A kinetic and equilibrium study of the inclusion of pyronine B by β- and γ-cyclodextrin

Abstract

Temperature-jump visible spectrophotometric studies show the dye pyronine B (PB) in its monovalent cationic form to be included by γ-cyclodextrin (γCD) in a fast step to form a 1:1 complex (PB·γCD), which is followed by the slower formation of the 2:1 complex (PB)₂·γCD, in which pyronine B dimerises: PB +gamma;CD [graphic omitted] PB·gamma;CD (K₁)(fast)(1), PB + PB·γCD [graphic omitted] (PB)₂·γCD (K₂)(slow)(2) where, in aqueous 1.00 mol dm^–3 NaCl at pH 5.70 and 298.2 K, K₁=(4.3 ± 0.1)× 10² dm³ mol^–1, K₂=(1.28 ± 0.04)× 10⁵ dm³ mol^–1, k₂=(8.2 ± 0.2)× 10⁸ dm³ mol^–1 s^–1 and k_–2=(6.40 ± 0.05)× 10³ s^–1. In the presence of βCD only the formation of PB·βCD is detected and is characterised by k₁=(1.1 ± 0.1)× 10⁸ dm³ mol^–1 s^–1 and k_–1=(1.5 ± 0.5)× 10⁴ s^–1. Equilibrium u.v.–visible spectrophotometric measurements are consistent with these kinetic data. No inclusion of PB by αCD was detected. These data are compared with those characterising related systems. The dimerisation of PB in the absence of γCD is fast, an observation which forms the basis for some of the mechanistic deductions concerning the formation of (PB)₂·γCD, and also for a reassessment of some mechanistic interpretion in earlier studies.