Fluorimetric study on molecular recognition of beta-cyclodextrin with 2-amino-9-fluorenone

The molecular recognition interaction of beta-cyclodextrin (beta-CDx) was investigated using 2-amino-9-fluorenone (2AFN) by UV, steady-state fluorescence and time-resolved fluorescence measurements in aqueous solution at various pH. The effect of acidity on the ground and excited state equilibria between the neutral and the monocationic forms of 2AFN in water and in beta-CDx environments are studied. Based on the change in the fluorescence spectrum and lifetimes of 2AFN by the addition of beta-CDx, it is found that the unsubstituted part of the 2AFN is encapsulated in the hydrophobic cavity of beta-CDx. The unusual red shift obtained for the protonation of amino group in water and beta-CDx solution is due to large solvent relaxation of the monocation. The structure of the 1:1 inclusion complex between 2AFN and beta-CDx has been proposed on the basis of ground and excited state pKa values and the bond distances obtained by MOPAC/AM 1 data.