Theoretical study of p- and n-type doping of the leucoemeraldine base form of polyaniline: Evolution of the geometric and electronic structure

Theoretical Studies of Conducting Polymers: A Mini Review

The present short review discusses the computational studies carried out on polyacetylene (PAc), polyaniline (PANI), polypyrrole (PPy), and other conducting polymers for predicting their electronic, optoelectronic and structural properties. Studies...

Ultrafast dynamics of polarons in conductive polyaniline: comparison of primary and secondary doped forms

Time- and wavelength-resolved pump-probe measurements are performed on the conductive, primary and secondary doped, forms of polyaniline in solution to investigate the relaxation dynamics of photoexcited polarons. Contrasting dynamics observed in the two forms allow investigation of electronic (and structural) relaxation of this material. Pump pulse at 800 nm photoexcites an electron from the valence to polaron band, and subsequent relaxation dynamics are probed by a white light continuum pulse. Three distinct features are observed in the primary doped sample: (1) absorption near time zero in the 900-1025 nm probe wavelength region; (2) delayed absorption from 850 to 1025 nm; (3) pronounced oscillations with frequencies of 165 and 210 cm(-1). The first two features are associated with intraband absorptions to higher-lying states in the polaron band from the initial excited and conformationally changed intermediate states. The oscillations reflect torsional motions associated with photoexcitation and relaxation. In the secondary doped material, only bleaching and stimulated emission are observed throughout the whole spectral region; neither transient absorption signals nor oscillatory dynamics are observed. Kinetic modeling is performed to establish the mechanism of relaxation. We propose that the excited polaron relaxes nonradiatively to the ground state through an intermediate state(s) with a twisted geometry. Our measurements and analysis allow us to describe the structure of the polaron bands for primary and secondary doped polyaniline; they are in the small and large polaron limits, respectively, and are consistent with a bandgap for the primary doped form and without a bandgap (i.e., metallic) for the secondary doped material.

Optical Properties of Emeraldine Salt Polymers from Ab Initio Calculations: Comparison with Recent Experimental Data

We present the absorption coefficient alpha(omega), the transverse dielectric function epsilon(omega), the optical conductivity sigma(omega), and the reflectance R(omega) calculated for an emeraldine salt conducting polymer in its crystalline 3D polaronic structure. We utilize Kohn-Sham density functional theory (DFT) electronic wavefunctions and energies implemented in the expression of the macroscopic transverse dielectric function in the framework of the band theory without the electron-hole interaction. Contributions of intra-band transitions are taken into account by adding a Drude-like term to the dielectric function calculated ab initio. Comparison with optical properties, recently measured on high-quality emeraldine salts (Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C.-W.; Lee, S. H. Nature 2006, 441, 65), and with optical absorption spectra, recorded on other emeraldine salts, is very satisfactory. The calculated spectra are discussed in terms of energy-band structure, density of states, inter- and intra-band transitions, and transverse dielectric function.

Theoretical Investigation of the Tunable Behavior of p−n Copolymers Based on Oligothiophenes and 1,4-Bis(oxadiazolyl)-benzene

Semiempirical calculations were carried out on several model oligomers to investigate the tunable behavior of p-n copolymers with the repeating units constructed by oligothiophenes as the pi-excessive type blocks and 1,4-bis(oxadiazolyl) benzene as the pi-deficient type block. The calculated evolutions of the HOMO and LUMO of the model oligomers were in good agreement with the experimental oxidation and reduction potentials of the corresponding polymers. The effect of the length of the oligothiophene on the electronic structures and optical properties was elucidated by analyzing the HOMO and LUMO spatial distribution patterns of the model oligomers. When the number of thiophene rings increases, the HOMO and LUMO are contributed mostly from the oligothiophene segments and either the introduced single positive or negative charge focuses on the oligothiophene segments. The absorption spectra of polymers were also simulated by performing calculations on the corresponding oligomers. Good matches were observed between the calculated absorption spectra and the experimental UV-vis spectra of the corresponding polymers. The study shows that the backbone modification of the p-n copolymer, that is, changing the number of thiophene unit in the p-n diblock copolymer, greatly modifies the optical properties of the polymer.

Isolation and Crystal Structures of Two Singlet Bis(Triarylamine) Dications with Nonquinoidal Geometries

We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4'-bis[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl)amino]styryl}-3,4-di(n-butoxy)thiophene, [1](2+) and [2](2+) respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray structures, together with (1)H NMR coupling constants for [2](2+), indicate geometries in which the bond lengths are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT calculations with closed-shell singlet configurations reproduce the observed structures well. Our results indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1 A between formally single and double bonds) on the basis of a limiting valence-bond representation of the structure can, in fact, show structures with significantly different patterns of bond lengths.

Density functional theory and empirical derived force fields for the delocalized polaron form of polyaniline. Application to properties determination of an isolated oligomer using molecular dynamics simulations

Normal mode analyses obtained from quantum chemical calculations at the DFT level of theory have been performed for the repetitive unit of the delocalized bipolaron form of polyaniline (PANI). Empirical molecular mechanics force field parameters were consecutively refined using the SPASIBA software and applied to the molecular dynamics properties of an isolated dodeca oligomer model of the leucoemeraldine form of PANI. It is shown that effects of protonation of the emeraldine base spread over the four rings constituting the repeating unit. Molecular dynamics simulations reveal alternative bendings of the whole chain with a time period of 18-20 ps for the model of PANI under study.

Optical properties of singly charged conjugated oligomers: A coupled-cluster equation of motion study

We have implemented a coupled-cluster equation of motion approach combined with the intermediate neglect of differential overlap parametrization and applied it to study the excited states and optical absorptions in positively and negatively charged conjugated oligomers. The method is found to be both reliable and efficient. The theoretical results are in very good agreement with experiments and confirm that there appear two subgap absorption peaks upon polaron formation. Interestingly, the relative intensities of the polaron-induced subgap absorptions can be related to the extent of the lattice geometry relaxations.