Langmuir 1999, 15, 1323-1327
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UV-Visible Spectroelectrochemistry of Conducting Polymers. Energy Linked to Conformational Changes Toribio F. Otero*,† and Miguel Bengoechea Facultad de Quı´micas, Lab. de Electroquı´mica, P.O. Box 1072, 20080 San Sebastia´ n, Spain Received December 1, 1997. In Final Form: October 28, 1998 Large and reverse hypsochromic and bathochromic shifts on either polaronic (1.29 eV) or bipolaronic (0.68 eV) bands have been determinated from electrochromic polypyrrole films by “in-situ” spectroelectrochemical measurements in the visible region during oxidation/reduction switching. Whatever the rate of the electrochemical reaction, or the electrochemical method used, the energy of the maxima changes linearly with the number of positive charges stored in the chains per monomeric unit. Any possible solvatochromic, ionochromic, or thermochromic effects, related to water or counterion interchange or to thermal heating by the Joule effect, have been experimentally studied and discarded as the origin of this great shift. Only the conformational changes of the polymeric chains during the electrochemically induced swelling/shrinking processes seem to be responsible for this new electrochemical way to store and release molecular energy in a reverse way, observed in polypyrrole.
Introduction Most of the biochemical processes linked to life occur through conformational changes of amorphous polymers: opening and closing of ionic channels through membranes, enzymatic reactions, movements of myosin heads along ketosyn fibbers in muscular contractions, etc.1 Those “in vivo” processes involve energetic transitions between different conformations of the amorphous polymeric structures. Those energies, until now, could not be measured by any direct applications of “in-situ” NMR or neutron diffraction techniques. Conducting polymers are multifunctional materials2 able to mimic natural muscles or membranes. Most of them, like polypyrrole, are practically amorphous and can be electrochemically oxidized in a reverse way.3 During oxidation a nonstoichiometric polymer-counterion compound is formed. The counterion content changes under electrochemical control from zero to 40-45% w/w in the solid. The oxidation deep (the counterion content) can be electrochemically stopped at any value or reversed from any value. Different properties change in parallel with the oxidation deep: the conductivity (from 10-7 to 103 S‚cm-1), the volume of the film,4 the color, the permeability to ions, the stored charge,5 etc. Films of a neutral polymer present a very compact and amorphous structure: they have a strong polymer-polymer interaction. The polymer oxidation generates polarons (radical cations) or bipolarons (dications) along chains. Coulombic repulsions give conformational changes with opening of channels and penetration of counterions (and solvent) in order to keep the electroneutrality in the solid. Opening of channels and increase of volume take place along oxidation. The oxidized polymer is a soft, amorphous and complex material, formed † Tel: (34 943) 44 81 86. Fax: (34 943) 21 22 36. E-mail:
[email protected].
(1) Stryer, L. In Biochemistry; Reverte´: Barcelona, 1988. (2) Otero, T. F. In Handbook of Organic Conductive Molecules and Polymers; Nalwa H. S., Ed.; John Wiley & Sons: Chichester, U.K., 1997; Vol. 4, p 517. (3) Diaz, A. Chem. Scr. 1981, 17, 145. (4) Slama, A.; Tanguy, J. Synth. Met. 1989, 28, C171. (5) Rodrı´guez, J.; Grande, H.; Otero, T. F. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley & Sons: Chichester, U.K., 1997; Vol. 2, p 415.
