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 (Keq = 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 pKa by about 2.6 units upward and about 0.7 units downward, in the two respective cases. These divergent pKa shifts of the dye arise from the differential affinity of the two host molecules to encapsulate the protonated and neutral form of the dye.

The binding affinity of Neutral Red with cucurbit[7]uril (CB7) can be fine-tuned by addition and competitive binding of metal ions, which leads also to a pK(a) shift of the dye; this can be exploited to relocate the dye from the macrocyclic cavity of CB7 to the biomolecular pocket of bovine serum albumin.

The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2-(2'-hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground-state absorption and steady-state and time-resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K approximately 2.4x10(6) M(-1)). At pH approximately 4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH approximately 8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited-state intramolecular proton-transfer process for the encapsulated dye. In both the ground and excited states, the two pK(a) values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7-complexed dye is represented by a six-state model, and the pH-dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pK(a) values using this six-state model match well with the values obtained experimentally. The changes in the pK(a) values in the presence of CB7 have been corroborated with the modulation of the proton-transfer process of the dye within the host cavity.

Supramolecular host-guest interactions of oxazine-1 dye with β- and γ-cyclodextrins (βCD and γCD, respectively) have been investigated in neutral aqueous solution (pH ∼ 7) at ambient temperature (∼25 °C) following absorption, fluorescence, and circular dichroism measurements. The dye forms inclusion complexes with both CDs, causing significant changes in its photophysical properties. Whereas fluorescence titration data for lower dye concentrations fit well with 1:1 stoichiometric complexes, the time-resolved fluorescence results indicate formation of a small extent of 1:2 (dye-host) complexes as well, especially at higher CD concentrations. The moderate range of the binding constant values for the present systems indicates the weaker hydrophobic interaction as responsible for the inclusion complex formation in these systems. It has also been observed that γCD facilitates dimerization of the dye, prominently indicated at the higher dye concentrations. On the contrary, βCD always assists deaggregation of the dye, even at very high dye concentrations. Time-resolved fluorescence anisotropy results qualitatively support the inclusion complex formation in the present systems. Results from quantum chemical calculations also nicely corroborate with the inferences drawn from photophysical studies. Observed results demonstrate that the size compatibility of the guest and the host cavity mainly determines the host-guest interaction in the present systems, much similar to the substrate-catalyst binding in many biological systems.

Interactions among macrocyclic hosts and dyes/drugs have been explored extensively for their direct usage in controlled uptake and release of large number of potential drug molecules. In this paper we report the non-covalent interaction of cucurbit[8]uril macrocycle (CB8) with a biologically important dye, neutral red, by absorption and fluorescence spectroscopy. A comparative analysis with the complexation behaviour of the dye with CB7, the lower homologue of CB8, indicates contrasting guest binding behaviour with significant changes in the photophysical characteristics of the dye. While CB7 interaction leads to a 1 ratio 1 stoichiometry resulting in approximately 6 fold enhancement in the fluorescence emission of the dye, CB8 displays signatures for a 1 ratio 2 host-guest stoichiometry with drastic reduction in the fluorescence emission. Apart from the evaluation of approximately 2 unit shift in the protolytic equilibrium on complexation (pK(a) shift), the measurements with tryptophan established a selective guest exchange to favour a co-localized dimer inside the CB8 cavity. In a protein medium (BSA), the 1 ratio 2 complex was converted to a 1 ratio 1 ratio 1 CB8-NRH(+)-BSA complex. The finding that NRH(+) can be transferred from CB8 to BSA, even though the binding constant for NRH(+)-CB8 is much higher than NRH(+)-BSA, is projected for a controlled slow release of NRH(+) towards BSA. Since the release and activity of drugs can be controlled by regulating the protolytic equilibrium, the macromolecular encapsulation and release of NRH(+) demonstrated here provide information relevant to host-guest based drug delivery systems and its applications.