Structural aspects of the neutral-to-ionic transition in mixed-stack charge-transfer complexes

Abstract

Original and intriguing features associated with the so-called neutral-to-ionic (N-I) transition have been examined in some organic mixed-stack charge-transfer (CT) complexes, involving simultaneously electronic, structural and magnetic aspects. The origin of the N-I transition and the essential parameters governing it are still controversial. Structural aspects of the N-I transition in the prototype compound tetrathiafulvalene- p-chloranil (TTF-CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. However, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume variation occurs at T N-I with an abrupt change in the cell parameters in directions perpendicular to the stacking axis. This clear three-dimensional aspect of the compound is also evident from several close interchain contacts. Intramolecular deformations observed at the transition have been directly related to the ionicity change and the symmetry breaking, in agreement with the evolution of the 35Cl NQR spectrum. The phase diagram has been determined and a clear change in the nature of the transition has been observed with a tendency towards second-order character at high pressures.