by polymer, counterions, and solvent. This is similar to living materials in membranes or cells.6 Transitions between compact, or neutral, films and open, or oxidized, films are controlled by electrochemical oxidation or reduction. Conformational changes required to open the structure can be the controlling process of the polymer oxidation, and the electrochemical responses only can be simulated through a conformational relaxation-diffusion electrochemical model.7,8 The energy involved in conformational changes can be translated to macroscopic devices giving artificial muscles working at some hundreds of millivolts, very close to natural muscles which uses a nervous pulse of 60-150 mV. Working muscles confirm the proposed swelling/shrinking processes during oxidation/reduction.2 Our aim now is to use an “in-situ” technique able to follow energetic changes along conformational variations. Taking into account that electronic transitions between fundamental levels and polaronic or bipolaronic levels involve the energy of photons from the visible region of the spectrum, an “in-situ” UV-vis spectroelectrochemical determination seems to be the most adequate technique.9 A compacted polypyrrole film requires increasing energy to generate free volume enough to lodge consecutive counterions and solvent. This energy is consumed to extract consecutive electrons from an already positively charged chain.10 This picture supposes increasing energies to enhance the population of polarons (or bipolarons): a large hypsochromic shift should be expected for both, polaronic and bipolaronic, bands during oxidation. Those shifts were predicted theoretically in the literature.9 This picture opposes to the usual consideration of polarons and bipolarons such as redox couples having: well-defined electronic levels, well-defined redox potentials, and specific (6) Otero, T. F. In Polymer Sensors and Actuators; De Rossi, D., Osada, Y., Eds.; Springer-Verlag: New York, in press. (7) Otero, T. F.; Grande, H.; Rodrı´guez, J. J. Electroanal. Chem. 1995, 394, 211. (8) Otero, T. F.; Grande, H.; Rodrı´guez, J. J. Phys. Chem. B 1997, 101, 8525. (9) Bre´das, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309. (10) Otero, T. F. In Modern Aspects of Electrochemistry; O′M Bockris, J., White, R. E., Conway, B. E., Eds.; Plenum Publish. Corp.: New York, in press.
10.1021/la971311z CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999
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Figure 1. Schema of the experimental set used for in-situ absorption-reflection. The beam is reflected (meanly) on the mirrorpolished platinum electrode passing two times through the polymer film.
λmax in the visible spectra, independently of the oxidation deep (population level). Despite of the amount of work performed by “in-situ” spectroelectrochemical measurements, those important hypsochromic shifts during the oxidation of conducting polymers (or bathochromic, along reduction) were not observed, were observed only among polymers obtained from modified monomers in order to change rigidity along the chain,11 or were observed from experiments performed at different potentials but never before studied and identified. The problem is linked, from our viewpoint, to the experimental conditions for the polymer synthesis or electrosynthesis. The generation of conducting polymers coexists with both chemical (proton initiated) polymerization and degradation processes.5 Once those parallel reactions were minimized important conformational changes took place and fast artificial muscles were constructed.2 Experimental Section Here we use the same conditions of synthesis optimized to produce artificial muscles. A mirror polished platinum electrode (1 × 1 cm) was used in an one compartment electrochemical cell having a quartz window. The cell was rinsed and then filled with 24 cm3 of 2.5 M LiClO4 aqueous solution. The electrode was fixed parallel to the quartz window at a distance of 0.5 cm. The white light produced by a Xe-Ne 66006 Oriel lamp was conducted to the quartz window through a 77562 Oriel optic fiber and focused by means of a lens on the electrode (Figure 1). The diameter of the incident surface was about 0.7 cm. The reflected light was focused on and collected through an optic fiber which drives the light to a photodiode array from Oriel model 77173. A spectrum was stored using a Mosaic PC 486 microcomputer through an Instaspec program: this will be the baseline. Pyrrole and water were added to the cell solution until a final concentration 0.1 M in pyrrole and 1-2% v/v (volume/volume) of water. A constant current of 1 mA‚cm-2 was passed through the cell during 135 s using as counter electrode a 8 cm2 stainless steel sheet lying on the cell bottom. A charge of 135 mC was consumed during electrogeneration. A nitrogen atmosphere was kept along the experiments. A second electrogeneration was performed in parallel using identical cell and electrodes. Here the working electrode once coated with the polypyrrole film was reduced to neutral polymer by polarization at -800 mV vs SCE during 30 s, to expel counterions and to close the polymeric structure. A PARC 273 potentiogalvanostat from EG&G controlled by an IBM PS microcomputer through PARC 273 software was used for electrochemical measurements. This second coated electrode was (11) Leclerc, M.; Faı¨d, K. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsembaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998; 695.
