Controlling the helicity of 2,2'-bipyridyl ruthenium(II) and zinc(II) hemicage complexes

Two enantiomers of a new 4,5-pineno-2,2'-bipyridine ligand were synthesized and subsequently incorporated into hemicage ligands through a phenyl linker to yield ligands (+)-L1 and (-)-L1 or through a mesityl linker to yield ligands (+)-L2 and (-)-L2. Complexation of these ligands to Ru(II) afforded diastereomerically pure Delta and Lambda isomers, as verified through circular dichroism and circularly polarized luminescence spectroscopy. Ligands (+)-L2 and (-)-L2 were further coordinated to Zn(II) to form a complex with intriguing photophysical properties. Whereas Zn(bpy)32+ was shown to be a fluorescent emitter outside the visible spectrum, the caging process provided an unprecedented enhancement of intersystem crossing and subsequent switching to the phosphorescent emission of blue light. Additionally, the chiroptical properties of the Zn(II) complexes were also studied.