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

Abstract

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···π halogen bonding, and C–H···π, C–H···I, or C–H···F hydrogen bonding as well as F···F and S···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···π in cocrystal 3 or π···XDIHA···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 nm (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 β-cyclodextrin aqueous solution, indicating modulatory phosphorescence emission by the molecular structure of emitters per se, weak bonding, and the cocrystal environment.