Complexation of Crystal Violet, Pyronine B, and Rhodamine B by Linked β-Cyclodextrin Trimers

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≤0.10moldm-3 has been studied by UV-Vis and 1H NMR spectroscopy. The complexation constants 10-3K1 (298.2K) are 4.68, 86.3, and 7.08dm3mol-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 ΔHo and TΔSo 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 (298.2K)≤2.14, 4.57, and 3.86dm3mol-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.