Ground State Phase Diagram of Mixed-Stack Compounds with Intermolecular Electron Transfer

The ground state energy for a chain of donor and acceptor molecules (mixed-stack architecture) is calculated within the three-state model. The model describes the intermolecular electron transfer and, in particular, stresses the role of the diagonal coupling of the electron to symmetry breaking molecular displacements and the local electric field. The modulation of the intermolecular Coulomb interaction is shown to have important consequences for the ground state and its dynamics. In particular, the ground state energy as a function of the displacement may show one, two or three minima with varied molecular ionicity. An analysis of the function gives a phase diagram which indicates a possibility for the coexistence of neutral (undistorted) and ionic (distorted) chains of molecules in the ground state. The function is illustrated by numerical calculations with parameters appropriate for the tetrathiafulvalene-chloranil crystal which undergoes a neutral-to-ionic phase transition induced by either temperature or pressure. The effect of the electron transfer on the lattice dynamics of the mixed-stack system is briefly considered. It is suggested that the thermodynamical phase diagram for tetrathiafulvalene-chloranil system can be understood as a result of two effects: pressure induced quantum mixing between diabatic states which determine a nature of components and temperature stimulated classical mixing of the components.