R.P. McCall

The oxidation state of “emeraldine” base

Abstract “Emeraldine” base prepared in air has an oxidation state as defined in the text of (1−y)∼0.6. whereas when prepared under an inert atmosphere, (1ȳ)∼0.4. The electronic spectrum of freshly prepared “emeraldine” base solutions in aqueous 80% acetic acid depends on the oxidation state: an absorption at λ max ∼670 nm (1.85 eV) is observed for the blue solutions of “emeraldine” base having (1−y)∼0.6, whereas the same absorption is shifted to λ max ∼760 nm (1.63 eV) for the green solutions of “emeraldine” base having (1−y)∼0.4. The blue solutions become green after ∼0.5 hours. On standing, the absorpt ion peaks of both solutions shift to an identical equilibrium value of λ max =787 nm (1.58 eV). These observations can be used as a convenient color “spot-test” to gain information concerning the oxidation state of “emeraldine” base. The infrared spectra of the solid samples are consistent with the above (1−y) values.

Polyaniline: Oriented Films and Fibers

Abstract The dependency of the conductivity of polyaniline (emeraldine oxidation state) on its molecular weight has been determined. Uni- and biaxially oriented films and uniaxially oriented fibers of emeraldine base have been studied and the dependency of their degree of crystallinity, tensile strength, and conductivity (after doping) on draw ratio has been determined.

IR absorption, photoinduced IR absorption, and photoconductivity of polyaniline

Abstract The infrared spectra, photoinduced infrared spectra, and photoconductivity of polyaniline are reported. Oxidative doping of leucoemeraldine base yields changes that are consistent with breaking the symmetry of the polymer and an overlap of the vibrational modes with a continuum of states. Small photoinduced changes in the vibrational modes of emeraldine base also indicate this symmetry breaking and the creation of polarons. The absence of significant photoconductivity suggests that the photoexcited polarons have low mobility. Calculations based on the amplitude mode formalism give a very large polaron mass in emeraldine base, consistent with low polaron mobility.

Solution studies of the emeraldine oxidation state of polyaniline

Abstract Optical studies of emeraldine base in solutions show a red shift in both 2eV and 3.8eV absorption peaks compared with the optical absorption for emeraldine films and powders. As for solids, the 2.0eV peak is absent for the salt form. Unlike the salt film, the strong absorption peak is narrow without the free electron absorption tail, suggestive of strong localization of polarons in the solution form. Dilution studies show there is no shift in the absorption peaks with concentration in both emeraldine base and salt, indicating that optical absorption is an intrachain interaction. Temperature dependent UV-VIS spectra show the first thermochromatic effect in a system without side groups that suggests possible ring flipping. Photoinduced optical studies of emeraldine base polymer in solution shows photoinduced absorption peaks at 1.4eV and 3.0eV and photoinduced bleaching at 1.8eV. The photoinduced absorption peak observed at 0.9eV in film is no longer present. Its disappearance implies the importance of interchain interactions for the formation of the electronic state. In contrast, the 1.4eV absorption is from the interchain excitation of polarons.

Highly conducting polyacetylene

Abstract “New”-(CH) x and “ARA” (i.e. A dditional R educing A gent)-(CH)x have been synthesized at room temperature. “New”-(CH)x is formed by polymerizing acetylene on flat polymeric or glass substrates using an AlEt3/Ti (n-BuO)4 catalyst in silicone oil which has previously been heat-treated. Additional reducing agents may be added to the catalyst after its thermal treatment to produce “ARA”-(CH)x. The stretch-oriented “New”-(CH)x polymerized on glass and “ARA”-(CH)x polymerized on polyethylene can be p-doped with I2/CCl4 to give conductivities of ∼50,000 and ∼75,000 S/cm respectively. Optical absorption, photoinduced infrared absorption, magnetic susceptibility and conductivity studies on selected forms of the above doped and undoped types of (CH)x are discussed. The electronic structure of highly conducting “New”-and “ARA”-(CH)x is observed to be essentially identical to that of conventional Shirakawa-(CH)x.

