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… DA−γDA−γ … of donors (D) and acceptors (A). Extrapolating exact results for finite rings shows that the magnetic gap ΔEm is finite for γ < γc = 0.68 ± 0.01 and vanishes for γ > γ c as expected in the paramagnetic ionic limit γ= 1. The small finite ΔEm 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 D D D … is discussed.

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