Bridged bis(β-cyclodextrin)s possessing coordinated metal center(s) and their inclusion complexation behavior with model substrates: Enhanced molecular binding ability by multiple recognition

To investigate quantitatively the cooperative binding ability of several β-cyclodextrin oligomers bearing single or multiligated metal center(s), the inclusion complexation behavior of four bis(βcyclodextrin)s (2-5) linked by 2,2′-bipyridine-4,4′-dicarboxy tethers and their copper(II) complexes (6-9) with representative dye guests, i.e., methyl orange (MO), acridine red (AR), rhodamine B (RhB), ammonium 8-anilino-1-naphthalenesulfonic acid (ANS), and sodium 6-(p-toludino)-2-naphthalenesulfonate (TNS), have been examined in aqueous solution at 25 °C by means of UV-vis, circular dichroism, fluorescence, and 2D NMR spectroscopy. The results obtained indicate that bis(β-cyclodextrin)s 2-5 can associate with one or three copper(II) ion(s) producing 2:1 or 2:3 bis-(β-cyclodextrin)-copper(II) complexes. These metal-ligated oligo(β-cyclodextrin)s can bind two model substrates to form intramolecular 2:2 host-guest inclusion complexes and thus significantly enhance the original binding abilities of parent β-cyclodextrin and bis(β-cyclodextrin) toward model substrates through the cooperative binding of two guest molecules by four tethered cyclodextrin moieties, as well as the additional binding effect supplied by ligated metal center(s). Host 6 showed the highest enhancement of the stability constant, up to 38.3 times for ANS as compared with parent β-cyclodextrin. The molecular binding mode and stability constant of substrates by bridged bis- and oligo(β-cyclodextrin)s 2-9 are discussed from the viewpoint of the size/shape-fit interaction and molecular multiple recognition between host and guest.