Emeraldine Base Form Of Polyaniline Research Articles

Picosecond photoinduced exciton bleaching in emeraldine base: Crossover in decay mechanisms.

We report picosecond photoinduced bleaching (PB) of the exciton absorption of the emeraldine base form of polyaniline. The temperature- (T-) independent dispersive decay, \ensuremath{\sim}${t}^{\mathrm{\ensuremath{-}}0.11}$, observed in this study differs markedly from the dynamics of earlier studied semiconductors. It also contrasts with long-time PB, which varies as ${t}^{\mathrm{\ensuremath{-}}\ensuremath{\beta}}$ with \ensuremath{\beta}\ensuremath{\sim}0.5, and is weakly T dependent. These results point to the likely role of rotations in ring-containing polymers.

Localization of charged excitations in polyaniline

Photogenerated polarons in the emeraldine base form of polyaniline have been extensively studied by using the infrared and optical photoexcitation, photoconductivity, and light-induced electron spin resonance (LESR) techniques. The photogenerated polarons in polyaniline are strongly localized and stable for the experimental time scales (≥ 1 msec.) at both 2.0 eV and 4.0 eV optical pumping energies as proved by infrared and LESR studies. The dynamic mass of the charged excitations in polyaniline is found to be two orders of magnitude larger than the electron band mass. Steady state photoconductivity measurements demonstrate that the photogenerated charges are highly immobile for both 2.0 eV and 4.0 eV pumping, consistent with the infrared studies. Significantly different results were obtained in the fast time scale (≤ 800 psec.) photoconductivity measurements. Photoexcitation at 4.0 eV leads to a measured photoconductivity one order of magnitude less than that of other conjugated polymers. The 2.0 eV optical pumping yields a factor of 30 smaller photoconductivity than that of the 4.0 eV pumping for equal number of incident photons. We propose that photogenerated holes are responsible for the transport, and the photogenerated electrons are mainly localized on the benzene ring (as monitored by the softening of the ring deformation mode caused by losing the aromaticity of the benzenoid structure).

Photoexcitations in emeraldine base

The emeraldine base form of polyaniline has been studied by photoinduced absorption (PA) spectroscopy and by light-induced electron spin resonance (LESR). The PA spectrum shows the presence of an induced bleaching at 2 eV, three photoinduced absorptions at 0.9, 1.4, and 3.0 eV, and an associated bleaching of the bandedge transition starting at 3.5 eV. The two induced absorptions at 1.4 and 3.0 eV are ascribed to the photogeneration of positive polarons. The origin of the “low energy” induced absorption at 0.9 eV is less certain, though it is possibly due to the formation of bipolarons or some other interchain species. Time-resolved studies of the exciton bleaching indicate that the excitons form within 10 ps or less after light strikes the polymer, and that their fast decay dynamics are independent of temperature. The measured LESR signal for pump light of energy ∼ 2.5 eV is consistent with the dissociation of some excitons into polarons.

Electronic phenomena in polyaniline

We 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 emeraldine base form of polyaniline leads to the occurrence of polaron absorption features. Protonation to the emeraldine salt form causes a transformation of the electronic structure of the chain into a polaron lattice. Studies of dopant-induced and photoinduced infrared active vibrations reveal that the polarons in this polymer system are massive (∼ 50 m e ), while an analysis of this data using the amplitude mode formalism suggests the presence of large pinning and confinement parameters. All of the infrared modes undergo a Fano-like enhancement upon doping to the metallic emeraldine 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 emeraldine polymer results in substantial phase segregation into protonated and nonprotonated regions with charge conduction dominated by charging energy limited tunneling among the granular polymeric metal regions. At very low protonation, charge conduction is dominated by hopping between polaron and bipolaron sites. The temperature dependence of the EPR linewidth is affected by both the metallic and textured character of the polymer.

Photoexcitation of polarons and molecular excitons in emeraldine base.

We report the first observation of the photoinduced absorption spectrum of the emeraldine base form of polyaniline. The spectrum contains both subgap photoinduced bleaching at 1.8 eV and photoinduced absorptions at 0.9, 1.4, and 3.0 eV, as well as band-edge bleaching above 3.5 eV. The existence of the subgap bleaching peak is consistent with a model of optically excited localized molecular excitons. The energies of the three photoinduced absorption peaks and the dependence of the peak amplitudes on pump power give evidence for the photoexcitation of polaron pairs that recombine bimolecularly.

