Co-crystallization turned on the phosphorescence of phenanthrene by C-Br•••π halogen bonding, π-hole•••π bonding and other assisting interactions

A novel cocrystal was successfully constructed by 1,4-dibromotetrafluorobenzene (1,4-DBrTFB) and phenanthrene (Phe) based on C-Br center dot center dot center dot pi halogen bonding, pi-hole center dot center dot center dot pi bonding, C-H center dot center dot center dot pi, C-H center dot center dot center dot Br and C-H center dot center dot center dot F hydrogen bonding, and was well characterized by XRD and powder XRD. The stoichiometry of Phe to 1,4-DBrTFB is 3 : 2. Due to the introduction of 1,4-DBrTFB, Phe exists as two states in the cocrystal. One is isolated by pi-hole center dot center dot center dot pi bonding between 1,4-DBrTFB and Phe; the other is delocalized by pi-pi conjugation between Phe molecules. Interestingly, the two different states are corresponding to different phosphorescent emissions: the isolated Phe spanning 565 to 741 nm with a longer lifetime; the delocalized Phe from 672 to 741 nm with shorter lifetime. The emission spectra and decay are well matched with its structure. The phosphorescence of Phe in the cocrystal is not only turned efficiently on but also well modulated. Moreover, the cocrystal as suspended micro-particles can be very easily assembled in the aqueous phase. It should be instructive to prepare micro- or nano-materials with predictable and modulated luminescent properties.