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Electronic phenomena in polyaniline
Synthetic Metals, 29 (1989) E395 E400
ELECTRONIC
PHENOMENA
E395
IN POLYANILINE
J.M. GINDER and A.J. EPSTEIN Department of Physics, The Ohio State University, Columbus, OH 43210 (USA) A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104 (USA)
ABSTRACT \Ve have carried out an extensive set of optical, photoinduced optical, magnetic, transport, electrochemical, and mechanical studies of the polyaniline system of polymers. The results of these studies demonstrate the essential role of molecular excitons and polarons in this system. Photoexcitation of the exciton absorption in the emeraldlne base form of polyaniline leads to the occurrence of polaron absorption features. Protonation to the emeraldine sMt ='orm c~uses a transformation of the electronic structure of the chain into a poiaron lattice. Studies of dopant-induced and photoinduccd infrared active vibrations reveal that the polarons in this polymer system are massive (~ 50 me), while an analysis of this data using the amplitude mode formalism suggests the presence of large pinning and confinement paraxnctcrs. All of the infrared modes undergo a Fano-likc enhancement upon doping to the metallic emcraldine salt state. The fast phototransformation of the electronic structure suggests the presence of a third order optical nonlinearity Im X (3) ~ 10 -8 esu. Partial protonation of the cmcraldine polymer results in substantial phase segregation into protonatcd and nonprotonated regions with charge conduction dominated by charging energy limited tunneling among the granular polymcrlc metal regions. At very low protonation, charge conduction is dominated by hopping between polaron and bipolaron sites. The temperature dependence of the E P R linewidth is affected by both the metallic and textured character of the polymer.
INTRODUCTION The polyaniline family of polymers is an excellent system for studying electronic phenomena in polymers, especially with regard to issues such as the role of the absence of charge conjugation symmetry, the presence of a large extrinsic energy gap in addition to that induced by the electron-phonon interaction (the Peierls gap) [1], the influence of dissimilar repeat units (an A-B polymer) [2], and the presence of other than half-filled ~r bands in the undoped forms. Other important issues within the polyaniline family of polymers are the 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
E396 relative effects of oxidative versus protonic acid doping; the relative stability of polarons (P) and bipolarons (BP); the effects of the torsional degrees of freedom on the dectronic, optical, and mechanical properties; and the effects of localization on transport and magnetic phenomena. The polyaniline system is readily transformed among three insulating forms: leucoemeraldine base (LE, or polyparaphenylene amine), emeraldine base (EB, or polyparaphenylene amine imine), and pernigraniline (PNA, or polyparaphenylene imine) [3,4], as shown in Figure 1. In the completely reduced LE form, the energy bands involve hybridization of doubly occupied nitrogen 2pz and benzene 7r orbitals, leading to relatively broad valence bands and narrow conduction bands (the antibonding orbitals of benzene have nodes at the parts carbon atoms, yielding small transfer integrals with the neighboring nitrogen atoms). Hence, there is substantial charge conjugation asymmetry and a large energy gap ~ 3.5 eV, essentially due to the separation between the filled and empty ~r orbitals of the benzene moieties on the leucoemeraldine backbone. On the other hand, the energy gap in the completely oxidized PNA form is likely to originate from the energy difference between the nitrogen and phenyl pz electron levels (A-B nature), as well as the dimerization gap in this (ideally) degenerate ground state system [5].
H
H
(rq'"O...o'N'o.),
(C) ~.N~. ~
N~
Figure 1: Schematic illustration of the (a) leucoemeraldine, (b) emeraldine, and (e) pernigraniline base forms of polyaniline. Protonatlon of the emeraldine base polymer [6-8] leads to the emeraldine salt polymer (ES) and a reorganization of the electronic structure to form a polsron metal [7,8]. The same final electronic state can be achieved by oxidation of the lencoemeraldine base by treatment with oxidizing agents such as C12 [9], shown in Figure 2. The quasilinear increase of the metallic Pauli susceptibility of emeraldine base with the degree of protonatlon lead to the proposal of segregation into metallic and nonmetallic regions [7,8]. Analysis of the Pauli susceptibility yidded estimates of a polaron bandwidth of ~ 0.4 eV and polaron width
E397 H
(o) \
ix
H
H
H
H
H
Figure 2: Schematic illustration of the transformation of the (a) emeraldine base and (c) leucoemeraldine base to the (b) emeraldine salt polymer, through protonation and oxidation respectively.
