Photocatalysis with Polyoxometalates Associated to Porphyrins under Visible Light: An Application of Charge Transfer in Electrostatic Complexes

Abstract

Absorption spectrum of derived Dawson sandwich polyoxometalates (POM) [M₄(H₂O)₂(P₂W₁₅O₅₆)₂]ⁿ⁻ with n = 16 for M = Zn^II, Ni^II, and n = 12 for M = Fe^III have been extended in the visible range forming electrostatic complexes with a chromophore, the zinc tetracationic porphyrin [ZnTMePyP]⁴⁺. Formation of such complexes was followed by steady-state absorption and luminescence spectroscopies. The electrostatic complexation gives in all cases a strong, neutral, and nonluminescent complex. A charge transfer between the two units was shown by transient absorption spectroscopy. Upon a visible excitation of the porphyrin subunit, an electron transfer from the porphyrin to the POM occurs and imparts it a catalytic activity. This has been demonstrated studying a model reaction such as the reduction of silver cations leading to nanoparticles. In all cases, the reduction of the silver cations takes place. We showed that the catalytic activity depends of the nature of the metal of the tetraoxometallic central cluster of the Dawson sandwich POM.