Fluorescent and Phosphorescent Co-Crystals Based on Halogen

The search for purely organic luminescent materials is an appealing field of research, not only from a theoretical point of view but also because of their potential applications in optoelectronic devices. Recently, a turning point in this direction has come from crystal engineering, which aims to control the properties of solids through the careful study and exploitation of the interactions that "glue" together the components, such as halogen bonds. Co-crystallization of organic emitting molecules with halogenated co-formers has proved to be successful in lighting up phosphorescence, but also in improving the fluorescence performances despite the heavy atom effect. This is due to the twofold role of the co-former which serves as "solid diluent", thus reducing self-quenching phenomena, and because of the heavy atoms presence is able to favor phosphorescence. We recently explored this approach by studying materials obtained by co-crystallization of different organic molecules (1,8-naphthalimide derivatives, aromatic hydrocarbons) with the co-former 1,4-diiodotetrafluorobenzene. The luminescence properties of the solid materials have been analyzed by means of steady-state spectroscopic techniques. The results will be here discussed, addressing the role of co-crystallization stoichiometry on the phosphorescence vs fluorescence output from the solid materials.