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

Abstract

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 β-cyclodextrin (β-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 β-CD, but binds strongly with CB7 (K_eq = 2.0 × 10⁵ 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 β-CD, but the binding constants are lower by more than two orders of magnitude compared to that of the AOH⁺–CB7 system. CB7 and β-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.