A negative temperature dependence of the electron self-exchange rates of zinc porphyrin π radical cations

The one-electron oxidation of ZnT(t-Bu)PP (T(t-Bu)PP²- = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin dianion) by one equivalent of Ru(bpy)₃³⁺ (bpy = 2,2′-bipyridine) results in quantitative formation of ZnT(t-Bu)PP^•+ which is detected by ESR. In the presence of excess ZnT(t-Bu)PP, the ESR line width becomes broader with increasing ZnT(t-Bu)PP concentration due to the electron self-exchange between ZnT(t-Bu)PP^•+ and ZnT(t-Bu)PP. The line width of the ESR signal of ZnT(t-Bu)PP^•+ becomes broader as the temperature is decreased from 313 to 233 K. This indicates that the electron self-exchange reaction becomes faster at a lower temperature. The substituent and solvent effects on such a negative temperature dependence of the electron self-exchange rates are reported.