Molecular Recognition Studies on Supramolecular Systems 34. Synthesis of Aromatic Diamino-bridged Bis(-cyclodextrin)s and their Inclusion Complexation with Dye Molecules

A series of 6,6′-bis(-cyclodextrin)s with rigid aromatic diamino tethers, i.e. p-phenylenediamino-bridged-bis(6-amino-6-deoxy--cyclodextrin) (3), 4,4′-bianilino-bridged-bis(6-amino-6-deoxy--cyclodextrin) (4) and 3,3′-bianilino-bridged-bis(6-amino-6-deoxy--cyclodextrin) (5), have been synthesized by the reaction of mono[6-O-(p-toluenesulfonyl)]-(-cyclodextrin) with corresponding materials. The inclusion complexation behavior of native -cyclodextrin (1), mono-(6-anilino-6-deoxy)--cyclodextrin (2), and novel bis(-cyclodextrin) 3-5 with some representative dyes, i.e. ammonium 8-anilino-1-naphthalenesulfonate (ANS), Brilliant Green, Methyl Orange, Acridine Red and Rhodamine B, was investigated at 25°C in aqueous phosphate buffer solution (pH 7.20) by means of fluorescence, ultraviolet, circular dichroism spectrometry as well as fluorescence lifetime measurement. The spectrophotometric titrations gave the complex stability constants (K_S) and Gibbs free energy changes (ΔG⁰) for the stoichiometric 1:1 inclusion complexation of hosts examined with dye molecules. As compared with 1 or 2, bridged bis(-cyclodextrin)s displayed significantly enhanced binding abilities towards these dyes. Typically, dimer 3 showed the highest binding ability upon inclusion complexation with acridine red affording 17 times higher K_S for 3 than for 1. The molecular binding abilities and selectivities of dyes by bridged bis(-cyclodextrin)s have been discussed from the viewpoint of induced-fit interaction and multipoint recognition mechanism.