Valence-bond analysis of extended Hubbard models: Charge-transfer excitations of molecular conductors

The low-energy charge transfer (CT) excitation characteristic of both π-molecular conductors and complex-ion-radical salts is interpreted as a nearest-neighbor Coulomb interaction V that is comparable to the bandwidth, $4|t|$. Partly filled segregated regular stacks in organic conductors are represented by extended Hubbard models, whose exact CT energies and intensities are obtained by diagrammatic valence-bond (VB) methods for four electrons on finite rings and chains, together with an approximate treatment of V in partly filled infinite stacks for infinite on-site correlations U. Finite V~4|t| yields an intense low-lying CT band, containing V and U-2V excitations, that depends weakly on the band filling. Finite V also splits the usual CT absorption around U for half filled bands into strong absorptions around U-V, weak ones around U, and much weaker bands around $U+V$ and $U+2V$. The CT spectra of mixed-valence tetrathiofulvalene (TTF) salts are modeled with V~0.4 eV, U~1.4 eV, and |t|~0.10-0.13 eV. Similar CT transitions in complex tetracyanoquinodimethane (TCNQ) salts are consistent with the insensitivity of the V peak's position to the filling or the structure. Restricting the basis to one valence state per site produces several general consequences for dipole-allowed optical transitions.