Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Synthetic Metals 116 (2001) 157±161 Electronic properties of the conducting form of polyaniline from electroabsorption measurements Lavanya Premvardhana, Linda A. Peteanua,*, Pen-Cheng Wangb, Alan G. MacDiarmidb a Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA b Abstract The polaron band at 1.4 eV of d,l-camphorsulfonic acid-doped polyaniline (()-HCSA-PANI) in a poly(methyl methacrylate) (PMMA) matrix was studied using electroabsorption (Stark-effect) spectroscopy at 298 K. A very small change in dipole moment on excitation (jDmj) Ê 3. The electroabsorption in the order of 1:4 0:2 D was measured as well as an average change in polarizability (Da) of only ÿ1:3 0:4 A signal of ()-HCSA-PANI in this region showed evidence for heterogeneity in the absorption band. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroabsorption; Acid-doped polyaniline; Polaron; Dipole; Polarizability 1. Introduction Electroabsorption (Stark-effect) spectroscopy has proven to be a very fruitful source of information regarding the electronic properties of conjugated polymers. For some applications see for example [1±5]. This technique provides a convenient means to measure both the degree of chargetransfer and the extent of electron delocalization in states that are optically coupled to the ground state. The chargetransfer properties of a given electronic state are re¯ected in the change in dipole moment following excitation (Dm). The extent of electron delocalization is correlated to the change in polarizability (Da). With input from electronic structure calculations, Da can also be analyzed to give the transition matrix elements between the electronic state being probed and the other states of the molecule. Equally importantly, comparison of the measured and calculated values of Dm and Da for a given electronic transition can frequently aid in assigning its orbital nature in cases where agreement does not exist in the literature. The polymer we have chosen for study here, polyaniline, exists in three states at various levels of oxidation: the fullyreduced leucoemeraldine base form, the half-oxidized emeraldine base (EB) form and the fully-oxidized pernigraniline base form. In general, the protonation of EB produces the emeraldine salt, which contains a stable delocalized polysemiquinone radical cation [6] with a half-®lled polaron * Corresponding author. E-mail address: [email protected] (L.A. Peteanu). conduction band [7] and can typically exhibit a 108 factor increase in conductivity over EB.1 Therefore, protonation of EB is considered to be a unique form of doping, which neither increases nor reduces the number of electrons associated with the polymer backbone. In this study, a localized polaron transition band at 1.4 eV exhibited by a doped PANI sample is investigated by electroabsorption spectroscopy. One unique aspect of this work is that we are studying the conductive doped PANI. This experiment is made possible by the fact that the doped PANI sample is dispersed within a matrix of poly(methyl methacrylate) (PMMA) in which the sample as a whole is suf®ciently insulating to allow a ®eld of 3 105 V/cm to be applied. As a result, the electric-®eld response of electronic transitions that are characteristic of the conducting form of our doped PANI may be measured. 2. Experimental 2.1. Preparation of d,l-HCSA-doped polyaniline (()HCSA-PANI) 128 mg of ()-HCSA (d,l-camphorsulfonic acid) was ®rst dissolved in 20 ml of chloroform. Then 100 mg of 1 The conductivity of EB is 10ÿ10 S/cm. Up to the time this paper was submitted, the conductivity of an emeraldine salt reported in the literature typically ranges from 10ÿ2 to 103 S/cm, depending on the protonic acid used and/or processing method. 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 7 7 - X 158 L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161 EB [8] was slowly added to the previous ()-HCSA chloroform solution. This new solution was stirred for 21 h to make a ()-HCSA-PANI solution. 2.2. Sample preparation A small quantity (0.5 ml) of the above ()-HCSA-PANI solution and 0.6 g of PMMA was added to 20 ml of chloroform. The solution as-made was then poured into an aluminum dish containing two pieces of ITO-coated quartz slides. After the solvent had evaporated, the ITO slides, on which a thin ®lm of the ()-HCSA-PANI/PMMA blend was deposited, were clamped together and heated in an oven at 1508C for 10±15 min. The blend sample cell thus prepared held off a suf®ciently high ®eld (1000±1500 V over a 50 mm sample thickness) for an electroabsorption signal to be measured. In order to obtain the absorption spectra, blanks consisting of PMMA alone were made by the same procedures outlined above. To determine the magnitude of the ®eld applied across the sample, the thickness of the sample is determined from the interference patterns in the near IR (900±2000 nm) obtained using an absorption spectrometer (Perkin-Elmer l900). The