One-electron Reduction and Electron Transfer Reaction Controlled by Cyclodextrin Inclusion Complexation

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Cyclodextrins (CDs) form inclusion complexes with various organic molecules in aqueous solution depending on the sizes of their hydrophobic cavities and of the guest molecules. The preset pulse radiolysis study is concerned with the one-electron reduction of aromatic carboxylates and sulfonates controlled by the inclusion comlexation. The pulse radiolysis of aqueous solutions of the aromatic electrolytes results in the one-electron reduction of the aromatic moieties of the solutes by the hydrated electron. The reaction is effectively retarded when the hydrophobic aromatic moieties are included in the CD cavities. Thus, the rate constants for the one-electron reduction of biphenyl-4-carboxylate (BPC), biphenyl-4-asetate (BPA) and biphenyl-4-sulfonate (BPS) are appreciably reduced by the complexation with beta-CD,whose cavity best fits to the biphenylyl group. Similarly, the one-electron reduction of pyrene-1-carboxylate (PyC), 4-pyren-1-ylbutyrate (PyB) and pyrene-1-sulfonate (PyS) is r … More etarded by the best fitting gamma-CD.The control of the one-electron reduction and electron transfer reactions has been examined for the solutions of mixtures of the sulfonates. The selective reduction of a mixed BPS and PyS solution has been achieved by using beta-and gamma-CDs. The electron transfer from the reduced BPS to PyS is also retarded effectively by the complexation of PyS with gamma-CD.The pH dependence of the effect of the complexation has demonstrated that a 1 ; 2 complex is formed between BPC and alpha-CD but not between BPC and alpha-CD because of the steric effect of the methylene group of BPA.The apparent retarding effect of alpha-CD on the one-electron reduction of BPC is observed at much higer host concentrations compared with that of beta-CD.PyC,PyB and PyS form 2 ; 2 complexes with gamma-CD,and the complexes of PyC and PyS undergo a structural change upon the one-electron reduction of the guests. The association constants of the 1 ; 1 complexes of BPC,BPA and BPS have been determined from the pulse radiolysis data. Less