The role of long-range Coulomb interaction in the neutral-to-ionic transition of quasi-one-dimensional charge transfer compounds

The neutral (N)-to-ionic (I) transition with dimerization, found in organic charge transfer compounds with mixed stacking, has been studied using a self-consistent theory which takes exact account of long-range Coulomb interactions but neglects interchain coupling. The unrestricted Hartree-Fock approximation gives a set of optimally mixed donor-acceptor bands with optical gap which bifurcates in the I phase. The dimerization is ascribed to the electrostatic instability, rather than to the distance dependence of the intermolecular electronic transfer energy as is usually done. The phase diagram for the first-order N-I transition with dimerization and discontinuous change of charge transfer is presented on the parameter plane of the redox potential and the transfer energy. The results of numerical calculations are consistent with the experimental data for tetrathiafulvalene-chloranil compound.