refractive index of PMMA (1.49) was used to determine the thickness of the PMMA ®lms. These were typically 50 mm thick with an error of 10 mm. 2.3. Instrumentation The instrumentation has been previously described in great detail [9]. The experiments were performed at a spectral resolution of 1.25 nm. 2.4. Theory of electroabsorption and data analysis procedures Our analysis [9] of the electroabsorption data follows the standard procedure as outlined by Liptay [10]. Essentially, the change in absorption due to the application of an external electric ®eld is ®t to the weighted sum of zeroth, ®rst and second derivatives of the zero-®eld absorption spectrum. The overall change in transmitted light intensity caused by the application of an electric ®eld is described by the following equation: @ A n @ 2 A n cw n 2 DA n F 2 aw A n bw n @n n n @n (1) The unperturbed absorption spectrum as a function of frequency (n) is designated A(n), and the change induced by the field is denoted DA(n). The effective field at the sample, F, includes the enhancement of the applied field due to the cavity field of the matrix. All spectra were obtained with an angle of 54.78 between the direction of the applied electric field and the electric field vector of the polarized light. Each of the coef®cients in Eq. (1), aw, bw, and cw, represent the magnitude of the contributions of the zeroth, ®rst, and second derivative of the absorption spectrum, respectively. These are related to molecular parameters in the following fashion. The aw term is proportional to the change in the oscillator strength or transition dipole of the absorption band due to the applied ®eld. In a rigid, isotropic sample, the bw coef®cient is related primarily to the average change in polarizability. The polarizability for a state i is given as X jmij j2 E ÿ Ej i6j i where mij represents the transition moment between electronic state i and all other states j and E represents the energy of the respective states. Here the average value of Da, denoted Da, is reported (i.e. Da 13 Tr Da). Finally, the coefficient cw represents the contribution to the signal due to broadening of the absorption spectrum by the applied field. This broadening, which results from interaction between the isotropically oriented ground and excited state dipole moments of the molecule and the field, is proportional to Dm F2 where Dm is the change in dipole moment on excitation. Because the molecules in the sample are not oriented, only the absolute value of Dm (denoted jDmj) is measured. For a more detailed discussion of these effects, see [10,11]. The contributions made by the zeroth, ®rst, and second derivatives of the absorption spectrum to the overall lineshape of the electroabsorption spectrum are extracted by means of a linear least-squares (LLSQ) ®t of the electroabsorption signal to the sum of the derivatives of A(n). If the resultant ®t to the absorption lineshape is not of high quality, the following factors may be responsible. There may be more than one transition (electronic or vibronic) underlying the absorption band, each of which has different electrooptical properties (jDmj and/or Da) and/or multiple groundstate conformers with differing properties may be present in the sample. As will be shown below, the electroabsorption spectrum of ()-HCSA-PANI shows evidence for heterogeneity of this type. There are a number of strategies available for ®tting such spectra [2,9,12]. These require deconvolving the absorption band into its constituent transitions and independently varying the ®tting parameters of each. At present, however, we do not have a unique method for deconvolving the absorption spectrum of ()-HCSAPANI so a ®t of this type is not reported. 3. Results 3.1. Electronic transitions of doped polyaniline The absorption spectrum of the samples of ()-HCSAPANI in PMMA used in this study exhibits several prominent transitions. There are two bands in the near UV, one of which has been assigned to a p±p transition (3.6 eV), and L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161 Fig. 1. (a) The absorption band of d,l-camphorsulfonic acid-doped polyaniline (()-HCSA-PANI) in the region of the polaron transition; (b) the electroabsorption spectrum of the polaron band (solid line) and the fit to the spectrum (dashed line); (c) the individual contributions of the first derivative (dotted) and second derivative (dashed) of the absorption spectrum to the lineshape of the electroabsorption spectrum; (d) scaling of the first derivative (dotted) and second derivative (dashed) contributions for comparison to the electroabsorption lineshape. This comparison indicates that the second derivative of the absorption band makes the predominant contribution to the fit of the electroabsorption spectrum. The y-axis units of the electroabsorption spectra, normalized by the applied field, are reported in (cm/V)2. the second of which is identi®ed as a polaron band (2.9 eV). Another transition at 1.4 eV has been assigned to a localized polaron that results from the positive charge being localized on the amine nitrogen (Fig. 1a) [7]. In addition, the absorption contains a small low-energy charge-carrier `tail' that begins at 1200 nm and extends beyond 2000 nm [13]. The absorption spectrum of the samples of ()-HCSA-PANI in PMMA used in these experiments closely resembles those of ()-HCSA-PANI with less expanded molecular conformation [14], indicating that the polymer is in a ``coil-like'' form in the blend sample [13]. 