A covalent bond to bromine in HBr-treated polyaniline from X-ray diffraction

When the emeraldine base form of polyaniline is doped by aqueous HBr, the conductivity is increased to 2 (ohm cm) −1 for a compressed pellet. X-ray diffraction results on the conducting polymer together with elemental analysis indicate that a significant portion of the bromine is covalently bound.

Polyaniline: Processability from aqueous solutions and effect of water vapor on conductivity

The chemically-synthesized emeraldine base form of polyaniline is soluble in aqueous acetic acid solutions, in which it presumably exists as the ‘doped’ emeraldine acetate salt. Films having a conductivity of ≈0.5 –2 S/cm may be cast from the solutions. These films may be converted to relatively flexible, free-standing, lustrous, copper-colored films of emeraldine base, which can in turn be converted to free-standing, lustrous, dark blue films of emeraldine hydrochloride ( θ≈1–2 S/ cm). The resistance of films of both emeraldine base and 50% protonated emeraldine hydrochloride decreases by a factor of ∼2 when exposed to ∼4 Torr of water vapor. The resistance increases only very slowly on removing the water vapor under dynamic vacuum, consistent with the postulated structure of emeraldine hydrochloride involving ‘islands’ of heavily-doped emeraldine base in a matrix of undoped or slightly-doped material.

Polyaniline: Electrochemistry and application to rechargeable batteries

The present study shows that: (1) reversible electrochemical oxidation of the leuucoemeraldine base form of polyaniline in the pH range ≈1 to ≈4 occurs initially with no deprotonation to give the emeraldine salt; continued oxidation occurs with spontaneous deprotonation to give the pernigraniline base and (2) a rechargeable battery employing the analytically pure emeraldine base form of polyaniline as the cathode can be constructed. This battery shows very promising features such as a capacity of ≈148 Ah/kg, an energy density of ≈340 Wh/kg, coulombic recovery > 98%, good recyclability and satisfactory stand life.

Polyaniline: non-oxidative doping of the emeraldine base form to the metallic regime

Evidence is provided which proves that the emeraldine base form of polyaniline is ‘doped’ by protonic acids to the metallic conducting regime by a process involving neither oxidation nor reduction of the polymer, thus introducing a new concept of doping to the conducting polymer field.

Nonlinear Optical Properties of Polyaniline

ABSTRACTPolyaniline is a family of polymers whose electronic and optical properties can be controlled through variation of the number of electrons and protons on the polymer chain. For example, the emeraldine base form of the polymer (EB) is insulating while the emeraldine salt form (ES) is metallic. We present here results for photoinduced absorption spectroscopy of the emeraldine base form of polyaniline. Unlike earlier studied polymers, the spectrum of emeraldine base contains both subgap photoinduced bleaching (at 1.8 eV) and photoinduced absorption (at 0.9, 1.4, and 3.0 eV). The existence of the bleaching peak is consistent with a model of optically excited localized molecular excitons in the emeraldine base polymer. The energies of the three photoinduced absorptions peaks and an associated interband bleaching (at energies greater than ∼3.5 eV) are in agreement with the photogeneration of polaron pairs. Laser intensity studies support that the excitons decay monomolecularly while the polarons decay bimolecularly. Picosecond time-resolved spectroscopy of the exciton decay demonstrates the rapid response of this system.

‘Polyaniline’: Protonic acid doping of the emeraldine form to the metallic regime

The emeraldine base form of polyaniline, which consists of equal numbers of reduced [(C 6H 4)N(H)(C 6H 4)N(H)] and oxidized [(C 6H 4)N(C 6H 4)N] repeat units, is doped to the metallic conducting regime by aqueous 1 M HCl. The product is believed to consist of equal numbers of the reduced repeat units and the diprotonated oxidized repeat units, ▪ The resulting material appears to have significant π delocalization in the polymer backbone. The doping is of a type not hitherto observed in a conducting polymer. It results in the formation of an environmentally stable nitrogen base salt rather than of a carbonium ion as in conventional oxidative p-doping of a polymer. Unlike all other conducting polymers, its conductivity depends on two variables, namely the degree of oxidation and the degree of protonation of the material. Various relationships between conductivity, pH of the acid dopant solutions, percentage doping and p K a of the protonated emeraldine salt are described and discussed.