Phosphorescent Cocrystals Assembled by 1,4-Diiodotetrafluorobenzene and Fluorene and Its Heterocyclic Analogues Based on C-I•••π Halogen Bonding

Three phosphorescent cocrystals were prepared by 1,4-diiodotetrafluorobenzene and fluorene (cocrystal 1) and its heterocyclic analogues, dibenzofuran (cocrystal 2) and dibenzothiophene (cocrystal 3), based on C-I center dot center dot center dot pi halogen bonding, and C-H center dot center dot center dot pi, C-H center dot center dot center dot I, or C-H center dot center dot center dot F hydrogen bonding as well as F center dot center dot center dot F and S center dot center dot center dot S contacts. They were well characterized by X-ray crystallography, infrared, Raman spectroscopy, and differential scanning calorimetry and thermogravimetric analysis. The calculated halogen and hydrogen bonding energies indicate that the synergistic double C-I center dot center dot center dot pi in cocrystal 3 or pi center dot center dot center dot I-XD(HA)center dot center dot center dot H patterns in cocrystal 2 really exist. 1,4-DITFB is a dual functional synthon: the cement to link luminescence molecules and a heavy atom perturber to enhance phosphorescence of emitters by spin-orbital coupling. Three cocrystals phosphoresce distinctively with well defined vibrational bands at 496 (0-0) and 531 nm (max) for 1, 496 (0-0) and 529 nm (max) for 2, and 520 (0-0) and 564 am (max) for 3, respectively. All the decays obey a monoexponential law with lifetimes 0.34, 0.51, and 2.50 ms, respectively. The phosphorescence spectra of fluorene, dibenzofuran, or dibenzothiophene in cocrystals are largely red-shifted by approximately 50-90 nm with respect to the free monomer in beta-cyclodextrin aqueous solution, indicating modulatory phosphorescence emission by the molecular structure of emitters per se, weak bonding, and the cocrystal environment.