of ~ 0.8 N-N spacings [7,8] in accord with the more recent band structure calculations of Stafstr6m, et aI. [4] and ENDOR studies of Lindgren, et al. [10]. RESULTS AND D I S C U S S I O N Studies of the temperature-dependent dc conductivity, electric field dependent conductivity, and thermopower of emeraldine polymer as a function of protonation level lead to the proposal of charging energy limited tunneling among metallic (ES) islands embedded in an insulating (EB) matrix [11]. More recent studies of the temperature dependent audio frequency (101 to l0 s Hz) conductivity of lightly protonated E B [12, 13] and microwave frequency (6.5 x 109 Hz) response as a function of protonation [13,14]indicate that at low protonation levelsz _< 0.13 (z --[CI-]/[N]),the complex transport coefficientsare dominated by the hopping of holes or electrons among fixed (pinned) polaron sites. The dielectricconstant reflectsboth the charge hopping among sites (frequenciesless that 103 Hz) and the pinning of the polaron itself(pinning frequency ~ 10 ]3 Hz) [12-14]. The doping level dependence of the microwave frequency conductivity and dielectricconstant reflects the granular metallic nature of the emeraldine salt and the "tcxturcd" nature of the electron potential within the metal islands and Icad to an cstimatc of ~ 350 S/cm for the "instrinsic" conductivity of the "metal islands" [13,14]. A more detailed picture of the magnetic properties and transport state is obtained from electron spin resonance studies of emeraldine [15,16]. Moisture and oxygen have a large effect on the measured electron spin resonance response [15]. For extensively pumped samples, the intrinsic resonance properties can be studied; the lineshape can then be decomposed into Gaussian and Lorentzima contributions [15]. With increasing protonation, Paull as well as Curie contributions to the susceptibilityoccur for the Lorentzian spin contribution, while the temperature dependence of the Gaussian line remains Curie-llke. A linear temperature
E398 dependence of the linewidth of the Lorentzian component at high temperature is in accord with the importance of phonon scattering in the metallic state [17,18], while the increase in lincwldth (relaxation rate) at low temperatures reflects the textured nature of polyaniline. The optical absorption of emeraldine base changes dramatically upon protonatlon [4,7]. Steady-state photolnduced optical studies of emeraldine [19,20] reveal that absorption of 2.0-2.7 eV photons into the molecular exciton band leads to a bleaching of the exclton and interband transitions and the production of polarons, as identified by the 1.5 and 3.0 eV photoinduced absorptions corresponding to the observed protonatlon-lnduced absorptions. An additional photoinduced absorption peak at 0.9 eV has very different temporal behavior than the other photoinduced features; its virtual absence in photoindueed absorption spectra of emeraldine base solutions [21] suggests that this "low energy" peak results from an interchain process. The energy of this peak is, perhaps, too high to correspond to expectations for an interchain bipolaron, as proposed by Zakhidov [22]. Since the interchain bandwidth is expected to be no larger than several tenths of an eV, if the "low energy" peak does correspond to an interchain blpolaron it implies the presence of a Coulomb repulsion ~ 0.7 eV for an intrachain bipolaron. Such a Coulomb repulsion could account for the stability of a polaron lattice over a blpolaron lattice in long polyaniline chains. In contrast, it is noted that on short oligomers of polyaniline, like the tetramer CsHs-(NH-CsH4)4-CsHs, the spinless bipolaron state is more stable [23]. The temporal evolution of photoinduced excitations provides important insight into the underlying interactions in polymers. Measurements of the dynamics of the photolndueed exclton bleaching show that the vast majority of the excltons recombine within several microseconds [24]. Using the initial change in optical absorption upon pumping with picosecond pulses into the exclton band, a resonant third-order optical nonlinearity Im X(3) ~ 10 - s esu is estimated [25], of the order of those calculated for polyacetylene [26] and polydiacetylenes [27]. Preliminary estimates based on degenerate four wave mixing experiments in emeraldine salt Rims suggest similarly large X(s) values [28]. At much longer times (~ 1 s) the strength of the photoindueed "low energy" peak in emeraldine base is negligible relative to the long-lived polaron absorptions at 1.5 and 3.0 eV and the exciton bleaching at 2 eV [29]. The decay of the long-llved excitations is very slow at low temperature and is thermally activated near room temperature [29], suggesting that the long lifetimes result from the relaxation of moetles associated with the exciton and polaron into new conformations [29-31]. The activation energy for recombination of longlived polarons is approximately equal to that observed for activation of free volume in NMR experiments on emeraldine base [31]. Infrared spectroscopy provides a direct means of probing the effects of electron-phonon interaction in conducting polymers. The two strong Raman-active modes in leucoemeraldine base [32] contribute to the infrared activity upon conversion (by stepwise oxidation and deprotonation) to emeraldlne base because of the resulting symmetry reduction [29]. Similar effects occur with the oxidative chlorine doping of leueoemeraldine base toward the emeraldlne salt form. In contrast, protonatlon of emeraldine base to form emeraldine salt does not lead to new infrared modes. Rather, there is a nearly uniform increase in oscillator strength of all infrared active modes that is well fit [29] by a Fano effect [33] resulting
E399 from the degeneracy of discrete modes with a continuum absorption; the continuum in this instance is associated with the "metallic" polaron band. Photoinduced infrared absorption studies clearly reveal at least two photoinduced modes associated with the generation of =barged polarons, P + . a n d P - [29]. These photoinduced infrared active modes (IRAV) are very long-lived at low temperature, consistent with the importance of thermally activated free volume for ring rotations observed in NMR [31] and dynamic mechanical [34] experiments. The photoinduced IRAV are much weaker than the corresponding electronic transitions, leading to an estimate of the polaron mass Nip ,-~ 50 me. Preliminary application of the amplitude mode theory [1] to the observed Raman and infrared modes yields a pinning parameter a ,,~ 0.8; an effective electron-phonon coupling parameter 2~ ,,~ 0.9; and, assuming an intrinsic gap roughly the same size as in polyacetylene (1.7 eV [1]), a confinement parameter 7 ~ 2. These unusual values point to the polyaniline family as an important model system to elucidate the breadth of phenomena possible in conducting polymer systems. SUMMARY A broad range of optical, magnetic, and transport studies reveals polya~filine to be a system whose physics is substantially different from polyacetylene and related polymers. Derivatization at nitrogen [35] and ring [36] positions provides the opportunity to "finetune" these phenomena and optimise them for applications such as nonlinear optics [25] and microwave absorption [13,14]. ACKNOWLEDGEMENT This work was supported in part by the Defense Advanced Research Projects Agency through a contract monitored by the U.S. Ofllce of Naval Research. REFERENCES 1. E. Ehrenfreund, Z. Vardeny, O. Brafman, and B. Horovitz, Phys. Rev. B, 36 (1987) 1535, and references therein. 2. M.J. Rice and E.J. Mele, Phys. Rev. Lett., 49 (1982) 1455. 3. A.G. MacDiarmld, J.C. Chiang, A.F. Richter, and A.J. Epstein, Synth. Met., 18 (1987) 285. 4. S. StafstrSm, J.L. Br~das, A.J. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang, and A.G. MacDiarmid, Phys. Rev. Lett., 59 (1987) 1464. 5. M. C. dos Santos and J. L. Br~das. Synth. Met.~ 29 (1989) E321 (these Proceedings) and references therein. 6. J.P. Travers, J. Chroboczek, F. Devreux, F. Genoud, M. Nechtsehein, A. Syed, E.M. Geni~s, and C. Tsintavis, Mol. Cryst. Liq. Cryst., 121 (1985) 195. 7. A.J. Epstein, J.M. Ginder, F. Zuo, 11. W. Bigelow, H.S. Woo, D.B. Tanner, A.F. Richter, W.S. Huang, and A.G. MacDiarmid, Synth. Met., 18 (1987) 303. 8. J.M. Ginder, A.F. Richter, A.G. MacDiarmid, and A.J. Epstein, Solid State Comrnun. 2 63 (1987) 97.
