Complexation of Synthetic Organic Dye Dapoxyl with cyclodextrins studied by fluorescence spectroscopy

Abstract: Cyclodextrins (CDs) are natural‐occurring oligomers with a hydrophobic cavity which allow them to form inclusion complexes in water with a variety of organic molecules. These complexes are generally stabilized by van der Waals and hydrophobic interactions, although specific host‐guest interactions can also play an important role.[1] In addition, geometrical and orientational requirements of both the guest and the host may control the association process, whereas the dissociation rate is determined by the strength of the interactions.[2] Therefore, a change of the cavity size or the rigidity of the host has dramatic effects on both the association and the dissociation rate constants and in consequence on the stability of the complexes.[3] Moreover, for a certain guest, stoichiometry and geometry of the inclusion complexes may also be much dependent on the host cavity size.[4] In this work we study the effects of CD cavity size on the stoichiometry, stability and structure of the complexes formed between the synthetic organic probe Dapoxyl and three natural cyclodextrins (α‐CD, β‐CD and γ‐CD) differing in the size of their inner cavity, using steady‐state and time‐resolved fluorescence techniques. The ditopic structure of this fluorophore together with its high sensitivity to the surroundings make it a useful model molecule to study the geometrical effects on the complexation. Complexation provokes strong changes in the fluorescent properties of Dapoxyl with a large blue shift of its emission spectrum and a great increase of the fluorescence quantum yield. The differences observed for the three CDs regarding stoichiometry, stability and fluorescence properties of the complexes are discussed on the basis of a size‐selective complexation of the Dapoxyl.