Valence Bond Theory of Organic Charge-Transfer Salts

Abstract

A valence bond (VB) description is introduced for organic charge-transfer (CT) salts based on mixed stacks… D^+γA^−γD^+γA^−γ … of donors (D) and acceptors (A). Extrapolating exact results for finite rings shows that the magnetic gap ΔE_m is finite for γ < γ_c = 0.68 ± 0.01 and vanishes for γ > γ _c as expected in the paramagnetic ionic limit γ= 1. The small finite ΔE_m in the “ionic” TMPD-TCNQ complex indicates incomplete CT, in agreement with the resonant Raman result of γ∼ 0.7. The extension of diagrammatic VB techniques to partly-filled segregated ion-radical stacks like … A ^−γ A ^−γ A ^−γ A ^−γ or … D ^+γ D ^+γ D ^+γ D ^+γ… is discussed.