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Aniline oligomers have been intensively studied in the past years. In particular, aniline oligomers substituted with electron-donor groups have been synthesized and its electronic properties calculated. However, when an electron-acceptor group is attached to the benzenoid ring of the oligoaniline, strong effects over its electronic properties are expected to happen. In this work some semiempirical quantum chemistry calculations of geometric structures, and energy level distribution of aniline and substituted anilines and its corresponding oligomer are presented. Geometry calculations of aniline and oligoanilines have been performed by using the PM3 and AM1 methods. Energy calculations and UV−vis spectra have been done by using the ZINDO/S-CI method. The studied substituents are methoxy, methyl, fluorine, nitro, and cyano groups, located at positions 2 or 3, in the benzenoid ring. This series of substituent groups involves a large range of σ−π electron-donor−acceptor capability. The methoxy and the nitro substituted tetranilines show an interaction between the oxygen of the substitutent and the nitrogen of the oligomer through an hydrogen atom. These hydrogen bonds modify largely the structure of the oligomers. Particularly nitro groups show the strongest electrostatic attraction between hydrogen and oxygen. The cyano and the nitro groups (σ−π acceptor) induce an increasing of the ionization potential. Theoretical analysis of the orbital energies of molecules substituted with electron-acceptor groups shows a lowering of the LUMO energy values larger than those in the HOMOs cases. A decreasing of the energy of the first optical transition when the electron acceptor capability of the substituent increases is shown. Tetranilines substituted with nitro groups display a band around 380 nm in the calculated UV−vis spectrum. Thus, oligoanilines substituted with electron-acceptor groups (especially nitro groups) show the lowest energy gap and they are the most encouraging material for semiconducting applications that we have studied.

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We present some semiempirical quantum chemistry calculations, geometric structures, charge distribution, gap energy, and enthalpy of formation (AHf) for aniline oligomers in the different oxidation states using the AM 1 method. A linear relationship between calculated optical transition values and the experimental reported ones was found. The effect of the interaction between the chloride counterion and these molecules was analyzed and indicates a decrease both in A 1t/of the aniline oligomers in the radical cation state and in AE(SOMO-LUMO). The withdrawal of one electron from the reduced aniline tetramer to form a radical cation in the presence of chloride (C1-) yields to the radical cation band, similar to the polaron band in the polyaniline case. Contrary to the expected results, our calculations show that CI-was able to transfer about 80% of its charge to the oligomers.

The structural and electronic properties of pyridine, its oligomers, and polypyridine (PPY) as obtained with density functional methods are presented in this work. Among the different exchange-correlation functionals used, B3LYP gives good structural results, whereas B3P88 predicts more accurately the electronic properties. The calculated first excitation energies of pyridine systems are in good agreement with experimental data. The coupling between the monomers in forming oligomers influences the structural and electronic properties of the system significantly. The trans head-to-head dimer is found to be the most stable form and the only one having a planar geometry. The introduction of a head-to-head or tail-to-tail coupling in order to break the regioregularity of a tetramer changes the frontier orbitals and the total energy of the system. The inclusion of a head-to-head coupling in the central units of a tetramer leads to a global stabilization of the system and lowers the HOMO, producing an increase in the first electronic excitation energy. Finally, the electronic properties of infinite PPY are obtained by extrapolations from those of finite oligomers. The calculated ionization potential, electronic affinity, and (π−π) transition are 6.3, 3.4, and 2.9 eV, respectively, in excellent agreement with previous experimental reports. Furthermore, the band structures and density of states of PPY are calculated using a DFT-LMTO method. The calculated density of states is in good qualitative and quantitative agreement with experimental UPS spectrum for this system.