Phosphorescent cocrystals constructed by 1,4-diiodotetrafluorobenzene and polyaromatic hydrocarbons based on C–I⋯π halogen bonding and other assisting weak interactions

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Abstract
The cocrystals were assembled with 1,4-diiodotetrafluorobenzene (1,4-DITFB) as the halogen bonding donor and the polycyclic aromatic hydrocarbons (PAH), biphenyl (Bp), naphthalene (Nap), phenanthrene (Phe), as the π-type halogen bonding acceptors, corresponding to cocrystals 3 (Bp-DITFB), 4 (Nap-DITFB) and 5 (Phe-DITFB), respectively. The single crystal X-ray diffraction analysis reveals that the two-dimensional layer in 3 or the one-dimensional chain structures in 4 and 5 are formed by C–I⋯π halogen bonds, and that the C–I⋯π bonding occurs in both above-the-carbon and above-the-bond models of DITFB to PAHs. Other intermolecular interactions in cocrystals such as C–F⋯F–C contacts, edge-to-edge type π–π stacking and C–H⋯I hydrogen bonding are also significant in maintaining the stability of supramolecular structures. The stoichiometry of all three cocrystals is 1 : 2 of PAH to DITFB. The PAHs with smaller conjugation systems and linear (not more than two benzene rings) or bent structures may be apt to form cocrystals with 1,4-DITFB by C–I⋯π halogen bonding. The cocrystals are well characterized by various spectroscopic methods. Additionally, the cocrystals 4 and 5, as expected, produce strong green and orange phosphorescence, respectively, due to strong spin-orbital coupling of iodine atoms. Phosphorescence of Phe in cocrystal 5 appears to be largely red-shifted compared with regular emission in solution or on general solid-supports. The phosphorescence decay of cocrystals 4 and 5 both obey a mono-exponential form with lifetimes of 0.067 ms and 1.449 ms, respectively.

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