Singular Solid-Liquid-Gas Like Phase Diagram of Neutral-to-Ionic Phase Transition

For the understanding of basic mechanisms in condensed matter, organic charge-transfer (CT) complexes are of general interest because of their particular electronic tunability strongly coupled with their structural properties. Thus they can refer simultaneously to the chemical physics of organic solid state, by the way of notions like molecular multistability or solid state electron transfer reactions, and to the physics of low-dimensional systems where non-linear structurally-relaxed electronic excitations play a tremendous role, but also to the physics of structural phase transitions for the cooperativity point of view. This is particularly true for the neutral-to-ionic (N-I) transition which takes place in most of mixed-stack CT complexes [1]. These systems are formed of stacks with alternating electron donor (D) and electron acceptor (A) molecules. The transition manifests itself by a change of the degree of CT and a dimerization distortion with the formation of (D+A-) singlet pairs along the stacking axis in the ionic phase [2]. It is known nowadays, that the transformation can be induced by temperature [3], and/or photo-irradiation [4], but it was pressure which allowed the first observation of the N-I transition [1]. After a brief description of the mechanism of the N-I transition, considering pressure as well as temperature effects, this paper will be focussed on the thermodynamics of its recognized ambassador, the tetrathiafulvalene-p-chloranil (TTF-CA) on the basis of on recent experimental results [6] supported by theoretical considerations [7].