3.2. Electroabsorption spectra of ()-HCSA-PANI Electroabsorption data obtained for the region of the polaron band are shown in Fig. 1b (solid lines) with the ®t to this band shown in dashed lines. The ®t is comprised of zeroth, ®rst and second derivatives of the experimentally 159 obtained absorption spectrum (see Analysis section above). The ®rst (dotted line) and second (dashed line) derivative components are reproduced in Fig. 1c. The zeroth derivative component is not shown because it makes a very small contribution to the ®t. The overall magnitude and shape of the spectrum are fairly well reproduced using this ®tting method. Therefore, we expect that the corresponding results provide a reasonable measure of the jDmj and of the Da of the polaron transition. The values thus obtained for ()-HCSA-PANI in PMMA are jDmj 1:4 D and Ê 3. 2 Da ÿ1:3 A In Fig. 1d, the ®rst (dotted line) and second (dashed line) derivatives of the absorption spectrum have been scaled up so that a direct comparison between both derivatives and the lineshape of the electroabsorption spectrum (solid line) can be made. The scaling factors used are given in the ®gure legend. This ®gure demonstrates that the dominant contribution to the electroabsorption spectrum arises from the second derivative of the absorption spectrum (jDmj). If we were to ®t the electroabsorption spectrum to the second derivative of the absorption spectrum alone, the value of jDmj extracted from this ®t would only be 10% larger than the result reported above, as jDmj increases by the square root of the scaling factor. However, if we compare the quality of such a ®t (Fig. 1d, dashed line) to one that is obtained by including a ®rst derivative component as well (Fig. 1b dashed line), we see that the latter is a better match to the experimental spectrum. Therefore, we conclude that the contribution of Da to the ®eld response of this transition, though small, is not negligible. Though the ®t to the electroabsorption spectrum of ()HCSA-PANI in PMMA in this spectral region is of fairly high quality, systematic and reproducible deviations between the experiment and the ®t (Fig. 1b) are seen. These deviations indicate that the absorption band consists of multiple underlying transitions that vary in their characteristic values of jDmj and/or Da. A ®t that is essentially indistinguishable from the experimental electroabsorption spectrum can be obtained by deconvolving the absorption band into two gaussians that model the underlying transitions and allowing the ®eld response of each (aw, bw, and cw for each) to vary independently in the LLSQ ®tting procedure. However, the results from this ®t (not shown) are very similar to those reported here. 4. Discussion We ®nd that there have been few experiments or calculations published to date with which we can directly compare 2 A ground-state dipole moment for a charged molecule such as ()HCSA-PANI is not uniquely defined because the moment due to the charge will depend on the origin chosen. However, the change in dipole moment upon excitation for such a molecule is defined because the difference measurement removes this dependence. 160 L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161 our results for ()-HCSA-PANI. Not surprisingly, comparison to the results of Monkman et al. for the EB form [15] as well as our own for EB in PMMA [16] demonstrates that there is a substantial change in the electronic properties of the material upon doping. Moreover, all of these materials demonstrate heterogeneity in their electronic properties as evidenced by deviations between the electroabsorption spectra and the corresponding ®ts [15,17]. In future work, we intend to examine the role of conformational heterogeneity in the electroabsorption spectrum of ()-HCSA-PANI by exposing the ®lms to m-cresol which will favor the formation of a more expanded polymer chain [13] as well as by obtaining spectra of shorter oligomers. In contrast to the electroabsorption results for the 1Bu exciton states of numerous conjugated systems [1±4], both the jDmj and the Da of the polaron band of ()-HCSA-PANI are quite small, indicating little change in either the spatial extent of charge-transfer or of electron delocalization upon excitation. The negative sign of Da implies that, if anything, electron delocalization is slightly decreased in the polaron excited state relative to the ground state. From another perspective, a negative Da implies that the transition moments between