Specific interactions in the inclusion complexes of pyronines Y and B with beta-cyclodextrin.

The aim of this work is to analyze the role of specific interactions in host-guest association processes. The formation of inclusion complexes between pyronines Y and B and beta-cyclodextrin and the nature of the interactions involved have been studied using absorption, steady-state fluorescence, and time-resolved fluorescence spectroscopies. The two pyronines form 1:1 complexes with beta-cyclodextrin, with the association equilibrium constant being much higher in the case of pyronine B. Complexation causes a slight red shift of the emission spectra of the pyronines but decreases significantly their fluorescence quantum yields and lifetimes. To explain this atypical behavior, the photophysical properties of the pyronines in different solvents were determined and compared with those of the complexes. The similarities observed between the pyronines in dioxane and in the interior of the cyclodextrin cavity suggest that there are important specific interactions of the pyronines with the electron-rich oxygens present in these media. A possible explanation for the increase in the nonradiative rate constants in these media involves the existence of a charge-transfer excited state with the location of the positive charge at the xanthene moiety, which would be stabilized by the mentioned interactions. The observed differences between pyronine Y and B can be understood on the basis of these specific interactions.