E400
9. A. Ray, G.E. Asturias, D.L. Kershner, A.F. Richter, A.G. MacDiarmid, and A.3. Epstein, Synth. Met., 29(1989) El41 (these Proceedings). 10. M. Lindgren, M. Sandberg, T. Hjertberg, A. Lund, and O. Wennerstr6m, preprint. 11. F. Zuo, M. Angelopoulos, A.G. MacDiarmid, and A.J. Epstein, phys. Rev. B, 36 (1987) 3475. 12. F. Zuo, M. Angelopoulos, A.G. MacDiarmid, and A.J. Epstein, submitted. 13. H. H. S. Javadi, F. Zuo, K. R. Cromack, M. Angelopoulos, A. G. MacDiarmid, and A. J. Epstein, Synth. Met., 29(1989) E409 (these Proceedings). 14. H.H.S. Javad], K.R. Cromack, A.G. MacDiarmid, and A.J. Epstein, submitted. 15. H.H.S. Javadi, R. Laversanne, A.J. Epstein, R.K. Kohli, E.M. Scherr, and A.G. MacDiarmid, Synth. Met, 29(1989) E439 (these Proceedings). 16. R.Laversanne, H.H.S. Javadi, A.G. MacDiarmld, and A.J. Epstein, to be published. 17. R.J. Elliot, Phys. Rev., 96 (1954) 266. 18. F. Rachdi and P. Bernier, Phys. Rev. B, 33 (1986) 7817. 19. M.G. Roe, J.M. Ginder, P.E. Wigen, A.J. Epstein, M. Angelopoulos, and A.G. MacDiarmid, Phys. Rev. Lett., 60 (1988) 2789. 20. M.G. Roe, J. M. Ginder, R. P. McCall, K. R. Cromack, A. J. Epstein, T. L. Gustafson, M. Angelopoulos, and A. G. MacDiarmid, Synth. Met, 29 (1989) E425 (these Proceedings). 21. F. Zuo, R. P. McCall, J. M. Ginder. M. G. Roe, J. M. Leng, A. J. Epstein, G. Asturias, S. P. Ermer, A. Ray, and A. G. MacDiarmid, Synth. Met., 29 (1989) E445 (these Proceedings). 22. A. Zakhidov, ~ Met., 27 (1988) A51 (these Proceedings). 23. tLH.S. Javadi, J.M. Giader, S.P.Treat, J.F. Wolf, and A.J. Epste'n, to be published. 24. M.G. Roe, J.M. Ginder, T.L. (lustafson, A.J. Epstein, M. Angelopoulos, and A.G. MacDiarmid, to be published. 25. J.M. Ginder, A.G. MacDiarmid, and A.J. Epstein, to be published. 26. M. Sinclair, D. Moses, A.J. Hceger, K. Vilhelmsson, B. Valk, and M. Salour, Solid State Commun., 61 (1987) 221. 27. B.I. Greene, J. Orenstein, R.R. Millard, and L.R. Williams, Phys. Rev. Left., 58 (1987) 2750. 28. 3.P. Kurmer, R. Schwerzcl, J.M. Ginder, E. Scherr, A.G. MacDiarmid, and A.J. MacDiarmid, to be published. 29. R.P. McCall, M.G. Roe, J.M. Ginder, T. Kusumoto, A.J. Epstein, G. Asturias, E. Scherr, and A. G. MacDiarmid, Synth. Met.2~_9(1989) E433 (theseProceedings). 30. C.B. Duke, E.M. Conwell, and A. Paton, Chem. Phys. Left., 131 (1986) 82. 31. S. Kaplan, E.M. Conwell, A.F. Richter, and A.G. MacDiarmid, Polymer Preprints, 29 (1988) 58; Synth. Met., 29 (1989) E235 (theseProceedings).
32. 33. 34. 35.
Y. Furukawa, T. Hara, Y. Hyodo, and I. Harada, Synth. Met., 16 (1986) 189. U. Fano, Phys. Rcv., 124 (1961) 1866. A.J. Epstein and A.G. MacDiarmid, to be published. S. Manohar, A. G. MacDiarmid, K. R. Cromack, J. M. Ginder, and A. J. Epstein, Synth. Met, 29
(1989) E349 (theseProceedings). 36. Y. Wei, W . W . Focke, G.E. Wnek, A. Ray, and A.G. MacDiarmid, J. Phys. Chem., in press.
