Pulse radiolysis of aromatic carboxylates in aqueous solution and effect of cyclodextrin complexation on the one-electron reduction by the hydrated electron

Abstract

Pulse radiolysis of Ar-saturated aqueous solutions of biphenyl-4-ylacetate (BPA^–), biphenyl-4-carboxylate (BPC^–), 4-pyren-1-ylbutyrate (PyB^–) and pyrene-1-carboxylate (PyC^–) results in the one-electron reduction of the solutes by the hydrated electron. The transient absorption spectra of the reduced aromatic carboxylates are analogous to those of the radical anions of the corresponding aromatic hydrocarbons, suggesting that the radical anion site is located on the aromatic moieties. The transient absorption spectrum of the reduced BPC^– is shifted to shorter wavelengths with time after the pulse at pH below ca. 10. The spectral shift has been attributed to the association of the reduced BPC^– with H⁺: (BP˙^–)COO^–+ H⁺→(BP˙^–)CO₂H, where BP denotes biphenylyl. In the case of biphenyl-2-carboxylate, no absorption bands like those of the biphenyl radical anion were observed. This means that the steric effect of the carboxy group prevents the formation of biphenyl-type radical anions, which have a more coplanar structure than the parent molecule. Effect of cyclodextrin (CD) complexation of the hydrophobic moieties of the carboxylates has been investigated by using α-, β- and γ-CDs. The rate constants for the one-electron reduction of BPA^– and BPC^– are effectively reduced by the addition of β-CD, whose cavity best fits to the biphenylyl group. Similarly, the one-electron reduction of PyB^– and PyC^– is retarded by the best fitting γ-CD. The retarding effect of the CD complexation is greater for BPA^– and PyB^– having –CH₂– and –(CH₂)₃–, respectively, between the aromatic and carboxy groups than for BPC^– and PyC^– not having such a spacer. The transient absorption spectrum of reduced PyC^– in the presence of γ-CD is shifted to shorter wavelengths with time after the pulse. Such a spectral shift is not observed for PyB^–. It has been proposed that the 2 : 2 complex of PyC^– with γ-CD undergoes a structural change upon one-electron reduction.