Dynamic Aspects in Host−Guest Interactions. 5. Kinetics of the Metal Complexation Reactions of 2-(5-Bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol Associated with Cyclodextrin Inclusion Reactions

Kinetic Study for the Inclusion Complex of Carboxylic Acids with Cyclodextrin by the Ultrasonic Relaxation Method

Ultrasonic absorption coefficients in the frequency range of 0.8-95 MHz were measured in aqueous solutions containing both beta-cyclodextrin (beta-CD) (host) and butanoic acid (in its dissociated form and undissociated one) (guest). A single relaxational phenomenon was observed only when the solutes were coexisting, although no relaxation was found in the beta-CD solution or in the acid solutions. The absorption was also measured in a solution of pentanoic acid (dissociated form) with beta-CD, and single relaxation was detected. The ultrasonic relaxation observed in these solutions was due to a perturbation of a chemical equilibrium related to a reaction of an inclusion complex formed by the host and guest. The equilibrium constant was obtained from the dependence of the maximum absorption per wavelength on the guest concentration. The rate constant for the inclusion process of the guest into a cavity of beta-CD and that for the leaving process from the cavity were determined from the obtained relaxation frequency and the equilibrium constant. The standard volume change of the reaction was also computed from the maximum absorption per wavelength. These results were compared with those in solutions containing both beta-CD and different guest molecules. It was found that the hydrophobicity of guest molecules played an important role in the formation of the inclusion complex and also that the charge on the carboxylic group had a considerable effect on the kinetic characteristics of the complexation reaction.

Assembly behavior of inclusion complexes of β-cyclodextrin with 4-hydroxyazobenzene and 4-aminoazobenzene

To further reveal the factors governing the supramolecular assembly of beta-cyclodextrin (beta-CD) inclusion complexes, two aggregates (1 and 2) were prepared from the inclusion complexes of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene, respectively, and their binding behavior were investigated by means of X-ray analysis, UV-vis, NMR, and circular dichroism spectra in both solution and the solid state. The obtained results indicated that the beta-CD/4-hydroxyazobenzene complex 1 could form head-to-head dimers (triclinic system, space group P1) in the solid state, which were further self-assembled to a linear supramolecular architecture by the intra- and interdimer hydrogen bond interactions as well as the intradimer pi-pi interactions. However, when the included guest 4-hydroxyazobenzene was switched to a 4-aminoazobenzene, the resultant beta-CD/4-aminoazobenzene complex 2 (monoclinic system, space group P2(1)) could be self-assembled to a wave-type supramolecular aggregate under similar conditions. Furthermore, the combination of crystallographic and spectral investigations jointly revealed the inclusion complexation geometry of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene in both solution and the solid state, which demonstrated that the disparity of substituents in the azobenzenes played an important role in the inclusion complexation and molecular assembly, affecting not only the structural features of aggregates but also the binding abilities of azobenzenes with beta-CD.