Crystal stability and optical properties of organic chain compounds

The solution to the long-standing problem of the cohesion of organic chain compounds is proposed. We consider the tight-binding dielectric matrix with two electronic bands per chain, determine the corresponding hybridised col lective modes, and show that three among them are considerably softened due to strong dipole-dipole and monopole-dipole interactions. By this we expla in the unusual low-frequency optical activity of TTF-TCNQ, including the ob served 10 meV anomaly. The softening of the modes also explains the cohesio n of the mixed-stack lattice, the fractional charge transfer almost indepen dent of the material, and the formation of the charged sheets in same compo unds.