the excited state being probed and higher-lying electronic states of the system are small compared to that to the ground state. Alternatively, it may indicate the presence of a low-lying state with a large transition moment to the polaron excited state. One possible reason why the values of jDmj and Da for the polaron band of ()-HCSA-PANI are measured to be small for our sample is that this molecule already exhibits considerable charge separation and/or electron delocalization in its ground state as a result of the doping process. The results are particularly striking when compared to those derived from electroabsorption measurements on the absorption bands of photogenerated polarons in samples of C60-doped poly(p-phenylene). For these systems, changes in polarizÊ 3 were measured as well as ability in the order of 107 A Ê 3 [17]. ground state polarizabilities of 104 A Having measured a relatively weak electroabsorption signals for ()-HCSA-PANI, particularly in comparison to those that have been observed for both EB and for the other conjugated polymers, we considered the possibility that the doped polymer itself sets up a ®eld that opposes the applied ®eld, such that the net ®eld strength on the sample is substantially diminished [17]. This opposing ®eld would arise by virtue of the fact that doped PANIs are conductors in their ground electronic states. We have begun to investigate this possibility using the following approach. We prepared a sample in which a dye (nile red) was incorporated into the PMMA matrix along with the ()-HCSA-PANI polymer [18]. Nile red was chosen because it exhibits a fairly strong electro-optic response and its lowest energy absorption bands are clearly separable from those due to ()-HCSA-PANI. The electroabsorption spectrum of nile red in the ()-HCSA-PANI/ PMMA blend appears to be approximately a factor of two smaller than that of nile red in PMMA alone (data not shown) [16]. From this observation we may conclude that, due to the presence of the conducting polymer, the strength of the applied ®eld may be diminished by approximately factor of two but it is not zero, at least within the region of the sample matrix accessible to the dye. 5. Conclusions Here, we have presented the electroabsorption spectrum of a conducting polymer in its doped form, ()-HCSA-PANI in a PMMA matrix. The electronic properties measured for the polaron band at 1.4 eV are jDmj 1:4 0:2 D and Ê 3. Future work is in progress to underDa ÿ1:3 0:4 A stand the effects of ground state conformation and conjugation length on these properties as well as to examine other electronic transitions in the spectrum. Acknowledgements We would like to thank Drs. David Yaron, Andrew Shreve, and Ian Norris for helpful discussions, the NSF-CAREER and POWRE programs for funding (LAP), and the Center for Molecular Analysis at Carnegie Mellon University for the use of the absorption spectrometer. A.G.M. would like to thank ONR for ®nancial support. References  . HorvaÂth, Electroabsorption spectroscopy on p[1] G. Weiser, A conjugated polymers, in: N.S. Sariciftci (Ed.), Primary Photoexcitations in p-Conjugated Polymers: Molecular Exciton versus Semiconductor Band Model, World Scientific, NJ, 1997, pp. 318±362. [2] M. Liess, S. Jeglinski, Z.V. Vardeny, M. Ozaki, K. Yoshino, Y. Ding, T. Barton, Phys. Rev. B 56 (1997) 15712±15724. [3] D. Guo, S. Mazumdar, S.N. Dixit, F. Kajzar, F. Jarka, Y. Kawabe, N. Peyghambarian, Phys. Rev. B 48 (1993) 1433±1459. [4] S.D. Phillips, R. Worland, G. Yu, T. Hagler, R. Freedman, Y. Cao, V. Yoon, J. Chiang, W.C. Walker, A.J. Heeger, Phys. Rev. B 40 (1989) 9751±9759. [5] F. Feller, A.P. Monkman, Phys. Rev. B 60 (1999) 8111±8116. [6] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, A.J. Epstein, Synth. Met. 18 (1987) 285±290. [7] S. StafstroÈm, J.L. BreÂdas, A.J. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang, A.G. MacDiarmid, Phys. Rev. Lett. 59 (1987) 1464± 1467. [8] W.-S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. 1 82 (1986) 2385±2400. [9] L.L. Premvardhan, L.A. Peteanu, J. Phys. Chem. A 103 (1999) 7506± 7514. [10] W. Liptay, Dipole moments and polarizabilities of molecules in excited electronic states, in: E.C. Lim (Ed.), Excited States, Academic Press, New York, 1974, pp. 129±229. [11] G.U. Bublitz, S.G. Boxer, Ann. Rev. Phys. Chem. 48 (1997) 213± 242. [12] W. Liptay, R. Wortmann, R. Bohm, N. Detzer, Chem. Phys. 120 (1988) 439±448. L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161 [13] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 (1994) 103± 116. [14] Y. Xia, A.G. MacDiarmid, A.J. Epstein, Macromolecules 27 (1994) 7212±7214. [15] S.J. Pomfret, E. Rebourt, A.P. Monkman, Synth. Met. 76 (1996) 19± 22. 161 [16] L.L. Premvardhan, L.A. Peteanu, D.J. Yaron, P.-C. Wang, A.G. MacDiarmid, submitted. [17] M. Liess, Z.V. Vardeny, P.A. Lane, Phys. Rev. B 59 (1999) 11053± 11061. [18] T.M. Brown, J.S. Kim, R.H. Friend, F. Cacialli, R. Daik, W.J. Feast, Appl. Phys. Lett. 75 (1999) 1679±1681.