ELECTRONIC
PHENOMENA
E395
IN POLYANILINE
J.M. GINDER and A.J. EPSTEIN Department of Physics, The Ohio State University, Columbus, OH 43210 (USA) A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104 (USA)
ABSTRACT \Ve have carried out an extensive set of optical, photoinduced optical, magnetic, transport, electrochemical, and mechanical studies of the polyaniline system of polymers. The results of these studies demonstrate the essential role of molecular excitons and polarons in this system. Photoexcitation of the exciton absorption in the emeraldlne base form of polyaniline leads to the occurrence of polaron absorption features. Protonation to the emeraldine sMt ='orm c~uses a transformation of the electronic structure of the chain into a poiaron lattice. Studies of dopant-induced and photoinduccd infrared active vibrations reveal that the polarons in this polymer system are massive (~ 50 me), while an analysis of this data using the amplitude mode formalism suggests the presence of large pinning and confinement paraxnctcrs. All of the infrared modes undergo a Fano-likc enhancement upon doping to the metallic emcraldine salt state. The fast phototransformation of the electronic structure suggests the presence of a third order optical nonlinearity Im X (3) ~ 10 -8 esu. Partial protonation of the cmcraldine polymer results in substantial phase segregation into protonatcd and nonprotonated regions with charge conduction dominated by charging energy limited tunneling among the granular polymcrlc metal regions. At very low protonation, charge conduction is dominated by hopping between polaron and bipolaron sites. The temperature dependence of the E P R linewidth is affected by both the metallic and textured character of the polymer.
INTRODUCTION The polyaniline family of polymers is an excellent system for studying electronic phenomena in polymers, especially with regard to issues such as the role of the absence of charge conjugation symmetry, the presence of a large extrinsic energy gap in addition to that induced by the electron-phonon interaction (the Peierls gap) [1], the influence of dissimilar repeat units (an A-B polymer) [2], and the presence of other than half-filled ~r bands in the undoped forms. Other important issues within the polyaniline family of polymers are the 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
E396 relative effects of oxidative versus protonic acid doping; the relative stability of polarons (P) and bipolarons (BP); the effects of the torsional degrees of freedom on the dectronic, optical, and mechanical properties; and the effects of localization on transport and magnetic phenomena. The polyaniline system is readily transformed among three insulating forms: leucoemeraldine base (LE, or polyparaphenylene amine), emeraldine base (EB, or polyparaphenylene amine imine), and pernigraniline (PNA, or polyparaphenylene imine) [3,4], as shown in Figure 1. In the completely reduced LE form, the energy bands involve hybridization of doubly occupied nitrogen 2pz and benzene 7r orbitals, leading to relatively broad valence bands and narrow conduction bands (the antibonding orbitals of benzene have nodes at the parts carbon atoms, yielding small transfer integrals with the neighboring nitrogen atoms). Hence, there is substantial charge conjugation asymmetry and a large energy gap ~ 3.5 eV, essentially due to the separation between the filled and empty ~r orbitals of the benzene moieties on the leucoemeraldine backbone. On the other hand, the energy gap in the completely oxidized PNA form is likely to originate from the energy difference between the nitrogen and phenyl pz electron levels (A-B nature), as well as the dimerization gap in this (ideally) degenerate ground state system [5].
H
H
(rq'"O...o'N'o.),
(C) ~.N~. ~
N~
Figure 1: Schematic illustration of the (a) leucoemeraldine, (b) emeraldine, and (e) pernigraniline base forms of polyaniline. Protonatlon of the emeraldine base polymer [6-8] leads to the emeraldine salt polymer (ES) and a reorganization of the electronic structure to form a polsron metal [7,8]. The same final electronic state can be achieved by oxidation of the lencoemeraldine base by treatment with oxidizing agents such as C12 [9], shown in Figure 2. The quasilinear increase of the metallic Pauli susceptibility of emeraldine base with the degree of protonatlon lead to the proposal of segregation into metallic and nonmetallic regions [7,8]. Analysis of the Pauli susceptibility yidded estimates of a polaron bandwidth of ~ 0.4 eV and polaron width
E397 H
(o) \
ix
H
H
H
H
H
Figure 2: Schematic illustration of the transformation of the (a) emeraldine base and (c) leucoemeraldine base to the (b) emeraldine salt polymer, through protonation and oxidation respectively.
