CDs during electropoly- merization process. The films were also characterized by cyclic voltammetry and FT-IR spectroscopy. © 2009 Elsevier B.V. All rights reserved.m/locate /saa 2 J. Arjomandi et al. / Spectrochimica Acta Part A 78 (2011) 1–6 cyclodextrin), [36–38] and polypyrrole/-cyclodextrin [39]. We have shown the role of -DMCD molecules in the polymeriza- tion process, optical absorption and the conductivity of PPy and PNMPy films in two differentmedia [40,41], and P3MPy in acetoni- trile, previo (UV–vis an aqueous sol The change modes of PP 2. Experim Cyclodex as received LiClO4 used uum. Cyclic potentiosta card operat metry (CV) ∼0.4 cm2) e Chemicals), as counter used as ref a BioRad FT with a liqui using the K the polyme electrode in a standard ment; a cuv wasplaced ingly positi the negativ (as indicate an ISA T640 detection sy ing laser lig 70 Series io with aCohe (polycrysta to confer S aqueous so (ORC) treatm with 25 sca effect are 5 ber of cycle found to be were rinsed under vacu ples were p experiment purged solu 2.1. Prepara A (1:1) ( tions were 1.33g (1m DMCD) com evaporator. 2.2. Polypy in aqueous s 0.05M ( solving 1.74 aman d on d nt. LiC rgin ning towards positive potentials from 0.00<ESCE < 1.80V at rate of 50mVs−1. The electrosynthesis was stopped after 35 The PPy film was also prepared in the same way. The elec- merizationmediumwas 69.1L (1mm) of themonomer in of 0.1M LiClO4 aqueous solution. ults and discussion 1 shows the Raman spectra of PPy films deposited on ubstrates with and without ORC treatment. No meaning- rmation can be obtained from the Raman spectroscopy if posits on a gold substrate without ORC pre-treatment. The spectra of PPy deposited on the roughened Au indicated marked increase in intensity and a better resolution. s of PPy and P(Py--DMCD) were deposited by cyclic metry. Suitable films with good transition of vibration S measurement can be prepared after 35 cycles from SCE <1.80V.Althoughat thepotential limit of 1.8V therewas f overoxidationof pyrrole, itwas a good choice for the sakeof rison with electropolymerization of (pyrrole-cyclodextrin) ex as also reported in the literature for the electropolymer- of PPy, PPy-sulfated--CD [37] and PPy, PPy--CDfilms [39] at a gold electrode at applied potentials up to ESCE = 1.80V ypical CVs of PPy and P(Py--DMCD) films in 0.1M LiClO4 n are shown in Fig. 2. Apparently the CVs seem to be of the ature. Nevertheless, differences can be observed especially position of the oxidation peaks (around E SCE = 0.14V). In e of P(Py--DMCD), the oxidation peak is shifted by 0.12V s higher potential values. This indicates that the presence of extrin not only increases the oxidation potential of PPy butusly [42]. In the present paper spectroelectrochemical d Raman) results of PPy and P(Py--DMCD) films in ution, using Kr+ ion laser (647.1nm) light, are reported. s in intensity and characteristics of various vibrational y and (Py--DMCD) films are discussed. ental trin (Wacker Chemie, Burghausen, Germany)was used . Pyrrole (Aldrich, 99%) was distilled under vacuum. as electrolyte (Heraeus, Germany)wasdriedunder vac- voltammograms were recorded with a custom-built t interfaced with a standard PC via an ADDA-converter ingwith custom-developed software. For cyclic voltam- , a gold electrode (99.99%, Schiefer, Hamburg-area mbedded in epoxy ARALDIT D/HY 956 (Ciba Special was used as working electrode. A gold sheet served electrode and a saturated calomel electrode SCE was erence electrode. Infrared spectra were recorded on S-40 Fourier transform infrared (FT-IR) spectrometer d-nitrogen-cooledmercury cadmium telluride detector Br pellet technique. UV–vis spectra were recorded with r films deposited on an optically transparent ITO-glass the supporting electrolyte solution (0.1M LiClO4) in 10mm cuvette using a Shimadzu UV 2101-PC instru- ette with the same solution and an uncoated ITO glass in the referencebeam. Spectrawere recordedat increas- ve potentials; in a few cases, spectra were recorded in e-going potential direction in order to test reversibility d in selected figures). Raman spectra were recorded on 00 spectrometer connected to a spectraview 2D CCD stem. SER spectra were recorded using 647.1nm excit- ht from Kr+ ion lasers, provided by a Coherent Innova n laser; the laser power wasmeasured at the laser head rent200powermeter. Rougheningof thegold electrode lline 99.99%, polished down to 0.3mAl2O3) employed ERS activity was performed in a separate cell with an lution of 0.1M KCL by 25 oxidation–reduction cycles entbetweenESCE =−0.28 toESCE = 1.22Vat500mVs−1 ns. The optimum cathodic and anodic scans for SERS 00 