Tipping the Balance with the Aid of Stoichiometry: Room Temperature Phosphorescence versus Fluorescence in Organic Cocrystals

The interaction of the coformer 1,4-diiodotetrafluorobenzene (I2F4) with the conjugated hydrocarbons diphenylacetylene (DPA) and trans-stilbene (tS) leads to formation of the novel cocrystals DPA.I2F4 (1), DPA.(I2F4)(2) (2), tS.I2F4 (3), and tS.(I2F4)(2) (4). These materials have been synthesized by mechanochemical methods and characterized by X-ray techniques and luminescence spectroscopy in the solid state. In the cocrystals, the DPA and tS molecules interact with the molecules of I2F4 via halogen bonds (XB) of the kind halogen(...)phenyl (Hlg(...)pi). As a result of the external heavy atom effect, and depending on the stoichiometry, these cocrystals exhibit both fluorescence and phosphorescence (1 and 3) or exclusive phosphorescence (2 and 4) at room temperature. Differences in the luminescence efficiencies between the DPA- and tS-containing materials are observed.