Supramolecular host-guest complexes between coumarin 460 and cyclodextrins: a matter of size

The role of the host size on stoichiometry, structure and strength of supramolecular association between a fluorescent probe, coumarin 460 (C460), and the three natural cyclodextrins (CDs) with increasing cavity sizes (a, ß and γ-cyclodextrin) is analyzed, using fluorescence spectroscopy. The variations of the fluorescence emission spectra of C460 with CD concentration can be explained with a 1:1 complexation equilibrium model for a-CD and ß-CD, but also a 1:2 complex has to be included in the case of γ-CD. Nevertheless, the photophysical properties and the stability of the complexes do not correlate in a simple way with the cavity size. For instance, the association between C460 and ß-CD, which is the CD with medium cavity size, is much stronger than with the other CDs but it does not provoke fluorescence enhancement as in those cases. Detailed analysis of fluorescence quantum yields and fluorescence lifetimes of the complexes help us to propose an explanation for this unexpected behaviour. This is the formation of two types of 1:1 complexes with ß-CD and γ-CD which differ in the side groups of C460 which are included in the CD cavity and how much effect this has on the photoinduced charge-transfer processes in the coumarins. This system is a nice example how subtle variations in the host-cavity size can induce big structural changes in the supramolecular complexes.