Properties of trans-polyacetylene with sp3 defects.

Abstract

The theoretical prediction that solitons, defects in the bond-alternation pattern characteristic of a conjugated polymer chain, are central in determining the properties of trans-polyacetylene has been verified experimentally in many ways. Recently additional tests of the theory have been attempted by incorporating into polyacetylene ``conjugation-interrupting'' defects with average spacing considerably less than the length of a soliton. The resulting samples were found to have ESR linewidths and optical properties comparable to samples without such incorporated defects at similar iodine doping concentrations. A possible conclusion is that the conjugation is not severely interrupted by these defects. We have investigated by means of modified neglect of differential overlap calculations the properties of a trans-polyacetylene chain with one such type of defect, an ${\mathrm{sp}}^{3}$ inclusion created by adding a second hydrogen atom to one of the carbon atoms on the chains, the two hydrogen atoms being located symmetrically about the carbon atom in the plane perpendicular to the backbone.Introduction of an ${\mathrm{sp}}^{3}$ defect is found to cause a long bond to be formed at both sides of the defect, reversing the phase of bond alternation to one side of the defect. The off-plane hydrogen atoms are found to give rise to a ${p}_{z}$-like orbital, made up of the antisymmetric combination of the two hydrogen 1s orbitals. Each ${\mathrm{sp}}^{3}$ defect creates two localized levels, one below the \ensuremath{\pi}-like band, the other above the ${\ensuremath{\pi}}^{\mathrm{*}}$-like band. Two findings give ample evidence that conjugation is severely interrupted by these defects, i.e., (1) the ${p}_{z}$ orbitals (except for the ${p}_{z}$-like one due to the H atoms) have very small amplitude in the neighborhood of the ${\mathrm{sp}}^{3}$ site, and (2) the energy levels of a chain with an ${\mathrm{sp}}^{3}$ defect at the center are (apart from the localized levels) very much like those of either of the half chains. The severity of the interruption is such that experiments in which an ${\mathrm{sp}}^{3}$ concentration of 17 at. % has been introduced would show much larger effects than have been seen, if the defects were uniformly distributed. From these considerations, plus the fact that the total energy is found to be a minimum when these defects are as close as possible, we conclude that in samples with a high concentration of ${\mathrm{sp}}^{3}$ defects there is a bimodal distribution of defect separations, peaked at average spacings of the order of those seen in pristine samples and at very small spacings.