Tuning the Electrochemistry of Free-Base Porphyrins in Acidic Nonaqueous Media: Influence of Solvent, Supporting Electrolyte, and Ring Substituents

A detailed study of reduction potentials, electroreduction mechanisms, and acid-base chemistry was carried out on two series of free-base porphyrins in nonaqueous media. The first series is represented by four -pyrrole-substituted tetraphenylporphyrin (TPP) derivatives, two of which are planar and two of which are nonplanar in their nonprotonated form. The second comprises porphyrins with 0-4 meso-phenyl groups on the macrocycle. Equilibrium constants for the conversion of each neutral porphyrin to its diprotic [H4P](2+) form were determined and the electrochemistry was then elucidated as a function of: 1)type of nonaqueous solvent, 2)anion of supporting electrolyte, 3)porphyrin ring substituents, and 4)concentration of acid added to solution. Spectroelectrochemistry was used to characterize absorption spectra of each electroreduced species and, when combined with results of the above studies, significantly improves our ability to tune redox reactivity of these types of compounds.