Host−Guest Complexation of Neutral Red with Macrocyclic Host Molecules:  Contrasting pK a Shifts and Binding Affinities for Cucurbit[7]uril and β-Cyclodextrin

• Published in: The journal of physical chemistry. B Vol. 110; no. 10; pp. 5132 - 5138
• Main Authors: Mohanty, J, Bhasikuttan, A. C, Nau, W. M, Pal, H
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.03.2006
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp056411p

The photophysical properties of the phenazine-based dye neutral red were investigated in aqueous solution in the presence of the macrocyclic host molecule cucurbit[7]uril (CB7) using ground-state absorption as well as steady-state and time-resolved fluorescence measurements. The results are contrasted to those previously obtained for β-cyclodextrin (β-CD; Singh et al. J. Phys. Chem. A 2004, 108, 1465). Both the neutral (NR) and cationic (NRH+) forms of the dye formed inclusion complexes with CB7, with the larger binding constant for the latter (K = 6.5 × 103 M-1 versus 6.0 × 105 M-1). This result differed from that for β-CD, where only the neutral form of the dye was reported to undergo sizable inclusion complex formation. From the difference in binding constants and the pK a value of protonated neutral red in the absence of CB7 (6.8), an increased pK a value of the dye when complexed by CB7 was projected (∼8.8). This shift differed again from the behavior of the dye with β-CD, where a decreased pK a value (ca. 6.1) was reported. The photophysical properties of both NR and NRH+ forms showed significant changes in the presence of CB7. Fluorescence anisotropy studies indicated that the inclusion complexes of both forms of the dye rotate as a whole, giving rotational relaxation times much larger than that expected for the free dye in aqueous solution. The thermodynamic parameters for the NRH+·CB7 complex were investigated in temperature-dependent binding studies, suggesting an entropic driving force for complexation related to desolvation of the cation and the removal of high-energy water molecules from the CB7 cavity.