The photoreactivity of the water-soluble zinc derivative of 5,10,15,20-tetra(N-methyl-pyridinium-3-yl)porphyrin [Zn(MePy)₄P⁴⁺] has been investigated using the laser-photolysis technique and shown to be the same as that of the pyridinium-4-yl isomer. Electron transfer from the triplet state of this porphyrin to methyl viologen is greatly influenced by coulombic effects. In the presence of chloride ions, the existence of (MV²⁺·Cl^–) ion pairs could explain the observed kinetics for direct electron transfer. Cupric ion quenches the triplet, and the second-order rate constant k_{T + Cu²⁺}=(1.5 + 0.1)× 10⁶ dm³ mol^–1 s^–1 is similar to that observed with methyl viologen. However, no indication of a photoredox process was found.

We have also measured the rate constants of the (T + T) reaction, 2k_TT=(3.1 ± 0.4)× 10⁷ dm³ mol^–1 s^–1, and the (T + O₂) reaction, k_{T + O2}=(1.2 ± 0.1)× 10⁹ dm³ mol^–1 s^–1.