rinsed with acetonitrile, and then it was dried and weighed on a Sartorius 4504 MP 8 ultramicrobalance (precision 10-7 g). The weight of the electrogenerated neutral polymer, once the platinum weight was subtracted, was 0.0483 mg. From the cell used for spectrochemical measurements, the polymerization solution was extracted using a probe connected to a vacuum pump. Then it was filled up with acetonitrile which was extracted again. This rinsing process was repeated two times. Then a 2.5 M LiClO4 aqueous solution was put in the cell. Both cell and working electrode kept the same position along those operations. Now the mirror-polished platinum electrode, coated with a polypyrrole film, is ready inside the background solution for spectroelectrochemical measurements using the stored baseline.
Results and Discussion To check both electroactivity and electrochromic changes of the coated polymer, it was submitted to a potential sweep from -900 mV (vs SCE) to 400 mV at 20 mV‚s-1. Along the potential sweep a spectrum was taken every 1 s. Figure 2a,b shows the evolution of polypyrrole absorption spectra, referred to the baseline taken by using the uncoated electrode in the background solution, as a function of the anodic and cathodic potentials, respectively. Figure 3 shows the parallel voltammogram. In good agreement with previous studies, the reduced polymer shows a maximum of absorbance at 425 nm related to the interband π-π* transitions of the aromatic form of neutral polypyrrole.12 When the oxidation starts, new polaronic and bipolaronic levels are generated inside this band gap. Those void levels are detected by promotion of electrons from fundamental levels to void polaronic levels which require the average energy supplied by photons of 660 nm. When the concentration of positive charges increases continuously from 0 until 0.28 per monomeric unit (obtained from the oxidation charge determined by integration of the anodic voltammogram and from the mass of the reduced polymer, considering -C4H3N- as the monomeric unit), this maximum experiences a continuous hypsochromic shift until 435 nm. This is an important energetic shift of 225 nm (1.29 eV or 127 kJ‚mol-1). Figure 4 shows the evolution of the λmax versus the number of positive charges per monomeric units along the potential sweep. A parallel bathochromic shift is observed along the reduction process (cathodic sweep). When the λmax is represented vs the number of positive charges per monomeric unit, hypsochromic and bathochromic shifts of polaronic bands overlap (Figure 4). (12) Fermı´n, D. J.; Teruel, H.; Scharifker, B. R. J. Electroanal. Chem. 1996, 401, 207.
Spectroelectrochemistry of Conducting Polymers
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Figure 4. Evolution of the wavelength associated with the maxima of polaronic (b) and bipolaronic (O) bands (from Figure 2a,b) as a function of the number of electrons extracted during oxidation per monomer unit taking part of the polymeric chains. Table 1. Wavelengths of the Polaronic Levels at the Beginning of the Oxidation (λmax,0) and Close to the End of the Oxidation (λmax,0.25), When 0.25 Positive Charges Are Present Per Monomeric Unit, in Different Solventsa
solvent
Figure 2. (a) Top: Evolution of the absorption spectra of an electrochromic polypyrrole film as a function on the oxidation potential (we represent here one spectrum every 80 mV) obtained along a voltammogram performed between -900 and 400 mV in 2.5 M LiClO4 aqueous solution at a scan rate of 20 mV‚s-1. Every spectrum requires 20 ms to be obtained. (b) Bottom: Evolution of the absorption spectra of an electrochromic polypyrrole along a cathodic voltammogram performed between 400 and -900 mV in 2.5 M LiClO4 aqueous solution at a scan rate of 20 mV‚s-1. One spectrum was obtained every 70 mV.
