Complexation of crystal violet, pyronine B, and rhodamine B by linked β-Cyclodextrin trimers

Abstract

The host-guest complexation by β-cyclodextrin (βCD), 1,3,5-N,N,N-tris-(6ᴬ-deoxy-6ᴬ-β-cyclodextrin)-benzene (βCD₃bz), and 1,3,5-N,N,N-tris(6ᴬ-(2-aminoethyl)amino-6ᴬ-deoxy-6ᴬ-β-cyclodextrin)-benzene (βCDen₃bz) of cationic crystal violet (CV⁺) and pyronine B (PB⁺) and zwitterionic rhodamine B (RB) in aqueous phosphate buffer at pH 7.0 and I = 0.10 mol dm⁻³ has been studied by UV-Vis and ¹H NMR spectroscopy. The complexation constants 10⁻³K₁ (298.2 K) are 4.68, 86.3, and 7.08 dm³ mol⁻¹ for the 1 : 1 host-guest complexes: βCD.CV⁺, βCD₃bz.CV⁺, βCDen₃bz.CV⁺, respectively, and those for the analogous PB⁺ and RB complexes have also been determined. The associated ΔH° and TΔS° for all nine complexes coincide with an entropy-enthalpy compensation plot for the formation of a wide range of βCD and modified βCD host-guest complexes reported in the literature. Crystal violet also forms (βCD)₂.CV⁺, (βCD₃bz)₂.CV⁺ and (βCDen₃bz)₂.CV⁺ complexes characterised by 10⁻²K₂ (298.2 K) = 2.14, 4.57, and 3.86 dm³ mol⁻¹ and analogous (βCD)₂.PB⁺, (βCDen₃bz)₂.PB⁺ and (βCDen₃bz)₂.RB complexes also form, but the (βCD₃bz)₂.PB⁺, (βCD)₂.RB, and (βCD₃bz)₂.RB complexes were not detected. The effects of the structures of the hosts and guests on the complexation processes are discussed.