Photoinduced Absorption and Erasable Optical Information Storage in Polyanilines

Abstract Photoinduced absorption spectroscopy has been used to study a variety of forms of polyaniline. Experiments on emeraldine base and leucoemeraldine base have led to a picture of the photoproduction and decay of massive, long-lived defect states that can be described in terms of conformational changes involving rotations of the C 6 H 4 rings. Photoexcitation spectra of other polyanilines, such as pernigraniline base and poly( o -toluidine), exhibit similar features including relatively weak photoinduced infrared vibrations and a relatively strong photoinduced absorption peak near 1.4–1.5 eV. In several of these materials, the photoinduced changes are very long-lived, leading to an investigation of the polyanilines as media for erasable optical information storage.

Photoexcitation spectroscopy of pernigraniline

Abstract Absorption and photoinduced absorption spectroscopies are reported for pernigraniline base (PNB). Direct absorption of PNB shows the band gap energy at 2.3 eV and π − π ∗ transition energies at 3.8 and 4.3 eV. The photoinduced absorption (PA) spectrum at room temperature shows three induced absorption peaks at 1.0 (LE), 1.5 (ME) and 3.0 eV (HE). At a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). The ME peak is very long-lived; in addition, there are very long-lived relatively weak infrared vibrations. Light-induced ESR studies show the long-lived 1.5 eV peak has spin-1/2. We suggest that the LE peak originates from A-B type soliton pairs, while the ME peak originates from massive (∼300 me) ring rotational polarons in amorphous regions.

Photoexcitations in emeraldine base

Abstract 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.

Photoinduced absorption in the LaCUO systems

Abstract Near-steady-state photoinduced absorption experiments on samples belonging to the high-T c families of materials give insight into the types of charged excitations in them, as well as the nature of their ground states and mechanisms of superconductivity. In powdered samples of La 2 CuO 4− δ at low temperature, excitation by 2.54 eV laser photons induces several features: bleaching above 2 eV, photoluminescence whose intensity increases rapidly above ∼ 2 eV, and two new absorptions at 0.5 and 1.4 eV. The behavior of these photoinduced features suggests that a bimolecular type process governs the decay of the induced excitations. The presence of the photoinduced bleaching and photoluminescence is consistent with a ∼ 2 eV energy gap, while the photoinduced absorptions suggest that defect states in the gap are created. Similar photoinduced experiments on thin films of YBa 2 Cu 3 O x also reveal several features. We examine our experimental results in light of recently developed theories that predict soliton, polaron, or bipolaron defects to be the low-lying excited states of these materials.

ABSORPTION AND PHOTOINDUCED ABSORPTION STUDIES OF La2CuO4 and YBa2Cu3O6+x: ROLE OF DEFECT STATES

ABSTRACT Linear and photoinduced optical absorption studies of YBa 2 Cu 3 O 6+x are described and compared with those of La 2 CuO 4 . Absorption studies of thin films of YBa 2 Cu 3 O 6+x show a series of optical absorptions at 0.5, 1.6, 3.0, 4.3, and 4.8 eV for x=0. As × increases, the energies of each of the peaks are unchanged except for the highest-energy peak, which shifts considerably with increasing oxygen concetration. Photoinduced optical absorptions studies of YBa 2 Cu 3 O 6+x reveal similar results for x=0 and x=0.3. These spectra differ from those of La 2 CuO 4 in that the crossover from photoinduced absorption to photoinduced bleaching is shifted down in energy from 2.0 eV to 1.5 eV. There remain two photoinduced electronic absorptions, though their energies and character differ substantially from those observed in La 2 CuO 4 . For x=0.0 samples, the high energy photoinduced absorption is very broad, with a weak maximum at ˜ 1.2 eV. The low energy peak is much sharper, with a maximum at ˜0.13 eV. The infrared spectrum of x=0.0 samples is similar to that previously reported. Photoinduced infrared studies demonstrate the important role of electron-phonon coupling. The phonon modes shift as expected with substitution of 18 O for 16 O. A new photoinduced mode at 315 cm −1 is observed for YBa 2 Cu 3 16 O 6.0 . This mode is associated with the breaking of the tetragonal symmetry to orthorhombic or lower symmetry upon photoexcitation of charge carriers. It is likely associated with the formation of O 1− sites upon photoexcited charge transfer.