Figure 3. Control voltammogram between -900 and 400 mV at 20 mV‚s-1 peformed to an electrochromic polypyrrole film in the background electrolyte (2.5 M LiClO4 aqueous solution).
Bipolarons are formed by oxidation of polarons. Electrons require an average lower energy to be transferred to those bipolaronic levels from the fundamental state: λmax > 850 nm. The increase of the population of charges until 0.28 positive charges per monomeric unit present in the polymer promotes, Figure 2a, a hypsochromic shift of the maximum until 580 nm. That means an increase on the band gap between fundamental levels and polaronic levels greater than 270 nm (0.68 eV or 66 kJ‚mol-1). The bathochromic shifts of bipolaronic species during the reduction process present a low hysteresis (Figure 4). The question now is about the origin of this important and reverse hypsochromic-bathochromic shift of the energetic levels along electrochemical oxidation-reduc-
PC methanol ketone DMSO water
slope/eV (no. of e-/ λmax,0.25/ ∆E0.25-0/ λmax,0/ nm monomeric unit)-1 nm eV 580 605 604 612 660
3.08 2.85 5.97 2.95 3.07
419 450 428 450 473
0.82 0.71 0.84 0.73 0.74
r 0.99 0.97 0.98 0.98 0.97
a Slopes and correlation coefficients are given of the straight lines determined by the shift of the maxima as a function of the oxidation deep. The overall energetic shift (in eV) of the polaronic maxima during the oxidation in the studied solvents is given.
tion. We found in the literature no so important shifts due to interferometric colors by swelling-shrinking processes in films.13 Our film swells under oxidation, as proved by electrochemomechanical devices, and shrinks during reduction.2 Nevertheless, λmax shifts in the direction opposite to that expected by interferometric colors during swelling-shrinking. If we consider the great influence of solvents on swelling-shrinking processes, we could attribute those shifts to a solvatochromic effect:14-16 entrance of solvent during swelling by oxidation and solvent expulsion along shrinking by reduction. Different experiments were performed using 0.1 M LiClO4 in water, propylene carbonate, ketone, dimethyl sulfoxide, and methanol. Initial polaronic bands (those formed at low oxidation deep) appear at different λmax when the film is oxidized in different solvents. The wavelengths corresponding to the initial polaronic bands are summarized in Table 1. These wavelengths contain the energetic information about the transition from π to the initial polaronic levels in each solvent. An energy difference of 0.26 eV is measured between those initial polaronic levels formed when the oxidation of the polymer starts in aqueous solution and those formed by oxidation in propylene carbonate, this being the larger observed difference. Taking into account that the only difference between both experiments was (13) Murao, K.; Suzuki, K. J. Electrochem. Soc. 1988, 135, 1415. (14) Ingana¨s, O.; Salaneck, W. R.; O ¨ sterholm, J. E.; Laakso, J. Synth. Met. 1988, 22, 395. (15) Gustafsson, G.; Ingana¨s, O.; Salaneck, W. R.; Laakso, J.; Loponen, M.; Taka, T.; O ¨ sterholm, J. E.; Stubb, H.; Hjertberg, T. In Conjugated Polymers; Bre´das, J. L., Silbey, R., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1991; p 315. (16) Da Silva, L.; Machado, C.; Rezende, M. C. J. Chem. Soc., Perkin Trans. 2 1995, 483.
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Figure 5. Evolution of the wavelength associated with the polaronic maxima of the absorption spectra obtained from 0.05 M Li salts of: ClO4- (O), NO3- (0), BF4- (b), or I- (×) in propylene carbonate as a function of the number of electrons extracted per monomeric unit.
