Photoelectrochemical oxidation of 2-mercaptoethanol at the surface of octacyanophthalocyanine thin film electrodes

Thin films of octacyanophthalocyanine metal complexes (1) on ITO substrates have been studied in photoelectrochemical experiments in comparison with tetrapyridotetraazaporphyrinatozinc (2), tetrapyrazinotetraazaporphyrinatozinc (3) and unsubstituted phthalocyaninatozinc (4) in contact with aqueous redox electrolytes containing 2-mercaptoethanol or hydroquinone. At electrodes of the zinc(II) complex la and at electrodes of 2 and 3 the current under illumination exceeds that in the dark by a factor of up to five, whereas no significant photocurrent is observed at electrodes of 4. The relative concentrations of electrons in the LUMO (CB) and defect-electrons in the HOMO (VB) are influenced by a changing electron density within the inner ring pi-system, leading to the observed differences in the semiconducting properties. Electrodes of la were investigated in further detail. In spite of the presence of thiyl-radicals the electrodes performed at a high stability. The thiolate anion is found to be the electroactive species. The dependence of the photocurrent on the reactant concentration could be attributed to an adsorption step. A model including charging and discharging processes of these surface states is presented for the analysis of photocurrent transients as obtained by a chopped light beam.