Fluorescent and Phosphorescent Co-Crystals Based on Halogen Bonds

The search for purely organic luminescent materials is an appealing field of research [1], 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 [2], which aims to control the properties of solids through the careful study and exploitation of the interactions that "glue" together the components. 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. Here we report on (i) newly synthesized 4-pyperidinyl-1,8-naphtalimide derivatives, which form co-crystals via halogen bonds [3] with the co-former 1,4-diiodotetrafluorobenzene, and on (ii) co-crystals obtained by reaction of aryl hydrocarbons with 1,4-diiodotetrafluorobenzene which, depending on the stoichiometry, exhibit both fluorescence and phosphorescence or exclusive phosphorescence at RT. These materials have been synthesized by mechanochemical methods and characterized by X-ray techniques and luminescence spectroscopy in the solid state. The effect of the molecular arrangement in the solid state on the luminescence properties of the compounds will be analyzed and discussed. References 1. J. D. Wuest, Nat. Chem., 2012, 4, 74-75. 2. D. Braga, F. Grepioni, A. G. Orpen, Crystal Engineering: from Molecules and Crystal to Materials, Kluwer Academic Publishers, Dordrecht, 1999. 3. G. R. Desiraju, P. S. Ho, L. Kloo, A. C. Legon, R. Marquardt, P. Metrangolo, P. Politzer, G. Resnati, K. Rissanen, Pure Appl. Chem., 2013, 85, 1711-1713.