Comparison of solid state voltammetry and membrane voltammetry: The reduction of 4,4′,4″,4‴-(21H,23H-porphine-5,10,15,20-tetrayl) tetrakis[1-octylpyridinium] tetrabromide immobilised as microcrystalline solid and accumulated into a mesoporous TiO₂ phytate membrane

The tetra-cationic free base porphyrin, 4,4′,4″,4‴-(21H,23H-porphine-5,10,15,20-tetrayl) tetrakis[1-octylpyridinium]⁴⁺ (PTTO⁴⁺) shares the properties of both a long alkyl chain containing molecule with hydrophobic shell and a highly charged water soluble molecule with hydrophilic core. As a result, PTTO⁴⁺ is only very sparingly water soluble and tends to aggregate on suitable surfaces. It is shown here that the electrochemical reduction of PTTO⁴⁺ in aqueous media can be studied either by 'solid state voltammetry' or by 'membrane voltammetry' methodology with complementary results. When immobilised by adhering the microcrystalline powder to the surface of a basal plane pyrolytic graphite electrode, PTTO⁴⁺ exhibits two separate one electron-one proton reduction responses, which are both coupled to chemical follow-up processes (assumed here is a H-shift reaction). Both reduction responses become reversible at sufficiently high scan rates. In contrast, when accumulated into a TiO₂ phytate membrane, PTTO⁴⁺ exhibits a chemically reversible 2 electron-2 proton reduction response. Upon increasing of the concentration of PTTO⁴⁺ in the membrane or upon increasing the thickness of the membrane more complex voltammetric responses are detected. High local 'concentration' conditions during solid state voltammetry experiments may be regarded as a limiting case of high concentration membrane voltammetry at very thin membranes.