Spectroelectrochemical Analysis of Charge Carriers as a Way of

Article pubs.acs.org/JPCC

Spectroelectrochemical Analysis of Charge Carriers as a Way of Improving Poly(p‑phenylene)-Based Electrochromic Windows Przemyslaw Data,*,†,‡,§ Radoslaw Motyka,† Mieczyslaw Lapkowski,†,‡ Jerzy Suwinski,‡ and Andrew P. Monkman§ †

Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland Center of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, 41-819 Zabrze, Poland § Physics Department, University of Durham, South Road, Durham DH1 3LE, United Kingdom Downloaded by NANYANG TECHNOLOGICAL UNIV on August 23, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.jpcc.5b06846

‡

S Supporting Information *

ABSTRACT: We show the electrochemical polymerization and characteristic of six pphenylene derivatives. The electroactive comonomers and polymers containing furan, thiophene, and selenophene units are studied using electrochemical techniques (differential pulse voltammetry, cyclic voltammetry, electrochemical deposition, and voltabsorptiometry) in order to characterize layers, charge carriers, and energy electronic levels. The possibility of using such polymers as electrochromic windows is presented. Simple evaluation of charge carriers helps to explain the long switching time of electrochromic window.

1. INTRODUCTION In the age of modern, rapidly developing electronics, novel materials such as conjugated molecules and polymers, featuring photo- and electroluminescent properties, are attracting exceptional interest in terms of the possibilities they offer for molecular optoelectronic applications, for example, as potential elements of electroluminescent diodes, displays, and photovoltaic cells.1−11 Materials for optoelectronic applications should be prepare as well-defined structures featuring desirable electronic properties with solubility in common organic solvents for inkjet printing or be able to be vapor deposition.12−16 Organic materials can already be found in prototypes and commercial chemical sensors, photovoltaic cells, and AMOLED (active matrix organic light-emitting diodes) display devices.17−21 Their major advantage is the possibility to tailor their properties, including the color of light emitted by modifying their molecular structure. Comonomers of a pphenylene type structure with heterocycle side groups exhibit electroluminescent properties and are readily polymerized by electrochemical methods because the positions α and α′ are free, and the conjugated molecule is so short that it can be relatively easily oxidized to the unstable cation.22−25 This type of reaction allows photoluminescent polymers to be obtained via electrochemical methods. The use of electrochemical methods for the synthesis of polymers has many advantages, including easy control of film thickness by a passed charge during the electropolymerization, control of the conjugation length of polymer by choosing a suitable solvent for electropolymerization, the possibility of obtaining uniform © XXXX American Chemical Society

layers with the complex surfaces of the electrodes, the homogeneity of the film thickness, and the uniformity of coverage electrodes, etc. Oligomers with a similar structure and polymers derived from them have been the focus of interest in recent years.25−30 There are several studies that have shown the usefulness of physicochemical properties using electrochemical methods to study this type of material.25−28 Analysis of the doping process and charge carrier formation is often challenging and requires a number of techniques coupled together. There are different types of charge carriers present in the doping of conjugated organic compounds, and it is necessary to distinguish between them, especially for polymers, oligomers, and macromolecules like star-shaped small molecules. Electrochemical and spectroelectrochemical methods allow relatively easy evaluation of properties such as HOMO and LUMO values, the kind of charge carriers responsible for the process of conductivity and other parameters for optoelectronic applications of both monomers (small molecules) and polymers.31−35 There are different types of charge carriers present in the doping of conjugated organic compounds. Analysis of the doping process and charge carrier formation is often difficult and requires a number of techniques coupled together. A very good method of analysis is the measurement of ESR (electron spin resonance) spectroscopy. ESR spectroscopy shows the Received: July 15, 2015 Revised: August 12, 2015

A

DOI: 10.1021/acs.jpcc.5b06846 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

