Insight into Unusual Downfield NMR Shifts in the Inclusion Complex of Acridine Orange with Cucurbit[7]uril

Abstract

AbstractThis paper describes the structure and properties of the complex of acridine orange (AO) with cucurbit[7]uril (CB[7]) in aqueous solutions, studied by 1H NMR spectroscopy, matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI–TOF) mass spectrometry, UV/Vis spectroscopy, and fluorescence spectroscopy with the aid of competitive binding methods. It was found that AO was included in the CB[7] cavity through strong ion–dipole interactions and hydrogen bonds. Theoretical studies based on calculations performed from first principles further confirmed this binding mode. Unusual downfield NMR shifts of the AO proton resonances in the presence of CB[7] were observed, in sharp contrast with cases of inclusion of other organic molecules in the CB[7] cavities, which resulted in upfield shifts of their proton resonances. The downfield shifts of the proton resonances of the AO·CB[7] complex were found to be the net result of small upfield shifts arising from the inclusion of AO in the CB[7] cavity and large downfield shifts resulting from the deaggregation of the AO aggregates, which were readily formed even at low concentrations in aqueous solutions. Further theoretical calculations explained the unusual downfield NMR shifts well. These research results are of importance for understanding of the structures and properties of complexes of related organic molecules with macrocyclic hosts. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)