Electronic Structure of Ion‐Radical Organic Solids and Polyenes

Abstract

AbstractThe electronic structures of organic ion‐radical salts, molecular conductors, and charge‐transfer complexes are described by solid‐state models with one valence state per site. Similar models occur in π‐electron theories of conjugated molecules or in spin Hamiltonians for magnetic insulators. A localized or valence bond (VB) representation is qualitatively adequate in narrow band systems. Diagrammatic VB methods in finite models yield convenient exact solutions to the resulting configuration interaction (CI) problem as correlated states based on linear combinations of VB diagrams. Also found are exact charges, bond orders, ionicity, correlation functions, transition moments or other properties of correlated electronic states. Particular attention is given to qualitative failures of conventional one‐electron descriptions, which often reflect degeneracies lifted by correlations. Correlated states give the correct ordering of the 21Ag and 11Bu states of trans‐trans octatetraene, clarify the midgap absorption in polyacetylene, rationalize the partial ionicity and electrostatic energy of ion‐radical organic complexes, demonstrate an exactly soluble magnetic analog to the Mott transition, and quantitatively fit the thermodynamics of several random‐exchange Heisenberg antiferromagnets. Correlated states remain central in simultaneously modeling optical, magnetic, and electrical excitations in π‐radical solids.