ments (Figure 3), a similar dependence was noticed. The value of potential of the first oxidation peaks increases in the series 1a < 1c < 1b < 2a < 2c < 2b which is exactly the same as that from the CV analysis. The first reduction peak decreases in the series 1c > 1b > 1a > 2c > 2b > 2a. Furan and thiophene have similar ionization potentials measured by theoretical calculation or from UPS experiments (in margin of error). In electrochemistry, where we have strong solvent interaction, the changes in oxidation potential could arise. It need to be marked that furan have smaller heteroatom and strong vibronic coupling than thiophene which will benefit with lower oxidation potential which was already shown in previous investigations.49−51 The first oxidation and first reduction process of comonomers were helpful to determine their electron affinity (EA) and ionization potential (IP) (Table 1 and Figure 3). The electrochemical band gap value for comonomers will be a range between those peaks. This means the selenophene comonomer with a vinyl linker had the lowest band gap in the investigated series. Comonomers with vinyl linkers have a lower potential than analogues without vinyl linkers, which is connected to a higher conjugation length in the comonomer. In the next step, synthesized comonomers were electropolymerized using a cyclic voltammetric process (Figure 2). In all cases, the electropolymerization occurred at first oxidation potentials. It was observed that the current grew in successive scans during the electropolymerization, and this corresponds to the formation of a conducting layer on the working electrode in the form of a polymer layer (Figure 2). In the case of comonomer 1a probably only the dimerization process occurred on the electrode because the growing oxidation peaks were too close to the oxidation peak of the comonomer.49 Obtained polymers were characterized as well as comonomers to first determine the stability and second to characterized the parameters resulting IP and EA values (Table 1, Figures 4 and 5). The electrochemical characterization of polymeric layers was investigated via CV analysis in a comonomer-free 1 M Bu4NBF4 DCM electrolyte. As shown in Figure 4, a poly(1a) with a bifuran unit has at least three oxidation peaks and three reduction peaks at potentials ca. −0.39, 0.14, 0.24 V and 0.44, 0.23, −0.34 V. There is a large shift in the oxidation and reduction potential of polymer 1a which indicates a problem with the doping−dedoping process as a result of counterion movement in the conductive layer net. The oxidation of poly(1a) started at a potential of −0.40 V, but it needed higher energy in the form of potential to insert a counterion and without that the polymer would not oxidize. In CV of poly(1b) with bithiophene, units oxidation occurred as broad peaks without a developed maximum. The oxidation and reduction peak of poly(1b) looks slightly symmetrical and without a hysteresis effect (unfortunately, the poly(1b) degrade rapidly on the electrode). A similar behavior occurred in poly(2b), but the maximum oxidation developed at 0.44 V and the polymer layer once formed was more stable in the doping− dedoping process. Poly(1c) had two developed oxidation peaks and one reduction peak at 0.08, 0.30, and 0.29 V. The oxidation polymers containing biselenophene units had the lowest oxidation peak potential in the series with and without vinyl linkers. Similar to the situation with thiophene analogues and previous investigations,49,52 the oxidation and reduction peaks presented only a little shift between oxidation and reduction potentials; the hysteresis was very low. In a poly(2c) similar

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formation of unpaired electrons, such as radicals, generated in the organic structure.36−38 During ESR measurement with a coupled potentiostat, formation of polaron and their transformation into bipolaron and biradical cation could be observed as a function of potential. ESR measurements are usually carried out in potentiostatic measurements because there are problems associated with counterion diffusion into the polymeric network.39−42 There are only a few techniques in which this can be applied to a dynamic electrochemical process like cyclic voltammetry.15,43−45 The second technique which allows the definition of indirectly generated charge carriers is UV−vis−NIR spectroelectrochemistry, which has been described in a few publications.46−48 The synthesis of the investigated compounds (Figure 1) was associated with examination of the influence of the heteroaryl

Figure 1. Structures of the investigated compounds.

and vinyl group on the molecule properties and electrochemical polymerization process and the optical change during electrochemical doping of the resultant polymeric layers.

2. RESULTS AND DISCUSSION Electrochemical Studies. The first step of the electrochemical analysis of the synthesized compounds was the determination of oxidation potentials and starting polymerization potentials. During cyclic voltammetry measurements of comonomers at a positive potential range (Figure 2), the curves show a multistep irreversible oxidation process, and both the shape of the curves and the individual oxidation potentials of peaks depend on the type of heteroatom and vinyl linker. The onset potentials, i.e., the voltage value at which the oxidation process starts, were lowest (−0.07 V, Figure 2) for comonomers with vinyl and furan groups (1a) and highest (0.47 V) for comonomers with thiophene and without a vinyl linker (2b). The onset potentials grow in a series of comonomers 1a < 1c < 2a < 1b < 2c < 2b in which the series 1 is with vinyl linker and set 2 without one. A similar dependence occurred for oxidation peaks; the lowest peaks (0.42 V) were for 1a comonomers, and the highest peak potentials (0.70 V) were for 2b comonomers. There is a slight change when comparing the oxidation peaks of the series of comonomers (1a < 1c < 1b < 2a < 2c < 2b); there is no overlapping of comonomers with and without vinyl linkers. After dedoping, in the next cycle, new oxidation peaks are observed at much lower potentials than the first oxidation peak for all comonomers, which indicates that the reactions result in products with a higher conjugation degree than the initial comonomer. In all cases, the electropolymerization occurred at first oxidation potentials. The first oxidation peaks are from heteroaryl groups like furan, thiophene, and selenophene, which, after oxidation, dimerized. When comparing peaks form CV spectra with absolute peaks of oxidation and reduction processes in DPV (differential pulse voltammetry) measureB


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Figure 2. Cyclic voltammetry of investigated comonomers of polymerization. (a−f) Respective comonomers. Measurement conditions: scan rate 50 mV/s, Ag/AgCl quasi-reference electrode, calibrated against a ferrocene/ferrocenium redox couple.

