Direct evidence of clay-mediated charge transfer

The charge transfer (CT) from montmorillonite K10 (iron-containing clay) was studied by monitoring the reaction of halides (Cl^-, Br^-, and I^-) with the intercalated photosensitizer, 10-methylacridinium (AcH⁺). The electrodes, which were fabricated by the AcH⁺-intercalated clays (denoted clay/AcH⁺), showed no current responses to bromide and chloride. However, there was a significant current response corresponding to the reduction of iodine and this response was enhanced by light irradiation. These differences in the electrochemical behavior suggested the electron transfer on the clay/AcH⁺ surface was regulated by the electrochemically active iron species in the clay. The lifetime of AcH⁺ is ca. 36 ns, whereas for AcH⁺_(clay), it is as fast as the instrumental response (<10 ns), indicating the clay can efficiently quench the excited acridinium, AcH⁺*. Moreover, a long-lived species, attributed to I₂^-., was recorded in KI solutions (pH 4) containing either AcH⁺ or clay/AcH⁺. In contrast, the transient species, presumed to be Br₂^-., was only observed in the solutions containing free AcH⁺. Both electrochemical and laser spectroscopic results indicated that the electron transfer between AcH⁺* and I^- is mediated by the iron species in montmorillonite K10.