Molecular conformation of polyaniline oligomers: Optical absorption and photoemission of three-phenyl molecules

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Abstract
To examine the conformation of polyanilines with different degrees of oxidation two three‐ring oligomers, N‐N’‐diphenyl‐1,4‐phenylenediamine (the ‘‘amine trimer’’) and N‐N’‐diphenyl‐1,4‐benzoquinodiimine (the ‘‘imine trimer’’) have been studied. The amine and imine trimers are model molecules for leucoemeraldine and pernigraniline, respectively. MNDO calculations for the cis and trans forms of the imine trimer predict the ground states to have almost identical energies. Both forms have the quinoid ring in the plane of the backbone and the torsion angle between the quinoid and benzenoid rings close to 90°. The density of valence states (DOVS) and optical absorption were calculated for different conformations of the amine and imine molecules using the CNDO/S3 model. Comparison of the calculated DOVS with photoemission data for the amine molecules indicates a conformation about the same as that predicted by MNDO for the ground state. The optically excited state for the amine, according to the comparison between calculated and measured optical absorption, could have the same geometry or possibly exhibit a larger torsion angle. In the case of the imine molecule, however, the existence of a low energy absorption at 2.8 eV requires the optically excited state to have a different geometry from that predicted by MNDO for the ground state. Analysis of the optical absorption using the CNDO/S3 model suggests that the quinoid ring is twisted out of the plane of the nitrogen backbone by ∼55° while the benzenoid ring is twisted by about the same angle in the opposite direction. The DOVS resulting from this conformation describes the photoemission data reasonably well, although these data are not well described by DOVS obtained using the ‘‘ground state’’ conformation predicted by the MNDO model.