Complexation Dynamics of Xanthone and Thioxanthone to β-Cyclodextrin Derivatives

The complexation dynamics of xanthone to substituted β-cyclodextrins (CDs) and of thioxanthone to β-CD was studied by directly following the relocation of the excited triplet-state ketones from the CD cavity to the aqueous phase. Equilibrium constants for the ground-state complexation of both ketones with CDs were determined from fluorescence studies. The complexation efficiency and dynamics of thioxanthone with β-CD were the same as previously observed for the xanthone/β-CD system. Substitution of some of the hydrogens on the alcohol groups by methyl moieties at both entrances to the β-CD cavity increased the equilibrium constant for ground-state xanthone, but decreased that for the excited triplet state, leading to a larger difference for the binding behavior of these two electronic states of the guest. Addition of one pendant arm containing a carboxyl moiety to the rim where xanthone is bound does not change the binding efficiency of the ground state or the binding dynamics of triplet xanthone. In contrast, when this arm contains an ethyl ester moiety, the binding efficiencies of both the ground and excited triplet states are increased. The higher binding constant for triplet xanthone is due to an increase of the association rate constant and a decrease for the dissociation rate constant.