polymers upon p- and n-doping at a scan rate of 300 mV s−1. The HOMO−LUMO energy levels and the electrochemical band gap of the electrochemically generated polymers are given in Table 1. The difference in onset potentials obtained from the p- and n-doping sides can be taken as a measure of the band gap (EG) of the polymer (Figure 5). The poly(1c) formed well conductive polymer which allow for quick response during reduction showing quasi-reversible n-doping process (Figure 5a). After electrochemical characterization, the thin polymeric films were deposited (Figure 6) on indium tin oxide (ITO)-

behavior was observed but with higher potentials. In the oxidation and reduction process we found five oxidation and four reduction peaks. The first couple (−0.34 VOX, −0.41 VRED) is from the generated σ-dimers in the polymer layer.53 The σdimers formation was not observed in poly(1c) because polymer was more conductive than poly(2c). Other redox couples are from the doping and dedoping process of the polymer layer. As a result of the electrochemical characterization of polymers by analyzing the oxidation and reduction processes, an estimation of the electrochemical band gap value is possible. Figure 5 represents the cyclic voltammogram of C


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Figure 3. DPV spectra of investigated comonomers: (a) comonomers with vinyl linker; (b) comonomers without vinyl linker. Measurement conditions: scan rate 50 mV/s, Ag/AgCl quasi-reference electrode, calibrated against a ferrocene/ferrocenium redox couple.

of the electrochemical doping during measurements, both the electronic structure of poly(1) and its optical behavior were changed. Neutral poly(1a) was characterized by one broad absorption peak located at λmax = 365 nm. During the oxidation process of poly(1a) film, the π−π* band at 300−500 nm diminished as a result of increasing potential and electrochemical doping and two new broad bands developed between 500 and 950 nm (polaronic band) with a maximum at 730 nm and 950−1700 nm (bipolaronic band) with a maximum at 1265 nm. An isosbestic point was not observed because degradation of the polymer was also observed. The neutral polymer 1b was characterized by two broad absorption peaks located at λmax1 = 363 nm and λmax2 = 488 nm relating to the interband π−π* transitions of the aromatic form of the neutral polythiophene derivative. When the oxidation starts, new polaronic and bipolaronic levels are generated. The UV−vis set of spectrum recorded during poly(1b) oxidation revealed an absorption band (300−550 nm) correlated with the polymer (Figure 7b), gradually losing its intensity as the applied potential increased. At the same time, a new defined absorption band occurred (550−950 nm) with maxima at 692 nm, during the formation of radical cations of bithiophene and p-phenylenevinylene. The isosbestic point of the doping process was formed at the 565 nm wavelength. The bipolaronic band was formed between the 950 and 1700 nm wavelengths with a maximum at 1438 nm. The absorption spectra of poly(2) films without vinyl linkers on ITO glass are presented in Figure 8. The neutral poly(2a) were characterized by vibronic structures and three overlapping absorption peaks located at λmax1 = 451 nm, λmax2 = 582 nm, and λmax3 = 514 nm. During the oxidation process of poly(2a) film, the band of neutral polymer at 350−550 nm diminished, and a new broad band of polarons developed between 550 and 850 nm (Figure 8a) with an isosbestic point at the 539 nm wavelength. Formation of a bipolaron band occurs with absorption band formation from 850 nm to beyond 1700 nm with a maximum at 1665 nm and a second isosbestic point at 764 nm. The second isosbestic point indicates formation of bipolarons directly from polarons. Electrons on average require lower energy to be transferred to those bipolaronic levels from the fundamental state λmax > 850 nm.

Table 1. Electrochemical and Optical Band Gap

1a 1b 1c 2a 2b 2c poly(1a) poly(1b) poly(1c) poly(2a) poly(2b) poly(2c)

EOx (V)a

Ered (V)b

IP (eV)c

EA (eV)d

EGel (eV)e

EGop (eV)f

0.32 0.44 0.43 0.47 0.65 0.51 0.08 0.10 −0.03 −0.24 −0.07 0.01

−2.45 −2.35 −2.30 −3.01 −2.73 −2.64 −1.93 −1.95 −1.73 −2.38 −2.21 −2.08

−5.42 −5.54 −5.53 −5.57 −5.75 −5.61 −5.18 −5.2 −5.07 −4.86 −5.03 −5.11

−2.65 −2.75 −2.80 −2.09 −2.37 −2.46 −3.17 −3.15 −3.37 −2.72 −2.89 −3.02

2.77 2.79 2.73 3.57 3.38 3.15 2.01 2.05 1.7 2.14 2.14 2.09

2.76 2.75 2.69 3.24 3.09 3.02 2.24 2.02 1.91 2.25 2.14 2.08

a

First oxidation potentials of comonomers from DPV and of polymers from CV measurements. bFirst reduction potentials of comonomers from DPV and of polymers from CV measurements. cEnergy of highest occupied molecular orbital calculated from EHOMO = −(Ep + 5.1), where Ep is DPV oxidation peak potential for comonomers and onset of CV oxidation potential versus Fc/Fc+.54−59 dEnergy of lowest unoccupied molecular orbital calculated from ELUMO = −(En + 5.1), where En is DPV reduction peak potential for comonomers and onset of CV reduction potential versus Fc/Fc+.54−59 eEnergy of band gap calculated from the difference between energy of HOMO and LUMO band. fEnergy of band gap calculated from UV−vis spectroscopy EGop = hc/λonset, where h is Planck’s constant, c is speed of light in a vacuum, and λonset is compound onset of absorption.54−59

