Simple model for nonconducting tetracyanoquinodimethane (TCNQ) salts with donor to acceptor ratio 2:1

We propose a simple model for those nonconducting tetracyanoquinodimethane (TCNQ) salts in which the ratio of the numbers of donor and acceptor chains is 2:1. We assume that each TCNQ chain can be subdivided into electrically neutral boxes containing two TCNQ molecules and one electron. Electron transfer between boxes is neglected. The dominant Coulomb and exchange interactions are identified. The free energy is calculated exactly in two limits, vanishing and infinite intrabox hopping. Depending on the signs and relative magnitude of the Coulomb interactions, we find in the first case singlet, or paramagnetic doublet, or triplet ground states. In the second case, we find ferromagnetic or antiferromagnetic ground states. We discuss also the intermediate cases.