Electrochemical and Spectroelectrochemical Study of Polyazulene

Article pubs.acs.org/JPCC

Electrochemical and Spectroelectrochemical Study of Polyazulene/ BBL-PEO Donor−Acceptor Composite Layers Rose-Marie Latonen,*,† Anna Ö sterholm,†,§ Carita Kvarnström,‡ and Ari Ivaska† †

Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, FI-20500, Åbo-Turku, Finland ‡ Laboratory of Materials Chemistry and Chemical Analysis, Department of Chemistry, Turku University Centre for Materials and Surfaces, University of Turku, Vatselankatu 2, FI-20014, Turku, Finland ABSTRACT: Donor−acceptor composite layers made of the electrically conducting polymers p-type polyazulene (PAz) and n-type polybenzimidazobenzophenanthroline-poly(ethylene oxide) (BBL-PEO) were studied. The water dispersible BBLPEO was solution-cast on PAz electropolymerized on Pt or ITO glass electrodes or on the bare electrode surfaces. The doping behavior within both the n- and p-doping potential regions was studied by cyclic voltammetry and in situ spectroelectrochemical techniques: UV−visible and FTIR-ATR spectroscopy. All of the results showed contributions from both polymers in the composites, besides the spectral behaviors in the n-doping potential region were mainly controlled by BBL-PEO and in the p-doping region by PAz. Rather stable donor−acceptor composites with the formation of charge carriers in both n- and p-doping regions were formed. In the BBL-PEO/PAz composite PAz was found to have a higher promoting effect on the formation of charge carriers than in the PAz/BBL-PEO composite. The composites stayed conducting within a broad negative potential region while some charges appeared to be trapped in the layers after the negative potential scan.

1. INTRODUCTION Research in the field of synthesis and characterization of electrically conducting polymers (ECPs) has attracted a lot of interest during at least the last 25 years due to their electrochemical, electronic, and optical properties which are useful in many promising application areas.1,2 New types of electrically conducting materials have been developed by forming composites or bilayers of ECPs with different inorganic semiconductors like TiO23−9 and ZnO,8−13 with fullerene C60 and its derivatives14−21 and carbon nanotubes22−25 as well as with other ECPs.26−29 Composite materials often have better performance than the single components alone and show some new properties as a result of synergistic effect between the single components. Formation of bilayers or composites of ECPs with other electrically active components may offer the possibility to improve the mechanical strength and thermal stability5,22,23,25 of the materials, induce some rectifying characteristics,27 increase the surface area, improve the crystallinity, or even decrease the band gap. Smaller band gaps could result in improved light harvesting and altered electrochromic properties6,10,18−21 and may even improve charge separation and charge transport within the material.7−9,11−13,15−17,24 The bilayers or composites have interesting properties in many different areas of application, such as electrochromic devices,6 light-emitting diodes,18 and photovoltaic devices recently reviewed in three articles.30−32 The electrochemical properties of the bilayers or composites depend on the energy levels of the individual layers and on the permeability of the ions in the solution through the outer layer. © 2012 American Chemical Society

It is of utmost importance to understand the fundamental redox switching processes of the bilayers or composites in order to obtain a successful application of these materials. Redox processes in ECPs lead to oxidized or reduced conducting states with mobile charge carriers accompanied by changes in the polymer chain around the charge causing a local distortion. These redox processes are described as p- and n-doping. The localized distortion gives rise to new electronic states in the region between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the band gap which leads to characteristic optical transitions. These transitions can be studied by UV−visible33 and FTIR-ATR34 spectroelectrochemical techniques by simultaneously measuring the electrochemical and spectroscopic signals. The infrared spectra of ECPs show, upon doping, a strong increase in absorbance in the high energy range of the mid-IR region (7000−1600 cm−1) related to the increased conductivity of the polymer. In addition, new and very intense infrared active vibration (IRAV) bands appear during doping in the low energy range of the mid-IR region (1600−600 cm−1) due to changes in the dipole moment of the vibrations related to the distorted conjugated system. Therefore, by in situ FTIRATR measurements it is possible to obtain information about the interaction of the fused rings in the polymeric structure Received: August 24, 2012 Revised: October 12, 2012 Published: October 19, 2012 23793

dx.doi.org/10.1021/jp308420s | J. Phys. Chem. C 2012, 116, 23793−23802

The Journal of Physical Chemistry C

Article

Figure 1. Cyclic voltammograms recorded during polymerization of 10 mM Az in 0.1 M TBAPF6-ACN electrolyte solution on (a) Pt/BBL-PEO(5 μL) electrode by three potential cycles and (b) on bare Pt by eight potential cycles. Scan rate 50 mV s−1; the potential values were measured against a Ag/AgCl pseudo reference electrode.