coated quartz glass substrates using potentiodynamic deposition in the electrochemical conditions presented in Figure 2. During spectroelectrochemical analysis relationships between absorption bands and the vinyl linker and heterocyclic groups in molecules were observed. Spectroelectrochemical Studies. After electrochemical deposition of a polymer, the ITO electrode revealed different types of color, and this is presented in Figure 6. The spectroelectrochemical analysis of polymers prepared on ITO-coated glass electrodes was investigated in a potentiodynamic system. The absorption spectra of poly(1) films with vinyl linkers on ITO glass are presented in Figure 7. As a result D


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Figure 4. Electrochemical doping−dedoping process of the electrodeposited polymers in a monomer-free medium. (a−f) Respective polymers. Measurement conditions: scan rate 50 mV/s, Ag/AgCl quasi-reference electrode, calibrated against a ferrocene/ferrocenium redox couple.

The neutral poly(2c) indicates one broad absorption peak located at λmax = 448 nm. During the oxidation process the absorption band from 300 to 550 nm correlated with the polymer (Figure 8c) gradually lost its intensity as the applied potential increased. At the same time, a new defined polaron absorption band was formed at 550−900 nm with a maximum at 695 nm and an isosbestic point at 543 nm. The next

oxidation step occurred by bipolaronic band formation between 900 and 1700 nm wavelengths with maxima at 1363 nm. The observed spectroelectrochemical processes were “classical”, and the bathochromic shifts occurring while doping was conserved. The wavelength of the isosbestic point increased for polymers with vinyl linkers in the series selenophene > thiophene > furan (Table 2, Figures 7 and 8). It could be E


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Figure 5. CV scans of the doping−dedoping process of polymers in a monomer-free medium. Measurement conditions: scan rate −300 mV/s, calibrated against a ferrocene/ferrocenium redox couple.

of information about the process of charge carrier formation. The polaronic absorption band typically should be between 500 and 1000 nm, while the bipolaronic band is above 850 nm. Comparing the two derivatives of furan, whose structure differs only in terms of vinyl links, it appears that in addition to the polymerization process, there are differences in the absorption band for both the neutral and oxidized polymer. The absorption π−π* band for the compound without a vinyl linker, poly(2a), is narrower, and the vibronic structure with three peaks at 451, 482, and 514 nm can be seen (Figure 9a). In the second case, poly(1a), the spectrum shows a broader peak with a relatively low wavelength of the absorption maximum at 365 nm. By analyzing the optical band gap, it can be seen that values are similar: 2.25 eV for poly(2a) and 2.24 eV for poly(1a). Taking into account the fact that comonomer compound 1a is more conjugated (Table 1) than comonomer compound 2a, polymer poly(1a) should also exhibit a higher conjugation. The fact that polymer conjugation poly(2a) is similar to polymer poly(1a) is caused by a high content of low molecular weight oligomers in the polymer layer, which could also be a factor in the low stability of the polymer layer. Polaronic bands of the compounds are 500−950 nm with a maximum at 730 nm for poly(1a) (Figure 9a) and 550−850 nm with a maximum at 650 nm for poly(2a). The bipolaronic band is above 950 nm with a maximum at 1265 nm for the compound poly(1a) and above 850 nm with a maximum at 1665 nm for poly(2a). To clarify processes involved during polymers oxidation, the absorption changes at three wavelengths of the three bands (π−π* neutral polymer, polaron, and bipolaron) were chosen, and these are shown in Figure 9. When comparing the timing of absorption changes due to the potential (Figure 9a,b), it could be observed that in the case of poly(1a) we are dealing with side reactions, and difficulties with the dedoping process of the polymer layer are observable (Figure 9a). It is also observed that the bipolaronic band (Figure 9a, green line) is formed together with the polaronic band (Figure 9a, red line). However, after reversing the potential, the changes are slow, and the dedoping process takes a long time. In the second case, compound poly(2a) (Figure 9b), a typical oxidation process can be observed.

