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···π).
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.