with the π-electron delocalization along the polymeric backbone. In this study the redox properties of composite layers of polyazulene (PAz) and polybenzimidazobenzophenanthrolinepoly(ethylene oxide) (BBL-PEO) have been characterized. PAz is an ECP with a coplanar fused-ring structure. This makes the backbone rigid and is expected to have a reducing effect on the band gap therefore increasing its absorbance within the UV and visible spectral regions.35,36 The absorption profile of PAz in the UV and visible spectral regions has also been shown to be broad3,4,35,37,38 and it is n-dopable to some extent. PAz has also been shown to have electron-donating properties.14,38−40 Furthermore, the easy electropolymerization process of Az3,4,35,37,38 based on the relatively low oxidation potential of Az, makes PAz a good candidate as an electron-donor material combined with an electron accepting material. BBL is a highly conjugated ladder-type polymer also possessing a rigid doublestranded backbone with a planar structure.41−43 BBL is thermally stable up to 600 °C and shows highly reversible ntype conductivity and has a high electron affinity. Thus, it is a potential candidate for use in different organic electronic applications. The main drawback of BBL is its insolubility in most solvents even in the strongest acidic conditions. Recently, water dispersible BBL derivatives with partial substitution of chain ends with poly(ethylene oxide) (PEO) making it easier to process have been made.44 In the present work, composite layers of PAz electrochemically polymerized from organic solution and BBL-PEO drop-cast in water solution have been studied. The doping behavior of the composite structures has been studied both electrochemically with cyclic voltammetry and spectroelectrochemically with in situ UV−visible and in situ FTIR-ATR spectroscopy.

each polymerization, and the ITO substrates were washed with chloroform and acetone for 30 min in an ultrasonic bath. A Pt wire was used as the counter electrode. The poly(azulene) (PAz) films in the PAz-polybenzimidazobenzophenanthrolinepoly(ethylene oxide) (PAz/BBL-PEO) composite layers were polymerized by potential cycling of 10 mM azulene (Az, Aldrich) between −0.6 and +1.2 V with a scan rate of 50 mV s−1 in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich) electrolyte salt dissolved in acetonitrile (ACN, Aldrich) either on a bare electrode or on an electrode covered with BBL-PEO (synthesized at University of Helsinki, Department of Chemistry). Az was used as received. A constant volume of 71.4 μL cm−2 of BBL-PEO was drop-cast on bare Pt or ZnSe/Pt electrodes or on the same electrodes covered with PAz. The ITO glass electrodes were dipped into the BBL-PEO solution a few times to form a thin layer on top of them. The BBL-PEO layer on the bare electrodes was dried at 75 °C for 2 h to make the drying process faster and to decrease the surface roughness. When applied on a PAz layer the annealing was omitted and the BBL-PEO layer was dried at room temperature for 5 h. The reason for this was the different hydrophobicities of PAz and BBL-PEO. All potential values were measured against a Ag wire covered with AgCl pseudo reference electrode (calibrated against ferrocene/ferrocenium, Eredox= 0.36 V in 0.1 M TBAPF6-ACN). The electrolyte salt TBAPF6 was dried at 80 °C under vacuum for 1 h. For the in situ FTIR-ATR studies, the BBL-PEO/PAz and PAz/BBL-PEO composite layers were synthesized on a ZnSe reflection element (size 10 mm ×10 mm ×2 mm) covered with a thin layer of Pt (∼30 nm) which served as the working electrode. This reflection element was attached to the spectroelectrochemical cell made of Teflon and constructed as a flow cell. The construction details have been described earlier.45 The reference electrode was the same as in the cyclic voltammetric (CV) experiments and a Pt spiral was used as the counter electrode. A beam condenser 4XF-BR3 (Harrick Scientific) served as the attachment to the FTIR spectrometer. The n- and p-doping processes of the polymer layers were studied in a monomer-free electrolyte solution by recording FTIR spectra in situ during a slow voltammetric scan (10 mV s−1) between 0 and −1.5 V and −0.4 and +1.1 V, respectively. For each spectrum 32 interferograms were coadded and the spectral resolution was 4 cm−1. The measured spectra were related to a reference spectrum taken prior to the studied

2. EXPERIMENTAL SECTION Polymerization of the studied composite layers and the charging−discharging experiments were carried out in a conventional three-electrode one-compartment electrochemical cell. The cell was connected to an Autolab PGSTAT100 potentiostat using General Purpose Electrochemical System (GPES) software. The working electrode was either a Pt-disk (A = 0.07 cm2) sealed in a Teflon body or an optically transparent indium tin oxide (ITO) glass (Präzions Glas & Optik GmbH, ≤ 10 Ω square−1). The Pt-disk electrode was polished mechanically with 0.03 μm alumina powder before 23794



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Figure 2. Cyclic voltammograms (3rd cycles) of (a) BBL-PEO/PAz composite and (b) PAz/BBL-PEO composite made as in Figure 1 and recorded in monomer-free electrolyte solution with five different scan rates (20, 40, 50, 80, and 100 mV s−1); the potential values were measured against a Ag/ AgCl pseudo reference electrode. The inset shows Ia vs v1/2.