of ~ 0.8 N-N spacings [7,8] in accord with the more recent band structure calculations of Stafstr6m, et aI. [4] and ENDOR studies of Lindgren, et al. [10]. RESULTS AND D I S C U S S I O N Studies of the temperature-dependent dc conductivity, electric field dependent conductivity, and thermopower of emeraldine polymer as a function of protonation level lead to the proposal of charging energy limited tunneling among metallic (ES) islands embedded in an insulating (EB) matrix [11]. More recent studies of the temperature dependent audio frequency (101 to l0 s Hz) conductivity of lightly protonated E B [12, 13] and microwave frequency (6.5 x 109 Hz) response as a function of protonation [13,14]indicate that at low protonation levelsz _< 0.13 (z --[CI-]/[N]),the complex transport coefficientsare dominated by the hopping of holes or electrons among fixed (pinned) polaron sites. The dielectricconstant reflectsboth the charge hopping among sites (frequenciesless that 103 Hz) and the pinning of the polaron itself(pinning frequency ~ 10 ]3 Hz) [12-14]. The doping level dependence of the microwave frequency conductivity and dielectricconstant reflects the granular metallic nature of the emeraldine salt and the "tcxturcd" nature of the electron potential within the metal islands and Icad to an cstimatc of ~ 350 S/cm for the "instrinsic" conductivity of the "metal islands" [13,14]. A more detailed picture of the magnetic properties and transport state is obtained from electron spin resonance studies of emeraldine [15,16]. Moisture and oxygen have a large effect on the measured electron spin resonance response [15]. For extensively pumped samples, the intrinsic resonance properties can be studied; the lineshape can then be decomposed into Gaussian and Lorentzima contributions [15]. With increasing protonation, Paull as well as Curie contributions to the susceptibilityoccur for the Lorentzian spin contribution, while the temperature dependence of the Gaussian line remains Curie-llke. A linear temperature
E398 dependence of the linewidth of the Lorentzian component at high temperature is in accord with the importance of phonon scattering in the metallic state [17,18], while the increase in lincwldth (relaxation rate) at low temperatures reflects the textured nature of polyaniline. The optical absorption of emeraldine base changes dramatically upon protonatlon [4,7]. Steady-state photolnduced optical studies of emeraldine [19,20] reveal that absorption of 2.0-2.7 eV photons into the molecular exciton band leads to a bleaching of the exclton and interband transitions and the production of polarons, as identified by the 1.5 and 3.0 eV photoinduced absorptions corresponding to the observed protonatlon-lnduced absorptions. An additional photoinduced absorption peak at 0.9 eV has very different temporal behavior than the other photoinduced features; its virtual absence in photoindueed absorption spectra of emeraldine base solutions [21] suggests that this "low energy" peak results from an interchain process. The energy of this peak is, perhaps, too high to correspond to expectations for an interchain bipolaron, as proposed by Zakhidov [22]. Since the interchain bandwidth is expected to be no larger than several tenths of an eV, if the "low energy" peak does correspond to an interchain blpolaron it implies the presence of a Coulomb repulsion ~ 0.7 eV for an intrachain bipolaron. Such a Coulomb repulsion could account for the stability of a polaron lattice over a blpolaron lattice in long polyaniline chains. In contrast, it is noted that on short oligomers of polyaniline, like the tetramer CsHs-(NH-CsH4)4-CsHs, the spinless bipolaron state is more stable [23]. The temporal evolution of photoinduced excitations provides important insight into the underlying interactions in polymers. Measurements of the dynamics of the photolndueed exclton bleaching show that the vast majority of the excltons recombine within several microseconds [24]. Using the initial change in optical absorption upon pumping with picosecond pulses into the exclton band, a resonant third-order optical nonlinearity Im X(3) ~ 10 - s esu is estimated [25], of the order of those calculated for polyacetylene [26] and polydiacetylenes [27]. Preliminary estimates based on degenerate four wave mixing experiments in emeraldine salt Rims suggest similarly large X(s) values [28]. At much longer times (~ 1 s) the strength of the photoindueed "low energy" peak in emeraldine base is negligible relative to the long-lived polaron absorptions at 1.5 and 3.0 eV and the exciton bleaching at 2 eV [29]. The decay of the long-llved excitations is very slow at low temperature and is thermally activated near room temperature [29], suggesting that the long lifetimes result from the relaxation of moetles associated with the exciton and polaron into new conformations [29-31]. The activation energy for recombination of longlived polarons is approximately equal to that observed for activation of free volume in NMR experiments on emeraldine base [31]. Infrared spectroscopy provides a direct means of probing the effects of electron-phonon interaction in conducting polymers. The two strong Raman-active modes in leucoemeraldine base [32] contribute to the infrared activity upon conversion (by stepwise oxidation and deprotonation) to emeraldlne base because of the resulting symmetry reduction [29]. Similar effects occur with the oxidative chlorine doping of leueoemeraldine base toward the emeraldlne salt form. In contrast, protonatlon of emeraldine base to form emeraldine salt does not lead to new infrared modes. Rather, there is a nearly uniform increase in oscillator strength of all infrared active modes that is well fit [29] by a Fano effect [33] resulting
E399 from the degeneracy of discrete modes with a continuum absorption; the continuum in this instance is associated with the "metallic" polaron band. Photoinduced infrared absorption studies clearly reveal at least two photoinduced modes associated with the generation of =barged polarons, P + . a n d P - [29]. These photoinduced infrared active modes (IRAV) are very long-lived at low temperature, consistent with the importance of thermally activated free volume for ring rotations observed in NMR [31] and dynamic mechanical [34] experiments. The photoinduced IRAV are much weaker than the corresponding electronic transitions, leading to an estimate of the polaron mass Nip ,-~ 50 me. Preliminary application of the amplitude mode theory [1] to the observed Raman and infrared modes yields a pinning parameter a ,,~ 0.8; an effective electron-phonon coupling parameter 2~ ,,~ 0.9; and, assuming an intrinsic gap roughly the same size as in polyacetylene (1.7 eV [1]), a confinement parameter 7 ~ 2. These unusual values point to the polyaniline family as an important model system to elucidate the breadth of phenomena possible in conducting polymer systems. SUMMARY A broad range of optical, magnetic, and transport studies reveals polya~filine to be a system whose physics is substantially different from polyacetylene and related polymers. Derivatization at nitrogen [35] and ring [36] positions provides the opportunity to "finetune" these phenomena and optimise them for applications such as nonlinear optics [25] and microwave absorption [13,14]. ACKNOWLEDGEMENT This work was supported in part by the Defense Advanced Research Projects Agency through a contract monitored by the U.S. Ofllce of Naval Research. REFERENCES 1. E. Ehrenfreund, Z. Vardeny, O. Brafman, and B. Horovitz, Phys. Rev. B, 36 (1987) 1535, and references therein. 2. M.J. Rice and E.J. Mele, Phys. Rev. Lett., 49 (1982) 1455. 3. A.G. MacDiarmld, J.C. Chiang, A.F. Richter, and A.J. Epstein, Synth. Met., 18 (1987) 285. 4. S. StafstrSm, J.L. Br~das, A.J. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang, and A.G. MacDiarmid, Phys. Rev. Lett., 59 (1987) 1464. 5. M. C. dos Santos and J. L. Br~das. Synth. Met.~ 29 (1989) E321 (these Proceedings) and references therein. 6. J.P. Travers, J. Chroboczek, F. Devreux, F. Genoud, M. Nechtsehein, A. Syed, E.M. Geni~s, and C. Tsintavis, Mol. Cryst. Liq. Cryst., 121 (1985) 195. 7. A.J. Epstein, J.M. Ginder, F. Zuo, 11. W. Bigelow, H.S. Woo, D.B. Tanner, A.F. Richter, W.S. Huang, and A.G. MacDiarmid, Synth. Met., 18 (1987) 303. 8. J.M. Ginder, A.F. Richter, A.G. MacDiarmid, and A.J. Epstein, Solid State Comrnun. 2 63 (1987) 97.
E400
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