Photoreduction of methyl viologen sensitised by the excited singlet state of a magnesium phthalocyanine

Abstract

Methyl viologen quenches the singlet excited state of a positively charged magnesium phthalocyanine but not the triplet excited state. In aqueous solution, the singlet quenching rate constant depends upon ionic strength but at zero ionic strength it approaches the diffusion-controlled limit. Overall, the quenching act leads to net electron transfer but the probability of separation of the singlet ion-pair is very low. The efficiency of this separation process is related to the electronic charge on the reactants, the spin-multiplicity of the ion-pair and to the amount of free energy that must be dissipated by the competing internal conversion process.

Although the system possesses the capability of collecting ca. 45% of the solar spectrum, irradiation in the presence of EDTA and a colloidal Pt catalyst does not produce H₂.