Charge-transfer-to-solvent photochemistry of electrode-confined ferrocene- and cobaltocene-based polymers: Photoelectrochemical reduction of halocarbons

Like the metallocenes themselves, metallocene-based polymers exhibit near-UV (280-370 nm) charge-transfer-to-solvent (CTTS) absorption in the presence of CCl₄, CHCl₃, CH₂Cl₂, CBr₄, and CHBr₃. The photoelectrochemistry of charge transfer complexes of two ferrocene-containing polymers and one cobaltocene-containing polymer has been studied. The polymers are poly(2-ferrocenylethyl methacrylate), poly(3-(octamethylferrocenyl)propyl methacrylate), and poly(1,1′-bis[((3-(triethoxysilyl)propyl)amino)carbonyl]cobaltocene). Photoexcitation of metallocenes and metallocene-based polymers in the presence of many halocarbons yields oxidation of the metallocene and reduction of the halocarbon. When the metallocene-based polymer is confined to the surface of an electrode that is held at a potential negative of the formal potential of the metallocene, near-UV excitation results in sustained cathodic current in electrolyte solutions containing halocarbons. The wavelength, acceptor, and potential dependences are in accord with a sustained current that is due to a metallocene-to-halocarbon CTTS absorption where the photoprocess results in the reduction of the halocarbon at an electrode potential significantly positive of where electrochemical reduction occurs in the dark. The octamethylferrocene-based system shows a more negative potential onset and a longer wavelength offset of photocurrent than the simple ferrocene-based system, consistent with the electron-releasing nature of the methyl substituents. The onset of photocurrent in the cobaltocene-based system occurs at the most negative potential of the three, consistent with the cobaltocene-based system having the most negative formal potential of the metallocenes studied.