Threading of Long End-Functionalized Organic Molecules into Cyclodextrins: Structural Analysis in Aqueous Solution by NMR Spectroscopy and in the Solid State by X-Ray Crystallography

The length of guest molecules determines the stoichiometry of inclusion complexes of long endfunctionalized molecules with cyclodextrins in aqueous solution. Molecules up to 10-12 Å form primarily 1:1 host:guest adducts, even if they have to curve inside the cavity, whereas longer molecules form mixtures of 1:1 and 2:1 adducts in equilibrium with each other. The width of the cyclodextrin cavity determines the host-guest recognition, therefore aliphatic chains have a preference for α-cyclodextrin, whereas aromatic rings fit better to β-cyclodextrin. If the size of the cyclodextrin cavity is large relative to the thickness of the guest, and the guest has a propensity to dimerise, then 2:2 host:guest adducts, may be observed. Out of the various mixtures in the aqueous solutions, crystallization of single-stoichiometry inclusion complexes takes place and the guest's end-groups play a key role in the crystal packing. It seems, therefore, that the length of the guest influences critically the first-order organization of cyclodextrins in the aqueous solution, whereas the end-functional groups influence heavily the further organization of the initial assemblies into larger supramolecular arrays, as expressed by the packing modes in the crystal.