and p-doping of the BBL-PEO/PAz composite layer. By increasing the potential cycles during polymerization of Az on Pt/BBL-PEO(5 μL) to six, only the charge involved during pdoping increased and the charging contribution from both polymers was not in balance any more. The PAz film on top of Pt has a hydrophobic character and, as a result, depositing the hydrophilic BBL-PEO water dispersion on top of PAz is difficult and deposition of an even layer is impossible. Because of the hydrophobic character of PAz, it is difficult to fabricate the PAz/BBL-PEO layers reproducibly. This is partly the reason for the lower charging capacity of BBL-PEO in the PAz/ BBL-PEO composite (Figure 2b) than in the BBL-PEO/PAz composite (Figure 2a). The insets in Figure 2a and 2b showing a linear increase in the oxidation peak current vs square root of the scan rate indicate that p-doping reaction in both composite layers is controlled by the rate of diffusion of ions in the electrolyte solution toward the electrode surface. In this paragraph the CV responses of pure BBL-PEO and PAz are compared to the response of the BBL-PEO/PAz composite. Figure 3 shows the CV of BBL-PEO (5 μL) recorded at 5 mV s−1, PAz synthesized by four potential cycles

reduction/oxidation reaction. The spectra shown in this work therefore describe the spectral differences from the reference state. The spectra were recorded on a Bruker IFS 66/S FTIR instrument equipped with an MCT detector. For the in situ UV−visible spectroscopic experiments, the BBL-PEO/PAz and PAz/BBL-PEO composite layers were formed on ITO glass substrates in the same way as described earlier. Three potential cycles during polymerization of Az were enough to form a uniform yet not too thick a layer for the measurements. The path length of the quartz cuvette used was 1 cm. The reference electrode was the same as in the CV experiments and a Pt wire was used as the counter electrode. The potential was held at a constant value for 60 s before each measurement and then increased by steps of 100 mV. The pdoping experiments were performed before the n-doping due to the poor stability of ITO glass at negative potentials. The spectra were recorded between 300 and 1100 nm with a scan rate of 480 nm min−1 on a Perkin-Elmer Lambda 25 spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetric Experiments. PAz films were electropolymerized either on a drop-cast BBL-PEO film (5 μL) on Pt or on a bare Pt electrode. Polymerization was made in 10 mM Az in 0.1 M TBAPF6-ACN solution by cycling the potential between −0.6 and 1.2 V with a scan rate of 50 mV s−1. The thickness of the PAz layer was controlled by the number of potential cycles made during polymerization. Polymerization of Az on Pt/BBL-PEO (5 μL) by three potential cycles and on bare Pt by eight potential cycles is shown in Figure 1a,b, respectively. The charge consumed during polymerization was higher when Az was polymerized on Pt/BBL-PEO than on bare Pt due to the higher surface area of the Pt/BBL-PEO electrode. The cyclic voltammograms (CVs) of both composites recorded in monomer-free electrolyte solution within the potential range −1.5 to +1.0 V with five different scan rates (20, 40, 50, 80, and 100 mV s−1) are shown in Figure 2a for Pt/BBL-PEO/PAz and in Figure 2b for Pt/PAz/BBL-PEO. Some degradation of the composite layers occurred during the series of CV experiments of altogether 15 potential cycles within the 2.5 V wide potential window. This was visible by a slight decrease in the charge involved during the potential scanning. Only three potential cycles during polymerization of Az on Pt/BBL-PEO were enough to accomplish almost equal charge involved during n-

Figure 3. Cyclic voltammograms (3rd cycles) of BBL-PEO (5 μL; scan rate 5 mV s−1), PAz synthesized by four potential cycles (scan rate 50 mV s−1), and the BBL-PEO/PAz composite layer prepared as in Figure 1a (scan rate 50 mV s−1). The potential values were measured against a Ag/AgCl pseudo reference electrode. 23795



