Charge instabilities in molecular materials: cooperative behaviour from electrostatic interactions

The interplay between electron transfer and electrostatic interactions leads in molecular materials to interesting charge instabilities. Here we discuss two families of materials where this phenomenon occurs, namely clusters of push-pull chromophores and charge transfer salts with a mixed stack motif. In the first case electrons are localized within each chromophore, whereas in the second case electrons are truly delocalized in one dimension. Cooperative and collective behavior results in the first class of materials from classical electrostatic intermolecular interactions, whereas in CT salts a complex interplay of electrostatic interactions, delocalization and lattice phonons governs the low-energy physics. We underline the similarity in the basic physics of the two systems, including the appearance of discontinuous phase transitions and of multielectron transfer. Collective and cooperative contribution to static polarizabilities are discussed in the two families of materials showing that supramolecular interactions can profoundly alter the material behavior, offering a powerful tool for the optimization of the material properties.