Figure 6. Pictures of polymer films on ITO substrates.

observed that polaronic band of poly(2a) furan derivatives without vinyl linker is at higher wavelength (lower energies) than poly(1a) with vinyl linker which is in opposite to selenophene and thiophene. Such behavior is casued by higher conjugation of poly(2a) species in compare to poly(2a), which was also in opposite to the thiophene and selenophene voltabsorptiometric (VAbs) spectra are often shown, but usually their analysis is not very detailed. Only if the measurement is made very carefully is it possible to get a lot F


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Figure 7. UV−vis spectra recorded during electrochemical oxidation of (a) poly(1a), (b) poly(1b), and (c) poly(1c) films.

the polymer film. From such a phenomenon can be concluded that the oxidation process of the polymer first forms polarons, and then slowly polarons are converted into bipolarons. Direct formation of bipolarons is not observed. Comparing the second group of derivatives containing a thiophene ring (poly(1b) and poly(2b)), the impact of the vinyl linker on the absorption spectrum could also be seen (Figure 7b, Figure 8b, and Figure S1). The absorption π−π* band for the neutral polymer poly(1b) is in the range of 300− 570 nm, and it could be specified by two absorption maxima at 363 and 488 nm (Figure 7b). For polymer poly(2b) the π−π* band is in the range 300−520 nm and can specify only one peak at 404 nm (Figure 8b). It should be noted that dependence of the optical band gap in thiophene derivatives has been preserved. When compared to a polymer containing a vinyl linker poly(1b) has a lower band gap value (2.02 eV) than the analogue without linker poly(2b) (2.14 eV). In the case of thiophene polymer, it could be assumed that the generation of the charge carriers in the molecule is associated with creation of a mixed polaronic and bipolaronic state (Figure S1). Decrease in the bipolaronic band together with an increase in the neutral polymer band occurred before any change in the polaronic band were observed with the switch of potential (Figure S1). The last polymer groups to be compared were compounds containing a selenophene ring (poly(1c) and poly(2c)). Here, a similar effect of the vinyl linker on the absorption spectrum (Figures 7c and 8c) to that observed in thiophene derivatives can be seen. On the other hand, in both cases we do not see deviation from the polaron and bipolaron changes (Figure 10). That mean in selenophene derivatives both charge carriers bipolarons and polarons are taking part in the process.

First, the molecular polaron charge carriers are formed (Figure 9b, red line), and then the bipolaron absorption band increased. After about 18 s, a decrease in the polaronic band and an increase in the bipolaronic band were observed. At this potential, all formed polarons are transformed into bipolarons. After about 24 s, the potential reversal occurred, and an almost symmetrical dedoping signal was observed. Further conclusions can be drawn from comparing the cyclovoltabsorptiometric spectra (CVAbs) with the cyclovoltammperometric spectra (Figure 9c,d). In the analysis of the compound poly(1a) (Figure 9c) at the beginning of the oxidation process (onset) the increasing of both bands (polaronic and bipolaronic) is observed. The swifter change occurred in the case of the bipolaronic band while the maximum of change correlates with the current maximum peak of the polymer CV. Above this peak (0.21 V) there followed a side reaction which did not generate additional charge carriers. There was no change in the CVAbs spectra after potential switched direction. During the analysis of the poly(2a) compound other changes could be observed. During oxidation polaronic and bipolaronic band changes are observed. The first CV peak (−0.06 V) is linked with the formation of polarons and bipolarons in molecules, while the second CV peak at 0.16 V is linked only with the bipolaronic band (Figure 9d, green line). The third CV peak at 0.25 V is the second peak of the polaronic band, and the fourth CV peak in the 0.63 V potential clearly shows the bipolaron forming in the molecule. One of the conclusions is that the polymer film consists of two main mass dispersions (bimodal polymer). The two peaks of the reduction process confirm this suggestion. Both CV reduction peaks (Figure 9d, 0.43 and 0.20 V) are linked with a decrease in the bipolaronic band. The reduction in the polaronic band is initiated by potential 0.20 V, and it correlated with the regrowth band of G


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Figure 8. UV−vis spectra recorded during electrochemical oxidation of (a) poly(2a), (b) poly(2b), and (c) poly(2c) films.

of potentiostatic switching between potentials −1.0 and 1.0 V made it possible to characterize optical parameters such as intensity of color change during electrochemical doping (contrast ratio CR) characterized for λmax of the undoped form, coloration efficiency (CE) defined as the amount of electrochromic color formed by the charge consumed, and optical density (ΔOD) which is the absorbance during doping (dedoping) process (Table 3). The polymers without a vinyl linker had a higher optical density value than polymers with a vinyl group (Table 3). A similar dependence occurred for contrast ratio values, and this was because these parameters are dependent only on transmittance. The changes are different for coloration efficiency because this parameter is dependent on charge density. The highest amount of charge needed to change color state was for polymer poly(2b). The large area of color change by a single charge was for polymers 1a, 1c, and 2c. Both selenophene derivatives were better from the others because charge density was much smaller than for other derivative. One of the possible reason for that could be the bigger distance between polymer chain improving the ion movement in the layer. Polymers 1c and 2c were suitable for optoelectronic applications on the basis of the optical density, coloration efficiency, and charge density, but the generated bipolarons in polymers influence high doping and dedoping times which would be affecting large area devices.