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and recorded at 50 mV s−1, and the BBL-PEO/PAz composite layer prepared as in Figure 1a and recorded by the scan rate of 50 mV s−1. During n-doping of the composite, the two reduction peaks visible in pure BBL-PEO merge together into a broad current wave, however, both reduction processes are still visible. The reduction peak at −0.34 V is not visible in pure BBL-PEO and could result from the uncharging of PAz during the negative potential scan. After decreasing the negative potential within the n-doping potential region, reoxidation of the reduced species in pure BBL-PEO is visible as two clear current peaks. These oxidation current peaks merge into one in the composite layer and occur at more positive potential than in BBL-PEO. This means that uncharging of the BBL-PEO/ PAz composite is more difficult compared to that for BBL-PEO alone and some negative charges may be trapped inside the composite film. In the p-doping potential region the biggest difference is the increased charging capacity of PAz in the composite. A likely reason for this could be that a larger amount of PAz is formed in the electropolymerization of Az on Pt/BBL-PEO due to the higher surface area of that substrate compared with bare Pt (see Figure 1). In thin PAz films a prepeak in the CV around 0.3 V is sometimes visible. This prepeak is, however, more evident in the BBL-PEO/PAz composite than in PAz. These kinds of prepeaks have also been found to appear in nanoporous PAz/TiO2 composite layers4 and in PAz/C60 composites.14 The origin of the redox reaction causing the prepeaks can be assigned to charge trapping.46−50 During the n-doping process, PAz is partially reduced and negative charge carriers are formed with the subsequent insertion of TBA+ from the electrolyte solution. Upon reversal of the potential scan only a portion of PAz will be neutralized due to its low electrical conductivity. Hence, some negative charge carriers are left isolated and are trapped in the composite layer. When a critical number of positive charge carriers are formed during p-doping, they can interact with the trapped negative charge carriers and cause the prepeak seen in the CV. Both the FTIR-ATR and UV−visible spectroscopic measurements also show that uncharging of both composite layers during n- and p-doping is not complete as is the case in single PAz and BBL-PEO films. 3.2. In Situ FTIR-ATR Spectroelectrochemistry. 3.2.1. nDoping of the Composite Layers. The n- and p-doping processes, i.e., reduction/oxidation of the composite layers by increasing the negative/positive electrode potential, of both BBL-PEO/PAz and PAz/BBL-PEO composite layers were studied by in situ FTIR-ATR spectroscopy. Figure 4a,b shows the spectra of BBL-PEO/PAz and PAz/BBL-PEO composite layers, respectively, recorded in 0.1 M TBAPF6-ACN electrolyte solution during n-doping. The CVs of both composites are shown in the insets of Figure 4. The arrows indicate at which potential the different spectra were measured. The spectra are separated for the sake of clarity. The spectra are dominated by an electronic absorbance between 8000 and 1726 cm−1. This absorption band is correlated to the formation of new electronic states within the band gap. The electronic absorbance increases continuously with the increase of the negative potential, and furthermore, this absorption band appears at a lower cathodic potential in the BBL-PEO/PAz composite compared to that for PAz/BBL-PEO. For the BBLPEO/PAz composite layer the absorption maximum is found at approximately 4300 cm−1 from −0.19 to −0.58 V. At −0.74 V and below a new absorption maximum appears at approximately 7280 cm−1 in accordance with the first reduction peak

Figure 4. In situ FTIR-ATR spectra of (a) the BBL-PEO/PAz and (b) the PAz/BBL-PEO composite layers recorded in 0.1 M TBAPF6-ACN electrolyte solution during n-doping. The cyclic voltammograms are shown in the insets. The reference spectrum for both composite layers was recorded at 0 V, and the potential values were measured against a Ag/AgCl pseudo reference electrode.

visible in the CV (see the inset in Figure 4a). This new absorption band does not grow at the expense of the band at 4300 cm−1, but rather a broad electronic absorbance without a well-defined maximum appears. In the same potential range PAz has been shown to have two maxima at approximately 3700 and 5300 cm−1.39 BBL-PEO, on the other hand, has two maxima at approximately 3500 and 6700 cm−1 (unpublished results). The behavior of the BBL-PEO/PAz composite seems to be different from the behavior of pure PAz and BBL-PEO films in the n-doping potential region. The appearance of two maxima in the high wavenumber region of the in situ FTIRATR spectra indicates formation of two different kinds of charge carriers in the polymeric material. Formation of the dianionic species of BBL-PEO obviously occurs in the lower wavenumber region (lower energy upon n-doping) and the anionic charge carriers at higher wavenumbers (higher energy upon n-doping). Finding the absorption maxima of BBL-PEO/ PAz at higher wavenumbers, compared with those for pure PAz and BBL-PEO, indicates that formation of negative charge carriers in the BBL-PEO/PAz requires more energy compared to that in pure polymers. However, formation of negative charge carriers occurs at a less negative potential (at −0.19 V) in BBL-PEO/PAz than in the pure polymers (at −1.7 V in PAz39 and at −0.25 V in BBL-PEO51). This indicates that PAz promotes formation of charge carriers in the BBL-PEO/PAz composite. One reason for this could be that the trapped charges in PAz serve as counterions during n-doping of the composite and in that way facilitate formation of charge carriers. On the other hand, the contribution of BBL-PEO to 23796



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Figure 5. (a) n-doping FTIR-ATR spectra of both composites and the spectrum of pure BBL-PEO recorded at −0.6, −1.1, and −1.3 V vs Ag/AgCl pseudo reference electrode. (b) The p-doping FTIR-ATR spectra of both composites compared to the spectrum of pure PAz recorded at 0.35, 0.7, and 1.1 V vs Ag/AgCl pseudo reference electrode.

