The phosphorescent co-crystals of 1,4-diiodotetrafluorobenzene and bent 3-ring-N-heterocyclic hydrocarbons by C–I⋯N and C–I⋯π halogen bonds

Abstract

The co-crystals 1–3 were successfully assembled from 1,4-diiodotetrafluorobenzene (1,4-DITFB) and bent 3-ring-N-heterocyclic hydrocarbons (3-R-NHHs) based on halogen bonds (XBs) and other weak interactions. The halogen bond as a good strategy of introducing a metal-free heavy-atom perturber at a suitable stoichiometry is further confirmed, which makes the spin–orbital coupling more efficient and the phosphorescence more observable. Herein, the selected 3-R-NHHs in co-crystals 1–3 show different emissions, of green, orange-yellow and orange phosphorescence with well defined vibrational bands at 544 nm (max) for 1, 592 nm (max) for 2 and 605 nm (max) for 3, whereas the pure 3-R-NHHs are not phosphorescent in the solid state. The reason can be mainly ascribed to the different N-positions within the three 3-R-NHHs that affect the C–I⋯π halogen bond properties, which influence further the energy level of the excited triplet state and meanwhile make the transition from T₁ to the higher vibrational level of the S₀ state become more probable with respect to the free monomer. It is estimated that in the co-crystals the energy level of the excited ππ* triplet state is lowered, as evidenced by the presence of a bathochromic effect in the maximum emission bands of 50–100 nm. The other interactions and local molecular environment can affect the phosphorescent behaviours, too. The different phosphorescent behaviours of three co-crystals should be suitable for the design of some appropriate π-type acceptors for the development of phosphorescent materials via the introduction of halogen bonds.