Color-tunable phosphorescence of 1,10-phenanthrolines by 4,7-methyl/-diphenyl/-dichloro substituents in cocrystals assembled via bifurcated C-I...N halogen bonds using 1,4-diiodotetrafluorobenzene as a bonding donor.

Single-crystal X-ray diffraction reveals a series of phosphorescent cocrystals which were assembled by 1,4-diiodotetrafluorobenzene (1,4-DITFB) and either 4,7-dimethyl-1,10-phenanthroline (DMPhe), 4,7-diphenyl-1,10-phenanthroline (DPPhe) or 4,7-dichloro-1,10-phenanthroline (DClPhe) via C-I...N halogen bonding. These cocrystals, labeled (1), (2) and (3), respectively, are phosphorescent and a distinct change in phosphorescent color can be observed from orange-yellow, green to yellow-green, with well defined vibrational band maxima at 587, 520 and 611 nm for (1), (2) and (3). Based on the dependence of halogen bonding in sites and strength, we discussed the impact of substituents with different electron-withdrawing effects and steric hindrance on intermolecular noncovalent interactions and phosphorescence. The method of inducing and modulating phosphorescence by halogen bonding and other weak non-covalent interactions through changing the substituent groups of molecules should be significant in both theory and the application of optical function materials with predictable and modulated luminescent properties.