EPR and pulse radiolysis studies on electron transfer in transition metal dithiolenes at the surface of colloidal TiO₂

One-electron reduction of several transition metal bis-dithiolenes at the surface of TiO₂ particles and in aqueous solutions were studied. Electron Paramagnetic Resonance (EPR) was used to study the reduction at the particle surface and the pulse radiolysis technique was employed for aqueous solutions. Variable temperature EPR measurements following band-gap excitation of the particles showed the disappearance of signals due to the paramagnetic Ni(III) and Cu(II) complexes of 4,5-dimercaptophthalic acid (DMPA), 3,4-dimercaptotoluene (TDT) and maleonitrile dithiolate (MNT), with the surface electron signals vanishing concomitantly. Similarly, the diamagnetic Cu(III) complex of TDT was reduced to the paramagnetic Cu(II) complex. The bimolecular rate constants for the reaction of Ni(III)DMPA and Ni(II)MNT with ea-qwere determined to be 4.3 × 10⁹ dm³ mol^-1 s^-1 and 1.3× 10¹⁰ dm³ mol^-1 s^-1, respectively. Pulse radiolysis results were used to distinguish between reduction with electron and · CH₂OH radical. It is inferred that ea-q · reduces the metal centre while · CH₂OH adds to the ligand.