in the composite layers. Formation of charge carriers in the PAz/BBL-PEO composite at −0.6 V seems to be the most difficult of the studied films. By increasing the negative potential to −1.1 V (corresponding to the second reduction peak in the CVs) formation of charge carriers is easier in the PAz/BBL-PEO composite than in BBL-PEO/PAz. At −1.3 V (corresponding to the third reduction peak in the CVs) the absorption bands no longer increase suggesting that the number of charge carriers did not increase any more in all the studied layers. However, in pure BBL-PEO the decrease in the electronic absorbance between 8000 and 4000 cm−1 begins already at −1.0 V compared to −1.3 V in both composites. This indicates that the composites stay electroactive within a broader negative potential range than the pure BBL-PEO. Between 4000 and 1700 cm−1 the increase in the electronic absorbance continues until −1.4 V in the pure BBL-PEO and in the BBLPEO/PAz composite. In the PAz/BBL-PEO composite both electronic absorbances begin to decrease at −1.3 V. This further indicates that PAz promotes the formation of charge carriers in the BBL-PEO/PAz composite but still indicates that BBL-PEO has a bigger effect on the n-doping behavior of the BBL-PEO/ PAz composite than on that of the PAz/BBL-PEO composite. Overall, it seems that BBL-PEO has a bigger effect than PAz on the n-doping behavior of both composite layers, but that PAz strengthens the charging capacity of the BBL-PEO/PAz composite more than that of the PAz/BBL-PEO composite. 3.2.2. p-Doping of the Composite Layers. The spectra recorded during the p-doping process of both the BBL-PEO/ PAz and the PAz/BBL-PEO composite layers are shown in Figure 6a,b, respectively. The CVs of both composites are

the n-doping properties of BBL-PEO/PAz is higher than that of PAz. In the spectra in Figure 4b of the composite film made by depositing BBL-PEO on PAz (PAz/BBL-PEO) the electronic absorbance maximum is found at 4100 cm−1. This band begins to shift to lower wavenumbers (3235 cm−1), i.e., lower energy upon n-doping of the film after −0.93 V. This absorbance continues to increase in intensity with increased negative potential. At higher wavenumbers a broad absorption wave is found whose maximum is outside the wavenumber range of the FTIR. This absorbance has a lower intensity compared to the absorbance at lower wavenumbers. For both composites it appears like BBL-PEO is the component that contributes most to the n-doping. These observations were also expected since BBL-PEO is an efficient acceptor material. Next, the n-doping behavior of both composites was compared to the behavior of pure BBL-PEO at three different potentials, −0.6, −1.1, and −1.3 V. The spectra are shown in Figure 5a and the corresponding CVs in the insets of Figure 4a,b (the bold arrows indicate the potentials at which the comparisons have been made). The differences in the intensities between the spectra are partly caused by the different thicknesses of the polymeric films. When a potential of −0.6 V was applied to the electrodes (this potential value is close to the first reduction peak in the CVs of both composite layers), the spectra of both composite layers look almost the same. The steeper increase of the electronic absorbance in the spectrum of the pure BBL-PEO layer shows that the energy differences between the mid gap states and the LUMO are smaller, and therefore, it is more difficult to form charge carriers 23797



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charge carriers at this potential (0.35 V) in the BBL-PEO/PAz composite than in the PAz/BBL-PEO composite. In the spectrum of PAz two maxima can be found, one at 4700 cm−1 and the other one at 7650 cm−1. At 0.7 V (at the current plateau in the CVs of both composites) the electronic absorbance in the high frequency region increases in the PAz/BBL-PEO composite in the same way as in the pure PAz; however, the increase in the pure PAz spectrum is more pronounced. At high positive potential (1.1 V) formation of new charge carriers in the composite layers has already begun decreasing compared to the pure PAz in which new charge carriers are still formed. In addition, the decrease occurs faster in the BBL-PEO/PAz composite than in the PAz/BBL-PEO composite. From the FTIR-ATR experiments it became clear that the composite layers do not return to their initial state once the n- and p-doping cycles are completed. This means that some charges are trapped in the composite layers after ndoping, which makes the p-doping more difficult and formation of charge carriers less efficient in the composite layers than in the pure PAz, which has not been n-doped before p-doping. In the BBL-PEO/PAz composite, where BBL-PEO is closer to the electrode surface than PAz, the still partly negatively charged BBL-PEO and PAz make oxidation of PAz during p-doping more difficult compared to oxidation of the same in the PAz/ BBL-PEO composite. This may be one reason for the faster decrease in the electronic absorbance in the FTIR-ATR spectra recorded during p-doping of the BBL-PEO/PAz composite. Another reason for this could be the low conductivity of BBLPEO in the p-doping potential region (0−1.1 V), which could make the trapped negative charges more difficult to be released and to combine with the formed positive charges during oxidation of PAz in the composite. 3.2.3. IRAV Bands during n- and p-Doping. Enlargements of the n- and p-doping spectra in the wavenumber region 2000−680 cm−1 are shown in Figure 7a,b for BBL-PEO/PAz and PAz/BBL-PEO composite layers, respectively. The ndoping spectra are recorded at −1.1 V and the p-doping spectra at 0.7 V applied to the electrode. New infrared active vibration (IRAV) bands appear which grow during n- and p-doping processes due to negative/positive charges inserted in the polymer chain resulting in changes in the dipole moment. The IRAV patterns of the composite layers are compared to the IRAV pattern of the pure BBL-PEO during n-doping (Figure 7a) and to the pure PAz during p-doping (Figure 7b). The IRAV bands are broad, which together with the high intensities indicates a rather high delocalization of the negative charge carriers along the chain.52 The pattern of the IRAV bands in the n-doping spectrum of the PAz/BBL-PEO composite follows fairly well the pattern of BBL-PEO, but the pattern of the BBLPEO/PAz composite differs slightly especially between 1250 and 800 cm−1. The biggest difference is the occurrence of the band at 834 cm−1 assigned to PF6− anions in the spectrum of the BBL-PEO/PAz composite. Anions inserted into PAz during polymerization should be expelled from the polymer film during n-doping which makes the polymer negatively charged. During n-doping of the BBL-PEO/PAz composite the BBLPEO layer being closest to the electrode surface attracts the electrons from it and becomes negatively charged. This could cause difficulties to reduce and remove PF6− anions from the PAz layer on top of it and therefore the peak is still visible in the spectrum at −1.1 V. In the CV of the BBL-PEO/PAz composite recorded in the FTIR-ATR spectroelectrochemical cell during n-doping process there is a reduction wave between

