Inclusion Complexes between Amphiphilic Phenyleneethynylene Fluorophores and Cyclodextrins in Aqueous Media

Abstract

Binding events of cyclodextrins (CyD's) in aqueous media are important for designing and explaining the host-guest chemistry applied in sensing and controlled release systems. A water-soluble tricationic compound (3N(+)) with three branches of phenyleneethynylene fluorescent moieties and its related amphiphilic compounds (3C(-), N(0)N(+), N(+), and 2N(+)) are employed as molecular probes in the systematic characterization of the supramolecular interactions with CyD's (α, β, and γ). The strong fluorescence enhancement, combined with induced circular dichroism (CD) signals and (1)H NMR data, is evidence of 1:1 static inclusion complexes of 3N(+)/γ-CyD and 2N(+)/γ-CyD. 3N(+) presents a structural design which can form inclusion complexation with γ-CyD with one of the highest binding constants of 3.0 × 10(4). The relatively moderate fluorescence enhancement, shift of (1)H NMR signals, and weak induced CD signals indicate fast exchange complexation of β-CyD with the amphiphilic guest molecules. The interaction with α-CyD is perceived only for N(0)N(+), the only nonbranched fluorescent guest model, via its strong fluorescence enhancement. However, the lack of (1)H NMR signal splitting and the lack of induced CD signals suggest the noninclusion mode of binding between N(0)N(+) and α-CyD.