Table 2. UV−Vis−NIR Characteristics isosbestic point 1 (nm) poly(1a) poly(1b) poly(1c) poly(2a) poly(2b) poly(2c)

565 611 539 522 543

isosbestic point 2 (nm)

maximum peak of π−π* band (nm)

maximum peak of polaronic band (nm)

maximum peak of bipolaronic band (nm)

764 747 814

365 488 510 514 404 448

730 692 859 648 671 695

1265 1438 1416 1665 1318 1363

The voltabsorptiometric analysis elucidates the charge carriers generated in the polymeric layers during oxidation (Figures 9 and 10, Figure S1). As unfavorable charge carriers in optoelectronic application are bipolarons, which are much slower than polaronic moieties. Thanks to such analysis, we were able to determine the working boundaries of possible operating optical devices. The values of absorption maxima showed possibility of application in electrochromic windows but oxidation of furan polymers resulting in high content of bipolarons and problems with dedoping process (Figure 9). Only in thiophene derivatives bipolarons were formed at higher potentials (Figure S1), in selenophene polymers, bipolarons are formed almost instantly with polarons (Figure 10), but both of those redox processes were stable. Electrochromic Windows. Based on the conclusion drawn in the spectroelectrochemical analysis, the electrochromic windows were formed from polymers. Transmittance analysis H


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Figure 9. Absorptiometric measurements during changing potential (a, b) and voltabsorptiometric spectra (c, d) of poly(1a) and poly(2a).

3. CONCLUSION All comonomers electropolymerized during oxidative polymerization and polymers thus formed are electroactive and optically active. The heteroatom and vinyl group in conjugated comonomers and polymers has a significant influence on photo- and electrochemical properties. The band gap decreased for comonomers and polymers in a furan > thiophene > selenophene series for compounds with vinyl and without vinyl. Which means, the conjugation length is smaller with the larger heteroatom. The knowledge of the process carried in electroactive layer is crucial. The voltabsorptiometric analysis gave a possibility to analyze the generation process of charge carriers in the polymeric layer. It was observed that the bipolaronic band was formed together with the polaronic band in furan derivative during the doping process. In thiophene derivatives, it was found a decrease in the polaronic band and then increase in the bipolaronic intensity. In one case, it was possible to distinguish between two polaronic and two bipolaronic CV peaks and to conclude that the polymer film consists of two main mass dispersions (bimodal polymer).

The voltabsorptiometric spectra were more useful for CV analysis and made it possible to distinguish easily between polaronic and bipolaronic processes. The polymers with selenophene group allow us to assume that polyselenophenes could be good substitutes for the conventional thiophene derivatives used in electrochromic windows on the basis of the optical density, coloration efficiency, and charge density.

4. EXPERIMENTAL SECTION Melting points (not corrected) were determined on a Boethius HMK apparatus. NMR spectra were taken in CDCl3, with TMS as an internal reference, using a Varian XL-300 at 300 MHz for 1 H and 75.5 MHz for 13C. The electrochemical investigation was carried out using Eco Chemie Company AUTOLAB potentiostats: PGSTAT20 and PGSTAT100. The electrochemical cell composed of platinum wire with a 1 mm diameter of working area as a working electrode and an ITO glass as a working electrode, an Ag/AgCl electrode as a reference electrode, and a platinum coil as an auxiliary electrode. Cyclic voltammetry measurements were conducted at room temperature at a potential rate of 50 mV/s and were calibrated against a ferrocene/ferrocenium redox couple. UV− I


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Figure 10. Absorptiometric measurements during changing potential (a, b) and voltabsorptiometric spectra (c, d) of poly(1c) and poly(2c).

Table 3. Electrochromic Characteristics poly(1a) poly(1b) poly(1c) poly(2a) poly(2b) poly(2c)

Tb (%)a

Tc (%)b

λmax (nm)c

ORTdop (s)d

ORTdedop (s)d

ΔODe

Qd (mC/cm2)f

CE (cm2/C)g

CRh

55.59 52.00 74.40 80.72 72.95 57.28

15.49 18.45 13.70 18.16 25.82 8.45

365 488 511 514 404 448

1.41 1.38 1.15 1.39 1.52 1.32

2.73 1.44 1.06 1.42 1.69 1.27

0.56 0.45 0.73 0.65 0.45 0.83

1.77 4.79 3.66 10.85 31.67 2.65

313.59 93.92 200.96 59.71 14.24 313.63

3.59 2.82 5.43 4.45 2.82 6.78

a Transmittance of oxidized polymer at λmax. bTransmittance of neutral polymer at λmax. cThe absorption peak of the neutral polymer film. dOptical response time of doping and dedoping process at 95% of the maximum transmittance difference. eOptical density ΔOD = log[Tb/Tc]. fCharge density calculated in chronoculometric experiment.60,61 gCE is the coloration efficiency.60,61 hThe contrast ratio, CR = Tb(λmax)/Tc(λmax), where Tb and Tc are the transmittance in the bleached and colored states at λmax of the longest wavelength absorption peak in the neutral state of the film.