Figure 6. In situ FTIR-ATR spectra of (a) the BBL-PEO/PAz and (b) the PAz/BBL-PEO composite layers recorded in 0.1 M TBAPF6-ACN electrolyte solution during p-doping. The cyclic voltammograms are shown in the insets. The reference spectrum for both composite layers was recorded at 0 V, and the potential values were measured against a Ag/AgCl pseudo reference electrode.

shown in the insets of Figure 6. Again, the arrows indicate at which potential the different spectra have been measured. The spectra are dominated by the electronic absorbance at high energy between 8000 and 2000 cm−1. The electronic absorbance continuously increases with the increased positive potential in both composite layers. In the p-doping spectra in Figure 6a of the BBL-PEO/PAz composite the absorption maximum is located approximately at 4800 cm−1 with a tail reaching toward the NIR region. This electronic absorbance appears after −0.16 V has been applied to the electrode. Formation of charge carriers in the PAz/BBL-PEO composite is slowed down during p-doping and the absorption maximum appears at approximately 5100 cm−1 with a second maximum appearing after 0.52 V at 7450 cm−1. The behavior of the PAz/ BBL-PEO composite during p-doping resembles that of PAz also exhibiting absorption maxima located at ∼5000 and ∼7570 cm−1. In order to acquire a better understanding of the differences in the behavior of both composite layers during p-doping the spectra were compared with the spectra of pure PAz measured at three different potentials, 0.35, 0.7, and 1.1 V. The spectra are shown in Figure 5b and the corresponding CVs in the insets of Figure 6a,b (the bold arrows indicate the potentials at which the comparisons have been made). The intensity differences in the spectra are, again, partly caused by the different thicknesses of the polymeric films. At 0.35 V (i.e., after the oxidation onset of PAz in the CVs of both composite layers) one absorption maximum appeared at approximately 4700 cm−1 in the spectra of the two composite layers. It seems like it is easier to form 23798



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the wavenumber region 1565−1390 cm−1 are assigned to aromatic CC ring stretching vibrations.53,54 This change in the internal ratio between the band intensities might be due to the different chain length of PAz synthesized on Pt than on Pt/ BBL-PEO. 3.3. In Situ UV−visible Spectroelectrochemistry. The BBL-PEO/PAz and PAz/BBL-PEO composite films were also characterized by UV−visible spectroelectrochemistry. The spectral changes in both the n- and p-doping regions were studied separately at different potentials applied to the electrode. The composite layers were n-doped by increasing the negative potential stepwise from 0 to −1.5 V (ΔE = 0.1 V). Before recording the spectra the potential was maintained at each value for 60 s in order to convert the material to the new electronic state. The spectral changes in the p-doping region were recorded in the same way as in the n-doping region but by increasing the potential stepwise from −0.4 to +1.0 V (ΔE = 0.1 V). Due to the rather complex nature of the spectra the changes occurring upon potential change in both BBL-PEO/ PAz and PAz/BBL-PEO composites are shown in Figure 8 as the absorbance values of the chosen absorption bands vs potential. Figure 8a shows the changes occurring during ndoping processes and Figure 8b during p-doping processes. The following discussion is in reference to Figure 8. 3.3.1. n-Doping of the Composite Layers. Upon negative charging of both composite layers a doping-induced band appears due to the formation of an anion of BBL-PEO at approximately 900 nm when −0.4 V was applied to the electrode. The doping-induced band becomes visible in the spectra before the first reduction peak of the pure BBL-PEO appears in the CV at −0.95 V. In the composite layers the first reduction peak which can be assigned to BBL-PEO reduction appears between −1.0 and −1.1 V (see the CVs in the insets in Figure 4). However, already before −1.0 V a reduction current is visible in the CVs of both composite layers between −0.3 and −0.9 V. In the CV of the pure BBL-PEO a broad wave between −0.3 and −0.7 V can also be observed. The currents, however, are lower than those in the composite layers. The spectral changes before reaching the potential of the first reduction of the pure BBL-PEO in the CV indicate that a Faradaic process occurs which could be related to the discharging of PAz and to the beginning of the charging of BBL-PEO in the composite layers. Doping-induced changes before the first reduction peak of the pure BBL-PEO have also been observed with in situ FTIR-ATR measurements. The doping induced band at 900 nm increases in intensity until −1.1 V in the BBL-PEO/PAz composite and until −1.2 V in the PAz/BBL-PEO composite after which the second reduction peak starts to appear in the CVs of both composite layers. This indicates that the 900 nm absorption band could be associated with the reaction responsible to the first BBL-PEO reduction peak and simultaneous discharging of PAz. At increasing negative potentials both the current and the absorption band intensities at 900 nm decrease but do not vanish completely. A second doping-induced absorption band becomes visible at 736 nm in the spectra of the BBL-PEO/PAz composite and at 795 nm in the spectra of the PAz/BBL-PEO composite. The absorbance at 900 nm gradually decreases while the second doping-induced band increases with increasing negative potential. This second doping-induced band is closely associated with the second reduction peak starting to decrease at −1.4 V at the CVs of both composite layers (see the inset in Figure 4).

