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Aggregation in the nondoped polyaniline, emeraldine base (EB), as observed by GPC in a N-methyl-2-pyrrolidinone (NMP) solution, was found to be closely related to the fraction of the imine nitrogen atoms in the nondoped polymer, i.e., to the oxidation state of the polymer. As EB was reduced to leucoemeraldine base (LEB), the degree of aggregation was significantly reduced. The results of gel permeation chromatography (GPC) and infrared studies suggest that the aggregation of the polymer is due to interchain hydrogen bonding between the imine and amine nitrogen sites on adjacent polymer molecules. The degree of hydrogen bonding and, thus, aggregation found in the polymer is greatly affected by solvents, LiCl, and the oxidation state of the polymer.

We report the far-infrared through visible photoinduced absorption (PA) spectra of polyaniline in the emeraldine-base and leucoemeraldine-base forms. The direct-absorption spectrum of emeraldine base (EB) has a broad absorption centered at 2 eV (the "exciton" band) and an absorption band at-3.6 eV (the m.-~* band gap). The PA spectra of EB for pumping into the "exciton" band and across the band gap are nearly identical, indicating the same types of charged defect states are created upon photoexcitation. The direct absorption spectrum of leucoemeraldine base (LB) shows only the~-m* band-gap absorption at-3.6 eV. For pumping into this absorption band, the PA spectrum of LB is very similar to that of EB, although important differences result from the lack of quinoid structures in this material. Based on our results, we propose a model for the photocreation of defect states in leucoemeraldine base and emeraldine base. The central roles of phenyl-ring rotations and of massive polarons are discussed.

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The insulator-metal transition (IMT) for a model quasi-one-dimensional (quasi-1D) conducting polymer (polyaniline) is probed at room temperature (RT) over an unusually broad frequency range (2 meV-6 eV) and also via T -dependent dc conductivity (s dc ). We determine that the IMT is not monotonic with increasing s dc ͑RT͒. The RT far infrared scattering time (t) becomes unusually long ($ 10 213 s) as s dc ͑RT͒ increases, even for samples on the insulating side of the IMT. We conclude that the IMT is due to percolation in the presence of inhomogeneous disorder and quasi-1D localization rather than 3D Anderson localization. [S0031-9007(96)01283-5] PACS numbers: 71.30.+h, 72.30.+q, 72.60.+g, 78.66.Qn The study of insulator-metal transitions (IMTs) has provided insight into a wide variety of phenomena in heavily doped semiconductors, metal ammonia solutions, and conducting polymers [1][2][3][4][5][6][7][8][9][10]. Generally, these studies focused on dc transport properties [1,2,5,[8][9][10], and the effects of pressure [1,2,5,9], magnetic field [1,2,4,5,9], and composition [1,2,4,5]. Systematic probes of the electronic response away from the Fermi level (E F ) would provide new insight into IMTs. High frequency studies are particularly useful for discerning the essential differences between a three-dimensional (3D) Anderson transition and an IMT due to percolation in the presence of inhomogeneous disorder and quasi-1D localization.A 3D Anderson transition occurs in the presence of large homogeneous (uniform statistical) disorder. A mobility edge (E C ) separates localized from extended states [1][2][3][4], and the transport scattering time (t) varies slowly with energy in the vicinity of E C [11]. At the IMT, the electronic localization length (L loc ) diverges. A monotonic development of the transport properties occurs as E F crosses E C into the delocalized states [1]. In particular, s dc grows in magnitude and weakens in temperature dependence [1], and the slope of the quantity W [ϵ d ln s dc ͑T͒͞d ln T ] [12] vs T changes sign from negative to positive. However, the large disorder required to localize electronic wave functions leads to the slowly varying t being short (typically ϳ10 215 s). The IoffeRegel condition [13] requires k F l ϳ 1, where k F is the Fermi wave vector and l is the mean free path. Approaching the IMT from the metallic side, the optical conductivity [s͑v͒] is monotonically suppressed due to the short t beneath the free carrier s Drude ͑v͒ at low energy [2,14]. For short t, localization corrections to the metallic Drude response result in a dielectric function [e͑v͒] that is positive in the far infrared [2,4,11,14], in contrast to the negative low energy e Drude ͑v͒ for usual metals [15].In contrast, a very different IMT is presented by an array of 3D (spatially [16,17] and electronically [18] anisotropic) metallic ellipsoids with an open Fermi surface [6,7,10,19] separated by a disordered quasi-1D medium (inhomogeneous disorder model). Conduction electrons in isolated 1D systems are localiz...

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