vis−NIR spectroscopy and spectroelectrochemistry were performed on an Ocean Optics QE6500 and NIRQuest apparatus. UV−vis spectroelectrochemical measurements were carried out on an ITO (indium tin oxide) quartz glass working electrode coated with polymers. Polymeric layers were synthesized on an ITO electrode in conditions similar to those pertaining during cyclic voltammetry measurements. All solvents for synthesis were dried and then distilled before use. Other commercially available substances and reagents were used without purification. Electrochemical measurements were

conducted in 1.0 mM concentrations of all comonomers for all cyclic voltammetry measurements. Electrochemical studies were undertaken in 0.1 M solutions of Bu4NBF4, 99% (Sigma-Aldrich) in dichloromethane (DCM) solvent, and CHROMASOLV, 99.9% (Sigma-Aldrich) at room temperature. All comonomers were synthesized according to our previously published procedures for (EE)-1,4-dibutoxy-2,5bis[2-(heteroar-2-yl)ethenyl]benzenes and 1,4-dibutoxy-2,5-bis(heteroar-2-yl)benzenes.24,25 J


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7.32 (dd, 4J = 0.90 Hz, 3J = 5.40 Hz, 2H, C4H3S, 2 × C5−H), 7.52 (dd, 4J = 0.90 Hz, 3J = 3.60 Hz, 2H, C4H3S, 2 × C3−H). 13 C NMR (75 MHz, CDCl3) δ (ppm): 22.30 (2 × −CH3), 72.22 (2 × −OCH⟨), 115.19 (2 × CAr−H), 124.39 (2 × CAr), 125.18 (C4H3S, 2 × C3), 125.77 (C4H3S, 2 × C4), 126.75 (C4H3S, 2 × C5), 139.62 (C4H3S, 2 × C2), 147.96 (2 × COAlk). UV−vis (CH2Cl2): λ = 358.5 nm, ε = (1.86 ± 0.12) × 104, λ = 305.5 nm, ε = (1.54 ± 0.10) × 104, λ = 256.0 nm, ε = (0.93 ± 0.06) × 104 [dm3/(mol cm)]. 1,4-Di(isopropoxy)-2,5-bis(selenophen-2-yl)benzene (2c). Light yellow solid with tm = 133−134 °C. 1H NMR (300 MHz, CDCl3) δ (ppm): 1.45 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.73 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 7.26 (s, 2H, 2 × HAr), 7.35 (dd, J = 4.50 Hz, J = 6.00 Hz, 2H, C4H3Se, 2 × C4− H), 7.69 (d, J = 4.50 Hz, 2H, C4H3Se, 2 × C3−H), 8.05 (d, J = 6.00 Hz, 2H, C4H3Se, 2 × C5−H). UV−vis (CH2Cl2): λ = 372.0 nm, ε = (1.95 ± 0.12) × 104, λ = 325.5 nm, ε = (1.35 ± 0.09) × 104, λ = 315.5 nm, ε = (1.30 ± 0.09) × 104, λ = 288.0 nm, ε = (0.93 ± 0.63) × 104 [dm3/(mol cm)].