Figure 7. IRAV bands recorded during (a) n-doping at −1.1 V and (b) p-doping at 0.7 V of BBL-PEO/PAz and PAz/BBL-PEO composite layers in the wavenumber region 2000−680 cm−1. The n-doping spectra are compared to pure BBL-PEO and the p-doping spectra to pure PAz. The potential values were measured against a Ag/AgCl pseudo reference electrode.

−0.5 and −1.0 V having a lower intensity than the corresponding wave in the CV of the PAz/BBL-PEO composite (see insets in Figure 4a,b). The higher current in the CV of the PAz/BBL-PEO composite may be due to a more efficient discharging of PAz in that composite compared with the current peak observed with the BBL-PEO/PAz composite due to the aforementioned reason. The IRAV bands recorded during p-doping at 0.7 V of both composites are compared with the bands of the pure PAz in Figure 7b. These IRAV bands are, similarly to the bands recorded during n-doping, broad and have high intensities again indicating rather high delocalization of the positive charges along the polymer chain.52 The pattern of the IRAV bands of both composites follows the pattern of pure PAz rather well. The IRAV band pattern of the PAz/BBLPEO composite, however, differs slightly from the patterns of the BBL-PEO/PAz composite and the pure PAz at the wavenumber region between 1550 and 1400 cm−1. The intensity of the peak at 1463 cm−1 is lower compared with the intensity of the neighboring peak at 1416 cm−1 in the spectrum of the PAz/BBL-PEO composite throughout the pdoping process. The intensities of the corresponding peaks in the BBL-PEO/PAz composite and in the pure PAz are vice versa. In the spectrum of a PAz-TiO2 nanoparticle composite studied earlier by us,3 the intensities of the corresponding peaks are in the same order as in the spectra of the BBL-PEO/PAz composite and in the pure PAz. The bands in neutral PAz in 23799



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Figure 8. Absorbance vs potential of the UV−visible absorption bands of the BBL-PEO/PAz composite and the PAz/BBL-PEO composite recorded during (a) n-doping and (b) p-doping processes. The potential values were measured against a Ag/AgCl pseudo reference electrode.

the BBL-PEO/PAz composite and up to −1.3 V in the PAz/ BBL-PEO composite. In the BBL-PEO/PAz composite where the BBL-PEO layer is closer to the electrode substrate than PAz its contribution to the spectral behavior is stronger than that of PAz, which would explain the decrease in absorbance at 420 nm at less negative potentials. 3.3.1. p-Doping of the Composite Layers. The UV−visible spectral changes occurring during p-doping of the composite layers are shown in Figure 8b as the absorbance values of the chosen absorption bands vs potential. The HOMO-LUMO transition in pure PAz at 412−420 nm3,4,39 is visible at 443 nm in the BBL-PEO/PAz composite and at 438 nm in the PAz/ BBL-PEO composite. These absorption bands stay almost unchanged until 0.3 V in both composite layers. Electrochemical p-doping of the composite layers results in a continuous growth of a rather broad absorption band already visible in the neutral state of the composites at 562 nm in the BBL-PEO/PAz composite and at around 585 nm in the PAz/ BBL-PEO composite. These spectral changes start to occur when a Faradaic current starts to be visible in the CV and the layers are converted to a conducting state. The reason for the occurrence of these bands already in the pristine forms of the composite layers could be that BBL-PEO in the neutral state has an absorption band at around the same wavelength. The band due to Az monomer at approximately 620 nm35 is also visible in the p-doping spectra of both composite layers and broadens the bands at 562 and 585 nm, respectively. Simultaneously with the growth of the bands at 562 and 585 nm in the two composite layers, a broad band at around 1000 nm grows. All of these bands grow in intensity until 1.0 V indicating formation of new charge carriers in the material. The corresponding doping induced absorption bands of PAz are

