Cooperativity at Neutral-Ionic Transformation

Abstract

Thermochromism and photochromism are associated with the thermo-and photo-control of a color change in molecular materials. These properties are often discussed in terms of local chemical processes. However, changes in molecular identity (electronic structure) are strongly coupled to different intra-and inter-molecular structural changes, and those processes at solid state are highly cooperative. Thus, in some molecular materials the relaxation of electronic excited states results in drastic structural changes involving a large number of electrons and molecules W. This situation is carried to extreme in the case of neutral-ionic (N-I) phase transformation in quasi-one-dimensional charge-transfer (CT) crystals. This exotic transformation, associated with a change of the degree of CT, may be induced by temperature, pressure and also light. Indeed, beside structural phase transitions at thermal equilibrium exists the possibility of out-ofequilibrium photo-induced phase transformations, i.e. when light triggers a macroscopic phase change. These electronic-structural phase transformations proceed via a cascade of cooperative phenomena: the formation of one-dimensional structurally relaxed CT strings along chains due to strong intra-chain interactions, their three-dimensional condensation and their three dimensional ordering (crystallization), both originating from weaker inter-chain interactions. The formation of lattice-relaxed CT strings is a profound example of self-organized electron-transfer. A richness of new physical properties originates from these non-linear excitations. Thus, three-dimensional cooperativity between the relaxed strings leads to singular pressure-temperature (P-T) phase diagrams and to a new mechanism of molecular ferroelectricity. The cooperativity is also the essence of out-of-equilibrium photo-induced phase transformations. The purpose of this paper is to give a brief overview of the cooperative phenomena that trigger N-I phase transformations, and relate them to recent experimental results.