and 1000mVs−1, respectively. The optimum num- for roughening the Au substrate in ORC treatment was 25 cycles [43]. After the ORC treatment the electrodes thoroughly with distilled water, and finally dried in a um dryer for 24h at room temperature. Then the sam- laced in a desiccator filled with nitrogen before use. All s were performed at room temperature with nitrogen- tions. tion of (pyrrole-cyclodextrin) complex mole–mole) pyrrole/2,6-dimethyl--cyclodextrin solu- prepared by mixing 69.1L (1mmole) of pyrrole and mole) -DMCD in 5mL methanol. The formed (Py-- plexwas isolated by removing themethanol in a rotary rrole and P(pyrrole-cyclodextrin) film electrosynthesis olution Py--DMCD) complex solution was prepared by dis- g (1.25mmol) solid (Py--DMCD) complex in 25mL Fig. 1. R deposite treatme of 0.1M gen pu by scan a scan cycles. tropoly 25mL 3. Res Fig. gold s ful info PPy de Raman both a Film voltam in SER 0.00<E a risk o compa compl ization grown [36]. T solutio same n in the the cas toward cyclodspectra of PPy films prepared in 0.1M LiClO4 aqueous solution and ifferent gold substrates (a) with ORC treatment and (b) without ORC lO4 aqueous solutions. After vigorousmixing and nitro- g (10min) cyclic voltammetric synthesis was initiated J. Arjomandi et al. / Spectrochimica Acta Part A 78 (2011) 1–6 3 Fig. 2. Cyclic v PPy (solid line solution. Scan also affects extent. In situ SE tion lineatd The general tra previou 632.8nm e gradually fr goes positi this region The double the C–H in- ESCE = 1.00V then remai increasing 1094 cm−1 1056 cm−1. to the ring band aroun ESCE = 0.80 u belonging t are attribut band aroun n situ e with solut SCE = retch ring dical seem to be valid in the present context as the band at −1 shows decrease in intensity with the potential increaseoltammograms of a gold substrate (with ORC treatment) coatedwith ) and P(Py--DMCD) films, (dashed line) in 0.1M LiClO4 aqueous rate =50mVs−1. the properties of the resulting polymer film to some RS spectra of PPy film obtained with 647.1nm excita- ifferentpotentials in theanodic scanare shown inFig. 3. appearance of the Raman spectra is similar to the spec- sly reported for PPy films in 0.1M LiClO4 solution with xcitation line [44]. The C C stretching band is shifted om 1599 to 1617 cm−1 when the electrode potential Fig. 3. I electrod aqueous from E ring st to the and ra ments 998 cmvely from ESCE = 0.00 to 1.80V. The wave number in is representative of the redox state of PPy [33,37,44]. peaks at about 1056 and 1094 cm−1 are attributed to plane deformation [27]. With applied potential up to the intensity of the peak at 1056 cm−1 decreases and ned nearly constant up to ESCE = 1.80V. However, with potential from ESCE = 1.00 to ESCE = 1.80V, the peak at is getting stronger as compared to the band around The peaks around 1332 and 1396 cm−1 are attributed -stretching mode of PPy [25,27]. The intensity of the d 1396 cm−1 increases with the potential increase from p to ESCE = 1.80V. Both the 1094 and 1396 cm−1 bands, o the C–H in-plane and the ring stretching, respectively, ed to the oxidized PPy [25,37]. The intensity of the d 1431 cm−1 also increases with increasing potential beyond a ce 940 cm−1 in at 931 cm−1 dopants [48 Thismight b at 940 cm−1 Fig. 4 sh ferent elect features as stretching f sity with t The ring de sity with th about 1431 cies and is o Table 1 Band assignments for Raman spectra of PPy and P(Py--DMCD) in 0.1M LiClO4 aq Assignment PPy/cm C–H out-of-plane deformation 940–946 Ring deformation 996–1001 C–H in-plane deformation 1056–1066 N–H/C–H in-plane deformation 1261–12,767 Ring stretching 1331–1343 1344–1357 C–N stretching 1390–1396 1427–1437 C C backbone stretching 1599–1650Raman spectra at 647.1nm excitation of PPy film on gold substrate ORC treatment taken at different potentials imposed in 0.1M LiClO4 ion. 0.00 to ESCE = 1.80V. This band is associated with the ing [45]. The bands about 940 and 998 cm−1, assigned deformation, are associated with dications (bipolaron) cations (polaron), respectively [46,47]. These assign-rtain value. However, the intensity of the band around creases gradually with the potential increase. The peak attributed to the symmetric stretchingmode of ClO4−1 ] does not show a well-defined potential dependency. e due to overlappingwith the peak of ring deformation . ows Raman spectra of a P(Py--DMCD) film at dif- rode potentials. Generally the spectra show the same that for PPy film (Fig. 3). The broad peak of C C bond rom1570 to 1641 cm−1 shows a slight increase in inten- he potential shifting from ESCE = 0.00 to ESCE = 1.80V. formation peak at about 940 cm−1 increases in inten- e