Complexation of acridine orange by cucurbit[7]uril and β-cyclodextrin:: photophysical effects and pKa shifts

The host-guest interactions of the neutral (AO) and cationic (AOH(+)) forms of the dye acridine orange with the macrocyclic hosts cucurbit[7]uril (CB7) and beta-cyclodextrin (beta-CD) were investigated by using ground-state absorption and steady-state as well as time-resolved fluorescence measurements. The cationic form undergoes no significant complexation with beta-CD, but binds strongly with CB7 (K-eq = 2.0 x 10(5) M-1), causing a large enhancement in fluorescence intensity and lifetime of the dye in the latter host. The strong and selective binding of AOH(+) with CB7 is attributed to ion-dipole interactions involving the ureido carbonyl rims of CB7 and the charged AOH(+). In contrast, the neutral AO form of the dye shows quite similar binding with both CB7 and beta-CD, but the binding constants are lower by more than two orders of magnitude compared to that of the AOH(+)-CB7 system. CB7 and beta-CD show a contrasting behavior in modifying the acid-base character of the dye, shifting its pK(a) by about 2.6 units upward and about 0.7 units downward, in the two respective cases. These divergent pK(a) shifts of the dye arise from the differential affinity of the two host molecules to encapsulate the protonated and neutral form of the dye.