Charge separation and photoreduction of zinc tetrakis(sulfonatophenyl)porphyrin by nitrobenzene and methylviologen in aqueous solutions

The photochemical reactions of zinc tetrakis(sulfonatophenyl)porphyrin (ZnTPPS^4-) in aqueous solutions containing nitrobenzene (NB) or methylviologen (MV²⁺) are reported. NB quenches the singlet state by electron transfer without charge separation. When the triplet is quenched by NB, charge-transfer products are observed. The rate constants for quenching and back electron transfer are reported. MV²⁺ quenches the singlet through a static effect of complexation. The stability constant of the complex is reported. In the presence of high concentrations of NaClO₄ and ZnSO₄ the complexation is avoided. Under these conditions the triplet is formed and quenched through electron transfer. Charge separation is reported. Similar results were obtained when a negative polyelectrolyte poly(vinyl sulfonate) (PVS) was introduced. PVS completely avoided the complexation and enabled charge separation after photoreduction of the triplet.