β-Cyclodextrin Inclusion Complexes of 2-Hydroxyfluorene and 2-Hydroxy-9-fluorenone: Differences in Stoichiometry and Excited State Prototropic Equilibrium

We report that 1:1 and 1:2 complexes are formed for 2-hydroxy-9-fluorenone with beta-cyclodextrin (beta-CD) and that there is an unusual red shift in emission at higher concentrations of beta-CD. Between different stoichiometries of the complexes the titrimetric curves for the neutral-anionic equilibria for the guests differ drastically and so do the excited state pK values. The formation of an 1:1 inclusion complex with 2-hydroxy-9-fluorenone (2HFN) as the guest in beta-CD with the binding constant (K) of 606.65 L center dot mol(-1) was determined. The ground and excited state pK (a) values for the neutral-mono-anion equilibrium are not affected by beta-CD. Hence the hydroxyl group is considered exposed in the aqueous environment. Two different types of inclusion complexes of 2HFN were observed in beta-CD. The 1:2 complex of 2HFN shows a red shift from the 1:1 complex and is less fluorescent that the 1:1 complex. The red shift reveals that the 1:2 complex is more stabilized than the 1:1 complex. The excited state pK (a) values in both complexes with beta-CD are higher that those in aqueous solution. This shows that the complexation makes the molecule less acidic in the S1 state. The beta-CD molecule is perceived as not able to encapsulate the 2HFN molecule fully, but the larger rim of the beta-CD comes closer to the C=O group. The other half of the 2HFN molecule is encapsulated by the second beta-CD molecule and thus there is formation of the 1:2 inclusion complex at higher concentrations of beta-CD.