Chiroptical detection of nonchromophoric, achiral guests by enantiopure alleno-acetylenic helicages

Enantiopure alleno-acetylenic ligands assemble diastereoselectively upon the addition of a zinc(II) salt to form triple-stranded helicates, which provide a sufficiently large helical cage ("helicage") for the encapsulation of guests. The inclusion complexation of heteroalicycles is confirmed by ROESY and DOSY NMR spectroscopy and quantified in 1H NMR binding titrations. The ECD spectra of the helicates, which showed strong Cotton effects and exciton coupling, were found to be extremely sensitive to the nature of the guest molecules. Consequently, a series of nonchromophoric, achiral guests of different sizes as well as regioisomers (1,3- and 1,4- dioxane) became distinguishable on the basis of their induced CD (ICD) spectra. Molecular dynamics (MD) simulations show the adaptability of the cavity size to individual guest molecules and support the selective ICD output. Particularly high affinity towards 1,4-dioxane allowed its selective detection at parts-per-million (ppm) levels in aqueous solutions.