title: Absorption spectra of model single chains of conducting polyaniline.
abstract: The theoretical study addresses the type and nature of the transitions in the absorption spectra of octamers, dodecamers, and hexadecamers of the emeraldine salt-the conducting form of polyaniline. Each of the fully protonated oligomers is modeled in its lowest singlet (bipolaronic form) and highest possible multiplicity (polaronic form). Two configurations of the chloride counterions with respect to the oligomer chains are considered. All structures are optimized with BLYP/6-31G*/PCM, while the spectra are evaluated with CIS/6-31G*/PCM. The optical behavior of the bipolaronic and polaronic forms of the investigated systems is discussed and compared to relevant experimental data. The optical transitions at about 400 and 800 nm characteristic for the emeraldine salt are registered for all model structures. Weighed against experimental and earlier theoretical findings the results prove that CIS gives qualitatively correct electronic spectra of these conjugated species. While the two configurations have almost identical spectra in the highest multiplicity, the singlets' absorption conduct turns out to be sensitive to the counterions position. In all cases the most intensive absorption is the longest wavelength one in the near-IR region, but the number and oscillator strengths of the polaronic and bipolaronic bands are noticeably dissimilar. The bands of the low-spin oligomers are grouped, while those of the high-spin species cover the entire visible region. Each extension of the chain with one elementary unit contributes systematically a set of new bands to the spectrum. The possibility for a solvatochromic effect is estimated.
authors.affiliation: Faculty of Chemistry and Pharmacy, University of Sofia, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria.
authors.forename: Jasmina
authors.initials: J
authors.lastname: Petrova
authors.forename: Julia
authors.initials: J
authors.lastname: Romanova
authors.forename: Galia
authors.initials: G
authors.lastname: Madjarova
authors.forename: Anela
authors.initials: A
authors.lastname: Ivanova
authors.forename: Alia
authors.initials: A
authors.lastname: Tadjer
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D004553:Electric Conductivity; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000814:Aniline Compounds; C416807:polyaniline