In the studied wavelength range (300−1100 nm) the pristine forms of PAz and BBL-PEO have absorption bands at 412− 4203,4,9 and 555 nm,51 respectively. Both of these absorption bands are also visible in the n-doping spectra of the composite layers, the PAz band at approximately 420 nm and the BBLPEO band at 558 nm in the BBL-PEO/PAz composite and at 580 nm in the PAz/BBL-PEO composite. In addition, a weak band due to Az monomer at approximately 620 nm35 appears in the spectra of both composite layers. Within the spectra of the BBL-PEO/PAz composite the band at 558 nm increases with increasing negative potential until −0.6 V after which its intensity decreases until −0.8 V and starts to increase again at −0.9 V. In the spectra of the PAz/BBL-PEO composite the corresponding band at 580 nm increases in intensity until −1.4 V. If these absorption bands would only be related to pristine BBL-PEO in the composites, their intensity should decrease with increasing negative potential indicating a diminishing contribution from the neutral form of the polymer. During ndoping of pure PAz an absorption band at 650 nm increases with increasing reduction potential indicating formation of new charge carriers in PAz.39 In the p-doping region between −0.4 and +1.0 V the BBL-PEO film is inactive, but in the p-doping spectra of the composite layers absorption bands at the same wavelengths as in the n-doping region (558 nm in BBL-PEO/ PAz and 580 nm in PAz/BBL-PEO) occur, which increase in intensity with increasing positive potential. This would suggest that conversion of PAz in the composites upon negative charging to an anionic and more conducting form would also contribute to the increasing absorbance at 558 nm in BBLPEO/PAz and at 580 nm in the PAz/BBL-PEO composite. The absorbance at 420 nm assigned to the HOMO−LUMO transition in neutral PAz, however, increases up to −1.0 V in 23800



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reported to be at 526 and 1300 nm35 and at 590 nm and around 1600 nm.40 As a conclusion it can be stated that the origin of the doping induced bands in the potential region from −0.4 to +1.0 V mainly originate from PAz in the composite layers. In addition, an absorption band at around 350 nm is visible in the p-doping spectra of both composite layers. This band increases slightly in intensity with increasing doping level. A more undefined absorbance around the same wavelength was also visible in the n-doping spectra of the composites. It is known that conjugated polymers form charge transfer complexes with a variety of electron acceptors. These kinds of charge transfer complexes usually show optical absorption bands, which corresponds to the transition from the ground state to the charge separated state of the complex.55 The absorption band at 350 nm in the UV−visible spectra of both composites may, therefore, be assigned to a charge transfer absorbance in a donor−acceptor complex composed of PAz and BBL-PEO. The increase in the intensity of that band with increasing doping level could be explained by an easier charge transfer at higher doping levels.

Present Address §

School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Georgia 30332-0400, Atlanta, United States. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

Professor Heikki Tenhu’s research group in Helsinki University, Laboratory of Polymer Chemistry and especially M.Sc. SamiPekka Hirvonen is acknowledged for the synthesis of the water dispersible BBL-PEO polymer used in this study. This work is part of the activities at the Åbo Akademi Process Chemistry Centre (ÅA-PCC) nominated by the Academy of Finland within the Finnish Center of Excellence Programme for the years 2000-2011.

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4. CONCLUSIONS Polyazulene/BBL-PEO donor−acceptor composite layers were successfully prepared on Pt and ITO electrode surfaces. All experiments made in this study clearly show contributions from both polymers in the composites. The spectral behavior of the BBL-PEO/PAz composite is mainly dominated by the absorbance of the BBL-PEO layer. The spectra of the PAz/ BBL-PEO composite, however, are dominated by the absorbance of the PAz layer. By the UV−visible and the FTIR-ATR spectroscopic measurements, a Faradaic reduction process was observed before any reduction current of BBLPEO appeared in the cyclic voltammogram. These spectral changes can be related to discharging of PAz and to the commencement of charging of BBL-PEO. So, formation of charge carriers in the composite layers during n-doping seems to require less energy, but the occurrence of the absorption maxima at higher wavenumbers in the in situ FTIR-ATR spectra indicates that the formation is slightly more difficult in the composites than in the pure polymers. PAz shows a higher promoting effect on the formation of charge carriers in the ndoping region in the BBL-PEO/PAz composite than in the PAz/BBL-PEO composite. The UV−visible and FTIR-ATR spectra also show that the composites stay in the conducting state within a broader negative potential region than the pure BBL-PEO. In contrast to the n-doping experiments at high negative potential, the p-doping experiments of the composite layers show a decrease in the mobility of charge carriers at a high positive potential. A probable reason for this may be charge trapping in the composites after n-doping and therefore formation of charge carriers during p-doping is more difficult. The charge trapping phenomenon was also clearly visible in the IRAV bands recorded during n-doping of the BBL-PEO/PAz composite, where the band assigned to PF6− doping anions in the composite layer was visible although the polymers in the composite were negatively charged.

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*Tel.: +35822154422. Fax: +35822154479. E-mail: rose-marie. latonen@abo.fi. 23801



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Electrochemical and Spectroelectrochemical Study of Polyazulene

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