(EE)-1,4-Di(isopropoxy)-2,5-bis[2-(furan-2-yl)ethenyl]benzene (1a). Orange solid with tm = 124−125.5 °C. 1H NMR (300 MHz, CDCl3) δ (ppm): 1.38 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.51 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 6.35 (d, J = 3.00 Hz, 2H, C4H3O, 2 × C3−H), 6.41 (dd, J = 1.80 Hz, J = 3.30 Hz, 2H, C4H3O, 2 × C4−H), 6.93 (d, J = 16.50 Hz, 2H, 2 × Hvinyl), 7.05 (s, 2H, 2 × HAr), 7.32 (d, J = 16.50 Hz, 2H, 2 × Hvinyl), 7.40 (d, J = 1.80 Hz, 2H, C4H3O, 2 × C5−H). 13C NMR (75 MHz, CDCl3) δ (ppm): 22.43 (4 × −CH3), 72.36 (2 × −OCH⟨), 108.35 (C4H3O, 2 × C3), 111.77 (C4H3O, 2 × C4), 113.46 (2 × Cvinyl), 116.88 (2 × CAr−H), 122.55 (2 × Cvinyl), 128.03 (2 × CAr), 142.18 (C4H3O, 2 × C5), 150.18 (2 × COAlk), 154.06 (C4H3O, 2 × C2). UV−vis (CH2Cl2): λ = 389.0 nm, ε = (4.40 ± 0.28) × 104, λ = 339.5 nm, ε = (2.59 ± 0.17) × 104 [dm3/(mol cm)]. (EE)-1,4-Di(isopropoxy)-2,5-bis[2-(tiophen-2-yl)ethenyl]benzene (1b). Yellow solid with tm = 136−138 °C. 1 H NMR (300 MHz, CDCl3) δ (ppm): 1.39 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.49 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 7.00 (dd, J = 3.60 Hz, J = 5.10 Hz, 2H, C4H3S, 2 × C4−H), 7.04−7.10 (m, 4H, 2 × HAr, 2 × C3−H), 7.18 (d, J = 5.10 Hz, 2H, C4H3S, 2 × C5−H), 7.21−7.25 (m, 4H, 4 × Hvinyl). 13C NMR (75 MHz, CDCl3) δ (ppm): 22.46 (4 × −CH3), 72.74 (2 × −OCH⟨), 113.75 (2 × CAr−H), 122.07 (2 × Cvinyl), 123.76 (2 × Cvinyl), 124.33 (C4H3S, 2 × C3), 125.84 (C4H3S, 2 × C4), 127.73 (2 × CAr), 128.20 (C4H3S, 2 × C5), 143.91 (C4H3S, 2 × C2), 150.24 (2 × COAlk). UV−vis (CH2Cl2): λ = 400,0 nm, ε = (3.59 ± 0.22) × 104, λ = 349.5 nm, ε = (1.81 ± 0.11) × 104, λ = 254.5 nm, ε = (0.96 ± 0.06) × 104 [dm3/(mol cm)]. (EE)-1,4-Di(isopropoxy)-2,5-bis[2-(selenophen-2-yl)ethenyl]benzene (1c). Yellow solid with tm = 149−150.5 °C. 1 H NMR (300 MHz, CDCl3) δ (ppm): 1.39 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.49 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 7.07 (s, 2H, 2 × HAr), 7.12 (d, J = 16.50 Hz, 2H, 2 × Hvinyl), 7.18−7.24 (m, 4H, C4H3Se, 2 × C3−H, 2 × C4−H), 7.28 (d, J = 16.50 Hz, 2H, 2 × Hvinyl), 7.82 (dd, J = 1.20 Hz, J = 3.60 Hz, 2H, C4H3Se, 2 × C5−H). 13C NMR (75 MHz, CDCl3) δ (ppm): 22.46 (4 × −CH3), 72.80 (2 × −OCH⟨), 113.86 (2 × CAr−H), 124.59 (2 × Cvinyl), 125.06 (2 × Cvinyl), 128.28 (2 × CAr), 128.62 (C4H3Se, 2 × C5), 128.84 (C4H3Se, 2 × C4), 130.29 (C4H3Se, 2 × C3), 150.12 (C4H3Se, 2 × C2), 150.27 (2 × COAlk). UV−vis (CH2Cl2): λ = 407.0 nm, ε = (4.50 ± 0.29) × 104, λ = 290.5 nm, ε = (0.94 ± 0.06) × 104, λ = 261.0 nm, ε = (1.01 ± 0.07) × 104 [dm3/(mol cm)]. 1,4-Di(isopropoxy)-2,5-bis(furan-2-yl)benzene (2a). Light green solid with tm = 110.5−113 °C. 1H NMR (300 MHz, CDCl3) δ (ppm): 1.42 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.73 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 6.49 (dd, J = 1.80 Hz, J = 3.30 Hz, 2H, C4H3O, 2 × C4−H), 7.06 (d, J = 3.30 Hz, 2H, C4H3O, 2 × C3−H), 7.35 (s, 2H, 2 × HAr), 7.45 (d, J = 1.80 Hz, 2H, C4H3O, 2 × C5−H). 13C NMR (75 MHz, CDCl3) δ (ppm): 22.44 (4 × −CH3), 71.12 (2 × −OCH⟨), 110.39 (C4H3O, 2 × C3), 111.52 (C4H3O, 2 × C4), 112.03 (2 × CAr− H), 120.12 (2 × CAr), 140.99 (C4H3O, 2 × C5), 147.70 (C4H3O, 2 × C2), 150.60 (2 × COAlk). UV−vis (CH2Cl2): λ = 353.5 nm, ε = (2.23 ± 0.14) × 104, λ = 310.0 nm, ε = (2.60 ± 0.16) × 104, λ = 296.0 nm, ε = (1.98 ± 0.13) × 104, λ = 249.5 nm, ε = (1.00 ± 0.07) × 104 [dm3/(mol cm)]. 1,4-Di(isopropoxy)-2,5-bis(thiophen-2-yl)benzene (2b). Light green solid with tm = 125−127 °C. 1H NMR (300 MHz, CDCl3) δ (ppm): 1.41 (d, J = 6.00 Hz, 12H, 4 × −CH3), 4.62 (sept, J = 6.00 Hz, 2H, 2 × −OCH⟨), 7.08 (dd, J = 3.60 Hz, J = 5.40 Hz, 2H, C4H3S, 2 × C4−H), 7.27 (s, 2H, 2 × HAr),

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06846. Spectroelectrochemical characteristics of thiophene derivatives (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Polish National Science Centre grant no. 2011/03/N/ST5/04362.

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REFERENCES

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