potential only after ESCE = 1.00V. The broad peak at cm−1 for PPy has been shifted towards higher frequen- bserved at about 1462 cm−1 in the present case. These ueous solution at =647.1nm excitation. P(Py--DMCD)/cm 939–939 986–994-weak 1052–1063 1256–1265 1311–1332 1351–1361 1380–1399 1462–1463 1570–1647 4 J. Arjomandi et al. / Spectrochimica Acta Part A 78 (2011) 1–6 Fig. 4. In situ Raman spectra at 647.1nm excitation of P(Py--DMCD) film on gold substrate electrode with ORC treatment taken at different potentials imposed in 0.1M LiClO4 aqueous solution. peaks are attributed to the C–N stretching. Surprisingly the inten- sity of this peak for P(Py--DMCD) film does not change with the applied potential. Raman peak frequencies of both polymer films with corresponding assignments are listed in Table 1. The slight differences in the position of the peaks and their potential dependence for PPy and P(Py--DMCD) films indi- cate some interaction between polymer chain and CDs during electropolymerization process. Thus by careful selection of the excitation wavelength, SERS technique not only can be used as a powerful tool for analyzing the changes in redox state of con- ducting polymers but also gives information about the interactions between host and guest during polymerization process. Fig. 5a and b shows the absorption spectra of a PPy film on ITO-glass electrode recorded after stepwise increases of poten- tial from ESCE = 0.0 to 1.80V and the potential dependence of the absorbance at three selected wavelengths, respectively. The spec- trawere recorded in 0.1M LiClO4 solution. Themain features in the spectra are essentially the same as reported before [49,50], where three absorption peaks, which depend in bothmagnitude and posi- tion on potential, can be distinguished. The highest absorption around=350nm is causedby the→* electronic transition and is characteristic of the reduced formof the polymer. The decrease in intensity of this bandwith increasing potential indicates reduction in concentration of the reduced sites because of their transforma- tions to oxidized sites. The absorption in the range of 490–530nm also shows sensitivity towards the potential shift. This absorption is assigned to the high-energy polaron transition [40,37,51,52]. The third absorptionattributed to the fully oxidized form is around860- 880nm. The spectra show that the absorption bands at about 530 and 880nm increases and shifts to 490 and 860nm, respectively, with the increase in potentials up to ESCE = 1.80V. Fig. 6a shows the UV–vis spectra of a P(Py--DMCD)-coated electrode recorded in 0.1M LiClO4 at different applied potentials successively increasing in the anodic direction. The spectral fea- tures of the different fro Fig. 5. (a) UV–vis spectra of a PPy film-coated ITO-glass electrode, obtained at different electrode potent plot for three selected wavelengths, derived from spectra displayed above.P(Py--DMCD)-coated electrode reflect characteristics m those of a simple PPy-coated electrode. Two absorp- ial values, ranging from ESCE = 0.0 to 1.80. (b) Absorbance vs. potential J. Arjomandi et al. / Spectrochimica Acta Part A 78 (2011) 1–6 5 Fig. 6. (a) UV– t electrode potential values, ranging from ESCE = 0.0 to 1.80. (b) Absorbance vs. potential p tion bands c than three about 860– of a broad case for P(P around360 sible ways. reported in 490–530nm spectra of the high-en polymer ch entire abso towards low at 360nm i →* tran Fig. 6a). Sim due to the shifted to 3 possibility decreases c supported b 400nm wh the involve of the polym the absorba potential. Fig. 7 sh DMCD) and IR spectra o ratory cond to the differ vibrational synthesized our previou to that of PPy formed in the absence of CD. The data for PPy ood agreement with values reported elsewhere [54,55–57]. broadpeakoccurringat 3417 cm−1 inPPy is attributed to the retch 5 cmvis spectra of a P(Py--DMCD) film-coated ITO-glass electrode, obtained at differen lot for two selected wavelengths, derived from spectra displayed above. an be observed in the spectra of P(Py--DMCD) rather absorption bands. The absorption band observed at 880nm for PPy (Fig. 5a) can be observed in the form absorption band at about 760–820nm in the present y--DMCD) film. The other absorption band is located –400nm.Thesedifferences canbeexplained in twopos- Firstly, like P(Py--DMCD) in nonaqueous solution as our previous work [40], the absorption band around similar are in g The N–Hst at 152in the spectra of PPy disappears completely in the P(Py--DMCD). The reason might be the absence of ergy polaron transition because of interaction between ain andCDsduringelectropolymerizatio