First-Principles Study of Soliton Hyperfine Interactions in Polyacetylene
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FIRST-PRINCIPLES STUDY OF SOLITON HYPERFINE INTERACTIONS IN POLYACETYLENE
C. T. WHITE,- F. W. KUTZLER,b J. W. MINTMIRE,a and M. R. COOKc "Theoretical Chemistry Section, Naval Research Laboratory, Washington, DC 20375 bDepartment of Chemistry, Tennessee Technical University, Cookeville, TN 38505 cDepartment of Chemical Engineering, University of Massachusetts, Amherst, MA 01003
ABSTRACT All-trans-polyacetylene is considered the prototypical broad band gap quasi onedimensional organic semiconductor. Intrinsic soliton defects have long been known to be important to the understanding of the observed properties of this system at low doping levels. Magnetic resonance techniques provide powerful experimental probes into the nature and environment of these neutral-radical defects. In an earlier work we showed that firstprinciples spin-polarized local density functional (LDF) methods reliably predict proton Fermi-contact coupling constants for planar, neutral, organic 7r-radicals. We have also used these methods to calculate the Fermi-contact proton coupling constants associated with the soliton defect in polyacetylene. Herein we compare the results of these earlier soliton calculations to results from recent electron-nuclear double-resonance (ENDOR) experiments. Our predicted ratio of the negative to positive spin densities is in good agreement with these ENDOR studies. The negative spin densities arise from spin-polarization effects which are found to cause the soliton level at midgap to be split by several tenths of an eV.
I. INTRODUCTION Ignoring occasional cross-links, good semiconducting polymers are typically composed of fairly long chains which are bound together by long range van der Waals-like forces and are usually separated by more than three Angstroms. Because of these large separations each chain will largely maintain its individual identity within the solid leading to highly anisotropic electronic properties in the response of oriented samples. The quasi-one-dimensional behavior of these materials suggests immediately that intrinsic defects will play a crucial role in understanding their properties at low doping levels because in a one-dimensional system any local perturbation-no matter how smallwill lead to localized states which can potentially serve as electron and/or hole traps. These materials are of course not truly one-dimensional and hence there will always be some cutoff in the magnitude of this perturbation below which localized states will not occur. This cutoff, however, should be lower than the corresponding cutoff for fully threedimensional solids. In addition, even if there were no intrinsic localized defects in these quasi-one-dimensional polymers, electrons or holes injected at the Fermi-level, eF, of these materials will often self-trap by inducing local distortions that produce defect states within the semiconducting gap. By far the most thoroughly studied intrinsic defect in quasi-one-dimensional semiconducting polymers is the soliton defect in all-trans-polyacetylene 1 . All-trans-poly
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