Cooperative halogen bonding and polarized π-stacking in the formation of coloured charge-transfer co-crystals

Red co-crystals are formed between the matched complementary electron rich halogen bond acceptors, isomeric 4-(N,N-dimethylamino)phenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene. The red 1:1 cocrystals exhibit strong halogen bonding and strong -stacking. The N center dot center dot center dot I distances range from 2.80 to 2.85 angstrom and the C-IN angles are between 169.9 and 175.8 degrees. In all four structures the donor and acceptor molecules are alternately -stacked with the centroid to centroid distances between the dinitrophenyl moiety and the dimethylaminophenyl moiety ranging from 3.61 to 3.73 angstrom. The calculated - stacking binding energy is -22.24 kcal mol(-1) for the complex between 4-[4-(N,N-dimethylamino)phenylethynyl]pyridine and 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the calculated halogen bond binding energy between the same couple is -7.97 kcal mol(-1).