Figure 6. Evolution of the wavelength associated with the polaronic maxima obtained from 0.05 M LiClO4 in propylene carbonate at different temperatures [30 (O), 20 (0), 10 (b), 0 (×), -5 °C (+)] as a function of the number of electrons extracted during oxidation per monomeric unit.
the solvent used, this energy is a solvatochromic effect due to the polarons-solvent interaction. During the electrochemical oxidation, increasingly charged polymeric chains are formed. From a molecular point of view this means that either polymer-solvent, polymer-polymer, or polymer-counterion interactions change at increasing oxidation deeps. As a result the concentrations of both solvent molecules and counterions increase in the film. This composition change modifies every one of the acting interaction forces and the subsequent energy of both ground and polaronic levels when the oxidation is performed in different solvents. This point is reflected in different electrochemical kinetics obtained for each solvent (consumed charge as a function of the potential, along a potential sweep). To normalize and to compare spectroscopic results from different solvents, the λmax (in eV) of the polaronic bands in different solvents were plotted as a function of the oxidation deep. They fit straight lines having correlation coefficients higher than 0.97 (Table 1). The slopes of these representations defined as electronvolts shifted by the λmax per electron extracted from every monomeric unit during oxidation are summarized in Table 1. Very similar slopes are obtained in the studied solvents; only ketone shows a different behavior. In this solvent the population of polarons saturates very fast when only 0.15 electrons were extracted per monomeric unit, giving the greater slope (5.97 eV (no. of e-/monomeric unit)-1). If we consider the overall shift, from the λmax,0 to the λmax when the polymer was full oxidized (0.25 electrons are extracted per monomeric unit, λmax,0.25), the overall shifts in eV are very similar in the different solvents (Table 1). Both similar slopes and similar overall shifts of the λmax discard solvents as the origin of the observed shift. We could consider the counterion-polymer interaction as responsible for those important λmax shifts17 (ionochromic effect). Experiments using different concentrations of counterions show the expected influence on the oxidation-reduction kinetics (a shift on the voltammograms). Nevertheless, if we consider normalized λmax, overlapping shifts were obtained when λmax were plotted vs the number of positive charges per monomeric unit. To check different counterions 0.05 M solutions of ClO4-, NO3-, I-, and BF4in propylene carbonate were studied. The evolution of the wavelength associated with the maximum of the polaronic band as a function of the number of electrons per monomeric unit for every counterion is shown in Figure
5. All the studied counterions show similar behavior, discarding an ionochromic origin of the studied shift. Finally we could consider the presence of local changes of temperature, due to Joule effects inside the film, as the origin of those important λmax shifts18-20 (thermochromic effect). To check this influence, different temperatures of 30, 20, 10, 0, and -5 °C in 0.1 M solutions of ClO4- in propylene carbonate were studied. The evolution of the wavelength associated with the maximum of the polaronic band as a function of the number of electrons per monomeric unit for every temperature is shown in Figure 6. No significant influence on the shift is obtained when the temperature varies between 30 and -5 °C, discarding a thermochromic effect. Only conformational changes and the increase of chain stress during the oxidation remain as responsible for the large λmax shifts. A high energy is involved with those changes, ranging between 0.8 and 1.29 eV. This is the molecular energy stored and released when the compacted and reduced polymeric chains having a coillike structure are oxidized, taking on a rodlike structure. Shifts of the maxima, and consecutive conformations, are related to the consecutive electronic loss starting from a neutral chain. Energies required to extract consecutive electrons increase with the number of positive charges present in the chain in good agreement with experimental results: polaronic and bipolaronic maxima shift toward higher energies at increasing oxidation deep. The position of the maxima is related to an average number of positive charges per chain, and this number increases when the oxidation deep rises. Every new electron extracted from a chain promotes new transitions from aromatic to quinoid structures. Both length and angles between consecutive monomeric units in a chain change giving new conformations linked to every new oxidation deep. Shifts of the maxima, oxidation deeps, and conformational changes are so related. This reverse and electrochemically associated molecular energy is the origin of the macroscopic motors, named artificial muscles, developed by our laboratory.2 Those soft and wet materials are able to trail a steel pin, until 1000 times the weight of the conducting polymer, adhered at the botton of a bilayer (conducting polymeradherent and flexible polymer) along an angular movement greater than 180° around the fixed top of the bilayer.
