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···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.