(17) Chen, S.-A.; Hua, M-Y. Macromolecules 1996, 29, 4919.
(18) Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J.; Wudl, F. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 1071. (19) Chen, S.-A.; Ni, J.-M. Macromolecules 1992, 25, 6081. (20) Iwasaki, K-I.; Fujimoto, H.; Matsuzaki, S. Synth. Met. 1994, 63, 101.
Spectroelectrochemistry of Conducting Polymers
Under those conditions the experimental energetic shifts can be attributed to electrochemically stimulated conformational changes along the chains. A final problem remains unsolved related to the copious literature about spectroelectrochemical measurements without significant detection of those shifts. The response seems related to the presence of a nonelectroactive polymer in the studied films. This induces strong hindrance on molecular and macroscopic movements (muscles does not work if 10% of the polymer is nonelectroactive). During electropolymerization parallel processes of chemical (proton induced) polymerization are present to yield nonelectroactive polymer while at the same time a degradation process is present.5 Electrodic material and chemical and electrical conditions of synthesis determine the final composition of the film. Conformational movements, as in living beings, require very specific conditions. We will proceed in a reverse way in order to confirm this hypothesis. An electroactive film was electrogenerated showing important λmax shifts (like those observed in Figure 2a) along reverse voltammetric oxidation-reduction between -750 and 350 mV. By integration of the anodic voltammogram we find that the oxidized polymer stores 20 mC, which means 0.28 e- per monomeric unit. The film is submitted to 20 consecutive potential cycles between -800 and 400 mV at 20 mV‚s-1. The film is then submitted to a potential sweep of control from -750 to 350 mV. By integration of the anodic voltammogram we find that only 10% of the ability to store charge was lost but the evolution of the parallel experimental spectra changes dramatically, being similar to those obtained in the literature: the reduced film exhibited a high absorption (0.5 along the visible region), the oxidized state shows a low adsorption (1 in the visible), and no significant shift is observed for the bands during oxidation. Conclusions Large and reverse hypsochromic and bathochromic shifts were observed when polypyrrole films, generated under improving conditions of electrosynthesis, were studied by electrochemical oxidation and reduction in aqueous solution. Lower energetic changes related to
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organic molecules and polymers in solution were attributed in the literature to thermochromic, solvatochromic, or ionochromic effects. Considering the strong interchange of ions and water molecules between the reduced polymer and the solution during oxidation, solvatochromic and ionochromic possibilities were studied and discarded using different solvents, different electrolytes, and different concentrations of electrolyte. Those effects act on the different energies of the initial polaronic or bypolaronic levels, but they do not affect the overall shift. Thermochromic effects could appear by a local Joule effect at the beginning of the oxidation by the lower conductivity of the reduced polymer. Studies performed at different temperatures discarded any thermochromic effect. The only way that the polymeric chains have to store a so important energetic change are consecutive conformational changes linked to the consecutive electronic loss (electrochemically stimulated conformational changes) from every chain. The conformational energy can be translated to macroscopic mechanical energy. Those energetic requirements can be the controlling step of the electrochemical applications of conducting polymers as batteries, smart windows, artificial muscles, actuators or sensors, drug release, interfaces between electronic conductors and nerves, etc. The degradation of 10% of the electroactive polymer forces the structure hindering conformational changes, swelling and shrinking processes, which can be applied when changes of volume have to be avoided (solid batteries) with low energetic loss. Finally the cheap and available technique of UV-visible spectroscopy allows a quantitative following of conformational changes in amorphous structures of conducting polymers able to mimic ionic channel in membranes, enzymatic processes, conformational changes in muscles, etc. This can open new ways for the understanding and mimicking of living processes. Acknowledgment. This work has been supported by the Ministerio de Educacio´n y Ciencia of the Spanish Government. LA971311Z
